LALIC_N_M

NEW ORAL ANTIDIABETIC DRUGS: DPP-4 INHIBITORS AND INCRETIN MIMETICS Nebojša M. Lalić

Introduction Treatment of hyperglycemia in type 2 diabetes involves a wide spectrum of different therapeutic approaches, particularly of oral agents, which are available to be used in an attempt to achieve normoglycemia otherwise postulated to be the major goal of this treatment. (1) However, the longterm studies have shown that the sustainable or “definitive” achievement of the persistent normoglycemia on the treatment with any of those particular agents or their combinations is not yet feasible. (2) In the recent years the studies have strongly emphasized the need for the recruitment of the new therapeutic agents and modalities which could bring in some advances in the possibilities of obtaining the persistent optimal glycoregulation. In this context, the agents acting on the level incretin effect, named as incretin mimetics and DPP-4 inhibitors, have been introduced in the treatment of type 2 diabetes, offering new therapeutic advantages which are still under intensive investigations of many ongoing studies. In this chapter we will discuss (a) the present state of the use of oral agents in type 2 diabetes treatment, (b) the incretin effect and the role of glucagon-like peptide 1 (GLP-1), (c) the therapeutic effects of incretin-based treatment: DPP-4 inhibitors and incretin mimetics and (d) the positioning of the incretin-based therapeutic agents in the recommendations for type 2 diabetes treatment.

The use of oral agents in type 2 diabetes treatment: present state The last decade has been characterized by a continuous upgrading of clinical recommendations for the treatment of hyperglycemia in type 2 diabetes in order to meet more efficiently the postulated requirement of obtaining near-normoglycemia. The process culminated in adoption of the consensus statement of the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) from 2009 which reaffirmed and synthesized the prevailing international recommendations in this area (3) The consensus statement has brought in some very important changes in the use of oral agents.

Following the previous versions of the consensus statement of 2006 and 2008 (4), this statement has reestablished the threshold value of the glycated hemoglobin (HbA1c) of 7% as a marker of the treatment failure when switching from one level of treatment to the more complex one. Also,

the consensus statement reemphasized the importance of the use of metformin immediately upon the diagnosis, which is to be done together with the lifestyle changes and the metformin dose has to be titrated very rapidly in order to obtain near-normoglycemia (3).

However, the consensus statement has proposed a novel approach in further treatment after the failure of metformin monotherapy. In the two tier concept, the authors have suggested in Tier 1 that the majority of the patients should be treated with “well-validated” add-on treatment to metformin, using either sulfonilureas for less hyperglycemic or basal insulin for more hyperglycemic diabetic patients. In Tier 2, after the metformin failure, the authors have recommended that the minority of patients could be treated with “less well-validated” treatment modalities comprising the add-on of either pioglitazon or GLP-1 mimetics, while the DPP-4 inhibitors were not included in these recommendations. In both tiers, after the failure in the second step, the use of insulin is recommended, starting with basal insulins and then being intensified according to different algorithms (3).

Both in that consensus statement, as well as in many previous recommendations, it has been evident that there is a lack of an optimal insulin secretagogue which would be active in the early phases of type 2 diabetes and, simultaneously, which would avoid the side effects of sulphonylurea treatment (hypoglycemia, weight gain) and possibly prevent the ongoing beta cell deterioration which remains to be inevitable during the treatment by any of the existing antihyperglycemic agents in type 2 diabetes (5). The need for the new types of oral agents has been confirmed especially by the follow-up findings from the United Kingdom Prospective Diabetes Study (UKPDS) demonstrating the progressive increases in HbA1c together with the loss of beta cell function even in the patients successful in achieving otherwise very strict target levels for blood glucose (2). Following the above mentioned need for the new therapeutic approaches, the researchers have rediscovered the incretin effect and became interested the possibilities of using the incretin concept in the treatment of type 2 diabetes.

The incretin effect and the role of GLP-1 It has been well known from the eighties that, in healthy subjects, orally ingested glucose elicits a much greater (around 60%) insulin response than an equivalent intravenous glucose load, a phenomenon termed the “incretin effect” (6-8). Incretin effect, the amplification of nutrient-induced insulin secretion, is mediated by the hormones from the gut, among which glucose-dependent

insulinotropic polypeptide (GIP), produced by K cells located predominantly in the duodenum (9) and GLP-1, secreted by L cells in the distal small intestine and colon (10), play a major role (11).

The well known stimulus for release of GLP-1 and GIP is oral or intraduodenal glucose administration. The first observed physiological effect of GLP-1 was its glucose-dependent insulinotropic activity (12-14). It has been shown that GLP-1, under hyperglycemic conditions in humans, stimulate insulin secretion by facilitating closure of β cell ATP-sensitive K+ (KATP) channels (15). However, it has been soon realized that GLP-1 not only stimulated secretion of preformed insulin from β cells, but also all phases in insulin biosynthesis and insulin gene expression, via increasing expression and activity of the transcription factor Pdx1 (16-18).

Experiments conducted in vitro, as well as in vivo in rodents, suggested that GLP-1 has a unique, trophic effect on β cells: not only does it inhibit their apoptosis (19), it also stimulates proliferation of preexisting (20,21) as well as differentiation of new β cells in the pancreatic duct epithelium (22). Experiments in rats demonstrated that even in old, glucose-intolerant rats, GLP-1 induced a reversal of the age-related decline in β cell function with subsequent restore of a first phase insulin response to glucose by sensitizing previously unresponsive β cell to glucose and by increasing the amount of insulin secreted per cell (23). There is evidence that GLP-1 promotes β cell growth through epidermal growth factor (EGF) receptor–dependent activation of Pi3k/Akt signaling [17, 24] and, on the other hand, stimulates FoxO1 phosphorylation and nuclear exclusion, which is believed to be required for the increase in β cell mass in response to the GLP-1 (24).

In humans, it has been shown that GLP-1 is able to inhibit the β cell apoptosis in vitro, which implied that it exerts the equivalent preservation of the insulin-secreting cells in human settings (25). However, until now, the data from studies in humans are not available that would demonstrate the regenerative potential of the administration of GLP-1 in human subjects.

Moreover, GLP-1 suppresses glucagon release in a glucose-dependent manner: glucagon is suppressed in the presence of hyperglycemia while is released with the reduction in plasma glucose levels (26). GLP-1 also have a strong effect on gastrointestinal function by inhibiting gastric emptying and acid secretion with powerful subsequent reduction effect on postprandial glucose excursions in both healthy subjects and patients with type 2 diabetes (27-29). In addition, GLP-1 was shown, both in healthy (30), and in individuals with type 2 diabetes (31), to decrease the food intake, reduce the feelings of hunger, as well as prospective food consumption, and increase the feeling of fullness in dose-dependent manner (29-31). These gastrointestinal effects of GLP-1 are known as ‘ileal brake’ (32). Furthermore, GLP-1 has been found to be very rapidly

degraded by an enzyme, dipeptidyl-peptidase 4 (DPP-4). The degradation occurs within minutes, and thus it represents an important determinant both regarding its physiological effect as well as regarding its pharmacological use (33).

The therapeutic effects of incretin-based treatment: DPP-4 inhibitors and incretin mimetics Type 2 diabetes is characterized by a marked blunting of the incretin effect, which is caused, at least in part, by decreased secretion of GLP-1 (34). This blunting of the incretin effect results in defective glucose-stimulated insulin secretion, reduced glucose clearance, increased levels of glucagon, and quicker gastric emptying (35).

It was eventually demonstrated in patients with type 2 diabetes that intravenous infusion of GLP-1 had dramatic effects on insulin secretion and was able to completely normalize fasting blood glucose level (36-39). In addition, although GLP-1 stimulates insulin release through the similar mechanism as sulfonylureas, GLP-1 remains glucose-dependent so that blood glucose levels are normalized without causing hypoglycaemia (40,41).

In the light of all these findings, GLP-1 became a suitable substance to be turned into a pharmacological agent for the therapy of type 2 diabetes. Nevertheless, it soon emerged that simple subcutaneous or intravenous injections were not effective enough; although there was an effect on insulin and blood glucose, both were transient and weak due to its rapid degradation by the enzyme DPP-4 (42,43). So, for practical diabetes treatment, the possibilities were: (a) to provide GLP-1 continuously; (b) to develop either stable analogues of GLP-1 or agonists of the GLP-1 receptors; and (c) to produce the inhibitor of the DPP-4 (44).

In early studies, GLP-1 was given to a type 2 diabetes patients as a continuous intravenous infusion at different infusion rates. (45,46). The lower infusion rates were practically free from side effects (nausea and vomiting) and effective in reducing diurnal blood glucose levels, but the termination of the infusion led to immediate return of glycemia to previous levels. Since intravenous infusions were clearly not of any clinical utility, Zander et al. tried to apply a continuous subcutaneous infusion (using insulin pumps) to type 2 diabetes patients for 6 weeks (47). After 6 weeks of treatment, fasting and average plasma glucose levels were lowered by approximately 5 mmol/L, HbA1c fell by 1.2%, free fatty acids were significantly lowered, and the patients lost 2 kg in weight. Furthermore, insulin sensitivity almost doubled while insulin secretion capacity greatly improved with a first-phase response restored. This work, confirmed by others (48), provided ‘proof-of-concept’ for the principle of GLP-1-based therapy of type 2 diabetes

mellitus. However, as with continuous subcutaneous infusion of insulin, this therapeutic approach was limited in its application, and alternative approaches were therefore explored (44).

Thus, in the last 20 years, we have seen the development of the two distinct groups of incretinbased pharmatocological agents: incretin mimetics and DPP-4 inhibitors. The incretin mimetics represents agents that are administered parenterally and the group involves exenatide, an agonist of GLP-1 receptor, and liraglutide, a GLP-1 analogue. The group of DPP-4 inhibitors, oral agents, includes also two drugs, sitagliptin and vildagliptin.

In 1992 molecule named exendin-4 was isolated from a lizard Gila Monster (Heloderma suspectum) (49). Almost a decade later, Edwards et al. demonstrated that this molecule was the full agonist for the GLP-1 receptor, resistant to DPP-4. (50) Exenatide, a synthetic exendin-4, seems to share all the effects of native GLP-1 (50). In addition, it is effective for 5 to 7 hours after a single subcutaneous injection (51) and is therefore given twice daily. This compound has been tested in several clinical trials as an add-on therapy for type 2 diabetic patients inadequately treated with sulfonylureas (52), metformin (53) or a combination of metformin and sulfonylureas (54). After 30 weeks of treatment, fasting blood glucose concentrations fell, HbA1c levels were reduced by approximately 0.5-1%, while adverse effects were mild and generally gastrointestinal.

Mild hypoglycemia was observed in up to one third of patients also receiving sulfonylurea. Another important result was a significant, dose-dependent and progressive weight loss of 1.6 kg (patients treated with sulfonylurea and sulfonylurea + metformin) and 2.8 kg (metformin treatment) from baseline. In open-label extensions of these studies, exenatide was given for a total of 82 weeks with continued effects on HbA1c and body weight. However, more than one third of patients developed low-titer antibodies against exenatide, and 6% developed antibodies with higher titers, followed by attenuation of exantides glucose-lowering effect in half of them.

Besides the agonists of GLP-1 receptors, stable analogues of human GLP-1 have also being developed. One of those is liraglutide, a slightly modified GLP-1 sequence with a palmitoyl chain attached, that binds to albumin, thereby acquiring its pharmacokinetic profile and protecting GLP-1 from DPP-4 and renal elimination. Since the plasma half life of liraglutide is 12 h, it provides exposure for more than 24 h after a single injection (55), and, moreover, this compound seems to possess all of the activities of native GLP-1 (56). When administered to patients with type 2 diabetes for 3 months, liraglutide lowered fasting blood glucose and obtained a similar decrease in HbA1c 0.5-1% and also significantly lowered body weight in dose-dependent manner. The adverse effects were very few, and, in addition, no antibody formation was observed (57).

Beyond their effects on glycemic control and weight, both exenatide and liraglutide have demonstrated pleiotropic effects that may enhance their therapeutic effect in patients with type 2 diabetes (58). It has been shown that exenatide treatment leads to substantial reductions in apolipoprotein B (ApoB) and triglyceride, together with increased HDL cholesterol and small reductions in total cholesterol and LDL cholesterol. In addition, exenatide appears to be associated with improvements in systolic and diastolic blood pressure after long-term treatment (59). These effects may, at least in part, be the result of the weight loss observed in patients who remained on treatment in the extension studies (58). Nevertheless, much shorter-term studies of liraglutide have also found substantial reductions in triglycerides levels as well as in blood pressure (60), suggesting that GLP-1 might have direct effects that are not simply the result of weight loss. Moreover, these studies have demonstrated reductions in emerging markers of cardiovascular risk and inflammation, including plasminogen activator inhibitor type 1, brain natiuretic peptide, and C-reactive protein (61) which is particularly important concerning elevated risk for morbidity and mortality from cardiovascular disease in patients with type 2 diabetes (58).

The treatment with GLP-1 mimetics is accompanied by nausea and vomiting as major side effects, which also can be manageable by the tapering of the dose (54). The studies have reported increased incidence of acute pancreatitis as an adverse effect in the exenatide treated group in comparison to the controls (62). However, the causality between the exenatide treatment and the appearance of pancreatitis could not be determined (62) DPP-4 inhibitors have been shown to completely prevent the N-terminal degradation of GLP-1 that occurs in vivo, resulting in significant enhancement of its insulinotropic activity (63). Sitagliptin was the first DPP-4 inhibitor to be approved for clinical use in type 2 diabetic patients which provided sustained glucose lowering in type 2 diabetes patients over the 24-h period, with significantly lower pre-meal glucose concentrations and smaller postprandial glucose excursions, HbA1c reduced by 0.5-0,8% when used as monotherapy, without significant changes in body weight and with improved β cell function assessed by the model-based analysis (64). The antihyperglycemic effect could be amplified in combination with metformin (65). Vildagliptin, a DPP-4 inhibitor approved in an increasing number of countries, has obtained similar HbA1c reduction without significant changes in body weight or in twenty-four hour plasma insulin profile. (66).

Regarding the side or adverse effects with the use of DPP-4 inhibitors, they are found to be very rare. The treatment with those agents is not accompanied with nausea and vomiting (58).

Although some concerns have been express regarding the possibility that the inhibition of DPP-4

might lead to the simultaneous inhibition and interference with other DPP isoforms present at different tissues (e.g. lymphocytes) and exerting some important functions (e.g. immune response), the adverse events in this area were not detected (58).

Incretin-based therapeutic agents in the recommendations for type 2 diabetes treatment As it has already been mentioned, the existing recommendation for the treatment of hyperglycemia in type 2 diabetes, including the ADA/EASD consensus statement, do not include the incretin-based therapeutic agents to a great extent, mostly due to the lack of sufficient data about different modalities of their therapeutic use. Thus, in the consensus statement, only GLP-1 mimetics are mentioned and positioned at Tier 2, i.e. among “less well-validated� therapeutic agents (3).

However, the increasing number of data keeps showing that both incretin mimetics and DPP-4 inhibitors exhibit a significant antihyperglycemic effect together with remarkably low incidence of hypoglycemic episodes, due to the unique glucose-dependent insulinotropic activity of GLP-1 (58).

In addition, DPP-4 inhibitors are shown to achieve a potent antihyperglycemic effect without increasing body weight, which is in contrast to sulphonylurea agents which frequently induce both hypoglycemia and weight gain. Thus, DPP-4 inhibitors have shown the ability to stimulate insulin secretion in an optimal way, which might be extremely important for the treatment in the initial phases of type 2 diabetes (65,66).

GLP-1 mimetics have demonstrated slightly more potent antihypeglycemic effect compared to DPP-4 inhibitors, although their implementation might be somehow restricted by the parentheral mode of application but also by nausea and vomiting as side effects, although the latter seem to be modifiable by the dose adjustments (67). Another important beneficial effect of GLP-1 mimetics is the weight loss achievable by those agents, which suggests even the possibilities of the use of these agents for weight reduction even apart from type 2 diabetes treatment.

Both groups of incretin-based antihyperglycemic agents have the prospect of the ability to protect Ă&#x; cell from further destruction. If proven to be of a therapeutic significance in type 2 diabetes patients, this characteristic of incretin-based agents would position them at a very important place in the treatment of type 2 diabetes, due to the fact that none of other existing therapeutic modalities can achieve this effect. Furthermore, pleiotropic beneficial effects of these agents, especially cardioprotective influences, could be of an extreme importance due to the fact that type 2 diabetes is proven to be cardiovascular disease equivalent (68).

Based on all these data, several associations have made an effort to define the role of the incretin-based agents in the treatment of type 2 diabetes. An initial input has been made by the ACE/AACE Diabetes Road Map Task Force which was followed by the guidelines from other countries (69). All those recommendations agree that these agents should be positioned in the initial phase of type 2 diabetes.

DPP-4 inhibitors are considered to be secretagogues of the first choice, which could exert an optimal glucose-dependent stimulation of insulin secretion as an add-on to the preexisting metformin treatment or sometimes together with metformin from the very beginning of the treatment. Usually, the use of DPP-4 inihibitors is recommended when initial HbA1c levels do not exceed 8-8.5% (69).

GLP-1 mimetics are also considered for the use while the endogenous insulin secretion capacity is still present, although their strong antihyperglycemic effect enables them to be used even later during the course of type 2 diabetes and the efficacy of different combinations with other antihypeglycemic agents are still under investigation (67).

Finally, the weight lowering effect of GLP-1 mimetics and weight neutral influence of DPP-4 inhibitors offer an opportunity to consider the use of these agents in attempts to prevent type 2 diabetes which is to be focused by the studies that are in progress.

References 1. Intensive blood glucose control with sulphonilureas or insulin compared with conventional treatment and risk of complications in patients with type 2 diabetes (UKPDS 33). UK Prospective Diabetes Study (UKPDS) Group.Lancet. 1998; 352 (9131): 837-53.

2. Effect of intensive blood-glucose control with metformin on complications in overweight patients with type 2 diabetes (UKPDS 34). UK Prospective Diabetes Study (UKPDS) Group. Lancet. 1998; 352 (9131): 854-65.

3. Nathan DM, Buse JB, Davidson MB, Ferrannini E, Holman RR, Sherwin R, Zinman B. Medical Management of Hyperglycemia in Type 2 Diabetes: A Consensus Algorithm for the Initiation and Adjustment of Therapy: A consensus statement of the American Diabetes Association and the European Association for the Study of Diabetes. Diabetes Care 2009; 32: 193-203.

4. Nathan DM, Buse JB, Davidson MB, Heine RJ, Holman RR, Sherwin R, Zinman B. Management of hyperglycaemia in type 2 diabetes: a consensus algorithm for the initiation and adjustment of therapy. A consensus statement from the American Diabetes Association and the European Association for the Study of Diabetes. Diabetologia 2006; 49: 1711–21.

5. U.K. prospective diabetes study 16. Overview of 6 years' therapy of type II diabetes: a progressive disease. U.K. Prospective Diabetes Study Group. Diabetes 1995; 44:1249-58.

6. Nauck MA, Homberger E, Siegel EG, Allen RC, Eaton RP, Ebert R, Creutzfeldt W. Incretin effects of increasing glucose loads in man calculated from venous insulin and C-peptide responses. J Clin Endocrinol Metab 1986; 63(2): 492-8 7. Efendic S, Portwood N. Overview of incretin hormones. Horm Metab Res 2004;36:742–6.

8. Meier JJ, Nauck MA. The potential role of glucagon-like peptide 1 in diabetes. Curr Opin Investig Drugs 2004; 5:402–10 9. Deacon CF. What do we know about the secretion and degradation of incretin hormones? Regul Pept 2005; 128:117–24.

10. Holst JJ. Evidence that glicentin contains the entire sequence of glucagon. Biochem J 1980; 187:337–43 11. Vilsboll T, Holst JJ. Incretins, insulin secretion and Type 2 diabetes mellitus. Diabetologia 2004; 47:357–66 12. Bell GI, Santerre RF, Mullenbach GT. Hamster preproglucagon contains the sequence of glucagon and two related peptides. Nature 1983; 302: 716–8

13. Holst JJ, Orskov C, Schwartz TW, Buhl T, Baldissera F. Proglucagon 78-107, a potent insulinotropic hormone from thelower small intestine. Diabetologia 1986; 29: 549A 14. Mojsov S, Weir GC, Habener JF. Insulinotropin: glucagon-like peptide I (7-37) co-encoded in the glucagons gene is a potent stimulator of insulin release in the perfused rat pancreas. J Clin Invest 1987; 79: 616–9 15. Doyle ME, Egan JM. Mechanisms of action of glucagon-like peptide 1 in the pancreas. Pharmacol Ther 2007; 113: 546–93.

16. Fehmann HC, Habener JF. Insulinotropic hormone glucagon-like peptide-i (7-37) stimulation of proinsulin gene expression and proinsulin biosynthesis in insulinoma beta TC-1 cells. Endocrinology 1992; 130:159–66 17. Buteau J, Roduit R, Susini S, Prentki M. Glucagon-like peptide-1 promotes DNA synthesis, activates phosphatidylinositol 3-kinase and increases transcription factor pancreatic and duodenal homeobox gene 1 (PDX1) DNA binding activity in beta (INS-1)-cells. Diabetologia 1999; 42: 856–64.

18. Stoffers DA, Kieffer TJ, Hussain MA et al. Insulinotropic glucagon-like peptide 1 agonists stimulate expression of homeodomain protein IDX-1 and increase islet size in mouse pancreas. Diabetes 2000; 49: 741–8 19. Buteau J, El-Assaad W, Rhodes CJ, Rosenberg L, Joly E, Prentki M. Glucagon-like peptide-1 prevents beta cell glucolipotoxicity. Diabetologia2004; 47: 806–15 20. Xu G, Stoffers DA, Habener JF, Bonner-Weir S. Exendin-4 stimulates both beta-cell replication and neogenesis, resulting in increased beta-cell mass and improved glucose tolerance in diabetic rats. Diabetes 1999; 48: 2270–6 21. Stoffers DA, Kieffer TJ, Hussain MA et al. Insulinotropic glucagon-like peptide 1 agonists stimulate expression of homeodomain protein IDX-1 and increase islet size in mouse pancreas. Diabetes 2000; 49:741–8 22. Zhou J, Wang X, Pineyro MA, Egan JM. Glucagon-like peptide 1 and exendin-4 convert pancreatic AR42J cells into glucagon- and insulin-producing cells. Diabetes 1999; 48:2358–66 23. Wang Y, Perfetti R, Greig NH, et al. Glucagon-like Peptide-1 can reverse the age-related decline in glucose tolerance in rats. J Clin Invest 1997; 99: 2883–9 24. Buteau J, Spatz ML, Accili D. Transcription factor FoxO1 mediates glucagon-like peptide-1 effects on pancreatic beta-cell mass. Diabetes 2006; 55: 1190–6 25. Farilla, L., Bulotta, A., Hirshberg, B., Li Calzi, S., Khoury, N., Noushmehr, H., et al. Glucagon-like peptide 1 inhibits cell apoptosis and improves glucose responsiveness of freshly isolated human islets. Endocrinology 2003; 144, 5149–58.

26. Nauck MA. Glucagon-like peptide 1 (GLP-1) in the treatment of diabetes. Horm Metab Res 2004; 36: 852–58.

27. Nauck MA, Niedereichholz U, Ettler R et al. Glucagon like peptide 1 inhibition of gastric emptying outweighs its insulinotropic effects in healthy humans. Am J Physiol 1997; 273: E981–E988 28. Willms B, Werner J, Holst JJ, Orskov C, Creutzfeldt W, Nauck MA. Gastric emptying, glucose responses and insulin secretion after a liquid test meal: effects of exogenous glucagon- like peptide-1 (GLP-1)-(7-36) amide in type 2 (noninsulin-dependent) diabetic patients. J Clin Endocrinol Metab 1996; 81:327–32 29. C. Verdich C, Flint A, Gutzwiller JP, Naslund E, Beglinger C, Hellstrom M et al. A meta-analysis of the effect of glucagone-like peptide-1 (7-36) amide on ad libitum energy intake in humans. J Clin Endocr Metab 2001; 86(9): 4382-9 30. Flint A, Raben A, Astrup A, Holst JJ. Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans. J Clin Invest 1998; 101:515–20 31. Gutzwiller JP, Drewe J, Goke B et al. Glucagon-like peptide-1 promotes satiety and reduces food intake in patients with diabetes mellitus type 2. Am J Physiol 1999; 276: R1541–4 32. Holst JJ. Glucagon-like peptide 1 (GLP-1): an intestinal hormone signaling nutritional abundance, with an unusual therapeutic potential. Trends Endocrinol Metab 1999; 10:229–34 33. Kieffer, T.J., McIntosh, C.H., and Pederson, R.A.. Degradation of glucose- dependent insulinotropic polypeptide and truncated glucagon-like peptide 1 in vitro and in vivo by dipeptidyl peptidase IV. Endocrinology 1995; 136, 3585–96.

34. Raz I, Hanefeld M, Xu L, Caria C, Williams- Herman D, Khatami H. Efficacy and safety of the dipeptidyl peptidase-4 inhibitor sitagliptin as monotherapy in patients with type 2 diabetes mellitus. Diabetologia. 2006; 49(11): 2564-71.

35. Aschner P, Kipnes MS, Lunceford JK, Sanchez M,Mickel C, Williams-Herman DE. Effect of the dipeptidyl peptidase-4 inhibitor sitagliptin as monotherapy on glycemic control in patients with type 2 diabetes. Diabetes Care. 2006; 29(12):2632-7.

36. Gutniak M, Orskov C, Holst JJ, Ahren B, Efendic S. Antidiabetogenic effect of glucagon-like peptide-1 (7-36) amide in normal subjects and patients with diabetes mellitus. N Engl J Med 1992; 326:1316–22 37. Nathan DM, Schreiber E, Fogel H, Mojsov S, Habener JF. Insulinotropic action of glucagonlike peptide-I-(7-37) in diabetic and nondiabetic subjects. Diabetes Care 1992; 15: 270–6

38. Nauck MA, Kleine N, Orskov C, Holst JJ, Willms B, Creutzfeldt W. Normalization of fasting hyperglycaemia by exogenous glucagon-like peptide 1 (7-36 amide) in type 2 (non-insulin-dependent) diabetic patients. Diabetologia 1993; 36: 741–4 39. Nauck MA, Heimesaat MM, Orskov C, Holst JJ, Ebert R, Creutzfeldt W. Preserved incretin activity of glucagonlike peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus. J Clin Invest 1993; 91:301–7 40. Wajchenberg BL. Beta-cell failure in diabetes and preservation by clinical treatment. Endocr Rev 2007; 28: 187– 218 41. Del Prato S, Bianchi C, Marchetti P. β cell function and anti-diabetic harmacotherapy. Diabetes Metab Res Rev 2007; 23: 518-27 42. Nauck MA, Wollschlager D, Werner J et al. Effects of subcutaneous glucagon-like peptide 1 (GLP-1 [7-36 amide]) in patients with NIDDM. Diabetologia 1996; 39:1546–53 43. Juntti-Berggren L, Pigon J, Karpe F et al. The antidiabetogenic effect of GLP-1 is maintained during a 7-day treatment period and improves diabetic dyslipoproteinemia in NIDDM patients. Diabetes Care 1996; 19: 1200–6 44. Holst JJ. Glucagone-like peptide-1: from extract to agent. The Claude Bernard Lecture, 2005. Diabetologia 2006; 49: 253-60 45. Larsen J, Jallad J, Damsbo P. One-week continuous infusion of GLP-1(7-37) improves glycemic control in NIDDM. Diabetes 1996; 45: 233A 46. Larsen J, Hylleberg B, Ng K, Damsbo P. Glucagon-like peptide-1 infusion must be maintained for 24 h/day to obtain acceptable glycemia in type 2 diabetic patients who are poorly controlled on sulphonylurea treatment.

Diabetes Care 2001; 24: 1416–21 47. Zander M, Madsbad S, Madsen JL, Holst JJ. Effect of 6-week course of glucagon-like peptide 1 on glycaemic control,insulin sensitivity, and beta-cell function in type 2 diabetes:a parallel-group study. Lancet 2002;359:824-30 48. Meneilly GS, Greig N, Tildesley H, Habener JF, Egan JM, Elahi D. Effects of 3 months of continuous subcutaneous administration of glucagon-like peptide 1 in elderly patients with type 2 diabetes. Diabetes Care 2003; 26:2835–41 49. Eng J, Kleinman WA, Singh L, Singh G, Raufman JP. Isolation and characterization of exendin-4, an exendin-3 analogue, from Heloderma suspectum venom. Further evidence for an exendin receptor on dispersed acini from guinea pig pancreas. J Biol Chem 1992; 267:7402–5

50. Edwards CM, Stanley SA, Davis R et al. Exendin-4 reduces fasting and postprandial glucose and decreases energy intake in healthy volunteers. Am J Physiol Endocrinol Metab 2001; 281:E155–61 51. Kolterman OG, Kim DD, Shen L et al. Pharmacokinetics, pharmacodynamics, and safety of exenatide in patients with type 2 diabetes mellitus. Am J Health Syst Pharm 2005; 62:173–81 52. Buse JB, Henry RR, Han J, Kim DD, Fineman MS, Baron AD. Effects of exenatide (exendin-4) on glycemic control over 30 weeks in sulfonylurea-treated patients with Type 2 diabetes. Diabetes Care 2004; 27:2628-35 53. Defronzo RA, Ratner RE, Han J, Kim DD, Fineman MS, Baron AD. Effects of exenatide (exendin-4) on glycemic control and weight over 30 weeks in metformin-treated patients with type 2 diabetes. Diabetes Care 2005; 28:1092–100 54. Kendall DM, Riddle MC, Rosenstock J et al. Effects of exenatide (exendin-4) on glycemic control over 30 weeks in patients with type 2 diabetes treated with metformin and sulfonylurea. Diabetes Care 2005; 28:1083–91 55. Degn KB, Juhl CB, Sturis J et al. One week’s treatment with the long-acting glucagon-like peptide 1 derivative liraglutide (NN2211) markedly improves 24-h glycemia and alphaand beta-cell function and reduces endogenous glucose release in patients with type 2 diabetes. Diabetes 2004; 53:1187–94 56. Knudsen LB, Agersø H, Bjenning C et al. GLP-1 analogs as novel compounds for the treatment of type 2 diabetes: selection of NN2211 for clinical development. Drugs of the Future 2001; 26:677–85 57. Madsbad S, Schmitz O, Ranstam J, Jakobsen G, Matthews DR. Improved glycemic control with no weight increase in patients with type 2 diabetes after once-daily treatment with the long-acting glucagon-like peptide 1 analog liraglutide (NN2211): a 12-week, double-blind, randomized, controlled trial. Diabetes Care 2004; 27:1335– 40 58. Pratley RE. Overview of glucagone-like peptide-1analogs and dipeptydil peptidase-4 inhibitors for type 2 diabetes.

Medscape J Med 2008; 10(7): 171.

59. Ratner RE, Maggs D, Nielsen LL, et al. Long-term effects of exenatide therapy over 82 weeks on glycaemic control and weight in over-weight metformin-treated patients with type 2 diabetes mellitus. Diabetes Obes Metab.

2006; 8: 419–28.

60. Vilsboll T, Zdravkovic M, Le-Thi T, et al. Liraglutide, a long-acting human glucagon-like peptide-1 analog, given as monotherapy significantly improves glycemic control and lowers body weight without risk of hypoglycemia in patients with type 2 diabetes. Diabetes Care. 2007;30:1608–10

61. Vilsboll T, Zdravkovic M, Le-Thi T, et al. Beneficial effect of the GLP-1 analogue liraglutide on blood pressure and cardiovascular risk markers in subjects with type 2 diabetes. Diabetic Med. 2006;23(Suppl 4):696.

62. Bain SC, Stephens JW. Exenatide and pancreatitis: an update. Expert Opin Drug Saf. 2008; 7(6): 643-4.

63. Deacon CF, Hughes TE, Holst JJ. Dipeptidyl peptidase IV inhibition potentiates the insulinotropic effect of glucagonlike peptide 1 in the anesthetized pig. Diabetes 1998; 47:764–9 64. Brazg R, Xu L, Dalla ManC, CobelliC, Thomas K, Stein PP. Effect of adding sitagliptin, a dipeptidyl peptidase-4 inhibitor, to metformin on 24-h glycaemic control and beta-cell function in patients with type 2 diabetes. Diabetes Obes Metab 2007; 9: 186–93.

65. Goldstein BJ, Feinglos MN, Lunceford JK, Johnson J, Williams-Herman DE. Effect of Initial Combination Therapy with Sitagliptin, a Dipeptidyl Peptidase-4 Inhibitor, and Metformin on Glycemic Control in Patients with Type 2 Diabetes. Diabetes Care 2007;30(8):1979-87.

66. Ahren B. Vildagliptin: an inhibitor of dipeptidyl peptidase-4 with antidiabetic properties. Expert Opin Investig Drugs 2006; 15: 431–42.

67. McGill JB. Insights from the Liraglutide Clinical Development Program--the Liraglutide Effect and Action in Diabetes (LEAD) studies. Postgrad Med. 2009;121(3):16-25.

68. Bose, A.K., Mocanu, M.M., Carr, R.D., Brand, C.L., and Yellon, D.M. Glucagon-like peptide-1 (GLP-1) can directly protect the heart against ischemia/ reperfusion injury. Diabetes 2005; 54: 146–51.

69. Jellinger PS, Davidson JA, Blonde L, et al; ACE/AACE Diabetes Road Map Task Force. Road maps to achieve glycemic control in type 2 diabetes mellitus: ACE/AACE Diabetes Road Map Task Force. Endocr Pract.

2007;13(3):260-8.