Identifying the Drugs Behind Unintended Bleeding Disorders

Identifying the drugs behind unintended bleeding disorders

By Elizabeth Bennett

Image credit: Pexels

Medications are a powerful and important tool at a physician’s disposal, but they can become problematic when the full extent of their behavior is unknown. Many drugs come with long lists of known side effects, yet unintended reactions to prescribed medications, called adverse drug reactions (ADRs), are a leading cause of death worldwide. Associated with numerous drug classes, uncontrollable bleeding is a particularly dangerous and common ADR presented by patients. Identifying the risk of drug-induced bleeding disorders associated with specific medications would represent a vital step toward reducing the hospitalizations and deaths associated with drug-induced bleeding disorders.1 Although antithrombotic drugs, or blood thinners, understandably confer the greatest risk of drug-induced bleeding, ADRs associated with the use of blood thinners tend to be predictable and manageable. Scientists know how these drugs interfere with coagulation, through targeting the vitamin K cycle, and understand how to counter their effects. In contrast, other drugs such as anti-inflammatory medications, antibiotics, anti-depressants, and chemotherapeutic agents pose bleeding risks that are not predictable or controllable.1 Understanding how these medications interfere with coagulation and which of them could cause bleeding disorders would save lives, rendering many ADRs preventable and controllable. The Stafford Lab at the University of North Carolina at Chapel Hill hopes to achieve this goal. The Stafford Lab seeks to investigate “the structure and function study of enzymes in the vitamin K cycle and…vitamin K-dependent proteins involved in blood coagulation.” 2 The lab, located in UNC’s Biology Department, used its own cell-based assay, developed in 2013 by Dr. Jack Tie & Dr. Da-yun Jin, to screen the drugs—a testament to years of carefully researching the vitamin K cycle.3 In a recent study led by Xuejie Chen, a postdoctoral researcher in the Stafford Lab, the group “established a cell-based, high-throughput approach for screening drugs that have bleeding risks caused by off-target inhibition of vitamin K redox cycling,” a process involved in blood coagulation.1

Vitamin K is a fat-soluble vitamin and a necessity for many of the body’s regulatory functions. Vitamin K-dependent proteins, including many responsible for coagulation, cannot properly function without vitamin K acting as a cofactor for their enzymatic modification. Yet despite the body’s need for vitamin K, its storage capacity for the nutrient is very small. Instead, the body recycles the vitamin through the vitamin K-epoxide cycle, which greatly reduces the amount of vitamin K an individual must intake each day.4 The vitamin K-epoxide cycle involves a series of redox reactions that takes vitamin K through three forms, only one of which, vitamin K epoxide (KO), is usable as a cofactor.4 After KO is used by the body’s proteins, VKR and VKOR enzymes transform the vitamin back into its reduced form, hydroquinone, in two steps; first VKOR reduces KO to vitamin K, and next both enzymes reduce vitamin K to its reduced form. From there, hydroquinone can be oxidized to KO.1 Anti-coagulants like Warfarin, the most commonly prescribed vitamin K antagonist, reduce clotting by inhibiting VKOR, disrupting the vitamin K cycle. Without VKOR to reduce KO back to vitamin K, the vitamin cannot progress through the cycle and return to its usable form. Therefore, Warfarin is intended to limit the body’s ability to produce coagulation factors.1

However, drugs that are not blood thinners can also act as vitamin K antagonists by “off-targeting” the vitamin K cycle. Identifying these would-be blood thinners represents an important aspect of “preventing and controlling bleeding disorders associated with ADRs.” 1 The Stafford Lab used their cell-based assay to screen compounds from the NIH Clinical Collection Library of drugs for unintended interference in vitamin K-dependent coagulation. Out of 727 compounds, the lab identified nine drugs as vitamin K antagonists. The drugs were also found to impact the cycle in three ways: they either interfered with KO reduction, vitamin K reduction, or the availability of vitamin K. 1 However, the identification of drugs capable of antagonizing the vitamin K-epoxide cycle and of inducing bleeding is only the first step towards reducing ADR-related deaths and hospitalizations. Chen and the Stafford Lab also study how to counter the effects of vitamin K antagonists. The method of interference matters: Chen’s study confirmed that the effects of VKOR-targeting drugs in the study—including Warfarin, Nitazoxanide, and Lansoprazole—can be countered by simultaneously giving patients vitamin K supplements, effectively bypassing VKOR’s reduction of KO. Unfortunately, drugs targeting VKR present a problem without a simple solution.1 Chen explained that the reduced form of vitamin K, “cannot be used to rescue VKR off-targeting, as it is unstable.” 2 An ongoing, NIH-funded study in the Stafford Lab seeks to address the problem of VKR targeting by synthesizing a form of vitamin K capable of rescuing VKR off-targeting. 2 The Stafford Lab also hopes “to build screening systems, to not only test drugs that target the vitamin K cycle directly, but to also test drugs that affect vitamin K absorption, transportation, and metabolism.” 2

For the present, experts in the field expressed in a commentary article that the cell-based assay system “could serve as a high-throughput platform to screen novel drugs for druginduced bleeding resulting from…interaction[s] with the VK cycle.” 5 The ability to screen for specific aspects of a drug’s behavior and flag detrimental interactions is crucial to ADR prevention; while vitamin K antagonists are powerful drugs physicians can use to help patients, their effects are dangerous if unintended. With the Stafford Lab’s tools in hand, physicians will have more options to rescue and reduce bleeding related to vitamin K cycle off targeting.

“Identifying the risk of drug-induced bleeding disorders associated with specific medications would represent a vital step toward reducing the hospitalizations and deaths associated with drug-induced bleeding disorders”

Figure 2. Drugs interfering with KO reduction stop the enzyme VKOR from reducing KO to vitamin K. Image sourced from PDBe (EBI), CC0, via Wikimedia Commons.

Figure 3. (left) Warfarin is a vitamin K antagonist and used as a blood thinner. Image sourced from Calvero, Public domain, via Wikimedia Commons.

References

1. Xuejie Chen, Caihong Li, Da-Yun Jin, Brian Ingram, Zhenyu Hao, Xue Bai…Jian-Ke Tie (2020). A cell-based high-throughput screen identifies drugs that cause bleeding disorders by off-targeting the vitamin K cycle. Blood, 136 (7): 898–908. https://doi-org.libproxy.lib.unc.edu/10.1182/ blood.2019004234 2. Interview with Xuejie Chen, Ph.D. 09/24/20 3. Tie J.K., Jin D.Y., Tie K., & Stafford D.W. (2013). Evaluation of warfarin resistance using transcription activator-like effector nucleases-mediated vitamin K epoxide reductase knockout HEK 293 cells. Journal of Thrombosis and Haemostasias. Aug;11(8):1556-64. 4. Higdon, J., Drake, V. J., & Delage, B. (2000). Vitamin K. Linus Pauling Institute. https://lpi.oregonstate.edu/mic/ vitamins/vitamin-K 5. Mettine H. A. Bos, Felix J. M. van der Meer (2020). Vitamin K therapy to reduce bleeding. Blood; 136 (7): 780–782. doi: https://doi.org/10.1182/blood.2020006563