Basic & clinical pharmacology 3

CHAPTER

39 Adrenocorticosteroids & Adrenocortical Antagonists George P. Chrousos, MD

CASE STUDY A 19-year-old man complains of anorexia, fatigue, dizziness, and weight loss of 8 months’ duration. The examining physician discovers postural hypotension and moderate vitiligo (depigmented areas of skin) and obtains routine blood tests. She finds hyponatremia, hyperkalemia, and acidosis and suspects Addison’s disease. She performs a standard ACTH 1–24 stimulation test, which reveals an insufficient plasma cortisol response, compatible with primary adrenal insufficiency. The diagnosis of autoimmune Addison’s disease is made, and the patient must start replacement of the hormones he cannot produce himself. How should this patient be treated? What precautions should he take?

The natural adrenocortical hormones are steroid molecules produced and released by the adrenal cortex. Both natural and synthetic corticosteroids are used for the diagnosis and treatment of disorders of adrenal function. They are also used—more often and in much larger doses—for treatment of a variety of inflammatory and immunologic disorders.

Secretion of adrenocortical steroids is controlled by the pituitary release of corticotropin (ACTH). Secretion of the salt-retaining hormone aldosterone is primarily under the influence of angiotensin. Corticotropin has some actions that do not depend on its effect on adrenocortical secretion. However, its pharmacologic value as an anti-inflammatory agent and its use in testing adrenal function depend on its secretory action. Its pharmacology is discussed in Chapter 37 and is reviewed only briefly here.

Inhibitors of the synthesis or antagonists of the action of the adrenocortical steroids are important in the treatment of several conditions. These agents are described at the end of this chapter.

ADRENOCORTICOSTEROIDS The adrenal cortex releases a large number of steroids into the circulation. Some have minimal biologic activity and function primarily as precursors, and there are some for which no function has been established. The hormonal steroids may be classified as those having important effects on intermediary metabolism and immune function (glucocorticoids), those having principally salt-retaining activity (mineralocorticoids), and those having androgenic or estrogenic activity (see Chapter 40). In humans, the major glucocorticoid is cortisol and the most important mineralocorticoid is aldosterone. Quantitatively, dehydroepiandrosterone (DHEA) in its sulfated form (DHEAS) is the major adrenal androgen. However, DHEA and two other adrenal androgens, androstenedione and androstenediol, are weak androgens and androstenediol is a potent estrogen. Androstenedione can be converted to testosterone and estradiol in extra-adrenal tissues (Figure 39–1). Adrenal androgens constitute the major endogenous precursors of estrogen in women after menopause and in younger patients in whom ovarian function is deficient or absent.

THE NATURALLY OCCURRING GLUCOCORTICOIDS; CORTISOL (HYDROCORTISONE) Pharmacokinetics Cortisol (also called hydrocortisone, compound F) exerts a wide range of physiologic effects, including regulation of intermediary metabolism, cardiovascular function, growth, and immunity. Its synthesis and secretion are tightly regulated by the central nervous system, which is very sensitive to negative feedback by the circulating cortisol and exogenous (synthetic) glucocorticoids. Cortisol is synthesized from cholesterol (as shown in Figure 39–1). The mechanisms controlling its secretion are discussed in Chapter 37.

In the normal adult, in the absence of stress, 10–20 mg of cortisol is secreted daily. The rate of secretion follows a circadian rhythm

(Figure 39–2) governed by pulses of ACTH that peak in the early morning hours and after meals. In plasma, cortisol is bound to circulating proteins. Corticosteroid-binding globulin (CBG), an α2 globulin synthesized by the liver, binds about 90% of the circulating hormone under normal circumstances. The remainder is free (about 5–10%) or loosely bound to albumin (about 5%) and is available to exert its effect on target cells. When plasma cortisol levels exceed 20–30 mcg/dL, CBG is saturated, and the concentration of free cortisol rises rapidly. CBG is increased in pregnancy and with estrogen administration and in hyperthyroidism. It is decreased by hypothyroidism, genetic defects in synthesis, and protein deficiency states. Albumin has a large capacity but low affinity for cortisol, and for practical purposes albumin-bound cortisol should be considered free. Synthetic corticosteroids such as dexamethasone are largely bound to albumin rather than CBG.

FIGURE 39–2 Circadian variation in plasma cortisol throughout the 24-hour day (upper panel). The sensitivity of tissues to glucocorticoids is also circadian but inverse to that of cortisol, with low sensitivity in the late morning and high sensitivity in the evening and early night (lower panel). The sensitivity of tissues to glucocorticoids is inversely related to that of glucocorticoid receptor (GR) acetylation by the transcription factor CLOCK; the acetylated receptor has decreased transcriptional activity. (Adapted, with permission, from Nader N, Chrousos GP, Kino T: Interactions of the circadian CLOCK system and the HPA axis. Trends Endocrinol Metab 2010;21:277. Copyright Elsevier.)

The half-life of cortisol in the circulation is normally about 60–90 minutes; it may be increased when hydrocortisone (the pharmaceutical preparation of cortisol) is administered in large amounts or when stress, hypothyroidism, or liver disease is present. Only 1% of cortisol is excreted unchanged in the urine as free cortisol; about 20% of cortisol is converted to cortisone by 11-hydroxysteroid dehydrogenase in the kidney and other tissues with mineralocorticoid receptors (see below) before reaching the liver. Most cortisol is metabolized in the liver. About one third of the cortisol produced daily is excreted in the urine as dihydroxy ketone metabolites and is measured as 17-hydroxysteroids (see Figure 39–3 for carbon numbering). Many cortisol metabolites are conjugated with glucuronic acid or sulfate at the C3 and C21 hydroxyls, respectively, in the liver; they are then excreted in the urine.

FIGURE 39–3 Chemical structures of several glucocorticoids. The acetonide-substituted derivatives (eg, triamcinolone acetonide) have increased surface activity and are useful in dermatology. Dexamethasone is identical to betamethasone except for the configuration of the methyl group at C16 : in betamethasone it is beta (projecting up from the plane of the rings); in dexamethasone it is alpha.

In some species (eg, the rat), corticosterone is the major glucocorticoid. It is less firmly bound to protein and therefore metabolized more rapidly. The pathways of its degradation are similar to those of cortisol.

Pharmacodynamics A. Mechanism of Action Most of the known effects of the glucocorticoids are mediated by widely distributed glucocorticoid receptors. These proteins are members of the superfamily of nuclear receptors, which includes steroid, sterol (vitamin D), thyroid, retinoic acid, and many other receptors with unknown or nonexistent ligands (orphan receptors). All these receptors interact with the promoters of—and regulate the transcription of—target genes (Figure 39–4). In the absence of the hormonal ligand, glucocorticoid receptors are primarily cytoplasmic, in oligomeric complexes with chaperone heat-shock proteins (hsp). The most important of these are two molecules of hsp90, although other proteins (eg, hsp40, hsp70, FKBP5) are certainly involved. Free hormone from the plasma and interstitial fluid enters the cell and binds to the receptor, inducing conformational changes that allow it to dissociate from the heat shock proteins. The dimeric ligand-bound receptor complex then is actively transported into the nucleus, where it interacts with DNA and nuclear proteins. As a homodimer, it binds to glucocorticoid receptor elements (GREs) in the promoters of responsive genes. The GRE is composed of two palindromic sequences that bind to the hormone receptor dimer.

FIGURE 39–4 A model of the interaction of a steroid, S (eg, cortisol), and its receptor, R, and the subsequent events in a target cell.

The steroid is present in the blood in bound form on the corticosteroid-binding globulin (CBG) but enters the cell as the free molecule.

The intracellular receptor is bound to stabilizing proteins, including two molecules of heat-shock protein 90 (hsp90) and several others including FKBP5, denoted as “X” in the figure. This receptor complex is incapable of activating transcription. When the complex binds a molecule of cortisol, an unstable complex is created and the hsp90 and associated molecules are released. The steroid-receptor complex is now able to dimerize, enter the nucleus, bind to a glucocorticoid response element (GRE) on the regulatory region of the gene, and regulate transcription by RNA polymerase II and associated transcription factors. A variety of regulatory factors (not shown) may participate in facilitating (coactivators) or inhibiting (corepressors) the steroid response. The resulting mRNA is edited and exported to the cytoplasm for the production of protein that brings about the final hormone response. An alternative to the steroid-receptor complex interaction with a GRE is an interaction with and altering the function of other transcription factors, such as NF-κB in the nucleus of cells.

In addition to binding to GREs, the ligand-bound receptor also forms complexes with and influences the function of other transcription factors, such as AP1 and nuclear factor kappa-B (NF-κB), which act on non-GRE-containing promoters, to contribute to the regulation of transcription of their responsive genes. These transcription factors have broad actions on the regulation of growth factors, proinflammatory cytokines, etc, and to a great extent mediate the anti-growth, anti-inflammatory, and immunosuppressive effects of glucocorticoids.

Two genes for the corticoid receptor have been identified: one encoding the classic glucocorticoid receptor (GR) and the other encoding the mineralocorticoid receptor (MR). Alternative splicing of human glucocorticoid receptor pre-mRNA generates two highly homologous isoforms, termed hGRα and hGRβ. Human GRα is the classic ligand-activated glucocorticoid receptor which, in the hormone-bound state, modulates the expression of glucocorticoid-responsive genes. In contrast, hGRβ does not bind glucocorticoids and

is transcriptionally inactive. However, hGRβ is able to inhibit the effects of hormone-activated hGRα on glucocorticoid-responsive genes, playing the role of a physiologically relevant endogenous inhibitor of glucocorticoid action. It was recently shown that the two hGR alternative transcripts have eight distinct translation initiation sites; ie, in a human cell there may be up to 16 GRα and GRβ isoforms, which may form up to 256 homodimers and heterodimers with different transcriptional and possibly nontranscriptional activities. This variability suggests that this important class of steroid receptors has complex stochastic activities. In addition, rare mutations in hGR may result in partial glucocorticoid resistance. Affected individuals have increased ACTH secretion because of reduced pituitary feedback and additional endocrine abnormalities (see below).

The prototype GR isoform is composed of about 800 amino acids and can be divided into three functional domains (see Figure 2–6).

The glucocorticoid-binding domain is located at the carboxyl terminal of the molecule. The DNA-binding domain is located in the middle of the protein and contains nine cysteine residues. This region folds into a “two-finger” structure stabilized by zinc ions connected to cysteines to form two tetrahedrons. This part of the molecule binds to the GREs that regulate glucocorticoid action on glucocorticoidregulated genes. The zinc fingers represent the basic structure by which the DNA-binding domain recognizes specific nucleic acid sequences. The amino-terminal domain is involved in the transactivation activity of the receptor and increases its specificity.

The interaction of glucocorticoid receptors with GREs or other transcription factors is facilitated or inhibited by several families of proteins called steroid receptor coregulators, divided into coactivators and corepressors. The coregulators do this by serving as bridges between the receptors and other nuclear proteins and by expressing enzymatic activities such as histone acetylase or deacetylase, which alter the conformation of nucleosomes and the transcribability of genes.

Between 10% and 20% of expressed genes in a cell are regulated by glucocorticoids. The number and affinity of receptors for the hormone, the complement of transcription factors and coregulators, and post-transcription events determine the relative specificity of these hormones’ actions in various cells. The effects of glucocorticoids are mainly due to proteins synthesized from mRNA transcribed from their target genes.

Some of the effects of glucocorticoids can be attributed to their binding to mineralocorticoid receptors. Indeed, MRs bind aldosterone and cortisol with similar affinity. A mineralocorticoid effect of the higher levels of cortisol is avoided in some tissues (eg, kidney, colon, salivary glands) by expression of 11β-hydroxysteroid dehydrogenase type 2, the enzyme responsible for biotransformation to its 11-keto derivative (cortisone), which has minimal action on aldosterone receptors.

The GR also interacts with other regulators of cell function. One such molecule is CLOCK/BMAL-1, a transcription factor dimer expressed in all tissues and generating the circadian rhythm of cortisol secretion (Figure 39–2) at the suprachiasmatic nucleus of the hypothalamus. CLOCK is an acetyltransferase that acetylates the hinge region of the GR, neutralizing its transcriptional activity and thus rendering target tissues resistant to glucocorticoids. As shown in Figure 39–2, lower panel, the glucocorticoid target tissue sensitivity rhythm generated is in reverse phase to that of circulating cortisol concentrations, explaining the increased sensitivity of the organism to evening administration of glucocorticoids. The GR also interacts with NF-κB, a regulator of production of cytokines and other molecules involved in inflammation.

Prompt effects such as initial feedback suppression of pituitary ACTH occur in minutes and are too rapid to be explained on the basis of gene transcription and protein synthesis. It is not known how these effects are mediated. Among the proposed mechanisms are direct effects on cell membrane receptors for the hormone or nongenomic effects of the classic hormone-bound glucocorticoid receptor. The putative membrane receptors might be entirely different from the known intracellular receptors. For example, recent studies implicate G protein-coupled membrane receptors in the response of glutamatergic neurons to glucocorticoids in rats. Furthermore, all steroid receptors (except the MRs) have been shown to have palmitoylation motifs that allow enzymatic addition of palmitate and increased localization of the receptors in the vicinity of plasma membranes. Such receptors are available for direct interactions with and effects on various membrane-associated or cytoplasmic proteins without the need for entry into the nucleus and induction of transcriptional actions.

B. Physiologic Effects The glucocorticoids have widespread effects because they influence the function of most cells in the body. The major metabolic consequences of glucocorticoid secretion or administration are due to direct actions of these hormones in the cell. However, some important effects are the result of homeostatic responses by insulin and glucagon. Although many of the effects of glucocorticoids are dose-related and become magnified when large amounts are administered for therapeutic purposes, there are also other effects—called permissive effects—without which many normal functions become deficient. For example, the response of vascular and bronchial smooth muscle to catecholamines is diminished in the absence of cortisol and restored by physiologic amounts of this glucocorticoid.

Similarly, the lipolytic responses of fat cells to catecholamines, ACTH, and growth hormone are attenuated in the absence of glucocorticoids.

C. Metabolic Effects The glucocorticoids have important dose-related effects on carbohydrate, protein, and fat metabolism. The same effects are responsible for some of the serious adverse effects associated with their use in therapeutic doses. Glucocorticoids stimulate and are required for gluconeogenesis and glycogen synthesis in the fasting state. They stimulate phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and glycogen synthase and the release of amino acids in the course of muscle catabolism.

Glucocorticoids increase serum glucose levels and thus stimulate insulin release and inhibit the uptake of glucose by muscle cells, while they stimulate hormone-sensitive lipase and thus lipolysis. The increased insulin secretion stimulates lipogenesis and to a lesser degree inhibits lipolysis, leading to a net increase in fat deposition combined with increased release of fatty acids and glycerol into the circulation.

The net results of these actions are most apparent in the fasting state, when the supply of glucose from gluconeogenesis, the release of amino acids from muscle catabolism, the inhibition of peripheral glucose uptake, and the stimulation of lipolysis all contribute to maintenance of an adequate glucose supply to the brain.

D. Catabolic and Antianabolic Effects Although glucocorticoids stimulate RNA and protein synthesis in the liver, they have catabolic and antianabolic effects in lymphoid and connective tissue, muscle, peripheral fat, and skin. Supraphysiologic amounts of glucocorticoids lead to decreased muscle mass and weakness and thinning of the skin. Catabolic and antianabolic effects on bone are the cause of osteoporosis in Cushing’s syndrome and impose a major limitation in the long-term therapeutic use of glucocorticoids. In children, glucocorticoids reduce growth. This effect may be partially prevented by administration of growth hormone in high doses, but this use of growth hormone is not recommended.

E. Anti-Inflammatory and Immunosuppressive Effects Glucocorticoids dramatically reduce the manifestations of inflammation. This is due to their profound effects on the concentration, distribution, and function of peripheral leukocytes and to their suppressive effects on the inflammatory cytokines and chemokines and on other mediators of inflammation. Inflammation, regardless of its cause, is characterized by the extravasation and infiltration of leukocytes into the affected tissue. These events are mediated by a complex series of interactions of white cell adhesion molecules with those on endothelial cells and are inhibited by glucocorticoids. After a single dose of a short-acting glucocorticoid, the concentration of neutrophils in the circulation increases while the lymphocytes (T and B cells), monocytes, eosinophils, and basophils decrease. The changes are maximal at 6 hours and are dissipated in 24 hours. The increase in neutrophils is due both to the increased influx into the blood from the bone marrow and to the decreased migration from the blood vessels, leading to a reduction in the number of cells at the site of inflammation. The reduction in circulating lymphocytes, monocytes, eosinophils, and basophils is primarily the result of their movement from the vascular bed to lymphoid tissue.

Glucocorticoids also inhibit the functions of tissue macrophages and other antigen-presenting cells. The ability of these cells to respond to antigens and mitogens is reduced. The effect on macrophages is particularly marked and limits their ability to phagocytose and kill microorganisms and to produce tumor necrosis factor-Îą, interleukin-1, metalloproteinases, and plasminogen activator. Both macrophages and lymphocytes produce less interleukin-12 and interferon-Îł, important inducers of TH1 cell activity, and cellular immunity.

In addition to their effects on leukocyte function, glucocorticoids influence the inflammatory response by inhibiting phospholipase A 2 thereby reducing the synthesis of arachidonic acid, the precursor of prostaglandins and leukotrienes, and of platelet-activating factor.

Finally, glucocorticoids reduce expression of cyclooxygenase-2, the inducible form of this enzyme, in inflammatory cells, thus reducing the amount of enzyme available to produce prostaglandins (see chapters 18 and 36).

Glucocorticoids cause vasoconstriction when applied directly to the skin, possibly by suppressing mast cell degranulation. They also decrease capillary permeability by reducing the amount of histamine released by basophils and mast cells.

The anti-inflammatory and immunosuppressive effects of glucocorticoids are largely due to the actions described above. In humans, complement activation is unaltered, but its effects are inhibited. Antibody production can be reduced by large doses of steroids, although it is unaffected by moderate doses (eg, 20 mg/d of prednisone).

The anti-inflammatory and immunosuppressive effects of these agents are widely useful therapeutically but are also responsible for some of their most serious adverse effects (see text that follows).

F. Other Effects Glucocorticoids have important effects on the nervous system. Adrenal insufficiency causes marked slowing of the alpha rhythm of the electroencephalogram and is associated with depression. Increased amounts of glucocorticoids often produce behavioral disturbances in humans: initially insomnia and euphoria and subsequently depression. Large doses of glucocorticoids may increase intracranial pressure (pseudotumor cerebri).

Glucocorticoids given chronically suppress the pituitary release of ACTH, growth hormone, thyroid-stimulating hormone, and luteinizing hormone.

Large doses of glucocorticoids have been associated with the development of peptic ulcer, possibly by suppressing the local immune response against Helicobacter pylori. They also promote fat redistribution in the body, with increase of visceral, facial, nuchal, and supraclavicular fat, and they appear to antagonize the effect of vitamin D on calcium absorption. The glucocorticoids also have important effects on the hematopoietic system. In addition to their effects on leukocytes, they increase the number of platelets and red blood cells.

Cortisol deficiency results in impaired renal function (particularly glomerular filtration), augmented vasopressin secretion, and diminished ability to excrete a water load.

Glucocorticoids have important effects on the development of the fetal lungs. Indeed, the structural and functional changes in the

lungs near term, including the production of pulmonary surface-active material required for air breathing (surfactant), are stimulated by glucocorticoids.

SYNTHETIC CORTICOSTEROIDS Glucocorticoids have become important agents for use in the treatment of many inflammatory, immunologic, hematologic, and other disorders. This has stimulated the development of many synthetic steroids with anti-inflammatory and immunosuppressive activity.

Pharmacokinetics Pharmaceutical steroids are usually synthesized from cholic acid obtained from cattle or steroid sapogenins found in plants. Further modifications of these steroids have led to the marketing of a large group of synthetic steroids with special characteristics that are pharmacologically and therapeutically important (Table 39–1; Figure 39–3).

TABLE 39–1 Some commonly used natural and synthetic corticosteroids for general use.

The metabolism of the naturally occurring adrenal steroids has been discussed above. The synthetic corticosteroids (Table 39–1) are in most cases rapidly and completely absorbed when given by mouth. Although they are transported and metabolized in a fashion similar

to that of the endogenous steroids, important differences exist.

Alterations in the glucocorticoid molecule influence its affinity for glucocorticoid and mineralocorticoid receptors as well as its protein-binding affinity, side chain stability, rate of elimination, and metabolic products. Halogenation at the 9 position, unsaturation of the δ1–2 bond of the A ring, and methylation at the 2 or 16 position prolong the half-life by more than 50%. The δ1 compounds are excreted in the free form. In some cases, the agent given is a prodrug; for example, prednisone is rapidly converted to the active product prednisolone in the body.

Pharmacodynamics The actions of the synthetic steroids are similar to those of cortisol (see above). They bind to the specific intracellular receptor proteins and produce the same effects but have different ratios of glucocorticoid to mineralocorticoid potency (Table 39–1).

CLINICAL PHARMACOLOGY A. Diagnosis and Treatment of Disturbed Adrenal Function 1. Adrenocortical insufficiency a. Chronic (Addison’s disease)—Chronic adrenocortical insufficiency is characterized by weakness, fatigue, weight loss, hypotension, hyperpigmentation, and inability to maintain the blood glucose level during fasting. In such individuals, minor noxious, traumatic, or infectious stimuli may produce acute adrenal insufficiency with circulatory shock and even death.

In primary adrenal insufficiency, about 20–30 mg of hydrocortisone must be given daily, with increased amounts during periods of stress. Although hydrocortisone has some mineralocorticoid activity, this must be supplemented by an appropriate amount of a saltretaining hormone such as fludrocortisone. Synthetic glucocorticoids that are long-acting and devoid of salt-retaining activity should not be administered to these patients. b. Acute—When acute adrenocortical insufficiency is suspected, treatment must be instituted immediately. Therapy consists of large amounts of parenteral hydrocortisone in addition to correction of fluid and electrolyte abnormalities and treatment of precipitating factors.

Hydrocortisone sodium succinate or phosphate in doses of 100 mg intravenously is given every 8 hours until the patient is stable. The dose is then gradually reduced, achieving maintenance dosage within 5 days.

The administration of salt-retaining hormone is resumed when the total hydrocortisone dosage has been reduced to 50 mg/d.

2. Adrenocortical hypo- and hyperfunction a. Congenital adrenal hyperplasia—This group of disorders is characterized by specific defects in the synthesis of cortisol. In pregnancies at high risk for congenital adrenal hyperplasia, fetuses can be protected from genital abnormalities by administration of dexamethasone to the mother.

The most common defect is a decrease in or lack of P450c21 (21α-hydroxylase) activity. * As can be seen in Figure 39–1, this would lead to a reduction in cortisol synthesis and thus produce a compensatory increase in ACTH release. The adrenal becomes hyperplastic and produces abnormally large amounts of precursors such as 17-hydroxyprogesterone that can be diverted to the androgen pathway, which leads to virilization and can result in ambiguous genitalia in the female fetus. Metabolism of this compound in the liver leads to pregnanetriol, which is characteristically excreted into the urine in large amounts in this disorder and can be used to make the diagnosis and to monitor efficacy of glucocorticoid substitution. However, the most reliable method of detecting this disorder is the increased response of plasma 17-hydroxyprogesterone to ACTH stimulation.

FIGURE 39–1 Outline of major pathways in adrenocortical hormone biosynthesis. The major secretory products are underlined.

Pregnenolone is the major precursor of corticosterone and aldosterone, and 17-hydroxypregnenolone is the major precursor of cortisol.

The enzymes and cofactors for the reactions progressing down each column are shown on the left and across columns at the top of the figure. When a particular enzyme is deficient, hormone production is blocked at the points indicated by the shaded bars. (Reproduced, with permission, from Ganong WF: Review of Medical Physiology, 22nd ed. McGraw-Hill, 2005. Copyright © The McGraw-Hill Companies, Inc.) If the defect is in 11-hydroxylation, large amounts of deoxycorticosterone are produced, and because this steroid has mineralocorticoid activity, hypertension with or without hypokalemic alkalosis ensues. When 17-hydroxylation is defective in the adrenals and gonads, hypogonadism is also present. However, increased amounts of 11-deoxycorticosterone are formed, and the signs and symptoms associated with mineralocorticoid excess—such as hypertension and hypokalemia—are also observed.

When first seen, the infant with congenital adrenal hyperplasia may be in acute adrenal crisis and should be treated as described above, using appropriate electrolyte solutions and an intravenous preparation of hydrocortisone in stress doses. Once the patient is stabilized, oral hydrocortisone, 12–18 mg/m2 /d in two unequally divided doses (two thirds in the morning, one third in late afternoon) is begun. The dosage is adjusted to allow normal growth and bone maturation and to prevent androgen excess. Alternate-day therapy with prednisone has also been used to achieve greater ACTH suppression without increasing growth inhibition. Fludrocortisone, 0.05–0.2 mg/d, should also be administered by mouth, with added salt to maintain normal blood pressure, plasma renin activity, and electrolytes. b. Cushing’s syndrome—Cushing’s syndrome is usually the result of bilateral adrenal hyperplasia secondary to an ACTH-secreting pituitary adenoma (Cushing’s disease) but occasionally is due to tumors or nodular hyperplasia of the adrenal gland or ectopic production of ACTH by other tumors. The manifestations are those associated with the chronic presence of excessive glucocorticoids. When glucocorticoid hypersecretion is marked and prolonged, a rounded, plethoric face and trunk obesity are striking in appearance. Protein loss may be significant and includes muscle wasting; thinning, purple striae, and easy bruising of the skin; poor wound healing; and osteoporosis. Other serious disturbances include mental disorders, hypertension, and diabetes. This disorder is treated by surgical removal of the tumor producing ACTH or cortisol, irradiation of the pituitary tumor, or resection of one or both adrenals. These patients must receive large doses of cortisol during and after the surgical procedure. Doses of up to 300 mg of soluble hydrocortisone may be given as a continuous intravenous infusion on the day of surgery. The dose must be reduced slowly to normal replacement levels, since rapid reduction in dose may produce withdrawal symptoms, including fever and joint pain. If adrenalectomy has been performed, long-term maintenance is similar to that outlined above for adrenal insufficiency. c. Primary generalized glucocorticoid resistance (Chrousos syndrome)—This rare sporadic or familial genetic condition is usually due to inactivating mutations of the glucocorticoid receptor gene. The hypothalamic-pituitary-adrenal (HPA) axis hyperfunctions in an attempt to compensate for the defect, and the increased production of ACTH leads to high circulating levels of cortisol and cortisol precursors such as corticosterone and 11-deoxycorticosterone with mineralocorticoid activity, as well as of adrenal androgens. These increased levels may result in hypertension with or without hypokalemic alkalosis and hyperandrogenism expressed as virilization and precocious puberty in children and acne, hirsutism, male pattern baldness, and menstrual irregularities (mostly oligo-amenorrhea and hypofertility) in women. The therapy of this syndrome is high doses of synthetic glucocorticoids such as dexamethasone with no inherent mineralocorticoid activity. These doses are titrated to normalize the production of cortisol, cortisol precursors, and adrenal androgens. d. Aldosteronism—Primary aldosteronism usually results from the excessive production of aldosterone by an adrenal adenoma.

However, it may also result from abnormal secretion by hyperplastic glands or from a malignant tumor. The clinical findings of hypertension, weakness, and tetany are related to the continued renal loss of potassium, which leads to hypokalemia, alkalosis, and elevation of serum sodium concentrations. This syndrome can also be produced in disorders of adrenal steroid biosynthesis by excessive secretion of deoxycorticosterone, corticosterone, or 18-hydroxycorticosterone—all compounds with inherent mineralocorticoid activity.

In contrast to patients with secondary aldosteronism (see text that follows), these patients have low (suppressed) levels of plasma renin activity and angiotensin II. When treated with fludrocortisone (0.2 mg twice daily orally for 3 days) or deoxycorticosterone acetate (20 mg/d intramuscularly for 3 days—but not available in the USA), patients fail to retain sodium and the secretion of aldosterone is not significantly reduced. When the disorder is mild, it may escape detection if serum potassium levels are used for screening. However, it may be detected by an increased ratio of plasma aldosterone to renin. Patients generally improve when treated with spironolactone, an aldosterone receptor-blocking agent, and the response to this agent is of diagnostic and therapeutic value.

3. Use of glucocorticoids for diagnostic purposes—It is sometimes necessary to suppress the production of ACTH to identify the source of a particular hormone or to establish whether its production is influenced by the secretion of ACTH. In these circumstances, it is advantageous to use a very potent substance such as dexamethasone because the use of small quantities reduces the possibility of confusion in the interpretation of hormone assays in blood or urine. For example, if complete suppression is achieved by the use of 50 mg of cortisol, the urinary 17-hydroxycorticosteroids will be 15–18 mg/24 h, since one third of the dose given will be recovered in urine as 17-hydroxycorticosteroid. If an equivalent dose of 1.5 mg of dexamethasone is used, the urinary excretion will be only 0.5 mg/24 h and blood levels will be low.

The dexamethasone suppression test is used for the diagnosis of Cushing’s syndrome and has also been used in the differential diagnosis of depressive psychiatric states. As a screening test, 1 mg dexamethasone is given orally at 11 PM, and a plasma sample is obtained the following morning. In normal individuals, the morning cortisol concentration is usually less than 3 mcg/dL, whereas in Cushing’s syndrome the level is usually greater than 5 mcg/dL. The results are not reliable in the patient with depression, anxiety, concurrent illness, and other stressful conditions or in the patient who is receiving a medication that enhances the catabolism of dexamethasone in the liver. To distinguish between hypercortisolism due to anxiety, depression, and alcoholism (pseudo-Cushing syndrome) and bona fide Cushing’s syndrome, a combined test is carried out, consisting of dexamethasone (0.5 mg orally every 6 hours for 2 days) followed by a standard corticotropin-releasing hormone (CRH) test (1 mg/kg given as a bolus intravenous infusion 2 hours after the last dose of dexamethasone).

In patients in whom the diagnosis of Cushing’s syndrome has been established clinically and confirmed by a finding of elevated free cortisol in the urine, suppression with large doses of dexamethasone will help to distinguish patients with Cushing’s disease from those with steroid-producing tumors of the adrenal cortex or with the ectopic ACTH syndrome. Dexamethasone is given in a dosage of 0.5 mg orally every 6 hours for 2 days, followed by 2 mg orally every 6 hours for 2 days, and the urine is then assayed for cortisol or its metabolites (Liddle’s test); or dexamethasone is given as a single dose of 8 mg at 11 PM, and the plasma cortisol is measured at 8 AM the following day. In patients with Cushing’s disease, the suppressant effect of dexamethasone usually produces a 50% reduction in hormone levels. In patients in whom suppression does not occur, the ACTH level will be low in the presence of a cortisol-producing adrenal tumor and elevated in patients with an ectopic ACTH-producing tumor.

B. Corticosteroids and Stimulation of Lung Maturation in the Fetus Lung maturation in the fetus is regulated by the fetal secretion of cortisol. Treatment of the mother with large doses of glucocorticoid reduces the incidence of respiratory distress syndrome in infants delivered prematurely. When delivery is anticipated before 34 weeks of gestation, intramuscular betamethasone, 12 mg, followed by an additional dose of 12 mg 18–24 hours later, is commonly used.

Betamethasone is chosen because maternal protein binding and placental metabolism of this corticosteroid is less than that of cortisol, allowing increased transfer across the placenta to the fetus. A study of over 10,000 infants born at 23 to 25 weeks of gestation indicated that exposure to exogenous corticosteroids before birth reduced the death rate and evidence of neurodevelopmental impairment.

C. Corticosteroids and Nonadrenal Disorders The synthetic analogs of cortisol are useful in the treatment of a diverse group of diseases unrelated to any known disturbance of adrenal function (Table 39–2). The usefulness of corticosteroids in these disorders is a function of their ability to suppress inflammatory and immune responses and to alter leukocyte function, as previously described (see also Chapter 55). These agents are useful in disorders in which host response is the cause of the major manifestations of the disease. In instances in which the inflammatory or immune response is important in controlling the pathologic process, therapy with corticosteroids may be dangerous but justified to prevent irreparable damage from an inflammatory response—if used in conjunction with specific therapy for the disease process.

TABLE 39–2 Some therapeutic indications for the use of glucocorticoids in nonadrenal disorders.

Since corticosteroids are not usually curative, the pathologic process may progress while clinical manifestations are suppressed.

Therefore, chronic therapy with these drugs should be undertaken with great care and only when the seriousness of the disorder warrants their use and when less hazardous measures have been exhausted.

In general, attempts should be made to bring the disease process under control using medium- to intermediate-acting glucocorticoids such as prednisone and prednisolone (Table 39–1), as well as all ancillary measures possible to keep the dose low. Where possible, alternate-day therapy should be used (see the following text). Therapy should not be decreased or stopped abruptly. When prolonged therapy is anticipated, it is helpful to obtain chest X-rays and a tuberculin test, since glucocorticoid therapy can reactivate dormant tuberculosis. The presence of diabetes, peptic ulcer, osteoporosis, and psychological disturbances should be taken into consideration, and cardiovascular function should be assessed.

Treatment for transplant rejection is a very important application of glucocorticoids. The efficacy of these agents is based on their ability to reduce antigen expression from the grafted tissue, delay revascularization, and interfere with the sensitization of cytotoxic T lymphocytes and the generation of primary antibody-forming cells.

Toxicity The benefits obtained from glucocorticoids vary considerably. Use of these drugs must be carefully weighed in each patient against their widespread effects on every part of the organism. The major undesirable effects of glucocorticoids are the result of their hormonal actions, which lead to the clinical picture of iatrogenic Cushing’s syndrome (see later in text).

When glucocorticoids are used for short periods (< 2 weeks), it is unusual to see serious adverse effects even with moderately large doses. However, insomnia, behavioral changes (primarily hypomania), and acute peptic ulcers are occasionally observed even after only a few days of treatment. Acute pancreatitis is a rare but serious acute adverse effect of high-dose glucocorticoids.

A. Metabolic Effects Most patients who are given daily doses of 100 mg of hydrocortisone or more (or the equivalent amount of synthetic steroid) for longer than 2 weeks undergo a series of changes that have been termed iatrogenic Cushingâ&#x20AC;&#x2122;s syndrome. The rate of development is a function of the dosage and the genetic background of the patient. In the face, rounding, puffiness, fat deposition, and plethora usually appear (moon facies). Similarly, fat tends to be redistributed from the extremities to the trunk, the back of the neck, and the supraclavicular fossae. There is an increased growth of fine hair over the face, thighs and trunk. Steroid-induced punctate acne may appear, and insomnia and increased appetite are noted. In the treatment of dangerous or disabling disorders, these changes may not require cessation of therapy. However, the underlying metabolic changes accompanying them can be very serious by the time they become obvious. The continuing breakdown of protein and diversion of amino acids to glucose production increase the need for insulin and over time result in weight gain; visceral fat deposition; myopathy and muscle wasting; thinning of the skin, with striae and bruising; hyperglycemia; and eventually osteoporosis, diabetes, and aseptic necrosis of the hip. Wound healing is also impaired under these circumstances. When diabetes occurs, it is treated with diet and insulin. These patients are often resistant to insulin but rarely develop ketoacidosis. In general, patients treated with corticosteroids should be on high-protein and potassium-enriched diets.

B. Other Complications Other serious adverse effects of glucocorticoids include peptic ulcers and their consequences. The clinical findings associated with certain disorders, particularly bacterial and mycotic infections, may be masked by the corticosteroids, and patients must be carefully monitored to avoid serious mishap when large doses are used. Severe myopathy is more frequent in patients treated with long-acting glucocorticoids. The administration of such compounds has been associated with nausea, dizziness, and weight loss in some patients.

These effects are treated by changing drugs, reducing dosage, and increasing potassium and protein intake.

Hypomania or acute psychosis may occur, particularly in patients receiving very large doses of corticosteroids. Long-term therapy with intermediate- and long-acting steroids is associated with depression and the development of posterior subcapsular cataracts.

Psychiatric follow-up and periodic slit-lamp examination is indicated in such patients. Increased intraocular pressure is common, and glaucoma may be induced. Benign intracranial hypertension also occurs. In dosages of 45 mg/m2 /d or more of hydrocortisone or its equivalent, growth retardation occurs in children. Medium-, intermediate-, and long-acting glucocorticoids have greater growthsuppressing potency than the natural steroid at equivalent doses.

When given in larger than physiologic amounts, steroids such as cortisone and hydrocortisone, which have mineralocorticoid effects in addition to glucocorticoid effects, cause some sodium and fluid retention and loss of potassium. In patients with normal cardiovascular and renal function, this leads to a hypokalemic, hypochloremic alkalosis and eventually to a rise in blood pressure. In patients with hypoproteinemia, renal disease, or liver disease, edema may also occur. In patients with heart disease, even small degrees of sodium retention may lead to heart failure. These effects can be minimized by using synthetic non-salt-retaining steroids, sodium restriction, and judicious amounts of potassium supplements.

C. Adrenal Suppression

When corticosteroids are administered for more than 2 weeks, adrenal suppression may occur. If treatment extends over weeks to months, the patient should be given appropriate supplementary therapy at times of minor stress (twofold dosage increases for 24–48 hours) or severe stress (up to tenfold dosage increases for 48–72 hours) such as accidental trauma or major surgery. If corticosteroid dosage is to be reduced, it should be tapered slowly. If therapy is to be stopped, the reduction process should be quite slow when the dose reaches replacement levels. It may take 2–12 months for the hypothalamic-pituitary-adrenal axis to function acceptably, and cortisol levels may not return to normal for another 6–9 months. The glucocorticoid-induced suppression is not a pituitary problem, and treatment with ACTH does not reduce the time required for the return of normal function.

If the dosage is reduced too rapidly in patients receiving glucocorticoids for a certain disorder, the symptoms of the disorder may reappear or increase in intensity. However, patients without an underlying disorder (eg, patients cured surgically of Cushing’s disease) also develop symptoms with rapid reductions in corticosteroid levels. These symptoms include anorexia, nausea or vomiting, weight loss, lethargy, headache, fever, joint or muscle pain, and postural hypotension. Although many of these symptoms may reflect true glucocorticoid deficiency, they may also occur in the presence of normal or even elevated plasma cortisol levels, suggesting glucocorticoid dependence.

Contraindications & Cautions A. Special Precautions Patients receiving glucocorticoids must be monitored carefully for the development of hyperglycemia, glycosuria, sodium retention with edema or hypertension, hypokalemia, peptic ulcer, osteoporosis, and hidden infections.

The dosage should be kept as low as possible, and intermittent administration (eg, alternate-day) should be used when satisfactory therapeutic results can be obtained on this schedule. Even patients maintained on relatively low doses of corticosteroids may require supplementary therapy at times of stress, such as when surgical procedures are performed or intercurrent illness or accidents occur.

B. Contraindications Glucocorticoids must be used with great caution in patients with peptic ulcer, heart disease or hypertension with heart failure, certain infectious illnesses such as varicella and tuberculosis, psychoses, diabetes, osteoporosis, or glaucoma.

Selection of Drug & Dosage Schedule Glucocorticoid preparations differ with respect to relative anti-inflammatory and mineralocorticoid effect, duration of action, cost, and dosage forms available (Table 39–1), and these factors should be taken into account in selecting the drug to be used.

A. ACTH versus Adrenocortical Steroids In patients with normal adrenals, ACTH was used in the past to induce the endogenous production of cortisol to obtain similar effects.

However, except when an increase in androgens is desirable, the use of ACTH as a therapeutic agent has been abandoned. Instances in which ACTH was claimed to be more effective than glucocorticoids were probably due to the administration of smaller amounts of corticosteroids than were produced by the dosage of ACTH. B. Dosage In determining the dosage regimen to be used, the physician must consider the seriousness of the disease, the amount of drug likely to be required to obtain the desired effect, and the duration of therapy. In some diseases, the amount required for maintenance of the desired therapeutic effect is less than the dose needed to obtain the initial effect, and the lowest possible dosage for the needed effect should be determined by gradually lowering the dose until a small increase in signs or symptoms is noted.

When it is necessary to maintain continuously elevated plasma corticosteroid levels to suppress ACTH, a slowly absorbed parenteral preparation or small oral doses at frequent intervals are required. The opposite situation exists with respect to the use of corticosteroids in the treatment of inflammatory and allergic disorders. The same total quantity given in a few doses may be more effective than that given in many smaller doses or in a slowly absorbed parenteral form.

Severe autoimmune conditions involving vital organs must be treated aggressively, and undertreatment is as dangerous as overtreatment. To minimize the deposition of immune complexes and the influx of leukocytes and macrophages, 1 mg/kg/d of prednisone in divided doses is required initially. This dosage is maintained until the serious manifestations respond. The dosage can then be gradually reduced.

When large doses are required for prolonged periods of time, alternate-day administration of the compound may be tried after control is achieved. When used in this manner, very large amounts (eg, 100 mg of prednisone) can sometimes be administered with less marked adverse effects because there is a recovery period between each dose. The transition to an alternate-day schedule can be made after the disease process is under control. It should be done gradually and with additional supportive measures between doses.

When selecting a drug for use in large doses, a medium- or intermediate-acting synthetic steroid with little mineralocorticoid effect is

advisable. If possible, it should be given as a single morning dose.

C. Special Dosage Forms Local therapy, such as topical preparations for skin disease, ophthalmic forms for eye disease, intra-articular injections for joint disease, inhaled steroids for asthma, and hydrocortisone enemas for ulcerative colitis, provides a means of delivering large amounts of steroid to the diseased tissue with reduced systemic effects.

Beclomethasone dipropionate, and several other glucocorticoids—primarily budesonide, flunisolide, and mometasone furoate, administered as aerosols—have been found to be extremely useful in the treatment of asthma (see Chapter 20).

Beclomethasone dipropionate, triamcinolone acetonide, budesonide, flunisolide, and others are available as nasal sprays for the topical treatment of allergic rhinitis. They are effective at doses (one or two sprays one, two, or three times daily) that in most patients result in plasma levels that are too low to influence adrenal function or have any other systemic effects.

Corticosteroids incorporated in ointments, creams, lotions, and sprays are used extensively in dermatology. These preparations are discussed in more detail in Chapter 61.

Recently, new timed-release hydrocortisone tablets were developed for the replacement treatment of addisonian and congenital adrenal hyperplasia patients. These tablets produce plasma cortisol levels that are similar to those secreted normally in a circadian fashion.

MINERALOCORTICOIDS (ALDOSTERONE, DEOXYCORTICOSTERONE, FLUDROCORTISONE) The most important mineralocorticoid in humans is aldosterone. However, small amounts of deoxycorticosterone (DOC) are also formed and released. Although the amount is normally insignificant, DOC was of some importance therapeutically in the past. Its actions, effects, and metabolism are qualitatively similar to those described below for aldosterone. Fludrocortisone, a synthetic corticosteroid, is the most commonly prescribed salt-retaining hormone.

Aldosterone Aldosterone is synthesized mainly in the zona glomerulosa of the adrenal cortex. Its structure and synthesis are illustrated in Figure 39–1.

The rate of aldosterone secretion is subject to several influences. ACTH produces a moderate stimulation of its release, but this effect is not sustained for more than a few days in the normal individual. Although aldosterone is no less than one third as effective as cortisol in suppressing ACTH, the quantities of aldosterone produced by the adrenal cortex and its plasma concentrations are insufficient to participate in any significant feedback control of ACTH secretion.

Without ACTH, aldosterone secretion falls to about half the normal rate, indicating that other factors, eg, angiotensin, are able to maintain and perhaps regulate its secretion (see Chapter 17). Independent variations between cortisol and aldosterone secretion can also be demonstrated by means of lesions in the nervous system such as decerebration, which decreases the secretion of cortisol while increasing the secretion of aldosterone.

A. Physiologic and Pharmacologic Effects Aldosterone and other steroids with mineralocorticoid properties promote the reabsorption of sodium from the distal part of the distal convoluted renal tubule and from the cortical collecting tubules, loosely coupled to the excretion of potassium and hydrogen ion. Sodium reabsorption in the sweat and salivary glands, gastrointestinal mucosa, and across cell membranes in general is also increased. Excessive levels of aldosterone produced by tumors or overdosage with synthetic mineralocorticoids lead to hypokalemia, metabolic alkalosis, increased plasma volume, and hypertension.

Mineralocorticoids act by binding to the mineralocorticoid receptor in the cytoplasm of target cells, especially principal cells of the distal convoluted and collecting tubules of the kidney. The drug-receptor complex activates a series of events similar to those described above for the glucocorticoids and illustrated in Figure 39–4. It is of interest that this receptor has the same affinity for cortisol, which is present in much higher concentrations in the extracellular fluid. The specificity for mineralocorticoids in the kidney appears to be conferred, at least in part, by the presence of the enzyme 11β-hydroxysteroid dehydrogenase type 2, which converts cortisol to cortisone.

The latter has low affinity for the receptor and is inactive as a mineralocorticoid or glucocorticoid in the kidney. The major effect of activation of the aldosterone receptor is increased expression of Na+/K+-ATPase and the epithelial sodium channel (ENaC).

B. Metabolism Aldosterone is secreted at the rate of 100–200 mcg/d in normal individuals with a moderate dietary salt intake. The plasma level in men (resting supine) is about 0.007 mcg/dL. The half-life of aldosterone injected in tracer quantities is 15–20 minutes, and it does not appear to be firmly bound to serum proteins.

The metabolism of aldosterone is similar to that of cortisol, about 50 mcg/24 h appearing in the urine as conjugated tetrahydroaldosterone. Approximately 5–15 mcg/24 h is excreted free or as the 3-oxo glucuronide.

Deoxycorticosterone (DOC) DOC, which also serves as a precursor of aldosterone (Figure 39–1), is normally secreted in amounts of about 200 mcg/d. Its half-life when injected into the human circulation is about 70 minutes. Preliminary estimates of its concentration in plasma are approximately 0.03 mcg/dL. The control of its secretion differs from that of aldosterone in that the secretion of DOC is primarily under the control of ACTH. Although the response to ACTH is enhanced by dietary sodium restriction, due to adaptations, a low-salt diet does not increase DOC secretion. The secretion of DOC may be markedly increased in abnormal conditions such as adrenocortical carcinoma and congenital adrenal hyperplasia with reduced P450c11 or P450c17 activity.

Fludrocortisone This compound, a potent steroid with both glucocorticoid and mineralocorticoid activity, is the most widely used mineralocorticoid. Oral doses of 0.1 mg two to seven times weekly have potent salt-retaining activity and are used in the treatment of adrenocortical insufficiency associated with mineralocorticoid deficiency. These dosages are too small to have important anti-inflammatory or antigrowth effects.

ADRENAL ANDROGENS The adrenal cortex secretes large amounts of DHEA and smaller amounts of androstenedione and testosterone. Although these androgens are thought to contribute to the normal maturation process, they do not stimulate or support major androgen-dependent pubertal changes in humans. Recent studies suggest that DHEA and its sulfate may have other important physiologic actions. If that is correct, these results are probably due to the peripheral conversion of DHEA to more potent androgens or to estrogens and interaction with androgen and estrogen receptors, respectively. Additional effects may be exerted through an interaction with the GABA A and glutamate receptors in the brain or with a nuclear receptor in several central and peripheral sites. The therapeutic use of DHEA in humans has been explored, but the substance has already been adopted with uncritical enthusiasm by members of the sports drug culture and the vitamin and food supplement culture.

The results of a placebo-controlled trial of DHEA in patients with systemic lupus erythematosus have been reported as well as those of a study of DHEA replacement in women with adrenal insufficiency. In both studies a small beneficial effect was seen, with significant improvement of the disease in the former and a clearly added sense of well-being in the latter. The androgenic or estrogenic actions of DHEA could explain the effects of the compound in both situations. In contrast, there is no evidence to support DHEA use to increase muscle strength or improve memory.

ANTAGONISTS OF ADRENOCORTICAL AGENTS SYNTHESIS INHIBITORS & GLUCOCORTICOID ANTAGONISTS Inhibitors of steroid synthesis act at several different steps and one glucocorticoid antagonist acts at the receptor level.

Aminoglutethimide Aminoglutethimide (Figure 39–5) blocks the conversion of cholesterol to pregnenolone (see Figure 39–1) and causes a reduction in the synthesis of all hormonally active steroids. It has been used in conjunction with dexamethasone or hydrocortisone to reduce or eliminate estrogen production in patients with carcinoma of the breast. In a dosage of 1 g/d it was well tolerated; however, with higher dosages, lethargy and skin rash were common effects. The use of aminoglutethimide in breast cancer patients has now been supplanted by tamoxifen or by another class of drugs, the aromatase inhibitors (see chapters 40 and 54). Aminoglutethimide can be used in conjunction with metyrapone or ketoconazole to reduce steroid secretion in patients with Cushing’s syndrome due to adrenocortical cancer who do not respond to mitotane.

FIGURE 39–5 Some adrenocortical blockers. Because of their toxicity, some of these compounds are no longer available in the USA. Aminoglutethimide also apparently increases the clearance of some steroids. It has been shown to enhance the metabolism of dexamethasone, reducing its half-life from 4–5 hours to 2 hours.

Ketoconazole Ketoconazole, an antifungal imidazole derivative (see Chapter 48), is a potent and rather nonselective inhibitor of adrenal and gonadal steroid synthesis. This compound inhibits the cholesterol side-chain cleavage, P450c17, C17,20-lyase, 3β-hydroxysteroid dehydrogenase, and P450c11 enzymes required for steroid hormone synthesis. The sensitivity of the P450 enzymes to this compound in mammalian tissues is much lower than that needed to treat fungal infections, so that its inhibitory effects on steroid biosynthesis are seen only at high doses.

Ketoconazole has been used in the treatment of patients with Cushing’s syndrome due to several causes. Dosages of 200–1200 mg/d have produced a reduction in hormone levels and clinical improvement in some patients. This drug has some hepatotoxicity and should be started at 200 mg/d and slowly increased by 200 mg/d every 2–3 days up to a total daily dose of 1000 mg.

Etomidate Etomidate [R-1-(1-ethylphenyl)imidazole-5-ethyl ester] is a unique drug used for induction of general anesthesia and sedation. At subhypnotic doses of 0.1 mg/kg/h this drug inhibits adrenal steroidogenesis at the level of 11β-hydroxylase and has been used as the only parenteral medication available in the treatment of severe Cushing’s syndrome.

Metyrapone Metyrapone (Figure 39–5) is a relatively selective inhibitor of steroid 11-hydroxylation, interfering with cortisol and corticosterone synthesis. In the presence of a normal pituitary gland, there is a compensatory increase in pituitary ACTH release and adrenal 11deoxycortisol secretion. This response is a measure of the capacity of the anterior pituitary to produce ACTH and has been adapted for clinical use as a diagnostic test. Although the toxicity of metyrapone is much lower than that of mitotane (see text that follows), the drug may produce transient dizziness and gastrointestinal disturbances. This agent has not been widely used in the treatment of Cushing’s syndrome. However, in doses of 0.25 g twice daily to 1 g four times daily, metyrapone can reduce cortisol production to normal levels in some patients with endogenous Cushing’s syndrome. Thus, it may be useful in the management of severe manifestations of cortisol excess while the cause of this condition is being determined or in conjunction with radiation or surgical treatment. Metyrapone is the only adrenal-inhibiting medication that can be administered to pregnant women with Cushing’s syndrome. The major adverse effects observed are salt and water retention and hirsutism resulting from diversion of the 11-deoxycortisol precursor to DOC and androgen synthesis.

Metyrapone is commonly used in tests of adrenal function. The blood levels of 11-deoxycortisol and the urinary excretion of 17hydroxycorticoids are measured before and after administration of the compound. Normally, there is a twofold or greater increase in the urinary 17-hydroxycorticoid excretion. A dosage of 300–500 mg every 4 hours for six doses is often used, and urine collections are made on the day before and the day after treatment. In patients with Cushing’s syndrome, a normal response to metyrapone indicates that the cortisol excess is not the result of a cortisol-secreting adrenal carcinoma or adenoma, since secretion by such tumors produces suppression of ACTH and atrophy of normal adrenal cortex.

Pituitary function may also be tested by administering metyrapone, 2–3 g orally at midnight and by measuring the level of ACTH or 11-deoxycortisol in blood drawn at 8 AM or by comparing the excretion of 17-hydroxycorticosteroids in the urine during the 24-hour periods preceding and following administration of the drug. In patients with suspected or known lesions of the pituitary, this procedure is a means of estimating the ability of the gland to produce ACTH. Metyrapone has been withdrawn from the market in the USA but is available on a compassionate basis.

Trilostane Trilostane is a 3β-17 hydroxysteroid dehydrogenase inhibitor that interferes with the synthesis of adrenal and gonadal hormones and is comparable to aminoglutethimide. Trilostane’s adverse effects are predominantly gastrointestinal; adverse effects occur in about 50% of patients with both trilostane and aminoglutethimide. There is no cross-resistance or crossover of side effects between these compounds.

Trilostane is not available in the USA.

Abiraterone Abiraterone is the newest of the steroid synthesis inhibitors to be approved. It blocks 17α-hydroxylase (P450c17) and 17,20-lyase (Figure 39–1), and predictably reduces synthesis of cortisol in the adrenal and gonadal steroids in the gonads. A compensatory increase occurs in ACTH and aldosterone synthesis, but this can be prevented by concomitant administration of dexamethasone. Abiraterone is an orally active steroid prodrug and is approved for the treatment of refractory prostate cancer.

Mifepristone (RU-486) The search for a glucocorticoid receptor antagonist finally succeeded in the early 1980s with the development of the 11β-aminophenylsubstituted 19-norsteroid called RU-486, later named mifepristone. Unlike the enzyme inhibitors previously discussed, mifepristone is a pharmacologic antagonist at the steroid receptor. This compound has strong antiprogestin activity and initially was proposed as a contraceptive-contragestive agent. High doses of mifepristone exert antiglucocorticoid activity by blocking the glucocorticoid receptor, since mifepristone binds to it with high affinity, causing (1) some stabilization of the hsp-glucocorticoid receptor complex and inhibition of the dissociation of the RU-486–bound glucocorticoid receptor from the hsp chaperone proteins; and (2) alteration of the interaction of the glucocorticoid receptor with coregulators, favoring the formation of a transcriptionally inactive complex in the cell nucleus. The result is inhibition of glucocorticoid receptor activation.

The mean half-life of mifepristone is 20 hours. This is longer than that of many natural and synthetic glucocorticoid agonists (dexamethasone has a half-life of 4–5 hours). Less than 1% of the daily dose is excreted in the urine, suggesting a minor role of kidneys in the clearance of the compound. The long plasma half-life of mifepristone results from extensive and strong binding to plasma proteins.

Less than 5% of the compound is found in the free form when plasma is analyzed by equilibrium dialysis. Mifepristone can bind to albumin and α1 -acid glycoprotein, but it has no affinity for corticosteroid-binding globulin.

In humans, mifepristone causes generalized glucocorticoid resistance. Given orally to several patients with Cushing’s syndrome due to ectopic ACTH production or adrenal carcinoma, it was able to reverse the cushingoid phenotype, to eliminate carbohydrate intolerance, normalize blood pressure, to correct thyroid and gonadal hormone suppression, and to ameliorate the psychological sequelae of hypercortisolism in these patients. At present, this use of mifepristone can only be recommended for inoperable patients with ectopic ACTH secretion or adrenal carcinoma who have failed to respond to other therapeutic manipulations. Its pharmacology and use in

women as a progesterone antagonist are discussed in Chapter 40.

Mitotane Mitotane (Figure 39–5), a drug related to the DDT class of insecticides, has a nonselective cytotoxic action on the adrenal cortex in dogs and to a lesser extent in humans. This drug is administered orally in divided doses up to 12 g daily. About one third of patients with adrenal carcinoma show a reduction in tumor mass. In 80% of patients, the toxic effects are sufficiently severe to require dose reduction.

These include diarrhea, nausea, vomiting, depression, somnolence, and skin rashes. The drug has been withdrawn from the market in the USA but is available on a compassionate basis.

MINERALOCORTICOID ANTAGONISTS In addition to agents that interfere with aldosterone synthesis (see above), there are steroids that compete with aldosterone for its receptor and decrease its effect peripherally. Progesterone is mildly active in this respect.

Spironolactone is a 7α-acetylthiospironolactone. Its onset of action is slow, and the effects last for 2–3 days after the drug is discontinued. It is used in the treatment of primary aldosteronism in dosages of 50–100 mg/d. This agent reverses many of the manifestations of aldosteronism. It has been useful in establishing the diagnosis in some patients and in ameliorating the signs and symptoms when surgical removal of an adenoma is delayed. When used diagnostically for the detection of aldosteronism in hypokalemic patients with hypertension, dosages of 400–500 mg/d for 4–8 days—with an adequate intake of sodium and potassium—restore potassium levels to or toward normal. Spironolactone is also useful in preparing these patients for surgery. Dosages of 300–400 mg/d for 2 weeks are used for this purpose and may reduce the incidence of cardiac arrhythmias.

Spironolactone is also an androgen antagonist and as such is sometimes used in the treatment of hirsutism in women. Dosages of 50– 200 mg/d cause a reduction in the density, diameter, and rate of growth of facial hair in patients with idiopathic hirsutism or hirsutism secondary to androgen excess. The effect can usually be seen in 2 months and becomes maximal in about 6 months.

Spironolactone as a diuretic is discussed in Chapter 15. The drug has benefits in heart failure greater than those predicted from its diuretic effects alone (see Chapter 13). Adverse effects reported for spironolactone include hyperkalemia, cardiac arrhythmia, menstrual abnormalities, gynecomastia, sedation, headache, gastrointestinal disturbances, and skin rashes.

Eplerenone, another aldosterone antagonist, is approved for the treatment of hypertension (see chapters 11 and 15). Like spironolactone, eplerenone has also been found to reduce mortality in heart failure. This aldosterone receptor antagonist is somewhat more selective than spironolactone and has no reported effects on androgen receptors. The standard dosage in hypertension is 50–100 mg/d. The most common toxicity is hyperkalemia, but this is usually mild.

Drospirenone, a progestin, in an oral contraceptive (see Chapter 40), also antagonizes the effects of aldosterone.

PREPARATIONS AVAILABLE*

REFERENCES Alesci S et al: Glucocorticoid-induced osteoporosis: From basic mechanisms to clinical aspects. Neuroimmunomodulation 2005;12:1.

Bamberger CM, Schulte HM, Chrousos GP: Molecular determinants of glucocorticoid receptor function and tissue sensitivity to glucocorticoids. Endocr Rev 1996;17:245.

Charmandari E et al: Peripheral CLOCK regulates target-tissue glucocorticoid receptor transcriptional activity in a circadian fashion in man. PLoS ONE 2011;6:e25612.

Charmandari E, Kino T : Chrousos syndrome: A seminal report, a phylogenetic enigma and the clinical implications of glucocorticoid signaling changes. Eur J Clin Invest 2010;40:932.

Charmandari E, T sigos C, Chrousos GP: Neuroendocrinology of stress. Ann Rev Physiol 2005;67:259.

Chrousos GP: Stress and disorders of the stress system. Nat Rev Endocrinol 2009;5:374.

Chrousos GP, Kino T : Glucocorticoid signaling in the cell: Expanding clinical implications to complex human behavioral and somatic disorders. In: Glucocorticoids and mood: Clinical manifestations, risk factors, and molecular mechanisms. Proc NY Acad Sci 2009;1179:153.

Elenkov IJ, Chrousos GP: Stress hormones, T H1/T H2 patterns, pro/anti-inflammatory cytokines and susceptibility to disease. T rends Endocrinol Metab 1999;10:359.

Elenkov IJ et al: Cytokine dysregulation, inflammation, and wellbeing. Neuroimmunomodulation 2005;12:255.

Franchimont D et al: Glucocorticoids and inflammation revisited: T he state of the art. Neuroimmunomodulation 2002â&#x20AC;&#x201C;03;10:247.

Graber AL et al: Natural history of pituitary-adrenal recovery following long-term suppression with corticosteroids. J Clin Endocrinol Metab 1965;25:11.

Hochberg Z, Pacak K, Chrousos GP: Endocrine withdrawal syndromes. Endocr Rev 2003;24:523.

Kalantaridou S, Chrousos GP: Clinical review 148: Monogenic disorders of puberty. J Clin Endocrinol Metab 2002;87:2481.

Kino T , Charmandari E, Chrousos G (editors): Glucocorticoid action: Basic and clinical implications. Ann NY Acad Sci 2004;1024 (entire volume).

Kino T et al: T he GT P-binding (G) protein β interacts with the activated glucocorticoid receptor and suppresses its transcriptional activity in the nucleus. J Cell Biol 2005;20:885.

Koch CA, Chrousos GP (editors): Endocrine hypertension: Underlying mechanisms and therapy. In: Contemporary Endocrinology, vol XIII. Springer, 2013.

Koch CA, Pacak K, Chrousos GP: T he molecular pathogenesis of hereditary and sporadic adrenocortical and adrenomedullary tumors. J Clin Endocrinol Metab 2002;87:5367.

Mao J, Regelson W, Kalimi M: Molecular mechanism of RU 486 action: A review. Mol Cellular Biochem 1992;109:1.

Marik PE et al: Clinical practice guidelines for the diagnosis and management of corticosteroid insufficiency in critical illness: Recommendations of an international task force. Crit Care Med 2008;36:1937.

Meduri GU et al: Steroid treatment in ARDS: A critical appraisal of the ARDS network trial and the recent literature. Intens Care Med 2008;34:61.

Meduri GU et al: Activation and regulation of systemic inflammation in ARDS: Rationale for prolonged glucocorticoid therapy. Chest 2009;136:1631.

Merke DP et al: Future directions in the study and management of congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Ann Intern Med 2002;136:320.

Nader N, Chrousos GP, Kino T : Interactions of the circadian CLOCK system and the HPA axis. T rends Endocrinol Metab 2010;21:277.

Pervanidou P, Kanaka-Gantenbein C, Chrousos GP: Assessment of metabolic profile in a clinical setting. Curr Opin Clin Nutr Metab Care 2006;9:589.

Preda VA et al: Etomidate in the management of hypercortisolaemia in Cushingâ&#x20AC;&#x2122;s syndrome: A review. Eur J Endocrinol 2012;167:137.

T sigos C, Chrousos GP: Differential diagnosis and management of Cushingâ&#x20AC;&#x2122;s syndrome. Annu Rev Med 1996; 47:443.

Whitaker MJ et al: An oral multiparticulate, modified-release, hydrocortisone replacement therapy that provides physiological cortisol exposure. Clin Endocrinol (Oxf) 2014;80:554.

CASE STUDY ANSWER The patient should be placed on replacement oral hydrocortisone at 10 mg/m2 /d and fludrocortisone at 75 mcg/d. He should be given a MedicAlert bracelet and instructions for minor and major stress glucocorticoid coverage at 2 times and 10 times replacement of hydrocortisone over 24 and 48 hours, respectively.

______________ *

Names for the adrenal steroid synthetic enzymes include the following: P450c11 (11b-hydroxylase), P450c17 (17α-hydroxylase), P450c21 (21α-hydroxylase).

CHAPTER

40 The Gonadal Hormones & Inhibitors George P. Chrousos, MD

CASE STUDY A 25-year-old woman with menarche at 13 years and menstrual periods until about 1 year ago complains of hot flushes, skin and vaginal dryness, weakness, poor sleep, and scanty and infrequent menstrual periods of a yearâ&#x20AC;&#x2122;s duration. She visits her gynecologist, who obtains plasma levels of follicle-stimulating hormone and luteinizing hormone, both of which are moderately elevated. She is diagnosed with premature ovarian failure, and estrogen and progesterone replacement therapy is recommended. A dual-energy absorptiometry scan (DEXA) reveals a bone density t-score of < 2.5 SD, ie, frank osteoporosis. How should the ovarian hormones she lacks be replaced? What extra measures should she take for her osteoporosis while receiving treatment?

THE OVARY (ESTROGENS, PROGESTINS, OTHER OVARIAN HORMONES, ORAL CONTRACEPTIVES, INHIBITORS & ANTAGONISTS, & OVULATIONINDUCING AGENTS) The ovary has important gametogenic functions that are integrated with its hormonal activity. In the human female, the gonad is relatively quiescent during childhood, the period of rapid growth and maturation. At puberty, the ovary begins a 30- to 40-year period of cyclic function called the menstrual cycle because of the regular episodes of bleeding that are its most obvious manifestation. It then fails to respond to gonadotropins secreted by the anterior pituitary gland, and the cessation of cyclic bleeding that occurs is called menopause.

The mechanism responsible for the onset of ovarian function at the time of puberty is thought to be neural in origin, because the immature gonad can be stimulated by gonadotropins already present in the pituitary and because the pituitary is responsive to exogenous hypothalamic gonadotropin-releasing hormone. The maturation of centers in the brain may withdraw a childhood-related inhibitory effect upon hypothalamic arcuate nucleus neurons, allowing them to produce gonadotropin-releasing hormone (GnRH) in pulses with the appropriate amplitude, which stimulates the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (see Chapter 37). At first, small amounts of the latter two hormones are released during the night, and the limited quantities of ovarian estrogen secreted in response start to cause breast development. Subsequently, FSH and LH are secreted throughout the day and night, causing secretion of higher amounts of estrogen and leading to further breast enlargement, alterations in fat distribution, and a growth spurt that culminates in epiphysial closure in the long bones. The change of ovarian function at puberty is called gonadarche.

A year or so after gonadarche, sufficient estrogen is produced to induce endometrial changes and periodic bleeding (menarche).

After the first few irregular cycles, which may be anovulatory, normal cyclic function is established.

At the beginning of each cycle, a variable number of follicles (vesicular follicles), each containing an ovum, begin to enlarge in response to FSH. After 5 or 6 days, one follicle, called the dominant follicle, begins to develop more rapidly. The outer theca and inner granulosa cells of this follicle multiply and, under the influence of LH, synthesize and release estrogens at an increasing rate. The estrogens appear to inhibit FSH release and may lead to regression of the smaller, less mature follicles. The mature dominant ovarian follicle consists of an ovum surrounded by a fluid-filled antrum lined by granulosa and theca cells. The estrogen secretion reaches a peak just before midcycle, and the granulosa cells begin to secrete progesterone. These changes stimulate the brief surge in LH and FSH release that precedes and causes ovulation. When the follicle ruptures, the ovum is released into the abdominal cavity near the opening of the uterine tube.

ACRONYMS

Following the above events, the cavity of the ruptured follicle fills with blood (corpus hemorrhagicum), and the luteinized theca and granulosa cells proliferate and replace the blood to form the corpus luteum. The cells of this structure produce estrogens and progesterone for the remainder of the cycle, or longer if pregnancy occurs.

If pregnancy does not occur, the corpus luteum begins to degenerate and ceases hormone production, eventually becoming a corpus albicans. The endometrium, which proliferated during the follicular phase and developed its glandular function during the luteal phase, is shed in the process of menstruation. These events are summarized in Figure 40â&#x20AC;&#x201C;1.

FIGURE 40â&#x20AC;&#x201C;1 The menstrual cycle, showing plasma levels of pituitary and ovarian hormones and histologic changes.

The ovary normally ceases its gametogenic and endocrine function with time. This change is accompanied by a cessation in uterine bleeding (menopause) and occurs at a mean age of 52 years in the USA. Although the ovary ceases to secrete estrogen, significant levels of estrogen persist in many women as a result of conversion of adrenal and ovarian steroids such as androstenedione to estrone and estradiol in adipose and possibly other nonendocrine tissues.

Disturbances in Ovarian Function

Disturbances of cyclic function are common even during the peak years of reproduction. A minority of these result from inflammatory or neoplastic processes that influence the functions of the uterus, ovaries, or pituitary. Many of the minor disturbances leading to periods of amenorrhea or anovulatory cycles are self-limited. They are often associated with emotional or physical stress and reflect temporary alterations in the stress centers in the brain that control the secretion of GnRH. Anovulatory cycles are also associated with eating disorders (bulimia, anorexia nervosa) and with severe exercise such as distance running and swimming. Among the more common organic causes of persistent ovulatory disturbances are pituitary prolactinomas and syndromes and tumors characterized by excessive ovarian or adrenal androgen production. Normal ovarian function can be modified by androgens produced by the adrenal cortex or tumors arising from it. The ovary also gives rise to androgen-producing neoplasms such as arrhenoblastomas, as well as to estrogenproducing granulosa cell tumors.

THE ESTROGENS Estrogenic activity is shared by a large number of chemical substances. In addition to the variety of steroidal estrogens derived from animal sources, numerous nonsteroidal estrogens have been synthesized. Many phenols are estrogenic, and estrogenic activity has been identified in such diverse forms of life as those found in ocean sediments. Estrogen-mimetic compounds (flavonoids) are found in many plants, including saw palmetto, and soybeans and other foods. Studies have shown that a diet rich in these plant products may cause slight estrogenic effects. Additionally, some compounds used in the manufacture of plastics (bisphenols, alkylphenols, phthalate phenols) have been found to be estrogenic. It has been proposed that these agents are associated with an increased breast cancer incidence in both women and men in the industrialized world.

Natural Estrogens The major estrogens produced by women are estradiol (estradiol-17β, E2 ), estrone (E1 ), and estriol (E3 ) (Figure 40â&#x20AC;&#x201C;2). Estradiol is the major secretory product of the ovary. Although some estrone is produced in the ovary, most estrone and estriol are formed in the liver from estradiol or in peripheral tissues from androstenedione and other androgens (see Figure 39â&#x20AC;&#x201C;1). As noted above, during the first part of the menstrual cycle estrogens are produced in the ovarian follicle by the theca and granulosa cells. After ovulation, the estrogens as well as progesterone are synthesized by the luteinized granulosa and theca cells of the corpus luteum, and the pathways of biosynthesis are slightly different.

FIGURE 40–2 Biosynthesis and metabolism of estrogens and testosterone.

During pregnancy, a large amount of estrogen is synthesized by the fetoplacental unit—consisting of the fetal adrenal zone, secreting androgen precursor, and the placenta, which aromatizes it into estrogen. The estriol synthesized by the fetoplacental unit is released into the maternal circulation and excreted into the urine. Repeated assay of maternal urinary estriol excretion has been used in the assessment of fetal well-being.

One of the most prolific natural sources of estrogenic substances is the stallion, which liberates more of these hormones than the pregnant mare or pregnant woman. The equine estrogens—equilenin and equilin—and their congeners are unsaturated in the B as well as the A ring and are excreted in large quantities in urine, from which they can be recovered and used for medicinal purposes.

In normal women, estradiol is produced at a rate that varies during the menstrual cycle, resulting in plasma levels as low as 50 pg/mL

in the early follicular phase to as high as 350–850 pg/mL at the time of the preovulatory peak (Figure 40–1).

Synthetic Estrogens A variety of chemical alterations have been applied to the natural estrogens. The most important effect of these alterations has been to increase their oral effectiveness. Some structures are shown in Figure 40–3. Those with therapeutic use are listed in Table 40–1.

FIGURE 40–3 Compounds with estrogenic activity.

TABLE 40–1 Commonly used estrogens.

In addition to the steroidal estrogens, a variety of nonsteroidal compounds with estrogenic activity have been synthesized and used clinically. These include dienestrol, diethylstilbestrol, benzestrol, hexestrol, methestrol, methallenestril, and chlorotrianisene.

Pharmacokinetics When released into the circulation, estradiol binds strongly to an α2 globulin (sex hormone-binding globulin [SHBG]) and with lower affinity to albumin. Bound estrogen is relatively unavailable for diffusion into cells, and it is the free fraction that is physiologically active.

Estradiol is converted by the liver and other tissues to estrone and estriol (Figure 40–2) and their 2-hydroxylated derivatives and conjugated metabolites (which are too insoluble in lipid to cross the cell membrane readily) and excreted in the bile. Estrone and estriol have low affinity for the estrogen receptor. However, the conjugates may be hydrolyzed in the intestine to active, reabsorbable compounds. Estrogens are also excreted in small amounts in the breast milk of nursing mothers.

Because significant amounts of estrogens and their active metabolites are excreted in the bile and reabsorbed from the intestine, the resulting enterohepatic circulation ensures that orally administered estrogens will have a high ratio of hepatic to peripheral effects. As noted below, the hepatic effects are thought to be responsible for some undesirable actions such as synthesis of increased clotting factors and plasma renin substrate. The hepatic effects of estrogen can be minimized by routes that avoid first-pass liver exposure, ie, vaginal, transdermal, or by injection.

Physiologic Effects A. Mechanism Estrogens in the blood and interstitial fluid are bound to SHBG, from which they dissociate to cross the cell membrane, enter the nucleus, and bind to their receptor. Two genes code for two estrogen receptor isoforms, α and β, which are members of the superfamily of steroid, sterol, retinoic acid, and thyroid receptors. Unlike glucocorticoid receptors, estrogen receptors are found predominantly in the

nucleus bound to heat shock proteins that stabilize them (see Figure 39–4).

Binding of the hormone to its receptor alters its conformation and releases it from the stabilizing proteins (predominantly Hsp90). The receptor-hormone complex forms dimers (usually ERα-ERα, ERβ-ERβ, or ERα-ERβ) that bind to a specific sequence of nucleotides, called estrogen response elements (EREs), in the regulatory regions of various genes and regulate their transcription. The ERE is composed of two half-sites arranged as a palindrome separated by a small group of nucleotides called the spacer. The interaction of a receptor dimer with the ERE also involves a number of nuclear proteins, the coregulators, as well as components of the transcription machinery. Complex interactions with various coregulators appear to be responsible for some of the tissue-specific effects that govern the actions of selective estrogen receptor modulators (SERMs, see below). The receptor may also bind to other transcription factors to influence the effects of these factors on their responsive genes. Interestingly, although ERβ has its own separate actions from ERα, it also acts as a dominant negative inhibitor of ERα. Thus, while ERα has many growth-promoting properties, ERβ has antigrowth effects. Many phytoestrogens act via the ERβ protecting cells from the pro-growth effects of ERα.

The relative concentrations and types of receptors, receptor coregulators, and transcription factors confer the cell specificity of the hormone’s actions. The genomic effects of estrogens are mainly due to proteins synthesized by translation of RNA transcribed from a responsive gene. Some of the effects of estrogens are indirect, mediated by the autocrine and paracrine actions of autacoids such as growth factors, lipids, glycolipids, and cytokines produced by the target cells in response to estrogen.

Rapid estrogen-induced effects such as granulosa cell Ca2+ uptake and increased uterine blood flow do not require gene activation.

These appear to be mediated by nongenomic effects of the classic estrogen receptor-estrogen complex, influencing several intracellular signaling pathways.

Recently, all steroid receptors except the mineralocorticoid receptors were shown to have palmitoylation motifs that allow enzymatic addition of palmitate and increased localization of the receptors in the vicinity of plasma membranes. Such receptors are available for direct interactions with, and effects on, various membrane-associated or cytoplasmic proteins without the need for entry into the nucleus and induction of transcriptional actions.

B. Female Maturation Estrogens are required for the normal sexual maturation and growth of the female. They stimulate the development of the vagina, uterus, and uterine tubes as well as the secondary sex characteristics. They stimulate stromal development and ductal growth in the breast and are responsible for the accelerated growth phase and the closing of the epiphyses of the long bones that occur at puberty. They contribute to the growth of axillary and pubic hair and alter the distribution of body fat to produce typical female body contours. Larger quantities also stimulate development of pigmentation in the skin, most prominent in the region of the nipples and areolae and in the genital region.

C. Endometrial Effects In addition to its growth effects on uterine muscle, estrogen plays an important role in the development of the endometrial lining. When estrogen production is properly coordinated with the production of progesterone during the normal human menstrual cycle, regular periodic bleeding and shedding of the endometrial lining occur. Continuous exposure to estrogens for prolonged periods leads to hyperplasia of the endometrium that is usually associated with abnormal bleeding patterns.

D. Metabolic and Cardiovascular Effects Estrogens have a number of important metabolic and cardiovascular effects. They seem to be partially responsible for maintenance of the normal structure and function of the skin and blood vessels in women. Estrogens also decrease the rate of resorption of bone by promoting the apoptosis of osteoclasts and by antagonizing the osteoclastogenic and pro-osteoclastic effects of parathyroid hormone and interleukin-6. Estrogens also stimulate adipose tissue production of leptin and are in part responsible for the higher levels of this hormone in women than in men.

In addition to stimulating the synthesis of enzymes and growth factors leading to uterine and breast growth and differentiation, estrogens alter the production and activity of many other proteins in the body. Metabolic alterations in the liver are especially important, so that there is a higher circulating level of proteins such as transcortin (corticosteroid-binding globulin [CBG]), thyroxine-binding globulin (TBG), SHBG, transferrin, renin substrate, and fibrinogen. This leads to increased circulating levels of thyroxine, estrogen, testosterone, iron, copper, and other substances.

Alterations in the composition of the plasma lipids caused by estrogens are characterized by an increase in the high-density lipoproteins (HDL), a slight reduction in the low-density lipoproteins (LDL), and a reduction in total plasma cholesterol levels. Plasma triglyceride levels are increased. Estrogens decrease hepatic oxidation of adipose tissue lipid to ketones and increase synthesis of triglycerides.

E. Effects on Blood Coagulation Estrogens enhance the coagulability of blood. Many changes in factors influencing coagulation have been reported, including increased

circulating levels of factors II, VII, IX, and X and decreased antithrombin III, partially as a result of the hepatic effects mentioned above.

Increased plasminogen levels and decreased platelet adhesiveness have also been found (see Hormonal Contraception, below).

F. Other Effects Estrogens induce the synthesis of progesterone receptors. They are responsible for estrous behavior in animals and may influence behavior and libido in humans. Administration of estrogens stimulates central components of the stress system, including the production of corticotropin-releasing hormone and the activity of the sympathetic system, and promotes a sense of well-being when given to women who are estrogen-deficient. They also facilitate the loss of intravascular fluid into the extracellular space, producing edema. The resulting decrease in plasma volume causes a compensatory retention of sodium and water by the kidney. Estrogens also modulate sympathetic nervous system control of smooth muscle function.

Clinical Uses* A. Primary Hypogonadism Estrogens have been used extensively for replacement therapy in estrogen-deficient patients. The estrogen deficiency may be due to primary failure of development of the ovaries, premature menopause, castration, or menopause.

Treatment of primary hypogonadism is usually begun at 11–13 years of age in order to stimulate the development of secondary sex characteristics and menses, to stimulate optimal growth, to prevent osteoporosis, and to avoid the psychological consequences of delayed puberty and estrogen deficiency. Treatment attempts to mimic the physiology of puberty. It is initiated with small doses of estrogen (0.3 mg conjugated estrogens or 5–10 mcg ethinyl estradiol) on days 1–21 each month and is slowly increased to adult doses and then maintained until the age of menopause (approximately 51 years of age). A progestin is added after the first uterine bleeding. When growth is completed, chronic therapy consists mainly of the administration of adult doses of both estrogens and progestins, as described below.

B. Postmenopausal Hormonal Therapy In addition to the signs and symptoms that follow closely upon the cessation of normal ovarian function—such as loss of menstrual periods, vasomotor symptoms, sleep disturbances, and genital atrophy—there are longer-lasting changes that influence the health and well-being of postmenopausal women. These include an acceleration of bone loss, which in susceptible women may lead to vertebral, hip, and wrist fractures; and lipid changes, which may contribute to the acceleration of atherosclerotic cardiovascular disease noted in postmenopausal women. The effects of estrogens on bone have been extensively studied, and the effects of hormone withdrawal have been well-characterized. However, the role of estrogens and progestins in the cause and prevention of cardiovascular disease, which is responsible for 350,000 deaths per year, and breast cancer, which causes 35,000 deaths per year, is less well understood.

When normal ovulatory function ceases and the estrogen levels fall after menopause, oophorectomy, or premature ovarian failure, there is an accelerated rise in plasma cholesterol and LDL concentrations, while LDL receptors decline. HDL is not much affected, and levels remain higher than in men. Very-low-density lipoprotein and triglyceride levels are also relatively unaffected. Since cardiovascular disorders account for most deaths in this age group, the risk for these disorders constitutes a major consideration in deciding whether or not hormonal “replacement” therapy (HRT, also correctly called HT) is indicated and influences the selection of hormones to be administered. Estrogen replacement therapy has a beneficial effect on circulating lipids and lipoproteins, and this was earlier thought to be accompanied by a reduction in myocardial infarction by about 50% and of fatal strokes by as much as 40%. These findings, however, have been disputed by the results of a large study from the Women’s Health Initiative (WHI) project showing no cardiovascular benefit from estrogen plus progestin replacement therapy in perimenopausal or older postmenopausal patients. In fact, there may be a small increase in cardiovascular problems as well as breast cancer in women who received the replacement therapy. Interestingly, a small protective effect against colon cancer was observed. Although current clinical guidelines do not recommend routine hormone therapy in postmenopausal women, the validity of the WHI report has been questioned. In any case, there is no increased risk for breast cancer if therapy is given immediately after menopause and for the first 7 years, while the cardiovascular risk depends on the degree of atherosclerosis at the onset of therapy. Transdermal or vaginal administration of estrogen may be associated with decreased cardiovascular risk because it bypasses the liver circulation. Women with premature menopause should definitely receive hormone therapy.

In some studies, a protective effect of estrogen replacement therapy against Alzheimer’s disease was observed. However, several other studies have not supported these results.

Progestins antagonize estrogen’s effects on LDL and HDL to a variable extent. However, one large study has shown that the addition of a progestin to estrogen replacement therapy does not influence the cardiovascular risk.

Optimal management of the postmenopausal patient requires careful assessment of her symptoms as well as consideration of her age and the presence of (or risks for) cardiovascular disease, osteoporosis, breast cancer, and endometrial cancer. Bearing in mind the effects of the gonadal hormones on each of these disorders, the goals of therapy can then be defined and the risks of therapy assessed and discussed with the patient.

If the main indication for therapy is hot flushes and sleep disturbances, therapy with the lowest dose of estrogen required for symptomatic relief is recommended. Treatment may be required for only a limited period of time and the possible increased risk for breast cancer avoided. In women who have undergone hysterectomy, estrogens alone can be given 5 days per week or continuously, since progestins are not required to reduce the risk for endometrial hyperplasia and cancer. Hot flushes, sweating, insomnia, and atrophic vaginitis are generally relieved by estrogens; many patients experience some increased sense of well-being; and climacteric depression and other psychopathologic states are improved.

The role of estrogens in the prevention and treatment of osteoporosis has been carefully studied (see Chapter 42). The amount of bone present in the body is maximal in the young active adult in the third decade of life and begins to decline more rapidly in middle age in both men and women. The development of osteoporosis also depends on the amount of bone present at the start of this process, on vitamin D and calcium intake, and on the degree of physical activity. The risk of osteoporosis is highest in smokers who are thin, Caucasian, and inactive and have a low calcium intake and a strong family history of osteoporosis. Depression also is a major risk factor for development of osteoporosis in women.

Estrogens should be used in the smallest dosage consistent with relief of symptoms. In women who have not undergone hysterectomy, it is most convenient to prescribe estrogen on the first 21–25 days of each month. The recommended dosages of estrogen are 0.3–1.25 mg/d of conjugated estrogen or 0.01–0.02 mg/d of ethinyl estradiol. Dosages in the middle of these ranges have been shown to be maximally effective in preventing the decrease in bone density occurring at menopause. From this point of view, it is important to begin therapy as soon as possible after the menopause for maximum effect. In these patients and others not taking estrogen, calcium supplements that bring the total daily calcium intake up to 1500 mg are useful.

Patients at low risk of developing osteoporosis who manifest only mild atrophic vaginitis can be treated with topical preparations. The vaginal route of application is also useful in the treatment of urinary tract symptoms in these patients. It is important to realize, however, that although locally administered estrogens escape the first-pass effect (so that some undesirable hepatic effects are reduced), they are almost completely absorbed into the circulation, and these preparations should be given cyclically.

As noted below, the administration of estrogen is associated with an increased risk of endometrial carcinoma. The administration of a progestational agent with the estrogen prevents endometrial hyperplasia and markedly reduces the risk of this cancer. When estrogen is given for the first 25 days of the month and the progestin medroxyprogesterone (10 mg/d) is added during the last 10–14 days, the risk is only half of that in women not receiving hormone replacement therapy. On this regimen, some women will experience a return of symptoms during the period off estrogen administration. In these patients, the estrogen can be given continuously. If the progestin produces sedation or other undesirable effects, its dose can be reduced to 2.5–5 mg for the last 10 days of the cycle with a slight increase in the risk for endometrial hyperplasia. These regimens are usually accompanied by bleeding at the end of each cycle. Some women experience migraine headaches during the last few days of the cycle. The use of a continuous estrogen regimen will often prevent their occurrence. Women who object to the cyclic bleeding associated with sequential therapy can also consider continuous therapy. Daily therapy with 0.625 mg of conjugated equine estrogens and 2.5–5 mg of medroxyprogesterone will eliminate cyclic bleeding, control vasomotor symptoms, prevent genital atrophy, maintain bone density, and show a favorable lipid profile with a small decrease in LDL and an increase in HDL concentrations. These women have endometrial atrophy on biopsy. About half of these patients experience breakthrough bleeding during the first few months of therapy. Seventy to 80 percent become amenorrheic after the first 4 months, and most remain so. The main disadvantage of continuous therapy is the need for uterine biopsy if bleeding occurs after the first few months.

As noted above, estrogens may also be administered vaginally or transdermally. When estrogens are given by these routes, the liver is bypassed on the first circulation, and the ratio of the liver effects to peripheral effects is reduced.

In patients in whom estrogen replacement therapy is contraindicated, such as those with estrogen-sensitive tumors, relief of vasomotor symptoms may be obtained by the use of clonidine.

C. Other Uses Estrogens combined with progestins can be used to suppress ovulation in patients with intractable dysmenorrhea or when suppression of ovarian function is used in the treatment of hirsutism and amenorrhea due to excessive secretion of androgens by the ovary. Under these circumstances, greater suppression may be needed, and oral contraceptives containing 50 mcg of estrogen or a combination of a low estrogen pill with GnRH suppression may be required.

Adverse Effects Adverse effects of variable severity have been reported with the therapeutic use of estrogens. Many other effects reported in conjunction with hormonal contraceptives may be related to their estrogen content. These are discussed below.

A. Uterine Bleeding Estrogen therapy is a major cause of postmenopausal uterine bleeding. Unfortunately, vaginal bleeding at this time of life may also be due to carcinoma of the endometrium. To avoid confusion, patients should be treated with the smallest amount of estrogen possible. It should

be given cyclically so that bleeding, if it occurs, will be more likely to occur during the withdrawal period. As noted above, endometrial hyperplasia can be prevented by administration of a progestational agent with estrogen in each cycle.

B. Cancer The relation of estrogen therapy to cancer continues to be the subject of active investigation. Although no adverse effect of short-term estrogen therapy on the incidence of breast cancer has been demonstrated, a small increase in the incidence of this tumor may occur with prolonged therapy. Although the risk factor is small (1.25), the impact may be great since this tumor occurs in 10% of women, and addition of progesterone does not confer a protective effect. Studies indicate that following unilateral excision of breast cancer, women receiving tamoxifen (an estrogen partial agonist, see below) show a 35% decrease in contralateral breast cancer compared with controls.

These studies also demonstrate that tamoxifen is well tolerated by most patients, produces estrogen-like alterations in plasma lipid levels, and stabilizes bone mineral loss. Studies bearing on the possible efficacy of tamoxifen and raloxifene in postmenopausal women at high risk for breast cancer show decreases of risk for at least 5 years, but of unknown further duration. A recent study shows that postmenopausal hormone replacement therapy with estrogens plus progestins was associated with greater breast epithelial cell proliferation and breast epithelial cell density than estrogens alone or no replacement therapy. Furthermore, with estrogens plus progestins, breast proliferation was localized to the terminal duct-lobular unit of the breast, which is the main site of development of breast cancer. Thus, further studies are needed to conclusively assess the possible association between progestins and breast cancer risk.

Many studies show an increased risk of endometrial carcinoma in patients taking estrogens alone. The risk seems to vary with the dose and duration of treatment: 15 times greater in patients taking large doses of estrogen for 5 or more years, in contrast with two to four times greater in patients receiving lower doses for short periods. However, as noted above, the concomitant use of a progestin prevents this increased risk and may in fact reduce the incidence of endometrial cancer to less than that in the general population.

There have been a number of reports of adenocarcinoma of the vagina in young women whose mothers were treated with large doses of diethylstilbestrol early in pregnancy. These cancers are most common in young women (ages 14–44). The incidence is less than 1 per 1000 women exposed—too low to establish a cause-and-effect relationship with certainty. However, the risks for infertility, ectopic pregnancy, and premature delivery are also increased. It is now recognized that there is no indication for the use of diethylstilbestrol during pregnancy, and it should be avoided. It is not known whether other estrogens have a similar effect or whether the observed phenomena are peculiar to diethylstilbestrol. This agent should be used only in the treatment of cancer (eg, of the prostate) or as a “morning after” contraceptive (see page 712).

C. Other Effects Nausea and breast tenderness are common and can be minimized by using the smallest effective dose of estrogen. Hyperpigmentation also occurs. Estrogen therapy is associated with an increase in frequency of migraine headaches as well as cholestasis, gallbladder disease, and hypertension.

Contraindications Estrogens should not be used in patients with estrogen-dependent neoplasms such as carcinoma of the endometrium or in those with—or at high risk for—carcinoma of the breast. They should be avoided in patients with undiagnosed genital bleeding, liver disease, or a history of thromboembolic disorder. In addition, the use of estrogens should be avoided by heavy smokers.

Preparations & Dosages The dosages of commonly used natural and synthetic preparations are listed in Table 40–1. Although all of the estrogens produce almost the same hormonal effects, their potencies vary both between agents and depending on the route of administration. As noted above, estradiol is the most active endogenous estrogen, and it has the highest affinity for the estrogen receptor. However, its metabolites estrone and estriol have weak uterine effects.

For a given level of gonadotropin suppression, oral estrogen preparations have more effect on the circulating levels of CBG, SHBG, and a host of other liver proteins, including angiotensinogen, than do transdermal preparations. The oral route of administration allows greater concentrations of hormone to reach the liver, thus increasing the synthesis of these proteins. Transdermal preparations were developed to avoid this effect. When administered transdermally, 50–100 mcg of estradiol has effects similar to those of 0.625–1.25 mg of conjugated oral estrogens on gonadotropin concentrations, endometrium, and vaginal epithelium. Furthermore, the transdermal estrogen preparations do not significantly increase the concentrations of renin substrate, CBG, and TBG and do not produce the characteristic changes in serum lipids. Combined oral preparations containing 0.625 mg of conjugated estrogens and 2.5 mg of medroxyprogesterone acetate are available for menopausal replacement therapy. Tablets containing 0.625 mg of conjugated estrogens and 5 mg of medroxyprogesterone acetate are available to be used in conjunction with conjugated estrogens in a sequential fashion. Estrogens alone are taken on days 1–14 and the combination on days 15–28.

THE PROGESTINS Natural Progestins: Progesterone Progesterone is the most important progestin in humans. In addition to having important hormonal effects, it serves as a precursor to the estrogens, androgens, and adrenocortical steroids. It is synthesized in the ovary, testis, and adrenal cortex from circulating cholesterol.

Large amounts are also synthesized and released by the placenta during pregnancy.

In the ovary, progesterone is produced primarily by the corpus luteum. Normal males appear to secrete 1–5 mg of progesterone daily, resulting in plasma levels of about 0.03 mcg/dL. The level is only slightly higher in the female during the follicular phase of the cycle, when only a few milligrams per day of progesterone are secreted. During the luteal phase, plasma levels range from 0.5 mcg/dL to more than 2 mcg/dL (Figure 40–1). Plasma levels of progesterone are further elevated and reach their peak levels in the third trimester of pregnancy.

Synthetic Progestins A variety of progestational compounds have been synthesized. Some are active when given by mouth. They are not a uniform group of compounds, and all of them differ from progesterone in one or more respects. Table 40–2 lists some of these compounds and their effects. In general, the 21-carbon compounds (hydroxyprogesterone, medroxyprogesterone, megestrol, and dimethisterone) are the most closely related, pharmacologically as well as chemically, to progesterone. A new group of third-generation synthetic progestins has been introduced, principally as components of oral contraceptives. These “19-nor, 13-ethyl” steroid compounds include desogestrel (Figure 40– 4), gestodene, and norgestimate. They are claimed to have lower androgenic activity than older synthetic progestins.

TABLE 40–2 Properties of some progestational agents.

FIGURE 40–4 Progesterone and some progestational agents in clinical use.

Pharmacokinetics Progesterone is rapidly absorbed following administration by any route. Its half-life in the plasma is approximately 5 minutes, and small amounts are stored temporarily in body fat. It is almost completely metabolized in one passage through the liver, and for that reason it is quite ineffective when the usual formulation is administered orally. However, high-dose oral micronized progesterone preparations have been developed that provide adequate progestational effect.

In the liver, progesterone is metabolized to pregnanediol and conjugated with glucuronic acid. It is excreted into the urine as pregnanediol glucuronide. The amount of pregnanediol in the urine has been used as an index of progesterone secretion. This measure has been very useful despite the fact that the proportion of secreted progesterone converted to this compound varies from day to day and from individual to individual. In addition to progesterone, 20α- and 20β-hydroxyprogesterone (20α- and 20β-hydroxy-4-pregnene-3-one) are also found. These compounds have about one fifth the progestational activity of progesterone in humans and other species. Little is known of their physiologic role, but 20α-hydroxyprogesterone is produced in large amounts in some species and may be of some importance biologically.

The usual routes of administration and durations of action of the synthetic progestins are listed in Table 40–2. Most of these agents are extensively metabolized to inactive products that are excreted mainly in the urine.

Physiologic Effects A. Mechanism The mechanism of action of progesterone—described in more detail above—is similar to that of other steroid hormones. Progestins enter the cell and bind to progesterone receptors that are distributed between the nucleus and the cytoplasm. The ligand-receptor complex binds to a progesterone response element (PRE) to activate gene transcription. The response element for progesterone appears to be similar to the corticosteroid response element, and the specificity of the response depends upon which receptor is present in the cell as well as upon other cell-specific receptor coregulators and interacting transcription factors. The progesterone-receptor complex forms a dimer before binding to DNA. Like the estrogen receptor, it can form heterodimers as well as homodimers between two isoforms, A and

B. These isoforms are produced by alternative splicing of the same gene.

B. Effects of Progesterone Progesterone has little effect on protein metabolism. It stimulates lipoprotein lipase activity and seems to favor fat deposition. The effects on carbohydrate metabolism are more marked. Progesterone increases basal insulin levels and the insulin response to glucose. There is usually no manifest change in carbohydrate tolerance. In the liver, progesterone promotes glycogen storage, possibly by facilitating the effect of insulin. Progesterone also promotes ketogenesis.

Progesterone can compete with aldosterone for the mineralocorticoid receptor of the renal tubule, causing a decrease in Na+ reabsorption. This leads to an increased secretion of aldosterone by the adrenal cortex (eg, in pregnancy). Progesterone increases body temperature in humans. The mechanism of this effect is not known, but an alteration of the temperature-regulating centers in the hypothalamus has been suggested. Progesterone also alters the function of the respiratory centers. The ventilatory response to CO2 is increased by progesterone but synthetic progestins with an ethinyl group do not have respiratory effects. This leads to a measurable reduction in arterial and alveolar P CO2 during pregnancy and in the luteal phase of the menstrual cycle. Progesterone and related steroids also have depressant and hypnotic effects on the brain.

Progesterone is responsible for the alveolobular development of the secretory apparatus in the breast. It also participates in the preovulatory LH surge and causes the maturation and secretory changes in the endometrium that are seen following ovulation (Figure 40–1).

Progesterone decreases the plasma levels of many amino acids and leads to increased urinary nitrogen excretion. It induces changes in the structure and function of smooth endoplasmic reticulum in experimental animals.

Other effects of progesterone and its analogs are noted below in the section, Hormonal Contraception.

C. Synthetic Progestins The 21-carbon progesterone analogs antagonize aldosterone-induced sodium retention (see above). The remaining compounds (“19nortestosterone” third-generation agents) produce a decidual change in the endometrial stroma, do not support pregnancy in test animals, are more effective gonadotropin inhibitors, and may have minimal estrogenic and androgenic or anabolic activity (Table 40–2; Figure 40– 4). They are sometimes referred to as “impeded androgens.” Progestins without androgenic activity include desogestrel, norgestimate, and gestodene. The first two of these compounds are dispensed in combination with ethinyl estradiol for oral contraception (Table 40–3) in the USA. Oral contraceptives containing the progestins cyproterone acetate (also an antiandrogen) in combination with ethinyl estradiol are investigational in the USA. TABLE 40–3 Some oral and implantable contraceptive agents in use.1

Clinical Uses A. Therapeutic Applications The major uses of progestational hormones are for hormone replacement therapy (see above) and hormonal contraception (see below).

In addition, they are useful in producing long-term ovarian suppression for other purposes. When used alone in large doses parenterally (eg, medroxyprogesterone acetate, 150 mg intramuscularly every 90 days), prolonged anovulation and amenorrhea result. This therapy has been employed in the treatment of dysmenorrhea, endometriosis, and bleeding disorders when estrogens are contraindicated, and for contraception. The major problem with this regimen is the prolonged time required in some patients for ovulatory function to return after cessation of therapy. It should not be used for patients planning a pregnancy in the near future. Similar regimens will relieve hot flushes in some menopausal women and can be used if estrogen therapy is contraindicated.

Medroxyprogesterone acetate, 10–20 mg orally twice weekly—or intramuscularly in doses of 100 mg/m2 every 1–2 weeks—will prevent menstruation, but it will not arrest accelerated bone maturation in children with precocious puberty.

Progestins do not appear to have any place in the therapy of threatened or habitual abortion. Early reports of the usefulness of these agents resulted from the unwarranted assumption that after several abortions the likelihood of repeated abortions was over 90%. When progestational agents were administered to patients with previous abortions, a salvage rate of 80% was achieved. It is now recognized that similar patients abort only 20% of the time even when untreated. On the other hand, progesterone was given experimentally to delay premature labor with encouraging results.

Progesterone and medroxyprogesterone have been used in the treatment of women who have difficulty in conceiving and who demonstrate a slow rise in basal body temperature. There is no convincing evidence that this treatment is effective.

Preparations of progesterone and medroxyprogesterone have been used to treat premenstrual syndrome. Controlled studies have not confirmed the effectiveness of such therapy except when doses sufficient to suppress ovulation have been used.

B. Diagnostic Uses Progesterone can be used as a test of estrogen secretion. The administration of progesterone, 150 mg/d, or medroxyprogesterone, 10 mg/d, for 5–7 days, is followed by withdrawal bleeding in amenorrheic patients only when the endometrium has been stimulated by estrogens. A combination of estrogen and progestin can be given to test the responsiveness of the endometrium in patients with amenorrhea.

Contraindications, Cautions, & Adverse Effects Studies of progestational compounds alone and with combination oral contraceptives indicate that the progestin in these agents may increase blood pressure in some patients. The more androgenic progestins also reduce plasma HDL levels in women. (See Hormonal Contraception, below.) Two recent studies suggest that combined progestin plus estrogen replacement therapy in postmenopausal women may increase breast cancer risk significantly compared with the risk in women taking estrogen alone. These findings require careful examination and if confirmed will lead to important changes in postmenopausal hormone replacement practice.

OTHER OVARIAN HORMONES The normal ovary produces small amounts of androgens, including testosterone, androstenedione, and dehydroepiandrosterone. Of these, only testosterone has a significant amount of biologic activity, although androstenedione can be converted to testosterone or estrone in peripheral tissues. The normal woman produces less than 200 mcg of testosterone in 24 hours, and about one third of this is probably formed in the ovary directly. The physiologic significance of these small amounts of androgens is not established, but they may be partly responsible for normal hair growth at puberty, for stimulation of female libido, and, possibly, for metabolic effects. Androgen production by the ovary may be markedly increased in some abnormal states, usually in association with hirsutism and amenorrhea as noted above.

The ovary also produces inhibin and activin. These peptides consist of several combinations of α and β subunits and are described in greater detail later. The αβ dimer (inhibin) inhibits FSH secretion while the ββ dimer (activin) increases FSH secretion. Studies in primates indicate that inhibin has no direct effect on ovarian steroidogenesis but that activin modulates the response to LH and FSH. For example, simultaneous treatment with activin and human FSH enhances FSH stimulation of progesterone synthesis and aromatase activity in granulosa cells. When combined with LH, activin suppressed the LH-induced progesterone response by 50% but markedly enhanced basal and LH-stimulated aromatase activity. Activin may also act as a growth factor in other tissues. The physiologic roles of these modulators are not fully understood.

Relaxin is another peptide that can be extracted from the ovary. The three-dimensional structure of relaxin is related to that of growth-promoting peptides and is similar to that of insulin. Although the amino acid sequence differs from that of insulin, this hormone, like insulin, consists of two chains linked by disulfide bonds, cleaved from a prohormone. It is found in the ovary, placenta, uterus, and blood. Relaxin synthesis has been demonstrated in luteinized granulosa cells of the corpus luteum. It has been shown to increase glycogen synthesis and water uptake by the myometrium and to decrease uterine contractility. In some species, it changes the mechanical properties of the cervix and pubic ligaments, facilitating delivery.

In women, relaxin has been measured by immunoassay. Levels were highest immediately after the LH surge and during menstruation. A physiologic role for this peptide has not been established.

Clinical trials with relaxin have been conducted in patients with dysmenorrhea. Relaxin has also been administered to patients in premature labor and during prolonged labor. When applied to the cervix of a woman at term, it facilitates dilation and shortens labor.

Several other nonsteroidal substances such as corticotropin-releasing hormone, follistatin, and prostaglandins are produced by the ovary. These probably have paracrine effects within the ovary.

HORMONAL CONTRACEPTION (ORAL, PARENTERAL, & IMPLANTED CONTRACEPTIVES) A large number of oral contraceptives containing estrogens or progestins (or both) are now available for clinical use (Table 40–3). These preparations vary chemically and pharmacologically and have many properties in common as well as definite differences important for the correct selection of the optimum agent.

Two types of preparations are used for oral contraception: (1) combinations of estrogens and progestins and (2) continuous progestin therapy without concomitant administration of estrogens. The combination agents are further divided into monophasic forms (constant dosage of both components during the cycle) and biphasic or triphasic forms (dosage of one or both components is changed once or twice during the cycle). The preparations for oral use are all adequately absorbed, and in combination preparations the pharmacokinetics of neither drug is significantly altered by the other.

Only one implantable contraceptive preparation is available at present in the USA. Etonogestrel, also used in some oral contraceptives, is available in the subcutaneous implant form listed in Table 40–3. Several hormonal contraceptives are available as vaginal rings or intrauterine devices. Intramuscular injection of large doses of medroxyprogesterone also provides contraception of long

duration.

Pharmacologic Effects A. Mechanism of Action The combinations of estrogens and progestins exert their contraceptive effect largely through selective inhibition of pituitary function that results in inhibition of ovulation. The combination agents also produce a change in the cervical mucus, in the uterine endometrium, and in motility and secretion in the uterine tubes, all of which decrease the likelihood of conception and implantation. The continuous use of progestins alone does not always inhibit ovulation. The other factors mentioned, therefore, play a major role in the prevention of pregnancy when these agents are used.

B. Effects on the Ovary Chronic use of combination agents depresses ovarian function. Follicular development is minimal, and corpora lutea, larger follicles, stromal edema, and other morphologic features normally seen in ovulating women are absent. The ovaries usually become smaller even when enlarged before therapy.

The great majority of patients return to normal menstrual patterns when these drugs are discontinued. About 75% will ovulate in the first posttreatment cycle and 97% by the third posttreatment cycle. About 2% of patients remain amenorrheic for periods of up to several years after administration is stopped.

The cytologic findings on vaginal smears vary depending on the preparation used. However, with almost all of the combined drugs, a low maturation index is found because of the presence of progestational agents.

C. Effects on the Uterus After prolonged use, the cervix may show some hypertrophy and polyp formation. There are also important effects on the cervical mucus, making it more like postovulation mucus, ie, thicker and less copious.

Agents containing both estrogens and progestins produce further morphologic and biochemical changes of the endometrial stroma under the influence of the progestin, which also stimulates glandular secretion throughout the luteal phase. The agents containing “19nor” progestins—particularly those with the smaller amounts of estrogen—tend to produce more glandular atrophy and usually less bleeding.

D. Effects on the Breast Stimulation of the breasts occurs in most patients receiving estrogen-containing agents. Some enlargement is generally noted. The administration of estrogens and combinations of estrogens and progestins tends to suppress lactation. When the doses are small, the effects on breast-feeding are not appreciable. Studies of the transport of the oral contraceptives into breast milk suggest that only small amounts of these compounds cross into the milk, and they have not been considered to be of importance.

E. Other Effects of Oral Contraceptives 1. Effects on the central nervous system—The central nervous system effects of the oral contraceptives have not been well studied in humans. A variety of effects of estrogen and progesterone have been noted in animals. Estrogens tend to increase excitability in the brain, whereas progesterone tends to decrease it. The thermogenic action of progesterone and some of the synthetic progestins is also thought to occur in the central nervous system.

It is very difficult to evaluate any behavioral or emotional effects of these compounds in humans. Although the incidence of pronounced changes in mood, affect, and behavior appears to be low, milder changes are commonly reported, and estrogens are being successfully employed in the therapy of premenstrual tension syndrome, postpartum depression, and climacteric depression.

2. Effects on endocrine function—The inhibition of pituitary gonadotropin secretion has been mentioned. Estrogens also alter adrenal structure and function. Estrogens given orally or at high doses increase the plasma concentration of the α2 globulin that binds cortisol (corticosteroid-binding globulin). Plasma concentrations may be more than double the levels found in untreated individuals, and urinary excretion of free cortisol is elevated.

These preparations cause alterations in the renin-angiotensin-aldosterone system. Plasma renin activity has been found to increase, and there is an increase in aldosterone secretion.

Thyroxine-binding globulin is increased. As a result, total plasma thyroxine (T 4 ) levels are increased to those commonly seen during pregnancy. Since more of the thyroxine is bound, the free thyroxine level in these patients is normal. Estrogens also increase the plasma level of SHBG and decrease plasma levels of free androgens by increasing their binding; large amounts of estrogen may decrease androgens by gonadotropin suppression.

3. Effects on blood—Serious thromboembolic phenomena occurring in women taking oral contraceptives gave rise to a great many studies of the effects of these compounds on blood coagulation. A clear picture of such effects has not yet emerged. The oral contraceptives do not consistently alter bleeding or clotting times. The changes that have been observed are similar to those reported in pregnancy. There is an increase in factors VII, VIII, IX, and X and a decrease in antithrombin III. Increased amounts of coumarin anticoagulants may be required to prolong prothrombin time in patients taking oral contraceptives.

There is an increase in serum iron and total iron-binding capacity similar to that reported in patients with hepatitis.

Significant alterations in the cellular components of blood have not been reported with any consistency. A number of patients have been reported to develop folic acid deficiency anemias.

4. Effects on the liver—These hormones also have profound effects on the function of the liver. Some of these effects are deleterious and will be considered below in the section on adverse effects. The effects on serum proteins result from the effects of the estrogens on the synthesis of the various α2 globulins and fibrinogen. Serum haptoglobins produced in the liver are depressed rather than increased by estrogen. Some of the effects on carbohydrate and lipid metabolism are probably influenced by changes in liver metabolism (see below).

Important alterations in hepatic drug excretion and metabolism also occur. Estrogens in the amounts seen during pregnancy or used in oral contraceptive agents delay the clearance of sulfobromophthalein and reduce the flow of bile. The proportion of cholic acid in bile acids is increased while the proportion of chenodeoxycholic acid is decreased. These changes may be responsible for the observed increase in cholelithiasis associated with the use of these agents.

5. Effects on lipid metabolism—As noted above, estrogens increase serum triglycerides and free and esterified cholesterol.

Phospholipids are also increased, as are HDL; levels of LDL usually decrease. Although the effects are marked with doses of 100 mcg of mestranol or ethinyl estradiol, doses of 50 mcg or less have minimal effects. The progestins (particularly the “19-nortestosterone” derivatives) tend to antagonize these effects of estrogen. Preparations containing small amounts of estrogen and a progestin may slightly decrease triglycerides and HDL. 6. Effects on carbohydrate metabolism—The administration of oral contraceptives produces alterations in carbohydrate metabolism similar to those observed in pregnancy. There is a reduction in the rate of absorption of carbohydrates from the gastrointestinal tract.

Progesterone increases the basal insulin level and the rise in insulin induced by carbohydrate ingestion. Preparations with more potent progestins such as norgestrel may cause progressive decreases in carbohydrate tolerance over several years. However, the changes in glucose tolerance are reversible on discontinuing medication.

7. Effects on the cardiovascular system—These agents cause small increases in cardiac output associated with higher systolic and diastolic blood pressure and heart rate. The pressure returns to normal when treatment is terminated. Although the magnitude of the pressure change is small in most patients, it is marked in a few. It is important that blood pressure be followed in each patient. An increase in blood pressure has been reported to occur in a few postmenopausal women treated with estrogens alone.

8. Effects on the skin—The oral contraceptives have been noted to increase pigmentation of the skin (chloasma). This effect seems to be enhanced in women with dark complexions and by exposure to ultraviolet light. Some of the androgen-like progestins might increase the production of sebum, causing acne in some patients. However, since ovarian androgen is suppressed, many patients note decreased sebum production, acne, and terminal hair growth. The sequential oral contraceptive preparations as well as estrogens alone often decrease sebum production.

Clinical Uses The most important use of combined estrogens and progestins is for oral contraception. A large number of preparations are available for this specific purpose, some of which are listed in Table 40–3. They are specially packaged for ease of administration. In general, they are very effective; when these agents are taken according to directions, the risk of conception is extremely small. The pregnancy rate with combination agents is estimated to be about 5–12 per 100 woman years at risk. (Under conditions of perfect adherence, the pregnancy rate would be 0.5–1 per 100 woman years.) Contraceptive failure has been observed in some patients when one or more doses are missed, if phenytoin is also being used (which may increase catabolism of the compounds), or if antibiotics are taken that alter enterohepatic cycling of metabolites.

Progestins and estrogens are also useful in the treatment of endometriosis. When severe dysmenorrhea is the major symptom, the suppression of ovulation with estrogen alone may be followed by painless periods. However, in most patients this approach to therapy is inadequate. The long-term administration of large doses of progestins or combinations of progestins and estrogens prevents the periodic breakdown of the endometrial tissue and in some cases will lead to endometrial fibrosis and prevent the reactivation of implants for prolonged periods.

As is true with most hormonal preparations, many of the undesired effects are physiologic or pharmacologic actions that are objectionable only because they are not pertinent to the situation for which they are being used. Therefore, the product containing the smallest effective amounts of hormones should be selected for use.

Adverse Effects The incidence of serious known toxicities associated with the use of these drugs is low—far lower than the risks associated with pregnancy. There are a number of reversible changes in intermediary metabolism. Minor adverse effects are frequent, but most are mild and many are transient. Continuing problems may respond to simple changes in pill formulation. Although it is not often necessary to discontinue medication for these reasons, as many as one third of all patients started on oral contraception discontinue use for reasons other than a desire to become pregnant.

A. Mild Adverse Effects 1. Nausea, mastalgia, breakthrough bleeding, and edema are related to the amount of estrogen in the preparation. These effects can often be alleviated by a shift to a preparation containing smaller amounts of estrogen or to agents containing progestins with more androgenic effects.

2. Changes in serum proteins and other effects on endocrine function (see above) must be taken into account when thyroid, adrenal, or pituitary function is being evaluated. Increases in sedimentation rate are thought to be due to increased levels of fibrinogen.

3. Headache is mild and often transient. However, migraine is often made worse and has been reported to be associated with an increased frequency of cerebrovascular accidents. When this occurs or when migraine has its onset during therapy with these agents, treatment should be discontinued.

4. Withdrawal bleeding sometimes fails to occur—most often with combination preparations—and may cause confusion with regard to pregnancy. If this is disturbing to the patient, a different preparation may be tried or other methods of contraception used.

B. Moderate Adverse Effects Any of the following may require discontinuance of oral contraceptives: 1. Breakthrough bleeding is the most common problem in using progestational agents alone for contraception. It occurs in as many as 25% of patients. It is more frequently encountered in patients taking low-dose preparations than in those taking combination pills with higher levels of progestin and estrogen. The biphasic and triphasic oral contraceptives (Table 40–3) decrease breakthrough bleeding without increasing the total hormone content.

2. Weight gain is more common with the combination agents containing androgen-like progestins. It can usually be controlled by shifting to preparations with less progestin effect or by dieting.

3. Increased skin pigmentation may occur, especially in dark-skinned women. It tends to increase with time, the incidence being about 5% at the end of the first year and about 40% after 8 years. It is thought to be exacerbated by vitamin B deficiency. It is often reversible upon discontinuance of medication but may disappear very slowly.

4. Acne may be exacerbated by agents containing androgen-like progestins (Table 40–2), whereas agents containing large amounts of estrogen usually cause marked improvement in acne.

5. Hirsutism may also be aggravated by the “19-nortestosterone” derivatives, and combinations containing nonandrogenic progestins are preferred in these patients.

6. Ureteral dilation similar to that observed in pregnancy has been reported, and bacteriuria is more frequent.

7. Vaginal infections are more common and more difficult to treat in patients who are using oral contraceptives.

8. Amenorrhea occurs in some patients. Following cessation of administration of oral contraceptives, 95% of patients with normal menstrual histories resume normal periods and all but a few resume normal cycles during the next few months. However, some patients remain amenorrheic for several years. Many of these patients also have galactorrhea. Patients who have had menstrual irregularities before taking oral contraceptives are particularly susceptible to prolonged amenorrhea when the agents are discontinued.

Prolactin levels should be measured in these patients, since many have prolactinomas.

C. Severe Adverse Effects 1. Vascular disorders —Thromboembolism was one of the earliest of the serious unanticipated effects to be reported and has been the most thoroughly studied. a. Venous thromboembolic disease —Superficial or deep thromboembolic disease in women not taking oral contraceptives occurs in about 1 patient per 1000 woman years. The overall incidence of these disorders in patients taking low-dose oral contraceptives is about threefold higher. The risk for this disorder is increased during the first month of contraceptive use and remains constant for several years or more. The risk returns to normal within a month when use is discontinued. The risk of venous thrombosis or pulmonary embolism is increased among women with predisposing conditions such as stasis, altered clotting factors such as antithrombin III, increased levels of homocysteine, or injury. Genetic disorders, including mutations in the genes governing the production of protein C (factor V Leiden), protein S, hepatic cofactor II, and others, markedly increase the risk of venous thromboembolism. The incidence of these disorders is too low for cost-effective screening by current methods, but prior episodes or a family history may be helpful in identifying patients with

increased risk.

The incidence of venous thromboembolism appears to be related to the estrogen but not the progestin content of oral contraceptives and is not related to age, parity, mild obesity, or cigarette smoking. Decreased venous blood flow, endothelial proliferation in veins and arteries, and increased coagulability of blood resulting from changes in platelet functions and fibrinolytic systems contribute to the increased incidence of thrombosis. The major plasma inhibitor of thrombin, antithrombin III, is substantially decreased during oral contraceptive use. This change occurs in the first month of treatment and lasts as long as treatment persists, reversing within a month thereafter. b. Myocardial infarction—The use of oral contraceptives is associated with a slightly higher risk of myocardial infarction in women who are obese, have a history of preeclampsia or hypertension, or have hyperlipoproteinemia or diabetes. There is a much higher risk in women who smoke. The risk attributable to oral contraceptives in women 30–40 years of age who do not smoke is about 4 cases per 100,000 users per year, as compared with 185 cases per 100,000 among women 40–44 who smoke heavily. The association with myocardial infarction is thought to involve acceleration of atherogenesis because of decreased glucose tolerance, decreased levels of HDL, increased levels of LDL, and increased platelet aggregation. In addition, facilitation of coronary arterial spasm may play a role in some of these patients. The progestational component of oral contraceptives decreases HDL cholesterol levels, in proportion to the androgenic activity of the progestin. The net effect, therefore, will depend on the specific composition of the pill used and the patient’s susceptibility to the particular effects. Recent studies suggest that risk of infarction is not increased in past users who have discontinued oral contraceptives. c. Cerebrovascular disease—The risk of stroke is concentrated in women over age 35. It is increased in current users of oral contraceptives but not in past users. However, subarachnoid hemorrhages have been found to be increased among both current and past users and may increase with time. The risk of thrombotic or hemorrhagic stroke attributable to oral contraceptives (based on older, higher-dose preparations) has been estimated to about 37 cases per 100,000 users per year.

In summary, available data indicate that oral contraceptives increase the risk of various cardiovascular disorders at all ages and among both smokers and nonsmokers. However, this risk appears to be concentrated in women 35 years of age or older who are heavy smokers. It is clear that these risk factors must be considered in each individual patient for whom oral contraceptives are being considered. Some experts have suggested that screening for coagulopathy should be performed before starting oral contraception.

2. Gastrointestinal disorders—Many cases of cholestatic jaundice have been reported in patients taking progestin-containing drugs.

The differences in incidence of these disorders from one population to another suggest that genetic factors may be involved. The jaundice caused by these agents is similar to that produced by other 17-alkyl-substituted steroids. It is most often observed in the first three cycles and is particularly common in women with a history of cholestatic jaundice during pregnancy. Jaundice and pruritus disappear 1–8 weeks after the drug is discontinued.

These agents have also been found to increase the incidence of symptomatic gallbladder disease, including cholecystitis and cholangitis. This is probably the result of the alterations responsible for jaundice and bile acid changes described above.

It also appears that the incidence of hepatic adenomas is increased in women taking oral contraceptives. Ischemic bowel disease secondary to thrombosis of the celiac and superior and inferior mesenteric arteries and veins has also been reported in women using these drugs.

3. Depression—Depression of sufficient degree to require cessation of therapy occurs in about 6% of patients treated with some preparations.

4. Cancer—The occurrence of malignant tumors in patients taking oral contraceptives has been studied extensively. It is now clear that these compounds reduce the risk of endometrial and ovarian cancer. The lifetime risk of breast cancer in the population as a whole does not seem to be affected by oral contraceptive use. Some studies have shown an increased risk in younger women, and it is possible that tumors that develop in younger women become clinically apparent sooner. The relation of risk of cervical cancer to oral contraceptive use is still controversial. It should be noted that a number of recent studies associate the use of oral contraceptives by women who are infected with human papillomavirus with an increased risk of cervical cancer.

5. Other—In addition to the above effects, a number of other adverse reactions have been reported for which a causal relation has not been established. These include alopecia, erythema multiforme, erythema nodosum, and other skin disorders.

Contraindications & Cautions These drugs are contraindicated in patients with thrombophlebitis, thromboembolic phenomena, and cardiovascular and cerebrovascular disorders or a past history of these conditions. They should not be used to treat vaginal bleeding when the cause is unknown. They should be avoided in patients with known or suspected tumors of the breast or other estrogen-dependent neoplasms. Since these preparations have caused aggravation of preexisting disorders, they should be avoided or used with caution in patients with liver disease, asthma,

eczema, migraine, diabetes, hypertension, optic neuritis, retrobulbar neuritis, or convulsive disorders.

The oral contraceptives may produce edema, and for that reason they should be used with great caution in patients in heart failure or in whom edema is otherwise undesirable or dangerous.

Estrogens may increase the rate of growth of fibroids. Therefore, for women with these tumors, agents with the smallest amounts of estrogen and the most androgenic progestins should be selected. The use of progestational agents alone for contraception might be especially useful in such patients (see below).

These agents are contraindicated in adolescents in whom epiphysial closure has not yet been completed.

Women using oral contraceptives must be made aware of an important interaction that occurs with antimicrobial drugs. Because the normal gastrointestinal flora increase the enterohepatic cycling (and bioavailability) of estrogens, antimicrobial drugs that interfere with these organisms may reduce the efficacy of oral contraceptives. Additionally, coadministration with potent inducers of the hepatic microsomal metabolizing enzymes, such as rifampin, may increase liver catabolism of estrogens or progestins and diminish the efficacy of oral contraceptives.

Contraception with Progestins Alone Small doses of progestins administered orally or by implantation under the skin can be used for contraception. They are particularly suited for use in patients for whom estrogen administration is undesirable. They are about as effective as intrauterine devices or combination pills containing 20–30 mcg of ethinyl estradiol. There is a high incidence of abnormal bleeding.

Effective contraception can also be achieved by injecting 150 mg of depot medroxyprogesterone acetate (DMPA) every 3 months.

After a 150 mg dose, ovulation is inhibited for at least 14 weeks. Almost all users experience episodes of unpredictable spotting and bleeding, particularly during the first year of use. Spotting and bleeding decrease with time, and amenorrhea is common. This preparation is not desirable for women planning a pregnancy soon after cessation of therapy because ovulation suppression can sometimes persist for as long as 18 months after the last injection. Long-term DMPA use reduces menstrual blood loss and is associated with a decreased risk of endometrial cancer. Suppression of endogenous estrogen secretion may be associated with a reversible reduction in bone density, and changes in plasma lipids are associated with an increased risk of atherosclerosis.

The progestin implant method utilizes the subcutaneous implantation of capsules containing etonogestrel. These capsules release one fifth to one third as much steroid as oral agents, are extremely effective, and last for 2–4 years. The low levels of hormone have little effect on lipoprotein and carbohydrate metabolism or blood pressure. The disadvantages include the need for surgical insertion and removal of capsules and some irregular bleeding rather than predictable menses. An association of intracranial hypertension with an earlier type of implant utilizing norgestrel was observed in a small number of women. Patients experiencing headache or visual disturbances should be checked for papilledema.

Contraception with progestins is useful in patients with hepatic disease, hypertension, psychosis or mental retardation, or prior thromboembolism. The side effects include headache, dizziness, bloating and weight gain of 1–2 kg, and a reversible reduction of glucose tolerance.

Postcoital Contraceptives Pregnancy can be prevented following coitus by the administration of estrogens alone, progestin alone, or in combination (“morning after” contraception). When treatment is begun within 72 hours, it is effective 99% of the time. Some effective schedules are shown in Table 40–4. The hormones are often administered with antiemetics, since 40% of patients have nausea or vomiting. Other adverse effects include headache, dizziness, breast tenderness, and abdominal and leg cramps. Considerable controversy has accompanied the proposal to make these agents available without a prescription in the USA. TABLE 40–4 Schedules for use of postcoital contraceptives.

Mifepristone, an antagonist at progesterone and glucocorticoid receptors, has a luteolytic effect and is effective as a postcoital contraceptive. When combined with a prostaglandin it is also an effective abortifacient.

Beneficial Effects of Oral Contraceptives It has become apparent that reduction in the dose of the constituents of oral contraceptives has markedly reduced mild and severe adverse effects, providing a relatively safe and convenient method of contraception for many young women. Treatment with oral contraceptives has also been shown to be associated with many benefits unrelated to contraception. These include a reduced risk of ovarian cysts, ovarian and endometrial cancer, and benign breast disease. There is a lower incidence of ectopic pregnancy. Iron deficiency and rheumatoid arthritis are less common, and premenstrual symptoms, dysmenorrhea, endometriosis, acne, and hirsutism may be ameliorated with their use.

ESTROGEN & PROGESTERONE INHIBITORS & ANTAGONISTS TAMOXIFEN & RELATED PARTIAL AGONIST ESTROGENS Tamoxifen, a competitive partial agonist inhibitor of estradiol at the estrogen receptor (Figure 40–5), was the first selective estrogen receptor modulator to be introduced. The mechanism of its mixed agonist/antagonist relations to the estrogen receptor has been intensively studied but is still not completely understood. Proposals include recruitment of different coregulators to the estrogen receptor when it binds tamoxifen rather than estrogen, differential activation of heterodimers (ERα-ERβ) versus homodimers, competition of ERα

by ERβ and others. Tamoxifen is extensively used in the palliative treatment of breast cancer in postmenopausal women and is approved for chemoprevention of breast cancer in high-risk women (see Chapter 54). It is a nonsteroidal agent (see structure below) that is given orally. Peak plasma levels are reached in a few hours. Tamoxifen has an initial half-life of 7–14 hours in the circulation and is predominantly excreted by the liver. It is used in doses of 10–20 mg twice daily. Hot flushes and nausea and vomiting occur in 25% of patients, and many other minor adverse effects are observed. Studies of patients treated with tamoxifen as adjuvant therapy for early breast cancer have shown a 35% decrease in contralateral breast cancer. However, adjuvant therapy extended beyond 5 years in patients with breast cancer has shown no further improvement in outcome. In fact, resistant lines of tumor cells may recognize tamoxifen as an agonist rather than an antagonist, perhaps due to changes in the coregulators that interact with the estrogen receptor. Toremifene is a structurally similar compound with very similar properties, indications, and toxicities.

FIGURE 40–5 Control of ovarian secretion and the actions of its hormones. In the follicular phase the ovary produces mainly estrogens; in the luteal phase it produces estrogens and progesterone. SERMs, selective estrogen receptor modulators. See text.

Prevention of the expected loss of lumbar spine bone density and plasma lipid changes consistent with a reduction in the risk for atherosclerosis have also been reported in tamoxifen-treated patients following spontaneous or surgical menopause. However, this agonist activity also affects the uterus and may increase the risk of endometrial cancer.

Raloxifene is another partial estrogen agonist-antagonist at some but not all target tissues. It has estrogenic effects on lipids and bone but appears not to stimulate the endometrium or breast. Although subject to a high first-pass effect, raloxifene has a very large volume of distribution and a long half-life (> 24 hours), so it can be taken once a day. Raloxifene has been approved in the USA for the prevention of postmenopausal osteoporosis and prophylaxis of breast cancer in women with risk factors. Newer SERMs have been developed and one, bazedoxifene, in combination with conjugated estrogens, is approved for treatment of menopausal symptoms and prophylaxis of postmenopausal osteoporosis.

Clomiphene is an older partial agonist, a weak estrogen that also acts as a competitive inhibitor of endogenous estrogens (Figure 40–5). It has found use as an ovulation-inducing agent (see below).

MIFEPRISTONE (RU 486) Mifepristone is a “19-norsteroid” that binds strongly to the progesterone receptor and inhibits the activity of progesterone. The drug has luteolytic properties in 80% of women when given in the midluteal period. The mechanism of this effect is unknown, but it may provide the basis for using mifepristone as a contraceptive (as opposed to an abortifacient). However, because the compound has a long half-life, large doses may prolong the follicular phase of the subsequent cycle and so make it difficult to use continuously for this purpose. A single dose of 600 mg is an effective emergency postcoital contraceptive, though it may result in delayed ovulation in the following cycle. As noted in Chapter 39, the drug also binds to and acts as an antagonist at the glucocorticoid receptor. Limited clinical studies suggest that mifepristone or other analogs with similar properties may be useful in the treatment of endometriosis, Cushing’s syndrome, breast cancer, and possibly other neoplasms such as meningiomas that contain glucocorticoid or progesterone receptors.

Mifepristone’s major use thus far has been to terminate early pregnancies. Doses of 400–600 mg/d for 4 days or 800 mg/d for 2 days successfully terminated pregnancy in over 85% of the women studied. The major adverse effect was prolonged bleeding that on most occasions did not require treatment. The combination of a single oral dose of 600 mg of mifepristone and a vaginal pessary containing 1 mg of prostaglandin E1 or oral misoprostol has been found to effectively terminate pregnancy in over 95% of patients treated during the first 7 weeks after conception. The adverse effects of the medications included vomiting, diarrhea, and abdominal or pelvic pain. As many as 5% of patients have vaginal bleeding requiring intervention. Because of these adverse effects, mifepristone is administered only by physicians at family planning centers. Note: In a very small number of cases, use of a vaginal tablet for the prostaglandin dose has been associated with sepsis, so it is recommended that both drugs be given by mouth in all patients.

ZK 98734 (lilopristone) is a potent experimental progesterone inhibitor and abortifacient in doses of 25 mg twice daily. Like mifepristone, it also appears to have antiglucocorticoid activity.

DANAZOL Danazol, an isoxazole derivative of ethisterone (17α-ethinyltestosterone) with weak progestational, androgenic, and glucocorticoid activities, is used to suppress ovarian function. Danazol inhibits the midcycle surge of LH and FSH and can prevent the compensatory increase in LH and FSH following castration in animals, but it does not significantly lower or suppress basal LH or FSH levels in normal women (Figure 40–5). Danazol binds to androgen, progesterone, and glucocorticoid receptors and can translocate the androgen receptor into the nucleus to initiate androgen-specific RNA synthesis. It does not bind to intracellular estrogen receptors, but it does bind to sex hormone-binding and corticosteroid-binding globulins. It inhibits P450scc (the cholesterol side chain-cleaving enzyme), 3β-hydroxysteroid dehydrogenase, 17α-hydroxysteroid dehydrogenase, P450c17 (17α-hydroxylase), P450c11 (11β-hydroxylase), and P450c21 (21βhydroxylase). However, it does not inhibit aromatase, the enzyme required for estrogen synthesis. It increases the mean clearance of progesterone, probably by competing with the hormone for binding proteins, and may have similar effects on other active steroid hormones. Ethisterone, a major metabolite of danazol, has both progestational and mild androgenic effects.

Danazol is slowly metabolized in humans, having a half-life of over 15 hours. This results in stable circulating levels when the drug is administered twice daily. It is highly concentrated in the liver, adrenals, and kidneys and is excreted in both feces and urine.

Danazol has been employed as an inhibitor of gonadal function and has found its major use in the treatment of endometriosis. For this purpose, it can be given in a dosage of 600 mg/d. The dosage is reduced to 400 mg/d after 1 month and to 200 mg/d in 2 months. About 85% of patients show marked improvement in 3–12 months.

Danazol has also been used in the treatment of fibrocystic disease of the breast and hematologic or allergic disorders, including hemophilia, Christmas disease, idiopathic thrombocytopenic purpura, and angioneurotic edema.

The major adverse effects are weight gain, edema, decreased breast size, acne and oily skin, increased hair growth, deepening of the voice, headache, hot flushes, changes in libido, and muscle cramps. Although mild adverse effects are very common, it is seldom necessary to discontinue the drug because of them. Occasionally, because of its inherent glucocorticoid activity, danazol may cause adrenal suppression.

Danazol should be used with great caution in patients with hepatic dysfunction, since it has been reported to produce mild to moderate hepatocellular damage in some patients, as evidenced by enzyme changes. It is also contraindicated during pregnancy and breast-feeding, as it may produce urogenital abnormalities in the offspring.

OTHER INHIBITORS Anastrozole, a selective nonsteroidal inhibitor of aromatase (the enzyme required for estrogen synthesis, Figures 40–2 and 40–5), is effective in some women whose breast tumors have become resistant to tamoxifen (see Chapter 54) . Letrozole is similar.

Exemestane, a steroid molecule, is an irreversible inhibitor of aromatase. Like anastrozole and letrozole, it is approved for use in women with advanced breast cancer (see Chapter 54).

Several other aromatase inhibitors are undergoing clinical trials in patients with breast cancer. Fadrozole is an oral nonsteroidal (triazole) inhibitor of aromatase activity. These compounds appear to be as effective as tamoxifen. In addition to their use in breast cancer, aromatase inhibitors have been successfully employed as adjuncts to androgen antagonists in the treatment of precocious puberty and as primary treatment in the excessive aromatase syndrome.

Fulvestrant is a pure estrogen receptor antagonist that has been somewhat more effective than those with partial agonist effects in some patients who have become resistant to tamoxifen. Fulvestrant is approved for use in breast cancer patients who have become resistant to tamoxifen. ICI 164,384 is a newer antagonist; it inhibits dimerization of the occupied estrogen receptor and interferes with its binding to DNA. GnRH and its analogs (nafarelin, buserelin, etc) have become important in both stimulating and inhibiting ovarian function. They are discussed in Chapter 37.

OVULATION-INDUCING AGENTS CLOMIPHENE Clomiphene citrate, a partial estrogen agonist, is closely related to the estrogen chlorotrianisene (Figure 40–3). This compound is well absorbed when taken orally. It has a half-life of 5–7 days and is excreted primarily in the urine. It exhibits significant protein binding and enterohepatic circulation and is distributed to adipose tissues.

Pharmacologic Effects A. Mechanisms of Action Clomiphene is a partial agonist at estrogen receptors. The estrogenic agonist effects are best demonstrated in animals with marked gonadal deficiency. Clomiphene has also been shown to effectively inhibit the action of stronger estrogens. In humans it leads to an increase in the secretion of gonadotropins and estrogens by inhibiting estradiol’s negative feedback effect on the gonadotropins (Figure 40–5).

B. Effects The pharmacologic importance of this compound rests on its ability to stimulate ovulation in women with oligomenorrhea or amenorrhea and ovulatory dysfunction. The majority of patients suffer from polycystic ovary syndrome, a common disorder affecting about 7% of women of reproductive age. The syndrome is characterized by gonadotropin-dependent ovarian hyperandrogenism associated with anovulation and infertility. The disorder is frequently accompanied by adrenal hyperandrogenism. Clomiphene probably blocks the feedback inhibitory influence of estrogens on the hypothalamus, causing a surge of gonadotropins, which leads to ovulation.

Clinical Use Clomiphene is used in the treatment of disorders of ovulation in patients who wish to become pregnant. Usually, a single ovulation is induced by a single course of therapy, and the patient must be treated repeatedly until pregnancy is achieved, since normal ovulatory function does not usually resume. The compound is of no value in patients with ovarian or pituitary failure.

When clomiphene is administered in a dosage of 100 mg/d for 5 days, a rise in plasma LH and FSH is observed after several days. In patients who ovulate, the initial rise is followed by a second rise of gonadotropin levels just prior to ovulation.

Adverse Effects The most common adverse effects in patients treated with this drug are hot flushes, which resemble those experienced by menopausal patients. They tend to be mild, and disappear when the drug is discontinued. There have been occasional reports of eye symptoms due to intensification and prolongation of afterimages. These are generally of short duration. Headache, constipation, allergic skin reactions, and reversible hair loss have been reported occasionally.

The effective use of clomiphene is associated with some stimulation of the ovaries and usually with ovarian enlargement. The degree of enlargement tends to be greater and its incidence higher in patients who have enlarged ovaries at the beginning of therapy.

A variety of other symptoms such as nausea and vomiting, increased nervous tension, depression, fatigue, breast soreness, weight

gain, urinary frequency, and heavy menses have also been reported. However, these appear to result from the hormonal changes associated with an ovulatory menstrual cycle rather than from the medication. The incidence of multiple pregnancy is approximately 10%. Clomiphene has not been shown to have an adverse effect when inadvertently given to women who are already pregnant.

Contraindications & Cautions Special precautions should be observed in patients with enlarged ovaries. These women are thought to be more sensitive to this drug and should receive small doses. Any patient who complains of abdominal symptoms should be examined carefully. Maximum ovarian enlargement occurs after the 5-day course has been completed, and many patients can be shown to have a palpable increase in ovarian size by the seventh to tenth days. Treatment with clomiphene for more than a year may be associated with an increased risk of lowgrade ovarian cancer; however, the evidence for this effect is not conclusive.

Special precautions must also be taken in patients who have visual symptoms associated with clomiphene therapy, since these symptoms may make activities such as driving more hazardous.

OTHER DRUGS USED IN OVULATORY DISORDERS In addition to clomiphene, a variety of other hormonal and nonhormonal agents are used in treating anovulatory disorders. They are discussed in Chapter 37.

THE TESTIS (ANDROGENS & ANABOLIC STEROIDS, ANTIANDROGENS, & MALE CONTRACEPTION) The testis, like the ovary, has both gametogenic and endocrine functions. The onset of gametogenic function of the testes is controlled largely by the secretion of FSH by the pituitary. High concentrations of testosterone locally are also required for continuing sperm production in the seminiferous tubules. The Sertoli cells in the seminiferous tubules may be the source of the estradiol produced in the testes via aromatization of locally produced testosterone. With LH stimulation, testosterone is produced by the interstitial or Leydig cells found in the spaces between the seminiferous tubules.

The Sertoli cells in the testis synthesize and secrete a variety of active proteins, including müllerian duct inhibitory factor, inhibin, and activin. As in the ovary, inhibin and activin appear to be the product of three genes that produce a common α subunit and two β subunits, A and B. Activin is composed of the two β subunits (β AβB). There are two inhibins (A and B), which contain the α subunit and one of the β subunits. Activin stimulates pituitary FSH release and is structurally similar to transforming growth factor-β, which also increases FSH. The inhibins in conjunction with testosterone and dihydrotestosterone are responsible for the feedback inhibition of pituitary FSH secretion.

ANDROGENS & ANABOLIC STEROIDS In humans, the most important androgen secreted by the testis is testosterone. The pathways of synthesis of testosterone in the testes are similar to those previously described for the adrenal gland and ovary (Figures 39–1 and 40–2).

In men, approximately 8 mg of testosterone is produced daily. About 95% is produced by the Leydig cells and only 5% by the adrenals. The testis also secretes small amounts of another potent androgen, dihydrotestosterone, as well as androstenedione and dehydroepiandrosterone, which are weak androgens. Pregnenolone and progesterone and their 17-hydroxylated derivatives are also released in small amounts. Plasma levels of testosterone in males are about 0.6 mcg/dL after puberty and appear to decline after age 50.

Testosterone is also present in the plasma of women in concentrations of approximately 0.03 mcg/dL and is derived in approximately equal parts from the ovaries and adrenals and by the peripheral conversion of other hormones.

About 65% of circulating testosterone is bound to sex hormone-binding globulin. SHBG is increased in plasma by estrogen, by thyroid hormone, and in patients with cirrhosis of the liver. It is decreased by androgen and growth hormone and is lower in obese individuals.

Most of the remaining testosterone is bound to albumin. Approximately 2% remains free and available to enter cells and bind to intracellular receptors.

Metabolism In many target tissues, testosterone is converted to dihydrotestosterone by 5α-reductase. In these tissues, dihydrotestosterone is the major active androgen. The conversion of testosterone to estradiol by P450 aromatase also occurs in some tissues, including adipose tissue, liver, and the hypothalamus, where it may be of importance in regulating gonadal function.

The major pathway for the degradation of testosterone in humans occurs in the liver, with the reduction of the double bond and ketone in the A ring, as is seen in other steroids with a Δ 4 -ketone configuration in the A ring. This leads to the production of inactive substances such as androsterone and etiocholanolone that are then conjugated and excreted in the urine.

Androstenedione, dehydroepiandrosterone (DHEA), and dehydroepiandrosterone sulfate (DHEAS) are also produced in significant amounts in humans, although largely in the adrenal gland rather than in the testes. They contribute slightly to the normal maturation process supporting other androgen-dependent pubertal changes in the human, primarily development of pubic and axillary hair and bone maturation. As noted in Chapter 39, some studies suggest that DHEA and DHEAS may have other central nervous system and metabolic effects and may prolong life in rabbits. In men they may improve the sense of well-being and inhibit atherosclerosis. In a placebo-controlled clinical trial in patients with systemic lupus erythematosus, DHEA demonstrated some beneficial effects (see Adrenal Androgens, Chapter 39). Adrenal androgens are to a large extent metabolized in the same fashion as testosterone. Both steroids—but particularly androstenedione—can be converted by peripheral tissues to estrone in very small amounts (1–5%). The P450 aromatase enzyme responsible for this conversion is also found in the brain and is thought to play an important role in development.

Physiologic Effects In the normal male, testosterone or its active metabolite 5α-dihydrotestosterone is responsible for the many changes that occur in puberty. In addition to the general growth-promoting properties of androgens on body tissues, these hormones are responsible for penile and scrotal growth. Changes in the skin include the appearance of pubic, axillary, and beard hair. The sebaceous glands become more active, and the skin tends to become thicker and oilier. The larynx grows and the vocal cords become thicker, leading to a lower-pitched voice. Skeletal growth is stimulated and epiphysial closure accelerated. Other effects include growth of the prostate and seminal vesicles, darkening of the skin, and increased skin circulation. Androgens play an important role in stimulating and maintaining sexual function in men. Androgens increase lean body mass and stimulate body hair growth and sebum secretion. Metabolic effects include the reduction of hormone binding and other carrier proteins and increased liver synthesis of clotting factors, triglyceride lipase, α1 -antitrypsin, haptoglobin, and sialic acid. They also stimulate renal erythropoietin secretion and decrease HDL levels.

Synthetic Steroids with Androgenic & Anabolic Action Testosterone, when administered by mouth, is rapidly absorbed. However, it is largely converted to inactive metabolites, and only about one sixth of the dose administered is available in active form. Testosterone can be administered parenterally, but it has a more prolonged absorption time and greater activity in the propionate, enanthate, undecanoate, or cypionate ester forms. These derivatives are hydrolyzed to release free testosterone at the site of injection. Testosterone derivatives alkylated at the 17 position, eg, methyltestosterone and fluoxymesterone, are active when given by mouth.

Testosterone and its derivatives have been used for their anabolic effects as well as in the treatment of testosterone deficiency.

Although testosterone and other known active steroids can be isolated in pure form and measured by weight, biologic assays are still used in the investigation of new compounds. In some of these studies in animals, the anabolic effects of the compound as measured by trophic effects on muscles or the reduction of nitrogen excretion may be dissociated from the other androgenic effects. This has led to the marketing of compounds claimed to have anabolic activity associated with only weak androgenic effects. Unfortunately, this dissociation is less marked in humans than in the animals used for testing (Table 40–5), and all are potent androgens.

TABLE 40–5 Androgens: Preparations available and relative androgenic:anabolic activity in animals.

Pharmacologic Effects A. Mechanism of Action Like other steroids, testosterone acts intracellularly in target cells. In skin, prostate, seminal vesicles, and epididymis, it is converted to 5αdihydrotestosterone by 5α-reductase. In these tissues, dihydrotestosterone is the dominant androgen. The distribution of this enzyme in the fetus is different and has important developmental implications.

Testosterone and dihydrotestosterone bind to the intracellular androgen receptor, initiating a series of events similar to those described above for estradiol and progesterone, leading to growth, differentiation, and synthesis of a variety of enzymes and other functional proteins.

B. Effects In the male at puberty, androgens cause development of the secondary sex characteristics (see above). In the adult male, large doses of testosterone—when given alone—or its derivatives suppress the secretion of gonadotropins and result in some atrophy of the interstitial tissue and the tubules of the testes. Since fairly large doses of androgens are required to suppress gonadotropin secretion, it has been postulated that inhibin, in combination with androgens, is responsible for the feedback control of secretion. In women, androgens are capable of producing changes similar to those observed in the prepubertal male. These include growth of facial and body hair, deepening of the voice, enlargement of the clitoris, frontal baldness, and prominent musculature. The natural androgens stimulate erythrocyte production.

The administration of androgens reduces the excretion of nitrogen into the urine, indicating an increase in protein synthesis or a decrease in protein breakdown within the body. This effect is much more pronounced in women and children than in normal men.

Clinical Uses A. Androgen Replacement Therapy in Men Androgens are used to replace or augment endogenous androgen secretion in hypogonadal men (Table 40–6). Even in the presence of pituitary deficiency, androgens are used rather than gonadotropin except when normal spermatogenesis is to be achieved. In patients with hypopituitarism, androgens are not added to the treatment regimen until puberty, at which time they are instituted in gradually increasing doses to achieve the growth spurt and the development of secondary sex characteristics. In these patients, therapy should be started with long-acting agents such as testosterone enanthate or cypionate in doses of 50 mg intramuscularly, initially every 4, then every 3, and finally every 2 weeks, with each change taking place at 3-month intervals. The dose is then doubled to 100 mg every 2 weeks until maturation is complete. Finally, it is changed to the adult replacement dose of 200 mg at 2-week intervals.

TABLE 40–6 Androgen preparations for replacement therapy.

Testosterone propionate, though potent, has a short duration of action and is not practical for long-term use. Testosterone undecanoate can be given orally, administering large amounts of the steroid twice daily (eg, 40 mg/d); however, this is not recommended because oral testosterone administration has been associated with liver tumors. Testosterone can also be administered transdermally; skin patches or gels are available for scrotal or other skin area application. Two applications daily are usually required for replacement therapy. Implanted pellets and other longer-acting preparations are under study. The development of polycythemia or hypertension may require some reduction in dose.

B. Gynecologic Disorders Androgens are used occasionally in the treatment of certain gynecologic disorders, but the undesirable effects in women are such that they must be used with great caution. Androgens have been used to reduce breast engorgement during the postpartum period, usually in conjunction with estrogens. The weak androgen danazol is used in the treatment of endometriosis (see above).

Androgens are sometimes given in combination with estrogens for replacement therapy in the postmenopausal period in an attempt to eliminate the endometrial bleeding that may occur when only estrogens are used and to enhance libido. They have been used for chemotherapy of breast tumors in premenopausal women.

C. Use as Protein Anabolic Agents Androgens and anabolic steroids have been used in conjunction with dietary measures and exercises in an attempt to reverse protein loss after trauma, surgery, or prolonged immobilization and in patients with debilitating diseases.

D. Anemia In the past, large doses of androgens were employed in the treatment of refractory anemias such as aplastic anemia, Fanconiâ&#x20AC;&#x2122;s anemia, sickle cell anemia, myelofibrosis, and hemolytic anemias. Recombinant erythropoietin has largely replaced androgens for this purpose.

E. Osteoporosis Androgens and anabolic agents have been used in the treatment of osteoporosis, either alone or in conjunction with estrogens. With the exception of substitution therapy in hypogonadism, bisphosphonates have largely replaced androgen use for this purpose.

F. Use as Growth Stimulators These agents have been used to stimulate growth in boys with delayed puberty. If the drugs are used carefully, these children will probably achieve their expected adult height. If treatment is too vigorous, the patient may grow rapidly at first but will not achieve full

predicted final stature because of the accelerated epiphysial closure that occurs. It is difficult to control this type of therapy adequately even with frequent X-ray examination of the epiphyses, since the action of the hormones on epiphysial centers may continue for many months after therapy is discontinued.

G. Anabolic Steroid and Androgen Abuse in Sports The use of anabolic steroids by athletes has received worldwide attention. Many athletes and their coaches believe that anabolic steroids —in doses 10–200 times larger than the daily normal physiologic production—increase strength and aggressiveness, thereby improving competitive performance. Such effects have been unequivocally demonstrated only in women. Furthermore, the adverse effects of these drugs clearly make their use inadvisable.

H. Aging Androgen production falls with age in men and may contribute to the decline in muscle mass, strength, and libido. Preliminary studies of androgen replacement in aging males with low androgen levels show an increase in lean body mass and hematocrit and a decrease in bone turnover. Longer studies will be required to assess the usefulness of this therapy.

Adverse Effects The adverse effects of these compounds are due largely to their masculinizing actions and are most noticeable in women and prepubertal children. In women, the administration of more than 200–300 mg of testosterone per month is usually associated with hirsutism, acne, amenorrhea, clitoral enlargement, and deepening of the voice. These effects may occur with even smaller doses in some women. Some of the androgenic steroids exert progestational activity, leading to endometrial bleeding upon discontinuation. These hormones also alter serum lipids and could conceivably increase susceptibility to atherosclerotic disease in women.

Except under the most unusual circumstances, androgens should not be used in infants. Recent studies in animals suggest that administration of androgens in early life may have profound effects on maturation of central nervous system centers governing sexual development, particularly in the female. Administration of these drugs to pregnant women may lead to masculinization or undermasculinization of the external genitalia in the female and male fetus, respectively. Although the above-mentioned effects may be less marked with the anabolic agents, they do occur.

Sodium retention and edema are not common but must be carefully watched for in patients with heart and kidney disease.

Most of the synthetic androgens and anabolic agents are 17-alkyl-substituted steroids. Administration of drugs with this structure is often associated with evidence of hepatic dysfunction. Hepatic dysfunction usually occurs early in the course of treatment, and the degree is proportionate to the dose. Bilirubin levels may increase until clinical jaundice is apparent. The cholestatic jaundice is reversible upon cessation of therapy, and permanent changes do not occur. In older males, prostatic hyperplasia may develop, causing urinary retention.

Replacement therapy in men may cause acne, sleep apnea, erythrocytosis, gynecomastia, and azoospermia. Supraphysiologic doses of androgens produce azoospermia and decrease in testicular size, both of which may take months to recover after cessation of therapy.

The alkylated androgens in high doses can produce peliosis hepatica, cholestasis, and hepatic failure. They lower plasma HDL and may increase LDL. Hepatic adenomas and carcinomas have also been reported. Behavioral effects include psychological dependence, increased aggressiveness, and psychotic symptoms.

Contraindications & Cautions The use of androgenic steroids is contraindicated in pregnant women or women who may become pregnant during the course of therapy.

Androgens should not be administered to male patients with carcinoma of the prostate or breast. Until more is known about the effects of these hormones on the central nervous system in developing children, they should be avoided in infants and young children.

Special caution is required in giving these drugs to children to produce a growth spurt. In most patients, the use of somatotropin is more appropriate (see Chapter 37).

Care should be exercised in the administration of these drugs to patients with renal or cardiac disease predisposed to edema. If sodium and water retention occurs, it will respond to diuretic therapy.

Methyltestosterone therapy is associated with creatinuria, but the significance of this finding is not known.

Caution: Several cases of hepatocellular carcinoma have been reported in patients with aplastic anemia treated with androgen anabolic therapy. Erythropoietin and colony-stimulating factors (see Chapter 33) should be used instead.

ANDROGEN SUPPRESSION & ANTIANDROGENS ANDROGEN SUPPRESSION

The treatment of advanced prostatic carcinoma often requires orchiectomy or large doses of estrogens to reduce available endogenous androgen. The psychological effects of the former and gynecomastia produced by the latter make these approaches undesirable. As noted in Chapter 37, the GnRH analogs such as goserelin, nafarelin, buserelin, and leuprolide acetate produce effective gonadal suppression when blood levels are continuous rather than pulsatile (see Chapter 37 and Figure 40â&#x20AC;&#x201C;6).

FIGURE 40–6 Control of androgen secretion and activity and some sites of action of antiandrogens: (1) competitive inhibition of GnRH receptors; (2) stimulation (+, pulsatile administration) or inhibition via desensitization of GnRH receptors (–, continuous administration); (3) decreased synthesis of testosterone in the testis; (4) decreased synthesis of dihydrotestosterone by inhibition of 5α-reductase; (5) competition for binding to cytosol androgen receptors.

ANTIANDROGENS The potential usefulness of antiandrogens in the treatment of patients producing excessive amounts of testosterone has led to the search for effective drugs that can be used for this purpose. Several approaches to the problem, especially inhibition of synthesis and receptor antagonism, have met with some success.

Steroid Synthesis Inhibitors Ketoconazole, used primarily in the treatment of fungal disease, is an inhibitor of adrenal and gonadal steroid synthesis, as described in Chapter 39. It does not affect ovarian aromatase, but it reduces human placental aromatase activity. It displaces estradiol and dihydrotestosterone from sex hormone-binding protein in vitro and increases the estradiol:testosterone ratio in plasma in vivo by a different mechanism. However, it does not appear to be clinically useful in women with increased androgen levels because of the toxicity associated with prolonged use of the 400–800 mg/d required. The drug has also been used experimentally to treat prostatic carcinoma, but the results have not been encouraging. Men treated with ketoconazole often develop reversible gynecomastia during therapy; this may be due to the demonstrated increase in the estradiol:testosterone ratio.

Conversion of Steroid Precursors to Androgens Several compounds have been developed that inhibit the 17-hydroxylation of progesterone or pregnenolone, thereby preventing the action of the side chain-splitting enzyme and the further transformation of these steroid precursors to active androgens. A few of these compounds have been tested clinically but have been too toxic for prolonged use. As noted in Chapter 39, abiraterone, a newer 17αhydroxylase inhibitor, has been approved for use in metastatic prostate cancer.

Since dihydrotestosterone—not testosterone—appears to be the essential androgen in the prostate, androgen effects in this and similar dihydrotestosterone-dependent tissues can be reduced by an inhibitor of 5α-reductase (Figure 40–6). Finasteride, a steroid-like inhibitor of this enzyme, is orally active and causes a reduction in dihydrotestosterone levels that begins within 8 hours after administration and lasts for about 24 hours. The half-life is about 8 hours (longer in elderly individuals). Forty to 50 percent of the dose is metabolized; more than half is excreted in the feces. Finasteride has been reported to be moderately effective in reducing prostate size in men with benign prostatic hyperplasia and is approved for this use in the USA. The dosage is 5 mg/d. Dutasteride is a similar orally active steroid derivative with a slow onset of action and a much longer half-life than finasteride. It is approved for treatment of benign prostatic hyperplasia at a dosage of 0.5 mg daily. These drugs are not approved for use in women or children, although finasteride has been used successfully in the treatment of hirsutism in women and is approved for treatment of early male pattern baldness in men (1 mg/d).

Receptor Inhibitors Cyproterone and cyproterone acetate are effective antiandrogens that inhibit the action of androgens at the target organ. The acetate form has a marked progestational effect that suppresses the feedback enhancement of LH and FSH, leading to a more effective

antiandrogen effect. These compounds have been used in women to treat hirsutism and in men to decrease excessive sexual drive and are being studied in other conditions in which the reduction of androgenic effects would be useful. Cyproterone acetate in a dosage of 2 mg/d administered concurrently with an estrogen is used in the treatment of hirsutism in women, doubling as a contraceptive pill; it has orphan drug status in the USA. Flutamide, a substituted anilide, is a potent antiandrogen that has been used in the treatment of prostatic carcinoma. Although not a steroid, it behaves like a competitive antagonist at the androgen receptor. It is rapidly metabolized in humans. It frequently causes mild gynecomastia (probably by increasing testicular estrogen production) and occasionally causes mild reversible hepatic toxicity.

Administration of this compound causes some improvement in most patients with prostatic carcinoma who have not had prior endocrine therapy. Preliminary studies indicate that flutamide is also useful in the management of excess androgen effect in women.

Bicalutamide, nilutamide, and enzalutamide are potent orally active antiandrogens that can be administered as a single daily dose and are used in patients with metastatic carcinoma of the prostate. Studies in patients with carcinoma of the prostate indicate that these agents are well tolerated. Bicalutamide is recommended (to reduce tumor flare) for use in combination with a GnRH analog and may have fewer gastrointestinal side effects than flutamide. A dosage of 150–200 mg/d (when used alone) is required to reduce prostatespecific antigen levels to those achieved by castration, but, in combination with a GnRH analog, 50 mg/d may be adequate. Nilutamide is administered in a dosage of 300 mg/d for 30 days followed by 150 mg/d. The dosage of enzalutamide is 160 mg/d orally.

Spironolactone, a competitive inhibitor of aldosterone (see Chapter 15), also competes with dihydrotestosterone for the androgen receptors in target tissues. It also reduces 17α-hydroxylase activity, lowering plasma levels of testosterone and androstenedione. It is used in dosages of 50–200 mg/d in the treatment of hirsutism in women and appears to be as effective as finasteride, flutamide, or cyproterone in this condition.

CHEMICAL CONTRACEPTION IN MEN Although many studies have been conducted, an effective and nontoxic oral contraceptive for men has not yet been found. For example, various androgens, including testosterone and testosterone enanthate, in a dosage of 400 mg per month, produced azoospermia in less than half the men treated. Minor adverse reactions, including gynecomastia and acne, were encountered. Testosterone in combination with danazol was well tolerated but no more effective than testosterone alone. Androgens in combination with a progestin such as medroxyprogesterone acetate were no more effective. However, preliminary studies indicate that the intramuscular administration of 100 mg of testosterone enanthate weekly together with 500 mg of levonorgestrel daily orally can produce azoospermia in 94% of men.

Retinoic acid is important in the maturation of sperm and the testis contains a unique isoform of the alcohol dehydrogenase enzyme that converts retinol to retinoic acid but no nontoxic inhibitor of this enzyme has been found to date.

Cyproterone acetate, a very potent progestin and antiandrogen, also produces oligospermia; however, it does not cause reliable contraception.

At present, pituitary hormones—and potent antagonist analogs of GnRH—are receiving increased attention. A GnRH antagonist in combination with testosterone has been shown to produce reversible azoospermia in nonhuman primates.

GOSSYPOL Extensive trials of this cottonseed derivative have been conducted in China. This compound destroys elements of the seminiferous epithelium but does not significantly alter the endocrine function of the testis.

In Chinese studies, large numbers of men were treated with 20 mg/d of gossypol or gossypol acetic acid for 2 months, followed by a maintenance dosage of 60 mg/wk. On this regimen, 99% of men developed sperm counts below 4 million/mL. Preliminary data indicate that recovery (return of normal sperm count) following discontinuance of gossypol administration is more apt to occur in men whose counts do not fall to extremely low levels and when administration is not continued for more than 2 years. Hypokalemia is the major adverse effect and may lead to transient paralysis. Because of low efficacy and significant toxicity, gossypol has been abandoned as a candidate male contraceptive.

PREPARATIONS AVAILABLE*

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CASE STUDY ANSWER The patient should be advised to start daily transdermal estradiol therapy (100 mcg/d) along with oral natural progesterone (200 mg/d) for the last 12 days of each 28-day cycle. On this regimen, her symptoms should disappear and normal monthly uterine bleeding resume. She should also be advised to get adequate exercise and increase her calcium and vitamin D intake as treatment for her osteoporosis.

_______________ * T he use of estrogens in contraception is discussed later in this chapter.

CHAPTER

41 Pancreatic Hormones & Antidiabetic Drugs Martha S. Nolte Kennedy, MD, & Umesh Masharani, MBBS, MRCP (UK)

CASE STUDY A 56-year-old Hispanic woman presents to her medical practitioner with symptoms of fatigue, increased thirst, frequent urination, and exercise intolerance with shortness of breath of many months’ duration. She does not get regular medical care and is unaware of any medical problems. Her family history is significant for obesity, diabetes, high blood pressure, and coronary artery disease in both parents and several siblings. She is not taking any medications. Five of her six children had a birthweight of over 9 pounds.

Physical examination reveals a BMI (body mass index) of 34, blood pressure of 150/90 mm Hg, and evidence of mild peripheral neuropathy. Laboratory tests reveal a random blood sugar of 261 mg/dL; this is confirmed with a fasting plasma glucose of 192 mg/dL. A fasting lipid panel reveals total cholesterol 264 mg/dL, triglycerides 255 mg/dL, high-density lipoproteins 43 mg/dL, and low-density lipoproteins 170 mg/dL. What type of diabetes does this woman have? What further evaluations should be obtained?

How would you treat her diabetes?

THE ENDOCRINE PANCREAS The endocrine pancreas in the adult human consists of approximately 1 million islets of Langerhans interspersed throughout the pancreatic gland. Within the islets, at least five hormone-producing cells are present (Table 41–1). Their hormone products include insulin, the storage and anabolic hormone of the body; islet amyloid polypeptide (IAPP, or amylin), which modulates appetite, gastric emptying, and glucagon and insulin secretion; glucagon, the hyperglycemic factor that mobilizes glycogen stores; somatostatin, a universal inhibitor of secretory cells; pancreatic peptide, a small protein that facilitates digestive processes by a mechanism not yet clarified; and ghrelin, a peptide known to increase pituitary growth hormone release.

TABLE 41–1 Pancreatic islet cells and their secretory products.

Diabetes mellitus is defined as an elevated blood glucose associated with absent or inadequate pancreatic insulin secretion, with or without concurrent impairment of insulin action. The disease states underlying the diagnosis of diabetes mellitus are now classified into four categories: type 1, type 2, other, and gestational diabetes mellitus.

Type 1 Diabetes Mellitus The hallmark of type 1 diabetes is selective beta cell (B cell) destruction and severe or absolute insulin deficiency. Type 1 diabetes is further subdivided into immune-mediated (type 1a) and idiopathic causes (type 1b). The immune form is the most common form of type 1 diabetes. Although most patients are younger than 30 years of age at the time of diagnosis, the onset can occur at any age. Type 1 diabetes is found in all ethnic groups, but the highest incidence is in people from northern Europe and from Sardinia. Susceptibility appears to involve a multifactorial genetic linkage, but only 10–15% of patients have a positive family history. Most patients with type 1 diabetes have one or more circulating antibodies to glutamic acid decarboxylase 65 (GAD 65), insulin autoantibody, tyrosine phosphatase IA2 (ICA 512), and zinc transporter 8 (ZnT8) at the time of diagnosis. These antibodies facilitate the diagnosis of type 1a diabetes and can also be used to screen family members at risk for developing the disease.

For persons with type 1 diabetes, insulin replacement therapy is necessary to sustain life. Pharmacologic insulin is administered by injection into the subcutaneous tissue using a manual injection device or an insulin pump that continuously infuses insulin under the skin.

Interruption of the insulin replacement therapy can be life-threatening and can result in diabetic ketoacidosis or death. Diabetic ketoacidosis is caused by insufficient or absent insulin and results from excess release of fatty acids and subsequent formation of toxic levels of ketoacids.

Some patients with type 1 diabetes have a more indolent autoimmune process and initially retain enough beta cell function to avoid ketosis. They can be treated at first with oral hypoglycemic agents but then need insulin as their beta cell function declines. Antibody studies in northern Europeans indicate that up to 10–15% of “type 2” patients may actually have this milder form of type 1 diabetes (latent autoimmune diabetes of adulthood; LADA).

Type 2 Diabetes Mellitus Type 2 diabetes is characterized by tissue resistance to the action of insulin combined with a relative deficiency in insulin secretion. A given individual may have more resistance or more beta-cell deficiency, and the abnormalities may be mild or severe. Although insulin is produced by the beta cells in these patients, it is inadequate to overcome the resistance, and the blood glucose rises. The impaired insulin action also affects fat metabolism, resulting in increased free fatty acid flux and triglyceride levels and reciprocally low levels of highdensity lipoprotein (HDL).

Individuals with type 2 diabetes may not require insulin to survive, but 30% or more will benefit from insulin therapy to control blood glucose. Although persons with type 2 diabetes ordinarily do not develop ketosis, ketoacidosis may occur as the result of stress such as infection or the use of medication that enhances resistance, eg, corticosteroids. Dehydration in individuals with untreated or poorly controlled type 2 diabetes can lead to a life-threatening condition called nonketotic hyperosmolar coma. In this condition, the blood

glucose may rise to 6–20 times the normal range and an altered mental state develops or the person loses consciousness. Urgent medical care and rehydration are required.

Other Specific Types of Diabetes Mellitus The “other” designation refers to multiple other specific causes of an elevated blood glucose: pancreatectomy, pancreatitis, nonpancreatic diseases, drug therapy, etc. For a detailed list the reader is referred to the reference Expert Committee, 2003.

Gestational Diabetes Mellitus Gestational diabetes (GDM) is defined as any abnormality in glucose levels noted for the first time during pregnancy. Gestational diabetes is diagnosed in approximately 7% of all pregnancies in the USA. During pregnancy, the placenta and placental hormones create an insulin resistance that is most pronounced in the last trimester. Risk assessment for diabetes is suggested starting at the first prenatal visit. High-risk women should be screened immediately. Screening may be deferred in lower-risk women until the 24th to 28th week of gestation.

INSULIN Chemistry Insulin is a small protein with a molecular weight in humans of 5808. It contains 51 amino acids arranged in two chains (A and B) linked by disulfide bridges; there are species differences in the amino acids of both chains. Proinsulin, a long single-chain protein molecule, is processed within the Golgi apparatus of beta cells and packaged into granules, where it is hydrolyzed into insulin and a residual connecting segment called C-peptide by removal of four amino acids (Figure 41–1).

FIGURE 41–1 Structure of human proinsulin (C-peptide plus A and B chains) and insulin. Insulin is shown as the shaded (orange color) peptide chains, A and B. Differences in the A and B chains and amino acid modifications for the rapid-acting insulin analogs (aspart, lispro, and glulisine) and long-acting insulin analogs (glargine and detemir) are discussed in the text. (Adapted, with permission, from Gardner DG, Shoback D [editors]: Greenspan’s Basic & Clinical Endocrinology, 9th ed. McGraw-Hill, 2011. Copyright © The McGraw-Hill Companies, Inc.) Insulin and C-peptide are secreted in equimolar amounts in response to all insulin secretagogues; a small quantity of unprocessed or partially hydrolyzed proinsulin is released as well. Although proinsulin may have some mild hypoglycemic action, C-peptide has no known physiologic function. Granules within the beta cells store the insulin in the form of crystals consisting of two atoms of zinc and six molecules of insulin. The entire human pancreas contains up to 8 mg of insulin, representing approximately 200 biologic units. Originally, the unit was defined on the basis of the hypoglycemic activity of insulin in rabbits. With improved purification techniques, the unit is presently defined on the basis of weight, and present insulin standards used for assay purposes contain 28 units per milligram.

Insulin Secretion Insulin is released from pancreatic beta cells at a low basal rate and at a much higher stimulated rate in response to a variety of stimuli, especially glucose. Other stimulants such as other sugars (eg, mannose), amino acids (especially gluconeogenic amino acids, eg, leucine,

arginine), hormones such as glucagon-like polypeptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), glucagon, cholecystokinin, high concentrations of fatty acids, and β-adrenergic sympathetic activity are recognized. Stimulatory drugs are sulfonylureas, meglitinide and nateglinide, isoproterenol, and acetylcholine. Inhibitory signals are hormones including insulin itself, somatostatin, and leptin; α-adrenergic sympathetic activity; chronically elevated glucose; and low concentrations of fatty acids. Inhibitory drugs include diazoxide, phenytoin, vinblastine, and colchicine.

One mechanism of stimulated insulin release is diagrammed in Figure 41–2. As shown in the figure, hyperglycemia results in increased intracellular ATP levels, which close the ATP-dependent potassium channels. Decreased outward potassium efflux results in depolarization of the beta cell and opening of voltage-gated calcium channels. The resulting increased intracellular calcium triggers secretion of the hormone. The insulin secretagogue drug group (sulfonylureas, meglitinides, and D-phenylalanine) exploits parts of this mechanism.

FIGURE 41–2 One model of control of insulin release from the pancreatic beta cell by glucose and by sulfonylurea drugs. In the resting cell with normal (low) ATP levels, potassium diffuses down its concentration gradient through ATP-gated potassium channels, maintaining the intracellular potential at a fully polarized, negative level. Insulin release is minimal. If glucose concentration rises, ATP production increases, potassium channels close, and depolarization of the cell results. As in muscle and nerve, voltage-gated calcium channels open in response to depolarization, allowing more calcium to enter the cell. Increased intracellular calcium results in increased insulin secretion. Insulin secretagogues close the ATP-dependent potassium channel, thereby depolarizing the membrane and causing increased insulin release by the same mechanism.

Insulin Degradation The liver and kidney are the two main organs that remove insulin from the circulation. The liver normally clears the blood of approximately 60% of the insulin released from the pancreas by virtue of its location as the terminal site of portal vein blood flow, with the kidney removing 35–40% of the endogenous hormone. However, in insulin-treated diabetics receiving subcutaneous insulin injections, this ratio is reversed, with as much as 60% of exogenous insulin being cleared by the kidney and the liver removing no more than 30– 40%. The half-life of circulating insulin is 3–5 minutes.

Circulating Insulin

Basal serum insulin values of 5–15 μU/mL (30–90 pmol/L) are found in normal humans, with a peak rise to 60–90 μU/mL (360–540 pmol/L) during meals.

The Insulin Receptor After insulin has entered the circulation, it diffuses into tissues, where it is bound by specialized receptors that are found on the membranes of most tissues. The biologic responses promoted by these insulin-receptor complexes have been identified in the primary target tissues regulating energy metabolism, ie, liver, muscle, and adipose tissue. The receptors bind insulin with high specificity and affinity in the picomolar range. The full insulin receptor consists of two covalently linked heterodimers, each containing an α subunit, which is entirely extracellular and constitutes the recognition site, and a β subunit that spans the membrane (Figure 41–3). The β subunit contains a tyrosine kinase. The binding of an insulin molecule to the α subunits at the outside surface of the cell activates the receptor and through a conformational change brings the catalytic loops of the opposing cytoplasmic β subunits into closer proximity. This facilitates mutual phosphorylation of tyrosine residues on the β subunits and tyrosine kinase activity directed at cytoplasmic proteins.

FIGURE 41–3 Schematic diagram of the insulin receptor heterodimer in the activated state. IRS, insulin receptor substrate; MAP, mitogen-activated protein; P, phosphate; Tyr, tyrosine.

The first proteins to be phosphorylated by the activated receptor tyrosine kinases are the docking proteins, insulin receptor substrates (IRS). After tyrosine phosphorylation at several critical sites, the IRS molecules bind to and activate other kinases subserving energy metabolism—most significantly phosphatidylinositol-3-kinase—which produce further phosphorylations. Alternatively, they may stimulate a mitogenic pathway and bind to an adaptor protein such as growth factor receptor-binding protein 2, which translates the insulin signal to

a guanine nucleotide-releasing factor that ultimately activates the GTP binding protein, Ras, and the mitogen-activated protein kinase (MAPK) system. The particular IRS-phosphorylated tyrosine kinases have binding specificity with downstream molecules based on their surrounding 4–5 amino acid sequences or motifs that recognize specific Src homology 2 (SH2) domains on the other protein. This network of phosphorylations within the cell represents insulin’s second message and results in multiple effects, including translocation of glucose transporters (especially GLUT 4, Table 41–2) to the cell membrane with a resultant increase in glucose uptake; increased glycogen synthase activity and increased glycogen formation; multiple effects on protein synthesis, lipolysis, and lipogenesis; and activation of transcription factors that enhance DNA synthesis and cell growth and division.

TABLE 41–2 Glucose transporters.

Various hormonal agents (eg, glucocorticoids) lower the affinity of insulin receptors for insulin; growth hormone in excess increases this affinity slightly. Aberrant serine and threonine phosphorylation of the insulin receptor β subunits or IRS molecules may result in insulin resistance and functional receptor down-regulation.

Effects of Insulin on Its Targets Insulin promotes the storage of fat as well as glucose (both sources of energy) within specialized target cells (Figure 41–4) and influences cell growth and the metabolic functions of a wide variety of tissues (Table 41–3).

FIGURE 41â&#x20AC;&#x201C;4 Insulin promotes synthesis (from circulating nutrients) and storage of glycogen, triglycerides, and protein in its major target tissues: liver, fat, and muscle. The release of insulin from the pancreas is stimulated by increased blood glucose, incretins, vagal nerve stimulation, and other factors (see text).

TABLE 41â&#x20AC;&#x201C;3 Endocrine effects of insulin.

Characteristics of Available Insulin Preparations Commercial insulin preparations differ in a number of ways, such as differences in the recombinant DNA production techniques, amino acid sequence, concentration, solubility, and the time of onset and duration of their biologic action.

A. Principal Types and Duration of Action of Insulin Preparations Four principal types of injected insulins are available: (1) rapid-acting, with very fast onset and short duration; (2) short-acting, with rapid onset of action; (3) intermediate-acting; and (4) long-acting, with slow onset of action (Figure 41–5, Table 41–4). Injected rapid-acting and short-acting insulins are dispensed as clear solutions at neutral pH and contain small amounts of zinc to improve their stability and shelf life. Injected intermediate-acting NPH insulins have been modified to provide prolonged action and are dispensed as a turbid suspension at neutral pH with protamine in phosphate buffer (neutral protamine Hagedorn [NPH] insulin). Insulin glargine and insulin detemir are clear, soluble long-acting insulins.

FIGURE 41–5 Extent and duration of action of various types of insulin as indicated by the glucose infusion rates (mg/kg/min) required to maintain a constant glucose concentration. The durations of action shown are typical of an average dose of 0.2–0.3 U/kg. The durations of regular and NPH insulin increase considerably when dosage is increased.

TABLE 41–4 Some insulin preparations available in the USA.1

The goal of subcutaneous insulin therapy is to replicate normal physiologic insulin secretion and replace the background or basal (overnight, fasting, and between-meal) as well as bolus or prandial (mealtime) insulin. An exact reproduction of the normal glycemic profile is not technically possible because of the limitations inherent in subcutaneous administration of insulin. Current regimens generally use insulin analogs because of their more predictable action. Intensive therapy (â&#x20AC;&#x153;tight controlâ&#x20AC;?) attempts to restore near-normal glucose patterns throughout the day while minimizing the risk of hypoglycemia.

Intensive regimens involving multiple daily injections (MDI) use long-acting insulin analogs to provide basal or background coverage, and rapid-acting insulin analogs to meet the mealtime requirements. The latter insulins are given as supplemental doses to correct transient hyperglycemia. The most sophisticated insulin regimen delivers rapid-acting insulin analogs through a continuous subcutaneous insulin infusion device. Conventional therapy consists of split-dose injections of mixtures of rapid- or short-acting and intermediate-acting insulins.

1. Rapid-acting insulin—Three injected rapid-acting insulin analogs—insulin lispro, insulin aspart, and insulin glulisine—are commercially available. The rapid-acting insulins permit more physiologic prandial insulin replacement because their rapid onset and early peak action more closely mimic normal endogenous prandial insulin secretion than regular insulin, and they have the additional benefit of allowing insulin to be taken immediately before the meal without sacrificing glucose control. Their duration of action is rarely more than 4–5 hours, which decreases the risk of late postmeal hypoglycemia. The injected rapid-acting insulins have the lowest variability of absorption (approximately 5%) of all available commercial insulins (compared with 25% for regular insulin and 25% to over 50% for long-acting analog formulations and intermediate insulin, respectively). They are the preferred insulins for use in continuous subcutaneous insulin infusion devices.

Insulin lispro, the first monomeric insulin analog to be marketed, is produced by recombinant technology wherein two amino acids near the carboxyl terminal of the B chain have been reversed in position: Proline at position B28 has been moved to B29, and lysine at position B29 has been moved to B28 (Figure 41–1). Reversing these two amino acids does not interfere in any way with insulin lispro’s binding to the insulin receptor, its circulating half-life, or its immunogenicity, which are similar to those of human regular insulin. However, the advantage of this analog is its very low propensity—in contrast to human insulin—to self-associate in antiparallel fashion and form dimers. To enhance the shelf life of insulin in vials, insulin lispro is stabilized into hexamers by a cresol preservative. When injected subcutaneously, the drug quickly dissociates into monomers and is rapidly absorbed with onset of action within 5–15 minutes and peak activity as early as 1 hour. The time to peak action is relatively constant, regardless of the dose.

Insulin aspart is created by the substitution of the B28 proline with a negatively charged aspartic acid (Figure 41–1). This modification reduces the normal ProB28 and GlyB23 monomer-monomer interaction, thereby inhibiting insulin self-aggregation. Its absorption and activity profile are similar to those of insulin lispro, and it is more reproducible than regular insulin, but it has binding properties, activity, and mitogenicity characteristics similar to those of regular insulin in addition to equivalent immunogenicity.

Insulin glulisine is formulated by substituting a lysine for asparagine at B3 and glutamic acid for lysine at B29. Its absorption, action, and immunologic characteristics are similar to those of other injected rapid-acting insulins. After high-dose insulin glulisine interaction with the insulin receptor, there may be downstream differences in IRS-2 pathway activation relative to native insulin. The clinical significance of such differences is unclear.

2. Short-acting insulin—Regular insulin is a short-acting soluble crystalline zinc insulin that is now made by recombinant DNA techniques to produce a molecule identical to human insulin. Its effect appears within 30 minutes, peaks between 2 and 3 hours after subcutaneous injection, and generally lasts 5–8 hours. In high concentrations, eg, in the vial, regular insulin molecules self-aggregate in antiparallel fashion to form dimers that stabilize around zinc ions to create insulin hexamers. The hexameric nature of regular insulin causes a delayed onset and prolongs the time to peak action. After subcutaneous injection, the insulin hexamers are too large and bulky to be transported across the vascular endothelium into the bloodstream. As the insulin depot is diluted by interstitial fluid and the concentration begins to fall, the hexamers break down into dimers and finally monomers. This results in three rates of absorption of the injected insulin, with the final monomeric phase having the fastest uptake out of the injection site.

The clinical consequence is that when regular insulin is administered at mealtime, the blood glucose rises faster than the insulin with resultant early postprandial hyperglycemia and an increased risk of late postprandial hypoglycemia. Therefore, regular insulin should be injected 30–45 or more minutes before the meal to minimize the mismatching. As with all older insulin formulations, the duration of action as well as the time of onset and the intensity of peak action increase with the size of the dose. Clinically, this is a critical issue because the pharmacokinetics and pharmacodynamics of small doses of regular and NPH insulins differ greatly from those of large doses. The delayed absorption, dose-dependent duration of action, and variability of absorption (˜ 25%) of regular human insulin frequently results in a mismatching of insulin availability with need, and its use is declining.

However, short-acting, regular soluble insulin is the only type that should be administered intravenously because the dilution causes the hexameric insulin to immediately dissociate into monomers. It is particularly useful for intravenous therapy in the management of diabetic ketoacidosis and when the insulin requirement is changing rapidly, such as after surgery or during acute infections.

3. Intermediate-acting and long-acting insulins a. NPH (neutral protamine Hagedorn, or isophane) insulin—NPH insulin is an intermediate-acting insulin whose absorption and onset of action are delayed by combining appropriate amounts of insulin and protamine so that neither is present in an uncomplexed form (“isophane”). After subcutaneous injection, proteolytic tissue enzymes degrade the protamine to permit absorption of insulin. NPH insulin has an onset of approximately 2–5 hours and duration of 4–12 hours (Figure 41–5); it is usually mixed with regular, lispro, aspart, or glulisine insulin and given two to four times daily for insulin replacement. The dose regulates the action profile; specifically, small doses have lower, earlier peaks and a short duration of action with the converse true for large doses. The action of NPH is highly unpredictable, and its variability of absorption is over 50%. The clinical use of NPH is waning because of its adverse pharmacokinetics

combined with the availability of long-acting insulin analogs that have a more predictable and physiologic action. b. Insulin glargine—Insulin glargine is a soluble, “peakless” (ie, having a broad plasma concentration plateau), long-acting insulin analog. This product was designed to provide reproducible, convenient, background insulin replacement. The attachment of two arginine molecules to the B-chain carboxyl terminal and substitution of a glycine for asparagine at the A21 position created an analog that is soluble in an acidic solution but precipitates in the more neutral body pH after subcutaneous injection. Individual insulin molecules slowly dissolve away from the crystalline depot and provide a low, continuous level of circulating insulin. Insulin glargine has a slow onset of action (1–1.5 hours) and achieves a maximum effect after 4–6 hours. This maximum activity is maintained for 11–24 hours or longer.

Glargine is usually given once daily, although some very insulin-sensitive or insulin-resistant individuals benefit from split (twice a day) dosing. To maintain solubility, the formulation is unusually acidic (pH 4.0), and insulin glargine should not be mixed with other insulins.

Separate syringes must be used to minimize the risk of contamination and subsequent loss of efficacy. The absorption pattern of insulin glargine appears to be independent of the anatomic site of injection, and this drug is associated with less immunogenicity than human insulin in animal studies. Glargine’s interaction with the insulin receptor is similar to that of native insulin and shows no increase in mitogenic activity in vitro. It has sixfold to sevenfold greater binding than native insulin to the insulin-like growth factor-1 (IGF-1) receptor, but the clinical significance of this is unclear. c. Insulin detemir—This insulin is the most recently developed long-acting insulin analog. The terminal threonine is dropped from the B30 position and myristic acid (a C-14 fatty acid chain) is attached to the terminal B29 lysine. These modifications prolong the availability of the injected analog by increasing both self-aggregation in subcutaneous tissue and reversible albumin binding. Insulin detemir has the most reproducible effect of the intermediate- and long-acting insulins, and its use is associated with less hypoglycemia than NPH insulin.

Insulin detemir has a dose-dependent onset of action of 1–2 hours and duration of action of more than 12 hours. It is given twice daily to obtain a smooth background insulin level.

4. Mixtures of insulins—Because intermediate-acting NPH insulins require several hours to reach adequate therapeutic levels, their use in diabetic patients usually requires supplements of rapid-or short-acting insulin before meals. For convenience, these are often mixed together in the same syringe before injection. Insulin lispro, aspart, and glulisine can be acutely mixed (ie, just before injection) with NPH insulin without affecting their rapid absorption. However, premixed preparations have thus far been unstable. To remedy this, intermediate insulins composed of isophane complexes of protamine with insulin lispro and insulin aspart have been developed. These intermediate insulins have been designated as “NPL” (neutral protamine lispro) and “NPA” (neutral protamine aspart) and have the same duration of action as NPH insulin. They have the advantage of permitting formulation as premixed combinations of NPL and insulin lispro, and as NPA and insulin aspart, and they have been shown to be safe and effective in clinical trials. The FDA has approved 75%/25% NPL/insulin lispro and 70%/30% NPA/insulin aspart premixed formulations. Additional ratios are available abroad. Insulin glargine and detemir must be given as separate injections. They are not miscible acutely or in a premixed preparation with any other insulin formulation.

Premixed formulations of 70%/30% NPH/regular continue to be available. These preparations have all the limitations of regular insulin, namely, highly dose-dependent pharmacokinetic and pharmacodynamic profiles, and variability in absorption.

B. Insulin Production Mass production of human insulin and insulin analogs by recombinant DNA techniques is carried out by inserting the human or a modified human proinsulin gene into Escherichia coli or yeast and treating the extracted proinsulin to form the insulin or insulin analog molecules.

C. Concentration All insulins in the USA and Canada are available in a concentration of 100 U/mL (U100). A limited supply of U500 regular human insulin is available for use in rare cases of severe insulin resistance in which larger doses of insulin are required.

Insulin Delivery Systems A. Standard Delivery The standard mode of insulin therapy is subcutaneous injection using conventional disposable needles and syringes.

B. Portable Pen Injectors To facilitate multiple subcutaneous injections of insulin, particularly during intensive insulin therapy, portable pen-sized injectors have been developed. These contain cartridges of insulin and replaceable needles.

Disposable insulin pens are also available for selected formulations. These are regular insulin, insulin lispro, insulin aspart, insulin glulisine, insulin glargine, insulin detemir, and several mixtures of NPH with regular, lispro, or aspart insulin (Table 41–4). They have been well accepted by patients because they eliminate the need to carry syringes and bottles of insulin to the workplace and while traveling.

C. Continuous Subcutaneous Insulin Infusion Devices (CSII, Insulin Pumps) Continuous subcutaneous insulin infusion devices are external open-loop pumps for insulin delivery. The devices have a userprogrammable pump that delivers individualized basal and bolus insulin replacement doses based on blood glucose self-monitoring results.

Normally, the 24-hour background basal rates are preprogrammed and relatively constant from day to day, although temporarily altered rates can be superimposed to adjust for a short-term change in requirement. For example, the basal delivery rate might need to be decreased for several hours because of the increased insulin sensitivity associated with strenuous activity.

Boluses are used to correct high blood glucose levels and to cover mealtime insulin requirements based on the carbohydrate content of the food and concurrent activity. Bolus amounts are either dynamically programmed or use preprogrammed algorithms. When the boluses are dynamically programmed, the user calculates the dose based on the amount of carbohydrate consumed and the current blood glucose level. Alternatively, the meal or snack dose algorithm (grams of carbohydrate covered by a unit of insulin) and insulin sensitivity or blood glucose correction factor (fall in blood glucose level in response to a unit of insulin) can be preprogrammed into the pump. If the user enters the carbohydrate content of the food and current blood glucose value, the insulin pump will calculate the most appropriate dose of insulin. Advanced insulin pumps also have an “insulin on board” feature that adjusts a high blood glucose correction dose to correct for residual activity of previous bolus doses.

The traditional pump—which contains an insulin reservoir, the program chip, the keypad, and the display screen—is about the size of a pager. It is usually placed on a belt or in a pocket, and the insulin is infused through thin plastic tubing that is connected to the subcutaneously inserted infusion set. The abdomen is the favored site for the infusion set, although flanks and thighs are also used. The insulin reservoir, tubing, and infusion set need to be changed using sterile techniques every 2 or 3 days. Currently, only one pump does not require tubing. In this model, the pump is attached directly to the infusion set. Programming is done through a hand-held unit that communicates wirelessly with the pump. CSII delivery is regarded as the most physiologic method of insulin replacement.

Use of these continuous infusion devices is encouraged for people who are unable to obtain target control while on multiple injection regimens and in circumstances in which excellent glycemic control is desired, such as during pregnancy. Optimal use of these devices requires responsible involvement and commitment by the patient. Insulin aspart, lispro, and glulisine all are specifically approved for pump use and are preferred pump insulins because their favorable pharmacokinetic attributes allow glycemic control without increasing the risk of hypoglycemia.

D. Inhaled Insulin A dry powder formulation of recombinant regular insulin (technosphere insulin, Afrezza) is now approved for use in adults with diabetes.

After inhalation from the small, single use device, peak levels are reached in 12 -15 minutes and decline to baseline in 3 hours, significantly faster in onset and shorter in duration than subcutaneous insulin. In trials, inhaled insulin combined with injected basal insulin was as effective in lowering glucose as injected rapid-acting insulin combined with basal insulin. The most common adverse effect of inhaled insulin was cough, affecting 27% of trial patients and pulmonary function should be monitored. The drug is contraindicated in smokers and patients with chronic obstructive pulmonary disease.

Treatment with Insulin The current classification of diabetes mellitus identifies a group of patients who have virtually no insulin secretion and whose survival depends on administration of exogenous insulin. This insulin-dependent group (type 1) represents 10–15% of the diabetic population in the USA. Most type 2 diabetics do not require exogenous insulin for survival, but many need exogenous supplementation of their endogenous secretion to achieve optimum health.

Benefit of Glycemic Control in Diabetes Mellitus The consensus of the American Diabetes Association is that intensive glycemic control and targeting normal or near-normal glucose control associated with comprehensive self-management training should become standard therapy in diabetic patients (see Box: Benefits of Tight Glycemic Control in Diabetes). Exceptions include patients with advanced renal disease and the elderly, because the risks of hypoglycemia may outweigh the benefit of normal or near-normal glycemic control in these groups. In children under 7 years, the extreme susceptibility of the developing brain to incur damage from hypoglycemia contraindicates attempts at intensive glycemic control.

Insulin Regimens A. Intensive Insulin Therapy Intensive insulin regimens are prescribed for almost everyone with type 1 diabetes—diabetes associated with a severe deficiency or absence of endogenous insulin production—as well as many with type 2 diabetes.

Generally, the total daily insulin requirement in units is equal to the weight in pounds divided by four, or 0.55 times the person’s weight in kilograms. Approximately half the total daily insulin dosage covers the background or basal insulin requirements, and the remainder

covers meal and snack requirement and high blood sugar corrections. This is an approximate calculation and has to be individualized.

Examples of reduced insulin requirement include newly diagnosed persons and those with ongoing endogenous insulin production, longstanding diabetes with insulin sensitivity, significant renal insufficiency, or other endocrine deficiencies. Increased insulin requirements typically occur with obesity, during adolescence, during the latter trimesters of pregnancy, and in individuals with type 2 diabetes.

In intensive insulin regimens, the meal or snack and high blood sugar correction boluses are prescribed by formulas. The patient uses the formulas to calculate the rapid-acting insulin bolus dose by considering how much carbohydrate is in the meal or snack, the current plasma glucose, and the target glucose. The formula for the meal or snack bolus is expressed as an insulin-to-carbohydrate ratio, which refers to how many grams of carbohydrate will be disposed of by 1 unit of rapid-acting insulin. The high blood sugar correction formula is expressed as the predicted fall in plasma glucose (in mg/dL) after 1 unit of rapid-acting insulin. Diurnal variations in insulin sensitivity can be accommodated by prescribing different basal rates and bolus insulin doses throughout the day. Continuous subcutaneous insulin infusion devices provide the most sophisticated and physiologic insulin replacement.

B. Conventional Insulin Therapy Conventional insulin therapy is usually prescribed only for certain people with type 2 diabetes who are felt not to benefit from intensive glucose control. The insulin regimen ranges from one injection per day to many injections per day, using intermediate- or long-acting insulin alone or with short- or rapid-acting insulin or premixed insulins. Referred to as sliding-scale regimens, conventional insulin regimens customarily fix the dose of the intermediate- or long-acting insulin, but vary the short- or rapid-acting insulin based on the plasma glucose level before the injection. This insulin replacement regimen assumes similar daily timing and carbohydrate content of meals

Insulin Treatment of Special Circumstances A. Diabetic Ketoacidosis Diabetic ketoacidosis (DKA) is a life-threatening medical emergency caused by inadequate or absent insulin replacement, which occurs in people with type 1 diabetes and infrequently in those with type 2 diabetes. It typically occurs in newly diagnosed type 1 patients or in those who have experienced interrupted insulin replacement, and rarely in people with type 2 diabetes who have concurrent unusually stressful conditions such as sepsis or pancreatitis or are on high-dose steroid therapy. Signs and symptoms include nausea, vomiting, abdominal pain, deep slow (Kussmaul) breathing, change in mental status, elevated blood and urinary ketones and glucose, an arterial blood pH lower than 7.3, and low bicarbonate (< 15 mmol/L).

The fundamental treatment for DKA includes aggressive intravenous hydration and insulin therapy and maintenance of potassium and other electrolyte levels. Fluid and insulin therapy is based on the patient’s individual needs and requires frequent reevaluation and modification. Close attention has to be given to hydration and renal status, the sodium and potassium levels, and the rate of correction of plasma glucose and plasma osmolality. Fluid therapy generally begins with normal saline. Regular human insulin should be used for intravenous therapy with a usual starting dosage of about 0.1 U/kg/h.

B. Hyperosmolar Hyperglycemic Syndrome Hyperosmolar hyperglycemic syndrome (HHS) is diagnosed in persons with type 2 diabetes and is characterized by profound hyperglycemia and dehydration. It is associated with inadequate oral hydration, especially in elderly patients, with other illnesses, the use of medication that elevates the blood sugar or causes dehydration, such as phenytoin, steroids, diuretics, and β blockers, and with peritoneal dialysis and hemodialysis. The diagnostic hallmarks are declining mental status and even seizures, a plasma glucose of over 600 mg/dL, and a calculated serum osmolality higher than 320 mmol/L. Persons with HHS are not acidotic unless DKA is also present.

Benefits of Tight Glycemic Control in Diabetes A long-term randomized prospective study involving 1441 type 1 patients in 29 medical centers reported in 1993 that “near normalization” of blood glucose resulted in a delay in onset and a major slowing of progression of microvascular and neuropathic complications of diabetes during follow-up periods of up to 10 years (Diabetes Control and Complications Trial [DCCT] Research Group, 1993). In the intensively treated group, mean glycated hemoglobin (HbA1c) of 7.2% (normal < 6%) and mean blood glucose of 155 mg/dL were achieved, whereas in the conventionally treated group, HbA 1c averaged 8.9% with mean blood glucose of 225 mg/dL. Over the study period, which averaged 7 years, a reduction of approximately 60% in risk of diabetic retinopathy, nephropathy, and neuropathy was noted in the tight control group compared with the standard control group.

The DCCT study, in addition, introduced the concept of glycemic memory, which comprises the long-term benefits of any significant period of glycemic control. During a 6-year follow-up period, both the intensively and the conventionally treated groups had similar levels of glycemic control, and both had progression of carotid intimal-medial thickness. However, the intensively treated cohort had significantly less progression of intimal thickness.

The United Kingdom Prospective Diabetes Study (UKPDS) was a very large randomized prospective study carried out to study the effects of intensive glycemic control with several types of therapies and the effects of blood pressure control in type 2 diabetic patients. A total of 3867 newly diagnosed type 2 diabetic patients were studied over 10 years. A significant fraction of these were overweight and hypertensive. Patients were given dietary treatment alone or intensive therapy with insulin, chlorpropamide, glyburide, or glipizide. Metformin was an option for patients with inadequate response to other therapies. Tight control of blood pressure was added as a variable, with an angiotensin-converting enzyme inhibitor, a β blocker, or in some cases, a calcium channel blocker available for this purpose.

Tight control of diabetes, with reduction of HbA 1c from 9.1% to 7%, was shown to reduce the risk of microvascular complications overall compared with that achieved with conventional therapy (mostly diet alone, which decreased HbA1c to 7.9%).

Cardiovascular complications were not noted for any particular therapy; metformin treatment alone reduced the risk of macrovascular disease (myocardial infarction, stroke). Epidemiologic analysis of the study suggested that every 1% decrease in the HbA1c achieved an estimated risk reduction of 37% for microvascular complications, 21% for any diabetes-related end point and death related to diabetes, and 14% for myocardial infarction.

Tight control of hypertension also had a surprisingly significant effect on microvascular disease (as well as more conventional hypertension-related sequelae) in these diabetic patients. Epidemiologic analysis of the results suggested that every 10 mm Hg decrease in the systolic pressure achieved an estimated risk reduction of 13% for diabetic microvascular complications, and 12% for any diabetes-related complication, 15% for death related to diabetes, and 11% for myocardial infarction.

Post-study monitoring showed that 5 years after the closure of the UKPDS, the benefits of intensive management on diabetic end points was maintained and the risk reduction for a myocardial infarction became significant. The benefits of metformin therapy were maintained.

These studies show that tight glycemic control benefits both type 1 and type 2 patients.

The treatment of HHS centers around aggressive rehydration and restoration of glucose and electrolyte homeostasis; the rate of correction of these variables must be monitored closely. Low-dose insulin therapy may be required.

Complications of Insulin Therapy A. Hypoglycemia 1. Mechanisms and diagnosis—Hypoglycemic reactions are the most common complication of insulin therapy. They usually result from inadequate carbohydrate consumption, unusual physical exertion, or too large a dose of insulin.

Rapid development of hypoglycemia in persons with intact hypoglycemic awareness causes signs of autonomic hyperactivity—both sympathetic (tachycardia, palpitations, sweating, tremulousness) and parasympathetic (nausea, hunger)—and may progress to convulsions and coma if untreated.

In persons exposed to frequent hypoglycemic episodes during tight glycemic control, autonomic warning signals of hypoglycemia are less common or even absent. This dangerous acquired condition is termed “hypoglycemic unawareness.” When patients lack the early warning signs of low blood glucose, they may not take corrective measures in time. In patients with persistent, untreated hypoglycemia, the manifestations of insulin excess may develop—confusion, weakness, bizarre behavior, coma, seizures—at which point they may not be able to procure or safely swallow glucose-containing foods. Hypoglycemic awareness may be restored by preventing frequent hypoglycemic episodes. An identification bracelet, necklace, or card in the wallet or purse, as well as some form of rapidly absorbed glucose, should be carried by every diabetic person who is receiving hypoglycemic drug therapy.

2. Treatment of hypoglycemia—All the manifestations of hypoglycemia are relieved by glucose administration. To expedite absorption, simple sugar or glucose should be given, preferably in liquid form. To treat mild hypoglycemia in a patient who is conscious and able to swallow, dextrose tablets, glucose gel, or any sugar-containing beverage or food may be given. If more severe hypoglycemia has produced unconsciousness or stupor, the treatment of choice is to give 20–50 mL of 50% glucose solution by intravenous infusion over a period of 2–3 minutes. If intravenous therapy is not available, 1 mg of glucagon injected either subcutaneously or intramuscularly may restore consciousness within 15 minutes to permit ingestion of sugar. If the patient is stuporous and glucagon is not available, small amounts of honey or syrup can be inserted into the buccal pouch. In general, however, oral feeding is contraindicated in unconscious patients. Emergency medical services should be called immediately for all episodes of severely impaired consciousness.

B. Immunopathology of Insulin Therapy At least five molecular classes of insulin antibodies may be produced in diabetics during the course of insulin therapy: IgA, IgD, IgE, IgG, and IgM. There are two major types of immune disorders in these patients: 1. Insulin allergy—Insulin allergy, an immediate type hypersensitivity, is a rare condition in which local or systemic urticaria results from histamine release from tissue mast cells sensitized by anti-insulin IgE antibodies. In severe cases, anaphylaxis results. Because

sensitivity is often to noninsulin protein contaminants, the human and analog insulins have markedly reduced the incidence of insulin allergy, especially local reactions.

2. Immune insulin resistance—A low titer of circulating IgG anti-insulin antibodies that neutralize the action of insulin to a negligible extent develops in most insulin-treated patients. Rarely, the titer of insulin antibodies leads to insulin resistance and may be associated with other systemic autoimmune processes such as lupus erythematosus.

C. Lipodystrophy at Injection Sites Injection of animal insulin preparations sometimes led to atrophy of subcutaneous fatty tissue at the site of injection. Since the development of human and analog insulin preparations of neutral pH, this type of immune complication is almost never seen. Injection of these newer preparations directly into the atrophic area often results in restoration of normal contours.

Hypertrophy of subcutaneous fatty tissue remains a problem if injected repeatedly at the same site. However, this may be corrected by avoiding the specific injection site or by liposuction.

D. Increased Cancer Risk An increased risk of cancer attributed to insulin resistance and hyperinsulinemia has been reported in individuals with insulin resistance, prediabetes, and type 2 diabetes. Treatment with insulin and sulfonylureas, which increase circulating insulin levels, but not metformin possibly exacerbates that risk. These epidemiologic observations are preliminary and have not changed prescribing guidelines.

ORAL ANTIDIABETIC AGENTS Several categories of oral antidiabetic agents are now available in the USA for the treatment of persons with type 2 diabetes: (1) agents that bind to the sulfonylurea receptor and stimulate insulin secretion (sulfonylureas, meglitinides, D-phenylalanine derivatives); (2) agents that lower glucose levels by their actions on liver, muscle, and adipose tissue (biguanides, thiazolidinediones); (3) agents that principally slow the intestinal absorption of glucose (α-glucosidase inhibitors); (4) agents that mimic incretin effect or prolong incretin action (glucagon-like peptide-1 [GLP-1] receptor agonists, dipeptidyl peptidase-4 [DPP-4] inhibitors), (5) agents that inhibit the reabsorption of glucose in the kidney (sodium-glucose co-transporter inhibitors [SGLTs]), and (6) agents that act by other or ill-defined mechanisms (pramlintide, bromocriptine, colesevelam).

DRUGS THAT PRIMARILY STIMULATE INSULIN RELEASE BY BINDING TO THE SULFONYLUREA RECEPTOR SULFONYLUREAS Mechanism of Action The major action of sulfonylureas is to increase insulin release from the pancreas (Table 41–5). They bind to a 140-kDa high-affinity sulfonylurea receptor that is associated with a beta-cell inward rectifier ATP-sensitive potassium channel ( Figure 41–2). Binding of a sulfonylurea inhibits the efflux of potassium ions through the channel and results in depolarization. Depolarization opens a voltage-gated calcium channel and results in calcium influx and the release of preformed insulin.

TABLE 41–5 Regulation of insulin release in humans.

Efficacy & Safety of the Sulfonylureas Sulfonylureas are metabolized by the liver and, with the exception of acetohexamide, the metabolites are either weakly active or inactive.

The metabolites are excreted by the kidney and in the case of the second-generation sulfonylureas, partly excreted in the bile.

Idiosyncratic reactions are rare, with skin rashes or hematologic toxicity (leukopenia, thrombocytopenia) occurring in less than 0.1% of cases. The second-generation sulfonylureas have greater affinity for their receptor compared with the first-generation agents. The correspondingly lower effective doses and plasma levels of the second-generation drugs therefore lower the risk of drug-drug interactions based on competition for plasma binding sites or hepatic enzyme action.

In 1970, the University Group Diabetes Program (UGDP) in the USA reported that the number of deaths due to cardiovascular disease in diabetic patients treated with tolbutamide was excessive compared with either insulin-treated patients or those receiving placebos. Owing to design flaws, this study and its conclusions were not generally accepted. In the United Kingdom, the UKPDS did not find an untoward cardiovascular effect of sulfonylurea usage in their large, long-term study. The sulfonylureas continue to be widely prescribed, and six are available in the USA (Table 41â&#x20AC;&#x201C;6).

TABLE 41â&#x20AC;&#x201C;6 Sulfonylureas.

FIRST-GENERATION SULFONYLUREAS Tolbutamide is well absorbed but rapidly metabolized in the liver. Its duration of effect is relatively short (6–10 hours), with an elimination half-life of 4–5 hours, and it is best administered in divided doses (eg, 500 mg before each meal). Some patients only need one or two tablets daily. The maximum dosage is 3000 mg daily. Because of its short half-life and inactivation by the liver, it is relatively safe in the elderly and in patients with renal impairment. Prolonged hypoglycemia has been reported rarely, mostly in patients receiving certain antibacterial sulfonamides (sulfisoxazole), phenylbutazone for arthralgias, or the oral azole antifungal medications to treat candidiasis.

These drugs inhibit the metabolism of tolbutamide in the liver and increase its circulating levels.

Chlorpropamide has a half-life of 32 hours and is slowly metabolized in the liver to products that retain some biologic activity; approximately 20–30% is excreted unchanged in the urine. The average maintenance dosage is 250 mg daily, given as a single dose in the morning. Prolonged hypoglycemic reactions are more common in elderly patients, and the drug is contraindicated in this group. Other adverse effects include a hyperemic flush after alcohol ingestion in genetically predisposed patients and hyponatremia due to its effect on vasopressin secretion and action.

Tolazamide is comparable to chlorpropamide in potency but has a shorter duration of action. Tolazamide is more slowly absorbed than the other sulfonylureas, and its effect on blood glucose does not appear for several hours. Its half-life is about 7 hours. Tolazamide is metabolized to several compounds that retain hypoglycemic effects. If more than 500 mg/d are required, the dosage should be divided and given twice daily.

Acetohexamide is no longer available in the United States. Its half-life is only about 1 hour but its more active metabolite, hydroxyhexamide, has a half-life of 4–6 hours; thus the drug duration of action is 8–24 hours. Where available, its dosage is 0.25–1.5 g/d as single dose or in two divided doses.

Chlorpropamide, tolazamide, and acetohexamide are now rarely used in clinical practice.

SECOND-GENERATION SULFONYLUREAS Glyburide, glipizide, gliclazide, and glimepiride are 100–200 times more potent than tolbutamide. They should be used with caution in patients with cardiovascular disease or in elderly patients, in whom hypoglycemia would be especially dangerous.

Glyburide is metabolized in the liver into products with very low hypoglycemic activity. The usual starting dosage is 2.5 mg/d or less, and the average maintenance dosage is 5–10 mg/d given as a single morning dose; maintenance dosages higher than 20 mg/d are not recommended. A formulation of “micronized” glyburide (Glynase PresTab) is available in a variety of tablet sizes. However, there is some question as to its bioequivalence with nonmicronized formulations, and the FDA recommends careful monitoring to re-titrate dosage when switching from standard glyburide doses or from other sulfonylurea drugs.

Glyburide has few adverse effects other than its potential for causing hypoglycemia. Flushing has rarely been reported after ethanol ingestion, and the compound slightly enhances free water clearance. Glyburide is contraindicated in the presence of hepatic impairment and in patients with renal insufficiency.

Glipizide has the shortest half-life (2–4 hours) of the more potent agents. For maximum effect in reducing postprandial hyperglycemia, this agent should be ingested 30 minutes before breakfast because absorption is delayed when the drug is taken with food. The recommended starting dosage is 5 mg/d, with up to 15 mg/d given as a single dose. When higher daily dosages are required, they should be divided and given before meals. The maximum total daily dosage recommended by the manufacturer is 40 mg/d, although some studies indicate that the maximum therapeutic effect is achieved by 15–20 mg of the drug. An extended-release preparation (Glucotrol XL) provides 24-hour action after a once-daily morning dose (maximum of 20 mg/d). However, this formulation appears to have sacrificed its lower propensity for severe hypoglycemia compared with longer-acting glyburide without showing any demonstrable therapeutic advantages over the latter (which can be obtained as a generic drug). At least 90% of glipizide is metabolized in the liver to inactive products, and the remainder is excreted unchanged in the urine. Glipizide therapy is therefore contraindicated in patients with significant hepatic impairment. Because of its lower potency and shorter duration for action, it is preferable to glyburide in the elderly.

Glimepiride is approved for once-daily use as monotherapy or in combination with insulin. Glimepiride achieves blood glucose lowering with the lowest dosage of any sulfonylurea compound. A single daily dose of 1 mg has been shown to be effective, and the recommended maximal daily dosage is 8 mg. Glimepiride’s half-life under multidose conditions is 5–9 hours. It is completely metabolized by the liver to metabolites with weak or no activity.

Gliclazide (not available in the United States) has a half-life of 10 hours. The recommended starting dosage is 40–80 mg daily with a maximum dosage of 320 mg daily. Higher dosages are usually divided and given twice a day. It is completely metabolized by the liver to inactive metabolites.

MEGLITINIDE ANALOGS Repaglinide is the first member of the meglitinide group of insulin secretagogues (Table 41–7). These drugs modulate beta-cell insulin

release by regulating potassium efflux through the potassium channels previously discussed. There is overlap with the sulfonylureas in their molecular sites of action because the meglitinides have two binding sites in common with the sulfonylureas and one unique binding site.

TABLE 41–7 Other insulin secretagogues.

Repaglinide has a fast onset of action, with a peak concentration and peak effect within approximately 1 hour after ingestion, but the duration of action is 4–7 hours. It is cleared by hepatic CYP3A4 with a plasma half-life of 1 hour. Because of its rapid onset, repaglinide is indicated for use in controlling postprandial glucose excursions. The drug should be taken just before each meal in doses of 0.25–4 mg (maximum 16 mg/d); hypoglycemia is a risk if the meal is delayed or skipped or contains inadequate carbohydrate. It can be used in patients with renal impairment and in the elderly. Repaglinide is approved as monotherapy or in combination with biguanides. There is no sulfur in its structure, so repaglinide may be used in type 2 diabetics with sulfur or sulfonylurea allergy.

Mitiglinide (not available in the United States) is a benzylsuccinic acid derivative that binds to the sulfonylurea receptor and is similar to repaglinide in its clinical effects. It has been approved for use in Japan.

D-PHENYLALANINE DERIVATIVE Nateglinide, a D-phenylalanine derivative, stimulates rapid and transient release of insulin from beta cells through closure of the ATPsensitive K+ channel. It is absorbed within 20 minutes after oral administration with a time to peak concentration of less than 1 hour and is metabolized in the liver by CYP2C9 and CYP3A4 with a half-life of about 1 hour. The overall duration of action is about 4 hours. It is taken before the meal and reduces the postprandial rise in blood glucose levels. It is available as 60 and 120 mg tablets. The lower dose is used in patients with mild elevations in HbA1c. Nateglinide is efficacious when given alone or in combination with nonsecretagogue oral agents (such as metformin). Hypoglycemia is the main adverse effect. It can be used in patients with renal impairment and in the elderly.

DRUGS THAT PRIMARILY LOWER GLUCOSE LEVELS BY THEIR ACTIONS ON THE LIVER, MUSCLE, & ADIPOSE TISSUE

BIGUANIDES The structure of metformin is shown below. Phenformin (an older biguanide) was discontinued in the USA because of its association with lactic acidosis. Metformin is the only biguanide currently available in the United States.

Mechanisms of Action A full explanation of the mechanism of action of the biguanides remains elusive, but their primary effect is to activate the enzyme AMPactivated protein kinase (AMPK) and reduce hepatic glucose production. Patients with type 2 diabetes have considerably less fasting hyperglycemia as well as lower postprandial hyperglycemia after administration of biguanides; however, hypoglycemia during biguanide therapy is rare. These agents are therefore more appropriately termed “euglycemic” agents.

Metabolism & Excretion Metformin has a half-life of 1.5–3 hours, is not bound to plasma proteins, is not metabolized, and is excreted by the kidneys as the active compound. As a consequence of metformin’s blockade of gluconeogenesis, the drug may impair the hepatic metabolism of lactic acid. In patients with renal insufficiency, biguanides accumulate and thereby increase the risk of lactic acidosis, which appears to be a doserelated complication. In the United States, metformin use is not recommended at or above a serum creatinine level of 1.4 mg/dL in women and 1.5 mg/dL in men. In the United Kingdom, it is recommended to reassess use if the serum creatinine exceeds 1.5 mg/dL (estimated glomerular filtration rate [GFR] < 45 mL/min/1.73 m2 ) and to stop if the serum creatinine exceeds 1.7 mg/dL (estimated GFR < 30 mL/min/1.73 m2 ).

Clinical Use Biguanides are recommended as first-line therapy for type 2 diabetes. Because metformin is an insulin-sparing agent and does not increase body weight or provoke hypoglycemia, it offers obvious advantages over insulin or sulfonylureas in treating hyperglycemia in such persons. The UKPDS reported that metformin therapy decreases the risk of macrovascular as well as microvascular disease; this is in contrast to the other therapies, which only modified microvascular morbidity. Biguanides are also indicated for use in combination with insulin secretagogues or thiazolidinediones in type 2 diabetics in whom oral monotherapy is inadequate. Metformin is useful in the prevention of type 2 diabetes; the landmark Diabetes Prevention Program concluded that metformin is efficacious in preventing the new onset of type 2 diabetes in middle-aged, obese persons with impaired glucose tolerance and fasting hyperglycemia. It is interesting that metformin did not prevent diabetes in older, leaner prediabetics.

Although the recommended maximal dosage is 2.55 g daily, little benefit is seen above a total dosage of 2000 mg daily. Treatment is initiated at 500 mg with a meal and increased gradually in divided doses. Common schedules would be 500 mg once or twice daily increased to 1000 mg twice daily. The maximal dosage is 850 mg three times a day. Epidemiologic studies suggest that metformin use may reduce the risk of some cancers. These data are still preliminary, and the speculative mechanism of action is a decrease in insulin (which also functions as a growth factor) levels as well as direct cellular effects mediated by AMPK. Other studies suggest a reduction in cardiovascular deaths in humans and an increase in longevity in mice (see Chapter 60).

Toxicities The most common toxic effects of metformin are gastrointestinal (anorexia, nausea, vomiting, abdominal discomfort, and diarrhea), which occur in up to 20% of patients. They are dose related, tend to occur at the onset of therapy, and are often transient. However, metformin may have to be discontinued in 3–5% of patients because of persistent diarrhea.

Metformin interferes with the calcium-dependent absorption of vitamin B12 -intrinsic factor complex in the terminal ileum, and vitamin B12 deficiency can occur after many years of metformin use. Periodic screening for vitamin B12 deficiency should be considered, especially in patients with peripheral neuropathy or macrocytic anemia. Increased intake of calcium may prevent the metformin-induced B12 malabsorption.

Lactic acidosis can sometimes occur with metformin therapy. It is more likely to occur in conditions of tissue hypoxia when there is

increased production of lactic acid and in renal failure when there is decreased clearance of metformin. Almost all reported cases have involved patients with associated risk factors that should have contraindicated its use (kidney, liver, or cardiorespiratory insufficiency; alcoholism). Radiocontrast administration can cause acute kidney failure in patients with diabetes and incipient nephropathy. Metformin therapy should therefore be temporarily halted on the day of radiocontrast use and restarted a day or two later after confirmation that renal function has not deteriorated.

THIAZOLIDINEDIONES Thiazolidinediones act to decrease insulin resistance. They are ligands of peroxisome proliferator-activated receptor-gamma (PPAR-γ), part of the steroid and thyroid superfamily of nuclear receptors. These PPAR receptors are found in muscle, fat, and liver.

PPAR-γ receptors modulate the expression of the genes involved in lipid and glucose metabolism, insulin signal transduction, and adipocyte and other tissue differentiation. Observed effects of the thiazolidinediones include increased glucose transporter expression (GLUT 1 and GLUT 4), decreased free fatty acid levels, decreased hepatic glucose output, increased adiponectin and decreased release of resistin from adipocytes, and increased differentiation of preadipocytes to adipocytes. Thiazolidinediones have also been shown to decrease levels of plasminogen activator inhibitor type 1, matrix metalloproteinase-9, C-reactive protein, and interleukin-6. Two thiazolidinediones are currently available: pioglitazone and rosiglitazone (Table 41–8). Their distinct side chains create differences in therapeutic action, metabolism, metabolite profile, and adverse effects. An earlier compound, troglitazone, was withdrawn from the market because of hepatic toxicity thought to be related to its side chain.

TABLE 41–8 Thiazolidinediones.

Pioglitazone has some PPAR-α as well as PPAR-γ activity. It is absorbed within 2 hours of ingestion; although food may delay uptake, total bioavailability is not affected. Absorption is decreased with concomitant use of bile acid sequestrants. Pioglitazone is metabolized by CYP2C8 and CYP3A4 to active metabolites. The bioavailability of numerous other drugs also degraded by these enzymes may be affected by pioglitazone therapy, including estrogen-containing oral contraceptives; additional methods of contraception are advised. Pioglitazone may be taken once daily; the usual starting dosage is 15–30 mg/d, and the maximum is 45 mg/d. Pioglitazone is approved as a monotherapy and in combination with metformin, sulfonylureas, and insulin for the treatment of type 2 diabetes.

Rosiglitazone is rapidly absorbed and highly protein-bound. It is metabolized in the liver to minimally active metabolites, predominantly by CYP2C8 and to a lesser extent by CYP2C9. It is administered once or twice daily; 2–8 mg is the usual total dosage.

Rosiglitazone is approved for use in type 2 diabetes as monotherapy, in double combination therapy with a biguanide or sulfonylurea, or in

quadruple combination with a biguanide, sulfonylurea, and insulin.

The combination of a thiazolidinedione and metformin has the advantage of not causing hypoglycemia.

These drugs also have some additional effects apart from glucose lowering. Pioglitazone lowers triglycerides and increases HDL cholesterol without affecting total cholesterol and low-density lipoprotein (LDL) cholesterol. Rosiglitazone increases total cholesterol, HDL cholesterol, and LDL cholesterol but does not have significant effect on triglycerides. These drugs have been shown to improve the biochemical and histologic features of nonalcoholic fatty liver disease. They seem to have a positive effect on endothelial function: pioglitazone reduces neointimal proliferation after coronary stent placement, and rosiglitazone has been shown to reduce microalbuminuria.

Safety concerns and troublesome side effects have significantly reduced the use of this class of drugs. A meta-analysis of 42 randomized clinical trials with rosiglitazone suggested that this drug increased the risk of angina pectoris or myocardial infarction. As a result, its use was suspended in Europe and severely restricted in the United States. A subsequent large prospective clinical trial (the RECORD study) failed to confirm the meta-analysis finding and so the United States restrictions have been lifted. The drug remains unavailable in Europe.

Fluid retention occurs in about 3–4 % patients on thiazolidinedione monotherapy and occurs more frequently (10–15%) in patients on concomitant insulin therapy. Heart failure can occur, and the drugs are contraindicated in patients with New York Heart Association class III and IV cardiac status (see Chapter 13). Macular edema is a rare side effect that improves when the drug is discontinued. Loss of bone mineral density and increased atypical extremity bone fractures in women are described for both compounds, which is postulated to be due to decreased osteoblast formation. Other side effects include anemia, which might be due to a dilutional effect of increased plasma volume rather than a reduction in red cell mass. Weight gain occurs, especially when used in combination with a sulfonylurea or insulin. Some of the weight gain is fluid retention but there is also an increase in total fat mass. In preclinical trials, bladder tumors were observed in male rats on pioglitazone. A planned interim analysis of a long-term observational cohort of patients treated with pioglitazone found an increased risk of bladder cancer with increased dosage and duration of drug use. Safety analysis of a study designed to evaluate the impact of pioglitazone on macrovascular events noted 14 cases of bladder cancer in the treated group and 5 cases in the placebo group, a significant difference. Although there are currently no recommendations regarding screening for bladder cancer, it should be considered in patients on long-term therapy.

Troglitazone, the first medication in this class, was withdrawn because of cases of fatal liver failure. Although rosiglitazone and pioglitazone have not been reported to cause liver injury, the drugs are not recommended for use in patients with active liver disease or pretreatment elevation of alanine aminotransferase (ALT) 2.5 times greater than normal. Liver function tests should be performed prior to initiation of treatment and periodically thereafter.

DRUGS THAT AFFECT ABSORPTION OF GLUCOSE The α-glucosidase inhibitors competitively inhibit the intestinal α-glucosidase enzymes and reduce postmeal glucose excursions by delaying the digestion and absorption of starch and disaccharides (Table 41–9) . Acarbose and miglitol are available in the United States. Voglibose is available in Japan, Korea, and India. Acarbose and miglitol are potent inhibitors of glucoamylase, α-amylase, and sucrase but have less effect on isomaltase and hardly any on trehalase and lactase. Acarbose has the molecular mass and structural features of a tetrasaccharide and very little is absorbed. In contrast, miglitol has structural similarity to glucose and is absorbed.

TABLE 41–9 Alpha-glucosidase inhibitors.

Acarbose treatment is initiated at a dosage of 50 mg twice daily with gradual increase to 100 mg three times a day. It lowers postprandial glucose levels by 30–50%. Miglitol therapy is initiated at a dosage of 25 mg three times a day. The usual maintenance dosage is 50 mg three times a day but some patients may need 100 mg three times a day. The drug is not metabolized and is cleared by the kidney. It should not be used in renal failure.

Prominent adverse effects of α-glucosidase inhibitors include flatulence, diarrhea, and abdominal pain and result from the appearance of undigested carbohydrate in the colon that is then fermented into short-chain fatty acids, releasing gas. These adverse effects tend to diminish with ongoing use because chronic exposure to carbohydrate induces the expression of α-glucosidase in the jejunum and ileum, increasing distal small intestine glucose absorption and minimizing the passage of carbohydrate into the colon. Although not a problem with monotherapy or combination therapy with a biguanide, hypoglycemia may occur with concurrent sulfonylurea treatment.

Hypoglycemia should be treated with glucose (dextrose) and not sucrose, whose breakdown may be blocked. An increase in hepatic aminotransferases has been noted in clinical trials with acarbose, especially with dosages greater than 300 mg/d. The abnormalities resolve on stopping the drug.

These drugs are infrequently prescribed in the United States because of their prominent gastrointestinal adverse effects and relatively modest glucose-lowering benefit.

DRUGS THAT MIMIC INCRETIN EFFECT OR PROLONG INCRETIN ACTION An oral glucose load provokes a higher insulin response compared with an equivalent dose of glucose given intravenously. This is because the oral glucose causes a release of gut hormones (“incretins”), principally glucagon-like peptide-1 (GLP-1) and glucosedependent insulinotropic peptide (GIP), that amplify the glucose-induced insulin secretion. When GLP-1 is infused in patients with type 2 diabetes, it stimulates insulin release and lowers glucose levels. The GLP-1 effect is glucose dependent in that the insulin release is more pronounced when glucose levels are elevated but less so when glucose levels are normal. For this reason, GLP-1 has a lower risk for hypoglycemia than the sulfonylureas. In addition to its insulin stimulatory effect, GLP-1 has a number of other biologic effects. It suppresses glucagon secretion, delays gastric emptying, and reduces apoptosis of human islets in culture. In animals, GLP-1 inhibits feeding by a central nervous system mechanism. Type 2 diabetes patients on GLP-1 therapy are less hungry. It is unclear whether this is mainly related to the deceleration of gastric emptying or whether there is a central nervous system effect as well.

GLP-1 is rapidly degraded by dipeptidyl peptidase-4 (DPP-4) and by other enzymes such as endopeptidase 24.11 and is also cleared by the kidney. The native peptide therefore cannot be used therapeutically. One approach to this problem has been to develop metabolically stable analogs or derivatives of GLP-1 that are not subject to the same enzymatic degradation or renal clearance. Four such GLP-1 receptor agonists, exenatide, liraglutide, albiglutide, and dulaglutide are available for clinical use. The other approach has been to develop inhibitors of DPP-4 and prolong the action of endogenously released GLP-1 and GIP. Four oral DPP-4 inhibitors, sitagliptin, saxagliptin, linagliptin, and alogliptin, are available in the United States. An additional inhibitor, vildagliptin, is available in

Europe.

GLUCAGON-LIKE PEPTIDE-1 (GLP-1) RECEPTOR AGONISTS Exenatide, a derivative of the exendin-4 peptide in Gila monster venom, has a 53% homology with native GLP-1, and a glycine substitution to reduce degradation by DPP-4. Exenatide is approved as an injectable, adjunctive therapy in persons with type 2 diabetes treated with metformin or metformin plus sulfonylureas who still have suboptimal glycemic control.

Exenatide is dispensed as fixed-dose pens (5 mcg and 10 mcg). It is injected subcutaneously within 60 minutes before breakfast and dinner. It reaches a peak concentration in approximately 2 hours with a duration of action of up to 10 hours. Therapy is initiated at 5 mcg twice daily for the first month and if tolerated can be increased to 10 mcg twice daily. Exenatide LAR is a once-weekly preparation that is dispensed as a powder (2 mg). It is suspended in the provided diluent just prior to injection. When exenatide is added to preexisting sulfonylurea therapy, the oral hypoglycemic dosage may need to be decreased to prevent hypoglycemia. The major adverse effect is nausea (about 44% of users), which is dose dependent and declines with time. Exenatide mono-and combination therapy results in HbA1c reductions of 0.2–1.2%. Weight loss in the range of 2–3 kg occurs and contributes to the improvement of glucose control. In comparative trials the long-acting (LAR) preparation lowers the HbA1c level a little more than the twice-daily preparation. Exenatide undergoes glomerular filtration, and the drug is not approved for use in patients with estimated GFR of less than 30 mL/min.

High-titer antibodies against exenatide develop in about 6% of patients and in half of these patients an attenuation of glycemic response has been seen.

Liraglutide is a soluble fatty acid-acylated GLP-1 analog. The half-life is approximately 12 hours permitting once-daily dosing. It is approved in patients with type 2 diabetes who achieve inadequate control with diet and exercise and are receiving concurrent treatment with metformin, sulfonylureas, or thiazolidinediones. Treatment is initiated at 0.6 mg and increased after 1 week to 1.2 mg daily. If needed the dosage can be increased to 1.8 mg daily. In clinical trials liraglutide results in a reduction of HbA 1c of 0.8–1.5%; weight loss ranges from none to 3.2 kg. The most frequent side effects are nausea (28%) and vomiting (10%).

Albiglutide is a human GLP-1 dimer fused to human albumin. The half-life of albiglutide is about 5 days and a steady state is reached after 4–5 weeks of once weekly administration. The usual dose is 30 mg weekly by subcutaneous injection. The drug is supplied in a self-injection pen containing a powder that is reconstituted just prior to administration. Weight loss is much less common than with exenatide and liraglutide. The most frequent adverse effects were nausea and injection site erythema.

Dulaglutide consists of two GLP-1 analog molecules covalently linked to an Fc fragment of human IgG4. The GLP-1 molecule has amino acid substitutions that resist DPP-4 action. The half-life of dulaglutide is about 5 days. The usual dose is 0.75 mg weekly by subcutaneous injection. The maximum recommended dose is 1.5 mg weekly. The most frequent adverse reactions were nausea, diarrhea and vomiting.

All of the GLP-1 receptor agonists may increase the risk of pancreatitis. Patients on these drugs should be counseled to seek immediate medical care if they experience unexplained persistent severe abdominal pain. Cases of renal impairment and acute renal injury have been reported in patients taking exenatide. Some of these patients had preexisting kidney disease or other risk factors for renal injury. A number of them reported having nausea, vomiting and diarrhea and it is possible that volume depletion contributed to the development of renal injury. Both exenatide and liraglutide stimulate thyroidal C-cell (parafollicular) tumors in rodents. Human thyroidal C-cells express very few GLP-1 receptors and the relevance to human therapy is unclear. The drugs, however, should not be used in persons with a past medical or family history of medullary thyroid cancer or multiple endocrine neoplasia (MEN) syndrome type 2.

DIPEPTIDYL PEPTIDASE-4 (DPP-4) INHIBITORS Sitagliptin is given orally as 100 mg once daily, has an oral bioavailability of over 85%, achieves peak concentrations within 1–4 hours, and has a half-life of approximately 12 hours. It is primarily excreted in the urine, in part by active tubular secretion of the drug. Hepatic metabolism is limited and mediated largely by the cytochrome CYP3A4 isoform and, to a lesser degree, by CYP2C8. The metabolites have insignificant activity. Dosage should be reduced in patients with impaired renal function (50 mg if estimated GFR is 30–50 mL/min and 25 mg if < 30 mL/min). Sitagliptin has been studied as monotherapy and in combination with metformin, sulfonylureas, and thiazolidinediones. Therapy with sitagliptin has resulted in HbA1c reductions of 0.5–1.0%.

Common adverse effects include nasopharyngitis, upper respiratory infections, and headaches. Hypoglycemia can occur when the drug is combined with insulin secretagogues or insulin. There have been postmarketing reports of acute pancreatitis (fatal and nonfatal) and severe allergic and hypersensitivity reactions. Sitagliptin should be immediately discontinued if pancreatitis or allergic and hypersensitivity reactions occur.

Saxagliptin is given orally as 2.5–5 mg daily. The drug reaches maximal concentrations within 2 hours (4 hours for its active metabolite). It is minimally protein bound and undergoes hepatic metabolism by CYP3A4/5. The major metabolite is active, and excretion is by both renal and hepatic pathways. The terminal plasma half-life is 2.5 hours for saxagliptin and 3.1 hours for its active metabolite.

Dosage adjustment is recommended for individuals with renal impairment and concurrent use of strong CYP3A4/5 inhibitors such as

antiviral, antifungal, and certain antibacterial agents. It is approved as monotherapy and in combination with biguanides, sulfonylureas, and thiazolidinediones. During clinical trials, mono- and combination therapy with saxagliptin resulted in an HbA1c reduction of 0.4–0.9%.

Adverse effects include an increased rate of infections (upper respiratory tract and urinary tract), headaches, and hypersensitivity reactions (urticaria, facial edema). The dosage of a concurrently administered insulin secretagogue or insulin may need to be lowered to prevent hypoglycemia.

Linagliptin lowers HbA1c by 0.4–0.6% when added to metformin, sulfonylurea, or pioglitazone. The dosage is 5 mg daily and since it is primarily excreted via the bile, no dosage adjustment is needed in renal failure.

Adverse reactions include nasopharyngitis and hypersensitivity reactions (urticaria, angioedema, localized skin exfoliation, bronchial hyperreactivity). The risk of pancreatitis may be increased.

Vildagliptin (not available in the United States) lowers HbA1c levels by 0.5–1% when added to the therapeutic regimen of patients with type 2 diabetes. The dosage is 50 mg once or twice daily. Adverse reactions include upper respiratory infections, nasopharyngitis, dizziness, and headache. Rarely, it can cause hepatitis and liver function tests should be performed quarterly in the first year of use and periodically thereafter.

In animal studies, high doses of DPP-4 inhibitors and GLP-1 agonists cause expansion of pancreatic ductal glands and generation of premalignant pancreatic intraepithelial (PanIN) lesions that have the potential to progress to pancreatic adenocarcinoma. The relevance to human therapy is unclear and currently there is no evidence that these drugs cause pancreatic cancer in humans.

SODIUM-GLUCOSE CO-TRANSPORTER 2 (SGLT2) INHIBITORS Glucose is freely filtered by the renal glomeruli and is reabsorbed in the proximal tubules by the action of sodium-glucose transporters (SGLTs). Sodium-glucose transporter 2 (SGLT2) accounts for 90% of glucose reabsorption, and its inhibition causes glycosuria and lowers glucose levels in patients with type 2 diabetes. The SGLT2 inhibitors canagliflozin, dapagliflozin, and empagliflozin are approved for clinical use.

Canagliflozin reduces the threshold for glycosuria from a plasma glucose threshold of approximately 180 mg/dL to 70–90 mg/dL. It has been shown to reduce HbA1c by 0.6–1% when used alone or in combination with other oral agents or insulin. It also results in modest weight loss of 2–5 kg. The usual dosage is 100 mg daily. Increasing the dosage to 300 mg daily in patients with normal renal function can lower the HbA1c by an additional 0.5%.

Dapagliflozin reduces HbA1c by 0.5–0.8% when used alone or in combination with other oral agents or insulin. It also results in modest weight loss of about 2–4 kg. The usual dosage is 10 mg daily but 5 mg daily is recommended initially in patients with hepatic failure.

Empagliflozin reduces HbA1c by 0.5–0.7% when used alone or in combination with other oral agents or insulin. It also results in modest weight loss of 2–3 kg. The usual dosage is 10 mg daily but 25 mg/d may be used.

As might be expected, the efficacy of the SGLT2 inhibitors is reduced in chronic kidney disease. Canagliflozin and empagliflozin are contraindicated in patients with estimated GFR less than 45 mL/min/1.73 m2 . Dapagliflozin is not recommended for use in patients with estimated GFR less than 60 mL/min/1.73 m2 . The main side effects are increased incidence of genital infections and urinary tract infections affecting about 8–9% of patients. The osmotic diuresis can also cause intravascular volume contraction and hypotension.

Canagliflozin and empagliflozin caused a modest increase in LDL cholesterol levels (4–8%). In clinical trials patients taking dapagliflozin had higher rates of breast cancer (nine cases versus none in comparator arms) and bladder cancer (nine cases versus one in placebo arm). These cancer rates exceeded the expected rates in an age-matched reference diabetes population.

OTHER HYPOGLYCEMIC DRUGS Pramlintide is an islet amyloid polypeptide (IAPP, amylin) analog. IAPP is a 37-amino-acid peptide present in insulin secretory granules and secreted with insulin. It has approximately 46% homology with the calcitonin gene-related peptide (CGRP; see Chapter 17) and physiologically acts as a negative feedback on insulin secretion. At pharmacologic doses, IAPP reduces glucagon secretion, slows gastric emptying by a vagally medicated mechanism, and centrally decreases appetite. Pramlintide is an IAPP analog with substitutions of proline at positions 25, 28, and 29. These modifications make pramlintide soluble and non–self-aggregating, and suitable for pharmacologic use. Pramlintide is approved for use in insulin-treated type 1 and type 2 patients who are unable to achieve their target postprandial blood glucose levels. It is rapidly absorbed after subcutaneous administration; levels peak within 20 minutes, and the duration of action is not more than 150 minutes. It is metabolized and excreted by the kidney, but even at low creatinine clearance there is no significant change in bioavailability. It has not been evaluated in dialysis patients.

Pramlintide is injected immediately before eating; dosages range from 15 to 60 mcg subcutaneously for type 1 patients and from 60 to 120 mcg for type 2 patients. Therapy with this agent should be initiated at the lowest dosage and titrated upward. Because of the risk of hypoglycemia, concurrent rapid- or short-acting mealtime insulin dosages should be decreased by 50% or more. Pramlintide should always be injected by itself using a separate syringe; it cannot be mixed with insulin. The major adverse effects of pramlintide are hypoglycemia and gastrointestinal symptoms, including nausea, vomiting, and anorexia. Since the drug slows gastric emptying, recovery

from hypoglycemia can be problematic because of the delay in absorption of fast-acting carbohydrates.

Selected patients with type 1 diabetes who have problems with postprandial hyperglycemia can use pramlintide effectively to control the glucose rise especially in the setting of a high-carbohydrate meal. The drug is not that useful in type 2 patients who can instead use the GLP-1 receptor agonists.

Colesevelam hydrochloride, the bile acid sequestrant and cholesterol-lowering drug, is approved as an antihyperglycemic therapy for persons with type 2 diabetes who are taking other medications or have not achieved adequate control with diet and exercise. The exact mechanism of action is unknown but presumed to involve an interruption of the enterohepatic circulation and a decrease in farnesoid X receptor (FXR) activation. FXR is a nuclear receptor with multiple effects on cholesterol, glucose, and bile acid metabolism.

Bile acids are natural ligands of the FXR. Additionally, the drug may impair glucose absorption. In clinical trials, it lowered the HbA 1c concentration 0.3–0.5%. Adverse effects include gastrointestinal complaints (constipation, indigestion, flatulence). It can also exacerbate the hypertriglyceridemia that commonly occurs in people with type 2 diabetes.

Bromocriptine, the dopamine agonist, in randomized placebo-controlled studies lowered HbA1c by 0–0.2% compared with baseline and by 0.4–0.5% compared with placebo. The mechanism by which it lowers glucose levels is not known. The main adverse events are nausea, fatigue, dizziness, vomiting, and headache.

Colesevelam and bromocriptine have very modest efficacy in lowering glucose levels and their use for this purpose is questionable.

COMBINATION THERAPY IN TYPE 2 DIABETES Failure to maintain a good response to therapy over the long term owing to a progressive decrease in beta-cell mass, reduction in physical activity, decline in lean body mass, or increase in ectopic fat deposition remains a disconcerting problem in the management of type 2 diabetes. Multiple medications may be required to achieve glycemic control (Figure 41–6). Unless there is a contraindication, medical therapy should be initiated with metformin. If clinical failure occurs with metformin monotherapy, a second agent is added. Options include sulfonylureas, repaglinide or nateglinide, pioglitazone, GLP-1 receptor agonists, DPP-4 inhibitors, SGLT2 inhibitors, and insulin. In the choice of the second agent consideration should be given to efficacy of the agent, hypoglycemic risk, effect on weight, side effects, and cost. In patients who experience hyperglycemia after a carbohydrate-rich meal (such as dinner), a short-acting secretagogue before that meal may suffice to control the glucose levels. Patients with severe insulin resistance may be candidates for pioglitazone. Patients who are very concerned about weight gain may benefit from a trial of a GLP-1 receptor agonist, a DPP-4 inhibitor, or an SGLT2 inhibitor. If two agents are inadequate a third agent is added, although data regarding efficacy of such combined therapy are limited.

FIGURE 41–6 Suggested algorithm for the treatment of type 2 diabetes. The seven main classes of agents are metformin, sulfonylureas (includes nateglinide, repaglinide), pioglitazone, GLP-1 receptor agonists, DPP-4 inhibitors, SGLT2 inhibitors, insulins. (α-Glucosidase

inhibitors, colesevelam, pramlintide, and bromocriptine not included because of limited efficacy and significant adverse reactions). (Data from the consensus panel of the American Diabetes Association/European Association for the Study of Diabetes, as described in Inzucchi SE et al: Diabetes Care 2012;35:1364.)

When the combination of oral agents and injectable GLP-1 receptor agonists fails to adequately control glucose levels, insulin therapy should be instituted. Various insulin regimens may be effective. Simply adding nighttime intermediate or long-acting insulin to the oral regimen may lead to improved fasting glucose levels and adequate control during the day. If daytime glucose levels are problematic, premixed insulins before breakfast and dinner may help. If such a regimen does not achieve adequate control or leads to unacceptable rates of hypoglycemia, a more intensive basal bolus insulin regimen (long-acting basal insulin) combined with rapid acting analog before meals can be instituted. Metformin has been shown to be effective when combined with insulin therapy and should be continued.

Pioglitazone can be used with insulin, but this combination is associated with more weight gain and peripheral edema. Continuing with sulfonylureas, GLP-1 receptor agonists, DPP-4 inhibitors, and SGLT2 inhibitors can be of benefit in selected patients. Cost, complexity, and risk for adverse events should be considered when deciding which drugs to continue once the patient starts on insulin therapy.

GLUCAGON Chemistry & Metabolism Glucagon is synthesized in the alpha cells of the pancreatic islets of Langerhans (Table 41–1). Glucagon is a peptide—identical in all mammals—consisting of a single chain of 29 amino acids, with a molecular weight of 3485. Selective proteolytic cleavage converts a large precursor molecule of approximately 18,000 MW to glucagon. One of the precursor intermediates consists of a 69-amino-acid peptide called glicentin, which contains the glucagon sequence interposed between peptide extensions.

Glucagon is extensively degraded in the liver and kidney as well as in plasma and at its tissue receptor sites. Because of its rapid inactivation by plasma, chilling of the collecting tubes and addition of inhibitors of proteolytic enzymes are necessary when samples of blood are collected for immunoassay of circulating glucagon. Its half-life in plasma is between 3 and 6 minutes, which is similar to that of insulin.

Pharmacologic Effects of Glucagon A. Metabolic Effects The first six amino acids at the amino terminal of the glucagon molecule bind to specific Gs protein-coupled receptors on liver cells. This leads to an increase in cAMP, which facilitates catabolism of stored glycogen and increases gluconeogenesis and ketogenesis. The immediate pharmacologic result of glucagon infusion is to raise blood glucose at the expense of stored hepatic glycogen. There is no effect on skeletal muscle glycogen, presumably because of the lack of glucagon receptors on skeletal muscle. Pharmacologic amounts of glucagon cause release of insulin from normal pancreatic beta cells, catecholamines from pheochromocytoma, and calcitonin from medullary carcinoma cells.

B. Cardiac Effects Glucagon has a potent inotropic and chronotropic effect on the heart, mediated by the cAMP mechanism described above. Thus, it produces an effect very similar to that of β-adrenoceptor agonists without requiring functioning β receptors.

C. Effects on Smooth Muscle Large doses of glucagon produce profound relaxation of the intestine. In contrast to the above effects of the peptide, this action on the intestine may be due to mechanisms other than adenylyl cyclase activation.

Clinical Uses A. Severe Hypoglycemia The major use of glucagon is for emergency treatment of severe hypoglycemic reactions in patients with type 1 diabetes when unconsciousness precludes oral feedings and intravenous glucose treatment is not possible. Recombinant glucagon is currently available in 1-mg vials for parenteral (IV, IM, or SC) use (Glucagon Emergency Kit). Nasal sprays have been developed for this purpose but have not yet received FDA approval.

B. Endocrine Diagnosis Several tests use glucagon to diagnose endocrine disorders. In patients with type 1 diabetes mellitus, a classic research test of pancreatic beta-cell secretory reserve uses 1 mg of glucagon administered as an intravenous bolus. Because insulin-treated patients develop

circulating anti-insulin antibodies that interfere with radioimmunoassays of insulin, measurements of C-peptide are used to indicate betacell secretion.

C. Beta-Adrenoceptor Blocker Overdose Glucagon is sometimes useful for reversing the cardiac effects of an overdose of β-blocking agents because of its ability to increase cAMP production in the heart. However, it is not clinically useful in the treatment of cardiac failure.

D. Radiology of the Bowel Glucagon has been used extensively in radiology as an aid to X-ray visualization of the bowel because of its ability to relax the intestine.

Adverse Reactions Transient nausea and occasional vomiting can result from glucagon administration. These are generally mild, and glucagon is relatively free of severe adverse reactions. It should not be used in a patient with pheochromocytoma.

SUMMARY Drugs Used for Diabetes

PREPARATIONS AVAILABLE*

REFERENCES Action to Control Cardiovascular Risks in Diabetes Study Group: Effects of intensive glucose lowering in type 2 diabetes. N Engl J Med 2008;358:2545.

Adler AI et al: Association of systolic blood pressure with macrovascular and microvascular complications of type 2 diabetes (UKPDS 36): Prospective observational study.

Br Med J 2000;321:412.

ADVANCE Collaborative Group: Intensive blood glucose control and vascular outcomes in patients with type 2 diabetes. N Engl J Med 2008;358:2560.

Ahmadian M et al: PPARγ signaling and metabolism: T he good, the bad and the future. Nat Med 2013;19:557.

American Diabetes Association: Diagnosis and classification of diabetes mellitus. Diabetes Care 2013;36(Suppl 1):S67.

Andrianesis V, Doupis J: T he role of kidney in glucose homeostasis—SGLT 2 inhibitors, a new approach in diabetes treatment. Expert Rev Clin Pharmacol 2013;6:519.

Bennett WL et al: Comparative effectiveness and safety of medications for type 2 diabetes: An update including new drugs and 2-drug combinations. Ann Intern Med 2011;154: 602. Erratum in: Ann Intern Med 2011;155:67.

Butler PC et al: A critical analysis of the clinical use of incretin-based therapies: Are the GLP-1 therapies safe? Diabetes Care 2013;36:2118.

Diabetes Prevention Program Research Group: Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. N Engl J Med 2002;346:393.

Expert Committee on the Diagnosis and Classification of Diabetes Mellitus: Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus.

Diabetes Care 2003;26(Suppl 1):S5.

Gaede P et al: Effect of a multifactorial intervention on mortality in type 2 diabetes. N Engl J Med 2008;358:580.

Guo S: Insulin signaling, resistance, and the metabolic syndrome: Insights from mouse models to disease mechanisms. J Endocrinol 2014;220:T 1.

Inzucchi SE et al: Management of hyperglycemia in type 2 diabetes: A patient-centered approach: Position statement of the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD). Diabetes Care 2012;35:1364.

Karagiannis T et al: Dipeptidyl peptidase-4 inhibitors for treatment of type 2 diabetes mellitus in the clinical setting: Systematic review and meta-analysis. BMJ 201212;344.

Kitabchi A et al: T hirty years of personal experience in hyperglycemic crises: Diabetic ketoacidosis and hyperglycemic hyperosmolar state. J Clin Endocrinol Metab 2008;93:1541.

Lefebvre P et al: Role of bile acids and bile acid receptors in metabolic regulation. Physiol Rev 2009;89:147.

Miyazaki Y, DeFronzo RA: Rosiglitazone and pioglitazone similarly improve insulin sensitivity and secretion, glucose tolerance and adipocytokines in type 2 diabetic patients. Diabetes Obes Metab 2008;10:1204.

Nauck MA: A critical analysis of the clinical use of incretin-based therapies: T he benefits by far outweigh the potential risks. Diabetes Care 2013;36:2126.

Nwose OM, Jones MR: Atypical mechanism of glucose modulation by colesevelam in patients with type 2 diabetes. Clin Med Insights Endocrinol Diabetes 2013;6:75.

Ratner RE et al: Amylin replacement with pramlintide as an adjunct to insulin therapy improves long term glycemic and weight control in type 1 diabetes mellitus: A 1-year randomized controlled trial. Diabetic Med 2004;21:1204.

Reitman ML et al: Pharmacogenetics of metformin response: A step in the path toward personalized medicine. J Clin Invest 2007;117:1226.

Rizzo M et al: Non-glycemic effects of pioglitazone and incretin-based therapies. Expert Opin T her T argets 2013;17:739.

Russell S: Incretin-based therapies for type 2 diabetes mellitus: A review of direct comparisons of efficacy, safety and patient satisfaction. Int J Clin Pharm 2013;35:159.

Standl E, Schnell O: Alpha-glucosidase inhibitors 2012—cardiovascular considerations and trial evaluation. Diab Vasc Dis Res 2012;9:163.

Switzer SM et al: Intensive insulin therapy in patients with type 1 diabetes mellitus. Endocrinol Metab Clin North Am 2012;41:89.

T urner RM et al: T hiazolidinediones and associated risk of bladder cancer: A systematic review and meta-analysis. Br J Clin Pharmacol 2013 Dec 10.

United Kingdom Prospective Diabetes Study (UKPDS) Group: Glycemic control with diet, sulfonylurea, metformin, or insulin in patients with type 2 diabetes mellitus: Progressive requirement for multiple therapies: UKPDS 49. JAMA 1999;281:2005.

United Kingdom Prospective Diabetes Study (UKPDS) Group: T ight blood pressure control and risk of macrovascular and microvascular complications in type 2 diabetes: UKPDS 38. Br Med J 1998;317:703.

CASE STUDY ANSWER This patient has multiple risk factors for type 2 diabetes. Although she does not have a prior history of fasting hyperglycemia, glucose intolerance, or gestational diabetes, other risk factors are present. Further evaluations that should be obtained include HbA1c concentration, dilated retinal examination, baseline laboratory tests, spot urine test for microalbumin/creatinine ratio, plasma creatinine level, and neurologic examination. The patient should be taught how to use a glucose meter and monitor her fingerstick blood glucose level, referred to a nutritionist for dietary instruction, and given diabetes self-management education. Assuming she has no renal or hepatic impairment, hygienic interventions (diet and exercise) and metformin would be the first line of treatment. If she is unable to achieve adequate glycemic control on metformin, an additional agent such as an insulin secretagogue (ie, a sulfonylurea, meglitinide, or nateglinide), insulin, or another antidiabetic medication could be added.

CHAPTER

42 Agents That Affect Bone Mineral Homeostasis Daniel D. Bikle, MD, PhD

CASE STUDY A 65-year-old man is referred to you from his primary care physician (PCP) for evaluation and management of possible osteoporosis. He saw his PCP for evaluation of low back pain. X-rays of the spine showed some degenerative changes in the lumbar spine plus several wedge deformities in the thoracic spine. The patient is a long-time smoker (up to two packs per day) and has two to four glasses of wine with dinner, more on the weekends. He has chronic bronchitis, presumably from smoking, and has been treated many times with oral prednisone for exacerbations of bronchitis. He is currently on 10 mg/d prednisone. Examination shows kyphosis of the thoracic spine, with some tenderness to fist percussion over the thoracic spine. The DEXA (dual-energy Xray absorptiometry) measurement of the lumbar spine is “within the normal limits,” but the radiologist noted that the reading may be misleading because of degenerative changes. The hip measurement shows a T score (number of standard deviations by which the patient’s measured bone density differs from that of a normal young adult) in the femoral neck of –2.2. What further workup should be considered, and what therapy should be initiated?

BASIC PHARMACOLOGY Calcium and phosphate, the major mineral constituents of bone, are also two of the most important minerals for general cellular function.

Accordingly, the body has evolved complex mechanisms to carefully maintain calcium and phosphate homeostasis (Figure 42–1).

Approximately 98% of the 1–2 kg of calcium and 85% of the 1 kg of phosphorus in the human adult are found in bone, the principal reservoir for these minerals. This reservoir is dynamic, with constant remodeling of bone and ready exchange of bone mineral with that in the extracellular fluid. Bone also serves as the principal structural support for the body and provides the space for hematopoiesis. This relationship is more than fortuitous as elements of the bone marrow affect skeletal processes just as skeletal elements affect hematopoeitic processes. During aging and in nutritional diseases such as anorexia nervosa and obesity, fat accumulates in the marrow, suggesting a dynamic interaction between marrow fat and bone. Abnormalities in bone mineral homeostasis can lead to a wide variety of cellular dysfunctions (eg, tetany, coma, muscle weakness), and to disturbances in structural support of the body (eg, osteoporosis with fractures) and loss of hematopoietic capacity (eg, infantile osteopetrosis).

FIGURE 42–1 Mechanisms contributing to bone mineral homeostasis. Serum calcium (Ca) and phosphorus (P) concentrations are controlled principally by three hormones, 1,25-dihydroxyvitamin D (D), fibroblast growth factor 23 (FGF23), and parathyroid hormone (PTH), through their action on absorption from the gut and from bone and on renal excretion. PTH and 1,25(OH)2 D increase the input of calcium and phosphorus from bone into the serum and stimulate bone formation. 1,25(OH)2 D also increases calcium and phosphate absorption from the gut. In the kidney, 1,25(OH)2 D decreases excretion of both calcium and phosphorus, whereas PTH reduces calcium but increases phosphorus excretion. FGF23 stimulates renal excretion of phosphate. Calcitonin (CT) is a less critical regulator of calcium homeostasis, but in pharmacologic concentrations can reduce serum calcium and phosphorus by inhibiting bone resorption and stimulating their renal excretion. Feedback may alter the effects shown; for example, 1,25(OH)2 D increases urinary calcium excretion indirectly through increased calcium absorption from the gut and inhibition of PTH secretion and may increase urinary phosphate excretion because of increased phosphate absorption from the gut and stimulation of FGF23 production.

Calcium and phosphate enter the body from the intestine. The average American diet provides 600–1000 mg of calcium per day, of which approximately 100–250 mg is absorbed. This amount represents net absorption, because both absorption (principally in the duodenum and upper jejunum) and secretion (principally in the ileum) occur. The quantity of phosphorus in the American diet is about the same as that of calcium. However, the efficiency of absorption (principally in the jejunum) is greater, ranging from 70% to 90%, depending on intake. In the steady state, renal excretion of calcium and phosphate balances intestinal absorption. In general, over 98% of filtered calcium and 85% of filtered phosphate is reabsorbed by the kidney. The movement of calcium and phosphate across the intestinal and renal epithelia is closely regulated. Dysfunction of the intestine (eg, nontropical sprue) or kidney (eg, chronic renal failure) can disrupt bone mineral homeostasis.

Three hormones serve as the principal regulators of calcium and phosphate homeostasis: parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and vitamin D via its active metabolite 1,25-dihydroxyvitamin D (1,25(OH)2 D (Figure 42–2). The role of calcitonin (CT) is less critical during adult life but may play a greater role during pregnancy and lactation. The term vitamin D, when used without a subscript, refers to both vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). This applies also to the metabolites of vitamin D2 and D3 . Vitamin D2 and its metabolites differ from vitamin D3 and its metabolites only in the side chain where they contain a double bond between C-22–23 and a methyl group at C-24 (Figure 42–3). Vitamin D is considered a prohormone because it must be further metabolized to gain biologic activity (Figure 42–3). Vitamin D3 is produced in the skin under ultraviolet B (UVB) radiation (eg, in sunlight) from its precursor, 7-dehydrocholesterol. The initial product, pre-vitamin D 3 , undergoes a temperature-sensitive isomerization to vitamin D3 . The precursor of vitamin D2 is ergosterol, found in plants and fungi (mushrooms). It undergoes a similar transformation to vitamin D2 with UVB radiation. Vitamin D 2 thus comes only from the diet, whereas vitamin D3 comes from the skin or the diet, or both.

The subsequent metabolism of these two forms of vitamin D is essentially the same and follows the illustration for vitamin D3 metabolism

in Figure 42â&#x20AC;&#x201C;3. The first step is the 25-hydroxylation of vitamin D to 25-hydroxyvitamin D (25[OH]D). A number of enzymes in the liver and other tissues perform this function, of which CYP2R1 is the most important. 25(OH)D is then metabolized to the active hormone 1,25-dihydroxyvitamin D (1,25[OH]2 D) in the kidney and elsewhere. PTH stimulates the production of 1,25(OH)2 D in the kidney, whereas FGF23 is inhibitory. Elevated levels of blood phosphate and calcium also inhibit 1,25(OH) 2 D production in part by their effects on FGF23 (high phosphate stimulates FGF23 production) and PTH (high calcium inhibits PTH production). 1,25(OH)2 D inhibits its own production but, at least as important, it stimulates the enzyme 24-hydroxyase (CYP24A1), which begins the catabolism of 1,25(OH)2 D, suppresses PTH production, and stimulates FGF23 production, all of which conspire to reduce 1,25(OH)2 D levels. Other tissues also produce 1,25(OH)2 D; the control of this production differs from that in the kidney, as will be discussed subsequently. The complex interplay among PTH, FGF23, and 1,25(OH)2 D is discussed in detail later.

FIGURE 42â&#x20AC;&#x201C;2 The hormonal interactions controlling bone mineral homeostasis. In the body (A), 1,25-dihydroxyvitamin D (1,25[OH]2 D) is produced by the kidney under the control of parathyroid hormone (PTH), which stimulates its production, and fibroblast growth factor 23 (FGF23), which inhibits its production. 1,25(OH)2 D in turn inhibits the production of PTH by the parathyroid glands and stimulates FGF23 release from bone. 1,25(OH)2 D is the principal regulator of intestinal calcium and phosphate absorption. At the level of the bone (B), both PTH and 1,25(OH)2 D regulate bone formation and resorption, with each capable of stimulating both processes. This is accomplished by their stimulation of preosteoblast proliferation and differentiation into osteoblasts, the bone-forming cell. PTH also stimulates osteoblast formation indirectly by inhibiting the osteocyteâ&#x20AC;&#x2122;s production of sclerostin, a protein that blocks osteoblast

proliferation by inhibiting the wnt pathway (not shown). PTH and 1,25(OH)2 D stimulate the expression of RANKL by the osteoblast, which, with MCSF, stimulates the differentiation and subsequent activation of osteoclasts, the bone-resorbing cell. OPG blocks RANKL action, and may be inhibited by PTH and 1,25(OH)2 D. FGF23 in excess leads to osteomalacia indirectly by inhibiting 1,25(OH)2 D production and lowering phosphate levels. MCSF, macrophage colony-stimulating factor; OPG, osteoprotegerin; RANKL, ligand for receptor for activation of nuclear factor-ÎşB.

FIGURE 42â&#x20AC;&#x201C;3 Conversion of 7-dehydrocholesterol to vitamin D3 in the skin and its subsequent metabolism to 25-hydroxyvitamin D3 (25[OH]D3 ) in the liver and to 1,25-dihydroxyvitamin D3 (1,25[OH]2 D3 ) and 24,25-dihydroxyvitamin D3 (24,25[OH]2 D3 ) in the kidney.

Control of vitamin D metabolism is exerted primarily at the level of the kidney, where high concentrations of serum phosphorus (P) and calcium (Ca) as well as fibroblast growth factor 23 (FGF23) inhibit production of 1,25(OH)2 D3 (indicated by a minus [−] sign), but promote that of 24,25(OH)2 D3 (indicated by a plus [+] sign). Parathyroid hormone (PTH), on the other hand, stimulates 1,25(OH)2 D3 production but inhibits 24,25(OH)2 D3 production. The insert (shaded) shows the side chain for ergosterol, vitamin D2 , and the active vitamin D2 metabolites. Ergosterol is converted to vitamin D2 (ergocalciferol) by UV radiation similar to the conversion of 7dehydrocholesterol to vitamin D3 . Vitamin D2 , in turn, is metabolized to 25-hydroxyvitamin D2 , 1,25-dihydroxyvitamin D2 , and 24,25dihydroxyvitamin D2 via the same enzymes that metabolize vitamin D3 . In humans, corresponding D2 and D3 metabolites have equivalent biologic effects, although they differ in pharmacokinetics. +, facilitation; –, inhibition; P, phosphorus; Ca, calcium; PTH, parathyroid hormone; FGF23, fibroblast growth factor 23.

To summarize: 1,25(OH) 2 D suppresses the production of PTH as does calcium, but stimulates the production of FGF23. Phosphate stimulates both PTH and FGF23 secretion. In turn PTH stimulates 1,25(OH)2 D production, whereas FGF23 is inhibitory. 1,25(OH) 2 D stimulates the intestinal absorption of calcium and phosphate. 1,25(OH)2 D and PTH promote both bone formation and resorption in part by stimulating the proliferation and differentiation of osteoblasts and osteoclasts. Both PTH and 1,25(OH)2 D enhance renal retention of calcium, but PTH promotes renal phosphate excretion as does FGF23, whereas 1,25(OH)2 D promotes renal reabsorption of phosphate.

Other hormones—calcitonin, prolactin, growth hormone, insulin, thyroid hormone, glucocorticoids, and sex steroids—influence calcium and phosphate homeostasis under certain physiologic circumstances and can be considered secondary regulators. Deficiency or excess of these secondary regulators within a physiologic range does not produce the disturbance of calcium and phosphate homeostasis that is observed in situations of deficiency or excess of PTH, FGF23, and vitamin D. However, certain of these secondary regulators— especially calcitonin, glucocorticoids, and estrogens—are useful therapeutically and discussed in subsequent sections.

In addition to these hormonal regulators, calcium and phosphate themselves, other ions such as sodium and fluoride, and a variety of drugs (bisphosphonates, plicamycin, and diuretics) also alter calcium and phosphate homeostasis.

PRINCIPAL HORMONAL REGULATORS OF BONE MINERAL HOMEOSTASIS PARATHYROID HORMONE Parathyroid hormone (PTH) is a single-chain peptide hormone composed of 84 amino acids. It is produced in the parathyroid gland in a precursor form of 115 amino acids, the remaining 31 amino terminal amino acids being cleaved off before secretion. Within the gland is a calcium-sensitive protease capable of cleaving the intact hormone into fragments, thereby providing one mechanism by which calcium limits the production of PTH. A second mechanism involves the calcium-sensing receptor (CaSR) which, when stimulated by calcium, reduces PTH production and secretion. The parathyroid gland also contains the vitamin D receptor (VDR) and the enzyme, CYP27B1, that produces 1,25(OH)2 D, thus enabling circulating or endogenously produced 1,25(OH)2 D to suppress PTH production. 1,25(OH)2 D also induces the CaSR, making the parathyroid gland more sensitive to suppression by calcium. Biologic activity resides in the amino terminal region of PTH such that synthetic PTH 1-34 (available as teriparatide) is fully active. Loss of the first two amino terminal amino acids eliminates most biologic activity.

The metabolic clearance of intact PTH is rapid, with a half-time of disappearance measured in minutes. Most of the clearance occurs in the liver and kidney. The inactive carboxyl terminal fragments produced by metabolism of the intact hormone have a much lower clearance, especially in renal failure. In the past, this accounted for the very high PTH values observed in patients with renal failure when the hormone was measured by radioimmunoassays directed against the carboxyl terminal region. Currently, most PTH assays differentiate between intact PTH 1-34 and large inactive fragments, so that it is possible to more accurately evaluate biologically active PTH status in patients with renal failure.

PTH regulates calcium and phosphate flux across cellular membranes in bone and kidney, resulting in increased serum calcium and decreased serum phosphate (Figure 42–1). In bone, PTH increases the activity and number of osteoclasts, the cells responsible for bone resorption (Figure 42–2). However, this stimulation of osteoclasts is not a direct effect. Rather, PTH acts on the osteoblast (the boneforming cell) to induce membrane-bound and secreted soluble forms of a protein called RANK ligand (RANKL). RANKL acts on osteoclasts and osteoclast precursors to increase both the numbers and activity of osteoclasts. This action increases bone remodeling, a specific sequence of cellular events initiated by osteoclastic bone resorption and followed by osteoblastic bone formation. Denosumab, an antibody that inhibits the action of RANKL, has been developed for the treatment of excess bone resorption in patients with osteoporosis and certain cancers. PTH also inhibits the production and secretion of sclerostin from osteocytes. Sclerostin is one of several proteins that blocks osteoblast proliferation by inhibiting the wnt pathway. Thus, PTH indirectly increases proliferation of osteoblasts, the cells responsible for bone formation. An antibody against sclerostin is in clinical trials for the treatment of osteoporosis.

Although both bone resorption and bone formation are enhanced by PTH, the net effect of excess endogenous PTH is to increase bone resorption. However, administration of exogenous PTH in low and intermittent doses increases bone formation without first stimulating bone resorption. This net anabolic action may be indirect, involving other growth factors such as insulin-like growth factor 1 (IGF-1) as well as inhibition of sclerostin as noted above. These anabolic actions have led to the approval of recombinant PTH 1-34 (teriparatide)

for the treatment of osteoporosis. In the kidney, PTH increases tubular reabsorption of calcium and magnesium but reduces reabsorption of phosphate, amino acids, bicarbonate, sodium, chloride, and sulfate. As noted earlier another important action of PTH on the kidney is stimulation of 1,25(OH)2 D production.

VITAMIN D Vitamin D is a secosteroid produced in the skin from 7-dehydrocholesterol under the influence of ultraviolet radiation. Vitamin D is also found in certain foods and is used to supplement dairy products and other foods. Both the natural form (vitamin D3 , cholecalciferol) and the plant-derived form (vitamin D2 , ergocalciferol) are present in the diet. As discussed earlier these forms differ in that ergocalciferol contains a double bond and an additional methyl group in the side chain (Figure 42–3). Ergocalciferol and its metabolites bind less well than cholecalciferol and its metabolites to vitamin D-binding protein (DBP), the major transport protein of these compounds in blood, and have a different path of catabolism. As a result their half-lives are shorter than those of the cholecalciferol metabolites. This influences treatment strategies, as will be discussed. However, the key steps in metabolism and biologic activities of the active metabolites are comparable, so with this exception the following comments apply equally well to both forms of vitamin D. Vitamin D is a precursor to a number of biologically active metabolites (Figure 42–3). Vitamin D is first hydroxylated in the liver and other tissues to form 25(OH)D(calcifediol). As noted earlier there are a number of enzymes with 25-hydroxylase activity. This metabolite is further converted in the kidney to a number of other forms, the best studied of which are 1,25(OH)2 D (calcitriol) and 24,25dihydroxyvitamin D (24,25[OH]2 D), by the enzymes CYP27B1 and CYP24A1, respectively. The regulation of vitamin D metabolism is complex, involving calcium, phosphate, and a variety of hormones, the most important of which are PTH, which stimulates, and FGF23, which inhibits the production of 1,25(OH)2 D by the kidney while reciprocally inhibiting or promoting the production of 24,25(OH)2 D. The importance of CYP24A1, the enzyme that 24-hydroxylates 25(OH)D and 1,25(OH)2 D, is well demonstrated in children lacking this enzyme who have high levels of calcium and 1,25(OH)2 D resulting in kidney damage from nephrocalcinosis and stones. Of the natural metabolites, only vitamin D and 1,25(OH)2 D (as calcitriol) are available for clinical use (Table 42–1). A number of analogs of 1,25(OH)2 D have been synthesized to extend the usefulness of this metabolite to a variety of nonclassic conditions. Calcipotriene (calcipotriol), for example, is being used to treat psoriasis, a hyperproliferative skin disorder (see Chapter 61) . Doxercalciferol and paricalcitol are approved for the treatment of secondary hyperparathyroidism in patients with chronic kidney disease. Eldecalcitol is in phase 3 clinical trials in Japan for the treatment of osteoporosis. Other analogs are being investigated for the treatment of various malignancies.

TABLE 42–1 Vitamin D and its major metabolites and analogs.

Vitamin D and its metabolites circulate in plasma tightly bound to the DBP. This α-globulin binds 25(OH)D and 24,25(OH) 2 D with comparable high affinity and vitamin D and 1,25(OH)2 D with lower affinity. There is increasing evidence that it is the free or unbound forms of these metabolites that have biologic activity. This is of clinical importance because there are several different forms of DBP in the population with different affinities for the vitamin D metabolites. Such individuals vary with respect to the fraction of free metabolite available. Moreover, as noted above, the affinity of DBP for the D 2 metabolites is less than that for the D3 metabolites. In normal subjects, the terminal half-life of injected calcifediol (25[OH]D) is around 23 days, whereas in anephric subjects it is around 42 days. The half-life of 24,25(OH)2 D is probably similar. Tracer studies with vitamin D have shown a rapid clearance from the blood. The liver appears to be the principal organ for clearance. Excess vitamin D is stored in adipose tissue. The metabolic clearance of calcitriol (1,25[OH]2 D) in humans likewise indicates a rapid turnover, with a terminal half-life measured in hours. Several of the 1,25(OH) 2 D analogs are bound poorly by DBP. As a result, their clearance is very rapid, with a terminal half-life of minutes. Such analogs have less hypercalcemic, hypercalciuric effects than calcitriol, an important aspect of their use in the management of conditions such as psoriasis and hyperparathyroidism.

The mechanism of action of the vitamin D metabolites remains under active investigation. However, 1,25(OH) 2 D is well established as the most potent stimulant of intestinal calcium and phosphate transport and bone resorption. 1,25(OH)2 D appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein and TRPV6, an intestinal calcium channel) and by modulation of calcium flux across the brush border and basolateral membranes by a process that does not require new protein synthesis.

The molecular action of 1,25(OH)2 D on bone has received less attention. However, like PTH, 1,25(OH) 2 D can induce RANKL in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2 D are far less potent stimulators of intestinal calcium and phosphate transport or bone resorption.

Specific receptors for 1,25(OH)2 D (VDR) exist in nearly all tissues, not just intestine, bone, and kidney; as a result much effort has been made to develop analogs of 1,25(OH)2 D that will target these non-classic tissues without increasing serum calcium. These nonclassic actions include regulation of the secretion of PTH, insulin, and renin; dendritic cell as well as T-cell differentiation; and proliferation and differentiation of a number of cancer cells. Thus, the clinical utility of 1,25(OH)2 D and its analogs is expanding.

FIBROBLAST GROWTH FACTOR 23 Fibroblast growth factor 23 (FGF23) is a single-chain protein with 251 amino acids, including a 24-amino-acid leader sequence. It inhibits 1,25(OH)2 D production and phosphate reabsorption (via the sodium phosphate co-transporters NaPi 2a and 2c) in the kidney, and can lead to both hypophosphatemia and inappropriately low levels of circulating 1,25(OH)2 D. Whereas FGF23 was originally identified in certain mesenchymal tumors, osteoblasts and osteocytes in bone appear to be its primary site of production. Other tissues can also produce FGF23, though at lower levels. FGF23 requires O-glycosylation for its secretion, a glycosylation mediated by the glycosyl transferase GALNT3. Mutations in GALNT3 result in abnormal deposition of calcium phosphate in periarticular tissues (tumoral calcinosis) with elevated phosphate and 1,25(OH)2 D. FGF23 is normally inactivated by cleavage at an RXXR site (amino acids 176– 179). Mutations in this site lead to excess FGF23, the underlying problem in autosomal dominant hypophosphatemic rickets. A similar disease, X-linked hypophosphatemic rickets, is due to mutations in PHEX, an endopeptidase, which initially was thought to cleave FGF23.

However, this concept has been shown to be invalid, and the mechanism by which PHEX mutations lead to increased FGF23 levels remains obscure. FGF23 binds to FGF receptors 1 and 3c in the presence of the accessory receptor Klotho. Both Klotho and the FGF receptor must be present for signaling. Mutations in Klotho disrupt FGF23 signaling, resulting in elevated phosphate and 1,25(OH)2 D levels, a phenotype quite similar to inactivating mutations in FGF23 or GALNT3. FGF23 production is stimulated by 1,25(OH)2 D and phosphate and directly or indirectly inhibited by the dentin matrix protein DMP1 found in osteocytes. Mutations in DMP1 lead to increased FGF23 levels and osteomalacia.

INTERACTION OF PTH, FGF23, & VITAMIN D A summary of the principal actions of PTH, FGF23, and vitamin D on the three main target tissues—intestine, kidney, and bone—is presented in Table 42–2. The net effect of PTH is to raise serum calcium and reduce serum phosphate; the net effect of FGF23 is to decrease serum phosphate; the net effect of vitamin D is to raise both. Regulation of calcium and phosphate homeostasis is achieved through important feedback loops. Calcium is one of two principal regulators of PTH secretion. It binds to a novel ion recognition site that is part of a Gq protein-coupled receptor called the calcium-sensing receptor (CaSR) that employs the phosphoinositide second messenger system to link changes in the extracellular calcium concentration to changes in the intracellular free calcium. As serum calcium levels rise and activate this receptor, intracellular calcium levels increase and inhibit PTH secretion. This inhibition by calcium of PTH secretion, along with inhibition of renin and atrial natriuretic peptide secretion, is the opposite of the effect in other tissues such as the beta cell of the pancreas, in which calcium stimulates secretion. Phosphate regulates PTH secretion directly and indirectly by forming complexes with calcium in the serum. Because it is the ionized free concentration of extracellular calcium that is detected by the

parathyroid gland, increases in serum phosphate levels reduce the ionized calcium, leading to enhanced PTH secretion. Such feedback regulation is appropriate to the net effect of PTH to raise serum calcium and reduce serum phosphate levels. Likewise, both calcium and phosphate at high levels reduce the amount of 1,25(OH)2 D produced by the kidney and increase the amount of 24,25(OH)2 D produced.

TABLE 42â&#x20AC;&#x201C;2 Actions of parathyroid hormone (PTH), vitamin D, and FGF23 on gut, bone, and kidney.

High serum calcium works directly and indirectly by reducing PTH secretion. High serum phosphate works directly and indirectly by increasing FGF23 levels. Since 1,25(OH)2 D raises serum calcium and phosphate, whereas 24,25(OH)2 D has less effect, such feedback regulation is again appropriate. 1,25(OH)2 D directly inhibits PTH secretion (independent of its effect on serum calcium) by a direct inhibitory effect on PTH gene transcription. This provides yet another negative feedback loop. In patients with chronic renal failure who frequently are deficient in producing 1,25(OH)2 D, loss of this 1,25(OH)2 D-mediated feedback loop coupled with impaired phosphate excretion and intestinal calcium absorption leads to secondary hyperparathyroidism. The ability of 1,25(OH)2 D to inhibit PTH secretion directly is being exploited with calcitriol analogs that have less effect on serum calcium because of their lesser effect on intestinal calcium absorption. Such drugs are proving useful in the management of secondary hyperparathyroidism accompanying chronic kidney disease and may be useful in selected cases of primary hyperparathyroidism. 1,25(OH)2 D also stimulates the production of FGF23. This completes the negative feedback loop in that FGF23 inhibits 1,25(OH)2 D production while promoting hypophosphatemia, which in turn inhibits FGF23 production and stimulates 1,25(OH)2 D production.

SECONDARY HORMONAL REGULATORS OF BONE MINERAL HOMEOSTASIS A number of hormones modulate the actions of PTH, FGF23, and vitamin D in regulating bone mineral homeostasis. Compared with that of PTH, FGF23, and vitamin D, the physiologic impact of such secondary regulation on bone mineral homeostasis is minor. However, in pharmacologic amounts, several of these hormones, including calcitonin, glucocorticoids, and estrogens, have actions on bone mineral homeostatic mechanisms that can be exploited therapeutically.

CALCITONIN The calcitonin secreted by the parafollicular cells of the mammalian thyroid is a single-chain peptide hormone with 32 amino acids and a

molecular weight of 3600. A disulfide bond between positions 1 and 7 is essential for biologic activity. Calcitonin is produced from a precursor with MW 15,000. The circulating forms of calcitonin are multiple, ranging in size from the monomer (MW 3600) to forms with an apparent MW of 60,000. Whether such heterogeneity includes precursor forms or covalently linked oligomers is not known. Because of its chemical heterogeneity, calcitonin preparations are standardized by bioassay in rats. Activity is compared to a standard maintained by the British Medical Research Council (MRC) and expressed as MRC units.

Human calcitonin monomer has a half-life of about 10 minutes. Salmon calcitonin has a longer half-life of 40â&#x20AC;&#x201C;50 minutes, making it more attractive as a therapeutic agent. Much of the clearance occurs in the kidney by metabolism; little intact calcitonin appears in the urine.

The principal effects of calcitonin are to lower serum calcium and phosphate by actions on bone and kidney. Calcitonin inhibits osteoclastic bone resorption. Although bone formation is not impaired at first after calcitonin administration, with time both formation and resorption of bone are reduced. In the kidney, calcitonin reduces both calcium and phosphate reabsorption as well as reabsorption of other ions, including sodium, potassium, and magnesium. Tissues other than bone and kidney are also affected by calcitonin. Calcitonin in pharmacologic amounts decreases gastrin secretion and reduces gastric acid output while increasing secretion of sodium, potassium, chloride, and water in the gut. Pentagastrin is a potent stimulator of calcitonin secretion (as is hypercalcemia), suggesting a possible physiologic relationship between gastrin and calcitonin. In the adult human, no readily demonstrable problem develops in cases of calcitonin deficiency (thyroidectomy) or excess (medullary carcinoma of the thyroid). However, the ability of calcitonin to block bone resorption and lower serum calcium makes it a useful drug for the treatment of Pagetâ&#x20AC;&#x2122;s disease, hypercalcemia, and osteoporosis, albeit a less efficacious drug than other available agents.

GLUCOCORTICOIDS Glucocorticoid hormones alter bone mineral homeostasis by antagonizing vitamin D-stimulated intestinal calcium transport, stimulating renal calcium excretion, and blocking bone formation. Although these observations underscore the negative impact of glucocorticoids on bone mineral homeostasis, these hormones have proved useful in reversing the hypercalcemia associated with lymphomas and granulomatous diseases such as sarcoidosis (in which unregulated ectopic production of 1,25[OH]2 D occurs) or in cases of vitamin D intoxication. Prolonged administration of glucocorticoids is a common cause of osteoporosis in adults and can cause stunted skeletal development in children.

ESTROGENS Estrogens can prevent accelerated bone loss during the immediate postmenopausal period and at least transiently increase bone in postmenopausal women.

The prevailing hypothesis advanced to explain these observations is that estrogens reduce the bone-resorbing action of PTH. Estrogen administration leads to an increased 1,25(OH)2 D level in blood, but estrogens have no direct effect on 1,25(OH)2 D production in vitro. The increased 1,25(OH)2 D levels in vivo following estrogen treatment may result from decreased serum calcium and phosphate and increased PTH. However, estrogens also increase DBP production by the liver, which increases the total concentrations of the vitamin D metabolites in circulation without necessarily increasing the free levels. Estrogen receptors have been found in bone, and estrogen has direct effects on bone remodeling. Case reports of men who lack the estrogen receptor or who are unable to produce estrogen because of aromatase deficiency noted marked osteopenia and failure to close epiphyses. This further substantiates the role of estrogen in bone development, even in men. The principal therapeutic application for estrogen administration in disorders of bone mineral homeostasis is the treatment or prevention of postmenopausal osteoporosis. However, long-term use of estrogen has fallen out of favor due to concern about adverse effects. Selective estrogen receptor modulators (SERMs) have been developed to retain the beneficial effects on bone while minimizing deleterious effects on breast, uterus, and the cardiovascular system (see Box: Newer Therapies for Osteoporosis and Chapter 40).

NONHORMONAL AGENTS AFFECTING BONE MINERAL HOMEOSTASIS BISPHOSPHONATES The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42â&#x20AC;&#x201C;4). Currently available bisphosphonates include etidronate, pamidronate, alendronate, risedronate, tiludronate, ibandronate, and zoledronate. With the development of the more potent bisphosphonates, etidronate is seldom used.

FIGURE 42â&#x20AC;&#x201C;4 The structure of pyrophosphate and of the first three bisphosphonatesâ&#x20AC;&#x201D;etidronate, pamidronate, and alendronateâ&#x20AC;&#x201D;that were approved for use in the USA. Results from animal and clinical studies indicate that less than 10% of an oral dose of these drugs is absorbed. Food reduces absorption even further, necessitating their administration on an empty stomach. A major adverse effect of oral forms of the bisphosphonates (risedronate, alendronate, ibandronate) is esophageal and gastric irritation, which limits the use of this route by patients with upper gastrointestinal disorders. This complication can be circumvented with infusions of pamidronate, zoledronate, and ibandronate.

Intravenous dosing also allows a larger amount of drug to enter the body and markedly reduces the frequency of administration (eg, zoledronate is infused once per year). Nearly half of the absorbed drug accumulates in bone; the remainder is excreted unchanged in the urine. Decreased renal function dictates a reduction in dosage. The portion of drug retained in bone depends on the rate of bone turnover; drug in bone often is retained for months to years.

Newer Therapies for Osteoporosis Bone undergoes a continuous remodeling process involving resorption and formation. Any process that disrupts this balance by increasing bone resorption relative to formation results in osteoporosis. Inadequate gonadal hormone production is a major cause of osteoporosis in men and women. Estrogen replacement therapy at menopause is a well-established means of preventing osteoporosis in the female, but many women fear its adverse effects, particularly the increased risk of breast cancer from continued estrogen use (the well-demonstrated increased risk of endometrial cancer is prevented by combining the estrogen with a progestin) and do not like the persistence of menstrual bleeding that often accompanies this form of therapy. Medical enthusiasm for this treatment has waned with the demonstration that it does not protect against and may increase the risk of heart disease. Raloxifene was the first of the selective estrogen receptor modulators (SERMs; see Chapter 40) to be approved for the prevention of osteoporosis. Raloxifene shares some of the beneficial effects of estrogen on bone without increasing the risk of breast or endometrial cancer (it may actually reduce the risk of breast cancer). Although not as effective as estrogen in increasing bone density, raloxifene has been shown to reduce vertebral fractures.

Nonhormonal forms of therapy for osteoporosis have been developed with proven efficacy in reducing fracture risk.

Bisphosphonates such as alendronate, risedronate, and ibandronate have been conclusively shown to increase bone density and reduce fractures over at least 5 years when used continuously at a dosage of 10 mg/d or 70 mg/wk for alendronate; 5 mg/d or 35 mg/wk for risedronate; 2.5 mg/d or 150 mg/month for ibandronate; and 5 mg annually for intravenous zoledronate. Side-by-side trials between alendronate and calcitonin (another approved nonestrogen drug for osteoporosis) indicated a greater efficacy of alendronate. Bisphosphonates are poorly absorbed and must be given on an empty stomach or infused intravenously. At the higher oral doses used in the treatment of Paget’s disease, alendronate causes gastric irritation, but this is not a significant problem at the doses recommended for osteoporosis when patients are instructed to take the drug with a glass of water and remain upright.

Denosumab is a human monoclonal antibody directed against RANKL, and is very effective in inhibiting osteoclastogenesis and activity. Denosumab is given in 60 mg doses subcutaneously every 6 months. All of these drugs inhibit bone resorption with secondary effects to inhibit bone formation. On the other hand, teriparatide, the recombinant form of PTH 1-34, directly stimulates bone formation as well as bone resorption. However, teriparatide is given daily by subcutaneous injection. Its efficacy in preventing fractures is at least as great as that of the bisphosphonates. In all cases, adequate intake of calcium and vitamin D needs to be maintained.

Furthermore, there are several other forms of therapy in development. In Europe, strontium ranelate, a drug that appears to stimulate bone formation and inhibit bone resorption, has been used for several years with favorable results in large clinical trials; approval for use in the USA is expected. Additional promising new treatments undergoing clinical trials include an antibody against sclerostin, that has been shown to stimulate bone formation, and inhibitors of cathepsin K, an enzyme in osteoclasts that facilitates bone resorption. In Japan, eldecalcitol, an analog of 1,25(OH)2 D, is showing promise for the treatment of osteoporosis with minimal effects on serum calcium.

The bisphosphonates exert multiple effects on bone mineral homeostasis, which make them useful for the treatment of hypercalcemia associated with malignancy, for Paget’s disease, and for osteoporosis (see Box: Newer Therapies for Osteoporosis). They owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. Some of the newer bisphosphonates appear to increase bone mineral density well beyond the 2-year period predicted for a drug whose effects are limited to slowing bone resorption. This may be due to their other cellular effects, which include inhibition of 1,25(OH)2 D production, inhibition of intestinal calcium transport, metabolic changes in bone cells such as inhibition of glycolysis, inhibition of cell growth, and changes in acid and alkaline phosphatase activity.

Amino bisphosphonates such as alendronate and risedronate inhibit farnesyl pyrophosphate synthase, an enzyme in the mevalonate pathway that appears to be critical for osteoclast survival. The cholesterol-lowering statin drugs (eg, lovastatin), which block mevalonate synthesis (see Chapter 35), stimulate bone formation, at least in animal studies. Thus, the mevalonate pathway appears to be important in bone cell function and provides new targets for drug development. The mevalonate pathway effects vary depending on the bisphosphonate used (only amino bisphosphonates have this property), and may account for some of the clinical differences observed in the effects of the various bisphosphonates on bone mineral homeostasis.

With the exception of the induction of a mineralization defect by higher than approved doses of etidronate and gastric and esophageal irritation by the oral bisphosphonates, these drugs have proved to be remarkably free of adverse effects when used at the doses recommended for the treatment of osteoporosis. Esophageal irritation can be minimized by taking the drug with a full glass of water and remaining upright for 30 minutes or by using the intravenous forms of these compounds. Of the other complications, osteonecrosis of the jaw has received considerable attention but is rare in patients receiving usual doses of bisphosphonates (perhaps 1/100,000 patient-years).

This complication is more frequent when high intravenous doses of zoledronate are used to control bone metastases and cancer-induced hypercalcemia. More recently, concern has been raised about over-suppressing bone turnover. This may underlie the occurrence of subtrochanteric femur fractures in patients on long-term bisphosphonate treatment. This complication appears to be rare, comparable to that of osteonecrosis of the jaw, but has led some authorities to recommend a “drug holiday” after 5 years of treatment if the clinical condition warrants it (ie, if the fracture risk of discontinuing the bisphosphonate is not deemed high).

DENOSUMAB Denosumab is a fully human monoclonal antibody that binds to and prevents the action of RANKL. As described earlier, RANKL is produced by osteoblasts and other cells, including T lymphocytes. It stimulates osteoclastogenesis via RANK, the receptor for RANKL that is present on osteoclasts and osteoclast precursors. By interfering with RANKL function, denosumab inhibits osteoclast formation and activity. It is at least as effective as the potent bisphosphonates in inhibiting bone resorption and has been approved for treatment of postmenopausal osteoporosis and some cancers (prostate and breast). The latter application is to limit the development of bone metastases or bone loss resulting from the use of drugs that suppress gonadal function. Denosumab is administered subcutaneously every 6 months. The drug appears to be well tolerated but three concerns remain. First, a number of cells in the immune system also express RANKL, suggesting that there could be an increased risk of infection associated with the use of denosumab. Second, because the suppression of bone turnover with denosumab is similar to that of the potent bisphosphonates, the risk of osteonecrosis of the jaw and

subtrochanteric fractures may be increased, although this has not been reported in the clinical trials leading to its approval by the FDA. Third, denosumab can lead to transient hypocalcemia, especially in patients with marked bone loss (and bone hunger) or compromised calcium regulatory mechanisms, including chronic kidney disease and vitamin D deficiency.

CALCIMIMETICS Cinacalcet is the first representative of a new class of drugs that activates the calcium-sensing receptor (CaSR) described above. CaSR is widely distributed but has its greatest concentration in the parathyroid gland. By activating the parathyroid gland CaSR, cinacalcet inhibits PTH secretion. Cinacalcet is approved for the treatment of secondary hyperparathyroidism in chronic kidney disease and for the treatment of parathyroid carcinoma. CaSR antagonists are also being developed, and may be useful in conditions of hypoparathyroidism or as a means to stimulate intermittent PTH secretion in the treatment of osteoporosis.

PLICAMYCIN (MITHRAMYCIN) Plicamycin is a cytotoxic antibiotic (see Chapter 54) that has been used clinically for two disorders of bone mineral metabolism: Paget’s disease and hypercalcemia. The cytotoxic properties of the drug appear to involve binding to DNA and interruption of DNA-directed RNA synthesis. The reasons for its usefulness in the treatment of Paget’s disease and hypercalcemia are unclear but may relate to the need for protein synthesis to sustain bone resorption. The doses required to treat Paget’s disease and hypercalcemia are about one tenth the amount required to achieve cytotoxic effects. With the development of other less toxic drugs for these purposes, the clinical use of plicamycin is seldom indicated.

THIAZIDE DIURETICS The chemistry and pharmacology of the thiazide family of drugs are discussed in Chapter 15. The principal application of thiazides in the treatment of bone mineral disorders is in reducing renal calcium excretion. Thiazides may increase the effectiveness of PTH in stimulating reabsorption of calcium by the renal tubules or may act on calcium reabsorption secondarily by increasing sodium reabsorption in the proximal tubule. In the distal tubule, thiazides block sodium reabsorption at the luminal surface, increasing the calcium-sodium exchange at the basolateral membrane and thus enhancing calcium reabsorption into the blood at this site (see Figure 15–4). Thiazides have proved to be useful in reducing the hypercalciuria and incidence of urinary stone formation in subjects with idiopathic hypercalciuria.

Part of their efficacy in reducing stone formation may lie in their ability to decrease urine oxalate excretion and increase urine magnesium and zinc levels, both of which inhibit calcium oxalate stone formation.

FLUORIDE Fluoride is well established as effective for the prophylaxis of dental caries and has previously been investigated for the treatment of osteoporosis. Both therapeutic applications originated from epidemiologic observations that subjects living in areas with naturally fluoridated water (1–2 ppm) had less dental caries and fewer vertebral compression fractures than subjects living in nonfluoridated water areas. Fluoride accumulates in bones and teeth, where it may stabilize the hydroxyapatite crystal. Such a mechanism may explain the effectiveness of fluoride in increasing the resistance of teeth to dental caries, but it does not explain its ability to promote new bone growth.

Fluoride in drinking water appears to be most effective in preventing dental caries if consumed before the eruption of the permanent teeth. The optimum concentration in drinking water supplies is 0.5–1 ppm. Topical application is most effective if done just as the teeth erupt. There is little further benefit to giving fluoride after the permanent teeth are fully formed. Excess fluoride in drinking water leads to mottling of the enamel proportionate to the concentration above 1 ppm.

Because of the paucity of agents that stimulate new bone growth in patients with osteoporosis, fluoride for this disorder has been examined (see Osteoporosis, below). Results of earlier studies indicated that fluoride alone, without adequate calcium supplementation, produced osteomalacia. More recent studies, in which calcium supplementation has been adequate, have demonstrated an improvement in calcium balance, an increase in bone mineral, and an increase in trabecular bone volume. Despite these promising effects of fluoride on bone mass, clinical studies have failed to demonstrate a reliable reduction in fractures, and some studies showed an increase in fracture rate. At present, fluoride is not approved by the FDA for treatment or prevention of osteoporosis, and it is unlikely to be.

Adverse effects observed—at the higher doses used for testing fluoride’s effect on bone—include nausea and vomiting, gastrointestinal blood loss, arthralgias, and arthritis in a substantial proportion of patients. Such effects are usually responsive to reduction of the dose or giving fluoride with meals (or both).

STRONTIUM RANELATE Strontium ranelate is composed of two atoms of strontium bound to an organic ion, ranelic acid. Although not yet approved for use in the USA, this drug is used in Europe for the treatment of osteoporosis. Strontium ranelate appears to block differentiation of osteoclasts while promoting their apoptosis, thus inhibiting bone resorption. At the same time, strontium ranelate appears to promote bone formation.

Unlike bisphosphonates, denosumab, or teriparatide, this drug increases bone formation markers while inhibiting bone resorption markers.

Large clinical trials have demonstrated its efficacy in increasing bone mineral density and decreasing fractures in the spine and hip.

Toxicities reported thus far are similar to placebo.

CLINICAL PHARMACOLOGY Individuals with disorders of bone mineral homeostasis usually present with abnormalities in serum or urine calcium levels (or both), often accompanied by abnormal serum phosphate levels. These abnormal mineral concentrations may themselves cause symptoms requiring immediate treatment (eg, coma in malignant hypercalcemia, tetany in hypocalcemia). More commonly, they serve as clues to an underlying disorder in hormonal regulators (eg, primary hyperparathyroidism), target tissue response (eg, chronic kidney disease), or drug misuse (eg, vitamin D intoxication). In such cases, treatment of the underlying disorder is of prime importance.

Since bone and kidney play central roles in bone mineral homeostasis, conditions that alter bone mineral homeostasis usually affect one or both of these tissues secondarily. Effects on bone can result in osteoporosis (abnormal loss of bone; remaining bone histologically normal), osteomalacia (abnormal bone formation due to inadequate mineralization), or osteitis fibrosa (excessive bone resorption with fibrotic replacement of resorption cavities and marrow). Biochemical markers of skeletal involvement include changes in serum levels of the skeletal isoenzyme of alkaline phosphatase, osteocalcin, and N- and C-terminal propeptides of type I collagen (reflecting osteoblastic activity), and serum and urine levels of tartrate-resistant acid phosphatase and collagen breakdown products (reflecting osteoclastic activity). The kidney becomes involved when the calcium × phosphate product in serum rises above the point at which ectopic calcification occurs (nephrocalcinosis) or when the calcium × oxalate (or phosphate) product in urine exceeds saturation, leading to nephrolithiasis. Subtle early indicators of such renal involvement include polyuria, nocturia, and hyposthenuria. Radiologic evidence of nephrocalcinosis and stones is not generally observed until later. The degree of the ensuing renal failure is best followed by monitoring the decline in creatinine clearance. On the other hand, chronic kidney disease can be a primary cause of bone disease because of altered handling of calcium and phosphate, decreased 1,25(OH)2 D production, and secondary hyperparathyroidism.

ABNORMAL SERUM CALCIUM & PHOSPHATE LEVELS HYPERCALCEMIA Hypercalcemia causes central nervous system depression, including coma, and is potentially lethal. Its major causes (other than thiazide therapy) are hyperparathyroidism and cancer, with or without bone metastases. Less common causes are hypervitaminosis D, sarcoidosis, thyrotoxicosis, milk-alkali syndrome, adrenal insufficiency, and immobilization. With the possible exception of hypervitaminosis D, the latter disorders seldom require emergency lowering of serum calcium. A number of approaches are used to manage the hypercalcemic crisis.

Saline Diuresis In hypercalcemia of sufficient severity to produce symptoms, rapid reduction of serum calcium is required. The first steps include rehydration with saline and diuresis with furosemide, although the efficacy of furosemide in this setting has not been proved and use of the drug for this purpose appears to be falling out of favor. Most patients presenting with severe hypercalcemia have a substantial component of prerenal azotemia owing to dehydration, which prevents the kidney from compensating for the rise in serum calcium by excreting more calcium in the urine. Therefore, the initial infusion of 500–1000 mL/h of saline to reverse the dehydration and restore urine flow can by itself substantially lower serum calcium. The addition of a loop diuretic such as furosemide following rehydration enhances urine flow and also inhibits calcium reabsorption in the ascending limb of the loop of Henle (see Chapter 15). Monitoring of central venous pressure is important to forestall the development of heart failure and pulmonary edema in predisposed subjects. In many subjects, saline diuresis suffices to reduce serum calcium to a point at which more definitive diagnosis and treatment of the underlying condition can be achieved. If this is not the case or if more prolonged medical treatment of hypercalcemia is required, the following agents are available (discussed in order of preference).

Bisphosphonates Pamidronate, 60–90 mg, infused over 2–4 hours, and zoledronate, 4 mg, infused over at least 15 minutes, have been approved for the treatment of hypercalcemia of malignancy and have largely replaced the less effective etidronate for this indication. The bisphosphonate

effects generally persist for weeks, but treatment can be repeated after a 7-day interval if necessary and if renal function is not impaired.

Some patients experience a self-limited flu-like syndrome after the initial infusion, but subsequent infusions generally do not have this side effect. Repeated doses of these drugs have been linked to renal deterioration and osteonecrosis of the jaw, but this adverse effect is rare.

Calcitonin Calcitonin has proved useful as ancillary treatment in some patients. Calcitonin by itself seldom restores serum calcium to normal, and refractoriness frequently develops. However, its lack of toxicity permits frequent administration at high doses (200 MRC units or more).

An effect on serum calcium is observed within 4–6 hours and lasts for 6–10 hours. Calcimar (salmon calcitonin) is available for parenteral and nasal administration.

Gallium Nitrate Gallium nitrate is approved by the FDA for the management of hypercalcemia of malignancy. This drug inhibits bone resorption. At a dosage of 200 mg/m2 body surface area per day given as a continuous intravenous infusion in 5% dextrose for 5 days, gallium nitrate proved superior to calcitonin in reducing serum calcium in cancer patients. Because of potential nephrotoxicity, patients should be well hydrated and have good renal output before starting the infusion.

Plicamycin (Mithramycin) Because of its toxicity, plicamycin (mithramycin) is not the drug of first choice for the treatment of hypercalcemia. However, when other forms of therapy fail, 25–50 mcg/kg of plicamycin given intravenously usually lowers serum calcium substantially within 24–48 hours.

This effect can last several days. This dose can be repeated as necessary. The most dangerous toxic effect is sudden thrombocytopenia followed by hemorrhage. Hepatic and renal toxicity can also occur. Hypocalcemia, nausea, and vomiting may limit therapy. Use of this drug must be accompanied by careful monitoring of platelet counts, liver and kidney function, and serum calcium levels.

Phosphate Intravenous phosphate administration is probably the fastest and surest way to reduce serum calcium, but it is a hazardous procedure if not done properly. Intravenous phosphate should be used only after other methods of treatment (bisphosphonates, calcitonin, and saline diuresis) have failed to control symptomatic hypercalcemia. Phosphate must be given slowly (50 mmol or 1.5 g elemental phosphorus over 6–8 hours) and the patient switched to oral phosphate (1–2 g/d elemental phosphorus, as one of the salts indicated below) as soon as symptoms of hypercalcemia have cleared. The risks of intravenous phosphate therapy include sudden hypocalcemia, ectopic calcification, acute renal failure, and hypotension. Oral phosphate can also lead to ectopic calcification and renal failure if serum calcium and phosphate levels are not carefully monitored, but the risk is less and the time of onset much longer. Phosphate is available in oral and intravenous forms as sodium or potassium salts. Amounts required to provide 1 g of elemental phosphorus are as follows: Intravenous: In-Phos, 40 mL; or Hyper-Phos-K, 15 mL Oral: Fleet Phospho-Soda, 6.2 mL; or Neutra-Phos, 300 mL; or K-Phos-Neutral, 4 tablets

Glucocorticoids Glucocorticoids have no clear role in the immediate treatment of hypercalcemia. However, the chronic hypercalcemia of sarcoidosis, vitamin D intoxication, and certain cancers may respond within several days to glucocorticoid therapy. Prednisone in oral doses of 30–60 mg daily is generally used, although equivalent doses of other glucocorticoids are effective. The rationale for the use of glucocorticoids in these diseases differs, however. The hypercalcemia of sarcoidosis is secondary to increased production of 1,25(OH) 2 D by the sarcoid tissue itself. Glucocorticoid therapy directed at the reduction of sarcoid tissue results in restoration of normal serum calcium and 1,25(OH)2 D levels. The treatment of hypervitaminosis D with glucocorticoids probably does not alter vitamin D metabolism significantly but is thought to reduce vitamin D-mediated intestinal calcium transport and increase renal excretion of calcium. An action of glucocorticoids to reduce vitamin D-mediated bone resorption has not been excluded, however. The effect of glucocorticoids on the hypercalcemia of cancer is probably twofold. The malignancies responding best to glucocorticoids (ie, multiple myeloma and related lymphoproliferative diseases) are sensitive to the lytic action of glucocorticoids. Therefore part of the effect may be related to decreased tumor mass and activity. Glucocorticoids have also been shown to inhibit the secretion or effectiveness of cytokines elaborated by

multiple myeloma and related cancers that stimulate osteoclastic bone resorption. Other causes of hypercalcemia—particularly primary hyperparathyroidism—do not respond to glucocorticoid therapy.

HYPOCALCEMIA The main features of hypocalcemia are neuromuscular—tetany, paresthesias, laryngospasm, muscle cramps, and seizures. The major causes of hypocalcemia in the adult are hypoparathyroidism, vitamin D deficiency, chronic kidney disease, and malabsorption.

Hypocalcemia can also accompany the infusion of potent bisphosphonates and denosumab for the treatment of osteoporosis, but this is seldom of clinical significance unless the patient is already hypocalcemic at the onset of the infusion. Neonatal hypocalcemia is a common disorder that usually resolves without therapy. The roles of PTH, vitamin D, and calcitonin in the neonatal syndrome are under investigation. Large infusions of citrated blood can produce hypocalcemia secondary to the formation of citrate-calcium complexes.

Calcium and vitamin D (or its metabolites) form the mainstay of treatment of hypocalcemia.

Calcium A number of calcium preparations are available for intravenous, intramuscular, and oral use. Calcium gluceptate (0.9 mEq calcium/mL), calcium gluconate (0.45 mEq calcium/mL), and calcium chloride (0.68–1.36 mEq calcium/mL) are available for intravenous therapy.

Calcium gluconate is preferred because it is less irritating to veins. Oral preparations include calcium carbonate (40% calcium), calcium lactate (13% calcium), calcium phosphate (25% calcium), and calcium citrate (21% calcium). Calcium carbonate is often the preparation of choice because of its high percentage of calcium, ready availability (eg, Tums), low cost, and antacid properties. In achlorhydric patients, calcium carbonate should be given with meals to increase absorption, or the patient should be switched to calcium citrate, which is somewhat better absorbed. Combinations of vitamin D and calcium are available, but treatment must be tailored to the individual patient and the individual disease, a flexibility lost by fixed-dosage combinations.

Treatment of severe symptomatic hypocalcemia can be accomplished with slow infusion of 5–20 mL of 10% calcium gluconate.

Rapid infusion can lead to cardiac arrhythmias. Less severe hypocalcemia is best treated with oral forms sufficient to provide approximately 400–1200 mg of elemental calcium (1–3 g calcium carbonate) per day. Dosage must be adjusted to avoid hypercalcemia and hypercalciuria.

Vitamin D When rapidity of action is required, 1,25(OH)2 D3 (calcitriol), 0.25–1 mcg daily, is the vitamin D metabolite of choice because it is capable of raising serum calcium within 24–48 hours. Calcitriol also raises serum phosphate, although this action is usually not observed early in treatment. The combined effects of calcitriol and all other vitamin D metabolites and analogs on both calcium and phosphate make careful monitoring of these mineral levels especially important to prevent ectopic calcification secondary to an abnormally high serum calcium × phosphate product. Since the choice of the appropriate vitamin D metabolite or analog for long-term treatment of hypocalcemia depends on the nature of the underlying disease, further discussion of vitamin D treatment is found under the headings of the specific diseases.

HYPERPHOSPHATEMIA Hyperphosphatemia is a common complication of renal failure and is also found in all types of hypoparathyroidism (idiopathic, surgical, and pseudohypoparathyroidism), vitamin D intoxication, and the rare syndrome of tumoral calcinosis (usually due to insufficient bioactive FGF23). Emergency treatment of hyperphosphatemia is seldom necessary but can be achieved by dialysis or glucose and insulin infusions. In general, control of hyperphosphatemia involves restriction of dietary phosphate plus phosphate-binding gels such as sevelamer, or lanthanum carbonate and calcium supplements. Because of their potential to induce aluminum-associated bone disease, aluminum-containing antacids should be used sparingly and only when other measures fail to control the hyperphosphatemia. In patients with chronic kidney disease enthusiasm for the use of large doses of calcium to control hyperphosphatemia has waned because of the risk of ectopic calcification.

HYPOPHOSPHATEMIA Hypophosphatemia is associated with a variety of conditions, including primary hyperparathyroidism, vitamin D deficiency, idiopathic hypercalciuria, conditions associated with increased bioactive FGF23 (eg, X-linked and autosomal dominant hypophosphatemic rickets and tumor-induced osteomalacia), other forms of renal phosphate wasting (eg, Fanconi’s syndrome), overzealous use of phosphate binders, and parenteral nutrition with inadequate phosphate content. Acute hypophosphatemia may cause a reduction in the intracellular

levels of high-energy organic phosphates (eg, ATP), interfere with normal hemoglobin-to-tissue oxygen transfer by decreasing red cell 2,3-diphosphoglycerate levels, and lead to rhabdomyolysis. However, clinically significant acute effects of hypophosphatemia are seldom seen, and emergency treatment is generally not indicated. The long-term effects include proximal muscle weakness and abnormal bone mineralization (osteomalacia). Therefore, hypophosphatemia should be avoided when using forms of therapy that can lead to it (eg, phosphate binders, certain types of parenteral nutrition) and treated in conditions that cause it, such as the various forms of hypophosphatemic rickets. Oral forms of phosphate are listed above.

SPECIFIC DISORDERS INVOLVING BONE MINERAL-REGULATING HORMONES PRIMARY HYPERPARATHYROIDISM This rather common disease, if associated with symptoms and significant hypercalcemia, is best treated surgically. Oral phosphate and bisphosphonates have been tried but cannot be recommended. Asymptomatic patients with mild disease often do not get worse and may be left untreated. The calcimimetic agent cinacalcet, discussed previously, has been approved for secondary hyperparathyroidism and is in clinical trials for the treatment of primary hyperparathyroidism. If such drugs prove efficacious and cost effective, medical management of this disease will need to be reconsidered. Primary hyperparathyroidism is often associated with low levels of 25(OH)D, suggesting that mild vitamin D deficiency may be contributing to the elevated PTH levels. Vitamin D supplementation in such situations has proved safe with respect to further elevations of serum and urine calcium levels, but calcium should be monitored nevertheless when vitamin D is provided.

HYPOPARATHYROIDISM In PTH deficiency (idiopathic or surgical hypoparathyroidism) or an abnormal target tissue response to PTH (pseudohypoparathyroidism), serum calcium falls and serum phosphate rises. In such patients, 1,25(OH)2 D levels are usually low, presumably reflecting the lack of stimulation by PTH of 1,25(OH)2 D production. The skeletons of patients with idiopathic or surgical hypoparathyroidism are normal except for a slow turnover rate. A number of patients with pseudohypoparathyroidism appear to have osteitis fibrosa, suggesting that the normal or high PTH levels found in such patients are capable of acting on bone but not on the kidney.

The distinction between pseudohypoparathyroidism and idiopathic hypoparathyroidism is made on the basis of normal or high PTH levels but deficient renal response (ie, diminished excretion of cAMP or phosphate) in patients with pseudohypoparathyroidism.

The principal therapeutic goal is to restore normocalcemia and normophosphatemia. Vitamin D (either D 2 or D3 ; 25,000–100,000 IU three times per week) and dietary calcium supplements have been used in the past. More rapid increments in serum calcium can be achieved with calcitriol. Many patients treated with vitamin D experience episodes of hypercalcemia and hypercalciuria. This complication is more rapidly reversible with cessation of calcitriol therapy than therapy with vitamin D. This would be of importance to the patient in whom such hypercalcemic crises are common. Full-length PTH (Natpara) has been developed for the treatment of hypoparathyroidism and has been shown in phase 3 trials to reduce the need for large doses of calcium and calcitriol with less risk of hypercalciuria. It is currently being evaluated by the FDA for this condition.

NUTRITIONAL VITAMIN D DEFICIENCY OR INSUFFICIENCY The level of vitamin D thought to be necessary for good health is being reexamined with the appreciation that vitamin D acts on a large number of cell types beyond those responsible for bone and mineral metabolism. A level of 25(OH)D above 10 ng/mL is necessary for preventing rickets or osteomalacia. However, substantial epidemiologic and some prospective trial data indicate that a higher level, such as 20–30 ng/mL, is required to optimize intestinal calcium absorption, optimize the accrual and maintenance of bone mass, reduce falls and fractures, and prevent a wide variety of diseases including diabetes mellitus, hyperparathyroidism, autoimmune diseases, and cancer.

An expert panel for the Institute of Medicine (IOM) has recently recommended that a level of 20 ng/mL (50 nM) was sufficient, although up to 50 ng/mL (125 nM) was considered safe. For individuals between the ages of 1 and 70 years, 600 IU/d vitamin D was thought to be sufficient to meet these goals, although up to 4000 IU was considered safe. These recommendations are based primarily on data from randomized placebo-controlled clinical trials (RCT) that evaluated falls and fractures; data supporting the nonskeletal effects of vitamin D were considered too preliminary to be used in their recommendations because of lack of RCT for these other actions. The lower end of these recommendations has been considered too low and the upper end too restrictive by a number of vitamin D experts, and the Endocrine Society has published a different set of recommendations suggesting that 30 ng/mL was a more appropriate lower limit. Nevertheless, the call for better clinical data from RCTs, especially for the nonskeletal actions, is appropriate. The IOM guidelines —at least with respect to the lower recommended levels of vitamin D supplementation—are unlikely to correct vitamin D deficiency in individuals with obesity, dark complexions, limited capacity for sunlight exposure, or malabsorption. Vitamin D deficiency or insufficiency can be treated by higher dosages (either D2 or D3 , 1000–4000 IU/d or 50,000 IU/wk for several weeks). No other vitamin D metabolite

is indicated. Because the half-life of vitamin D3 metabolites in blood is greater than that of vitamin D2 , there are advantages to using vitamin D3 rather than vitamin D2 supplements, although when administered on a daily or weekly schedule these differences may be moot. The diet should also contain adequate amounts of calcium as several studies indicate a synergism between calcium and vitamin D with respect to a number of their actions.

CHRONIC KIDNEY DISEASE The major sequelae of chronic kidney disease that impact bone mineral homeostasis are deficient 1,25(OH)2 D production, retention of phosphate with an associated reduction in ionized calcium levels, and the secondary hyperparathyroidism that results from the parathyroid gland response to lowered serum ionized calcium and low 1,25(OH)2 D. FGF23 levels are also increased in this disorder in part due to the increased phosphate, and this can further reduce 1,25(OH)2 D production by the kidney. Although still investigational, antibodies to FGF23 in the early stages of renal failure result in normalization of 1,25(OH)2 D levels. With impaired 1,25(OH) 2 D production, less calcium is absorbed from the intestine and less bone is resorbed under the influence of PTH. As a result hypocalcemia usually develops, furthering the development of secondary hyperparathyroidism. The bones show a mixture of osteomalacia and osteitis fibrosa.

In contrast to the hypocalcemia that is more often associated with chronic kidney disease, some patients may become hypercalcemic from overzealous treatment with calcium. However, the most common cause of hypercalcemia is the development of severe secondary (sometimes referred to as tertiary) hyperparathyroidism. In such cases, the PTH level in blood is very high. Serum alkaline phosphatase levels also tend to be high. Treatment often requires parathyroidectomy. A less common circumstance leading to hypercalcemia is development of a form of bone disease characterized by a profound decrease in bone cell activity and loss of the calcium buffering action of bone (adynamic bone disease). In the absence of kidney function, any calcium absorbed from the intestine accumulates in the blood.

Such patients are very sensitive to the hypercalcemic action of 1,25(OH)2 D. These individuals generally have a high serum calcium but nearly normal alkaline phosphatase and PTH levels. The bone in such patients may have a high aluminum content, especially in the mineralization front, which blocks normal bone mineralization. These patients do not respond favorably to parathyroidectomy.

Deferoxamine, an agent used to chelate iron (see Chapter 57), also binds aluminum and is being used to treat this disorder. However, with the reduction in use of aluminum-containing phosphate binders, most cases of adynamic bone disease are not associated with aluminum deposition but are attributed to overzealous suppression of PTH secretion.

Vitamin D Preparations The choice of vitamin D preparation to be used in the setting of chronic kidney disease depends on the type and extent of bone disease and hyperparathyroidism. Individuals with vitamin D deficiency or insufficiency should first have their 25(OH)D levels restored to normal (20–30 ng/mL) with vitamin D. 1,25(OH)2 D3 (calcitriol) rapidly corrects hypocalcemia and at least partially reverses secondary hyperparathyroidism and osteitis fibrosa. Many patients with muscle weakness and bone pain gain an improved sense of well-being.

Two analogs of calcitriol—doxercalciferol and paricalcitol—are approved in the USA for the treatment of secondary hyperparathyroidism of chronic kidney disease. (In Japan, maxacalcitol [22-oxa-calcitriol] and falecalcitriol [26,27 F6 -1,25(OH)2 D3 ] are approved for this purpose.) Their principal advantage is that they are less likely than calcitriol to induce hypercalcemia for any given reduction in PTH (less true for falecalcitriol). Their greatest impact is in patients in whom the use of calcitriol may lead to unacceptably high serum calcium levels.

Regardless of the drug used, careful attention to serum calcium and phosphate levels is required. A calcium × phosphate product (in mg/dL units) less than 55 is desired with both calcium and phosphate in the normal range. Calcium adjustments in the diet and dialysis bath and phosphate restriction (dietary and with oral ingestion of phosphate binders) should be used along with vitamin D metabolites.

Monitoring of serum PTH and alkaline phosphatase levels is useful in determining whether therapy is correcting or preventing secondary hyperparathyroidism. In patients on dialysis, a PTH value of approximately twice the upper limits of normal is considered desirable to prevent adynamic bone disease. Although not generally available, percutaneous bone biopsies for quantitative histomorphometry may help in choosing appropriate therapy and following the effectiveness of such therapy, especially in cases suspected of adynamic bone disease.

Unlike the rapid changes in serum values, changes in bone morphology require months to years. Monitoring of serum vitamin D metabolite levels is useful for determining adherence, absorption, and metabolism.

INTESTINAL OSTEODYSTROPHY A number of gastrointestinal and hepatic diseases cause disordered calcium and phosphate homeostasis, which ultimately leads to bone disease. As bariatric surgery becomes more common, this problem is likely to increase. The bones in such patients show a combination of osteoporosis and osteomalacia. Osteitis fibrosa does not occur, in contrast to renal osteodystrophy. The important common feature in this group of diseases appears to be malabsorption of calcium and vitamin D. Liver disease may, in addition, reduce the production of 25(OH)D from vitamin D, although its importance in patients other than those with terminal liver failure remains in dispute. The

malabsorption of vitamin D is probably not limited to exogenous vitamin D as the liver secretes into bile a substantial number of vitamin D metabolites and conjugates that are normally reabsorbed in (presumably) the distal jejunum and ileum. Interference with this process could deplete the body of endogenous vitamin D metabolites in addition to limiting absorption of dietary vitamin D. In mild forms of malabsorption, high doses of vitamin D (25,000â&#x20AC;&#x201C;50,000 IU three times per week) should suffice to raise serum levels of 25(OH)D into the normal range. Many patients with severe disease do not respond to vitamin D. Clinical experience with the other metabolites is limited, but both calcitriol and calcifediol have been used successfully in doses similar to those recommended for treatment of renal osteodystrophy. Theoretically, calcifediol should be the drug of choice under these conditions, because no impairment of the renal metabolism of 25(OH)D to 1,25(OH)2 D and 24,25(OH)2 D exists in these patients. However, calcifediol is no longer available in the USA. Both calcitriol and 24,25(OH)2 D may be of importance in reversing the bone disease. Intramuscular injections of vitamin D would be an alternative form of therapy, but there are currently no FDA-approved intramuscular preparations available in the USA. The skin remains a good source of vitamin D production, although care is needed to prevent UVB overexposure (ie, by avoiding sunburn) to reduce the risk of photoaging and skin cancer.

As in the other diseases discussed, treatment of intestinal osteodystrophy with vitamin D and its metabolites should be accompanied by appropriate dietary calcium supplementation and monitoring of serum calcium and phosphate levels.

OSTEOPOROSIS Osteoporosis is defined as abnormal loss of bone predisposing to fractures. It is most common in postmenopausal women but also occurs in men. The annual direct medical cost of fractures in older women and men in the USA is estimated to be at least 20 billion dollars per year, and is increasing as our population ages. Osteoporosis is most commonly associated with loss of gonadal function as in menopause but may also occur as an adverse effect of long-term administration of glucocorticoids or other drugs, including those that inhibit sex steroid production; as a manifestation of endocrine disease such as thyrotoxicosis or hyperparathyroidism; as a feature of malabsorption syndrome; as a consequence of alcohol abuse and cigarette smoking; or without obvious cause (idiopathic). The ability of some agents to reverse the bone loss of osteoporosis is shown in Figure 42â&#x20AC;&#x201C;5. The postmenopausal form of osteoporosis may be accompanied by lower 1,25(OH)2 D levels and reduced intestinal calcium transport. This form of osteoporosis is due to reduced estrogen production and can be treated with estrogen (combined with a progestin in women with a uterus to prevent endometrial carcinoma). However, concern that estrogen increases the risk of breast cancer and fails to reduce or may actually increase the development of heart disease has reduced enthusiasm for this form of therapy, at least in older individuals.

FIGURE 42â&#x20AC;&#x201C;5 Typical changes in bone mineral density with time after the onset of menopause, with and without treatment. In the untreated condition, bone is lost during aging in both men and women. Strontium (Sr2+), parathyroid hormone (PTH), and vitamin D promote bone formation and can increase bone mineral density in subjects who respond to them throughout the period of treatment, although PTH and vitamin D in high doses also activate bone resorption. In contrast, estrogen, calcitonin, denosumab, and bisphosphonates block bone resorption. This leads to a transient increase in bone mineral density because bone formation is not initially decreased. However, with time, both bone formation and bone resorption decrease with these pure antiresorptive agents, and bone mineral density reaches a new plateau.

Bisphosphonates are potent inhibitors of bone resorption. They increase bone density and reduce the risk of fractures in the hip, spine, and other locations. Alendronate, risedronate, ibandronate, and zoledronate are approved for the treatment of osteoporosis, using daily dosing schedules of alendronate, 10 mg/d, risedronate, 5 mg/d, or ibandronate, 2.5 mg/d; or weekly schedules of alendronate, 70 mg/wk, or risedronate, 35 mg/wk; or monthly schedules of ibandronate, 150 mg/month; or quarterly (every 3 months) injections of ibandronate, 3 mg; or annual infusions of zoledronate, 5 mg. These drugs are effective in men as well as women and for various causes of osteoporosis.

As previously noted, estrogen-like SERMs (selective estrogen receptor modulators, Chapter 40) have been developed that prevent the increased risk of breast and uterine cancer associated with estrogen while maintaining the benefit to bone. The SERM raloxifene is approved for treatment of osteoporosis. Like tamoxifen, raloxifene reduces the risk of breast cancer. It protects against spine fractures but not hip fractures—unlike bisphosphonates, denosumab, and teriparatide, which protect against both. Raloxifene does not prevent hot flushes and imposes the same increased risk of venous thromboembolism as estrogen. To counter the reduced intestinal calcium transport associated with osteoporosis, vitamin D therapy is often used in combination with dietary calcium supplementation. In several large studies, vitamin D supplementation (800 IU/d) with calcium has been shown to improve bone density, reduce falls, and prevent fractures.

Calcitriol and its analog, 1α(OH)D3 , have also been shown to increase bone mass and reduce fractures. Use of these agents for osteoporosis is not FDA-approved, although they are used for this purpose in other countries.

Teriparatide, the recombinant form of PTH 1-34, is approved for treatment of osteoporosis. It is given in a dosage of 20 mcg subcutaneously daily. Teriparatide stimulates new bone formation, but unlike fluoride, this new bone appears structurally normal and is associated with a substantial reduction in the incidence of fractures. The drug is approved for only 2 years of use. Trials examining the sequential use of teriparatide followed by a bisphosphonate after 1 or 2 years are in progress and look promising. Use of the drug with a bisphosphonate has not shown greater efficacy than the bisphosphonate alone.

Calcitonin is approved for use in the treatment of postmenopausal osteoporosis. It has been shown to increase bone mass and reduce fractures, but only in the spine. It does not appear to be as effective as bisphosphonates or teriparatide.

Denosumab, the RANKL inhibitor, has recently been approved for treatment of postmenopausal osteoporosis. It is given subcutaneously every 6 months in 60 mg doses. Like the bisphosphonates it suppresses bone resorption and secondarily bone formation.

Denosumab reduces the risk of both vertebral and nonvertebral fractures with comparable effectiveness to the potent bisphosphonates.

Strontium ranelate has not been approved in the USA for the treatment of osteoporosis but is being used in Europe, generally at a dose of 2 g/d.

X-LINKED & AUTOSOMAL DOMINANT HYPOPHOSPHATEMIA & RELATED DISEASES These disorders usually manifest in childhood as rickets and hypophosphatemia, although they may first present in adults. In both X-linked and autosomal dominant hypophosphatemia, biologically active FGF23 accumulates, leading to phosphate wasting in the urine and hypophosphatemia. In autosomal dominant hypophosphatemia, mutations in the FGF23 gene replace an arginine required for proteolysis and result in increased FGF23 stability. X-linked hypophosphatemia is caused by mutations in the gene encoding the PHEX protein, an endopeptidase. Initially, it was thought that FGF23 was a direct substrate for PHEX, but this no longer appears to be the case. Tumorinduced osteomalacia is a phenotypically similar but acquired syndrome in adults that results from overexpression of FGF23 in tumor cells. The current concept for all of these diseases is that FGF23 blocks the renal uptake of phosphate and blocks 1,25(OH)2 D production leading to rickets in children and osteomalacia in adults. Phosphate is critical to normal bone mineralization; when phosphate stores are deficient, a clinical and pathologic picture resembling vitamin D–dependent rickets develops. However, affected children fail to respond to the standard doses of vitamin D used in the treatment of nutritional rickets. A defect in 1,25(OH) 2 D production by the kidney contributes to the phenotype as 1,25(OH)2 D levels are low relative to the degree of hypophosphatemia observed. This combination of low serum phosphate and low or low-normal serum 1,25(OH)2 D provides the rationale for treating these patients with oral phosphate (1– 3 g daily) and calcitriol (0.25–2 mcg daily). Reports of such combination therapy are encouraging in this otherwise debilitating disease, although prolonged treatment often leads to secondary hyperparathyroidism.

PSEUDOVITAMIN D DEFICIENCY RICKETS & HEREDITARY VITAMIN D– RESISTANT RICKETS These distinctly different autosomal recessive diseases present as childhood rickets that do not respond to conventional doses of vitamin D. Pseudovitamin D deficiency rickets is due to an isolated deficiency of 1,25(OH)2 D production caused by mutations in 25(OH)-D-1αhydroxylase (CYP27B1). This condition is treated with calcitriol (0.25–0.5 mcg daily). Hereditary vitamin D–resistant rickets (HVDRR) is caused by mutations in the gene for the vitamin D receptor. The serum levels of 1,25(OH) 2 D are very high in HVDRR but inappropriately low for the level of calcium in pseudovitamin D–deficient rickets. Treatment with large doses of calcitriol has been

claimed to be effective in restoring normocalcemia in some HVDRR patients, presumably those with a partially functional vitamin D receptor, although many patients are completely resistant to all forms of vitamin D. Calcium and phosphate infusions have been shown to correct the rickets in some children, similar to studies in mice in which the VDR gene has been deleted. These diseases are rare.

NEPHROTIC SYNDROME Patients with nephrotic syndrome can lose vitamin D metabolites in the urine, presumably by loss of the vitamin D-binding protein. Such patients may have very low 25(OH)D levels. Some of them develop bone disease. It is not yet clear what value vitamin D therapy has in such patients, because therapeutic trials with vitamin D (or any vitamin D metabolite) have not yet been carried out. Because the problem is not related to vitamin D metabolism, one would not anticipate any advantage in using the more expensive vitamin D metabolites in place of vitamin D.

IDIOPATHIC HYPERCALCIURIA Individuals with idiopathic hypercalciuria, characterized by hypercalciuria and nephrolithiasis with normal serum calcium and PTH levels, have been divided into three groups: (1) hyperabsorbers, patients with increased intestinal absorption of calcium, resulting in high-normal serum calcium, low-normal PTH, and a secondary increase in urine calcium; (2) renal calcium leakers, patients with a primary decrease in renal reabsorption of filtered calcium, leading to low-normal serum calcium and high-normal serum PTH; and (3) renal phosphate leakers, patients with a primary decrease in renal reabsorption of phosphate, leading to increased 1,25(OH)2 D production, increased intestinal calcium absorption, increased ionized serum calcium, low-normal PTH levels, and a secondary increase in urine calcium. There is some disagreement about this classification, and many patients are not readily categorized. Many such patients present with mild hypophosphatemia, and oral phosphate has been used with some success in reducing stone formation. However, a clear role for phosphate in the treatment of this disorder has not been established.

Therapy with hydrochlorothiazide, up to 50 mg twice daily, or chlorthalidone, 50–100 mg daily, is recommended. Loop diuretics such as furosemide and ethacrynic acid should not be used because they increase urinary calcium excretion. The major toxicity of thiazide diuretics, besides hypokalemia, hypomagnesemia, and hyperglycemia, is hypercalcemia. This is seldom more than a biochemical observation unless the patient has a disease such as hyperparathyroidism in which bone turnover is accelerated. Accordingly, one should screen patients for such disorders before starting thiazide therapy and monitor serum and urine calcium when therapy has begun.

An alternative to thiazides is allopurinol. Some studies indicate that hyperuricosuria is associated with idiopathic hypercalcemia and that a small nidus of urate crystals could lead to the calcium oxalate stone formation characteristic of idiopathic hypercalcemia.

Allopurinol, 100–300 mg daily, may reduce stone formation by reducing uric acid excretion.

OTHER DISORDERS OF BONE MINERAL HOMEOSTASIS PAGET’S DISEASE OF BONE Paget’s disease is a localized bone disorder characterized by uncontrolled osteoclastic bone resorption with secondary increases in poorly organized bone formation. The cause of Paget’s disease is obscure, although some studies suggest that a measles-related virus may be involved. The disease is fairly common, although symptomatic bone disease is less common. Recent studies indicate that this infection may produce a factor that increases the stimulation of bone resorption by 1,25(OH)2 D. The biochemical parameters of elevated serum alkaline phosphatase and urinary hydroxyproline are useful for diagnosis. Along with the characteristic radiologic and bone scan findings, these biochemical determinations provide good markers by which to follow therapy.

The goal of treatment is to reduce bone pain and stabilize or prevent other problems such as progressive deformity, fractures, hearing loss, high-output cardiac failure, and immobilization hypercalcemia. Calcitonin and bisphosphonates are the first-line agents for this disease. Treatment failures may respond to plicamycin. Calcitonin is administered subcutaneously or intramuscularly in doses of 50–100 MRC (Medical Research Council) units every day or every other day. Nasal inhalation at 200–400 units/d is also effective. Higher or more frequent doses have been advocated when this initial regimen is ineffective. Improvement in bone pain and reduction in serum alkaline phosphatase and urine hydroxyproline levels require weeks to months. Often a patient who responds well initially loses the response to calcitonin. This refractoriness is not correlated with the development of antibodies.

Sodium etidronate, alendronate, risedronate, and tiludronate are the bisphosphonates currently approved for clinical use in Paget’s disease of bone in the USA. Other bisphosphonates, including pamidronate, are being used in other countries. The recommended dosages of bisphosphonates are etidronate, 5 mg/kg/d; alendronate, 40 mg/d; risedronate, 30 mg/d; and tiludronate, 400 mg/d. Long-term (months to years) remission may be expected in patients who respond to a bisphosphonate. Treatment should not exceed 6 months per course but can be repeated after 6 months if necessary. The principal toxicity of etidronate is the development of osteomalacia and an increased incidence of fractures when the dosage is raised substantially above 5 mg/kg/d. The newer bisphosphonates such as risedronate and alendronate do not share this adverse effect. Some patients treated with etidronate develop bone pain similar in nature to the bone pain of

osteomalacia. This subsides after stopping the drug. The principal adverse effect of alendronate and the newer bisphosphonates is gastric irritation when used at these high doses. This is reversible on cessation of the drug.

The use of a potentially lethal cytotoxic drug such as plicamycin in a generally benign disorder such as Paget’s disease is recommended only when other less toxic agents (calcitonin, alendronate) have failed and the symptoms are debilitating. Clinical data on long-term use of plicamycin are insufficient to determine its usefulness for extended therapy. However, short courses involving 15–25 mcg/kg/d intravenously for 5–10 days followed by 15 mcg/kg intravenously each week have been used to control the disease.

ENTERIC OXALURIA Patients with short bowel syndromes and associated fat malabsorption can present with renal stones composed of calcium and oxalate.

Such patients characteristically have normal or low urine calcium levels but elevated urine oxalate levels. The reasons for the development of oxaluria in such patients are thought to be twofold: first, in the intestinal lumen, calcium (which is now bound to fat) fails to bind oxalate and no longer prevents its absorption; second, enteric flora, acting on the increased supply of nutrients reaching the colon, produce larger amounts of oxalate. Although one would ordinarily avoid treating a patient with calcium oxalate stones with calcium supplementation, this is precisely what is done in patients with enteric oxaluria. The increased intestinal calcium binds the excess oxalate and prevents its absorption. One to 2 g of calcium carbonate can be given daily in divided doses, with careful monitoring of urinary calcium and oxalate to be certain that urinary oxalate falls without a dangerous increase in urinary calcium.

SUMMARY Major Drugs Used in Diseases of Bone Mineral Homeostasis

PREPARATIONS AVAILABLE

REFERENCES Becker DJ, Kilgore ML, Morrisey MA: T he societal burden of osteoporosis. Curr Rheumatol Rep 2010;12:186.

Bhattacharyya N et al: Fibroblast growth factor 23: State of the field and future directions. T rends Endocrinol Metab 2012;23:610.

Bikle DD: Nonclassic actions of vitamin D. J Clin Endocrinol Metabol 2009;94:26.

Clines GA: Mechanisms and treatment of hypercalcemia of malignancy. Curr Opin Endocrinol Diabetes Obes 2011;18:339.

Cooper C et al: Long-term treatment of osteoporosis in postmenopausal women: a review from the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) and the International Osteoporosis Foundation (IOF). Curr Med Res Opin 2012;28:475.

Cummings SR et al: Denosumab for prevention of fractures in postmenopausal women with osteoporosis. N Engl J Med 2009;361:756.

Favus MJ: Bisphosphonates for osteoporosis. N Engl J Med 2010;363:2027.

Fukagawa M et al: Vitamin D supplementation in renal disease: Is calcitriol all that is needed? Scand J Clin Lab Invest Suppl 2012;243:120.

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Hagino H: Eldecalcitol: Newly developed active vitamin D3 analog for the treatment of osteoporosis. Expert Opin Pharmacother 2013;14:817.

Holick MF: Vitamin D deficiency. N Engl J Med 2007;357:266.

Holick MF et al: Evaluation, treatment, and prevention of vitamin D deficiency: An Endocrine Sociey Clinical Practice Guideline J Clin Endocrinol Metab 2011;96:1911.

Mosekilde L et al: T he pathogenesis, treatment, and prevention of osteoporosis in men. Drugs 2013;73:15.

Neer RM et al: Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis. N Engl J Med 2001;344:1434.

Nemeth EF, Shoback D: Calcimimetic and calcilytic drugs for treating bone and mineral-related disorders. Best Pract Res Clin Endocrinol Metab 2013;27:373.

Pettifor JM: Rickets and vitamin D deficiency in children and adolescents. Endocrinol Metab Clin North Am 2005;34:537.

Qazi RA, Martin KJ: Vitamin D in kidney disease: Pathophysiology and the utility of treatment. Endocrinol Metab Clin North Am 2010;39:355.

Rizzoli R et al: Vitamin D supplementation in elderly or postmenopausal women: A 2013 update of the 2008 recommendations from the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO). Curr Med Res Opin 2013;29:305.

Ross AC et al: T he 2011 report on dietary reference intakes for calcium and vitamin D from the Institute of Medicine: What clinicians need to know. J Clin Endocrinol Metab 2011;96:53.

Zwolak P, Dudek AZ: Antineoplastic activity of zoledronic acid and denosumab. Anticancer Res 2013;33:2981.

CASE STUDY ANSWER There are multiple reasons for this patientâ&#x20AC;&#x2122;s osteoporosis, including a heavy smoking history, possible alcoholism, and chronic inflammatory disease treated with glucocorticoids. High levels of cytokines from the chronic inflammation activate osteoclasts.

Glucocorticoids increase urinary losses of calcium, suppress bone formation, and inhibit intestinal calcium absorption as well as decreasing gonadotropin production, leading to hypogonadism. Management should include measurement of serum testosterone, calcium, 25(OH)D, and the 24-hour urine calcium and creatinine levels (to verify completeness of collection), with treatment as appropriate for these secondary causes, plus initiation of bisphosphonate or denosumab therapy as primary treatment.

SECTION VIII CHEMOTHERAPEUTIC DRUGS

INTRODUCTION TO ANTIMICROBIAL AGENTS Antimicrobial agents provide some of the most dramatic examples of the advances of modern medicine. Many infectious diseases once considered incurable and lethal are now amenable to treatment with a few doses of antibiotics. The remarkably powerful and specific activity of antimicrobial drugs is due to their selectivity for targets that are either unique to prokaryote and fungal microorganisms or much more important in these organisms than in humans. Among these targets are bacterial and fungal cell wall-synthesizing enzymes (Chapters 43 and 48), the bacterial ribosome (Chapters 44 and 45), the enzymes required for nucleotide synthesis and DNA replication (Chapter 46), and the machinery of viral replication (Chapter 49). The special group of drugs used in mycobacterial infections is discussed in Chapter 47. The cytotoxic antiseptics and disinfectants are discussed in Chapter 50. The clinical uses of many antimicrobial agents are reviewed in Chapter 51.

The major problem threatening the continued success of antimicrobial drugs is the development of resistant organisms. Bacteria â&#x20AC;&#x153;inventedâ&#x20AC;? antibiotics billions of years ago, and resistance is primarily the result of bacterial adaptation to antibiotic exposure over millennia. Antibiotic resistance mechanisms existed before the clinical use of antibiotics, even to synthetic drugs that were created in the 20th century. Since resistance mechanisms are already present in nature, an inevitable consequence of antimicrobial use is the selection of resistant microorganisms, one of the clearest examples of evolution in action. Over the last 70 years, antibiotic use in patients and animals has fueled a major increase in the prevalence of drug-resistant pathogens. In recent years, highly resistant gram-negative organisms with novel mechanisms of resistance have been increasingly reported. Some of these strains have spread over vast geographic areas as a result of patients seeking medical care in different countries.

Much attention has been focused on eliminating the misuse of antibiotics to slow the tide of resistance. Antibiotics are misused in a variety of ways, including use in patients who are unlikely to have bacterial infections, use over unnecessarily prolonged periods, and use of multiple agents or broad-spectrum agents when not needed. Large quantities of antibiotics have been used in agriculture to stimulate growth and prevent infection in livestock, and this has added to the selection pressure that results in resistant organisms. In December 2013, the FDA announced a program to phase out the nontherapeutic use of antibiotics in livestock. However, even if this program is successful, it will take years before the benefits are apparent.

Antibiotic resistance has many negative consequences. The prevalence of resistant organisms drives the use of broader-spectrum, less efficacious, or more toxic antibiotics. Not surprisingly, infections caused by antibiotic-resistant pathogens are associated with increased costs, morbidity, and mortality. Each year in the United States, at least 2 million people acquire serious infections with resistant bacteria. At least 23,000 people die each year as a direct result of these antibiotic-resistant infections. The total economic cost of antibiotic resistance to the US economy has been difficult to calculate. Estimates vary but have ranged as high as $20 billion in excess direct healthcare costs.

Unfortunately, as the need has grown in recent years, development of novel antibiotics has slowed. Several of the largest pharmaceutical companies have abandoned research and development in this area because of diminished success and profits; the resulting reduction in new drug introductions is shown in the figure below, which shows new systemic antibacterial agents approved by the FDA per 5-year period through 2012. The most vulnerable molecular targets of antimicrobial drugs have been identified and, in many cases, crystallized and characterized. Pending the identification of new targets and compounds, it seems likely that over the next decade we will have to rely on currently available families of drugs. In the face of continuing development of resistance, considerable effort will be required to maintain the effectiveness of these drug groups.

Decline in the number of new systemic antibacterial drugs approved by the FDA over a 30-year period. (Reproduced, with permission, from Boucher HW et al: 10 Ă&#x2014; 20 progress -- development of new drugs active against gram-negative bacilli: An update from the Infectious Diseases Society of America. Clin Infect Dis 2013;56:1685. By permission of Oxford University Press on behalf of the Infectious Diseases Society of America. Modified, with permission, from Spellberg B et al: T rends in antimicrobial drug development: Implications for the future. Clin Infect Dis 2004;38:1279. By permission of Oxford University Press.)

CHAPTER

43 Beta-Lactam & Other Cell Wall- & Membrane-Active Antibiotics Daniel H. Deck, PharmD, & Lisa G. Winston, MD*

CASE STUDY A 55-year-old man is brought to the local hospital emergency department by ambulance. His wife reports that he had been in his normal state of health until 3 days ago when he developed a fever and a productive cough. During the last 24 hours he has complained of a headache and is increasingly confused. His wife reports that his medical history is significant only for hypertension, for which he takes hydrochlorothiazide and lisinopril, and that he is allergic to amoxicillin. She says that he developed a rash many years ago when prescribed amoxicillin for bronchitis. In the emergency department, the man is febrile (38.7°C [101.7°F]), hypotensive (90/54 mm Hg), tachypneic (36/min), and tachycardic (110/min). He has no signs of meningismus but is oriented only to person. A stat chest x-ray shows a left lower lung consolidation consistent with pneumonia. The plan is to start empiric antibiotics and perform a lumbar puncture to rule out bacterial meningitis. What antibiotic regimen should be prescribed to treat both pneumonia and meningitis? Does the history of amoxicillin rash affect the antibiotic choice? Why or why not?

BETA-LACTAM COMPOUNDS PENICILLINS The penicillins share features of chemistry, mechanism of action, pharmacology, and immunologic characteristics with cephalosporins, monobactams, carbapenems, and β-lactamase inhibitors. All are β-lactam compounds, so named because of their four-membered lactam ring.

Chemistry All penicillins have the basic structure shown in Figure 43–1. A thiazolidine ring (A) is attached to a β-lactam ring (B) that carries a secondary amino group (RNH–). Substituents (R; examples shown in Figure 43–2) can be attached to the amino group. Structural integrity of the 6-aminopenicillanic acid nucleus (rings A plus B) is essential for the biologic activity of these compounds. Hydrolysis of the β-lactam ring by bacterial β-lactamases yields penicilloic acid, which lacks antibacterial activity.

FIGURE 43–1 Core structures of four β-lactam antibiotic families. The ring marked B in each structure is the β-lactam ring. The penicillins are susceptible to bacterial metabolism and inactivation by amidases and lactamases at the points shown. Note that the carbapenems have a different stereochemical configuration in the lactam ring that imparts resistance to most common β lactamases.

Substituents for the penicillin and cephalosporin families are shown in Figures 43â&#x20AC;&#x201C;2 and 43â&#x20AC;&#x201C;6, respectively.

FIGURE 43–2 Side chains of some penicillins (R groups).

A. Classification Substituents of the 6-aminopenicillanic acid moiety determine the essential pharmacologic and antibacterial properties of the resulting molecules. Penicillins can be assigned to one of three groups (below). Within each of these groups are compounds that are relatively stable to gastric acid and suitable for oral administration, eg, penicillin V, dicloxacillin, and amoxicillin. The side chains of some representatives of each group are shown in Figure 43–2, with a few distinguishing characteristics.

1. Penicillins (eg, penicillin G)—These have greatest activity against gram-positive organisms, gram-negative cocci, and non-βlactamase-producing anaerobes. However, they have little activity against gram-negative rods, and they are susceptible to hydrolysis by β-lactamases.

2. Antistaphylococcal penicillins (eg, nafcillin)—These penicillins are resistant to staphylococcal β-lactamases. They are active against staphylococci and streptococci but not against enterococci, anaerobic bacteria, and gram-negative cocci and rods.

3. Extended-spectrum penicillins (aminopenicillins and antipseudomonal penicillins)—These drugs retain the antibacterial spectrum of penicillin and have improved activity against gram-negative organisms. Like penicillin, however, they are relatively susceptible to hydrolysis by β-lactamases.

B. Penicillin Units and Formulations The activity of penicillin G was originally defined in units. Crystalline sodium penicillin G contains approximately 1600 units per mg (1 unit = 0.6 mcg; 1 million units of penicillin = 0.6 g). Semisynthetic penicillins are prescribed by weight rather than units. The minimum inhibitory concentration (MIC) of any penicillin (or other antimicrobial) is usually given in mcg/mL. Most penicillins are formulated as the sodium or potassium salt of the free acid. Potassium penicillin G contains about 1.7 mEq of K+ per million units of penicillin (2.8 mEq/g). Nafcillin contains Na+, 2.8 mEq/g. Procaine salts and benzathine salts of penicillin G provide repository forms for intramuscular injection. In dry crystalline form, penicillin salts are stable for years at 4°C. Solutions lose their activity rapidly (eg, 24 hours at 20°C) and must be prepared fresh for administration.

Mechanism of Action Penicillins, like all β-lactam antibiotics, inhibit bacterial growth by interfering with the transpeptidation reaction of bacterial cell wall synthesis. The cell wall is a rigid outer layer that completely surrounds the cytoplasmic membrane (Figure 43–3), maintains cell shape and integrity, and prevents cell lysis from high osmotic pressure. The cell wall is composed of a complex, cross-linked polymer of polysaccharides and polypeptides, peptidoglycan (also known as murein or mucopeptide). The polysaccharide contains alternating amino sugars, N-acetylglucosamine and N-acetylmuramic acid (Figure 43–4). A five-amino-acid peptide is linked to the N-acetylmuramic acid sugar. This peptide terminates in D-alanyl-D-alanine. Penicillin-binding protein (PBP, an enzyme) removes the terminal alanine in the process of forming a cross-link with a nearby peptide. Cross-links give the cell wall its structural rigidity. Beta-lactam antibiotics, structural analogs of the natural D-Ala-D-Ala substrate, covalently bind to the active site of PBPs. This binding inhibits the transpeptidation reaction (Figure 43–5) and halts peptidoglycan synthesis, and the cell dies. The exact mechanism of cell death is not completely understood, but autolysins and disruption of cell wall morphogenesis are involved. Beta-lactam antibiotics kill bacterial cells only when they are actively growing and synthesizing cell wall.

FIGURE 43â&#x20AC;&#x201C;3 A highly simplified diagram of the cell envelope of a gram-negative bacterium. The outer membrane, a lipid bilayer, is present in gram-negative but not gram-positive organisms. It is penetrated by porins, proteins that form channels providing hydrophilic access to the cytoplasmic membrane. The peptidoglycan layer is unique to bacteria and is much thicker in gram-positive organisms than in gramâ&#x20AC;&#x201C;negative ones. Together, the outer membrane and the peptidoglycan layer constitute the cell wall. Penicillin-binding proteins (PBPs) are membrane proteins that cross-link peptidoglycan. Beta lactamases, if present, reside in the periplasmic space or on the outer surface of the cytoplasmic membrane, where they may destroy β-lactam antibiotics that penetrate the outer membrane.

FIGURE 43–4 The transpeptidation reaction in Staphylococcus aureus that is inhibited by β-lactam antibiotics. The cell wall of grampositive bacteria is made up of long peptidoglycan polymer chains consisting of the alternating aminohexoses N-acetylglucosamine (G) and N-acetylmuramic acid (M) with pentapeptide side chains linked (in S aureus) by pentaglycine bridges. The exact composition of the side chains varies among species. The diagram illustrates small segments of two such polymer chains and their amino acid side chains.

These linear polymers must be cross-linked by transpeptidation of the side chains at the points indicated by the asterisk to achieve the strength necessary for cell viability.

FIGURE 43–5 The biosynthesis of cell wall peptidoglycan, showing the sites of action of five antibiotics (shaded bars; 1 = fosfomycin, 2 = cycloserine, 3 = bacitracin, 4 = vancomycin, 5 = β-lactam antibiotics). Bactoprenol (BP) is the lipid membrane carrier that transports

building blocks across the cytoplasmic membrane; M, N-acetylmuramic acid; Glc, glucose; NAcGlc or G, N-acetylglucosamine.

Resistance Resistance to penicillins and other β-lactams is due to one of four general mechanisms: (1) inactivation of antibiotic by β-lactamase, (2) modification of target PBPs, (3) impaired penetration of drug to target PBPs, and (4) antibiotic efflux. Beta-lactamase production is the most common mechanism of resistance. Hundreds of different β-lactamases have been identified. Some, such as those produced by Staphylococcus aureus, Haemophilus influenzae, and Escherichia coli, are relatively narrow in substrate specificity, preferring penicillins to cephalosporins. Other β-lactamases, eg, AmpC β-lactamase produced by Pseudomonas aeruginosa and Enterobacter sp, and extended–spectrum β-lactamases (ESBLs), hydrolyze both cephalosporins and penicillins. Carbapenems are highly resistant to hydrolysis by penicillinases and cephalosporinases, but they are hydrolyzed by metallo-β lactamase and carbapenemases.

Altered target PBPs are the basis of methicillin resistance in staphylococci and of penicillin resistance in pneumococci and enterococci. These resistant organisms produce PBPs that have low affinity for binding β-lactam antibiotics, and consequently, they are not inhibited except at relatively high, often clinically unachievable, drug concentrations.

Resistance due to impaired penetration of antibiotic to target PBPs occurs only in gram-negative species because of the impermeable outer membrane of their cell wall, which is absent in gram-positive bacteria. Beta-lactam antibiotics cross the outer membrane and enter gram-negative organisms via outer membrane protein channels called porins. Absence of the proper channel or down-regulation of its production can greatly impair drug entry into the cell. Poor penetration alone is usually not sufficient to confer resistance because enough antibiotic eventually enters the cell to inhibit growth. However, this barrier can become important in the presence of a β-lactamase, even a relatively inactive one, as long as it can hydrolyze drug faster than it enters the cell. Gram-negative organisms also may produce an efflux pump, which consists of cytoplasmic and periplasmic protein components that efficiently transport some β-lactam antibiotics from the periplasm back across the cell wall outer membrane.

Pharmacokinetics Absorption of orally administered drug differs greatly for different penicillins, depending in part on their acid stability and protein binding.

Gastrointestinal absorption of nafcillin is erratic, so it is not suitable for oral administration. Dicloxacillin, ampicillin, and amoxicillin are acid-stable and relatively well absorbed, producing serum concentrations in the range of 4–8 mcg/mL after a 500-mg oral dose.

Absorption of most oral penicillins (amoxicillin being an exception) is impaired by food, and the drugs should be administered at least 1–2 hours before or after a meal.

Intravenous administration of penicillin G is preferred to the intramuscular route because of irritation and local pain from intramuscular injection of large doses. Serum concentrations 30 minutes after an intravenous injection of 1 g of penicillin G (equivalent to approximately 1.6 million units) are 20–50 mcg/mL. Only a fraction of the total drug in serum is present as free drug, the concentration of which is determined by protein binding. Highly protein-bound penicillins (eg, nafcillin) generally achieve lower free-drug concentrations in serum than less protein-bound penicillins (eg, penicillin G or ampicillin). Protein binding becomes clinically relevant when the proteinbound percentage is approximately 95% or more. Penicillins are widely distributed in body fluids and tissues with a few exceptions. They are polar molecules, so intracellular concentrations are well below those found in extracellular fluids.

Benzathine and procaine penicillins are formulated to delay absorption, resulting in prolonged blood and tissue concentrations. A single intramuscular injection of 1.2 million units of benzathine penicillin maintains serum levels above 0.02 mcg/mL for 10 days, sufficient to treat β-hemolytic streptococcal infection. After 3 weeks, levels still exceed 0.003 mcg/mL, which is enough to prevent β-hemolytic streptococcal infection. A 600,000 unit dose of procaine penicillin yields peak concentrations of 1–2 mcg/mL and clinically useful concentrations for 12–24 hours after a single intramuscular injection.

Penicillin concentrations in most tissues are equal to those in serum. Penicillin is also excreted into sputum and breast milk to levels 3–15% of those in the serum. Penetration into the eye, the prostate, and the central nervous system is poor. However, with active inflammation of the meninges, as in bacterial meningitis, penicillin concentrations of 1–5 mcg/mL can be achieved with a daily parenteral dose of 18–24 million units. These concentrations are sufficient to kill susceptible strains of pneumococci and meningococci.

Penicillin is rapidly excreted by the kidneys; small amounts are excreted by other routes. Tubular secretion accounts for about 90% of renal excretion, and glomerular filtration accounts for the remainder. The normal half-life of penicillin G is approximately 30 minutes; in renal failure, it may be as long as 10 hours. Ampicillin and the extended-spectrum penicillins are secreted more slowly than penicillin G and have half-lives of 1 hour. For penicillins that are cleared by the kidney, the dose must be adjusted according to renal function, with approximately one fourth to one third the normal dose being administered if creatinine clearance is 10 mL/min or less (Table 43–1).

TABLE 43–1 Guidelines for dosing of some commonly used penicillins.

Nafcillin is primarily cleared by biliary excretion. Oxacillin, dicloxacillin, and cloxacillin are eliminated by both the kidney and biliary excretion; no dosage adjustment is required for these drugs in renal failure. Because clearance of penicillins is less efficient in the newborn, doses adjusted for weight alone result in higher systemic concentrations for longer periods than in the adult.

Clinical Uses Except for amoxicillin, oral penicillins should be given 1–2 hours before or after a meal; they should not be given with food to minimize binding to food proteins and acid inactivation. Amoxicillin may be given without regard to meals. Blood levels of all penicillins can be raised by simultaneous administration of probenecid, 0.5 g (10 mg/kg in children) every 6 hours orally, which impairs renal tubular secretion of weak acids such as β-lactam compounds. Penicillins should never be used for viral infections and should be prescribed only when there is reasonable suspicion of, or documented infection with, susceptible organisms.

A. Penicillin Penicillin G is a drug of choice for infections caused by streptococci, meningococci, some enterococci, penicillin-susceptible pneumococci, non-β-lactamase-producing staphylococci, Treponema pallidum and certain other spirochetes, some Clostridium species, Actinomyces and certain other gram-positive rods, and non-β-lactamase-producing gram-negative anaerobic organisms. Depending on the organism, the site, and the severity of infection, effective doses range between 4 and 24 million units per day administered intravenously in four to six divided doses. High-dose penicillin G can also be given as a continuous intravenous infusion.

Penicillin V, the oral form of penicillin, is indicated only in minor infections because of its relatively poor bioavailability, the need for dosing four times a day, and its narrow antibacterial spectrum. Amoxicillin (see below) is often used instead.

Benzathine penicillin and procaine penicillin G for intramuscular injection yield low but prolonged drug levels. A single intramuscular injection of benzathine penicillin, 1.2 million units, is effective treatment for β-hemolytic streptococcal pharyngitis; given intramuscularly once every 3–4 weeks, it prevents reinfection. Benzathine penicillin G, 2.4 million units intramuscularly once a week for 1–3 weeks, is effective in the treatment of syphilis. Procaine penicillin G was once a commonly used treatment for uncomplicated pneumococcal pneumonia and gonorrhea; however, it is rarely used now because many gonococcal strains are penicillin-resistant, and many pneumococci require higher doses of penicillin G or the use of more potent β-lactams.

B. Penicillins Resistant to Staphylococcal Beta Lactamase (Methicillin, Nafcillin, and Isoxazolyl Penicillins) These semisynthetic penicillins are indicated for infections caused by β-lactamase-producing staphylococci, although penicillin susceptible strains of streptococci and pneumococci are also susceptible to these agents. Listeria monocytogenes, enterococci, and methicillinresistant strains of staphylococci are resistant. In recent years the empirical use of these drugs has decreased substantially because of increasing rates of methicillin-resistance in staphylococci. However, for infections caused by methicillin-susceptible and penicillinresistant strains of staphylococci, these are considered the drugs of choice.

An isoxazolyl penicillin such as cloxacillin or dicloxacillin, 0.25–0.5 g orally every 4–6 hours (15–25 mg/kg/d for children), is suitable for treatment of mild to moderate localized staphylococcal infections. They are relatively acid-stable and have reasonable bioavailability.

However, food interferes with absorption, and the drugs should be administered 1 hour before or after meals.

Methicillin, the first antistaphylococcal penicillin to be developed, is no longer used clinically due to high rates of adverse effects.

Oxacillin and nafcillin, 8–12 g/d, given by intermittent intravenous infusion of 1–2 g every 4–6 hours (50–100 mg/kg/d for children), are considered the drugs of choice for serious systemic staphylococcal infections.

C. Extended-Spectrum Penicillins (Aminopenicillins, Carboxypenicillins, and Ureidopenicillins) These drugs have greater activity than penicillin against gram-negative bacteria because of their enhanced ability to penetrate the gramnegative outer membrane. Like penicillin G, they are inactivated by many β lactamases.

The aminopenicillins, ampicillin and amoxicillin, have very similar spectrums of activity, but amoxicillin is better absorbed orally.

Amoxicillin, 250–500 mg three times daily, is equivalent to the same amount of ampicillin given four times daily. Amoxicillin is given orally to treat urinary tract infections, sinusitis, otitis, and lower respiratory tract infections. Ampicillin and amoxicillin are the most active of the oral β-lactam antibiotics against pneumococci with elevated MICs to penicillin and are the preferred β-lactam antibiotics for treating infections suspected to be caused by these strains. Ampicillin (but not amoxicillin) is effective for shigellosis. Ampicillin, at dosages of 4– 12 g/d intravenously, is useful for treating serious infections caused by susceptible organisms, including anaerobes, enterococci, L monocytogenes, and β-lactamase-negative strains of gram-negative cocci and bacilli such as E coli, and Salmonella sp. Non-βlactamase-producing strains of H influenzae are generally susceptible, but strains that are resistant because of altered PBPs are emerging. Due to production of β lactamases by gram-negative bacilli, ampicillin can no longer be used for empirical therapy of urinary tract infections and typhoid fever. Ampicillin is not active against Klebsiella sp, Enterobacter sp, P aeruginosa, Citrobacter sp, Serratia marcescens, indole-positive proteus species, and other gram-negative aerobes that are commonly encountered in hospitalacquired infections. These organisms intrinsically produce β lactamases that inactivate ampicillin.

Carbenicillin, the first antipseudomonal carboxypenicillin, is no longer used in the USA, because there are more active, better

tolerated alternatives. A carboxypenicillin with activity similar to that of carbenicillin is ticarcillin. It is less active than ampicillin against enterococci. The ureidopenicillins, piperacillin, mezlocillin, and azlocillin, are also active against selected gram-negative bacilli, such as Klebsiella pneumoniae. Although supportive clinical data are lacking for superiority of combination therapy over single-drug therapy, because of the propensity of P aeruginosa to develop resistance during treatment, an antipseudomonal penicillin is sometimes used in combination with an aminoglycoside or fluoroquinolone for pseudomonal infections outside the urinary tract.

Ampicillin, amoxicillin, ticarcillin, and piperacillin are also available in combination with one of several β-lactamase inhibitors: clavulanic acid, sulbactam, or tazobactam. The addition of a β-lactamase inhibitor extends the activity of these penicillins to include βlactamase-producing strains of S aureus as well as some β-lactamase-producing gram-negative bacteria (see Beta-Lactamase Inhibitors).

Adverse Reactions The penicillins are generally well tolerated, and, unfortunately, this may encourage inappropriate use. Most of the serious adverse effects are due to hypersensitivity. The antigenic determinants are degradation products of penicillins, particularly penicilloic acid and products of alkaline hydrolysis bound to host protein. A history of a penicillin reaction is not reliable; about 5–8% of people claim such a history, but only a small number of these will have a serious reaction when given penicillin. Less than 1% of persons who previously received penicillin without incident will have an allergic reaction when given penicillin. Because of the potential for anaphylaxis, however, penicillin should be administered with caution or a substitute drug given if the person has a history of serious penicillin allergy. Penicillin skin testing may also be used to evaluate Type I hypersensitivity. If skin testing is negative, most patients can safely receive penicillin.

Allergic reactions include anaphylactic shock (very rare—0.05% of recipients); serum sickness-type reactions (now rare—urticaria, fever, joint swelling, angioneurotic edema, intense pruritus, and respiratory compromise occurring 7–12 days after exposure); and a variety of skin rashes. Oral lesions, fever, interstitial nephritis (an autoimmune reaction to a penicillin-protein complex), eosinophilia, hemolytic anemia and other hematologic disturbances, and vasculitis may also occur. Most patients allergic to penicillins can be treated with alternative drugs. However, if necessary (eg, treatment of enterococcal endocarditis or neurosyphilis in a patient with serious penicillin allergy), desensitization can be accomplished with gradually increasing doses of penicillin.

In patients with renal failure, penicillin in high doses can cause seizures. Nafcillin is associated with neutropenia; oxacillin can cause hepatitis; and methicillin causes interstitial nephritis (and is no longer used for this reason). Large doses of penicillins given orally may lead to gastrointestinal upset, particularly nausea, vomiting, and diarrhea. Ampicillin has been associated with pseudomembranous colitis.

Secondary infections such as vaginal candidiasis may occur. Ampicillin and amoxicillin can be associated with skin rashes when prescribed in the setting of viral illnesses, particularly noted during acute Epstein-Barr virus infection, but the incidence of rash may be lower than originally reported.

CEPHALOSPORINS & CEPHAMYCINS Cephalosporins are similar to penicillins but more stable to many bacterial β lactamases and, therefore, have a broader spectrum of activity. However, strains of E coli and Klebsiella sp expressing extended-spectrum β lactamases that can hydrolyze most cephalosporins are a growing clinical concern. Cephalosporins are not active against L monocytogenes, and of the available cephalosporins, only ceftaroline has some activity against enterococci.

Chemistry The nucleus of the cephalosporins, 7-aminocephalosporanic acid (Figure 43–6), bears a close resemblance to 6-aminopenicillanic acid (Figure 43–1). The intrinsic antimicrobial activity of natural cephalosporins is low, but the attachment of various R 1 and R2 groups has yielded hundreds of potent compounds, many with low toxicity. Cephalosporins can be classified into four major groups or generations, depending mainly on the spectrum of antimicrobial activity.

FIGURE 43–6 Structures of some cephalosporins. R1 and R2 structures are substituents on the 7-aminocephalosporanic acid nucleus pictured at the top. Other structures (cefoxitin and below) are complete in themselves. 1 Additional substituents not shown.

FIRST-GENERATION CEPHALOSPORINS First-generation cephalosporins include cefazolin, cefadroxil, cephalexin, cephalothin, cephapirin, and cephradine. These drugs are very active against gram-positive cocci, such as streptococci and staphylococci. Traditional cephalosporins are not active against methicillin-resistant strains of staphylococci; however, new compounds have been developed that have activity against methicillinresistant strains (see below). E coli, K pneumoniae, and Proteus mirabilis are often sensitive, but activity against P aeruginosa, indole-positive proteus species, Enterobacter sp, S marcescens, Citrobacter sp, and Acinetobacter sp is poor. Anaerobic cocci (eg, peptococci, peptostreptococci) are usually sensitive, but Bacteroides fragilis is not.

Pharmacokinetics & Dosage A. Oral Cephalexin, cephradine, and cefadroxil are absorbed from the gut to a variable extent. After oral doses of 500 mg, serum levels are 15– 20 mcg/mL. Urine concentration is usually very high, but in most tissues levels are variable and generally lower than in serum.

Cephalexin and cephradine are given orally in dosages of 0.25–0.5 g four times daily (15–30 mg/kg/d) and cefadroxil in dosages of 0.5–1 g twice daily. Excretion is mainly by glomerular filtration and tubular secretion into the urine. Drugs that block tubular secretion, eg, probenecid, may increase serum levels substantially. In patients with impaired renal function, dosage must be reduced (Table 43–2).

TABLE 43–2 Guidelines for dosing of some commonly used cephalosporins and other cell-wall inhibitor antibiotics.

B. Parenteral Cefazolin is the only first-generation parenteral cephalosporin still in general use. After an intravenous infusion of 1 g, the peak level of cefazolin is 90–120 mcg/mL. The usual intravenous dosage of cefazolin for adults is 0.5–2 g intravenously every 8 hours. Cefazolin can also be administered intramuscularly. Excretion is via the kidney, and dose adjustments must be made for impaired renal function.

Clinical Uses Oral drugs may be used for the treatment of urinary tract infections and staphylococcal or streptococcal infections, including cellulitis or soft tissue abscess. However, oral cephalosporins should not be relied on in serious systemic infections.

Cefazolin penetrates well into most tissues. It is a drug of choice for surgical prophylaxis. Cefazolin may also be a choice in infections for which it is the least toxic drug (eg, penicillinase-producing E coli or K pneumoniae) and in individuals with staphylococcal or streptococcal infections who have a history of penicillin allergy other than immediate hypersensitivity. Cefazolin does not penetrate the central nervous system and cannot be used to treat meningitis. Cefazolin is an alternative to an antistaphylococcal penicillin for patients who have mild allergic reactions to penicillin, and it has been shown to be effective for serious staphylococcal infections, eg, bacteremia.

SECOND-GENERATION CEPHALOSPORINS Members of the second-generation cephalosporins include cefaclor, cefamandole, cefonicid, cefuroxime, cefprozil, loracarbef, and ceforanide; and the structurally related cephamycins cefoxitin, cefmetazole, and cefotetan, which have activity against anaerobes.

This is a heterogeneous group with marked individual differences in activity, pharmacokinetics, and toxicity. In general, secondgeneration cephalosporins are active against organisms inhibited by first-generation drugs, but in addition they have extended gramnegative coverage. Klebsiella sp (including those resistant to cephalothin) are usually sensitive. Cefamandole, cefuroxime, cefonicid, ceforanide, and cefaclor are active against H influenzae but not against serratia or B fragilis. In contrast, cefoxitin, cefmetazole, and cefotetan are active against B fragilis and some serratia strains but are less active against H influenzae. As with first-generation agents, no member of this group is active against enterococci or P aeruginosa. Second-generation cephalosporins may exhibit in vitro activity against Enterobacter sp, but resistant mutants that constitutively express a chromosomal β lactamase that hydrolyzes these compounds (and third-generation cephalosporins) are readily selected, and they should not be used to treat enterobacter infections.

Pharmacokinetics & Dosage A. Oral Cefaclor, cefuroxime axetil, cefprozil, and loracarbef can be given orally. The usual dosage for adults is 10–15 mg/kg/d in two to four divided doses; children should be given 20–40 mg/kg/d up to a maximum of 1 g/d. Except for cefuroxime axetil, these drugs are not predictably active against penicillin-non-susceptible pneumococci and are not generally used for pneumococcal infections. Cefaclor is more susceptible to β-lactamase hydrolysis compared with the other agents, and its usefulness is correspondingly diminished.

B. Parenteral After a 1 g intravenous infusion, serum levels are 75–125 mcg/mL for most second-generation cephalosporins. Intramuscular administration is painful and should be avoided. Doses and dosing intervals vary depending on the specific agent (Table 43–2). There are marked differences in half-life, protein binding, and interval between doses. All are renally cleared and require dosage adjustment in renal failure.

Clinical Uses The oral second-generation cephalosporins are active against β-lactamase-producing H influenzae or Moraxella catarrhalis and have been primarily used to treat sinusitis, otitis, and lower respiratory tract infections, in which these organisms have an important role.

Because of their activity against anaerobes (including many B fragilis strains), cefoxitin, cefotetan, or cefmetazole can be used to treat mixed anaerobic infections such as peritonitis, diverticulitis, and pelvic inflammatory disease. Cefuroxime is used to treat communityacquired pneumonia because it is active against β-lactamase-producing H influenzae and K pneumoniae and also most pneumococci.

Although cefuroxime crosses the blood-brain barrier, it is less effective in treatment of meningitis than ceftriaxone or cefotaxime and should not be used.

THIRD-GENERATION CEPHALOSPORINS Third-generation agents include cefoperazone, cefotaxime, ceftazidime, ceftizoxime, ceftriaxone, cefixime, cefpodoxime

proxetil, cefdinir, cefditoren pivoxil, ceftibuten, and moxalactam.

Antimicrobial Activity Compared with second-generation agents, these drugs have expanded gram-negative coverage, and some are able to cross the bloodbrain barrier. Third-generation drugs are often active against Citrobacter, S marcescens, and Providencia. They are also effective against β-lactamase-producing strains of haemophilus and neisseria. Ceftazidime and cefoperazone are the only two drugs with useful activity against P aeruginosa. Like the second-generation drugs, third-generation cephalosporins are hydrolyzed by constitutively produced AmpC β lactamase, and they are not reliably active against Enterobacter species. Serratia, Providencia, and Citrobacter also produce a chromosomally encoded cephalosporinase that, when constitutively expressed, can confer resistance to third-generation cephalosporins. Ceftizoxime and moxalactam are active against B fragilis. Cefixime, cefdinir, ceftibuten, and cefpodoxime proxetil are oral agents possessing similar activity except that cefixime and ceftibuten are much less active against pneumococci and have poor activity against S aureus.

Pharmacokinetics & Dosage Intravenous infusion of 1 g of a parenteral cephalosporin produces serum levels of 60–140 mcg/mL. Third-generation cephalosporins penetrate body fluids and tissues well and, with the exception of cefoperazone and all oral cephalosporins, achieve levels in the cerebrospinal fluid sufficient to inhibit most susceptible pathogens.

The half-lives of these drugs and the necessary dosing intervals vary greatly: ceftriaxone (half-life 7–8 hours) can be injected once every 24 hours at a dosage of 15–50 mg/kg/d. A single daily 1 g dose is sufficient for most serious infections, with 2 g every 12 hours recommended for treatment of meningitis. Cefoperazone (half-life 2 hours) can be infused every 8–12 hours in a dosage of 25–100 mg/kg/d. The remaining drugs in the group (half-life 1–1.7 hours) can be infused every 6–8 hours in dosages between 2 and 12 g/d, depending on the severity of infection. Cefixime can be given orally (200 mg twice daily or 400 mg once daily) for urinary tract infections. Due to increasing resistance, cefixime is no longer recommended for the treatment of uncomplicated gonococcal urethritis and cervicitis. Intramuscular ceftriaxone, now used in combination with another antibiotic, is the drug of choice for treating gonococcal infections. The adult dose for cefpodoxime proxetil or cefditoren pivoxil is 200–400 mg twice daily; for ceftibuten, 400 mg once daily; and for cefdinir, 300 mg/12 h. The excretion of cefoperazone and ceftriaxone is mainly through the biliary tract, and no dosage adjustment is required in renal insufficiency. The others are excreted by the kidney and therefore require dosage adjustment in renal insufficiency.

Clinical Uses Third-generation cephalosporins are used to treat a wide variety of serious infections caused by organisms that are resistant to most other drugs. Strains expressing extended-spectrum β lactamases, however, are not susceptible. Third-generation cephalosporins should be avoided in treatment of enterobacter infections—even if the clinical isolate appears susceptible in vitro—because of emergence of resistance. Ceftriaxone and cefotaxime are approved for treatment of meningitis, including meningitis caused by pneumococci, meningococci, H influenzae, and susceptible enteric gram-negative rods, but not by L monocytogenes. Ceftriaxone and cefotaxime are the most active cephalosporins against penicillin-non-susceptible strains of pneumococci and are recommended for empirical therapy of serious infections that may be caused by these strains. Meningitis caused by strains of pneumococci with penicillin MICs > 1 mcg/mL may not respond even to these agents, and addition of vancomycin is recommended. Other potential indications include empirical therapy of sepsis in both the immunocompetent and the immunocompromised patient and treatment of infections for which a cephalosporin is the least toxic drug available. In neutropenic, febrile immunocompromised patients, ceftazidime is often used in combination with other antibiotics.

FOURTH-GENERATION CEPHALOSPORINS Cefepime is an example of a so-called fourth-generation cephalosporin. It is more resistant to hydrolysis by chromosomal β lactamases (eg, those produced by Enterobacter). However, like the third-generation compounds, it is hydrolyzed by extended-spectrum β lactamases. Cefepime has good activity against P aeruginosa, Enterobacteriaceae, S aureus, and S pneumoniae. It is highly active against Haemophilus and Neisseria sp. It penetrates well into cerebrospinal fluid. It is cleared by the kidneys and has a half-life of 2 hours, and its pharmacokinetic properties are very similar to those of ceftazidime. Unlike ceftazidime, however, cefepime has good activity against most penicillin-non-susceptible strains of streptococci, and it is useful in treatment of enterobacter infections.

Cephalosporins Active against Methicillin-Resistant Staphylococci Beta-lactam antibiotics with activity against methicillin-resistant staphylococci are currently under development. Ceftaroline fosamil, the prodrug of the active metabolite ceftaroline, is the first such drug to be approved for clinical use in the USA. Ceftaroline has increased

binding to penicillin-binding protein 2a, which mediates methicillin resistance in staphylococci, resulting in bactericidal activity against these strains. It has some activity against enterococci and a broad gram-negative spectrum similar to ceftriaxone. It is not active against AmpC or extended-spectrum β-lactamase-producing organisms. Ceftaroline is currently approved for the treatment of skin and soft tissue infections and community-acquired pneumonia.

ADVERSE EFFECTS OF CEPHALOSPORINS A. Allergy Cephalosporins are sensitizing and may elicit a variety of hypersensitivity reactions that are identical to those of penicillins, including anaphylaxis, fever, skin rashes, nephritis, granulocytopenia, and hemolytic anemia. Patients with documented penicillin anaphylaxis have an increased risk of reacting to cephalosporins compared with patients without a history of penicillin allergy. However, the chemical nucleus of cephalosporins is sufficiently different from that of penicillins, so that many individuals with a history of penicillin allergy tolerate cephalosporins. Overall the frequency of cross-allergenicity between the two groups of drugs is low (~1%). Cross-allergenicity appears to be most common among penicillin, aminopenicillins, and early generation cephalosporins. Penicillin, aminopenicillins, and early generation cephalosporins share similar R-1 side chains; this is thought to increase the risk of cross-reactivity. Patients with a history of anaphylaxis to penicillins should not receive first- or second-generation cephalosporins, while third- and fourth-generation cephalosporins should be administered with caution, preferably in a monitored setting.

B. Toxicity Local irritation can produce pain after intramuscular injection and thrombophlebitis after intravenous injection. Renal toxicity, including interstitial nephritis and tubular necrosis, has been demonstrated with several cephalosporins and caused the withdrawal of cephaloridine from clinical use.

Cephalosporins that contain a methylthiotetrazole group (cefamandole, cefmetazole, cefotetan, and cefoperazone) may cause hypoprothrombinemia and bleeding disorders. Oral administration of vitamin K1 , 10 mg twice weekly, can prevent this uncommon problem. Drugs with the methylthiotetrazole ring can also cause severe disulfiram-like reactions; consequently, alcohol and alcoholcontaining medications must be avoided.

OTHER BETA-LACTAM DRUGS MONOBACTAMS Monobactams are drugs with a monocyclic β-lactam ring (Figure 43–1). Their spectrum of activity is limited to aerobic gram-negative rods (including P aeruginosa). Unlike other β-lactam antibiotics, they have no activity against gram-positive bacteria or anaerobes.

Aztreonam is the only monobactam available in the USA. It has structural similarities to ceftazidime, and its gram-negative spectrum is similar to that of the third-generation cephalosporins. It is stable to many β lactamases with the notable exceptions being AmpC β lactamases and extended-spectrum β lactamases. It penetrates well into the cerebrospinal fluid. Aztreonam is given intravenously every 8 hours in a dose of 1–2 g, providing peak serum levels of 100 mcg/mL. The half-life is 1–2 hours and is greatly prolonged in renal failure.

Penicillin-allergic patients tolerate aztreonam without reaction. Occasional skin rashes and elevations of serum aminotransferases occur during administration of aztreonam, but major toxicity is uncommon. In patients with a history of penicillin anaphylaxis, aztreonam may be used to treat serious infections such as pneumonia, meningitis, and sepsis caused by susceptible gram-negative pathogens.

BETA-LACTAMASE INHIBITORS (CLAVULANIC ACID, SULBACTAM, & TAZOBACTAM) These substances resemble β-lactam molecules (Figure 43–7), but they have very weak antibacterial action. They are potent inhibitors of many but not all bacterial β lactamases and can protect hydrolyzable penicillins from inactivation by these enzymes. Beta-lactamase inhibitors are most active against Ambler class A β lactamases (plasmid-encoded transposable element [TEM] β lactamases in particular), such as those produced by staphylococci, H influenzae, N gonorrhoeae, salmonella, shigella, E coli, and K pneumoniae.

They are not good inhibitors of class C β lactamases, which typically are chromosomally encoded and inducible, produced by Enterobacter sp, Citrobacter sp, S marcescens, and P aeruginosa, but they do inhibit chromosomal β lactamases of B fragilis and M catarrhalis.

FIGURE 43–7 Beta-lactamase inhibitors.

The three inhibitors differ slightly with respect to pharmacology, stability, potency, and activity, but these differences usually are of little therapeutic significance. Beta-lactamase inhibitors are available only in fixed combinations with specific penicillins. The antibacterial spectrum of the combination is determined by the companion penicillin, not the β-lactamase inhibitor. (The fixed combinations available in the USA are listed in Preparations Available.) An inhibitor extends the spectrum of a penicillin provided that the inactivity of the penicillin is due to destruction by β lactamase and that the inhibitor is active against the β lactamase that is produced. Thus, ampicillin-sulbactam is active against β-lactamase-producing S aureus and H influenzae but not against serratia, which produces a β lactamase that is not inhibited by sulbactam. Similarly, if a strain of P aeruginosa is resistant to piperacillin, it is also resistant to piperacillin-tazobactam because tazobactam does not inhibit the chromosomal β lactamase produced by P aeruginosa.

The indications for penicillin-β-lactamase inhibitor combinations are empirical therapy for infections caused by a wide range of potential pathogens in both immunocompromised and immunocompetent patients and treatment of mixed aerobic and anaerobic infections, such as intra-abdominal infections. Doses are the same as those used for the single agents except that the recommended dosage of piperacillin in the piperacillin-tazobactam combination is 3–4 g every 6 hours. Adjustments for renal insufficiency are made based on the penicillin component.

CARBAPENEMS The carbapenems are structurally related to other β-lactam antibiotics (Figure 43–1) . Doripenem, ertapenem, imipenem, and meropenem are licensed for use in the USA. Imipenem, the first drug of this class, has a wide spectrum with good activity against many gram-negative rods, including P aeruginosa, gram-positive organisms, and anaerobes. It is resistant to most β lactamases but not carbapenemases or metallo-β lactamases. Enterococcus faecium, methicillin-resistant strains of staphylococci, Clostridium difficile, Burkholderia cepacia, and Stenotrophomonas maltophilia are resistant. Imipenem is inactivated by dehydropeptidases in renal tubules, resulting in low urinary concentrations. Consequently, it is administered together with an inhibitor of renal dehydropeptidase, cilastatin, for clinical use. Doripenem and meropenem are similar to imipenem but have slightly greater activity against gram-negative aerobes and slightly less activity against gram-positives. They are not significantly degraded by renal dehydropeptidase and do not require an inhibitor. Ertapenem is less active than the other carbapenems against P aeruginosa and Acinetobacter species. It is not degraded by renal dehydropeptidase.

Carbapenems penetrate body tissues and fluids well, including the cerebrospinal fluid. All are cleared renally, and the dose must be reduced in patients with renal insufficiency. The usual dosage of imipenem is 0.25–0.5 g given intravenously every 6–8 hours (half-life 1 hour). The usual adult dosage of meropenem is 0.5–1 g intravenously every 8 hours. The usual adult dosage of doripenem is 0.5 g administered as a 1- or 4-hour infusion every 8 hours. Ertapenem has the longest half-life (4 hours) and is administered as a once-daily dose of 1 g intravenously or intramuscularly. Intramuscular ertapenem is irritating, and the drug is formulated with 1% lidocaine for administration by this route.

A carbapenem is indicated for infections caused by susceptible organisms that are resistant to other available drugs, eg, P aeruginosa, and for treatment of mixed aerobic and anaerobic infections. Carbapenems are active against many penicillin-nonsusceptible strains of pneumococci. Carbapenems are highly active in the treatment of enterobacter infections because they are resistant to destruction by the β lactamase produced by these organisms. Clinical experience suggests that carbapenems are also the treatment of choice for serious infections caused by extended-spectrum β-lactamase-producing gram-negative bacteria. Ertapenem is insufficiently active against P aeruginosa and should not be used to treat infections caused by that organism. Imipenem, meropenem, or doripenem, with or without an aminoglycoside, may be effective treatment for febrile neutropenic patients.

The most common adverse effects of carbapenems—which tend to be more common with imipenem—are nausea, vomiting, diarrhea, skin rashes, and reactions at the infusion sites. Excessive levels of imipenem in patients with renal failure may lead to seizures.

Meropenem, doripenem, and ertapenem are much less likely to cause seizures than imipenem. Patients allergic to penicillins may be

allergic to carbapenems, but the incidence of cross-reactivity is low.

GLYCOPEPTIDE ANTIBIOTICS VANCOMYCIN Vancomycin is an antibiotic produced by Streptococcus orientalis and Amycolatopsis orientalis. It is active only against gram-positive bacteria. Vancomycin is a glycopeptide of molecular weight 1500. It is water soluble and quite stable.

Mechanisms of Action & Basis of Resistance Vancomycin inhibits cell wall synthesis by binding firmly to the D-Ala-D-Ala terminus of nascent peptidoglycan pentapeptide (Figure 43– 5). This inhibits the transglycosylase, preventing further elongation of peptidoglycan and cross-linking. The peptidoglycan is thus weakened, and the cell becomes susceptible to lysis. The cell membrane is also damaged, which contributes to the antibacterial effect.

Resistance to vancomycin in enterococci is due to modification of the D-Ala-D-Ala binding site of the peptidoglycan building block in which the terminal D-Ala is replaced by D-lactate. This results in the loss of a critical hydrogen bond that facilitates high-affinity binding of vancomycin to its target and loss of activity. This mechanism is also present in vancomycin-resistant S aureus strains (MIC ≥ 16 mcg/mL), which have acquired the enterococcal resistance determinants. The underlying mechanism for reduced vancomycin susceptibility in vancomycin-intermediate strains (MICs = 4–8 mcg/mL) of S aureus is not fully known. However, these strains have altered cell wall metabolism that results in a thickened cell wall with increased numbers of D-Ala-D-Ala residues, which serve as deadend binding sites for vancomycin. Vancomycin is sequestered within the cell wall by these false targets and may be unable to reach its site of action.

Antibacterial Activity Vancomycin is bactericidal for gram-positive bacteria in concentrations of 0.5–10 mcg/mL. Most pathogenic staphylococci, including those producing β lactamase and those resistant to nafcillin and methicillin, are killed by 2 mcg/mL or less. Vancomycin kills staphylococci relatively slowly and only if cells are actively dividing; the rate is less than that of the penicillins both in vitro and in vivo.

Vancomycin is synergistic in vitro with gentamicin and streptomycin against Enterococcus faecium and Enterococcus faecalis strains that do not exhibit high levels of aminoglycoside resistance. Vancomycin is active against many gram-positive anaerobes including C difficile.

Pharmacokinetics Vancomycin is poorly absorbed from the intestinal tract and is administered orally only for the treatment of colitis caused by C difficile.

Parenteral doses must be administered intravenously. A 1-hour intravenous infusion of 1 g produces blood levels of 15–30 mcg/mL for 1–2 hours. The drug is widely distributed in the body. Cerebrospinal fluid levels 7–30% of simultaneous serum concentrations are achieved if there is meningeal inflammation. Ninety percent of the drug is excreted by glomerular filtration. In the presence of renal insufficiency, striking accumulation may occur (Table 43–2). In functionally anephric patients, the half-life of vancomycin is 6–10 days.

A significant amount (roughly 50%) of vancomycin is removed during a standard hemodialysis run when a modern, high-flux membrane is used.

Clinical Uses Important indications for parenteral vancomycin are bloodstream infections and endocarditis caused by methicillin-resistant staphylococci.

However, vancomycin is not as effective as an antistaphylococcal penicillin for treatment of serious infections such as endocarditis caused by methicillin-susceptible strains. Vancomycin in combination with gentamicin is an alternative regimen for treatment of enterococcal endocarditis in a patient with serious penicillin allergy. Vancomycin (in combination with cefotaxime, ceftriaxone, or rifampin) is also recommended for treatment of meningitis suspected or known to be caused by a penicillin-resistant strain of pneumococcus (ie, penicillin MIC > 1 mcg/mL). The recommended dosage in a patient with normal renal function is 30–60 mg/kg/d in two or three divided doses. The traditional dosing regimen in adults with normal renal function is 1 g every 12 hours (~ 30 mg/kg/d); however, this dose will not typically achieve the trough concentrations (15–20 mcg/mL) recommended for serious infections. For serious infections (see below), a starting dose of 45–60 mg/kg/d should be given with titration of the dose to achieve trough levels of 15–20 mcg/mL. The dosage in children is 40 mg/kg/d in three or four divided doses. Clearance of vancomycin is directly proportional to creatinine clearance, and the dosage is reduced accordingly in patients with renal insufficiency. For patients receiving hemodialysis, a common dosing regimen is a 1 g loading dose followed by 500 mg after each dialysis session. Patients receiving a prolonged course of therapy should have serum trough concentrations checked. Recommended trough concentrations are 10–15 mcg/mL for mild to

moderate infections such as cellulitis and 15–20 mcg/mL for more serious infections such as endocarditis, meningitis, and necrotizing pneumonia.

Oral vancomycin, 0.125–0.25 g every 6 hours, is used to treat colitis caused by C difficile. Because of the emergence of vancomycin-resistant enterococci and the potential selective pressure of oral vancomycin for these resistant organisms, metronidazole had been preferred as initial therapy over the last two decades. However, use of oral vancomycin does not appear to be a significant risk factor for acquisition of vancomycin-resistant enterococci. Additionally, recent clinical data suggest that vancomycin is associated with a better clinical response than metronidazole for more severe cases of C difficile colitis. Therefore, oral vancomycin may be used as a first line treatment for severe cases or for cases that fail to respond to metronidazole.

Adverse Reactions Adverse reactions are encountered in about 10% of cases. Most reactions are relatively minor and reversible. Vancomycin is irritating to tissue, resulting in phlebitis at the site of injection. Chills and fever may occur. Ototoxicity is rare and nephrotoxicity uncommon with current preparations. However, administration with another ototoxic or nephrotoxic drug, such as an aminoglycoside, increases the risk of these toxicities. Ototoxicity can be minimized by maintaining peak serum concentrations below 60 mcg/mL. Among the more common reactions is the so-called “red man” syndrome. This infusion-related flushing is caused by release of histamine. It can be largely prevented by prolonging the infusion period to 1–2 hours or pretreatment with an antihistamine such as diphenhydramine.

TEICOPLANIN Teicoplanin is a glycopeptide antibiotic that is very similar to vancomycin in mechanism of action and antibacterial spectrum. Unlike vancomycin, it can be given intramuscularly as well as intravenously. Teicoplanin has a long half-life (45–70 hours), permitting once-daily dosing. This drug is available in Europe but has not been approved for use in the United States.

TELAVANCIN Telavancin is a semisynthetic lipoglycopeptide derived from vancomycin. Telavancin is active versus gram-positive bacteria and has in vitro activity against many strains with reduced susceptibility to vancomycin. Telavancin has two mechanisms of action. Like vancomycin, telavancin inhibits cell wall synthesis by binding to the D-Ala-D-Ala terminus of peptidoglycan in the growing cell wall. In addition, it disrupts the bacterial cell membrane potential and increases membrane permeability. The half-life of telavancin is approximately 8 hours, which supports once-daily intravenous dosing. The drug is approved for treatment of complicated skin and soft tissue infections and hospital-acquired pneumonia at a dose of 10 mg/kg IV daily. Unlike vancomycin therapy, monitoring of serum telavancin levels is not required. Telavancin is potentially teratogenic, so administration to pregnant women must be avoided.

DALBAVANCIN Dalbavancin is a semisynthetic lipoglycopeptide derived from teicoplanin. Dalbavancin shares the same mechanism of action as vancomycin and teicoplanin but has improved activity against many gram-positive bacteria including methicillin-resistant and vancomycinintermediate S aureus. It is not active against most strains of vancomycin-resistant enterococci. Dalbavancin has an extremely long halflife of 6–11 days, which allows for once-weekly intravenous administration. Dalbavancin has been studied for the treatment of skin and soft tissue infections and catheter-associated bloodstream infections. It is being reviewed for approval in the USA.

OTHER CELL WALL- OR MEMBRANE-ACTIVE AGENTS DAPTOMYCIN Daptomycin is a novel cyclic lipopeptide fermentation product of Streptomyces roseosporus (Figure 43–8). Its spectrum of activity is similar to that of vancomycin except that it may be active against vancomycin-resistant strains of enterococci and S aureus. In vitro, it has more rapid bactericidal activity than vancomycin. The precise mechanism of action is not fully understood, but it is known to bind to the cell membrane via calcium-dependent insertion of its lipid tail. This results in depolarization of the cell membrane with potassium efflux and rapid cell death (Figure 43–9). Daptomycin is cleared renally. The approved doses are 4 mg/kg/dose for treatment of skin and soft tissue infections and 6 mg/kg/dose for treatment of bacteremia and endocarditis once daily in patients with normal renal function and every other day in patients with creatinine clearance of less than 30 mL/min. For serious infections, many experts recommend using 8–10 mg/kg/dose. These higher doses appear to be safe and well tolerated, although evidence supporting increased efficacy is lacking. In

clinical trials powered for noninferiority, daptomycin was equivalent in efficacy to vancomycin. It can cause myopathy, and creatine phosphokinase levels should be monitored weekly. Pulmonary surfactant antagonizes daptomycin, and it should not be used to treat pneumonia. Daptomycin can also cause an allergic pneumonitis in patients receiving prolonged therapy (>2 weeks). Treatment failures have been reported in association with an increase in daptomycin MIC during therapy. Daptomycin is an effective alternative to vancomycin, and its role continues to unfold.

FIGURE 43â&#x20AC;&#x201C;8 Structure of daptomycin. (Kyn, deaminated tryptophan.)

FIGURE 43–9 Proposed mechanism of action of daptomycin. Daptomycin first binds to the cytoplasmic membrane (step 1) and then forms complexes in a calcium-dependent manner (steps 2 and 3). Complex formation causes a rapid loss of cellular potassium, possibly by pore formation, and membrane depolarization. This is followed by arrest of DNA, RNA, and protein synthesis resulting in cell death.

Cell lysis does not occur.

FOSFOMYCIN Fosfomycin trometamol, a stable salt of fosfomycin (phosphonomycin), inhibits a very early stage of bacterial cell wall synthesis (Figure 43–5). An analog of phosphoenolpyruvate, it is structurally unrelated to any other antimicrobial agent. It inhibits the cytoplasmic enzyme enolpyruvate transferase by covalently binding to the cysteine residue of the active site and blocking the addition of phosphoenolpyruvate to UDP-N-acetylglucosamine. This reaction is the first step in the formation of UDP-N-acetylmuramic acid, the precursor of Nacetylmuramic acid, which is found only in bacterial cell walls. The drug is transported into the bacterial cell by glycerophosphate or glucose 6-phosphate transport systems. Resistance is due to inadequate transport of drug into the cell.

Fosfomycin is active against both gram-positive and gram-negative organisms at concentrations ≥ 125 mcg/mL. Susceptibility tests should be performed in growth medium supplemented with glucose 6-phosphate to minimize false-positive indications of resistance. In vitro synergism occurs when fosfomycin is combined with β-lactam antibiotics, aminoglycosides, or fluoroquinolones.

Fosfomycin trometamol is available in both oral and parenteral formulations, although only the oral preparation is approved for use in the USA. Oral bioavailability is approximately 40%. Peak serum concentrations are 10 mcg/mL and 30 mcg/mL following a 2 g or 4 g oral dose, respectively. The half-life is approximately 4 hours. The active drug is excreted by the kidney, with urinary concentrations exceeding MICs for most urinary tract pathogens.

Fosfomycin is approved for use as a single 3-g dose for treatment of uncomplicated lower urinary tract infections in women. The drug appears to be safe for use in pregnancy.

BACITRACIN Bacitracin is a cyclic peptide mixture first obtained from the Tracy strain of Bacillus subtilis in 1943. It is active against gram-positive microorganisms. Bacitracin inhibits cell wall formation by interfering with dephosphorylation in cycling of the lipid carrier that transfers peptidoglycan subunits to the growing cell wall (Figure 43–5). There is no cross-resistance between bacitracin and other antimicrobial drugs.

Bacitracin is highly nephrotoxic when administered systemically and is only used topically (Chapter 61). Bacitracin is poorly absorbed. Topical application results in local antibacterial activity without systemic toxicity. Bacitracin, 500 units/g in an ointment base (often combined with polymyxin or neomycin), is indicated for the suppression of mixed bacterial flora in surface lesions of the skin, in wounds, or on mucous membranes. Solutions of bacitracin containing 100–200 units/mL in saline can be used for irrigation of joints, wounds, or the pleural cavity.

CYCLOSERINE Cycloserine is an antibiotic produced by Streptomyces orchidaceous . It is water soluble and very unstable at acid pH. Cycloserine inhibits many gram-positive and gram-negative organisms, but it is used almost exclusively to treat tuberculosis caused by strains of Mycobacterium tuberculosis resistant to first-line agents. Cycloserine is a structural analog of D-alanine and inhibits the incorporation of D-alanine into peptidoglycan pentapeptide by inhibiting alanine racemase, which converts L-alanine to D-alanine, and D-alanyl-D-alanine ligase (Figure 43–5). After ingestion of 0.25 g of cycloserine blood levels reach 20–30 mcg/mL—sufficient to inhibit many strains of mycobacteria and gram-negative bacteria. The drug is widely distributed in tissues. Most of the drug is excreted in active form into the urine. The dosage for treating tuberculosis is 0.5 to 1 g/d in two or three divided doses.

Cycloserine causes serious dose-related central nervous system toxicity with headaches, tremors, acute psychosis, and convulsions.

If oral dosages are maintained below 0.75 g/d, such effects can usually be avoided.

SUMMARY Beta-Lactam & Other Cell Wall- & Membrane-Active Antibiotics

PREPARATIONS AVAILABLE

REFERENCES Antibiotic resistance threats in the Unites States, 2013. Centers for Disease Control and Prevention (CDC). http://www.cdc.gov/drugresistance/threat-report-2013/.

Biek D et al: Ceftaroline fosamil: A novel broad-spectrum cephalosporin with expanded Gram-positive activity. J Antimicrob Chemother 2010;65 (Suppl 4):iv9.

Billeter M et al: Dalbavancin: A novel once-weekly lipoglycopeptide antibiotic. Clin Infect Dis 2008;46:577.

Carpenter CF, Chambers HF: Daptomycin: Another novel agent for treating infections due to drug-resistant gram-positive pathogens. Clin Infect Dis 2004;38:994.

Chang C et al: Overview of penicillin allergy. Clinic Rev Allerg Immunol 2012;43:84.

Chovel-Sella A et al: T he incidence of rash after amoxicillin treatment in children with infectious mononucleosis. Pediatrics 2013;131:1424.

DePestel DD et al: Cephalosporin use in treatment of patients with penicillin allergies. J Am Pharm Assoc 2008;48:530.

Fowler VG et al: Daptomycin versus standard therapy for bacteremia and endocarditis caused by Staphylococcus aureus. N Engl J Med 2006;355:653.

Jacoby GA, Munoz-Price LS: T he new beta-lactamases. N Engl J Med 2005;352:380.

Keating GM, Perry CM: Ertapenem: A review of its use in the treatment of bacterial infections. Drugs 2005;65:2151.

Leonard SN, Rybak MJ: T elavancin: An antimicrobial with a multifunctional mechanism of action for the treatment of serious gram-positive infections. Pharmacotherapy 2008;28:458.

Mandell L: Doripenem: A new carbapenem in the treatment of nosocomial infections. Clin Infect Dis 2009;49(Suppl 1):S1.

Noskin GA et al: National trends in Staphylococcus aureus infection rates: Impact on economic burden and mortality over a 6-year period. Clin Infect Dis 2007;45:1132.

Rybak M et al: T herapeutic monitoring of vancomycin in adult patients: A consensus review of the American Society of Health-System Pharmacists, the Infectious Diseases Society of America, and the Society of Infectious Diseases Pharmacists. Am J Health Syst Pharm 2009;66:82.

Sievart DM et al: Vancomycin-resistant Staphylococcus aureus in the United States, 2002-2006. Clin Infect Dis 2008;46:668.

T amma PD et al: T he use of cefepime for treating AmpC beta-lactamase-producing enterobacteriaceae. Clin Infect Dis 2013;57:781.

Zar FA et al: A comparison of vancomycin and metronidazole for the treatment of Clostridium difficile-associated diarrhea. Clin Infect Dis 2007;45:302.

CASE STUDY ANSWER An intravenous third-generation cephalosporin (ceftriaxone or cefotaxime) with adequate penetration into inflamed meninges that is active against the common bacteria that cause community-acquired pneumonia and meningitis (pneumococcus, meningococcus, Haemophilus) should be ordered. Vancomycin should also be administered until culture and sensitivity results are available in case the patient is infected with a resistant pneumococcus. Although the patient has a history of rash to amoxicillin, the presentation was not consistent with an anaphylactic reaction. The aminopenicillins are frequently associated with rashes that are not caused by Type I hypersensitivity. In this instance, cross-reactivity with a cephalosporin is unlikelyâ&#x20AC;&#x201D;particularly with a third-generation drugâ&#x20AC;&#x201D;and the patient presents with life-threatening illness necessitating appropriate and proven antibiotic coverage.

_______________ * T he authors thank Dr. Henry F. Chambers for his contributions to this chapter in previous editions.

CHAPTER

44 Tetracyclines, Macrolides, Clindamycin, Chloramphenicol, Streptogramins, & Oxazolidinones Daniel H. Deck, PharmD, & Lisa G. Winston, MD

CASE STUDY A 19-year-old woman with no significant past medical history presents to her college medical clinic complaining of a 2-week history of vaginal discharge. She denies any fever or abdominal pain but does report vaginal bleeding after sexual intercourse. When questioned about her sexual activity, she reports having vaginal intercourse, at times unprotected, with two men in the last 6 months.

A pelvic examination is performed and is positive for mucopurulent discharge from the endocervical canal. No cervical motion tenderness is present. A first-catch urine specimen is obtained to be tested for chlamydia and gonorrhea. A pregnancy test is also ordered as the patient reports she â&#x20AC;&#x153;missed her last period.â&#x20AC;? Pending these results, the decision is made to treat her empirically for gonococcal and chlamydial cervicitis. What are two potential treatment options for her possible chlamydial infection? How does her potential pregnancy affect the treatment decision?

The drugs described in this chapter inhibit bacterial protein synthesis by binding to and interfering with ribosomes. Most are bacteriostatic, but a few are bactericidal against certain organisms. Because of overuse, tetracycline and macrolide resistance is common. Except for tigecycline and the streptogramins, these antibiotics are usually administered orally.

TETRACYCLINES All of the tetracyclines have the basic structure shown at right:

Free tetracyclines are crystalline amphoteric substances of low solubility. They are available as hydrochlorides, which are more soluble. Such solutions are acid and, with the exception of chlortetracycline, fairly stable. Tetracyclines chelate divalent metal ions, which can interfere with their absorption and activity. A newer tetracycline analog, tigecycline, is a glycylcycline and a semisynthetic derivative of minocycline.

Mechanism of Action & Antimicrobial Activity Tetracyclines are broad-spectrum bacteriostatic antibiotics that inhibit protein synthesis. Tetracyclines enter microorganisms in part by passive diffusion and in part by an energy-dependent process of active transport. Susceptible organisms concentrate the drug intracellularly. Once inside the cell, tetracyclines bind reversibly to the 30S subunit of the bacterial ribosome, blocking the binding of aminoacyl-tRNA to the acceptor site on the mRNA-ribosome complex (Figure 44â&#x20AC;&#x201C;1). This prevents addition of amino acids to the growing peptide.

FIGURE 44â&#x20AC;&#x201C;1 Steps in bacterial protein synthesis and targets of several antibiotics. Amino acids are shown as numbered circles. The 70S ribosomal mRNA complex is shown with its 50S and 30S subunits. In step 1, the charged tRNA unit carrying amino acid 6 binds to the acceptor site on the 70S ribosome. The peptidyl tRNA at the donor site, with amino acids 1 through 5, then binds the growing amino

acid chain to amino acid 6 (peptide bond formation, step 2). The uncharged tRNA left at the donor site is released (step 3), and the new 6-amino acid chain with its tRNA shifts to the peptidyl site (translocation, step 4). The antibiotic binding sites are shown schematically as triangles. Chloramphenicol (C) and macrolides (M) bind to the 50S subunit and block peptide bond formation (step 2). The tetracyclines (T) bind to the 30S subunit and prevent binding of the incoming charged tRNA unit (step 1).

Tetracyclines are active against many gram-positive and gram-negative bacteria, including certain anaerobes, rickettsiae, chlamydiae, and mycoplasmas. The antibacterial activities of most tetracyclines are similar except that tetracycline-resistant strains may be susceptible to doxycycline, minocycline, and tigecycline, all of which are poor substrates for the efflux pump, if that is the mechanism of resistance. Differences in clinical efficacy for susceptible organisms are minor and attributable largely to features of absorption, distribution, and excretion of individual drugs.

Resistance Three mechanisms of resistance to tetracycline analogs have been described: (1) impaired influx or increased efflux by an active transport protein pump; (2) ribosome protection due to production of proteins that interfere with tetracycline binding to the ribosome; and (3) enzymatic inactivation. The most important of these are production of an efflux pump and ribosomal protection. Tet(AE) efflux pumpexpressing gram-negative species are resistant to the older tetracyclines, doxycycline, and minocycline. They are susceptible, however, to tigecycline, which is not a substrate of these pumps. Similarly, the Tet(K) efflux pump of staphylococci confers resistance to tetracycline, but not to doxycycline, minocycline, or tigecycline, none of which are pump substrates. The Tet(M) ribosomal protection protein expressed by gram-positives produces resistance to tetracycline, doxycycline, and minocycline, but not to tigecycline, which, because of its bulky t-butylglycylamido substituent, has a steric hindrance effect on Tet(M) binding to the ribosome. Tigecycline is a substrate of the chromosomally encoded multidrug efflux pumps of Proteus sp and Pseudomonas aeruginosa, accounting for their intrinsic resistance to all tetracyclines including tigecycline.

Pharmacokinetics Tetracyclines differ in their absorption after oral administration and in their elimination. Absorption after oral administration is approximately 30% for chlortetracycline; 60–70% for tetracycline, oxytetracycline, demeclocycline, and methacycline; and 95–100% for doxycycline and minocycline. Tigecycline is poorly absorbed orally and must be administered intravenously. A portion of an orally administered dose of tetracycline remains in the gut lumen, alters intestinal flora, and is excreted in the feces. Absorption occurs mainly in the upper small intestine and is impaired by food (except doxycycline and minocycline); by multivalent cations (Ca2+, Mg2+, Fe2+, Al3+); by dairy products and antacids, which contain multivalent cations; and by alkaline pH. Specially buffered tetracycline solutions are formulated for intravenous administration.

Tetracyclines are 40–80% bound by serum proteins. Oral dosages of 500 mg every 6 hours of tetracycline hydrochloride or oxytetracycline produce peak blood levels of 4–6 mcg/mL. Intravenously injected tetracyclines give somewhat higher levels, but only temporarily. Peak levels of 2–4 mcg/mL are achieved with a 200 mg dose of doxycycline or minocycline. Steady-state peak serum concentrations of tigecycline are 0.6 mcg/mL at the standard dosage. Tetracyclines are distributed widely to tissues and body fluids except for cerebrospinal fluid, where concentrations are 10–25% of those in serum. Minocycline reaches very high concentrations in tears and saliva, which makes it useful for eradication of the meningococcal carrier state. Tetracyclines cross the placenta to reach the fetus and are also excreted in breast milk. As a result of chelation with calcium, tetracyclines are bound to—and damage—growing bones and teeth. Carbamazepine, phenytoin, barbiturates, and chronic alcohol ingestion may shorten the half-life of doxycycline by 50% due to induction of hepatic enzymes that metabolize the drug.

Tetracyclines are excreted mainly in bile and urine. Concentrations in bile exceed those in serum tenfold. Some of the drug excreted in bile is reabsorbed from the intestine (enterohepatic circulation) and may contribute to maintenance of serum levels. Ten to fifty percent of various tetracyclines is excreted into the urine, mainly by glomerular filtration. Ten to forty percent of the drug is excreted in feces.

Doxycycline and tigecycline, in contrast to other tetracyclines, are eliminated by nonrenal mechanisms, do not accumulate significantly, and require no dosage adjustment in renal failure.

Tetracyclines are classified as short-acting (chlortetracycline, tetracycline, oxytetracycline), intermediate-acting (demeclocycline and methacycline), or long-acting (doxycycline and minocycline) based on serum half-lives of 6–8 hours, 12 hours, and 16–18 hours, respectively. Tigecycline has a half-life of 36 hours. The almost complete absorption and slow excretion of doxycycline and minocycline allow for once-daily dosing for certain indications, but, by convention, these two drugs are usually dosed twice daily.

Clinical Uses A tetracycline is the drug of choice in the treatment of infections caused by rickettsiae. Tetracyclines are also excellent drugs for the treatment of Mycoplasma pneumonia, chlamydiae, and some spirochetes. They are used in combination regimens to treat gastric and duodenal ulcer disease caused by Helicobacter pylori. They may be used in various gram-positive and gram-negative bacterial

infections, including vibrio infections, provided the organism is not resistant. In cholera, tetracyclines rapidly stop the shedding of vibrios, but tetracycline resistance has appeared during epidemics. Tetracyclines remain effective in most chlamydial infections, including sexually transmitted infections. Doxycycline, in combination with ceftriaxone, is an alternative treatment for gonococcal disease. A tetracycline—in combination with other antibiotics—is indicated for plague, tularemia, and brucellosis. Tetracyclines are sometimes used in the treatment or prophylaxis of protozoal infections, eg, those due to Plasmodium falciparum (see Chapter 52). Other uses include treatment of acne, exacerbations of bronchitis, community-acquired pneumonia, Lyme disease, relapsing fever, leptospirosis, and some nontuberculous mycobacterial infections (eg, Mycobacterium marinum). Tetracyclines formerly were used for a variety of common infections, including bacterial gastroenteritis and urinary tract infections. However, many strains of bacteria causing these infections are now resistant, and other agents have largely supplanted tetracyclines.

Minocycline, 200 mg orally daily for 5 days, can eradicate the meningococcal carrier state, but because of side effects and resistance of many meningococcal strains, ciprofloxacin or rifampin is preferred. Demeclocycline inhibits the action of antidiuretic hormone in the renal tubule and has been used in the treatment of inappropriate secretion of antidiuretic hormone or similar peptides by certain tumors (see Chapter 15).

Tigecycline, the first glycylcycline to reach clinical practice, has several unique features that warrant its consideration apart from the older tetracyclines. Many tetracycline-resistant strains are susceptible to tigecycline because it is not affected by the common resistance determinants. Its spectrum is very broad. Coagulase-negative staphylococci and Staphylococcus aureus, including methicillin-resistant, vancomycin-intermediate, and vancomycin-resistant strains; streptococci, penicillin-susceptible and resistant; enterococci, including vancomycin-resistant strains; gram-positive rods; Enterobacteriaceae; multidrug-resistant strains of Acinetobacter sp; anaerobes, both gram-positive and gramnegative; rickettsiae, Chlamydia sp, and Legionella pneumophila; and rapidly growing mycobacteria all are susceptible. Proteus sp and P aeruginosa, however, are intrinsically resistant.

Tigecycline, formulated for intravenous administration only, is given as a 100 mg loading dose, then 50 mg every 12 hours. As with all tetracyclines, tissue and intracellular penetration is excellent; consequently, the volume of distribution is quite large and peak serum concentrations are low. Elimination is primarily biliary, and no dosage adjustment is needed for patients with renal insufficiency. In addition to the tetracycline class effects, the chief adverse effect of tigecycline is nausea, which occurs in up to one third of patients, and occasionally vomiting. Neither nausea nor vomiting usually requires discontinuation of the drug.

Tigecycline is approved for treatment of skin and skin-structure infection, intra-abdominal infections, and community-acquired pneumonia. However, in a meta-analysis of clinical trials, tigecycline was associated with a small but significant increase in the risk of death compared with other antibiotics used to treat these infections. This has led the FDA to issue a black box warning that tigecycline should be reserved for situations where alternative treatments are not suitable. Because active drug concentrations in the urine are relatively low, tigecycline may not be effective for urinary tract infections and has no indication for this use. Tigecycline has in vitro activity against a wide variety of multidrug-resistant nosocomial pathogens (eg, methicillin-resistant S aureus, extended-spectrum βlactamase-producing gram-negatives, and Acinetobacter sp); however, its clinical efficacy in infections with multidrug-resistant organisms, compared with other agents, is unproven.

A. Oral Dosage The oral dosage for rapidly excreted tetracyclines, equivalent to tetracycline hydrochloride, is 0.25–0.5 g four times daily for adults and 20–40 mg/kg/d for children (8 years of age and older). For severe systemic infections, the higher dosage is indicated, at least for the first few days. The daily dose is 600 mg for demeclocycline or methacycline, 100 mg once or twice daily for doxycycline, and 100 mg twice daily for minocycline. Doxycycline is the oral tetracycline of choice because it can be given twice daily, and its absorption is not significantly affected by food. All tetracyclines chelate with metals, and none should be orally administered with milk, antacids, or ferrous sulfate. To avoid deposition in growing bones or teeth, tetracyclines should be avoided in pregnant women and children younger than 8 years.

B. Parenteral Dosage Several tetracyclines are available for intravenous injection in doses of 0.1–0.5 g every 6–12 hours (similar to oral doses) but doxycycline is the usual preferred agent, at a dosage of 100 mg every 12–24 hours. Intramuscular injection is not recommended because of pain and inflammation at the injection site.

Adverse Reactions Hypersensitivity reactions (drug fever, skin rashes) to tetracyclines are uncommon. Most adverse effects are due to direct toxicity of the drug or to alteration of microbial flora.

A. Gastrointestinal Adverse Effects Nausea, vomiting, and diarrhea are the most common reasons for discontinuing tetracyclines. These effects are attributable to direct local irritation of the intestinal tract. Nausea, anorexia, and diarrhea can usually be controlled by administering the drug with food or

carboxymethylcellulose, reducing drug dosage, or discontinuing the drug.

Tetracyclines alter the normal gastrointestinal flora, with suppression of susceptible coliform organisms and overgrowth of pseudomonas, proteus, staphylococci, resistant coliforms, clostridia, and candida. This can result in intestinal functional disturbances, anal pruritus, vaginal or oral candidiasis, or Clostridium difficile-associated colitis. However, the risk of C difficile colitis may be lower with tetracyclines than with other antibiotics.

B. Bony Structures and Teeth Tetracyclines are readily bound to calcium deposited in newly formed bone or teeth in young children. When a tetracycline is given during pregnancy, it can be deposited in the fetal teeth, leading to fluorescence, discoloration, and enamel dysplasia. It can also be deposited in bone, where it may cause deformity or growth inhibition. Because of these effects, tetracyclines are generally avoided in pregnancy. If the drug is given for long periods to children younger than 8 years, similar changes can result.

C. Other Toxicities Tetracyclines can impair hepatic function, especially during pregnancy, in patients with preexisting hepatic insufficiency and when high doses are given intravenously. Hepatic necrosis has been reported with daily doses of 4 g or more intravenously.

Renal tubular acidosis and Fanconi syndrome have been attributed to the administration of outdated tetracycline preparations.

Tetracyclines given along with diuretics may cause nephrotoxicity. Tetracyclines other than doxycycline may accumulate to toxic levels in patients with impaired kidney function.

Intravenous injection can lead to venous thrombosis. Intramuscular injection produces painful local irritation and should be avoided.

Systemically administered tetracycline, especially demeclocycline, can induce sensitivity to sunlight or ultraviolet light, particularly in fair-skinned persons.

Dizziness, vertigo, nausea, and vomiting have been noted, particularly with minocycline. With dosages of 200â&#x20AC;&#x201C;400 mg/d of minocycline, 35â&#x20AC;&#x201C;70% of patients will have these reactions. These symptoms may also occur with higher doses of doxycycline.

MACROLIDES The macrolides are a group of closely related compounds characterized by a macrocyclic lactone ring (usually containing 14 or 16 atoms) to which deoxy sugars are attached. The prototype drug, erythromycin, which consists of two sugar moieties attached to a 14-atom lactone ring, was obtained in 1952 from Streptomyces erythreus . Clarithromycin and azithromycin are semisynthetic derivatives of erythromycin.

ERYTHROMYCIN Chemistry The general structure of erythromycin is shown with the macrolide ring and the sugars desosamine and cladinose. It is poorly soluble in water (0.1%) but dissolves readily in organic solvents. Solutions are fairly stable at 4°C but lose activity rapidly at 20°C and at acid pH. Erythromycins are usually dispensed as various esters and salts.

Mechanism of Action & Antimicrobial Activity The antibacterial action of erythromycin and other macrolides may be inhibitory or bactericidal, particularly at higher concentrations, for susceptible organisms. Activity is enhanced at alkaline pH. Inhibition of protein synthesis occurs via binding to the 50S ribosomal RNA. The binding site is near the peptidyltrans-ferase center, and peptide chain elongation (ie, transpeptidation) is prevented by blocking of the polypeptide exit tunnel. As a result, peptidyl-tRNA is dissociated from the ribosome. Erythromycin also inhibits the formation of the 50S ribosomal subunit (Figure 44–1).

Erythromycin is active against susceptible strains of gram-positive organisms, especially pneumococci, streptococci, staphylococci, and corynebacteria. Mycoplasma pneumoniae, L pneumophila, Chlamydia trachomatis, Chlamydia psittaci, Chlamydia pneumoniae, H pylori, Listeria monocytogenes, and certain mycobacteria (Mycobacterium kansasii, Mycobacterium scrofulaceum) are also susceptible. Gram-negative organisms such as Neisseria sp, Bordetella pertussis, Bartonella henselae, and Bartonella quintana as well as some Rickettsia species, Treponema pallidum, and Campylobacter species are susceptible.

Haemophilus influenzae is somewhat less susceptible.

Resistance to erythromycin is usually plasmid-encoded. Three mechanisms have been identified: (1) reduced permeability of the cell membrane or active efflux; (2) production (by Enterobacteriaceae) of esterases that hydrolyze macrolides; and (3) modification of the ribosomal binding site (so-called ribosomal protection) by chromosomal mutation or by a macrolide-inducible or constitutive methylase.

Efflux and methylase production are the most important resistance mechanisms in gram-positive organisms. Cross-resistance is complete between erythromycin and the other macrolides. Constitutive methylase production also confers resistance to structurally unrelated but mechanistically similar compounds such as clindamycin and streptogramin B (so-called macrolide-lincosamide-streptogramin, or MLStype B, resistance), which share the same ribosomal binding site. Because nonmacrolides are poor inducers of the methylase, strains expressing an inducible methylase will appear susceptible in vitro. However, constitutive mutants that are resistant can be selected out and emerge during therapy with clindamycin.

Pharmacokinetics Erythromycin base is destroyed by stomach acid and must be administered with enteric coating. Food interferes with absorption.

Stearates and esters are fairly acid-resistant and somewhat better absorbed. The lauryl salt of the propionyl ester of erythromycin (erythromycin estolate) is the best-absorbed oral preparation. Oral dosage of 2 g/d results in serum erythromycin base and ester concentrations of approximately 2 mcg/mL. However, only the base is microbiologically active, and its concentration tends to be similar regardless of the formulation. A 500 mg intravenous dose of erythromycin lactobionate produces serum concentrations of 10 mcg/mL 1 hour after dosing. The serum half-life is approximately 1.5 hours normally and 5 hours in patients with anuria. Adjustment for renal failure is not necessary. Erythromycin is not removed by dialysis. Large amounts of an administered dose are excreted in the bile and lost in feces, and only 5% is excreted in the urine. Absorbed drug is distributed widely except to the brain and cerebrospinal fluid.

Erythromycin is taken up by polymorphonuclear leukocytes and macrophages. It traverses the placenta and reaches the fetus.

Clinical Uses Erythromycin is a traditional drug of choice in corynebacterial infections (diphtheria, corynebacterial sepsis, erythrasma) and in respiratory, neonatal, ocular, or genital chlamydial infections. While it was used in treatment of community-acquired pneumonia because its spectrum of activity includes pneumococcus, M pneumoniae, and L pneumophila, newer macrolides are now more commonly selected. Macrolide resistance is also increasing in pneumococci and M pneumoniae. Erythromycin had also been useful as a penicillin substitute in penicillin-allergic individuals with infections caused by staphylococci and streptococci. Emergence of erythromycin resistance in staphylococci and in strains of group A streptococci has made macrolides less attractive as first-line agents for treatment of pharyngitis and skin and soft tissue infections. Erythromycin has been recommended as prophylaxis against endocarditis during dental procedures in individuals with valvular heart disease, but clindamycin, which is better tolerated, has largely replaced it. Although erythromycin estolate is the best-absorbed salt, it imposes the greatest risk of adverse reactions. Therefore, the stearate or succinate salt may be preferred.

The oral dosage of erythromycin base, stearate, or estolate is 0.25–0.5 g every 6 hours (for children, 40 mg/kg/d). The dosage of erythromycin ethylsuccinate is 0.4–0.6 g every 6 hours. Oral erythromycin base (1 g) is sometimes combined with oral neomycin or kanamycin for preoperative preparation of the colon. The intravenous dosage of erythromycin gluceptate or lactobionate is 0.5–1.0 g

every 6 hours for adults and 20–40 mg/kg/d for children. The higher dosage is recommended when treating pneumonia caused by L pneumophila.

Adverse Reactions Anorexia, nausea, vomiting, and diarrhea are common. Gastrointestinal intolerance, which is due to a direct stimulation of gut motility, is the most common reason for discontinuing erythromycin and substituting another antibiotic.

Erythromycins, particularly the estolate, can produce acute cholestatic hepatitis (fever, jaundice, impaired liver function), probably as a hypersensitivity reaction. Most patients recover from this, but hepatitis recurs if the drug is readministered. Other allergic reactions include fever, eosinophilia, and rashes.

Erythromycin metabolites inhibit cytochrome P450 enzymes and, thus, increase the serum concentrations of numerous drugs, including theophylline, warfarin, cyclosporine, and methylprednisolone. Erythromycin increases serum concentrations of oral digoxin by increasing its bioavailability.

CLARITHROMYCIN Clarithromycin is derived from erythromycin by addition of a methyl group and has improved acid stability and oral absorption compared with erythromycin. Its mechanism of action is the same as that of erythromycin. Clarithromycin and erythromycin are similar with respect to antibacterial activity except that clarithromycin is more active against Mycobacterium avium complex (see Chapter 47).

Clarithromycin also has activity against Mycobacterium leprae, Toxoplasma gondii, and H influenzae. Erythromycin-resistant streptococci and staphylococci are also resistant to clarithromycin.

A 500 mg dose of clarithromycin produces serum concentrations of 2–3 mcg/mL. The longer half-life of clarithromycin (6 hours) compared with erythromycin permits twice-daily dosing. The recommended dosage is 250–500 mg twice daily or 1000 mg of the extended-release formulation once daily. Clarithromycin penetrates most tissues well, with concentrations equal to or exceeding serum concentrations.

Clarithromycin is metabolized in the liver. The major metabolite is 14-hydroxyclarithromycin, which also has antibacterial activity.

Portions of active drug and this major metabolite are eliminated in the urine, and dosage reduction (eg, a 500 mg loading dose, then 250 mg once or twice daily) is recommended for patients with creatinine clearances less than 30 mL/min. Clarithromycin has drug interactions similar to those described for erythromycin.

The advantages of clarithromycin compared with erythromycin are lower incidence of gastrointestinal intolerance and less frequent dosing.

AZITHROMYCIN Azithromycin, a 15-atom lactone macrolide ring compound, is derived from erythromycin by addition of a methylated nitrogen into the lactone ring. Its spectrum of activity, mechanism of action, and clinical uses are similar to those of clarithromycin. Azithromycin is active against M avium complex and T gondii. Azithromycin is slightly less active than erythromycin and clarithromycin against staphylococci and streptococci and slightly more active against H influenzae. Azithromycin is highly active against Chlamydia sp.

Azithromycin differs from erythromycin and clarithromycin mainly in pharmacokinetic properties. A 500 mg dose of azithromycin produces relatively low serum concentrations of approximately 0.4 mcg/mL. However, azithromycin penetrates into most tissues (except cerebrospinal fluid) and phagocytic cells extremely well, with tissue concentrations exceeding serum concentrations by 10- to 100-fold.

The drug is slowly released from tissues (tissue half-life of 2–4 days) to produce an elimination half-life approaching 3 days. These unique properties permit once-daily dosing and shortening of the duration of treatment in many cases. For example, a single 1-g dose of azithromycin is as effective as a 7-day course of doxycycline for chlamydial cervicitis and urethritis. Community-acquired pneumonia can be treated with azithromycin given as a 500 mg loading dose, followed by a 250 mg single daily dose for the next 4 days.

Azithromycin is rapidly absorbed and well tolerated orally. It should be administered 1 hour before or 2 hours after meals. Aluminum and magnesium antacids do not alter bioavailability but delay absorption and reduce peak serum concentrations. Because it has a 15member (not 14-member) lactone ring, azithromycin does not inactivate cytochrome P450 enzymes and, therefore, is free of the drug interactions that occur with erythromycin and clarithromycin.

Macrolide antibiotics prolong the QT interval due to an effect on potassium ion channels. Prolongation of the QT interval can lead to the torsades de pointes arrhythmia. Recent studies have suggested that azithromycin may be associated with a small increased risk of cardiac death.

KETOLIDES

Ketolides are semisynthetic 14-membered-ring macrolides, differing from erythromycin by substitution of a 3-keto group for the neutral sugar L-cladinose. Telithromycin is approved for limited clinical use. It is active in vitro against Streptococcus pyogenes, S pneumoniae, S aureus, H influenzae, Moraxella catarrhalis, Mycoplasma sp, L pneumophila, Chlamydia sp, H pylori, Neisseria gonorrhoeae, B fragilis, T gondii, and certain nontuberculosis mycobacteria. Many macrolide-resistant strains are susceptible to ketolides because the structural modification of these compounds renders them poor substrates for efflux pump-mediated resistance, and they bind to ribosomes of some bacterial species with higher affinity than macrolides.

Oral bioavailability of telithromycin is 57%, and tissue and intracellular penetration is generally good. Telithromycin is metabolized in the liver and eliminated by a combination of biliary and urinary routes of excretion. It is administered as a once-daily dose of 800 mg, which results in peak serum concentrations of approximately 2 mcg/mL. It is a reversible inhibitor of the CYP3A4 enzyme system and may slightly prolong the QTc interval. In the USA, telithromycin is now indicated only for treatment of community-acquired bacterial pneumonia. Other respiratory tract infections were removed as indications when it was recognized that use of telithromycin can result in hepatitis and liver failure. Telithromycin is also contraindicated in patients with myasthenia gravis because it may exacerbate this condition.

CLINDAMYCIN Clindamycin is a chlorine-substituted derivative of lincomycin, an antibiotic that is elaborated by Streptomyces lincolnensis.

Mechanism of Action & Antibacterial Activity Clindamycin, like erythromycin, inhibits protein synthesis by interfering with the formation of initiation complexes and with aminoacyl translocation reactions. The binding site for clindamycin on the 50S subunit of the bacterial ribosome is identical with that for erythromycin. Streptococci, staphylococci, and pneumococci are inhibited by clindamycin, 0.5–5 mcg/mL. Enterococci and gram-negative aerobic organisms are resistant. Bacteroides sp and other anaerobes, both gram-positive and gram-negative, are usually susceptible.

Resistance to clindamycin, which generally confers cross-resistance to macrolides, is due to (1) mutation of the ribosomal receptor site; (2) modification of the receptor by a constitutively expressed methylase (see section on erythromycin resistance, above); and (3) enzymatic inactivation of clindamycin. Gram-negative aerobic species are intrinsically resistant because of poor permeability of the outer membrane.

Pharmacokinetics Oral dosages of clindamycin, 0.15–0.3 g every 8 hours (10–20 mg/kg/d for children), yield serum levels of 2–3 mcg/mL. When administered intravenously, 600 mg of clindamycin every 8 hours gives levels of 5–15 mcg/mL. The drug is about 90% protein-bound.

Clindamycin penetrates well into most tissues, with brain and cerebrospinal fluid being important exceptions. It penetrates well into abscesses and is actively taken up and concentrated by phagocytic cells. Clindamycin is metabolized by the liver, and both active drug and active metabolites are excreted in bile and urine. The half-life is about 2.5 hours in normal individuals, increasing to 6 hours in patients with anuria. No dosage adjustment is required for renal failure.

Clinical Use Clindamycin is indicated for the treatment of skin and soft-tissue infections caused by streptococci and staphylococci. It is often active against community-acquired strains of methicillin-resistant S aureus, an increasingly common cause of skin and soft tissue infections.

Clindamycin is also indicated for treatment of infections caused by Bacteroides sp and other anaerobes. Clindamycin, sometimes in combination with an aminoglycoside or cephalosporin, is used to treat penetrating wounds of the abdomen and the gut; infections originating in the female genital tract, eg, septic abortion, pelvic abscesses, or pelvic inflammatory disease; and lung abscesses.

Clindamycin is now recommended rather than erythromycin for prophylaxis of endocarditis in patients with specific valvular heart disease who are undergoing certain dental procedures and have significant penicillin allergies. Clindamycin plus primaquine is an effective alternative to trimethoprim-sulfamethoxazole for moderate to moderately severe Pneumocystis jiroveci pneumonia in AIDS patients. It is also used in combination with pyrimethamine for AIDS-related toxoplasmosis of the brain.

Adverse Effects Common adverse effects are diarrhea, nausea, and skin rashes. Impaired liver function (with or without jaundice) and neutropenia sometimes occur. Administration of clindamycin is a risk factor for diarrhea and colitis due to C difficile.

STREPTOGRAMINS MECHANISM OF ACTION & ANTIBACTERIAL ACTIVITY Quinupristin-dalfopristin is a combination of two streptogramins—quinupristin, a streptogramin B, and dalfopristin, a streptogramin A —in a 30:70 ratio. The streptogramins share the same ribosomal binding site as the macrolides and clindamycin and thus inhibit protein synthesis in an identical manner. Quinupristin-dalfopristin is rapidly bactericidal for most susceptible organisms except Enterococcus faecium, which is killed slowly. Quinupristin-dalfopristin is active against gram-positive cocci, including multidrug-resistant strains of streptococci, penicillin-resistant strains of S pneumoniae, methicillin-susceptible and resistant strains of staphylococci, and E faecium (but not Enterococcus faecalis). Resistance is due to modification of the quinupristin binding site (MLS-B type resistance), enzymatic inactivation of dalfopristin, or efflux.

Pharmacokinetics Quinupristin-dalfopristin is administered intravenously at a dosage of 7.5 mg/kg every 8–12 hours. Peak serum concentrations following an infusion of 7.5 mg/kg over 60 minutes are 3 mcg/mL for quinupristin and 7 mcg/mL for dalfopristin. Quinupristin and dalfopristin are rapidly metabolized, with half-lives of 0.85 and 0.7 hours, respectively. Elimination is principally by the fecal route. Dose adjustment is not necessary for renal failure, peritoneal dialysis, or hemodialysis. Patients with hepatic insufficiency may not tolerate the drug at usual doses, however, because of increased area under the concentration curve of both parent drugs and metabolites. This may necessitate a dose reduction to 7.5 mg/kg every 12 hours or 5 mg/kg every 8 hours. Quinupristin and dalfopristin significantly inhibit CYP3A4, which metabolizes warfarin, diazepam, astemizole, terfenadine, cisapride, non-nucleoside reverse transcriptase inhibitors, and cyclosporine, among others. Dosage reduction of cyclosporine may be necessary.

Clinical Uses & Adverse Effects Quinupristin-dalfopristin is approved for treatment of infections caused by staphylococci or by vancomycin-resistant strains of E faecium, but not E faecalis, which is intrinsically resistant, probably because of an efflux-type resistance mechanism. The principal toxicities are infusion-related events, such as pain at the infusion site, and an arthralgia-myalgia syndrome.

CHLORAMPHENICOL Crystalline chloramphenicol is a neutral, stable compound with the following structure:

It is soluble in alcohol but poorly soluble in water. Chloramphenicol succinate, which is used for parenteral administration, is highly water-soluble. It is hydrolyzed in vivo with liberation of free chloramphenicol.

Mechanism of Action & Antimicrobial Activity Chloramphenicol is a potent inhibitor of microbial protein synthesis. It binds reversibly to the 50S subunit of the bacterial ribosome (Figure 44–1) and inhibits peptide bond formation (step 2). Chloramphenicol is a bacteriostatic broad-spectrum antibiotic that is active against both aerobic and anaerobic gram-positive and gram-negative organisms. It is active also against Rickettsiae but not Chlamydiae. Most gram-positive bacteria are inhibited at concentrations of 1–10 mcg/mL, and many gram-negative bacteria are inhibited by concentrations of 0.2–5 mcg/mL. H influenzae, Neisseria meningitidis, and some strains of bacteroides are highly susceptible, and for these organisms, chloramphenicol may be bactericidal.

Low-level resistance to chloramphenicol may emerge from large populations of chloramphenicol-susceptible cells by selection of mutants that are less permeable to the drug. Clinically significant resistance is due to production of chloramphenicol acetyltransferase, a plasmid-encoded enzyme that inactivates the drug.

Pharmacokinetics The usual dosage of chloramphenicol is 50–100 mg/kg/d. After oral administration, crystalline chloramphenicol is rapidly and completely absorbed. A 1 g oral dose produces blood levels between 10 and 15 mcg/mL. Chloramphenicol palmitate is a prodrug that is hydrolyzed in the intestine to yield free chloramphenicol. The parenteral formulation is a prodrug, chloramphenicol succinate, which is hydrolyzed to yield free chloramphenicol, giving blood levels somewhat lower than those achieved with orally administered drug. Chloramphenicol is widely distributed to virtually all tissues and body fluids, including the central nervous system and cerebrospinal fluid, such that the concentration of chloramphenicol in brain tissue may be equal to that in serum. The drug penetrates cell membranes readily.

Most of the drug is inactivated either by conjugation with glucuronic acid (principally in the liver) or by reduction to inactive aryl amines. Active chloramphenicol, about 10% of the total dose administered, and its inactive degradation products are eliminated in the urine. A small amount of active drug is excreted into bile and feces. The systemic dosage of chloramphenicol need not be altered in renal insufficiency, but it must be reduced markedly in hepatic failure. Newborns less than a week old and premature infants also clear chloramphenicol less well, and the dosage should be reduced to 25 mg/kg/d.

Clinical Uses Because of potential toxicity, bacterial resistance, and the availability of many other effective alternatives, chloramphenicol is rarely used in the United States. It may be considered for treatment of serious rickettsial infections such as typhus and Rocky Mountain spotted fever. It is an alternative to a β-lactam antibiotic for treatment of bacterial meningitis occurring in patients who have major hypersensitivity reactions to penicillin. The dosage is 50–100 mg/kg/d in four divided doses.

Chloramphenicol is used topically in the treatment of eye infections because of its broad spectrum and its penetration of ocular tissues and the aqueous humor. It is not effective for chlamydial infections.

Adverse Reactions Adults occasionally develop gastrointestinal disturbances, including nausea, vomiting, and diarrhea. These symptoms are rare in children.

Oral or vaginal candidiasis may occur as a result of alteration of normal microbial flora.

Chloramphenicol commonly causes a dose-related reversible suppression of red cell production at dosages exceeding 50 mg/kg/d after 1–2 weeks. Aplastic anemia, a rare consequence (1 in 24,000 to 40,000 courses of therapy) of chloramphenicol administration by any route, is an idiosyncratic reaction unrelated to dose, although it occurs more frequently with prolonged use. The anemia tends to be irreversible and can be fatal, although it may respond to bone marrow transplantation or immunosuppressive therapy.

Newborn infants lack an effective glucuronic acid conjugation mechanism for the degradation and detoxification of chloramphenicol.

Consequently, when infants are given dosages above 50 mg/kg/d, the drug may accumulate, resulting in the gray baby syndrome, with vomiting, flaccidity, hypothermia, gray color, shock, and vascular collapse. To avoid this toxic effect, chloramphenicol should be used with caution in infants and the dosage limited to 50 mg/kg/d (or less during the first week of life) in full-term infants more than 1 week old and 25 mg/kg/d in premature infants.

Chloramphenicol inhibits hepatic microsomal enzymes that metabolize several drugs. Half-lives of these drugs are prolonged, and the serum concentrations of phenytoin, tolbutamide, chlorpropamide, and warfarin are increased.

OXAZOLIDINONES MECHANISM OF ACTION & ANTIMICROBIAL ACTIVITY Linezolid is a member of the oxazolidinones, a newer class of synthetic antimicrobials. It is active against gram-positive organisms including staphylococci, streptococci, enterococci, gram-positive anaerobic cocci, and gram-positive rods such as corynebacteria,

Nocardia sp, and L monocytogenes. It is primarily a bacteriostatic agent but is bactericidal against streptococci. It is also active against Mycobacterium tuberculosis.

Linezolid inhibits protein synthesis by preventing formation of the ribosome complex that initiates protein synthesis. Its unique binding site, located on 23S ribosomal RNA of the 50S subunit, results in no cross-resistance with other drug classes. Resistance is caused by mutation of the linezolid binding site on 23S ribosomal RNA.

Pharmacokinetics Linezolid is 100% bioavailable after oral administration and has a half-life of 4â&#x20AC;&#x201C;6 hours. It is metabolized by oxidative metabolism, yielding two inactive metabolites. It is neither an inducer nor an inhibitor of cytochrome P450 enzymes. Peak serum concentrations average 18 mcg/mL following a 600 mg oral dose. The recommended dosage for most indications is 600 mg twice daily, either orally or intravenously.

Clinical Uses Linezolid is approved for vancomycin-resistant E faecium infections, health care-associated pneumonia, community-acquired pneumonia, and both complicated and uncomplicated skin and soft tissue infections caused by susceptible gram-positive bacteria. Off-label uses of linezolid include treatment of multidrug-resistant tuberculosis and Nocardia infections.

Adverse Effects The principal toxicity of linezolid is hematologic; the effects are reversible and generally mild. Thrombocytopenia is the most common manifestation (seen in approximately 3% of treatment courses), particularly when the drug is administered for longer than 2 weeks.

Anemia and neutropenia may also occur, most commonly in patients with a predisposition to or underlying bone marrow suppression.

Cases of optic and peripheral neuropathy and lactic acidosis have been reported with prolonged courses of linezolid. These side effects are thought to be related to linezolid-induced inhibition of mitochondrial protein synthesis. There are case reports of serotonin syndrome (see Chapter 16) occurring when linezolid is co-administered with serotonergic drugs, most frequently selective serotonin reuptake inhibitor antidepressants. The FDA has issued a warning regarding the use of the drug with serotonergic agents.

Tedizolid is the active moiety of the prodrug tedizolid phosphate, a next-generation oxazolidinone, with high potency against grampositive bacteria, including methicillin-resistant S aureus. It is currently in the late stages of clinical development for the treatment of skin and soft tissue infection and health care-associated pneumonia. Potential advantages over linezolid include increased potency against staphylococci and once-daily dosing.

SUMMARY Tetracyclines, Macrolides, Clindamycin, Chloramphenicol, Streptogramins, & Oxazolidinones

PREPARATIONS AVAILABLE

REFERENCES Chopra I, Roberts M: T etracycline antibiotics: Mode of action, applications, molecular biology, and epidemiology of bacterial resistance. Microbiol Mol Biol Rev 2001;65:232.

De Vriese AS et al: Linezolid-induced inhibition of mitochondrial protein synthesis. Clin Infect Dis 2006;42:1111.

Dryden MS: Linezolid pharmacokinetics and pharmacodynamics in clinical treatment. 2011;66(Suppl 4):S7.

Hancock RE: Mechanisms of action of newer antibiotics for gram-positive pathogens. Lancet Infect Dis 2005;5:209.

Leclerq R: Mechanisms of resistance to macrolides and lincosamides: Nature of the resistance elements and their clinical implications. Clin Infect Dis 2002;34:482.

Lee M et al: Linezolid for treatment of chronic extensively drug-resistant tuberculosis. N Engl J Med 2012;367:1508.

Livermore DM: T igecycline: What is it, and where should it be used? J Antimicrob Chemother 2005;56:611.

Moran GJ et al: Methicillin-resistant S aureus infections among patients in the emergency department. N Engl J Med 2006;355:666.

Prokocimer P et al: T edizolid phosphate vs linezolid for treatment of acute bacterial skin and skin structure infections. JAMA 2013;309:559.

T asina E et al: Efficacy and safety of tigecycline for the treatment of infectious diseases: A meta-analysis. Lancet Infect Dis 2011;11:834.

Wayne RA et al: Azithromycin and risk of cardiovascular death. N Engl J Med 2012;366:1881.

Woytowish MR, Rowe AS: Clinical relevance of linezolid-associated serotonin toxicity. Ann Pharmacother 2013;47:388.

Zuckerman JM: Macrolides and ketolides: Azithromycin, clarithromycin, telithromycin. Infect Dis Clin North Am 2004;18:621.

CASE STUDY ANSWER A tetracycline or a macrolide is effective in the treatment of chlamydial cervicitis. Doxycycline at a dose of 100 mg PO bid for 7 days is the preferred tetracycline, while azithromycin as a single 1 g dose is the preferred macrolide. If the patient is pregnant, then tetracyclines would be contraindicated and she should receive azithromycin, which is safe in pregnancy.

CHAPTER

45 Aminoglycosides & Spectinomycin Daniel H. Deck, PharmD, & Lisa G. Winston, MD*

CASE STUDY A 45-year-old man with no medical history was admitted to the intensive care unit (ICU) 10 days ago after suffering third-degree burns over 40% of his body. He had been relatively stable until the last 24 hours. Now he is febrile (39.5°C [103.1°F]), and his white blood cell count has risen from 8500 to 20,000/mm3 . He has also had an episode of hypotension (86/50 mm Hg) that responded to a fluid bolus. Blood cultures were obtained at the time of his fever and results are pending. The ICU attending physician is concerned about sepsis and decides to treat with empiric combination therapy directed against Pseudomonas. The combination therapy includes tobramycin. The patient weighs 70 kg (154 lb) and has an estimated creatinine clearance of 90 mL/min. How should tobramycin be dosed using once-daily and conventional dosing strategies? How should each regimen be monitored for efficacy and toxicity?

The drugs described in this chapter are bactericidal inhibitors of protein synthesis that interfere with ribosomal function. These agents are useful mainly against aerobic gram-negative microorganisms.

AMINOGLYCOSIDES The aminoglycosides include streptomycin, neomycin, kanamycin, amikacin, gentamicin, tobramycin, sisomicin, netilmicin, and others. They are used most widely in combination with a β-lactam antibiotic in serious infections with gram-negative bacteria, in combination with vancomycin or a β-lactam antibiotic for gram-positive endocarditis, and for treatment of tuberculosis.

General Properties of Aminoglycosides A. Physical and Chemical Properties Aminoglycosides have a hexose ring, either streptidine (in streptomycin) or 2-deoxystreptamine (in other aminoglycosides), to which various amino sugars are attached by glycosidic linkages (Figures 45–1 and 45–2). They are water-soluble, stable in solution, and more active at alkaline than at acid pH.

FIGURE 45â&#x20AC;&#x201C;1 Structure of streptomycin.

FIGURE 45–2 Structures of several important aminoglycoside antibiotics. Ring II is 2-deoxystreptamine. The resemblance between kanamycin and amikacin and between gentamicin, netilmicin, and tobramycin can be seen. The circled numerals on the kanamycin molecule indicate points of attack of plasmid-mediated bacterial transferase enzymes that can inactivate this drug. , , and , acetyltransferase;

, phosphotransferase;

, adenylyltransferase. Amikacin is resistant to modification at

,

,

, and

.

B. Mechanism of Action The mode of action of streptomycin has been studied more closely than that of other aminoglycosides, but all aminoglycosides act similarly. Aminoglycosides are irreversible inhibitors of protein synthesis, but the precise mechanism for bactericidal activity is not known.

The initial event is passive diffusion via porin channels across the outer membrane (see Figure 43–3). Drug is then actively transported across the cell membrane into the cytoplasm by an oxygen-dependent process. The transmembrane electrochemical gradient supplies the energy for this process, and transport is coupled to a proton pump. Low extracellular pH and anaerobic conditions inhibit transport by reducing the gradient. Transport may be enhanced by cell wall-active drugs such as penicillin or vancomycin; this enhancement may be the basis of the synergism of these antibiotics with aminoglycosides.

Inside the cell, aminoglycosides bind to specific 30S-subunit ribosomal proteins. Protein synthesis is inhibited by aminoglycosides in at least three ways (Figure 45–3): (1) interference with the initiation complex of peptide formation; (2) misreading of mRNA, which causes incorporation of incorrect amino acids into the peptide and results in a nonfunctional protein; and (3) breakup of polysomes into nonfunctional monosomes. These activities occur more or less simultaneously, and the overall effect is irreversible and leads to cell death.

FIGURE 45–3 Putative mechanisms of action of the aminoglycosides in bacteria. Normal protein synthesis is shown in the top panel. At least three aminoglycoside effects have been described, as shown in the bottom panel: block of formation of the initiation complex; miscoding of amino acids in the emerging peptide chain due to misreading of the mRNA; and block of translocation on mRNA. Block of movement of the ribosome may occur after the formation of a single initiation complex, resulting in an mRNA chain with only a single ribosome on it, a so-called monosome. (Reproduced, with permission, from Trevor AT, Katzung BG, Masters SB: Pharmacology: Examination & Board Review, 6th ed. McGraw-Hill, 2002. Copyright © The McGraw-Hill Companies, Inc.)

C. Mechanisms of Resistance Three principal mechanisms have been established: (1) production of a transferase enzyme or enzymes inactivates the aminoglycoside by adenylylation, acetylation, or phosphorylation. This is the principal type of resistance encountered clinically. (Specific transferase enzymes are discussed below.) (2) There is impaired entry of aminoglycoside into the cell. This may be genotypic, resulting from mutation or deletion of a porin protein or proteins involved in transport and maintenance of the electrochemical gradient; or phenotypic, eg, resulting from growth conditions under which the oxygen-dependent transport process described above is not functional. (3) The receptor protein on the 30S ribosomal subunit may be deleted or altered as a result of a mutation.

D. Pharmacokinetics and Once-Daily Dosing Aminoglycosides are absorbed very poorly from the intact gastrointestinal tract, and almost the entire oral dose is excreted in feces after oral administration. However, the drugs may be absorbed if ulcerations are present. Aminoglycosides are usually administered intravenously as a 30–60 minute infusion. After intramuscular injection, aminoglycosides are well absorbed, giving peak concentrations in blood within 30–90 minutes. After a brief distribution phase, peak serum concentrations are identical to those following intravenous injection. The normal half-life of aminoglycosides in serum is 2–3 hours, increasing to 24–48 hours in patients with significant impairment of renal function. Aminoglycosides are only partially and irregularly removed by hemodialysis—eg, 40–60% for gentamicin—and even less effectively by peritoneal dialysis. Aminoglycosides are highly polar compounds that do not enter cells readily. They are largely excluded from the central nervous system and the eye. In the presence of active inflammation, however, cerebrospinal fluid levels reach 20% of plasma levels, and in neonatal meningitis, the levels may be higher. Intrathecal or intraventricular injection is required for high levels in cerebrospinal fluid. Even after parenteral administration, concentrations of aminoglycosides are not high in most tissues except the renal cortex. Concentration in most secretions is also modest; in the bile, the level may reach 30% of that in blood. With prolonged therapy, diffusion into pleural or synovial fluid may result in concentrations 50–90% of that of plasma.

Traditionally, aminoglycosides have been administered in two or three equally divided doses per day in patients with normal renal function. However, administration of the entire daily dose in a single injection may be preferred in many clinical situations, for two reasons. Aminoglycosides have concentration-dependent killing; that is, higher concentrations kill a larger proportion of bacteria and at a more rapid rate. They also have a significant postantibiotic effect, such that the antibacterial activity persists beyond the time during which measurable drug is present. The postantibiotic effect of aminoglycosides can last several hours. Because of these properties, a given total amount of aminoglycoside may have better efficacy when administered as a single large dose than when administered as multiple smaller doses.

When administered with a cell wall-active antibiotic (a β lactam or vancomycin), aminoglycosides exhibit synergistic killing against certain bacteria. The effect of the drugs in combination is greater than the anticipated effect of each individual drug, ie, the killing effect of the combination is more than additive. This synergy is important in certain clinical situations, such as endocarditis.

Adverse effects from aminoglycosides are both time- and concentration-dependent. Toxicity is unlikely to occur until a certain threshold concentration is reached, but, once that concentration is achieved, the time beyond this threshold becomes critical. This threshold is not precisely defined, but a trough concentration above 2 mcg/mL is predictive of toxicity. At clinically relevant doses, the total time above this threshold is greater with multiple smaller doses of drug than with a single large dose.

Numerous clinical studies demonstrate that a single daily dose of aminoglycoside is just as effective—and probably less toxic—than multiple smaller doses. Therefore, many authorities now recommend that aminoglycosides be administered as a single daily dose in many clinical situations. However, the efficacy of once-daily aminoglycoside dosing in combination therapy of enterococcal and staphylococcal endocarditis remains to be defined, and the standard lower-dose, thrice-daily administration is still recommended. In contrast, limited data do support once-daily dosing in streptococcal endocarditis. The role of once-daily dosing in pregnancy and in neonates also is not well defined.

Once-daily dosing has potential practical advantages. For example, repeated determinations of serum concentrations are unnecessary unless aminoglycoside is given for more than 3 days. A drug administered once a day rather than three times a day is less labor intensive.

And, once-a-day dosing is more feasible for outpatient therapy.

Aminoglycosides are cleared by the kidney, and excretion is directly proportional to creatinine clearance. To avoid accumulation and toxic levels, once-daily dosing of aminoglycosides is generally avoided if renal function is impaired. Rapidly changing renal function, which may occur with acute kidney injury, must also be monitored to avoid overdosing or underdosing. Provided these pitfalls are avoided, once-daily aminoglycoside dosing is safe and effective. If the creatinine clearance is > 60 mL/min, then a single daily dose of 5– 7 mg/kg of gentamicin or tobramycin is recommended (15 mg/kg for amikacin). For patients with creatinine clearance < 60 mL/min, traditional dosing as described below is recommended. With once-daily dosing, serum concentrations need not be routinely checked until the second or third day of therapy, depending on the stability of renal function and the anticipated duration of therapy. It is unnecessary to check peak concentrations because they will be high. The goal is to administer drug so that concentrations of less than 1 mcg/mL are present between 18 and 24 hours after dosing. This provides a sufficient period of time for washout of drug to occur before the next dose is given. Several nomograms have been developed and validated to assist clinicians with once-daily dosing (eg, Freeman reference).

With traditional dosing, adjustments must be made to prevent accumulation of drug and toxicity in patients with renal insufficiency.

Either the dose of drug is kept constant and the interval between doses is increased, or the interval is kept constant and the dose is

reduced. Nomograms and formulas have been constructed relating serum creatinine levels to adjustments in traditional treatment regimens. Because aminoglycoside clearance is directly proportional to the creatinine clearance, a method for determining the aminoglycoside dose is to estimate creatinine clearance using the Cockcroft-Gault formula described in Chapter 60. For a traditional twice- or thrice-daily dosing regimen, peak serum concentrations should be determined from a blood sample obtained 30–60 minutes after a dose, and trough concentrations from a sample obtained just before the next dose. Doses of gentamicin and tobramycin should be adjusted to maintain peak levels between 5 and 10 mcg/mL and trough levels < 2 mcg/mL (< 1 mcg/mL is optimal).

E. Adverse Effects All aminoglycosides are ototoxic and nephrotoxic. Ototoxicity and nephrotoxicity are more likely to be encountered when therapy is continued for more than 5 days, at higher doses, in the elderly, and in the setting of renal insufficiency. Concurrent use with loop diuretics (eg, furosemide, ethacrynic acid) or other nephrotoxic antimicrobial agents (eg, vancomycin or amphotericin) can potentiate nephrotoxicity and should be avoided if possible. Ototoxicity can manifest either as auditory damage, resulting in tinnitus and highfrequency hearing loss initially, or as vestibular damage with vertigo, ataxia, and loss of balance. Nephrotoxicity results in rising serum creatinine levels or reduced creatinine clearance, although the earliest indication often is an increase in trough serum aminoglycoside concentrations. Neomycin, kanamycin, and amikacin are the most ototoxic agents. Streptomycin and gentamicin are the most vestibulotoxic. Neomycin, tobramycin, and gentamicin are the most nephrotoxic.

In very high doses, aminoglycosides can produce a curare-like effect with neuromuscular blockade that results in respiratory paralysis. This paralysis is usually reversible by calcium gluconate (given promptly) or neostigmine. Hypersensitivity occurs infrequently.

F. Clinical Uses Aminoglycosides are mostly used against aerobic gram-negative bacteria, especially when the isolate may be drug-resistant and when there is suspicion of sepsis. They are almost always used in combination with a β-lactam antibiotic to extend coverage to include potential gram-positive pathogens and to take advantage of the synergism between these two classes of drugs. Penicillin-aminoglycoside combinations also are used to achieve bactericidal activity in treatment of enterococcal endocarditis and to shorten duration of therapy for viridans streptococcal and some cases of staphylococcal endocarditis. Which aminoglycoside and what dose should be used depend on the infection being treated and the susceptibility of the isolate.

STREPTOMYCIN Streptomycin (Figure 45–1) was isolated from a strain of Streptomyces griseus. The antimicrobial activity of streptomycin is typical of that of other aminoglycosides, as are the mechanisms of resistance. Resistance has emerged in most species, restricting the current usefulness of streptomycin, with the exceptions listed below. Ribosomal resistance to streptomycin develops readily, limiting its role as a single agent.

Clinical Uses A. Mycobacterial Infections Streptomycin is mainly used as a second-line agent for treatment of tuberculosis. The dosage is 0.5–1 g/d (7.5–15 mg/kg/d for children), which is given intramuscularly or intravenously. It should be used only in combination with other agents to prevent emergence of resistance. See Chapter 47 for additional information regarding the use of streptomycin in mycobacterial infections.

B. Nontuberculous Infections In plague, tularemia, and sometimes brucellosis, streptomycin, 1 g/d (15 mg/kg/d for children), is given intramuscularly in combination with an oral tetracycline.

Penicillin plus streptomycin is effective for enterococcal endocarditis and 2-week therapy of viridans streptococcal endocarditis.

Gentamicin has largely replaced streptomycin for these indications. Streptomycin remains a useful agent for treating enterococcal infections as some isolates that are resistant to gentamicin (and therefore resistant to netilmicin, tobramycin, and amikacin) will remain susceptible to streptomycin.

Adverse Reactions Fever, skin rashes, and other allergic manifestations may result from hypersensitivity to streptomycin. This occurs most frequently with a prolonged course of treatment (eg, for tuberculosis).

Pain at the injection site is common but usually not severe. The most serious toxic effect with streptomycin is disturbance of vestibular function—vertigo and loss of balance. The frequency and severity of this disturbance are in proportion to the age of the

patient, the blood levels of the drug, and the duration of administration. Vestibular dysfunction may follow a few weeks of unusually high blood levels (eg, in individuals with impaired renal function) or months of relatively low blood levels. Vestibular toxicity tends to be irreversible. Streptomycin given during pregnancy can cause deafness in the newborn and, therefore, is relatively contraindicated.

GENTAMICIN Gentamicin is a mixture of three closely related constituents, C1 , C1A, and C2 (Figure 45–2) isolated from Micromonospora purpurea .

It is effective against both gram-positive and gram-negative organisms, and many of its properties resemble those of other aminoglycosides.

Antimicrobial Activity Gentamicin sulfate, 2–10 mcg/mL, inhibits in vitro many strains of staphylococci and coliforms and other gram-negative bacteria. It is active alone, but also as a synergistic companion with β-lactam antibiotics, against gram-negative rods that may be resistant to multiple other antibiotics. Like all aminoglycosides, it has no activity against anaerobes.

Resistance Streptococci and enterococci are relatively resistant to gentamicin owing to failure of the drug to penetrate into the cell. However, gentamicin in combination with vancomycin or a penicillin produces a potent bactericidal effect, which in part is due to enhanced uptake of drug that occurs with inhibition of cell wall synthesis. Resistance to gentamicin rapidly emerges in staphylococci during monotherapy owing to selection of permeability mutants. Ribosomal resistance is rare. Among gram-negative bacteria, resistance is most commonly due to plasmid-encoded aminoglycoside-modifying enzymes. Gram-negative bacteria that are gentamicin-resistant usually are susceptible to amikacin, which is much more resistant to modifying enzyme activity. The enterococcal enzyme that modifies gentamicin is a bifunctional enzyme that also inactivates amikacin, netilmicin, and tobramycin, but not streptomycin; the latter is modified by a different enzyme. This is why some gentamicin-resistant enterococci are susceptible to streptomycin.

Clinical Uses A. Intramuscular or Intravenous Administration Gentamicin is used mainly in severe infections caused by gram-negative bacteria that are likely to be resistant to other drugs, especially P aeruginosa, Enterobacter sp, Serratia marcescens, Proteus sp, Acinetobacter sp, and Klebsiella sp. It usually is used in combination with a second agent because an aminoglycoside alone may not be effective for infections outside the urinary tract. For example, gentamicin should not be used as a single agent to treat staphylococcal infections because resistance develops rapidly. Aminoglycosides also should not be used for single-agent therapy of pneumonia because penetration of infected lung tissue is poor and local conditions of low pH and low oxygen tension contribute to poor activity. Gentamicin 5–6 mg/kg/d traditionally is given intravenously in three equal doses, but once-daily administration is just as effective for some organisms and less toxic (see above).

Gentamicin, in combination with a cell wall-active antibiotic, is also indicated in the treatment of endocarditis caused by gram-positive bacteria (streptococci, staphylococci, and enterococci). The synergistic killing achieved by combination therapy may achieve bactericidal activity necessary for cure or allow for the shortening of the duration of therapy. The doses of gentamicin used for synergy against gram-positive bacteria are lower than traditional doses. Typically the drug is administered at a dose of 3 mg/kg/d in three divided doses.

Peak levels should be approximately 3 mcg/mL, while trough levels should be < 1 mcg/mL. There are limited data to support administering the 3 mg/kg dose as a single daily injection in the treatment of streptococcal endocarditis.

B. Topical and Ocular Administration Creams, ointments, and solutions containing 0.1–0.3% gentamicin sulfate have been used for the treatment of infected burns, wounds, or skin lesions and in attempts to prevent intravenous catheter infections. The effectiveness of topical preparations for these indications is unclear. Topical gentamicin is partly inactivated by purulent exudates. Ten mg can be injected subconjunctivally for treatment of ocular infections.

C. Intrathecal Administration Meningitis caused by gram-negative bacteria has been treated by the intrathecal injection of gentamicin sulfate, 1–10 mg/d. However, neither intrathecal nor intraventricular gentamicin was beneficial in neonates with meningitis, and intraventricular gentamicin was toxic, raising questions about the usefulness of this form of therapy. Moreover, the availability of third-generation cephalosporins for gramnegative meningitis has rendered this therapy obsolete in most cases.

Adverse Reactions Nephrotoxicity is usually reversible. It occurs in 5–25% of patients receiving gentamicin for longer than 3–5 days. Such toxicity requires, at the very least, adjustment of the dosing regimen and should prompt reconsideration of the need for the drug, particularly if there is a less toxic alternative agent. Measurement of gentamicin serum levels is essential. Ototoxicity, which tends to be irreversible, manifests itself mainly as vestibular dysfunction. Loss of hearing can also occur. Ototoxicity is in part genetically determined, having been linked to point mutations in mitochondrial DNA, and occurs in 1–5% for patients receiving gentamicin for more than 5 days. Hypersensitivity reactions to gentamicin are uncommon.

TOBRAMYCIN This aminoglycoside (Figure 45–2) has an antibacterial spectrum similar to that of gentamicin. Although there is some cross-resistance between gentamicin and tobramycin, it is unpredictable in individual strains. Separate laboratory susceptibility tests are therefore necessary.

The pharmacokinetic properties of tobramycin are virtually identical with those of gentamicin. The daily dose of tobramycin is 5–6 mg/kg intramuscularly or intravenously, traditionally divided into three equal amounts and given every 8 hours. Monitoring blood levels in renal insufficiency is an essential guide to proper dosing.

Tobramycin has almost the same antibacterial spectrum as gentamicin with a few exceptions. Gentamicin is slightly more active against S marcescens, whereas tobramycin is slightly more active against P aeruginosa; Enterococcus faecalis is susceptible to both gentamicin and tobramycin, but E faecium is resistant to tobramycin. Gentamicin and tobramycin are otherwise interchangeable clinically.

Like other aminoglycosides, tobramycin is ototoxic and nephrotoxic. Nephrotoxicity of tobramycin may be slightly less than that of gentamicin.

Tobramycin is also formulated in solution (300 mg in 5 mL) for inhalation for treatment of P aeruginosa lower respiratory tract infections complicating cystic fibrosis. The drug is recommended as a 300 mg dose regardless of the patient’s age or weight for administration twice daily in repeated cycles of 28 days on therapy, followed by 28 days off therapy. Serum concentrations 1 hour after inhalation average 1 mcg/mL; consequently, nephrotoxicity and ototoxicity rarely occur. Caution should be used when administering tobramycin to patients with preexisting renal, vestibular, or hearing disorders.

AMIKACIN Amikacin is a semisynthetic derivative of kanamycin; it is less toxic than the parent molecule (Figure 45–2). It is resistant to many enzymes that inactivate gentamicin and tobramycin, and therefore can be used against some microorganisms resistant to the latter drugs.

Many gram-negative bacteria, including many strains of Proteus, Pseudomonas, Enterobacter, and Serratia, are inhibited by 1–20 mcg/mL amikacin in vitro. After injection of 500 mg of amikacin every 12 hours (15 mg/kg/d) intramuscularly, peak levels in serum are 10–30 mcg/mL. Strains of multidrug-resistant Mycobacterium tuberculosis, including streptomycin-resistant strains, are usually susceptible to amikacin. Kanamycin-resistant strains may be cross-resistant to amikacin. The dosage of amikacin for tuberculosis is 7.5–15 mg/kg/d as a once-daily or two to three times weekly injection and always in combination with other drugs to which the isolate is susceptible.

Like all aminoglycosides, amikacin is nephrotoxic and ototoxic (particularly for the auditory portion of the eighth nerve). Serum concentrations should be monitored. Target peak serum concentrations for an every-12-hours dosing regimen are 20–40 mcg/mL, and troughs should be maintained between 4 and 8 mcg/mL.

NETILMICIN Netilmicin shares many characteristics with gentamicin and tobramycin. However, the addition of an ethyl group to the 1-amino position of the 2-deoxystreptamine ring (ring II, Figure 45–2) sterically protects the netilmicin molecule from enzymatic degradation at the 3amino (ring II) and 2-hydroxyl (ring III) positions. Consequently, netilmicin may be active against some gentamicin-resistant and tobramycin-resistant bacteria.

The dosage (5–7 mg/kg/d) and the routes of administration are the same as for gentamicin. Netilmicin is largely interchangeable with gentamicin or tobramycin but is no longer available in the United States.

NEOMYCIN & KANAMYCIN Neomycin and kanamycin are closely related. Paromomycin is another member of this group. All have similar properties.

Antimicrobial Activity & Resistance Drugs of the neomycin group are active against gram-positive and gram-negative bacteria and some mycobacteria. P aeruginosa and streptococci are generally resistant. Mechanisms of antibacterial action and resistance are the same as with other aminoglycosides. The widespread use of these drugs in bowel preparation for elective surgery has resulted in the selection of resistant organisms and some outbreaks of enterocolitis in hospitals. Cross-resistance between kanamycin and neomycin is complete.

Pharmacokinetics Drugs of the neomycin group are poorly absorbed from the gastrointestinal tract. After oral administration, the intestinal flora is suppressed or modified, and the drug is excreted in the feces. Excretion of any absorbed drug is mainly through glomerular filtration into the urine.

Clinical Uses Neomycin and kanamycin are now limited to topical and oral use. Neomycin is too toxic for parenteral use. With the advent of more potent and less toxic aminoglycosides, parenteral administration of kanamycin has also been largely abandoned. Paromomycin has recently been shown to be effective against visceral leishmaniasis when given parenterally (see Chapter 52), and this serious infection may represent an important new use for this drug. Paromomycin can be used for intestinal Entamoeba histolytica infection and is sometimes used for intestinal infections with other parasites.

A. Topical Administration Solutions containing 1–5 mg/mL are used on infected surfaces or injected into joints, the pleural cavity, tissue spaces, or abscess cavities where infection is present. The total amount of drug given in this fashion must be limited to 15 mg/kg/d because at higher doses enough drug may be absorbed to produce systemic toxicity. Whether topical application for active infection adds anything to appropriate systemic therapy is questionable. Ointments, often formulated as a neomycin-polymyxin-bacitracin combination, can be applied to infected skin lesions or in the nares for suppression of staphylococci but they are largely ineffective.

B. Oral Administration In preparation for elective bowel surgery, 1 g of neomycin is given orally every 6–8 hours for 1–2 days, often combined with 1 g of erythromycin base. This reduces the aerobic bowel flora with little effect on anaerobes. In hepatic encephalopathy, coliform flora can be suppressed by giving 1 g every 6–8 hours together with reduced protein intake, thus reducing ammonia production. Use of neomycin for hepatic encephalopathy has been largely supplanted by lactulose and other medications that are less toxic. Use of paromomycin in the treatment of protozoal infections is discussed in Chapter 52.

Adverse Reactions All members of the neomycin group have significant nephrotoxicity and ototoxicity. Auditory function is affected more than vestibular function. Deafness has occurred, especially in adults with impaired renal function and prolonged elevation of drug levels.

The sudden absorption of postoperatively instilled kanamycin from the peritoneal cavity (3–5 g) has resulted in curare-like neuromuscular blockade and respiratory arrest. Calcium gluconate and neostigmine can act as antidotes.

Although hypersensitivity is not common, prolonged application of neomycin-containing ointments to skin and eyes has resulted in severe allergic reactions.

SPECTINOMYCIN Spectinomycin is an aminocyclitol antibiotic that is structurally related to aminoglycosides. It lacks amino sugars and glycosidic bonds.

Spectinomycin is active in vitro against many gram-positive and gram-negative organisms, but it is used almost solely as an alternative treatment for drug-resistant gonorrhea or gonorrhea in penicillin-allergic patients. The majority of gonococcal isolates are inhibited by 6 mcg/mL of spectinomycin. Strains of gonococci may be resistant to spectinomycin, but there is no cross-resistance with other drugs used in gonorrhea. Spectinomycin is rapidly absorbed after intramuscular injection. A single dose of 40 mg/kg up to a maximum of 2 g is given.

There is pain at the injection site and, occasionally, fever and nausea. Nephrotoxicity and anemia have been observed rarely.

Spectinomycin is no longer available for use in the United States but may be available elsewhere.

SUMMARY Aminoglycosides

PREPARATIONS AVAILABLE

REFERENCES Busse H-J, Wöstmann C, Bakker EP: T he bactericidal action of streptomycin: Membrane permeabilization caused by the insertion of mistranslated proteins into the cytoplasmic membrane of Escherichia coli and subsequent caging of the antibiotic inside the cells due to degradation of these proteins. J Gen Microbiol 1992;138:551.

Cheer SM, Waugh J, Noble S: Inhaled tobramycin (T OBI): A review of its use in the management of Pseudomonas aeruginosa infections in patients with cystic fibrosis.

Drugs 2003;63:2501.

Freeman CD et al: Once-daily dosing of aminoglycosides: Review and recommendations for clinical practice. J Antimicrob Chemother 1997;39:677.

Jackson J et al: Aminogylcosides: How should we use them in the 21st century. Curr Opin Infect Dis 2013;26:516.

Le T , Bayer AS: Combination antibiotic therapy for infective endocarditis. Clin Infect Dis 2003;36:615.

Olsen KM et al: Effect of once-daily dosing vs. multiple daily dosing of tobramycin on enzyme markers of nephrotoxicity. Crit Care Med 2004;32:1678.

Paul M et al: Beta-lactam monotherapy versus beta-lactam-aminoglycoside combination therapy in cancer patients with neutropenia. Cochrane Database Syst Rev 2013 Jun 29;6:CD003038.

Pena C et al: Effect of adequate single-drug versus combination antimicrobial therapy on mortality in Pseudomonas aeruginosa bloodstream infections. Clin Infect Dis 2013;57:208.

Poole K: Aminoglycoside resistance in Pseudomonas aeruginosa. Antimicrob Agents Chemother 2005;49:479.

CASE STUDY ANSWER The patient has normal renal function and thus qualifies for once-daily dosing. Tobramycin could be administered as a single oncedaily injection at a dose of 350–490 mg (5–7 mg/kg). A serum level between 1.5 and 6 mcg/mL measured 8 hours after infusion correlates with an appropriate trough level. Alternatively, the same total daily dose could be divided and administered every 8 hours, as a conventional dosing strategy. With conventional dosing, peak and trough concentrations should be monitored with the target peak concentration of 5–10 mcg/mL and the target trough concentration of < 2 mcg/mL.

_______________ * T he authors thank Dr. Henry F. Chambers for his contributions to previous editions.

CHAPTER

46 Sulfonamides, Trimethoprim, & Quinolones Daniel H. Deck, PharmD, & Lisa G. Winston, MD*

CASE STUDY A 59-year-old woman presents to an urgent care clinic with a 4-day history of frequent and painful urination. She has had fevers, chills, and flank pain for the past 2 days. Her physician advised her to come immediately to the clinic for evaluation. In the clinic she is febrile (38.5°C [101.3°F]) but otherwise stable and states she is not experiencing any nausea or vomiting. Her urine dipstick test is positive for leukocyte esterase. Urinalysis and urine culture are ordered. Her past medical history is significant for three urinary tract infections in the past year. Each episode was uncomplicated, treated with trimethoprim-sulfamethoxazole, and promptly resolved. She also has osteoporosis for which she takes a daily calcium supplement. The decision is made to treat her with oral antibiotics for a complicated urinary tract infection with close follow-up. Given her history, what would be a reasonable empiric antibiotic choice? Depending on the antibiotic choice are there potential drug interactions?

ANTIFOLATE DRUGS SULFONAMIDES Chemistry The basic formulas of the sulfonamides and their structural similarity to p-aminobenzoic acid (PABA) are shown in Figure 46–1.

Sulfonamides with varying physical, chemical, pharmacologic, and antibacterial properties are produced by attaching substituents to the amido group (–SO2 –NH–R) or the amino group (–NH2 ) of the sulfanilamide nucleus. Sulfonamides tend to be much more soluble at alkaline than at acid pH. Most can be prepared as sodium salts, which are used for intravenous administration.

FIGURE 46â&#x20AC;&#x201C;1 Structures of some sulfonamides and p-aminobenzoic acid.

Mechanism of Action & Antimicrobial Activity Sulfonamide-susceptible organisms, unlike mammals, cannot use exogenous folate but must synthesize it from PABA. This pathway (Figure 46â&#x20AC;&#x201C;2) is thus essential for production of purines and nucleic acid synthesis. As structural analogs of PABA, sulfonamides inhibit dihydropteroate synthase and folate production. Sulfonamides inhibit both gram-positive and gram-negative bacteria, Nocardia sp, Chlamydia trachomatis, and some protozoa. Some enteric bacteria, such as Escherichia coli, Klebsiella pneumoniae, Salmonella, Shigella, and Enterobacter sp are also inhibited. It is interesting that rickettsiae are not inhibited by sulfonamides but are instead stimulated in their growth. Activity is poor against anaerobes. Pseudomonas aeruginosa is intrinsically resistant to sulfonamide antibiotics.

FIGURE 46–2 Actions of sulfonamides and trimethoprim.

Combination of a sulfonamide with an inhibitor of dihydrofolate reductase (trimethoprim or pyrimethamine) provides synergistic activity because of sequential inhibition of folate synthesis (Figure 46–2).

Resistance Some bacteria lack the enzymes required for folate synthesis from PABA and, like mammals, depend on exogenous sources of folate; therefore, they are not susceptible to sulfonamides. Sulfonamide resistance may also occur as a result of mutations that (1) cause overproduction of PABA, (2) cause production of a folic acid-synthesizing enzyme that has low affinity for sulfonamides, or (3) impair permeability to the sulfonamide. Dihydropteroate synthase with low sulfonamide affinity is often encoded on a plasmid that is transmissible and can disseminate rapidly and widely. Sulfonamide-resistant dihydropteroate synthase mutants also can emerge under selective pressure.

Pharmacokinetics Sulfonamides can be divided into three major groups: (1) oral, absorbable; (2) oral, nonabsorbable; and (3) topical. The oral, absorbable sulfonamides can be classified as short-, intermediate-, or long-acting on the basis of their half-lives (Table 46–1). They are absorbed from the stomach and small intestine and distributed widely to tissues and body fluids (including the central nervous system and cerebrospinal fluid), placenta, and fetus. Protein binding varies from 20% to over 90%. Therapeutic concentrations are in the range of 40–100 mcg/mL of blood. Blood levels generally peak 2–6 hours after oral administration.

TABLE 46–1 Pharmacokinetic properties of some sulfonamides and pyrimidines.

A portion of absorbed drug is acetylated or glucuronidated in the liver. Sulfonamides and inactive metabolites are then excreted into the urine, mainly by glomerular filtration. In significant renal failure, the dosage of sulfonamides must be reduced.

Clinical Uses Sulfonamides are infrequently used as single agents. Many strains of formerly susceptible species, including meningococci, pneumococci, streptococci, staphylococci, and gonococci, are now resistant. The fixed-drug combination of trimethoprim-sulfamethoxazole is the drug of choice for infections such as Pneumocystis jiroveci (formerly P carinii) pneumonia, toxoplasmosis, nocardiosis, and occasionally other bacterial infections.

A. Oral Absorbable Agents Sulfisoxazole and sulfamethoxazole are short- to medium-acting agents used almost exclusively to treat urinary tract infections. The usual adult dosage is 1 g of sulfisoxazole four times daily or 1 g of sulfamethoxazole two or three times daily.

Sulfadiazine in combination with pyrimethamine is first-line therapy for treatment of acute toxoplasmosis. The combination of sulfadiazine with pyrimethamine, a potent inhibitor of dihydrofolate reductase, is synergistic because these drugs block sequential steps in the folate synthesis pathway (Figure 46â&#x20AC;&#x201C;2). The dosage of sulfadiazine is 1 g four times daily, with pyrimethamine given as a 75 mg loading dose followed by a 25 mg once-daily dose. Folinic acid, 10 mg orally each day, should also be administered to minimize bone marrow suppression.

Sulfadoxine is a long-acting sulfonamide that is coformulated with pyrimethamine (Fansidar). This combination is no longer

commercially available in the USA but may be found in other parts of the world where it is used as a second-line treatment of malaria (see Chapter 52).

B. Oral Nonabsorbable Agents Sulfasalazine (salicylazosulfapyridine) is widely used in ulcerative colitis, enteritis, and other inflammatory bowel disease (see Chapter 62).

C. Topical Agents Sodium sulfacetamide ophthalmic solution or ointment is effective in the treatment of bacterial conjunctivitis and as adjunctive therapy for trachoma. Another sulfonamide, mafenide acetate, is used topically but can be absorbed from burn sites. The drug and its primary metabolite inhibit carbonic anhydrase and can cause metabolic acidosis, a side effect that limits its usefulness. Silver sulfadiazine is a less toxic topical sulfonamide and is preferred to mafenide for prevention of infection of burn wounds.

Adverse Reactions Historically, drugs containing a sulfonamide moiety, including antimicrobial sulfas, diuretics, diazoxide, and the sulfonylurea hypoglycemic agents, were considered to be cross-allergenic. However, recent evidence suggests cross-reactivity is uncommon and patients who are allergic to nonantibiotic sulfonamides may safely receive sulfonamide antibiotics. The most common adverse effects are fever, skin rashes, exfoliative dermatitis, photosensitivity, urticaria, nausea, vomiting, diarrhea, and difficulties referable to the urinary tract (see below). Stevens-Johnson syndrome, although relatively uncommon (< 1% of treatment courses), is a particularly serious and potentially fatal type of skin and mucous membrane eruption associated with sulfonamide use. Other unwanted effects include stomatitis, conjunctivitis, arthritis, hematopoietic disturbances (see below), hepatitis, and, rarely, polyarteritis nodosa and psychosis.

A. Urinary Tract Disturbances Sulfonamides may precipitate in urine, especially at neutral or acid pH, producing crystalluria, hematuria, or even obstruction. This is rarely a problem with the more soluble sulfonamides (eg, sulfisoxazole). Sulfadiazine when given in large doses, particularly if fluid intake is poor, can cause crystalluria. Crystalluria is treated by administration of sodium bicarbonate to alkalinize the urine and fluids to increase urine flow. Sulfonamides have also been implicated in various types of nephrosis and in allergic nephritis.

B. Hematopoietic Disturbances Sulfonamides can cause hemolytic or aplastic anemia, granulocytopenia, thrombocytopenia, or leukemoid reactions. Sulfonamides may provoke hemolytic reactions in patients with glucose-6-phosphate dehydrogenase deficiency. Sulfonamides taken near the end of pregnancy increase the risk of kernicterus in newborns.

TRIMETHOPRIM & TRIMETHOPRIM-SULFAMETHOXAZOLE MIXTURES Mechanism of Action Trimethoprim, a trimethoxybenzylpyrimidine, selectively inhibits bacterial dihydrofolic acid reductase, which converts dihydrofolic acid to tetrahydrofolic acid, a step leading to the synthesis of purines and ultimately to DNA (Figure 46â&#x20AC;&#x201C;2). Trimethoprim is a much less efficient inhibitor of mammalian dihydrofolic acid reductase. Pyrimethamine, another benzylpyrimidine, selectively inhibits dihydrofolic acid reductase of protozoa compared with that of mammalian cells. As noted above, trimethoprim or pyrimethamine in combination with a sulfonamide blocks sequential steps in folate synthesis, resulting in marked enhancement (synergism) of the activity of both drugs. The combination often is bactericidal, compared with the bacteriostatic activity of a sulfonamide alone.

Resistance Resistance to trimethoprim can result from reduced cell permeability, overproduction of dihydrofolate reductase, or production of an altered reductase with reduced drug binding. Resistance can emerge by mutation, although more commonly it is due to plasmid-encoded trimethoprim-resistant dihydrofolate reductases. These resistant enzymes may be coded within transposons on conjugative plasmids that exhibit a broad host range, accounting for rapid and widespread dissemination of trimethoprim resistance among numerous bacterial species.

Pharmacokinetics Trimethoprim is usually given orally, alone or in combination with sulfamethoxazole, which has a similar half-life. Trimethoprimsulfamethoxazole can also be given intravenously. Trimethoprim is well absorbed from the gut and distributed widely in body fluids and tissues, including cerebrospinal fluid.

Because trimethoprim is more lipid-soluble than sulfamethoxazole, it has a larger volume of distribution than the latter drug.

Therefore, when 1 part of trimethoprim is given with 5 parts of sulfamethoxazole (the ratio in the formulation), the peak plasma concentrations are in the ratio of 1:20, which is optimal for the combined effects of these drugs in vitro. About 30–50% of the sulfonamide and 50–60% of the trimethoprim (or their respective metabolites) are excreted in the urine within 24 hours. The dose should be reduced by half for patients with creatinine clearances of 15–30 mL/min.

Trimethoprim (a weak base) concentrates in prostatic fluid and in vaginal fluid, which are more acidic than plasma. Therefore, it has more antibacterial activity in prostatic and vaginal fluids than many other antimicrobial drugs.

Clinical Uses A. Oral Trimethoprim Trimethoprim can be given alone (100 mg twice daily) in acute urinary tract infections. Many community-acquired organisms are susceptible to the high concentrations that are found in the urine (200–600 mcg/mL).

B. Oral Trimethoprim-Sulfamethoxazole (TMP-SMZ) A combination of trimethoprim-sulfamethoxazole is effective treatment for a wide variety of infections including P jiroveci pneumonia, shigellosis, systemic salmonella infections, urinary tract infections, prostatitis, and some nontuberculous mycobacterial infections. It is active against most Staphylococcus aureus strains, both methicillin-susceptible and methicillin-resistant, and against respiratory tract pathogens such as pneumococcus, Haemophilus sp, Moraxella catarrhalis, and K pneumoniae (but not Mycoplasma pneumoniae).

However, the increasing prevalence of strains of E coli (up to 30% or more) and pneumococci that are resistant to trimethoprimsulfamethoxazole must be considered before using this combination for empiric therapy of upper urinary tract infections or pneumonia.

One double-strength tablet (each tablet contains trimethoprim 160 mg plus sulfamethoxazole 800 mg) given every 12 hours is effective treatment for urinary tract infections and prostatitis. One half of the regular (single-strength) tablet given three times weekly may serve as prophylaxis in recurrent urinary tract infections of some women. One double-strength tablet every 12 hours is effective treatment for infections caused by susceptible strains of shigella and salmonella. The dosage for children treated for shigellosis, urinary tract infection, or otitis media is 8 mg/kg trimethoprim and 40 mg/kg sulfamethoxazole every 12 hours.

Infections with P jiroveci and some other pathogens can be treated orally with high doses of the combination (dosed on the basis of the trimethoprim component at 15–20 mg/kg/d) or can be prevented in immunosuppressed patients by one double-strength tablet daily or three times weekly.

C. Intravenous Trimethoprim-Sulfamethoxazole A solution of the mixture containing 80 mg trimethoprim plus 400 mg sulfamethoxazole per 5 mL diluted in 125 mL of 5% dextrose in water can be administered by intravenous infusion over 60–90 minutes. It is the agent of choice for moderately severe to severe pneumocystis pneumonia. It may be used for gram-negative bacterial sepsis, including that caused by some multidrug-resistant species such as Enterobacter and Serratia; shigellosis; typhoid fever; or urinary tract infection caused by a susceptible organism when the patient is unable to take the drug by mouth. The dosage is 10–20 mg/kg/d of the trimethoprim component.

D. Oral Pyrimethamine with Sulfonamide Pyrimethamine and sulfadiazine are used in the treatment of toxoplasmosis. In falciparum malaria, the combination of pyrimethamine with sulfadoxine (Fansidar) has been used (see Chapter 52).

Adverse Effects Trimethoprim produces the predictable adverse effects of an antifolate drug, especially megaloblastic anemia, leukopenia, and granulocytopenia. The combination trimethoprim-sulfamethoxazole may cause all of the untoward reactions associated with sulfonamides. Nausea and vomiting, drug fever, vasculitis, renal damage, and central nervous system disturbances occasionally occur also. Patients with AIDS and pneumocystis pneumonia have a particularly high frequency of untoward reactions to trimethoprimsulfamethoxazole, especially fever, rashes, leukopenia, diarrhea, elevations of hepatic aminotransferases, hyperkalemia, and hyponatremia.

DNA GYRASE INHIBITORS FLUOROQUINOLONES The important quinolones are synthetic fluorinated analogs of nalidixic acid (Figure 46–3). They are active against a variety of grampositive and gram-negative bacteria.

Mechanism of Action Quinolones block bacterial DNA synthesis by inhibiting bacterial topoisomerase II (DNA gyrase) and topoisomerase IV. Inhibition of DNA gyrase prevents the relaxation of positively supercoiled DNA that is required for normal transcription and replication. Inhibition of topoisomerase IV interferes with separation of replicated chromosomal DNA into the respective daughter cells during cell division.

Antibacterial Activity Earlier quinolones such as nalidixic acid did not achieve systemic antibacterial levels and were useful only in the treatment of lower urinary tract infections. Fluorinated derivatives (ciprofloxacin, levofloxacin, and others; Figure 46–3 and Table 46–2) have greatly improved antibacterial activity compared with nalidixic acid and achieve bactericidal levels in blood and tissues.

FIGURE 46–3 Structures of nalidixic acid and some fluoroquinolones.

TABLE 46–2 Pharmacokinetic properties of some fluoroquinolones.

Fluoroquinolones were originally developed because of their excellent activity against gram-negative aerobic bacteria; they had limited activity against gram-positive organisms. Several newer agents have improved activity against gram-positive cocci. This relative activity against gram-negative versus gram-positive species is useful for classification of these agents. Norfloxacin is the least active of the fluoroquinolones against both gram-negative and gram-positive organisms, with minimum inhibitory concentrations (MICs) fourfold to eightfold higher than those of ciprofloxacin. Ciprofloxacin, enoxacin, lomefloxacin, levofloxacin, ofloxacin, and pefloxacin comprise a second group of similar agents possessing excellent gram-negative activity and moderate to good activity against gram-positive bacteria.

MICs for gram-negative cocci and bacilli, including Enterobacter sp, P aeruginosa, Neisseria meningitidis, Haemophilus sp, and Campylobacter jejuni, are 1â&#x20AC;&#x201C;2 mcg/mL and often less. Methicillin-susceptible strains of S aureus are generally susceptible to these fluoroquinolones, but methicillin-resistant strains of staphylococci are often resistant. Streptococci and enterococci tend to be less susceptible than staphylococci, and efficacy in infections caused by these organisms is limited. Ciprofloxacin is the most active agent of this group against gram-negative organisms, P aeruginosa in particular. Levofloxacin, the L-isomer of ofloxacin, has superior activity against gram-positive organisms, including Streptococcus pneumoniae.

Gatifloxacin, gemifloxacin, and moxifloxacin make up a third group of fluoroquinolones with improved activity against gram-positive organisms, particularly S pneumoniae and some staphylococci. Gemifloxacin is active in vitro against ciprofloxacin-resistant strains of S pneumoniae, but in vivo efficacy is unproven. Although MICs of these agents for staphylococci are lower than those of ciprofloxacin (and the other compounds mentioned in the paragraph above), it is not known whether the enhanced activity is sufficient to permit use of these agents for treatment of infections caused by ciprofloxacin-resistant strains. In general, none of these agents is as active as ciprofloxacin against gram-negative organisms. Fluoroquinolones also are active against agents of atypical pneumonia (eg, mycoplasmas and chlamydiae) and against intracellular pathogens such as Legionella and some mycobacteria, including Mycobacterium tuberculosis and Mycobacterium avium complex. Moxifloxacin also has modest activity against anaerobic bacteria. Because of toxicity, gatifloxacin is no longer available in the United States.

Resistance During fluoroquinolone therapy, resistant organisms emerge in about one of every 10 7 â&#x20AC;&#x201C;109 organisms, especially among staphylococci, P aeruginosa, and Serratia marcescens. Resistance is due to one or more point mutations in the quinolone binding region of the target enzyme or to a change in the permeability of the organism. However, this does not account for the relative ease with which resistance develops in exquisitely susceptible bacteria. More recently two types of plasmid-mediated resistance have been described. The first type utilizes Qnr proteins, which protect DNA gyrase from the fluoroquinolones. The second is a variant of an aminoglycoside acetyltransferase capable of modifying ciprofloxacin. Both mechanisms confer low-level resistance that may facilitate the point mutations that confer high-level resistance. Resistance to one fluoroquinolone, particularly if it is of high level, generally confers cross-resistance to all other members of this class.

Pharmacokinetics After oral administration, the fluoroquinolones are well absorbed (bioavailability of 80–95%) and distributed widely in body fluids and tissues (Table 46–2). Serum half-lives range from 3 to 10 hours. The relatively long half-lives of levofloxacin, gemifloxacin, gatifloxacin, and moxifloxacin permit once-daily dosing. Oral absorption is impaired by divalent and trivalent cations, including those in antacids.

Therefore, oral fluoroquinolones should be taken 2 hours before or 4 hours after any products containing these cations. Serum concentrations of intravenously administered drug are similar to those of orally administered drug. Most fluoroquinolones are eliminated by renal mechanisms, either tubular secretion or glomerular filtration (Table 46–2). Dosage adjustment is required for patients with creatinine clearances less than 50 mL/min, the exact adjustment depending on the degree of renal impairment and the specific fluoroquinolone being used. Dosage adjustment for renal failure is not necessary for moxifloxacin. Nonrenally cleared fluoroquinolones are relatively contraindicated in patients with hepatic failure.

Clinical Uses Fluoroquinolones (other than moxifloxacin, which achieves relatively low urinary levels) are effective in urinary tract infections caused by many organisms, including P aeruginosa. These agents are also effective for bacterial diarrhea caused by Shigella, Salmonella, toxigenic E coli, and Campylobacter. Fluoroquinolones (except norfloxacin, which does not achieve adequate systemic concentrations) have been used in infections of soft tissues, bones, and joints and in intra-abdominal and respiratory tract infections, including those caused by multidrug-resistant organisms such as Pseudomonas and Enterobacter. Ciprofloxacin is a drug of choice for prophylaxis and treatment of anthrax, although the newer fluoroquinolones are active in vitro and very likely in vivo as well.

Ciprofloxacin and levofloxacin are no longer recommended for the treatment of gonococcal infection in the United States as resistance is now common. However, both drugs are effective in treating chlamydial urethritis or cervicitis. Ciprofloxacin, levofloxacin, or moxifloxacin is occasionally used for treatment of tuberculosis and atypical mycobacterial infections. These agents are suitable for eradication of meningococci from carriers and for prophylaxis of infection in neutropenic cancer patients.

With their enhanced gram-positive activity and activity against atypical pneumonia agents (chlamydiae, Mycoplasma, and Legionella), levofloxacin, gatifloxacin, gemifloxacin, and moxifloxacin—so-called respiratory fluoroquinolones—are effective and used increasingly for treatment of upper and lower respiratory tract infections.

Adverse Effects Fluoroquinolones are generally well tolerated. The most common effects are nausea, vomiting, and diarrhea. Occasionally, headache, dizziness, insomnia, skin rash, or abnormal liver function tests develop. Photosensitivity has been reported with lomefloxacin and pefloxacin. Prolongation of the QTc interval may occur with gatifloxacin, levofloxacin, gemifloxacin, and moxifloxacin; these drugs should be avoided or used with caution in patients with known QTc interval prolongation or uncorrected hypokalemia; in those receiving class 1A (eg, quinidine or procainamide) or class 3 antiarrhythmic agents (sotalol, ibutilide, amiodarone); and in patients receiving other agents known to increase the QTc interval (eg, erythromycin, tricyclic antidepressants). Gatifloxacin has been associated with hyperglycemia in diabetic patients and with hypoglycemia in patients also receiving oral hypoglycemic agents. Because of these serious effects (including some fatalities), gatifloxacin was withdrawn from sale in the United States in 2006.

Fluoroquinolones may damage growing cartilage and cause an arthropathy. Thus, these drugs are not routinely recommended for patients under 18 years of age. However, the arthropathy is reversible, and there is a growing consensus that fluoroquinolones may be used in children in some cases (eg, for treatment of pseudomonal infections in patients with cystic fibrosis). Tendonitis, a rare complication that has been reported in adults, is potentially more serious because of the risk of tendon rupture. Risk factors for tendonitis include advanced age, renal insufficiency, and concurrent steroid use. Fluoroquinolones should be avoided during pregnancy in the absence of specific data documenting their safety. Oral or intravenously administered fluoroquinolones have also been associated with peripheral neuropathy. Neuropathy can occur at any time during treatment with fluoroquinolones and may persist for months to years after the drug is stopped. In some cases it may be permanent.

SUMMARY Sulfonamides, Trimethoprim, and Fluoroquinolones

PREPARATIONS AVAILABLE

REFERENCES Briasoulis A et al: QT prolongation and torsade de pointes induced by fluoroquinolones: Infrequent side effects from commonly used medications. Cardiology 2011;120:103.

Cohen JS: Peripheral neuropathy associated with fluoroquinolones. Ann Pharmacother 2001;35:1540.

Davidson R et al: Resistance to levofloxacin and failure of treatment of pneumococcal pneumonia. N Engl J Med 2002;346:747.

Gupta K et al: International clinical practice guidelines for the treatment of acute uncomplicated cystitis and pyelonephritis in women. Clin Infect Dis 2011;52:103.

Keating GM, Scott LJ: Moxifloxacin: A review of its use in the management of bacterial infections. Drugs 2004;64:2347.

Mandell LA et al: Infectious Disease Society of America/American T horacic Society consensus guidelines on the management of community-acquired pneumonia. Clin Infect Dis 2007;44:S27.

Mwenya DM et al: Impact of cotrimoxazole on carriage and antibiotic resistance of Streptococcus pneumoniae and Haemophilus influenzae in HIV-infected children in Zambia. Antimicrob Agents Chemother 2010;54:3756.

Nouira S et al: Standard versus newer antibacterial agents in the treatment of severe acute exacerbation of chronic obstructive pulmonary disease: A randomized trial of trimethoprim-sulfamethoxazole versus ciprofloxacin. Clin Infect Dis 2010;51:143.

Rodriguez-Martinez JM et al: Plasmid-mediated quinolone resistance: An update. J Infect Chemother 2011;17:149.

Scheld WM: Maintaining fluoroquinolone class efficacy: Review of influencing factors. Emerg Infect Dis 2003;9:1.

Schmitz GR et al: Randomized controlled trial of trimethoprim-sulfamethoxazole for uncomplicated skin abscesses in patients at risk for community-associated methicillinresistant Staphylococcus aureus infection. Ann Emerg Med 2010;56:283.

Strom BL et al: Absence of cross-reactivity between sulfonamide antibiotics and sulfonamide nonantibiotics. N Engl J Med 2003;349:1628.

T alan DA et al: Prevalence of and risk factor analysis of trimethoprim-sulfamethoxazole- and fluoroquinolone-resistant E. coli infection among emergency department patients with pyelonephritis. Clin Infect Dis 2008; 47:1150.

Ziganshina LE et al: Fluoroquinolones for treating tuberculosis (presumed drug sensitive). Cochrane Database Syst Rev 2013;(6):CD004795.

CASE STUDY ANSWER A fluoroquinolone that achieves good urinary levels (ciprofloxacin or levofloxacin) would be a reasonable choice for empiric treatment of this patientâ&#x20AC;&#x2122;s complicated urinary tract infection. Her recent exposure to multiple courses of trimethoprimsulfamethoxazole increases her chances of having a urinary tract infection with an isolate that is resistant to this antibiotic, making empiric trimethoprim-sulfamethoxazole a poor choice. The patient should be told to take the oral fluoroquinolone 2 hours before or 4 hours after her calcium supplement as divalent and trivalent cations can significantly impair the absorption of oral fluoroquinolones.

_______________ * T he authors thank Henry F. Chambers, MD, for his contributions to previous editions.

CHAPTER

47 Antimycobacterial Drugs Daniel H. Deck, PharmD, & Lisa G. Winston, MD*

CASE STUDY A 45-year-old homeless man presents to the emergency department complaining of a 2-month history of fatigue, weight loss (10 kg), fevers, night sweats, and a productive cough. He is currently living on the street and has spent time in homeless shelters and prison in the last several years. He reports drinking 2–3 pints of hard alcohol per day for the last 15 years and also reports a history of intravenous drug use. In the emergency department, a chest X-ray shows a right apical infiltrate. Given the high suspicion for pulmonary tuberculosis, the patient is placed in respiratory isolation. His first sputum smear shows many acid-fast bacilli, and a rapid HIV antibody test returns with a positive result. What drugs should be started for treatment of presumptive pulmonary tuberculosis?

Does the patient have a heightened risk of developing medication toxicity? If so, which medication(s) would be likely to cause toxicity?

Mycobacteria are intrinsically resistant to most antibiotics. Because they grow more slowly than other bacteria, antibiotics that are most active against rapidly growing cells are relatively ineffective. Mycobacterial cells can also be dormant and thus completely resistant to many drugs or killed only very slowly. The lipid-rich mycobacterial cell wall is impermeable to many agents. Mycobacterial species are intracellular pathogens, and organisms residing within macrophages are inaccessible to drugs that penetrate these cells poorly. Finally, mycobacteria are notorious for their ability to develop resistance. Combinations of two or more drugs are required to overcome these obstacles and to prevent emergence of resistance during the course of therapy. The response of mycobacterial infections to chemotherapy is slow, and treatment must be administered for months to years, depending on which drugs are used. The drugs used to treat tuberculosis, atypical mycobacterial infections, and leprosy are described in this chapter.

DRUGS USED IN TUBERCULOSIS Isoniazid (INH), rifampin (or other rifamycin), pyrazinamide, ethambutol, and streptomycin are the traditional five first-line agents for treatment of tuberculosis (Table 47–1). Streptomycin is no longer recommended as first-line therapy in most settings. Isoniazid and rifampin are the most active drugs. An isoniazid-rifampin combination administered for 9 months will cure 95–98% of cases of tuberculosis caused by susceptible strains. The addition of pyrazinamide to an isoniazid-rifampin combination for the first 2 months allows the total duration of therapy to be reduced to 6 months without loss of efficacy (Table 47–2). In practice, therapy is usually initiated with a four-drug regimen of isoniazid, rifampin, pyrazinamide, and ethambutol until susceptibility of the clinical isolate has been determined.

Neither ethambutol nor other drugs such as streptomycin adds substantially to the overall activity of the regimen (ie, the duration of treatment cannot be further reduced if another drug is used), but the fourth drug provides additional coverage if the isolate proves to be resistant to isoniazid, rifampin, or both. The prevalence of isoniazid resistance among clinical isolates in the United States is approximately 10%. Prevalence of resistance to both isoniazid and rifampin (which is termed multidrug resistance) is about 3%. Resistance to rifampin alone is rare.

TABLE 47–1 Antimicrobials used in the treatment of tuberculosis.

TABLE 47â&#x20AC;&#x201C;2 Recommended duration of therapy for tuberculosis.

ISONIAZID Isoniazid is the most active drug for the treatment of tuberculosis caused by susceptible strains. It is a small molecule (MW 137) that is freely soluble in water. The structural similarity to pyridoxine is shown below.

In vitro, isoniazid inhibits most tubercle bacilli at a concentration of 0.2 mcg/mL or less and is bactericidal for actively growing tubercle bacilli. It is less effective against atypical mycobacterial species. Isoniazid penetrates into macrophages and is active against both extracellular and intracellular organisms.

Mechanism of Action & Basis of Resistance Isoniazid inhibits synthesis of mycolic acids, which are essential components of mycobacterial cell walls. Isoniazid is a prodrug that is activated by KatG, the mycobacterial catalase-peroxidase. The activated form of isoniazid forms a covalent complex with an acyl carrier protein (AcpM) and KasA, a beta-ketoacyl carrier protein synthetase, which blocks mycolic acid synthesis. Resistance to isoniazid is associated with mutations resulting in overexpression of inhA, which encodes an NADH-dependent acyl carrier protein reductase;

mutation or deletion of the katG gene; promoter mutations resulting in overexpression of ahpC, a gene involved in protection of the cell from oxidative stress; and mutations in kasA. Overproducers of inhA express low-level isoniazid resistance and cross-resistance to ethionamide. KatG mutants express high-level isoniazid resistance and often are not cross-resistant to ethionamide.

Drug-resistant mutants are normally present in susceptible mycobacterial populations at about 1 bacillus in 106 . Since tuberculous lesions often contain more than 108 tubercle bacilli, resistant mutants are readily selected if isoniazid or any other drug is given as a single agent. The use of two independently acting drugs in combination is much more effective. The probability that a bacillus is initially resistant to both drugs is approximately 1 in 106 × 106 , or 1 in 1012 , several orders of magnitude greater than the number of infecting organisms. Thus, at least two (or more in certain cases) active agents should always be used to treat active tuberculosis to prevent emergence of resistance during therapy.

Pharmacokinetics Isoniazid is readily absorbed from the gastrointestinal tract. A 300 mg oral dose (5 mg/kg in children) achieves peak plasma concentrations of 3–5 mcg/mL within 1–2 hours. Isoniazid diffuses readily into all body fluids and tissues. The concentration in the central nervous system and cerebrospinal fluid ranges between 20% and 100% of simultaneous serum concentrations.

Metabolism of isoniazid, especially acetylation by liver N-acetyltransferase, is genetically determined (see Chapter 4). The average plasma concentration of isoniazid in rapid acetylators is about one third to one half of that in slow acetylators, and average half-lives are less than 1 hour and 3 hours, respectively. More rapid clearance of isoniazid by rapid acetylators is usually of no therapeutic consequence when appropriate doses are administered daily, but subtherapeutic concentrations may occur if drug is administered as a once-weekly dose or if there is malabsorption.

Isoniazid metabolites and a small amount of unchanged drug are excreted mainly in the urine. The dosage need not be adjusted in renal failure. Dose adjustment is not well defined in patients with severe preexisting hepatic insufficiency and should be guided by serum concentrations if a reduction in dose is contemplated.

Clinical Uses The typical dosage of isoniazid is 5 mg/kg/d; a typical adult dose is 300 mg given once daily. Up to 10 mg/kg/d may be used for serious infections or if malabsorption is a problem. A 15 mg/kg dose, or 900 mg, may be used in a twice-weekly dosing regimen in combination with a second antituberculous agent (eg, rifampin, 600 mg). Pyridoxine, 25–50 mg/d, is recommended for those with conditions predisposing to neuropathy, an adverse effect of isoniazid. Isoniazid is usually given by mouth but can be given parenterally in the same dosage.

Isoniazid as a single agent is also indicated for treatment of latent tuberculosis. The dosage is 300 mg/d (5 mg/kg/d) or 900 mg twice weekly, and the duration is usually 9 months.

Adverse Reactions The incidence and severity of untoward reactions to isoniazid are related to dosage and duration of administration.

A. Immunologic Reactions Fever and skin rashes are occasionally seen. Drug-induced systemic lupus erythematosus has been reported.

B. Direct Toxicity Isoniazid-induced hepatitis is the most common major toxic effect. This is distinct from the minor increases in liver aminotransferases (up to three or four times normal), which do not require cessation of the drug and which are seen in 10–20% of patients, who usually are asymptomatic. Clinical hepatitis with loss of appetite, nausea, vomiting, jaundice, and right upper quadrant pain occurs in 1% of isoniazid recipients and can be fatal, particularly if the drug is not discontinued promptly. There is histologic evidence of hepatocellular damage and necrosis. The risk of hepatitis depends on age. It occurs rarely under age 20, in 0.3% of those aged 21–35, 1.2% of those aged 36–50, and 2.3% for those aged 50 and above. The risk of hepatitis is greater in individuals with alcohol dependence and possibly during pregnancy and the postpartum period. Development of isoniazid hepatitis contraindicates further use of the drug.

Peripheral neuropathy is observed in 10–20% of patients given dosages greater than 5 mg/kg/d, but it is infrequently seen with the standard 300 mg adult dose. Peripheral neuropathy is more likely to occur in slow acetylators and patients with predisposing conditions such as malnutrition, alcoholism, diabetes, AIDS, and uremia. Neuropathy is due to a relative pyridoxine deficiency. Isoniazid promotes excretion of pyridoxine, and this toxicity is readily reversed by administration of pyridoxine in a dosage as low as 10 mg/d. Central nervous system toxicity, which is less common, includes memory loss, psychosis, and seizures. These effects may also respond to pyridoxine.

Miscellaneous other reactions include hematologic abnormalities, provocation of pyridoxine deficiency anemia, tinnitus, and

gastrointestinal discomfort. Isoniazid can reduce the metabolism of phenytoin, increasing its blood level and toxicity.

RIFAMPIN Rifampin is a semisynthetic derivative of rifamycin, an antibiotic produced by Streptomyces mediterranei. It is active in vitro against gram-positive and gram-negative cocci, some enteric bacteria, mycobacteria, and chlamydiae. Susceptible organisms are inhibited by less than 1 mcg/mL. Resistant mutants are present in all microbial populations at approximately 1 in 106 organisms and are rapidly selected out if rifampin is used as a single drug, especially in a patient with active infection. There is no cross-resistance to other classes of antimicrobial drugs, but there is cross-resistance to other rifamycin derivatives, eg, rifabutin and rifapentine.

Mechanism of Action, Resistance, & Pharmacokinetics Rifampin binds to the β subunit of bacterial DNA-dependent RNA polymerase and thereby inhibits RNA synthesis. Resistance results from any one of several possible point mutations in rpoB, the gene for the β subunit of RNA polymerase. These mutations result in reduced binding of rifampin to RNA polymerase. Human RNA polymerase does not bind rifampin and is not inhibited by it. Rifampin is bactericidal for mycobacteria. It readily penetrates most tissues and penetrates into phagocytic cells. It can kill organisms that are poorly accessible to many other drugs, such as intracellular organisms and those sequestered in abscesses and lung cavities.

Rifampin is well absorbed after oral administration and excreted mainly through the liver into bile. It then undergoes enterohepatic recirculation, with the bulk excreted as a deacylated metabolite in feces and a small amount excreted in the urine. Dosage adjustment for renal or hepatic insufficiency is not necessary. Usual doses result in serum levels of 5â&#x20AC;&#x201C;7 mcg/mL. Rifampin is distributed widely in body fluids and tissues. The drug is relatively highly protein-bound, and adequate cerebrospinal fluid concentrations are achieved only in the presence of meningeal inflammation.

Clinical Uses A. Mycobacterial Infections Rifampin, usually 600 mg/d (10 mg/kg/d) orally, must be administered with isoniazid or other antituberculous drugs to patients with active tuberculosis to prevent emergence of drug-resistant mycobacteria. In some short-course therapies, 600 mg of rifampin is given twice weekly. Rifampin, 600 mg daily or twice weekly for 6 months, also is effective in combination with other agents in some atypical mycobacterial infections and in leprosy. Rifampin, 600 mg daily for 4 months as a single drug, is an alternative to isoniazid for patients with latent tuberculosis who are unable to take isoniazid or who have had exposure to a case of active tuberculosis caused by an isoniazid-resistant, rifampin-susceptible strain.

B. Other Indications Rifampin has other uses in bacterial infections. An oral dosage of 600 mg twice daily for 2 days can eliminate meningococcal carriage.

Rifampin, 20 mg/kg/d for 4 days, is used as prophylaxis in contacts of children with Haemophilus influenzae type b disease. Rifampin combined with a second agent is used to eradicate staphylococcal carriage. Rifampin combination therapy is also indicated for treatment of serious staphylococcal infections such as osteomyelitis and prosthetic valve endocarditis.

Adverse Reactions Rifampin imparts a harmless orange color to urine, sweat, and tears (soft contact lenses may be permanently stained). Occasional adverse effects include rashes, thrombocytopenia, and nephritis. Rifampin may cause cholestatic jaundice and occasionally hepatitis, and it commonly causes light-chain proteinuria. If administered less often than twice weekly, rifampin may cause a flu-like syndrome characterized by fever, chills, myalgias, anemia, and thrombocytopenia. Its use has been associated with acute tubular necrosis. Rifampin strongly induces most cytochrome P450 isoforms (CYP1A2, 2C9, 2C19, 2D6, and 3A4), which increases the elimination of numerous other drugs including methadone, anticoagulants, cyclosporine, some anticonvulsants, protease inhibitors, some nonnucleoside reverse transcriptase inhibitors, contraceptives, and a host of others (see Chapters 4 and 66). Co-administration of rifampin results in significantly lower serum levels of these drugs.

ETHAMBUTOL Ethambutol is a synthetic, water-soluble, heat-stable compound, the dextro-isomer of the structure shown below, dispensed as the dihydrochloride salt.

Mechanism of Action & Clinical Uses Susceptible strains of Mycobacterium tuberculosis and other mycobacteria are inhibited in vitro by ethambutol, 1–5 mcg/mL. Ethambutol inhibits mycobacterial arabinosyl transferases, which are encoded by the embCAB operon. Arabinosyl transferases are involved in the polymerization reaction of arabinoglycan, an essential component of the mycobacterial cell wall. Resistance to ethambutol is due to mutations resulting in overexpression of emb gene products or within the embB structural gene.

Ethambutol is well absorbed from the gut. After ingestion of 25 mg/kg, a blood level peak of 2–5 mcg/mL is reached in 2–4 hours.

About 20% of the drug is excreted in feces and 50% in urine in unchanged form. Ethambutol accumulates in renal failure, and the dose should be reduced by half if creatinine clearance is less than 10 mL/min. Ethambutol crosses the blood-brain barrier only when the meninges are inflamed. Concentrations in cerebrospinal fluid are highly variable, ranging from 4% to 64% of serum levels in the setting of meningeal inflammation.

As with all antituberculous drugs, resistance to ethambutol emerges rapidly when the drug is used alone. Therefore, ethambutol is always given in combination with other antituberculous drugs.

Ethambutol hydrochloride, 15–25 mg/kg, is usually given as a single daily dose in combination with isoniazid or rifampin for the treatment of active tuberculosis. The higher dose may be used for treatment of tuberculous meningitis. The dose of ethambutol is 50 mg/kg when a twice-weekly dosing schedule is used.

Adverse Reactions Hypersensitivity to ethambutol is rare. The most common serious adverse event is retrobulbar neuritis, resulting in loss of visual acuity and red-green color blindness. This dose-related adverse effect is more likely to occur at dosages of 25 mg/kg/d continued for several months. At 15 mg/kg/d or less, visual disturbances are very rare. Periodic visual acuity testing is desirable if the 25 mg/kg/d dosage is used. Ethambutol is relatively contraindicated in children too young to permit assessment of visual acuity and red-green color discrimination.

PYRAZINAMIDE Pyrazinamide (PZA) is a relative of nicotinamide, and it is used only for treatment of tuberculosis. It is stable and slightly soluble in water. It is inactive at neutral pH, but at pH 5.5 it inhibits tubercle bacilli at concentrations of approximately 20 mcg/mL. The drug is taken up by macrophages and exerts its activity against mycobacteria residing within the acidic environment of lysosomes.

Mechanism of Action & Clinical Uses Pyrazinamide is converted to pyrazinoic acid—the active form of the drug—by mycobacterial pyrazinamidase, which is encoded by pncA. Pyrazinoic acid disrupts mycobacterial cell membrane metabolism and transport functions. Resistance may be due to impaired uptake of pyrazinamide or mutations in pncA that impair conversion of PZA to its active form.

Serum concentrations of 30–50 mcg/mL at 1–2 hours after oral administration are achieved with dosages of 25 mg/kg/d.

Pyrazinamide is well absorbed from the gastrointestinal tract and widely distributed in body tissues, including inflamed meninges. The half-life is 8–11 hours. The parent compound is metabolized by the liver, but metabolites are renally cleared; therefore, PZA should be administered at 25–35 mg/kg three times weekly (not daily) in hemodialysis patients and those in whom the creatinine clearance is less

than 30 mL/min. In patients with normal renal function, a dose of 40–50 mg/kg is used for thrice-weekly or twice-weekly treatment regimens.

Pyrazinamide is an important front-line drug used in conjunction with isoniazid and rifampin in short-course (ie, 6-month) regimens as a “sterilizing” agent active against residual intracellular organisms that may cause relapse. Tubercle bacilli develop resistance to pyrazinamide fairly readily, but there is no cross-resistance with isoniazid or other antimycobacterial drugs.

Adverse Reactions Major adverse effects of PZA include hepatotoxicity (in 1–5% of patients), nausea, vomiting, drug fever, and hyperuricemia. The latter occurs uniformly and is not a reason to halt therapy. Hyperuricemia may provoke acute gouty arthritis.

STREPTOMYCIN The mechanism of action and other pharmacologic features of streptomycin are discussed in Chapter 45. The typical adult dosage is 1 g/d (15 mg/kg/d). If the creatinine clearance is less than 30 mL/min or the patient is on hemodialysis, the dosage is 15 mg/kg two or three times per week. Most tubercle bacilli are inhibited by streptomycin, 1–10 mcg/mL, in vitro. Nontuberculosis species of mycobacteria other than Mycobacterium avium complex (MAC) and Mycobacterium kansasii are resistant. All large populations of tubercle bacilli contain some streptomycin-resistant mutants. On average, 1 in 108 tubercle bacilli can be expected to be resistant to streptomycin at levels of 10–100 mcg/mL. Resistance may be due to a point mutation in either the rpsL gene encoding the S12 ribosomal protein or the rrs gene encoding 16S ribosomal RNA, which alters the ribosomal binding site.

Streptomycin penetrates into cells poorly and is active mainly against extracellular tubercle bacilli. The drug crosses the blood-brain barrier and achieves therapeutic concentrations with inflamed meninges.

Clinical Use in Tuberculosis Streptomycin sulfate is used when an injectable drug is needed or desirable and in the treatment of infections resistant to other drugs.

The usual dosage is 15 mg/kg/d intramuscularly or intravenously daily for adults (20–40 mg/kg/d, not to exceed 1–1.5 g for children) for several weeks, followed by 1–1.5 g two or three times weekly for several months. Serum concentrations of approximately 40 mcg/mL are achieved 30–60 minutes after intramuscular injection of a 15 mg/kg dose. Other drugs are always given in combination to prevent emergence of resistance.

Adverse Reactions Streptomycin is ototoxic and nephrotoxic. Vertigo and hearing loss are the most common adverse effects and may be permanent.

Toxicity is dose-related, and the risk is increased in the elderly. As with all aminoglycosides, the dose must be adjusted according to renal function (see Chapter 45). Toxicity can be reduced by limiting therapy to no more than 6 months whenever possible.

SECOND-LINE DRUGS FOR TUBERCULOSIS The alternative drugs listed below are usually considered only (1) in case of resistance to first-line agents; (2) in case of failure of clinical response to conventional therapy; and (3) in case of serious treatment-limiting adverse drug reactions. Expert guidance to deal with the toxic effects of these second-line drugs is desirable. For many drugs listed in the following text, the dosage, emergence of resistance, and long-term toxicity have not been fully established.

Ethionamide Ethionamide is chemically related to isoniazid and similarly blocks the synthesis of mycolic acids. It is poorly water soluble and available only in oral form. It is metabolized by the liver.

Most tubercle bacilli are inhibited in vitro by ethionamide, 2.5 mcg/mL or less. Some other species of mycobacteria also are inhibited by ethionamide, 10 mcg/mL. Serum concentrations in plasma and tissues of approximately 20 mcg/mL are achieved by a dosage of 1 g/d.

Cerebrospinal fluid concentrations are equal to those in serum.

Ethionamide is administered at an initial dose of 250 mg once daily, which is increased in 250-mg increments to the recommended dosage of 1 g/d (or 15 mg/kg/d), if possible. The 1 g/d dosage, though theoretically desirable, is poorly tolerated because of gastric irritation and neurologic symptoms, often limiting the tolerable daily dose to 500–750 mg. Ethionamide is also hepatotoxic. Neurologic symptoms may be alleviated by pyridoxine.

Resistance to ethionamide as a single agent develops rapidly in vitro and in vivo. There can be low-level cross-resistance between isoniazid and ethionamide.

Capreomycin Capreomycin is a peptide protein synthesis inhibitor antibiotic obtained from Streptomyces capreolus . Daily injection of 1 g intramuscularly results in blood levels of 10 mcg/mL or more. Such concentrations in vitro are inhibitory for many mycobacteria, including multidrug-resistant strains of M tuberculosis.

Capreomycin (15 mg/kg/d) is an important injectable agent for treatment of drug-resistant tuberculosis. Strains of M tuberculosis that are resistant to streptomycin or amikacin usually are susceptible to capreomycin. Resistance to this drug, when it occurs, may be due to an rrs mutation.

Capreomycin is nephrotoxic and ototoxic. Tinnitus, deafness, and vestibular disturbances occur. The injection causes significant local pain, and sterile abscesses may develop.

Dosing of capreomycin is the same as that of streptomycin. Toxicity is reduced if 1 g is given two or three times weekly after an initial response has been achieved with a daily dosing schedule.

Cycloserine Cycloserine is an inhibitor of cell wall synthesis and is discussed in Chapter 43. Concentrations of 15–20 mcg/mL inhibit many strains of M tuberculosis. The dosage of cycloserine in tuberculosis is 0.5–1 g/d in two divided oral doses. This drug is cleared renally, and the dose should be reduced by half if creatinine clearance is less than 50 mL/min.

The most serious toxic effects are peripheral neuropathy and central nervous system dysfunction, including depression and psychotic reactions. Pyridoxine, 150 mg/d, should be given with cycloserine because this ameliorates neurologic toxicity. Adverse effects, which are most common during the first 2 weeks of therapy, occur in 25% or more of patients, especially at higher doses. Adverse effects can be minimized by monitoring peak serum concentrations. The peak concentration is reached 2–4 hours after dosing. The recommended range of peak concentrations is 20–40 mcg/mL.

Aminosalicylic Acid (PAS) Aminosalicylic acid is a folate synthesis antagonist that is active almost exclusively against M tuberculosis. It is structurally similar to pamino-benzoic acid (PABA) and to the sulfonamides (see Chapter 46).

Tubercle bacilli are usually inhibited in vitro by aminosalicylic acid, 1–5 mcg/mL. Aminosalicylic acid is readily absorbed from the gastrointestinal tract. Serum levels are 50 mcg/mL or more after a 4 g oral dose. The dosage is 8–12 g/d orally for adults and 300 mg/kg/d for children. The drug is widely distributed in tissues and body fluids except the cerebrospinal fluid. Aminosalicylic acid is rapidly excreted in the urine, in part as active PAS and in part as the acetylated compound and other metabolic products. Very high concentrations of aminosalicylic acid are reached in the urine, which can result in crystalluria.

Aminosalicylic acid is used infrequently because other oral drugs are better tolerated. Gastrointestinal symptoms are common and may be diminished by giving the drug with meals and with antacids. Peptic ulceration and hemorrhage may occur. Hypersensitivity reactions manifested by fever, joint pains, skin rashes, hepatosplenomegaly, hepatitis, adenopathy, and granulocytopenia often occur after 3–8 weeks of PAS therapy, making it necessary to stop administration temporarily or permanently.

Kanamycin & Amikacin The aminoglycoside antibiotics are discussed in Chapter 45. Kanamycin had been used for treatment of tuberculosis caused by streptomycin-resistant strains, but the availability of less toxic alternatives (eg, capreomycin and amikacin) has rendered it obsolete.

Amikacin is playing a greater role in the treatment of tuberculosis due to the prevalence of multidrug-resistant strains. Prevalence of amikacin-resistant strains is low (< 5%), and most multidrug-resistant strains remain amikacin-susceptible. M tuberculosis is inhibited at concentrations of 1 mcg/mL or less. Amikacin is also active against atypical mycobacteria. There is no cross-resistance between streptomycin and amikacin, but kanamycin resistance often indicates resistance to amikacin as well. Serum concentrations of 30–50 mcg/mL are achieved 30–60 minutes after a 15 mg/kg intravenous infusion. Amikacin is indicated for treatment of tuberculosis suspected or known to be caused by streptomycin-resistant or multidrug-resistant strains. This drug must be used in combination with at least one and preferably two or three other drugs to which the isolate is susceptible for treatment of drug-resistant cases. The recommended dosages are the same as those for streptomycin.

Fluoroquinolones In addition to their activity against many gram-positive and gram-negative bacteria (discussed in Chapter 46), ciprofloxacin, levofloxacin, gatifloxacin, and moxifloxacin inhibit strains of M tuberculosis at concentrations less than 2 mcg/mL. They are also active against atypical mycobacteria. Moxifloxacin is the most active against M tuberculosis in vitro. Levofloxacin tends to be slightly more active than ciprofloxacin against M tuberculosis, whereas ciprofloxacin is slightly more active against atypical mycobacteria.

Fluoroquinolones are an important addition to the drugs available for tuberculosis, especially for strains that are resistant to first-line agents. Resistance, which may result from one of several single point mutations in the gyrase A subunit, develops rapidly if a fluoroquinolone is used as a single agent; thus, the drug must be used in combination with two or more other active agents. The standard dosage of ciprofloxacin is 750 mg orally twice a day. The dosage of levofloxacin is 500–750 mg once a day. The dosage of moxifloxacin is 400 mg once a day.

Linezolid Linezolid (discussed in Chapter 44) inhibits strains of M tuberculosis in vitro at concentrations of 4–8 mcg/mL. It achieves good intracellular concentrations, and it is active in murine models of tuberculosis. Linezolid has been used in combination with other secondand third-line drugs to treat patients with tuberculosis caused by multidrug-resistant strains. Conversion of sputum cultures to negative was associated with linezolid use in these cases. Significant adverse effects, including bone marrow suppression and irreversible peripheral and optic neuropathy, have been reported with the prolonged courses of therapy that are necessary for treatment of tuberculosis. A 600 mg (adult) dose administered once a day (half of that used for treatment of other bacterial infections) seems to be sufficient and may limit the occurrence of these adverse effects. Although linezolid may prove to be an important new agent for treatment of tuberculosis, at this point it should only be used for multidrug-resistant strains that also are resistant to several other firstand second-line agents.

Rifabutin Rifabutin is derived from rifamycin and is related to rifampin. It has significant activity against M tuberculosis, MAC, and Mycobacterium fortuitum (see below). Its activity is similar to that of rifampin, and cross-resistance with rifampin is virtually complete.

Some rifampin-resistant strains may appear susceptible to rifabutin in vitro, but a clinical response is unlikely because the molecular basis of resistance, rpoB mutation, is the same. Rifabutin is both substrate and inducer of cytochrome P450 enzymes. Because it is a less potent inducer, rifabutin is indicated in place of rifampin for treatment of tuberculosis in patients with HIV infection who are receiving antiretroviral therapy with a protease inhibitor or a nonnucleoside reverse transcriptase inhibitor (eg, efavirenz), drugs that also are cytochrome P450 substrates.

The typical dosage of rifabutin is 300 mg/d unless the patient is receiving a protease inhibitor, in which case the dosage should be reduced. If efavirenz (also a cytochrome P450 inducer) is used, the recommended dosage of rifabutin is 450 mg/d.

Rifapentine Rifapentine is an analog of rifampin. It is active against both M tuberculosis and MAC. As with all rifamycins, it is a bacterial RNA polymerase inhibitor, and cross-resistance between rifampin and rifapentine is complete. Like rifampin, rifapentine is a potent inducer of cytochrome P450 enzymes, and it has the same drug interaction profile. Toxicity is similar to that of rifampin. Rifapentine and its microbiologically active metabolite, 25-desacetylrifapentine, have an elimination half-life of 13 hours. Rifapentine, 600 mg (10 mg/kg) once or twice weekly, is indicated for treatment of tuberculosis caused by rifampin-susceptible strains during the continuation phase only (ie, after the first 2 months of therapy and ideally after conversion of sputum cultures to negative). Rifapentine should not be used to treat patients with HIV infection because of an unacceptably high relapse rate with rifampin-resistant organisms. Rifapentine, given once weekly for 3 months in combination with isoniazid, is an effective short course treatment for latent tuberculosis infection.

Bedaquiline Bedaquiline, a diarylquinoline, is the first drug with a novel mechanism of action against M tuberculosis to be approved since 1971.

Bedaquiline inhibits adenosine 5′-triphosphate (ATP) synthase in mycobacteria, has in vitro activity against both replicating and nonreplicating bacilli, and has bactericidal and sterilizing activity in the murine model of tuberculosis. No cross-resistance has been found between bedaquiline and other medications used to treat tuberculosis.

Peak plasma concentration and plasma exposure of bedaquiline increase approximately twofold when administered with high-fat food. Bedaquiline is highly protein-bound (> 99%), is metabolized chiefly through the cytochrome P450 system, and is excreted primarily via the feces. The mean terminal half-life of bedaquiline and its major metabolite (M2), which is four to six times less active in terms of antimycobacterial potency, is approximately 5.5 months. This long elimination phase probably reflects slow release of bedaquiline and M2 from peripheral tissues. CYP3A4 is the major isoenzyme involved in the metabolism of bedaquiline and potent inhibitors or inducers of this enzyme cause clinically significant drug interactions.

Current recommendations state that bedaquiline, in combination with at least three other active medications, may be used for 24 weeks of treatment in adults with laboratory-confirmed pulmonary tuberculosis if the isolate is resistant to both isoniazid and rifampin.

The recommended dosage for bedaquiline is 400 mg once daily orally for 2 weeks, followed by 200 mg three times a week for 22 weeks taken orally with food in order to maximize absorption. Bedaquiline has been associated with both hepatotoxicity and cardiac toxicity (prolongation of the QTc interval), so patients must be closely monitored during treatment.

DRUGS ACTIVE AGAINST ATYPICAL MYCOBACTERIA Many mycobacterial infections seen in clinical practice in the United States are caused by nontuberculous or “atypical” mycobacteria.

These organisms have distinctive laboratory characteristics, are present in the environment, and are generally not communicable from person to person. As a rule, these mycobacterial species are less susceptible than M tuberculosis to antituberculous drugs. On the other hand, agents such as macrolides, sulfonamides, and tetracyclines, which are not active against M tuberculosis, may be effective for infections caused by atypical mycobacteria. Emergence of resistance during therapy is also a problem with these mycobacterial species, and active infection should be treated with combinations of drugs. M kansasii is susceptible to rifampin and ethambutol, partially susceptible to isoniazid, and completely resistant to pyrazinamide. A three-drug combination of isoniazid, rifampin, and ethambutol is the conventional treatment for M kansasii infection. A few representative pathogens, with the clinical presentation and the drugs to which they are often susceptible, are given in Table 47–3.

TABLE 47–3 Clinical Features and treatment options for infections with atypical mycobacteria.

M avium complex (MAC), which includes both M avium and M intracellulare, is an important and common cause of disseminated disease in late stages of AIDS (CD4 counts < 50/μL). MAC is much less susceptible than M tuberculosis to most antituberculous drugs.

Combinations of agents are required to suppress the infection. Azithromycin, 500 mg once daily, or clarithromycin, 500 mg twice daily, plus ethambutol, 15–25 mg/kg/d, is an effective and well-tolerated regimen for treatment of disseminated disease. Some authorities recommend use of a third agent, especially rifabutin, 300 mg once daily. Other agents that may be useful are listed in Table 47–3.

Azithromycin and clarithromycin are the prophylactic drugs of choice for preventing disseminated MAC in AIDS patients with CD4 cell counts less than 50/μL. Rifabutin in a single daily dose of 300 mg has been shown to reduce the incidence of MAC bacteremia but is less effective than macrolides.

DRUGS USED IN LEPROSY Mycobacterium leprae has never been grown in vitro, but animal models, such as growth in injected mouse footpads, have permitted laboratory evaluation of drugs. Only those drugs with the widest clinical use are presented here. Because of increasing reports of dapsone resistance, treatment of leprosy with combinations of the drugs listed below is recommended.

DAPSONE & OTHER SULFONES Several drugs closely related to the sulfonamides have been used effectively in the long-term treatment of leprosy. The most widely used is dapsone (diaminodiphenylsulfone). Like the sulfonamides, it inhibits folate synthesis. Resistance can emerge in large populations of M leprae, eg, in lepromatous leprosy, particularly if low doses are given. Therefore, the combination of dapsone, rifampin, and clofazimine is recommended for initial therapy of lepromatous leprosy. A combination of dapsone plus rifampin is commonly used for leprosy with a lower organism burden. Dapsone may also be used to prevent and treat Pneumocystis jiroveci pneumonia in AIDS patients.

Sulfones are well absorbed from the gut and widely distributed throughout body fluids and tissues. Dapsone’s half-life is 1–2 days, and drug tends to be retained in skin, muscle, liver, and kidney. Skin heavily infected with M leprae may contain several times more drug than normal skin. Sulfones are excreted into bile and reabsorbed in the intestine. Excretion into urine is variable, and most excreted drug is acetylated. In renal failure, the dose may have to be adjusted. The usual adult dosage in leprosy is 100 mg daily. For children, the dose is proportionately less, depending on weight.

Dapsone is usually well tolerated. Many patients develop some hemolysis, particularly if they have glucose-6-phosphate dehydrogenase deficiency. Methemoglobinemia is common but usually is not a problem clinically. Gastrointestinal intolerance, fever, pruritus, and various rashes occur. During dapsone therapy of lepromatous leprosy, erythema nodosum leprosum often develops. It is sometimes difficult to distinguish reactions to dapsone from manifestations of the underlying illness. Erythema nodosum leprosum may be suppressed by thalidomide (see Chapter 55).

RIFAMPIN Rifampin (see earlier discussion) in a dosage of 600 mg daily is highly effective in leprosy and is given with at least one other drug to prevent emergence of resistance. Even a dose of 600 mg per month may be beneficial in combination therapy.

CLOFAZIMINE Clofazimine is a phenazine dye used in the treatment of multibacillary leprosy, which is defined as having a positive smear from any site of infection. Its mechanism of action has not been clearly established. Absorption of clofazimine from the gut is variable, and a major portion of the drug is excreted in feces. Clofazimine is stored widely in reticuloendothelial tissues and skin, and its crystals can be seen inside phagocytic reticuloendothelial cells. It is slowly released from these deposits, so the serum half-life may be 2 months. A common dosage of clofazimine is 100 mg/d orally. The most prominent untoward effect is discoloration of the skin and conjunctivae.

Gastrointestinal side effects are also common.

SUMMARY First-Line Antimycobacterial Drugs

PREPARATIONS AVAILABLE

REFERENCES Gillespie SH et al: Early bactericidal activity of a moxifloxacin and isoniazid combination in smear-positive pulmonary tuberculosis. J Antimicrob Chemother 2005;56:1169.

Griffith DE et al: An official AT S/IDSA statement: Diagnosis, treatment, and prevention of nontuberculous mycobacterial disease. Am J Respir Crit Care Med 2007;175:367.

Hugonnet J-E et al: Meropenem-clavulanate is effective against extensively drug-resistant Mycobacterium tuberculosis. Science 2009;323:1215.

Jasmer RM, Nahid P, Hopewell PC: Latent tuberculosis infection. N Engl J Med 2002;347:1860.

Kinzig-Schippers M et al: Should we use N-acetyltransferase type 2 genotyping to personalize isoniazid doses? Antimicrob Agents Chemother 2005;49:1733.

Lee M et al: Linezolid for treatment of chronic extensively drug-resistant tuberculosis. N Engl J Med 2012;367:1508.

Mitnick CD et al: Comprehensive treatment of extensively drug-resistant tuberculosis. N Engl J Med 2008;359:563.

Provisional CDC Guidelines for the use and safety monitoring of bedaquiline fumarate (Sirturo) for the treatment of multidrug-resistant tuberculosis. MMWR Morb Mortal Wkly Rep 2013;62:1.

Recommendations for use of an isoniazid-rifapentine regimen with direct observation to treat latent Mycobacterium tuberculosis infection. MMWR Morb Mortal Wkly Rep 2011;60:1650.

Sulochana S, Rahman F, Paramasivan CN: In vitro activity of fluoroquinolones against Mycobacterium tuberculosis. J Chemother 2005;17:169.

T argeted tuberculin testing and treatment of latent tuberculosis infection. Am J Respir Crit Care Med 2000;161(4 Part 2):S221.

Update: Adverse event data and revised American T horacic Society/CDC recommendations against the use of rifampin and pyrazinamide for treatment of latent tuberculosis infectionâ&#x20AC;&#x201D;United States, 2003. MMWR Morb Mortal Wkly Rep 2003;52:735.

Zhang Y, Yew WW: Mechanisms of drug resistance in Mycobacterium tuberculosis. Int J T uberc Lung Dis 2009;13:1320.

Zumla A et al: Current conceptsâ&#x20AC;&#x201D;T uberculosis. N Engl J Med 2013;368:745.

CASE STUDY ANSWER The patient should be started on four-drug therapy with rifampin, isoniazid, pyrazinamide, and ethambutol. If a protease-inhibitorbased antiretroviral regimen is used to treat his HIV, rifabutin should replace rifampin because of the serious drug-drug interactions between rifampin and protease inhibitors. The patient is at increased risk of developing hepatotoxicity from both isoniazid and pyrazinamide given his history of chronic alcohol dependence.

_______________ * T he authors thank Henry F. Chambers, MD, for his contributions to previous editions.

CHAPTER

48 Antifungal Agents Don Sheppard, MD, & Harry W. Lampiris, MD

CASE STUDY A 55-year-old man presents to the emergency department with a 2-week history of an expanding ulcer on his left lower leg. He has a history of chronic neutropenia and transfusion-dependent anemia secondary to myelodysplastic syndrome requiring chronic therapy with deferoxamine for hepatic iron overload. He first noticed a red bump on his leg while fishing at his cabin in the woods and thought it was a bug bite. It rapidly enlarged, first as a red swollen area, and then began to ulcerate. He was given dicloxacillin orally, but with no improvement. In the emergency department he is febrile to 39°C (102.2°F), and looks unwell. On his left leg he has a 6 by 12 cm black ulcer with surrounding swelling and erythema that is quite tender. His complete blood count demonstrates an absolute neutrophil count of 300 and a total white blood cell count of 1000. An immediate operative debridement yields pathologic specimens demonstrating broad club-like nonseptate hyphae and extensive tissue necrosis. What initial medical therapy would be most appropriate?

Human fungal infections have increased dramatically in incidence and severity in recent years, owing mainly to advances in surgery, cancer treatment, treatment of patients with solid organ and bone marrow transplantation, the HIV epidemic, and increasing use of broad-spectrum antimicrobial therapy in critically ill patients. These changes have resulted in increased numbers of patients at risk for fungal infections.

For many years, amphotericin B was the only efficacious antifungal drug available for systemic use. While highly effective in many serious infections, it is also quite toxic. In the last several decades, pharmacotherapy of fungal disease has been revolutionized by the introduction of the relatively nontoxic azole drugs (both oral and parenteral formulations) and the echinocandins (only available for parenteral administration). The new agents in these classes offer more targeted, less toxic therapy than older agents such as amphotericin B for patients with serious systemic fungal infections. Combination therapy is being reconsidered, and new formulations of old agents are becoming available. Unfortunately, the appearance of azole-resistant organisms, as well as the rise in the number of patients at risk for mycotic infections, has created new challenges The antifungal drugs presently available fall into the following categories: systemic drugs (oral or parenteral) for systemic infections, oral systemic drugs for mucocutaneous infections, and topical drugs for mucocutaneous infections.

SYSTEMIC ANTIFUNGAL DRUGS FOR SYSTEMIC INFECTIONS

AMPHOTERICIN B Amphotericin A and B are antifungal antibiotics produced by Streptomyces nodosus. Amphotericin A is not in clinical use.

Chemistry & Pharmacokinetics Amphotericin B is an amphoteric polyene macrolide (polyene = containing many double bonds; macrolide = containing a large lactone ring of 12 or more atoms). It is nearly insoluble in water and is therefore prepared as a colloidal suspension of amphotericin B and sodium desoxycholate for intravenous injection. Several formulations have been developed in which amphotericin B is packaged in a lipidassociated delivery system (Table 48–1 and Box: Lipid Formulation of Amphotericin B).

TABLE 48–1 Properties of conventional amphotericin B and some lipid formulations.1

Amphotericin B is poorly absorbed from the gastrointestinal tract. Oral amphotericin B is thus effective only on fungi within the lumen of the tract and cannot be used for treatment of systemic disease. The intravenous injection of 0.6 mg/kg/d of amphotericin B results in average blood levels of 0.3–1 mcg/mL; the drug is more than 90% bound by serum proteins. Although it is mostly metabolized, some amphotericin B is excreted slowly in the urine over a period of several days. The serum half-life is approximately 15 days. Hepatic impairment, renal impairment, and dialysis have little impact on drug concentrations, and therefore no dose adjustment is required. The drug is widely distributed in most tissues, but only 2–3% of the blood level is reached in cerebrospinal fluid, thus occasionally necessitating intrathecal therapy for certain types of fungal meningitis.

Mechanisms of Action & Resistance Amphotericin B is selective in its fungicidal effect because it exploits the difference in lipid composition of fungal and mammalian cell membranes. Ergosterol, a cell membrane sterol, is found in the cell membrane of fungi, whereas the predominant sterol of bacteria and human cells is cholesterol. Amphotericin B binds to ergosterol and alters the permeability of the cell by forming amphotericin Bassociated pores in the cell membrane (Figure 48–1). As suggested by its chemistry, amphotericin B combines avidly with lipids (ergosterol) along the double bond-rich side of its structure and associates with water molecules along the hydroxyl-rich side. This amphipathic characteristic facilitates pore formation by multiple amphotericin molecules, with the lipophilic portions around the outside of the pore and the hydrophilic regions lining the inside. The pore allows the leakage of intracellular ions and macromolecules, eventually leading to cell death. Some binding to human membrane sterols does occur, probably accounting for the drug’s prominent toxicity.

FIGURE 48â&#x20AC;&#x201C;1 Targets of antifungal drugs. Except for flucytosine (and possibly griseofulvin, not shown), all currently available antifungals target the fungal cell membrane or cell wall.

Lipid Formulation of Amphotericin B Therapy with amphotericin B is often limited by toxicity, especially drug-induced renal impairment. This has led to the development of lipid drug formulations on the assumption that lipid-packaged drug binds to the mammalian membrane less readily, permitting the use of effective doses of the drug with lower toxicity. Liposomal amphotericin preparations package the active drug in lipid delivery vehicles, in contrast to the colloidal suspensions, which were previously the only available forms. Amphotericin binds to the lipids in these vehicles with an affinity between that for fungal ergosterol and that for human cholesterol. The lipid vehicle then serves as an amphotericin reservoir, reducing nonspecific binding to human cell membranes. This preferential binding allows for a reduction of toxicity without sacrificing efficacy and permits use of larger doses. Furthermore, some fungi contain lipases that may liberate free amphotericin B directly at the site of infection.

Three such formulations are now available and have differing pharmacologic properties as summarized in Table 48â&#x20AC;&#x201C;1. Although clinical trials have demonstrated different renal and infusion-related toxicities for these preparations compared with regular amphotericin B, there are no trials comparing the different formulations with each other. Limited studies have suggested at best a moderate improvement in the clinical efficacy of the lipid formulations compared with conventional amphotericin B. Because the lipid preparations are much more expensive, their use is usually restricted to patients intolerant to, or not responding to, conventional amphotericin treatment.

Resistance to amphotericin B occurs if ergosterol binding is impaired, either by decreasing the membrane concentration of ergosterol or by modifying the sterol target molecule to reduce its affinity for the drug.

Antifungal Activity & Clinical Uses Amphotericin B remains the antifungal agent with the broadest spectrum of action. It has activity against the clinically significant yeasts, including Candida albicans and Cryptococcus neoformans; the organisms causing endemic mycoses, including Histoplasma capsulatum, Blastomyces dermatitidis, and Coccidioides immitis; and the pathogenic molds, such as Aspergillus fumigatus and the agents of mucormycosis. Some fungal organisms such as Candida lusitaniae and Pseudallescheria boydii display intrinsic amphotericin B resistance.

Owing to its broad spectrum of activity and fungicidal action, amphotericin B remains a useful agent for nearly all life-threatening mycotic infections, although newer, less toxic agents have largely replaced it for most conditions. Amphotericin B is often used as the initial induction regimen to rapidly reduce fungal burden and then replaced by one of the newer azole drugs (described below) for chronic therapy or prevention of relapse. Such induction therapy is especially important for immunosuppressed patients and those with severe fungal pneumonia, severe cryptococcal meningitis, or disseminated infections with one of the endemic mycoses such as histoplasmosis or coccidioidomycosis. Once a clinical response has been elicited, these patients then often continue maintenance therapy with an azole; therapy may be lifelong in patients at high risk for disease relapse. For treatment of systemic fungal disease, amphotericin B is given by slow intravenous infusion at a dosage of 0.5â&#x20AC;&#x201C;1 mg/kg/d. Intrathecal therapy for fungal meningitis is poorly tolerated and fraught with difficulties related to maintaining cerebrospinal fluid access. Thus, intrathecal therapy with amphotericin B is being increasingly supplanted by other therapies but remains an option in cases of fungal central nervous system infections that have not responded to other agents.

Local or topical administration of amphotericin B has been used with success. Mycotic corneal ulcers and keratitis can be cured with topical drops as well as by direct subconjunctival injection. Fungal arthritis has been treated with adjunctive local injection directly into the joint. Candiduria responds to bladder irrigation with amphotericin B, and this route has been shown to produce no significant systemic toxicity.

Adverse Effects The toxicity of amphotericin B can be divided into two broad categories: immediate reactions, related to the infusion of the drug, and those occurring more slowly.

A. Infusion-Related Toxicity Infusion-related reactions are nearly universal and consist of fever, chills, muscle spasms, vomiting, headache, and hypotension. They can be ameliorated by slowing the infusion rate or decreasing the daily dose. Premedication with antipyretics, antihistamines, meperidine, or corticosteroids can be helpful. When starting therapy, many clinicians administer a test dose of 1 mg intravenously to gauge the severity of the reaction. This can serve as a guide to an initial dosing regimen and premedication strategy.

B. Cumulative Toxicity Renal damage is the most significant toxic reaction. Renal impairment occurs in nearly all patients treated with clinically significant doses of amphotericin. The degree of azotemia is variable and often stabilizes during therapy, but it can be serious enough to necessitate dialysis. A reversible component is associated with decreased renal perfusion and represents a form of prerenal renal failure. An irreversible component results from renal tubular injury and subsequent dysfunction. The irreversible form of amphotericin nephrotoxicity usually occurs in the setting of prolonged administration (> 4 g cumulative dose). Renal toxicity commonly manifests as renal tubular acidosis and severe potassium and magnesium wasting. There is some evidence that the prerenal component can be attenuated with sodium loading, and it is common practice to administer normal saline infusions with the daily doses of amphotericin B. Abnormalities of liver function tests are occasionally seen, as is a varying degree of anemia due to reduced erythropoietin production by damaged renal tubular cells. After intrathecal therapy with amphotericin, seizures and a chemical arachnoiditis may develop, often with serious neurologic sequelae.

FLUCYTOSINE Chemistry & Pharmacokinetics Flucytosine (5-FC) was discovered in 1957 during a search for novel antineoplastic agents. Though devoid of anti-cancer properties, it became apparent that it was a potent antifungal agent. Flucytosine is a water-soluble pyrimidine analog related to the chemotherapeutic agent 5-fluorouracil (5-FU). Its spectrum of action is much narrower than that of amphotericin B.

Flucytosine is currently available in North America only in an oral formulation. The dosage is 100–150 mg/kg/d in patients with normal renal function. It is well absorbed (> 90%), with serum concentrations peaking 1–2 hours after an oral dose. It is poorly proteinbound and penetrates well into all body fluid compartments, including the cerebrospinal fluid. It is eliminated by glomerular filtration with a half-life of 3–4 hours and is removed by hemodialysis. Levels rise rapidly with renal impairment and can lead to toxicity. Toxicity is more likely to occur in AIDS patients and those with renal insufficiency. Peak serum concentrations should be measured periodically in patients with renal insufficiency and maintained between 50 and 100 mcg/mL.

Mechanisms of Action & Resistance Flucytosine is taken up by fungal cells via the enzyme cytosine permease. It is converted intracellularly first to 5-FU and then to 5fluorodeoxyuridine monophosphate (FdUMP) and fluorouridine triphosphate (FUTP), which inhibit DNA and RNA synthesis, respectively (Figure 48–1). Human cells are unable to convert the parent drug to its active metabolites, resulting in selective toxicity.

Synergy with amphotericin B has been demonstrated in vitro and in vivo. It may be related to enhanced penetration of the flucytosine through amphotericin-damaged fungal cell membranes. In vitro synergy with azole drugs has also been seen, although the mechanism is unclear.

Resistance is thought to be mediated through altered metabolism of flucytosine, and, though uncommon in primary isolates, it develops rapidly in the course of flucytosine monotherapy.

Clinical Uses & Adverse Effects The spectrum of activity of flucytosine is restricted to C neoformans, some Candida sp, and the dematiaceous molds that cause chromoblastomycosis. Flucytosine is not used as a single agent because of its demonstrated synergy with other agents and to avoid the development of secondary resistance. Clinical use at present is confined to combination therapy, either with amphotericin B for cryptococcal meningitis or with itraconazole for chromoblastomycosis.

The adverse effects of flucytosine result from metabolism (possibly by intestinal flora) to the toxic antineoplastic compound fluorouracil. Bone marrow toxicity with anemia, leukopenia, and thrombocytopenia are the most common adverse effects, with derangement of liver enzymes occurring less frequently. A form of toxic enterocolitis can occur. There seems to be a narrow therapeutic window, with an increased risk of toxicity at higher drug levels and resistance developing rapidly at subtherapeutic concentrations. The use of drug concentration measurements may be helpful in reducing the incidence of toxic reactions, especially when flucytosine is combined with nephrotoxic agents such as amphotericin B.

AZOLES Chemistry & Pharmacokinetics Azoles are synthetic compounds that can be classified as either imidazoles or triazoles according to the number of nitrogen atoms in the five-membered azole ring, as indicated below. The imidazoles consist of ketoconazole, miconazole, and clotrimazole (Figure 48–2). The latter two drugs are now used only in topical therapy. The triazoles include itraconazole, fluconazole, voriconazole, and posaconazole.

Other triazoles are currently under investigation.

FIGURE 48â&#x20AC;&#x201C;2 Structural formulas of some antifungal azoles.

The pharmacology of each of the azoles is unique and accounts for some of the variations in clinical use. Table 48â&#x20AC;&#x201C;2 summarizes the differences among five of the azoles.

TABLE 48â&#x20AC;&#x201C;2 Pharmacologic properties of five systemic azole drugs.

Mechanisms of Action & Resistance The antifungal activity of azole drugs results from the reduction of ergosterol synthesis by inhibition of fungal cytochrome P450 enzymes (Figure 48â&#x20AC;&#x201C;1). The selective toxicity of azole drugs results from their greater affinity for fungal than for human cytochrome P450 enzymes. Imidazoles exhibit a lesser degree of selectivity than the triazoles, accounting for their higher incidence of drug interactions and adverse effects.

Resistance to azoles occurs via multiple mechanisms. Once rare, increasing numbers of resistant strains are being reported, suggesting that increasing use of these agents for prophylaxis and therapy may be selecting for clinical drug resistance in certain settings.

Clinical Uses, Adverse Effects, & Drug Interactions The spectrum of action of azole medications is broad, including many species of Candida, C neoformans, the endemic mycoses (blastomycosis, coccidioidomycosis, histoplasmosis), the dermatophytes, and, in the case of itraconazole and voriconazole, even Aspergillus infections. They are also useful in the treatment of intrinsically amphotericin-resistant organisms such as P boydii.

As a group, the azoles are relatively nontoxic. The most common adverse reaction is relatively minor gastrointestinal upset. All azoles have been reported to cause abnormalities in liver enzymes and, very rarely, clinical hepatitis. Adverse effects specific to individual agents are discussed below.

All azole drugs are prone to drug interactions because they affect the mammalian cytochrome P450 system of enzymes to some extent. The most significant reactions are indicated below.

KETOCONAZOLE Ketoconazole was the first oral azole introduced into clinical use. It is distinguished from triazoles by its greater propensity to inhibit mammalian cytochrome P450 enzymes; that is, it is less selective for fungal P450 than are the newer azoles. As a result, systemic ketoconazole has fallen out of clinical use in the USA and is not discussed in any detail here. Its dermatologic use is discussed in Chapter 61.

ITRACONAZOLE Itraconazole is available in oral and intravenous formulations and is used at a dosage of 100â&#x20AC;&#x201C;400 mg/d. Drug absorption is increased by food and by low gastric pH. Like other lipid-soluble azoles, it interacts with hepatic microsomal enzymes, though to a lesser degree than ketoconazole. An important drug interaction is reduced bioavailability of itraconazole when taken with rifamycins (rifampin, rifabutin, rifapentine). It does not affect mammalian steroid synthesis, and its effects on the metabolism of other hepatically cleared medications are much less than those of ketoconazole. While itraconazole displays potent antifungal activity, effectiveness can be limited by reduced bioavailability. Newer formulations, including an oral liquid and an intravenous preparation, have utilized cyclodextran as a carrier

molecule to enhance solubility and bioavailability. Like ketoconazole, itraconazole penetrates poorly into the cerebrospinal fluid.

Itraconazole is the azole of choice for treatment of disease due to the dimorphic fungi Histoplasma, Blastomyces, and Sporothrix.

Itraconazole has activity against Aspergillus sp, but it has been replaced by voriconazole as the azole of choice for aspergillosis.

Itraconazole is used extensively in the treatment of dermatophytoses and onychomycosis.

FLUCONAZOLE Fluconazole displays a high degree of water solubility and good cerebrospinal fluid penetration. Unlike ketoconazole and itraconazole, its oral bioavailability is high. Drug interactions are also less common because fluconazole has the least effect of all the azoles on hepatic microsomal enzymes. Because of fewer hepatic enzyme interactions and better gastrointestinal tolerance, fluconazole has the widest therapeutic index of the azoles, permitting more aggressive dosing in a variety of fungal infections. The drug is available in oral and intravenous formulations and is used at a dosage of 100â&#x20AC;&#x201C;800 mg/d.

Fluconazole is the azole of choice in the treatment and secondary prophylaxis of cryptococcal meningitis. Intravenous fluconazole has been shown to be equivalent to amphotericin B in treatment of candidemia in ICU patients with normal white blood cell counts, although echinocandins may have superior activity for this indication. Fluconazole is the agent most commonly used for the treatment of mucocutaneous candidiasis. Activity against the dimorphic fungi is limited to coccidioidal disease, and in particular for meningitis, where high doses of fluconazole often obviate the need for intrathecal amphotericin B. Fluconazole displays no activity against Aspergillus or other filamentous fungi.

Prophylactic use of fluconazole has been demonstrated to reduce fungal disease in bone marrow transplant recipients and AIDS patients, but the emergence of fluconazole-resistant fungi has raised concerns about this indication.

VORICONAZOLE Voriconazole is available in intravenous and oral formulations. The recommended dosage is 400 mg/d. The drug is well absorbed orally, with a bioavailability exceeding 90%, and it exhibits less protein binding than itraconazole. Metabolism is predominantly hepatic.

Voriconazole is a clinically relevant inhibitor of mammalian CYP3A4, and dose reduction of a number of medications is required when voriconazole is started. These include cyclosporine, tacrolimus, and HMG-CoA reductase inhibitors. Observed toxicities include rash and elevated hepatic enzymes. Visual disturbances are common, occurring in up to 30% of patients receiving intravenous voriconazole, and include blurring and changes in color vision or brightness. These visual changes usually occur immediately after a dose of voriconazole and resolve within 30 minutes. Photosensitivity dermatitis is commonly observed in patients receiving chronic oral therapy.

Voriconazole is similar to itraconazole in its spectrum of action, having excellent activity against Candida sp (including fluconazoleresistant species such as Candida krusei) and the dimorphic fungi. Voriconazole is less toxic than amphotericin B and is the treatment of choice for invasive aspergillosis and some environmental molds (see Box: Iatrogenic Fungal Meningitis). Measurement of voriconazole levels may predict toxicity and clinical efficacy, especially in immunocompromised patients. Therapeutic trough levels should be between 1 and 5 mcg/mL.

Iatrogenic Fungal Meningitis In September 2012, the U.S. Centers for Disease Control (CDC) in Atlanta received reports of a number of cases of fungal meningitis in patients who had received injections with the corticosteroid methylprednisolone. An investigation revealed a multistate outbreak of septic arthritis, paraspinal infections, and meningitis due to environmental molds, with the black mold Exserohilum rostratum being the most commonly isolated species. The outbreak was traced to the injection of methylprednisolone that was contaminated during its preparation by a compounding pharmacy facility in New England. Methylprednisolone injections are commonly given to patients with joint or back arthritis, and in the affected cases the patients were not only inadvertently injected with spores of environmental molds, but the normal immune response to this infection was inhibited by the potent immunosuppressive effect of the corticosteroid. While the outbreak investigation is ongoing, as of November 2013 more than 750 cases of fungal infection had been identified in 20 states, with over 60 deaths. Treatment of these infections is challenging, and the CDC has recommended the use of intravenous voriconazole as first-line therapy, with the addition of liposomal amphotericin B in cases of severe infection.

POSACONAZOLE Posaconazole is the newest triazole to be licensed in the USA. It is available only in a liquid oral formulation and is used at a dosage of 800 mg/d, divided into two or three doses. Absorption is improved when taken with meals high in fat. An intravenous form of

posaconazole and a tablet form with higher bioavailability have been evaluated in trials and may soon be available. Posaconazole is rapidly distributed to the tissues, resulting in high tissue levels but relatively low blood levels. Measurement of posaconazole levels is recommended in patients with serious invasive fungal infections (especially mold infections); steady-state posaconazole levels should be between 0.5 and 1.5 mcg/mL. Drug interactions with increased levels of CYP3A4 substrates such as tacrolimus and cyclosporine have been documented.

Posaconazole is the broadest spectrum member of the azole family, with activity against most species of Candida and Aspergillus.

It is the only azole with significant activity against the agents of mucormycosis. It is currently licensed for salvage therapy in invasive aspergillosis, as well as prophylaxis of fungal infections during induction chemotherapy for leukemia, and for allogeneic bone marrow transplant patients with graft-versus-host disease.

ECHINOCANDINS Chemistry & Pharmacokinetics Echinocandins are the newest class of antifungal agents to be developed. They are large cyclic peptides linked to a long-chain fatty acid.

Caspofungin, micafungin, and anidulafungin are the only licensed agents in this category of antifungals, although other drugs are under active investigation. These agents are active against Candida and Aspergillus, but not C neoformans or the agents of zygomycosis and mucormycosis.

Echinocandins are available only in intravenous formulations. Caspofungin is administered as a single loading dose of 70 mg, followed by a daily dose of 50 mg. Caspofungin is water soluble and highly protein-bound. The half-life is 9–11 hours, and the metabolites are excreted by the kidneys and gastrointestinal tract. Dosage adjustments are required only in the presence of severe hepatic insufficiency.

Micafungin displays similar properties with a half-life of 11–15 hours and is used at a dose of 150 mg/d for treatment of esophageal candidiasis, 100 mg/d for treatment of candidemia, and 50 mg/d for prophylaxis of fungal infections. Anidulafungin has a half-life of 24– 48 hours. For esophageal candidiasis, it is administered intravenously at 100 mg on the first day and 50 mg/d thereafter for 14 days. For candidemia, a loading dose of 200 mg is recommended with 100 mg/d thereafter for at least 14 days after the last positive blood culture.

Mechanism of Action Echinocandins act at the level of the fungal cell wall by inhibiting the synthesis of β(1–3)-glucan (Figure 48–1). This results in disruption of the fungal cell wall and cell death.

Clinical Uses & Adverse Effects Caspofungin is currently licensed for disseminated and mucocutaneous candidal infections, as well as for empiric antifungal therapy during febrile neutropenia, and has largely replaced amphotericin B for the latter indication. Of note, caspofungin is licensed for use in invasive aspergillosis only as salvage therapy in patients who have failed to respond to amphotericin B, and not as primary therapy.

Micafungin is licensed for mucocutaneous candidiasis, candidemia, and prophylaxis of candidal infections in bone marrow transplant patients. Anidulafungin is approved for use in esophageal candidiasis and invasive candidiasis, including candidemia.

Echinocandin agents are extremely well tolerated, with minor gastrointestinal side effects and flushing reported infrequently. Elevated liver enzymes have been noted in several patients receiving caspofungin in combination with cyclosporine, and this combination should be avoided. Micafungin has been shown to increase levels of nifedipine, cyclosporine, and sirolimus. Anidulafungin does not seem to have significant drug interactions, but histamine release may occur during intravenous infusion.

ORAL SYSTEMIC ANTIFUNGAL DRUGS FOR MUCOCUTANEOUS INFECTIONS

GRISEOFULVIN Griseofulvin is a very insoluble fungistatic drug derived from a species of penicillium. Its only use is in the systemic treatment of dermatophytosis (see Chapter 61). It is administered in a micro-crystalline form at a dosage of 1 g/d. Absorption is improved when it is given with fatty foods. Griseofulvin’s mechanism of action at the cellular level is unclear, but it is deposited in newly forming skin where it binds to keratin, protecting the skin from new infection. Because its action is to prevent infection of these new skin structures, griseofulvin must be administered for 2–6 weeks for skin and hair infections to allow the replacement of infected keratin by the resistant structures. Nail infections may require therapy for months to allow regrowth of the new protected nail and is often followed by relapse.

Adverse effects include an allergic syndrome much like serum sickness, hepatitis, and drug interactions with warfarin and phenobarbital.

Griseofulvin has been largely replaced by newer antifungal medications such as itraconazole and terbinafine.

TERBINAFINE Terbinafine is a synthetic allylamine that is available in an oral formulation and is used at a dosage of 250 mg/d. It is used in the treatment of dermatophytoses, especially onychomycosis (see Chapter 61). Like griseofulvin, terbinafine is a keratophilic medication, but unlike griseofulvin, it is fungicidal. Like the azole drugs, it interferes with ergosterol biosynthesis, but rather than interacting with the P450 system, terbinafine inhibits the fungal enzyme squalene epoxidase (Figure 48â&#x20AC;&#x201C;1). This leads to the accumulation of the sterol squalene, which is toxic to the organism. One tablet given daily for 12 weeks achieves a cure rate of up to 90% for onychomycosis and is more effective than griseofulvin or itraconazole. Adverse effects are rare, consisting primarily of gastrointestinal upset and headache.

Terbinafine does not seem to affect the P450 system and has demonstrated no significant drug interactions to date.

TOPICAL ANTIFUNGAL THERAPY

NYSTATIN Nystatin is a polyene macrolide much like amphotericin B. It is too toxic for parenteral administration and is only used topically. Nystatin is currently available in creams, ointments, suppositories, and other forms for application to skin and mucous membranes. It is not absorbed to a significant degree from skin, mucous membranes, or the gastrointestinal tract. As a result, nystatin has little toxicity, although oral use is often limited by the unpleasant taste.

Nystatin is active against most Candida sp and is most commonly used for suppression of local candidal infections. Some common indications include oropharyngeal thrush, vaginal candidiasis, and intertriginous candidal infections.

TOPICAL AZOLES The two azoles most commonly used topically are clotrimazole and miconazole; several others are available (see Preparations Available).

Both are available over-the-counter and are often used for vulvovaginal candidiasis. Oral clotrimazole troches are available for treatment of oral thrush and are a pleasant-tasting alternative to nystatin. In cream form, both agents are useful for dermatophytic infections, including tinea corporis, tinea pedis, and tinea cruris. Absorption is negligible, and adverse effects are rare.

Topical and shampoo forms of ketoconazole are also available and useful in the treatment of seborrheic dermatitis and pityriasis versicolor. Several other azoles are available for topical use (see Preparations Available).

TOPICAL ALLYLAMINES Terbinafine and naftifine are allylamines available as topical creams (see Chapter 61). Both are effective for treatment of tinea cruris and tinea corporis. These are prescription drugs in the USA.

SUMMARY Antifungal Drugs

PREPARATIONS AVAILABLE

REFERENCES Andes D et al: Antifungal therapeutic drug monitoring: Established and emerging indications. Antimicrob Agents Chemother 2009;53:24.

BrĂźggemann RJ et al: Clinical relevance of the pharmacokinetic interactions of azole antifungal drugs with other coadministered agents. Clin Infect Dis 2009;48:1441.

Cornely OA et al: Posazonacole vs. fluconazole or itraconazole prophylaxis in patients with neutropenia. N Engl J Med 2007;356:348.

Diekema DJ et al: Activities of caspofungin, itraconazole, posaconazole, ravuconazole, voriconazole, and amphotericin B against 448 recent clinical isolates of filamentous fungi. J Clin Microbiol 2003;41:3623.

Groll A, Piscitelli SC, Walsh T J: Clinical pharmacology of systemic antifungal agents: A comprehensive review of agents in clinical use, current investigational compounds, and putative targets for antifungal drug development. Adv Pharmacol 1998;44:343.

Herbrecht R et al: Voriconazole versus amphotericin B for primary therapy of invasive aspergillosis. N Engl J Med 2002;347:408.

Kim R, Khachikian D, Reboli AC: A comparative evaluation of properties and clinical efficacy of the echinocandins. Expert Opin Pharmacother 2007;8:1479.

Pasqualotto AC, Denning DW: New and emerging treatments for fungal infection. J Antimicrob Chemoth 2008;61(Suppl 1):i19.

Rogers T R: T reatment of zygomycosis: Current and new options. J Antimicrob Chemother 2008;61(Suppl 1):i35.

Wong-Beringer A, Jacobs RA, Guglielmo BJ: Lipid formulations of amphotericin B: Clinical efficacy and toxicities. Clin Infect Dis 1998;27:603.

CASE STUDY ANSWER The club-like nonseptate hyphae observed in cultures of intraoperative specimens from this patient are characteristic of Rhizopus, one of the agents of mucormycosis. This patient should be treated with an initial, prolonged course of therapy with liposomal amphotericin B and subsequent chronic suppressive therapy with posaconazole.

CHAPTER

49 Antiviral Agents Sharon Safrin, MD

CASE STUDY A 35-year-old white woman who recently tested seropositive for both HIV and hepatitis B virus surface antigen is referred for evaluation. She is feeling well overall but reports a 25-pack-year smoking history. She drinks 3-4 beers per week and has no known medication allergies. She has a history of heroin use and is currently receiving methadone. Physical examination reveals normal vital signs and no abnormalities. White blood cell count is 5800 cells/mm3 with a normal differential, hemoglobin is 11.8 g/dL, all liver tests are within normal limits, CD4 cell count is 278 cells/mm3 , and viral load (HIV RNA) is 110,000 copies/mL. What other laboratory tests should be ordered? Which antiretroviral medications would you begin?

Viruses are obligate intracellular parasites; their replication depends primarily on synthetic processes of the host cell. Therefore, to be effective, antiviral agents must either block viral entry into or exit from the cell or be active inside the host cell. As a corollary, nonselective inhibitors of virus replication may interfere with host cell function and result in toxicity.

Progress in antiviral chemotherapy began in the early 1950s, when the search for anti-cancer drugs generated several new compounds capable of inhibiting viral DNA synthesis. The two first-generation antiviral agents, 5-iododeoxyuridine and trifluorothymidine, had poor specificity (ie, they inhibited host cell DNA as well as viral DNA) that rendered them too toxic for systemic use. However, both agents are effective when used topically for the treatment of herpes keratitis.

Knowledge of the mechanisms of viral replication has provided insights into critical steps in the viral life cycle that can serve as potential targets for antiviral therapy. Recent research has focused on identifying agents with greater selectivity, higher potency, in vivo stability, and reduced toxicity. Antiviral therapy is now available for herpesviruses, hepatitis C virus (HCV), hepatitis B virus (HBV), papillomavirus, influenza, human immunodeficiency virus (HIV), and respiratory syncytial virus (RSV). Antiviral drugs share the common property of being virustatic; they are active only against replicating viruses and do not affect latent virus. Whereas some infections require monotherapy for brief periods of time (eg, acyclovir for herpes simplex virus), others require dual therapy for prolonged periods of time (interferon alfa/ribavirin for HCV), whereas still others require multiple drug therapy for indefinite periods (HIV). In chronic illnesses such as viral hepatitis and HIV infection, potent inhibition of viral replication is crucial in limiting the extent of systemic damage.

ACRONYMS & OTHER NAMES

Viral replication requires several steps (Figure 49â&#x20AC;&#x201C;1): (1) attachment of the virus to receptors on the host cell surface; (2) entry of the

virus through the host cell membrane; (3) uncoating of viral nucleic acid; (4) synthesis of early regulatory proteins, eg, nucleic acid polymerases; (5) synthesis of new viral RNA or DNA; (6) integration into the nuclear genome; (7) synthesis of late, structural proteins; (8) assembly (maturation) of viral particles; and (9) release from the cell. Antiviral agents can potentially target any of these steps.

FIGURE 49–1 The major sites of antiviral drug action. Note: Interferon alfas are speculated to have multiple sites of action. (Modified and reproduced, with permission, from T revor AJ, Katzung BG, Masters SB: Pharmacology: Examination & Board Review, 9th ed. McGraw-Hill, 2010. Copyright © T he McGraw-Hill Companies, Inc.)

AGENTS TO TREAT HERPES SIMPLEX VIRUS (HSV) & VARICELLA-ZOSTER VIRUS (VZV) INFECTIONS Three oral nucleoside analogs are licensed for the treatment of HSV and VZV infections: acyclovir, valacyclovir, and famciclovir. They have similar mechanisms of action and comparable indications for clinical use; all are well tolerated. Acyclovir has been the most extensively studied; it was licensed first and is the only one of the three that is available for intravenous use in the United States.

Comparative trials have demonstrated similar efficacies of these three agents for the treatment of HSV but modest superiority of famciclovir and valacyclovir for the treatment of herpes zoster infections.

ACYCLOVIR Acyclovir (eFigure 49–1.1) is an acyclic guanosine derivative with clinical activity against HSV-1, HSV-2, and VZV, but it is

approximately 10 times more potent against HSV-1 and HSV-2 than against VZV. In vitro activity against Epstein-Barr virus (EBV), cytomegalovirus (CMV), and human herpesvirus-6 (HHV-6) is present but weaker.

Acyclovir requires three phosphorylation steps for activation. It is converted first to the monophosphate derivative by the virusspecified thymidine kinase and then to the di- and triphosphate compounds by host cell enzymes (Figure 49–2). Because it requires the viral kinase for initial phosphorylation, acyclovir is selectively activated—and the active metabolite accumulates—only in infected cells.

Acyclovir triphosphate inhibits viral DNA synthesis by two mechanisms: competition with deoxyGTP for the viral DNA polymerase, resulting in binding to the DNA template as an irreversible complex; and chain termination following incorporation into the viral DNA.

FIGURE 49–2 Mechanism of action of antiherpes agents.

The bioavailability of oral acyclovir is low (15–20%) and is unaffected by food. An intravenous formulation is available. Topical formulations produce high concentrations in herpetic lesions, but systemic concentrations are undetectable by this route.

Acyclovir is cleared primarily by glomerular filtration and tubular secretion. The half-life is 2.5–3 hours in patients with normal renal function and 20 hours in patients with anuria. Acyclovir diffuses readily into most tissues and body fluids. Cerebrospinal fluid concentrations are 20–50% of serum values.

Oral acyclovir has multiple uses. In first episodes of genital herpes, oral acyclovir shortens the duration of symptoms by approximately 2 days, the time to lesion healing by 4 days, and the duration of viral shedding by 7 days. In recurrent anogenital herpes, the time course is shortened by 1–2 days. Treatment of first-episode genital herpes does not alter the frequency or severity of recurrent outbreaks. Long-term suppression with oral acyclovir in patients with frequent recurrences of genital herpes decreases the frequency of

symptomatic recurrences and of asymptomatic viral shedding, thus decreasing the rate of sexual transmission. However, outbreaks may resume upon discontinuation of suppressive acyclovir. Oral acyclovir is only modestly beneficial in recurrent herpes labialis. In contrast, acyclovir therapy significantly decreases the total number of lesions, duration of symptoms, and viral shedding in patients with varicella (if begun within 24 hours after the onset of rash) or cutaneous zoster (if begun within 72 hours); the risk of post-herpetic neuralgia is also reduced if treatment is initiated early. However, because VZV is less susceptible to acyclovir than HSV, higher doses are required (Table 49â&#x20AC;&#x201C;1). When given prophylactically to patients undergoing organ transplantation, oral or intravenous acyclovir prevents reactivation of HSV and VZV infection. Evidence from clinical trials suggests that the use of daily acyclovir (400 mg twice daily) may reduce the plasma viral load of HIV-1 and the risk of HIV-associated disease progression in individuals dually infected with HSV-2 and HIV-1.

TABLE 49â&#x20AC;&#x201C;1 Agents to treat or prevent herpes simplex virus (HSV) and varicella-zoster virus (VZV) infections.

Intravenous acyclovir is the treatment of choice for herpes simplex encephalitis, neonatal HSV infection, and serious HSV or VZV infections (Table 49–1). In neonates with central nervous system HSV, oral acyclovir suppression for 6 months following acute treatment improves neurodevelopmental outcomes. In immunocompromised patients with VZV infection, intravenous acyclovir reduces the incidence of cutaneous and visceral dissemination.

Topical acyclovir cream is substantially less effective than oral therapy for primary HSV infection. It is of no benefit in treating recurrent genital herpes.

Resistance to acyclovir can develop in HSV or VZV through alteration in either the viral thymidine kinase or the DNA polymerase, and clinically resistant infections have been reported in immunocompromised hosts. Most clinical isolates are resistant on the basis of deficient thymidine kinase activity and thus are cross-resistant to valacyclovir, famciclovir, and ganciclovir. Agents such as foscarnet, cidofovir, and trifluridine do not require activation by viral thymidine kinase and thus have preserved activity against the most prevalent acyclovir-resistant strains (Figure 49–2).

Acyclovir is generally well tolerated, although nausea, diarrhea, and headache may occur. Intravenous infusion may be associated with reversible renal toxicity (ie, crystalline nephropathy or interstitial nephritis) or neurologic effects (eg, tremors, delirium, seizures).

However, these are uncommon with adequate hydration and avoidance of rapid infusion rates. High doses of acyclovir cause chromosomal damage and testicular atrophy in rats, but there has been no evidence of teratogenicity, reduction in sperm production, or cytogenetic alterations in peripheral blood lymphocytes in patients receiving daily suppression of genital herpes for more than 10 years. A recent study found no evidence of increased birth defects in 1150 infants who were exposed to acyclovir during the first trimester. In fact, the American College of Obstetricians and Gynecologists recommends suppressive acyclovir therapy beginning at week 36 in pregnant women with active recurrent genital herpes to reduce the risk of recurrence at delivery and possibly the need for cesarean section. The impact of this intervention on neonatal infection has not been established.

Concurrent use of nephrotoxic agents may enhance the potential for nephrotoxicity. Probenecid and cimetidine decrease acyclovir clearance and increase exposure. Somnolence and lethargy may occur in patients receiving concomitant zidovudine and acyclovir.

VALACYCLOVIR Valacyclovir is the L-valyl ester of acyclovir. It is rapidly converted to acyclovir after oral administration via first-pass enzymatic hydrolysis in the liver and intestine, resulting in serum levels that are three to five times greater than those achieved with oral acyclovir and approximate those achieved with intravenous acyclovir. Oral bioavailability is 54–70%, and cerebrospinal fluid levels are about 50% of those in serum. Elimination half-life is 2.5–3.3 hours.

Twice-daily valacyclovir is effective for treatment of first or recurrent genital herpes and varicella and zoster infections; it is approved for use as a 1-day treatment for orolabial herpes and as suppression of frequently recurring genital herpes (Table 49–1). Oncedaily dosing of valacyclovir for chronic suppression in persons with recurrent genital herpes has been shown to markedly decrease the risk of sexual transmission. In comparative trials with acyclovir for the treatment of patients with zoster, rates of cutaneous healing were similar, but valacyclovir was associated with a shorter duration of zoster-associated pain. Higher doses of valacyclovir (2 g four times daily) are effective in preventing CMV disease after organ transplantation and suppressive valacyclovir prevents VZV reactivation after hematopoietic stem cell transplantation.

Valacyclovir is generally well tolerated, although nausea, headache, vomiting, or rash may occur. At high doses, confusion, hallucinations, and seizures have been reported. AIDS patients who received high-dosage valacyclovir chronically (ie, 8 g/d) had increased gastrointestinal intolerance as well as thrombotic thrombocytopenic purpura/hemolytic-uremic syndrome; this dose has also been associated with confusion and hallucinations in transplant patients. In a recent study, there was no evidence of increased birth defects in 181 infants who were exposed to valacyclovir during the first trimester.

FAMCICLOVIR Famciclovir is the diacetyl ester prodrug of 6-deoxypenciclovir, an acyclic guanosine analog (eFigure 49–2.1). After oral administration, famciclovir is rapidly deacetylated and oxidized by first-pass metabolism to penciclovir. It is active in vitro against HSV-1, HSV-2, VZV, EBV, and HBV. As with acyclovir, activation by phosphorylation is catalyzed by the virus-specified thymidine kinase in infected cells, followed by competitive inhibition of the viral DNA polymerase to block DNA synthesis. Unlike acyclovir, however, penciclovir does not cause chain termination. Penciclovir triphosphate has lower affinity for the viral DNA polymerase than acyclovir triphosphate, but it achieves higher intracellular concentrations. The most commonly encountered clinical mutants of HSV are thymidine kinase-deficient; these are cross-resistant to acyclovir and famciclovir.

The bioavailability of penciclovir from orally administered famciclovir is 70%. The intracellular half-life of penciclovir triphosphate is prolonged, at 7–20 hours. Penciclovir is excreted primarily in the urine.

Oral famciclovir is effective for the treatment of first and recurrent genital herpes, for chronic daily suppression of genital herpes, for treatment of herpes labialis, and for the treatment of acute zoster (Table 49–1). One-day usage of famciclovir significantly accelerates time to healing of recurrent genital herpes and of herpes labialis. Comparison of famciclovir to valacyclovir for treatment of herpes zoster

in immunocompetent patients showed similar rates of cutaneous healing and pain resolution; both agents shortened the duration of zosterassociated pain compared with acyclovir.

Oral famciclovir is generally well tolerated, although headache, nausea, or diarrhea may occur. As with acyclovir, testicular toxicity has been demonstrated in animals receiving repeated doses. However, men receiving daily famciclovir (250 mg every 12 hours) for 18 weeks had no changes in sperm morphology or motility. In a recent study, there was no evidence of increased birth defects in 32 infants who were exposed to famciclovir during the first trimester. The incidence of mammary adenocarcinoma was increased in female rats receiving famciclovir for 2 years.

PENCICLOVIR The guanosine analog penciclovir, the active metabolite of famciclovir, is available for topical use. Penciclovir cream (1%) shortened the median duration of recurrent herpes labialis by ~ 17 hours compared to placebo when applied within 1 hour of the onset of prodromal symptoms and continued every 2 hours during waking hours for 4 days. Adverse effects are uncommon, other than application site reactions in ~1%.

DOCOSANOL Docosanol is a saturated 22-carbon aliphatic alcohol that inhibits fusion between the host cell plasma membrane and the HSV envelope, thereby preventing viral entry into cells and subsequent viral replication. Topical docosanol 10% cream is available without a prescription.

When applied within 12 hours of the onset of prodromal symptoms, five times daily, median healing time was shortened by 18 hours compared with placebo in recurrent orolabial herpes. Application site reactions occur in ~2%.

TRIFLURIDINE Trifluridine (trifluorothymidine) is a fluorinated pyrimidine nucleoside that inhibits viral DNA synthesis in HSV-1, HSV-2, CMV, vaccinia, and some adenoviruses. It is phosphorylated intracellularly by host cell enzymes, and then competes with thymidine triphosphate for incorporation by the viral DNA polymerase (Figure 49–2). Incorporation of trifluridine triphosphate into both viral and host DNA prevents its systemic use. Application of a 1% solution is effective in treating keratoconjunctivitis and recurrent epithelial keratitis due to HSV-1 or HSV-2. Cutaneous application of trifluridine solution, alone or in combination with interferon alfa, has been used successfully in the treatment of acyclovir-resistant HSV infections.

INVESTIGATIONAL AGENTS Valomaciclovir is an inhibitor of the viral DNA polymerase; it is currently under clinical evaluation for the treatment of patients with acute zoster and acute EBV infection (infectious mononucleosis).

AGENTS TO TREAT CYTOMEGALOVIRUS (CMV) INFECTIONS CMV infections occur primarily in the setting of advanced immunosuppression and are typically due to reactivation of latent infection.

Dissemination of infection results in end-organ disease, including retinitis, colitis, esophagitis, central nervous system disease, and pneumonitis. Although the incidence in HIV-infected patients has markedly decreased with the advent of potent anti-retroviral therapy, clinical reactivation of CMV infection after organ transplantation is still prevalent.

The availability of oral valganciclovir has decreased the use of intravenous ganciclovir, intravenous foscarnet, and intravenous cidofovir for the prophylaxis and treatment of end-organ CMV disease (Table 49–2). Oral valganciclovir has replaced oral ganciclovir because of its lower pill burden.

TABLE 49–2 Agents to treat cytomegalovirus (CMV) infection.

GANCICLOVIR Ganciclovir is an acyclic guanosine analog (eFigure 49–2.1) that requires activation by triphosphorylation before inhibiting the viral DNA polymerase. Initial phosphorylation is catalyzed by the virus-specified protein kinase phosphotransferase UL97 in CMV-infected cells.

The activated compound competitively inhibits viral DNA polymerase and causes termination of viral DNA elongation ( Figure 49–2).

Ganciclovir has in vitro activity against CMV, HSV, VZV, EBV, HHV-6, and HHV-8. Its activity against CMV is up to 100 times greater than that of acyclovir.

Ganciclovir is administered intravenously; the bioavailability of oral ganciclovir is poor, and it is no longer available in the US. Ganciclovir gel is available for the treatment of acute herpetic keratitis. Cerebrospinal fluid concentrations are approximately 50% of serum concentrations. The elimination half-life is 4 hours, and the intracellular half-life is prolonged at 16–24 hours. Clearance of the drug is linearly related to creatinine clearance. Ganciclovir is readily cleared by hemodialysis.

Intravenous ganciclovir has been shown to delay progression of CMV retinitis in immunocompromised patients. Dual therapy with foscarnet and ganciclovir is more effective in delaying progression of retinitis than either drug alone in patients with AIDS (see Foscarnet), although adverse effects are compounded. Intravenous ganciclovir is also used to treat CMV colitis, esophagitis, and pneumonitis (the latter often in combination with intravenous cytomegalovirus immunoglobulin) in immunocompromised patients.

Intravenous ganciclovir, followed by either oral ganciclovir or high-dose oral acyclovir, reduced the risk of CMV infection in transplant recipients. Limited data in infants with symptomatic congenital neurologic CMV disease suggest that treatment with IV ganciclovir may reduce hearing loss. The risk of Kaposi’s sarcoma is reduced in AIDS patients receiving long-term ganciclovir, presumably because of activity against HHV-8.

Intravitreal injections of ganciclovir may be used to treat CMV retinitis. Concurrent therapy with a systemic anti-CMV agent is necessary to prevent other sites of end-organ CMV disease. The intraocular ganciclovir implant is no longer available in the USA. Resistance to ganciclovir increases with duration of use. The more common mutation, in UL97, results in decreased levels of the triphosphorylated (ie, active) form of ganciclovir. The less common UL54 mutation in DNA polymerase results in higher levels of resistance and potential cross-resistance with cidofovir and foscarnet. Antiviral susceptibility testing is recommended in patients in whom resistance is suspected clinically.

The most common adverse effect of intravenous ganciclovir treatment is myelosuppression, which although reversible may be doselimiting. Myelosuppression may be additive in patients receiving concurrent zidovudine, azathioprine, or mycophenolate mofetil. Other potential adverse effects are nausea, diarrhea, fever, rash, headache, insomnia, and peripheral neuropathy. Central nervous system toxicity (confusion, seizures, psychiatric disturbance) and hepatotoxicity have been rarely reported. Intravitreal ganciclovir has been associated with vitreous hemorrhage and retinal detachment. Ganciclovir is mutagenic in mammalian cells and carcinogenic and embryotoxic at high doses in animals and causes aspermatogenesis; the clinical significance of these preclinical data is unclear.

Levels of ganciclovir may rise in patients concurrently taking probenecid or trimethoprim. Concurrent use of ganciclovir with didanosine may result in increased levels of didanosine.

VALGANCICLOVIR Valganciclovir is an L-valyl ester prodrug of ganciclovir that exists as a mixture of two diastereomers. After oral administration, both diastereomers are rapidly hydrolyzed to ganciclovir by esterases in the intestinal wall and liver.

Valganciclovir is well absorbed; the bioavailability of oral valganciclovir is 60% and it is recommended that the drug be taken with food. The AUC 0–24h resulting from valganciclovir (900 mg once daily) is similar to that after 5 mg/kg once daily of intravenous ganciclovir and approximately 1.65 times that of oral ganciclovir. The major route of elimination is renal, through glomerular filtration and active tubular secretion. Plasma concentrations of valganciclovir are reduced approximately 50% by hemodialysis.

Valganciclovir is as effective as intravenous ganciclovir for the treatment of CMV retinitis and is also indicated for the prevention of CMV disease in high-risk solid organ and bone marrow transplant recipients. Adverse effects, drug interactions, and resistance patterns are the same as those associated with ganciclovir.

FOSCARNET Foscarnet (phosphonoformic acid) is an inorganic pyrophosphate analog that inhibits herpesvirus DNA polymerase, RNA polymerase, and HIV reverse transcriptase directly without requiring activation by phosphorylation. Foscarnet blocks the pyrophosphate binding site of these enzymes and inhibits cleavage of pyrophosphate from deoxynucleotide triphosphates. It has in vitro activity against HSV, VZV, CMV, EBV, HHV-6, HHV-8, HIV-1, and HIV-2.

Foscarnet is available in an intravenous formulation only; poor oral bioavailability and gastrointestinal intolerance preclude oral use.

Cerebrospinal fluid concentrations are 43–67% of steady-state serum concentrations. Although the mean plasma half-life is 3–7 hours, up to 30% of foscarnet may be deposited in bone, with a half-life of several months. The clinical repercussions of this are unknown.

Clearance of foscarnet is primarily renal and is directly proportional to creatinine clearance. Serum drug concentrations are reduced approximately 50% by hemodialysis.

Foscarnet is effective in the treatment of end-organ CMV disease (ie, retinitis, colitis, and esophagitis), including ganciclovir-resistant disease; it is also effective against acyclovir-resistant HSV and VZV infections. The dosage of foscarnet must be titrated according to the patient’s calculated creatinine clearance before each infusion. Use of an infusion pump to control the rate of infusion is important to prevent toxicity, and large volumes of fluid are required because of the drug’s poor solubility. The combination of ganciclovir and foscarnet is synergistic in vitro against CMV and has been shown to be superior to either agent alone in delaying progression of retinitis; however, toxicity is also increased when these agents are administered concurrently. As with ganciclovir, a decrease in the incidence of Kaposi’s sarcoma has been observed in patients who have received long-term foscarnet.

Foscarnet has been administered intravitreally for the treatment of CMV retinitis in patients with AIDS, but data regarding efficacy and safety are incomplete.

Resistance to foscarnet in HSV and CMV isolates is due to point mutations in the DNA polymerase gene and is typically associated with prolonged or repeated exposure to the drug. Mutations in the HIV-1 reverse transcriptase gene have also been described. Although foscarnet-resistant CMV isolates are typically cross-resistant to ganciclovir, foscarnet activity is usually maintained against ganciclovirand cidofovir-resistant isolates of CMV. Potential adverse effects of foscarnet include renal impairment, hypo- or hypercalcemia, hypo- or hyperphosphatemia, hypokalemia, and hypomagnesemia. Saline preloading helps prevent nephrotoxicity, as does avoidance of concomitant administration of drugs with nephrotoxic potential (eg, amphotericin B, pentamidine, aminoglycosides). The risk of severe hypocalcemia, caused by chelation of divalent cations, is increased with concomitant use of pentamidine. Genital ulcerations associated with foscarnet therapy may be due to high levels of ionized drug in the urine. Nausea, vomiting, anemia, elevation of liver enzymes, and fatigue have been reported; the risk of anemia may be additive in patients receiving concurrent zidovudine. Central nervous system toxicity includes headache, hallucinations, and seizures; the risk of seizures may be increased with concurrent use of imipenem. Foscarnet caused chromosomal damage in preclinical studies.

CIDOFOVIR

Cidofovir (eFigure 49–2.1) is a cytosine nucleotide analog with in vitro activity against CMV, HSV-1, HSV-2, VZV, EBV, HHV-6, HHV-8, adenovirus, poxviruses, polyomaviruses, and human papillomavirus. In contrast to ganciclovir, phosphorylation of cidofovir to the active diphosphate is independent of viral enzymes (Figure 49–2); thus activity is maintained against thymidine kinase-deficient or -altered strains of CMV or HSV. Cidofovir diphosphate acts both as a potent inhibitor of and as an alternative substrate for viral DNA polymerase, competitively inhibiting DNA synthesis and becoming incorporated into the viral DNA chain. Cidofovir-resistant isolates tend to be cross-resistant with ganciclovir but retain susceptibility to foscarnet.

Although the terminal half-life of cidofovir is approximately 2.6 hours, the active metabolite cidofovir diphosphate has a prolonged intracellular half-life of 17–65 hours, thus allowing infrequent dosing. A separate metabolite, cidofovir phosphocholine, has a half-life of at least 87 hours and may serve as an intracellular reservoir of active drug. Cerebrospinal fluid penetration is poor. Elimination is by active renal tubular secretion. High-flux hemodialysis reduces serum levels of cidofovir by approximately 75%.

Intravenous cidofovir is effective for the treatment of CMV retinitis and is used experimentally to treat adenovirus, human papillomavirus, BK polyomavirus, vaccinia, and poxvirus infections. Intravenous cidofovir must be administered with high-dose probenecid (2 g at 3 hours before the infusion and 1 g at 2 and 8 hours after), which blocks active tubular secretion and decreases nephrotoxicity. Before each infusion, cidofovir dosage must be adjusted for alterations in the calculated creatinine clearance or for the presence of urine protein, and aggressive adjunctive hydration is required. Initiation of cidofovir therapy is contraindicated in patients with existing renal insufficiency. Direct intravitreal administration of cidofovir is not recommended because of ocular toxicity.

The primary adverse effect of intravenous cidofovir is a dose-dependent proximal tubular nephrotoxicity, which may be reduced with prehydration using normal saline. Proteinuria, azotemia, metabolic acidosis, and Fanconi’s syndrome may occur. Concurrent administration of other potentially nephrotoxic agents (eg, amphotericin B, aminoglycosides, nonsteroidal anti-inflammatory drugs, pentamidine, foscarnet) should be avoided. Prior administration of foscarnet may increase the risk of nephrotoxicity. Other potential adverse effects include uveitis, ocular hypotony, and neutropenia (15–24%). Concurrent probenecid use may result in other toxicities or drug-drug interactions (see Chapter 36). Cidofovir is mutagenic, gonadotoxic, and embryotoxic, and causes hypospermia and mammary adenocarcinomas in animals.

ANTIRETROVIRAL AGENTS Substantial advances have been made in antiretroviral therapy since the introduction of the first agent, zidovudine, in 1987. Six classes of antiretroviral agents are currently available for use: nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs), non-nucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (PIs), fusion inhibitors, CCR5 co-receptor antagonists (also called entry inhibitors), and HIV integrase strand transfer inhibitors (INSTIs) (Table 49–3). These agents inhibit HIV replication at different parts of the cycle (Figure 49–3). Structures of some of these drugs are shown in eFigure 49–3.1.

TABLE 49–3 Currently available antiretroviral agents.

FIGURE 49–3 Life cycle of HIV. Binding of viral glycoproteins to host cell CD4 and chemokine receptors leads to fusion of the viral and host cell membranes via gp41 and entry of the virion into the cell. After uncoating, reverse transcription copies the single-stranded HIV RNA genome into double-stranded DNA, which is integrated into the host cell genome. Gene transcription by host cell enzymes produces messenger RNA, which is translated into proteins that assemble into immature noninfectious virions that bud from the host cell membrane. Maturation into fully infectious virions is through proteolytic cleavage. NNRTIs, nonnucleoside reverse transcriptase inhibitors; NRTIs, nucleoside/nucleotide reverse transcriptase inhibitors.

Greater knowledge of viral dynamics through the use of viral load and resistance testing has made it clear that combination therapy with maximally potent agents will reduce viral replication to the lowest possible level, thereby reducing the number of cumulative mutations and decreasing the likelihood of emergence of resistance. Thus, administration of combination antiretroviral therapy, typically including at least three antiretroviral agents with differing susceptibility patterns, has become the standard of care. Viral susceptibility to specific agents varies among patients and may change with time. Therefore, such combinations must be chosen with care and tailored to the individual, as must changes to a given regimen. In addition to potency and susceptibility, important factors in the selection of agents for any given patient are tolerability, convenience, and optimization of adherence. As new agents have become available, several older ones have had diminished usage, because of either suboptimal safety or inferior antiviral efficacy. Zalcitabine (ddC; dideoxycytidine), for example, is no longer marketed.

Decrease of the circulating viral load by antiretroviral therapy is correlated with enhanced survival as well as decreased morbidity.

Also, recent evidence suggests that in addition to providing clinical benefits for the patient, the use of antiretroviral therapy strongly reduces the risk for heterosexual HIV transmission.

Discussion of antiretroviral agents in this chapter is specific to HIV-1. Patterns of susceptibility of HIV-2 to these agents may vary; however, there is generally innate resistance to the NNRTIs and lower barriers of resistance to NRTIs and PIs; data regarding maraviroc are inconclusive.

NUCLEOSIDE & NUCLEOTIDE REVERSE TRANSCRIPTASE INHIBITORS (NRTIs) NRTIs are considered the “backbone” of antiretroviral therapy and are generally used in combination with other classes of agents, such as an NNRTI, PI, or integrase inhibitor. NRTIs are usually given in pairs, and many are available as coformulations in order to decrease pill burden and improve adherence. However, certain NRTI combinations should be avoided, due to either drug-drug interactions (eg, didanosine plus tenofovir; see Table 49–4), similar resistance patterns (eg, lamivudine plus emtricitabine) or overlapping toxicities (eg, stavudine plus didanosine).

TABLE 49–4 Clinically significant drug-drug interactions pertaining to two-drug antiretroviral combinations.1

The NRTIs act by competitive inhibition of HIV-1 reverse transcriptase; incorporation into the growing viral DNA chain causes premature chain termination due to inhibition of binding with the incoming nucleotide (Figure 49â&#x20AC;&#x201C;3). Each agent requires intracytoplasmic activation via phosphorylation by cellular enzymes to the triphosphate form.

Typical resistance mutations include M184V, L74V, D67N, and M41L. Lamivudine or emtricitabine therapy tends to select rapidly for the M184V mutation in regimens that are not fully suppressive. While the M184V mutation confers reduced susceptibility to abacavir, didanosine, and zalcitabine, its presence may restore phenotypic susceptibility to zidovudine. The K65R/N mutation is associated with

reduced susceptibility to tenofovir, abacavir, lamivudine, and emtricitabine.

All NRTIs may be associated with mitochondrial toxicity, probably owing to inhibition of mitochondrial DNA polymerase gamma.

Less commonly, lactic acidosis with hepatic steatosis may occur, which can be fatal. NRTI treatment should be suspended in the setting of rapidly rising aminotransferase levels, progressive hepatomegaly, or metabolic acidosis of unknown cause. The thymidine analogs zidovudine and stavudine may be particularly associated with dyslipidemia and insulin resistance. Also, some evidence suggests an increased risk of myocardial infarction in patients receiving abacavir; this remains unproven.

ABACAVIR Abacavir is a guanosine analog that is well absorbed following oral administration (83%) and is unaffected by food. The serum half-life is 1.5 hours. The drug undergoes hepatic glucuronidation and carboxylation. Since the drug is metabolized by alcohol dehydrogenase, serum levels of abacavir may be increased with concurrent alcohol (ie, ethanol) ingestion. Cerebrospinal fluid levels are approximately one-third those of plasma. Abacavir is available in a fixed dose formulation with lamivudine and also with zidovudine plus lamivudine.

High-level resistance to abacavir appears to require at least two or three concomitant mutations and thus tends to develop slowly.

Hypersensitivity reactions, occasionally fatal, have been reported in up to 8% of patients receiving abacavir and may be more severe in association with once-daily dosing. Symptoms, which generally occur within the first 6 weeks of therapy, include fever, fatigue, nausea, vomiting, diarrhea, and abdominal pain. Respiratory symptoms such as dyspnea, pharyngitis, and cough may also be present, and skin rash occurs in about 50% of patients. The laboratory abnormalities of a mildly elevated serum aminotransferase or creatine kinase level may be present but are nonspecific. Although the syndrome tends to resolve quickly with discontinuation of medication, rechallenge with abacavir results in return of symptoms within hours and may be fatal. Screening for HLA-B*5701 before initiation of abacavir therapy is recommended to identify patients with a markedly increased risk for abacavir-associated hypersensitivity reaction. Although the positive predictive value of this test is only about 50%, it has a negative predictive value approaching 100%.

Other potential adverse events are rash, fever, nausea, vomiting, diarrhea, headache, dyspnea, fatigue, and pancreatitis (rare). In some studies but not in others, abacavir has been associated with a higher risk of myocardial infarction. Since abacavir may lower methadone levels, patients receiving these two agents concurrently should be monitored for signs of opioid withdrawal and may require an increased dose of methadone.

DIDANOSINE Didanosine (ddI) is a synthetic analog of deoxyadenosine. Oral bioavailability is approximately 40%. Dosing on an empty stomach is optimal, but buffered formulations are necessary to prevent inactivation by gastric acid (Table 49–3). Cerebrospinal fluid concentrations of the drug are approximately 20% of serum concentrations. Serum half-life is 1.5 hours, but the intracellular half-life of the activated compound is as long as 20–24 hours. The drug is eliminated by both cellular metabolism and renal excretion.

The major clinical toxicity associated with didanosine therapy is dose-dependent pancreatitis. Other risk factors for pancreatitis (eg, alcohol abuse, hypertriglyceridemia) are relative contraindications, and concurrent use of drugs with the potential to cause pancreatitis, including zalcitabine, stavudine, ribavirin, and hydroxyurea, should be avoided (Table 49–3). The risk of peripheral distal sensory neuropathy, another potential toxicity, may be increased with concurrent use of stavudine, isoniazid, vincristine, or ribavirin. Other reported adverse effects include diarrhea (particularly with the buffered formulation), hepatitis, esophageal ulceration, cardiomyopathy, central nervous system toxicity (headache, irritability, insomnia), and hypertriglyceridemia. Due to an increased risk of lactic acidosis and hepatic steatosis when combined with stavudine, this combination should be avoided, especially during pregnancy. Previously asymptomatic hyperuricemia may precipitate attacks of gout in susceptible individuals; concurrent use of allopurinol may increase levels of didanosine. Reports of retinal changes and optic neuritis in patients receiving didanosine, particularly in adults receiving high doses and in children, mandate periodic retinal examinations. Lipoatrophy appears to be more common in patients receiving didanosine or other thymidine analogs.

The buffer in didanosine tablets interferes with absorption of indinavir, delavirdine, atazanavir, dapsone, itraconazole, and fluoroquinolone agents; therefore, administration should be separated in time. Serum levels of didanosine are increased when coadministered with tenofovir or ganciclovir, and are decreased by atazanavir, delavirdine, ritonavir, tipranavir, and methadone ( Table 49– 4). Didanosine should not be used in combination with ribavirin.

EMTRICITABINE Emtricitabine (FTC) is a fluorinated analog of lamivudine with a long intracellular half-life (> 24 hours), allowing for once-daily dosing.

Oral bioavailability of the capsules is 93% and is unaffected by food, but penetration into the cerebrospinal fluid is low. Elimination is by both glomerular filtration and active tubular secretion. The serum half-life is about 10 hours.

The oral solution, which contains propylene glycol, is contraindicated in young children, pregnant women, patients with renal or

hepatic failure, and those using metronidazole or disulfiram. Also, because of its activity against HBV, patients co-infected with HIV and HBV should be closely monitored if treatment with emtricitabine is interrupted or discontinued, owing to the likelihood of hepatitis flare.

Emtricitabine is available in a fixed-dose formulation with tenofovir, either alone or in combination with efavirenz, rilpivirine, or elvitegravir plus cobicistat (a boosting agent). Based on results of clinical trials, the combination of tenofovir and emtricitabine is now recommended as pre-exposure prophylaxis to reduce HIV acquisition in men who have sex with men, in heterosexually active men and women, and in injection drug users.

Like lamivudine, the M184V/I mutation is most frequently associated with emtricitabine use and may emerge rapidly in patients receiving regimens that are not fully suppressive. Because of their similar mechanisms of action and resistance profiles, the combination of lamivudine and emtricitabine is not recommended.

The most common adverse effects observed in patients receiving emtricitabine are headache, insomnia, nausea, and rash. In addition, hyperpigmentation of the palms or soles may be observed (~ 3%), particularly in African-Americans (up to 13%).

LAMIVUDINE Lamivudine (3TC) is a cytosine analog (eFigure 49–3.1) with in vitro activity against HIV-1 that is synergistic with a variety of antiretroviral nucleoside analogs—including zidovudine and stavudine—against both zidovudine-sensitive and zidovudine-resistant HIV-1 strains. As with emtricitabine, lamivudine has activity against HBV; therefore, discontinuation in patients that are co-infected with HIV and HBV may be associated with a flare of hepatitis. Lamivudine therapy rapidly selects for the M184V mutation in regimens that are not fully suppressive.

Oral bioavailability exceeds 80% and is not food-dependent. In children, the average cerebrospinal fluid:plasma ratio of lamivudine was 0.2. Serum half-life is 2.5 hours, whereas the intracellular half-life of the triphosphorylated compound is 11–14 hours. Most of the drug is eliminated unchanged in the urine. Lamivudine remains one of the recommended antiretroviral agents in pregnant women (Table 49–5). Lamivudine is available in a fixed-dose formulation with zidovudine and also with abacavir.

TABLE 49–5 The use of antiretroviral agents in pregnancy.1

Potential adverse effects are headache, dizziness, insomnia, fatigue, dry mouth, and gastrointestinal discomfort, although these are typically mild and infrequent. Lamivudine’s bioavailability increases when it is co-administered with trimethoprim-sulfamethoxazole.

Lamivudine and zalcitabine may inhibit the intracellular phosphorylation of one another; therefore, their concurrent use should be avoided if possible.

STAVUDINE

The thymidine analog stavudine (d4T) has high oral bioavailability (86%) that is not food-dependent. The serum half-life is 1.1 hours, the intracellular half-life is 3.0–3.5 hours, and mean cerebrospinal fluid concentrations are 55% of those of plasma. Excretion is by active tubular secretion and glomerular filtration.

The major toxicity is a dose-related peripheral sensory neuropathy. The incidence of neuropathy may be increased when stavudine is administered with other potentially neurotoxic drugs such as didanosine, vincristine, isoniazid, or ribavirin, or in patients with advanced immunosuppression. Symptoms typically resolve upon discontinuation of stavudine; in such cases, a reduced dosage may be cautiously restarted. Other potential adverse effects are pancreatitis, arthralgias, and elevation in serum aminotransferases. Lactic acidosis with hepatic steatosis, as well as lipodystrophy, appear to occur more frequently in patients receiving stavudine than in those receiving other NRTI agents. Moreover, because the co-administration of stavudine and didanosine may increase the incidence of lactic acidosis and pancreatitis, concurrent use should be avoided. This combination has been implicated in several deaths in HIV-infected pregnant women.

A rare adverse effect is a rapidly progressive ascending neuromuscular weakness. Since zidovudine may reduce the phosphorylation of stavudine, these two drugs should not be used together.

TENOFOVIR Tenofovir is an acyclic nucleoside phosphonate (ie, nucleotide) analog of adenosine. Like the nucleoside analogs, tenofovir competitively inhibits HIV reverse transcriptase and causes chain termination after incorporation into DNA. However, only two rather than three intracellular phosphorylations are required for active inhibition of DNA synthesis. Tenofovir is also approved for the treatment of patients with HBV infection.

Tenofovir disoproxil fumarate is a water-soluble prodrug of active tenofovir. The oral bioavailability in fasted patients is approximately 25% and increases to 39% after a high-fat meal. The prolonged serum (12–17 hours) and intracellular half-lives allow once-daily dosing.

Elimination occurs by both glomerular filtration and active tubular secretion, and dosage adjustment in patients with renal insufficiency is recommended.

Tenofovir is available in several fixed-dose formulations with emtricitabine, either alone or in combination with efavirenz, rilpivirine, and elvitegravir plus cobicistat. Based on results of several clinical trials, the combination of tenofovir and emtricitabine is now recommended as pre-exposure prophylaxis to reduce HIV acquisition in men who have sex with men, in heterosexually active men and women, and in injection drug users. The primary mutations associated with resistance to tenofovir are K65R/N and K70E. Gastrointestinal complaints (eg, nausea, diarrhea, vomiting, flatulence) are the most common adverse effects but rarely require discontinuation of therapy. Since tenofovir is formulated with lactose, these may occur more frequently in patients with lactose intolerance. Other potential adverse effects include headache, rash, dizziness, and asthenia. Cumulative loss of renal function has been observed, possibly increased with concurrent use of boosted PI regimens. Acute renal failure and Fanconi’s syndrome have also been reported. For this reason, tenofovir should be used with caution in patients at risk for renal dysfunction. Serum creatinine levels should be monitored during therapy and tenofovir discontinued for new proteinuria, glycosuria, or calculated glomerular filtration rate < 30 mL/min.

Tenofovir-associated proximal renal tubulopathy causes excessive renal phosphate and calcium losses and 1-hydroxylation defects of vitamin D. Osteomalacia has been demonstrated in several animal species, and tenofovir use has been an independent risk factor for bone fracture in some studies. Therefore, monitoring of bone mineral density should be considered with long-term use in those with risk factors for (or known) osteoporosis, as well as in children; additionally, alternative agents could be considered in post-menopausal women. Tenofovir may compete with other drugs that are actively secreted by the kidneys, such as cidofovir, acyclovir, and ganciclovir.

Concurrent use of atazanavir or lopinavir/ritonavir may increase serum levels of tenofovir (Table 49–4).

ZIDOVUDINE Zidovudine (azidothymidine; AZT) is a deoxythymidine analog that is well absorbed (63%) and distributed to most body tissues and fluids, including the cerebrospinal fluid, where drug levels are 60–65% of those in serum. Although the serum half-life averages 1 hour, the intracellular half-life of the phosphorylated compound is 3–4 hours, allowing twice-daily dosing. Zidovudine is eliminated primarily by renal excretion following glucuronidation in the liver.

Zidovudine is available in a fixed-dose formulation with lamivudine, either alone or in combination with abacavir.

Zidovudine was the first antiretroviral agent to be approved and has been well studied. The drug has been shown to decrease the rate of clinical disease progression and prolong survival in HIV-infected individuals. Efficacy has also been demonstrated in the treatment of HIV-associated dementia and thrombocytopenia. Studies evaluating the use of zidovudine during pregnancy, labor, and postpartum showed significant reductions in the rate of vertical transmission, and zidovudine remains one of the first-line agents for use in pregnant women (Table 49–5).

High-level zidovudine resistance is generally seen in strains with three or more of the five most common mutations: M41L, D67N, K70R, T215F, and K219Q. However, the emergence of certain mutations that confer decreased susceptibility to one drug (eg, L74V for didanosine and M184V for lamivudine) may enhance zidovudine susceptibility in previously zidovudine-resistant strains. Withdrawal of zidovudine may permit the reversion of zidovudine-resistant HIV-1 isolates to the susceptible wild-type phenotype.

The most common adverse effect of zidovudine is myelosuppression, resulting in macrocytic anemia (1–4%) or neutropenia (2–8%).

Gastrointestinal intolerance, headaches, and insomnia may occur but tend to resolve during therapy. Lipoatrophy appears to be more common in patients receiving zidovudine or other thymidine analogs. Less common toxicities include thrombocytopenia, hyperpigmentation of the nails, and myopathy. High doses can cause anxiety, confusion, and tremulousness.

Increased serum levels of zidovudine may occur with concomitant administration of probenecid, phenytoin, methadone, fluconazole, atovaquone, valproic acid, and lamivudine, either through inhibition of first-pass metabolism or through decreased clearance. Zidovudine may decrease phenytoin levels. Hematologic toxicity may be increased during co-administration of other myelosuppressive drugs such as ganciclovir, ribavirin, and cytotoxic agents. Combination regimens containing zidovudine and stavudine should be avoided due to in vitro antagonism.

NONNUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS (NNRTIs) The NNRTIs bind directly to HIV-1 reverse transcriptase ( Figure 49–3), resulting in allosteric inhibition of RNA- and DNA-dependent DNA polymerase activity. The binding site of NNRTIs is near to but distinct from that of NRTIs. Unlike the NRTI agents, NNRTIs neither compete with nucleoside triphosphates nor require phosphorylation to be active.

Baseline genotypic testing is recommended prior to initiating NNRTI treatment because primary resistance rates range from approximately 2% to 8%. NNRTI resistance occurs rapidly with monotherapy and can result from a single mutation. The K103N and Y181C mutations confer resistance to the first-generation NNRTIs, but not to the newer agents (ie, etravirine, rilpivirine). Other mutations (eg, L100I, Y188C, G190A) may also confer cross-resistance among the NNRTI class. However, there is no cross-resistance between the NNRTIs and the NRTIs; in fact, some nucleoside-resistant viruses display hypersusceptibility to NNRTIs. As a class, NNRTI agents tend to be associated with varying levels of gastrointestinal intolerance and skin rash, the latter of which may infrequently be serious (eg, Stevens-Johnson syndrome). A further limitation to use of NNRTI agents as a component of antiretroviral therapy is their metabolism by the CYP450 system, leading to innumerable potential drug-drug interactions (Tables 49–3 and 49–4). All NNRTI agents are substrates for CYP3A4 and can act as inducers (nevirapine), inhibitors (delavirdine), or mixed inducers and inhibitors (efavirenz, etravirine). Given the large number of non-HIV medications that are also metabolized by this pathway (see Chapter 4), drug-drug interactions must be expected and looked for; dosage adjustments are frequently required and some combinations are contraindicated.

DELAVIRDINE Delavirdine has an oral bioavailability of about 85%, but this is reduced by antacids or H2 -blockers. It is extensively bound (~ 98%) to plasma proteins and has correspondingly low cerebrospinal fluid levels. Serum half-life is approximately 6 hours.

Skin rash occurs in up to 38% of patients receiving delavirdine; it typically occurs during the first 1–3 weeks of therapy and does not preclude rechallenge. However, severe rash such as erythema multiforme and Stevens-Johnson syndrome have rarely been reported.

Other possible adverse effects are headache, fatigue, nausea, diarrhea, and increased serum aminotransferase levels. Delavirdine has been shown to be teratogenic in rats, causing ventricular septal defects and other malformations at dosages not unlike those achieved in humans. Thus, pregnancy should be avoided when taking delavirdine.

Delavirdine is extensively metabolized by the CYP3A and CYP2D6 enzymes and also inhibits CYP3A4 and 2C9. Therefore, there are numerous potential drug-drug interactions to consider (Tables 49–3 and 49–4). The concurrent use of delavirdine with fosamprenavir and rifabutin is not recommended because of decreased delavirdine levels. Other medications likely to alter delavirdine levels include didanosine, lopinavir, nelfinavir, and ritonavir. Co-administration of delavirdine with indinavir or saquinavir prolongs the elimination half-life of these protease inhibitors, thus allowing them to be dosed twice rather than thrice daily.

EFAVIRENZ Efavirenz can be given once daily because of its long half-life (40–55 hours). It is moderately well absorbed following oral administration (45%). Since toxicity may increase owing to increased bioavailability after a high-fat meal, efavirenz should be taken on an empty stomach. Efavirenz is principally metabolized by CYP3A4 and CYP2B6 to inactive hydroxylated metabolites; the remainder is eliminated in the feces as unchanged drug. It is highly bound to albumin (~ 99%), and cerebrospinal fluid levels range from 0.3% to 1.2% of plasma levels.

The principal adverse effects of efavirenz involve the central nervous system. Dizziness, drowsiness, insomnia, nightmares, and headache tend to diminish with continued therapy; dosing at bedtime may also be helpful. Psychiatric symptoms such as depression, mania, and psychosis have been observed and may necessitate discontinuation. Skin rash has also been reported early in therapy in up to 28% of patients; the rash is usually mild to moderate in severity and typically resolves despite continuation. Rarely, rash has been severe

or life-threatening. Other potential adverse reactions are nausea, vomiting, diarrhea, crystalluria, elevated liver enzymes, and an increase in total serum cholesterol by 10–20%. High rates of fetal abnormalities, such as neural tube defects, occurred in pregnant monkeys exposed to efavirenz in doses roughly equivalent to the human dosage; several cases of congenital anomalies have been reported in humans. Therefore, efavirenz should be avoided in pregnant women, particularly in the first trimester.

As both an inducer and an inhibitor of CYP3A4, efavirenz induces its own metabolism and interacts with the metabolism of many other drugs (Tables 49–3 and 49–4). Since efavirenz may lower methadone levels, patients receiving these two agents concurrently should be monitored for signs of opioid withdrawal and may require an increased dose of methadone.

ETRAVIRINE Etravirine was designed to be effective against strains of HIV that had developed resistance to first-generation NNRTIs, due to mutations such as K103N and Y181C, and is recommended for treatment-experienced patients that have resistance to other NNRTIs. Although etravirine has a higher genetic barrier to resistance than the other NNRTIs, mutations selected by etravirine usually are associated with resistance to efavirenz, nevirapine, and delavirdine.

Etravirine should be taken with a meal to increase systemic exposure. It is highly protein-bound and is primarily metabolized by the liver. Mean terminal half-life is approximately 41 hours.

The most common adverse effects of etravirine are rash, nausea, and diarrhea. The rash is typically mild and usually resolves after 1–2 weeks without discontinuation of therapy. Rarely, rash has been severe or life-threatening. Laboratory abnormalities include elevations in serum cholesterol, triglyceride, glucose, and hepatic transaminase levels. Transaminase elevations are more common in patients with HBV or HCV co-infection.

Etravirine is a substrate as well as an inducer of CYP3A4 and an inhibitor of CYP2C9 and CYP2C19; it has many therapeutically significant drug-drug interactions (Tables 49–3 and 49–4). Some of the interactions are difficult to predict. For example, etravirine may decrease itraconazole and ketoconazole concentrations but increase voriconazole concentrations. Etravirine should not be given with other NNRTIs, unboosted protease inhibitors, atazanavir/ritonavir, fosamprenavir/ritonavir, or tipranavir/ritonavir.

NEVIRAPINE The oral bioavailability of nevirapine is excellent (> 90%) and is not food-dependent. The drug is highly lipophilic and achieves cerebrospinal fluid levels that are 45% of those in plasma. Serum half-life is 25–30 hours. It is extensively metabolized by the CYP3A isoform to hydroxylated metabolites and then excreted, primarily in the urine.

A single dose of nevirapine (200 mg) is effective in the prevention of transmission of HIV from mother to newborn when administered to women at the onset of labor and followed by a 2 mg/kg oral dose to the neonate within 3 days after delivery, and nevirapine remains one of the recommended agents in pregnant women (Table 49–5). There is no evidence of human teratogenicity.

However, resistance has been documented after this single dose.

Rash, usually a maculopapular eruption that spares the palms and soles, occurs in up to 20% of patients, usually in the first 4–6 weeks of therapy. Although typically mild and self-limited, rash is dose-limiting in about 7% of patients. Women appear to have an increased incidence of rash. When initiating therapy, gradual dose escalation over 14 days is recommended to decrease the incidence of rash.

Severe and life-threatening skin rashes have been rarely reported, including Stevens-Johnson syndrome and toxic epidermal necrolysis.

Nevirapine therapy should be immediately discontinued in patients with severe rash and in those with accompanying constitutional symptoms; since rash may accompany hepatotoxicity, liver tests should be assessed. Symptomatic liver toxicity may occur in up to 4% of patients, may be severe, and is more frequent in those with higher pretherapy CD4 cell counts (ie, > 250 cells/mm3 in women and > 400 cells/mm3 in men), in women, and in those with HBV or HCV co-infection. Fulminant, life-threatening hepatitis has been reported, typically within the first 18 weeks of therapy. Other adverse effects include fever, nausea, headache, and somnolence.

Nevirapine is a moderate inducer of CYP3A metabolism, resulting in decreased levels of amprenavir, indinavir, lopinavir, saquinavir, efavirenz, and methadone. Drugs that induce the CYP3A system, such as rifampin, rifabutin, and St. John’s wort, can decrease levels of nevirapine, whereas those that inhibit CYP3A activity, such as fluconazole, ketoconazole, and clarithromycin, can increase nevirapine levels. Since nevirapine may lower methadone levels, patients receiving these two agents concurrently should be monitored for signs of opioid withdrawal and may require an increased dose of methadone.

RILPIVIRINE Rilpivirine is recommended only in treatment-naive patients with HIV-1 RNA ≤100,000 copies/mL, and only in combination with at least 2 other antiretroviral agents. It is available in a fixed dose formulation with emtricitabine and tenofovir.

Rilpivirine must be administered with a meal (preferably high fat or > 400 kcal). Its oral bioavailability can be significantly reduced in

the presence of acid lowering agents. It should be used with caution with antacids and H2 -receptor antagonists. Rilpivirine use with proton-pump inhibitors (PPIs) is contraindicated. The drug is highly protein bound and the terminal elimination half-life is 50 hours.

The E138K substitution emerged most frequently during rilpivirine treatment, commonly in combination with the M184I substitution.

There is cross-resistance with other NNRTIs, and the combination of rilpivirine with other NNRTIs is not recommended.

Rilpivirine is primarily metabolized by CYP3A4, and drugs that induce or inhibit CYP3A4 may thus affect the clearance of rilpivirine.

However, clinically significant drug-drug interactions with other antiretroviral agents have not been identified to date. Concurrent use of carbamazepine, dexamethasone, phenobarbital, phenytoin, proton pump inhibitors, rifabutin, rifampin, rifapentine, and St John’s wort is contraindicated. Methadone withdrawal may be precipitated with concurrent usage.

The most common adverse effects associated with rilpivirine therapy are rash, depression, headache, insomnia, and increased serum aminotransferases. Increased serum cholesterol, and fat redistribution syndrome have been reported. Higher doses have been associated with QTc prolongation.

PROTEASE INHIBITORS (PIs) During the later stages of the HIV growth cycle, the gag and gag-pol gene products are translated into polyproteins, and these become immature budding particles. The HIV protease is responsible for cleaving these precursor molecules to produce the final structural proteins of the mature virion core. By preventing post-translational cleavage of the Gag-Pol polyprotein, protease inhibitors (PIs) prevent the processing of viral proteins into functional conformations, resulting in the production of immature, noninfectious viral particles (Figure 49–3). Unlike the NRTIs, PIs do not need intracellular activation.

Specific genotypic alterations that confer phenotypic resistance are fairly common with these agents, thus contraindicating monotherapy. Some of the most common mutations conferring broad resistance to PIs are substitutions at the 10, 46, 54, 82, 84, and 90 codons; the number of mutations may predict the level of phenotypic resistance. The I50L substitution emerging during atazanavir therapy has been associated with increased susceptibility to other PIs. Darunavir and tipranavir appear to have improved virologic activity in patients harboring HIV-1 resistant to other PIs. As a class, PIs are associated with mild-to-moderate nausea, diarrhea, and dyslipidemia. A syndrome of redistribution and accumulation of body fat that results in central obesity, dorsocervical fat enlargement (buffalo hump), peripheral and facial wasting, breast enlargement, and a cushingoid appearance has been observed, perhaps less commonly with atazanavir (see below). Concurrent increases in triglyceride and low-density lipoprotein levels, along with hyperglycemia and insulin resistance, have also been noted.

Abacavir, lopinavir/ritonavir, and fosamprenavir/ritonavir have been associated with an increased risk of cardiovascular disease in some, but not all, studies. All PIs may be associated with cardiac conduction abnormalities, including PR or QT interval prolongation or both. A baseline electrocardiogram and avoidance of other agents causing prolonged PR or QT intervals should be considered. Drug-induced hepatitis and rare severe hepatotoxicity have been reported to varying degrees with all PIs; the frequency of hepatic events is higher with tipranavir/ritonavir than with other PIs. Whether PI agents are associated with bone loss and osteoporosis after long-term use is under investigation. PIs have been associated with increased spontaneous bleeding in patients with hemophilia A or B; an increased risk of intracranial hemorrhage has been reported in patients receiving tipranavir with ritonavir.

All of the antiretroviral PIs are extensively metabolized by CYP3A4, with ritonavir having the most pronounced inhibitory effect and saquinavir the least. Some PI agents, such as amprenavir and ritonavir, are also inducers of specific CYP isoforms. As a result, there is enormous potential for drug-drug interactions with other antiretroviral agents and other commonly used medications (Tables 49–3 and 49–4). Expert resources about drug-drug interactions should be consulted, as dosage adjustments are frequently required and some combinations are contraindicated. It is noteworthy that the potent CYP3A4 inhibitory properties of ritonavir are used to clinical advantage by having it “boost” the levels of other PI agents when given in combination, thus acting as a pharmacokinetic enhancer rather than an antiretroviral agent. Ritonavir boosting increases drug exposure, thereby prolonging the drug’s half-life and allowing reduction in frequency; in addition, the genetic barrier to resistance is raised.

ATAZANAVIR Atazanavir (eFigure 49–3.1) is an azapeptide PI with a pharmacokinetic profile that allows once-daily dosing. Atazanavir requires an acidic medium for absorption and exhibits pH-dependent aqueous solubility; therefore, it should be taken with meals and separation of ingestion from acid-reducing agents by at least 12 hours is recommended; concurrent proton pump inhibitors are contraindicated.

Atazanavir is able to penetrate both the cerebrospinal and seminal fluids. The plasma half-life is 6–7 hours, which increases to approximately 11 hours when co-administered with ritonavir. The primary route of elimination is biliary; atazanavir should not be given to patients with severe hepatic insufficiency. Atazanavir is one of the recommended antiretroviral agents for pregnant women (Table 49–5).

Resistance to atazanavir has been associated with various known PI mutations as well as with the novel I50L substitution. Whereas some atazanavir resistance mutations have been associated in vitro with decreased susceptibility to other PIs, the I50L mutation has been associated with increased susceptibility to other PIs. The most common adverse effects in patients receiving atazanavir are diarrhea and nausea; vomiting, abdominal pain, headache,

peripheral neuropathy, and skin rash may also occur. As with indinavir, indirect hyperbilirubinemia with overt jaundice may occur in approximately 10% of patients, owing to inhibition of the UGT1A1 glucuronidation enzyme. Elevation of hepatic enzymes has also been observed, usually in patients with underlying HBV or HCV co-infection. Nephrolithiasis has been described in association with atazanavir use, and prolonged use of boosted atazanavir is associated with cumulative loss of renal function. In contrast to the other PIs, atazanavir does not appear to be associated with dyslipidemia, fat redistribution, or the metabolic syndrome.

As an inhibitor of CYP3A4 and CYP2C9, the potential for drug-drug interactions with atazanavir is great (Tables 49–3 and 49–4).

Atazanavir AUC is reduced by up to 76% when combined with a proton pump inhibitor; thus, this combination is to be avoided. In addition, co-administration of atazanavir with other drugs that inhibit UGT1A1, such as irinotecan, may increase its levels. Tenofovir and efavirenz should not be co-administered with atazanavir unless ritonavir is added to boost levels.

DARUNAVIR Darunavir is licensed as a PI that must be co-administered with ritonavir. Darunavir should be taken with meals to improve bioavailability. It is highly protein-bound and primarily metabolized by the liver.

Symptomatic adverse effects of darunavir include diarrhea, nausea, headache, and rash. Laboratory abnormalities include dyslipidemia (though possibly less frequent than with other boosted PI regimens) and increases in amylase and hepatic transaminase levels. Liver toxicity, including severe hepatitis, has been reported in some patients taking darunavir; the risk of hepatotoxicity may be higher for persons with HBV, HCV, or other chronic liver disease. Darunavir contains a sulfonamide moiety and may cause a hypersensitivity reaction, particularly in patients with sulfa allergy.

Darunavir both inhibits and is metabolized by the CYP3A enzyme system, conferring many possible drug-drug interactions (Tables 49–3 and 49–4). In addition, the co-administered ritonavir is a potent inhibitor of CYP3A and CYP2D6, and an inducer of other hepatic enzyme systems.

FOSAMPRENAVIR Fosamprenavir is a prodrug of amprenavir that is rapidly hydrolyzed by enzymes in the intestinal epithelium. Because of its significantly lower daily pill burden, fosamprenavir tablets have replaced amprenavir capsules for adults. Fosamprenavir is most often administered in combination with low-dose ritonavir.

After hydrolysis of fosamprenavir, amprenavir is rapidly absorbed from the gastrointestinal tract, and its prodrug can be taken with or without food. However, high-fat meals decrease absorption and thus should be avoided. The plasma half-life is relatively long (7–11 hours). Amprenavir is metabolized in the liver and should be used with caution in the setting of hepatic insufficiency.

The most common adverse effects of fosamprenavir are headache, nausea, diarrhea, perioral paresthesias, depression.

Fosamprenavir contains a sulfa moiety and may cause a rash in up to 3% of patients, sometimes severe enough to warrant drug discontinuation.

Amprenavir is both an inducer and an inhibitor of CYP3A4 and is contraindicated with numerous drugs (Tables 49–3 and 49–4). The oral solution, which contains propylene glycol, is contraindicated in young children, pregnant women, patients with renal or hepatic failure, and those using metronidazole or disulfiram. Also, the oral solutions of amprenavir and ritonavir should not be co-administered because the propylene glycol in one and the ethanol in the other may compete for the same metabolic pathway, leading to accumulation of either.

Because the oral solution also contains vitamin E at several times the recommended daily dosage, supplemental vitamin E should be avoided. Amprenavir, a sulfonamide, is contraindicated in patients with a history of sulfa allergy. Lopinavir/ritonavir should not be coadministered with amprenavir owing to decreased amprenavir and altered lopinavir exposures. An increased dosage of amprenavir is recommended when co-administered with efavirenz (with or without the addition of ritonavir to boost levels).

INDINAVIR Indinavir requires an acidic environment for optimum solubility and therefore must be consumed on an empty stomach or with a small, low-fat, low-protein meal for maximal absorption (60–65%). The serum half-life is 1.5–2 hours, protein binding is approximately 60%, and the drug has a high level of cerebrospinal fluid penetration (up to 76% of serum levels). Excretion is primarily fecal. An increase in AUC by 60% and in half-life to 2.8 hours in the setting of hepatic insufficiency necessitates dose reduction.

The most common adverse effects of indinavir are indirect hyperbilirubinemia and nephrolithiasis due to urinary crystallization of the drug. Nephrolithiasis can occur within days after initiating therapy, with an estimated incidence of approximately 10%. Consumption of at least 48 ounces of water daily is important to maintain adequate hydration. Thrombocytopenia, elevations of serum aminotransferase levels, nausea, diarrhea, insomnia, dry throat, dry skin, and indirect hyperbilirubinemia have also been reported. Insulin resistance may be more common with indinavir than with the other PIs, occurring in 3–5% of patients. There have also been rare cases of acute hemolytic anemia.

Since indinavir is an inhibitor of CYP3A4, numerous and complex drug interactions can occur (Tables 49–3 and 49–4). Combination with ritonavir (boosting) allows for twice-daily rather than thrice-daily dosing and eliminates the food restriction associated with use of indinavir. However, there is potential for an increase in nephrolithiasis with this combination compared with indinavir alone; thus, a high fluid intake (1.5–2 L/d) is advised.

LOPINAVIR Lopinavir is currently available only in combination with ritonavir, which inhibits the CYP3A-mediated metabolism of lopinavir, thereby resulting in increased exposure to lopinavir. In addition to improved patient compliance due to reduced pill burden, lopinavir/ritonavir is generally well tolerated.

Lopinavir is highly protein bound (98–99%), and its half-life is 5–6 hours. It is extensively metabolized by CYP3A, which is inhibited by ritonavir. Serum levels of lopinavir may be increased in patients with hepatic impairment. Lopinavir/ritonavir is one of the recommended antiretroviral agents for use in pregnant women (Table 49–5).

The most common adverse effects of lopinavir are diarrhea, abdominal pain, nausea, vomiting, and asthenia. Ritonavir-boosted lopinavir may be more commonly associated with gastrointestinal adverse events than other PIs. Elevations in serum cholesterol and triglycerides are common. Prolonged use of boosted lopinavir is associated with cumulative loss of renal function, and lopinavir use has been an independent risk factor for bone fracture in some (but not all) studies.

Potential drug-drug interactions are extensive (Tables 49–3 and 49–4). Increased dosage of lopinavir/ritonavir is recommended when co-administered with efavirenz or nevirapine, which induce lopinavir metabolism. Concurrent use of fosamprenavir should be avoided owing to altered exposure to lopinavir with decreased levels of amprenavir. Also, concomitant use of lopinavir/ritonavir and rifampin is contraindicated due to an increased risk for hepatotoxicity. Since the oral solution of lopinavir/ritonavir contains alcohol, concurrent disulfiram and metronidazole are contraindicated.

NELFINAVIR Nelfinavir has high absorption in the fed state (70–80%), undergoes metabolism by CYP3A, and is excreted primarily in the feces. The plasma half-life in humans is 3.5–5 hours, and the drug is more than 98% protein-bound.

The most common adverse effects associated with nelfinavir are diarrhea and flatulence. Diarrhea often responds to antidiarrheal medications but can be dose-limiting. Nelfinavir is an inhibitor of the CYP3A system, and multiple drug interactions may occur (Tables 49–3 and 49–4). An increased dosage of nelfinavir is recommended when co-administered with rifabutin (with a decreased dose of rifabutin), whereas a decrease in saquinavir dose is suggested with concurrent nelfinavir. Co-administration with efavirenz should be avoided due to decreased nelfinavir levels.

RITONAVIR Ritonavir (eFigure 49–3.1) has a high bioavailability (about 75%) that increases with food. It is 98% protein-bound and has a serum halflife of 3–5 hours. Metabolism to an active metabolite occurs via the CYP3A and CYP2D6 isoforms; excretion is primarily in the feces.

Caution is advised when administering the drug to persons with impaired hepatic function. Ritonavir is one of the recommended antiretroviral agents for use in pregnant women (Table 49–5).

Potential adverse effects of ritonavir, particularly when administered at full dosage, are gastrointestinal disturbances, paresthesias (circumoral or peripheral), elevated serum aminotransferase levels, altered taste, headache, and elevations in serum creatine kinase.

Nausea, vomiting, diarrhea, or abdominal pain typically occur during the first few weeks of therapy but may diminish over time or if the drug is taken with meals. Dose escalation over 1–2 weeks is recommended to decrease the dose-limiting side effects.

Ritonavir is a potent inhibitor of CYP3A4, resulting in many potential drug interactions (Tables 49–3 and 49–4). However, this characteristic has been used to great advantage when ritonavir is administered in low doses (100–200 mg twice daily) in combination with any of the other PI agents, in that increased blood levels of the latter agents permit lower or less frequent dosing (or both) with greater tolerability as well as the potential for greater efficacy against resistant virus. Therapeutic levels of digoxin and theophylline should be monitored when co-administered with ritonavir owing to a likely increase in their concentrations. The concurrent use of saquinavir and ritonavir is contraindicated due to an increased risk of QT prolongation (with torsades de pointes arrhythmia) and PR interval prolongation.

SAQUINAVIR In its original formulation as a hard gel capsule, oral saquinavir was poorly bioavailable (only about 4% after food). However, reformulation of saquinavir for once-daily dosing in combination with low-dose ritonavir has both improved antiviral efficacy and

decreased gastrointestinal adverse effects. A previous formulation of saquinavir in soft gel capsules is no longer available.

Saquinavir should be taken within 2 hours after a fatty meal for enhanced absorption. Saquinavir is 97% protein-bound, and serum half-life is approximately 2 hours. Saquinavir has a large volume of distribution, but penetration into the cerebrospinal fluid is negligible.

Excretion is primarily in the feces. Reported adverse effects include gastrointestinal discomfort (nausea, diarrhea, abdominal discomfort, dyspepsia) and rhinitis. When administered in combination with low-dose ritonavir, there appears to be less dyslipidemia or gastrointestinal toxicity than with some of the other boosted PI regimens. However, the concurrent use of saquinavir and ritonavir may confer an increased risk of QT prolongation (with torsades de pointes arrhythmia) and PR interval prolongation.

Saquinavir is subject to extensive first-pass metabolism by CYP3A4 and functions as a CYP3A4 inhibitor as well as a substrate; thus, there are many potential drug-drug interactions (Tables 49–3 and 49–4). A decreased dose of saquinavir is recommended when coadministered with nelfinavir. Increased saquinavir levels when co-administered with omeprazole necessitate close monitoring for toxicities. Digoxin levels may increase if co-administered with saquinavir and should therefore be monitored. Liver tests should be monitored if saquinavir is co-administered with delavirdine or rifampin.

TIPRANAVIR Tipranavir is a newer PI indicated for use in treatment-experienced patients who harbor strains resistant to other PI agents. It is used in combination with ritonavir to achieve effective serum levels.

Bioavailability is poor but is increased when taken with a high-fat meal. The drug is metabolized by the liver microsomal system and is contraindicated in patients with hepatic insufficiency. Tipranavir contains a sulfonamide moiety and should not be administered to patients with known sulfa allergy.

The most common adverse effects of tipranavir are diarrhea, nausea, vomiting, and abdominal pain. An urticarial or maculopapular rash is more common in women and may be accompanied by systemic symptoms or desquamation. Liver toxicity, including lifethreatening hepatic decompensation, has been observed and may be more common than with other PIs, particularly in patients with chronic HBV or HCV infection. Tipranavir should be discontinued in patients who have increased serum transaminase levels that are more than 10 times the upper limit of normal or more than 5 times normal in combination with increased serum bilirubin. Because of an increased risk for intracranial hemorrhage in patients receiving tipranavir/ritonavir, the drug should be avoided in patients with head trauma or bleeding diathesis. Other potential adverse effects include depression, elevation in amylase, and decreased white blood cell count.

Tipranavir both inhibits and induces the CYP3A4 system. When used in combination with ritonavir, its net effect is inhibition.

Tipranavir also induces P-glycoprotein transporter and thus may alter the disposition of many other drugs (Table 49–4). Concurrent administration of tipranavir with fosamprenavir or saquinavir should be avoided owing to decreased blood levels of the latter drugs.

Tipranavir/ritonavir may also decrease serum levels of valproic acid and omeprazole. Levels of lovastatin, simvastatin, atorvastatin, and rosuvastatin may be increased, increasing the risk for rhabdomyolysis and myopathy.

ENTRY INHIBITORS The process of HIV-1 entry into host cells is complex; each step presents a potential target for inhibition. Viral attachment to the host cell entails binding of the viral envelope glycoprotein complex gp160 (consisting of gp120 and gp41) to its cellular receptor CD4. This binding induces conformational changes in gp120 that enable access to the chemokine receptors CCR5 or CXCR4. Chemokine receptor binding induces further conformational changes in gp120, allowing exposure to gp41 and leading to fusion of the viral envelope with the host cell membrane and subsequent entry of the viral core into the cellular cytoplasm.

ENFUVIRTIDE Enfuvirtide is a synthetic 36-amino-acid peptide fusion inhibitor that blocks HIV entry into the cell (Figure 49–3). Enfuvirtide binds to the gp41 subunit of the viral envelope glycoprotein, preventing the conformational changes required for the fusion of the viral and cellular membranes. It is administered in combination with other antiretroviral agents in treatment-experienced patients with evidence of viral replication despite ongoing antiretroviral therapy.

Enfuvirtide, which must be administered by subcutaneous injection, is the only parenterally administered antiretroviral agent.

Metabolism appears to be by proteolytic hydrolysis without involvement of the CYP450 system. Elimination half-life is 3.8 hours.

Resistance to enfuvirtide can result from mutations in gp41; the frequency and significance of this phenomenon are being investigated. However, enfuvirtide lacks cross-resistance with the other currently approved antiretroviral drug classes.

The most common adverse effects associated with enfuvirtide therapy are local injection site reactions, consisting of painful erythematous nodules. Although frequent, these are typically mild-to-moderate and rarely lead to discontinuation. Other side effects may include insomnia, headache, dizziness, and nausea. Hypersensitivity reactions may rarely occur, are of varying severity, and may recur on

rechallenge. Eosinophilia is the primary laboratory abnormality seen with enfuvirtide administration. No drug-drug interactions have been identified that would require the alteration of the dosage of concomitant antiretroviral or other drugs.

MARAVIROC Maraviroc (eFigure 49–3.1) is approved for use in combination with other antiretroviral agents in treatment-experienced adult patients infected with only CCR5-tropic HIV-1 detectable who are resistant to other antiretroviral agents. Maraviroc binds specifically and selectively to the host protein CCR5, one of two chemokine receptors necessary for entrance of HIV into CD4+ cells. Since maraviroc is active against HIV that uses the CCR5 co-receptor exclusively, and not against HIV strains with CXCR4, dual, or mixed tropism, coreceptor tropism should be determined by specific testing before maraviroc is started, using the enhanced sensitivity tropism assay.

Substantial proportions of patients, particularly those with advanced HIV infection, are likely to have virus that is not exclusively CCR5tropic.

The absorption of maraviroc is rapid but variable, with the time to maximum absorption generally being 1–4 hours after ingestion of the drug. Most of the drug (≥ 75%) is excreted in the feces, whereas approximately 20% is excreted in urine. The recommended dose of maraviroc varies according to renal function and the concomitant use of CYP3A inducers or inhibitors. Maraviroc is contraindicated in patients with severe or end-stage renal impairment who are taking concurrent CYP3A inhibitors or inducers, and caution is advised when used in patients with preexisting hepatic impairment and in those co-infected with HBV or HCV. Maraviroc has excellent penetration into the cervicovaginal fluid, with levels almost four times higher than the corresponding concentrations in blood plasma.

Resistance to maraviroc is associated with one or more mutations in the V3 loop of gp120. There appears to be no cross-resistance with drugs from any other class, including the fusion inhibitor enfuvirtide. However, emergence of CXCR4 virus (either previously undetected or newly developed) appears to be a more common cause of virologic failure than the development of resistance mutations.

Maraviroc is a substrate for CYP3A4 and therefore requires adjustment in the presence of drugs that interact with these enzymes (Tables 49–3 and 49–4). It is also a substrate for P-glycoprotein, which limits intracellular concentrations of the drug. The dosage of maraviroc must be decreased if it is co-administered with strong CYP3A inhibitors (eg, delavirdine, ketoconazole, itraconazole, clarithromycin, or any protease inhibitor other than tipranavir) and must be increased if co-administered with CYP3A inducers (eg, efavirenz, etravirine, rifampin, carbamazepine, phenytoin, or St. John’s wort).

Potential adverse effects of maraviroc include cough, upper respiratory tract infections, muscle and joint pain, diarrhea, sleep disturbance, and elevations in serum aminotransferases. Hepatotoxicity has been reported, which may be preceded by a systemic allergic reaction (ie, pruritic rash, eosinophilia, or elevated IgE); discontinuation of maraviroc should be prompt if this constellation occurs. Also, caution should be used in patients with pre-existing liver dysfunction or who are co-infected with HBV or HCV. Myocardial ischemia and infarction have been observed in patients receiving maraviroc; therefore caution is advised in patients at increased cardiovascular risk.

There has been concern that blockade of the chemokine CCR5 receptor—a human protein—may result in decreased immune surveillance, with a subsequent increased risk of malignancy (eg, lymphoma) or infection. To date, however, there has been no evidence of an increased risk of either malignancy or infection in patients receiving maraviroc.

INTEGRASE STRAND TRANSFER INHIBITORS (INSTIs) This class of agents binds integrase, a viral enzyme essential to the replication of both HIV-1 and HIV-2. By doing so, it inhibits strand transfer, the third and final step of provirus integration, thus interfering with the integration of reverse-transcribed HIV DNA into the chromosomes of host cells (Figure 49–3). As a class, these agents tend to be well tolerated, with headache and gastrointestinal effects being the most commonly reported adverse events. Other nervous system (including neuropsychiatric) effects are often reported but are milder and less frequent than with efavirenz. Limited data suggest that effects upon lipid metabolism are favorable compared with efavirenz and PIs, with more variable findings for elvitegravir than raltegravir and dolutegravir due to co-administration with the boosting agent cobicistat. Rare and severe events include systemic hypersensitivity reactions and rhabdomyolysis.

DOLUTEGRAVIR Dolutegravir may be taken with or without food. The absolute oral bioavailability has not been established. Dolutegravir should be taken 2 hours before or 6 hours after taking cation-containing antacids or laxatives, sucralfate, oral iron supplements, oral calcium supplements, or buffered medications. The terminal half-life is approximately 14 hours.

Dolutegravir is primarily metabolized via UGT1A1 with some contribution from CYP3A. Therefore, drug-drug interactions may occur (Table 49–4). Co-administration with the metabolic inducers phenytoin, phenobarbital, carbamazepine, and St. John’s wort should be avoided. Dolutegravir inhibits the renal organic cation transporter OCT2, thereby increasing plasma concentrations of drugs eliminated via OCT2 such as dofetilide and metformin. For this reason, co-administration with dofetilide is contraindicated and close monitoring, with

potential for dose adjustment, is recommended for co-administration with metformin.

Current evidence suggests that dolutegravir retains activity against some viruses resistant to both raltegravir and elvitegravir.

The most common adverse reactions associated with dolutegravir are insomnia and headache. Hypersensitivity reactions characterized by rash, constitutional findings, and sometimes organ dysfunction, including liver injury, have been reported and may be lifethreatening. The drug should be discontinued immediately if this occurs and not restarted. Other reported side effects include elevations in serum aminotransferases and the fat redistribution syndrome.

ELVITEGRAVIR Elvitegravir requires boosting with an additional drug, such as cobicistat (a pharmacokinetic enhancer that inhibits CYP3A4 as well as certain intestinal transport proteins) or ritonavir. Elvitegravir is therefore available only as a component of a fixed-dose combination, with cobicistat, emtricitabine, and tenofovir. The combined formulation should be taken with food.

Cobicistat can inhibit renal tubular secretion of creatinine, causing increases in serum creatinine that may not be clinically significant; in the fixed-dose formulation it may be difficult to distinguish between cobicistat effect and tenofovir-induced nephrotoxicity. The recommendation is that the fixed-dose combination elvitegravir/cobicistat/tenofovir/emtricitabine should not be initiated in patients with calculated creatinine clearance < 70 mL/min and should be discontinued in those with creatinine clearance < 50 mL/min; discontinuation should be considered if the serum creatinine increases by 0.4 mg/dL or more.

RALTEGRAVIR Absolute bioavailability of the pyrimidinone analog raltegravir has not been established but does not appear to be food-dependent. The drug does not interact with the cytochrome P450 system but is metabolized by glucuronidation, particularly UGT1A1. Inducers or inhibitors of UGT1A1 may affect serum levels of raltegravir. For example, since concurrent use of rifampin substantially decreases raltegravir concentrations, the dose of raltegravir should be increased. Since polyvalent cations (eg, magnesium, calcium, and iron) may bind integrase inhibitors and interfere with their activity, antacids should be used cautiously and ingestion separated by at least 4 hours from raltegravir. The chewable tablets may contain phenylalanine, which can be harmful to patients with phenylketonuria.

Although virologic failure has been uncommon in clinical trials of raltegravir to date, in vitro resistance requires only a single point mutation (eg, at codons 148 or 155). The low genetic barrier to resistance emphasizes the importance of combination therapies and of adherence. Integrase mutations are not expected to affect sensitivity to other classes of antiretroviral agents.

Potential adverse effects of raltegravir include insomnia, headache, dizziness, diarrhea, nausea, fatigue, and muscle aches. Increases in pancreatic amylase, serum aminotransferases, and creatine kinase (with rhabdomyolysis) may occur. Severe, potentially lifethreatening and fatal skin reactions have been reported, including Stevens-Johnson syndrome, hypersensitivity reaction, and toxic epidermal necrolysis.

ANTIHEPATITIS AGENTS INTERFERON ALFA Interferons are host cytokines that exert complex antiviral, immunomodulatory, and antiproliferative actions (see Chapter 55) and some have proven useful in both HBV and HCV. Interferon alfa appears to function by induction of intracellular signals following binding to specific cell membrane receptors, resulting in inhibition of viral penetration, translation, transcription, protein processing, maturation, and release, as well as increased host expression of major histocompatibility complex antigens, enhanced phagocytic activity of macrophages, and augmentation of the proliferation and survival of cytotoxic T cells.

Injectable preparations of interferon alfa are available for treatment of both HBV and HCV infections (Table 49â&#x20AC;&#x201C;6). Interferon alfa2a and interferon alfa-2b may be administered either subcutaneously or intramuscularly; half-life is 2â&#x20AC;&#x201C;5 hours, depending on the route of administration. Alfa interferons are filtered at the glomerulus and undergo rapid proteolytic degradation during tubular reabsorption, such that detection in the systemic circulation is negligible. Liver metabolism and subsequent biliary excretion are considered minor pathways.

TABLE 49â&#x20AC;&#x201C;6 Drugs used to treat viral hepatitis.

The use of pegylated (polyethylene glycol-complexed) interferon alfa-2a and pegylated interferon alfa-2b results in slower clearance, longer terminal half-lives, and steadier concentrations, thus allowing for less frequent dosing. Renal elimination accounts for about 30% of clearance, and clearance is approximately halved in subjects with impaired renal function; dosage must therefore be adjusted.

The adverse effects of interferon alfa include a flu-like syndrome (ie, headache, fevers, chills, myalgias, and malaise) that typically occurs within 6 hours after dosing; this syndrome occurs in more than 30% of patients during the first week of therapy and tends to resolve upon continued administration. Transient hepatic enzyme elevations may occur in the first 8–12 weeks of therapy and appear to be more common in responders. Potential adverse effects during chronic therapy include neurotoxicities (mood disorders, depression, somnolence, confusion, seizures), myelosuppression, profound fatigue, weight loss, rash, cough, myalgia, alopecia, tinnitus, reversible hearing loss, retinopathy, pneumonitis, and possibly cardiotoxicity. Induction of autoantibodies may occur, causing exacerbation or unmasking of autoimmune disease (particularly thyroiditis). The polyethylene glycol molecule is a nontoxic polymer that is readily excreted in the urine.

Contraindications to interferon alfa therapy include hepatic decompensation, autoimmune disease, and history of cardiac arrhythmia.

Caution is advised in the setting of psychiatric disease, epilepsy, thyroid disease, ischemic cardiac disease, severe renal insufficiency, and cytopenia. Alfa interferons are abortifacient in primates and should not be administered in pregnancy. Potential drug-drug interactions include increased theophylline and methadone levels. Co-administration with didanosine is not recommended because of a risk of hepatic failure, and co-administration with zidovudine may exacerbate cytopenias.

TREATMENT OF HEPATITIS B VIRUS INFECTION The goals of chronic HBV therapy are the suppression of HBV DNA to undetectable levels, seroconversion of HBeAg (or more rarely, HBsAg) from positive to negative, and reduction in elevated hepatic transaminase levels. These end points are correlated with improvement in necroinflammatory disease, a decreased risk of hepatocellular carcinoma and cirrhosis, and a decreased need for liver transplantation. All of the currently licensed therapies achieve these goals. However, because current therapies suppress HBV replication without eradicating the virus, initial responses may not be durable. The covalently closed circular (ccc) viral DNA exists in stable form indefinitely within the cell, serving as a reservoir for HBV throughout the life of the cell and resulting in the capacity to reactivate. Relapse is more common in patients co-infected with HBV and hepatitis D virus.

As of 2013 seven drugs were approved for treatment of chronic HBV infection in the United States: five oral nucleoside/nucleotide analogs (lamivudine, adefovir dipivoxil, tenofovir, entecavir, telbivudine) and two injectable interferon drugs (interferon alfa-2b, pegylated interferon alfa-2a) (Table 49–6). The use of interferon has been supplanted by long-acting pegylated interferon, allowing once-weekly rather than daily or thrice-weekly dosing. In general, nucleoside/nucleotide analog therapies have better tolerability and produce a higher response rate than the interferons and are now considered the first line of therapy. Combination therapies may reduce the development of resistance. The optimal duration of therapy remains unknown.

Several anti-HBV agents have anti-HIV activity as well, including tenofovir, lamivudine, and adefovir dipivoxil. Emtricitabine, an NRTI used in HIV infection, has resulted in excellent biochemical, virologic, and histologic improvement in patients with chronic HBV infection, although it is not approved for this indication. Agents with dual HBV and HIV activity are particularly useful as part of a firstline regimen in co-infected patients. However, it is important to note that acute exacerbation of hepatitis may occur upon discontinuation or interruption of these agents; this may be severe or even fatal.

ADEFOVIR DIPIVOXIL Although initially and abortively developed for treatment of HIV infection, adefovir dipivoxil gained approval, at lower and less toxic doses, for treatment of HBV infection. Adefovir dipivoxil is the diester prodrug of adefovir, an acyclic phosphonated adenine nucleotide analog (eFigure 49–4.1). It is phosphorylated by cellular kinases to the active diphosphate metabolite and then competitively inhibits HBV DNA polymerase and causes chain termination after incorporation into viral DNA. Adefovir is active in vitro against a wide range of DNA and RNA viruses, including HBV, HIV, and herpesviruses.

FIGURE 49â&#x20AC;&#x201C;4 Life cycle of HCV and mechanisms of drug action. (Adapted, with permission, from Asselah T, Marcellin P: Directacting antivirals for the treatment of chronic hepatitis C: One pill a day for tomorrow. Liver Int 2012;32 Suppl 1:88.)

Oral bioavailability of adefovir dipivoxil is about 59% and is unaffected by meals; it is rapidly and completely hydrolyzed to the parent compound by intestinal and blood esterases. Protein binding is low (< 5%). The intracellular half-life of the diphosphate is prolonged, ranging from 5 to 18 hours in various cells; this makes once-daily dosing feasible. Adefovir is excreted by a combination of glomerular filtration and active tubular secretion and requires dose adjustment for renal dysfunction; however, it may be administered to patients with decompensated liver disease.

Of the oral agents, adefovir may be slower to suppress HBV DNA levels and the least likely to induce HBeAg seroconversion.

Emergence of resistance is 20% to 30% after 5 years of use. Naturally occurring (ie, primary) adefovir-resistant rt233 HBV mutants have been described. There is no cross-resistance between adefovir and lamivudine or entecavir.

Adefovir is well tolerated. A dose-dependent nephrotoxicity, manifested by increased serum creatinine and decreased serum phosphorous, may occur and is more common with use of higher doses (30-60 mg/d) or pre-existing azotemia. Other potential adverse effects are headache, diarrhea, asthenia, and abdominal pain. As with other NRTI agents, lactic acidosis and hepatic steatosis are considered a risk owing to mitochondrial dysfunction. Pivalic acid, a by-product of adefovir metabolism, can esterify free carnitine and result in decreased carnitine levels. However, it is not necessary to administer carnitine supplementation with the low doses used to treat patients with HBV (10 mg/d). Severe acute exacerbations of hepatitis have been reported in up to 25% of patients who discontinued adefovir. Adefovir is embryotoxic in rats at high doses and is genotoxic in preclinical studies.

ENTECAVIR

Entecavir is an orally administered guanosine nucleoside analog. that competitively inhibits all three functions of HBV DNA polymerase, including base priming, reverse transcription of the negative strand, and synthesis of the positive strand of HBV DNA. Oral bioavailability approaches 100% but is decreased by food; therefore, entecavir should be taken on an empty stomach. The intracellular half-life of the active phosphorylated compound is 15 hours and plasma half-life is prolonged at 128-149 hours, allowing once-daily dosing. It is excreted by the kidney, undergoing both glomerular filtration and net tubular secretion.

Suppression of HBV DNA levels was greater with entecavir than with lamivudine or adefovir in comparative trials. Entecavir appears to have a higher barrier to the emergence of resistance than lamivudine but resistance may be more likely in the setting of lamivudine resistance. Although selection of resistant isolates with the S202G mutation has been documented during therapy, clinical resistance is rare (< 1% at 4 years). Entecavir has weak anti-HIV activity and can induce development of the M184V variant in HBV/HIV co-infected patients, resulting in resistance to emtricitabine and lamivudine.

Entecavir is well tolerated. Potential adverse events are headache, fatigue, dizziness, nausea, rash, and fever. Lung adenomas and carcinomas in mice, hepatic adenomas and carcinomas in rats and mice, vascular tumors in mice, and brain gliomas and skin fibromas in rats have been observed at varying exposures. Co-administration of entecavir with drugs that reduce renal function or compete for active tubular secretion may increase serum concentrations of either entecavir or the co-administered drug.

LAMIVUDINE The pharmacokinetics of lamivudine are described earlier in this chapter (see section, Nucleoside and Nucleotide Reverse Transcriptase Inhibitors). The more prolonged intracellular half-life in HBV cell lines (17â&#x20AC;&#x201C;19 hours) than in HIV-infected cell lines (10.5â&#x20AC;&#x201C;15.5 hours) allows for lower doses and less frequent administration. Lamivudine can be safely administered to patients with decompensated liver disease. Prolonged treatment has been shown to decrease clinical progression of HBV, as well as development of hepatocellular cancer by approximately 50%. Also, lamivudine has been effective in preventing vertical transmission of HBV from mother to newborn when given in the last 4 weeks of gestation.

Lamivudine inhibits HBV DNA polymerase and HIV reverse transcriptase by competing with deoxycytidine triphosphate for incorporation into the viral DNA, resulting in chain termination. Although lamivudine initially results in rapid and potent virus suppression, chronic therapy is limited by the emergence of lamivudine-resistant HBV isolates (eg, L180M or M204I/V), estimated to occur in 15â&#x20AC;&#x201C; 30% of patients at 1 year and 70% at 5 years of therapy. Resistance has been associated with flares of hepatitis and progressive liver disease. Cross-resistance between lamivudine and emtricitabine or entecavir may occur; however, adefovir and tenofovir maintain activity against lamivudine-resistant strains of HBV. In the doses used for HBV infection, lamivudine has an excellent safety profile. Headache, nausea, diarrhea, dizziness, myalgia, and malaise are rare. Co-infection with HIV may increase the risk of pancreatitis.

TELBIVUDINE Telbivudine is a thymidine nucleoside analog with activity against HBV DNA polymerase. It is phosphorylated by cellular kinases to the active triphosphate form, which has an intracellular half-life of 14 hours. The phosphorylated compound competitively inhibits HBV DNA polymerase, resulting in incorporation into viral DNA and chain termination. It is not active in vitro against HIV-1.

Oral bioavailability is unaffected by food. Plasma protein-binding is low (3%) and distribution wide. The serum half-life is approximately 15 hours and excretion is renal. There are no known metabolites and no known interactions with the CYP450 system or other drugs.

Telbivudine induced greater rates of virologic response than either lamivudine or adefovir in comparative trials. However, emergence of resistance, typically due to the M204I mutation, may occur in up to 22% of patients with durations of therapy exceeding 1 year, and may result in virologic rebound. Telbivudine is not effective in patients with lamivudine-resistant HBV. Adverse effects are mild; they include fatigue, headache, cough, nausea, diarrhea, rash, and fever. Both uncomplicated myalgia and myopathy have been reported, concurrent with increased creatine kinase levels, as has peripheral neuropathy. As with other nucleoside analogs, lactic acidosis and severe hepatomegaly with steatosis may occur during therapy as well as flares of hepatitis after discontinuation.

TENOFOVIR Tenofovir, a nucleotide analog of adenosine in use as an antiretroviral agent, has potent activity against HBV. The characteristics of tenofovir are described earlier in this chapter. Tenofovir maintains activity against lamivudine- and entecavir-resistant hepatitis virus isolates but has reduced activity against adefovir-resistant strains. Although similar in structure to adefovir dipivoxil, comparative trials show a higher rate of virologic response and histologic improvement, and a lower rate of emergence of resistance to tenofovir in patients with chronic HBV infection. The most common adverse effects of tenofovir in patients with HBV infection are nausea, abdominal pain,

diarrhea, dizziness, fatigue, and rash; other potential adverse effects are those listed earlier.

TREATMENT OF HEPATITIS C INFECTION In contrast to the treatment of patients with chronic HBV infection, the primary goal of treatment in patients with HCV infection is viral eradication. In clinical trials, the primary efficacy end point is typically achievement of sustained viral response (SVR), defined as the absence of detectable viremia 24 weeks after completion of therapy. SVR is associated with improvement in liver histology, reduction in risk of hepatocellular carcinoma, and, occasionally, with regression of cirrhosis as well. Late relapse occurs in less than 5% of patients who achieve SVR. In acute hepatitis C, the rate of clearance of the virus without therapy is estimated at 15–30%. In one (uncontrolled) study, treatment of acute infection with interferon alfa-2b, in doses higher than those used for chronic hepatitis C, resulted in a sustained rate of clearance of 95% at 6 months. Therefore, if HCV RNA testing documents persistent viremia 12 weeks after initial seroconversion, antiviral therapy is recommended.

Treatment of patients with chronic HCV infection is recommended for those with an increased risk for progression to cirrhosis. The parameters for selection are complex. In those who are to be treated, the traditional standard treatment is once-weekly pegylated interferon alfa in combination with daily oral ribavirin. Pegylated interferon alfa-2a and -2b have replaced their unmodified interferon alfa counterparts because of superior efficacy in combination with ribavirin, regardless of genotype. It is also clear that combination therapy with oral ribavirin is more effective than monotherapy with either interferon or ribavirin alone. Therefore, monotherapy with pegylated interferon alfa is recommended only in patients who cannot tolerate ribavirin. Interferon plus ribavirin therapy is active against all genotypes of HCV infection, with SVR rates of 70 to 80% among patients with HCV genotype 2 or 3 infection and rates of 45 to 70% among patients with any of the other genotypes. A genetic variant near the gene encoding interferon-lambda-3 ( IL28B rs12979860) is a strong predictor of response to peginterferon alfa and ribavirin.

However, the recent advent of NS3/4A protease inhibitors and the NS5B polymerase inhibitors is changing the face of chronic HCV therapy. Administration of boceprevir, simeprevir, or telaprevir, in combination with peginterferon and ribavirin, dramatically increased the rate of viral clearance in patients with HCV genotype 1; sofosbuvir is effective against HCV genotypes 1, 2, 3, and 4. Although all four of these new agents are licensed to be administered in combination with peginterferon and ribavirin, recent results of clinical trials have provided evidence that one or more of them may be effective in interferon- and ribavirin-free regimens.

POLYMERASE INHIBITORS Sofosbuvir Sofosbuvir is a nucleotide analog that inhibits the HCV NS5B RNA-dependent RNA polymerase in patients infected with HCV genotype 1, 2, 3, or 4. It is administered once daily, with or without food, in combination with peginterferon alfa and ribavirin, for a total of 12–24 weeks (the longer duration is recommended in patients infected with HCV genotype 3). Very high cure rates are reported but the drug is extraordinarily expensive.

Sofosbuvir is 61–65% bound to plasma proteins and is metabolized in the liver to form the active nucleoside analog triphosphate GS461203. Elimination is by renal clearance, and safety has not been established in patients with severe renal insufficiency.

Sofosbuvir is a substrate of drug transporter P-gp; therefore, potent P-gp inducers in the intestine should not be co-administered.

Commonly reported adverse effects are fatigue and headache.

PROTEASE INHIBITORS Three oral protease NS3/4A inhibitors have recently become available for the treatment of HCV genotype 1 infection, in combination with peginterferon and ribavirin: boceprevir, simeprevir, and telaprevir. These agents inhibit HCV replication directly by binding to the NS3/4A protease that cleaves HCV-encoded polyproteins ( Figure 49–4). Of concern is the enhanced toxicity when used in combination with peginterferon and ribavirin, the high potential for drug-drug interactions, and the low genetic barrier to resistance, which may develop as early as 4 days after initiation of therapy when administered as monotherapy. Use of these agents in the treatment of other HCV genotypes is not recommended. Cross-resistance is expected among NS3/4A protease inhibitors.

All three agents are inhibitors and substrates of CYP3A inhibitors. Drug-drug interactions are to be expected with many concurrent agents, particularly the NNRTIs and PIs in patients with HIV/HCV co-infection. Co-administration with strong CYP3A4 inducers (including rifampin) is contraindicated due to potential decrease in serum levels of the anti-HCV agent, and co-administration with statin agents is contraindicated due to increased serum levels of the statin agent. The effectiveness of hormonal contraceptives may be reduced by co-administration with boceprevir or telaprevir.

Since boceprevir, simeprevir, and telaprevir are always co-administered with ribavirin, their use in pregnant women and in men with

pregnant partners is contraindicated.

Boceprevir Boceprevir therapy is initiated after the administration of peginterferon and ribavirin therapy for 4 weeks. The duration of therapy is dependent on the achievement of undetectable virus.

Boceprevir should be taken with food to maximize absorption. It is ~75% protein-bound and has a mean plasma half-life of approximately 3.4 hours. Boceprevir is metabolized by the aldo-keto-reductase and CYP3A4/5 pathways and is an inhibitor of CYP3A4/5 and P-glycoprotein transporter. Co-administration of boceprevir with numerous drugs is contraindicated, including carbamazepine, phenobarbital, phenytoin, rifampin, ergot derivatives, cisapride, lovastatin, simvastatin, St. John’s wort, drospirenone, alfuzosin, sildenafil or tadalafil when used for pulmonary hypertension, pimozide, triazolam, midazolam, and efavirenz.

The most commonly reported adverse effects associated with boceprevir therapy are fatigue, anemia, neutropenia, nausea, headache, and dysgeusia. Rates of anemia are higher in patients taking boceprevir with peginterferon and ribavirin than in those taking peginterferon and ribavirin alone (~ 50% vs 25%, respectively); rates of neutropenia are also higher.

Simeprevir Simeprevir is administered once daily in combination with peginterferon and ribavirin for a total of 12 weeks in patients with compensated liver disease (including cirrhosis) that are infected with HCV genotype 1.

Simeprevir must be taken with food to maximize absorption. It is extensively bound to plasma proteins (> 99%), metabolized in the liver by CYP3A pathways, and undergoes biliary excretion. Its safety in patients with moderate to severe liver insufficiency has not been established. Mean simeprevir exposures are more than threefold higher in Asian patients compared with Caucasians, leading to potentially higher frequencies of adverse events. Simeprevir is a substrate and mild inhibitor of CYP3A and a substrate and inhibitor of P-gp and OATP1B1/3. Co-administration with moderate or strong inhibitors or inducers of CYP3A may significantly increase or decrease the plasma concentration of simeprevir.

The presence of the NS3 Q80K polymorphism at baseline is associated with reduced efficacy of therapy, and screening is recommended prior to the initiation of therapy. Emergence of amino acid substitutions resulting in decreased drug susceptibility has been documented during therapy and may be associated with reduced responsiveness.

Reported adverse events include photosensitivity reaction and rash (most common within the first 4 weeks of therapy). Since simeprevir contains a sulfa moiety, caution should be used in patients with a history of sulfa allergy.

Telaprevir Therapy with telaprevir plus peginterferon and ribavirin is administered for at least 12 weeks in treatment-naïve patients with HCV infection. As with boceprevir, the duration of therapy is dependent on the achievement of undetectable virus.

Telaprevir must be taken with food to maximize absorption. It is 59–76% bound to plasma proteins and the effective half-life at steady state is 9–11 hours. Telaprevir is metabolized by the CYP pathways in the liver and is an inhibitor of CYP3A4 and Pglycoprotein. Co-administration of telaprevir with numerous drugs is contraindicated, including rifampin, ergot derivatives, cisapride, lovastatin, simvastatin, alfuzosin, sildenafil or tadalafil when used for pulmonary hypertension, pimozide, St. John’s wort, triazolam, and midazolam. The dosage of telaprevir must be increased when co-administered with efavirenz, due to lowered levels of telaprevir.

The most commonly reported adverse effects associated with telaprevir therapy are rash (30–55%), anemia, fatigue, pruritus, nausea, and anorectal discomfort. Severe rash or Stevens-Johnson syndrome has been reported; in these patients, the drug should be stopped and not restarted. Rates of anemia are higher in patients taking telaprevir with peginterferon and ribavirin than in those taking peginterferon and ribavirin alone (~ 36% vs 17%, respectively). Leukopenia, thrombocytopenia, increased serum bilirubin levels, hyperuricemia, and anorectal burning may also occur.

RIBAVIRIN Ribavirin is a guanosine analog that is phosphorylated intracellularly by host cell enzymes. Although its mechanism of action has not been fully elucidated, it appears to interfere with the synthesis of guanosine triphosphate, to inhibit capping of viral messenger RNA, and to inhibit the viral RNA-dependent polymerase of certain viruses. Ribavirin triphosphate inhibits the replication of a wide range of DNA and RNA viruses, including influenza A and B, parainfluenza, respiratory syncytial virus, paramyxoviruses, HCV, and HIV-1.

The absolute oral bioavailability of ribavirin is 45–64%, increases with high-fat meals, and decreases with co-administration of antacids. Plasma protein binding is negligible, volume of distribution is large, and cerebrospinal fluid levels are about 70% of those in plasma. Ribavirin elimination is primarily through the urine; therefore, clearance is decreased in patients with creatinine clearances less than 30 mL/min.

Higher doses of ribavirin (ie, 1000–1200 mg/d, according to weight, rather than 800 mg/d) or a longer duration of therapy or both may be more efficacious in those with a lower likelihood of response to therapy (eg, those with genotype 1 or 4) or in those who have relapsed. This must be balanced with an increased likelihood of toxicity. A dose-dependent hemolytic anemia occurs in 10–20% of patients. Other potential adverse effects are depression, fatigue, irritability, rash, cough, insomnia, nausea, and pruritus. Contraindications to ribavirin therapy include anemia, end-stage renal failure, ischemic vascular disease, and pregnancy. Ribavirin is teratogenic and embryotoxic in animals as well as mutagenic in mammalian cells. Patients exposed to the drug should not conceive children for at least 6 months thereafter.

NEW & INVESTIGATIONAL AGENTS Second-generation NS3/NS4A protease inhibitors (eg, faldaprevir, simeprevir, asunaprevir), nucleoside/nucleotide NS5B polymerase inhibitors (eg, sofosbuvir, see above), and non-nucleoside NS5B polymerase inhibitors (eg, deleobuvir) are currently under clinical investigation. The goal is to identify potent and well tolerated regimens that do not require concurrent administration of interferon or ribavirin; in addition agents are needed with activity against HCV genotypes other than 1 (such as sofosbuvir). Other classes of agents in development include NS5A inhibitors (eg, daclatasvir), p7 and NS4B inhibitors, cyclophilin inhibitors, and antisense oligonucleotides inhibiting miR122 (eg, miravirsen).

ANTI-INFLUENZA AGENTS Influenza virus strains are classified by their core proteins (ie, A, B, or C), species of origin (eg, avian, swine), and geographic site of isolation. Influenza A, the only strain that causes pandemics, is classified into 16 H (hemagglutinin) and 9 N (neuraminidase) known subtypes based on surface proteins. Although influenza B viruses usually infect only people, influenza A viruses can infect a variety of animal hosts. Current influenza A subtypes that are circulating among worldwide populations include H1N1, H1N2, and H3N2. Fifteen subtypes are known to infect birds, providing an extensive reservoir. Although avian influenza subtypes are typically highly speciesspecific, they have on rare occasions crossed the species barrier to infect humans and cats. Viruses of the H5 and H7 subtypes (eg, H5N1, H7N7, and H7N3) may rapidly mutate within poultry flocks from a low to high pathogenic form and have recently expanded their host range to cause both avian and human disease. Of particular concern is the avian H5N1 virus, which first caused human infection (including severe disease and death) in 1997 and has become endemic in Southeast Asian poultry since 2003. To date, the spread of H5N1 virus from person to person has been rare, limited, and unsustained. However, the emergence of the 2009 H1N1 influenza virus (previously called “swine flu”) in 2009–2010 caused the first influenza pandemic (ie, global outbreak of disease caused by a new flu virus) in more than 40 years.

OSELTAMIVIR & ZANAMIVIR The neuraminidase inhibitors oseltamivir and zanamivir, analogs of sialic acid, interfere with release of progeny influenza virus from infected host cells, thus halting the spread of infection within the respiratory tract. These agents competitively and reversibly interact with the active enzyme site to inhibit viral neuraminidase activity at low nanomolar concentrations. Inhibition of viral neuraminidase results in clumping of newly released influenza virions to each other and to the membrane of the infected cell. Unlike amantadine and rimantadine, oseltamivir and zanamivir have activity against both influenza A and influenza B viruses. Early administration is crucial because replication of influenza virus peaks at 24–72 hours after the onset of illness. Initiation of a 5-day course of therapy within 48 hours after the onset of illness decreases the duration of symptoms, viral shedding and transmission, and the rate of complications such as pneumonia, asthma, hospitalization, and mortality. Once-daily prophylaxis is 70–90% effective in preventing disease after exposure.

Oseltamivir is an orally administered prodrug that is activated by hepatic esterases and widely distributed throughout the body. The dosage is 75 mg twice daily for 5 days for treatment and 75 mg once daily for prevention. Oral bioavailability is approximately 80%, plasma protein binding is low, and concentrations in the middle ear and sinus fluid are similar to those in plasma. The half-life of oseltamivir is 6–10 hours, and excretion is by glomerular filtration and tubular secretion. Probenecid reduces renal clearance of oseltamivir by 50%. Serum concentrations of oseltamivir carboxylate, the active metabolite of oseltamivir, increase with declining renal function; therefore, dosage should be adjusted in patients with renal insufficiency. Potential adverse effects include nausea, vomiting, and headache. Taking oseltamivir with food does not interfere with absorption and may decrease nausea and vomiting. Fatigue and diarrhea have also been reported and appear to be more common with prophylactic use. Rash is rare. Neuropsychiatric events (self-injury or delirium) have been reported, particularly in adolescents and adults living in Japan.

Zanamivir is administered directly to the respiratory tract via inhalation. Ten to twenty percent of the active compound reaches the lungs, and the remainder is deposited in the oropharynx. The concentration of the drug in the respiratory tract is estimated to be more than 1000 times the 50% inhibitory concentration for neuraminidase, and the pulmonary half-life is 2.8 hours. Five to fifteen percent of the total dose (10 mg twice daily for 5 days for treatment and 10 mg once daily for prevention) is absorbed and excreted in the urine with minimal metabolism. Potential adverse effects include cough, bronchospasm (occasionally severe), reversible decrease in pulmonary

function, and transient nasal and throat discomfort. Zanamivir administration is not recommended for patients with underlying airway disease. Both oseltamivir and zanamivir are available in intravenous formulations on a compassionate use basis from the manufacturer.

Although resistance to oseltamivir and zanamivir may emerge during therapy and be transmissible, nearly 100% of strains of H1N1, H3N2, and influenza B virus tested by the Centers for Diseases Control for the 2012-2013 season retained susceptibility to both agents.

Oseltamivir resistance, however, has been documented in strains of the novel avian H7N9 virus, in one instance appearing to emerge during treatment.

AMANTADINE & RIMANTADINE Amantadine (1-aminoadamantane hydrochloride) and its α-methyl derivative, rimantadine, are tricyclic amines of the adamantane family that block the M2 proton ion channel of the virus particle and inhibit uncoating of the viral RNA within infected host cells, thus preventing its replication. They are active against influenza A only. Rimantadine is four to ten times more active than amantadine in vitro.

Amantadine is well absorbed and 67% protein-bound. Its plasma half-life is 12–18 hours and varies by creatinine clearance. Rimantadine is about 40% protein-bound and has a half-life of 24–36 hours. Nasal secretion and salivary levels approximate those in the serum, and cerebrospinal fluid levels are 52–96% of those in the serum; nasal mucus concentrations of rimantadine average 50% higher than those in plasma. Amantadine is excreted unchanged in the urine, whereas rimantadine undergoes extensive metabolism by hydroxylation, conjugation, and glucuronidation before urinary excretion. Dose reductions are required for both agents in the elderly and in patients with renal insufficiency, and for rimantadine in patients with marked hepatic insufficiency.

In the absence of resistance, both amantadine and rimantadine, at 100 mg twice daily or 200 mg once daily, are 70–90% protective in the prevention of clinical illness when initiated before exposure. When begun within 1–2 days after the onset of illness, the duration of fever and systemic symptoms is reduced by 1–2 days. However, due to high rates of resistance in both H1N1 and H3N2 viruses, these agents are no longer recommended for the prevention or treatment of influenza.

The most common adverse effects are gastrointestinal (nausea, anorexia) and central nervous system (nervousness, difficulty in concentrating, insomnia, light-headedness). More serious side effects (eg, marked behavioral changes, delirium, hallucinations, agitation, and seizures) may be due to alteration of dopamine neurotransmission (see Chapter 28); are less frequent with rimantadine than with amantadine; are associated with high plasma concentrations; may occur more frequently in patients with renal insufficiency, seizure disorders, or advanced age; and may increase with concomitant antihistamines, anticholinergic drugs, hydrochlorothiazide, and trimethoprim-sulfamethoxazole. Clinical manifestations of anticholinergic activity tend to be present in acute amantadine overdose. Both agents are teratogenic and embryotoxic in rodents, and birth defects have been reported after exposure during pregnancy.

INVESTIGATIONAL AGENTS The neuraminidase inhibitor peramivir, a cyclopentane analog, has activity against both influenza A and B viruses. Peramivir received temporary emergency use authorization by FDA for intravenous administration in November 2009 due to the H1N1 pandemic, but is not now approved for use in the USA. Reported side effects include diarrhea, nausea, vomiting, and neutropenia. A long-acting neuraminidase inhibitor, laninamivir octanoate, may retain activity against oseltamivir-resistant virus. DAS181 is a host-directed antiviral agent that acts by removing the virus receptor, sialic acid, from adjacent glycan structures.

OTHER ANTIVIRAL AGENTS INTERFERONS Interferons have been studied for numerous clinical indications. In addition to HBV and HCV infections (see Antihepatitis Agents), intralesional injection of interferon alfa-2b or alfa-n3 may be used for treatment of condylomata acuminata (see Chapter 61).

RIBAVIRIN In addition to oral administration for HCV infection in combination with interferon alfa (see Antihepatitis Agents), aerosolized ribavirin is administered by nebulizer (20 mg/mL for 12–18 hours per day) to children and infants with severe respiratory syncytial virus (RSV) bronchiolitis or pneumonia to reduce the severity and duration of illness. Aerosolized ribavirin has also been used to treat influenza A and B infections but has not gained widespread use. Systemic absorption is low (< 1%). Aerosolized ribavirin may cause conjunctival or bronchial irritation and the aerosolized drug may precipitate on contact lenses. Ribavirin is teratogenic and embryotoxic. Health care workers and pregnant women should be protected against extended inhalation exposure.

Intravenous ribavirin decreases mortality in patients with Lassa fever and other viral hemorrhagic fevers if started early. High

concentrations inhibit West Nile virus in vitro, but clinical data are lacking. Clinical benefit has been reported in cases of severe measles pneumonitis and certain encephalitides, and continuous infusion of ribavirin has decreased virus shedding in several patients with severe lower respiratory tract influenza or parainfluenza infections. At steady state, cerebrospinal fluid levels are about 70% of those in plasma.

PALIVIZUMAB Palivizumab is a humanized monoclonal antibody directed against an epitope in the A antigen site on the F surface protein of RSV. It is licensed for the prevention of RSV infection in high-risk infants and children, such as premature infants and those with bronchopulmonary dysplasia or congenital heart disease. A placebo-controlled trial using once-monthly intramuscular injections (15 mg/kg) for 5 months beginning at the start of the RSV season demonstrated a 55% reduction in the risk of hospitalization for RSV in treated patients, as well as decreases in the need for supplemental oxygen, the illness severity score, and the need for intensive care. Although resistant strains have been isolated in the laboratory, no resistant clinical isolates have yet been identified. Potential adverse effects include upper respiratory tract infection, fever, rhinitis, rash, diarrhea, vomiting, cough, otitis media, and elevation in serum aminotransferase levels.

Agents under investigation for the treatment or prophylaxis of patients with RSV infection include the RNA interference (RNAi) therapeutic ALN-RSV01and the benzodiazepine RSV604.

IMIQUIMOD Imiquimod is an immune response modifier shown to be effective in the topical treatment of external genital and perianal warts (ie, condyloma acuminatum; see Chapter 61). The 5% cream is applied three times weekly and washed off 6â&#x20AC;&#x201C;10 hours after each application. Recurrences appear to be less common than after ablative therapies. Imiquimod may also be effective against molluscum contagiosum but is not licensed in the United States for this indication. Local skin reactions are the most common adverse effect; these tend to resolve within weeks after therapy. However, pigmentary skin changes may persist. Systemic adverse effects such as fatigue and influenza-like syndrome have occasionally been reported.

PREPARATIONS AVAILABLE

REFERENCES Antiviral drugs. Med Lett Drugs T her 2013;11:19.

Hsu J et al: Antivirals for treatment of influenza. A systematic review and meta-analysis of observational studies. Ann Intern Med 2012;156:512.

Liang T J, Ghany MG: Current and future therapies for hepatitis C virus infection. N Engl J Med 2013;368:1907.

Panel on Antiretroviral Guidelines for Adults and Adolescents: Guidelines for the use of antiretroviral agents in HIV-1 infected adults and adolescents. Department of Health and Human Services. http://aidsinfo.nih.gov/ContentFiles.AdultandAdolescentGL.pdf.

Panel on T reatment of HIV-Infected Pregnant Women and Prevention of Perinatal T ransmission: Recommendations for Use of Antiretroviral Drugs in Pregnant HIV-1Infected Women for Maternal Health and Interventions to Reduce Perinatal HIV T ransmission in the United States. July 31, 2012. http://aidsinfo.nih.gov/ContentFiles/PerinatalGL.pdf.

T hompson MA et al: Antiretroviral treatment of adult HIV infection: 2012 recommendations of the International Antiviral Society - USA panel. JAMA 2012;308(4):387.

RELEVANT WEBSITES http://www.aidsinfo.nih.gov http://www.hiv-druginteractions.org http://www.hivinsite.com http://www.iasusa.org

CASE STUDY ANSWER Combination antiviral therapy against both HIV and hepatitis B virus (HBV) is indicated in this patient, given the high viral load and low CD4 cell count. However, the use of methadone and possibly excessive alcohol consumption necessitate caution. Tenofovir and emtricitabine (two nucleoside/nucleotide reverse transcriptase inhibitors) would be excellent choices as components of an initial regimen, since both are active against HIV-1 and HBV, do not interact with methadone, and are available in a once-daily, fixeddose combination. Efavirenz, a nonnucleoside reverse transcriptase inhibitor, could be added and still maintain a once-daily regimen.

There are several other alternatives as well. Prior to initiation of this regimen, renal function should be checked, HBV DNA level should be assessed, and a bone mineral density test should be considered. Pregnancy should be ruled out, and the patient should be counseled that efavirenz should not be taken during pregnancy. Avoidance of alcohol should be recommended. The potential for lowered methadone levels with efavirenz necessitates close monitoring and possibly an increased dose of methadone. Finally, the patient should be made aware that abrupt cessation of these medications may precipitate an acute flare of hepatitis.

CHAPTER

50 Miscellaneous Antimicrobial Agents; Disinfectants, Antiseptics, & Sterilants Daniel H. Deck, PharmD, & Lisa G. Winston, MD*

CASE STUDY A 56-year-old man is admitted to the intensive care unit of a hospital for treatment of community-acquired pneumonia. He receives ceftriaxone and azithromycin upon admission, rapidly improves, and is transferred to a semiprivate ward room. On day 7 of his hospitalization, he develops copious diarrhea with eight bowel movements but is otherwise clinically stable. Clostridium difficile infection is confirmed by stool testing. What is an acceptable treatment for the patient’s diarrhea? The patient is transferred to a single-bed room. The housekeeping staff asks what product should be used to clean the patient’s old room. Why?

METRONIDAZOLE, MUPIROCIN, POLYMYXINS, & URINARY ANTISEPTICS METRONIDAZOLE Metronidazole is a nitroimidazole antiprotozoal drug (see Chapter 52) that also has potent antibacterial activity against anaerobes, including Bacteroides and Clostridium species. Metronidazole is selectively absorbed by anaerobic bacteria and sensitive protozoa.

Once taken up by anaerobes, it is nonenzymatically reduced by reacting with reduced ferredoxin. This reduction results in products that are toxic to anaerobic cells and allows for their selective accumulation in anaerobes. The metabolites of metronidazole are taken up into bacterial DNA, forming unstable molecules. This action only occurs when metronidazole is partially reduced, and, because this reduction usually happens only in anaerobic cells, it has relatively little effect on human cells or aerobic bacteria.

Metronidazole is well absorbed after oral administration, is widely distributed in tissues, and reaches serum levels of 4–6 mcg/mL after a 250 mg oral dose. It can also be given intravenously. The drug penetrates well into the cerebrospinal fluid and brain, reaching levels similar to those in serum. Metronidazole is metabolized in the liver and may accumulate in hepatic insufficiency.

Metronidazole is indicated for treatment of anaerobic or mixed intra-abdominal infections (in combination with other agents with activity against aerobic organisms), vaginitis (trichomonas infection, bacterial vaginosis), Clostridium difficile infection, and brain abscess. The typical dosage is 500 mg three times daily orally or intravenously (30 mg/kg/d). Vaginitis may respond to a single 2 g dose.

A vaginal gel is available for topical use.

Adverse effects include nausea, diarrhea, stomatitis, and peripheral neuropathy with prolonged use. Metronidazole has a disulfiramlike effect, and patients should be instructed to avoid alcohol. Although teratogenic in some animals, metronidazole has not been associated with this effect in humans. Other properties of metronidazole are discussed in Chapter 52.

A structurally similar agent, tinidazole, is a once-daily drug approved for treatment of trichomonas infection, giardiasis, amebiasis, and bacterial vaginosis. It also is active against anaerobic bacteria, but is not approved for treatment of anaerobic infections.

MUPIROCIN Mupirocin (pseudomonic acid) is a natural substance produced by Pseudomonas fluorescens. It is rapidly inactivated after absorption, and systemic levels are undetectable. It is available as an ointment for topical application.

Mupirocin is active against gram-positive cocci, including methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus. Mupirocin inhibits staphylococcal isoleucyl tRNA synthetase. Low-level resistance, defined as a minimum inhibitory

concentration (MIC) of up to 100 mcg/mL, is due to point mutation in the gene of the target enzyme. Low-level resistance has been observed after prolonged use. However, local concentrations achieved with topical application are well above this MIC, and this level of resistance does not lead to clinical failure. High-level resistance, with MICs exceeding 1000 mcg/mL, is due to the presence of a second isoleucyl tRNA synthetase gene, which is plasmid-encoded. High-level resistance results in complete loss of activity. Strains with highlevel resistance have caused hospital-associated outbreaks of staphylococcal infection and colonization. Although higher rates of resistance are encountered with intensive use of mupirocin, most staphylococcal isolates are still susceptible.

Mupirocin is indicated for topical treatment of minor skin infections, such as impetigo (see Chapter 61). Topical application over large infected areas, such as decubitus ulcers or open surgical wounds, is an important factor leading to emergence of mupirocin-resistant strains and is not recommended. Mupirocin temporarily eliminates S aureus nasal carriage by patients or health care workers, but results are mixed with respect to its ability to prevent subsequent staphylococcal infection.

POLYMYXINS The polymyxins are a group of basic peptides active against gram-negative bacteria and include polymyxin B and polymyxin E (colistin). Polymyxins act as cationic detergents. They attach to and disrupt bacterial cell membranes. They also bind and inactivate endotoxin. Gram-positive organisms, Proteus sp, and Neisseria sp are resistant.

Owing to their significant toxicity with systemic administration (especially nephrotoxicity), polymyxins were, until recently, largely restricted to topical use. Ointments containing polymyxin B, 0.5 mg/g, in mixtures with bacitracin or neomycin (or both) are commonly applied to infected superficial skin lesions. Emergence of strains of Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae that are resistant to all other agents has renewed interest in polymyxins as parenteral agents for salvage therapy of infections caused by these organisms.

FIDAXOMICIN Fidaxomicin is a narrow-spectrum, macrocyclic antibiotic that is active against gram-positive aerobes and anaerobes but lacks activity against gram-negative bacteria. Fidaxomicin inhibits bacterial protein synthesis by binding to the sigma subunit of RNA polymerase.

When administered orally, systemic absorption is negligible but fecal concentrations are high. Fidaxomicin has been approved by the FDA for the treatment for C difficile infection in adults. It is as effective as oral vancomycin and may be associated with lower rates of relapsing disease. Fidaxomicin is administered orally as a 200 mg tablet twice daily for 10 days.

URINARY ANTISEPTICS Urinary antiseptics are oral agents that exert antibacterial activity in the urine but have little or no systemic antibacterial effect. Their usefulness is limited to lower urinary tract infections.

Nitrofurantoin At therapeutic doses, nitrofurantoin is bactericidal for many gram-positive and gram-negative bacteria; however, P aeruginosa and many strains of Proteus are inherently resistant. Nitrofurantoin has a complex mechanism of action that is not fully understood.

Antibacterial activity appears to correlate with rapid intracellular conversion of nitrofurantoin to highly reactive intermediates by bacterial reductases. These intermediates react nonspecifically with many ribosomal proteins and disrupt the synthesis of proteins, RNA, DNA, and metabolic processes. It is not known which of the multiple actions of nitrofurantoin is primarily responsible for its bactericidal activity.

There is no cross-resistance between nitrofurantoin and other antimicrobial agents, and resistance emerges slowly. As resistance to trimethoprim-sulfamethoxazole and fluoroquinolones has become more common in Escherichia coli, nitrofurantoin has become an important alternative oral agent for treatment of uncomplicated urinary tract infection.

Nitrofurantoin is well absorbed after ingestion. It is metabolized and excreted so rapidly that no systemic antibacterial action is achieved. The drug is excreted into the urine by both glomerular filtration and tubular secretion. With average daily doses, concentrations of 200 mcg/mL are reached in urine. In renal failure, urine levels are insufficient for antibacterial action, but high blood levels may cause toxicity. Nitrofurantoin is contraindicated in patients with significant renal insufficiency (creatinine clearance < 60 mL/min).

The dosage for urinary tract infection in adults is 100 mg orally taken four times daily. A long-acting formulation (Macrobid) can be taken twice daily. Each long-acting capsule contains two forms of nitrofurantoin. Twenty-five percent is macrocrystalline nitrofurantoin, which has slower dissolution and absorption than nitrofurantoin monohydrate. The remaining 75% is nitrofurantoin monohydrate contained in a powder blend, which upon exposure to gastric and intestinal fluids forms a gel matrix that releases nitrofurantoin over time.

The drug should not be used to treat upper urinary tract infection. It is desirable to keep urinary pH below 5.5, which greatly enhances drug activity. A single daily dose of nitrofurantoin, 100 mg, can prevent recurrent urinary tract infections in some women.

Anorexia, nausea, and vomiting are the principal side effects of nitrofurantoin. Neuropathies and hemolytic anemia occur in patients with glucose-6-phosphate dehydrogenase deficiency. Nitrofurantoin antagonizes the action of nalidixic acid. Rashes, pulmonary infiltration and fibrosis, and other hypersensitivity reactions have been reported.

Methenamine Mandelate & Methenamine Hippurate Methenamine mandelate is the salt of mandelic acid and methenamine and possesses properties of both of these urinary antiseptics.

Methenamine hippurate is the salt of hippuric acid and methenamine. Below pH 5.5, methenamine releases formaldehyde, which is antibacterial (see Aldehydes, below). Mandelic acid or hippuric acid taken orally is excreted unchanged in the urine, in which these drugs are bactericidal for some gram-negative bacteria when pH is less than 5.5.

Methenamine mandelate, 1 g four times daily, or methenamine hippurate, 1 g twice daily by mouth (children, 50 mg/kg/d or 30 mg/kg/d, respectively), is used only as a urinary antiseptic to suppress, not treat, urinary tract infection. Acidifying agents (eg, ascorbic acid, 4–12 g/d) may be given to lower urinary pH below 5.5. Sulfonamides should not be given at the same time because they may form an insoluble compound with the formaldehyde released by methenamine. Persons taking methenamine mandelate may exhibit falsely elevated tests for catecholamine metabolites.

DISINFECTANTS, ANTISEPTICS, & STERILANTS Disinfectants are chemical agents or physical procedures that inhibit or kill microorganisms (Table 50–1). Antiseptics are disinfecting chemical agents with sufficiently low toxicity for host cells that they can be used directly on skin, mucous membranes, or wounds.

Sterilants kill both vegetative cells and spores when applied to materials for appropriate times and temperatures. Some of the terms used in this context are defined in Table 50–2.

TABLE 50–1 Activities of disinfectants.

TABLE 50â&#x20AC;&#x201C;2 Commonly used terms related to chemical and physical killing of microorganisms.

Disinfection prevents infection by reducing the number of potentially infective organisms by killing, removing, or diluting them.

Disinfection can be accomplished by application of chemical agents or use of physical agents such as ionizing radiation, dry or moist heat, or superheated steam (autoclave, 120°C) to kill microorganisms. Often a combination of agents is used, eg, water and moderate heat over time (pasteurization); ethylene oxide and moist heat (a sterilant); or addition of disinfectant to a detergent. Prevention of infection also can be achieved by washing, which dilutes the potentially infectious organism, or by establishing a barrier, eg, gloves, condom, or respirator, which prevents the pathogen from gaining entry to the host.

Hand hygiene is probably the most important means of preventing transmission of infectious agents from person to person or from regions of high microbial load, eg, mouth, nose, or gut, to potential sites of infection. Alcohol-based hand rubs and soap and warm water are used to remove bacteria. Skin disinfectants along with detergent and water are usually used preoperatively as a surgical scrub for surgeons’ hands.

Evaluation of effectiveness of antiseptics, disinfectants, and sterilants, although seemingly simple in principle, is very complex. Factors in any evaluation include the intrinsic resistance of the microorganism, the number of microorganisms present, mixed populations of organisms, amount of organic material present (eg, blood, feces, tissue), concentration and stability of disinfectant or sterilant, time and temperature of exposure, pH, and hydration and binding of the agent to surfaces. Specific, standardized assays of activity are defined for each use. Toxicity for humans also must be evaluated. In the United States, the Environmental Protection Agency (EPA) regulates disinfectants and sterilants and the FDA regulates antiseptics.

Users of antiseptics, disinfectants, and sterilants need to consider their short-term and long-term toxicity because they may have general biocidal activity and may accumulate in the environment or in the body. Disinfectants and antiseptics may also become contaminated by resistant microorganisms—eg, spores, P aeruginosa, or Serratia marcescens—and actually transmit infection. Most topical antiseptics interfere with wound healing to some degree. Cleansing of wounds with soap and water may be less damaging than the application of antiseptics.

Some of the chemical classes of antiseptics, disinfectants, and sterilants are described briefly in the text that follows. The reader is referred to the general references for descriptions of physical disinfection and sterilization methods.

ALCOHOLS The two alcohols most frequently used for antisepsis and disinfection are ethanol and isopropyl alcohol (isopropanol). They are rapidly active, killing vegetative bacteria, Mycobacterium tuberculosis, and many fungi, and inactivating lipophilic viruses. The optimum bactericidal concentration is 60–90% by volume in water. They probably act by denaturation of proteins. They are not used as sterilants because they are not sporicidal, do not penetrate protein-containing organic material, and may not be active against hydrophilic viruses.

Their skin-drying effect can be alleviated by addition of emollients to the formulation. Use of alcohol-based hand rubs has been shown to reduce transmission of health care–associated bacterial pathogens and is recommended by the Centers for Disease Control and Prevention (CDC) as the preferred method of hand decontamination in health care settings. Alcohol-based hand rubs are ineffective against spores of C difficile, and assiduous handwashing with soap and water is still required for decontamination after caring for a patient with infection from this organism.

Alcohols are flammable and must be stored in cool, well-ventilated areas. They must be allowed to evaporate before cautery, electrosurgery, or laser surgery. Alcohols may be damaging if applied directly to corneal tissue. Therefore, instruments such as tonometers that have been disinfected in alcohol should be rinsed with sterile water, or the alcohol should be allowed to evaporate before they are used.

CHLORHEXIDINE Chlorhexidine is a cationic biguanide with very low water solubility. Water-soluble chlorhexidine digluconate is used in water-based formulations as an antiseptic. It is active against vegetative bacteria and mycobacteria and has variable activity against fungi and viruses.

It strongly adsorbs to bacterial membranes, causing leakage of small molecules and precipitation of cytoplasmic proteins. It is active at pH 5.5–7.0. Chlorhexidine gluconate is slower in its action than alcohols, but, because of its persistence, it has residual activity when used repeatedly, producing bactericidal action equivalent to alcohols. It is most effective against gram-positive cocci and less active against gram-positive and gram-negative rods. Spore germination is inhibited by chlorhexidine. Chlorhexidine digluconate is resistant to inhibition by blood and organic materials. However, anionic and nonionic agents in moisturizers, neutral soaps, and surfactants may neutralize its action. Chlorhexidine digluconate formulations of 4% concentration have slightly greater antibacterial activity than newer 2% formulations. The combination of chlorhexidine gluconate in 70% alcohol, available in some countries including the United States, is the preferred agent for skin antisepsis in many surgical and percutaneous procedures. The advantage of this combination over povidoneiodine may derive from its more rapid action after application, its retained activity after exposure to body fluids, and its persistent activity on the skin. Chlorhexidine has a very low skin-sensitizing or irritating capacity. Oral toxicity is low because it is poorly absorbed from the alimentary tract. Chlorhexidine must not be used during surgery on the middle ear because it causes sensorineural deafness. Similar neural toxicity may be encountered during neurosurgery.

HALOGENS Iodine Iodine in a 1:20,000 solution is bactericidal in 1 minute and kills spores in 15 minutes. Tincture of iodine USP contains 2% iodine and 2.4% sodium iodide in alcohol. It is the most active antiseptic for intact skin. It is not commonly used because of serious hypersensitivity reactions that may occur and because of its staining of clothing and dressings.

Iodophors Iodophors are complexes of iodine with a surface-active agent such as polyvinyl pyrrolidone (PVP; povidone-iodine). Iodophors retain the activity of iodine. They kill vegetative bacteria, mycobacteria, fungi, and lipid-containing viruses. They may be sporicidal upon prolonged exposure. Iodophors can be used as antiseptics or disinfectants, the latter containing more iodine. The amount of free iodine is low, but it is released as the solution is diluted. An iodophor solution must be diluted according to the manufacturer’s directions to obtain full activity.

Iodophors are less irritating and less likely to produce skin hypersensitivity than tincture of iodine. They require drying time on skin before becoming active, which can be a disadvantage. Although iodophors have a somewhat broader spectrum of activity than chlorhexidine, including sporicidal action, they lack its persistent activity on skin.

Chlorine Chlorine is a strong oxidizing agent and universal disinfectant that is commonly provided as a 5.25% sodium hypochlorite solution, a typical formulation for household bleach. Because formulations may vary, the exact concentration should be verified on the label. A

1:10 dilution of household bleach (producing a 0.525% concentration) provides 5000 ppm of available chlorine. The CDC recommends this concentration for disinfection of blood spills. Less than 5 ppm kills vegetative bacteria, whereas up to 5000 ppm is necessary to kill spores. A concentration of 1000–10,000 ppm is tuberculocidal. One hundred ppm kills vegetative fungal cells in 1 hour, but fungal spores require 500 ppm. Viruses are inactivated by 200–500 ppm. Dilutions of sodium hypochlorite made up in pH 7.5–8.0 tap water retain their activity for months when kept in tightly closed, opaque containers. Frequent opening and closing of the container reduces the activity markedly.

Because chlorine is inactivated by blood, serum, feces, and protein-containing materials, surfaces should be cleaned before chlorine disinfectant is applied. Undissociated hypochlorous acid (HOCl) is the active biocidal agent. When pH is increased, the less active hypochlorite ion, OCl– , is formed. When hypochlorite solutions contact formaldehyde, the carcinogen bischloromethyl is formed. Rapid evolution of irritating chlorine gas occurs when hypochlorite solutions are mixed with acid and urine. Solutions are corrosive to aluminum, silver, and stainless steel.

Alternative chlorine-releasing compounds include chlorine dioxide and chloramine T. These agents retain chlorine longer and have a prolonged bactericidal action.

PHENOLICS Phenol itself (perhaps the oldest of the surgical antiseptics) is no longer used even as a disinfectant because of its corrosive effect on tissues, its toxicity when absorbed, and its carcinogenic effect. These adverse actions are diminished by forming derivatives in which a functional group replaces a hydrogen atom in the aromatic ring. The phenolic agents most commonly used are o-phe nylphe nol, obenzyl-p-chlorophenol, and p-tertiary amylphenol. Mixtures of phenolic derivatives are often used. Some of these are derived from coal tar distillates, eg, cresols and xylenols. Skin absorption and skin irritation still occur with these derivatives, and appropriate care is necessary in their use. Detergents are often added to formulations to clean and remove organic material that may decrease the activity of a phenolic compound.

Phenolic compounds disrupt cell walls and membranes, precipitate proteins, and inactivate enzymes. They are bactericidal (including mycobacteria) and fungicidal and are capable of inactivating lipophilic viruses. They are not sporicidal. Dilution and time of exposure recommendations of the manufacturer must be followed.

Phenolic disinfectants are used for hard surface decontamination in hospitals and laboratories, eg, floors, beds, and counter or bench tops. They are not recommended for use in nurseries and especially near infants, where their use has been associated with hyperbilirubinemia. Use of hexachlorophene as a skin disinfectant has caused cerebral edema and convulsions in premature infants and, occasionally, in adults.

QUATERNARY AMMONIUM COMPOUNDS The quaternary ammonium compounds (“quats”) are cationic surface-active detergents. The active cation has at least one long waterrepellent hydrocarbon chain, which causes the molecules to concentrate as an oriented layer on the surface of solutions and colloidal or suspended particles. The charged nitrogen portion of the cation has high affinity for water and prevents separation out of solution. The bactericidal action of quaternary compounds has been attributed to inactivation of energy-producing enzymes, denaturation of proteins, and disruption of the cell membrane. These agents are fungistatic and sporistatic and also inhibit algae. They are bactericidal for grampositive bacteria and moderately active against gram-negative bacteria. Lipophilic viruses are inactivated. They are not tuberculocidal or sporicidal, and they do not inactivate hydrophilic viruses. Quaternary ammonium compounds bind to the surface of colloidal protein in blood, serum, and milk and to the fibers in cotton, mops, cloths, and paper towels used to apply them, which can cause inactivation of the agent by removing it from solution. They are inactivated by anionic detergents (soaps), by many nonionic detergents, and by calcium, magnesium, ferric, and aluminum ions.

Quaternary compounds are used for sanitation of noncritical surfaces (floors, bench tops, etc). Their low toxicity has led to their use as sanitizers in food production facilities. The CDC recommends that quaternary ammonium compounds such as benzalkonium chloride not be used as antiseptics because several outbreaks of infections have occurred that were due to growth of Pseudomonas and other gram-negative bacteria in quaternary ammonium antiseptic solutions.

ALDEHYDES Formaldehyde and glutaraldehyde are used for disinfection or sterilization of instruments such as fiberoptic endoscopes, respiratory therapy equipment, hemodialyzers, and dental handpieces that cannot withstand exposure to the high temperatures of steam sterilization.

They are not corrosive for metal, plastic, or rubber. These agents have a broad spectrum of activity against microorganisms. They act by alkylation of chemical groups in proteins and nucleic acids. Failures of disinfection or sterilization can occur as a result of dilution below the known effective concentration, the presence of organic material, and the failure of liquid to penetrate into small channels in the

instruments. Automatic circulating baths are available that increase penetration of aldehyde solution into the instrument while decreasing exposure of the operator to irritating fumes.

Formaldehyde is available as a 40% weight per volume solution in water (100% formalin). An 8% formaldehyde solution in water has a broad spectrum of activity against bacteria, fungi, and viruses. Sporicidal activity may take as long as 18 hours. Its rapidity of action is increased by solution in 70% isopropanol. Formaldehyde solutions are used for high-level disinfection of hemodialyzers, preparation of vaccines, and preservation and embalming of tissues. The 4% formaldehyde (10% formalin) solutions used for fixation of tissues and embalming may not be mycobactericidal.

Glutaraldehyde is a dialdehyde (1,5-pentanedial). Solutions of 2% weight per volume glutaraldehyde are most commonly used. The solution must be alkalinized to pH 7.4–8.5 for activation. Activated solutions are bactericidal, sporicidal, fungicidal, and virucidal for both lipophilic and hydrophilic viruses. Glutaraldehyde has greater sporicidal activity than formaldehyde, but its tuberculocidal activity may be less. Lethal action against mycobacteria and spores may require prolonged exposure. Once activated, solutions have a shelf life of 14 days, after which polymerization reduces activity. Other means of activation and stabilization can increase the shelf life. Because glutaraldehyde solutions are frequently reused, the most common reason for loss of activity is dilution and exposure to organic material.

Test strips to measure residual activity are recommended.

Formaldehyde has a characteristic pungent odor and is highly irritating to respiratory mucous membranes and eyes at concentrations of 2–5 ppm. The U.S. Occupational Safety and Health Administration (OSHA) has declared that formaldehyde is a potential carcinogen and has established an employee exposure standard that limits the 8-hour time-weighted average (TWA) exposure to 0.75 ppm.

Protection of health care workers from exposure to glutaraldehyde concentrations greater than 0.2 ppm is advisable. Increased air exchange, enclosure in hoods with exhausts, tight-fitting lids on exposure devices, and use of protective personal equipment such as goggles, respirators, and gloves may be necessary to achieve these exposure limits.

Ortho-phthalaldehyde (OPA) is a phenolic dialdehyde chemical sterilant with a spectrum of activity comparable to glutaraldehyde, although it is several times more rapidly bactericidal. OPA solution typically contains 0.55% OPA. Its label claim is that high-level disinfection can be achieved in 12 minutes at room temperature compared with 45 minutes for 2.4% glutaraldehyde. Unlike glutaraldehyde, OPA requires no activation, is less irritating to mucous membranes, and does not require exposure monitoring. It has good materials compatibility and an acceptable environmental safety profile. OPA is useful for disinfection or sterilization of endoscopes, surgical instruments, and other medical devices.

SUPEROXIDIZED WATER Electrolysis of saline yields a mixture of oxidants, primarily hypochlorous acid and chlorine, with potent disinfectant and sterilant properties. The solution generated by the process, which has been commercialized and marketed as Sterilox for disinfection of endoscopes and dental materials, is rapidly bactericidal, fungicidal, tuberculocidal, and sporicidal. High-level disinfection is achieved with a contact time of 10 minutes. The solution is nontoxic and nonirritating and requires no special disposal precautions.

PEROXYGEN COMPOUNDS The peroxygen compounds, hydrogen peroxide and peracetic acid, have high killing activity and a broad spectrum against bacteria, spores, viruses, and fungi when used in appropriate concentration. They have the advantage that their decomposition products are not toxic and do not injure the environment. They are powerful oxidizers that are used primarily as disinfectants and sterilants.

Hydrogen peroxide is a very effective disinfectant when used for inanimate objects or materials with low organic content such as water. Organisms with the enzymes catalase and peroxidase rapidly degrade hydrogen peroxide. The innocuous degradation products are oxygen and water. Concentrated solutions containing 90% weight per volume H 2 O2 are prepared electrochemically. When diluted in high-quality deionized water to 6% and 3% and put into clean containers, they remain stable. Concentrations of 10–25% hydrogen peroxide are sporicidal. Vapor phase hydrogen peroxide (VPHP) is a cold gaseous sterilant that has the potential to replace the toxic or carcinogenic gases ethylene oxide and formaldehyde. VPHP does not require a pressurized chamber and is active at temperatures as low as 4°C and concentrations as low as 4 mg/L. It is incompatible with liquids and cellulose products. It penetrates the surface of some plastics. Automated equipment using vaporized hydrogen peroxide or hydrogen peroxide mixed with formic acid is available for sterilizing endoscopes.

Peracetic acid (CH3 COOOH) is prepared commercially from 90% hydrogen peroxide, acetic acid, and sulfuric acid as a catalyst. It is explosive in the pure form. It is usually used in dilute solution and transported in containers with vented caps to prevent increased pressure as oxygen is released. Peracetic acid is more active than hydrogen peroxide as a bactericidal and sporicidal agent.

Concentrations of 250–500 ppm are effective against a broad range of bacteria in 5 minutes at pH 7.0 at 20°C. Bacterial spores are inactivated by 500–30,000 ppm peracetic acid. Only slightly increased concentrations are necessary in the presence of organic matter.

Viruses require variable exposures. Enteroviruses require 2000 ppm for 15–30 minutes for inactivation.

An automated machine that uses buffered peracetic acid liquid of 0.1–0.5% concentration has been developed for sterilization of

medical, surgical, and dental instruments. Peracetic acid sterilization systems have also been adopted for hemodialyzers. The food processing and beverage industries use peracetic acid extensively because the breakdown products in high dilution do not produce objectionable odor, taste, or toxicity, and rinsing is not necessary.

Peracetic acid is a potent tumor promoter but a weak carcinogen. It is not mutagenic in the Ames test.

HEAVY METALS Heavy metals, principally mercury and silver, are now rarely used as disinfectants. Mercury is an environmental hazard, and some pathogenic bacteria have developed plasmid-mediated resistance to mercurials. Hypersensitivity to thimerosal is common, possibly in up to 40% of the population. These compounds are absorbed from solution by rubber and plastic closures. Thimerosal 0.001–0.004% is still used safely as a preservative of vaccines, antitoxins, and immune sera. Although a causative link to autism has never been established, thimerosal-free vaccines are available for use in children and pregnant woman.

Inorganic silver salts are strongly bactericidal. Silver nitrate, 1:1000, had been most commonly used, particularly as a preventive for gonococcal ophthalmitis in newborns. Antibiotic ointments have replaced silver nitrate for this indication. Silver sulfadiazine slowly releases silver and is used to suppress bacterial growth in burn wounds (see Chapter 46).

STERILANTS For many years, pressurized steam (autoclaving) at 120°C for 30 minutes has been the basic method for sterilizing instruments and other heat-resistant materials. When autoclaving is not possible, as with lensed instruments and materials containing plastic and rubber, ethylene oxide—diluted with either fluorocarbon or carbon dioxide to diminish explosive hazard—has been used at 440–1200 mg/L at 45–60°C with 30–60% relative humidity. The higher concentrations have been used to increase penetration.

Ethylene oxide is classified as a mutagen and carcinogen. The OSHA permissible exposure limit (PEL) for ethylene oxide is 1 ppm calculated as a time-weighted average. Alternative sterilants now being used increasingly include vapor phase hydrogen peroxide, peracetic acid, ozone, gas plasma, chlorine dioxide, formaldehyde, and propylene oxide. Each of these sterilants has potential advantages and problems. Automated peracetic acid systems are being used increasingly for high-level decontamination and sterilization of endoscopes and hemodialyzers because of their effectiveness, automated features, and the low toxicity of the residual products of sterilization.

PRESERVATIVES Disinfectants are used as preservatives to prevent the overgrowth of bacteria and fungi in pharmaceutical products, laboratory sera and reagents, cosmetic products, and contact lenses. Multi-use vials of medication that may be reentered through a rubber diaphragm, and eye and nose drops, require preservatives. Preservatives should not be irritating or toxic to tissues to which they will be applied, they must be effective in preventing growth of microorganisms likely to contaminate solutions, and they must have sufficient solubility and stability to remain active.

Commonly used preservative agents include organic acids such as benzoic acid and salts, the parabens, (alkyl esters of phydroxybenzoic acid), sorbic acid and salts, phenolic compounds, quaternary ammonium compounds, alcohols, and mercurials such as thimerosal in 0.001–0.004% concentration.

SUMMARY Miscellaneous Antimicrobials

PREPARATIONS AVAILABLE

REFERENCES Bischoff WE et al: Handwashing compliance by health care workers: T he impact of introducing an accessible, alcohol-based hand antiseptic. Arch Intern Med 2000;160:1017.

Chambers HF, Winston LG: Mupirocin prophylaxis misses by a nose. Ann Intern Med 2004;140:484.

Gordin FM et al: Reduction in nosocomial transmission of drug-resistant bacteria after introduction of an alcohol-based hand-rub. Infect Control Hosp Epidemiol 2005;26:650.

Humphreys PN: T esting standards for sporicides. J Hosp Infect 2011;77:193.

Louie T J et al: Fidaxomicin versus vancomycin for Clostridium difficile infection. N Engl J Med 2011;364:422.

Meyer GW: Endoscope disinfection. UpT oDate 2012.

Noorani A et al: Systematic review and meta-analysis of preoperative antisepsis with chlorhexidine versus povidone-iodine in clean-contaminated surgery. Br J Surg 2010;97:1614.

Rutala WA, Weber DJ: New disinfection and sterilization methods. Emerg Infect Dis 2001;7:348.

T inidazole. Med Lett Drugs T her 2004;46:70.

Widmer AF, Frei R: Decontamination, disinfection, and sterilization. In: Murray PR et al (editors): Manual of Clinical Microbiology, 7th ed. American Society for Microbiology, 1999.

CASE STUDY ANSWER The patient may be treated with oral metronidazole, which is an appropriate drug for mild to moderate cases of C difficileassociated infection. Oral vancomycin is also a reasonable alternative. The room should be cleaned with a bleach solution (5000 ppm) because it is sporicidal. Other sporicidal disinfectants may also be effective.

_______________ * T he authors thank Henry F. Chambers, MD, the author of this chapter in previous editions, for his contributions.

CHAPTER

51 Clinical Use of Antimicrobial Agents Harry W. Lampiris, MD, & Daniel S. Maddix, PharmD

CASE STUDY A 51-year-old alcoholic patient presents to the emergency department with fever, headache, neck stiffness, and altered mental status for 12 hours. Vital signs are blood pressure 90/55 mm Hg, pulse 120/min, respirations 30/min, temperature 40°C [104°F] rectal. The patient is minimally responsive to voice and does not follow commands. Examination is significant for a right third cranial nerve palsy and nuchal rigidity. Laboratory results show a white blood cell count of 24,000/mm3 with left shift, but other hematologic and chemistry values are within normal limits. An emergency CT scan of the head is normal. Blood cultures are obtained, and a lumbar puncture reveals the following cerebrospinal fluid (CSF) values: white blood cells 5000/mm3 , red blood cells 10/mm3 , protein 200 mg/dL, glucose 15 mg/dL (serum glucose 96 taken at same time). CSF Gram stain reveals gram-positive cocci in pairs. What is the most likely diagnosis in this patient? What organisms should be treated empirically? Are there other pharmacologic interventions to consider before initiating antimicrobial therapy?

The development of antimicrobial drugs represents one of the most important advances in therapeutics, both in the control or cure of serious infections and in the prevention and treatment of infectious complications of other therapeutic modalities such as cancer chemotherapy, immunosuppression, and surgery. However, evidence is overwhelming that antimicrobial agents are vastly overprescribed in outpatient settings in the United States, and the availability of antimicrobial agents without prescription in many developing countries has—by facilitating the development of resistance—already severely limited therapeutic options in the treatment of life-threatening infections. Therefore, the clinician should first determine whether antimicrobial therapy is warranted for a given patient. The specific questions one should ask include the following: 1. Is an antimicrobial agent indicated on the basis of clinical findings? Or is it prudent to wait until such clinical findings become apparent?

2. Have appropriate clinical specimens been obtained to establish a microbiologic diagnosis?

3. What are the likely etiologic agents for the patient’s illness?

4. What measures should be taken to protect individuals exposed to the index case to prevent secondary cases, and what measures should be implemented to prevent further exposure?

5. Is there clinical evidence (eg, from well-executed clinical trials) that antimicrobial therapy will confer clinical benefit for the patient?

Once a specific cause is identified based on specific microbiologic tests, the following further questions should be considered: 1.

2.

3.

4.

5.

If a specific microbial pathogen is identified, can a narrower-spectrum agent be substituted for the initial empiric drug?

Is one agent or a combination of agents necessary?

What are the optimal dose, route of administration, and duration of therapy?

What specific tests (eg, susceptibility testing) should be undertaken to identify patients who will not respond to treatment?

What adjunctive measures can be undertaken to eradicate the infection? For example, is surgery feasible for removal of devitalized tissue or foreign bodies—or drainage of an abscess—into which antimicrobial agents may be unable to penetrate? Is it possible to decrease the dosage of immunosuppressive therapy in patients who have undergone organ transplantation? Is it possible to reduce morbidity or mortality due to the infection by reducing host immunologic response to the infection (eg, by the use of corticosteroids for the treatment of severe Pneumocystis jiroveci pneumonia or meningitis due to Streptococcus pneumoniae)?

EMPIRIC ANTIMICROBIAL THERAPY Antimicrobial agents are frequently used before the pathogen responsible for a particular illness or the susceptibility to a particular antimicrobial agent is known. This use of antimicrobial agents is called empiric (or presumptive) therapy and is based on experience with a particular clinical entity. The usual justification for empiric therapy is the hope that early intervention will improve the outcome; in the best cases, this has been established by placebo-controlled, double-blind prospective clinical trials. For example, treatment of febrile episodes in neutropenic cancer patients with empiric antimicrobial therapy has been demonstrated to have impressive morbidity and mortality benefits even though the specific bacterial agent responsible for fever is determined for only a minority of such episodes.

Finally, there are many clinical entities, such as certain episodes of community-acquired pneumonia, in which it is difficult to identify a specific pathogen. In such cases, a clinical response to empiric therapy may be an important clue to the likely pathogen.

Frequently, the signs and symptoms of infection diminish as a result of empiric therapy, and microbiologic test results become available that establish a specific microbiologic diagnosis. At the time that the pathogenic organism responsible for the illness is identified, empiric therapy is optimally modified to definitive therapy, which is typically narrower in coverage and is given for an appropriate duration based on the results of clinical trials or experience when clinical trial data are not available.

Approach to Empiric Therapy Initiation of empiric therapy should follow a specific and systematic approach.

A. Formulate a Clinical Diagnosis of Microbial Infection Using all available data, the clinician should determine that there is a clinical syndrome compatible with infection (eg, pneumonia, cellulitis, sinusitis).

B. Obtain Specimens for Laboratory Examination Examination of stained specimens by microscopy or simple examination of an uncentrifuged sample of urine for white blood cells and bacteria may provide important immediate etiologic clues. Cultures of selected anatomic sites (blood, sputum, urine, cerebrospinal fluid, and stool) and nonculture methods (antigen testing, polymerase chain reaction, and serology) may also confirm specific etiologic agents.

C. Formulate a Microbiologic Diagnosis The history, physical examination, and immediately available laboratory results (eg, Gram stain of urine or sputum) may provide highly specific information. For example, in a young man with urethritis and a Gram-stained smear from the urethral meatus demonstrating intracellular gram-negative diplococci, the most likely pathogen is Neisseria gonorrhoeae. In the latter instance, however, the clinician should be aware that a significant number of patients with gonococcal urethritis have negative Gram stains for the organism and that a significant number of patients with gonococcal urethritis harbor concurrent chlamydial infection that is not demonstrated on the Gramstained smear.

D. Determine the Necessity for Empiric Therapy Whether or not to initiate empiric therapy is an important clinical decision based partly on experience and partly on data from clinical trials. Empiric therapy is indicated when there is a significant risk of serious morbidity or mortality if therapy is withheld until a specific pathogen is detected by the clinical laboratory.

In other settings, empiric therapy may be indicated for public health reasons rather than for demonstrated superior outcome of therapy in a specific patient. For example, urethritis in a young sexually active man usually requires treatment for N gonorrhoeae and Chlamydia trachomatis despite the absence of microbiologic confirmation at the time of diagnosis. Because the risk of noncompliance with follow-up visits in this patient population may lead to further transmission of these sexually transmitted pathogens, empiric therapy is warranted.

E. Institute Treatment Selection of empiric therapy may be based on the microbiologic diagnosis or a clinical diagnosis without available microbiologic clues. If no microbiologic information is available, the antimicrobial spectrum of the agent or agents chosen must necessarily be broader, taking into account the most likely pathogens responsible for the patientâ&#x20AC;&#x2122;s illness.

Choice of Antimicrobial Agent Selection from among several drugs depends on host factors that include the following: (1) concomitant disease states (eg, AIDS, neutropenia due to the use of cytotoxic chemotherapy, organ transplantation, severe chronic liver or kidney disease) or the use of

immunosuppressive medications; (2) prior adverse drug effects; (3) impaired elimination or detoxification of the drug (may be genetically predetermined but more frequently is associated with impaired renal or hepatic function due to underlying disease); (4) age of the patient; (5) pregnancy status; and (6) epidemiologic exposure (eg, exposure to a sick family member or pet, recent hospitalization, recent travel, occupational exposure, or new sexual partner).

Pharmacologic factors include (1) the kinetics of absorption, distribution, and elimination; (2) the ability of the drug to be delivered to the site of infection; (3) the potential toxicity of an agent; and (4) pharmacokinetic or pharmacodynamic interactions with other drugs.

Knowledge of the susceptibility of an organism to a specific agent in a hospital or community setting is important in the selection of empiric therapy. Pharmacokinetic differences among agents with similar antimicrobial spectrums may be exploited to reduce the frequency of dosing (eg, ceftriaxone, ertapenem, or daptomycin may be conveniently given once every 24 hours). Finally, increasing consideration is being given to the cost of antimicrobial therapy, especially when multiple agents with comparable efficacy and toxicity are available for a specific infection. Changing from intravenous to oral antibiotics for prolonged administration can be particularly costeffective.

Brief guides to empiric therapy based on presumptive microbial diagnosis and site of infection are given in Tables 51â&#x20AC;&#x201C;1 and 51â&#x20AC;&#x201C;2.

TABLE 51â&#x20AC;&#x201C;1 Empiric antimicrobial therapy based on microbiologic etiology.

TABLE 51â&#x20AC;&#x201C;2 Empiric antimicrobial therapy based on site of infection.

ANTIMICROBIAL THERAPY OF INFECTIONS WITH KNOWN ETIOLOGY INTERPRETATION OF CULTURE RESULTS Properly obtained and processed specimens for culture frequently yield reliable information about the cause of infection. The lack of a confirmatory microbiologic diagnosis may be due to the following: 1. Sample error, eg, obtaining cultures after antimicrobial agents have been administered, inadequate volume or quantity of specimen obtained, or contamination of specimens sent for culture 2. Noncultivable or slow-growing organisms (Histoplasma capsulatum, Bartonella or Brucella species), in which cultures are often discarded before sufficient growth has occurred for detection 3. Requesting bacterial cultures when infection is due to other organisms 4. Not recognizing the need for special media or isolation techniques (eg, charcoal yeast extract agar for isolation of Legionella species, shell-vial tissue culture system for rapid isolation of cytomegalovirus) Even in the setting of a classic infectious disease for which isolation techniques have been established for decades (eg, pneumococcal pneumonia, pulmonary tuberculosis, streptococcal pharyngitis), the sensitivity of the culture technique may be inadequate to identify all cases of the disease.

GUIDING ANTIMICROBIAL THERAPY OF ESTABLISHED INFECTIONS Susceptibility Testing Testing bacterial pathogens in vitro for their susceptibility to antimicrobial agents is extremely valuable in confirming susceptibility, ideally to a narrow-spectrum nontoxic antimicrobial drug. Tests measure the concentration of drug required to inhibit growth of the organism (minimal inhibitory concentration [MIC]) or to kill the organism (minimal bactericidal concentration [MBC]). The results of these tests can then be correlated with known drug concentrations in various body compartments. Only MICs are routinely measured in most infections, whereas in infections in which bactericidal therapy is required for eradication of infection (eg, meningitis, endocarditis, sepsis in the granulocytopenic host), MBC measurements occasionally may be useful.

Specialized Assay Methods A. Beta-Lactamase Assay For some bacteria (eg, Haemophilus species), the susceptibility patterns of strains are similar except for the production of β lactamase.

In these cases, extensive susceptibility testing may not be required, and a direct test for β lactamase using a chromogenic β-lactam substrate (nitrocephin disk) may be substituted.

B. Synergy Studies Synergy studies are in vitro tests that attempt to measure synergistic, additive, indifferent, or antagonistic drug interactions. In general, these tests have not been standardized and have not correlated well with clinical outcome. (See section on Antimicrobial Drug Combinations for details.)

MONITORING THERAPEUTIC RESPONSE: DURATION OF THERAPY The therapeutic response may be monitored microbiologically or clinically. Cultures of specimens taken from infected sites should eventually become sterile or demonstrate eradication of the pathogen and are useful for documenting recurrence or relapse. Follow-up cultures may also be useful for detecting superinfections or the development of resistance. Clinically, the patient’s systemic manifestations of infection (malaise, fever, leukocytosis) should abate, and the clinical findings should improve (eg, as shown by clearing of radiographic infiltrates or lessening hypoxemia in pneumonia).

The duration of definitive therapy required for cure depends on the pathogen, the site of infection, and host factors (immunocompromised patients generally require longer courses of treatment). Precise data on duration of therapy exist for some infections (eg, streptococcal pharyngitis, syphilis, gonorrhea, tuberculosis, and cryptococcal meningitis). In many other situations, duration of therapy is determined empirically. For recurrent infections (eg, sinusitis, urinary tract infections), longer courses of antimicrobial therapy or surgical intervention are frequently necessary for eradication.

Clinical Failure of Antimicrobial Therapy When the patient has an inadequate clinical or microbiologic response to antimicrobial therapy selected by in vitro susceptibility testing, systematic investigation should be undertaken to determine the cause of failure. Errors in susceptibility testing are rare, but the original results should be confirmed by repeat testing. Drug dosing and absorption should be scrutinized and tested directly using serum measurements, pill counting, or directly observed therapy.

The clinical data should be reviewed to determine whether the patientâ&#x20AC;&#x2122;s immune function is adequate and, if not, what can be done to maximize it. For example, are adequate numbers of granulocytes present and is undiagnosed immunodeficiency, malignancy, or malnutrition present? The presence of abscesses or foreign bodies should also be considered. Finally, culture and susceptibility testing should be repeated to determine whether superinfection has occurred with another organism or whether the original pathogen has developed drug resistance.

ANTIMICROBIAL PHARMACODYNAMIC The time course of drug concentration is closely related to the antimicrobial effect at the site of infection and to any toxic effects.

Pharmacodynamic factors include pathogen susceptibility testing, drug bactericidal versus bacteriostatic activity, drug synergism, antagonism, and postantibiotic effects. Together with pharmacokinetics, pharmacodynamic information permits the selection of optimal antimicrobial dosage regimens.

Bacteriostatic versus Bactericidal Activity Antibacterial agents may be classified as bacteriostatic or bactericidal (Table 51â&#x20AC;&#x201C;3). For agents that are primarily bacteriostatic, inhibitory drug concentrations are much lower than bactericidal drug concentrations. In general, cell wall-active agents are bactericidal, and drugs that inhibit protein synthesis are bacteriostatic.

TABLE 51â&#x20AC;&#x201C;3 Bactericidal and bacteriostatic antibacterial agents.

The classification of antibacterial agents as bactericidal or bacteriostatic has limitations. Some agents that are considered to be bacteriostatic may be bactericidal against selected organisms. On the other hand, enterococci are inhibited but not killed by vancomycin, penicillin, or ampicillin used as single agents.

Bacteriostatic and bactericidal agents are equivalent for the treatment of most infectious diseases in immunocompetent hosts.

Bactericidal agents should be selected over bacteriostatic ones in circumstances in which local or systemic host defenses are impaired.

Bactericidal agents are required for treatment of endocarditis and other endovascular infections, meningitis, and infections in neutropenic cancer patients.

Bactericidal agents can be divided into two groups: agents that exhibit concentration-dependent killing (eg, aminoglycosides and quinolones) and agents that exhibit time-dependent killing (eg, β lactams and vancomycin). For drugs whose killing action is concentration-dependent, the rate and extent of killing increase with increasing drug concentrations. Concentration-dependent killing is one of the pharmacodynamic factors responsible for the efficacy of once-daily dosing of aminoglycosides. For drugs whose killing action is time-dependent, bactericidal activity continues as long as serum concentrations are greater than the MBC.

Postantibiotic Effect Persistent suppression of bacterial growth after limited exposure to an antimicrobial agent is known as the postantibiotic effect (PAE).

The PAE can be expressed mathematically as follows: PAE = T - C where T is the time required for the viable count in the test (in vitro) culture to increase tenfold above the count observed immediately before drug removal and C is the time required for the count in an untreated culture to increase tenfold above the count observed

immediately after completion of the same procedure used on the test culture. The PAE reflects the time required for bacteria to return to logarithmic growth.

Proposed mechanisms include (1) slow recovery after reversible nonlethal damage to cell structures; (2) persistence of the drug at a binding site or within the periplasmic space; and (3) the need to synthesize new enzymes before growth can resume. Most antimicrobials possess significant in vitro PAEs (≥ 1.5 hours) against susceptible gram-positive cocci (Table 51–4). Antimicrobials with significant PAEs against susceptible gram-negative bacilli are limited to carbapenems and agents that inhibit protein or DNA synthesis.

TABLE 51–4 Antibacterial agents with in vitro postantibiotic effects ≥ 1.5 hours.

In vivo PAEs are usually much longer than in vitro PAEs. This is thought to be due to postantibiotic leukocyte enhancement (PALE) and exposure of bacteria to subinhibitory antibiotic concentrations. The efficacy of once-daily dosing regimens is in part due to

the PAE. Aminoglycosides and quinolones possess concentration-dependent PAEs; thus, high doses of aminoglycosides given once daily result in enhanced bactericidal activity and extended PAEs. This combination of pharmacodynamic effects allows aminoglycoside serum concentrations that are below the MICs of target organisms to remain effective for extended periods of time.

PHARMACOKINETIC CONSIDERATIONS Route of Administration Many antimicrobial agents have similar pharmacokinetic properties when given orally or parenterally (ie, tetracyclines, trimethoprimsulfamethoxazole, quinolones, metronidazole, clindamycin, rifampin, linezolid, and fluconazole). In most cases, oral therapy with these drugs is equally effective, is less costly, and results in fewer complications than parenteral therapy.

The intravenous route is preferred in the following situations: (1) for critically ill patients; (2) for patients with bacterial meningitis or endocarditis; (3) for patients with nausea, vomiting, gastrectomy, ileus, or diseases that may impair oral absorption; and (4) when giving antimicrobials that are poorly absorbed following oral administration.

Conditions That Alter Antimicrobial Pharmacokinetics Various diseases and physiologic states alter the pharmacokinetics of antimicrobial agents. Impairment of renal or hepatic function may result in decreased elimination. Table 51â&#x20AC;&#x201C;5 lists drugs that require dosage reduction in patients with renal or hepatic insufficiency. Failure to reduce antimicrobial agent dosage in such patients may cause toxic effects. Conversely, patients with burns, cystic fibrosis, or trauma may have increased dosage requirements for selected agents. The pharmacokinetics of antimicrobials is also altered in the elderly (see Chapter 60), in neonates (see Chapter 59), and in pregnancy.

TABLE 51â&#x20AC;&#x201C;5 Antimicrobial agents that require dosage adjustment or are contraindicated in patients with renal or hepatic impairment.

Drug Concentrations in Body Fluids Most antimicrobial agents are well distributed to most body tissues and fluids. Penetration into the cerebrospinal fluid is an exception.

Most do not penetrate uninflamed meninges to an appreciable extent. In the presence of meningitis, however, the cerebrospinal fluid concentrations of many antimicrobials increase (Table 51â&#x20AC;&#x201C;6).

TABLE 51â&#x20AC;&#x201C;6 Cerebrospinal fluid (CSF) penetration of selected antimicrobials.

Monitoring Serum Concentrations of Antimicrobial Agents For most antimicrobial agents, the relation between dose and therapeutic outcome is well established, and serum concentration monitoring is unnecessary for these drugs. To justify routine serum concentration monitoring, it should be established (1) that a direct relationship exists between drug concentrations and efficacy or toxicity; (2) that substantial interpatient variability exists in serum concentrations on standard doses; (3) that a small difference exists between therapeutic and toxic serum concentrations; (4) that the clinical efficacy or toxicity of the drug is delayed or difficult to measure; and (5) that an accurate assay is available.

In clinical practice, serum concentration monitoring is routinely performed on patients receiving aminoglycosides or vancomycin.

Flucytosine serum concentration monitoring has been shown to reduce toxicity when doses are adjusted to maintain peak concentrations below 100 mcg/mL.

MANAGEMENT OF ANTIMICROBIAL DRUG TOXICITY Owing to the large number of antimicrobials available, it is usually possible to select an effective alternative in patients who develop serious drug toxicity (Table 51–1). However, for some infections there are no effective alternatives to the drug of choice. For example, in patients with neurosyphilis who have a history of anaphylaxis to penicillin, it is necessary to perform skin testing and desensitization to penicillin. It is important to obtain a clear history of drug allergy and other adverse drug reactions. A patient with a documented antimicrobial allergy should carry a card with the name of the drug and a description of the reaction. Cross-reactivity between penicillins and cephalosporins is less than 10%. Cephalosporins may be administered to patients with penicillin-induced maculopapular rashes but should be avoided in patients with a history of penicillin-induced immediate hypersensitivity reactions. On the other hand, aztreonam does not cross-react with penicillins and can be safely administered to patients with a history of penicillin-induced anaphylaxis. For mild reactions, it may be possible to continue therapy with use of adjunctive agents or dosage reduction.

Adverse reactions to antimicrobials occur with increased frequency in several groups, including neonates, geriatric patients, renal failure patients, and AIDS patients. Dosage adjustment of the drugs listed in Table 51–5 is essential for the prevention of adverse effects in patients with renal failure. In addition, several agents are contraindicated in patients with renal impairment because of increased rates of serious toxicity (Table 51–5). See the preceding chapters for discussions of specific drugs.

ANTIMICROBIAL DRUG COMBINATIONS RATIONALE FOR COMBINATION ANTIMICROBIAL THERAPY Most infections should be treated with a single antimicrobial agent. Although indications for combination therapy exist, antimicrobial combinations are often overused in clinical practice. The unnecessary use of antimicrobial combinations increases toxicity and costs and may occasionally result in reduced efficacy due to antagonism of one drug by another. Antimicrobial combinations should be selected for one or more of the following reasons: 1. To provide broad-spectrum empiric therapy in seriously ill patients.

2. To treat polymicrobial infections (such as intra-abdominal abscesses, which typically are due to a combination of anaerobic and aerobic gram-negative organisms, and enterococci). The antimicrobial combination chosen should cover the most common known or suspected pathogens but need not cover all possible pathogens. The availability of antimicrobials with excellent polymicrobial coverage (eg, β-lactamase inhibitor combinations or carbapenems) may reduce the need for combination therapy in the setting of polymicrobial infections.

3. To decrease the emergence of resistant strains. The value of combination therapy in this setting has been clearly demonstrated for tuberculosis.

4. To decrease dose-related toxicity by using reduced doses of one or more components of the drug regimen. The use of flucytosine in combination with amphotericin B for the treatment of cryptococcal meningitis in non–HIV-infected patients allows for a reduction in amphotericin B dosage with decreased amphotericin B-induced nephrotoxicity.

5. To obtain enhanced inhibition or killing. This use of antimicrobial combinations is discussed in the paragraphs that follow.

SYNERGISM & ANTAGONISM When the inhibitory or killing effects of two or more antimicrobials used together are significantly greater than expected from their effects when used individually, synergism is said to result. Synergism is marked by a fourfold or greater reduction in the MIC or MBC of each drug when used in combination versus when used alone. Antagonism occurs when the combined inhibitory or killing effects of two or more antimicrobial drugs are significantly less than observed when the drugs are used individually.

Mechanisms of Synergistic Action The need for synergistic combinations of antimicrobials has been clearly established for the treatment of enterococcal endocarditis.

Bactericidal activity is essential for the optimal management of bacterial endocarditis. Penicillin or ampicillin in combination with gentamicin or streptomycin is superior to monotherapy with a penicillin or vancomycin. When tested alone, penicillins and vancomycin are only bacteriostatic against susceptible enterococcal isolates. When these agents are combined with an aminoglycoside, however, bactericidal activity results. The addition of gentamicin or streptomycin to penicillin allows for a reduction in the duration of therapy for selected patients with viridans streptococcal endocarditis. Some evidence exists that synergistic combinations of antimicrobials may be of benefit in the treatment of gram-negative bacillary infections in febrile neutropenic cancer patients and in systemic infections caused by Pseudomonas aeruginosa.

Other synergistic antimicrobial combinations have been shown to be more effective than monotherapy with individual components.

Trimethoprim-sulfamethoxazole has been successfully used in the treatment of bacterial infections and P jiroveci (carinii) pneumonia.* β-Lactamase inhibitors restore the activity of intrinsically active but hydrolyzable β lactams against organisms such as Staphylococcus aureus and Bacteroides fragilis. Three major mechanisms of antimicrobial synergism have been established: 1. Blockade of sequential steps in a metabolic sequence: Trimethoprim-sulfamethoxazole is the best-known example of this mechanism of synergy (see Chapter 46). Blockade of the two sequential steps in the folic acid pathway by trimethoprimsulfamethoxazole results in a much more complete inhibition of growth than achieved by either component alone.

2. Inhibition of enzymatic inactivation: Enzymatic inactivation of β-lactam antibiotics is a major mechanism of antibiotic resistance.

Inhibition of β lactamase by β-lactamase inhibitor drugs (eg, sulbactam) results in synergism.

3. Enhancement of antimicrobial agent uptake: Penicillins and other cell wall-active agents can increase the uptake of aminoglycosides by a number of bacteria, including staphylococci, enterococci, streptococci, and P aeruginosa. Enterococci are thought to be intrinsically resistant to aminoglycosides because of permeability barriers. Similarly, amphotericin B is thought to enhance the uptake of flucytosine by fungi.

Mechanisms of Antagonistic Action There are few clinically relevant examples of antimicrobial antagonism. The most striking example was reported in a study of patients with pneumococcal meningitis. Patients who were treated with the combination of penicillin and chlortetracycline had a mortality rate of 79% compared with a mortality rate of 21% in patients who received penicillin monotherapy (illustrating the first mechanism set forth below).

The use of an antagonistic antimicrobial combination does not preclude other potential beneficial interactions. For example, rifampin may antagonize the action of anti-staphylococcal penicillins or vancomycin against staphylococci. However, the aforementioned antimicrobials may prevent the emergence of resistance to rifampin.

Two major mechanisms of antimicrobial antagonism have been established: 1. Inhibition of cidal activity by static agents: Bacteriostatic agents such as tetracyclines and chloramphenicol can antagonize the action of bactericidal cell wall-active agents because cell wall-active agents require that the bacteria be actively growing and dividing.

2. Induction of enzymatic inactivation: Some gram-negative bacilli, including enterobacter species, P aeruginosa, Serratia marcescens, and Citrobacter freundii, possess inducible β lactamases. β-Lactam antibiotics such as imipenem, cefoxitin, and ampicillin are potent inducers of β-lactamase production. If an inducing agent is combined with an intrinsically active but hydrolyzable β lactam such as piperacillin, antagonism may result.

ANTIMICROBIAL PROPHYLAXIS Antimicrobial agents are effective in preventing infections in many settings. Antimicrobial prophylaxis should be used in circumstances in which efficacy has been demonstrated and benefits outweigh the risks of prophylaxis. Antimicrobial prophylaxis may be divided into surgical prophylaxis and nonsurgical prophylaxis.

Surgical Prophylaxis Surgical wound infections are a major category of nosocomial infections. The estimated annual cost of surgical wound infections in the United States is more than $1.5 billion.

The National Research Council (NRC) Wound Classification Criteria have served as the basis for recommending antimicrobial prophylaxis. NRC criteria consist of four classes (see Box: National Research Council [NRC] Wound Classification Criteria).

The Study of the Efficacy of Nosocomial Infection Control (SENIC) identified four independent risk factors for postoperative wound infections: operations on the abdomen, operations lasting more than 2 hours, contaminated or dirty wound classification, and at least three medical diagnoses. Patients with at least two SENIC risk factors who undergo clean surgical procedures have an increased risk of developing surgical wound infections and should receive antimicrobial prophylaxis.

Surgical procedures that necessitate the use of antimicrobial prophylaxis include contaminated and clean-contaminated operations, selected operations in which postoperative infection may be catastrophic such as open heart surgery, clean procedures that involve placement of prosthetic materials, and any procedure in an immunocompromised host. The operation should carry a significant risk of postoperative site infection or cause significant bacterial contamination.

General principles of antimicrobial surgical prophylaxis include the following: 1. The antibiotic should be active against common surgical wound pathogens; unnecessarily broad coverage should be avoided.

2. The antibiotic should have proved efficacy in clinical trials.

3. The antibiotic must achieve concentrations greater than the MIC of suspected pathogens, and these concentrations must be present at the time of incision.

4. The shortest possible course—ideally a single dose—of the most effective and least toxic antibiotic should be used.

5. The newer broad-spectrum antibiotics should be reserved for therapy of resistant infections.

6. If all other factors are equal, the least expensive agent should be used.

The proper selection and administration of antimicrobial prophylaxis are of utmost importance. Common indications for surgical prophylaxis are shown in Table 51–7. Cefazolin is the prophylactic agent of choice for head and neck, gastroduodenal, biliary tract, gynecologic, and clean procedures. Local wound infection patterns should be considered when selecting antimicrobial prophylaxis. The selection of vancomycin over cefazolin may be necessary in hospitals with high rates of methicillin-resistant S aureus or S epidermidis infections. The antibiotic should be present in adequate concentrations at the operative site before incision and throughout the procedure; initial dosing is dependent on the volume of distribution, peak levels, clearance, protein binding, and bioavailability. Parenteral agents should be administered during the interval beginning 60 minutes before incision. In cesarean section, the antibiotic is administered after umbilical cord clamping. For many antimicrobial agents, doses should be repeated if the procedure exceeds 2–6 hours in duration. Singledose prophylaxis is effective for most procedures and results in decreased toxicity and antimicrobial resistance.

TABLE 51–7 Recommendations for surgical antimicrobial prophylaxis.

National Research Council (NRC) Wound Classification Criteria Clean: Elective, primarily closed procedure; respiratory, gastrointestinal, biliary, genitourinary, or oropharyngeal tract not entered; no acute inflammation and no break in technique; expected infection rate â&#x2030;¤ 2%.

Clean contaminated: Urgent or emergency case that is otherwise clean; elective, controlled opening of respiratory, gastrointestinal, biliary, or oropharyngeal tract; minimal spillage or minor break in technique; expected infection rate â&#x2030;¤ 10%.

Contaminated: Acute nonpurulent inflammation; major technique break or major spill from hollow organ; penetrating trauma less than 4 hours old; chronic open wounds to be grafted or covered; expected infection rate about 20%.

Dirty: Purulence or abscess; preoperative perforation of respiratory, gastrointestinal, biliary, or oropharyngeal tract; penetrating

trauma more than 4 hours old; expected infection rate about 40%.

Improper administration of antimicrobial prophylaxis leads to excessive surgical wound infection rates. Common errors in antibiotic prophylaxis include selection of the wrong antibiotic, administering the first dose too early or too late, failure to repeat doses during prolonged procedures, excessive duration of prophylaxis, and inappropriate use of broad-spectrum antibiotics.

Nonsurgical Prophylaxis Nonsurgical prophylaxis includes the administration of antimicrobials to prevent colonization or asymptomatic infection as well as the administration of drugs following colonization by or inoculation of pathogens but before the development of disease. Nonsurgical prophylaxis is indicated in individuals who are at high risk for temporary exposure to selected virulent pathogens and in patients who are at increased risk for developing infection because of underlying disease (eg, immunocompromised hosts). Prophylaxis is most effective when directed against organisms that are predictably susceptible to antimicrobial agents. Common indications and drugs for nonsurgical prophylaxis are listed in Table 51â&#x20AC;&#x201C;8.

TABLE 51â&#x20AC;&#x201C;8 Recommendations for nonsurgical antimicrobial prophylaxis.

REFERENCES American T horacic Society: Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia. Am J Respir Crit Care Med 2005;171:388.

Baddour LM et al: Infective endocarditis: Diagnosis, antimicrobial therapy, and management of complications. Circulation 2005;111:3167.

Baron EJ et al: Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2013 recommendations by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM). Clin Infect Dis 2013;57:e22.

Blumberg HM et al: American T horacic Society/Centers for Disease Control and Prevention/Infectious Diseases Society of America: T reatment of tuberculosis. Am J Respir Crit Care Med 2003;167:603.

Boucher HW: 10 × 20 Progress—Development of new drugs active against gram-negative bacilli: An update from the Infectious Diseases Society of America. Clin Infect Dis 2013;56:1685.

Bratzler DW et al: Clinical practice guidelines for antimicrobial prophylaxis in surgery. Am J Health Syst Pharm 2013;70:195.

Gonzales R et al: Principles of appropriate antibiotic use for treatment of acute respiratory tract infections in adults: Background, specific aims, and methods. Ann Intern Med 2001;134:479.

Gruchalla RS et al: Antibiotic allergy. N Engl J Med 2006;354:601.

Jones RN, Pfaller MA: Bacterial resistance: A worldwide problem. Diagn Microbiol Infect Dis 1998;31:379.

Kaye KS, Kaye D: Antibacterial therapy and newer agents. Infect Dis Clin North Am 2009;23:757.

Mandell LA et al: Infectious Diseases Society of America/American T horacic Society Consensus guidelines on the management of community-acquired pneumonia in adults.

Clin Infect Dis 2007;44:S27.

Mazuski JE: Surgical infections. Surg Clin North Am 2009;89:295.

National Nosocomial Infections Surveillance (NNIS) System Report, Data Summary from January 1992–June 2004, issued October 2004. Am J Infect Control 2004;32:470.

Panel on Opportunistic Infections in HIV-Infected Adults and Adolescents: Guidelines for the prevention and treatment of opportunistic infections in HIV-infected adults and adolescents: Recommendations from the Centers for Disease Control and Prevention, the National Institutes of Health, and the HIV Medicine Association of the Infectious Diseases Society of America. Available at: http://aidsinfo.nih.gov/contentfiles/lvguidelines/adult_oi.pdf. Accessed August 18, 2013.

Sexually transmitted diseases treatment guidelines 2010. Centers for Disease Control and Prevention. MMWR Morb Mortal Wkly Rep 2010;59(RR-12):1.

Simons FE: Anaphylaxis. J Allergy Clin Immunol 2010;125(Suppl 2):S161.

Spellberg B et al: T he future of antibiotics and resistance. N Engl J Med 2013;368:299.

T unkel AR et al: Practice guidelines for the management of bacterial meningitis. Clin Infect Dis 2004;39:1267.

Wilson W et al: Prevention of infective endocarditis: Guidelines from the American Heart Association. Circulation 2007;116:1736.

CASE STUDY ANSWER The most likely diagnosis for this patient is Streptococcus pneumoniae meningitis, the most common bacterial cause of meningitis in adults. Other possible microbiologic etiologies include Neisseria meningitidis, Listeria monocytogenes, and enteric gramnegative bacilli. Intravenous antimicrobials to which local strains of these organisms are sensitive should be started while awaiting culture and sensitivity results. In addition, the use of dexamethasone has been demonstrated to reduce mortality in adults with pneumococcal meningitis in conjunction with appropriate antimicrobial therapy.

CHAPTER

52 Antiprotozoal Drugs Philip J. Rosenthal, MD

CASE STUDY A 5-year-old American girl presents with a 1-week history of intermittent chills, fever, and sweats. She had returned home 2 weeks earlier after leaving the United States for the first time to spend 3 weeks with her grandparents in Nigeria. She received all standard childhood immunizations, but no additional treatment before travel, since her parents have returned to their native Nigeria frequently without medical consequences. Three days ago, the child was seen in an outpatient clinic and diagnosed with a viral syndrome.

Examination reveals a lethargic child, with a temperature of 39.8°C (103.6°F) and splenomegaly. She has no skin rash or lymphadenopathy. Initial laboratory studies are remarkable for hematocrit 29.8%, platelets 45,000/mm3 , creatinine 2.5 mg/dL (220 μmol/L), and mildly elevated bilirubin and transaminases. A blood smear shows ring forms of Plasmodium falciparum at 1.5% parasitemia. What treatment should be started?

MALARIA Malaria is the most important parasitic disease of humans and causes hundreds of millions of illnesses per year. Four species of plasmodium typically cause human malaria: Plasmodium falciparum, P vivax, P malariae, and P ovale. A fifth species, P knowlesi, is primarily a pathogen of monkeys but has recently been recognized to cause illness, including severe disease, in humans in Asia. Although all of the latter species may cause significant illness, P falciparum is responsible for the majority of serious complications and deaths.

Drug resistance is an important therapeutic problem, most notably with P falciparum.

PARASITE LIFE CYCLE An anopheline mosquito inoculates plasmodium sporozoites to initiate human infection (Figure 52–1). Circulating sporozoites rapidly invade liver cells, and exoerythrocytic stage tissue schizonts mature in the liver. Merozoites are subsequently released from the liver and invade erythrocytes. Only erythrocytic parasites cause clinical illness. Repeated cycles of infection can lead to the infection of many erythrocytes and serious disease. Sexual stage gametocytes also develop in erythrocytes before being taken up by mosquitoes, where they develop into infective sporozoites.

FIGURE 52–1 Life cycle of malaria parasites. Only the asexual erythrocytic stage of infection causes clinical malaria. All effective antimalarial treatments are blood schizonticides that kill this stage. (Reproduced from Baird JK: Effectiveness of antimalarial drugs. N Engl J Med 2005;352:1565.)

In P falciparum and P malariae infection, only one cycle of liver cell invasion and multiplication occurs, and liver infection ceases spontaneously in less than 4 weeks. Thus, treatment that eliminates erythrocytic parasites will cure these infections. In P vivax and P ovale infections, a dormant hepatic stage, the hypnozoite, is not eradicated by most drugs, and relapses can occur after therapy directed against erythrocytic parasites. Eradication of both erythrocytic and hepatic parasites is required to cure these infections.

DRUG CLASSIFICATION Several classes of antimalarial drugs are available (Table 52–1 and Figure 52–2). Drugs that eliminate developing or dormant liver forms are called tissue schizonticides; those that act on erythrocytic parasites are blood schizonticides; and those that kill sexual stages and prevent transmission to mosquitoes are gametocides. No single available agent can reliably effect a radical cure, ie, eliminate both hepatic and erythrocytic stages. Few available agents are causal prophylactic drugs, ie, capable of preventing erythrocytic infection.

However, all effective chemoprophylactic agents kill erythrocytic parasites before they increase sufficiently in number to cause clinical disease.

TABLE 52–1 Major antimalarial drugs.

FIGURE 52–2 Structural formulas of some antimalarial drugs.

CHEMOPROPHYLAXIS & TREATMENT When patients are counseled on the prevention of malaria, it is imperative to emphasize measures to prevent mosquito bites (eg, with insect repellents, insecticides, and bed nets), because parasites are increasingly resistant to multiple drugs and no chemoprophylactic regimen is fully protective. Current recommendations from the Centers for Disease Control and Prevention (CDC) include the use of chloroquine for chemoprophylaxis in the few areas infested by only chloroquine-sensitive malaria parasites (principally Hispaniola and Central America west of the Panama Canal), and mefloquine, Malarone,* or doxycycline for most other malarious areas, with doxycycline preferred for areas with a high prevalence of multidrug-resistant falciparum malaria (principally border areas of Thailand) (Table 52–2). CDC recommendations should be checked regularly (Phone: 770-488-7788; after hours 770-488-7100; Internet: http://www.cdc.gov/malaria), because these may change in response to changing resistance patterns and increasing experience with new drugs. In some circumstances, it may be appropriate for travelers to carry supplies of drugs with them in case they develop a febrile illness when medical attention is unavailable. Regimens for self-treatment include new artemisinin-based combination therapies (see below), which are widely available internationally (and, in the case of Coartem** , in the USA); Malarone; mefloquine; and quinine. Most authorities do not recommend routine terminal chemoprophylaxis with primaquine to eradicate dormant hepatic stages of P vivax and P ovale after travel, but this may be appropriate in some circumstances, especially for travelers with major exposure to these parasites.

TABLE 52–2 Drugs for the prevention of malaria in travelers.1

Multiple drugs are available for the treatment of malaria that presents in the USA (Table 52–3). Most nonfalciparum infections and falciparum malaria from areas without known resistance should be treated with chloroquine. For vivax malaria from areas with suspected chloroquine resistance, including Indonesia and Papua New Guinea, other therapies effective against falciparum malaria may be used.

Vivax and ovale malaria should subsequently be treated with primaquine to eradicate liver forms. Uncomplicated falciparum malaria from

most areas is most often treated with Malarone, but new artemisinin-based combinations are increasingly the international standard of care, and one combination, Coartem, is now available in the USA. Other agents that are generally effective against resistant falciparum malaria include mefloquine, quinine, and halofantrine, all of which have toxicity concerns at treatment dosages. Severe falciparum malaria is treated with intravenous artesunate, quinidine, or quinine (intravenous quinine is not available in the USA).

TABLE 52â&#x20AC;&#x201C;3 Treatment of malaria.

CHLOROQUINE Chloroquine has been a drug of choice for both treatment and chemoprophylaxis of malaria since the 1940s, but its usefulness against P falciparum has been seriously compromised by drug resistance. It remains the drug of choice in the treatment of sensitive P falciparum and other species of human malaria parasites.

Chemistry & Pharmacokinetics Chloroquine is a synthetic 4-aminoquinoline (Figure 52–2) formulated as the phosphate salt for oral use. It is rapidly and almost completely absorbed from the gastrointestinal tract, reaches maximum plasma concentrations in about 3 hours, and is rapidly distributed to the tissues. It has a very large apparent volume of distribution of 100–1000 L/kg and is slowly released from tissues and metabolized.

Chloroquine is principally excreted in the urine with an initial half-life of 3–5 days but a much longer terminal elimination half-life of 1–2 months.

Antimalarial Action & Resistance When not limited by resistance, chloroquine is a highly effective blood schizonticide. It is also moderately effective against gametocytes of P vivax, P ovale, and P malariae but not against those of P falciparum. Chloroquine is not active against liver stage parasites. The drug probably acts by concentrating in parasite food vacuoles, preventing the biocrystallization of the hemoglobin breakdown product, heme, into hemozoin, and thus eliciting parasite toxicity due to the buildup of free heme.

Resistance to chloroquine is now very common among strains of P falciparum and uncommon but increasing for P vivax. In P falciparum, mutations in a putative transporter, PfCRT, have been correlated with resistance. Chloroquine resistance can be reversed by certain agents, including verapamil, desipramine, and chlorpheniramine, but the clinical value of resistance-reversing drugs is not established.

Clinical Uses 1. Treatment—Chloroquine is the drug of choice in the treatment of uncomplicated nonfalciparum and sensitive falciparum malaria. It rapidly terminates fever (in 24–48 hours) and clears parasitemia (in 48–72 hours) caused by sensitive parasites. Chloroquine has been replaced by other drugs, principally artemisinin-based combination therapies, as the standard therapy to treat falciparum malaria in most endemic countries. Chloroquine does not eliminate dormant liver forms of P vivax and P ovale, and for that reason primaquine must be added for the radical cure of these species.

2. Chemoprophylaxis—Chloroquine is the preferred chemoprophylactic agent in malarious regions without resistant falciparum malaria.

Eradication of P vivax and P ovale requires a course of primaquine to clear hepatic stages.

3. Amebic liver abscess—Chloroquine reaches high liver concentrations and may be used for amebic abscesses that fail initial therapy with metronidazole (see below).

Adverse Effects Chloroquine is usually very well tolerated, even with prolonged use. Pruritus is common, primarily in Africans. Nausea, vomiting, abdominal pain, headache, anorexia, malaise, blurring of vision, and urticaria are uncommon. Dosing after meals may reduce some adverse effects. Rare reactions include hemolysis in glucose-6-phosphate dehydrogenase (G6PD)-deficient persons, impaired hearing, confusion, psychosis, seizures, agranulocytosis, exfoliative dermatitis, alopecia, bleaching of hair, hypotension, and electrocardiographic changes (QRS widening, T-wave abnormalities). The long-term administration of high doses of chloroquine for rheumatologic diseases (see Chapter 36) can result in irreversible ototoxicity, retinopathy, myopathy, and peripheral neuropathy, but these are rarely seen with standard-dose weekly chemoprophylaxis. Intramuscular injections or intravenous infusions of chloroquine hydrochloride can result in severe hypotension and respiratory and cardiac arrest, and should be avoided.

Contraindications & Cautions Chloroquine is contraindicated in patients with psoriasis or porphyria. It should generally not be used in those with retinal or visual field abnormalities or myopathy. Chloroquine should be used with caution in patients with liver, neurologic, or hematologic disorders. The antidiarrheal agent kaolin and calcium- and magnesium-containing antacids interfere with the absorption of chloroquine and should not be co-administered. Chloroquine is considered safe in pregnancy and for young children.

OTHER QUINOLINES Amodiaquine is closely related to chloroquine, and it probably shares mechanisms of action and resistance. Amodiaquine has been widely used to treat malaria because of its low cost, limited toxicity, and, in some areas, effectiveness against chloroquine-resistant strains of P falciparum, but toxicities, including agranulocytosis, aplastic anemia, and hepatotoxicity, have limited its use. However, recent reevaluation has shown that serious toxicity from amodiaquine is uncommon. The most important current use of amodiaquine is in combination therapy. The World Health Organization (WHO) lists artesunate plus amodiaquine as a recommended therapy for falciparum malaria (Table 52–4). This combination is now available as a single tablet (ASAQ, Arsucam, Coarsucam) and is the first-line therapy for the treatment of uncomplicated falciparum malaria in many countries in Africa. Another combination, amodiaquine plus sulfadoxine-pyrimethamine, remains reasonably effective for the treatment of falciparum malaria. Long-term chemoprophylaxis with amodiaquine is best avoided because of its apparent increased toxicity with long-term use, but short-term seasonal malaria chemoprevention with amodiaquine plus sulfadoxine-pyrimethamine (monthly treatment doses for 3–4 months during the transmission season) is now recommended by the WHO for the Sahel sub-region of Africa.

TABLE 52–4 WHO recommendations for the treatment of falciparum malaria.

Piperaquine is a bisquinoline that was used widely to treat chloroquine-resistant falciparum malaria in China in the 1970s–1980s, but its use waned after resistance became widespread. Recently, piperaquine combined with dihydroartemisinin (Artekin, Duocotecxin) has shown excellent efficacy and safety for the treatment of falciparum malaria, without apparent drug resistance. Piperaquine has a longer half-life (~ 28 days) than amodiaquine (~ 14 days), mefloquine (~ 14 days), or lumefantrine (~ 4 days), leading to a longer period of posttreatment prophylaxis with dihydroartemisinin-piperaquine than with the other leading artemisinin-based combinations; this feature should be particularly advantageous in high transmission areas. Dihydroartemisinin-piperaquine is now the first-line therapy for the treatment of uncomplicated falciparum malaria in some countries in Asia.

ARTEMISININ & ITS DERIVATIVES Artemisinin (qinghaosu) is a sesquiterpene lactone endoperoxide (Figure 52–2), the active component of an herbal medicine that has been used as an antipyretic in China for over 2000 years. Artemisinin is insoluble and can only be used orally. Analogs have been synthesized to increase solubility and improve antimalarial efficacy. The most important of these analogs are artesunate (water-soluble; oral, intravenous, intramuscular, and rectal administration), artemether (lipid-soluble; oral, intramuscular, and rectal administration), and dihydroartemisinin (water-soluble; oral administration).

Chemistry & Pharmacokinetics Artemisinin and its analogs are rapidly absorbed, with peak plasma levels occurring promptly. Half-lives after oral administration are 30– 60 minutes for artesunate and dihydroartemisinin, and 2–3 hours for artemether. Artemisinin, artesunate, and artemether are rapidly metabolized to the active metabolite dihydroartemisinin. Drug levels appear to decrease after a number of days of therapy.

Antimalarial Action & Resistance The artemisinins are now widely available, but monotherapy for the treatment of uncomplicated malaria is strongly discouraged. Rather, co-formulated artemisinin-based combination therapies are recommended to improve efficacy and prevent the selection of artemisininresistant parasites. The oral combination regimen Coartem (artemether-lumefantrine) was approved by the FDA in 2009, and may be considered the new first-line therapy in the USA for uncomplicated falciparum malaria, although it may not be widely available.

Intravenous artesunate was made available by the CDC in 2007; use of the drug is initiated by contacting the CDC, which will release it for appropriate indications (falciparum malaria with signs of severe disease or inability to take oral medications) from stocks stored around the USA. Artemisinin and its analogs are very rapidly acting blood schizonticides against all human malaria parasites. Artemisinins have no effect on hepatic stages. The antimalarial activity of artemisinins appears to result from the production of free radicals that follows the iron-catalyzed cleavage of the artemisinin endoperoxide bridge. Artemisinin resistance is not yet a widespread problem, but delayed clearance of P falciparum infections and decreased treatment efficacy in parts of Southeast Asia demonstrate a worrisome focus of resistance.

Clinical Uses Artemisinin-based combination therapy is now the standard of care for treatment of uncomplicated falciparum malaria in nearly all areas endemic for falciparum malaria. The leading regimens are highly efficacious, safe, and well tolerated. These regimens were developed because the short plasma half-lives of the artemisinins led to unacceptably high recrudescence rates after short-course therapy, which were reversed by inclusion of longer-acting drugs. Combination therapy also helps to protect against the selection of artemisinin resistance. However, with completion of dosing after 3 days, the artemisinin components are rapidly eliminated, and so selection of resistance to partner drugs is of concern.

The WHO recommends five artemisinin-based combinations for the treatment of uncomplicated falciparum malaria (Table 52–4).

One of these, artesunate-sulfadoxine-pyrimethamine is not recommended in many areas owing to unacceptable levels of resistance to sulfadoxine-pyrimethamine, but it is the first-line therapy in some countries. The other recommended regimens are available as combination formulations, although manufacturing standards may vary. Artesunate-mefloquine is highly effective in Southeast Asia, where resistance to many antimalarials is common; it is the first-line therapy in some countries in Southeast Asia and South America.

This regimen is less practical for other areas, particularly Africa, because of its relatively high cost and poor tolerability. Either artesunate-amodiaquine or artemether-lumefantrine is the standard treatment for uncomplicated falciparum malaria in most countries in Africa and some additional endemic countries on other continents. Dihydroartemisinin-piperaquine is a newer regimen that has shown excellent efficacy; it is a first-line therapy for falciparum malaria in parts of Southeast Asia.

Artemisinins also have outstanding efficacy in the treatment of complicated falciparum malaria. Large randomized trials and metaanalyses have shown that intramuscular artemether has an efficacy equivalent to that of quinine and that intravenous artesunate is superior to intravenous quinine in terms of parasite clearance time and—most important—patient survival. Intravenous artesunate also has a superior side-effect profile when compared with intravenous quinine or quinidine. Thus, intravenous artesunate has replaced quinine as the standard of care for the treatment of severe falciparum malaria, although it is not yet available in many areas. Artesunate and artemether have also been effective in the treatment of severe malaria when administered rectally, offering a valuable treatment modality when parenteral therapy is not available.

Adverse Effects & Cautions Artemisinins are generally very well tolerated. The most commonly reported adverse effects are nausea, vomiting, diarrhea, and

dizziness, and these may often be due to underlying malaria rather than the medications. Rare serious toxicities include neutropenia, anemia, hemolysis, elevated liver enzymes, and allergic reactions. Irreversible neurotoxicity has been seen in animals, but only after doses much higher than those used to treat malaria. Artemisinins have been embryotoxic in animal studies, but rates of congenital abnormalities, stillbirths, and abortions were not elevated, compared with those of controls, in women who received artemisinins during pregnancy.

Based on this information and the significant risk of malaria during pregnancy, the WHO recommends artemisinin-based combination therapies for the treatment of uncomplicated falciparum malaria during the second and third trimesters of pregnancy (quinine plus clindamycin is recommended during the first trimester), intravenous artesunate or quinine for the treatment of severe malaria during the first trimester, and intravenous artesunate for treatment of severe malaria during the second and third trimesters.

QUININE & QUINIDINE Quinine and quinidine remain important therapies for falciparum malaria—especially severe disease—although toxicity may complicate therapy.

Chemistry & Pharmacokinetics Quinine is derived from the bark of the cinchona tree, a traditional remedy for intermittent fevers from South America. The alkaloid quinine was purified in 1820, and has been used in the treatment and prevention of malaria since that time. Quinidine, the dextrorotatory stereoisomer of quinine, is at least as effective as parenteral quinine in the treatment of severe falciparum malaria. After oral administration, quinine is rapidly absorbed, reaches peak plasma levels in 1–3 hours, and is widely distributed in body tissues. The use of a loading dose in severe malaria allows the achievement of peak levels within a few hours. The pharmacokinetics of quinine varies among populations. Individuals with malaria develop higher plasma levels of the drug than healthy controls, but toxicity is not increased, apparently because of increased protein binding. The half-life of quinine also is longer in those with severe malaria (18 hours) than in healthy controls (11 hours). Quinidine has a shorter half-life than quinine, mostly as a result of decreased protein binding. Quinine is primarily metabolized in the liver and excreted in the urine.

Antimalarial Action & Resistance Quinine is a rapid-acting, highly effective blood schizonticide against the four species of human malaria parasites. The drug is gametocidal against P vivax and P ovale but not P falciparum. It is not active against liver stage parasites. The mechanism of action of quinine is unknown. Resistance to quinine is common in some areas of Southeast Asia, especially border areas of Thailand, where the drug may fail if used alone to treat falciparum malaria. However, quinine still provides at least a partial therapeutic effect in most patients.

Clinical Uses 1. Parenteral treatment of severe falciparum malaria—For many years quinine dihydrochloride or quinidine gluconate were the treatments of choice for severe falciparum malaria, although intravenous artesunate is now preferred. Quinine can be administered slowly intravenously or, in a dilute solution, intramuscularly, but parenteral preparations are not available in the USA. Quinidine is available (although not always readily accessible) in the USA for the parenteral treatment of severe falciparum malaria. Quinidine can be administered in divided doses or by continuous intravenous infusion; treatment should begin with a loading dose to achieve effective plasma concentrations promptly. Because of its cardiac toxicity and the relative unpredictability of its pharmacokinetics, intravenous quinidine should be administered slowly with cardiac monitoring. Therapy should be changed to an effective oral agent as soon as the patient has improved and can tolerate oral medications.

2. Oral treatment of falciparum malaria—Quinine sulfate is appropriate therapy for uncomplicated falciparum malaria except when the infection was transmitted in an area without documented chloroquine resistance. Quinine is commonly used with a second drug (most often doxycycline or, in children, clindamycin) to shorten the duration of use (usually to 3 days) and limit toxicity. Quinine is not generally used to treat nonfalciparum malaria, because it is less effective than chloroquine against these parasites and is more toxic.

3. Malarial chemoprophylaxis—Quinine is not generally used in chemoprophylaxis owing to its toxicity, although a daily dose of 325 mg is effective.

4. Babesiosis—Quinine is first-line therapy, in combination with clindamycin, in the treatment of infection with Babesia microti or other human babesial infections.

Adverse Effects Therapeutic dosages of quinine and quinidine commonly cause tinnitus, headache, nausea, dizziness, flushing, and visual disturbances, a constellation of symptoms termed cinchonism. Mild symptoms of cinchonism do not warrant the discontinuation of therapy. More severe findings, often after prolonged therapy, include more marked visual and auditory abnormalities, vomiting, diarrhea, and abdominal pain. Hypersensitivity reactions include skin rashes, urticaria, angioedema, and bronchospasm. Hematologic abnormalities include hemolysis (especially with G6PD deficiency), leukopenia, agranulocytosis, and thrombocytopenia. Therapeutic doses may cause hypoglycemia through stimulation of insulin release; this is a particular problem in severe infections and in pregnant patients, who have increased sensitivity to insulin. Quinine can stimulate uterine contractions, especially in the third trimester. However, this effect is mild, and quinine and quinidine remain appropriate for treatment of severe falciparum malaria even during pregnancy. Intravenous infusions of the drugs may cause thrombophlebitis.

Severe hypotension can follow too-rapid intravenous infusions of quinine or quinidine. Electrocardiographic abnormalities (QT interval prolongation) are fairly common with intravenous quinidine, but dangerous arrhythmias are uncommon when the drug is administered appropriately in a monitored setting.

Blackwater fever is a rare severe illness that includes marked hemolysis and hemoglobinuria in the setting of quinine therapy for malaria. It appears to be due to a hypersensitivity reaction to the drug, although its pathogenesis is uncertain.

Contraindications & Cautions Quinine (or quinidine) should be discontinued if signs of severe cinchonism, hemolysis, or hypersensitivity occur. It should be avoided if possible in patients with underlying visual or auditory problems. It must be used with great caution in those with underlying cardiac abnormalities. Quinine should not be given concurrently with mefloquine and should be used with caution in a patient with malaria who has previously received mefloquine chemoprophylaxis. Absorption may be blocked by aluminum-containing antacids. Quinine can raise plasma levels of warfarin and digoxin. Dosage must be reduced in renal insufficiency.

MEFLOQUINE Mefloquine is effective therapy for many chloroquine-resistant strains of P falciparum and against other species. Although toxicity is a concern, mefloquine is one of the recommended chemoprophylactic drugs for use in most malaria-endemic regions with chloroquineresistant strains.

Chemistry & Pharmacokinetics Mefloquine hydrochloride is a synthetic 4-quinoline methanol that is chemically related to quinine. It can only be given orally because severe local irritation occurs with parenteral use. It is well absorbed, and peak plasma concentrations are reached in about 18 hours.

Mefloquine is highly protein-bound, extensively distributed in tissues, and eliminated slowly, allowing a single-dose treatment regimen. The terminal elimination half-life is about 20 days, allowing weekly dosing for chemoprophylaxis. With weekly dosing, steady-state drug levels are reached over a number of weeks. Mefloquine and its metabolites are slowly excreted, mainly in the feces.

Antimalarial Action & Resistance Mefloquine has strong blood schizonticidal activity against P falciparum and P vivax, but it is not active against hepatic stages or gametocytes. The mechanism of action is unknown. Sporadic resistance to mefloquine has been reported from many areas. At present, resistance appears to be uncommon except in regions of Southeast Asia with high rates of multidrug resistance (especially border areas of Thailand). Mefloquine resistance appears to be associated with resistance to quinine and halofantrine, but not with resistance to chloroquine.

Clinical Uses 1. Chemoprophylaxisâ&#x20AC;&#x201D;Mefloquine is effective in prophylaxis against most strains of P falciparum and probably all other human malarial species. Mefloquine is therefore among the drugs recommended by the CDC for chemoprophylaxis in all malarious areas except those with no chloroquine resistance (where chloroquine is preferred) and some rural areas of Southeast Asia with a high prevalence of mefloquine resistance. As with chloroquine, eradication of P vivax and P ovale requires a course of primaquine.

2. Treatmentâ&#x20AC;&#x201D;Mefloquine is effective in treating uncomplicated falciparum malaria. The drug is not appropriate for treating individuals with severe or complicated malaria, since quinine, quinidine, and artemisinins are more rapidly active, and since drug resistance is less likely with those agents. The combination of artesunate plus mefloquine showed excellent antimalarial efficacy in regions of Southeast

Asia with some resistance to mefloquine, and this regimen is now one of the combination therapies recommended by the WHO for the treatment of uncomplicated falciparum malaria (Table 52–4). Artesunate-mefloquine is the first-line therapy for uncomplicated falciparum malaria in a number of countries in Asia and South America.

Adverse Effects Weekly dosing with mefloquine for chemoprophylaxis may cause nausea, vomiting, dizziness, sleep and behavioral disturbances, epigastric pain, diarrhea, abdominal pain, headache, rash, and dizziness. Neuropsychiatric toxicities have received a good deal of publicity, but despite frequent anecdotal reports of seizures and psychosis, a number of controlled studies have found the frequency of serious adverse effects from mefloquine to be similar to that with other common antimalarial chemoprophylactic regimens. However, concern about reported long-term effects of short-term use of mefloquine led in 2013 to the FDA adding a black box warning regarding potential neurologic and psychiatric toxicities. Leukocytosis, thrombocytopenia, and aminotransferase elevations have also been reported.

Adverse effects are more common with the higher dosages of mefloquine required for treatment. These effects may be lessened by administering the drug in two doses separated by 6–8 hours. The incidence of neuropsychiatric symptoms appears to be about ten times greater than with chemoprophylactic dosing, with widely varying frequencies of up to about 50% reported. Serious neuropsychiatric toxicities (depression, confusion, acute psychosis, or seizures) have been reported in less than 1 in 1000 treatments, but some authorities believe that these toxicities are actually more common. Mefloquine can also alter cardiac conduction, and arrhythmias and bradycardia have been reported.

Contraindications & Cautions Mefloquine is contraindicated in a patient with a history of epilepsy, psychiatric disorders, arrhythmia, cardiac conduction defects, or sensitivity to related drugs. It should not be co-administered with quinine, quinidine, or halofantrine, and caution is required if quinine or quinidine is used to treat malaria after mefloquine chemoprophylaxis. The CDC no longer advises against mefloquine use in patients receiving β-adrenoceptor antagonists. Mefloquine is also now considered safe in young children, and it is the only chemoprophylactic other than chloroquine approved for children weighing less than 5 kg and for pregnant women. Available data suggest that mefloquine is safe throughout pregnancy, although experience in the first trimester is limited. An older recommendation to avoid mefloquine use in those requiring fine motor skills (eg, airline pilots) is controversial. Mefloquine chemoprophylaxis should be discontinued if significant neuropsychiatric symptoms develop.

PRIMAQUINE Primaquine is the drug of choice for the eradication of dormant liver forms of P vivax and P ovale and can also be used for chemoprophylaxis against all malarial species.

Chemistry & Pharmacokinetics Primaquine phosphate is a synthetic 8-aminoquinoline (Figure 52–2). The drug is well absorbed orally, reaching peak plasma levels in 1–2 hours. The plasma half-life is 3–8 hours. Primaquine is widely distributed to the tissues, but only a small amount is bound there. It is rapidly metabolized and excreted in the urine. Its three major metabolites appear to have less antimalarial activity but more potential for inducing hemolysis than the parent compound.

Antimalarial Action & Resistance Primaquine is active against hepatic stages of all human malaria parasites. It is the only available agent active against the dormant hypnozoite stages of P vivax and P ovale. The drug is also gametocidal against the four human malaria species and it has weak activity against erythrocytic stage parasites. The mechanism of antimalarial action is unknown.

Some strains of P vivax in New Guinea, Southeast Asia, Central and South America, and other areas are relatively resistant to primaquine. Liver forms of these strains may not be eradicated by a single standard treatment and may require repeated therapy.

Because of decreasing efficacy, the standard dosage of primaquine for radical cure of P vivax infection was doubled in 2005 to 30 mg base daily for 14 days.

Clinical Uses 1. Therapy (radical cure) of acute vivax and ovale malaria—Standard therapy for these infections includes chloroquine to eradicate erythrocytic forms and primaquine to eradicate liver hypnozoites and prevent a subsequent relapse. Chloroquine is given acutely, and therapy with primaquine is withheld until the G6PD status of the patient is known. If the G6PD level is normal, a 14-day course of

primaquine is given. Prompt evaluation of the G6PD level is helpful, since primaquine appears to be most effective when instituted before completion of dosing with chloroquine.

2. Terminal prophylaxis of vivax and ovale malaria—Standard chemoprophylaxis does not prevent a relapse of vivax or ovale malaria, because the hypnozoite forms of these parasites are not eradicated by chloroquine or other available blood schizonticides. To markedly diminish the likelihood of relapse, some authorities advocate the use of primaquine after the completion of travel to an endemic area.

3. Chemoprophylaxis of malaria—Primaquine has been studied as a daily chemoprophylactic agent. Daily treatment with 30 mg (0.5 mg/kg) of base provided good levels of protection against falciparum and vivax malaria. However, potential toxicities of long-term use remain a concern, and primaquine is generally recommended for this purpose only when mefloquine, Malarone, and doxycycline cannot be used.

4. Gametocidal action—A single dose of primaquine (45 mg base) renders P falciparum gametocytes noninfective to mosquitoes.

Gametocidal activity may be achieved with much lower dosages, and mass administration or short treatments with low doses of primaquine are under study to improve control of falciparum malaria.

5. Pneumocystis jiroveci infection—The combination of clindamycin and primaquine is an alternative regimen in the treatment of pneumocystosis, particularly mild to moderate disease. This regimen offers improved tolerance compared with high-dose trimethoprimsulfamethoxazole or pentamidine, although its efficacy against severe pneumocystis pneumonia is not well studied.

Adverse Effects Primaquine in recommended doses is generally well tolerated. It infrequently causes nausea, epigastric pain, abdominal cramps, and headache, and these symptoms are more common with higher dosages and when the drug is taken on an empty stomach. More serious but rare adverse effects are leukopenia, agranulocytosis, leukocytosis, and cardiac arrhythmias. Standard doses of primaquine may cause hemolysis or methemoglobinemia (manifested by cyanosis), especially in persons with G6PD deficiency or other hereditary metabolic defects.

Contraindications & Cautions Primaquine should be avoided in patients with a history of granulocytopenia or methemoglobinemia, in those receiving potentially myelosuppressive drugs (eg, quinidine), and in those with disorders that commonly include myelosuppression. It is never given parenterally because it may cause marked hypotension.

Patients should be tested for G6PD deficiency before primaquine is prescribed. When a patient is deficient in G6PD, treatment strategies may consist of withholding therapy and treating subsequent relapses, if they occur, with chloroquine; treating patients with standard dosing, paying close attention to their hematologic status; or treating with weekly primaquine (45 mg base) for 8 weeks. G6PDdeficient individuals of Mediterranean and Asian ancestry are most likely to have severe deficiency, whereas those of African ancestry usually have a milder biochemical defect. This difference can be taken into consideration in choosing a treatment strategy. In any event, primaquine should be discontinued if there is evidence of hemolysis or anemia. Primaquine should be avoided in pregnancy because the fetus is relatively G6PD-deficient and thus at risk of hemolysis.

ATOVAQUONE Atovaquone, a hydroxynaphthoquinone (Figure 52–2), was initially developed as an antimalarial agent, and as a component of Malarone is recommended for treatment and prevention of malaria. Atovaquone has also been approved by the FDA for the treatment of mild to moderate P jiroveci pneumonia.

The drug is only administered orally. Its bioavailability is low and erratic, but absorption is increased by fatty food. The drug is heavily protein-bound and has a half-life of 2–3 days. Most of the drug is eliminated unchanged in the feces. Atovaquone acts against plasmodia by disrupting mitochondrial electron transport. It is active against tissue and erythrocytic schizonts, allowing chemoprophylaxis to be discontinued only 1 week after the end of exposure (compared with 4 weeks for mefloquine or doxycycline, which lack activity against tissue schizonts).

Initial use of atovaquone to treat malaria led to disappointing results, with frequent failures due to the selection of resistant parasites during therapy. In contrast, Malarone, a fixed combination of atovaquone (250 mg) and proguanil (100 mg), is highly effective for both the treatment and chemoprophylaxis of falciparum malaria, and it is now approved for both indications in the USA. For chemoprophylaxis, Malarone must be taken daily (Table 52–2). It has an advantage over mefloquine and doxycycline in requiring shorter periods of treatment before and after the period at risk for malaria transmission, but it is more expensive than the other agents. It should

be taken with food.

Atovaquone is an alternative therapy for P jiroveci infection, although its efficacy is lower than that of trimethoprimsulfamethoxazole. Standard dosing is 750 mg taken with food twice daily for 21 days. Adverse effects include fever, rash, nausea, vomiting, diarrhea, headache, and insomnia. Serious adverse effects appear to be minimal, although experience with the drug remains limited. Atovaquone has also been effective in small numbers of immunocompromised patients with toxoplasmosis unresponsive to other agents, although its role in this disease is not yet defined.

Malarone is generally well tolerated. Adverse effects include abdominal pain, nausea, vomiting, diarrhea, headache, and rash, and these are more common with the higher dosage required for treatment. Reversible elevations in liver enzymes have been reported. The safety of atovaquone in pregnancy is unknown, and its use is not advised in pregnant women. It is considered safe for use in children with body weight above 5 kg. Plasma concentrations of atovaquone are decreased about 50% by co-administration of tetracycline or rifampin.

INHIBITORS OF FOLATE SYNTHESIS Inhibitors of enzymes involved in folate metabolism are used, generally in combination regimens, in the treatment and prevention of malaria.

Chemistry & Pharmacokinetics Pyrimethamine is a 2,4-diaminopyrimidine related to trimethoprim (see Chapter 46). Proguanil is a biguanide derivative (Figure 52–2).

Both drugs are slowly but adequately absorbed from the gastrointestinal tract. Pyrimethamine reaches peak plasma levels 2–6 hours after an oral dose, is bound to plasma proteins, and has an elimination half-life of about 3.5 days. Proguanil reaches peak plasma levels about 5 hours after an oral dose and has an elimination half-life of about 16 hours. Therefore, proguanil must be administered daily for chemoprophylaxis, whereas pyrimethamine can be given once a week. Pyrimethamine is extensively metabolized before excretion.

Proguanil is a prodrug; only its triazine metabolite, cycloguanil, is active. Fansidar, a fixed combination of the sulfonamide sulfadoxine (500 mg per tablet) and pyrimethamine (25 mg per tablet), is well absorbed. Its components display peak plasma levels within 2–8 hours and are excreted mainly by the kidneys. The average half-life of sulfadoxine is about 170 hours.

Antimalarial Action & Resistance Pyrimethamine and proguanil act slowly against erythrocytic forms of susceptible strains of all four human malaria species. Proguanil also has some activity against hepatic forms. Neither drug is adequately gametocidal or effective against the persistent liver stages of P vivax or P ovale. Sulfonamides and sulfones are weakly active against erythrocytic schizonts but not against liver stages or gametocytes. They are not used alone as antimalarials but are effective in combination with other agents.

The mechanism of action of pyrimethamine and proguanil involves selective inhibition of plasmodial dihydrofolate reductase, a key enzyme in the pathway for synthesis of folate. Sulfonamides and sulfones inhibit another enzyme in the folate pathway, dihydropteroate synthase. As described in Chapter 46, combinations of inhibitors of these two enzymes provide synergistic activity (see Figure 46–2).

Resistance to folate antagonists and sulfonamides is common in many areas for P falciparum and less common for P vivax.

Resistance is due primarily to mutations in dihydrofolate reductase and dihydropteroate synthase, with increasing numbers of mutations leading to increasing levels of resistance. At present, resistance seriously limits the efficacy of sulfadoxine-pyrimethamine (Fansidar) for the treatment of malaria in most areas, but in Africa most parasites exhibit an intermediate level of resistance, such that antifolates may continue to offer some preventive efficacy against malaria. Because different mutations may mediate resistance to different agents, cross-resistance is not uniformly seen.

Clinical Uses 1. Chemoprophylaxis—Chemoprophylaxis with single folate antagonists is no longer recommended because of frequent resistance, but a number of agents are used in combination regimens. The combination of chloroquine (500 mg weekly) and proguanil (200 mg daily) was previously widely used, but with increasing resistance to both agents it is no longer recommended. Fansidar and Maloprim (the latter is a combination of pyrimethamine and the sulfone dapsone) are both effective against sensitive parasites with weekly dosing, but they are no longer recommended because of resistance and toxicity. Trimethoprim-sulfamethoxazole, an antifolate combination that is more active against bacteria than malaria parasites, is increasingly used as a daily prophylactic therapy for HIV-infected patients in developing countries. Although it is administered primarily to prevent typical HIV opportunistic and bacterial infections, this regimen offers partial preventive efficacy against malaria in Africa.

2. Intermittent preventive therapy—A new strategy for malaria control is intermittent preventive therapy, in which high-risk patients receive intermittent treatment for malaria, regardless of their infection status. This practice is most accepted in pregnancy, with the use

of two or more doses of sulfadoxine-pyrimethamine after the first trimester now standard policy in Africa. In children intermittent preventive therapy has not been widely accepted, but the WHO now recommends seasonal malaria chemoprevention with amodiaquine plus sulfadoxine-pyrimethamine in the Sahel sub-region of Africa, where malaria is highly seasonal and resistance to antifolates is relatively uncommon. Unfortunately, in most other areas drug resistance seriously limits the preventive efficacy of antifolates.

3. Treatment of chloroquine-resistant falciparum malaria—Until recently Fansidar was commonly used to treat uncomplicated falciparum malaria. Advantages of Fansidar are ease of administration (a single oral dose) and low cost. However, due to unacceptable levels of resistance, Fansidar is no longer a recommended therapy. In particular, Fansidar should not be used for severe malaria, since it is slower-acting than other available agents. Fansidar is also not reliably effective in vivax malaria, and its usefulness against P ovale and P malariae has not been adequately studied. Artesunate plus sulfadoxine-pyrimethamine is recommended by the WHO to treat falciparum malaria (Table 52–4), but resistance limits the efficacy of this regimen more than the other recommended combinations.

4. Toxoplasmosis—Pyrimethamine, in combination with sulfadiazine, is first-line therapy in the treatment of toxoplasmosis, including acute infection, congenital infection, and disease in immunocompromised patients. For immunocompromised patients, high-dose therapy is required followed by chronic suppressive therapy. Folinic acid is included to limit myelosuppression. Toxicity from the combination is usually due primarily to sulfadiazine. The replacement of sulfadiazine with clindamycin provides an effective alternative regimen.

5. Pneumocystosis—P jiroveci is the cause of human pneumocystosis and is now recognized to be a fungus, but this organism is discussed in this chapter because it responds to antiprotozoal drugs, not antifungals. (The related species P carinii is now recognized to be the cause of animal infections.) First-line therapy of pneumocystosis is trimethoprim plus sulfamethoxazole (see also Chapter 46).

Standard treatment includes high-dose intravenous or oral therapy (15 mg/kg trimethoprim and 75 mg/kg sulfamethoxazole per day in three or four divided doses) for 21 days. High-dose therapy entails significant toxicity, especially in patients with AIDS. Important toxicities include nausea, vomiting, fever, rash, leukopenia, hyponatremia, elevated hepatic enzymes, azotemia, anemia, and thrombocytopenia. Less common effects include severe skin reactions, mental status changes, pancreatitis, and hypocalcemia.

Trimethoprim-sulfamethoxazole is also the standard chemoprophylactic drug for the prevention of P jiroveci infection in immunocompromised individuals. Dosing is one double-strength tablet daily or three times per week. The chemoprophylactic dosing schedule is much better tolerated than high-dose therapy in immunocompromised patients, but rash, fever, leukopenia, or hepatitis may necessitate changing to another drug.

Adverse Effects & Cautions Most patients tolerate pyrimethamine and proguanil well. Gastrointestinal symptoms, skin rashes, and itching are rare. Mouth ulcers and alopecia have been described with proguanil. Fansidar is no longer recommended for chemoprophylaxis because of uncommon but severe cutaneous reactions, including erythema multiforme, Stevens-Johnson syndrome, and toxic epidermal necrolysis. Severe reactions appear to be much less common with single-dose or intermittent therapy, and use of the drug has been justified by the risks associated with falciparum malaria.

Rare adverse effects with a single dose of Fansidar are those associated with other sulfonamides, including hematologic, gastrointestinal, central nervous system, dermatologic, and renal toxicity. Maloprim is no longer recommended for chemoprophylaxis because of unacceptably high rates of agranulocytosis. Folate antagonists should be used cautiously in the presence of renal or hepatic dysfunction. Although pyrimethamine is teratogenic in animals, Fansidar has been safely used in pregnancy. Proguanil is considered safe in pregnancy. Folate supplements should be routinely administered during pregnancy, but in women receiving Fansidar preventive therapy, high-dose folate supplementation (eg, 5 mg daily) should be replaced by the standard recommended dosage (0.4–0.6 mg daily) to avoid potential loss of protective efficacy.

ANTIBIOTICS A number of antibiotics in addition to the folate antagonists and sulfonamides are modestly active antimalarials. The antibiotics that are bacterial protein synthesis inhibitors appear to act against malaria parasites by inhibiting protein synthesis in a plasmodial prokaryote-like organelle, the apicoplast. None of the antibiotics should be used as single agents in the treatment of malaria because their action is much slower than that of standard antimalarials.

Tetracycline and doxycycline (see Chapter 44) are active against erythrocytic schizonts of all human malaria parasites. They are not active against liver stages. Doxycycline is used in the treatment of falciparum malaria in conjunction with quinine, allowing a shorter and better-tolerated course of that drug. Doxycycline is also used to complete treatment courses after initial treatment of severe malaria with intravenous quinine, quinidine, or artesunate. In all of these cases a 1-week treatment course of doxycycline is carried out. Doxycycline has also become a standard chemoprophylactic drug, especially for use in areas of Southeast Asia with high rates of resistance to other antimalarials, including mefloquine. Doxycycline adverse effects include gastrointestinal symptoms, candidal vaginitis, and photosensitivity. Its safety in long-term chemoprophylaxis has not been extensively evaluated.

Clindamycin (see Chapter 44) is slowly active against erythrocytic schizonts and can be used after treatment courses of quinine, quinidine, or artesunate in those for whom doxycycline is not recommended, such as children and pregnant women. Antimalarial activity of azithromycin and fluoroquinolones has also been demonstrated, but efficacy for the therapy or chemoprophylaxis of malaria has been suboptimal.

Antibiotics are also active against other protozoans. Tetracycline and erythromycin are alternative therapies for the treatment of intestinal amebiasis. Clindamycin, in combination with other agents, is effective therapy for toxoplasmosis, pneumocystosis, and babesiosis. Spiramycin is a macrolide antibiotic that is used to treat primary toxoplasmosis acquired during pregnancy. Treatment lowers the risk of the development of congenital toxoplasmosis.

HALOFANTRINE & LUMEFANTRINE Halofantrine hydrochloride, a phenanthrene-methanol, is effective against erythrocytic (but not other) stages of all four human malaria species. Oral absorption is variable and enhanced by food. Because of toxicity concerns, it should not be taken with meals. Plasma levels peak 16 hours after dosing, and the half-life is about 4 days. Excretion is mainly in the feces. The mechanism of action is unknown.

Halofantrine is not available in the USA (although it has been approved by the FDA), but it is widely available in malaria-endemic countries.

Halofantrine (three 500 mg doses at 6-hour intervals, repeated in 1 week for nonimmune individuals) is rapidly effective against P falciparum, but its use is limited by cardiac toxicity. It should not be used for chemoprophylaxis. Halofantrine is generally well tolerated.

The most common adverse effects are abdominal pain, diarrhea, vomiting, cough, rash, headache, pruritus, and elevated liver enzymes.

Of greater concern, the drug alters cardiac conduction, with dose-related prolongation of QT and PR intervals that is exacerbated by prior mefloquine therapy. Rare instances of dangerous arrhythmias and deaths have been reported. The drug is contraindicated in patients who have cardiac conduction defects or who have recently taken mefloquine. Halofantrine is embryotoxic in animals and therefore contraindicated in pregnancy.

Lumefantrine, an aryl alcohol related to halofantrine, is available only as a fixed-dose combination with artemether (Coartem, Riamet), which is now the first-line therapy for uncomplicated falciparum malaria in many countries. In addition, Coartem is approved in many nonendemic countries, including the USA. The half-life of lumefantrine, when used in combination, is 3â&#x20AC;&#x201C;4 days. Drug levels may be altered by interactions with other drugs, including those that affect CYP3A4 metabolism. As with halofantrine, oral absorption is variable and improved when the drug is taken with food. Since lumefantrine does not engender the dangerous toxicity concerns of halofantrine, Coartem should be administered with fatty food to maximize antimalarial efficacy. Coartem is highly effective in the treatment of falciparum malaria when administered twice daily for 3 days. Coartem can cause minor prolongation of the QT interval, but this appears to be clinically insignificant, and the drug does not carry the risk of dangerous arrhythmias seen with halofantrine and quinidine. Indeed, Coartem is very well tolerated. The most commonly reported adverse events in drug trials have been gastrointestinal disturbances, headache, dizziness, rash, and pruritus, and in many cases these toxicities may have been due to underlying malaria or concomitant medications rather than to Coartem.

AMEBIASIS Amebiasis is infection with Entamoeba histolytica. This organism can cause asymptomatic intestinal infection, mild to moderate colitis, severe intestinal infection (dysentery), ameboma, liver abscess, and other extraintestinal infections. The choice of drugs for amebiasis depends on the clinical presentation (Table 52â&#x20AC;&#x201C;5).

TABLE 52â&#x20AC;&#x201C;5 Treatment of amebiasis. Not all preparations are available in the USA.1

Treatment of Specific Forms of Amebiasis 1. Asymptomatic intestinal infection—Asymptomatic carriers generally are not treated in endemic areas, but in nonendemic areas they are treated with a luminal amebicide. A tissue amebicidal drug is unnecessary. Standard luminal amebicides are diloxanide furoate, iodoquinol, and paromomycin. Each drug eradicates carriage in about 80–90% of patients with a single course of treatment. Therapy with a luminal amebicide is also required in the treatment of all other forms of amebiasis.

2. Amebic colitis—Metronidazole plus a luminal amebicide is the treatment of choice for amebic colitis and dysentery. Tetracyclines and erythromycin are alternative drugs for moderate colitis but are not effective against extraintestinal disease. Dehydroemetine or emetine can also be used, but are best avoided because of toxicity.

3. Extraintestinal infections—The treatment of choice for extraintestinal infections is metronidazole plus a luminal amebicide. A 10-day course of metronidazole cures over 95% of uncomplicated liver abscesses. For unusual cases in which initial therapy with metronidazole has failed, aspiration of the abscess and the addition of chloroquine to a repeat course of metronidazole should be considered.

Dehydroemetine and emetine are toxic alternative drugs.

METRONIDAZOLE & TINIDAZOLE Metronidazole, a nitroimidazole (Figure 52–3), is the drug of choice in the treatment of extraluminal amebiasis. It kills trophozoites but not cysts of E histolytica and effectively eradicates intestinal and extraintestinal tissue infections. Tinidazole, a related nitroimidazole, appears to have similar activity and a better toxicity profile. It offers simpler dosing regimens and can be substituted for the indications listed below.

FIGURE 52–3 Structural formulas of other antiprotozoal drugs.

Pharmacokinetics & Mechanism of Action Oral metronidazole and tinidazole are readily absorbed and permeate all tissues by simple diffusion. Intracellular concentrations rapidly approach extracellular levels. Peak plasma concentrations are reached in 1–3 hours. Protein binding of both drugs is low (10–20%); the half-life of unchanged drug is 7.5 hours for metronidazole and 12–14 hours for tinidazole. Metronidazole and its metabolites are excreted mainly in the urine. Plasma clearance of metronidazole is decreased in patients with impaired liver function. The nitro group of metronidazole is chemically reduced in anaerobic bacteria and sensitive protozoans. Reactive reduction products appear to be responsible for antimicrobial activity. The mechanism of tinidazole is assumed to be the same.

Clinical Uses 1. Amebiasis—Metronidazole or tinidazole is the drug of choice in the treatment of all tissue infections with E histolytica. Neither drug

is reliably effective against luminal parasites and so must be used with a luminal amebicide to ensure eradication of the infection.

2. Giardiasis—Metronidazole is the treatment of choice for giardiasis. The dosage for giardiasis is much lower—and the drug thus better tolerated—than that for amebiasis. Efficacy after a single treatment is about 90%. Tinidazole is at least equally effective.

3. Trichomoniasis—Metronidazole is the treatment of choice. A single dose of 2 g is effective. Metronidazole-resistant organisms can lead to treatment failures. Tinidazole may be effective against some of these resistant organisms.

Adverse Effects & Cautions Nausea, headache, dry mouth, or a metallic taste in the mouth occurs commonly. Infrequent adverse effects include vomiting, diarrhea, insomnia, weakness, dizziness, thrush, rash, dysuria, dark urine, vertigo, paresthesias, and neutropenia. Taking the drug with meals lessens gastrointestinal irritation. Pancreatitis and severe central nervous system toxicity (ataxia, encephalopathy, seizures) are rare.

Metronidazole has a disulfiram-like effect, so that nausea and vomiting can occur if alcohol is ingested during therapy. The drug should be used with caution in patients with central nervous system disease. Intravenous infusions have rarely caused seizures or peripheral neuropathy. The dosage should be adjusted for patients with severe liver or renal disease. Tinidazole has a similar adverse-effect profile, although it appears to be somewhat better tolerated than metronidazole.

Metronidazole has been reported to potentiate the anticoagulant effect of coumarin-type anticoagulants. Phenytoin and phenobarbital may accelerate elimination of the drug, whereas cimetidine may decrease plasma clearance. Lithium toxicity may occur when the drug is used with metronidazole. Metronidazole and its metabolites are mutagenic in bacteria. Chronic administration of large doses is tumorigenic in mice. Data on teratogenicity are inconsistent. Metronidazole is thus best avoided in pregnant or nursing women, although congenital abnormalities have not clearly been associated with use in humans.

IODOQUINOL Iodoquinol (diiodohydroxyquin) is a halogenated hydroxyquinoline. It is an effective luminal amebicide. Pharmacokinetic data are incomplete but 90% of the drug is retained in the intestine and excreted in the feces. The remainder enters the circulation, has a half-life of 11–14 hours, and is excreted in the urine as glucuronides. The mechanism of action of iodoquinol against trophozoites is unknown. It is effective against organisms in the bowel lumen but not against trophozoites in the intestinal wall or extraintestinal tissues.

Infrequent adverse effects include diarrhea—which usually stops after several days—anorexia, nausea, vomiting, abdominal pain, headache, rash, and pruritus. The drug may increase protein-bound serum iodine, leading to a decrease in measured 131 I uptake that persists for months. Some halogenated hydroxyquinolines can produce severe neurotoxicity with prolonged use at greater than recommended doses. Iodoquinol is not known to produce these effects at its recommended dosage, and this dosage should never be exceeded. Iodoquinol should be taken with meals to limit gastrointestinal toxicity. It should be used with caution in patients with optic neuropathy, renal or thyroid disease, or nonamebic hepatic disease. The drug should be discontinued if it produces persistent diarrhea or signs of iodine toxicity (dermatitis, urticaria, pruritus, fever). It is contraindicated in patients with intolerance to iodine.

DILOXANIDE FUROATE Diloxanide furoate is a dichloroacetamide derivative. It is an effective luminal amebicide but is not active against tissue trophozoites. In the gut, diloxanide furoate is split into diloxanide and furoic acid; about 90% of the diloxanide is rapidly absorbed and then conjugated to form the glucuronide, which is promptly excreted in the urine. The unabsorbed diloxanide is the active antiamebic substance. The mechanism of action of diloxanide furoate is unknown. It is not available commercially in the USA but can be obtained from some compounding pharmacies. It is used with a tissue amebicide, usually metronidazole, to treat serious intestinal and extraintestinal infections. Diloxanide furoate does not produce serious adverse effects. Flatulence is common, but nausea and abdominal cramps are infrequent and rashes are rare. The drug is not recommended in pregnancy.

PAROMOMYCIN SULFATE Paromomycin sulfate is an aminoglycoside antibiotic (see also Chapter 45) that is not significantly absorbed from the gastrointestinal tract. It is used as a luminal amebicide and has no effect against extraintestinal organisms. The small amount absorbed is slowly excreted unchanged, mainly by glomerular filtration. However, the drug may accumulate with renal insufficiency and contribute to renal toxicity.

Paromomycin appears to have similar efficacy and less toxicity than other luminal agents; in a recent study, it was superior to diloxanide furoate in clearing asymptomatic infections. As it is readily available, paromomycin can be considered the antiamebic luminal agent of choice in the USA. Adverse effects include occasional abdominal distress and diarrhea. Parenteral paromomycin is now used to treat

visceral leishmaniasis and is discussed separately in the text that follows.

EMETINE & DEHYDROEMETINE Emetine, an alkaloid derived from ipecac, and dehydroemetine, a synthetic analog, are effective against tissue trophozoites of E histolytica, but because of major toxicity concerns their use is limited to unusual circumstances in which severe amebiasis requires effective therapy and metronidazole cannot be used. Dehydroemetine is preferred because of its somewhat better toxicity profile. The drugs should be used for the minimum period needed to relieve severe symptoms (usually 3–5 days) and should be administered subcutaneously (preferred) or intramuscularly in a supervised setting. Adverse effects, which are generally mild with use for 3–5 days, increase over time and include pain, tenderness, and sterile abscesses at the injection site; diarrhea, nausea, and vomiting; muscle weakness and discomfort; and minor electrocardiographic changes. Serious toxicities include cardiac arrhythmias, heart failure, and hypotension.

OTHER ANTIPROTOZOAL DRUGS The primary drugs used to treat African trypanosomiasis are listed in Table 52–6, and those for other protozoal infections are listed in Table 52–7. Important drugs that are not covered elsewhere in this or other chapters are discussed below.

TABLE 52–6 Treatment of African trypanosomiasis.

TABLE 52–7 Treatment of other protozoal infections. Not all preparations are available in the USA.1

PENTAMIDINE Pentamidine has activity against trypanosomatid protozoans and against P jiroveci, but toxicity is significant.

Chemistry & Pharmacokinetics Pentamidine is an aromatic diamidine (Figure 52â&#x20AC;&#x201C;3) formulated as an isethionate salt. The drug is only administered parenterally. It leaves the circulation rapidly, with an initial half-life of about 6 hours, but is bound avidly by tissues. Pentamidine thus accumulates and is eliminated very slowly, with a terminal elimination half-life of about 12 days. Only trace amounts of pentamidine appear in the central nervous system, so it is not effective against CNS African trypanosomiasis. Pentamidine can also be inhaled as a nebulized powder for the prevention of pneumocystosis. Absorption into the systemic circulation after inhalation appears to be minimal. The mechanism of action of pentamidine is unknown.

Clinical Uses 1. Pneumocystosisâ&#x20AC;&#x201D;Pentamidine is a well-established alternative therapy for pulmonary and extrapulmonary disease caused by P jiroveci. The drug has somewhat lower efficacy and greater toxicity than trimethoprim-sulfamethoxazole. The standard dosage is 3 mg/kg/d intravenously for 21 days. Significant adverse reactions are common, and with multiple regimens now available to treat P jiroveci infection, pentamidine is best reserved for patients with severe disease who cannot tolerate or fail other drugs.

Pentamidine is also an alternative agent for primary or secondary prophylaxis against pneumocystosis in immunocompromised individuals, including patients with advanced AIDS. For this indication, pentamidine is administered as an inhaled aerosol (300 mg inhaled monthly). The drug is well tolerated in this form. Its efficacy is good but less than that of daily trimethoprim-sulfamethoxazole. Because of its cost and ineffectiveness against nonpulmonary disease, it is best reserved for patients who cannot tolerate oral chemoprophylaxis

with other drugs.

2. African trypanosomiasis (sleeping sickness)—Pentamidine has been used since 1940 and is the drug of choice to treat the early hemolymphatic stage of disease caused by Trypanosoma brucei gambiense (West African sleeping sickness). The drug is inferior to suramin for the treatment of early East African sleeping sickness. Pentamidine should not be used to treat late trypanosomiasis with central nervous system involvement. A number of dosing regimens have been described, generally providing 2–4 mg/kg daily or on alternate days for a total of 10–15 doses. Pentamidine has also been used for chemoprophylaxis against African trypanosomiasis, with dosing of 4 mg/kg every 3–6 months.

3. Leishmaniasis—Pentamidine is an alternative to sodium stibogluconate and newer agents for the treatment of visceral leishmaniasis.

The drug has been successful in some cases that have failed therapy with antimonials. The dosage is 2–4 mg/kg intramuscularly daily or every other day for up to 15 doses, and a second course may be necessary. Pentamidine has also shown success against cutaneous leishmaniasis, but it is not routinely used for this purpose.

Adverse Effects & Cautions Pentamidine is a highly toxic drug, with adverse effects noted in about 50% of patients receiving 4 mg/kg/d. Rapid intravenous administration can lead to severe hypotension, tachycardia, dizziness, and dyspnea, so the drug should be administered slowly (over 2 hours), and patients should be recumbent and monitored closely during treatment. With intramuscular administration, pain at the injection site is common, and sterile abscesses may develop.

Pancreatic toxicity is common. Hypoglycemia due to inappropriate insulin release often appears 5–7 days after onset of treatment, can persist for days to several weeks, and may be followed by hyperglycemia. Reversible renal insufficiency is also common. Other adverse effects include rash, metallic taste, fever, gastrointestinal symptoms, abnormal liver function tests, acute pancreatitis, hypocalcemia, thrombocytopenia, hallucinations, and cardiac arrhythmias. Inhaled pentamidine is generally well tolerated but may cause cough, dyspnea, and bronchospasm.

SODIUM STIBOGLUCONATE Pentavalent antimonials, including sodium stibogluconate (pentostam; Figure 52–3) and meglumine antimoniate, are first-line agents for cutaneous and visceral leishmaniasis except in parts of India, where the efficacy of these drugs has diminished greatly. The drugs are rapidly absorbed and distributed after intravenous (preferred) or intramuscular administration and eliminated in two phases, with a short initial (about 2-hour) half-life and a much longer terminal (> 24-hour) half-life. Treatment is given at a dosage of 20 mg/kg once daily intravenously or intramuscularly for 20 days in cutaneous leishmaniasis and 28 days in visceral and mucocutaneous disease.

The mechanism of action of the antimonials is unknown. Their efficacy against different species may vary, possibly based on local drug resistance patterns. Cure rates are generally quite good, but resistance to sodium stibogluconate is increasing in some endemic areas, notably in India where other agents (eg, amphotericin or miltefosine) are generally recommended.

Few adverse effects occur initially, but the toxicity of stibogluconate increases over the course of therapy. Most common are gastrointestinal symptoms, fever, headache, myalgias, arthralgias, and rash. Intramuscular injections can be very painful and lead to sterile abscesses. Electrocardiographic changes may occur, most commonly T-wave changes and QT prolongation. These changes are generally reversible, but continued therapy may lead to dangerous arrhythmias. Thus, the electrocardiogram should be monitored during therapy. Hemolytic anemia and serious liver, renal, and cardiac effects are rare.

NITAZOXANIDE Nitazoxanide is a nitrothiazolyl-salicylamide prodrug. Nitazoxanide was recently approved in the USA for use against Giardia lamblia and Cryptosporidium parvum. It is rapidly absorbed and converted to tizoxanide and tizoxanide conjugates, which are subsequently excreted in both urine and feces. The active metabolite, tizoxanide, inhibits the pyruvate-ferredoxin oxidoreductase pathway.

Nitazoxanide appears to have activity against metronidazole-resistant protozoal strains and is well tolerated. Unlike metronidazole, nitazoxanide and its metabolites appear to be free of mutagenic effects. Other organisms that may be susceptible to nitazoxanide include E histolytica, Helicobacter pylori, Ascaris lumbricoides, several tapeworms, and Fasciola hepatica. The recommended adult dosage is 500 mg twice daily for 3 days.

OTHER DRUGS FOR TRYPANOSOMIASIS & LEISHMANIASIS Available therapies for all forms of trypanosomiasis are seriously deficient in efficacy, safety, or both. Availability of these therapies is also a concern, since they are supplied mainly through donation or nonprofit production by pharmaceutical companies. For visceral

leishmaniasis, three new promising therapies are liposomal amphotericin, miltefosine, and paromomycin, and combinations of these agents have shown particularly promising results.

A. Suramin Suramin is a sulfated naphthylamine that was introduced in the 1920s. It is the first-line therapy for early hemolymphatic East African trypanosomiasis (T brucei rhodesiense infection), but because it does not enter the central nervous system, it is not effective against advanced disease. Suramin is less effective than pentamidine for early West African trypanosomiasis. The drug’s mechanism of action is unknown. It is administered intravenously and displays complex pharmacokinetics with very tight protein binding. Suramin has a short initial half-life but a terminal elimination half-life of about 50 days. The drug is slowly cleared by renal excretion.

Suramin is administered after a 200-mg intravenous test dose. Regimens that have been used include 1 g on days 1, 3, 7, 14, and 21 or 1 g each week for 5 weeks. Combination therapy with pentamidine may improve efficacy. Suramin can also be used for chemoprophylaxis against African trypanosomiasis. Adverse effects are common. Immediate reactions can include fatigue, nausea, vomiting, and, more rarely, seizures, shock, and death. Later reactions include fever, rash, headache, paresthesias, neuropathies, renal abnormalities including proteinuria, chronic diarrhea, hemolytic anemia, and agranulocytosis.

B. Melarsoprol Melarsoprol is a trivalent arsenical that has been available since 1949 and is first-line therapy for advanced central nervous system East African trypanosomiasis, and second-line therapy (after eflornithine) for advanced West African trypanosomiasis. After intravenous administration it is excreted rapidly, but clinically relevant concentrations accumulate in the central nervous system within 4 days.

Melarsoprol is administered in propylene glycol by slow intravenous infusion at a dosage of 3.6 mg/kg/d for 3–4 days, with repeated courses at weekly intervals, if needed. A new regimen of 2.2 mg/kg daily for 10 days had efficacy and toxicity similar to what was observed with three courses over 26 days. Melarsoprol is extremely toxic. The use of such a toxic drug is justified only by the severity of advanced trypanosomiasis and the lack of available alternatives. Immediate adverse effects include fever, vomiting, abdominal pain, and arthralgias. The most important toxicity is a reactive encephalopathy that generally appears within the first week of therapy (in 5–10% of patients) and is probably due to disruption of trypanosomes in the central nervous system. Common consequences of the encephalopathy include cerebral edema, seizures, coma, and death. Other serious toxicities include renal and cardiac disease and hypersensitivity reactions. Failure rates with melarsoprol appear to have increased recently in parts of Africa, suggesting drug resistance.

C. Eflornithine Eflornithine (difluoromethylornithine), an inhibitor of ornithine decarboxylase, is the only new drug registered to treat African trypanosomiasis in the last half-century. It is now the first-line drug for advanced West African trypanosomiasis, but is not effective for East African disease. Eflornithine is less toxic than melarsoprol but not as widely available. The drug had very limited availability until recently, when it was developed for use as a topical depilatory cream, leading to donation of the drug for the treatment of trypanosomiasis. Eflornithine is administered intravenously, and good central nervous system drug levels are achieved. The elimination half-life is about 3 hours. The usual regimen is 100 mg/kg intravenously every 6 hours for 7–14 days (14 days was superior for a newly diagnosed infection). Eflornithine appears to be as effective as melarsoprol against advanced T brucei gambiense infection, but its efficacy against T brucei rhodesiense is limited by drug resistance. Combining eflornithine with a 10-day course of nifurtimox afforded efficacy against West African trypanosomiasis similar to a 14-day regimen of eflornithine alone, with simpler and shorter treatment (injections every 12 hours for 7 days). Toxicity from eflornithine is significant, but considerably less than that from melarsoprol. Adverse effects include diarrhea, vomiting, anemia, thrombocytopenia, leukopenia, and seizures. These effects are generally reversible. Increased experience with eflornithine and increased availability of the compound in endemic areas may lead to its replacement of suramin, pentamidine, and melarsoprol in the treatment of T brucei gambiense infection.

D. Benznidazole Benznidazole is an orally administered nitroimidazole for the treatment of American trypanosomiasis (Chagas disease) that probably has improved efficacy and safety compared to nifurtimox. The ability of both of these drugs to eliminate parasites and prevent progression to or treat the serious syndromes associated with chronic Chagas disease is suboptimal. Drug availability was a major problem until recently. Standard dosage is 5 mg/kg/d in two or three divided doses for 60 days, given with meals. Important toxicities, which are generally reversible, include rash (in 20–30% of those treated), peripheral neuropathy, gastrointestinal symptoms, and myelosuppression.

E. Nifurtimox Nifurtimox, a nitrofuran, is a standard drug for Chagas disease. Nifurtimox is also under study in the treatment of African trypanosomiasis in combination with eflornithine. Nifurtimox is well absorbed after oral administration and eliminated with a plasma halflife of about 3 hours. The drug is administered at a dosage of 8–10 mg/kg/d in three divided doses with meals for 60–90 days. Toxicity related to nifurtimox is common. Adverse effects include nausea, vomiting, abdominal pain, fever, rash, headache, restlessness, insomnia, neuropathies, and seizures. These effects are generally reversible but often lead to cessation of therapy before completion of a standard

course.

F. Amphotericin This important antifungal drug (see Chapter 48) is an alternative therapy for visceral leishmaniasis, especially in parts of India with highlevel resistance to sodium stibogluconate. Liposomal amphotericin has shown excellent efficacy at a dosage of 3 mg/kg/d intravenously on days 1–5, 14, and 21. Nonliposomal amphotericin (1 mg/kg intravenously every other day for 30 days) is much less expensive, also efficacious, and widely used in India. However, in an Indian trial a single infusion of liposomal amphotericin showed noninferior efficacy and decreased cost compared to a standard 30-day course of amphotericin. Amphotericin is also used for cutaneous leishmaniasis in some areas. The use of amphotericin, and especially liposomal preparations, is limited in developing countries by difficulty of administration, cost, and toxicity.

G. Miltefosine Miltefosine is an alkylphosphocholine analog that is the first effective oral drug for visceral leishmaniasis. It has recently shown excellent efficacy in the treatment of visceral leishmaniasis in India, where it is administered orally (2.5 mg/kg/d with varied dosing schedules) for 28 days. It was also recently shown to be effective in regimens including a single dose of liposomal amphotericin followed by 7–14 days of miltefosine. A 28-day course of miltefosine (2.5 mg/kg/d) was also effective for the treatment of New World cutaneous leishmaniasis.

Vomiting and diarrhea are common but generally short-lived toxicities. Transient elevations in liver enzymes and nephrotoxicity are also seen. The drug should be avoided in pregnancy (or in women who may become pregnant within 2 months of treatment) because of its teratogenic effects. Miltefosine is registered for the treatment of visceral leishmaniasis in India and some other countries, and— considering the serious limitations of other drugs, including parenteral administration, toxicity, and resistance—it may become the treatment of choice for that disease. Resistance to miltefosine develops readily in vitro.

H. Paromomycin Paromomycin sulfate is an aminoglycoside antibiotic that until recently was used in parasitology only for oral therapy of intestinal parasitic infections (see previous text). It has recently been developed for the treatment of visceral leishmaniasis. It is much less expensive than amphotericin or miltefosine. A trial in India showed excellent efficacy, with a daily intramuscular dosage of 11 mg/kg for 21 days yielding a 95% cure rate, and noninferiority compared with amphotericin. The drug was registered for the treatment of visceral leishmaniasis in India in 2006. However, a recent trial showed poorer efficacy in Africa, with the cure rate for paromomycin significantly inferior to that with sodium stibogluconate. In initial studies, paromomycin was well tolerated, with common mild injection pain, uncommon ototoxicity and reversible liver enzyme elevations, and no nephrotoxicity. Paromomycin has also shown good efficacy when topically applied, alone or with gentamicin, for the treatment of cutaneous leishmaniasis.

I. Drug Combinations Used in the Treatment of Visceral Leishmaniasis The use of drug combinations to improve treatment efficacy, shorten treatment courses, and reduce the selection of resistant parasites has been an active area of research. In a recent trial in India, compared to a standard 30-day (treatment on alternate days) course of amphotericin, noninferior efficacy and decreased adverse events were seen with a single dose of liposomal amphotericin plus a 7-day course of miltefosine, a single dose of liposomal amphotericin plus a 10-day course of paromomycin, or a 10-day course of miltefosine plus paromomycin. In a trial in East Africa, compared to a standard 30-day course of sodium stibogluconate, similar efficacy was seen with a 17-day course of sodium stibogluconate plus paromomycin.

PREPARATIONS AVAILABLE

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CASE STUDY ANSWER This child has acute falciparum malaria, and her lethargy and abnormal laboratory tests are consistent with progression to severe disease. She should be hospitalized and treated urgently with intravenous artesunate or, if this is unavailable, intravenous quinine or quinidine. She should be followed closely for progression of severe malaria, in particular neurologic, renal, or pulmonary complications, and if treated with quinine or quinidine should have cardiac monitoring for potential toxicities.

_______________ * Malarone is a proprietary formulation of atovaquone plus proguanil. ** Coartem is a proprietary formulation of artemether and lumefantrine.

CHAPTER

53 Clinical Pharmacology of the Antihelminthic Drugs Philip J. Rosenthal, MD

CASE STUDY A 29-year-old Peruvian man presents with the incidental finding of a 10 by 8 by 8 cm liver cyst on an abdominal computed tomography (CT) scan. The patient had noted 2 days of abdominal pain and fever, and his clinical evaluation and CT scan were consistent with appendicitis. His clinical findings resolved after laparoscopic appendectomy. Ten years ago, the patient immigrated to the United States from a rural area of Peru where his family trades in sheepskins. His father and sister have undergone resection of abdominal masses, but details of their diagnoses are unavailable. What is your differential diagnosis? What are your diagnostic and therapeutic plans?

CHEMOTHERAPY OF HELMINTHIC INFECTIONS Helminths (worms) are multicellular organisms that infect very large numbers of humans and cause a broad range of diseases. Over 1 billion people are infected with intestinal nematodes, and many millions are infected with filarial nematodes, flukes, and tapeworms. They are also a large problem in domestic animals. Many drugs, directed against a number of different targets, are available to treat helminthic infections. In many cases, especially in the developing world, the goal is control of infection, with elimination of most parasites, alleviating disease symptoms, and decreasing the transmission of infection. In other cases, complete elimination of parasites is the goal of therapy, although this goal can be challenging with certain helminthic infections, because of both limited efficacy of drugs and frequent reinfection after therapy in endemic areas.

Table 53â&#x20AC;&#x201C;1 lists the major helminthic infections and provides a guide to the drug of choice and alternative drugs for each infection. In the text that follows, these drugs are arranged alphabetically. In general, parasites should be identified before treatment is started.

TABLE 53â&#x20AC;&#x201C;1 Drugs for the treatment of helminthic infections.1

ALBENDAZOLE Albendazole, a broad-spectrum oral antihelminthic, is the drug of choice and is approved in the USA for treatment of hydatid disease and cysticercosis. It is also used in the treatment of pinworm and hookworm infections, ascariasis, trichuriasis, and strongyloidiasis.

Basic Pharmacology Albendazole is a benzimidazole carbamate. After oral administration, it is erratically absorbed (increased with a fatty meal) and then rapidly undergoes first-pass metabolism in the liver to the active metabolite albendazole sulfoxide. It reaches variable maximum plasma concentrations about 3 hours after a 400 mg oral dose, and its plasma half-life is 8–12 hours. The sulfoxide is mostly protein-bound, distributes well to tissues, and enters bile, cerebrospinal fluid, and hydatid cysts. Albendazole metabolites are excreted in the urine.

Benzimidazoles are thought to act against nematodes by inhibiting microtubule synthesis. Albendazole also has larvicidal effects in hydatid disease, cysticercosis, ascariasis, and hookworm infection and ovicidal effects in ascariasis, ancylostomiasis, and trichuriasis.

Clinical Uses Albendazole is administered on an empty stomach when used against intraluminal parasites but with a fatty meal when used against tissue parasites.

1. Ascariasis, trichuriasis, and hookworm and pinworm infections—For adults and children older than 2 years with ascariasis and pinworm infections, the treatment for ascariasis is a single dose of 400 mg orally (repeated for 2–3 days for heavy infections and in 2 weeks for pinworm infections). These treatments typically achieve good cure rates and marked reduction in egg counts in those not cured. For hookworm infections and trichuriasis, albendazole at 400 mg orally once daily for 3 days is now recommended, with albendazole showing improved efficacy over mebendazole. In addition, combination of either mebendazole or albendazole with ivermectin to treat trichuriasis markedly improved treatment outcomes.

2. Hydatid disease—Albendazole is the treatment of choice for medical therapy and is a useful adjunct to surgical removal or aspiration of cysts. It is more active against Echinococcus granulosus than against Echinococcus multilocularis. Dosing is 400 mg twice daily with meals for 1 month or longer. Daily therapy for up to 6 months has been well tolerated. One reported therapeutic strategy is to treat with albendazole and praziquantel, to assess response after 1 month or more and, depending on the response, to then manage the patient with continued chemotherapy or combined surgical and drug therapy.

3. Neurocysticercosis—Indications for medical therapy for neurocysticercosis are controversial, since antihelminthic therapy is not clearly superior to therapy with corticosteroids alone and may exacerbate neurologic disease. Therapy is probably most appropriate for symptomatic parenchymal or intraventricular cysts. Corticosteroids are usually given with the antihelminthic drug to decrease inflammation caused by dying organisms. Albendazole is now generally considered the drug of choice over praziquantel because of its shorter course, lower cost, improved penetration into the subarachnoid space, and increased drug levels (as opposed to decreased levels of praziquantel) when administered with corticosteroids. Albendazole is given in a dosage of 400 mg twice daily for up to 21 days.

4. Other infections—Albendazole is the drug of choice in the treatment of cutaneous larva migrans (400 mg daily for 3 days), visceral larva migrans (400 mg twice daily for 5 days), intestinal capillariasis (400 mg daily for 10 days), microsporidial infections (400 mg twice daily for 2 weeks or longer), and gnathostomiasis (400 mg twice daily for 3 weeks). It also has activity against taeniasis (400 mg daily for 3 days), trichinosis (400 mg twice daily for 1–2 weeks), and clonorchiasis (400 mg twice daily for 1 week). There have been reports of effectiveness in treatment of opisthorchiasis, toxocariasis, and loiasis. Albendazole is included in programs to control lymphatic filariasis.

It appears to be less active than diethylcarbamazine or ivermectin for this purpose, but it is included in combination with either of those drugs in control programs. Albendazole has been recommended as empiric therapy to treat those who return from the tropics with persistent unexplained eosinophilia. Considering protozoal infections, albendazole has shown efficacy similar to that of metronidazole, with less toxicity, against giardiasis.

Adverse Reactions, Contraindications, & Cautions When used for 1–3 days, albendazole is nearly free of significant adverse effects. Mild and transient epigastric distress, diarrhea, headache, nausea, dizziness, lassitude, and insomnia can occur. In long-term use for hydatid disease, albendazole is well tolerated, but it can cause abdominal distress, headaches, fever, fatigue, alopecia, increases in liver enzymes, and pancytopenia.

Blood counts and liver function should be monitored during long-term therapy. The drug should not be given to patients with known hypersensitivity to other benzimidazole drugs or to those with cirrhosis. The safety of albendazole in pregnancy and in children younger than 2 years has not been established.

BITHIONOL Bithionol is an alternative to triclabendazole for the treatment of fascioliasis (sheep liver fluke) and an alternative to praziquantel for the treatment of paragonimiasis.

Basic Pharmacology & Clinical Uses After ingestion, bithionol reaches peak blood levels in 4–8 hours. Excretion appears to be mainly via the kidney.

For treatment of paragonimiasis and fascioliasis, the dosage of bithionol is 30–50 mg/kg in two or three divided doses, given orally after meals on alternate days for 10–15 doses. For pulmonary paragonimiasis, cure rates are over 90%. For cerebral paragonimiasis, repeat courses may be necessary.

Adverse Reactions, Contraindications, & Cautions Adverse effects, which occur in up to 40% of patients, are generally mild and transient, but occasionally their severity requires interruption of therapy. These problems include diarrhea, abdominal cramps, anorexia, nausea, vomiting, dizziness, and headache. Skin rashes may occur after a week or more of therapy, suggesting a reaction to antigens released from dying worms. Bithionol should be used with caution in children younger than 8 years because there has been limited experience in this age group.

DIETHYLCARBAMAZINE CITRATE Diethylcarbamazine is a drug of choice in the treatment of filariasis, loiasis, and tropical eosinophilia. It has been replaced by ivermectin for the treatment of onchocerciasis.

Basic Pharmacology Diethylcarbamazine, a synthetic piperazine derivative, is rapidly absorbed from the gastrointestinal tract; after a 0.5 mg/kg dose, peak plasma levels are reached within 1–2 hours. The plasma half-life is 2–3 hours in the presence of acidic urine but about 10 hours if the urine is alkaline, a Henderson-Hasselbalch trapping effect (see Chapter 1). The drug rapidly equilibrates with all tissues except fat. It is excreted, principally in the urine, as unchanged drug and the N-oxide metabolite. Dosage should be reduced in patients with renal impairment.

Diethylcarbamazine immobilizes microfilariae and alters their surface structure, displacing them from tissues and making them more susceptible to destruction by host defense mechanisms. The mode of action against adult worms is unknown.

Clinical Uses The drug should be taken after meals.

1. Wuchereria bancrofti, Brugia malayi, Brugia timori, and Loa loa—Diethylcarbamazine is the drug of choice for treatment of infections with these parasites because of its efficacy and lack of serious toxicity. Microfilariae of all species are rapidly killed; adult parasites are killed more slowly, often requiring several courses of treatment. The drug is highly effective against adult L loa. The extent to which W bancrofti and B malayi adults are killed is not known, but after appropriate therapy microfilariae do not reappear in the majority of patients. Lymphatic filariasis is treated with 2 mg/kg three times a day for 12 days, and loiasis is treated with the same regimen for 2–3 weeks. Antihistamines may be given for the first few days of therapy to limit allergic reactions, and corticosteroids should be started and doses of diethylcarbamazine lowered or interrupted if severe reactions occur. Cures may require several courses of treatment. For patients with high L loa worm burdens (more than 2500 circulating parasites/mL), strategies to decrease risks of severe toxicity include (a) apheresis, if available, to remove microfilariae before treatment with diethycarbamazine, or (b) therapy with albendazole, which is slower acting and better tolerated, followed by therapy with diethylcarbamazine or ivermectin. Diethylcarbamazine may also be used for chemoprophylaxis against filarial infections (300 mg weekly or 300 mg on 3 successive days each month for loiasis; 50 mg monthly for bancroftian and Malayan filariasis).

2. Other uses—For tropical eosinophilia, diethylcarbamazine is given orally at a dosage of 2 mg/kg three times daily for 2–3 weeks.

Diethylcarbamazine is effective in Mansonella streptocerca infections, since it kills both adults and microfilariae. Limited information suggests that the drug is not effective, however, against adult Mansonella ozzardi or Mansonella perstans and that it has limited activity against microfilariae of these parasites. An important application of diethylcarbamazine has been mass treatment to reduce the prevalence of W bancrofti infection, generally in combination with ivermectin or albendazole. This strategy has led to excellent progress in disease control in a number of countries.

Adverse Reactions, Contraindications, & Cautions Reactions to diethylcarbamazine, which are generally mild and transient, include headache, malaise, anorexia, weakness, nausea, vomiting, and dizziness. Adverse effects also occur as a result of the release of proteins from dying microfilariae or adult worms.

Reactions are particularly severe with onchocerciasis, but diethylcarbamazine is no longer commonly used for this infection, because ivermectin is equally efficacious and less toxic. Reactions to dying microfilariae are usually mild in W bancrofti, more intense in B malayi, and occasionally severe in L loa infections. Reactions include fever, malaise, papular rash, headache, gastrointestinal symptoms, cough, chest pain, and muscle or joint pain. Leukocytosis is common and eosinophilia may increase with treatment. Proteinuria may also occur. Symptoms are most likely to occur in patients with heavy loads of microfilariae. Retinal hemorrhages and, rarely, encephalopathy have been described. Local reactions may occur in the vicinity of dying adult or immature worms. These include lymphangitis with localized swellings in W bancrofti and B malayi, small wheals in the skin in L loa, and flat papules in M streptocerca infections.

Patients with attacks of lymphangitis due to W bancrofti or B malayi should be treated during a quiescent period between attacks.

Caution is advised when using diethylcarbamazine in patients with hypertension or renal disease.

DOXYCYCLINE This tetracycline antibiotic is described in more detail in Chapter 44. Doxycycline has recently been shown to have significant macrofilaricidal activity against W bancrofti, suggesting better activity than any other available drug against adult worms. Activity is also seen against onchocerciasis. Doxycycline acts indirectly, by killing Wolbachia, an intracellular bacterial symbiont of filarial parasites. It may prove to be an important drug for filariasis, both for treatment of active disease and in mass chemotherapy campaigns.

IVERMECTIN Ivermectin is the drug of choice in strongyloidiasis and onchocerciasis. It is also an alternative drug for a number of other helminthic infections (Table 53-1).

Basic Pharmacology Ivermectin, a semisynthetic macrocyclic lactone derived from the soil actinomycete Streptomyces avermitilis, is a mixture of avermectin B1a and B1b. Ivermectin is available only for oral administration in humans. The drug is rapidly absorbed, reaching maximum plasma concentrations 4 hours after a 12 mg dose. Ivermectin has a wide tissue distribution and a volume of distribution of about 50 L. Its halflife is about 16 hours. Excretion of the drug and its metabolites is almost exclusively in the feces.

Ivermectin appears to paralyze nematodes and arthropods by intensifying γ-aminobutyric acid (GABA)-mediated transmission of signals in peripheral nerves. In onchocerciasis, ivermectin is microfilaricidal. It does not effectively kill adult worms but blocks the release of microfilariae for some months after therapy. After a single standard dose, microfilariae in the skin diminish rapidly within 2–3 days, remain low for months, and then gradually increase; microfilariae in the anterior chamber of the eye decrease slowly over months, eventually clear, and then gradually return. With repeated doses of ivermectin, the drug appears to have a low-level macrofilaricidal action and to permanently reduce microfilarial production.

Clinical Uses 1. Onchocerciasis—Treatment is with a single oral dose of ivermectin, 150 mcg/kg, with water on an empty stomach. Doses are repeated; regimens vary from monthly to less frequent (every 6–12 months) dosing schedules. After acute therapy, treatment is repeated at 12-month intervals until the adult worms die, which may take 10 years or longer. With the first treatment only, patients with microfilariae in the cornea or anterior chamber may be treated with corticosteroids to avoid inflammatory eye reactions.

Ivermectin also now plays a key role in onchocerciasis control. Annual mass treatments have led to major reductions in disease transmission. However, evidence of diminished responsiveness after mass administration of ivermectin has raised concern regarding selection of drug-resistant parasites.

2. Strongyloidiasis—Treatment consists of 200 mcg/kg once daily for 2 days. In immunosuppressed patients with disseminated infection, repeated treatment is often needed, and cure may not be possible. In this case, suppressive therapy—ie, once monthly—may be helpful.

3. Other parasites—Ivermectin reduces microfilariae in B malayi and M ozzardi infections but not in M perstans infections. It has been used with diethylcarbamazine and albendazole for the control of W bancrofti, but it does not kill adult worms. In loiasis, although the drug reduces microfilaria concentrations, it can occasionally induce severe reactions and appears to be more dangerous in this regard than diethylcarbamazine. Ivermectin is also effective in controlling scabies, lice, and cutaneous larva migrans and in eliminating a large

proportion of ascarid worms.

Adverse Reactions, Contraindications, & Cautions In strongyloidiasis treatment, infrequent adverse effects include fatigue, dizziness, nausea, vomiting, abdominal pain, and rashes. In onchocerciasis treatment, adverse effects are principally from the killing of microfilariae and can include fever, headache, dizziness, somnolence, weakness, rash, increased pruritus, diarrhea, joint and muscle pains, hypotension, tachycardia, lymphadenitis, lymphangitis, and peripheral edema. This reaction starts on the first day and peaks on the second day after treatment. It occurs in 5–30% of persons and is generally mild, but it may be more frequent and more severe in individuals who are not long-term residents of onchocerciasisendemic areas. A more intense reaction occurs in 1–3% of persons and a severe reaction in 0.1%, including high fever, hypotension, and bronchospasm. Corticosteroids are indicated in these cases, at times for several days. Toxicity diminishes with repeated dosing. Swellings and abscesses occasionally occur at 1–3 weeks, presumably at sites of adult worms. Some patients develop corneal opacities and other eye lesions several days after treatment. These are rarely severe and generally resolve without corticosteroid treatment. It is best to avoid concomitant use of ivermectin with other drugs that enhance GABA activity, eg, barbiturates, benzodiazepines, and valproic acid.

Ivermectin should not be used during pregnancy. Safety in children younger than 5 years has not been established.

MEBENDAZOLE Mebendazole is a synthetic benzimidazole that has a wide spectrum of antihelminthic activity and a low incidence of adverse effects.

Basic Pharmacology Less than 10% of orally administered mebendazole is absorbed. The absorbed drug is protein-bound (> 90%), is rapidly converted to inactive metabolites (primarily during its first pass in the liver), and has a half-life of 2–6 hours. It is excreted mostly in the urine, principally as decarboxylated derivatives, as well as in the bile. Absorption is increased if the drug is ingested with a fatty meal.

Mebendazole probably acts by inhibiting microtubule synthesis; the parent drug appears to be the active form. Efficacy of the drug varies with gastrointestinal transit time, with intensity of infection, and perhaps with the strain of parasite. The drug kills hookworm, ascaris, and trichuris eggs.

Clinical Uses Mebendazole is indicated for use in ascariasis, trichuriasis, hookworm and pinworm infections, and certain other helminthic infections. It can be taken before or after meals; the tablets should be chewed before swallowing. For pinworm infection, the dose is 100 mg once, repeated at 2 weeks. For ascariasis, trichuriasis, hookworm, and trichostrongylus infections, a dosage of 100 mg twice daily for 3 days is used for adults and for children older than 2 years. Cure rates are good for pinworm infections and ascariasis, but have been disappointing in recent studies of trichuriasis, although efficacy for trichuriasis is better than that of albendazole. Cure rates are also lower for hookworm infections, but a marked reduction in the worm burden occurs in those not cured. For intestinal capillariasis, mebendazole is used at a dosage of 200 mg twice daily for 21 or more days. In trichinosis, limited reports suggest efficacy against adult worms in the intestinal tract and tissue larvae. Treatment is three times daily, with fatty foods, at 200–400 mg per dose for 3 days and then 400–500 mg per dose for 10 days; corticosteroids should be co-administered for severe infections.

Adverse Reactions, Contraindications, & Cautions Short-term mebendazole therapy for intestinal nematodes is nearly free of adverse effects. Mild nausea, vomiting, diarrhea, and abdominal pain have been reported infrequently. Rare side effects, usually with high-dose therapy, are hypersensitivity reactions (rash, urticaria), agranulocytosis, alopecia, and elevation of liver enzymes.

Mebendazole is teratogenic in animals and therefore contraindicated in pregnancy. It should be used with caution in children younger than 2 years because of limited experience and rare reports of convulsions in this age group. Plasma levels may be decreased by concomitant use of carbamazepine or phenytoin and increased by cimetidine. Mebendazole should be used with caution in patients with cirrhosis.

METRIFONATE (TRICHLORFON) Metrifonate is a safe, low-cost alternative drug for the treatment of Schistosoma haematobium infections. It is not active against Schistosoma mansoni or Schistosoma japonicum. It is not available in the USA.

Basic Pharmacology Metrifonate, an organophosphate compound, is rapidly absorbed after oral administration. After the standard oral dose, peak blood levels are reached in 1–2 hours; the half-life is about 1.5 hours. Clearance appears to be through nonenzymatic transformation to dichlorvos, its active metabolite. Metrifonate and dichlorvos are well distributed to the tissues and are completely eliminated in 24–48 hours.

The mode of action is thought to be cholinesterase inhibition. This inhibition temporarily paralyzes adult worms, resulting in their shift from the bladder venous plexus to small arterioles of the lungs, where they are killed. The drug is not effective against S haematobium eggs; live eggs continue to pass in the urine for several months after all adult worms have been killed.

Clinical Uses In the treatment of S haematobium, an oral dose of 7.5–10 mg/kg is given three times at 14-day intervals. Cure rates on this schedule are 44–93%, with marked reductions in egg counts in those not cured. Metrifonate was also effective as a prophylactic agent when given monthly to children in a highly endemic area, and it has been used in mass treatment programs. In mixed infections with S haematobium and S mansoni, metrifonate has been successfully combined with oxamniquine.

Adverse Reactions, Contraindications, & Cautions Some studies note mild and transient cholinergic symptoms, including nausea and vomiting, diarrhea, abdominal pain, bronchospasm, headache, sweating, fatigue, weakness, dizziness, and vertigo. These symptoms may begin within 30 minutes and persist up to 12 hours.

Metrifonate should not be used after recent exposure to insecticides or drugs that might potentiate cholinesterase inhibition. Metrifonate is contraindicated in pregnancy.

NICLOSAMIDE Niclosamide is a second-line drug for the treatment of most tapeworm infections, but it is not available in the USA.

Basic Pharmacology Niclosamide is a salicylamide derivative. It appears to be minimally absorbed from the gastrointestinal tract—neither the drug nor its metabolites have been recovered from the blood or urine. Adult worms (but not ova) are rapidly killed, presumably due to inhibition of oxidative phosphorylation or stimulation of ATPase activity.

Clinical Uses The adult dose of niclosamide is 2 g once, given in the morning on an empty stomach. The tablets must be chewed thoroughly and then swallowed with water.

1. Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Diphyllobothrium latum (fish tapeworm)—A single 2 g dose of niclosamide results in cure rates of over 85% for D latum and about 95% for T saginata. It is probably equally effective against T solium. Cysticercosis can theoretically occur after treatment of T solium infections, because viable ova are released into the gut lumen after digestion of segments, but no such cases have been reported.

2. Other tapeworms—Most patients treated with niclosamide for Hymenolepsis diminuta and Dipylidium caninum infections are cured with a 7-day course of treatment; a few require a second course. Praziquantel is superior for Hymenolepis nana (dwarf tapeworm) infection. Niclosamide is not effective against cysticercosis or hydatid disease.

3. Intestinal fluke infections—Niclosamide can be used as an alternative drug in the treatment of Fasciolopsis buski, Heterophyes heterophyes, and Metagonimus yokogawai infections. The standard dose is given every other day for three doses.

Adverse Reactions, Contraindications, & Cautions Infrequent, mild, and transitory adverse events include nausea, vomiting, diarrhea, and abdominal discomfort. The consumption of alcohol should be avoided on the day of treatment and for 1 day afterward. The safety of the drug has not been established in pregnancy or for children younger than 2 years.

OXAMNIQUINE Oxamniquine is an alternative to praziquantel for the treatment of S mansoni infections. It has also been used extensively for mass treatment. It is not effective against S haematobium or S japonicum. It is not available in the USA.

Basic Pharmacology Oxamniquine, a semisynthetic tetrahydroquinoline, is readily absorbed orally; it should be taken with food. Its plasma half-life is about 2.5 hours. The drug is extensively metabolized to inactive metabolites and excreted in the urine—up to 75% in the first 24 hours. Intersubject variations in serum concentration have been noted, which may explain some treatment failures.

Oxamniquine is active against both mature and immature stages of S mansoni but does not appear to be cercaricidal. The mechanism of action is unknown. Contraction and paralysis of the worms results in detachment from terminal venules in the mesentery and transit to the liver, where many die; surviving females return to the mesenteric vessels but cease to lay eggs. Strains of S mansoni in different parts of the world vary in susceptibility. Oxamniquine has been effective in instances of praziquantel resistance.

Clinical Uses Oxamniquine is safe and effective in all stages of S mansoni disease, including advanced hepatosplenomegaly. The drug is generally less effective in children, who require higher doses than adults. It is better tolerated with food.

Optimal dosage schedules vary for different regions of the world. In the western hemisphere and western Africa, the adult oxamniquine dosage is 12–15 mg/kg given once. In northern and southern Africa, standard schedules are 15 mg/kg twice daily for 2 days. In eastern Africa and the Arabian peninsula, standard dosage is 15–20 mg/kg twice in 1 day. Cure rates are 70–95%, with marked reduction in egg excretion in those not cured. In mixed schistosome infections, oxamniquine has been successfully used in combination with metrifonate.

Adverse Reactions, Contraindications, & Cautions Mild symptoms, starting about 3 hours after a dose and lasting for several hours, occur in more than one third of patients. Central nervous system symptoms (dizziness, headache, drowsiness) are most common; nausea and vomiting, diarrhea, colic, pruritus, and urticaria also occur. Infrequent adverse effects are low-grade fever, an orange to red discoloration of the urine, proteinuria, microscopic hematuria, and a transient decrease in leukocytes. Seizures have been reported rarely.

Since the drug makes many patients dizzy or drowsy, it should be used with caution in patients whose work or activity requires mental alertness (eg, no driving for 24 hours). It should be used with caution in those with a history of epilepsy. Oxamniquine is contraindicated in pregnancy.

PIPERAZINE Piperazine is an alternative for the treatment of ascariasis, with cure rates over 90% when taken for 2 days, but it is not recommended for other helminth infections. Piperazine is available as the hexahydrate and as a variety of salts. It is readily absorbed, and maximum plasma levels are reached in 2–4 hours. Most of the drug is excreted unchanged in the urine in 2–6 hours, and excretion is complete within 24 hours.

Piperazine causes paralysis of ascaris by blocking acetylcholine at the myoneural junction; live worms are expelled by peristalsis.

For ascariasis, the dosage of piperazine (as the hexahydrate) is 75 mg/kg (maximum dose, 3.5 g) orally once daily for 2 days. For heavy infections, treatment should be continued for 3–4 days or repeated after 1 week.

Occasional mild adverse effects include nausea, vomiting, diarrhea, abdominal pain, dizziness, and headache. Neurotoxicity and allergic reactions are rare. Piperazine compounds should not be given to pregnant women, patients with impaired renal or hepatic function, or those with a history of epilepsy or chronic neurologic disease.

PRAZIQUANTEL Praziquantel is effective in the treatment of schistosome infections of all species and most other trematode and cestode infections, including cysticercosis. The drug’s safety and effectiveness as a single oral dose have also made it useful in mass treatment of several infections.

Basic Pharmacology

Praziquantel is a synthetic isoquinoline-pyrazine derivative. It is rapidly absorbed, with a bioavailability of about 80% after oral administration. Peak serum concentrations are reached 1–3 hours after a therapeutic dose. Cerebrospinal fluid concentrations of praziquantel reach 14–20% of the drug’s plasma concentration. About 80% of the drug is bound to plasma proteins. Most of the drug is rapidly metabolized to inactive mono- and polyhydroxylated products after a first pass in the liver. The half-life is 0.8–1.5 hours.

Excretion is mainly via the kidneys (60–80%) and bile (15–35%). Plasma concentrations of praziquantel increase when the drug is taken with a high-carbohydrate meal or with cimetidine; bioavailability is markedly reduced with some antiepileptics (phenytoin, carbamazepine) or with corticosteroids.

Praziquantel appears to increase the permeability of trematode and cestode cell membranes to calcium, resulting in paralysis, dislodgement, and death. In schistosome infections of experimental animals, praziquantel is effective against adult worms and immature stages, and it has a prophylactic effect against cercarial infection.

Clinical Uses Praziquantel tablets are taken with liquid after a meal; they should be swallowed without chewing because their bitter taste can induce retching and vomiting.

1. Schistosomiasis—Praziquantel is the drug of choice for all forms of schistosomiasis. The dosage is 20 mg/kg per dose for two (S mansoni and S haematobium) or three (S japonicum and S mekongi) doses at intervals of 4–6 hours. High cure rates (75–95%) are achieved when patients are evaluated at 3–6 months; there is marked reduction in egg counts in those not cured. The drug is effective in adults and children and is generally well tolerated by patients in the hepatosplenic stage of advanced disease. There is no standard regimen for acute schistosomiasis (Katayama syndrome), but standard doses as described above, often with corticosteroids to limit inflammation from the acute immune response and dying worms, are recommended. Increasing evidence indicates rare S mansoni drug resistance, which may be countered with extended courses of therapy (eg, 3–6 days at standard dosing) or treatment with oxamniquine.

Effectiveness of praziquantel for chemoprophylaxis has not been established.

2. Clonorchiasis, opisthorchiasis, and paragonimiasis—Standard dosing is 25 mg/kg three times daily for 2 days for each of these fluke infections.

3. Taeniasis and diphyllobothriasis—A single dose of praziquantel, 5–10 mg/kg, results in nearly 100% cure rates for T saginata, T solium, and D latum infections. Because praziquantel does not kill eggs, it is theoretically possible that larvae of T solium released from eggs in the large bowel could penetrate the intestinal wall and give rise to cysticercosis, but this hazard is probably minimal.

4. Neurocysticercosis—Albendazole is now the preferred drug, but when it is not appropriate or available, praziquantel has similar efficacy. Indications for praziquantel are similar to those for albendazole. The praziquantel dosage is 100 mg/kg/d in three divided doses for 1 day, then 50 mg/kg/d to complete a 2- to 4-week course. Clinical responses to therapy vary from dramatic improvements of seizures and other neurologic findings to no response and even progression of the disease. Praziquantel—but not albendazole—has diminished bioavailability when taken concurrently with a corticosteroid. Recommendations on use of both antihelminthics and corticosteroids in neurocysticercosis vary.

5. Hymenolepis nana—Praziquantel is the drug of choice for H nana infections and the first drug to be highly effective. A single dose of 25 mg/kg is taken initially and repeated in 1 week.

6. Hydatid disease—In hydatid disease, praziquantel kills protoscoleces but does not affect the germinal membrane. Praziquantel is being evaluated as an adjunct with albendazole pre- and postsurgery. In addition to its direct action, praziquantel enhances the plasma concentration of albendazole.

7. Other parasites—Limited trials showed effectiveness of praziquantel at a dosage of 25 mg/kg three times daily for 1–2 days against fasciolopsiasis, metagonimiasis, and other forms of heterophyiasis. Praziquantel was not effective for fascioliasis, however, even at dosages as high as 25 mg/kg three times daily for 3–7 days.

Adverse Reactions, Contraindications, & Cautions Mild and transient adverse effects are common. They begin within hours after ingestion of praziquantel and may persist for about 1 day.

Most common are headache, dizziness, drowsiness, and lassitude; others include nausea, vomiting, abdominal pain, loose stools, pruritus, urticaria, arthralgia, myalgia, and low-grade fever. Mild and transient elevations of liver enzymes have been reported. Several days after starting praziquantel, low-grade fever, pruritus, and skin rashes (macular and urticarial), sometimes associated with worsened eosinophilia, may occur, probably due to the release of proteins from dying worms rather than direct drug toxicity. The intensity and frequency of adverse effects increase with dosage such that they occur in up to 50% of patients who receive 25 mg/kg three times daily.

In neurocysticercosis, neurologic abnormalities may be exacerbated by inflammatory reactions around dying parasites. Common findings in patients who do not receive corticosteroids, usually presenting during or shortly after therapy, are headache, meningismus, nausea, vomiting, mental changes, and seizures (often accompanied by increased cerebrospinal fluid pleocytosis). More serious reactions, including arachnoiditis, hyperthermia, and intracranial hypertension, may also occur. Corticosteroids are commonly used with praziquantel in the treatment of neurocysticercosis to decrease the inflammatory response, but this is controversial and complicated by knowledge that corticosteroids decrease the plasma level of praziquantel up to 50%. Praziquantel is contraindicated in ocular cysticercosis, because parasite destruction in the eye may cause irreparable damage. Some workers also caution against use of the drug in spinal neurocysticercosis.

Praziquantel is safe and well tolerated in children. The drug should be avoided in pregnancy if possible. Because the drug induces dizziness and drowsiness, patients should not drive during therapy and should be warned regarding activities requiring particular physical coordination or alertness.

PYRANTEL PAMOATE Pyrantel pamoate is a broad-spectrum antihelminthic highly effective for the treatment of pinworm, ascaris, and Trichostrongylus orientalis infections. It is moderately effective against both species of hookworm. It is not effective in trichuriasis or strongyloidiasis.

Oxantel pamoate, an analog of pyrantel not available in the USA, has been used successfully in the treatment of trichuriasis; the two drugs have been combined for their broad-spectrum antihelminthic activity.

Basic Pharmacology Pyrantel pamoate is a tetrahydropyrimidine derivative. It is poorly absorbed from the gastrointestinal tract and active mainly against luminal organisms. Peak plasma levels are reached in 1â&#x20AC;&#x201C;3 hours. Over half of the administered dose is recovered unchanged in the feces. Pyrantel is effective against mature and immature forms of susceptible helminths within the intestinal tract but not against migratory stages in the tissues or against ova. The drug is a neuromuscular blocking agent that causes release of acetylcholine and inhibition of cholinesterase; this results in paralysis of worms, followed by expulsion.

Clinical Uses The standard dose is 11 mg (base)/kg (maximum, 1 g), given orally once with or without food. For pinworm, the dose is repeated in 2 weeks, and cure rates are greater than 95%. The drug is available in the USA without prescription for this indication.

For ascariasis, a single dose yields cure rates of 85â&#x20AC;&#x201C;100%. Treatment should be repeated if eggs are found 2 weeks after treatment.

For hookworm infections, a single dose is effective against light infections; but for heavy infections, especially with Necator americanus, a 3-day course is necessary to achieve 90% cure rates. A course of treatment can be repeated in 2 weeks.

Adverse Reactions, Contraindications, & Cautions Adverse effects are infrequent, mild, and transient. They may include nausea, vomiting, diarrhea, abdominal cramps, dizziness, drowsiness, headache, insomnia, rash, fever, and weakness. Pyrantel should be used with caution in patients with liver dysfunction, as transient aminotransferase elevations have been noted. Experience with the drug in pregnant women and children younger than 2 years is limited.

THIABENDAZOLE Thiabendazole is an alternative to ivermectin or albendazole for the treatment of strongyloidiasis and cutaneous larva migrans.

Basic Pharmacology Thiabendazole is a benzimidazole compound. Although it is a chelating agent that forms stable complexes with a number of metals, including iron, it does not bind calcium. Thiabendazole is rapidly absorbed after ingestion. With a standard dose, drug concentrations in plasma peak within 1â&#x20AC;&#x201C;2 hours; the half-life is 1.2 hours. The drug is almost completely metabolized in the liver to the 5-hydroxy form; 90% is excreted in the urine in 48 hours, largely as the glucuronide or sulfonate conjugate. Thiabendazole can also be absorbed from the skin. The mechanism of action of thiabendazole is probably the same as that of other benzimidazoles (inhibition of microtubule synthesis).

The drug has ovicidal effects against some parasites.

Clinical Uses The standard dosage, 25 mg/kg (maximum 1.5 g) twice daily, should be given after meals. Tablets should be chewed. For strongyloides infection, treatment is for 2 days. Cure rates are reportedly 93%. A course can be repeated in 1 week if indicated. In patients with hyperinfection syndrome, the standard dose is continued twice daily for 5â&#x20AC;&#x201C;7 days. For cutaneous larva migrans, thiabendazole cream can be applied topically, or the oral drug can be given for 2 days (although albendazole is less toxic and therefore preferred).

Adverse Reactions, Contraindications, & Cautions Thiabendazole is much more toxic than other benzimidazoles and more toxic than ivermectin, so other agents are now preferred for most indications. Common adverse effects include dizziness, anorexia, nausea, and vomiting. Less common problems are epigastric pain, abdominal cramps, diarrhea, pruritus, headache, drowsiness, and neuropsychiatric symptoms. Irreversible liver failure and fatal StevensJohnson syndrome have been reported. Experience with thiabendazole is limited in children weighing less than 15 kg. The drug should not be used in pregnancy or in the presence of hepatic or renal disease

PREPARATIONS AVAILABLE1

REFERENCES Bagheri H et al: Adverse drug reactions to anthelmintics. Ann Pharmacother 2004;38:383.

Basáñez MG et al: Effect of single-dose ivermectin on Onchocerca volvulus: A systematic review and meta-analysis. Lancet Infect Dis 2008;8:310.

Bethony J et al: Soil-transmitted helminth infections: Ascariasis, trichuriasis, and hookworm. Lancet 2006;367:1521.

Bockarie MJ et al: Efficacy of single-dose diethylcarbamazine compared with diethylcarbamazine combined with albendazole against Wuchereria bancrofti infection in Papua New Guinea. Am J T rop Med Hyg 2007;76:62.

Brunetti E, White AC Jr. Cestode infestations: Hydatid disease and cysticercosis. Infect Dis Clin North Am 2012;26:421.

Cabada MM, White AC Jr: New developments in epidemiology, diagnosis, and treatment of fascioliasis. Curr Opin Infect Dis 2012;25:518.

Craig P, Ito A: Intestinal cestodes. Curr Opin Infect Dis 2007;20:524.

Danso-Appiah A et al: Drugs for treating Schistosoma mansoni infection. Cochrane Database Syst Rev. 2013;2:CD000528.

Drugs for parasitic infections. Med Lett Drugs T her 2013;Supplement.

Fox LM: Ivermectin: Uses and impact 20 years on. Curr Opin Infect Dis 2006;19:588.

Fürst T et al: T rematode infections: Liver and lung flukes. Infect Dis Clin North Am 2012;26:399.

Fürst T et al: Manifestation, diagnosis, and management of foodborne trematodiasis. BMJ 2012;344:e4093.

Gryseels B: Schistosomiasis. Infect Dis Clin North Am 2012;26:383.

Kappagoda S, Singh U, Blackburn BG: Antiparasitic therapy. Mayo Clin Proc 2011;86:561.

Keiser J, Utzinger J: Efficacy of current drugs against soil-transmitted helminth infections: Systematic review and meta-analysis. JAMA 2008;299:1937.

Knopp S et al: Albendazole and mebendazole administered alone or in combination with ivermectin against T richuris trichiura: A randomized controlled trial. Clin Infect Dis 2010;51:1420.

Knopp S et al: Nematode infections: Filariases. Infect Dis Clin North Am 2012;26:359.

Knopp S et al: Nematode infections: Soil-transmitted helminths and trichinella. Infect Dis Clin North Am 2012;26:341.

Matthaiou DK et al: Albendazole versus praziquantel in the treatment of neurocysticercosis: A meta-analysis of comparative trials. PLoS Negl T rop Dis 2008;2:e194.

McManus DP et al: Diagnosis, treatment, and management of echinococcosis. BMJ 2012;344:e3866.

Mejia R, Nutman T B: Screening, prevention, and treatment for hyperinfection syndrome and disseminated infections caused by Strongyloides stercoralis. Curr Opin Infect Dis 2012;25:458.

Nash T E, Garcia HH: Diagnosis and treatment of neurocysticercosis. Nat Rev Neurol 2011;7:584.

Osei-Atweneboana MY: Prevalence and intensity of Onchocerca volvulus infection and efficacy of ivermectin in endemic communities in Ghana: A two-phase epidemiological study. Lancet 2007;369:2021.

Reddy M et al: Oral drug therapy for multiple neglected tropical diseases: A systematic review. JAMA 2007;298:1911.

Soukhathammavong PA et al: Low efficacy of single-dose albendazole and mebendazole against hookworm and effect on concomitant helminth infection in Lao PDR. PLoS Negl T rop Dis 2012;6:e1417.

Steinmann P et al: Efficacy of single-dose and triple-dose albendazole and mebendazole against soil-transmitted helminths and T aenia spp.: a randomized controlled trial.

PLoS One 2011;6:e25003.

Supali T et al: Doxycycline treatment of Brugia malayi-infected persons reduces microfilaremia and adverse reactions after diethylcarbamazine and albendazole treatment.

Clin Infect Dis 2008;46:1385.

T aylor MJ et al: Lymphatic filariasis and onchocerciasis. Lancet 2010 Oct 2;376:1175.

T isch DJ, Michael E, Kazura JW: Mass chemotherapy options to control lymphatic filariasis: A systematic review. Lancet Infect Dis 2005;5:514.

Udall DN: Recent updates on onchocerciasis: Diagnosis and treatment. Clin Infect Dis 207;44:53.

CASE STUDY ANSWER The presentation is highly suggestive of cystic hydatid disease (infection with Echinococcus granulosus), which is transmitted by eggs from the feces of dogs in contact with livestock. Other causes of liver fluid collections include amebic and pyogenic abscesses, but these are usually not cystic in appearance. For echinococcosis, a typical cystic lesion and positive serology support the diagnosis, and treatment generally entails albendazole in conjunction with cautious surgery or percutaneous aspiration. One approach entails treatment with albendazole followed by aspiration to confirm the diagnosis and, if it is confirmed, to remove most of the infecting worms.

CHAPTER

54 Cancer Chemotherapy Edward Chu, MD, & Alan C. Sartorelli, PhD

CASE STUDY A 55-year-old man presents with increasing fatigue, 15-pound weight loss, and a microcytic anemia. Colonoscopy identifies a mass in the ascending colon, and biopsy specimens reveal well-differentiated colorectal cancer (CRC). He undergoes surgical resection and is found to have high-risk stage III CRC with five positive lymph nodes. After surgery, he feels entirely well with no symptoms.

Of note, he has no other comorbid illnesses. What is this patient’s prognosis? Should he receive adjuvant chemotherapy? The patient receives a combination of 5-fluorouracil (5-FU), leucovorin, and oxaliplatin as adjuvant therapy. One week after receiving the first cycle of therapy, he experiences significant toxicity in the form of myelosuppression, diarrhea, and altered mental status.

What is the most likely explanation for this increased toxicity? Is there any role for genetic testing to determine the etiology of this level of toxicity?

In 2014, approximately 1.6 million new cancer cases will be diagnosed in the USA, and nearly 580,000 individuals will die from this disease. Cancer is the second most common cause of death in the United States, accounting for 1 in 4 deaths. It is a disease characterized by a defect in the normal control mechanisms that govern cell survival, proliferation, and differentiation. Cells that have undergone neoplastic transformation usually express cell surface antigens that may be of normal fetal type, and they may display other signs of apparent immaturity, and may exhibit qualitative or quantitative chromosomal abnormalities, including various translocations and the appearance of amplified gene sequences. It is now well established that a small subpopulation of cells, referred to as tumor stem cells, reside within a tumor mass. They retain the ability to undergo repeated cycles of proliferation as well as to migrate to distant sites in the body to colonize various organs in the process called metastasis. Such tumor stem cells thus can express clonogenic (colony-forming) capability, and they are characterized by chromosome abnormalities reflecting their genetic instability, which leads to progressive selection of subclones that can survive more readily in the multicellular environment of the host. This genetic instability also allows them to become resistant to chemotherapy and radiotherapy. The invasive and metastatic processes as well as a series of metabolic abnormalities associated with the cancer result in tumor-related symptoms and eventual death of the patient unless the neoplasm can be eradicated with treatment.

CAUSES OF CANCER The incidence, geographic distribution, and behavior of specific types of cancer are related to multiple factors, including sex, age, race, genetic predisposition, and exposure to environmental carcinogens. Of these factors, environmental exposure is probably most important. Exposure to ionizing radiation has been well documented as a significant risk factor for a number of cancers, including acute leukemias, thyroid cancer, breast cancer, lung cancer, soft tissue sarcoma, and basal cell and squamous cell skin cancers. Chemical carcinogens (particularly those in tobacco smoke) as well as azo dyes, aflatoxins, asbestos, benzene, and radon have all been well documented as leading to a wide range of human cancers.

Several viruses have been implicated in the etiology of various human cancers. For example, hepatitis B (HBV) and hepatitis C (HCV) are associated with the development of hepatocellular cancer; HIV is associated with Hodgkin’s and non-Hodgkin’s lymphomas; human papillomavirus (HPV) is associated with cervical cancer, anal and penile cancers, and oropharyngeal head and neck cancer; and Ebstein-Barr virus, also known as human herpesvirus 4 (HHV-4), is associated with nasopharyngeal cancer, Burkitt’s lymphoma, and Hodgkin’s lymphoma. Expression of virus-induced neoplasia may also depend on additional host and environmental factors that modulate the transformation process. Cellular genes are known that are homologous to the transforming genes of the retroviruses, a family of RNA viruses, and induce oncogenic transformation. These mammalian cellular genes, known as oncogenes, have been shown to code

for specific growth factors and their corresponding receptors. These genes may be amplified (increased number of gene copies) or mutated, both of which can lead to constitutive overexpression in malignant cells. The bcl-2 family of genes represents a series of prosurvival genes that promotes survival by directly inhibiting apoptosis, a key pathway of programmed cell death.

ACRONYMS

Another class of genes, known as tumor suppressor genes, may be deleted or mutated, which gives rise to the neoplastic phenotype. The p53 gene is the best-established tumor suppressor gene identified to date, and the normal wild-type gene appears to play an important role in suppressing malignant transformation. Of note, p53 is mutated in up to 50% of all human solid tumors, including liver, breast, colon, lung, cervix, bladder, prostate, and skin.

CANCER TREATMENT MODALITIES With present methods of treatment, when the tumor remains localized at the time of diagnosis, about one-third of patients are cured with local treatment strategies, such as surgery or radiotherapy. Earlier diagnosis might lead to increased cure rates with such local treatment.

In the remaining cases, however, early micrometastasis is a characteristic feature, indicating that a systemic approach with chemotherapy is required for effective cancer management. In patients with locally advanced disease, chemotherapy is often combined with radiotherapy to allow for subsequent surgical resection to take place, and such a combined modality approach has led to improved clinical outcomes. At present, about 50% of patients who are initially diagnosed with cancer can be cured. In contrast, chemotherapy alone is able to cure less than 10% of all cancer patients when the tumor is diagnosed at an advanced stage.

Chemotherapy is presently used in three main clinical settings: (1) primary induction treatment for advanced disease or for cancers for which there are no other effective treatment approaches, (2) neoadjuvant treatment for patients who present with localized disease, for whom local forms of therapy such as surgery or radiation, or both, are inadequate by themselves, (3) adjuvant treatment to local methods of treatment, including surgery, radiation therapy, or both.

Primary chemotherapy refers to chemotherapy administered as the primary treatment in patients who present with advanced cancer for which no alternative treatment exists. This has been the main approach in treating patients with advanced metastatic disease, and in most cases, the goals of therapy are to relieve tumor-related symptoms, improve overall quality of life, and prolong time to tumor progression. Studies in a wide range of solid tumors have shown that chemotherapy in patients with advanced disease confers survival benefit when compared with supportive care, providing sound rationale for the early initiation of drug treatment. However, cancer chemotherapy can be curative in only a small subset of patients who present with advanced disease. In adults, these curable cancers include Hodgkin’s and non-Hodgkin’s lymphoma, acute myelogenous leukemia, germ cell cancer, and choriocarcinoma, while the curable childhood cancers include acute lymphoblastic leukemia, Burkitt’s lymphoma, Wilms’ tumor, and embryonal rhabdomyosarcoma.

Neoadjuvant chemotherapy refers to the use of chemotherapy in patients who present with localized cancer for which alternative local therapies, such as surgery, exist but which are less than completely effective. At present, neoadjuvant therapy is most often administered in the treatment of anal cancer, bladder cancer, breast cancer, esophageal cancer, laryngeal cancer, locally advanced nonsmall cell lung cancer (NSCLC), and osteogenic sarcoma. For some of these diseases, such as anal cancer, gastroesophageal cancer, laryngeal cancer, and NCSLC, optimal clinical benefit is derived when chemotherapy is administered with radiation therapy either concurrently or sequentially. The goal of the neoadjuvant approach is to reduce the size of the primary tumor so that surgical resection can then be made easier. In addition, in some cases such as with rectal cancer and laryngeal cancer, the administration of combined modality therapy prior to surgery can result in sparing of vital organs such as the rectum or larynx. In most cases, additional chemotherapy is given after surgery has been performed.

One of the most important roles for cancer chemotherapy is as an adjuvant to local treatment modalities such as surgery, and this has been termed adjuvant chemotherapy. In this setting, chemotherapy is administered after surgery has been performed, and the goal of chemotherapy is to reduce the incidence of both local and systemic recurrence and to improve the overall survival of patients. In general, chemotherapy regimens with clinical activity against advanced disease may have curative potential following surgical resection of the primary tumor, provided the appropriate dose and schedule are administered. Adjuvant chemotherapy is effective in prolonging both disease-free survival (DFS) and overall survival (OS) in patients with breast cancer, colon cancer, gastric cancer, NCSLC, Wilms’ tumor, anaplastic astrocytoma, and osteogenic sarcoma. Patients with primary malignant melanoma at high risk of local recurrence or systemic metastases derive clinical benefit from adjuvant treatment with the biologic agent α-interferon, although this treatment must be given for 1 year’s duration for maximal clinical efficacy. Finally, the antihormonal agents tamoxifen, anastrozole, and letrozole are effective in the adjuvant therapy of postmenopausal women with early-stage breast cancer whose breast tumors express the estrogen receptor (see Chapter 40 for additional details). However, because these agents are cytostatic rather than cytocidal, they must be administered on a long-term basis, with the standard recommendation being 5 years’ duration.

ROLE OF CELL CYCLE KINETICS & ANTI-CANCER EFFECT The key principles of cell cycle kinetics were initially developed using the murine L1210 leukemia as the experimental model system (Figure 54–1). However, drug treatment of human cancers requires a clear understanding of the differences between the characteristics of this rodent leukemia and of human cancers, as well as an understanding of the differences in growth rates of normal target tissues between mice and humans. For example, L1210 is a rapidly growing leukemia with a high percentage of cells synthesizing DNA, as measured by the uptake of tritiated thymidine (the labeling index). Because L1210 leukemia has a growth fraction of 100% (ie, all its cells are actively progressing through the cell cycle), its life cycle is consistent and predictable. Based on the murine L1210 model, the

cytotoxic effects of anti-cancer drugs follow log cell-kill kinetics. As such, a given agent would be predicted to kill a constant fraction of cells as opposed to a constant number.

FIGURE 54–1 Log-kill hypothesis—relationship of tumor cell number to time of diagnosis, symptoms, treatment, and survival. Three alternative approaches to drug treatment are shown for comparison with the course of tumor growth when no treatment is given (dashed line). In the protocol diagrammed at top, treatment (indicated by the arrows) is given infrequently, and the result is manifested as prolongation of survival but with recurrence of symptoms between courses of treatment and eventual death of the patient. The combination chemotherapy treatment diagrammed in the middle section is begun earlier and is more intensive. Tumor cell kill exceeds regrowth, drug resistance does not develop, and “cure” results. In this example, treatment has been continued long after all clinical evidence of cancer has disappeared (1–3 years). This approach has been established as effective in the treatment of childhood acute leukemia, testicular cancers, and Hodgkin’s lymphoma. In the treatment diagrammed near the bottom of the graph, early surgery has been employed to remove the primary tumor and intensive adjuvant chemotherapy has been administered long enough (up to 1 year) to eradicate the remaining tumor cells that comprise the occult micrometastases.

Thus, if a particular dose of an individual drug leads to a 3-log kill of cancer cells and reduces the tumor burden from 1010 to 107 cells, the same dose used at a tumor burden of 105 cells reduces the tumor mass to 102 cells. Cell kill is, therefore, proportional, regardless of tumor burden. The cardinal rule of chemotherapy—the invariable inverse relation between cell number and curability—was established with this model, and this relationship is applicable to other hematologic malignancies.

Although growth of murine leukemias simulates exponential cell kinetics, mathematical modeling data suggest that most human solid tumors do not grow in such an exponential manner. Taken together, the experimental data in human solid cancers support a Gompertzian model of tumor growth and regression. The critical distinction between Gompertzian and exponential growth is that the growth fraction of the tumor is not constant with Gompertzian kinetics but instead decreases exponentially with time (exponential growth is matched by exponential retardation of growth, due to blood supply limitations and other factors). The growth fraction peaks when the tumor is approximately one-third its maximum size. Under the Gompertzian model, when a patient with advanced cancer is treated, the tumor mass is larger, its growth fraction is low, and the fraction of cells killed is, therefore, small. An important feature of Gompertzian growth is that response to chemotherapy in drug-sensitive tumors depends, in large measure, on where the tumor is in its particular growth curve.

Information on cell and population kinetics of cancer cells explains, in part, the limited effectiveness of most available anticancer drugs. A schematic summary of cell cycle kinetics is presented in Figure 54–2. This information is relevant to the mode of action, indications, and scheduling of cell cycle-specific (CCS) and cell cycle-nonspecific (CCNS) drugs. Agents falling into these two major classes are summarized in Table 54–1.

TABLE 54–1 Cell cycle effects of major classes of anti-cancer drugs.

FIGURE 54–2 Cell cycle and cancer. A conceptual depiction of the cell cycle phases that all cells—normal and neoplastic—must traverse before and during cell division. The percentages given represent the approximate percentage of time spent in each phase by a typical malignant cell; the duration of G1 , however, can vary markedly. Many of the effective anti-cancer drugs exert their action on cells traversing the cell cycle and are called cell cycle-specific (CCS) drugs (see Table 54–1). A second group of agents called cell cyclenonspecific (CCNS) drugs can sterilize tumor cells whether they are cycling or resting in the G0 compartment. CCNS drugs can kill both G0 and cycling cells (although cycling cells are more sensitive).

The Role of Drug Combinations With rare exceptions (eg, choriocarcinoma and Burkitt’s lymphoma), single drugs at clinically tolerable doses have been unable to cure cancer. In the 1960s and early 1970s, drug combination regimens were developed based on the known biochemical actions of available anti-cancer drugs rather than on their clinical efficacy. Such regimens were, however, largely ineffective. The era of effective combination chemotherapy began when a number of active drugs from different classes became available for use in combination in the treatment of the acute leukemias and lymphomas. Following this initial success with hematologic malignancies, combination chemotherapy was extended to the treatment of solid tumors.

The use of combination chemotherapy is important for several reasons. First, it provides maximal cell kill within the range of toxicity tolerated by the host for each drug as long as dosing is not compromised. Second, it provides a broader range of interaction between drugs and tumor cells with different genetic abnormalities in a heterogeneous tumor population. Finally, it may prevent or slow the subsequent development of cellular drug resistance. The same principles apply to the therapy of chronic infections, such as HIV and tuberculosis.

Certain principles have guided the selection of drugs in the most effective drug combinations, and they provide a paradigm for the development of new drug therapeutic programs.

1. Efficacy: Only drugs known to be somewhat effective when used alone against a given tumor should be selected for use in combination. If available, drugs that produce complete remission in some fraction of patients are preferred to those that produce only partial responses.

2. Toxicity: When several drugs of a given class are available and are equally effective, a drug should be selected on the basis of toxicity that does not overlap with the toxicity of other drugs in the combination. Although such selection leads to a wider range of adverse effects, it minimizes the risk of a lethal effect caused by multiple insults to the same organ system by different drugs and allows dose intensity to be maximized.

3. Optimum scheduling: Drugs should be used in their optimal dose and schedule, and drug combinations should be given at consistent intervals. Because long intervals between cycles negatively affect dose intensity, the treatment-free interval between cycles should be the shortest time necessary for recovery of the most sensitive normal target tissue, which is usually the bone marrow.

4. Mechanism of interaction: There should be a clear understanding of the biochemical, molecular, and pharmacokinetic mechanisms of interaction between the individual drugs in a given combination, to allow for maximal effect. Omission of a drug from a combination may allow overgrowth by a tumor clone sensitive to that drug alone and resistant to other drugs in the combination.

5. Avoidance of arbitrary dose changes: An arbitrary reduction in the dose of an effective drug in order to add other less effective drugs may reduce the dose of the most effective agent below the threshold of effectiveness and destroy the ability of the combination to cure disease in a given patient.

Dosage Factors Dose intensity is one of the main factors limiting the ability of chemotherapy or radiation therapy to achieve cure. As described in Chapter 2, the dose-response curve in biologic systems is usually sigmoidal in shape, with a threshold, a linear phase, and a plateau phase.

For chemotherapy, therapeutic selectivity is dependent on the difference between the dose-response curves of normal and tumor tissues.

In experimental animal models, the dose-response curve is usually steep in the linear phase, and a reduction in dose when the tumor is in the linear phase of the dose-response curve almost always results in a loss in the capacity to cure the tumor effectively before a reduction in the antitumor activity is observed. Although complete remissions continue to be observed with dose reduction down to as low as 20% of the optimal dose, residual tumor cells may not be entirely eliminated, thereby allowing for eventual relapse. Because anticancer drugs are associated with toxicity, it is often appealing for clinicians to avoid acute toxicity by simply reducing the dose or by increasing the time interval between each cycle of treatment. However, such empiric modifications in dose represent a major cause of treatment failure in patients with drug-sensitive tumors.

A positive relationship between dose intensity and clinical efficacy has been documented in several solid tumors, including advanced ovarian, breast, lung, and colon cancers, as well as in hematologic malignancies, such as the lymphomas. At present, there are three main approaches to dose-intense delivery of chemotherapy. The first approach, dose escalation, involves increasing the doses of the respective anti-cancer agents. The second strategy is administration of anti-cancer agents in a dose-intense manner by reducing the interval between treatment cycles, while the third approach involves sequential scheduling of either single agents or of combination regimens. Each of these strategies is presently being applied to a wide range of solid cancers, including breast, colorectal, and NSCLC, and in general, such dose-intense regimens have significantly improved clinical outcomes.

DRUG RESISTANCE A fundamental problem in cancer chemotherapy is the development of cellular drug resistance. Primary, or inherent resistance refers to drug resistance in the absence of prior exposure to available standard agents. The presence of inherent drug resistance was first proposed by Goldie and Coleman in the early 1980s and was thought to result from the genomic instability associated with the development of most cancers. For example, mutations in the p53 tumor suppressor gene occur in up to 50% of all human tumors.

Preclinical and clinical studies have shown that loss of p53 function leads to resistance to radiation therapy as well as resistance to a wide range of anti-cancer agents. Defects in the mismatch repair enzyme family, which are tightly linked to the development of familial and sporadic colorectal cancer, are associated with resistance to several unrelated anti-cancer agents, including the fluoropyrimidines, the thiopurines, and cisplatin/carboplatin. In contrast to primary resistance, acquired resistance develops in response to exposure to a given anti-cancer agent. Experimentally, drug resistance can be highly specific to a single drug and is usually based on a specific change in the genetic machinery of a given tumor cell with amplification or increased expression of one or more genes. In other instances, a multidrugresistant phenotype occurs, associated with increased expression of the MDR1 gene, which encodes a cell surface transporter glycoprotein (P-glycoprotein, see Chapter 5). This form of drug resistance leads to enhanced drug efflux and reduced intracellular accumulation of a broad range of structurally unrelated anti-cancer agents, including the anthracyclines, vinca alkaloids, taxanes, camptothecins, epipodophyllotoxins, and even small molecule inhibitors, such as imatinib.

BASIC PHARMACOLOGY OF CANCER CHEMOTHERAPEUTIC DRUGS ALKYLATING AGENTS The major clinically useful alkylating agents (Figure 54â&#x20AC;&#x201C;3) have a structure containing a bis(chloroethyl)amine, ethyleneimine, or nitrosourea moiety, and they are classified in several different groups. Among the bis(chloroethyl)amines, cyclophosphamide, mechlorethamine, melphalan, and chlorambucil are the most useful. Ifosfamide is closely related to cyclophosphamide but has a somewhat different spectrum of activity and toxicity. Thiotepa and busulfan are used to treat breast and ovarian cancer, and chronic myeloid leukemia, respectively. The major nitrosoureas are carmustine (BCNU) and lomustine (CCNU).

FIGURE 54â&#x20AC;&#x201C;3 Structures of major classes of alkylating agents.

Mechanism of Action As a class, the alkylating agents exert their cytotoxic effects via transfer of their alkyl groups to various cellular constituents. Alkylations of DNA within the nucleus probably represent the major interactions that lead to cell death. However, these drugs react chemically with sulfhydryl, amino, hydroxyl, carboxyl, and phosphate groups of other cellular nucleophiles as well. The general mechanism of action of these drugs involves intramolecular cyclization to form an ethyleneimonium ion that may directly or through formation of a carbonium ion transfer an alkyl group to a cellular constituent (Figure 54â&#x20AC;&#x201C;4). In addition to alkylation, a secondary mechanism that occurs with nitrosoureas involves carbamoylation of lysine residues of proteins through formation of isocyanates.

FIGURE 54â&#x20AC;&#x201C;4 Mechanism of alkylation of DNA guanine. A bis(chloroethyl)amine forms an ethyleneimonium ion that reacts with a base such as N7 of guanine in DNA, producing an alkylated purine. Alkylation of a second guanine residue, through the illustrated mechanism, results in cross-linking of DNA strands.

The major site of alkylation within DNA is the N7 position of guanine; however, other bases are also alkylated albeit to lesser degrees, including N1 and N3 of adenine, N3 of cytosine, and O6 of guanine, as well as phosphate atoms and proteins associated with DNA. These interactions can occur on a single strand or on both strands of DNA through cross-linking, as most major alkylating agents are bifunctional, with two reactive groups. Alkylation of guanine can result in miscoding through abnormal base pairing with thymine or in depurination by excision of guanine residues. The latter effect leads to DNA strand breakage through scission of the sugar-phosphate backbone of DNA. Cross-linking of DNA appears to be of major importance to the cytotoxic action of alkylating agents, and replicating cells are most susceptible to these drugs. Thus, although alkylating agents are not cell cycle-specific, cells are most susceptible to alkylation in late G1 and S phases of the cell cycle.

Resistance The mechanism of acquired resistance to alkylating agents may involve increased capability to repair DNA lesions through increased expression and activity of DNA repair enzymes, decreased transport of the alkylating drug into the cell, and increased expression or activity of glutathione and glutathione-associated proteins, which are needed to conjugate the alkylating agent, or increased glutathione Stransferase activity, which catalyzes the conjugation.

Adverse Effects The adverse effects usually associated with alkylating agents are generally dose-related and occur primarily in rapidly growing tissues such as bone marrow, gastrointestinal tract, and reproductive system. Nausea and vomiting can be a serious issue with a number of these agents. In addition, they are potent vesicants and can damage tissues at the site of administration as well as produce systemic toxicity.

As a class, alkylating agents are carcinogenic in nature, and there is an increased risk of secondary malignancies, especially acute myelogenous leukemia.

Cyclophosphamide is one of the most widely used alkylating agents. One of the potential advantages of this compound relates to its high oral bioavailability. As a result, it can be administered via the oral and intravenous routes with equal clinical efficacy. It is inactive in its parent form, and must be activated to cytotoxic forms by liver microsomal enzymes (Figure 54â&#x20AC;&#x201C;5). The cytochrome P450 mixed-

function oxidase system converts cyclophosphamide to 4-hydroxycyclophosphamide, which is in equilibrium with aldophosphamide. These active metabolites are delivered to both tumor and normal tissue, where nonenzymatic cleavage of aldophosphamide to the cytotoxic forms—phosphoramide mustard and acrolein—occurs. The liver appears to be protected through the enzymatic formation of the inactive metabolites 4-ketocyclophosphamide and carboxyphosphamide.

FIGURE 54–5 Cyclophosphamide metabolism.

The major toxicities of the individual alkylating agents are outlined in Table 54–2 and discussed below.

TABLE 54–2 Alkylating agents and platinum analogs: Clinical activity and toxicities.

NITROSOUREAS These drugs appear to be non-cross-resistant with other alkylating agents; all require biotransformation, which occurs by nonenzymatic decomposition, to metabolites with both alkylating and carbamoylating activities. The nitrosoureas are highly lipid-soluble and are able to readily cross the blood-brain barrier, making them effective in the treatment of brain tumors. Although the majority of alkylations by the nitrosoureas are on the N7 position of guanine in DNA, the critical alkylation responsible for cytotoxicity appears to be on the O6 position of guanine, which leads to G-C crosslinks in DNA. After oral administration of lomustine, peak plasma levels of metabolites appear within 1–4 hours; central nervous system concentrations reach 30–40% of the activity present in the plasma. Urinary excretion appears to be the major route of elimination from the body. One naturally occurring sugar-containing nitrosourea, streptozocin, is interesting because it has minimal bone marrow toxicity. This agent has activity in the treatment of insulin-secreting islet cell carcinoma of the pancreas.

NONCLASSIC ALKYLATING AGENTS Several other compounds have mechanisms of action that involve DNA alkylation as their cytotoxic mechanism of action. These agents include procarbazine, dacarbazine, and bendamustine. Their clinical activities and toxicities are listed in Table 54–2.

Procarbazine Procarbazine is an orally active methylhydrazine derivative, and in the clinical setting, it is used in combination regimens for Hodgkin’s and non-Hodgkin’s lymphoma as well as brain tumors.

The precise mechanism of action of procarbazine is uncertain; however, it inhibits DNA, RNA, and protein biosynthesis; prolongs interphase; and produces chromosome breaks. Oxidative metabolism of this drug by microsomal enzymes generates azoprocarbazine and H2 O2 , which may be responsible for DNA strand scission. A variety of other drug metabolites are formed that may be cytotoxic. One metabolite is a weak monoamine oxidase (MAO) inhibitor, and adverse events can occur when procarbazine is given with other MAO inhibitors as well as with sympathomimetic agents, tricyclic antidepressants, antihistamines, central nervous system depressants, antidiabetic agents, alcohol, and tyramine-containing foods.

There is an increased risk of secondary cancers in the form of acute leukemia, and its carcinogenic potential is thought to be higher than that of most other alkylating agents.

Dacarbazine Dacarbazine is a synthetic compound that functions as an alkylating agent following metabolic activation in the liver by oxidative Ndemethylation to the monomethyl derivative. This metabolite spontaneously decomposes to diazomethane, which generates a methyl carbonium ion that is believed to be the key cytotoxic species. Dacarbazine is administered parenterally and is used in the treatment of malignant melanoma, Hodgkin’s lymphoma, soft tissue sarcomas, and neuroblastoma. The main dose-limiting toxicity is myelosuppression, but nausea and vomiting can be severe in some cases. This agent is a potent vesicant, and care must be taken to avoid extravasation during drug administration.

Bendamustine Bendamustine is a bifunctional alkylating agent consisting of a purine benzimidazole ring and a nitrogen mustard moiety. As with other alkylating agents, it forms cross-links with DNA resulting in single- and double-stranded breaks, leading to inhibition of DNA synthesis and function. This molecule also inhibits mitotic checkpoints and induces mitotic catastrophe, which leads to cell death. Of note, the cross-resistance between bendamustine and other alkylating agents is only partial, thereby providing a rationale for its clinical activity despite the development of resistance to other alkylating agents. This agent is approved for use in chronic lymphocytic leukemia, with activity also observed in Hodgkin’s and non-Hodgkin’s lymphoma, multiple myeloma, and breast cancer. The main dose-limiting toxicities include myelosuppression and mild nausea and vomiting. Hypersensitivity infusion reactions, skin rash, and other skin reactions occur rarely.

PLATINUM ANALOGS Three platinum analogs are currently used in clinical practice: cisplatin, carboplatin, and oxaliplatin. Cisplatin (cisdiamminedichloroplatinum [II]) is an inorganic metal complex that was initially discovered through a serendipitous observation that neutral platinum complexes inhibited division and filamentous growth of Escherichia coli. Several platinum analogs were subsequently synthesized. Although the precise mechanism of action of the platinum analogs is unclear, they are thought to exert their cytotoxic effects

in the same manner as alkylating agents. As such, they kill tumor cells in all stages of the cell cycle and bind DNA through the formation of intrastrand and interstrand cross-links, thereby leading to inhibition of DNA synthesis and function. The primary binding site is the N7 position of guanine, but covalent interaction with the N3 position of adenine and O6 position of cytosine can also occur. In addition to targeting DNA, the platinum analogs have been shown to bind to both cytoplasmic and nuclear proteins, which may also contribute to their cytotoxic and antitumor effects. The platinum complexes appear to synergize with certain other anti-cancer drugs, including alkylating agents, fluoropyrimidines, and taxanes. The major toxicities of the individual platinum analogs are outlined in Table 54â&#x20AC;&#x201C;2.

Cisplatin has major antitumor activity in a broad range of solid tumors, including non-small cell and small cell lung cancer, esophageal and gastric cancer, cholangiocarcinoma, head and neck cancer, and genitourinary cancers, particularly testicular, ovarian, and bladder cancer. When used in combination regimens, cisplatin-based therapy has led to the cure of nonseminomatous testicular cancer. Cisplatin and the other platinum analogs are extensively cleared by the kidneys and excreted in the urine. As a result, dose modification is required in patients with renal dysfunction.

Carboplatin is a second-generation platinum analog whose mechanisms of cytotoxic action, mechanisms of resistance, and clinical pharmacology are identical to those described for cisplatin. As with cisplatin, carboplatin has broad-spectrum activity against a wide range of solid tumors. However, in contrast to cisplatin, it exhibits significantly less renal toxicity and gastrointestinal toxicity. Its main dose-limiting toxicity is myelosuppression. It has therefore been widely used in transplant regimens to treat refractory hematologic malignancies. Moreover, since vigorous intravenous hydration is not required for carboplatin therapy, carboplatin is viewed as an easier agent to administer to patients, and as such, it has replaced cisplatin in various combination chemotherapy regimens.

Oxaliplatin is a third-generation diaminocyclohexane platinum analog. Its mechanism of action and clinical pharmacology are identical to those of cisplatin and carboplatin. However, tumors that are resistant to cisplatin or carboplatin on the basis of mismatch repair defects are not cross-resistant to oxaliplatin, and this finding may explain the activity of this platinum compound in colorectal cancer. Oxaliplatin was initially approved for use as second-line therapy in combination with the fluoropyrimidine 5-fluorouracil (5-FU) and leucovorin, termed the FOLFOX regimen, for metastatic colorectal cancer. There are various iterations of the FOLFOX regimen, which have now become the most widely used combination regimens in the first-line treatment of advanced colorectal cancer. In addition, this regimen is widely used in the adjuvant therapy of stage III colon cancer and high-risk stage II colon cancer. Clinical activity has also been observed in other gastrointestinal cancers, such as pancreatic, gastroesophageal, and hepatocellular cancer. Neurotoxicity is the main dose-limiting toxicity, and it is manifested by a peripheral sensory neuropathy. There are two forms of neurotoxicity, an acute form that is often triggered and worsened by exposure to cold, and a chronic form that is dose-dependent. Although this chronic form is dependent on the cumulative dose of drug administered, it tends to be reversible, in contrast to cisplatin-induced neurotoxicity.

ANTIMETABOLITES The development of drugs with actions on intermediary metabolism of proliferating cells has been important both conceptually and clinically. While biochemical properties unique to all cancer cells have yet to be discovered, there are a number of quantitative differences in metabolism between cancer cells and normal cells that render cancer cells more sensitive to the antimetabolites. Many of these agents have been rationally designed and synthesized based on knowledge of critical cellular processes involved in DNA biosynthesis. The individual antimetabolites and their respective clinical spectrum and toxicities are presented in Table 54â&#x20AC;&#x201C;3 and are discussed below.

TABLE 54â&#x20AC;&#x201C;3 Antimetabolites: Clinical activity and toxicities.

ANTIFOLATES Methotrexate Methotrexate (MTX) is a folic acid analog that binds with high affinity to the active catalytic site of dihydrofolate reductase (DHFR).

This results in inhibition of the synthesis of tetrahydrofolate (THF), the key one-carbon carrier for enzymatic processes involved in de novo synthesis of thymidylate, purine nucleotides, and the amino acids serine and methionine. Inhibition of these metabolic processes thereby interferes with the formation of DNA, RNA, and key cellular proteins (see Figure 33–3). Intracellular formation of polyglutamate metabolites, with the addition of up to 5–7 glutamate residues, is critically important for the therapeutic action of MTX, and this process is catalyzed by the enzyme folylpolyglutamate synthase (FPGS). MTX polyglutamates are selectively retained within cancer cells, and they display increased inhibitory effects on enzymes involved in de novo purine nucleotide and thymidylate biosynthesis, making them important determinants of MTX’s cytotoxic action.

Several resistance mechanisms to MTX have been identified, and they include (1) decreased drug transport via the reduced folate carrier or folate receptor protein, (2) decreased formation of cytotoxic MTX polyglutamates, (3) increased levels of the target enzyme DHFR through gene amplification and other genetic mechanisms, and (4) altered DHFR protein with reduced affinity for MTX. Recent studies have suggested that decreased accumulation of drug through activation of the multidrug resistance transporter P170 glycoprotein may also result in drug resistance.

MTX is administered by the intravenous, intrathecal, or oral route. However, oral bioavailability is saturable and erratic at doses

greater than 25 mg/m2 . Renal excretion is the main route of elimination and is mediated by glomerular filtration and tubular secretion. As a result, dose modification is required in the setting of renal dysfunction. Care must also be taken when MTX is used in the presence of drugs such as aspirin, nonsteroidal anti-inflammatory agents, penicillin, and cephalosporins, as these agents inhibit the renal excretion of MTX. The biologic effects of MTX can be reversed by administration of the reduced folate leucovorin (5-formyltetrahydrofolate) or by L-leucovorin, which is the active enantiomer. Leucovorin rescue is used in conjunction with high-dose MTX therapy to rescue normal cells from undue toxicity, and it has also been used in cases of accidental drug overdose. The main adverse effects are listed in Table 54–3.

Pemetrexed Pemetrexed is a pyrrolopyrimidine antifolate analog with activity in the S phase of the cell cycle. As in the case of MTX, it is transported into the cell via the reduced folate carrier and requires activation by FPGS to yield higher polyglutamate forms. While this agent targets DHFR and enzymes involved in de novo purine nucleotide biosynthesis, its main mechanism of action is inhibition of thymidylate synthase (TS). At present, this antifolate is approved for use in combination with cisplatin in the treatment of mesothelioma, as a single agent in the second-line therapy of NSCLC, in combination with cisplatin for the first-line treatment of NSCLC, and most recently, as maintenance therapy in patients with NSCLC whose disease has not progressed after four cycles of platinum-based chemotherapy. As with MTX, pemetrexed is mainly excreted in the urine, and dose modification is required in patients with renal dysfunction. The main adverse effects include myelosuppression, skin rash, mucositis, diarrhea, fatigue, and hand-foot syndrome. Of note, vitamin supplementation with folic acid and vitamin B12 appears to reduce the toxicities associated with pemetrexed, while not interfering with clinical efficacy. The handfoot syndrome is manifested by painful erythema and swelling of the hands and feet, and dexamethasone treatment has been shown to be effective in reducing the incidence and severity of this toxicity.

Pralatrexate Pralatrexate is a 10-deaza-aminopterin antifolate analog, and as in the case of MTX, it is transported into the cell via the reduced folate carrier (RFC) and requires activation by FPGS to yield higher polyglutamate forms. However, this molecule was designed to be a more potent substrate for the RFC-1 carrier protein as well as an improved substrate for FPGS. It inhibits DHFR, inhibits enzymes involved in de novo purine nucleotide biosynthesis, and also inhibits TS. Although pralatrexate was originally developed for NSCLC, it is presently approved for use in the treatment of relapsed or refractory peripheral T-cell lymphoma. As with the other antifolate analogs, pralatrexate is mainly excreted in the urine, and dose modification is required in renal dysfunction. The main adverse effects include myelosuppression, skin rash, mucositis, diarrhea, and fatigue. Vitamin supplementation with folic acid and vitamin B 12 appear to reduce the toxicities associated with pralatrexate, while not interfering with clinical efficacy.

FLUOROPYRIMIDINES 5-Fluorouracil 5-Fluorouracil (5-FU) is inactive in its parent form and requires activation via a complex series of enzymatic reactions to ribosyl and deoxyribosyl nucleotide metabolites. One of these metabolites, 5-fluoro-2′-deoxyuridine-5′-monophosphate (FdUMP), forms a covalently bound ternary complex with the enzyme TS and the reduced folate 5,10-methylenetetrahydrofolate, a reaction critical for the de novo synthesis of thymidylate. This results in inhibition of DNA synthesis through “thymineless death.” 5-FU is converted to 5-fluorouridine-5′triphosphate (FUTP), which is then incorporated into RNA, where it interferes with RNA processing and mRNA translation. 5-FU is also converted to 5-fluorodeoxyuridine-5′-triphosphate (FdUTP), which can be incorporated into cellular DNA, resulting in inhibition of DNA synthesis and function. Thus, the cytotoxicity of 5-FU is thought to be the result of combined effects on both DNA- and RNAmediated events.

5-FU is administered intravenously, and the clinical activity of this drug is highly schedule-dependent. Because of its extremely short half-life, on the order of 10–15 minutes, infusional schedules of administration have been generally favored over bolus schedules. Up to 80–85% of an administered dose of 5-FU is catabolized by the enzyme dihydropyrimidine dehydrogenase (DPD). Of note, a pharmacogenetic syndrome involving partial or complete deficiency of the DPD enzyme is seen in up to 5% of cancer patients. In this particular setting, severe toxicity in the form of myelosuppression, diarrhea, nausea and vomiting, and neurotoxicity is observed.

5-FU remains the most widely used agent in the treatment of colorectal cancer, both as adjuvant therapy and for advanced disease. It also has activity against a wide variety of solid tumors, including cancers of the breast, stomach, pancreas, esophagus, liver, head and neck, and anus. Major toxicities include myelosuppression, gastrointestinal toxicity in the form of mucositis and diarrhea, skin toxicity manifested by the hand-foot syndrome, and neurotoxicity.

Capecitabine Capecitabine is a fluoropyrimidine carbamate prodrug with 70–80% oral bioavailability. As with 5-FU, capecitabine is inactive in its parent form and undergoes extensive metabolism in the liver by the enzyme carboxylesterase to an intermediate, 5′-deoxy-5fluorocytidine. This metabolite is then converted to 5′-deoxy-5-fluorouridine by the enzyme cytidine deaminase. These two initial steps occur mainly in the liver. The 5′-deoxy-5-fluorouridine metabolite is finally hydrolyzed by thymidine phosphorylase to 5-FU directly in the tumor. The expression of thymidine phosphorylase has been shown to be significantly higher in a broad range of solid tumors than in corresponding normal tissue, particularly in breast cancer and colorectal cancer.

Capecitabine is used in the treatment of metastatic breast cancer either as a single agent or in combination with other anti-cancer agents, including docetaxel, paclitaxel, lapatinib, ixabepilone, and trastuzumab. It is also approved for use in the adjuvant therapy of stage III and high-risk stage II colon cancer as well as for treatment of metastatic colorectal cancer as monotherapy. At this time, significant efforts are directed at combining this agent with other active cytotoxic agents, including irinotecan and oxaliplatin. The capecitabine/oxaliplatin (XELOX) regimen is now widely used for the first-line treatment of metastatic colorectal cancer. The main toxicities of capecitabine include diarrhea and the hand-foot syndrome. While myelosuppression, nausea and vomiting, and mucositis are also observed with this agent, their incidence is significantly less than that observed with intravenous 5-FU.

DEOXYCYTIDINE ANALOGS Cytarabine Cytarabine (ara-C) is an S phase-specific antimetabolite that is converted by deoxycytidine kinase to the 5′-mononucleotide (ara-CMP).

Ara-CMP is further metabolized to the diphosphate and triphosphate metabolites, and the ara-CTP triphosphate is felt to be the main cytotoxic metabolite. Ara-CTP competitively inhibits DNA polymerase-α and DNA polymerase-β, thereby resulting in blockade of DNA synthesis and DNA repair, respectively. This metabolite is also incorporated into RNA and DNA. Incorporation into DNA leads to interference with chain elongation and defective ligation of fragments of newly synthesized DNA. The cellular retention of ara-CTP appears to correlate with its lethality to malignant cells.

After intravenous administration, the drug is cleared rapidly, with most of an administered dose being deaminated to inactive forms.

The stoichiometric balance between the level of activation and catabolism of cytarabine is important in determining its eventual cytotoxicity.

The clinical activity of cytarabine is highly schedule-dependent and because of its rapid degradation, it is usually administered via continuous infusion over a 5–7 day period. Its activity is limited exclusively to hematologic malignancies, including acute myelogenous leukemia and non-Hodgkin’s lymphoma. This agent has absolutely no activity in solid tumors. The main adverse effects associated with cytarabine therapy include myelosuppression, mucositis, nausea and vomiting, and neurotoxicity when high-dose therapy is administered.

Gemcitabine Gemcitabine is a fluorine-substituted deoxycytidine analog that is phosphorylated initially by the enzyme deoxycytidine kinase to the monophosphate form and then by other nucleoside kinases to the diphosphate and triphosphate nucleotide forms. The antitumor effect is considered to result from several mechanisms: inhibition of ribonucleotide reductase by gemcitabine diphosphate, which reduces the level of deoxyribonucleoside triphosphates required for DNA synthesis; inhibition by gemcitabine triphosphate of DNA polymerase-α and DNA polymerase-β, thereby resulting in blockade of DNA synthesis and DNA repair; and incorporation of gemcitabine triphosphate into DNA, leading to inhibition of DNA synthesis and function. Following incorporation of the gemcitabine triphosphate into DNA, only one additional nucleotide can be added to the growing DNA strand, resulting in chain termination.

In contrast to cytarabine, which is inactive in solid tumors, gemcitabine has broad-spectrum activity against solid tumors and hematologic malignancies. This nucleoside analog was initially approved for use in advanced pancreatic cancer but is now widely used to treat a broad range of malignancies, including NSCLC, bladder cancer, ovarian cancer, soft tissue sarcoma, and non-Hodgkinâ&#x20AC;&#x2122;s lymphoma. Myelosuppression in the form of neutropenia is the principal dose-limiting toxicity. Nausea and vomiting occur in 70% of patients and a flu-like syndrome has also been observed. In rare cases, renal microangiopathy syndromes, including hemolytic-uremic syndrome and thrombotic thrombocytopenic purpura have been reported.

PURINE ANTAGONISTS 6-Thiopurines 6-Mercaptopurine (6-MP) was the first of the thiopurine analogs found to be effective in cancer therapy. This agent is used primarily in the treatment of childhood acute leukemia, and a closely related analog, azathioprine, is used as an immunosuppressive agent (see Chapter 55). As with other thiopurines, 6-MP is inactive in its parent form and must be metabolized by hypoxanthine-guanine phosphoribosyl transferase (HGPRT) to form the monophosphate nucleotide 6-thioinosinic acid, which in turn inhibits several enzymes of de novo purine nucleotide synthesis (Figure 54â&#x20AC;&#x201C;6). The monophosphate form is eventually metabolized to the triphosphate form, which can then be incorporated into both RNA and DNA. Significant levels of thioguanylic acid and 6-methylmercaptopurine ribotide (MMPR) are also formed from 6-MP. These metabolites may contribute to its cytotoxic action.

FIGURE 54–6 Mechanism of action of 6-mercaptopurine and 6-thioguanine.

6-Thioguanine (6-TG) also inhibits several enzymes in the de novo purine nucleotide biosynthetic pathway (Figure 54–6). Various metabolic lesions result, including inhibition of purine nucleotide interconversion; decrease in intracellular levels of guanine nucleotides, which leads to inhibition of glycoprotein synthesis; interference with the formation of DNA and RNA; and incorporation of thiopurine nucleotides into both DNA and RNA. 6-TG has a synergistic action when used together with cytarabine in the treatment of adult acute leukemia.

6-MP is converted to an inactive metabolite (6-thiouric acid) by an oxidation reaction catalyzed by xanthine oxidase, whereas 6-TG undergoes deamination. This is an important issue because the purine analog allopurinol, a potent xanthine oxidase inhibitor, is frequently used as a supportive care measure in the treatment of acute leukemias to prevent the development of hyperuricemia that often occurs with tumor cell lysis. Because allopurinol inhibits xanthine oxidase, simultaneous therapy with allopurinol and 6-MP would result in increased levels of 6-MP, thereby leading to excessive toxicity. In this setting, the dose of mercaptopurine must be reduced by 50–75%.

In contrast, such an interaction does not occur with 6-TG, which can be used in full doses with allopurinol.

The thiopurines are also metabolized by the enzyme thiopurine methyltransferase (TPMT), in which a methyl group is attached to the thiopurine ring. Patients who have a pharmacogenetic syndrome involving partial or complete deficiency of this enzyme are at increased risk for developing severe toxicities in the form of myelosuppression and gastrointestinal toxicity with mucositis and diarrhea.

Fludarabine Fludarabine phosphate is rapidly dephosphorylated to 2-fluoro-arabinofuranosyladenosine and then phosphorylated intracellularly by deoxycytidine kinase to the monophosphate, which is eventually converted to the triphosphate. Fludarabine triphosphate interferes with the processes of DNA synthesis and DNA repair through inhibition of DNA polymerase-α and DNA polymerase-β. The triphosphate form can also be directly incorporated into DNA, resulting in inhibition of DNA synthesis and function. The diphosphate metabolite of fludarabine inhibits ribonucleotide reductase, leading to inhibition of essential deoxyribonucleotide triphosphates. Finally, fludarabine induces apoptosis in susceptible cells through as yet undetermined mechanisms. This purine nucleotide analog is used mainly in the treatment of low-grade non-Hodgkin’s lymphoma and chronic lymphocytic leukemia (CLL). It is given parenterally, and up to 25–30% of parent drug is excreted in the urine. The main dose-limiting toxicity is myelosuppression. This agent is a potent immunosuppressant with inhibitory effects on CD4 and CD8 T cells. Patients are at increased risk for opportunistic infections, including fungi, herpes, and Pneumocystis jiroveci pneumonia (PCP). Patients should receive PCP prophylaxis with trimethoprim-sulfamethoxazole (double strength) at least three times a week, and this should continue for up to 1 year after stopping fludarabine therapy.

Cladribine Cladribine (2-chlorodeoxyadenosine) is a purine nucleoside analog with high specificity for lymphoid cells. Inactive in its parent form, it is initially phosphorylated by deoxycytidine kinase to the monophosphate form and eventually metabolized to the triphosphate form, which can then be incorporated into DNA. The triphosphate metabolite can also interfere with DNA synthesis and DNA repair by inhibiting DNA polymerase-α and DNA polymerase-β, respectively. Cladribine is indicated for the treatment of hairy cell leukemia, with activity in other low-grade lymphoid malignancies such as CLL and low-grade non-Hodgkin’s lymphoma. It is normally administered as a single continuous 7-day infusion; under these conditions, it has a very manageable safety profile with the main toxicity consisting of transient myelosuppression. As with other purine nucleoside analogs, it has immunosuppressive effects, and a decrease in CD4 and CD8 T cells, lasting for over 1 year, is observed in patients.

NATURAL PRODUCT CANCER CHEMOTHERAPY DRUGS VINCA ALKALOIDS Vinblastine Vinblastine is an alkaloid derived from the periwinkle plant Vinca rosea . Its mechanism of action involves inhibition of tubulin polymerization, which disrupts assembly of microtubules, an important part of the cytoskeleton and the mitotic spindle. This inhibitory effect results in mitotic arrest in metaphase, bringing cell division to a halt, which then leads to cell death. Vinblastine and other vinca alkaloids are metabolized by the liver P450 system, and the majority of the drug is excreted in feces via the hepatobiliary system. As such, dose modification is required in the setting of liver dysfunction. The main adverse effects are outlined in Table 54–4, and they include nausea and vomiting, bone marrow suppression, and alopecia. This agent is also a potent vesicant, and care must be taken in its administration. It has clinical activity in the treatment of Hodgkin’s and non-Hodgkin’s lymphomas, breast cancer, and germ cell cancer.

TABLE 54–4 Natural product cancer chemotherapy drugs: Clinical activity and toxicities.

Vincristine Vincristine is another alkaloid derivative of V rosea and is closely related in structure to vinblastine. Its mechanism of action, mechanism of resistance, and clinical pharmacology are identical to those of vinblastine. Despite these similarities to vinblastine, vincristine has a strikingly different spectrum of clinical activity and safety profile.

Vincristine has been effectively combined with prednisone for remission induction in acute lymphoblastic leukemia in children. It is also active in various hematologic malignancies such as Hodgkin’s and non-Hodgkin’s lymphomas, and multiple myeloma, and in several pediatric tumors including rhabdomyosarcoma, neuroblastoma, Ewing’s sarcoma, and Wilms’ tumor.

The main dose-limiting toxicity is neurotoxicity, usually expressed as a peripheral sensory neuropathy, although autonomic nervous system dysfunction with orthostatic hypotension, urinary retention, and paralytic ileus or constipation, cranial nerve palsies, ataxia, seizures, and coma have been observed. While myelosuppression occurs, it is generally milder and much less significant than with vinblastine. The other adverse effect that may develop is the syndrome of inappropriate secretion of antidiuretic hormone (SIADH).

Vinorelbine Vinorelbine is a semisynthetic derivative of vinblastine whose mechanism of action is identical to that of vinblastine and vincristine, ie, inhibition of mitosis of cells in the M phase through inhibition of tubulin polymerization. This agent has activity in NSCLC, breast cancer, and ovarian cancer. Myelosuppression with neutropenia is the dose-limiting toxicity, but other adverse effects include nausea and vomiting, transient elevations in liver function tests, neurotoxicity, and SIADH.

TAXANES & OTHER ANTI-MICROTUBULE DRUGS Paclitaxel is an alkaloid ester derived from the Pacific yew (Taxus brevifolia ) and the European yew (Taxus baccata). The drug

functions as a mitotic spindle poison through high-affinity binding to microtubules with enhancement of tubulin polymerization. This promotion of microtubule assembly by paclitaxel occurs in the absence of microtubule-associated proteins and guanosine triphosphate and results in inhibition of mitosis and cell division.

Paclitaxel has significant activity in a broad range of solid tumors, including ovarian, advanced breast, NSCLC and small cell lung cancer (SCLC), head and neck, esophageal, prostate, and bladder cancers and AIDS-related Kaposi’s sarcoma. It is metabolized extensively by the liver P450 system, and nearly 80% of the drug is excreted in feces via the hepatobiliary route. Dose reduction is required in patients with liver dysfunction. The primary dose-limiting toxicities are listed in Table 54–4. Hypersensitivity reactions may be observed in up to 5% of patients, but the incidence is significantly reduced by premedication with dexamethasone, diphenhydramine, and an H2 blocker.

A novel albumin-bound paclitaxel formulation (Abraxane) is approved for use in metastatic breast cancer. In contrast to paclitaxel, this formulation is not associated with hypersensitivity reactions, and premedication to prevent such reactions is not required. Moreover, this agent has significantly reduced myelosuppressive effects compared with paclitaxel, and the neurotoxicity that results appears to be more readily reversible than is typically observed with paclitaxel.

Docetaxel is a semisynthetic taxane derived from the European yew tree. Its mechanism of action, metabolism, and elimination are identical to those of paclitaxel. It is approved for use as second-line therapy in advanced breast cancer and NSCLC, and it also has major activity in head and neck cancer, small cell lung cancer, gastric cancer, advanced platinum-refractory ovarian cancer, and bladder cancer. Its major toxicities are listed in Table 54–4.

Cabazitaxel is a semisynthetic taxane produced from a precursor extracted from the yew tree. Its mechanism of action, metabolism, and elimination are identical to those of the other taxanes. However, unlike other taxanes, cabazitaxel is a poor substrate for the multidrug resistance P-glycoprotein efflux pump and may therefore be useful for treating multidrug-resistant tumors. It is approved for use in combination with prednisone in the second-line therapy of hormone-refractory metastatic prostate cancer previously treated with a docetaxel-containing regimen. Its major toxicities include myelosuppression, neurotoxicity, and allergic reactions.

Although not strictly a taxane, ixabepilone is a semisynthetic epothilone B analog that functions as a microtubule inhibitor and binds directly to β-tubulin subunits on microtubules, leading to inhibition of normal microtubule dynamics. As such, it is active in the M phase of the cell cycle. This agent is presently approved for metastatic breast cancer in combination with the oral fluoropyrimidine capecitabine or as monotherapy. Of note, this agent continues to have activity in drug-resistant tumors that overexpress P-glycoprotein or tubulin mutations. The main adverse effects include myelosuppression, hypersensitivity reactions, and neurotoxicity in the form of peripheral sensory neuropathy. Eribulin is a synthetic analog of halichondrin B, and it inhibits microtubule function, leading to a block in the G2 -M phase of the cell cycle. This agent appears to be less sensitive to the multidrug resistance-mediated P-glycoprotein efflux pump, and continues to have activity in drug-resistant tumors that overexpress P-glycoprotein. It is presently approved for the treatment of patients with metastatic breast cancer.

EPIPODOPHYLLOTOXINS Etoposide is a semisynthetic derivative of podophyllotoxin, which is extracted from the mayapple root (Podophyllum peltatum).

Intravenous and oral formulations of etoposide are approved for clinical use in the USA. The oral bioavailability is about 50%, requiring the oral dose to be twice that of an intravenous dose. Up to 30–50% of drug is excreted in the urine, and dose reduction is required in patients with renal dysfunction. The main site of action is inhibition of the DNA enzyme topoisomerase II. Etoposide has clinical activity in germ cell cancer, small cell and NSCLC, Hodgkin’s and non-Hodgkin’s lymphomas, and gastric cancer. Major toxicities are listed in Table 54–4.

CAMPTOTHECINS The camptothecins are natural products derived from the Camptotheca acuminata tree originally found in China; they inhibit the activity of topoisomerase I, the key enzyme responsible for cutting and religating single DNA strands. Inhibition of this enzyme results in DNA damage. Topotecan and irinotecan are the two camptothecin analogs used in clinical practice in the USA. Although they both inhibit the same molecular target, their spectrum of clinical activity is quite different.

Topotecan is indicated in the treatment of advanced ovarian cancer as second-line therapy following initial treatment with platinumbased chemotherapy. It is also approved as second-line therapy of small cell lung cancer. The main route of elimination is renal excretion, and dosage must be adjusted in patients with renal impairment. Irinotecan is a prodrug that is converted mainly in the liver by the carboxylesterase enzyme to the SN-38 metabolite, which is 1000-fold more potent as an inhibitor of topoisomerase I than the parent compound. In contrast to topotecan, irinotecan and SN-38 are mainly eliminated in bile and feces, and dose reduction is required in the setting of liver dysfunction. Irinotecan was originally approved as second-line monotherapy in patients with metastatic colorectal cancer who had failed fluorouracil-based therapy. It is now approved as first-line therapy when used in combination with 5-FU and leucovorin.

Myelosuppression and diarrhea are the two most common adverse events (Table 54–4). There are two forms of diarrhea: an early form

that occurs within 24 hours after administration and is thought to be a cholinergic event effectively treated with atropine, and a late form that usually occurs 2â&#x20AC;&#x201C;10 days after treatment. The late diarrhea can be severe, leading to significant electrolyte imbalance and dehydration in some cases.

ANTITUMOR ANTIBIOTICS Screening of microbial products led to the discovery of a number of growth-inhibiting compounds that have proved to be clinically useful in cancer chemotherapy. Many of these antibiotics bind to DNA through intercalation between specific bases and block the synthesis of RNA, DNA, or both; cause DNA strand scission; and interfere with cell replication. All of the anti-cancer antibiotics now being used in clinical practice are products of various strains of the soil microbe Streptomyces. These include the anthracyclines, bleomycin, and mitomycin.

ANTHRACYCLINES The anthracycline antibiotics, isolated from Streptomyces peucetius var caesius, are among the most widely used cytotoxic anti-cancer drugs. The structures of two congeners, doxorubicin and daunorubicin, are shown below. Several other anthracycline analogs have entered clinical practice, including idarubicin, epirubicin, and mitoxantrone. The anthracyclines exert their cytotoxic action through four major mechanisms: (1) inhibition of topoisomerase II; (2) high-affinity binding to DNA through intercalation, with consequent blockade of the synthesis of DNA and RNA, and DNA strand scission; (3) generation of semiquinone free radicals and oxygen free radicals through an iron-dependent, enzyme-mediated reductive process; and (4) binding to cellular membranes to alter fluidity and ion transport. While the precise mechanisms by which the anthracyclines exert their cytotoxic effects remain to be defined (and may depend upon the specific tumor type), it is now well-established that the free radical mechanism is the cause of the cardiotoxicity associated with the anthracyclines (Table 54â&#x20AC;&#x201C;4).

Anthracyclines are administered via the intravenous route. They are metabolized extensively in the liver, with reduction and hydrolysis of the ring substituents. The hydroxylated metabolite is an active species, whereas the aglycone is inactive. Up to 50% of drug is eliminated in the feces via biliary excretion, and dose reduction is required in patients with liver dysfunction. Although anthracyclines are usually administered on an every-3-week schedule, alternative schedules such as low-dose weekly or 72- to 96-hour continuous infusions have been shown to yield equivalent clinical efficacy with reduced toxicity.

Doxorubicin is one of the most important anti-cancer drugs in clinical practice, with major clinical activity in cancers of the breast, endometrium, ovary, testicle, thyroid, stomach, bladder, liver, and lung; in soft tissue sarcomas; and in several childhood cancers, including neuroblastoma, Ewingâ&#x20AC;&#x2122;s sarcoma, osteosarcoma, and rhabdomyosarcoma. It also has clinical activity in hematologic malignancies, including acute lymphoblastic leukemia, multiple myeloma, and Hodgkinâ&#x20AC;&#x2122;s and non-Hodgkinâ&#x20AC;&#x2122;s lymphomas. It is generally used in combination with other anti-cancer agents (eg, cyclophosphamide, cisplatin, and 5-FU), and clinical activity is improved with combination regimens as opposed to single-agent therapy.

Daunorubicin was the first agent in this class to be isolated, and it is still used in the treatment of acute myeloid leukemia. In contrast to doxorubicin, its efficacy in solid tumors is limited.

Idarubicin is a semisynthetic anthracycline glycoside analog of daunorubicin, and it is approved for use in combination with cytarabine for induction therapy of acute myeloid leukemia. When combined with cytarabine, idarubicin appears to be more active than daunorubicin in producing complete remissions and in improving survival in patients with acute myelogenous leukemia.

Epirubicin is an anthracycline analog whose mechanism of action and clinical pharmacology are identical to those of all other anthracyclines. It was initially approved for use as a component of adjuvant therapy in early-stage, node-positive breast cancer but is also used in the treatment of metastatic breast cancer and gastroesophageal cancer.

Mitoxantrone (dihydroxyanthracenedione) is an anthracene compound whose structure resembles the anthracycline ring. It binds to DNA to produce strand breakage and inhibits both DNA and RNA synthesis. It is currently used in the treatment of advanced, hormone-

refractory prostate cancer and low-grade non-Hodgkin’s lymphoma. It is also indicated in breast cancer and in pediatric and adult acute myeloid leukemias. Myelosuppression with leukopenia is the dose-limiting toxicity, and mild nausea and vomiting, mucositis, and alopecia also occur. Although the drug is thought to be less cardiotoxic than doxorubicin, both acute and chronic cardiac toxicities are observed. A blue discoloration of the fingernails, sclera, and urine is observed 1–2 days after drug administration.

The main dose-limiting toxicity of all anthracyclines is myelosuppression, with neutropenia more commonly observed than thrombocytopenia. In some cases, mucositis is dose-limiting. Two forms of cardiotoxicity are observed. The acute form occurs within the first 2–3 days and presents as arrhythmias and conduction abnormalities, other electrocardiographic changes, pericarditis, and myocarditis. This form is usually transient and in most cases is asymptomatic. The chronic form results in a dose-dependent, dilated cardiomyopathy associated with heart failure. The chronic cardiac toxicity appears to result from increased production of free radicals within the myocardium. This effect is rarely seen at total doxorubicin dosages below 500–550 mg/m2 . Use of lower weekly doses or continuous infusions of doxorubicin appear to reduce the incidence of cardiac toxicity. In addition, treatment with the iron-chelating agent dexrazoxane (ICRF-187) is currently approved to prevent or reduce anthracycline-induced cardiotoxicity in women with metastatic breast cancer who have received a total cumulative dose of doxorubicin of 300 mg/m2 . The anthracyclines can also produce a “radiation recall reaction,” with erythema and desquamation of the skin observed at sites of prior radiation therapy.

MITOMYCIN Mitomycin (mitomycin C) is an antibiotic isolated from Streptomyces caespitosus. It undergoes metabolic activation through an enzymemediated reduction to generate an alkylating agent that cross-links DNA. Hypoxic tumor stem cells of solid tumors exist in an environment conducive to reductive reactions and are more sensitive to the cytotoxic effects of mitomycin than normal cells and oxygenated tumor cells. It is active in all phases of the cell cycle, and is the best available drug for use in combination with radiation therapy to attack hypoxic tumor cells. Its main clinical use is in the treatment of squamous cell cancer of the anus in combination with 5FU and radiation therapy. In addition, it is used in combination chemotherapy for squamous cell carcinoma of the cervix and for breast, gastric, and pancreatic cancer. One special application of mitomycin has been in the intravesical treatment of superficial bladder cancer.

Because virtually none of the agent is absorbed, there is little to no systemic toxicity when used in this way.

The common toxicities of mitomycin are outlined in Table 54–4. Hemolytic-uremic syndrome, manifested as microangiopathic hemolytic anemia, thrombocytopenia, and renal failure, as well as occasional instances of interstitial pneumonitis have been reported.

BLEOMYCIN Bleomycin is a small peptide that contains a DNA-binding region and an iron-binding domain at opposite ends of the molecule. It acts by binding to DNA, which results in single- and double-strand breaks following free radical formation, and inhibition of DNA biosynthesis.

The fragmentation of DNA is due to oxidation of a DNA-bleomycin-Fe(II) complex and leads to chromosomal aberrations. Bleomycin is a cell cycle-specific drug that causes accumulation of cells in the G2 phase of the cell cycle.

Bleomycin is indicated for the treatment of Hodgkin’s and non-Hodgkin’s lymphomas, germ cell tumor, head and neck cancer, and squamous cell cancer of the skin, cervix, and vulva. One advantage of this agent is that it can be administered subcutaneously, intramuscularly, or intravenously. Elimination of bleomycin is mainly via renal excretion, and dose modification is recommended in patients with renal dysfunction.

Pulmonary toxicity is dose-limiting for bleomycin and usually presents as pneumonitis with cough, dyspnea, dry inspiratory crackles on physical examination, and infiltrates on chest X-ray. The incidence of pulmonary toxicity is increased in patients older than 70 years of age, in those who receive cumulative doses greater than 400 units, in those with underlying pulmonary disease, and in those who have received prior mediastinal or chest irradiation. In rare cases, pulmonary toxicity can be fatal. Other toxicities are listed in Table 54–4.

MISCELLANEOUS ANTI-CANCER DRUGS A large number of anti-cancer drugs that do not fit traditional categories have been approved for clinical use; they are listed in Table 54– 5.

TABLE 54–5 Miscellaneous anti-cancer drugs: Clinical activity and toxicities.

IMATINIB & OTHER TYROSINE KINASE INHIBITORS (TKIs) Imatinib is an inhibitor of the tyrosine kinase domain of the Bcr-Abl oncoprotein and prevents phosphorylation of the kinase substrate by ATP. It is indicated for the treatment of chronic myelogenous leukemia (CML), a pluripotent hematopoietic stem cell disorder characterized by the t(9:22) Philadelphia chromosomal translocation. This translocation results in the Bcr-Abl fusion protein, the causative agent in CML, and is present in up to 95% of patients with this disease. This agent also inhibits other receptor tyrosine kinases for platelet-derived growth factor receptor (PDGFR), and c-kit.

Imatinib is well absorbed orally, and it is metabolized in the liver, with elimination of metabolites occurring mainly in feces via biliary excretion. This agent is approved for use as first-line therapy in chronic phase CML, in blast crisis, and as second-line therapy for chronic phase CML that has progressed on prior interferon-alfa therapy. Imatinib is also effective in the treatment of gastrointestinal stromal tumors expressing the c-kit tyrosine kinase. The main adverse effects are listed in Table 54–5.

Dasatinib is an oral inhibitor of several tyrosine kinases, including Bcr-Abl, Src, c-kit, and PDGFR-α. It differs from imatinib in that it binds to the active and inactive conformations of the Abl kinase domain and overcomes imatinib resistance resulting from mutations in the Bcr-Abl kinase. It is approved for use in CML and Philadelphia (Ph) chromosome-positive acute lymphoblastic leukemia (ALL) with resistance or intolerance to imatinib therapy.

Nilotinib is a second-generation phenylamino-pyrimidine molecule that inhibits Bcr-Abl, c-kit, and PDGFR-β tyrosine kinases. It has a higher binding affinity (up to 20- to 50-fold) for the Abl kinase when compared with imatinib, and it overcomes imatinib resistance resulting from Bcr-Abl mutations. It was originally approved for chronic phase and accelerated phase CML with resistance or intolerance to prior therapy that included imatinib and was recently approved as first-line therapy of chronic phase CML. Bosutinib is a potent inhibitor of the Bcr-Abl tyrosine kinase, and it retains activity in 16 of 18 imatinib-resistant Bcr-Abl mutations.

However, it is not effective against T315I and V299L mutations, which reside within the ATP-binding domain of the Abl tyrosine kinase.

It is currently approved for the treatment of adult patients with chronic, accelerated, or blast phase Ph chromosome-positive CML with resistance or intolerance to prior therapy.

Imatinib and the other TKIs are metabolized in the liver, mainly by the CYP3A4 liver microsomal enzyme. A large fraction of each drug is eliminated in feces via the hepatobiliary route. It is important to review the patient’s current list of prescription and nonprescription drugs because these agents have potential drug-drug interactions, especially with those that are also metabolized by the CYP3A4 system.

In addition, patients should avoid grapefruit products and the use of St. John’s Wort, as they may alter the metabolism of these small molecule inhibitors (see Chapter 4).

GROWTH FACTOR RECEPTOR INHIBITORS Cetuximab & Panitumumab The epidermal growth factor receptor (EGFR) is a member of the erb-B family of growth factor receptors, and it is overexpressed in a number of solid tumors, including colorectal cancer, head and neck cancer, NSCLC, and pancreatic cancer. Activation of the EGFR signaling pathway results in downstream activation of several key cellular events involved in cellular growth and proliferation, invasion and metastasis, and angiogenesis. In addition, this pathway inhibits the cytotoxic activity of various anti-cancer agents and radiation therapy, presumably through suppression of key apoptotic mechanisms, thereby leading to the development of cellular drug resistance.

Cetuximab is a chimeric monoclonal antibody directed against the extracellular domain of the EGFR, and it is presently approved for use in combination with irinotecan for metastatic colon cancer in the refractory setting or as monotherapy in patients who are deemed to be irinotecan-refractory. Because cetuximab is of the G1 isotype, its antitumor activity may also be mediated, in part, by immunologicmediated mechanisms. There is growing evidence that cetuximab can be effectively and safely combined with irinotecan- and oxaliplatinbased chemotherapy in the first-line treatment of metastatic colorectal cancer as well. Of note, the efficacy of cetuximab is restricted to only those patients whose tumors express wild-type KRAS. Regimens combining cetuximab with cytotoxic chemotherapy may be of particular benefit in the neoadjuvant therapy of patients with liver-limited disease. Although this antibody was initially approved to be administered on a weekly schedule, pharmacokinetic studies have shown that an every-2-week schedule provides the same level of clinical activity as the weekly schedule. This agent is also approved for use in combination with radiation therapy in patients with locally advanced head and neck cancer. Cetuximab is well tolerated, with the main adverse effects being an acneiform skin rash, hypersensitivity infusion reaction, and hypomagnesemia.

Panitumumab is a fully human monoclonal antibody directed against the EGFR and works through inhibition of the EGFR signaling pathway. In contrast to cetuximab, this antibody is of the G2 isotype, and as such, it would not be expected to exert any immunologicmediated effects. Presently, panitumumab is approved for patients with refractory metastatic colorectal cancer who have been treated with all other active agents, and as with cetuximab, this antibody is only effective in patients whose tumors express wild-type KRAS. Recent clinical studies have shown that this antibody can be effectively and safely combined with oxaliplatin- and irinotecan-based chemotherapy in the first- and second-line treatment of metastatic colorectal cancer. Acneiform skin rash and hypomagnesemia are the two main adverse effects associated with its use. Despite being a fully human antibody, infusion-related reactions are observed albeit

rarely.

Erlotinib Erlotinib is a small molecule inhibitor of the tyrosine kinase domain associated with the EGFR. It is now approved as first-line treatment of metastatic NSCLC in patients whose tumors have EGFR exon 19 deletions or exon 21 (L858R) mutations, and are refractory to at least one prior chemotherapy regimen. It is also approved for maintenance therapy of patients with metastatic NCSLC whose disease has not progressed after four cycles of platinum-based chemotherapy. Patients who are nonsmokers and who have a bronchoalveolar histologic subtype appear to be more responsive to these agents. In addition, erlotinib has been approved for use in combination with gemcitabine for the treatment of advanced pancreatic cancer. It is metabolized in the liver by the CYP3A4 enzyme system, and elimination is mainly hepatic with excretion in feces. Caution must be taken when using these agents with drugs that are also metabolized by the liver CYP3A4 system, such as phenytoin and warfarin, and the use of grapefruit products should be avoided. An acneiform skin rash, diarrhea, and anorexia and fatigue are the most common adverse effects observed with these small molecules (Table 54–5).

Bevacizumab, Ziv-aflibercept, Sorafenib, Sunitinib, & Pazopanib The vascular endothelial growth factor (VEGF) is one of the most important angiogenic growth factors. The growth of both primary and metastatic tumors requires an intact vasculature. As a result, the VEGF-signaling pathway represents an attractive target for chemotherapy. Several approaches have been taken to inhibit VEGF signaling; they include inhibition of VEGF interactions with its receptor by targeting either the VEGF ligand with antibodies or soluble chimeric decoy receptors, or by direct inhibition of the VEGF receptor-associated tyrosine kinase activity by small molecule inhibitors.

Bevacizumab is a recombinant humanized monoclonal antibody that targets all forms of VEGF-A. This antibody binds to and prevents VEGF-A from interacting with the target VEGF receptors. Bevacizumab can be safely and effectively combined with 5-FU-, irinotecan-, and oxaliplatin-based chemotherapy in the treatment of metastatic colorectal cancer. Bevacizumab is FDA approved as a first-line treatment for metastatic colorectal cancer in combination with any intravenous fluoropyrimidine-containing regimen and is now also approved in combination with chemotherapy for metastatic NSCLC and breast cancer. One potential advantage of this antibody is that it does not appear to exacerbate the toxicities typically observed with cytotoxic chemotherapy. The main safety concerns associated with bevacizumab include hypertension, an increased incidence of arterial thromboembolic events (transient ischemic attack, stroke, angina, and myocardial infarction), wound healing complications, gastrointestinal perforations, and proteinuria.

Ziv-aflibercept is a recombinant fusion protein made up of portions of the extracellular domains of human VEGF receptors (VEGFR) 1 and 2 fused to the Fc portion of the human IgG1 molecule. This molecule serves as a soluble receptor to VEGF-A, VEGF-B, and placental growth factor (PlGF), and it binds with significantly higher affinity to VEGF-A than bevacizumab. Presumably, binding of the VEGF ligands prevents their subsequent interactions with the target VEGF receptors, which then results in inhibition of downstream VEGFR signaling. This agent is FDA-approved in combination with the FOLFIRI regimen for patients with metastatic colorectal cancer that has progressed on oxaliplatin-based chemotherapy. The main adverse effects are similar to what has been observed with bevacizumab.

Sorafenib is a small molecule that inhibits multiple receptor tyrosine kinases (RTKs), especially VEGF-R2 and VEGF-R3, plateletderived growth factor-β (PDGFR-β), and raf kinase. It was initially approved for advanced renal cell cancer and is also approved for advanced hepatocellular cancer.

Sunitinib is similar to sorafenib in that it inhibits multiple RTKs, although the specific types are somewhat different. They include PDGFR-α and PDGFR-β, VEGF-R1, VEGF-R2, VEGF-R3, and c-kit. It is approved for the treatment of advanced renal cell cancer and for the treatment of gastrointestinal stromal tumors (GIST) after disease progression on or with intolerance to imatinib.

Pazopanib is a small molecule that inhibits multiple RTKs, especially VEGF-R2 and VEGF-R3, PDGFR-β, and raf kinase. This oral agent is approved for the treatment of advanced renal cell cancer.

Sorafenib, sunitinib, and pazopanib are metabolized in the liver by the CYP3A4 system, and elimination is primarily hepatic with excretion in feces. Therefore, each of these agents has potential interactions with drugs that are also metabolized by the CYP3A4 system, especially warfarin. In addition, patients should avoid grapefruit products and the use of St. John’s Wort, as they may alter the clinical activity of these agents. Hypertension, bleeding complications, and fatigue are the most common adverse effects seen with these drugs. With respect to sorafenib, skin rash and the hand-foot syndrome are observed in up to 30–50% of patients. For sunitinib, there is also an increased risk of cardiac dysfunction, which in some cases can lead to congestive heart failure.

ASPARAGINASE Asparaginase (L-asparagine amidohydrolase) is an enzyme occasionally used to treat childhood acute lymphoblastic leukemia (ALL). It hydrolyzes circulating L-asparagine to aspartic acid and ammonia. Because tumor cells in ALL lack asparagine synthetase, they require an exogenous source of L-asparagine. Thus, depletion of L-asparagine results in effective inhibition of protein synthesis. In contrast, normal cells can synthesize L-asparagine and thus are less susceptible to the cytotoxic action of asparaginase. The main adverse effect

of this agent is a hypersensitivity reaction manifested by fever, chills, nausea and vomiting, skin rash, and urticaria. Severe cases can present with bronchospasm, respiratory failure, and hypotension.

CLINICAL PHARMACOLOGY OF CANCER CHEMOTHERAPEUTIC DRUGS A complete knowledge of the kinetics of tumor cell proliferation along with an understanding of the pharmacology and mechanism of action of cancer chemotherapeutic agents is important in designing optimal regimens for patients with cancer. The strategy for developing drug regimens also requires knowledge of the specific characteristics of individual tumors. Is there a high growth fraction? Is there a high spontaneous cell death rate? Are most of the cells in G0 ? Is a significant fraction of the tumor composed of hypoxic stem cells? Are their normal counterparts under hormonal control? Similarly, an understanding of the pharmacology of specific drugs is important. Are the tumor cells sensitive to the drug? Is the drug cell cycle-specific? Does the drug require activation in certain normal tissue such as the liver (cyclophosphamide), or is it activated in the tumor tissue itself (capecitabine)? Knowledge of specific pathway abnormalities (eg, EGFR mutations, KRAS mutations) for intracellular signaling may prove important for the next generation of anticancer drugs.

For some tumor types, knowledge of receptor expression is important. In patients with breast cancer, analysis of the tumor for expression of estrogen or progesterone receptors is important in guiding therapy with selective estrogen receptor modulators. In addition, analysis of breast cancer for expression of the HER-2/neu growth factor receptor can determine whether the humanized monoclonal anti-HER-2/neu antibody, trastuzumab, would be appropriate therapy. In the case of prostate cancer, chemical suppression of androgen secretion with gonadotropin-releasing hormone agonists or antagonists is important. The basic pharmacology of hormonal therapy is discussed in Chapter 40. The use of specific cytotoxic and biologic agents for each of the main cancers is discussed in this section.

THE LEUKEMIAS ACUTE LEUKEMIA Childhood Leukemia Acute lymphoblastic leukemia (ALL) is the main form of leukemia in childhood, and it is the most common form of cancer in children.

Children with this disease have a relatively good prognosis. A subset of patients with neoplastic lymphocytes expressing surface antigenic features of T lymphocytes has a poor prognosis (see Chapter 55). A cytoplasmic enzyme expressed by normal thymocytes, terminal deoxycytidyl transferase (terminal transferase), is also expressed in many cases of ALL. T-cell ALL also expresses high levels of the enzyme adenosine deaminase (ADA). This led to interest in the use of the ADA inhibitor pentostatin (deoxycoformycin) for treatment of such T-cell cases. Until 1948, the median length of survival in ALL was 3 months. With the advent of methotrexate, the length of survival was greatly increased. Subsequently, corticosteroids, 6-mercaptopurine, cyclophosphamide, vincristine, daunorubicin, and asparaginase have all been found to be active against this disease. A combination of vincristine and prednisone plus other agents is currently used to induce remission. Over 90% of children enter complete remission with this therapy with only minimal toxicity. However, circulating leukemic cells often migrate to sanctuary sites located in the brain and testes. The value of prophylactic intrathecal methotrexate therapy for prevention of central nervous system leukemia (a major mechanism of relapse) has been clearly demonstrated.

Intrathecal therapy with methotrexate should therefore be considered as a standard component of the induction regimen for children with ALL.

Adult Leukemia Acute myelogenous leukemia (AML) is the most common leukemia in adults. The single most active agent for AML is cytarabine; however, it is best used in combination with an anthracycline, which leads to complete remissions in about 70% of patients. While there are several anthracyclines that can be effectively combined with cytarabine, idarubicin is preferred.

Patients often require intensive supportive care during the period of induction chemotherapy. Such care includes platelet transfusions to prevent bleeding, the granulocyte colony-stimulating factor filgrastim to shorten periods of neutropenia, and antibiotics to combat infections. Younger patients (eg, age < 55) who are in complete remission and have an HLA-matched donor are candidates for allogeneic bone marrow transplantation. The transplant procedure is preceded by high-dose chemotherapy and total body irradiation followed by immunosuppression. This approach may cure up to 35â&#x20AC;&#x201C;40% of eligible patients. Patients over age 60 respond less well to chemotherapy, primarily because their tolerance for aggressive therapy and resistance to infection are lower.

Once remission of AML is achieved, consolidation chemotherapy is required to maintain a durable remission and to induce cure.

CHRONIC MYELOGENOUS LEUKEMIA

Chronic myelogenous leukemia (CML) arises from a chromosomally abnormal hematopoietic stem cell in which a balanced translocation between the long arms of chromosomes 9 and 22, t(9:22), is observed in 90–95% of cases. This translocation results in constitutive expression of the Bcr-Abl fusion oncoprotein with a molecular weight of 210 kDa. The clinical symptoms and course are related to the white blood cell count and its rate of increase. Most patients with white cell counts over 50,000/μL should be treated. The goals of treatment are to reduce the granulocytes to normal levels, to raise the hemoglobin concentration to normal, and to relieve disease-related symptoms. The tyrosine kinase inhibitor imatinib is considered as standard first-line therapy in previously untreated patients with chronic phase CML. Nearly all patients treated with imatinib exhibit a complete hematologic response, and up to 40–50% of patients show a complete cytogenetic response. As described previously, this drug is generally well tolerated and is associated with relatively minor adverse effects. Initially, dasatinib and nilotinib were approved for patients who were intolerant or resistant to imatinib; each shows clinical activity, and both are now also indicated as first-line treatment of chronic phase CML. In addition to these tyrosine kinase inhibitors, other treatment options include interferon-α, busulfan, other oral alkylating agents, and hydroxyurea.

CHRONIC LYMPHOCYTIC LEUKEMIA Patients with early-stage chronic lymphocytic leukemia (CLL) have a relatively good prognosis, and therapy has not changed the course of the disease. However, in the setting of high-risk disease or in the presence of disease-related symptoms, treatment is indicated.

Chlorambucil and cyclophosphamide are the two most widely used alkylating agents for this disease. Chlorambucil is frequently combined with prednisone, although there is no clear evidence that the combination yields better response rates or survival compared with chlorambucil alone. In most cases, cyclophosphamide is combined with vincristine and prednisone (COP), or it can also be given with these same drugs along with doxorubicin (CHOP). Bendamustine is the newest alkylating agent to be approved for use in this disease, either as monotherapy or in combination with prednisone. The purine nucleoside analog fludarabine is also effective in treating CLL. This agent can be given alone, in combination with cyclophosphamide and with mitoxantrone and dexamethasone, or combined with rituximab. Monoclonal antibody-targeted therapies are being widely used in CLL, especially in relapsed or refractory disease. Rituximab is an anti-CD20 antibody that has documented clinical activity in this setting. This chimeric antibody appears to enhance the antitumor effects of cytotoxic chemotherapy and is also effective in settings in which resistance to chemotherapy has developed. Ofatumumab is a fully human IgG1 antibody that binds to a different CD20 epitope than rituximab. Of note, it maintains activity in rituximab-resistant tumors, and it is presently approved for CLL that is refractory to fludarabine and alemtuzumab therapy.

HODGKIN’S & NON-HODGKIN’S LYMPHOMAS HODGKIN’S LYMPHOMA The treatment of Hodgkin’s lymphoma has undergone dramatic evolution over the last 40 years. This lymphoma is now widely recognized as a B-cell neoplasm in which the malignant Reed-Sternberg cells have rearranged VH genes. In addition, the Epstein-Barr virus genome has been identified in up to 80% of tumor specimens.

Complete staging evaluation is required before a definitive treatment plan can be made. For patients with stage I and stage IIA disease, there has been a significant change in the treatment approach. Initially, these patients were treated with extended-field radiation therapy. However, given the well-documented late effects of radiation therapy, which include hypothyroidism, an increased risk of secondary cancers, and coronary artery disease, combined-modality therapy with a brief course of combination chemotherapy and involved field radiation therapy is now the recommended approach. The main advance for patients with advanced stage III and IV Hodgkin’s lymphoma came with the development of MOPP (mechlorethamine, vincristine, procarbazine, and prednisone) chemotherapy in the 1960s. This regimen resulted initially in high complete response rates, on the order of 80–90%, with cures in up to 60% of patients.

More recently, the anthracycline-containing regimen termed ABVD (doxorubicin, bleomycin, vinblastine, and dacarbazine) has been shown to be more effective and less toxic than MOPP, especially with regard to the incidence of infertility and secondary malignancies.

In general, four cycles of ABVD are given to patients. An alternative regimen, termed Stanford V, utilizes a 12-week course of combination chemotherapy (doxorubicin, vinblastine, mechlorethamine, vincristine, bleomycin, etoposide, and prednisone), followed by involved radiation therapy.

With all of these regimens, over 80% of previously untreated patients with advanced Hodgkin’s lymphoma (stages III and IV) are expected to go into complete remission, with disappearance of all disease-related symptoms and objective evidence of disease. In general, approximately 50–60% of all patients with Hodgkin’s lymphoma are cured of their disease.

NON-HODGKIN’S LYMPHOMA Non-Hodgkin’s lymphoma is a heterogeneous disease, and the clinical characteristics of non-Hodgkin’s lymphoma subsets are related to the underlying histopathologic features and the extent of disease involvement. In general, the nodular (or follicular) lymphomas have a far better prognosis, with a median survival up to 7 years, compared with the diffuse lymphomas, which have a median survival of about 1–2

years.

Combination chemotherapy is the treatment standard for patients with diffuse non-Hodgkin’s lymphoma. The anthracyclinecontaining regimen CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) has been considered the best treatment in terms of initial therapy. Randomized phase III clinical studies have now shown that the combination of CHOP with rituximab results in improved response rates, disease-free survival, and overall survival compared with CHOP chemotherapy alone.

The nodular follicular lymphomas are low-grade, relatively slow-growing tumors that tend to present in an advanced stage and are usually confined to lymph nodes, bone marrow, and spleen. This form of non-Hodgkin’s lymphomas, when presenting at an advanced stage, is considered incurable, and treatment is generally palliative. To date, there is no evidence that immediate treatment with combination chemotherapy offers clinical benefit over close observation and “watchful waiting” with initiation of chemotherapy at the onset of disease symptoms.

MULTIPLE MYELOMA This plasma cell malignancy is one of the models of neoplastic disease in humans as it arises from a single tumor stem cell. Moreover, the tumor cells produce a marker protein (myeloma immunoglobulin) that allows the total body burden of tumor cells to be quantified.

Multiple myeloma principally involves the bone marrow and bone, causing bone pain, lytic lesions, bone fractures, and anemia as well as an increased susceptibility to infection.

Most patients with multiple myeloma are symptomatic at the time of initial diagnosis and require treatment with cytotoxic chemotherapy. Treatment with the combination of the alkylating agent melphalan and prednisone (MP protocol) has been a standard regimen for nearly 30 years. About 40% of patients respond to the MP combination, and the median duration of remission is 2–2.5 years.

In patients who are considered candidates for high-dose therapy with stem cell transplantation, melphalan and other alkylating agents are to be avoided, as they can affect the success of stem cell harvesting.

Thalidomide is a well-established agent for treating refractory or relapsed disease, and about 30% of patients will achieve a response to this therapy. More recently, thalidomide has been used in combination with dexamethasone, and response rates approaching 65% have been observed. Studies are now under way to directly compare the combination of vincristine, doxorubicin, and dexamethasone (VAD protocol) with the combination of thalidomide and dexamethasone. In some patients, especially those with poor performance status, single-agent pulse dexamethasone administered on a weekly basis can be effective in palliating symptoms.

Lenalidomide and pomalidomide are two immunomodulatory analogs (IMiDs) of thalidomide. Lenalidomide is approved in combination with dexamethasone for multiple myeloma patients who have received at least one prior therapy, and clinical data show that this combination is effective as first-line therapy. Pomalidomide is the most recent IMiD to receive approval and this drug may be able to overcome resistance to thalidomide and lenalidomide. The side effect profiles of these IMiDs appear to be similar although neurotoxicity is observed more commonly with thalidomide, somewhat less often with pomalidomide, and rarely with lenalidomide.

Bortezomib was first approved for use in relapsing or refractory multiple myeloma and is now widely used as first-line therapy. This agent is thought to exert its main cytotoxic effects through inhibition of the 26S proteosome, resulting in down-regulation of the nuclear factor kappa B (NF-κB) signaling pathway, which is felt to be a major signaling pathway for this disease. Of note, inhibition of NF-κB has also been shown to restore chemosensitivity. Based on this mechanism of action, further efforts have focused on developing bortezomib in various combination regimens. Carfilzomib is an epoxyketone 26S proteosome inhibitor that is approved for patients with multiple myeloma who have received at least two prior therapies, including bortezomib and an immunomodulatory agent. This agent is important as it is able to overcome resistance to bortezomib, and preclinical and clinical studies suggest that it has broad-spectrum activity in hematologic malignancies as well as in solid tumors.

BREAST CANCER STAGE I & STAGE II DISEASE The management of primary breast cancer has undergone a remarkable evolution as a result of major efforts at early diagnosis (through encouragement of self-examination as well as through the use of cancer detection centers) and the implementation of combined modality approaches incorporating systemic chemotherapy as an adjuvant to surgery and radiation therapy. Women with stage I disease (small primary tumors and negative axillary lymph node dissections) are currently treated with surgery alone, and they have an 80% chance of cure.

Women with node-positive disease have a high risk of both local and systemic recurrence. Thus, lymph node status directly indicates the risk of occult distant micrometastasis. In this situation, postoperative use of systemic adjuvant chemotherapy with six cycles of cyclophosphamide, methotrexate, and fluorouracil (CMF protocol) or of fluorouracil, doxorubicin, and cyclophosphamide (FAC) has been shown to significantly reduce the relapse rate and prolong survival. Alternative regimens with equivalent clinical benefit include four cycles of doxorubicin and cyclophosphamide and six cycles of fluorouracil, epirubicin, and cyclophosphamide (FEC). Each of these chemotherapy regimens has benefited women with stage II breast cancer with one to three involved lymph nodes. Women with four or

more involved nodes have had limited benefit thus far from adjuvant chemotherapy. Long-term analysis has clearly shown improved survival rates in node-positive premenopausal women who have been treated aggressively with multiagent combination chemotherapy.

The results from three randomized clinical trials clearly show that the addition of trastuzumab, a monoclonal antibody directed against the HER-2/neu receptor, to anthracycline- and taxane-containing adjuvant chemotherapy benefits women with HER-2-overexpressing breast cancer with respect to disease-free and overall survival.

Breast cancer was the first neoplasm shown to be responsive to hormonal manipulation. Tamoxifen is beneficial in postmenopausal women when used alone or in combination with cytotoxic chemotherapy. The present recommendation is to administer tamoxifen for 5 years of continuous therapy after surgical resection. Longer durations of tamoxifen therapy do not appear to offer additional clinical benefit. Postmenopausal women who complete 5 years of tamoxifen therapy should be placed on an aromatase inhibitor such as anastrozole for at least 2.5 years, although the optimal duration is unknown. In women who have completed 2–3 years of tamoxifen therapy, treatment with an aromatase inhibitor for a total of 5 years of hormonal therapy is now recommended (see Chapter 40).

Results from several randomized trials for breast cancer have established that adjuvant chemotherapy for premenopausal women and adjuvant tamoxifen for postmenopausal women are of benefit to women with stage I (node-negative) breast cancer. While this group of patients has the lowest overall risk of recurrence after surgery alone (about 35–50% over 15 years), this risk can be further reduced with adjuvant therapy.

STAGE III & STAGE IV DISEASE The approach to women with advanced breast cancer remains a major challenge, as current treatment options are only palliative.

Combination chemotherapy, endocrine therapy, or a combination of both results in overall response rates of 40–50%, but only a 10–20% complete response rate. Breast cancers expressing estrogen receptors (ER) or progesterone receptors (PR) retain the intrinsic hormonal sensitivities of the normal breast—including the growth-stimulatory response to ovarian, adrenal, and pituitary hormones. Patients who show improvement with hormonal ablative procedures also respond to the addition of tamoxifen. The aromatase inhibitors anastrozole and letrozole are now approved as first-line therapy in women with advanced breast cancer whose tumors are hormone-receptor positive. In addition, these agents and exemestane are approved as second-line therapy following treatment with tamoxifen.

Patients with significant involvement of the lung, liver, or brain and those with rapidly progressive disease rarely benefit from hormonal maneuvers, and initial systemic chemotherapy is indicated in such cases. For the 25–30% of breast cancer patients whose tumors express the HER-2/neu cell surface receptor, the humanized monoclonal anti-HER-2/neu antibody, trastuzumab, is available for therapeutic use alone or in combination with cytotoxic chemotherapy.

About 50–60% of patients with metastatic disease respond to initial chemotherapy. A broad range of anti-cancer agents have activity in this disease, including the anthracyclines (doxorubicin, mitoxantrone, and epirubicin), the taxanes (docetaxel, paclitaxel, and albuminbound paclitaxel) along with the microtubule inhibitor ixabepilone, navelbine, capecitabine, gemcitabine, cyclophosphamide, methotrexate, and cisplatin. The anthracyclines and the taxanes are two of the most active classes of cytotoxic drugs. Combination chemotherapy has been found to induce higher and more durable remissions in up to 50–80% of patients, and anthracycline-containing regimens are now considered the standard of care in first-line therapy. With most combination regimens, partial remissions have a median duration of about 10 months and complete remissions have a duration of about 15 months. Unfortunately, only 10–20% of patients achieve complete remissions with any of these regimens, and as noted, complete remissions are usually not long-lasting.

PROSTATE CANCER Prostate cancer was the second cancer shown to be responsive to hormonal manipulation. The treatment of choice for patients with metastatic prostate cancer is elimination of testosterone production by the testes through either surgical or chemical castration. Bilateral orchiectomy or estrogen therapy in the form of diethylstilbestrol was previously used as first-line therapy. Presently, the use of luteinizing hormone-releasing hormone (LHRH) agonists—including leuprolide and goserelin agonists, alone or in combination with an antiandrogen (eg, flutamide, bicalutamide, or nilutamide)—is the preferred approach. There appears to be no survival advantage of total androgen blockade using a combination of LHRH agonist and antiandrogen agent compared with single-agent therapy.

Abiraterone, an inhibitor of steroid synthesis (see Chapter 39), has recently been approved. Hormonal treatment reduces symptoms— especially bone pain—in 70–80% of patients and may cause a significant reduction in the prostate-specific antigen (PSA) level, which is now widely accepted as a surrogate marker for response to treatment in prostate cancer. Although initial hormonal manipulation is able to control symptoms for up to 2 years, patients usually develop progressive disease. Second-line hormonal therapies include aminoglutethimide plus hydrocortisone, the antifungal agent ketoconazole plus hydrocortisone, or hydrocortisone alone.

Unfortunately, nearly all patients with advanced prostate cancer eventually become refractory to hormone therapy. A regimen of mitoxantrone and prednisone is approved in patients with hormone-refractory prostate cancer because it provides effective palliation in those who experience significant bone pain. Estramustine is an antimicrotubule agent that produces an almost 20% response rate as a single agent. However, when used in combination with either etoposide or a taxane such as docetaxel or paclitaxel, response rates are more than doubled to 40–50%. The combination of docetaxel and prednisone was recently shown to confer survival advantage when compared with the mitoxantrone-prednisone regimen, and this combination has now become the standard of care for hormone-refractory

prostate cancer.

GASTROINTESTINAL CANCERS Colorectal cancer (CRC) is the most common type of gastrointestinal malignancy. Nearly 150,000 new cases are diagnosed each year in the USA; worldwide, nearly 1.2 million cases are diagnosed annually. At the time of initial presentation, only about 40–45% of patients are potentially curable with surgery. Patients presenting with high-risk stage II disease and stage III disease are candidates for adjuvant chemotherapy with an oxaliplatin-based regimen in combination with 5-FU plus leucovorin (FOLFOX or FLOX) or with oral capecitabine (XELOX) and are generally treated for 6 months following surgical resection. Treatment with this combination regimen reduces the recurrence rate after surgery by 35% and clearly improves overall patient survival compared with surgery alone.

Significant advances have been made over the past 10 years with respect to treatment of metastatic CRC. There are four active cytotoxic agents—5-FU, the oral fluoropyrimidine capecitabine, oxaliplatin, and irinotecan; and 5 active biologic agents—the anti-VEGF antibody bevacizumab; the recombinant fusion protein ziv-aflibercept that targets VEGF-A, VEGF-B, and PlGF; the anti-EGFR antibodies cetuximab and panitumumab; and the small molecule TKI inhibitor regorafenib. In general, a fluoropyrimidine with either intravenous 5-FU or oral capecitabine serves as the main foundation of cytotoxic chemotherapy regimens. Recent clinical studies have shown that in tumors with wild-type V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS), FOLFOX/FOLFIRI regimens in combination with the anti-VEGF antibody bevacizumab or with the anti-EGFR antibody cetuximab or panitumumab result in significantly improved clinical efficacy with no worsening of the toxicities normally observed with chemotherapy. In order for patients to derive maximal benefit, they should be treated with each of these active agents in a continuum of care approach. Using this strategy, median overall survival is now in the 24- to 28-month range, and in some cases, approaches 3 years.

The incidence of gastric cancer, esophageal cancer, and pancreatic cancer is much lower than for CRC, but these malignancies tend to be more aggressive and result in greater tumor-related symptoms. In most cases, they cannot be completely resected surgically, as most patients present with either locally advanced or metastatic disease at the time of their initial diagnosis. 5-FU-based chemotherapy, using either intravenous 5-FU or oral capecitabine, is generally considered the main backbone for regimens targeting gastroesophageal cancers. In addition, cisplatin-based regimens in combination with either irinotecan or one of the taxanes (paclitaxel or docetaxel) also exhibit clinical activity. Response rates in the 40–50% range are now being reported. Recent studies have shown that the addition of the biologic agent trastuzumab to cisplatin-containing chemotherapy regimens provides significant clinical benefit in gastric cancer patients whose tumors overexpress the HER-2/neu receptor. Although gemcitabine is approved for use as a single agent in metastatic pancreatic cancer, the overall response rate is less than 10%, with complete responses being quite rare. Intense efforts continue to be placed on incorporating gemcitabine into various combination regimens and on identifying novel agents that target signal transduction pathways thought to be critical for the growth of pancreatic cancer. One such agent is erlotinib. This agent is now approved for use in combination with gemcitabine in locally advanced or metastatic pancreatic cancer although the improvement in clinical benefit is relatively small.

There is also evidence to support the use of adjuvant chemotherapy with either single-agent gemcitabine or 5-FU/leucovorin in patients with early-stage pancreatic cancer who have undergone successful surgical resection.

LUNG CANCER Lung cancer is divided into two main histopathologic subtypes, non-small cell and small cell. Non-small cell lung cancer (NSCLC) makes up about 75–80% of all cases of lung cancer, and this group includes adenocarcinoma, squamous cell cancer, and large cell cancer, while small cell lung cancer (SCLC) makes up the remaining 20–25%. When NSCLC is diagnosed in an advanced stage with metastatic disease, the prognosis is extremely poor, with a median survival of about 8 months. It is clear that prevention (primarily through avoidance of cigarette smoking) and early detection remain the most important means of control. When diagnosed at an early stage, surgical resection results in patient cure. Moreover, recent studies have shown that adjuvant platinum-based chemotherapy provides a survival benefit in patients with pathologic stage IB, II, and IIIA disease. However, in most cases, distant metastases have occurred at the time of diagnosis. In certain instances, radiation therapy can be offered for palliation of pain, airway obstruction, or bleeding and to treat patients whose performance status would not allow for more aggressive treatments.

In patients with advanced disease, systemic chemotherapy is generally recommended. Combination regimens that include a platinum agent (“platinum doublets”) appear superior to non-platinum doublets, and either cisplatin or carboplatin are appropriate platinum agents for such regimens. For the second drug, paclitaxel and vinorelbine appear to have activity independent of histology, while the antifolate pemetrexed should be used for non-squamous cell cancer, and gemcitabine for squamous cell cancer. For patients with good performance status and those with non-squamous histology, the combination of the anti-VEGF antibody bevacizumab with carboplatin and paclitaxel is a standard treatment option. In patients deemed not to be appropriate candidates for bevacizumab therapy and those with squamous cell histology, a platinum-based chemotherapy regimen in combination with the anti-EGFR antibody cetuximab is a reasonable treatment strategy. Maintenance chemotherapy with pemetrexed is now used in patients with non-squamous NSCLC whose disease has not progressed after four cycles of platinum-based first-line chemotherapy. Finally, first-line therapy with erlotinib significantly improves outcomes in NSCLC patients with sensitizing EGFR mutations.

Small cell lung cancer is the most aggressive form of lung cancer. It is usually exquisitely sensitive, at least initially, to platinum-based combination regimens, including cisplatin and etoposide or cisplatin and irinotecan. Unfortunately, drug resistance eventually develops in

nearly all patients with extensive disease. When diagnosed at an early stage, this disease is potentially curable using a combined modality approach of chemotherapy and radiation therapy. Topotecan is used as second-line monotherapy in patients who have failed a platinumbased regimen.

OVARIAN CANCER In the majority of patients, ovarian cancer remains occult and becomes symptomatic only after it has already metastasized to the peritoneal cavity. At this stage, it usually presents with malignant ascites. It is important to accurately stage this cancer with laparoscopy, ultrasound, and CT scanning. Patients with stage I disease appear to benefit from whole-abdomen radiotherapy and may receive additional benefit from combination chemotherapy with cisplatin and cyclophosphamide.

Combination chemotherapy is the standard approach to stage III and stage IV disease. Randomized clinical studies have shown that the combination of paclitaxel and cisplatin provides survival benefit compared with the previous standard combination of cisplatin plus cyclophosphamide. More recently, carboplatin plus paclitaxel has become the treatment of choice. In patients who present with recurrent disease, topotecan, altretamine, or liposomal doxorubicin are used as single agent monotherapy.

TESTICULAR CANCER The introduction of platinum-based combination chemotherapy has made an impressive change in the treatment of patients with advanced testicular cancer. Presently, chemotherapy is recommended for patients with stage IIC or stage III seminomas and nonseminomatous disease. Over 90% of patients respond to chemotherapy and, depending upon the extent and severity of disease, complete remissions are observed in 70â&#x20AC;&#x201C;80% of patients. Over 50% of patients achieving complete remission are cured with chemotherapy. In patients with good risk features, three cycles of cisplatin, etoposide, and bleomycin (PEB protocol) or four cycles of cisplatin and etoposide yield virtually identical results. In patients with high-risk disease, the combination of cisplatin, etoposide, and ifosfamide can be used as well as etoposide and bleomycin with high-dose cisplatin.

MALIGNANT MELANOMA Malignant melanoma is curable with surgical resection when it presents locally (see also Chapter 61). However, once metastasis has occurred, it is one of the most difficult cancers to treat because of drug resistance. While dacarbazine, temozolomide, and cisplatin are the most active cytotoxic agents for this disease, the overall response rates to these agents remain low. Biologic agents, including interferon-Îą and interleukin-2 (IL-2), have greater activity than traditional cytotoxic agents, and treatment with high-dose IL-2 has led to cures, albeit in a relatively small subset of patients. Ipilimumab is the most recent biologic agent to have been approved for metastatic melanoma. This molecule binds to cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), which is expressed on the surface of activated CD4 and CD8 T-cells. CTLA-4 normally acts as a brake on T-cell antitumor activity. Binding of ipilimumab results in inhibition of the interaction between CTLA-4 and its target ligands CD80/CD86 and thus enhances T-cell immune responses, which include T-cell activation and proliferation. Impressive responses have been reported in small numbers of patients but the drug has significant toxicity.

Studies are presently investigating the combination of IL-2 plus ipilimumab as well as combination regimens that include ipilimumab and other immune checkpoint inhibitors that target the programmed death-1 (PD-1) receptor/programmed death ligand 1 (PD-L1) signaling pathway.

The BRAF:V600E mutation has been identified in the large majority of melanomas. This mutation results in constitutive activation of BRAF kinase, which then leads to activation of downstream signaling pathways involved in cell growth and proliferation. Since 2011, two oral and highly selective small molecule inhibitors of BRAF:V600E have been approved for metastatic melanoma (vemurafenib and dabrafenib). Studies are ongoing to determine their activity in combination with other cytotoxic and biologic agents for metastatic melanoma as well as their potential role in the adjuvant and neoadjuvant therapy of early stage melanoma.

A novel agent trametinib was approved for patients with metastatic melanoma whose tumors express the BRAF:V600E or V600K mutation. This small molecule acts as a mitogen-activated, extracellular signal-regulated kinase (MEK) inhibitor, and while it is approved for use as monotherapy, the clinical studies conducted to date suggest that the most promising clinical activity is seen when it is used in combination with a BRAF inhibitor.

BRAIN CANCER In general, chemotherapy has had only limited efficacy in the treatment of malignant gliomas. Given their ability to cross the blood-brain barrier, the nitrosoureas have historically been the most active agents in this disease. Carmustine (BCNU) has been used as a single agent, or lomustine (CCNU) can be used in combination with procarbazine and vincristine (PCV regimen). In addition, the alkylating agent temozolomide is active when combined with radiotherapy and is also used in patients with newly diagnosed glioblastoma multiforme (GBM) as well as in those with recurrent disease. The histopathologic subtype oligodendroglioma has been shown to be especially chemosensitive, and the PCV combination regimen is the treatment of choice for this disease. It is now well-established that the anti-VEGF antibody bevacizumab alone or in combination with chemotherapy has documented clinical activity in adult GBM.

Bevacizumab is presently approved as a single agent for adult GBM in the setting of progressive disease following first-line chemotherapy.

SECONDARY MALIGNANCIES & CANCER CHEMOTHERAPY The development of secondary malignancies is a late complication of the alkylating agents and the epipodophyllotoxin etoposide. For both drug classes, the most frequent secondary malignancy is acute myelogenous leukemia (AML). In general, AML develops in up to 15% of patients with Hodgkin’s lymphoma who have received radiotherapy plus MOPP chemotherapy and in patients with multiple myeloma, ovarian carcinoma, or breast carcinoma treated with melphalan. The increased risk of AML is observed as early as 2–4 years after the initiation of chemotherapy and typically peaks at 5 and 9 years. With improvements in the clinical efficacy of various combination chemotherapy regimens resulting in prolonged survival and in some cases actual cure of cancer, the issue of how second cancers may affect long-term survival assumes greater importance. There is already evidence that certain alkylating agents (eg, cyclophosphamide) may be less carcinogenic than others (eg, melphalan). In addition to AML, other secondary malignancies have been well-described, including non-Hodgkin’s lymphoma and bladder cancer, the latter most typically associated with cyclophosphamide therapy.

SUMMARY Anti-cancer Drugs See Tables 54–2, –3, –4, –5

PREPARATIONS AVAILABLE

REFERENCES Books & Monographs Abeloff MD et al: Clinical Oncology, 5th ed. Elsevier, 2014.

Barakat RR et al: Principles and Practice of Gynecologic Oncology, 5th ed. Lippincott Williams & Wilkins, 2009.

Chabner BA, Longo DL: Cancer Chemotherapy and Biotherapy: Principles and Practice, 5th ed. Lippincott Williams & Wilkins, 2011.

Chu E, DeVita VT Jr: Cancer Chemotherapy Drug Manual 2014, 14th ed. Jones & Bartlett, 2013.

DeVita VT Jr, Hellman S, Rosenberg SA: Cancer: Principles and Practice of Oncology, 9th ed. Lippincott Williams & Wilkins, 2011.

Harris JR et al: Diseases of the Breast, 4th ed. Lippincott Williams & Wilkins, 2009.

Hoppe R et al: Textbook of Radiation Oncology, 3rd ed. Elsevier, 2010.

Kantoff PW et al: Prostate Cancer: Principles and Practice. Lippincott Williams & Wilkins, 2001.

Kelsen DP et al: Gastrointestinal Oncology: Principles and Practices, 2nd ed. Lippincott Williams & Wilkins, 2007.

Kufe D et al: Cancer Medicine, 7th ed. BC Decker, 2006.

Mendelsohn J et al: The Molecular Basis of Cancer, 3rd ed. Saunders, 2008.

Pass HI et al: Principles and Practice of Lung Cancer: The Official Reference Text of the International Association for the Study of Lung Cancer (IASLC), 4th ed. Lippincott Williams & Wilkins, 2010.

Pizzo PA, Poplack AG: Principles and Practice of Pediatric Oncology, 6th ed. Lippincott Williams & Wilkins, 2010.

Weinberg RA: Biology of Cancer, 2nd ed. T aylor & Francis, 2013.

Articles & Reviews DeVita VT , Chu E: T he history of cancer chemotherapy. Cancer Res 2008;68:8643.

Redmond KM et al: Resistance mechanisms to cancer chemotherapy. Front Biosci 2008;13:5138.

CASE STUDY ANSWER The 5-year survival rate for patients with high-risk stage III CRC is on the order of 25â&#x20AC;&#x201C;30%. Because the patient has no symptoms after surgery and has no comorbid illnesses, he would be an appropriate candidate to receive aggressive adjuvant chemotherapy.

Adjuvant chemotherapy is usually begun 4â&#x20AC;&#x201C;6 weeks after surgery to allow sufficient time for the surgical wound to heal. The usual recommendation would be to administer 6 months of oxaliplatin-based chemotherapy using either infusional 5-FU or oral capecitabine as the fluoropyrimidine base in combination with oxaliplatin.

Patients with partial or complete deficiency in the enzyme dihydropyrimidine dehydrogenase (DPD) experience an increased incidence of severe toxicity to fluoropyrimidines in the form of myelosuppression, gastrointestinal toxicity in the form of mucositis and diarrhea, and neurotoxicity. Although mutations in DPD can be identified in peripheral blood mono-nuclear cells, nearly 50% of patients who exhibit severe 5-FU toxicity do not have a defined mutation in the DPD gene. In addition, such mutations may not result in reduced expression of the DPD protein or in altered enzymatic activity. For this reason, genetic testing is not recommended at this time as part of routine clinical practice. There is now an immunoassay that can measure 5-FU drug levels in the peripheral blood that can help guide 5-FU dosing even in patients with DPD deficiency.

CHAPTER

55 Immunopharmacology Douglas F. Lake, PhD & Adrienne D. Briggs, MD

CASE STUDY A 30-year-old woman has one living child, age 6. Her child and her husband are Rh positive and she is Rh o (D) and Du negative.

She is now in her ninth month of pregnancy and is in the labor room having frequent contractions. Her Rh antibody test taken earlier in the pregnancy was negative. What immunotherapy is appropriate for this patient? When and how should it be administered?

Agents that suppress the immune system play an important role in preventing the rejection of organ or tissue grafts and in the treatment of certain diseases that arise from dysregulation of the immune response. While precise details of the mechanisms of action of a number of these agents are still obscure, knowledge of the elements of the immune system is useful in understanding their effects. Agents that augment the immune response or selectively alter the balance of various components of the immune system are also becoming important in the management of certain diseases such as cancer, AIDS, and autoimmune or inflammatory diseases. A growing number of other conditions (infections, cardiovascular diseases, organ transplantation) may also be candidates for immune manipulation.

ELEMENTS OF THE IMMUNE SYSTEM NORMAL IMMUNE RESPONSES The immune system has evolved to protect the host from invading pathogens and to eliminate disease. When functioning at its best, the immune system is exquisitely responsive to invading pathogens while retaining the capacity to recognize self tissues and antigens to which it is tolerant. Protection from infection and disease is provided by the collaborative efforts of the innate and adaptive immune systems.

The Innate Immune System The innate immune system is the first line of defense against invading pathogens (eg, bacteria, viruses, fungi, parasites) and consists of mechanical, biochemical, and cellular components. Mechanical components include skin/epidermis and mucus; biochemical components include antimicrobial peptides and proteins (eg, defensins), complement, enzymes (eg, lysozyme, acid hydrolases), interferons, acidic pH, and free radicals (eg, hydrogen peroxide, superoxide anions); cellular components include neutrophils, monocytes, macrophages, natural killer (NK), and natural killer-T (NKT) cells. Unlike adaptive immunity, the innate immune response exists prior to infection, is not enhanced by repeated infection, and is generally not antigen-specific. An intact skin or mucosa is the first barrier to infection. When this barrier is breached, an immediate innate immune response, referred to as â&#x20AC;&#x153;inflammationâ&#x20AC;? is provoked that ultimately leads to destruction of the pathogen. The process of pathogen destruction can be accomplished, for example, by biochemical components such as lysozyme (which breaks down bacterial peptidoglycan cell walls) and complement activation. Complement components (Figure 55â&#x20AC;&#x201C;1) enhance macrophage and neutrophil phagocytosis by acting as opsonins (C3b) and chemoattractants (C3a, C5a), which recruit immune cells from the bloodstream to the site of infection. The activation of complement eventually leads to pathogen lysis via the generation of a membrane attack complex that creates holes in the pathogen membrane, killing it.

FIGURE 55–1 Role of complement in innate immunity. Complement is made up of nine proteins (C1–C9), which are split into fragments during activation. A: Complement components (C3a, C5a) attract phagocytes (1) to inflammatory sites (2), where they ingest and degrade pathogens (3). B: Complement components C5b, C6, C7, C8, and C9 associate to form a membrane attack complex (MAC) that lyses bacteria, causing their destruction. C: Complement component C3b is an opsonin that coats bacteria (1) and facilitates their ingestion (2) and digestion (3) by phagocytes.

During the inflammatory response triggered by infection, neutrophils and monocytes enter the tissue sites from the peripheral circulation. This cellular influx is mediated by the action of chemoattractant cytokines (chemokines) (eg, interleukin-8 [IL-8; CXCL8], macrophage chemotactic protein-1 [MCP-1; CCL2], and macrophage inflammatory protein-1α [MIP-1α CCL3]) released from activated endothelial cells and immune cells (mostly tissue macrophages) at the inflammatory site. Egress of the immune cells from blood vessels into the inflammatory site is mediated by adhesive interactions between cell surface receptors (eg, L-selectin, integrins) on the immune cells and ligands (eg, sialyl-Lewis x, intercellular adhesion molecule-1 [ICAM-1]) on the activated endothelial cell surface. The tissue macrophages as well as dendritic cells express pattern recognition receptors (PRRs) that include Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), scavenger receptors, mannose receptors, and

lipopolysaccharide (LPS)-binding protein, which recognize key evolutionarily conserved pathogen components referred to as pathogenassociated molecular patterns (PAMPs). Examples of PAMPs include microbe-derived unmethylated CpG DNA, flagellin, doublestranded RNA, peptidoglycan, and LPS. The PRRs recognize PAMPs in various components of pathogens and stimulate the release of proinflammatory cytokines, chemokines, and interferons. If the innate immune response is successfully executed, the invading pathogen is ingested, degraded, and eliminated, and disease is either prevented or is of short duration.

ACRONYMS

In addition to monocytes and neutrophils, natural killer (NK), natural killer-T (NKT), and gamma-delta T (γδT) cells recruited to the inflammatory site contribute to the innate response by secreting interferon-gamma (IFN-γ) and interleukin-17 (IL-17),* which activate resident tissue macrophages and dendritic cells and recruit neutrophils respectively to successfully eliminate invading pathogens.

NK cells are so called because they are able to recognize and destroy virus-infected normal cells as well as tumor cells without prior stimulation. This activity is regulated by “killer cell immunoglobulin-like receptors” (KIRs) on the NK cell surface that are specific for major histocompatibility complex (MHC) class I molecules. When NK cells bind self MHC class I proteins (expressed on all nucleated cells), these receptors deliver inhibitory signals, preventing them from killing normal host cells. Tumor cells or virus-infected cells that have down-regulated MHC class I expression do not engage these KIRs, resulting in activation of NK cells and subsequent destruction of the target cell. NK cells kill target cells by releasing cytotoxic granules such as perforins and granzymes that induce programmed cell death.

NKT cells express T-cell receptors as well as receptors commonly found on NK cells. NKT cells recognize microbial lipid antigens presented by a unique class of MHC-like molecules known as CD1 and have been implicated in host defense against microbial agents, autoimmune diseases, and tumors.

The Adaptive Immune System The adaptive immune system is mobilized by cues from the innate response when the innate processes are incapable of coping with an infection. The adaptive immune system has a number of characteristics that contribute to its success in eliminating pathogens. These include the ability to (1) respond to a variety of antigens, each in a specific manner; (2) discriminate between foreign (“non-self”) antigens (pathogens) and self antigens of the host; and (3) respond to a previously encountered antigen in a learned way by initiating a vigorous memory response. This adaptive response culminates in the production of antibodies, which are the effectors of humoral immunity; and the activation of T lymphocytes, which are the effectors of cell-mediated immunity.

The induction of specific adaptive immunity requires the participation of professional antigen-presenting cells (APCs), which include dendritic cells (DCs), macrophages, and B lymphocytes. These cells play pivotal roles in the induction of an adaptive immune response because of their capacity to phagocytize particulate antigens (eg, pathogens) or endocytose protein antigens, and enzymatically digest them to generate peptides, which are then loaded onto class I or class II MHC proteins and “presented” to the cell surface T-cell receptor (TCR) (Figure 55–2). CD8 T cells recognize class I-MHC peptide complexes while CD4 T cells recognize class II-MHC peptide complexes. At least two signals are necessary for the activation of T cells. The first signal is delivered following engagement of the TCR with peptide-bound MHC molecules. In the absence of a second signal, the T cells become unresponsive (anergic) or undergo apoptosis. The second signal involves binding of costimulatory molecules (CD40, CD80 [also known as B7-1], and CD86 [also known as B7-2]) on the APC to their respective ligands (CD40L for CD40, CD28 for CD80 or CD86). Activation of T cells is regulated via a negative feedback loop involving another molecule known as T-lymphocyte-associated antigen 4 (CTLA-4). Following engagement of CD28 with CD80 or CD86, CTLA-4 in the cytoplasm is mobilized to the cell surface where, because of its higher affinity of binding to CD80 and CD86, it outcompetes or displaces CD28 resulting in suppression of T-cell activation and proliferation. This property of CTLA-4 has been exploited as a strategy for sustaining a desirable immune response such as that directed against cancer. A recombinant humanized antibody (ipilimumab) that binds CTLA-4 prevents its association with CD80/CD86. In so doing, the activated state of T cells is sustained. Recently completed vaccine trials in metastatic melanoma patients receiving anti-CTLA-4 antibody reported objective and durable clinical responses in a few patients. Unfortunately, these beneficial responses were associated with the development of autoimmune toxicity in some patients, raising concern about this approach.

FIGURE 55â&#x20AC;&#x201C;2 T-cell activation by an antigen-presenting cell requires engagement of the T-cell receptor by the MHC-peptide complex (signal 1) and binding of the costimulatory molecules (CD80, CD86) on the dendritic cell to CD28 on the T cell (signal 2). The activation signals are strengthened by CD40/CD40L and ICAM-1/LFA-1 interactions. In a normal immune response, T-cell activation is regulated

by T-cell-derived CTLA-4, which binds to CD80 or CD86 with higher affinity than CD28 and sends inhibitory signals to the nucleus of the T cell.

T lymphocytes develop and learn to recognize self and non-self antigens in the thymus; those T cells that bind with high affinity to self antigens in the thymus undergo apoptosis (negative selection), while those that are capable of recognizing foreign antigens in the presence of self MHC molecules are retained and expanded (positive selection) for export to the periphery (lymph nodes, spleen, mucosa-associated lymphoid tissue, peripheral blood), where they become activated after encountering MHC-presented peptides (Figures 55â&#x20AC;&#x201C;2 and 55â&#x20AC;&#x201C;3).

FIGURE 55â&#x20AC;&#x201C;3 Scheme of cellular interactions during the generation of cell-mediated and humoral immune responses (see text). The

cell-mediated arm of the immune response involves the ingestion and digestion of antigen by antigen-presenting cells such as macrophages. Activated Th cells secrete IL-2, which causes proliferation and activation of cytotoxic T lymphocytes, and Th1 and Th2 cell subsets. Th1 cells also produce IFN-™ and TNF-™, which can directly activate macrophages and NK cells. The humoral response is triggered when B lymphocytes bind antigen via their surface immunoglobulin. They are then induced by Th2-derived IL-4 and IL-5 to proliferate and differentiate into memory cells and antibody-secreting plasma cells. Regulatory cytokines such as IFN-γ and IL-10 downregulate Th2 and Th1 responses, respectively.

Studies using murine T-cell clones have demonstrated the presence of two subsets of T helper lymphocytes (Th1 and Th2) based on the cytokines they secrete after activation. The TH1 subset characteristically produces IFN-γ, IL-2, and IL-12 and induces cell-mediated immunity by activation of macrophages, cytotoxic T cells (CTLs), and NK cells. The TH2 subset produces IL-4, IL-5, IL-6, and IL-10 (and sometimes IL-13), which induce B-cell proliferation and differentiation into antibody-secreting plasma cells. IL-10 produced by TH2 cells inhibits cytokine production by TH1 cells via the down-regulation of MHC expression by APCs. Conversely, IFN-γ produced by TH1 cells inhibits the proliferation of TH2 cells (Figure 55–3). Although these subsets have been well described in vitro, the nature of the antigenic challenge that elicits a TH1 or TH2 phenotype is less clear. Extracellular bacteria typically cause the elaboration of T H2 cytokines, culminating in the production of neutralizing or opsonic antibodies. In contrast, intracellular organisms (eg, mycobacteria) elicit the production of TH1 cytokines, which lead to the activation of effector cells such as macrophages. A less well-defined T-cell subset (TH3) has been described that produces transforming growth factor-β (TGF-β), whose numerous functions include down-regulation of proliferation and differentiation of T lymphocytes.

Recently, a subset of CD4 T cells that secrete IL-17 (T H17) has been implicated in neutrophil recruitment to sites of inflammation.

The regulatory T (Treg) cells comprise a population of CD4 T cells that is essential for preventing autoimmunity and allergy as well as maintaining homeostasis and tolerance to self antigens. This cell population exists as natural Treg (nTreg), derived directly from the thymus, and induced (adaptive) Treg (iTreg), generated from naïve CD4 T cells in the periphery. Both populations have also been shown to inhibit antitumor immune responses and are implicated in fostering tumor growth and progression. Recent attempts to distinguish both populations have resulted in the discovery of a transcription factor, Helios, in nTreg but not in iTreg cells.

CD8 T lymphocytes recognize endogenously processed peptides presented by virus-infected cells or tumor cells. These peptides are usually nine-amino-acid fragments derived from virus or protein tumor antigens in the cytoplasm and are loaded onto MHC class I molecules (Figure 55–2) in the endoplasmic reticulum. In contrast, class II MHC molecules present peptides (usually 11–22 amino acids) derived from extracellular (exogenous) pathogens to CD4 T helper cells. In some instances, exogenous antigens, upon ingestion by APCs, can be presented on class I MHC molecules to CD8 T cells. This phenomenon, referred to as “cross-presentation,” involves retro-translocation of antigens from the endosome to the cytosol for peptide generation in the proteosome and is thought to be useful in generating effective immune responses against infected host cells that are incapable of priming T lymphocytes. Upon activation, CD8 T cells induce target cell death via lytic granule enzymes (“granzymes”), perforin, and the Fas-Fas ligand (Fas-FasL) apoptosis pathways.

B lymphocytes undergo selection in the bone marrow, during which self-reactive B lymphocytes are clonally deleted while B-cell clones specific for foreign antigens are retained and expanded. The repertoire of antigen specificities by T cells is genetically determined and arises from T-cell receptor gene rearrangement while the specificities of B cells arise from immunoglobulin gene rearrangement; for both types of cells, these determinations occur prior to encounters with antigen. Upon an encounter with antigen, a mature B cell binds the antigen, internalizes and processes it, and presents its peptide—bound to class II MHC—to CD4 helper cells, which in turn secrete IL-4 and IL-5. These interleukins stimulate B-cell proliferation and differentiation into memory B cells and antibody-secreting plasma cells. The primary antibody response consists mostly of IgM-class immunoglobulins. Subsequent antigenic stimulation results in a vigorous “booster” response accompanied by class (isotype) switching to produce IgG, IgA, and IgE antibodies with diverse effector functions (Figure 55–3). These antibodies also undergo affinity maturation, which allows them to bind more efficiently to the antigen.

With the passage of time, this results in accelerated elimination of microorganisms in subsequent infections. Antibodies mediate their functions by acting as opsonins to enhance phagocytosis and cellular cytotoxicity and by activating complement to elicit an inflammatory response and induce bacterial lysis (Figure 55–4).

FIGURE 55â&#x20AC;&#x201C;4 Antibody has multiple functions. The prototypical antibody consists of two heavy (H) and two light (L) chains, each subdivided into constant (CL, CH) and variable (VL, VH) domains. The structure is held together by intra- and interchain disulfide bridges.

A: The complementarity-determining region (CDR) of the antigen-binding portion of the antibody engages the antigenic determinant (epitope) in a lock and key fashion. B: Antigen-antibody complexes activate complement to produce split complement components that

cause bacterial lysis. C: The Fc portion of antibodies binds to Fc receptors on phagocytes (eg, macrophages, neutrophils) and facilitates uptake of bacteria (opsonization).

ABNORMAL IMMUNE RESPONSES Whereas the normally functioning immune response can successfully neutralize toxins, inactivate viruses, destroy transformed cells, and eliminate pathogens, inappropriate responses can lead to extensive tissue damage (hypersensitivity) or reactivity against self antigens (autoimmunity); conversely, impaired reactivity to appropriate targets (immunodeficiency) may occur and abrogate essential defense mechanisms.

Hypersensitivity Hypersensitivity can be classified as antibody-mediated or cell-mediated. Three types of hypersensitivity are antibody-mediated (types I– III), while the fourth is cell-mediated (type IV). Hypersensitivity occurs in two phases: the sensitization phase and the effector phase.

Sensitization occurs upon initial encounter with an antigen; the effector phase involves immunologic memory and results in tissue pathology upon a subsequent encounter with that antigen.

1. Type I—Immediate, or type I, hypersensitivity is IgE-mediated, with symptoms usually occurring within minutes following the patient’s reencounter with antigen. Type I hypersensitivity results from cross-linking of membrane-bound IgE on blood basophils or tissue mast cells by antigen. This cross-linking causes cells to degranulate, releasing substances such as histamine, leukotrienes, and eosinophil chemotactic factor, which induce anaphylaxis, asthma, hay fever, or urticaria (hives) in affected individuals ( Figure 55–5). A severe type I hypersensitivity reaction such as systemic anaphylaxis (eg, from insect envenomation, ingestion of certain foods, or drug hypersensitivity) requires immediate medical intervention.

FIGURE 55–5 Mechanism of type I hypersensitivity. Initial exposure to allergen (sensitization phase) leads to production of IgE by plasma cells differentiated from allergen-specific B cells (not shown). The secreted IgE binds IgE-specific receptors (Fc?R) on blood basophils and tissue mast cells. Re-exposure to allergen leads to cross-linking of membrane-bound IgE (effector phase). This crosslinking causes degranulation of cytoplasmic granules and release of mediators that induce vasodilation, smooth muscle contraction, and increased vascular permeability. These effects lead to the clinical symptoms characteristic of type I hypersensitivity.

2. Type II—Type II hypersensitivity results from the formation of antigen-antibody complexes between foreign antigen and IgM or IgG immunoglobulins. One example of this type of hypersensitivity is a blood transfusion reaction that can occur if blood is not cross-matched properly. Preformed antibodies bind to red blood cell membrane antigens that activate the complement cascade, generating a membrane attack complex that lyses the transfused red blood cells. In hemolytic disease of the newborn, anti-Rh IgG antibodies produced by an Rhnegative mother cross the placenta, bind to red blood cells of an Rh-positive fetus, and damage them. The disease is prevented in subsequent pregnancies by the administration of anti-Rh antibodies to the mother 24–48 hours after delivery (see Immunosuppressive

Antibodies, below). Type II hypersensitivity can also be drug-induced and may occur during the administration of penicillin (for example) to allergic patients. In these patients, penicillin binds to red blood cells or other host tissue to form a neoantigen that evokes production of antibodies capable of inducing complement-mediated red cell lysis. In some circumstances, subsequent administration of the drug can lead to systemic anaphylaxis (type I hypersensitivity).

3. Type III—Type III hypersensitivity is due to the presence of elevated levels of antigen-antibody complexes in the circulation that ultimately deposit on basement membranes in tissues and vessels. Immune complex deposition activates complement to produce components with anaphylatoxic and chemotactic activities (C5a, C3a, C4a) that increase vascular permeability and recruit neutrophils to the site of complex deposition. Complex deposition and the action of lytic enzymes released by neutrophils can cause skin rashes, glomerulonephritis, and arthritis in these individuals. If patients have type III hypersensitivity against a particular antigen, clinical symptoms usually occur 3–4 days after exposure to the antigen.

4. Type IV: Delayed-type hypersensitivity —Unlike type I, II, and III hypersensitivities, delayed-type hypersensitivity (DTH) is cellmediated, and responses occur 2–3 days after exposure to the sensitizing antigen. DTH is caused by antigen-specific DTH TH1 cells and induces a local inflammatory response that causes tissue damage characterized by the influx of antigen-nonspecific inflammatory cells, especially macrophages. These cells are recruited under the influence of TH1-produced cytokines (Figure 55–6), which chemoattract circulating monocytes and neutrophils, induce myelopoiesis, and activate macrophages. The activated macrophages are primarily responsible for the tissue damage associated with DTH. Although widely considered to be deleterious, DTH responses are very effective in eliminating infections caused by intracellular pathogens such as Mycobacterium tuberculosis and Leishmania species. Clinical manifestations of DTH include tuberculin and contact hypersensitivities. Tuberculosis exposure is determined using a DTH skin test.

Positive responses show erythema and induration caused by accumulation of macrophages and DTH T (TDT H) cells at the site of the tuberculin injection. Poison ivy is the most common cause of contact hypersensitivity, in which pentadecacatechol, the lipophilic chemical in poison ivy, modifies cellular tissue and results in a DTH T-cell response.

FIGURE 55–6 Mechanism of type IV hypersensitivity (DTH). In the sensitization phase, the processed allergen (eg, from poison ivy) is presented to CD4 Th1 cells by antigen-presenting cells in association with class II MHC. T cells are induced to express IL-2 receptors and are stimulated to proliferate and differentiate into memory TDT H cells. Secondary contact with antigen triggers the effector phase, in which memory TDT H cells release cytokines that attract and activate nonspecific inflammatory macrophages and neutrophils. These

cells display increased phagocytic and microbicidal activities and release large quantities of lytic enzymes that cause extensive tissue damage.

Autoimmunity Autoimmune disease arises when the body mounts an immune response against itself due to failure to distinguish self tissues and cells from foreign (nonself) antigens or loss of tolerance to self. This phenomenon derives from the activation of self-reactive T and B lymphocytes that generate cell-mediated or humoral immune responses directed against self antigens. The pathologic consequences of this reactivity constitute several types of autoimmune diseases. Autoimmune diseases are highly complex due to MHC genetics, environmental conditions, infectious entities, and dysfunctional immune regulation. Examples of such diseases include rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, and insulin-dependent diabetes mellitus (type 1 diabetes). In rheumatoid arthritis, IgM antibodies (rheumatoid factors) are produced that react with the Fc portion of IgG and may form immune complexes that activate the complement cascade, causing chronic inflammation of the joints and kidneys. In systemic lupus erythematosus, antibodies are made against DNA, histones, red blood cells, platelets, and other cellular components. In multiple sclerosis and type 1 diabetes, cell-mediated autoimmune attack destroys myelin surrounding nerve cells and insulin-producing islet beta cells of the pancreas, respectively. In type 1 diabetes, activated CD4 TDT H cells that infiltrate the islets of Langerhans and recognize self islet beta cell peptides are thought to produce cytokines that stimulate macrophages to produce lytic enzymes, which destroy islet beta cells. Autoantibodies directed against the islet beta cell antigens are produced, but do not contribute significantly to disease.

A number of mechanisms have been proposed to explain autoimmunity: 1. Exposure of antigens previously sequestered from the immune system (eg, lens protein, myelin basic protein) to self-reactive T lymphocytes.

2. Molecular mimicry by invading pathogens, in which immune responses are directed at antigenic determinants on pathogens that share identical or similar epitopes with normal host tissue. This phenomenon occurs in rheumatic fever following Streptococcus pyogenes infection, in which heart damage is thought to arise from an immune response directed against streptococcal antigens shared with heart muscle. The suggested viral etiology of autoimmune diseases has been ascribed to immune responses (both cell-mediated and humoral) directed against virus epitopes that mimic self antigens.

3. Inappropriate expression of class II MHC molecules on the membranes of cells that normally do not express class II MHC (eg, islet beta cells). Increased expression of MHC II may increase presentation of self peptides to T helper cells, which in turn induce CTL, TDT H, and B-lymphocyte cells that react against self antigens.

Immunodeficiency Diseases Immunodeficiency diseases result from inadequate function in the immune system; the consequences include increased susceptibility to infections and prolonged duration and severity of disease. Immunodeficiency diseases are either congenital or arise from extrinsic factors such as bacterial or viral infections or drug treatment. Affected individuals frequently succumb to infections caused by opportunistic organisms of low pathogenicity for the immunocompetent host. Examples of congenitally acquired immunodeficiency diseases include Xlinked agammaglobulinemia, DiGeorgeâ&#x20AC;&#x2122;s syndrome, and severe combined immunodeficiency disease (SCID) due to adenosine deaminase (ADA) deficiency.

X-linked agammaglobulinemia is a disease affecting males that is characterized by a failure of immature B lymphocytes to mature into antibody-producing plasma cells. These individuals are susceptible to recurrent bacterial infections, although the cell-mediated responses directed against viruses and fungi are preserved. DiGeorgeâ&#x20AC;&#x2122;s syndrome is due to failure of the thymus to develop, resulting in diminished T-cell responses (TDT H, CTL), while the humoral response remains functional, but does not benefit from T-cell help.

The ADA enzyme normally prevents the accumulation of toxic deoxy-ATP in cells. Deoxy-ATP is particularly toxic to lymphocytes, and leads to death of T and B cells. Absence of the enzyme therefore results in SCID. Infusion of the purified enzyme (pegademase, from bovine sources) and transfer of ADA gene-modified lymphocytes have both been used successfully to treat this disease.

AIDS represents the classic example of immunodeficiency disease caused by extrinsic viral infection, in this instance the human immunodeficiency virus (HIV). This virus exhibits a strong tropism for CD4 T helper cells; these become depleted, giving rise to increased frequency of opportunistic infections and malignancies in infected individuals. AIDS is also characterized by an imbalance in TH1 and TH2 cells, and the ratios of cells and their functions are skewed toward TH2. This results in loss of cytotoxic T-lymphocyte activity, loss of delayed hypersensitivity, and hypergammaglobulinemia.

IMMUNOSUPPRESSIVE THERAPY Immunosuppressive agents have proved very useful in minimizing the occurrence or impact of deleterious effects of exaggerated or inappropriate immune responses. Unfortunately, these agents also have the potential to cause disease and to increase the risk of infection

and malignancies.

GLUCOCORTICOIDS Glucocorticoids (corticosteroids) were the first hormonal agents recognized as having lympholytic properties. Administration of any glucocorticoid reduces the size and lymphoid content of the lymph nodes and spleen, although it has no toxic effect on proliferating myeloid or erythroid stem cells in the bone marrow.

Glucocorticoids are thought to interfere with the cell cycle of activated lymphoid cells. The mechanism of their action is described in Chapter 39. Glucocorticoids are quite cytotoxic to certain subsets of T cells, but their immunologic effects are probably due to their ability to modify cellular functions rather than to direct cytotoxicity. Although cellular immunity is more affected than humoral immunity, the primary antibody response can be diminished, and with continued use, previously established antibody responses are also decreased.

Additionally, continuous administration of corticosteroid increases the fractional catabolic rate of IgG, the major class of antibody immunoglobulins, thus lowering the effective concentration of specific antibodies. Contact hypersensitivity mediated by DTH T cells, for example, is usually abrogated by glucocorticoid therapy.

Glucocorticoids are used in a wide variety of conditions (Table 55â&#x20AC;&#x201C;1). It is thought that the immunosuppressive and anti-inflammatory properties of corticosteroids account for their beneficial effects in diseases like idiopathic thrombocytopenic purpura and rheumatoid arthritis. Glucocorticoids modulate allergic reactions and are useful in the treatment of diseases like asthma or as premedication for other agents (eg, blood products, chemotherapy) that might cause undesirable immune responses. Glucocorticoids are first-line immunosuppressive therapy for both solid organ and hematopoietic stem cell transplant recipients, with variable results. The toxicities of long-term glucocorticoid therapy can be severe and are discussed in Chapter 39.

TABLE 55â&#x20AC;&#x201C;1 Clinical uses of immunosuppressive agents.

CALCINEURIN INHIBITORS Cyclosporine Cyclosporine (cyclosporin A, CSA) is an immunosuppressive agent with efficacy in human organ transplantation, in the treatment of graft-versus-host (GVH) disease after hematopoietic stem cell transplantation, and in the treatment of selected autoimmune disorders.

Cyclosporine is a peptide antibiotic that appears to act at an early stage in the antigen receptor-induced differentiation of T cells and blocks their activation. Cyclosporine binds to cyclophilin, a member of a class of intracellular proteins called immunophilins. Cyclosporine and cyclophilin form a complex that inhibits the cytoplasmic phosphatase, calcineurin, which is necessary for the activation of a T-cellspecific transcription factor. This transcription factor, NF-AT, is involved in the synthesis of interleukins (eg, IL-2) by activated T cells.

In vitro studies have indicated that cyclosporine inhibits the gene transcription of IL-2, IL-3, IFN-γ, and other factors produced by antigen-stimulated T cells, but it does not block the effect of such factors on primed T cells nor does it block interaction with antigen.

Cyclosporine may be given intravenously or orally, though it is slowly and incompletely absorbed (20–50%). The absorbed drug is primarily metabolized by the P450 3A enzyme system in the liver with resultant multiple drug interactions. This propensity for drug interactions contributes to significant interpatient variability in bioavailability, such that cyclosporine requires individual patient dosage adjustments based on steady-state blood levels and the desired therapeutic ranges for the drug. Cyclosporine ophthalmic solution is now available for severe dry eye syndrome, as well as ocular GVH disease. Inhaled cyclosporine is being investigated for use in lung transplantation.

Toxicities are numerous and include nephrotoxicity, hypertension, hyperglycemia, liver dysfunction, hyperkalemia, altered mental status, seizures, and hirsutism. Cyclosporine causes very little bone marrow toxicity. While an increased incidence of lymphoma and other cancers (Kaposi’s sarcoma, skin cancer) have been observed in transplant recipients receiving cyclosporine, other immunosuppressive agents may also predispose recipients to cancer. Some evidence suggests that tumors may arise after cyclosporine treatment because the drug induces TGF-β, which promotes tumor invasion and metastasis.

Cyclosporine may be used alone or in combination with other immunosuppressants, particularly glucocorticoids. It has been used successfully as the sole immunosuppressant for cadaveric transplantation of the kidney, pancreas, and liver, and it has proved extremely useful in cardiac transplantation as well. In combination with methotrexate, cyclosporine is a standard prophylactic regimen to prevent GVH disease after allogeneic stem cell transplantation. Cyclosporine has also proved useful in a variety of autoimmune disorders, including uveitis, rheumatoid arthritis, psoriasis, and asthma. Its combination with newer agents is showing considerable efficacy in clinical and experimental settings where effective and less toxic immunosuppression is needed. Newer formulations of cyclosporine have been developed that are improving patient compliance (smaller, better tasting pills), and increasing bioavailability.

Tacrolimus Tacrolimus (FK 506) is an immunosuppressant macrolide antibiotic produced by Streptomyces tsukubaensis. It is not chemically related to cyclosporine, but their mechanisms of action are similar. Both drugs bind to cytoplasmic peptidylprolyl isomerases that are abundant in all tissues. While cyclosporine binds to cyclophilin, tacrolimus binds to the immunophilin FK-binding protein (FKBP). Both complexes inhibit calcineurin, which is necessary for the activation of the T-cell-specific transcription factor NF-AT. On a weight basis, tacrolimus is 10–100 times more potent than cyclosporine in inhibiting immune responses. Tacrolimus is utilized for the same indications as cyclosporine, particularly in organ and stem cell transplantation. Multicenter studies in the USA and in Europe indicate that both graft and patient survival are similar for the two drugs. Tacrolimus has proven to be effective therapy for preventing rejection in solid-organ transplant patients even after failure of standard rejection therapy, including anti-T-cell antibodies. It is now considered a standard prophylactic agent (usually in combination with methotrexate or mycophenolate mofetil) for GVH disease.

Tacrolimus can be administered orally or intravenously. The half-life of the intravenous form is approximately 9–12 hours. Like cyclosporine, tacrolimus is metabolized primarily by P450 enzymes in the liver, and there is potential for drug interactions. The dosage is determined by trough blood level at steady state. Its toxic effects are similar to those of cyclosporine and include nephrotoxicity, neurotoxicity, hyperglycemia, hypertension, hyperkalemia, and gastrointestinal complaints.

Because of the effectiveness of systemic tacrolimus in some dermatologic diseases, a topical preparation is now available.

Tacrolimus ointment is currently used in the therapy of atopic dermatitis and psoriasis.

PROLIFERATION SIGNAL INHIBITORS A new class of immunosuppressive agents called proliferation-signal inhibitors (PSIs) includes sirolimus (rapamycin) and its derivative everolimus. The mechanism of action of PSIs differs from that of the calcineurin inhibitors. PSIs bind the circulating immunophilin FK506-binding protein 12, resulting in an active complex that blocks the molecular target of rapamycin (mTOR). The mTOR is a key component of a complex intracellular signaling pathway involved in cellular processes such as cell growth and proliferation, angiogenesis, and metabolism. Thus, blockade of mTOR ultimately can lead to inhibition of interleukin-driven T-cell proliferation. Both everolimus and sirolimus also may inhibit B-cell proliferation and immunoglobulin production.

Sirolimus is available only as an oral drug. Its half-life is about 60 hours, while that of everolimus is about 43 hours. Both drugs are rapidly absorbed and elimination is similar to that of cyclosporine and tacrolimus, being substrates for both cytochrome P450 3A and Pglycoprotein. Hence, significant drug interactions can occur. For example, use with cyclosporine can increase the plasma levels of both sirolimus and everolimus such that drug levels need to be monitored. Target dose-ranges of these drugs vary depending on clinical use.

Sirolimus has been used effectively alone and in combination with other immunosuppressants (corticosteroids, cyclosporine, tacrolimus, and mycophenolate mofetil) to prevent rejection of solid organ allografts. It is used as prophylaxis and as therapy for steroidrefractory acute and chronic GVH disease in hematopoietic stem cell transplant recipients. Topical sirolimus is also used in some dermatologic disorders and, in combination with cyclosporine, in the management of uveoretinitis. Recently, sirolimus-eluting coronary stents have been shown to reduce restenosis and additional adverse cardiac events in patients with severe coronary artery disease, due to the drug’s antiproliferative effects. Everolimus is a newer drug that has shown clinical efficacy similar to sirolimus in solid organ transplant recipients; it is under investigation as an additional therapeutic agent for the treatment of chronic cardiac allograft vasculopathy.

Toxicities of the PSIs can include profound myelosuppression (especially thrombocytopenia), hepatotoxicity, diarrhea, hypertriglyceridemia, pneumonitis, and headache. Because nephrotoxicity is of major concern when administering calcineurin inhibitors, and since renal toxicity is less common with PSIs, there is interest in increased early use of the latter agents. However, increased use in stem cell transplantation regimens as GVH disease prophylaxis, particularly when combined with tacrolimus, has revealed an increased incidence of hemolytic-uremic syndrome.

MYCOPHENOLATE MOFETIL Mycophenolate mofetil (MMF) is a semisynthetic derivative of mycophenolic acid, isolated from the mold Penicillium glaucus. In vitro, it inhibits T- and B-lymphocyte responses, including mitogen and mixed lymphocyte responses, probably by inhibition of de novo synthesis of purines. Mycophenolate mofetil is hydrolyzed to mycophenolic acid, the active immunosuppressive moiety; it is synthesized and administered as MMF to enhance bioavailability.

Mycophenolate mofetil is available in both oral and intravenous forms. The oral form is rapidly metabolized to mycophenolic acid.

Although the cytochrome P450 3A system is not involved, some drug interactions still occur. Plasma drug levels are frequently monitored.

Mycophenolate mofetil is used in solid organ transplant patients for refractory rejection and, in combination with prednisone, as an alternative to cyclosporine or tacrolimus in patients who do not tolerate those drugs. Its antiproliferative properties make it the first-line drug for preventing or reducing chronic allograft vasculopathy in cardiac transplant recipients. Mycophenolate mofetil is used as prophylaxis for and treatment of both acute and chronic GVH disease in hematopoietic stem cell transplant patients. Newer immunosuppressant applications for MMF include lupus nephritis, rheumatoid arthritis, inflammatory bowel disease, and some dermatologic disorders.

Toxicities include gastrointestinal disturbances (nausea and vomiting, diarrhea, abdominal pain) headache, hypertension, and reversible myelosuppression (primarily neutropenia).

THALIDOMIDE Thalidomide is an oral sedative drug that was withdrawn from the market in the 1960s because of disastrous teratogenic effects when used during pregnancy. Nevertheless, it has significant immunomodulatory actions and is currently in active use or in clinical trials for over 40 different illnesses. Thalidomide inhibits angiogenesis and has anti-inflammatory and immunomodulatory effects. It inhibits tumor necrosis factor-alpha (TNF-α), reduces phagocytosis by neutrophils, increases production of IL-10, alters adhesion molecule expression, and enhances cell-mediated immunity via interactions with T cells. The complex actions of thalidomide continue to be studied as its clinical use evolves.

Thalidomide is currently used in the treatment of multiple myeloma at initial diagnosis and for relapsed-refractory disease. Patients generally show signs of response within 2–3 months of starting the drug, with response rates of 20–70%. When combined with dexamethasone, the response rates in myeloma are 90% or more in some studies. Many patients have durable responses—up to 12–18 months in refractory disease and even longer in some patients treated at diagnosis. The success of thalidomide in myeloma has led to numerous clinical trials in other diseases such as myelodysplastic syndrome, acute myelogenous leukemia, and GVH disease, as well as in solid tumors like colon cancer, renal cell carcinoma, melanoma, and prostate cancer, with variable results to date. Thalidomide has been used for many years in the treatment of some manifestations of leprosy and has been reintroduced in the USA for erythema nodosum leprosum; it is also useful in management of the skin manifestations of lupus erythematosus.

The adverse effect profile of thalidomide is extensive. The most important toxicity is teratogenesis. Because of this effect, thalidomide prescription and use is closely regulated by the manufacturer. Other adverse effects of thalidomide include peripheral neuropathy, constipation, rash, fatigue, hypothyroidism, and increased risk of deep vein thrombosis. Thrombosis is sufficiently frequent, particularly in the hematologic malignancy population, that most patients are placed on some type of anticoagulant when thalidomide

treatment is initiated.

Owing to thalidomide’s serious toxicity profile, considerable effort has been expended in the development of analogs.

Immunomodulatory derivatives of thalidomide are termed IMiDs. Some IMiDs are much more potent than thalidomide in regulating cytokines and affecting T-cell proliferation. Lenalidomide is an oral IMiD that in animal and in vitro studies has been shown to be similar to thalidomide in action, but with less toxicity, especially teratogenicity. Lenalidomide was approved by the FDA when trials showed its effectiveness in the treatment of the myelodysplastic syndrome with the chromosome 5q31 deletion. Clinical trials using lenalidomide to treat multiple myeloma showed similar efficacy, leading to approval for both primary and relapsed/refractory myeloma.

Pomalidomide (originally called CC-4047) is the newest oral IMiD to be FDA approved. Like the other IMiDs, it has myriad mechanisms of actions including antiangiogenic activity, inhibition of TNF-α, and stimulation of apoptosis and cytotoxic T-cell activity.

Most clinical trials of pomalidomide have targeted patients with relapsed/refractory multiple myeloma, for which the FDA approved the drug in 2013. Both lenalidomide and pomalidomide have side effect profiles similar to thalidomide.

CYTOTOXIC AGENTS Azathioprine Azathioprine is a prodrug of mercaptopurine and, like mercaptopurine, functions as an antimetabolite (see Chapter 54). Although its action is presumably mediated by conversion to mercaptopurine and further metabolites, it has been more widely used than mercaptopurine for immunosuppression in humans. These agents represent prototypes of the antimetabolite group of cytotoxic immunosuppressive drugs, and many other agents that kill proliferative cells appear to work at a similar level in the immune response.

Azathioprine is well absorbed from the gastrointestinal tract and is metabolized primarily to mercaptopurine. Xanthine oxidase converts much of the active material to 6-thiouric acid prior to excretion in the urine. After administration of azathioprine, small amounts of unchanged drug and mercaptopurine are also excreted by the kidney, and as much as a twofold increase in toxicity may occur in anephric or anuric patients. Since much of the drug’s inactivation depends on xanthine oxidase, patients who are also receiving allopurinol (see Chapters 36 and 54) for control of hyperuricemia should have the dose of azathioprine reduced to one-fourth to one-third the usual amount to prevent excessive toxicity.

Azathioprine and mercaptopurine appear to produce immunosuppression by interfering with purine nucleic acid metabolism at steps that are required for the wave of lymphoid cell proliferation that follows antigenic stimulation. The purine analogs are thus cytotoxic agents that destroy stimulated lymphoid cells. Although continued messenger RNA synthesis is necessary for sustained antibody synthesis by plasma cells, these analogs appear to have less effect on this process than on nucleic acid synthesis in proliferating cells.

Cellular immunity as well as primary and secondary serum antibody responses can be blocked by these agents.

Azathioprine and mercaptopurine appear to be of definite benefit in maintaining renal allografts and may be of value in transplantation of other tissues. These antimetabolites have also been used with some success in the management of acute glomerulonephritis, in the renal component of systemic lupus erythematosus, and in some cases of rheumatoid arthritis, Crohn’s disease, and multiple sclerosis. The drugs have been of occasional use in prednisone-resistant antibody-mediated idiopathic thrombocytopenic purpura and autoimmune hemolytic anemias.

The chief toxic effect of azathioprine and mercaptopurine is bone marrow suppression, usually manifested as leukopenia, although anemia and thrombocytopenia may occur. Skin rashes, fever, nausea and vomiting, and sometimes diarrhea occur, with the gastrointestinal symptoms seen mainly at higher dosages. Hepatic dysfunction, manifested by very high serum alkaline phosphatase levels and mild jaundice, occurs occasionally, particularly in patients with preexisting hepatic dysfunction.

Cyclophosphamide The alkylating agent cyclophosphamide is one of the most efficacious immunosuppressive drugs available. Cyclophosphamide destroys proliferating lymphoid cells (see Chapter 54) but also appears to alkylate some resting cells. It has been observed that very large doses (eg, > 120 mg/kg intravenously over several days) may induce an apparent specific tolerance to a new antigen if the drug is administered simultaneously with, or shortly after, the antigen. In smaller doses, it has been effective against autoimmune disorders (including systemic lupus erythematosus) and in patients with acquired factor XIII antibodies and bleeding syndromes, autoimmune hemolytic anemia, antibody-induced pure red cell aplasia, and Wegener’s granulomatosis.

Treatment with large doses of cyclophosphamide carries considerable risk of pancytopenia and therefore is generally combined with stem cell rescue (transplant) procedures. Although cyclophosphamide appears to induce tolerance for marrow or immune cell grafting, its use does not prevent the subsequent GVH syndrome, which may be serious or lethal if the donor is a poor histocompatibility match (despite the severe immunosuppression induced by high doses of cyclophosphamide). The drug may also cause hemorrhagic cystitis, which can be prevented or treated with mesna. Other adverse effects of cyclophosphamide include nausea, vomiting, cardiac toxicity, and electrolyte disturbances.

Pyrimidine Synthesis Inhibitors Leflunomide is a prodrug of an inhibitor of pyrimidine synthesis. Teriflunomide is the principal active metabolite of leflunomide. They both reversibly inhibit the mitochondrial enzyme dihydroorotate dehydrogenase, which is involved in pyrimidine synthesis and ultimately results in decreased lymphocyte activation. They have anti-inflammatory activity in addition to immunomodulatory properties.

Leflunomide is orally active, and the active metabolite has a long half-life of several weeks. Thus, the drug should be started with a loading dose, but it can be taken once daily after reaching steady state. It is approved only for rheumatoid arthritis at present, though studies are underway combining leflunomide with mycophenolate mofetil for a variety of autoimmune and inflammatory skin disorders, as well as preservation of allografts in solid organ transplantation. Leflunomide also appears (from murine data) to have antiviral activity.

Toxicities include elevation of liver enzymes with some risk of liver damage and renal impairment. Patients with severe liver disease should not receive leflunomide. This drug is teratogenic and contraindicated in pregnancy. A low frequency of cardiovascular effects (angina, tachycardia) has been reported.

Teriflunomide is FDA-approved for the treatment of relapsing-remitting multiple sclerosis. Although immunomodulatory, its exact mechanism of action in the treatment of multiple sclerosis is unclear. It is hypothesized to decrease the number of activated lymphocytes in the central nervous system. It is a once-daily oral drug that, unlike leflunomide, does not require a loading dose. Teriflunomideâ&#x20AC;&#x2122;s side effect profile is similar to that of leflunomide and it is contraindicated in pregnancy and severe liver disease. The incidence of neutropenia in patients taking the drug is 15%, and 10% of patients have a decrease in platelet counts.

Hydroxychloroquine Hydroxychloroquine is an antimalarial agent with immunosuppressant properties. It is thought to suppress intracellular antigen processing and loading of peptides onto MHC class II molecules by increasing the pH of lysosomal and endosomal compartments, thereby decreasing T-cell activation.

Because of these immunosuppressant activities, hydroxychloroquine is used to treat some autoimmune disorders (see Chapter 36), eg, rheumatoid arthritis and systemic lupus erythematosus. It has also been used to both treat and prevent GVH disease after allogeneic stem cell transplantation.

Other Cytotoxic Agents Other cytotoxic agents, including methotrexate, vincristine, and cytarabine (see Chapter 54), also have immunosuppressive properties. Methotrexate has been used extensively in rheumatoid arthritis (see Chapter 36) and in the treatment of GVH disease.

Although the other agents can be used for immunosuppression, their use has not been as widespread as the purine antagonists, and their indications for immunosuppression are less certain. The use of methotrexate (which can be given orally) appears reasonable in patients with idiosyncratic reactions to purine antagonists. The antibiotic dactinomycin has also been used with some success at the time of impending renal transplant rejection. Vincristine appears to be quite useful in idiopathic thrombocytopenic purpura refractory to prednisone. The related vinca alkaloid vinblastine has been shown to prevent mast cell degranulation in vitro by binding to microtubule units within the cell and to prevent release of histamine and other vasoactive compounds.

Pentostatin is an adenosine deaminase inhibitor that has been used mainly as an antineoplastic agent for lymphoid malignancies; it produces a profound lymphopenia. It is now frequently used for steroid-resistant GVH disease after allogeneic stem cell transplantation, as well as in preparative regimens prior to those transplants to provide severe immunosuppression to prevent allograft rejection.

Miscellaneous Agents Three other FDA-approved immunomodulators are used exclusively in the treatment of relapsing remitting multiple sclerosis.

Dimethyl fumarate (DMF) is the methyl ester of fumaric acid. Its exact mechanism of action is unknown, though it appears to activate the nuclear factor (erythroid-derived)-like-2 (NFR-2) transcriptional pathway. Activation of the NFR-2 pathway results in reduction of the oxidative stress that contributes to demyelination; it also appears to help protect the nerve cells from inflammation. DMF is given orally. Lymphopenia may be significant, so blood counts must be monitored regularly and the drug may be withheld if active infection is present. Flushing is common with treatment initiation and usually improves with time. Other less common adverse effects include nausea, diarrhea, abdominal pain, increased hepatic enzymes, and eosinophilia.

Glatiramer acetate (GA) is a mixture of synthetic polypeptides and four amino acids (L-glutamic acid, L-alanine, L-lysine and Ltyrosine) in a fixed molar ratio. Its mechanism of immunomodulation in multiple sclerosis is unknown. Studies suggest that GA downregulates the immune response to myelin antigens by induction and activation of suppressor T-cells that migrate to the central nervous system. It is given as a subcutaneous injection (not intravenously) in variable dosages and schedules. Toxicities include skin hypersensitivity, and rarely lipoatrophy and skin necrosis at the injection site. Other adverse effects include flushing, chest pain, dyspnea, throat constriction, and palpitations, all of which are usually mild and self-limited.

Fingolimod hydrochloride (FH) is an orally active sphingosine 1-phosphate (S1P) receptor modulator that is derived from the

fungal metabolite myriocin. The S1P receptor (subtype 1) controls the release of lymphocytes from lymph nodes and the thymus. FH is metabolized to fingolimod phosphate, which subsequently binds the S1P receptor and ultimately decreases circulating lymphocyte numbers in the periphery and central nervous system. S1P receptors are also expressed on neurons, such that FH may also be affecting neurodegeneration, gliosis, and endogenous repair mechanisms as well as resulting in lymphopenia to modify disease activity in multiple sclerosis. FH can cause serious cardiac toxicity including bradycardia, prolongation of the QT interval, and other arrhythmias. Because of these potential complications, the drug requires cardiac monitoring for 6 hours after the first dose is given. FH is contraindicated in patients with preexisting conditions such as type II or III heart block, prolonged QTc, recent myocardial infarction, or heart failure. Less common adverse effects include macular edema, elevated hepatic enzymes, headache, diarrhea, and cough. The drug is metabolized primarily by the cytochrome P450 system; thus caution is needed when it is used in combination with other drugs metabolized in the same manner.

IMMUNOSUPPRESSIVE ANTIBODIES The development of hybridoma technology by Milstein and Köhler in 1975 revolutionized the antibody field and radically increased the purity and specificity of antibodies used in the clinic and for diagnostic tests in the laboratory. Hybridomas are B cells fused to immortal plasmacytoma cells that secrete monoclonal antibodies specific for a target antigen. Large-scale hybridoma culture facilities are employed by the pharmaceutical industry to produce diagnostic and clinical grade monoclonal antibodies.

More recently, molecular biology has been used to develop monoclonal antibodies. Combinatorial libraries of cDNAs encoding immunoglobulin heavy and light chains expressed on bacteriophage surfaces are screened against purified antigens. The result is an antibody fragment with specificity and high affinity for the antigen of interest. This technique has been used to develop antibodies specific for viruses (eg, HIV), bacterial proteins, tumor antigens, and even cytokines. Several antibodies developed in this manner are FDAapproved for use in humans.

Other genetic engineering techniques involve production of chimeric and humanized versions of murine monoclonal antibodies in order to reduce their antigenicity and increase the half-life of the antibody in the patient. Murine antibodies administered as such to human patients elicit production of human antimouse antibodies (HAMAs), which clear the original murine proteins very rapidly. Humanization involves replacing most of the murine antibody with equivalent human regions while keeping only the variable, antigen-specific regions intact. Chimeric mouse-human antibodies have similar properties with less complete replacement of the murine components. The current naming convention for these engineered substances uses the suffix “-umab” or “-zumab” for humanized antibodies, and “-imab” or “ximab” for chimeric products. These procedures have been successful in reducing or preventing HAMA production for many of the antibodies discussed below.

Antilymphocyte & Antithymocyte Antibodies, & Chimeric Molecules Antisera directed against lymphocytes have been prepared sporadically for over 100 years. With the advent of human organ transplantation as a realistic therapeutic option, heterologous antilymphocyte globulin (ALG) took on new importance. ALG and antithymocyte globulin (ATG) are now in clinical use in many medical centers, especially in transplantation programs. The antiserum is usually obtained by immunization of horses, sheep, or rabbits with human lymphoid cells.

ALG acts primarily on the small, long-lived peripheral lymphocytes that circulate between the blood and lymph. With continued administration, “thymus-dependent” (T) lymphocytes from lymphoid follicles are also depleted, as they normally participate in the recirculating pool. As a result of the destruction or inactivation of T cells, an impairment of delayed hypersensitivity and cellular immunity occurs while humoral antibody formation remains relatively intact. ALG and ATG are useful for suppressing certain major compartments (ie, T cells) of the immune system and play a definite role in the management of solid organ and bone marrow transplantation.

Monoclonal antibodies directed against specific cell surface proteins such as CD2, CD3, CD25, CD40, and various integrins much more selectively influence T-cell subset function. The high specificity of these antibodies improves selectivity and reduces toxicity of the therapy, and alters the disease course in several different autoimmune disorders.

In the management of transplants, ALG and monoclonal antibodies can be used in the induction of immunosuppression, in the treatment of initial rejection, and in the treatment of steroid-resistant rejection. There has been some success in the use of ALG and ATG plus cyclosporine to prepare recipients for bone marrow transplantation. In this procedure, the recipient is treated with ALG or ATG in large doses for 7–10 days prior to transplantation of bone marrow cells from the donor. ALG appears to destroy the T cells in the donor marrow graft, and the probability of severe GVH disease is reduced.

The adverse effects of ALG are mostly those associated with injection of a foreign protein. Local pain and erythema often occur at the injection site (type III hypersensitivity). Since the humoral antibody response remains active in the recipient, skin-reactive and precipitating antibodies may be formed against the foreign ALG. Similar reactions occur with monoclonal antibodies of murine origin caused by the release of cytokines by T cells and monocytes.

Anaphylactic and serum sickness reactions to ALG and murine monoclonal antibodies have been observed and usually require cessation of therapy. Complexes of host antibodies with horse ALG may precipitate and localize in the glomeruli of the kidneys causing kidney damage.

Immune Globulin Intravenous (IGIV) A different approach to immunomodulation is the intravenous use of polyclonal human immunoglobulin. This immunoglobulin preparation (usually IgG) is prepared from pools of thousands of healthy donors, and no single, specific antigen is the target of the “therapeutic antibody.” Rather, one expects that the pool of different antibodies will have a normalizing effect upon the patient’s immune networks.

IGIV in high doses (2 g/kg) has proved effective in a variety of different applications ranging from immunoglobulin deficiencies to autoimmune disorders to HIV disease to bone marrow transplantation. In patients with Kawasaki’s disease, it has been shown to be safe and effective, reducing systemic inflammation and preventing coronary artery aneurysms. It has also brought about good clinical responses in systemic lupus erythematosus and refractory idiopathic thrombocytopenic purpura. Possible mechanisms of action of IGIV include a reduction of T helper cells, increase of regulatory T cells, decreased spontaneous immunoglobulin production, Fc receptor blockade, increased antibody catabolism, and idiotypic-anti-idiotypic interactions with “pathologic antibodies.” Although its precise mechanism of action is still unknown, IGIV brings undeniable clinical benefit to many patients with a variety of immune syndromes.

Rho(D) Immune Globulin One of the earliest major advances in immunopharmacology was the development of a technique for preventing Rh hemolytic disease of the newborn. The technique is based on the observation that a primary antibody response to a foreign antigen can be blocked if specific antibody to that antigen is administered passively at the time of exposure to antigen. Rho (D) immune globulin is a concentrated (15%) solution of human IgG containing high titer antibodies against the Rho (D) antigen of the red cell.

Sensitization of Rh-negative mothers to the D antigen occurs usually at the time of birth of an Rho (D)-positive or Du-positive infant, when fetal red cells leak into the mother’s bloodstream. Sensitization might also occur occasionally with miscarriages or ectopic pregnancies. In subsequent pregnancies, maternal antibody against Rh-positive cells is transferred to the fetus during the third trimester, leading to the development of erythroblastosis fetalis (hemolytic disease of the newborn).

If an injection of Rho (D) antibody is administered to the Rh-negative mother within 24–72 hours after the birth of an Rh-positive infant, the mother’s own antibody response to the foreign Rho (D)-positive cells is suppressed because the infant’s red cells are cleared from circulation before the mother can generate a B-cell response against Rho (D). Therefore she has no memory B cells that can activate upon subsequent pregnancies with an Rho (D)-positive fetus.

When the mother has been treated in this fashion, Rh hemolytic disease of the newborn has not been observed in subsequent pregnancies. For this prophylactic treatment to be successful, the mother must be Rho (D)-negative and Du-negative and must not already be immunized to the Rho (D) factor. Treatment is also often advised for Rh-negative mothers antepartum at 26–28 weeks’ gestation who have had miscarriages, ectopic pregnancies, or abortions, when the blood type of the fetus is unknown. Note: Rh o (D) immune globulin is administered to the mother and must not be given to the infant.

The usual dose of Rho (D) immune globulin is 2 mL intramuscularly, containing approximately 300 mcg anti-Rh o (D) IgG. Adverse reactions are infrequent and consist of local discomfort at the injection site or, rarely, a slight temperature elevation.

Hyperimmune Immunoglobulins Hyperimmune immunoglobulins are IGIV preparations made from pools of selected human or animal donors with high titers of antibodies against particular agents of interest such as viruses or toxins (see also Appendix). Various hyperimmune IGIVs are available for treatment of respiratory syncytial virus, cytomegalovirus, varicella zoster, human herpesvirus 3, hepatitis B virus, rabies, tetanus, and digoxin overdose. Intravenous administration of the hyperimmune globulins is a passive transfer of high titer antibodies that either reduces risk or reduces the severity of infection. Rabies hyperimmune globulin is injected around the wound and given intravenously. Tetanus hyperimmune globulin is administered intravenously when indicated for prophylaxis. Rattlesnake and coral snake hyperimmune globulins (antivenoms) are of equine or ovine origin and are effective for North and South American rattlesnakes and some coral snakes (but not Arizona coral snake). Equine and ovine antivenoms are available for rattlesnake envenomations, but only equine antivenom is available for coral snake bite. An Arizona bark scorpion antivenom is also available as equine (Fab)™2. This preparation prevents neurologic manifestations of scorpion envenomation and is generally used in young children and infants.

MONOCLONAL ANTIBODIES (MABS) Advances in the ability to manipulate the genes for immunoglobulins have resulted in development of a wide array of humanized and chimeric monoclonal antibodies directed against therapeutic targets. As described above, the only murine elements of humanized monoclonal antibodies are the complementarity-determining regions in the variable domains of immunoglobulin heavy and light chains.

Complementarity-determining regions are primarily responsible for the antigen-binding capacity of antibodies. Chimeric antibodies typically contain antigen-binding murine variable regions and human constant regions. The following are brief descriptions of the engineered antibodies that have been approved for clinical use; they are presented alphabetically.

Antitumor MABs Alemtuzumab is a humanized IgG1 with a kappa chain that binds to CD52 found on normal and malignant B and T lymphocytes, NK cells, monocytes, macrophages, and a small population of granulocytes. Alemtuzumab was previously approved for the treatment of Bcell chronic lymphocytic leukemia (CLL) in patients who have been treated with alkylating agents and have failed fludarabine therapy.

Alemtuzumab appears to deplete leukemic (and normal) cells by direct antibody-dependent lysis. More recently, alemtuzumab was approved by the EU for the treatment of patients diagnosed with relapsing remitting multiple sclerosis. In the latter, alemtuzumab depletes autoimmune inflammatory T and B cells while the drug is in the circulation. Repopulating lymphocytes appear to temporarily rebalance the immune system. Patients receiving this antibody become lymphopenic and may also become neutropenic, anemic, and thrombocytopenic. As a result patients should be closely monitored for opportunistic infections and hematologic toxicity.

Bevacizumab is a humanized IgG1 monoclonal antibody that binds to vascular endothelial growth factor (VEGF) and inhibits VEGF from binding to its receptor, especially on endothelial cells. It is an antiangiogenic drug that has been shown to inhibit growth of blood vessels (angiogenesis) in tumors. It is approved for first- and second-line treatment of patients with metastatic colorectal cancer alone or in combination with appropriate chemotherapy. It is also approved for treatment of non-small cell lung cancer, glioblastoma multiforme that has progressed after prior treatment, and metastatic kidney cancer when used with interferon-alfa. Since bevacizumab is antiangiogenic, it should not be administered until patients heal from surgery. Patients taking the drug should be watched for hemorrhage, gastrointestinal perforations, and wound healing problems. Bevacizumab has also been used off label by intravitreal injection to slow progression of neovascular macular degeneration (see ranibizumab under Other MABs, below).

Catumaxomab is a recombinant bi-specific trifunctional rat-mouse IgG hybrid monoclonal antibody that targets the epithelial cell adhesion molecule (EpCAM) on tumor cells and the CD3 protein on T cells. This bi-specific monoclonal antibody is approved in the EU and in the USA as an orphan drug for treating abdominal ascites in ovarian and gastric cancers. The rationale behind the bi-specific characteristics of catumaxomab is that it brings CD3-expressing anti-tumor T cells into close proximity of tumor cells expressing EpCAM. The Fc portion of the antibody also recruits phagocytic cells that mediate antibody-dependent cellular cytotoxicity and complement resulting in complement-dependent cytotoxicity of tumor cells.

Cetuximab is a human-mouse chimeric monoclonal antibody that targets epidermal growth factor receptor (EGFR). Binding of cetuximab to EGFR inhibits tumor cell growth by a variety of mechanisms, including decreases in kinase activity, matrix metalloproteinase activity, and growth factor production, and increased apoptosis. It is approved for use in patients with EGFR-positive head and neck squamous cell carcinoma in combination with radiotherapy or appropriate chemotherapy. It is also approved for treatment of kRas-negative, EGFR-positive metastatic colorectal cancer in combination with radiotherapy or appropriate chemotherapy, or as a single agent in patients who cannot tolerate certain chemotherapies. Cetuximab may be administered in combination with irinotecan or alone in patients who cannot tolerate irinotecan. HAMAs are generated by about 4% of patients being treated with cetuximab.

Ofatumumab is a human IgG1 monoclonal antibody directed against an epitope on CD20 on lymphocytes. Rituximab, the first approved CD20 monoclonal antibody (see below), binds a different CD20 epitope. Ofatumumab is approved for patients with CLL who are refractory to fludarabine and alemtuzumab. Ofatumumab binds to all B cells including B-CLL. It is thought to lyse B-CLL cells in the presence of complement and to mediate antibody-dependent cellular cytotoxicity. There is a slight risk of hepatitis B virus reactivation in patients taking ofatumumab.

Panitumumab is a fully human IgG2 kappa light chain monoclonal antibody. It is approved for the treatment of EGFR-expressing metastatic colorectal carcinoma with disease progression on or following fluoropyrimidine-, oxaliplatin-, and irinotecan-containing chemotherapy regimens. Panitumumab binds to EGFR (similar to cetuximab), inhibiting epidermal growth factor from binding to its receptor, and prevents ligand-induced receptor autophosphorylation and activation of receptor-associated kinases. It inhibits cell growth, induces apoptosis, decreases vascular growth factor production, and suppresses internalization of the EGFR. Although dermatologic and infusion-related toxicities are common following infusion of panitumumab, the distinct advantage over cetuximab is that it is fully human (ie, does not elicit HAMAs) and thus has an extended half-life in circulation. This is the first FDA-approved monoclonal antibody produced from transgenic mice expressing the human immunoglobulin gene loci.

Pertuzumab is a recombinant humanized IgG1 monoclonal antibody. It is approved for the treatment of metastatic or locally advanced HER-2/neu-positive breast cancer in combination with trastuzumab (see below) and docetaxel as neoadjuvant therapy. This antibody suppresses tumor growth by preventing heterodimerization of the human epidermal growth factor receptor HER-2/neu with other HER family members, thus inhibiting ligand-mediated intracellular signaling through MAP kinase and PI3 kinase pathways.

Pertuzumab also mediates antibody-dependent cell-mediated cytotoxicity on HER-2/neu-positive tumor cells.

Rituximab is a chimeric murine-human monoclonal IgG1 (human Fc) that binds to the CD20 molecule on normal and malignant B lymphocytes and is approved for the therapy of patients with CD20-positive large-B-cell diffuse non-Hodgkinâ&#x20AC;&#x2122;s lymphoma, and relapsed or refractory low-grade or follicular B-cell non-Hodgkinâ&#x20AC;&#x2122;s lymphoma as a single agent or in combination with appropriate chemotherapy.

It is approved for treatment of CLL in combination with chemotherapy. It is also approved for the treatment of rheumatoid arthritis in combination with methotrexate in patients for whom anti-TNF-α therapy has failed. The most recent indication for rituximab is for the treatment of Wegener’s granulomatosis and microscopic polyangiitis. The mechanism of action includes complement-mediated lysis, antibody-dependent cellular cytotoxicity, and induction of apoptosis in malignant lymphoma cells and in B cells involved in the pathogenesis of rheumatoid arthritis and granulomatosis and polyangiitis. In lymphoma this drug appears to be synergistic with chemotherapy (eg, fludarabine, CHOP, see Chapter 54). Anemia or neutropenia is an important adverse effect, which can be countered with granulocyte colony-stimulating factor (G-CSF). Other adverse effects include hypotension, rash, gastrointestinal disturbance, fever, and fatigue.

Trastuzumab is a recombinant DNA-derived, humanized monoclonal antibody that binds to the extracellular domain of HER-2/neu.

This antibody blocks the natural ligand from binding and down-regulates the receptor. Trastuzumab is approved for the treatment of HER-2/neu-positive tumors in patients with breast cancer and patients with metastatic gastric or gastroesophageal junction adenocarcinoma. As a single agent it induces remission in 15–20% of breast cancer patients; in combination with chemotherapy, it increases response rates and duration as well as 1-year survival. Trastuzumab is under investigation for other tumors that express HER2/neu (see Chapter 54). Patients should be monitored for potential cardiomyopathy while taking this drug.

MABs Used to Deliver Isotopes & Toxins to Tumors Ado-trastuzumab emtansine is an antibody-drug conjugate in which the anti-HER-2/neu antibody, trastuzumab (see above), is chemically linked to the cytotoxic agent, mertansine, a microtubule disruptor. Ado-trastuzumab emtansine is approved for patients with HER-2/neu-positive breast cancer who have previously received trastuzumab and a taxane separately or in combination, and whose disease recurred or progressed during prior treatment. Toxicities are identical to trastuzumab alone, but also include hepatotoxicity due to emtansine.

Arcitumomab is a murine Fab fragment from an anti-carcinoembryonic antigen (CEA) antibody labeled with technetium 99m 99m ( Tc) that is used for imaging patients with metastatic colorectal carcinoma (immunoscintigraphy) to determine extent of disease. CEA is often upregulated in patients with gastrointestinal carcinomas. The use of the Fab fragment decreases the immunogenicity of the agent so that it can be given more than once; intact murine monoclonal antibodies would elicit stronger HAMA. Brentuximab vedotin is a new antibody-drug conjugate that binds CD30, a cell surface marker in the TNF receptor superfamily that is expressed on anaplastic large T-cell lymphomas and on Reed-Sternberg cells in Hodgkin lymphoma; it may also be expressed on activated leukocytes. Brentuximab vedotin consists of a chimeric (mouse-human) IgG1 linked to monomethylauristatin E (MMAE), a microtubule-disrupting agent that induces cell cycle arrest and apoptosis. When this ADC binds CD30 on the cell surface, the complex is internalized followed by proteolytic cleavage of MMAE from the IgG. Brentuximab is approved for treatment of patients with Hodgkin’s lymphoma after failure of autologous stem cell transplantation or after failure of at least two previous chemotherapy regimens. It is also approved for patients with systemic anaplastic large cell lymphoma after failure of at least one previous multiagent chemotherapy regimen. Patients taking brentuximab vedotin should be monitored primarily for neutropenia and peripheral sensory neuropathy.

Capromab pendetide is a murine monoclonal antibody specific for prostate specific membrane antigen. It is coupled to isotopic indium (111 In) and is used in immunoscintigraphy for patients with biopsy-confirmed prostate cancer and post-prostatectomy in patients with rising prostate specific antibody level to determine extent of disease.

Ibritumomab tiuxetan is an anti-CD20 murine monoclonal antibody labeled with isotopic yttrium (90 Y) or 111 In. The radiation of the isotope coupled to the antibody provides the major antitumor activity of this drug. Ibritumomab is approved for use in patients with relapsed or refractory low-grade, follicular, or B-cell non-Hodgkin’s lymphoma, including patients with rituximab-refractory follicular disease. It is used in conjunction with rituximab in a two-step therapeutic regimen.

MABs Used as Immunosuppressants & Anti-Inflammatory Agents Adalimumab, certolizumab pegol, etanercept, golimumab, and infliximab are antibodies that bind TNF-α, a proinflammatory cytokine that is important in rheumatoid arthritis and similar inflammatory diseases. Abatacept is a recombinant fusion protein composed of the extracellular domain of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) fused to hinge, CH 2 , and CH3 domains of the Fc domains of human IgG1 . Tocilizumab is recombinant humanized IgG1 that binds to soluble and membrane-associated IL-6 receptors. It inhibits IL-6-mediated signaling on lymphocytes, suppressing inflammatory processes. These drugs are approved for use in rheumatoid and other forms of arthritis and are discussed in Chapter 36.

Alefacept is an engineered protein consisting of the CD2-binding portion of leukocyte-function-associated antigen-3 (LFA-3) fused to a human IgG1 Fc region (hinge, CH2 , and CH3 ). It is approved for the treatment of plaque psoriasis. It inhibits activation of T cells by binding to cell surface CD2, inhibiting the normal CD2/LFA-3 interaction (Figure 55–7). Treatment of patients with alefacept also results in a dose-dependent reduction of the total number of circulating T cells, especially CD4 and CD8 memory effector subsets that predominate in psoriatic plaques. Peripheral T-cell counts of patients receiving alefacept must be monitored and the drug discontinued if CD4 lymphocyte levels fall below 250 cells/μL.

FIGURE 55–7 Actions of some monoclonal antibodies (shown in red). CTLA-4-lgFc fusion protein (CTLA-4-lg, abatacept) binds to CD80/86 on DC and inhibits T-cell costimulation. Alefacept inhibits activation of T cells by blocking the interaction of LFA-3 and CD2.

Basiliximab and daclizumab block IL-2 from binding to the IL-2 receptor (CD25) on T cells, preventing activation; CD25 is also important for the survival of T regulatory cells. T-cell activation can be maintained or restored if CTLA-4 interaction with CD80/86 is blocked using an anti-CTLA-4 antibody (ipilimumab); ipilimumab inhibits CTLA-4 signaling and prolongs activation.

Basiliximab is a chimeric mouse-human IgG1 that binds to CD25, the IL-2 receptor alpha chain on activated lymphocytes.

Daclizumab is a humanized IgG1 that also binds to the α subunit of the IL-2 receptor. Both agents function as IL-2 antagonists, blocking IL-2 from binding to activated lymphocytes, and are therefore immunosuppressive. They are indicated for prophylaxis of acute organ rejection in renal transplant patients and either drug may be used as part of an immunosuppressive regimen that also includes glucocorticoids and cyclosporine.

Canakinumab is a human IgG kappa chain monoclonal antibody that prevents IL-1β from binding to its receptor. It is approved for cryopyrin-associated periodic syndromes (CAPS) in adults and children 4 years old and older. CAPS includes familial cold autoinflammatory syndrome, Muckle-Wells syndrome, and systemic juvenile idiopathic arthritis in children 2 years old or older. These diseases are caused by mutations in a gene (NLRP-3) that encodes cryopyrin, an important component of the inflammasome. NLRP-3 mutations cause excessive release of IL-1β, causing autoimmune inflammation resulting in fever, urticarial-like rash, arthralgia, myalgia, fatigue, and conjunctivitis.

Natalizumab is a humanized IgG4 monoclonal antibody that binds to the α4-subunit of α4β1 and α4β7 integrins expressed on the surfaces of all leukocytes except neutrophils, and inhibits the α4-mediated adhesion of leukocytes to their cognate receptor. It is indicated for patients with multiple sclerosis and Crohn’s disease who have not tolerated or had inadequate responses to conventional treatments.

Natalizumab should not be used with any of the anti-TNF-α drugs listed above.

Omalizumab is an anti-IgE recombinant humanized monoclonal antibody that is approved for the treatment of allergic asthma in adult and adolescent patients whose symptoms are refractory to inhaled corticosteroids (see Chapter 20). The drug is also approved for chronic urticaria. The antibody blocks the binding of IgE to the high-affinity Fcε receptor on basophils and mast cells, which suppresses IgE-mediated release of type I allergy mediators such as histamine and leukotrienes. Total serum IgE levels may remain elevated in patients for up to 1 year after administration of omalizumab.

Ustekinumab is a human IgG1 monoclonal antibody that binds to the p40 subunit of IL-12 and IL-23 cytokines. It blocks IL-12 and IL-23 from binding to their receptors, therefore inhibiting receptor-mediated signaling in lymphocytes. Ustekinumab is indicated for adult patients with moderate to severe plaque psoriasis either alone or with methotrexate. The advantage of ustekinumab over anti-TNF-α drugs for psoriasis is faster and longer term improvement in symptoms along with very infrequent dosing.

Vedolizumab is a humanized monoclonal antibody that targets the α4β7 integrin in the gastrointestinal tract. It does not appear to induce systemic immunosuppression of other α4β7 integrin-binding antibodies such as natalizumab because it does not bind to the majority of α4β7 integrin on lymphocytes. It has been recommended for approval for the treatment of Crohn’s disease and ulcerative colitis.

Other MABs Abciximab is a Fab fragment of a murine-human monoclonal antibody that binds to the integrin GPIIb/IIIa receptor on activated platelets and inhibits fibrinogen, von Willebrand factor, and other adhesion molecules from binding to activated platelets, thus preventing their aggregation. It is indicated as an adjunct to percutaneous coronary intervention in combination with aspirin and heparin for the prevention of cardiac ischemic complications. See Chapter 34 for additional details.

Denosumab is a human IgG2 monoclonal antibody specific for human RANKL (receptor activator of nuclear factor kappa-B ligand; see Chapter 42). By binding RANKL it inhibits the maturation of osteoclasts, the cells responsible for bone resorption. Denosumab is indicated for treatment of postmenopausal women with osteoporosis at high risk for fracture. Before starting denosumab, patients must be evaluated to be sure they are not hypocalcemic. During treatment, patients should receive supplements of calcium and vitamin D. Eculizumab is a humanized IgG monoclonal antibody that binds the C5 complement component, inhibiting its cleavage into C5a and C5b thereby inhibiting the terminal pore-forming lytic activity of complement. Eculizumab is approved for patients with paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). It dramatically reduces the need for red blood cell transfusions. It prevents PNH symptoms of anemia, fatigue, thrombosis, and hemoglobinemia by inhibiting intravascular hemolysis.

Similarly in aHUS eculizumab prevents complement-mediated thrombotic microangiopathy. Clinicians must be aware of increased risk of meningococcal infection in patients receiving this anti-C5 monoclonal antibody.

Palivizumab is a humanized IgG1 monoclonal antibody that binds to the fusion protein of respiratory syncytial virus (RSV), preventing serious lower respiratory tract disease. It is used in neonates at risk for this viral infection and reduces the frequency of infection and hospitalization by about 50% (see Chapter 49).

Ranibizumab is a recombinant human IgG1 Fab that binds to VEGF-A. It prevents new blood vessel formation by blocking VEGF from binding to its receptor. Ranibizumab is approved for intravitreal injection in patients with neovascular age-related macular degeneration, diabetic macular edema, and sudden blurring or vision loss secondary to macular edema following retinal vein occlusion.

Pegaptanib is a pegylated oligonucleotide that binds extracellular VEGF and is also given by intravitreous injection to slow macular degeneration.

Raxibacumab is a human IgG1 lambda chain monoclonal antibody that binds to the PA protein of Bacillus anthracis, preventing cellular entry of the anthrax toxins (lethal and edema factors). Raxibacumab is approved for the treatment or prophylaxis of adults and children with inhalational anthrax in combination with appropriate antibacterial drugs. Interestingly, raxibacumab was not tested in humans because exposing a control cohort to inhalational anthrax is unethical and there are too few naturally infected persons to conduct a proper clinical trial.

CLINICAL USES OF IMMUNOSUPPRESSIVE DRUGS Immunosuppressive agents are commonly used in two clinical circumstances: transplantation and autoimmune disorders. The agents used differ somewhat for the specific disorders treated (see specific agents and Table 55–1), as do administration schedules. Because autoimmune disorders are very complex, optimal treatment schedules have yet to be established in many of them.

SOLID ORGAN & BONE MARROW TRANSPLANTATION In organ transplantation, tissue typing—based on donor and recipient histocompatibility matching with the human leukocyte antigen (HLA) haplotype system—is required. Close histocompatibility matching reduces the likelihood of graft rejection and may also reduce the requirements for intensive immunosuppressive therapy. Prior to transplant, patients may receive an immunosuppressive regimen, including antithymocyte globulin, daclizumab, or basiliximab. Four types of rejection can occur in a solid organ transplant recipient: hyperacute, accelerated, acute, and chronic. Hyperacute rejection is due to preformed antibodies against the donor organ, such as anti-blood group antibodies. Hyperacute rejection occurs within hours of the transplant and cannot be stopped with immunosuppressive drugs. It results in rapid necrosis and failure of the transplanted organ. Accelerated rejection is mediated by both antibodies and T cells, and it also cannot be stopped by immunosuppressive drugs. Acute rejection of an organ occurs within days to months and involves mainly cellular immunity. Reversal of acute rejection is usually possible with general immunosuppressive drugs such as azathioprine, mycophenolate mofetil, cyclosporine, tacrolimus, glucocorticoids, cyclophosphamide, methotrexate, and sirolimus. Recently, biologic agents such as anti-CD3 monoclonal antibodies have been used to stem acute rejection. Chronic rejection usually occurs months or even years after transplantation. It is characterized by thickening and fibrosis of the vasculature of the transplanted organ, involving both cellular and humoral immunity. Chronic rejection is treated with the same drugs as those used for acute rejection.

Allogeneic hematopoietic stem cell transplantation is a well-established treatment for many malignant and nonmalignant diseases. An HLA-matched donor, usually a family member, is located, patients are conditioned with high-dose chemotherapy or radiation therapy, and then donor stem cells are infused. The conditioning regimen is used not only to kill cancer cells in the case of malignant disease, but also to totally suppress the immune system so that the patient does not reject the donor stem cells. As patients’ blood counts recover (after reduction by the conditioning regimen) they develop a new immune system that is created from the donor stem cells. Rejection of donor

stem cells is uncommon, and can only be treated by infusion of more stem cells from the donor.

GVH disease, however, is very common, occurring in the majority of patients who receive an allogeneic transplant. GVH disease occurs because donor T cells fail to recognize the patient’s skin, liver, and gut (usually) as self and attack those tissues. Although patients are given immunosuppressive therapy (cyclosporine, methotrexate, and others) early in the transplant course to help prevent this development, it usually occurs despite these medications. Acute GVH disease occurs within the first 100 days, and is usually manifested as a skin rash, severe diarrhea, or hepatotoxicity. Additional medications are added, invariably starting with high-dose corticosteroids, and adding drugs such as mycophenolate mofetil, sirolimus, tacrolimus, daclizumab, and others, with variable success rates. Patients generally progress to chronic GVH disease (after 100 days) and require therapy for variable periods thereafter. Unlike solid organ transplant patients, however, most stem cell transplant patients are able to eventually discontinue immunosuppressive drugs as GVH disease resolves (usually 1–2 years after their transplant).

AUTOIMMUNE DISORDERS The effectiveness of immunosuppressive drugs in autoimmune disorders varies widely. Nonetheless, with immunosuppressive therapy, remissions can be obtained in many instances of autoimmune hemolytic anemia, idiopathic thrombocytopenic purpura, type 1 diabetes, Hashimoto’s thyroiditis, and temporal arteritis. Improvement is also often seen in patients with systemic lupus erythematosus, acute glomerulonephritis, acquired factor VIII inhibitors (antibodies), rheumatoid arthritis, inflammatory myopathy, scleroderma, and certain other autoimmune states.

Immunosuppressive therapy is utilized in chronic severe asthma, where cyclosporine is often effective and sirolimus is another alternative. Omalizumab (anti-IgE antibody) has been approved for the treatment of severe asthma (see previous section). Tacrolimus is currently under clinical investigation for the management of autoimmune chronic active hepatitis and of multiple sclerosis, where IFN-β has a definitive role.

IMMUNOMODULATION THERAPY The development of agents that modulate the immune response rather than suppress it has become an important area of pharmacology.

The rationale underlying this approach is that such drugs may increase the immune responsiveness of patients who have either selective or generalized immunodeficiency. The major potential uses are in immunodeficiency disorders, chronic infectious diseases, and cancer.

The AIDS epidemic has greatly increased interest in developing more effective immunomodulating drugs.

CYTOKINES The cytokines are a large and heterogeneous group of proteins with diverse functions. Some are immunoregulatory proteins synthesized within lymphoreticular cells and play numerous interacting roles in the function of the immune system and in the control of hematopoiesis.

The cytokines that have been clearly identified are summarized in Table 55–2. In most instances, cytokines mediate their effects through receptors on relevant target cells and appear to act in a manner similar to the mechanism of action of hormones. In other instances, cytokines may have antiproliferative, antimicrobial, and antitumor effects.

TABLE 55–2 The cytokines.

The first group of cytokines discovered, the interferons (IFNs), were followed by the colony-stimulating factors (CSFs, discussed in Chapter 33). The latter regulate the proliferation and differentiation of bone marrow progenitor cells. Most of the more recently discovered cytokines have been classified as interleukins (ILs) and numbered in the order of their discovery. Pharmaceutical cytokines are produced using gene cloning techniques.

Most cytokines (including TNF-α, IFN-γ, IL-2, G-CSF, and granulocyte-macrophage colony-stimulating factor [GM-CSF]) have very short serum half-lives (minutes). The usual subcutaneous route of administration provides slower release into the circulation and a longer duration of action. Each cytokine has its own unique toxicity, but some toxicities are shared. For example, IFN-α, IFN-β, IFN-γ, IL-2, and TNF-α all induce fever, flu-like symptoms, anorexia, fatigue, and malaise.

Interferons are proteins that are currently grouped into three families: IFN-α, IFN-β, and IFN-γ. The IFN-α and IFN-β families comprise type I IFNs, ie, acid-stable proteins that act on the same receptor on target cells. IFN-γ, a type II IFN, is acid-labile and acts on a separate receptor on target cells. Type I IFNs are usually induced by virus infections, with leukocytes producing IFN-α. Fibroblasts and epithelial cells produce IFN-β. IFN-γ is usually the product of activated T lymphocytes.

IFNs interact with cell receptors to produce a wide variety of effects that depend on the cell and IFN types. IFNs, particularly IFN-γ, display immune-enhancing properties, which include increased antigen presentation and macrophage, NK cell, and cytotoxic Tlymphocyte activation. IFNs also inhibit cell proliferation. In this respect, IFN-α and IFN-β are more potent than IFN-γ. Another striking IFN action is increased expression of MHC molecules on cell surfaces. While all three types of IFN induce MHC class I molecules, only IFN-γ induces class II expression. In glial cells, IFN-β antagonizes this effect and may, in fact, decrease antigen presentation within the nervous system.

IFN-α is approved for the treatment of several neoplasms, including hairy cell leukemia, chronic myelogenous leukemia, malignant melanoma, and Kaposi’s sarcoma, and for use in hepatitis B and C infections. It has also shown activity as an anti-cancer agent in renal cell carcinoma, carcinoid syndrome, and T-cell leukemia. IFN-β is approved for use in relapsing-type multiple sclerosis. IFN-γ is approved for the treatment of chronic granulomatous disease and IL-2, for metastatic renal cell carcinoma and malignant melanoma.

Clinical investigations of other cytokines, including IL-1, -3, -4, -6, -10, -11, and -12, are ongoing. Toxicities of IFNs, which include fever, chills, malaise, myalgias, myelosuppression, headache, and depression, can severely restrict their clinical use.

TNF-α has been extensively tested in the therapy of various malignancies, but results have been disappointing due to dose-limiting toxicities. One exception is the use of intra-arterial high-dose TNF-α for malignant melanoma and soft tissue sarcoma of the extremities.

In these settings, response rates greater than 80% have been noted.

Cytokines have been under clinical investigation as adjuvants to vaccines, and IFNs and IL-2 have shown some positive effects in the response of human subjects to hepatitis B vaccine. Denileukin diftitox is IL-2 fused to diphtheria toxin, used for the treatment of patients with CD25+ cutaneous T-cell lymphomas. IL-12 and GM-CSF have also shown adjuvant effects with vaccines. GM-CSF is of particular interest because it promotes recruitment of professional antigen-presenting cells such as the dendritic cells required for priming naive antigen-specific T-lymphocyte responses. There are some claims that GM-CSF can itself stimulate an antitumor immune response, resulting in tumor regression in melanoma and prostate cancer.

It is important to emphasize that cytokine interactions with target cells often result in the release of a cascade of different endogenous cytokines, which exert their effects sequentially or simultaneously. For example, IFN-γ exposure increases the number of cell-surface receptors on target cells for TNF-α. Therapy with IL-2 induces the production of TNF-α, while therapy with IL-12 induces the production of IFN-γ.

CYTOKINE INHIBITORS An important application of immunomodulation therapy involves the use of cytokine inhibitors for inflammatory diseases (see Chapter 36) and septic shock, conditions in which cytokines such as IL-1 and TNF-α (see above) are involved in the pathogenesis. Drugs now in use or under investigation include anticytokine antibodies and soluble cytokine receptors. Anakinra is a recombinant form of the naturally occurring IL-1 receptor antagonist that prevents IL-1 from binding to its receptor, stemming the cascade of cytokines that would otherwise be released. Anakinra is approved for use in adult rheumatoid arthritis patients who have failed treatment with one or more disease-modifying antirheumatic drugs but is no longer much used for this indication. As stated above, canakinumab is a recombinant human anti-IL-1β monoclonal antibody. It binds to human IL-1β and prevents it from binding to IL-1 receptors. Rilonacept is a dimeric fusion protein consisting of the ligand-binding domains of the extracellular portions of the human interleukin-1 receptor component (IL1RI) and IL-1 receptor accessory protein (IL-1RAcP) fused to the Fc portion of human IgG1 . These molecules are indicated for treatment of cryopyrin-associated periodic syndromes.

Patients must be carefully monitored for serious infections or malignancies if they are also taking an anti-TNF-α drug, have chronic infections, or are otherwise immunosuppressed.

IMMUNOLOGIC REACTIONS TO DRUGS & DRUG ALLERGY The basic immune mechanism and the ways in which it can be suppressed or stimulated by drugs have been discussed in previous

sections of this chapter. Drugs also activate the immune system in undesirable ways that are manifested as adverse drug reactions.

These reactions are generally grouped in a broad classification as “drug allergy.” Indeed, many drug reactions such as those to penicillin, iodides, phenytoin, and sulfonamides are allergic in nature. These drug reactions are manifested as skin eruptions, edema, anaphylactoid reactions, glomerulonephritis, fever, and eosinophilia.

Drug reactions mediated by immune responses can have several different mechanisms. Thus, any of the four major types of hypersensitivity discussed earlier in this chapter (pages 950–951) can be associated with allergic drug reactions: • Type I: IgE-mediated acute allergic reactions to stings, pollens, and drugs, including anaphylaxis, urticaria, and angioedema. IgE is fixed to tissue mast cells and blood basophils, and after interaction with antigen the cells release potent mediators.

• Type II: Drugs often modify host proteins, thereby eliciting antibody responses to the modified protein. These allergic responses involve IgG or IgM in which the antibody becomes fixed to a host cell, which is then subject to complement-dependent lysis or to antibody-dependent cellular cytotoxicity.

• Type III: Drugs may cause serum sickness, which involves immune complexes containing IgG complexed with a foreign antigen and is a multisystem complement-dependent vasculitis that may also result in urticaria.

• Type IV: Cell-mediated allergy is the mechanism involved in allergic contact dermatitis from topically applied drugs or induration of the skin at the site of an antigen injected intradermally.

In some drug reactions, several of these hypersensitivity responses may occur simultaneously. Some adverse reactions to drugs may be mistakenly classified as allergic or immune when they are actually genetic deficiency states or are idiosyncratic and not mediated by immune mechanisms (eg, hemolysis due to primaquine in glucose-6-phosphate dehydrogenase deficiency, or aplastic anemia caused by chloramphenicol).

IMMEDIATE (TYPE I) DRUG ALLERGY Type I (immediate) sensitivity allergy to certain drugs occurs when the drug, not capable of inducing an immune response by itself, covalently links to a host carrier protein (hapten). When this happens, the immune system detects the drug-hapten conjugate as “modified self” and responds by generating IgE antibodies specific for the drug-hapten. It is not known why some people mount an IgE response to a drug, while others mount IgG responses. Under the influence of IL-4, -5, and -13 secreted by TH2 cells, B cells specific for the drug secrete IgE antibody. The mechanism for IgE-mediated immediate hypersensitivity is diagrammed in Figure 55–5.

Fixation of the IgE antibody to high-affinity Fc receptors (FcεRs) on blood basophils or their tissue equivalent (mast cells) sets the stage for an acute allergic reaction. The most important sites for mast cell distribution are skin, nasal epithelium, lung, and gastrointestinal tract. When the offending drug is reintroduced into the body, it binds and cross-links basophil and mast cell-surface IgE to signal release of the mediators (eg, histamine, leukotrienes; see Chapters 16 and 18) from granules. Mediator release is associated with calcium influx and a fall in intracellular cAMP within the mast cell. Many of the drugs that block mediator release appear to act through the cAMP mechanism (eg, catecholamines, glucocorticoids, theophylline), others block histamine release, and still others block histamine receptors.

Other vasoactive substances such as kinins may also be generated during histamine release. These mediators initiate immediate vascular smooth muscle relaxation, increased vascular permeability, hypotension, edema, and bronchoconstriction.

Drug Treatment of Immediate Allergy One can test an individual for possible sensitivity to a drug by a simple scratch test, ie, by applying an extremely dilute solution of the drug to the skin and making a scratch with the tip of a needle. If allergy is present, an immediate (within 10–15 minutes) wheal (edema) and flare (increased blood flow) will occur. However, skin tests may be negative in spite of IgE hypersensitivity to a hapten or to a metabolic product of the drug, especially if the patient is taking steroids or antihistamines.

Drugs that modify allergic responses act at several links in this chain of events. Prednisone, often used in severe allergic reactions, is immunosuppressive; it blocks proliferation of the IgE-producing clones and inhibits IL-4 production by T helper cells in the IgE response, since glucocorticoids are generally toxic to lymphocytes. In the efferent limb of the allergic response, isoproterenol, epinephrine, and theophylline reduce the release of mediators from mast cells and basophils and produce bronchodilation. Epinephrine opposes histamine; it relaxes bronchiolar smooth muscle and contracts vascular muscle, relieving both bronchospasm and hypotension. As noted in Chapter 8, epinephrine is the drug of choice in anaphylactic reactions. The antihistamines competitively inhibit histamine, which would otherwise produce bronchoconstriction and increased capillary permeability in end organs. Glucocorticoids may also act to reduce tissue injury and edema in the inflamed tissue, as well as facilitating the actions of catecholamines in cells that may have become refractory to epinephrine or isoproterenol. Several agents directed toward the inhibition of leukotrienes may be useful in acute allergic and inflammatory disorders (see Chapter 20).

Desensitization to Drugs

When reasonable alternatives are not available, certain drugs (eg, penicillin, insulin) must be used for life-threatening illnesses even in the presence of known allergic sensitivity. In such cases, desensitization (also called hyposensitization) can sometimes be accomplished by starting with very small doses of the drug and gradually increasing the dose over a period of hours or days to the full therapeutic range (see Chapter 43). This practice is hazardous and must be performed under direct medical supervision with epinephrine available for immediate injection, as anaphylaxis may occur before desensitization has been achieved. It is thought that slow and progressive administration of the drug gradually binds all available IgE on mast cells, triggering a gradual release of granules. Once all of the IgE on the mast cell surfaces has been bound and the cells have been degranulated, therapeutic doses of the offending drug may be given with minimal further immune reaction. Therefore, a patient is only desensitized during administration of the drug.

AUTOIMMUNE (TYPE II) REACTIONS TO DRUGS Certain autoimmune syndromes can be induced by drugs. Examples include systemic lupus erythematosus following hydralazine or procainamide therapy, “lupoid hepatitis” due to cathartic sensitivity, autoimmune hemolytic anemia resulting from methyldopa administration, thrombocytopenic purpura due to quinidine, and agranulocytosis due to a variety of drugs. As indicated in other chapters of this book, a number of drugs are associated with type I and type II reactions. In these drug-induced autoimmune states, IgG antibodies bind to drug-modified tissue and are destroyed by the complement system or by phagocytic cells with Fc receptors. Fortunately, autoimmune reactions to drugs usually subside within several months after the offending drug is withdrawn. Immunosuppressive therapy is warranted only when the autoimmune response is unusually severe.

SERUM SICKNESS & VASCULITIC (TYPE III) REACTIONS Immunologic reactions to drugs resulting in serum sickness are more common than immediate anaphylactic responses, but type II and type III hypersensitivities often overlap. The clinical features of serum sickness include urticarial and erythematous skin eruptions, arthralgia or arthritis, lymphadenopathy, glomerulonephritis, peripheral edema, and fever. The reactions generally last 6–12 days and usually subside once the offending drug is eliminated. Antibodies of the IgM or IgG class are usually involved. The mechanism of tissue injury is immune complex formation and deposition on basement membranes (eg, lung, kidney), followed by complement activation and infiltration of leukocytes, causing tissue destruction. Glucocorticoids are useful in attenuating severe serum sickness reactions to drugs. In severe cases plasmapheresis can be used to remove the offending drug and immune complexes from circulation.

Immune vasculitis can also be induced by drugs. The sulfonamides, penicillin, thiouracil, anticonvulsants, and iodides have all been implicated in the initiation of hypersensitivity angiitis. Erythema multiforme is a relatively mild vasculitic skin disorder that may be secondary to drug hypersensitivity. Stevens-Johnson syndrome is probably a more severe form of this hypersensitivity reaction and consists of erythema multiforme, arthritis, nephritis, central nervous system abnormalities, and myocarditis. It has frequently been associated with sulfonamide therapy. Administration of nonhuman monoclonal or polyclonal antibodies such as rattlesnake antivenom may cause serum sickness.

PREPARATIONS AVAILABLE*

REFERENCES General Immunology Bonneville M et al: γδ T cell effector functions: A blend of innate programming and acquired plasticity. Nat Rev Immunol 2010;10:467.

Kumar H, Kawai T , Akira S: T oll-like receptors and innate immunity. Biochem Biophys Res Comm 2009;388:621.

Levinson WE: Review of Medical Microbiology and Immunology, 13th ed. McGraw-Hill, 2014.

Murphy KM, T ravers P, Walport M (editors): Janeway’s Immunobiology, 7th ed. Garland Science, 2008.

T hornton AM et al: Expression of helios, an ikaros transcription factor family member, differentiates thymic-derived from peripherally induced Foxp3+ T regulatory cells. J Immunol 2010;184:3433.

Hypersensitivity Hausmann O: Drug hypersensitivity reactions involving skin. Handbk Exper Pharmacol 2010;196:29.

Phillips EJ: Pharmacogenetics of drug hypersensitivity. Pharmacogenomics 2010;11:973.

Autoimmunity Bousvaros A: Use of immunomodulators and biologic therapies in children with inflammatory bowel disease. Expert Rev Clin Immunol 2010;6:659.

Carroll WM: Clinical trials of multiple sclerosis therapies: Improvements to demonstrate long-term patient benefit. Mult Scler 2009;15:951.

Kircik LH et al: How and when to use biologics in psoriasis. J Drugs Dermatol 2010;9:s106.

La Cava A: Anticytokine therapies in systemic lupus erythematosus. Immunotherapy 2010;2:575.

Ma MH et al: Remission in early rheumatoid arthritis. J Rheumatol 2010;37:1444.

Immunodeficiency Diseases Wood PM: Primary antibody deficiency syndromes. Curr Opin Hematol 2010;17:356.

Immunosuppressive Agents Braun J: Optimal administration and dosage of methotrexate. Clin Exp Rheumatol 2010;28:S46.

Filippini G et al: Immunomodulators and immunosuppressants for multiple sclerosis: A network meta-analysis. Cochrane Database Syst Rev 2013;(6):CD008933.

Galustian C: T he anticancer agents lenalidomide and pomalidomide inhibit the proliferation and function of T regulatory cells. Cancer Immunol Immunother 2009;58:1033.

Li S, Gill N, Lentzch S: Recent advances of IMiDs in cancer therapy. Curr Opin Oncol 2010;22:579.

Ponticelli C: Calcineurin inhibitors in renal transplantation still needed but in reduced doses: A review. T ransplant Proc 2010;42:2205.

Vicari-Christensen M: T acrolimus: Review of pharmacokinetics, pharmacodynamics, and pharmacogenetics to facilitate practitioners’ understanding and offer strategies for educating patients and promoting adherence. Prog T ransplant 2009;19:277.

Zhou H: Updates of mT OR inhibitors. Anticancer Agents Med Chem 2010;10:571.

Antilymphocyte Globulin & Monoclonal Antibodies Cummings SR: Denosumab for prevention of fractures in postmenopausal women with osteoporosis. N Engl J Med 2009;361:756.

Gürcan HM: Information for healthcare providers on general features of IGIV with emphasis on differences between commercially available products. Autoimmunity Rev 2010;9:553.

Nelson AL: Development trends for human monoclonal antibody therapeutics. Nat Rev Drug Discov 2010;9:767.

Stone JH: Overview of biologic agents in the rheumatic diseases. UpT oDate 2014; topic 7966.

T aylor PC: Pharmacology of T NF blockade in rheumatoid arthritis and other chronic inflammatory diseases. Curr Opin Pharmacol 2010;10:308.

Weiner LM: Monoclonal antibodies: Versatile platforms for cancer immunotherapy. Nat Rev Immunol 2010;10:317.

Cytokines Foster GR: Pegylated interferons for the treatment of chronic hepatitis C: Pharmacological and clinical differences between peginterferon-alpha-2a and peginterferon-alpha2b. Drugs 2010;70:147.

Gabay C: IL-1 pathways in inflammation and human diseases. Nat Rev Rheumatol 2010;6:232.

Drug Allergy Castells MC, Solensky R: Rapid drug desensitization for immediate hypersensitivity reactions. UpT oDate 2014; topic 2071.

Hamilton RG: Human IgE antibody serology: A primer for the practicing North American allergist/immunologist. J Allergy Clin Immunol 2010;126:33.

Khan DA: Drug allergy. J Allergy Clin Immunol 2010;125:S126.

CASE STUDY ANSWER Within 24â&#x20AC;&#x201C;72 hours postpartum, the woman should be given a 2 mL intramuscular injection of 300 mcg anti-Rh o (D) immune globulin. This will clear any fetal Rh-positive red cells from her circulation so she does not generate anti-Rho (D) B cells that might jeopardize any future pregnancy.

SECTION IX TOXICOLOGY

CHAPTER

56 Introduction to Toxicology: Occupational & Environmental Daniel T. Teitelbaum, MD*

Humans live in a chemical world. They inhale, ingest, and absorb through the skin many of these chemicals. The occupationalenvironmental toxicologist is primarily concerned with adverse effects in humans resulting from exposure to chemicals encountered at work or in the general environment. In clinical practice, the occupational-environmental toxicologist must identify and treat the adverse health effects of these exposures. In addition, the trained occupational-environmental toxicologist will be called upon to assess and identify hazards associated with chemicals used in the workplace or introduced into the human environment.

Occupational and environmental toxicology cases present unusually complex problems. Occupational and environmental exposure is rarely limited to a single type of molecule. Most workplace or environmental materials are compounds or mixtures, and the ingredients are often poorly described in the documentation that is available for physician review. Moreover, although regulatory agencies in many countries have requirements for disclosure of hazardous materials and their health impacts, proprietary information exclusions often make it difficult for those who treat occupationally and environmentally poisoned patients to understand the nature and scope of the presenting illness. Because many of these illnesses have long latency periods before they become manifest, it is often a matter of detective work, when patients finally present with disease, to ascertain exposure and relate it to clinical effect. Monitoring of exposure concentrations both in the workplace and in the general environment has become more common, but it is far from widespread, and so it is often very difficult to establish the extent of exposure, its duration, and its dose rate when this information is critical to the identification of the toxic disorder and its management.

Occupational Toxicology Occupational toxicology deals with the chemicals found in the workplace. The major emphasis of occupational toxicology is to identify the agents of concern, identify the acute and chronic diseases that they cause, define the conditions under which they may be used safely, and prevent absorption of harmful amounts of these chemicals. The occupational toxicologist will also be called upon to treat the diseases caused by these chemicals if he or she is a physician. Occupational toxicologists may also define and carry out programs for the surveillance of exposed workers and the environment in which they work. They frequently work hand in hand with occupational hygienists, certified safety professionals, and occupational health nurses in their activities.

Regulatory limits and voluntary guidelines have been elaborated to establish safe ambient air concentrations for many chemicals found in the workplace. Governmental and supragovernmental bodies throughout the world have generated workplace health and safety rules, including short- and long-term exposure limits for workers. These permissible exposure limits (PELs) have the power of law in the United States. Copies of the U.S. Occupational Safety and Health Administration (OSHA) standards may be found on OSHAâ&#x20AC;&#x2122;s website a t http://www.osha.gov. Copies of the U.S. Mine Safety and Health Administration (MSHA) standards may be found at http://www.msha.gov. In addition to the PELs that appear in the OSHA publications and on the website, OSHA promulgates standards for specific materials of particularly serious toxicity. These standards are developed following extensive scientific study, stakeholder input at hearings, public comment, and other steps such as publication in the Federal Register. Such standards have the force of law and employers who use these materials are obligated to comply with the standards. OSHA standards may be found in full on the OSHA website at http://www.osha.gov.

Voluntary organizations, such as the American Conference of Governmental Industrial Hygienists (ACGIH), periodically prepare lists of recommended threshold limit values (TLVs) for many chemicals. These guidelines are periodically updated. Regulatory imperatives in the United States may also be updated from time to time when new information about toxicity becomes available.

However, this process is slow and requires input from many sources except under certain extraordinary circumstances. In those cases, emergency alterations to standards may be made and an emergency temporary standard may be promulgated after appropriate regulatory procedures. The ACGIH TLV guidelines are useful as reference points in the evaluation of potential workplace exposures.

Compliance with these voluntary guidelines is not a substitute for compliance with the OSHA requirements in the United States. TLVs do not have the force of law. Current TLV lists may be obtained from the ACGIH at http://www.acgih.org.

Environmental Toxicology Environmental toxicology deals with the potentially deleterious impact of chemicals, present as pollutants of the environment, on living organisms. The term environment includes all the surroundings of an individual organism, but particularly the air, soil, and water.

Although humans are considered a target species of particular interest, other species are of considerable importance as potential biologic targets. Scientific study of signal occurrences in animals often provides early warning of impending human events as a result of ecotoxic impacts.

Air pollution is usually a product of industrialization, technologic development, and increased urbanization. On rare occasions, natural phenomena such as volcanic eruptions may result in air pollution with gases, vapors, or particulates that are harmful to humans. Humans may also be exposed to chemicals used in the agricultural environment as pesticides or in food processing that may persist as residues or ingredients in food products. Air contaminants are regulated in the United States by the Environmental Protection Agency (EPA) based on both health and esthetic considerations. Tables of primary and secondary regulated air contaminants and other regulatory issues that relate to air contaminants in the United States may be found at http://www.epa.gov. Many states within the USA also have individual air contaminant regulations that may be more rigorous than those of the EPA. Many other nations and some supragovernmental organizations regulate air contaminants. In the case of adjoining countries, transborder air and water pollution problems have been of concern in recent years. Particulates, radionuclides, acid rain, and similar problems have resulted in cross-contamination of air and water in different countries. Maritime contamination, too, has raised concern about oceanic pollution and has had an impact on the fisheries of some countries. This type of pollution is now the subject of much research and of new international treaties.

The United Nations Food and Agriculture Organization and the World Health Organization (FAO/WHO) Joint Expert Commission on Food Additives adopted the term acceptable daily intake (ADI) to denote the daily intake of a chemical from food that, during an entire lifetime, appears to be without appreciable risk. These guidelines are reevaluated as new information becomes available. In the United States, the FDA and the Department of Agriculture are responsible for the regulation of contaminants such as pesticides, drugs, and chemicals in foods. Major international problems have occurred because of traffic among nations in contaminated or adulterated foods from countries whose regulations and enforcement of pure food and drug laws are lax or nonexistent. Recently, for example, both human and animal illnesses have resulted from ingestion of products imported from China that contained melamine.

Ecotoxicology Ecotoxicology is concerned with the toxic effects of chemical and physical agents on populations and communities of living organisms within defined ecosystems; it includes the transfer pathways of those agents and their interactions with the environment. Traditional toxicology is concerned with toxic effects on individual organisms; ecotoxicology is concerned with the impact on populations of living organisms or on ecosystems. Ecotoxicology research has become one of the foremost areas of study for toxicologists.

TOXICOLOGIC TERMS & DEFINITIONS Hazard & Risk Hazard is the ability of a chemical agent to cause injury in a given situation or setting; the conditions of use and exposure are primary considerations. To assess hazard, one needs to have knowledge about both the inherent toxicity of the substance and the amounts to which individuals are liable to be exposed. Hazard is often a description based on subjective estimates rather than objective evaluation.

Risk is defined as the expected frequency of the occurrence of an undesirable effect arising from exposure to a chemical or physical agent. Estimation of risk makes use of dose-response data and extrapolation from the observed relationships to the expected responses at doses occurring in actual exposure situations. The quality and suitability of the biologic data used in such estimates are major limiting factors. Risk assessment has become an integral part of the regulatory process in most countries. However, many of the assumptions of risk assessment scientists remain unproven, and only long-term observation of population causes and outcomes will provide the basis for validation of newer risk assessment technologies.

Routes of Exposure The route of entry for chemicals into the body differs in different exposure situations. In the industrial setting, inhalation is the major route of entry. The transdermal route is also quite important, but oral ingestion is a relatively minor route. Consequently, primary prevention should be designed to reduce or eliminate absorption by inhalation or by topical contact. Atmospheric pollutants gain entry by inhalation and by dermal contact. Water and soil pollutants are absorbed through inhalation, ingestion, and dermal contact.

Quantity, Duration, & Intensity of Exposure Toxic reactions may differ depending on the quantity of exposure, its duration, and the rate at which the exposure takes place. An

exposure to a toxic substance that is absorbed by the target human or animal results in a dose. A single exposure, or multiple exposures that occur over a brief period from seconds to 1-2 days, represents acute exposure. Intense, rapidly absorbed acute doses of substances that may ordinarily be detoxified by enzymatic mechanisms in small doses may overwhelm the body’s ability to detoxify the substance and may result in serious or even fatal toxicity. The same amount of the substance, absorbed slowly, may result in little or no toxicity.

This is the case with cyanide exposure. Rhodanese, a mitochondrial enzyme present in humans, effectively detoxifies cyanide to relatively nontoxic thiocyanate when cyanide is presented in small amounts, but the enzyme is overwhelmed by large, rapidly encountered cyanide doses, with lethal effect.

Single or multiple exposures over a longer period of time represent chronic exposure. In the occupational setting, both acute (eg, accidental discharge) and chronic (eg, repetitive handling of a chemical) exposures occur. Exposures to chemicals found in the environment such as air and water pollutants are often chronic, resulting in chronic disease, as in the Minamata Bay, Japan, methyl mercury disaster. Sudden large chemical releases may result in acute massive population exposure with serious or lethal consequences.

The tragedy in Bhopal, India, was such an event, in which methyl isocyanate was released into a crowded population area, resulting in almost 4000 deaths and more than half a million injuries. The release of dioxin in Seveso, Italy, contaminated a populated area with a persistent organic chemical having both acute and long-term chronic effects. More recently, the massive oil spill caused by the explosion of BP’s Deepwater Horizon drilling rig in the Gulf of Mexico highlighted the potential for long-term ecotoxic impacts involving widespread geographic areas.

ENVIRONMENTAL CONSIDERATIONS Certain chemical and physical characteristics are important for the estimation of the potential hazard of environmental toxicants. Data on toxic effects of different organisms, along with knowledge about degradability, bioaccumulation, and transport and biomagnification through food chains, help in this estimation. (See Box: Bioaccumulation & Biomagnification, for a classic example involving the Great Lakes.) Poorly degraded chemicals (by abiotic or biotic pathways) exhibit environmental persistence and can accumulate. Such chemicals include the persistent organic pollutants (POPs), polychlorinated biphenyls, dioxins and furans, and similar substances.

Lipophilic substances such as the largely banned or abandoned organochlorine pesticides tend to bioaccumulate in body fat. This results in tissue residues that are slowly released over time. These residues and their metabolites may have chronic adverse effects such as endocrine disruption. When the toxicant is incorporated into the food chain, biomagnification occurs as one species feeds on others. This concentrates the chemical in organisms higher on the food chain. Humans stand at the apex of the food chain. They may be exposed to highly concentrated pollutant loads as bioaccumulation and biomagnification occur. The pollutants that have the widest environmental impact are poorly degradable; are relatively mobile in air, water, and soil; exhibit bioaccumulation; and also exhibit biomagnification.

SPECIFIC CHEMICALS AIR POLLUTANTS Air pollution may result from vapors, aerosols, smokes, particulates, and individual chemicals. Five major substances have been said to account for about 98% of air pollution: carbon monoxide (about 52%); sulfur oxides (about 14%); hydrocarbons (about 14%); nitrogen oxides (about 14%) and ozone, their breakdown product; and particulate matter (about 4%). Agriculture, especially industrial-scale farming, contributes a variety of air pollutants: dusts as particulates, pesticidal chemicals, hydrogen sulfide, and others. Sources of pollutants include fossil fuel burning, transportation, manufacturing, other industrial activities, generation of electric power, space heating, refuse disposal, and others. Studies in Helsinki and other cities have shown that uncatalyzed automobile traffic emissions are larger contributors to ground-level air pollution than any other source. The introduction of catalytic converters on automobiles and their mandatory use in many countries has greatly reduced automobile-released air pollution. In addition, the ban on tetraethyl lead in gasoline has eliminated a major source of lead contamination and childhood lead poisoning in urban environments. In emerging economies, the use of transport based on two-cycle engines creates heavy ground-level air pollution in very crowded cities. The introduction of “clean, lowsulfur” diesel fuels is helping to reduce urban and highway pollutants such as sulfur oxides.

Bioaccumulation & Biomagnification If the intake of a long-lasting contaminant by an organism exceeds the latter’s ability to metabolize or excrete the substance, the chemical accumulates within the tissues of the organism. This is called bioaccumulation.

Although the concentration of a contaminant may be virtually undetectable in water, it may be magnified hundreds or thousands of times as the contaminant passes up the food chain. This is called biomagnification.

The biomagnification of polychlorinated biphenyls (PCBs) in the Great Lakes of North America is illustrated by the following residue values available from a classic Environment Canada report published by the Canadian government, and elsewhere.

The biomagnification for this substance in the food chain, beginning with phytoplankton and ending with the herring gull, is nearly

50,000-fold. Domestic animals and humans may eat fish from the Great Lakes, resulting in PCB residues in these species as well.

Sulfur dioxide and smoke from incomplete combustion of coal have been associated with acute adverse effects among children, the elderly, and individuals with preexisting cardiac or respiratory disease. Ambient air pollution has been implicated as a cause of cardiac disease, bronchitis, obstructive ventilatory disease, pulmonary emphysema, bronchial asthma, and airway or lung cancer. Extensive basic science and clinical epidemiologic literature on air pollutant toxicology has been published and has led to modifications of regulatory standards for air pollutants. EPA standards for these substances apply to the general environment, and OSHA standards apply to workplace exposure. Ambient air standards for carbon monoxide and five other harmful pollutants—particulate matter, nitrogen dioxide, ozone, sulfur dioxide, and lead—may be found at http://www.epa.gov/air/criteria.html.

Carbon Monoxide Carbon monoxide (CO) is a colorless, tasteless, odorless, and non-irritating gas, a byproduct of incomplete combustion. The average concentration of CO in the atmosphere is about 0.1 ppm; in heavy traffic, the concentration may exceed 100 ppm. Current recommended permissible exposure limit (PEL) values are shown in Table 56–1 (see also http://www.osha.gov, 1910.1000, Table Z-1).

TABLE 56–1 Examples of permissible exposure limit values (PELs) of some common air pollutants and solvents in parts per million (ppm).1

1. Mechanism of action—CO combines tightly but reversibly with the oxygen-binding sites of hemoglobin and has an affinity for hemoglobin that is about 220 times that of oxygen. The product formed—carboxyhemoglobin—cannot transport oxygen. Furthermore, the presence of carboxyhemoglobin interferes with the dissociation of oxygen from the remaining oxyhemoglobin as a result of the Bohr effect. This reduces the transfer of oxygen to tissues. Organs with the highest oxygen demand (the brain, heart, and kidneys) are most seriously affected. Normal nonsmoking adults have carboxyhemoglobin levels of less than 1% saturation (1% of total hemoglobin is in the form of carboxyhemoglobin); this has been attributed to the endogenous formation of CO from heme catabolism. Smokers may exhibit 5– 10% CO saturation. The level depends on their smoking habits. A person who breathes air that contains 0.1% CO (1000 ppm) would have a carboxyhemoglobin level of about 50% in a short period of time.

2. Clinical effects—The principal signs of CO intoxication are those of hypoxia. They progress in the following sequence: (1) psychomotor impairment; (2) headache and tightness in the temporal area; (3) confusion and loss of visual acuity; (4) tachycardia, tachypnea, syncope, and coma; and (5) deep coma, convulsions, shock, and respiratory failure. There is great variability in individual responses to carboxyhemoglobin concentration. Carboxyhemoglobin levels below 15% may produce headache and malaise; at 25% many workers complain of headache, fatigue, decreased attention span, and loss of fine motor coordination. Collapse and syncope may appear at around 40%; and with levels above 60%, death may ensue as a result of irreversible damage to the brain and myocardium. The clinical effects may be aggravated by heavy labor, high altitudes, and high ambient temperatures. CO intoxication is usually thought of as a form of acute toxicity. There is evidence that chronic exposure to low CO levels may lead to adverse cardiac effects, neurologic disturbance, and emotional disorders. The developing fetus is quite susceptible to the effects of CO exposure. Exposure of a pregnant woman to elevated CO levels at critical periods of fetal development may cause fetal death or serious and irreversible but survivable birth defects.

3. Treatment—Patients who have been exposed to CO must be removed from the exposure source immediately. Respiration must be maintained and high flow and concentration of oxygen—the specific antagonist to CO—should be administered promptly. If respiratory failure is present, mechanical ventilation is required, High concentrations of oxygen may be toxic and may contribute to the development of acute respiratory distress syndrome. Therefore, patients should be treated with high concentrations only for a short period. With room air at 1 atm, the elimination half-time of CO is about 320 minutes; with 100% oxygen, the half-time is about 80 minutes; and with hyperbaric oxygen (2–3 atm), the half-time can be reduced to about 20 minutes. Although some controversy exists about hyperbaric oxygen for CO poisoning, it may be used if it is readily available. It is particularly recommended for the management of pregnant women exposed to CO. Hypothermic therapy to reduce metabolic demand of the brain has also been useful. Cerebral edema that results from CO poisoning does not seem to respond to either mannitol or steroid therapy and may be persistent. Progressive recovery from treated CO poisoning, even of a severe degree can be complete but some patients manifest neuropsychological and motor dysfunction for a long time after recovery from acute CO poisoning.

Sulfur Dioxide Sulfur dioxide (SO2 ) is a colorless irritant gas generated primarily by the combustion of sulfur-containing fossil fuels. The current OSHA PEL (Table 56–1) is given on the OSHA website (see http://www.osha.gov, 1910.1000, Table Z-1).

1. Mechanism of action—At room temperature, the solubility of SO2 is approximately 200 g SO2 /L of water. Because of its high solubility, when SO 2 contacts moist membranes, it transiently forms sulfurous acid. This acid has severe irritant effects on the eyes, mucous membranes, and skin. Approximately 90% of inhaled SO 2 is absorbed in the upper respiratory tract, the site of its principal effect. The inhalation of SO2 causes bronchial constriction and produces profuse bronchorrhea; parasympathetic reflexes and altered smooth muscle tone appear to be involved. The clinical outcome is an acute irritant asthma. Exposure to 5 ppm SO2 for 10 minutes leads to increased resistance to airflow in most humans. Exposures of 5–10 ppm are reported to cause severe bronchospasm; 10–20% of the healthy young adult population is estimated to be reactive to even lower concentrations. The phenomenon of adaptation to irritating concentrations has been reported in workers. However, current studies have not confirmed this phenomenon. Asthmatic individuals are especially sensitive to SO2 .

2. Clinical effects and treatment—The signs and symptoms of intoxication include irritation of the eyes, nose, and throat, reflex bronchoconstriction, and increased bronchial secretions. In asthmatic subjects, exposure to SO2 may result in an acute asthmatic episode.

If severe exposure has occurred, delayed-onset pulmonary edema may be observed. Cumulative effects from chronic low-level exposure to SO2 are not striking, particularly in humans, but these effects have been associated with aggravation of chronic cardiopulmonary disease. When combined exposure to high respirable particulate loads and SO2 occurs, the mixed irritant load may increase the toxic respiratory response. Treatment is not specific for SO 2 but depends on therapeutic maneuvers used to treat irritation of the respiratory tract and asthma. In some severely polluted urban air basins, elevated SO2 concentrations combined with elevated particulate loads has led to air pollution emergencies and a marked increase in cases of acute asthmatic bronchitis. Children and the elderly seem to be at greatest risk. The principal source of urban SO2 is the burning of coal, both for domestic heating and in coal-fired power plants. Highsulfur transportation fuels also contribute. Both also contribute to the respirable fine particulate load and to increased urban cardiorespiratory morbidity and mortality.

Nitrogen Oxides Nitrogen dioxide (NO2 ) is a brownish irritant gas sometimes associated with fires. It is formed also from fresh silage; exposure of farmers to NO2 in the confines of a silo can lead to silo-filler’s disease, a severe and potentially lethal form of acute respiratory distress syndrome. The disorder is uncommon today. Miners who are regularly exposed to diesel equipment exhaust have been particularly affected by nitrogen oxide emissions with serious respiratory effects. Today, the most common source of human exposure to oxides of nitrogen, including NO2 , is automobile and truck traffic emissions. Recent air pollution inventories in cities with high traffic congestion have demonstrated the important role that internal combustion engines have in the increasing NO2 urban air pollution. A variety of disorders of the respiratory system, cardiovascular system, and other problems have been linked to NO2 exposure.

1. Mechanism of action—NO2 is a relatively insoluble deep lung irritant. It is capable of producing pulmonary edema and acute adult respiratory distress syndrome (ARDS). Inhalation damages the lung infrastructure that produces the surfactant necessary to allow smooth and low effort lung alveolar expansion. The type I cells of the alveoli appear to be the cells chiefly affected by acute low to moderate inhalation exposure. At higher exposure, both type I and type II alveolar cells are damaged. If only type I cells are damaged, after an acute period of severe distress, it is likely that treatment with modern ventilation equipment and medications will result in recovery. Some patients develop nonallergic asthma, or “twitchy airway” disease, after such a respiratory insult. If severe damage to the type I and type II alveolar cells occurs, replacement of the type I cells may be impaired; progressive fibrosis may ensue that eventually

leads to bronchial ablation and alveolar collapse. This can result in permanent restrictive respiratory disease. In addition to the direct deep lung effect, long-term exposure to lower concentrations of nitrogen dioxide has been linked to cardiovascular disease, increased incidence of stroke, and other chronic disease.

The current PEL for NO 2 is given in Table 56–1. Exposure to 25 ppm of NO2 is irritating to some individuals; 50 ppm is moderately irritating to the eyes and nose. Exposure for 1 hour to 50 ppm can cause pulmonary edema and perhaps subacute or chronic pulmonary lesions; 100 ppm can cause pulmonary edema and death.

2. Clinical effects—The signs and symptoms of acute exposure to NO2 include irritation of the eyes and nose, cough, mucoid or frothy sputum production, dyspnea, and chest pain. Pulmonary edema may appear within 1–2 hours. In some individuals, the clinical signs may subside in about 2 weeks; the patient may then pass into a second stage of abruptly increasing severity, including recurring pulmonary edema and fibrotic destruction of terminal bronchioles (bronchiolitis obliterans). Chronic exposure of laboratory animals to 10–25 ppm NO2 has resulted in emphysematous changes; thus, chronic effects in humans are of concern.

3. Treatment—There is no specific treatment for acute intoxication by NO2 ; therapeutic measures for the management of deep lung irritation and noncardiogenic pulmonary edema are used. These measures include maintenance of gas exchange with adequate oxygenation and alveolar ventilation. Drug therapy may include bronchodilators, sedatives, and antibiotics. New approaches to the management of NO2 -induced ARDS have been developed and considerable controversy now exists about the precise respiratory protocol to use in any given patient.

Ozone & Other Oxides Ozone (O3 ) is a bluish irritant gas found in the earth’s atmosphere, where it is an important absorbent of ultraviolet light at high altitude.

At ground level, ozone is an important pollutant. Atmospheric ozone pollution is derived from photolysis of oxides of nitrogen, volatile organic compounds, and CO. These compounds are produced primarily when fossil fuels such as gasoline, oil, or coal are burned or when some chemicals (eg, solvents) evaporate. Nitrogen oxides are emitted from power plants, motor vehicles, and other sources of high-heat combustion. Volatile organic compounds are emitted from motor vehicles, chemical plants, refineries, factories, gas stations, paint, and other sources. An EPA fact sheet on ground-level ozone, its sources, and consequences may be found at http://www.epa.gov/glo/.

Ozone can be generated in the workplace by high-voltage electrical equipment, and around ozone-producing devices used for air and water purification. Agricultural sources of ozone are also important. There is a near-linear gradient between exposure to ozone (1-hour level, 20–100 ppb) and bronchial smooth muscle response. See Table 56–1 for the current PEL for ozone.

1. Mechanism of action and clinical effects—Ozone is an irritant of mucous membranes. Mild exposure produces upper respiratory tract irritation. Severe exposure can cause deep lung irritation, with pulmonary edema when inhaled at sufficient concentrations. Ozone penetration in the lung depends on tidal volume; consequently, exercise can increase the amount of ozone reaching the distal lung. Some of the effects of O3 resemble those seen with radiation, suggesting that O3 toxicity may result from the formation of reactive free radicals. The gas causes shallow, rapid breathing and a decrease in pulmonary compliance. Enhanced sensitivity of the lung to bronchoconstrictors is also observed. Exposure around 0.1 ppm O3 for 10–30 minutes causes irritation and dryness of the throat; above 0.1 ppm, one finds changes in visual acuity, substernal pain, and dyspnea. Pulmonary function is impaired at concentrations exceeding 0.8 ppm.

Airway hyperresponsiveness and airway inflammation have been observed in humans. The response of the lung to O3 is a dynamic one. The morphologic and biochemical changes are the result of both direct injury and secondary responses to the initial damage. Longterm exposure in animals results in morphologic and functional pulmonary changes. Chronic bronchitis, bronchiolitis, fibrosis, and emphysematous changes have been reported in a variety of species, including humans, exposed to concentrations above 1 ppm.

Increased visits to hospital emergency departments for cardiopulmonary disease during ozone alerts have been reported. A study of the basic physiologic responses of humans to ozone exposure and the biomarkers evoked provides useful insight into the fundamental toxicologic impacts of ozone.

2. Treatment—There is no specific treatment for acute O3 intoxication. Management depends on therapeutic measures used for deep lung irritation and noncardiogenic pulmonary edema that have resulted in ARDS. Current national ambient air quality standards for ozone are listed at http://www.epa.gov/air/criteria.html.

SOLVENTS Halogenated Aliphatic Hydrocarbons These “halohydrocarbon” agents once found wide use as industrial solvents, degreasing agents, and cleaning agents. The substances

include carbon tetrachloride, chloroform, trichloroethylene, tetrachloroethylene (perchloroethylene), and 1,1,1-trichloroethane (methyl chloroform). Many halogenated aliphatic hydrocarbons are classified as known or probable human carcinogens. Carbon tetrachloride and trichloroethylene have largely been removed from the workplace. Perchloroethylene and trichloroethane are still in use for dry cleaning and solvent degreasing, but it is likely that their use will be very limited in the future. The EPA now considers perchloroethylene a likely human carcinogen. The EPA data sheet may be found at http://www.epa.gov/ttnatw01/hlthef/tet-ethy.html. Dry cleaning as an occupation is listed as a class 2B carcinogenic activity by the International Agency for Research on Cancer (IARC). The Canadian Center for Occupational Health and Safety lists occupations and exposures to occupational carcinogens at http://www.ccohs.ca/oshanswers/diseases/carcinogen_occupation.html.

Fluorinated aliphatics such as the freons and closely related compounds have also been used in the workplace, in consumer goods, and in stationary and mobile air conditioning systems. Because of the severe damage they cause to the ozone layer in the troposphere, their use has been limited or eliminated by international treaty agreements. The common halogenated aliphatic solvents also create serious problems as persistent water pollutants. They are widely found in both groundwater and drinking water as a result of poor disposal practices.

Table 56–1 includes recommended OSHA PELs for several of these compounds (see also http://www.osha.gov, Table Z-1).

1. Mechanism of action and clinical effects—In laboratory animals, the halogenated hydrocarbons cause central nervous system (CNS) depression, liver injury, kidney injury, and some degree of cardiotoxicity. Several are also carcinogenic in animals and are considered probable human carcinogens. Trichloroethylene and tetrachloroethylene are listed as “reasonably anticipated to be a human carcinogen” by the U.S. National Toxicology Program, and as class 2A probable human carcinogens by IARC. These substances are depressants of the CNS in humans. Chronic workplace exposure to halogenated hydrocarbon solvents can cause significant neurotoxicity with impaired memory and peripheral neuropathy. All halohydrocarbon solvents can cause cardiac arrhythmias in humans, particularly in situations involving sympathetic excitation and norepinephrine release.

Hepatotoxicity is also a common toxic effect that can occur in humans after acute or chronic halohydrocarbon exposures.

Nephrotoxicity can occur in humans exposed to carbon tetrachloride, chloroform, and trichloroethylene. Chloroform, carbon tetrachloride, trichloroethylene, and tetrachloroethylene carcinogenicity have been observed in lifetime exposure studies performed in rats and mice and in some human epidemiologic studies. Dichloromethane (methylene chloride) is a potent neurotoxin, a generator of CO in humans, and a probable human carcinogen. It has been widely used as a paint stripper, plastic glue, and for other purposes. Epidemiologic studies of workers who have been exposed to aliphatic hydrocarbon solvents that include dichloromethane, trichloroethylene, and tetrachloroethylene have found significant associations between the agents and renal, prostate, and testicular cancer. Trichloroethylene is now considered a class 1, known human carcinogen by IARC; renal cancers and non-Hodgkin’s lymphoma have been reported. Other cancers are increased but their incidence has not reached statistical significance.

2. Treatment—There is no specific treatment for acute intoxication resulting from exposure to halogenated hydrocarbons. Management depends on the organ system involved.

Aromatic Hydrocarbons Benzene is used for its solvent properties and as an intermediate in the synthesis of other chemicals. It remains an important component of gasoline. Benzene may be found in premium gasolines at concentrations of about 1.5%. In cold climates such as Alaska, benzene concentrations in gasoline may reach 5% in order to provide an octane boost. It is one of the most widely used industrial chemicals in the world. The current PEL is 1.0 ppm in the air (see Table 56–1 and http://www.osha.gov, Table Z-1), and a 5 ppm limit is recommended for skin exposure. The National Institute for Occupational Safety and Health (NIOSH) and others have recommended that the exposure limits for benzene be further reduced to 0.1 ppm because excess blood cancers occur at the current PEL. The acute toxic effect of benzene is depression of the CNS. Exposure to 7500 ppm for 30 minutes can be fatal. Exposure to concentrations larger than 3000 ppm may cause euphoria, nausea, locomotor problems, and coma. Vertigo, drowsiness, headache, and nausea may occur at concentrations ranging from 250 to 500 ppm. No specific treatment exists for the acute toxic effect of benzene.

Chronic exposure to benzene can result in very serious toxic effects, the most significant of which is bone marrow injury. Aplastic anemia, leukopenia, pancytopenia, and thrombocytopenia occur, as does leukemia. Chronic exposure to low levels of benzene has been associated with leukemia of several types as well as lymphomas, myeloma, and myelodysplastic syndrome. Recent studies have shown the occurrence of leukemia following exposures as low as 2 ppm-years. The pluri-potent bone marrow stem cells appear to be targets of benzene or its metabolites and other stem cells may also be targets.

Benzene has long been known to be a potent clastogen, ie, a mutagen that acts by causing chromosomal breakage. Recent studies have suggested specific chromosome reorganization and genomic patterns that are associated with benzene-induced leukemia.

Epidemiologic data confirm a causal association between benzene exposure and leukemia and other bone marrow cancers in workers.

IARC classifies benzene as a class 1, known human carcinogen. Most national and international organizations classify benzene as a known human carcinogen.

Toluene (methylbenzene) does not possess the myelotoxic properties of benzene, nor has it been associated with leukemia. It is not

carcinogenic and is listed as class 3 by IARC. It is, however, a CNS depressant and a skin and eye irritant. It is also fetotoxic. See Table 56–1 and OSHA Tables Z-1, and Z-2 ( http://www.osha.gov) for the PELs. Exposure to 800 ppm can lead to severe fatigue and ataxia; 10,000 ppm can produce rapid loss of consciousness. Chronic effects of long-term toluene exposure are unclear because human studies indicating behavioral effects usually concern exposures to several solvents. In limited occupational studies, however, metabolic interactions and modification of toluene’s effects have not been observed in workers also exposed to other solvents. Less refined grades of toluene contain benzene. If technical grade toluene is to be used where there is human contact or exposure, analysis of the material for benzene content is advisable.

Xylene (dimethylbenzene) has been substituted for benzene in many solvent degreasing operations. Like toluene, the three xylenes do not possess the myelotoxic properties of benzene, nor have they been associated with leukemia. Xylene is a CNS depressant and a skin irritant. Less refined grades of xylene contain benzene. Estimated TLV-TWA and TLV-STEL are 100 and 150 ppm, respectively.

The current OSHA PELs may be found at http://www.osha.gov, Table Z-1.

PESTICIDES Organochlorine Pesticides These agents are usually classified into four groups: DDT (chlorophenothane) and its analogs, benzene hexachlorides, cyclodienes, and toxaphenes (Table 56–2). They are aryl, carbocyclic, or heterocyclic compounds containing chlorine substituents. The individual compounds differ widely in their biotransformation and capacity for storage in tissues; toxicity and storage are not always correlated.

They can be absorbed through the skin as well as by inhalation or oral ingestion. There are, however, important quantitative differences between the various derivatives; DDT in solution is poorly absorbed through the skin, whereas dieldrin absorption from the skin is very efficient. Organochlorine pesticides have largely been abandoned because they cause severe environmental damage. They are now known to be endocrine disrupters in animals and humans. DDT continues to have very restricted use for domestic mosquito elimination in malaria-infested areas of Africa. This use is controversial, but it is very effective and is likely to remain in place for the foreseeable future. Organochlorine pesticide residues in humans, animals, and the environment present long-term problems that are not yet fully understood.

TABLE 56–2 Organochlorine pesticides.

1. Human toxicology—The acute toxic properties of all the organochlorine pesticides in humans are qualitatively similar. These agents interfere with inactivation of the sodium channel in excitable membranes and cause rapid repetitive firing in most neurons. Calcium ion transport is inhibited. These events affect repolarization and enhance the excitability of neurons. The major effect is CNS stimulation.

With DDT, tremor may be the first manifestation, possibly continuing to convulsions, whereas with the other compounds convulsions often appear as the first sign of intoxication. There is no specific treatment for the acute intoxicated state, and management is symptomatic.

The potential carcinogenic properties of organochlorine pesticides have been extensively studied, and results indicate that chronic administration to laboratory animals over long periods results in enhanced carcinogenesis. Endocrine pathway disruption is the postulated mechanism. Numerous mechanisms for xenoestrogen (estrogen-like) carcinogenesis have been postulated. To date, however, several large epidemiologic studies in humans have not found a significant association between the risk of cancer and specific compounds or serum levels of organochlorine pesticide metabolites. The results of a case-control study conducted to investigate the relation between dichlorodiphenyldichloroethylene (DDE, the primary metabolite of DDT) and DDT breast adipose tissue levels and breast cancer risk did not confirm a positive association. In contrast, recent work supports an association between prepubertal exposure to DDT and brain cancer. Recent studies also suggest that the risk of testicular cancer and non-Hodgkin’s lymphoma is increased in persons with elevated organochlorine levels. Noncancer end points are also of concern. Recent work associates cryptorchidism and hypospadias in newborns with maternal adipose levels of chlordane metabolites. These residues are also linked to testicular cancer.

2. Environmental toxicology—The organochlorine pesticides are considered persistent chemicals. Degradation is quite slow when compared with other pesticides, and bioaccumulation, particularly in aquatic ecosystems, is well documented. Their mobility in soil depends on the composition of the soil; the presence of organic matter favors the adsorption of these chemicals onto the soil particles, whereas adsorption is poor in sandy soils. Once adsorbed, they do not readily desorb. These compounds induce significant abnormalities in the endocrine balance of sensitive animal and bird species, in addition to their adverse impact on humans. Since the early 1960s, when Rachel Carson’s work and subsequent book, Silent Spring, brought attention to the issue, the organochlorine pesticides have been recognized as pernicious environmental toxins. Their use is banned in most jurisdictions.

Organophosphorus Pesticides These agents, some of which are listed in Table 56–3, are used to combat a large variety of pests. They are useful pesticides when in direct contact with insects or when used as plant systemics, where the agent is translocated within the plant and exerts its effects on insects that feed on the plant. The many varieties currently in use are applied by spray techniques including hand, tractor, and aerial methods. They are often spread widely by wind and weather and are subject to widespread drift. The organophosphate pesticides are based on compounds such as soman, sarin, and tabun, which were developed for use as war gases. Some of the less toxic organophosphorus compounds are used in human and veterinary medicine as local or systemic antiparasitics (see Chapters 7 and 53).

The compounds are absorbed by the skin as well as by the respiratory and gastrointestinal tracts. Biotransformation is rapid, particularly when compared with the rates observed with the chlorinated hydrocarbon pesticides. Storm and collaborators reviewed current and suggested human inhalation occupational exposure limits for 30 organophosphate pesticides (see References).

TABLE 56–3 Organophosphorus pesticides.

1. Human toxicology—In mammals as well as insects, the major effect of these agents is inhibition of acetylcholinesterase through phosphorylation of the esteratic site. The signs and symptoms that characterize acute intoxication are due to inhibition of this enzyme and accumulation of acetylcholine; some of the agents also possess direct cholinergic activity. Specific treatment with antidotes and useful antagonists is available. In addition, pretreatment with physostigmine and other short-acting compounds may provide protection against these pesticides or their war gas analogs if used in timely fashion. These effects and their treatment are described in Chapters 7 and 8 of this book. Altered neurologic and cognitive functions, as well as psychological symptoms of variable duration, have been associated with exposure to these pesticides. Furthermore, there is some indication of an association of low arylesterase activity with neurologic symptom complexes in Gulf War veterans.

In addition to—and independently of—inhibition of acetylcholinesterase, some of these agents are capable of phosphorylating another enzyme present in neural tissue, the so-called neuropathy target esterase (NTE). This results in progressive demyelination of the longest nerves. Associated with paralysis and axonal degeneration, this lesion is sometimes called organophosphorus ester-induced

delayed polyneuropathy (OPIDP). Delayed central and autonomic neuropathy may occur in some poisoned patients. Hens are particularly sensitive to these properties and have proved very useful for studying the pathogenesis of the lesion and for identifying potentially neurotoxic organophosphorus derivatives. There is no specific treatment for NTE toxicity.

In humans, progressive chronic axonal neurotoxicity has been observed with triorthocresyl phosphate (TOCP), a noninsecticidal organophosphorus compound. It is also thought to occur with the pesticides dichlorvos, trichlorfon, leptophos, methamidophos, mipafox, trichloronat, and others. The polyneuropathy usually begins as burning and tingling sensations, particularly in the feet, with motor weakness occurring a few days later. Sensory and motor difficulties may extend to the legs and hands. Gait is affected, and ataxia may be present. Central nervous system and autonomic changes may develop still later. There is no specific treatment for this form of delayed neurotoxicity. The long-term prognosis of NTE inhibition is highly variable. Reports of this type of neuropathy (and other toxicities) in pesticide manufacturing workers and in agricultural pesticide applicators have been published (see References).

Recent clinical observation has also defined an intermediate syndrome in severely organophosphate-poisoned patients. This syndrome is characterized by neuromuscular transmission failure, and cardiac failure more typical of nicotinic than muscarinic poisoning.

Progressive neuromuscular failure leads to weakness of the respiratory muscles and eventually to death. The physiologic abnormalities are complex but involve a progressive decrement in neuromuscular junction transmission efficiency. Patients who develop this intermediate syndrome are at great risk of cardiorespiratory failure and may require mechanical ventilation. Because organophosphorus poisoning frequently occurs in less developed parts of the world where medical resources are very limited, the development of the intermediate syndrome is frequently a lethal complication. It is not effectively treated with the usual management protocol for organophosphate pesticide poisoning.

2. Environmental toxicologyâ&#x20AC;&#x201D;Organophosphorus pesticides are not considered to be persistent pesticides. They are relatively unstable and break down in the environment as a result of hydrolysis and photolysis. As a class they are considered to have a small permanent impact on the environment, in spite of their acute effects on organisms.

Carbamate Pesticides These compounds (Table 56â&#x20AC;&#x201C;4) inhibit acetylcholinesterase by carbamoylation of the esteratic site. Thus, they possess the toxic properties associated with inhibition of this enzyme as described for the organophosphorus pesticides. However, as described in Chapters 7 and 8, the binding is relatively weak, dissociation occurs after minutes to hours, and clinical effects are of shorter duration than those observed with organophosphorus compounds. Spontaneous reactivation of cholinesterase is more rapid after inhibition by the carbamates.

The therapeutic index, the ratio of the doses that cause severe toxicity or death to those that result in minor intoxication, is larger with carbamates than with the organophosphorus agents. Although the clinical approach to carbamate poisoning is similar to that for organophosphates, the use of pralidoxime is not recommended.

TABLE 56â&#x20AC;&#x201C;4 Carbamate pesticides.

The carbamates are considered to be nonpersistent pesticides. They exert only a small impact on the environment.

Botanical Pesticides Pesticides derived from natural sources include nicotine, rotenone, and pyrethrum. Nicotine is obtained from the dried leaves of Nicotiana tabacum and N rustica. It is rapidly absorbed from mucosal surfaces; the free alkaloid, but not the salt, is readily absorbed from the skin. Nicotine reacts with the acetylcholine receptor of the postsynaptic membrane (sympathetic and parasympathetic ganglia, neuromuscular junction), resulting in depolarization of the membrane. Toxic doses cause stimulation rapidly followed by blockade of transmission. These actions are described in Chapter 7. Treatment is directed toward maintenance of vital signs and suppression of convulsions. Nicotine analogs (neonicotinoids) have been developed for use as agricultural pesticides and have been accused of a role in bee colony collapse.

Rotenone (Figure 56â&#x20AC;&#x201C;1) is obtained from Derris elliptica, D mallaccensis, Lonchocarpus utilis, and L urucu. The oral ingestion of rotenone produces gastrointestinal irritation. Conjunctivitis, dermatitis, pharyngitis, and rhinitis can also occur. Treatment is symptomatic.

FIGURE 56â&#x20AC;&#x201C;1 Chemical structures of selected herbicides and pesticides.

Pyrethrum consists of six known insecticidal esters: pyrethrin I (Figure 56â&#x20AC;&#x201C;1), pyrethrin II, cinerin I, cinerin II, jasmolin I, and jasmolin II. Synthetic pyrethroids account for an increasing percentage of worldwide pesticide usage. Pyrethrum may be absorbed after inhalation

or ingestion. When absorbed in sufficient quantities, the major site of toxic action is the CNS; excitation, convulsions, and tetanic paralysis can occur. Voltage-gated sodium, calcium, and chloride channels are considered targets, as well as peripheral-type benzodiazepine receptors. Treatment of exposure is usually directed at management of symptoms. Anticonvulsants are not consistently effective. The chloride channel agonist, ivermectin, is of use, as are pentobarbital and mephenesin. The pyrethroids are highly irritating to the eyes, skin, and respiratory tree. They may cause irritant asthma and, potentially, reactive airways dysfunction syndrome (RADS) and even anaphylaxis. The most common injuries reported in humans result from their allergenic and irritant effects on the airways and skin.

Cutaneous paresthesias have been observed in workers spraying synthetic pyrethroids. The use of persistent synthetic pyrethroids to exterminate insects on aircraft has caused respiratory and skin problems as well as some neurologic complaints in flight attendants and other aircraft workers. Severe occupational exposures to synthetic pyrethroids in China resulted in marked effects on the CNS, including convulsions. Other previously unreported toxic manifestations have been manifest in pyrethrin-exposed individuals.

HERBICIDES Chlorophenoxy Herbicides 2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and their salts and esters have been used as herbicides for the destruction of weeds (Figure 56â&#x20AC;&#x201C;1). These compounds are of relatively low acute human toxicity. However, despite their low acute hazard, they cause serious long-term human and environmental toxicity. 2,4-D remains in wide commercial and domestic use for lawn weed control. 2,4,5-T had similar uses but was infamously incorporated into Agent Orange, used as a defoliant during the Vietnam conflict. Agent Orange was contaminated with 2,3,7,8-tetrachlorodibenzo- p-dioxin (a potent animal carcinogen and likely human carcinogen) and other toxic, persistent, and undesirable polychlorinated compounds. When this toxicity was discovered, the U.S. Department of Agriculture canceled the domestic pesticide registrations for trichlorophenoxy herbicides, and these compounds are no longer used. However, other, less thoroughly studied compounds, eg, chlorinated xanthenes, are present in both the dichlorophenoxy and trichlorophenoxy herbicides (see below).

In humans, 2,4-D in large doses can cause coma and generalized muscle hypotonia. Rarely, muscle weakness and marked hypotonia may persist for several weeks. In laboratory animals, signs of liver and kidney dysfunction have also been reported with chlorphenoxy herbicides. Several epidemiologic studies performed by the U.S. National Cancer Institute confirmed the causal link between 2,4-D and non-Hodgkinâ&#x20AC;&#x2122;s lymphoma. Evidence for a causal link to soft tissue sarcoma, however, is considered equivocal.

The dichlorophenoxy and related herbicides have been found to contain and to generate dimethylnitrosamine (Nnitrosodimethylamine; NDMA), a potent human carcinogen, during environmental transformation as well as non-chlorine water disinfection. Studies by Environment Canada and others have questioned the use of this compound because of water contamination.

Studies of related nitrosamine-forming herbicidal compounds have raised questions about the suitability of these compounds for general weed control. Because of the extremely high economic value of herbicides to the agricultural community, however, long-term decisions on their use have been delayed.

Glyphosate Glyphosate (N-[phosphonomethyl] glycine, Figure 56â&#x20AC;&#x201C;1), the principle ingredient in Roundup, is now the most widely used herbicide in the world. It functions as a contact herbicide and is absorbed through the leaves and roots of plants. It is generally formulated with surfactant to enhance its intended effect on noxious plants. Because it is nonselective, it may damage important crops and desirable ornamental plants even when used as directed. Therefore, genetically modified plants such as soybean, corn, and cotton that are glyphosate-resistant have been developed and patented. They are widely grown throughout the world. Almost all soybean crops and many corn crops grown today are of the glyphosate-resistant type. These genetically modified (GMO) crops are grown from patented seeds and have great economic value to growers, contributing to the food supply in a meaningful way. However, in some jurisdictions their use is highly controversial. While there is no evidence that the modified crops are toxic or dangerous to humans or animals, the long-term agricultural impact of widespread use of glyphosate herbicides on resistant crops remains to be determined. Additionally, the impact of effective weed elimination on the food supply and habitat of critical insect species, eg, bees, and some migrating birds has been a source of increasing concern.

Because of the widespread availability and use of this herbicide, glyphosate-surfactant poisonings are common. Many of the observed ingestions and reports of poisoning are from developing countries, where suicide by pesticide is common. Many injuries are minor, but some serious and lethal poisonings have been reported. Glyphosate is a significant eye and skin irritant. When ingested it can cause mild to moderate esophageal erosion. It also causes aspiration pneumonia and renal failure. There have been some reports of teratogenic outcomes in workers who handle and apply glyphosate, but the epidemiologic evidence is not clear. There is a growing literature on management of acute glyphosate poisoning. Treatment is symptomatic and no specific protocol is indicated. Hemodialysis has been used with success in cases of renal failure.

Although glyphosate seems to have little persistence and lower toxicity than other herbicides, the commercial formulations often contain surfactants and other active compounds that complicate the toxicity of the product. Some of the toxic effects are related to the

surfactant material.

Bipyridyl Herbicides Paraquat is the most important agent of this class (Figure 56–1). Its mechanism of action is said to be similar in plants and animals and involves single-electron reduction of the herbicide to free radical species. Ingestion (accidental or suicidal) is among the most serious and potentially lethal pesticide poisonings. Many serious exposures take place in developing countries where limited treatment resources are available. Paraquat accumulates slowly in the lung by an active process and causes lung edema, alveolitis, and progressive fibrosis. It probably inhibits superoxide dismutase, resulting in intracellular free-radical oxygen toxicity.

In humans, the first signs and symptoms after oral exposure are hematemesis and bloody stools. Within a few days, however, delayed toxicity occurs, with respiratory distress and the development of congestive hemorrhagic pulmonary edema accompanied by widespread cellular proliferation. During the acute period, oxygen should be used cautiously to combat dyspnea or cyanosis, because it may aggravate the pulmonary lesions. Hepatic, renal, or myocardial involvement may develop. The interval between ingestion and death may be several weeks.

Because of the delayed pulmonary toxicity, prompt immobilization of the paraquat to prevent absorption is important. Adsorbents (eg, activated charcoal, Fuller’s earth) are routinely given to bind the paraquat and minimize its absorption. Gastric lavage is not recommended as it may promote aspiration from the stomach into the lungs. Once the paraquat is absorbed, treatment is successful in fewer than 50% of cases. Monitoring of plasma and urine paraquat concentrations is useful for prognostic assessment. CT scanning has also been used to follow the pulmonary lesions as they develop and to help with prognosis. The pulmonary proliferative phase begins 1–2 weeks after paraquat ingestion. Although a few reports indicate some success with dialysis, hemodialysis and hemoperfusion rarely change the clinical course. Many approaches have been used to slow or stop the progressive pulmonary fibrosis. Immunosuppression using corticosteroids and cyclophosphamide is widely practiced, but evidence for efficacy is weak. Antioxidants such as acetylcysteine and salicylate might be beneficial through free radical-scavenging, anti-inflammatory, and nuclear factor kappa-B inhibitory actions.

However, there are no published human trials. The case fatality rate is high in all centers despite large variations in treatment. Patients require prolonged observation and treatment for respiratory and renal insufficiency if they survive the acute stage of poisoning.

ENVIRONMENTAL POLLUTANTS Polychlorinated & Polybrominated Biphenyls Highly halogenated biphenyl compounds, which have desirable properties for insulation, fire retardancy, and many other uses, were manufactured in large quantities during the mid-20th century. The quantities produced and the almost universal dispersion of the materials in which they were incorporated have produced an enormous environmental problem. Both chlorinated and brominated biphenyls are environmentally dangerous and significantly toxic, and are now banned from use.

The polychlorinated biphenyls (PCBs, coplanar biphenyls) were used as dielectric and heat transfer fluids, lubricating oils, plasticizers, wax extenders, and flame retardants. Their industrial use and manufacture in the USA were terminated by 1977. The chlorinated products used commercially were actually mixtures of PCB isomers and homologs containing 12–68% chlorine. These chemicals are very stable, highly lipophilic, poorly metabolized, and very resistant to environmental degradation; thus they bioaccumulate in food chains.

Food is the major source of PCB residues in humans. Accumulation of PCB in fish species led Canada and the USA to restrict commercial fishing and to limit consumption of fish from the Great Lakes of North America (see Box: Bioaccumulation & Biomagnification, earlier). In addition, large industrial site contamination, illegal dumping, migration from hazardous waste sites and other large-scale sources, and widespread use of PCBs in electrical transformers has led to multiple localized areas of contamination and human exposure. Leakage of transformer dielectric fluids in neighborhoods and backyards has caused significant numbers of serious but highly localized PCB exposure events.

There are numerous reports of large population exposures to PCBs. A serious exposure to PCBs—lasting several months—occurred in Japan in 1968 as a result of cooking oil contamination with PCB-containing transfer medium (Yusho disease). A similar outbreak called Yucheng disease occurred at about the same time in Taiwan. Effects on the fetus and on the development of the offspring of poisoned women were reported. It is now known that the contaminated cooking oil contained not only PCBs but also polychlorinated dibenzofurans (PCDFs) and polychlorinated quaterphenyls (PCQs). It is likely that the effects initially attributed to the PCBs were actually caused by a mixture of contaminants. Workers occupationally exposed to PCBs develop dermatologic problems that include chloracne, folliculitis, erythema, dryness, rash, hyperkeratosis, and hyperpigmentation. Some hepatic abnormalities have been found in PCB poisoning, and plasma triglycerides are elevated.

Information about the effects of PCBs on reproduction and development is accumulating. The halogenated pesticides are potent endocrine disrupters and there is widespread concern about the persistent estrogenic effect of these chemicals. Adverse reproductive impacts of PCBs have been found in many animal studies. Direct teratogenic effects in humans have yet to be established: studies in workers and in the general population exposed to moderate or to very high levels of PCBs have not been conclusive. Some adverse

behavioral effects in infants have been reported. An association between prenatal exposure to PCBs and deficits in childhood intellectual function was described for children born to mothers who had eaten large quantities of contaminated fish. Epidemiologic studies have established increases in various cancers including melanoma, breast, pancreatic, and thyroid cancers. These findings and animal studies provided a sufficient basis for the IARC to classify some co-planar PCBs as class 1, carcinogenic to humans, in volume 100 of the IARC monographs. A comprehensive EPA fact sheet on PCBs may be found at http://www.epa.gov/epawaste/hazard/tsd/pcbs/index.htm.

The polybrominated biphenyls (PBBs) and their esters (PBDEs) share many of the toxic and environmentally damaging persistent qualities of PCBs. They were introduced as fire retardants in the 1950s and have been used in massive quantities since that time. The biphenyls are no longer produced and may no longer be used, but the biphenyl esters remain in use as fire retardants in plastics for bedding and in automobile upholstery. PBB fire retardant contamination has been extensive in the Great Lakes region, resulting in large exposure to the population. PBBs are considered IARC class 2a: probable human carcinogens. PDBEs are not classified. An EPA technical fact sheet on PBB and PBDEs may be found at http://www2.epa.gov/fedfac/technical-fact-sheet-polybrominated-diphenylethers-pbdes-and-polybrominated-biphenyls-pbbs.

T h e polychlorinated dibenzo-p-dioxins (PCDDs), or dioxins, are a group of halogenated congeners of which tetrachlorodibenzodioxin (TCDD) has been the most carefully studied. There is a large group of dioxin-like compounds, including polychlorinated dibenzofurans (PCDFs) and coplanar biphenyls. While PCBs were used commercially, PCDDs and PCDFs are unwanted byproducts that appear in the environment and in manufactured products as contaminants because of improperly controlled combustion processes. They are also produced when unexpected heating to temperatures over 600° C occurs as in lightning strikes or electrical fires in PCB-containing transformers. Like PCBs, these chemicals are very stable and highly lipophilic. They are poorly metabolized and very resistant to environmental degradation. Several significant environmental contamination episodes involving dioxins and furans from industrial sites have occurred. Recent publications have demonstrated an elevated incidence of subsequent chronic diseases (eg, diabetes, metabolic syndrome, and obesity) in exposed persons. Laboratory studies of the blood concentrations of TCDD and its metabolites have provided insight into the persistence and metabolism of the contaminants.

In laboratory animals, TCDD has produced a variety of toxic effects. Wasting syndrome (severe weight loss accompanied by reduction of muscle mass and adipose tissue), thymic atrophy, epidermal changes, hepatotoxicity, immunotoxicity, effects on reproduction and development, teratogenicity, and carcinogenicity have been produced. The effects observed in workers involved in the manufacture of 2,4,5-T (and therefore presumably exposed to TCDD) consisted of contact dermatitis and chloracne. In severely TCDD-intoxicated patients, discrete chloracne may be the only manifestation.

The presence of TCDD in 2,4,5-T, commercially known as Silvex, was believed to be responsible for other human toxicities associated with the herbicide. There is epidemiologic evidence for an association between occupational exposure to the phenoxy herbicides and an excess incidence of non-Hodgkin’s lymphoma. The TCDD contaminant in these herbicides seems to play a role in a number of cancers such as soft tissue sarcomas, lung cancer, Hodgkin’s lymphomas, and others. TCDD is considered an IARC class 1, known human carcinogen. Other halogenated compounds of this type are not currently classifiable as to carcinogenicity; they are listed as IARC class 3.

Perfluorinated Compounds (PFCs) Fluorinated hydrocarbon chemicals have been of commercial interest since the mid-20th century. Their uses have included coolant materials in air conditioning systems; artificial oxygen-carrying substances in experimental clinical studies; and heat-, stain-, and stickresistant coatings for cookware, fabrics, and other materials. The fluorocarbons were produced in very large quantities and have become widespread in the environment. When it later became apparent that migration of lower molecular weight fluorocarbons to the troposphere had a deleterious effect on the protective ozone layer, they were banned from use. The higher molecular weight, more highly fluorinated compounds, now called perfluorinated substances (eg, Teflon), have remained in broad use. Like the heavily chlorinated and brominated hydrocarbons, their commercial usefulness has been complicated by a recognition of adverse environmental and suspected human toxic impacts that resemble some of the adverse qualities of the other halogenated hydrocarbons. A useful reference is the Centers for Disease Control (CDC) fact sheet on PFCs. It is found at http://www.cdc.gov/biomonitoring/pdf/PFCs_FactSheet.pdf.

1. Human toxicology—Concerns about the toxicology of PFCs have centered on their estrogenic properties and accumulation and persistence in humans. Human exposure to perfluoro compounds takes place through ingestion and inhalation. Since these compounds enter the food chain and water sources and are persistent, ingestion of contaminated food and water products is a major source of human accumulation. The human half-life of PFOA is estimated to be about 3 years. As a persistent chemical and an endocrine disrupter, it is likely that it has some long-term adverse impact on reproductive function, cellular proliferation, and other cellular homeostatic mechanisms. Several PFCs (but not perfluoro compounds derived from PFOA) have been found to act as proliferators of breast cancer cells. However, a large epidemiologic study recently demonstrated a statistically significant association between high and very high serum PFOA levels in workers and kidney cancer, and possibly prostate cancer, ovarian cancer, and non-Hodgkin’s lymphoma. There also may be modest associations with cholesterol elevation and uric acid abnormalities. Finally, an acute pulmonary disorder, polymer fume fever, is caused by the pyrolysis of PFOA. Like metal fume fever, seen in welders as a result of cadmium vaporization, polymer fume fever has an acute onset several hours after exposure to the vaporized PFOA and may cause severe respiratory distress. The onset of

constitutional symptoms, malaise, chills and fever, and respiratory distress is characteristic of fume fevers. While polymer fume fever is usually mild and self-limited, noncardiogenic pulmonary edema has occurred. Whenever PFOA is heated above 350–400° C, toxic fumes capable of causing polymer fume fever are emitted. Overheated household cookware or burning of coated fabrics present this risk.

Other human effects are not clearly defined, although animal studies have shown toxic effects on immune, liver, and endocrine function, and some increase in tumors and neonatal deaths. A useful American Cancer Society fact sheet on the subject may be found at http://www.cancer.org/cancer/cancercauses/othercarcinogens/athome/teflon-and-perfluorooctanoic-acid--pfoa.

2. Environmental toxicology—Perfluoro compounds are persistent environmental chemicals having a broad environmental impact.

PFOA and related compounds are now found widely in water, soil, and many terrestrial and avian species. Aquatic organisms have accumulated significant loads of PFCs. An extensive risk assessment of the perfluoro chemicals has been carried out by Environment Canada, and guidelines have been developed for the management of PFOA and related compounds. These may be found at http://www.ec.gc.ca/ese-ees/default.asp?lang=En&n=451C95ED-1.

Endocrine Disruptors As described above, the potential hazardous effects of some chemicals in the environment are receiving considerable attention because of their estrogen-like or antiandrogenic properties. Compounds that affect thyroid function are also of concern. Since 1998, the process of prioritization, screening, and testing of chemicals for such actions has been undergoing worldwide development. These chemicals mimic, enhance, or inhibit a hormonal action. They include a number of plant constituents (phytoestrogens) and some mycoestrogens as well as industrial chemicals, persistent organochlorine agents (eg, DDT), PCBs, and brominated flame retardants. Concerns exist because of their increasing contamination of the environment, the appearance of bioaccumulation, and their potential for toxicity. In vitro assays alone are unreliable for regulatory purposes, and animal studies are considered indispensable. Modified endocrine responses in reptiles and marine invertebrates have been observed. In humans, however, a causal relation between exposure to a specific environmental agent and an adverse health effect due to endocrine modulation has not been fully established. Epidemiologic studies of populations exposed to higher concentrations of endocrine-disrupting environmental chemicals are underway. There are indications that breast and other reproductive cancers are increased in these patients. Prudence dictates that exposure to environmental chemicals that disrupt endocrine function should be reduced.

Asbestos Asbestos in many of its forms has been widely used in industry for over 100 years. All forms of asbestos have been shown to cause progressive fibrotic lung disease (asbestosis), lung cancer, and mesothelioma. Every form of asbestos, including chrysotile asbestos, causes an increase in lung cancer and mesothelioma. Lung cancer occurs in people exposed at fiber concentrations well below concentrations that produce asbestosis. Very large scale studies of insulation workers have shown that cigarette smoking and exposure to radon daughters increase the incidence of asbestos-caused lung cancer in a synergistic fashion. Asbestos exposure and smoking is a very hazardous combination.

All forms of asbestos cause mesothelioma of the pleura or peritoneum at very low doses. Other cancers (colon, laryngeal, stomach, and perhaps even lymphoma) are increased in asbestos-exposed patients. The mechanism for asbestos-caused cancer is not yet delineated. Arguments that chrysotile asbestos does not cause mesothelioma are contradicted by many epidemiologic studies of worker populations. Recognition that all forms of asbestos are dangerous and carcinogenic has led many countries to ban all uses of asbestos.

Countries such as Canada, Zimbabwe, Russia, Brazil, and others that still produce asbestos argue that asbestos can be used safely with careful workplace environmental controls. However, studies of industrial practice make the “safe use” of asbestos highly improbable.

Recent attempts to limit international trade in asbestos have been thwarted by heavy pressure from the asbestos industry and the producing countries. Information on countries that currently ban asbestos and the International Ban Asbestos movement may be found at http://ibasecretariat.org/alpha_ban_list.php.

METALS Occupational and environmental poisoning with metals, metalloids, and metal compounds is a major health problem. Toxic metal exposure occurs in many industries, in the home, and elsewhere in the nonoccupational environment. The classic metal poisons (arsenic, lead, and mercury) continue to be widely used. (Treatment of their toxicities is discussed in Chapter 57.) Occupational exposure and poisoning due to beryllium, cadmium, manganese, and uranium are relatively new occupational problems.

Beryllium Beryllium (Be) is a light alkaline metal that confers special properties on the alloys and ceramics in which it is incorporated. Berylliumcopper alloys find use as components of computers, in the encasement of the first stage of nuclear weapons, in devices that require

hardening such as missile ceramic nose cones, and in heat shield tiles used in space vehicles. Because of the use of beryllium in dental appliances, dentists and dental appliance makers are often exposed to beryllium dust in toxic concentrations and may develop beryllium disease.

Beryllium is highly toxic by inhalation and is classified by the IARC as a class 1, known human carcinogen. Inhalation of beryllium particles produces both acute beryllium disease and chronic disease characterized by progressive pulmonary fibrosis. Skin disease also develops in workers exposed to beryllium. The pulmonary disease is called chronic beryllium disease (CBD) and is a chronic granulomatous pulmonary fibrosis. In the 5â&#x20AC;&#x201C;15% of the population that is immunologically sensitive to beryllium, CBD is the result of activation of an autoimmune attack on the skin and lungs. The disease is progressive and may lead to severe disability, cancer, and death.

Although some treatment approaches to CBD show promise, the prognosis is poor in most cases.

The current permissible exposure levels for beryllium of 0.01 mcg/m3 averaged over a 30-day period or 2 mcg/m3 over an 8-hour period are insufficiently protective to prevent CBD. Both NIOSH and the ACGIH have recommended that the 8-hour PEL and TLV be reduced to 0.05 mcg/m3 . These recommendations have not yet been implemented. Current OSHA information on beryllium appears at https://www.osha.gov/SLTC/beryllium/index.html.

Environmental beryllium exposure is not generally thought to be a hazard to human health except in the vicinity of industrial sites where air, water, and soil pollution have occurred.

Cadmium Cadmium (Cd) is a transition metal widely used in industry. Workers are exposed to cadmium in the manufacture of nickel cadmium batteries, pigments, low-melting-point eutectic materials; in solder; in television phosphors; and in plating operations. It is also used extensively in semiconductors and in plastics as a stabilizer. Cadmium smelting is often done from residual dust from lead smelting operations, and cadmium smelter workers often face both lead and cadmium toxicity.

Cadmium is toxic by inhalation and by ingestion. When metals that have been plated with cadmium or welded with cadmiumcontaining materials are vaporized by the heat of torches or cutting implements, the fine dust and fumes released produce an acute respiratory disorder called cadmium fume fever. This disorder, common in welders, is usually characterized by shaking chills, cough, fever, and malaise. Although it may produce pneumonia, it is usually transient. However, chronic exposure to cadmium dust produces a far more serious progressive pulmonary fibrosis. Cadmium also causes severe kidney damage, including renal failure if exposure continues. Cadmium is a human carcinogen and is listed as a class 1, known human carcinogen by the IARC. The current OSHA PEL for cadmium is 5 mcg/m 3 but is insufficiently protective of worker health. The OSHA cadmium standard may be found at https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=10035.

Nanomaterials Nanomaterials are defined as any material, natural or manufactured, that has at least one dimension that lies between 1 and 100 nanometers (nm) in size. The Stanford University Health and Safety Department gives a more precise definition at http://www.stanford.edu/dept/EHS/prod/researchlab/IH/nano/what_are_nanomaterials.html.

Nanomaterials have been of increasing commercial interest and are now used for an extraordinary range of purposes. In the pharmaceutical manufacturing industry, nanoparticles are being tested and used to deliver cancer chemotherapeutic and other drugs.

Currently produced nanomaterials include gold, silver, cadmium, germanium, ceramic, and aluminum oxide nanowires; carbon, silicon, and germanium nanotubes; zinc oxide nanocrystals; gold nanowafers; and copper oxide nanocubes. The increasing use of nanomaterials has led to release of these nanoscale substances into the workplace and the general environment. Because nanomaterials behave in unique patterns of chemical and physical reactivity, their toxicology is often novel and there is insufficient information on the likely human or environmental impact of dispersal of these manufactured products in the environment. The University of North Carolina laboratory safety and health manual outlines the problems of working with nanomaterials in the laboratory and their safe use at http://ehs.unc.edu/manuals/laboratory/docs/lsm18.pdf.

1. Human toxicologyâ&#x20AC;&#x201D;Inhalation, oral ingestion, dermal absorption, and parenteral administration of nanomaterials have been the sources of human exposure. Because of the unique physicochemical properties of nanomaterials, their toxicity may be similar to or very different from the larger, bulk materials encountered in traditional toxicology studies. The nature of the exposure will impact the likelihood that nanomaterials will reach target organs or cells. Nanomaterials can cross cellular membranes, penetrate nuclear material and genetic information, and may impact cellular response at a nanoscale. Silica nanoparticles have been demonstrated to produce kidney toxicity in humans, and zinc oxide nanoparticles are toxic to human liver cells. Multiwalled carbon nanotubes have been found to be cytotoxic in human lung cells. Titanium dioxide nanoparticles that are widely used in sunscreens, other cosmetics, pharmaceuticals, and many other products have been noted to be toxic in the lungs and elsewhere.

2. Environmental toxicologyâ&#x20AC;&#x201D;Nanomaterials can enter the environment at all stages of their industrial life cycle, including manufacturing, delivery, use, and disposal. When nanomaterials are placed into waste streams they may enter water systems, or be

carried by wind or soils, and enter the food chain. An EPA fact sheet on nanomaterials in the environment is available at http://www.epa.gov/athens/research/nano.html.

The increasing production of nanomaterials and their multiple uses has led to environmental contamination. Many species, including bacteria, small mammals, and fish and other aquatic organisms have been studied in laboratory assessments of nanomaterial toxicity. The ecotoxicology of nanomaterials remains an area of deep concern and ongoing research.

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_______________ * T he author thanks the late Gabriel L. Plaa, PhD, previous author of this chapter, for his enduring contributions.

CHAPTER

57 Heavy Metal Intoxication & Chelators Michael J. Kosnett, MD, MPH

CASE STUDY A 48-year-old painter is referred for evaluation of recent onset of severe abdominal pains, headaches, and myalgias. For the last week, he has been removing old paint from an iron bridge using grinding tools and a blow torch. His employer states that all the bridge workers are provided with the equivalent of “hazmat” (hazardous materials) suits. What tests should be carried out?

Assuming positive test results, what therapy would be appropriate?

Some metals such as iron are essential for life, whereas others such as lead are present in all organisms but serve no useful biologic purpose. Some of the oldest diseases of humans can be traced to heavy metal poisoning associated with metal mining, refining, and use.

Even with the present recognition of the hazards of heavy metals, the incidence of intoxication remains significant, and the need for preventive strategies and effective therapy remains high. Toxic heavy metals interfere with the function of essential cations, cause enzyme inhibition, generate oxidative stress, and alter gene expression. As a result, multisystem signs and symptoms are a hallmark of heavy metal intoxication.

When intoxication occurs, chelator molecules (from chela “claw”), or their in vivo biotransformation products, may be used to bind the metal and facilitate its excretion from the body. Chelator drugs are discussed in the second part of this chapter.

TOXICOLOGY OF HEAVY METALS LEAD Lead poisoning is one of the oldest occupational and environmental diseases in the world. Despite its recognized hazards, lead continues to have widespread commercial application, including production of storage batteries (nearly 90% of US consumption), ammunition, metal alloys, solder, glass, plastics, pigments, and ceramics. Corrosion of lead plumbing in older buildings or supply lines may increase the lead concentration of tap water. Environmental lead exposure, ubiquitous by virtue of the anthropogenic distribution of lead to air, water, and food, has declined considerably in the last three decades as a result of the elimination of lead as an additive in gasoline, as well as diminished contact with lead-based paint and other lead-containing consumer products, such as lead solder in cans used as food containers. Lead continues to be used in some formulations of aviation gasoline for piston-engine aircraft. The presence of lead in certain folk medicines (eg, the Mexican remedies azarcon and greta, and certain Ayurvedic preparations) and in cosmetics (eg, kohl utilized around the eyes in certain African and Asian communities) has contributed to lead exposure to children and adults. Although public health measures, together with improved workplace conditions, have decreased the incidence of serious overt lead poisoning, there remains considerable concern over the effects of low-level lead exposure. Extensive evidence indicates that low levels of lead exposure may have subtle subclinical adverse effects on neurocognitive function in children and may contribute to hypertension and cardiovascular disease in adults. Lead serves no useful purpose in the human body. In key target organs such as the developing central nervous system, no level of lead exposure has been shown to be without deleterious effects.

Pharmacokinetics Inorganic lead is slowly but consistently absorbed via the respiratory and gastrointestinal tracts. It is poorly absorbed through the skin.

Absorption of lead dust via the respiratory tract is the most common cause of industrial poisoning. The intestinal tract is the primary route of entry in nonindustrial exposure (Table 57–1). Absorption via the gastrointestinal tract varies with the nature of the lead compound, but

in general, adults absorb about 10â&#x20AC;&#x201C;15% of the ingested amount, whereas young children absorb up to 50%. Low dietary calcium, iron deficiency, and ingestion on an empty stomach all have been associated with increased lead absorption.

TABLE 57â&#x20AC;&#x201C;1 Toxicology of selected arsenic, lead, and mercury compounds.

Once absorbed from the respiratory or gastrointestinal tract, lead enters the bloodstream, where approximately 99% is bound to erythrocytes and 1% is present in the plasma. Lead is subsequently distributed to soft tissues such as the bone marrow, brain, kidney, liver, muscle, and gonads; then to the subperiosteal surface of bone; and later to bone matrix. Lead also crosses the placenta and poses a potential hazard to the fetus. The kinetics of lead clearance from the body follows a multicompartment model, composed predominantly of the blood and soft tissues, with a half-life of 1–2 months; and the skeleton, with a half-life of years to decades. Approximately 70% of the lead that is eliminated appears in the urine, with lesser amounts excreted through the bile, skin, hair, nails, sweat, and breast milk. The fraction not undergoing prompt excretion, approximately half of the absorbed lead, may be incorporated into the skeleton, the repository of more than 90% of the body lead burden in most adults. In patients with high bone lead burdens, slow release from the skeleton may elevate blood lead concentrations for years after exposure ceases, and pathologic high bone turnover states such as hyperthyroidism or prolonged immobilization may result in frank lead intoxication. Migration of retained lead bullet fragments into a joint space or adjacent to bone has been associated with the development of lead poisoning signs and symptoms years or decades after an initial gunshot injury.

Pharmacodynamics Lead exerts multisystemic toxic effects that are mediated by multiple modes of action, including inhibition of enzymatic function; interference with the action of essential cations, particularly calcium, iron, and zinc; generation of oxidative stress; changes in gene expression; alterations in cell signaling; and disruption of the integrity of membranes in cells and intracellular organelles.

A. Nervous System The developing central nervous system of the fetus and young child is the most sensitive target organ for lead’s toxic effect.

Epidemiologic studies suggest that blood lead concentrations less than 5 mcg/dL may result in subclinical deficits in neurocognitive function in lead-exposed young children, with no demonstrable threshold or “no effect” level. The dose response between low blood lead concentrations and cognitive function in young children is nonlinear, such that the decrement in intelligence associated with an increase in blood lead from less than 1 to 10 mcg/dL (6.2 IQ points) exceeds that associated with a change from 10 to 30 mcg/dL (3.0 IQ points).

Adults are less sensitive to the CNS effects of lead, but long-term exposure to blood lead concentrations in the range of 10–30 mcg/dL may be associated with subclinical effects on neurocognitive function. At blood lead concentrations higher than 30 mcg/dL, behavioral and neurocognitive signs or symptoms may gradually emerge, including irritability, fatigue, decreased libido, anorexia, sleep disturbance, impaired visual-motor coordination, and slowed reaction time. Headache, arthralgias, and myalgias are also common complaints. Tremor occurs but is less common. Lead encephalopathy, usually occurring at blood lead concentrations higher than 100 mcg/dL, is typically accompanied by increased intracranial pressure and may cause ataxia, stupor, coma, convulsions, and death. Recent epidemiological studies suggest that lead may accentuate an age-related decline in cognitive function in older adults. In experimental animals, developmental lead exposure, possibly acting through epigenetic mechanisms, has been associated with increased expression of beta-amyloid, increased phosphorylated tau protein, oxidative DNA damage, and Alzheimer’s-type pathology in the aging brain. There is wide interindividual variation in the magnitude of lead exposure required to cause overt lead-related signs and symptoms.

Overt peripheral neuropathy may appear after chronic high-dose lead exposure, usually following months to years of blood lead concentrations higher than 100 mcg/dL. Predominantly motor in character, the neuropathy may present clinically with painless weakness of the extensors, particularly in the upper extremity, resulting in classic wrist-drop. Preclinical signs of lead-induced peripheral nerve dysfunction may be detectable by electrodiagnostic testing.

B. Blood Lead can induce an anemia that may be either normocytic or microcytic and hypochromic. Lead interferes with heme synthesis by blocking the incorporation of iron into protoporphyrin IX and by inhibiting the function of enzymes in the heme synthesis pathway, including aminolevulinic acid dehydratase and ferrochelatase. Within 2–8 weeks after an elevation in blood lead concentration (generally to 30–50 mcg/dL or greater), increases in heme precursors, notably free erythrocyte protoporphyrin or its zinc chelate, zinc protoporphyrin, may be detectable in whole blood. Lead also contributes to anemia by increasing erythrocyte membrane fragility and decreasing red cell survival time. Frank hemolysis may occur with high exposure. Basophilic stippling on the peripheral blood smear, thought to be a consequence of lead inhibition of the enzyme 3™,5™-pyrimidine nucleotidase, is sometimes a suggestive—albeit insensitive and nonspecific—diagnostic clue to the presence of lead intoxication.

C. Kidneys Chronic high-dose lead exposure, usually associated with months to years of blood lead concentrations greater than 80 mcg/dL, may result in renal interstitial fibrosis and nephrosclerosis. Lead nephropathy may have a latency period of years. Lead may alter uric acid excretion by the kidney, resulting in recurrent bouts of gouty arthritis (“saturnine gout”). Acute high-dose lead exposure sometimes produces transient azotemia, possibly as a consequence of intrarenal vasoconstriction. Studies conducted in general population samples have documented an association between blood lead concentration and measures of renal function, including serum creatinine and creatinine clearance. The presence of other risk factors for renal insufficiency, including hypertension and diabetes, may increase

susceptibility to lead-induced renal dysfunction.

D. Reproductive Organs High-dose lead exposure is a recognized risk factor for stillbirth or spontaneous abortion. Epidemiologic studies of the impact of low-level lead exposure on reproductive outcome such as low birth weight, preterm delivery, or spontaneous abortion have yielded mixed results.

However, a well-designed nested case-control study detected an odds ratio for spontaneous abortion of 1.8 (95% CI 1.1–3.1) for every 5 mcg/dL increase in maternal blood lead across an approximate range of 5–20 mcg/dL. Recent studies have linked prenatal exposure to low levels of lead (eg, maternal blood lead concentrations of 5–15 mcg/dL) to decrements in physical and cognitive development assessed during the neonatal period and early childhood. In males, blood lead concentrations higher than 40 mcg/dL have been associated with diminished or aberrant sperm production.

E. Gastrointestinal Tract Moderate lead poisoning may cause loss of appetite, constipation, and, less commonly, diarrhea. At high dosage, intermittent bouts of severe colicky abdominal pain (“lead colic”) may occur. The mechanism of lead colic is unclear but is believed to involve spasmodic contraction of the smooth muscles of the intestinal wall, mediated by alteration in synaptic transmission at the smooth muscleneuromuscular junction. In heavily exposed individuals with poor dental hygiene, the reaction of circulating lead with sulfur ions released by microbial action may produce dark deposits of lead sulfide at the gingival margin (“gingival lead lines”). Although frequently mentioned as a diagnostic clue in the past, in recent times this has been a relatively rare sign of lead exposure.

F. Cardiovascular System Epidemiologic, experimental, and in vitro mechanistic data indicate that lead exposure elevates blood pressure in experimental animals and in susceptible humans. The pressor effect of lead may be mediated by an interaction with calcium-mediated contraction of vascular smooth muscle, as well as generation of oxidative stress and an associated interference in nitric oxide signaling pathways. In populations with environmental or occupational lead exposure, blood lead concentration is linked with increases in systolic and diastolic blood pressure. Studies of middle-aged and elderly men and women have identified relatively low levels of lead exposure sustained by the general population to be an independent risk factor for hypertension. Lead exposure has also been associated with prolongation of the QTc interval on the electrocardiogram. Of considerable public health concern, recent epidemiologic studies suggest that low to moderate levels of lead exposure are risk factors for increased cardiovascular mortality.

Major Forms of Lead Intoxication A. Inorganic Lead Poisoning (Table 57–1) 1. Acute—Acute inorganic lead poisoning is uncommon today. It usually results from industrial inhalation of large quantities of lead oxide fumes or, in small children, from ingestion of a large oral dose of lead in the form of lead-based paint chips; small objects, eg, toys coated or fabricated from lead; or contaminated food or drink. The onset of severe symptoms usually requires several days or weeks of recurrent exposure and manifests as signs and symptoms of encephalopathy or colic. Evidence of hemolytic anemia (or anemia with basophilic stippling if exposure has been subacute) and elevated hepatic aminotransferases may be present.

The diagnosis of acute inorganic lead poisoning may be difficult, and depending on the presenting symptoms, the condition has sometimes been mistaken for appendicitis, peptic ulcer, biliary colic, pancreatitis, or infectious meningitis. Subacute presentation, featuring headache, fatigue, intermittent abdominal cramps, myalgias, and arthralgias, has often been mistaken for a flu-like viral illness. When there has been recent ingestion of lead-containing paint chips, glazes, pellets, or weights, radiopacities may be visible on abdominal radiographs.

2. Chronic—The patient with symptomatic chronic lead intoxication typically presents with multisystemic findings, including complaints of anorexia, fatigue, and malaise; neurologic complaints, including headache, difficulty in concentrating, and irritability or depressed mood; weakness, arthralgias, or myalgias; and gastrointestinal symptoms. Lead poisoning should be strongly suspected in any patient presenting with headache, abdominal pain, and anemia; and less commonly with motor neuropathy, gout, and renal insufficiency.

Chronic lead intoxication should be considered in any child with neurocognitive deficits, growth retardation, or developmental delay. It is important to recognize that adverse effects of lead that are of considerable public health significance, such as subclinical decrements in neurodevelopment in children and hypertension in adults, are usually nonspecific and may not come to medical attention.

The diagnosis of lead intoxication is best confirmed by measuring lead in whole blood. Although this test reflects lead currently circulating in blood and soft tissues and is not a reliable marker of either recent or cumulative lead exposure, most patients with leadrelated disease have blood lead concentrations higher than the normal range. Average background blood lead concentrations in North America and Europe have declined by 90% in recent decades, and the geometric mean blood lead concentration in the United States in 2009–2010 was estimated to be 1.12 mcg/dL. Though predominantly a research tool, the concentration of lead in bone assessed by

noninvasive K X-ray fluorescence measurement of lead has been correlated with long-term cumulative lead exposure, and its relationship to numerous lead-related disorders is the subject of ongoing investigation. Measurement of lead excretion in the urine after a single dose of a chelating agent (sometimes called a “chelation challenge test”) primarily reflects the lead content of soft tissues and may not be a reliable marker of long-term lead exposure, remote past exposure, or skeletal lead burden. Accordingly, this test is rarely indicated in clinical practice. Because of the lag time associated with lead-induced elevations in circulating heme precursors, the finding of a blood lead concentration of 30 mcg/dL or more with no concurrent increase in zinc protoporphyrin suggests that the lead exposure was of recent onset.

B. Organolead Poisoning Poisoning from organolead compounds is now very rare, in large part because of the worldwide phase-out of tetraethyl and tetra-methyl lead as antiknock additives in gasoline. However, organolead compounds such as lead stearate or lead naphthenate are still used in certain commercial processes. Because of their volatility or lipid solubility, organolead compounds tend to be well absorbed through either the respiratory tract or the skin. Organolead compounds predominantly target the CNS, producing dose-dependent effects that may include neurocognitive deficits, insomnia, delirium, hallucinations, tremor, convulsions, and death.

Treatment A. Inorganic Lead Poisoning Treatment of inorganic lead poisoning involves immediate termination of exposure, supportive care, and the judicious use of chelation therapy. (Chelation is discussed later in this chapter.) Lead encephalopathy is a medical emergency that requires intensive supportive care. Cerebral edema may improve with corticosteroids and mannitol, and anticonvulsants may be required to treat seizures.

Radiopacities on abdominal radiographs may suggest the presence of retained lead objects requiring gastrointestinal decontamination.

Adequate urine flow should be maintained, but overhydration should be avoided. Intravenous edetate calcium disodium (CaNa2 EDTA) is administered at a dosage of 1000–1500 mg/m2 /d (approximately 30–50 mg/kg/d) by continuous infusion for up to 5 days. Some clinicians advocate that chelation treatment for lead encephalopathy be initiated with an intramuscular injection of dimercaprol, followed in 4 hours by concurrent administration of dimercaprol and EDTA. Parenteral chelation is limited to 5 or fewer days, at which time oral treatment with another chelator, succimer, may be instituted. In symptomatic lead intoxication without encephalopathy, treatment may sometimes be initiated with succimer. The end point for chelation is usually resolution of symptoms or return of the blood lead concentration to the premorbid range. In patients with chronic exposure, cessation of chelation may be followed by an upward rebound in blood lead concentration as the lead re-equilibrates from bone lead stores.

Although most clinicians support chelation for symptomatic patients with elevated blood lead concentrations, the decision to chelate asymptomatic subjects is more controversial. Since 1991, the Centers for Disease Control and Prevention (CDC) has recommended chelation for all children with blood lead concentrations of 45 mcg/dL or greater. However, a randomized, double-blind, placebocontrolled clinical trial of succimer in children with blood lead concentrations between 25 mcg/dL and 44 mcg/dL found no benefit on neurocognitive function or long-term blood lead reduction. Prophylactic use of chelating agents in the workplace should never be a substitute for reduction or prevention of excessive exposure.

Management of elevated blood lead levels in children and adults should include a conscientious effort to identify and reduce all potential sources of future lead exposure. Many local, state, or national governmental agencies maintain lead poisoning prevention programs that can assist in case management. Blood lead screening of family members or coworkers of a lead poisoning patient is often indicated to assess the scope of the exposure. In 2012, the CDC adopted a new policy that recognized childhood blood lead concentrations at or exceeding a reference value of 5 mcg/dL to be elevated and to merit clinical follow-up and environmental investigation. Although the US Occupational Safety and Health Administration (OSHA) lead regulations introduced in the late 1970s mandate that workers be removed from lead exposure for blood lead levels higher than 50–60 mcg/dL, an expert panel in 2007 recommended that removal be initiated for a single blood lead level greater than 30 mcg/dL, or when two successive blood lead levels measured over a 4-week interval are 20 mcg/dL or more. The longer-term goal should be for workers to maintain blood lead levels less than 10 mcg/dL, and for pregnant women to avoid occupational or avocational exposure that would result in blood lead levels higher than 5 mcg/dL. Environmental Protection Agency (EPA) regulations effective since 2010 require that contractors who perform renovation, repair, and painting projects that disturb lead-based paint in pre-1978 residences and child-occupied facilities must be certified and must follow specific work practices to prevent lead contamination.

B. Organic Lead Poisoning Initial treatment consists of decontaminating the skin and preventing further exposure. Treatment of seizures requires appropriate use of anticonvulsants. Empiric chelation may be attempted if high blood lead concentrations are present.

Arsenic

Arsenic is a naturally occurring element in the earth’s crust with a long history of use as a constituent of commercial and industrial products, as a component in pharmaceuticals, and as an agent of deliberate poisoning. Recent commercial applications of arsenic include its use in the manufacture of semiconductors, wood preservatives for industrial applications (eg, marine timbers or utility poles), nonferrous alloys, glass, herbicides, and nitarsone, an organoarsenical pharmaceutical used in certain poultry. In some regions of the world, groundwater may contain high levels of arsenic that has leached from natural mineral deposits. Arsenic in drinking water in the Ganges delta of India and Bangladesh is now recognized as one of the world’s most pressing environmental health problems.

Environmental risk assessments have suggested that arsenic migrating from coal combustion wastes (eg, coal ash) deposited in unlined landfills may contaminate underlying groundwater. Arsine, an arsenous hydride (AsH 3 ) gas with potent hemolytic effects, is manufactured predominantly for use in the semiconductor industry but may also be generated accidentally when arsenic-containing ores come in contact with acidic solutions.

It is of historical interest that Fowler’s solution, which contains 1% potassium arsenite, was widely used as a medicine for many conditions from the eighteenth century through the mid-twentieth century. Organic arsenicals were the first pharmaceutical antimicrobials* and were widely used for the first half of the twentieth century until supplanted by sulfonamides and other more effective and less toxic agents.

Other organoarsenicals, most notably lewisite (dichloro-[2-chlorovinyl]arsine), were developed in the early 20th century as chemical warfare agents. Arsenic trioxide was reintroduced into the United States Pharmacopeia in 2000 as an orphan drug for the treatment of relapsed acute promyelocytic leukemia and is finding expanded use in experimental cancer treatment protocols (see Chapter 54).

Melarsoprol, another trivalent arsenical, is used in the treatment of advanced African trypanosomiasis (see Chapter 52).

Pharmacokinetics Soluble arsenic compounds are well absorbed through the respiratory and gastrointestinal tracts (Table 57–1). Percutaneous absorption is limited but may be clinically significant after heavy exposure to concentrated arsenic reagents. Most of the absorbed inorganic arsenic undergoes methylation, mainly in the liver, to monomethyl-arsonic acid and dimethylarsinic acid, which are excreted, along with residual inorganic arsenic, in the urine. When chronic daily absorption is less than 1000 mcg of soluble inorganic arsenic, approximately two thirds of the absorbed dose is excreted in the urine within 2–3 days. After massive ingestions, the elimination half-life is prolonged. Inhalation of arsenic compounds of low solubility may result in prolonged retention in the lung and may not be reflected by urinary arsenic excretion.

Arsenic binds to sulfhydryl groups present in keratinized tissue, and following cessation of exposure, hair, nails, and skin may contain elevated levels after urine values have returned to normal. However, arsenic in hair and nails as a result of external deposition may be indistinguishable from that incorporated after internal absorption.

Pharmacodynamics Arsenic compounds are thought to exert their toxic effects by several modes of action. Interference with enzyme function may result from sulfhydryl group binding by trivalent arsenic or by substitution for phosphate. Inorganic arsenic or its metabolites may induce oxidative stress, alter gene expression, and interfere with cell signal transduction. Although on a molar basis, inorganic trivalent arsenic (As3+, arsenite) is generally two to ten times more acutely toxic than inorganic pentavalent arsenic (As5+, arsenate), in vivo interconversion is known to occur, and the full spectrum of arsenic toxicity has occurred after sufficient exposure to either form. Recent studies suggest that the trivalent form of the methylated metabolites (eg, monomethylarsonous acid [MMAIII]) may be more toxic than the inorganic parent compounds. Reduced efficiency in the methylation of MMA to DMA, resulting in an elevated percentage of MMA in the urine, has been associated with an increased risk of chronic adverse effects. Arsenic methylation requires S-adenosylmethionine, a universal methyl donor in the body, and arsenic-associated perturbations in one-carbon metabolism may underlie some arsenic-induced epigenetic effects such as altered gene expression.

Arsine gas is oxidized in vivo and exerts a potent hemolytic effect associated with alteration of ion flux across the erythrocyte membrane; it also disrupts cellular respiration in other tissues. Arsenic is a recognized human carcinogen and has been associated with cancer of the lung, skin, and bladder. Marine organisms may contain large amounts of a well-absorbed trimethylated organoarsenic, arsenobetaine, as well as a variety of arsenosugars and arsenolipids. Arsenobetaine exerts no known toxic effects when ingested by mammals and is excreted in the urine unchanged; arsenosugars are partially metabolized to dimethylarsinic acid. Thio-dimethylarsinic acid has recently been identified as a common but minor human arsenic metabolite of uncertain toxicological significance.

Major Forms of Arsenic Intoxication A. Acute Inorganic Arsenic Poisoning Within minutes to hours after exposure to high doses (tens to hundreds of milligrams) of soluble inorganic arsenic compounds, many systems are affected. Initial gastrointestinal signs and symptoms include nausea, vomiting, diarrhea, and abdominal pain. Diffuse capillary leak, combined with gastrointestinal fluid loss, may result in hypotension, shock, and death. Cardiopulmonary toxicity, including congestive

cardiomyopathy, cardiogenic or noncardiogenic pulmonary edema, and ventricular arrhythmias, may occur promptly or after a delay of several days. Pancytopenia usually develops within 1 week, and basophilic stippling of erythrocytes may be present soon after. Central nervous system effects, including delirium, encephalopathy, and coma, may occur within the first few days of intoxication. An ascending sensorimotor peripheral neuropathy may begin to develop after a delay of 2–6 weeks. This neuropathy may ultimately involve the proximal musculature and result in neuromuscular respiratory failure. Months after an acute poisoning, transverse white striae (AldrichMees lines) may be visible in the nails.

Acute inorganic arsenic poisoning should be considered in an individual presenting with abrupt onset of gastroenteritis in combination with hypotension and metabolic acidosis. Suspicion should be further heightened when these initial findings are followed by cardiac dysfunction, pancytopenia, and peripheral neuropathy. The diagnosis may be confirmed by demonstration of elevated amounts of inorganic arsenic and its metabolites in the urine (typically in the range of several thousand micrograms in the first 2–3 days after acute symptomatic poisoning). Arsenic disappears rapidly from the blood, and except in anuric patients, blood arsenic levels should not be used for diagnostic purposes. Treatment is based on appropriate gut decontamination, intensive supportive care, and prompt chelation with unithiol, 3–5 mg/kg intravenously every 4–6 hours, or dimercaprol, 3–5 mg/kg intramuscularly every 4–6 hours. In animal studies, the efficacy of chelation has been highest when it is administered within minutes to hours after arsenic exposure; therefore, if diagnostic suspicion is high, treatment should not be withheld for the several days to weeks often required to obtain laboratory confirmation.

Succimer has also been effective in animal models and has a higher therapeutic index than dimercaprol. However, because it is available in the United States only for oral administration, its use may not be advisable in the initial treatment of acute arsenic poisoning, when severe gastroenteritis and splanchnic edema may limit absorption by this route.

B. Chronic Inorganic Arsenic Poisoning Chronic inorganic arsenic poisoning also results in multisystemic signs and symptoms. Overt noncarcinogenic effects may be evident after chronic absorption of more than 0.01 mg/kg/d (~ 500–1000 mcg/d in adults). The time to appearance of symptoms varies with dose and interindividual tolerance. Constitutional symptoms of fatigue, weight loss, and weakness may be present, along with anemia, nonspecific gastrointestinal complaints, and a sensorimotor peripheral neuropathy, particularly featuring a stocking glove pattern of dysesthesia. Skin changes—among the most characteristic effects—typically develop after years of exposure and include a “raindrop” pattern of hyperpigmentation, and hyperkeratoses involving the hands and feet (Figure 57–1). Peripheral vascular disease and noncirrhotic portal hypertension may also occur. Epidemiologic studies suggest a possible link to hypertension, cardiovascular disease mortality, diabetes, chronic nonmalignant respiratory disease, and adverse reproductive outcomes. Cancer of the lung, skin, bladder, and possibly other sites, may appear years after exposure to doses of arsenic that are not high enough to elicit other acute or chronic effects.

Some studies suggest that tobacco smoking may interact synergistically with arsenic in increasing the risk of certain adverse health outcomes.

FIGURE 57â&#x20AC;&#x201C;1 Dermatologic lesions associated with chronic ingestion of arsenic in drinking water. (Photo courtesy of Dipankar Chakraborti, PhD.) Administration of arsenite in cancer chemotherapy regimens, often at a daily dose of 10â&#x20AC;&#x201C;20 mg for weeks to a few months, has been associated with prolongation of the QT interval on the electrocardiogram and occasionally has resulted in malignant ventricular arrhythmias such as torsades de pointes.

The diagnosis of chronic arsenic poisoning involves integration of the clinical findings with confirmation of exposure. The urine concentration of the sum of inorganic arsenic and its primary metabolites MMA and DMA is less than 20 mcg/L in the general population. High urine levels associated with overt adverse effects may return to normal within days to weeks after exposure ceases.

Because it may contain large amounts of nontoxic organoarsenic, all seafood should be avoided for at least 3 days before submission of a urine sample for diagnostic purposes. The arsenic content of hair and nails (normally less than 1 ppm) may sometimes reveal past elevated exposure, but results should be interpreted cautiously in view of the potential for external contamination.

Management of chronic arsenic poisoning consists primarily of termination of exposure and nonspecific supportive care. Although empiric short-term oral chelation with unithiol or succimer for symptomatic individuals with elevated urine arsenic concentrations may be considered, it has no proven benefit beyond removal from exposure alone. Preliminary studies suggest that dietary supplementation of

folate—thought to be a cofactor in arsenic methylation—might be of value in arsenic-exposed individuals, particularly men, who are also deficient in folate.

C. Arsine Gas Poisoning Arsine gas poisoning produces a distinctive pattern of intoxication dominated by profound hemolytic effects. After a latent period that may range from 2 to 24 hours postinhalation (depending on the magnitude of exposure), massive intravascular hemolysis may occur.

Initial symptoms may include malaise, headache, dyspnea, weakness, nausea, vomiting, abdominal pain, jaundice, and hemoglobinuria.

Oliguric renal failure, a consequence of hemoglobin deposition in the renal tubules, often appears within 1–3 days. In massive exposures, lethal effects on cellular respiration may occur before renal failure develops. Urinary arsenic levels are elevated but are seldom available to confirm the diagnosis during the critical period of illness. Intensive supportive care—including exchange transfusion, vigorous hydration, and, in the case of acute renal failure, hemodialysis—is the mainstay of therapy. Currently available chelating agents have not been demonstrated to be of clinical value in arsine poisoning.

MERCURY Metallic mercury as “quicksilver”—the only metal that is liquid under ordinary conditions—has attracted scholarly and scientific interest from antiquity. The mining of mercury was early recognized as being hazardous to health. As industrial use of mercury became common during the last 200 years, new forms of toxicity were recognized that were found to be associated with various transformations of the metal. In the early 1950s, a mysterious epidemic of birth defects and neurologic disease occurred in the Japanese fishing village of Minamata. The causative agent was determined to be methylmercury in contaminated seafood, traced to industrial discharges into the bay from a nearby factory. In addition to elemental mercury and alkylmercury (including methylmercury), other key mercurials include inorganic mercury salts and aryl mercury compounds, each of which exerts a relatively unique pattern of clinical toxicity.

Mercury is mined predominantly as HgS in cinnabar ore and is then converted commercially to a variety of chemical forms. Key industrial and commercial applications of mercury are found in the electrolytic production of chlorine and caustic soda; the manufacture of electrical equipment, thermometers, and other instruments; fluorescent lamps; and dental amalgam. The widespread use of elemental mercury in artisanal gold production is a growing problem in many developing countries. Beginning in 2014, an international treaty established through the United Nations severely restricted the international transfer of elemental mercury. Mercury use in pharmaceuticals and in biocides has declined substantially in recent years, but occasional use in antiseptics and folk medicines is still encountered. Thimerosal, an organomercurial preservative that is metabolized in part to ethylmercury, has been removed from almost all the vaccines in which it was formerly present. Environmental releases of mercury from the burning of fossil fuels, which contributes to the bioaccumulation of methylmercury in fish, remains a concern in some regions of the world. Low-level exposure to mercury released from dental amalgam fillings occurs, but systemic toxicity from this source has not been established.

Pharmacokinetics The absorption of mercury varies considerably depending on the chemical form of the metal. Elemental mercury is quite volatile and can be absorbed from the lungs (Table 57–1). It is poorly absorbed from the intact gastrointestinal tract. Inhaled mercury is the primary source of occupational exposure. Organic short-chain alkylmercury compounds are volatile and potentially harmful by inhalation as well as by ingestion. Percutaneous absorption of metallic mercury and inorganic mercury can be of clinical concern following massive acute or long-term chronic exposure. Alkylmercury compounds appear to be well absorbed through the skin, and acute contact with a few drops of dimethylmercury has resulted in severe, delayed toxicity. After absorption, mercury is distributed to the tissues within a few hours, with the highest concentration occurring in the kidney. Inorganic mercury is excreted through the urine and feces. Excretion of inorganic mercury follows a multicompartment model: most is excreted within weeks to months, but a fraction may be retained in the kidneys and brain for years. After inhalation of elemental mercury vapor, urinary mercury levels decline with a half-life of approximately 1–3 months. Methylmercury, which has a blood and whole body half-life of approximately 50 days, undergoes biliary excretion and enterohepatic circulation, with more than two thirds eventually excreted in the feces. Mercury binds to sulfhydryl groups in keratinized tissue, and as with lead and arsenic, traces appear in the hair and nails.

Major Forms of Mercury Intoxication Mercury interacts with sulfhydryl groups in vivo, inhibiting enzymes and altering cell membranes. The pattern of clinical intoxication from mercury depends to a great extent on the chemical form of the metal and the route and severity of exposure.

A. Acute Acute inhalation of elemental mercury vapors may cause chemical pneumonitis and noncardiogenic pulmonary edema. Acute gingivostomatitis may occur, and neurologic sequelae (see following text) may also ensue. Acute ingestion of inorganic mercury salts,

such as mercuric chloride, can result in a corrosive, potentially life-threatening hemorrhagic gastroenteritis followed within hours to days by acute tubular necrosis and oliguric renal failure.

B. Chronic Chronic poisoning from inhalation of mercury vapor results in a classic triad of tremor, neuropsychiatric disturbance, and gingivostomatitis. The tremor usually begins as a fine intention tremor of the hands, but the face may also be involved, and progression to choreiform movements of the limbs may occur. Neuropsychiatric manifestations, including memory loss, fatigue, insomnia, and anorexia, are common. There may be an insidious change in mood to shyness, withdrawal, and depression along with explosive anger or blushing (a behavioral pattern referred to as erethism). Recent studies suggest that low-dose exposure may produce subclinical neurologic effects.

Gingivostomatitis, sometimes accompanied by loosening of the teeth, may be reported after high-dose exposure. Evidence of peripheral nerve damage may be detected on electrodiagnostic testing, but overt peripheral neuropathy is rare. Acrodynia is an uncommon idiosyncratic reaction to subacute or chronic mercury exposure and occurs mainly in children. It is characterized by painful erythema of the extremities and may be associated with hypertension, diaphoresis, anorexia, insomnia, irritability or apathy, and a miliary rash. Chronic exposure to inorganic mercury salts, sometimes via topical application in cosmetic skin-lightening creams, has been associated with neurological symptoms and renal toxicity in case reports and case series.

Methylmercury intoxication affects mainly the CNS and results in paresthesias, ataxia, hearing impairment, dysarthria, and progressive constriction of the visual fields. Signs and symptoms of methylmercury intoxication may first appear several weeks or months after exposure begins. Methylmercury is a reproductive toxin. High-dose prenatal exposure to methylmercury may produce mental retardation and a cerebral palsy-like syndrome in the offspring. Low-level prenatal exposures to methylmercury have been associated with a risk of subclinical neurodevelopmental deficits.

A 2004 report by the Institute of Medicineâ&#x20AC;&#x2122;s Immunization Safety Review Committee concluded that available evidence favored rejection of a causal relation between thimerosal-containing vaccines and autism. In like manner, a recent retrospective cohort study conducted by the CDC did not support a causal association between early prenatal or postnatal exposure to mercury from thimerosalcontaining vaccines and neuropsychological functioning later in childhood.

Dimethylmercury is a rarely encountered but extremely neurotoxic form of organomercury that may be lethal in small quantities.

The diagnosis of mercury intoxication involves integration of the history and physical findings with confirmatory laboratory testing or other evidence of exposure. In the absence of occupational exposure, the urine mercury concentration is usually less than 5 mcg/L, and whole blood mercury is less than 5 mcg/L. In 1990, the Biological Exposure Index (BEI) Committee of the American Conference of Governmental Industrial Hygienists (ACGIH) recommended that workplace exposures should result in urinary mercury concentrations less than 35 mcg per gram of creatinine and end-of-work-week whole blood mercury concentrations less than 15 mcg/L. To minimize the risk of developmental neurotoxicity from methylmercury, the EPA and the FDA have advised pregnant women, women who might become pregnant, nursing mothers, and young children to avoid consumption of fish with high mercury levels (eg, swordfish) and to limit consumption of fish with lower levels of mercury to no more than 12 ounces (340 g, or two average meals) per week.

Treatment A. Acute Exposure In addition to intensive supportive care, prompt chelation with oral or intravenous unithiol, intramuscular dimercaprol, or oral succimer may be of value in diminishing nephrotoxicity after acute exposure to inorganic mercury salts. Vigorous hydration may help to maintain urine output, but if acute renal failure ensues, days to weeks of hemodialysis or hemodiafiltration in conjunction with chelation may be necessary. Because the efficacy of chelation declines with time since exposure, treatment should not be delayed until the onset of oliguria or other major systemic effects.

B. Chronic Exposure Unithiol and succimer increase urine mercury excretion following acute or chronic elemental mercury inhalation, but the impact of such treatment on clinical outcome is unknown. Dimercaprol has been shown to redistribute mercury to the central nervous system from other tissue sites, and since the brain is a key target organ, dimercaprol should not be used in treatment of exposure to elemental or organic mercury. Limited data suggest that succimer, unithiol, and N-acetyl-L-cysteine (NAC) may enhance body clearance of methylmercury.

PHARMACOLOGY OF CHELATORS Chelating agents are drugs used to prevent or reverse the toxic effects of a heavy metal on an enzyme or other cellular target, or to accelerate the elimination of the metal from the body. By forming a complex with the heavy metal, the chelating agent renders the metal unavailable for toxic interactions with functional groups of enzymes or other proteins, coenzymes, cellular nucleophiles, and membranes.

Chelating agents contain one or more coordinating atoms, usually oxygen, sulfur, or nitrogen, which donate a pair of electrons to a cationic metal ion to form one or more coordinate-covalent bonds. Depending on the number of metal-ligand bonds, the complex may be

referred to as mono-, bi-, or polydentate. Figure 57â&#x20AC;&#x201C;2 depicts the hexadentate chelate formed by interaction of edetate (ethylenediaminetetraacetate) with a metal atom, such as lead.

FIGURE 57–2 Salt and chelate formation with edetate (ethylenediaminetetraacetate, EDTA). A: In a solution of calcium disodium salt of EDTA, the sodium and hydrogen ions are chemically and biologically available. B: In solutions of calcium disodium edetate, calcium is bound by coordinate-covalent bonds with nitrogens as well as by the usual ionic bonds. C: In the lead–edetate chelate, lead is incorporated into five heterocyclic rings. (Adapted, with permission, from Meyers FH, Jawetz E, Goldfien A: Review of Medical Pharmacology, 7th ed. Originally published by Lange Medical Publications. McGraw-Hill, 1980. Copyright © The McGraw-Hill Companies, Inc.) In some cases, the metal-mobilizing effect of a therapeutic chelating agent may not only enhance that metal’s excretion—a desired effect—but may also redistribute some of the metal to other vital organs. This has been demonstrated for dimercaprol, which redistributes mercury and arsenic to the brain while also enhancing urinary mercury and arsenic excretion. Although several chelating agents have the capacity to mobilize cadmium, their tendency to redistribute cadmium to the kidney and increase nephrotoxicity has negated their therapeutic value in cadmium intoxication.

In addition to removing the target metal that is exerting toxic effects on the body, some chelating agents may enhance excretion of essential cations, such as zinc in the case of calcium EDTA and diethylenetriaminepentaacetic acid (DTPA), and zinc and copper in the case of succimer. No clinical significance of this effect has been demonstrated, although some animal data suggest the possibility of adverse developmental impact. If prolonged chelation during the prenatal period or early childhood period is necessary, judicious supplementation of the diet with zinc might be considered.

The longer the half-life of a metal in a particular organ, the less effectively it will be removed by chelation. For example, in the case of lead chelation with calcium EDTA or succimer, or of plutonium chelation with DTPA, the metal is more effectively removed from soft tissues than from bone, where incorporation into bone matrix results in prolonged retention.

In most cases, the capacity of chelating agents to prevent or reduce the adverse effects of toxic metals appears to be greatest when such agents are administered very soon after an acute metal exposure. Use of chelating agents days to weeks after an acute metal exposure ends—or their use in the treatment of chronic metal intoxication—may still be associated with increased metal excretion.

However, at that point, the capacity of such enhanced excretion to mitigate the pathologic effect of the metal exposure may be reduced.

The most important chelating agents currently in use in the USA are described below.

DIMERCAPROL (2,3-DIMERCAPTOPROPANOL, BAL) Dimercaprol (Figure 57–3), an oily, colorless liquid with a strong mercaptan-like odor, was developed in Great Britain during World War II as a therapeutic antidote against poisoning by the arsenic-containing warfare agent lewisite. It thus became known as British antilewisite, or BAL. Because aqueous solutions of dimercaprol are unstable and oxidize readily, it is dispensed in 10% solution in peanut oil and must be administered by intramuscular injection, which is often painful.

FIGURE 57–3 Chemical structures of several chelators. Ferroxamine (ferrioxamine) without the chelated iron is deferoxamine. It is represented here to show the functional groups; the iron is actually held in a caged system. The structures of the in vivo metal-chelator complexes for dimercaprol, succimer, penicillamine, and unithiol (see text) are not known and may involve the formation of mixed disulfides with amino acids. (Adapted, with permission from Meyers FH, Jawetz E, and Goldfien A: Review of Medical Pharmacology, 7th ed. Originally published by Lange Medical Publications. McGraw-Hill, 1980. Copyright © The McGraw-Hill Companies, Inc.) In animal models, dimercaprol prevents and reverses arsenic-induced inhibition of sulfhydryl-containing enzymes and, if given soon after exposure, may protect against the lethal effects of inorganic and organic arsenicals. Human data indicate that it can increase the rate of excretion of arsenic and lead and may offer therapeutic benefit in the treatment of acute intoxication by arsenic, lead, and mercury.

Indications & Toxicity Dimercaprol is FDA-approved as single-agent treatment of acute poisoning by arsenic and inorganic mercury and for the treatment of severe lead poisoning when used in conjunction with edetate calcium disodium (EDTA; see below). Although studies of its metabolism in humans are limited, intramuscularly administered dimercaprol appears to be readily absorbed, metabolized, and excreted by the kidney within 4–8 hours. Animal models indicate that it may also undergo biliary excretion, but the role of this excretory route in humans and other details of its biotransformation are uncertain.

When used in therapeutic doses, dimercaprol is associated with a high incidence of adverse effects, including hypertension, tachycardia, nausea, vomiting, lacrimation, salivation, fever (particularly in children), and pain at the injection site. Its use has also been associated with thrombocytopenia and increased prothrombin time—factors that may limit intramuscular injection because of the risk of hematoma formation at the injection site. Despite its protective effects in acutely intoxicated animals, dimercaprol may redistribute arsenic and mercury to the central nervous system, and it is not advocated for treatment of chronic poisoning. Water-soluble analogs of dimercaprol—unithiol and succimer—have higher therapeutic indices and have replaced dimercaprol in many settings.

SUCCIMER (DIMERCAPTOSUCCINIC ACID, DMSA) Succimer is a water-soluble analog of dimercaprol, and like that agent it has been shown in animal studies to prevent and reverse metalinduced inhibition of sulfhydryl-containing enzymes and to protect against the acute lethal effects of arsenic. In humans, treatment with succimer is associated with an increase in urinary lead excretion and a decrease in blood lead concentration. It may also decrease the

mercury content of the kidney, a key target organ of inorganic mercury salts. In the USA, succimer is formulated exclusively for oral use, but intravenous formulations have been used successfully elsewhere. It is absorbed rapidly but somewhat variably after oral administration. Peak blood levels of succimer occur at approximately 3 hours. The drug binds in vivo to the amino acid cysteine to form 1:1 and 1:2 mixed disulfides, possibly in the kidney, and it may be these complexes that are the active chelating moieties. Experimental data suggest that multidrug-resistance protein 2 (Mrp2), one of a group of transporter proteins involved in the cellular excretion of xenobiotics, facilitates the renal excretion of mercury compounds that are bound to the transformed succimer and to unithiol. The elimination half-time of transformed succimer is approximately 2–4 hours.

Indications & Toxicity Succimer is currently FDA-approved for the treatment of children with blood lead concentrations greater than 45 mcg/dL, but it is also commonly used in adults. The typical dosage is 10 mg/kg orally three times a day. Oral administration of succimer is comparable to parenteral EDTA in reducing blood lead concentration and has supplanted EDTA in outpatient treatment of patients who are capable of absorbing the oral drug. However, despite the demonstrated capacity of both succimer and EDTA to enhance lead elimination, their value in reversing established lead toxicity or in otherwise improving therapeutic outcome has yet to be established by a placebocontrolled clinical trial. In a recent study in lead-exposed juvenile rats, high-dose succimer did reduce lead-induced neurocognitive impairment when administered to animals with moderate- and high-dose lead exposure. Conversely, when administered to the control group that was not lead exposed, succimer was associated with a decrement in neurocognitive performance. Based on its protective effects against arsenic in animals and its ability to mobilize mercury from the kidney, succimer has also been used in the treatment of arsenic and mercury poisoning.

In limited clinical trials, succimer has been well tolerated. It has a negligible impact on body stores of calcium, iron, and magnesium. It induces a mild increase in urinary excretion of zinc and, less consistently, copper. This effect on trace metal balance has not been associated with overt adverse effects, but its long-term impact on neurodevelopment is uncertain. Gastrointestinal disturbances, including anorexia, nausea, vomiting, and diarrhea, are the most common side effects, occurring in less than 10% of patients. Rashes, sometimes requiring discontinuation of the medication, have been reported in less than 5% of patients. Mild, reversible increases in liver aminotransferases have been noted in 6–10% of patients, and isolated cases of mild to moderate neutropenia have been reported.

EDETATE CALCIUM DISODIUM (ETHYLENEDIAMINETETRAACETIC ACID, EDTA) Ethylenediaminetetraacetic acid (Figure 57–2) is an efficient chelator of many divalent and trivalent metals in vitro. To prevent potentially life-threatening depletion of calcium, treatment of metal intoxication should only be performed with the calcium disodium salt form of EDTA (edetate calcium disodium).

EDTA penetrates cell membranes relatively poorly and therefore chelates extracellular metal ions much more effectively than intracellular ions.

The highly polar ionic character of EDTA limits its oral absorption. Moreover, oral administration may increase lead absorption from the gut. Consequently, EDTA should be administered by intravenous infusion. In patients with normal renal function, EDTA is rapidly excreted by glomerular filtration, with 50% of an injected dose appearing in the urine within 1 hour. EDTA mobilizes lead from soft tissues, causing a marked increase in urinary lead excretion and a corresponding decline in blood lead concentration. In patients with renal insufficiency, excretion of the drug—and its metal-mobilizing effects—may be delayed.

Indications & Toxicity Edetate calcium disodium is indicated chiefly for the chelation of lead, but it may also have usefulness in poisoning by zinc, manganese, and certain heavy radionuclides. A recent randomized, double-blind, placebo-controlled prospective trial of edetate disodium (not edetate calcium disodium) observed a significant decrease in cardiovascular events in a subgroup consisting of diabetic patients with a prior history of myocardial infarction. Further study is indicated to replicate the findings and explore potential mechanisms of benefit.

Because the drug and the mobilized metals are excreted via the urine, the drug is relatively contraindicated in anuric patients. In such instances, the use of low doses of EDTA in combination with high-flux hemodialysis or hemofiltration has been described. Nephrotoxicity from EDTA has been reported, but in most cases can be prevented by maintenance of adequate urine flow, avoidance of excessive doses, and limitation of a treatment course to 5 or fewer consecutive days. EDTA may result in temporary zinc depletion that is of uncertain clinical significance. Analogs of EDTA, the calcium and zinc disodium salts of DTPA, pentetate, have been used for removal (“decorporation”) of certain transuranic, rare earth, and transition metal radioisotopes, and in 2004 were approved by the FDA for treatment of contamination with plutonium, americium, and curium.

UNITHIOL (DIMERCAPTOPROPANESULFONIC ACID, DMPS) Unithiol, a dimercapto chelating agent that is a water-soluble analog of dimercaprol, has been available in the official formularies of Russia and other former Soviet countries since 1958 and in Germany since 1976. It has been legally available from compounding pharmacies in the USA since 1999. Unithiol can be administered orally and intravenously. Bioavailability by the oral route is approximately 50%, with peak blood levels occurring in approximately 4 hours. Over 80% of an intravenous dose is excreted in the urine, mainly as cyclic DMPS sulfides. The elimination half-time of total unithiol (parent drug and its transformation products) is approximately 20 hours. Unithiol exhibits protective effects against the toxic action of mercury and arsenic in animal models, and it increases the excretion of mercury, arsenic, and lead in humans. Animal studies and a few case reports suggest that unithiol may also have usefulness in the treatment of poisoning by bismuth compounds.

Indications & Toxicity Unithiol has no FDA-approved indications, but experimental studies and its pharmacologic and pharmacodynamic profile suggest that intravenous unithiol offers advantages over intramuscular dimercaprol or oral succimer in the initial treatment of severe acute poisoning by inorganic mercury or arsenic. Aqueous preparations of unithiol (usually 50 mg/mL in sterile water) can be administered at a dosage of 3–5 mg/kg every 4 hours by slow intravenous infusion over 20 minutes. If a few days of treatment are accompanied by stabilization of the patient’s cardiovascular and gastrointestinal status, it may be possible to change to oral administration of 4–8 mg/kg every 6–8 hours.

Oral unithiol may also be considered as an alternative to oral succimer in the treatment of lead intoxication.

Unithiol has been reported to have a low overall incidence of adverse effects (< 4%). Self-limited dermatologic reactions (drug exanthems or urticaria) are the most commonly reported adverse effects, although isolated cases of major allergic reactions, including erythema multiforme and Stevens-Johnson syndrome, have been reported. Because rapid intravenous infusion may cause vasodilation and hypotension, unithiol should be infused slowly over 15–20 minutes.

PENICILLAMINE (D-DIMETHLCYSTEINE) Penicillamine (Figure 57–3) is a white crystalline, water-soluble derivative of penicillin. D-Penicillamine is less toxic than the L-isomer and consequently is the preferred therapeutic form. Penicillamine is readily absorbed from the gut and is resistant to metabolic degradation.

Indications & Toxicity Penicillamine is used chiefly for treatment of poisoning with copper or to prevent copper accumulation, as in Wilson’s disease (hepatolenticular degeneration). It is also used occasionally in the treatment of severe rheumatoid arthritis (see Chapter 36). Its ability to increase urinary excretion of lead and mercury had occasioned its use in outpatient treatment for intoxication with these metals, but succimer, with its stronger metal-mobilizing capacity and lower adverse-effect profile, has generally replaced penicillamine for these purposes.

Adverse effects have been seen in up to one third of patients receiving penicillamine. Hypersensitivity reactions include rash, pruritus, and drug fever, and the drug should be used with extreme caution, if at all, in patients with a history of penicillin allergy. Nephrotoxicity with proteinuria has also been reported, and protracted use of the drug may result in renal insufficiency. Pancytopenia has been associated with prolonged drug intake. Pyridoxine deficiency is a frequent toxic effect of other forms of the drug but is rarely seen with the D isomer. An acetylated derivative, N-acetylpenicillamine, has been used experimentally in mercury poisoning and may have superior metal-mobilizing capacity, but it is not commercially available.

DEFEROXAMINE Deferoxamine is isolated from Streptomyces pilosus. It binds iron avidly (Figure 57–3) but binds essential trace metals poorly.

Furthermore, though competing for loosely bound iron in iron-carrying proteins (hemosiderin and ferritin), it fails to compete for biologically chelated iron, as in microsomal and mitochondrial cytochromes and hemoproteins. Consequently, it is the parenteral chelator of choice for iron poisoning (see Chapters 33 and 58). Deferoxamine plus hemodialysis may also be useful in the treatment of aluminum toxicity in renal failure. Deferoxamine is poorly absorbed when administered orally and may increase iron absorption when given by this

route. It should therefore be administered intramuscularly or, preferably, intravenously. It is believed to be metabolized, but the pathways are unknown. The iron-chelator complex is excreted in the urine, often turning the urine an orange-red color.

Rapid intravenous administration may result in hypotension. Adverse idiosyncratic responses such as flushing, abdominal discomfort, and rash have also been observed. Pulmonary complications (eg, acute respiratory distress syndrome) have been reported in some patients undergoing deferoxamine infusions lasting longer than 24 hours, and neurotoxicity and increased susceptibility to certain infections (eg, with Yersinia enterocolitica ) have been described after long-term therapy of iron overload conditions (eg, thalassemia major).

DEFERASIROX Deferasirox is a tridentate chelator with a high affinity for iron and low affinity for other metals, eg, zinc and copper. It is orally active and well absorbed. In the circulation, it binds iron, and the complex is excreted in the bile. Deferasirox was approved by the FDA in 2005 for the oral treatment of iron overload caused by blood transfusions, a problem in the treatment of thalassemia and myelodysplastic syndrome. More than five years of clinical experience suggest that daily long-term usage is generally well tolerated, with the most common adverse effects consisting of mild to moderate gastrointestinal disturbances (< 15% of patients) and skin rash (≈ 5% of patients).

PRUSSIAN BLUE (FERRIC HEXACYANOFERRATE) Ferric hexacyanoferrate (insoluble Prussian blue) is a hydrated crystalline compound in which Fe2+ and Fe3+atoms are coordinated with cyanide groups in a cubic lattice structure. Although used as a dark blue commercial pigment for nearly 300 years, it was only three decades ago that its potential usefulness as a pharmaceutical chelator was recognized. Primarily by ion exchange, and secondarily by mechanical trapping or adsorption, the compound has high affinity for certain univalent cations, particularly cesium and thallium. Used as an oral drug, insoluble Prussian blue undergoes minimal gastrointestinal absorption (< 1%). Because the complexes it forms with cesium or thallium are nonabsorbable, oral administration of the chelator diminishes intestinal absorption or interrupts enterohepatic and enteroenteric circulation of these cations, thereby accelerating their elimination in the feces. In clinical case series, the use of Prussian blue has been associated with a decline in the biologic half-life (ie, in vivo retention) of radioactive cesium and thallium.

Indications & Toxicity In 2003, the FDA approved Prussian blue for the treatment of contamination with radioactive cesium ( 137 Cs) and intoxication with thallium salts. Approval was prompted by concern over potential widespread human contamination with radioactive cesium caused by terrorist use of a radioactive dispersal device (“dirty bomb”). The drug is part of the Strategic National Stockpile of pharmaceuticals and medical material maintained by the CDC (http://www.bt.cdc.gov/stockpile/#material). (Note: Although soluble forms of Prussian blue, such as potassium ferric hexacyanoferrate, may have better utility in thallium poisoning, only the insoluble form is currently available as a pharmaceutical.)

After exposure to 137 Cs or thallium salts, the approved adult dosage is 3 g orally three times a day; the corresponding pediatric dosage (2–12 years of age) is 1 g orally three times a day. Serial monitoring of urine and fecal radioactivity ( 137 Cs) and urinary thallium concentrations can guide the recommended duration of therapy. Adjunctive supportive care for possible acute radiation illness ( 137 Cs) or systemic thallium toxicity should be instituted as needed.

Prussian blue has not been associated with significant adverse effects. Constipation, which may occur in some cases, should be treated with laxatives or increased dietary fiber.

PREPARATIONS AVAILABLE

REFERENCES Lead Advisory Committee on Childhood Lead Poisoning Prevention of the Centers for Disease Control and Prevention. Low Level Lead Exposure Harms Children: A Renewed Call for Primary Prevention. CDC: Atlanta, GA. 2012. http://www.cdc.gov/nceh/lead/ACCLPP/Final_Document_030712.pdf.

Brubaker CJ et al: T he influence of age of lead exposure on adult gray matter volume. Neurotoxicology 2010;31:259.

Carlisle JC et al: A blood lead benchmark for assessing risks from childhood lead exposure. J Environ Sci Health Part A 2009;44:1200.

Centers for Disease Control and Prevention (CDC): Guidelines for the Identification and Management of Lead Exposure in Pregnant and Lactating Women. CDC, 2010. http://www.cdc.gov/nceh/lead/publications/LeadandPregnancy2010.pdf.

Environmental Protection Agency. Integrated Science Assessment for Lead. EPA: Research T riangle Park, NC. 2013. Available at: http://epa.gov/ncea/isa/lead.htm.

Eum KD et al: Prospective cohort study of lead exposure and electrocardiographic conduction disturbances in the Department of Veterans Affairs Normative Aging Study.

Environ Health Perspect 2011;119:940.

Kosnett MJ et al: Recommendations for medical management of adult lead exposure. Environ Health Perspect 2007;115:463.

Lanphear BP et al: Low-level environmental lead exposure and children’s intellectual development: An international pooled analysis. Environ Health Perspect 2005;113:894.

Weisskopf MG et al: A prospective study of bone lead concentration and death from all causes, cardiovascular diseases, and cancer in the Department of Veterans Affairs Normative Aging Study. Circulation 2009;120:1056.

Arsenic Caldwell KL et al: Levels of urinary total and speciated arsenic in the US population: National Health and Nutrition Examination Survey 2003–2004. J Exp Sci Environ Epid 2009;19:59.

Chen Y et al: Arsenic exposure from drinking water and mortality from cardiovascular disease in Bangladesh: Prospective cohort study. BMJ 2011;342:d2431.

Gamble MV: Folate and arsenic metabolism: A double-blind, placebo-controlled folic acid supplementation trial in Bangladesh. Am J Clin Nutr 2006;84:1093.

National Research Council: Critical Aspects of EPA’s IRIS Assessment of Inorganic Arsenic: Interim Report. Washington, DC: T he National Academies Press, 2013.

Naujokas MF et al: T he broad scope of health effects from chronic arsenic exposure: update on a worldwide public health problem. Environ Health Perspect 2013;121:295.

Parvez F et al: A prospective study of respiratory symptoms associated with chronic arsenic exposure in Bangladesh: findings from the Health Effects of Arsenic Longitudinal Study (HEALS). T horax 2010;65:528.

Vahter M: Effects of arsenic on maternal and fetal health. Annu Rev Nutr 2009;29:381

Mercury Bellinger DC et al: Dental amalgam restorations and children’s neuropsychological function: T he New England Children’s Amalgam T rial. Environ Health Perspect 2007;115:440.

Caldwell KL et al: T otal blood mercury concentrations in the U.S. population: 1999–2006. Int J Hyg Environ Health 2009;212:588.

Centers for Disease Control and Prevention (CDC): Mercury exposure among household users and nonusers of skin-lightening creams produced in Mexico—California and Virginia, 2010. MMWR Morb Mortal Wkly Rep 2012;61:33.

Environmental Protection Agency. What you need to know about mercury in fish and shellfish. http://water.epa.gov/scitech/swguidance/fishshellfish/outreach/advice_index.cfm.

Franzblau A et al: Low-level mercury exposure and peripheral nerve function. Neurotoxicology 2012;33:299.

Grandjean P et al: Adverse effects of methylmercury: Environmental health research implications. Environ Health Perspect 2010;118:1137.

Hertz-Picciotto I et al: Blood mercury concentrations in CHARGE study children with and without autism. Environ Health Perspect 2010;118:161.

Lederman SA et al: Relation between cord blood mercury levels and early child development in a World T rade Center cohort. Environ Health Perspect 2008;116:1085.

Yorifuji T et al: Long-term exposure to methylmercury and neurologic signs in Minamata and neighboring communities. Epidemiology 2008;19:3.

Chelating Agents Agarwal MB: Deferasirox: Oral, once daily iron chelator—an expert opinion. Indian J Pediatr 2010;77:185.

Bradberry S, Vale A: A comparison of sodium calcium edetate (edetate calcium disodium) and succimer (DMSA) in the treatment of inorganic lead poisoning. Clin T oxicol 2009;47:841.

Dargan PI et al: Case report: Severe mercuric sulphate poisoning treated with 2,3-dimercaptopropane-1-sulphonate and haemodiafiltration. Crit Care 2003;7:R1.

Escolar E et al: T he effect of an EDT A-based chelation regimen on patients with diabetes mellitus and prior myocardial infarction in the T rial to Assess Chelation T herapy (T ACT ). Circ Cardiovasc Qual Outcomes 2014;7(1):15.

Kosnett MJ: Chelation for heavy metals (arsenic, lead, and mercury): Protective or perilous? Clin Pharmacol T her 2010;88:412.

Kosnett MJ: T he role of chelation in the treatment of arsenic and mercury poisoning. J Med T oxicol 2013;9:347.

Smith D et al: T he scientific basis for chelation: animal studies and lead chelation. J Med T oxicol 2013;9:326.

T hompson DF, Called ED: Soluble or insoluble prussian blue for radiocesium and thallium poisoning? Ann Pharmacother 2004;38:1509.

CASE STUDY ANSWER This scenario is highly suspicious for acute lead intoxication. Lead-based paints have been commonly used as anticorrosion coatings on iron and steel structures, and grinding and torch cutting can result in high-dose exposure to inhaled lead dust and fumes.

Measurement of a whole blood lead concentration would be a key diagnostic test. If an elevated blood lead concentration is confirmed, the primary therapeutic intervention will be removal of the individual from further work exposure until blood lead concentration has declined and symptoms resolved. If the blood lead concentration is in excess of 80 mcg/dL (~ 4 μmol/L), treatment with a chelating agent, such as oral succimer or parenteral edetate calcium disodium, should be strongly considered. Upon return to work, use of proper respiratory protection and adherence to protective work practices are essential.

_______________ * Paul Ehrlich’s “ magic bullet” for syphilis (arsphenamine, Salvarsan) was an arsenical.

CHAPTER

58 Management of the Poisoned Patient Kent R. Olson, MD

CASE STUDY A 62-year-old woman with a history of depression is found in her apartment in a lethargic state. An empty bottle of bupropion is on the bedside table. In the emergency department, she is unresponsive to verbal and painful stimuli. She has a brief generalized seizure, followed by a respiratory arrest. The emergency physician performs endotracheal intubation and administers a drug intravenously, followed by another substance via a nasogastric tube. The patient is admitted to the intensive care unit for continued supportive care and recovers the next morning. What drug might be used intravenously to prevent further seizures? What substance is commonly used to adsorb drugs still present in the gastrointestinal tract?

Over 1 million cases of acute poisoning occur in the USA each year, although only a small number are fatal. Most deaths are due to intentional suicidal overdose by an adolescent or adult. Childhood deaths due to accidental ingestion of a drug or toxic household product have been markedly reduced in the last 40 years as a result of safety packaging and effective poisoning prevention education.

Even with a serious exposure, poisoning is rarely fatal if the victim receives prompt medical attention and good supportive care.

Careful management of respiratory failure, hypotension, seizures, and thermoregulatory disturbances has resulted in improved survival of patients who reach the hospital alive.

This chapter reviews the basic principles of poisoning, initial management, and specialized treatment of poisoning, including methods of increasing the elimination of drugs and toxins.

TOXICOKINETICS & TOXICODYNAMICS The term toxicokinetics denotes the absorption, distribution, excretion, and metabolism of toxins, toxic doses of therapeutic agents, and their metabolites. The term toxicodynamics is used to denote the injurious effects of these substances on body functions. Although many similarities exist between the pharmacokinetics and toxicokinetics of most substances, there are also important differences. The same caution applies to pharmacodynamics and toxicodynamics.

SPECIAL ASPECTS OF TOXICOKINETICS Volume of Distribution The volume of distribution (Vd) is defined as the apparent volume into which a substance is distributed in the body (see Chapter 3). A large V implies that the drug is not readily accessible to measures aimed at purifying the blood, such as hemodialysis. Examples of drugs with large volumes of distribution (> 5 L/kg) include antidepressants, antipsychotics, antimalarials, opioids, propranolol, and verapamil.

Drugs with a relatively small V (< 1 L/kg) include salicylate, ethanol, phenobarbital, lithium, valproic acid, and phenytoin (see Table 3â&#x20AC;&#x201C;1).

Clearance Clearance is a measure of the volume of plasma that is cleared of drug per unit time (see Chapter 3). The total clearance for most drugs is the sum of clearances via excretion by the kidneys and metabolism by the liver. In planning a detoxification strategy, it is important to know the contribution of each organ to total clearance. For example, if a drug is 95% cleared by liver metabolism and only 5% cleared by renal excretion, even a dramatic increase in urinary concentration of the drug will have little effect on overall elimination.

Overdosage of a drug can alter the usual pharmacokinetic processes, and this must be considered when applying kinetics to poisoned patients. For example, dissolution of tablets or gastric emptying time may be slowed so that absorption and peak toxic effects are delayed. Drugs may injure the epithelial barrier of the gastrointestinal tract and thereby increase absorption. If the capacity of the liver to metabolize a drug is exceeded, the first-pass effect will be reduced and more drug will be delivered to the circulation. With a dramatic increase in the concentration of drug in the blood, protein-binding capacity may be exceeded, resulting in an increased fraction of free drug and greater toxic effect. At normal dosage, most drugs are eliminated at a rate proportional to the plasma concentration (first-order kinetics). If the plasma concentration is very high and normal metabolism is saturated, the rate of elimination may become fixed (zeroorder kinetics). This change in kinetics may markedly prolong the apparent serum half-life and increase toxicity.

SPECIAL ASPECTS OF TOXICODYNAMICS The general dose-response principles described in Chapter 2 are relevant when estimating the potential severity of an intoxication. When considering quantal dose-response data, both the therapeutic index and the overlap of therapeutic and toxic response curves must be considered. For instance, two drugs may have the same therapeutic index but unequal safe dosing ranges if the slopes of their doseresponse curves are not the same. For some drugs, eg, sedative-hypnotics, the major toxic effect is a direct extension of the therapeutic action, as shown by their graded dose-response curve (see Figure 22–1). In the case of a drug with a linear dose-response curve (drug A), lethal effects may occur at 10 times the normal therapeutic dose. In contrast, a drug with a curve that reaches a plateau (drug B) may not be lethal at 100 times the normal dose.

For many drugs, at least part of the toxic effect may be different from the therapeutic action. For example, intoxication with drugs that have atropine-like effects (eg, tricyclic antidepressants) reduces sweating, making it more difficult to dissipate heat. In tricyclic antidepressant intoxication, there may also be increased muscular activity or seizures; the body’s production of heat is thus enhanced, and lethal hyperpyrexia may result. Overdoses of drugs that depress the cardiovascular system, eg, β blockers or calcium channel blockers, can profoundly alter not only cardiac function but all functions that are dependent on blood flow. These include renal and hepatic elimination of the toxin and that of any other drugs that may be given.

APPROACH TO THE POISONED PATIENT HOW DOES THE POISONED PATIENT DIE? An understanding of common mechanisms of death due to poisoning can help prepare the care-giver to treat patients effectively. Many toxins depress the central nervous system (CNS), resulting in obtundation or coma. Comatose patients frequently lose their airway protective reflexes and their respiratory drive. Thus, they may die as a result of airway obstruction by the flaccid tongue, aspiration of gastric contents into the tracheobronchial tree, or respiratory arrest. These are the most common causes of death due to overdoses of narcotics and sedative-hypnotic drugs (eg, barbiturates and alcohol).

Cardiovascular toxicity is also frequently encountered in poisoning. Hypotension may be due to depression of cardiac contractility; hypovolemia resulting from vomiting, diarrhea, or fluid sequestration; peripheral vascular collapse due to blockade of α-adrenoceptormediated vascular tone; or cardiac arrhythmias. Hypothermia or hyperthermia due to exposure as well as the temperature-dysregulating effects of many drugs can also produce hypotension. Lethal arrhythmias such as ventricular tachycardia and fibrillation can occur with overdoses of many cardioactive drugs such as ephedrine, amphetamines, cocaine, digitalis, and theophylline; and drugs not usually considered cardioactive, such as tricyclic antidepressants, antihistamines, and some opioid analogs.

Cellular hypoxia may occur in spite of adequate ventilation and oxygen administration when poisoning is due to cyanide, hydrogen sulfide, carbon monoxide, and other poisons that interfere with transport or utilization of oxygen. Such patients may not be cyanotic, but cellular hypoxia is evident by the development of tachycardia, hypotension, severe lactic acidosis, and signs of ischemia on the electrocardiogram.

Seizures, muscular hyperactivity, and rigidity may result in death. Seizures may cause pulmonary aspiration, hypoxia, and brain damage. Hyperthermia may result from sustained muscular hyperactivity and can lead to muscle breakdown and myoglobinuria, renal failure, lactic acidosis, and hyperkalemia. Drugs and poisons that often cause seizures include antidepressants, isoniazid (INH), diphenhydramine, cocaine, and amphetamines.

Other organ system damage may occur after poisoning and is sometimes delayed in onset. Paraquat attacks lung tissue, resulting in pulmonary fibrosis, beginning several days after ingestion. Massive hepatic necrosis due to poisoning by acetaminophen or certain mushrooms results in hepatic encephalopathy and death 48–72 hours or longer after ingestion.

Finally, some patients may die before hospitalization because the behavioral effects of the ingested drug may result in traumatic injury. Intoxication with alcohol and other sedative-hypnotic drugs is a common contributing factor to motor vehicle accidents. Patients under the influence of hallucinogens such as phencyclidine (PCP) or lysergic acid diethylamide (LSD) may suffer trauma when they become combative or fall from a height.

INITIAL MANAGEMENT OF THE POISONED PATIENT The initial management of a patient with coma, seizures, or otherwise altered mental status should follow the same approach regardless of the poison involved: supportive measures are the basics (“ABCDs”) of poisoning treatment.

First, the airway should be cleared of vomitus or any other obstruction and an oral airway or endotracheal tube inserted if needed.

For many patients, simple positioning in the lateral, leftside-down position is sufficient to move the flaccid tongue out of the airway.

Breathing should be assessed by observation and pulse oximetry and, if in doubt, by measuring arterial blood gases. Patients with respiratory insufficiency should be intubated and mechanically ventilated. The circulation should be assessed by continuous monitoring of pulse rate, blood pressure, urinary output, and evaluation of peripheral perfusion. An intravenous line should be placed and blood drawn for serum glucose and other routine determinations.

At this point, every patient with altered mental status should receive a challenge with concentrated dextrose, unless a rapid bedside blood glucose test demonstrates that the patient is not hypoglycemic. Adults are given 25 g (50 mL of 50% dextrose solution) intravenously, children 0.5 g/kg (2 mL/kg of 25% dextrose). Hypoglycemic patients may appear to be intoxicated, and there is no rapid and reliable way to distinguish them from poisoned patients. Alcoholic or malnourished patients should also receive 100 mg of thiamine intramuscularly or in the intravenous infusion solution at this time to prevent Wernicke’s syndrome.

The opioid antagonist naloxone may be given in a dose of 0.4–2 mg intravenously. Naloxone reverses respiratory and CNS depression due to all varieties of opioid drugs (see Chapter 31). It is useful to remember that these drugs cause death primarily by respiratory depression; therefore, if airway and breathing assistance have already been instituted, naloxone may not be necessary. Larger doses of naloxone may be needed for patients with overdose involving propoxyphene, codeine, and some other opioids. The benzodiazepine antagonist flumazenil (see Chapter 22) may be of value in patients with suspected benzodiazepine overdose, but it should not be used if there is a history of tricyclic antidepressant overdose or a seizure disorder, as it can induce convulsions in such patients.

History & Physical Examination Once the essential initial ABCD interventions have been instituted, one can begin a more detailed evaluation to make a specific diagnosis.

This includes gathering any available history and performing a toxicologically oriented physical examination. Other causes of coma or seizures such as head trauma, meningitis, or metabolic abnormalities should be sought and treated. Some common intoxications are described under Common Toxic Syndromes.

A. History Oral statements about the amount and even the type of drug ingested in toxic emergencies may be unreliable. Even so, family members, police, and fire department or paramedical personnel should be asked to describe the environment in which the toxic emergency occurred and should bring to the emergency department any syringes, empty bottles, household products, or over-the-counter medications in the immediate vicinity of the possibly poisoned patient.

B. Physical Examination A brief examination should be performed, emphasizing those areas most likely to give clues to the toxicologic diagnosis. These include vital signs, eyes and mouth, skin, abdomen, and nervous system.

1. Vital signs—Careful evaluation of vital signs (blood pressure, pulse, respirations, and temperature) is essential in all toxicologic emergencies. Hypertension and tachycardia are typical with amphetamines, cocaine, and antimuscarinic (anticholinergic) drugs.

Hypotension and bradycardia are characteristic features of overdose with calcium channel blockers, β blockers, clonidine, and sedative hypnotics. Hypotension with tachycardia is common with tricyclic antidepressants, trazodone, quetiapine, vasodilators, and β agonists.

Rapid respirations are typical of salicylates, carbon monoxide, and other toxins that produce metabolic acidosis or cellular asphyxia.

Hyperthermia may be associated with sympathomimetics, anticholinergics, salicylates, and drugs producing seizures or muscular rigidity.

Hypothermia can be caused by any CNS-depressant drug, especially when accompanied by exposure to a cold environment.

2. Eyes—The eyes are a valuable source of toxicologic information. Constriction of the pupils (miosis) is typical of opioids, clonidine, phenothiazines, and cholinesterase inhibitors (eg, organophosphate insecticides), and deep coma due to sedative drugs. Dilation of the pupils (mydriasis) is common with amphetamines, cocaine, LSD, and atropine and other anticholinergic drugs. Horizontal nystagmus is characteristic of intoxication with phenytoin, alcohol, barbiturates, and other sedative drugs. The presence of both vertical and horizontal nystagmus is strongly suggestive of phencyclidine poisoning. Ptosis and ophthalmoplegia are characteristic features of botulism.

3. Mouth—The mouth may show signs of burns due to corrosive substances, or soot from smoke inhalation. Typical odors of alcohol, hydrocarbon solvents, or ammonia may be noted. Poisoning due to cyanide can be recognized by some examiners as an odor like bitter almonds.

4. Skin—The skin often appears flushed, hot, and dry in poisoning with atropine and other antimuscarinics. Excessive sweating occurs with organophosphates, nicotine, and sympathomimetic drugs. Cyanosis may be caused by hypoxemia or by methemoglobinemia. Icterus may suggest hepatic necrosis due to acetaminophen or Amanita phalloides mushroom poisoning.

5. Abdomen—Abdominal examination may reveal ileus, which is typical of poisoning with antimuscarinic, opioid, and sedative drugs.

Hyperactive bowel sounds, abdominal cramping, and diarrhea are common in poisoning with organophosphates, iron, arsenic, theophylline, A phalloides, and A muscaria.

6. Nervous system—A careful neurologic examination is essential. Focal seizures or motor deficits suggest a structural lesion (eg, intracranial hemorrhage due to trauma) rather than toxic or metabolic encephalopathy. Nystagmus, dysarthria, and ataxia are typical of phenytoin, carbamazepine, alcohol, and other sedative intoxication. Twitching and muscular hyperactivity are common with atropine and other anticholinergic agents, and cocaine and other sympathomimetic drugs. Muscular rigidity can be caused by haloperidol and other antipsychotic agents, and by strychnine or by tetanus. Generalized hypertonicity of muscles and lower extremity clonus are typical of serotonin syndrome. Seizures are often caused by overdose with antidepressants (especially tricyclic antidepressants and bupropion [as in the case study]), cocaine, amphetamines, theophylline, isoniazid, and diphenhydramine. Flaccid coma with absent reflexes and even an isoelectric electroencephalogram may be seen with deep coma due to sedative-hypnotic or other CNS depressant intoxication and may be mistaken for brain death.

Laboratory & Imaging Procedures A. Arterial Blood Gases Hypoventilation results in an elevated P CO2 (hypercapnia) and a low P O2 (hypoxia). The P O2 may also be low in a patient with aspiration pneumonia or drug-induced pulmonary edema. Poor tissue oxygenation due to hypoxia, hypotension, or cyanide poisoning will result in metabolic acidosis. The P O2 measures only oxygen dissolved in the plasma and not total blood oxygen content or oxyhemoglobin saturation and may appear normal in patients with severe carbon monoxide poisoning. Pulse oximetry may also give falsely normal results in carbon monoxide intoxication.

B. Electrolytes Sodium, potassium, chloride, and bicarbonate should be measured. The anion gap is then calculated by subtracting the measured anions from cations: Anion gap = (Na+ + K+) – (HCO3 − + Cl−) Normally, the sum of the cations exceeds the sum of the anions by no more than 12–16 mEq/L (or 8–12 mEq/L if the formula used for estimating the anion gap omits the potassium level). A larger than expected anion gap is caused by the presence of unmeasured anions (lactate, etc) accompanying metabolic acidosis. This may occur with numerous conditions, such as diabetic ketoacidosis, renal failure, or shock-induced lactic acidosis. Drugs that may induce an elevated anion gap metabolic acidosis (Table 58–1) include aspirin, metformin, methanol, ethylene glycol, isoniazid, and iron.

TABLE 58–1 Examples of drug-induced anion gap acidosis.

Alterations in the serum potassium level are hazardous because they can result in cardiac arrhythmias. Drugs that may cause hyperkalemia despite normal renal function include potassium itself, β blockers, digitalis glycosides, potassium-sparing diuretics, and fluoride. Drugs associated with hypokalemia include barium, β agonists, caffeine, theophylline, and thiazide and loop diuretics.

C. Renal Function Tests Some toxins have direct nephrotoxic effects; in other cases, renal failure is due to shock or myoglobinuria. Blood urea nitrogen and creatinine levels should be measured and urinalysis performed. Elevated serum creatine kinase (CK) and myoglobin in the urine suggest muscle necrosis due to seizures or muscular rigidity. Oxalate crystals in large numbers in the urine suggest ethylene glycol poisoning.

D. Serum Osmolality The calculated serum osmolality is dependent mainly on the serum sodium and glucose and the blood urea nitrogen and can be estimated from the following formula:

This calculated value is normally 280–290 mOsm/L. Ethanol and other alcohols may contribute significantly to the measured serum osmolality but, since they are not included in the calculation, cause an osmol gap:

Table 58–2 lists the concentration and expected contribution to the serum osmolality in ethanol, methanol, ethylene glycol, and isopropanol poisonings.

TABLE 58–2 Some substances that can cause an osmol gap.

E. Electrocardiogram Widening of the QRS complex duration (to more than 100 milliseconds) is typical of tricyclic antidepressant and quinidine overdoses (Figure 58–1). The QTc interval may be prolonged (to more than 440 milliseconds) in many poisonings, including quinidine, antidepressants and antipsychotics, lithium, and arsenic (see also https://www.crediblemeds.org/everyone/composite-list-all-qtdrugs/).

Variable atrioventricular (AV) block and a variety of atrial and ventricular arrhythmias are common with poisoning by digoxin and other cardiac glycosides. Hypoxemia due to carbon monoxide poisoning may result in ischemic changes on the electrocardiogram.

FIGURE 58–1 Changes in the electrocardiogram in tricyclic antidepressant overdosage. A: Slowed intraventricular conduction results in prolonged QRS interval (0.18 s; normal, 0.08 s). B and C: Supraventricular tachycardia with progressive widening of QRS complexes mimics ventricular tachycardia. (Reproduced, with permission, from Benowitz NL, Goldschlager N: Cardiac disturbances. In: Haddad LM, Shannon MW, Winchester JF [editors]. Clinical Management of Poisoning and Drug Overdose, 3rd ed. WB Saunders, 1998. © Elsevier.)

F. Imaging Findings A plain film of the abdomen may be useful because some tablets, particularly iron and potassium, may be radiopaque. Chest radiographs may reveal aspiration pneumonia, hydrocarbon pneumonia, or pulmonary edema. When head trauma is suspected, a computed tomography (CT) scan is recommended.

Toxicology Screening Tests It is a common misconception that a broad toxicology “screen” is the best way to diagnose and manage an acute poisoning.

Unfortunately, comprehensive toxicology screening is time-consuming and expensive and results of tests may not be available for days.

Moreover, many highly toxic drugs such as calcium channel blockers, β blockers, and isoniazid are not included in the screening process.

The clinical examination of the patient and selected routine laboratory tests are usually sufficient to generate a tentative diagnosis and an appropriate treatment plan. Although screening tests may be helpful in confirming a suspected intoxication or for ruling out intoxication as a cause of apparent brain death, they should not delay needed treatment.

When a specific antidote or other treatment is under consideration, quantitative laboratory testing may be indicated. For example, determination of the acetaminophen level is useful in assessing the need for antidotal therapy with acetylcysteine. Serum levels of salicylate (aspirin), ethylene glycol, methanol, theophylline, carbamazepine, lithium, valproic acid, and other drugs and poisons may indicate the need for hemodialysis (Table 58–3).

TABLE 58–3 Hemodialysis in drug overdose and poisoning.1

Decontamination Decontamination procedures should be undertaken simultaneously with initial stabilization, diagnostic assessment, and laboratory evaluation. Decontamination involves removing toxins from the skin or gastrointestinal tract.

A. Skin Contaminated clothing should be completely removed and double-bagged to prevent illness in health care providers and for possible

laboratory analysis. Wash contaminated skin with soap and water.

B. Gastrointestinal Tract Controversy remains regarding the efficacy of gut emptying by emesis or gastric lavage, especially when treatment is initiated more than 1 hour after ingestion. For most ingestions, clinical toxicologists recommend simple administration of activated charcoal to bind ingested poisons in the gut before they can be absorbed (as in the case study). In unusual circumstances, induced emesis or gastric lavage may also be used.

1. Emesis—Emesis can be induced with ipecac syrup (never extract of ipecac), and this method was previously used to treat some childhood ingestions at home under telephone supervision of a physician or poison control center personnel. However, the risks involved with inappropriate use outweighed the unproven benefits, and this treatment is rarely used in the home or hospital. Ipecac should not be used if the suspected intoxicant is a corrosive agent, a petroleum distillate, or a rapid-acting convulsant. Previously popular methods of inducing emesis such as fingertip stimulation of the pharynx, salt water, and apomorphine are ineffective or dangerous and should not be used.

2. Gastric lavage—If the patient is awake or if the airway is protected by an endotracheal tube, gastric lavage may be performed using an orogastric or nasogastric tube—as large a tube as possible. Lavage solutions (usually 0.9% saline) should be at body temperature to prevent hypothermia.

3. Activated charcoal—Owing to its large surface area, activated charcoal can adsorb many drugs and poisons. It is most effective if given in a ratio of at least 10:1 of charcoal to estimated dose of toxin by weight. Charcoal does not bind iron, lithium, or potassium, and it binds alcohols and cyanide only poorly. It does not appear to be useful in poisoning due to corrosive mineral acids and alkali. Studies suggest that oral activated charcoal given alone may be just as effective as gut emptying (eg, ipecac-induced emesis or gastric lavage) followed by charcoal. Repeated doses of oral activated charcoal may enhance systemic elimination of some drugs (including carbamazepine, dapsone, and theophylline) by a mechanism referred to as “gut dialysis,” although the clinical benefit is unproved.

4. Cathartics—Administration of a cathartic (laxative) agent may hasten removal of toxins from the gastrointestinal tract and reduce absorption, although no controlled studies have been done. Whole bowel irrigation with a balanced polyethylene glycol-electrolyte solution (GoLYTELY, CoLyte) can enhance gut decontamination after ingestion of iron tablets, enteric-coated medicines, illicit drug-filled packets, and foreign bodies. The solution is administered orally at 1–2 L/h (500 mL/h in children) for several hours until the rectal effluent is clear.

Specific Antidotes There is a popular misconception that there is an antidote for every poison. Actually, selective antidotes are available for only a few classes of toxins. The major antidotes and their characteristics are listed in Table 58–4.

TABLE 58–4 Examples of specific antidotes.

Methods of Enhancing Elimination of Toxins After appropriate diagnostic and decontamination procedures and administration of antidotes, it is important to consider whether measures for enhancing elimination, such as hemodialysis or urinary alkalinization, can improve the clinical outcome. Table 58–3 lists intoxications for which dialysis may be beneficial.

A. Dialysis Procedures 1. Peritoneal dialysis—Although it is a relatively simple and available technique, peritoneal dialysis is inefficient in removing most drugs.

2. Hemodialysis—Hemodialysis is more efficient than peritoneal dialysis and has been well studied. It assists in correction of fluid and electrolyte imbalance and may also enhance removal of toxic metabolites (eg, formic acid in methanol poisoning; oxalic and glycolic acids in ethylene glycol poisoning). The efficiency of both peritoneal dialysis and hemodialysis is a function of the molecular weight, water solubility, protein binding, endogenous clearance, and distribution in the body of the specific toxin. Hemodialysis is especially useful in overdose cases in which the precipitating drug can be removed and fluid and electrolyte imbalances are present and can be corrected (eg, salicylate intoxication).

B. Forced Diuresis and Urinary pH Manipulation Previously popular but of unproved value, forced diuresis may cause volume overload and electrolyte abnormalities and is not recommended. Renal elimination of a few toxins can be enhanced by alteration of urinary pH. For example, urinary alkalinization is useful in cases of salicylate overdose. Acidification may increase the urine concentration of drugs such as phencyclidine and amphetamines but is not advised because it may worsen renal complications from rhabdomyolysis, which often accompanies the intoxication.

COMMON TOXIC SYNDROMES ACETAMINOPHEN Acetaminophen is one of the drugs commonly involved in suicide attempts and accidental poisonings, both as the sole agent and in combination with other drugs. Acute ingestion of more than 150–200 mg/kg (children) or 7 g total (adults) is considered potentially toxic.

A highly toxic metabolite is produced in the liver (see Figure 4–5).

Initially, the patient is asymptomatic or has mild gastrointestinal upset (nausea, vomiting). After 24–36 hours, evidence of liver injury appears, with elevated aminotransferase levels and hypoprothrombinemia. In severe cases, fulminant liver failure occurs, leading to hepatic encephalopathy and death. Renal failure may also occur.

The severity of poisoning is estimated from a serum acetaminophen concentration measurement. If the level is greater than 150–200 mg/L approximately 4 hours after ingestion, the patient is at risk for liver injury. (Chronic alcoholics or patients taking drugs that enhance P450 production of toxic metabolites are at risk with lower levels.) The antidote acetylcysteine acts as a glutathione substitute, binding the toxic metabolite as it is produced. It is most effective when given early and should be started within 8–10 hours if possible. Liver transplantation may be required for patients with fulminant hepatic failure.

AMPHETAMINES & OTHER STIMULANTS Stimulant drugs commonly abused in the USA include methamphetamine (“crank,” “crystal”), methylenedioxymethamphetamine (MDMA, “ecstasy”), and cocaine (“crack”) as well as pharmaceuticals such as pseudoephedrine (Sudafed) and ephedrine (as such and in the herbal agent Ma-huang) (see Chapter 32). Caffeine is often added to dietary supplements sold as “metabolic enhancers” or “fatburners.” Newer synthetic analogs of amphetamines such as 3,4-methylenedioxypyrovalerone (MDPV) and various derivatives of methcathinone are becoming popular drugs of abuse, often sold on the street as “bath salts” with names like “Ivory Wave,” “Bounce,” “Bubbles,” “Mad Cow,” and “Meow Meow.”

At the doses usually used by stimulant abusers, euphoria and wakefulness are accompanied by a sense of power and well-being. At higher doses, restlessness, agitation, and acute psychosis may occur, accompanied by hypertension and tachycardia. Prolonged muscular hyperactivity or seizures may contribute to hyperthermia and rhabdomyolysis. Body temperatures as high as 42°C (107.6°F) have been recorded. Hyperthermia can cause brain damage, hypotension, coagulopathy, and renal failure.

Treatment for stimulant toxicity includes general supportive measures as outlined earlier. There is no specific antidote. Seizures and hyperthermia are the most dangerous manifestations and must be treated aggressively. Seizures are usually managed with intravenous benzodiazepines (eg, lorazepam). Temperature is reduced by removing clothing, spraying with tepid water, and encouraging evaporative cooling with fanning. For very high body temperatures (eg, > 40–41°C [104–105.8°F]), neuromuscular paralysis is used to abolish muscle activity quickly.

ANTICHOLINERGIC AGENTS A large number of prescription and nonprescription drugs, as well as a variety of plants and mushrooms, can inhibit the effects of acetylcholine at muscarinic receptors. Some drugs used for other purposes (eg, antihistamines) also have anticholinergic effects, in addition to other potentially toxic actions. For example, antihistamines such as diphenhydramine can cause seizures; tricyclic antidepressants, which have anticholinergic, quinidine-like, and α-blocking effects, can cause severe cardiovascular toxicity.

The classic anticholinergic (technically, “antimuscarinic”) syndrome is remembered as “red as a beet” (skin flushed), “hot as a hare” (hyperthermia), “dry as a bone” (dry mucous membranes, no sweating), “blind as a bat” (blurred vision, cycloplegia), and “mad as a hatter” (confusion, delirium). Patients usually have sinus tachycardia, and the pupils are usually dilated (see Chapter 8). Agitated delirium or coma may be present. Muscle twitching is common, but seizures are unusual unless the patient has ingested an antihistamine or a tricyclic antidepressant. Urinary retention is common, especially in older men.

Treatment for anticholinergic syndrome is largely supportive. Agitated patients may require sedation with a benzodiazepine or an antipsychotic agent (eg, haloperidol). The specific antidote for peripheral and central anticholinergic syndrome is physostigmine, which has a prompt and dramatic effect and is especially useful for patients who are very agitated. Physostigmine is given in small intravenous doses (0.5–1 mg) with careful monitoring, because it can cause bradycardia and seizures if given too rapidly. Physostigmine should not be given to a patient with suspected tricyclic antidepressant overdose because it can aggravate cardiotoxicity, resulting in heart block or asystole. Catheterization may be needed to prevent excessive distention of the bladder.

ANTIDEPRESSANTS Tricyclic antidepressants (eg, amitriptyline, desipramine, doxepin, many others; see Chapter 30) are among the most common prescription drugs involved in life-threatening drug overdose. Ingestion of more than 1 g of a tricyclic (or about 15–20 mg/kg) is considered potentially lethal.

Tricyclic antidepressants are competitive antagonists at muscarinic cholinergic receptors, and anticholinergic findings (tachycardia, dilated pupils, dry mouth) are common even at moderate doses. Some tricyclics are also strong α blockers, which can lead to vasodilation. Centrally mediated agitation and seizures may be followed by depression and hypotension. Most important is the fact that tricyclics have quinidine-like depressant effects on the cardiac sodium channel that cause slowed conduction with a wide QRS interval and depressed cardiac contractility. This cardiac toxicity may result in serious arrhythmias (Figure 58–1), including ventricular conduction block and ventricular tachycardia.

Treatment of tricyclic antidepressant overdose includes general supportive care as outlined earlier. Endotracheal intubation and assisted ventilation may be needed. Intravenous fluids are given for hypotension, and dopamine or norepinephrine is added if necessary.

Many toxicologists recommend norepinephrine as the initial drug of choice for tricyclic-induced hypotension. The antidote for quinidinelike cardiac toxicity (manifested by a wide QRS complex) is sodium bicarbonate: a bolus of 50–100 mEq (or 1–2 mEq/kg) provides a rapid increase in extracellular sodium that helps overcome sodium channel blockade. Do not use physostigmine! Although physostigmine does effectively reverse anticholinergic symptoms, it can aggravate depression of cardiac conduction and cause seizures.

Monoamine oxidase inhibitors (eg, tranylcypromine, phenelzine) are older antidepressants that are occasionally used for resistant depression. They can cause severe hypertensive reactions when interacting foods or drugs are taken (see Chapters 9 and 30), and they can interact with the selective serotonin reuptake inhibitors (SSRIs).

Newer antidepressants (eg, fluoxetine, paroxetine, citalopram, venlafaxine) are mostly SSRIs and are generally safer than the tricyclic antidepressants and monoamine oxidase inhibitors, although they can cause seizures. Bupropion (not an SSRI) has caused seizures even in therapeutic doses. Some antidepressants have been associated with QT prolongation and torsades de pointes arrhythmia.

SSRIs may interact with each other or especially with monoamine oxidase inhibitors to cause the serotonin syndrome, characterized by agitation, muscle hyperactivity, and hyperthermia (see Chapter 16).

ANTIPSYCHOTICS Antipsychotic drugs include the older phenothiazines and butyrophenones, as well as newer atypical drugs. All of these can cause CNS depression, seizures, and hypotension. Some can cause QT prolongation. The potent dopamine D2 blockers are also associated with parkinsonian movement disorders (dystonic reactions) and in rare cases with the neuroleptic malignant syndrome, characterized by “leadpipe” rigidity, hyperthermia, and autonomic instability (see Chapters 16 and 29).

ASPIRIN (SALICYLATE) Salicylate poisoning (see Chapter 36) is a much less common cause of childhood poisoning deaths since the introduction of child-resistant containers and the reduced use of children’s aspirin. It still accounts for numerous suicidal and accidental poisonings. Acute ingestion of

more than 200 mg/kg is likely to produce intoxication. Poisoning can also result from chronic overmedication; this occurs most commonly in elderly patients using salicylates for chronic pain who become confused about their dosing. Poisoning causes uncoupling of oxidative phosphorylation and disruption of normal cellular metabolism.

The first sign of salicylate toxicity is often hyperventilation and respiratory alkalosis due to medullary stimulation. Metabolic acidosis follows, and an increased anion gap results from accumulation of lactate as well as excretion of bicarbonate by the kidney to compensate for respiratory alkalosis. Arterial blood gas testing often reveals a mixed respiratory alkalosis and metabolic acidosis. Body temperature may be elevated owing to uncoupling of oxidative phosphorylation. Severe hyperthermia may occur in serious cases. Vomiting and hyperpnea as well as hyperthermia contribute to fluid loss and dehydration. With very severe poisoning, profound metabolic acidosis, seizures, coma, pulmonary edema, and cardiovascular collapse may occur. Absorption of salicylate and signs of toxicity may be delayed after very large overdoses or ingestion of enteric coated tablets.

General supportive care is essential. After massive aspirin ingestions (eg, more than 100 tablets), aggressive gut decontamination is advisable, including gastric lavage, repeated doses of activated charcoal, and consideration of whole bowel irrigation. Intravenous fluids are used to replace fluid losses caused by tachypnea, vomiting, and fever. For moderate intoxications, intravenous sodium bicarbonate is given to alkalinize the urine and promote salicylate excretion by trapping the salicylate in its ionized, polar form. For severe poisoning (eg, patients with severe acidosis, coma, and serum salicylate level > 100 mg/dL), emergency hemodialysis is performed to remove the salicylate more quickly and restore acid-base balance and fluid status.

BETA BLOCKERS In overdose, β blockers inhibit both β1 and β2 adrenoceptors; selectivity, if any, is lost at high dosage. The most toxic β blocker is propranolol. As little as two to three times the therapeutic dose can cause serious toxicity. This may be because propranolol in high doses may cause sodium channel blocking effects similar to those seen with quinidine-like drugs, and it is lipophilic, allowing it to enter the CNS (see Chapter 10).

Bradycardia and hypotension are the most common manifestations of toxicity. Agents with partial agonist activity (eg, pindolol) can cause tachycardia and hypertension. Seizures and cardiac conduction block (wide QRS complex) may be seen with propranolol overdose.

General supportive care should be provided as outlined earlier. The usual measures used to raise the blood pressure and heart rate, such as intravenous fluids, β-agonist drugs, and atropine, are generally ineffective. Glucagon is a useful antidote that—like β agonists— acts on cardiac cells to raise intracellular cAMP but does so independent of β adrenoceptors. It can improve heart rate and blood pressure when given in high doses (5–20 mg intravenously).

CALCIUM CHANNEL BLOCKERS Calcium antagonists can cause serious toxicity or death with relatively small overdoses. These channel blockers depress sinus node automaticity and slow AV node conduction (see Chapter 12). They also reduce cardiac output and blood pressure. Serious hypotension is mainly seen with nifedipine and related dihydropyridines, but in severe overdose all of the listed cardiovascular effects can occur with any of the calcium channel blockers.

Treatment requires general supportive care. Since most ingested calcium antagonists are in sustained-release form, it may be possible to expel them before they are completely absorbed; initiate whole bowel irrigation and oral activated charcoal as soon as possible, before calcium antagonist-induced ileus intervenes. Calcium, given intravenously in doses of 2–10 g, is a useful antidote for depressed cardiac contractility but less effective for nodal block or peripheral vascular collapse. Other treatments reported to be helpful in managing hypotension associated with calcium channel blocker poisoning include glucagon and high-dose insulin (0.5–1 unit/kg/h) plus glucose supplementation to maintain euglycemia. Recently case reports have suggested benefit from administration of lipid emulsion (Intralipid, normally used as an intravenous dietary fat supplement) for severe verapamil overdose.

CARBON MONOXIDE & OTHER TOXIC GASES Carbon monoxide (CO) is a colorless, odorless gas that is ubiquitous because it is created whenever carbon-containing materials are burned. Carbon monoxide poisoning is the leading cause of death due to poisoning in the USA. Most cases occur in victims of fires, but accidental and suicidal exposures are also common. The diagnosis and treatment of carbon monoxide poisoning are described in Chapter 56. Many other toxic gases are produced in fires or released in industrial accidents (Table 58–5).

TABLE 58–5 Characteristics of poisoning with some gases.

CHOLINESTERASE INHIBITORS Organophosphate and carbamate cholinesterase inhibitors (see Chapter 7) are widely used to kill insects and other pests. Most cases of serious organophosphate or carbamate poisoning result from intentional ingestion by a suicidal person, but poisoning has also occurred at work (pesticide application or packaging) or, rarely, as a result of food contamination or terrorist attack (eg, release of the chemical warfare nerve agent sarin in the Tokyo subway system in 1995).

Stimulation of muscarinic receptors causes abdominal cramps, diarrhea, excessive salivation, sweating, urinary frequency, and increased bronchial secretions (see Chapters 6 and 7). Stimulation of nicotinic receptors causes generalized ganglionic activation, which can lead to hypertension and either tachycardia or bradycardia. Muscle twitching and fasciculations may progress to weakness and respiratory muscle paralysis. CNS effects include agitation, confusion, and seizures. The mnemonic DUMBELS (diarrhea, urination, miosis and muscle weakness, bronchospasm, excitation, lacrimation, and seizures, sweating, and salivation) helps recall the common findings. Blood testing may be used to document depressed activity of red blood cell (acetylcholinesterase) and plasma (butyrylcholinesterase) enzymes, which provide an indirect estimate of synaptic cholinesterase activity.

General supportive care should be provided as outlined above. Precautions should be taken to ensure that rescuers and health care providers are not poisoned themselves by exposure to contaminated clothing or skin. This is especially critical for the most potent substances such as parathion or nerve gas agents. Antidotal treatment consists of atropine and pralidoxime (see Table 58â&#x20AC;&#x201C;4). Atropine is an effective competitive inhibitor at muscarinic sites but has no effect at nicotinic sites. Pralidoxime given early enough may be capable of restoring the cholinesterase activity and is active at both muscarinic and nicotinic sites.

CYANIDE

Cyanide (CN−) salts and hydrogen cyanide (HCN) are highly toxic chemicals used in chemical synthesis, as rodenticides (eg, “gopher getter”), formerly as a method of execution, and as agents of suicide or homicide. Hydrogen cyanide is formed from the burning of plastics, wool, and many other synthetic and natural products. Cyanide is also released after ingestion of various plants (eg, cassava) and seeds (eg, apple, peach, and apricot).

Cyanide binds readily to cytochrome oxidase, inhibiting oxygen utilization within the cell and leading to cellular hypoxia and lactic acidosis. Symptoms of cyanide poisoning include shortness of breath, agitation, and tachycardia followed by seizures, coma, hypotension, and death. Severe metabolic acidosis is characteristic. The venous oxygen content may be elevated because oxygen is not being taken up by cells.

Treatment of cyanide poisoning includes rapid administration of activated charcoal (although charcoal binds cyanide poorly, it can reduce absorption) and general supportive care. The conventional antidote kit available in the USA includes two forms of nitrite (amyl nitrite and sodium nitrite) and sodium thiosulfate. The nitrites induce methemoglobinemia, which binds CN−, creating the less toxic cyanomethemoglobin; thiosulfate is a cofactor in the enzymatic conversion of CN− to the much less toxic thiocyanate (SCN−).

In 2006 the FDA approved a new cyanide antidote, a concentrated form of hydroxocobalamin, which is now available as the Cyanokit (EMD Pharmaceuticals, Durham, North Carolina). Hydroxocobalamin (one form of vitamin B12 ) combines rapidly with CN− to form nontoxic cyanocobalamin (another form of vitamin B12 ).

DIGOXIN Digitalis and other cardiac glycosides and cardenolides are found in many plants (see Chapter 13) and in the skin of some toads. Toxicity may occur as a result of acute overdose or from accumulation of digoxin in a patient with renal insufficiency or from taking a drug that interferes with digoxin elimination. Patients receiving long-term digoxin treatment are often also taking diuretics, which can lead to electrolyte depletion (especially potassium).

Vomiting is common in patients with digitalis overdose. Hyperkalemia may be caused by acute digitalis overdose or severe poisoning, whereas hypokalemia may be present in patients as a result of long-term diuretic treatment. (Digitalis does not cause hypokalemia.) A variety of cardiac rhythm disturbances may occur, including sinus bradycardia, AV block, atrial tachycardia with block, accelerated junctional rhythm, premature ventricular beats, bidirectional ventricular tachycardia, and other ventricular arrhythmias.

General supportive care should be provided. Atropine is often effective for bradycardia or AV block. The use of digoxin antibodies (see Chapter 13) has revolutionized the treatment of digoxin toxicity; they should be administered intravenously in the dosage indicated in the package insert. Symptoms usually improve within 30–60 minutes after antibody administration. Digoxin antibodies may also be tried in cases of poisoning by other cardiac glycosides (eg, digitoxin, oleander), although larger doses may be needed due to incomplete crossreactivity.

ETHANOL & SEDATIVE-HYPNOTIC DRUGS Overdosage with ethanol and sedative-hypnotic drugs (eg, benzodiazepines, barbiturates, γ-hydroxybutyrate [GHB], carisoprodol [Soma]; see Chapters 22 and 23) occurs frequently because of their common availability and use.

Patients with ethanol or other sedative-hypnotic overdose may be euphoric and rowdy (“drunk”) or in a state of stupor or coma (“dead drunk”). Comatose patients often have depressed respiratory drive. Depression of protective airway reflexes may result in pulmonary aspiration of gastric contents, leading to pneumonia. Hypothermia may be present because of environmental exposure and depressed shivering. Ethanol blood levels greater than 300 mg/dL usually cause deep coma, but regular users are often tolerant to the effects of ethanol and may be ambulatory despite even higher levels. Patients with GHB overdose are often deeply comatose for 3–4 hours and then awaken fully in a matter of minutes.

General supportive care should be provided. With careful attention to protecting the airway (including endotracheal intubation) and assisting ventilation, most patients recover as the drug effects wear off. Hypotension usually responds to intravenous fluids, body warming if cold, and, if needed, dopamine. Patients with isolated benzodiazepine overdose may awaken after intravenous flumazenil, a benzodiazepine antagonist. However, this drug is not widely used as empiric therapy for drug overdose because it may precipitate seizures in patients who are addicted to benzodiazepines or who have ingested a convulsant drug (eg, a tricyclic antidepressant). There are no antidotes for ethanol, barbiturates, or most other sedative-hypnotics.

ETHYLENE GLYCOL & METHANOL Ethylene glycol and methanol are alcohols that are important toxins because of their metabolism to highly toxic organic acids (see Chapter 23). They are capable of causing CNS depression and a drunken state similar to ethanol overdose. In addition, their products of metabolism—formic acid (from methanol) or hippuric, oxalic, and glycolic acids (from ethylene glycol)—cause a severe metabolic

acidosis and can lead to coma and blindness (in the case of formic acid) or renal failure (from oxalic acid and glycolic acid). Initially, the patient appears drunk, but after a delay of up to several hours, a severe anion gap metabolic acidosis becomes apparent, accompanied by hyperventilation and altered mental status. Patients with methanol poisoning may have visual disturbances ranging from blurred vision to blindness.

Metabolism of ethylene glycol and methanol to their toxic products can be blocked by inhibiting the enzyme alcohol dehydrogenase with a competing drug, such as fomepizole (4-methylpyrazole). Ethanol is also an effective antidote, but it can be difficult to achieve a safe and effective blood level.

IRON & OTHER METALS Iron is widely used in over-the-counter vitamin preparations and is a leading cause of childhood poisoning deaths. As few as 10–12 prenatal multivitamins with iron may cause serious illness in a small child. Poisoning with other metals (lead, mercury, arsenic) is also important, especially in industry. See Chapters 33, 56, and 57 for detailed discussions of poisoning by iron and other metals.

OPIOIDS Opioids (opium, morphine, heroin, meperidine, methadone, etc) are common drugs of abuse (see Chapters 31 and 32), and overdose is a common result of using the poorly standardized preparations sold on the street. See Chapter 31 for a detailed discussion of opioid overdose and its treatment.

RATTLESNAKE ENVENOMATION In the USA, rattlesnakes are the most common venomous reptiles. Bites are rarely fatal, and 20% do not involve envenomation.

However, about 60% of bites cause significant morbidity due to the destructive digestive enzymes found in the venom. Evidence of rattlesnake envenomation includes severe pain, swelling, bruising, hemorrhagic bleb formation, and obvious fang marks. Systemic effects include nausea, vomiting, muscle fasciculations, tingling and metallic taste in the mouth, shock, and systemic coagulopathy with prolonged clotting time and reduced platelet count.

Studies have shown that emergency field remedies such as incision and suction, tourniquets, and ice packs are far more damaging than useful. Avoidance of unnecessary motion, on the other hand, does help to limit the spread of the venom. Definitive therapy relies on intravenous antivenom (also known as antivenin) and this should be started as soon as possible.

THEOPHYLLINE Although it has been largely replaced by inhaled β agonists, theophylline continues to be used for the treatment of bronchospasm by some patients with asthma and bronchitis (see Chapter 20). A dose of 20–30 tablets can cause serious or fatal poisoning. Chronic or subacute theophylline poisoning can also occur as a result of accidental overmedication or use of a drug that interferes with theophylline metabolism (eg, cimetidine, ciprofloxacin, erythromycin; see Chapter 4).

In addition to sinus tachycardia and tremor, vomiting is common after overdose. Hypotension, tachycardia, hypokalemia, and hyperglycemia may occur, probably owing to β2 -adrenergic activation. The cause of this activation is not fully understood, but the effects can be ameliorated by β blockers (see below). Cardiac arrhythmias include atrial tachycardias, premature ventricular contractions, and ventricular tachycardia. In severe poisoning (eg, acute overdose with serum level > 100 mg/L), seizures often occur and are usually resistant to common anticonvulsants. Toxicity may be delayed in onset for many hours after ingestion of sustained-release tablet formulations.

General supportive care should be provided. Aggressive gut decontamination should be carried out using repeated doses of activated charcoal and whole bowel irrigation. Propranolol or other β blockers (eg, esmolol) are useful antidotes for β-mediated hypotension and tachycardia. Phenobarbital is preferred over phenytoin for convulsions; most anticonvulsants are ineffective. Hemodialysis is indicated for serum concentrations greater than 100 mg/L and for intractable seizures in patients with lower levels.

REFERENCES Dart RD (editor): Medical Toxicology, 3rd ed. Lippincott Williams & Wilkins, 2004.

Goldfrank LR et al (editors): Goldfrank’s Toxicologic Emergencies, 9th ed. McGraw-Hill, 2010.

Olson KR et al (editors): Poisoning & Drug Overdose, 6th ed. McGraw-Hill, 2011.

POISINDEX. (Revised Quarterly.) T homson/Micromedex.

CASE STUDY ANSWER Overdose of bupropion can cause seizures that are often recurrent or prolonged. Drug-induced seizures are treated with an intravenous benzodiazepine such as lorazepam or diazepam. If this is not effective, phenobarbital or another more potent central nervous system depressant may be used. To prevent ingested drugs and poisons from being absorbed systemically, a slurry of activated charcoal is often given orally or by nasogastric tube.

SECTION X SPECIAL TOPICS

CHAPTER

59 Special Aspects of Perinatal & Pediatric Pharmacology Gideon Koren, MD*

The effects of drugs on the fetus and newborn infant are based on the general principles set forth in Chapters 1â&#x20AC;&#x201C;4 of this book.

However, the physiologic contexts in which these pharmacologic laws operate are different in pregnant women and in rapidly maturing infants. At present, the special pharmacokinetic factors operative in these patients are beginning to be understood, whereas information regarding pharmacodynamic differences (eg, receptor characteristics and responses) is still incomplete.

DRUG THERAPY IN PREGNANCY Pharmacokinetics Most drugs taken by pregnant women can cross the placenta and expose the developing embryo and fetus to their pharmacologic and teratogenic effects. Critical factors affecting placental drug transfer and drug effects on the fetus include the following: (1) the physicochemical properties of the drug; (2) the rate at which the drug crosses the placenta and the amount of drug reaching the fetus; (3) the duration of exposure to the drug; (4) distribution characteristics in different fetal tissues; (5) the stage of placental and fetal development at the time of exposure to the drug; and (6) the effects of drugs used in combination.

A. Lipid Solubility As is true also of other biologic membranes, drug passage across the placenta is dependent on lipid solubility and the degree of drug ionization. Lipophilic drugs tend to diffuse readily across the placenta and enter the fetal circulation. For example, thiopental, a drug commonly used for cesarean sections, crosses the placenta almost immediately and can produce sedation or apnea in the newborn infant.

Highly ionized drugs such as succinylcholine and tubocurarine, also used for cesarean sections, cross the placenta slowly and achieve very low concentrations in the fetus. Impermeability of the placenta to polar compounds is relative rather than absolute. If high enough maternal-fetal concentration gradients are achieved, polar compounds cross the placenta in measurable amounts. Salicylate, which is almost completely ionized at physiologic pH, crosses the placenta rapidly. This occurs because the small amount of salicylate that is not ionized is highly lipid-soluble.

B. Molecular Size and pH The molecular weight of the drug also influences the rate of transfer and the amount of drug transferred across the placenta. Drugs with molecular weights of 250â&#x20AC;&#x201C;500 can cross the placenta easily, depending upon their lipid solubility and degree of ionization; those with molecular weights of 500â&#x20AC;&#x201C;1000 cross the placenta with more difficulty; and those with molecular weights greater than 1000 cross very poorly. An important clinical application of this property is the choice of heparin as an anticoagulant in pregnant women. Because it is a very large (and polar) molecule, heparin is unable to cross the placenta. Unlike warfarin, which is teratogenic and should be avoided during the first trimester and even beyond (as the brain continues to develop), heparin may be safely given to pregnant women who need anticoagulation. Yet the placenta contains drug transporters, which can carry larger molecules to the fetus. For example, a variety of maternal antibodies cross the placenta and may cause fetal morbidity, as in Rh incompatibility.

Because maternal blood has a pH of 7.4 whereas the fetal blood is 7.3, basic drugs with a pKa above 7.4 will be more ionized in the fetal compartment, leading to ion trapping and, hence, to higher fetal levels (see Chapter 1, Ionization of Weak Acids and Weak Bases).

C. Placental Transporters During the last decade, many drug transporters have been identified in the placenta, with increasing recognition of their effects on drug transfer to the fetus. For example, the P-glycoprotein transporter encoded by the MDR1 gene pumps back into the maternal circulation a variety of drugs, including cancer drugs (eg, vinblastine, doxorubicin) and other agents. Similarly, viral protease inhibitors, which are

substrates to P-glycoprotein, achieve only low fetal concentrationsâ&#x20AC;&#x201D;an effect that may increase the risk of vertical HIV infection from the mother to the fetus. The hypoglycemic drug glyburide has much lower plasma levels in the fetus as compared with the mother.

Recent work has documented that this agent is effluxed from the fetal circulation by the BCRP transporter as well as by the MRP3 transporter located in the placental brush border membrane. In addition, very high maternal protein binding of glyburide (> 98.8%) also contributes to lower fetal levels as compared with maternal concentrations.

D. Protein Binding The degree to which a drug is bound to plasma proteins (particularly albumin) may also affect the rate of transfer and the amount transferred. However, if a compound is very lipid-soluble (eg, some anesthetic gases), it will not be affected greatly by protein binding.

Transfer of these more lipid-soluble drugs and their overall rates of equilibration are more dependent on (and proportionate to) placental blood flow. This is because very lipid-soluble drugs diffuse across placental membranes so rapidly that their overall rates of equilibration do not depend on the free drug concentrations becoming equal on both sides. If a drug is poorly lipid-soluble and is ionized, its transfer is slow and will probably be impeded by its binding to maternal plasma proteins. Differential protein binding is also important since some drugs exhibit greater protein binding in maternal plasma than in fetal plasma because of a lower binding affinity of fetal proteins. This has been shown for sulfonamides, barbiturates, phenytoin, and local anesthetic agents.

E. Placental and Fetal Drug Metabolism Two mechanisms help protect the fetus from drugs in the maternal circulation: (1) The placenta itself plays a role both as a semipermeable barrier and as a site of metabolism of some drugs passing through it. Several different types of aromatic oxidation reactions (eg, hydroxylation, N-dealkylation, demethylation) have been shown to occur in placental tissue. Pentobarbital is oxidized in this way. Conversely, it is possible that the metabolic capacity of the placenta may lead to creation of toxic metabolites, and the placenta may therefore augment toxicity (eg, ethanol, benzpyrenes). (2) Drugs that have crossed the placenta enter the fetal circulation via the umbilical vein. About 40â&#x20AC;&#x201C;60% of umbilical venous blood flow enters the fetal liver; the remainder bypasses the liver and enters the general fetal circulation. A drug that enters the liver may be partially metabolized there before it enters the fetal circulation. In addition, a large proportion of drug present in the umbilical artery (returning to the placenta) may be shunted through the placenta back to the umbilical vein and into the liver again. It should be noted that metabolites of some drugs may be more active than the parent compound and may affect the fetus adversely.

Pharmacodynamics A. Maternal Drug Actions The effects of drugs on the reproductive tissues (breast, uterus, etc) of the pregnant woman are sometimes altered by the endocrine environment appropriate for the stage of pregnancy. Drug effects on other maternal tissues (heart, lungs, kidneys, central nervous system, etc) are not changed significantly by pregnancy, although the physiologic context (cardiac output, renal blood flow, etc) may be altered, requiring the use of drugs that are not needed by the same woman when she is not pregnant. For example, cardiac glycosides and diuretics may be needed for heart failure precipitated by the increased cardiac workload of pregnancy, or insulin may be required for control of blood glucose in pregnancy-induced diabetes.

B. Therapeutic Drug Actions in the Fetus Fetal therapeutics is an emerging area in perinatal pharmacology. This involves drug administration to the pregnant woman with the fetus as the target of the drug. At present, corticosteroids are used to stimulate fetal lung maturation when preterm birth is expected.

Phenobarbital, when given to pregnant women near term, can induce fetal hepatic enzymes responsible for the glucuronidation of bilirubin, and the incidence of jaundice is lower in newborns when mothers are given phenobarbital than when phenobarbital is not used.

Before phototherapy became the preferred mode of therapy for neonatal indirect hyperbilirubinemia, phenobarbital was used for this indication. Administration of phenobarbital to the mother was suggested recently as a means of decreasing the risk of intracranial bleeding in preterm infants. However, large randomized studies failed to confirm this effect. Antiarrhythmic drugs have also been given to mothers for treatment of fetal cardiac arrhythmias. Although their efficacy has not yet been established by controlled studies, digoxin, flecainide, procainamide, verapamil, and other antiarrhythmic agents have been shown to be effective in case series. During the last two decades it has been shown that maternal use of zidovudine decreases by two thirds transmission of HIV from the mother to the fetus, and use of combinations of three antiretroviral agents can eliminate fetal infection almost entirely (see Chapter 49).

C. Predictable Toxic Drug Actions in the Fetus Chronic use of opioids by the mother may produce dependence in the fetus and newborn. This dependence may be manifested after delivery as a neonatal withdrawal syndrome. A less well understood fetal drug toxicity is caused by the use of angiotensin-converting enzyme inhibitors during pregnancy. These drugs can result in significant and irreversible renal damage in the fetus and are therefore

contraindicated in pregnant women. Adverse effects may also be delayed, as in the case of female fetuses exposed to diethylstilbestrol, who may be at increased risk for adenocarcinoma of the vagina after puberty.

D. Teratogenic Drug Actions A single intrauterine exposure to a drug can affect the fetal structures undergoing rapid development at the time of exposure.

Thalidomide is an example of a drug that may profoundly affect the development of the limbs after only brief exposure. This exposure, however, must be at a critical time in the development of the limbs. The thalidomide phocomelia risk occurs during the fourth through the seventh weeks of gestation because it is during this time that the arms and legs develop (Figure 59–1).

FIGURE 59–1 Schematic diagram of critical periods of human development. (Reproduced, with permission, from Moore KL: The Developing Human: Clinically Oriented Embryology, 4th ed. Saunders, 1988. © Elsevier.)

1. Teratogenic mechanisms—The mechanisms by which different drugs produce teratogenic effects are poorly understood and are probably multifactorial. For example, drugs may have a direct effect on maternal tissues with secondary or indirect effects on fetal tissues. Drugs may interfere with the passage of oxygen or nutrients through the placenta and therefore have effects on the most rapidly metabolizing tissues of the fetus. Finally, drugs may have important direct actions on the processes of differentiation in developing tissues.

For example, vitamin A (retinol) has been shown to have important differentiation-directing actions in normal tissues. Several vitamin A analogs (isotretinoin, etretinate) are powerful teratogens, suggesting that they alter the normal processes of differentiation. Finally, deficiency of a critical substance appears to play a role in some types of abnormalities. For example, folic acid supplementation during pregnancy appears to reduce the incidence of neural tube defects (eg, spina bifida).

Continued exposure to a teratogen may produce cumulative effects or may affect several organs going through varying stages of development. Chronic consumption of high doses of ethanol during pregnancy, particularly during the first and second trimesters, may result in the fetal alcohol syndrome (see Chapter 23). In this syndrome, the central nervous system, growth, and facial development may be affected.

2. Defining a teratogen—To be considered teratogenic, a candidate substance or process should (1) result in a characteristic set of malformations, indicating selectivity for certain target organs; (2) exert its effects at a particular stage of fetal development, eg, during the limited time period of organogenesis of the target organs (Figure 59–1); and (3) show a dose-dependent incidence. Some drugs with known teratogenic or other adverse effects in pregnancy are listed in Table 59–1. Teratogenic effects are not limited only to major malformations, but also include intrauterine growth restriction (eg, cigarette smoking), miscarriage (eg, alcohol), stillbirth (eg, cigarette smoke), and neurocognitive delay (eg, alcohol, valproic acid).

TABLE 59–1 Drugs with significant teratogenic or other adverse effects on the fetus.

The widely cited FDA system for teratogenic potential (Table 59–2) is an attempt to quantify teratogenic risk from A (safe) to X (definite human teratogenic risk). This system has been criticized as inaccurate and impractical. For example, several drugs have been labeled “X” despite extensive opposite human safety data (eg, oral contraceptives). Diazepam and other benzodiazepines are labeled as “D” despite lack of positive evidence of human fetal risk. Presently the FDA is changing its system from the A, B, C grading system to narrative statements that will summarize evidence-based knowledge about each drug in terms of fetal risk and safety.

TABLE 59–2 FDA teratogenic risk categories.

3. Counseling women about teratogenic risk—Since the thalidomide disaster, medicine has been practiced as if every drug were a potential human teratogen when, in fact, fewer than 30 such drugs have been identified, with hundreds of agents proved safe for the unborn. Owing to high levels of anxiety among pregnant women—and because half of the pregnancies in North America are unplanned —every year many thousands of women need counseling about fetal exposure to drugs, chemicals, and radiation. In the Motherisk program in Toronto, thousands of women are counseled every month, and the ability of appropriate counseling to prevent unnecessary abortions has been documented. Clinicians who wish to provide such counsel to pregnant women must ensure that their information is upto-date and evidence-based and that the woman understands that the baseline teratogenic risk in pregnancy (ie, the risk of a neonatal abnormality in the absence of any known teratogenic exposure) is about 3%. It is also critical to address the maternal-fetal risks of the untreated condition if a medication is avoided. Recent studies show serious morbidity in women who discontinued selective serotonin reuptake inhibitor therapy for depression in pregnancy.

DRUG THERAPY IN INFANTS & CHILDREN Physiologic processes that influence pharmacokinetic variables in the infant change significantly in the first year of life, particularly during the first few months. Therefore, special attention must be paid to pharmacokinetics in this age group. Pharmacodynamic differences between pediatric and other patients have not been explored in great detail and are probably small except for those specific target tissues that mature at birth or immediately thereafter (eg, the ductus arteriosus).

Drug Absorption Drug absorption in infants and children follows the same general principles as in adults. Unique factors that influence drug absorption

include blood flow at the site of administration, as determined by the physiologic status of the infant or child; and, for orally administered drugs, gastrointestinal function, which changes rapidly during the first few days after birth. Age after birth also influences the regulation of drug absorption.

A. Blood Flow at the Site of Administration Absorption after intramuscular or subcutaneous injection depends mainly, in neonates as in adults, on the rate of blood flow to the muscle or subcutaneous area injected. Physiologic conditions that might reduce blood flow to these areas are cardiovascular shock, vasoconstriction due to sympathomimetic agents, and heart failure. However, sick preterm infants requiring intramuscular injections may have very little muscle mass. This is further complicated by diminished peripheral perfusion to these areas. In such cases, absorption becomes irregular and difficult to predict, because the drug may remain in the muscle and be absorbed more slowly than expected. If perfusion suddenly improves, there can be a sudden and unpredictable increase in the amount of drug entering the circulation, resulting in high and potentially toxic concentrations of drug. Examples of drugs especially hazardous in such situations are cardiac glycosides, aminoglycoside antibiotics, and anticonvulsants.

B. Gastrointestinal Function Significant biochemical and physiologic changes occur in the neonatal gastrointestinal tract shortly after birth. In full-term infants, gastric acid secretion begins soon after birth and increases gradually over several hours. In preterm infants, the secretion of gastric acid occurs more slowly, with the highest concentrations appearing on the fourth day of life. Therefore, drugs that are partially or totally inactivated by the low pH of gastric contents should not be administered orally.

Gastric emptying time is prolonged (up to 6 or 8 hours) in the first day or so after delivery. Therefore, drugs that are absorbed primarily in the stomach may be absorbed more completely than anticipated. In the case of drugs absorbed in the small intestine, therapeutic effect may be delayed. Peristalsis in the neonate is irregular and may be slow. The amount of drug absorbed in the small intestine may therefore be unpredictable; more than the usual amount of drug may be absorbed if peristalsis is slowed, and this could result in potential toxicity from an otherwise standard dose. Table 59–3 summarizes data on oral bioavailability of various drugs in neonates compared with older children and adults. An increase in peristalsis, as in diarrheal conditions, tends to decrease the extent of absorption, because contact time with the large absorptive surface of the intestine is decreased.

TABLE 59–3 Oral drug absorption (bioavailability) of various drugs in the neonate compared with older children and adults.

Gastrointestinal enzyme activities tend to be lower in the newborn than in the adult. Activities of α-amylase and other pancreatic enzymes in the duodenum are low in infants up to 4 months of age. Neonates also have low concentrations of bile acids and lipase, which may decrease the absorption of lipid-soluble drugs.

Drug Distribution As body composition changes with development, the distribution volumes of drugs are also changed. The neonate has a higher percentage of its body weight in the form of water (70–75%) than does the adult (50–60%). Differences can also be observed between the full-term neonate (70% of body weight as water) and the small preterm neonate (85% of body weight as water). Similarly, extracellular water is 40% of body weight in the neonate, compared with 20% in the adult. Most neonates will experience diuresis in the first 24–48 hours of life. Since many drugs are distributed throughout the extracellular water space, the size (volume) of the extracellular water compartment may be important in determining the concentration of drug at receptor sites. This is especially important for watersoluble drugs (such as aminoglycosides) and less crucial for lipid-soluble agents.

Preterm infants have much less fat than full-term infants. Total body fat in preterm infants is about 1% of total body weight, compared with 15% in full-term neonates. Therefore, organs that generally accumulate high concentrations of lipid-soluble drugs in adults and older children may accumulate smaller amounts of these agents in less mature infants.

Another major factor determining drug distribution is drug binding to plasma proteins. Albumin is the plasma protein with the greatest binding capacity. In general, protein binding of drugs is reduced in the neonate. This has been seen with local anesthetic drugs, diazepam, phenytoin, ampicillin, and phenobarbital. Therefore, the concentration of free (unbound) drug in plasma is increased initially. Because the free drug exerts the pharmacologic effect, this can result in greater drug effect or toxicity despite a normal or even low plasma concentration of total drug (bound plus unbound). Consider a therapeutic dose of a drug (eg, diazepam) given to a patient. The concentration of total drug in the plasma is 300 mcg/L. If the drug is 98% protein-bound in an older child or adult, then 6 mcg/L is the concentration of free drug. Assume that this concentration of free drug produces the desired effect in the patient without producing toxicity. However, if this drug is given to a preterm infant in a dosage adjusted for body weight and it produces a total drug concentration of 300 mcg/L—and protein binding is only 90%—then the free drug concentration will be 30 mcg/L, or five times higher. Although the higher free concentration may result in faster elimination (see Chapter 3), this concentration may be quite toxic initially.

Some drugs compete with serum bilirubin for binding to albumin. Drugs given to a neonate with jaundice can displace bilirubin from albumin. Because of the greater permeability of the neonatal blood-brain barrier, substantial amounts of bilirubin may enter the brain and cause kernicterus. This was in fact observed when sulfonamide antibiotics were given to preterm neonates as prophylaxis against sepsis.

Conversely, as the serum bilirubin rises for physiologic reasons or because of a blood group incompatibility, bilirubin can displace a drug from albumin and substantially raise the free drug concentration. This may occur without altering the total drug concentration and would result in greater therapeutic effect or toxicity at normal concentrations. This has been shown to happen with phenytoin.

Drug Metabolism The metabolism of most drugs occurs in the liver (see Chapter 4). The drug-metabolizing activities of the cytochrome P450-dependent mixed-function oxidases and the conjugating enzymes are substantially lower (50–70% of adult values) in early neonatal life than later.

The point in development at which enzymatic activity is maximal depends upon the specific enzyme system in question. Glucuronide formation reaches adult values (per kilogram body weight) between the third and fourth years of life. Because of the neonate’s decreased ability to metabolize drugs, many drugs have slow clearance rates and prolonged elimination half-lives. If drug doses and dosing schedules are not altered appropriately, this immaturity predisposes the neonate to adverse effects from drugs that are metabolized by the liver. Table 59–4 demonstrates how neonatal and adult drug elimination half-lives can differ and how the half-lives of phenobarbital and phenytoin decrease as the neonate grows older. The process of maturation must be considered when administering drugs to this age group, especially in the case of drugs administered over long periods.

TABLE 59–4 Comparison of elimination half-lives of various drugs in neonates and adults.

Another consideration for the neonate is whether or not the mother was receiving drugs (eg, phenobarbital) that can induce early maturation of fetal hepatic enzymes. In this case, the ability of the neonate to metabolize certain drugs will be greater than expected, and one may see less therapeutic effect and lower plasma drug concentrations when the usual neonatal dose is given. During toddlerhood (12–36 months), the metabolic rate of many drugs exceeds adult values, often necessitating larger doses per kilogram than later in life.

Drug Excretion The glomerular filtration rate is much lower in newborns than in older infants, children, or adults, and this limitation persists during the first few days of life. Calculated on the basis of body surface area, glomerular filtration in the neonate is only 30–40% of the adult value. The glomerular filtration rate is even lower in neonates born before 34 weeks of gestation. Function improves substantially during the first week of life. At the end of the first week, the glomerular filtration rate and renal plasma flow have increased 50% from the first day. By the end of the third week, glomerular filtration is 50–60% of the adult value; by 6–12 months, it reaches adult values (per unit surface area). Subsequently, during toddlerhood, it exceeds adult values, often necessitating larger doses per kilogram than in adults, as described previously for drug-metabolic rate. Therefore, drugs that depend on renal function for elimination are cleared from the body very slowly in the first weeks of life.

Penicillins, for example, are cleared by preterm infants at 17% of the adult rate based on comparable surface area and 34% of the adult rate when adjusted for body weight. The dosage of ampicillin for a neonate less than 7 days old is 50–100 mg/kg/d in two doses at 12-hour intervals. The dosage for a neonate over 7 days old is 100–200 mg/kg/d in three doses at 8-hour intervals. A decreased rate of renal elimination in the neonate has also been observed with aminoglycoside antibiotics (kanamycin, gentamicin, neomycin, and streptomycin). The dosage of gentamicin for a neonate less than 7 days old is 5 mg/kg/d in two doses at 12-hour intervals. The dosage for a neonate over 7 days old is 7.5 mg/kg/d in three doses at 8-hour intervals. Total body clearance of digoxin is directly dependent upon adequate renal function, and accumulation of digoxin can occur when glomerular filtration is decreased. Since renal function in a sick infant may not improve at the predicted rate during the first weeks and months of life, appropriate adjustments in dosage and dosing

schedules may be very difficult. In this situation, adjustments are best made on the basis of plasma drug concentrations determined at intervals throughout the course of therapy.

Although great focus is naturally concentrated on the neonate, it is important to remember that toddlers may have shorter elimination half-lives of drugs than older children and adults, due probably to increased renal elimination and metabolism. For example, the dose per kilogram of digoxin is much higher in toddlers than in adults. The mechanisms for these developmental changes are still poorly understood.

Special Pharmacodynamic Features in the Neonate The appropriate use of drugs has made possible the survival of neonates with severe abnormalities who would otherwise die within days or weeks after birth. For example, administration of indomethacin (see Chapter 36) causes the rapid closure of a patent ductus arteriosus, which would otherwise require surgical closure in an infant with a normal heart. Infusion of prostaglandin E1 , on the other hand, causes the ductus to remain open, which can be lifesaving in an infant with transposition of the great vessels or tetralogy of Fallot (see Chapter 18). An unexpected effect of such infusion has been described when the drug caused antral hyperplasia with gastric outlet obstruction as a clinical manifestation in 6 of 74 infants who received it. This phenomenon appears to be dose-dependent. Neonates are also more sensitive to the central depressant effects of opioids than are older children and adults, necessitating extra caution when they are exposed to some narcotics (eg, codeine) through breast milk.

At birth, the function of drug transporters may be very low; for example, P-glycoprotein, which pumps morphine from the blood-brain barrier back to the systemic circulation. Low-level function of P-glycoprotein at birth may explain why neonates are substantially more sensitive than older children to the central nervous system depressant effects of morphine.

PEDIATRIC DOSAGE FORMS & COMPLIANCE The form in which a drug is manufactured and the way in which the parent dispenses the drug to the child determine the actual dose administered. Many drugs prepared for children are in the form of elixirs or suspensions. Elixirs are alcoholic solutions in which the drug molecules are dissolved and evenly distributed. No shaking is required, and unless some of the vehicle has evaporated, the first dose from the bottle and the last dose should contain equivalent amounts of drug. Suspensions contain undissolved particles of drug that must be distributed throughout the vehicle by shaking. If shaking is not thorough each time a dose is given, the first doses from the bottle may contain less drug than the last doses, with the result that less than the expected plasma concentration or effect of the drug may be achieved early in the course of therapy. Conversely, toxicity may occur late in the course of therapy, when it is not expected. This uneven distribution is a potential cause of inefficacy or toxicity in children taking phenytoin suspensions. It is thus essential that the prescriber know the form in which the drug will be dispensed and provide proper instructions to the pharmacist and patient or parent.

Compliance may be more difficult to achieve in pediatric practice than otherwise, since it involves not only the parent’s conscientious effort to follow directions but also such practical matters as measuring errors, spilling, and spitting out. For example, the measured volume of “teaspoons” ranges from 2.5 to 7.8 mL. The parents should obtain a calibrated medicine spoon or syringe from the pharmacy.

These devices improve the accuracy of dose measurements and simplify administration of drugs to children.

When evaluating compliance, it is often helpful to ask if an attempt has been made to give a further dose after the child has spilled half of what was offered. The parents may not always be able to say with confidence how much of a dose the child actually received.

The parents must be told whether or not to wake the infant for its every-6-hour dose day or night. These matters should be discussed and made clear, and no assumptions should be made about what the parents may or may not do. Noncompliance frequently occurs when antibiotics are prescribed to treat otitis media or urinary tract infections and the child feels well after 4 or 5 days of therapy. The parents may not feel there is any reason to continue giving the medicine even though it was prescribed for 10 or 14 days. This common situation should be anticipated so the parents can be told why it is important to continue giving the medicine for the prescribed period even if the child seems to be “cured.”

Practical and convenient dosage forms and dosing schedules should be chosen to the extent possible. The easier it is to administer and take the medicine and the easier the dosing schedule is to follow, the more likely it is that compliance will be achieved.

Consistent with their ability to comprehend and cooperate, children should also be given some responsibility for their own health care and for taking medications. This should be discussed in appropriate terms both with the child and with the parents. Possible adverse effects and drug interactions with over-the-counter medicines or foods should also be discussed. Whenever a drug does not achieve its therapeutic effect, the possibility of noncompliance should be considered. There is ample evidence that in such cases parents’ or children’s reports may be grossly inaccurate. Random pill counts and measurement of serum concentrations may help disclose noncompliance. The use of computerized pill containers, which record each lid opening, has been shown to be very effective in measuring compliance.

Because many pediatric doses are calculated—eg, using body weight—rather than simply read from a list, major dosing errors may result from incorrect calculations. Typically, tenfold errors due to incorrect placement of the decimal point have been described. In the case of digoxin, for example, an intended dose of 0.1 mL containing 5 mcg of drug, when replaced by 1.0 mL—which is still a small

volume—can result in fatal overdosage. A good rule for avoiding such “decimal point” errors is to use a leading “0” plus decimal point when dealing with doses less than “1” and to avoid using a zero after a decimal point (see Chapter 65).

DRUG USE DURING LACTATION Despite the fact that most drugs are excreted into breast milk in amounts too small to adversely affect neonatal health, thousands of women taking medications do not breast-feed because of misperception of risk. Unfortunately, physicians contribute heavily to this bias.

It is important to remember that formula feeding is associated with higher morbidity and mortality in all socioeconomic groups.

Most drugs administered to lactating women are detectable in breast milk. Fortunately, the concentration of drugs achieved in breast milk is usually low (Table 59–5). Therefore, the total amount the infant would receive in a day is substantially less than what would be considered a “therapeutic dose.” If the nursing mother must take medications and the drug is a relatively safe one, she should optimally take it 30–60 minutes after nursing and 3–4 hours before the next feeding. In some cases this may allow time for drugs to be partially cleared from the mother’s blood, and the concentrations in breast milk will be relatively low. Most antibiotics taken by nursing mothers can be detected in breast milk. Tetracycline concentrations in breast milk are approximately 70% of maternal serum concentrations and present a risk of permanent tooth staining in the infant. Isoniazid rapidly reaches equilibrium between breast milk and maternal blood. The concentrations achieved in breast milk are high enough so that signs of pyridoxine deficiency may occur in the infant if the mother is not given pyridoxine supplements.

TABLE 59–5 Drugs often used during lactation and possible effects on the nursing infant.

Most sedatives and hypnotics achieve concentrations in breast milk sufficient to produce a pharmacologic effect in some infants.

Barbiturates taken in hypnotic doses by the mother can produce lethargy, sedation, and poor suck reflexes in the infant. Chloral hydrate can produce sedation if the infant is fed at peak milk concentrations. Diazepam can have a sedative effect on the nursing infant, but, most importantly, its long half-life can result in significant drug accumulation.

Opioids such as heroin, methadone, and morphine enter breast milk in quantities potentially sufficient to prolong the state of neonatal narcotic dependence if the drug was taken chronically by the mother during pregnancy. If conditions are well controlled and there is a good relationship between the mother and the physician, an infant could be breast-fed while the mother is taking methadone. She should not, however, stop taking the drug abruptly; the infant can be tapered off the methadone as the mother’s dose is tapered. The infant should be watched for signs of narcotic withdrawal. Although codeine has been believed to be safe, a recent case of neonatal death from opioid toxicity revealed that the mother was an ultra rapid metabolizer of cytochrome 2D6 substrates, producing substantially higher amounts of morphine. Hence, polymorphism in maternal drug metabolism may affect neonatal exposure and safety. A subsequent case control study has shown that this situation is not rare. The FDA has published a warning to lactating mothers to exert extra caution while using painkillers containing codeine.

Minimal use of alcohol by the mother has not been reported to harm nursing infants. Excessive amounts of alcohol, however, can produce alcohol effects in the infant. Nicotine concentrations in the breast milk of smoking mothers are low and do not produce effects in the infant. Very small amounts of caffeine are excreted in the breast milk of coffee-drinking mothers.

Lithium enters breast milk in concentrations equal to those in maternal serum. Clearance of this drug is almost completely dependent upon renal elimination, and women who are receiving lithium may expose the infant to relatively large amounts of the drug.

Radioactive substances such as iodinated 125 I albumin and radioiodine can cause thyroid suppression in infants and may increase the risk of subsequent thyroid cancer as much as tenfold. Breast-feeding is contraindicated after large doses and should be withheld for days to weeks after small doses. Similarly, breast-feeding should be avoided in mothers receiving cancer chemotherapy or being treated with cytotoxic or immunomodulating agents for collagen diseases such as lupus erythematosus or after organ transplantation.

PEDIATRIC DRUG DOSAGE Because of differences in pharmacokinetics in infants and children, simple proportionate reduction in the adult dose may not be adequate to determine a safe and effective pediatric dose. The most reliable pediatric dose information is usually that provided by the manufacturer in the package insert. However, such information is not available for the majority of products, even when studies have been published in the medical literature, reflecting the reluctance of manufacturers to label their products for children. Recently, the FDA has moved toward more explicit expectations that manufacturers test their new products in infants and children. Still, most drugs in the common formularies, eg, Physicians’ Desk Reference , are not specifically approved for children, in part because manufacturers often lack the economic incentive to evaluate drugs for use in the pediatric market.

Most drugs approved for use in children have recommended pediatric doses, generally stated as milligrams per kilogram or per pound.

In the absence of explicit pediatric dose recommendations, an approximation can be made by any of several methods based on age, weight, or surface area. These rules are not precise and should not be used if the manufacturer provides a pediatric dose. When pediatric doses are calculated (either from one of the methods set forth below or from a manufacturer’s dose), the pediatric dose should never exceed the adult dose.

The current epidemic proportions of childhood obesity calls for a fresh and careful look at pediatric drug dosages. Studies in adults indicate that dosing based on per-kilogram body weight may constitute overdosing, because in obese subjects, drugs are distributed based on lean body weight.

Surface Area, Age, & Weight Calculations of dosage based on age or weight (see below) are conservative and tend to underestimate the required dose. Doses based on surface area (Table 59–6) are more likely to be adequate.

TABLE 59–6 Determination of drug dosage from surface area.1

In spite of these approximations, only by conducting studies in children can safe and effective doses for a given age group and condition

be determined.

REFERENCES American Heart Association (AHA) guidelines for cardiopulmonary resuscitation (CPR) and emergency cardiovascular care (ECC) of pediatric and neonatal patients: Pediatric basic life support. Circulation. 2005;112(24 Suppl):IV1.

Briggs GG, Freeman RK, Yaffe SJ: Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk, 9th ed. Williams & Wilkins, 2011. de Wildt SN et al: Ontogeny of midazolam glucuronidation in preterm infants. Eur J Clin Pharmacol 2010;66:165.

Gavin PJ, Yogev R: T he role of protease inhibitor therapy in children with HIV infection. Paediatr Drugs 2002;4:581.

Hansten PD, Horn JR: Drug Interactions, Analysis and Management. Facts & Comparisons. [Quarterly.]

Iqbal MM, Sohhan T , Mahmud SZ: T he effects of lithium, valproic acid, and carbamazepine during pregnancy and lactation. J T oxicol Clin T oxicol 2001;39:381.

Ito S: Drug therapy for breast feeding women. N Engl J Med 2000;343:118.

Kearns GL et al: Developmental pharmacologyâ&#x20AC;&#x201D;drug disposition, action and therapy in infants and children. N Engl J Med 2003;349:1157.

Koren G: Medication Safety during Pregnancy and Breastfeeding; A Clinicianâ&#x20AC;&#x2122;s Guide, 4th ed. McGraw-Hill, 2006.

Koren G, Klinger G, Ohlsson A: Fetal pharmacotherapy. Drugs 2002;62:757.

Koren G, Nordeng H: Antidepressant use during pregnancy: T he benefit-risk ratio. Am J Obstet Gynecol 2012;207:157.

Koren G, Pastuszak A: Prevention of unnecessary pregnancy terminations by counseling women on drug, chemical, and radiation exposure during the first trimester.

T eratology 1990;41:657.

Koren G, Pastuszak A, Ito E: Drugs in pregnancy. N Engl J Med 1998;338:1128.

Koren G et al: Sex differences in the pharmacokinetics and bioequivalence of the delayed-release combination of doxylamine succinate-pyridoxine hydrochloride; Implications for pharmacotherapy in pregnancy. J Clin Pharmacol 2013;53:1268.

Loebstein R, Koren G: Clinical pharmacology and therapeutic drug monitoring in neonates and children. Pediatr Rev 1998;19:423.

Madadi P et al: Pharmacogenetics of neonatal opioid toxicity following maternal use of codeine during breastfeeding: A case control study. Clin Pharmacol T her 2009;85:31.

Namouz-Haddad S, Koren G: Fetal pharmacotherapy 2: Fetal arrhythmia. J Obstet Gynaecol Can 2013;35:1023.

Neubert D: Reproductive toxicology: T he science today. T eratog Carcinog Mutagen 2002;22:159.

Peled N et al: Gastric-outlet obstruction induced by prostaglandin therapy in neonates. N Engl J Med 1992;327:505.

SickKids Drug Handbook and Formulary 2013/2014. T he Hospital for Sick Children, T oronto.

T etelbaum M et al:. Back to basics: Understanding drugs in children: Pharmacokinetic maturation. Pediatr Rev 2005;26:321.

Van Lingen RA et al: T he effects of analgesia in the vulnerable infant during the perinatal period. Clin Perinatol 2002;29:511.

_______________ * Supported by grants from the Canadian Institutes for Health Research, T he Research Leadership for Better Pharmacotherapy During Pregnancy and Lactation, and Shoppers Drug Mart, Canada.

CHAPTER

60 Special Aspects of Geriatric Pharmacology Bertram G. Katzung, MD, PhD

CASE STUDY A 77-year-old man comes to your office at his wife’s insistence. He has had documented moderate hypertension for 18 years but does not like to take his medications. He says he has no real complaints, but his wife remarks that he has become much more forgetful lately and has almost stopped reading the newspaper and watching television. A Mini-Mental State Examination reveals that he is oriented as to name and place but is unable to give the month or year. He cannot remember the names of his three adult children nor three random words (eg, tree, flag, chair) for more than 2 minutes. No cataracts are visible, but he is unable to read standard newsprint without a powerful magnifier. Why doesn’t he take his antihypertensive medications? What therapeutic measures are available for the treatment of Alzheimer’s disease? How might macular degeneration be treated?

Society has traditionally classified everyone over 65 as “elderly,” but most authorities consider the field of geriatrics to apply to persons over 75—even though this too is an arbitrary definition. Furthermore, chronologic age is only one determinant of the changes pertinent to drug therapy that occur in older people. In addition to the chronic diseases of adulthood, the elderly have an increased incidence of many conditions, including Alzheimer’s disease, Parkinson’s disease, and vascular dementia; stroke; visual impairment, especially cataracts and macular degeneration; atherosclerosis, coronary and peripheral vascular disease, and heart failure; diabetes; arthritis, osteoporosis, and fractures; cancer; and incontinence. As a result, the need for drug treatment is great in this age group. And as the average life span approaches (and in some countries, already exceeds) 80, this need will increase dramatically.

When all confounders are accounted for, age itself is still the strongest risk factor for cardiovascular and neurodegenerative diseases and most forms of cancer. Research into the molecular basis of aging has answered a few questions and opened many more. It has long been known that caloric restriction alone can prolong the life span of animals, including mammals. Some evidence suggests that calorically restricted mice also remain healthier for a longer time. Drugs that mimic caloric restriction have been shown to increase lifespan in the nematode Caenorhabditis elegans, as well as other species, including mice. Metformin and rapamycin each increase life span alone and appear to have synergistic effects when given together. Sirtuins, a class of endogenous protein deacetylase enzymes, may be linked to life span in some species, but activators (such as resveratrol) of certain sirtuins have not been shown to prolong life in mice. Assuming that safer alternatives to metformin or rapamycin can be found, should everyone over the age of 40 or 60 years take such a drug? Few would maintain that a simple increase in the years of life—life span—is desirable unless accompanied by an increase in the years of healthy life—“health span.”

Important changes in responses to some drugs occur with increasing age in many individuals. For other drugs, age-related changes are minimal, especially in the “healthy old.” Drug usage patterns also change as a result of the increasing incidence of disease with age and the tendency to prescribe heavily for patients in nursing homes. General changes in the lives of older people have significant effects on the way drugs are used. Among these changes are the increased incidence with advancing age of several simultaneous diseases, nutritional problems, reduced financial resources, and—in some patients—decreased dosing adherence (also called compliance) for a variety of reasons. The health practitioner should be aware of the changes in pharmacologic responses that may occur in older people and should know how to deal with these changes.

PHARMACOLOGIC CHANGES ASSOCIATED WITH AGING In the general population, measurements of functional capacity of most of the major organ systems show a decline beginning in young adulthood and continuing throughout life. As shown in Figure 60–1, there is no “middle-age plateau” but rather a linear decrease beginning no later than age 45. However, these data reflect the mean and do not apply to every person above a certain age;

approximately one third of healthy subjects have no age-related decrease in, for example, creatinine clearance up to the age of 75. Thus, the elderly do not lose specific functions at an accelerated rate compared with young and middle-aged adults but rather accumulate more deficiencies with the passage of time. Some of these changes result in altered pharmacokinetics. For the pharmacologist and the clinician, the most important of these is the decrease in renal function. Other changes and concurrent diseases may alter the pharmacodynamic characteristics of particular drugs in certain patients.

FIGURE 60–1 Effect of age on some physiologic functions. (Adapted, with permission, from Kohn RR: Principles of Mammalian Aging. Copyright © 1978 by Prentice-Hall, Inc. Used by permission of Pearson Education, Inc.)

Pharmacokinetic Changes A. Absorption There is little evidence of any major alteration in drug absorption with age. However, conditions associated with age may alter the rate at which some drugs are absorbed. Such conditions include altered nutritional habits, greater consumption of nonprescription drugs (eg, antacids and laxatives), and changes in gastric emptying, which is often slower in older persons, especially in older diabetics.

B. Distribution Compared with young adults, the elderly have reduced lean body mass, reduced body water, and increased fat as a percentage of body mass. Some of these changes are shown in Table 60–1. There is usually a decrease in serum albumin, which binds many drugs, especially weak acids. There may be a concurrent increase in serum orosomucoid (α-acid glycoprotein), a protein that binds many basic drugs. Thus, the ratio of bound to free drug may be significantly altered. As explained in Chapter 3, these changes may alter the appropriate loading dose of a drug. However since both the clearance and the effects of drugs are related to the free concentration, the steady-state effects of a maintenance dosage regimen should not be altered by these factors alone. For example, the loading dose of digoxin in an elderly patient with heart failure should be reduced (if used at all) because of the decreased apparent volume of distribution.

The maintenance dose may have to be reduced because of reduced clearance of the drug.

TABLE 60–1 Some changes related to aging that affect pharmacokinetics of drugs.

C. Metabolism The capacity of the liver to metabolize drugs does not appear to decline consistently with age for all drugs. Animal studies and some clinical studies have suggested that certain drugs are metabolized more slowly in the elderly; some of these drugs are listed in Table 60– 2. The greatest changes are in phase I reactions, ie, those carried out by microsomal P450 systems. There are much smaller changes in the ability of the liver to carry out conjugation (phase II) reactions (see Chapter 4). Some of these changes may be caused by decreased liver blood flow (Table 60–1), an important variable in the clearance of drugs that have a high hepatic extraction ratio. In addition, there is a decline with age of the liver’s ability to recover from injury, eg, that caused by alcohol or viral hepatitis. Therefore, a history of recent liver disease in an older person should lead to caution in dosing with drugs that are cleared primarily by the liver, even after apparently complete recovery from the hepatic insult. Finally, malnutrition and diseases that affect hepatic function—eg, heart failure—are more common in the elderly. Heart failure may dramatically alter the ability of the liver to metabolize drugs by reducing hepatic blood flow.

Similarly, severe nutritional deficiencies, which occur more often in old age, may impair hepatic function.

TABLE 60–2 Effects of age on hepatic clearance of some drugs.

D. Elimination Because the kidney is the major organ for clearance of drugs from the body, the age-related decline of renal functional capacity is very important. The decline in creatinine clearance occurs in about two thirds of the population. It is important to note that this decline is not reflected in an equivalent rise in serum creatinine because the production of creatinine is also reduced as muscle mass declines with age; therefore, serum creatinine alone is not an adequate measure of renal function. The practical result of this change is marked prolongation of the half-life of many drugs, and the possibility of accumulation to toxic levels if dosage is not reduced in size or frequency. Dosing recommendations for the elderly often include an allowance for reduced renal clearance. If only the young adult dosage is known for a drug that requires renal clearance, a rough correction can be made by using the Cockcroft-Gault formula, which is applicable to patients from ages 40 through 80: