New Anticoagulants

Prophylaxis of thromboembolism during therapy with asparaginase in adults with acute lymphoblastic leukaemia

BACKGROUND

The risk of venous thromboembolism is increased in adults and enhanced by asparaginase-based chemotherapy, and venous thromboembolism introduces a secondary risk of treatment delay and premature discontinuation of key anti-leukaemic agents, potentially compromising survival. Yet, the trade-off between benefits and harms of primary thromboprophylaxis in adults with acute lymphoblastic leukaemia (ALL) treated according to asparaginase-based regimens is uncertain.  OBJECTIVES: The primary objectives were to assess the benefits and harms of primary thromboprophylaxis for first-time symptomatic venous thromboembolism in adults with ALL receiving asparaginase-based therapy compared with placebo or no thromboprophylaxis. The secondary objectives were to compare the benefits and harms of different groups of primary systemic thromboprophylaxis by stratifying the main results per type of drug (heparins, vitamin K antagonists, synthetic pentasaccharides, parenteral direct thrombin inhibitors, direct oral anticoagulants, and blood-derived products for antithrombin substitution).

SEARCH METHODS

We conducted a comprehensive literature search on 02 June 2020, with no language restrictions, including (1) electronic searches of Pubmed/MEDLINE; Embase/Ovid; Scopus/Elsevier; Web of Science Core Collection/Clarivate Analytics; and Cochrane Central Register of Controlled Trials (CENTRAL) and (2) handsearches of (i) reference lists of identified studies and related reviews; (ii) clinical trials registries (ClinicalTrials.gov registry; the International Standard Randomized Controlled Trial Number (ISRCTN) registry; the World Health Organisation's International Clinical Trials Registry Platform (ICTRP); and pharmaceutical manufacturers of asparaginase including Servier, Takeda, Jazz Pharmaceuticals, Ohara Pharmaceuticals, and Kyowa Pharmaceuticals), and (iii) conference proceedings (from the annual meetings of the American Society of Hematology (ASH); the European Haematology Association (EHA); the American Society of Clinical Oncology (ASCO); and the International Society on Thrombosis and Haemostasis (ISTH)). We conducted all searches from 1970 (the time of introduction of asparaginase in ALL treatment). We contacted the authors of relevant studies to identify any unpublished material, missing data, or information regarding ongoing studies.

SELECTION CRITERIA

Randomised controlled trials (RCTs); including quasi-randomised, controlled clinical, cross-over, and cluster-randomised trial designs) comparing any parenteral/oral preemptive anticoagulant or mechanical intervention with placebo or no thromboprophylaxis, or comparing two different pre-emptive anticoagulant interventions in adults aged at least 18 years with ALL treated according to asparaginase-based chemotherapy regimens. For the description of harms, non-randomised observational studies with a control group were eligible for inclusion.  DATA COLLECTION AND ANALYSIS: Using a standardised data collection form, two review authors independently screened and selected studies, extracted data, assessed risk of bias for each outcome using standardised tools (RoB 2.0 tool for RCTs and ROBINS-I tool for non-randomised studies) and the certainty of evidence for each outcome using the GRADE approach. Primary outcomes included first-time symptomatic venous thromboembolism, all-cause mortality, and major bleeding. Secondary outcomes included asymptomatic venous thromboembolism, venous thromboembolism-related mortality, adverse events (i.e. clinically relevant non-major bleeding and heparin-induced thrombocytopenia for trials using heparins), and quality of life. Analyses were performed according to the guidelines of the Cochrane Handbook for Systematic Reviews of Interventions. For non-randomised studies, we evaluated all studies (including studies judged to be at critical risk of bias in at least one of the ROBINS-I domains) in a sensitivity analysis exploring confounding.  MAIN RESULTS: We identified 23 non-randomised studies that met the inclusion criteria of this review, of which 10 studies provided no outcome data for adults with ALL. We included the remaining 13 studies in the 'Risk of bias' assessment, in which we identified invalid control group definition in two studies and judged outcomes of nine studies to be at critical risk of bias in at least one of the ROBINS-I domains and outcomes of two studies at serious risk of bias. We did not assess the benefits of thromboprophylaxis, as no RCTs were included. In the main descriptive analysis of harms, we included two retrospective non-randomised studies with outcomes judged to be at serious risk of bias. One study evaluated antithrombin concentrates compared to no antithrombin concentrates. We are uncertain whether antithrombin concentrates have an effect on all-cause mortality (risk ratio (RR) 0.55, 95% confidence interval (CI) 0.26 to 1.19 (intention-to-treat analysis); one study, 40 participants; very low certainty of evidence). We are uncertain whether antithrombin concentrates have an effect on venous thromboembolism-related mortality (RR 0.10, 95% CI 0.01 to 1.94 (intention-to-treat analysis); one study, 40 participants; very low certainty of evidence). We do not know whether antithrombin concentrates have an effect on major bleeding, clinically relevant non-major bleeding, and quality of life in adults with ALL treated with asparaginase-based chemotherapy, as data were insufficient. The remaining study (224 participants) evaluated prophylaxis with low-molecular-weight heparin versus no prophylaxis. However, this study reported insufficient data regarding harms including all-cause mortality, major bleeding, venous thromboembolism-related mortality, clinically relevant non-major bleeding, heparin-induced thrombocytopenia, and quality of life. In the sensitivity analysis of harms, exploring the effect of confounding, we also included nine non-randomised studies with outcomes judged to be at critical risk of bias primarily due to uncontrolled confounding. Three studies (179 participants) evaluated the effect of antithrombin concentrates and six studies (1224 participants) evaluated the effect of prophylaxis with different types of heparins. When analysing all-cause mortality; venous thromboembolism-related mortality; and major bleeding (studies of heparin only) including all studies with extractable outcomes for each comparison (antithrombin and low-molecular-weight heparin), we observed small study sizes; few events; wide CIs crossing the line of no effect; and substantial heterogeneity by visual inspection of the forest plots. Although the observed heterogeneity could arise through the inclusion of a small number of studies with differences in participants; interventions; and outcome assessments, the likelihood that bias due to uncontrolled confounding was the cause of heterogeneity is inevitable. Subgroup analyses were not possible due to insufficient data.  AUTHORS' CONCLUSIONS: We do not know from the currently available evidence, if thromboprophylaxis used for adults with ALL treated according to asparaginase-based regimens is associated with clinically appreciable benefits and acceptable harms. The existing research on this question is solely of non-randomised design, seriously to critically confounded, and underpowered with substantial imprecision. Any estimates of effect based on the existing insufficient evidence is very uncertain and is likely to change with future research.

Assessment of an Extended Interval Fondaparinux Dosing Regimen for Venous Thromboembolism Prophylaxis in Critically Ill Patients with Severe Renal Dysfunction Using Antifactor Xa Levels

OBJECTIVE

Pharmacologic options for venous thromboembolism (VTE) prophylaxis are often limited in critically ill patients due to thrombocytopenia and multisystem organ dysfunction. Fondaparinux offers potential advantages in the critically ill; however, it is currently contraindicated in severe renal dysfunction (SRD). We evaluated anti-factor Xa levels in critically ill patients with SRD who were receiving an extended interval dosing regimen of fondaparinux for VTE prophylaxis.

METHODS

A prospective, single-arm, interventional study was conducted at two academic hospitals of the Detroit Medical Center. Eligible patients were in the intensive care unit, had an estimated creatinine clearance of less than 30 ml/minute, and had either acute kidney injury or end-stage renal disease; several patients were taking renal replacement therapy. Fondaparinux was administered at an extended interval dosing regimen of 2.5 mg subcutaneously every 48 hours. Fondaparinux peak and trough anti-factor Xa levels were obtained. Lower extremity venous duplex studies were performed at baseline and study completion to assess for deep vein thrombosis (DVT), and patients were monitored for bleeding complications.

RESULTS

Thirty-two patients were enrolled. Patients received a median of four doses (interquartile range two to five) of fondaparinux. Fondaparinux peak (n=98) and trough (n=86) anti-factor Xa levels were 0.36 ± 0.18 mg/L and 0.17 ± 0.11 mg/L (mean ± SD), respectively, and were similar to levels reported in patients with normal renal function receiving conventional once-daily dosing. No lower extremity DVTs or suspected VTE events occurred. Two (6%) patients had significant bleeding events.

CONCLUSIONS

In critically ill patients with SRD, an extended interval fondaparinux dosing regimen of 2.5 mg every 48 hours for VTE prophylaxis achieved peak and trough anti-factor Xa levels similar to those reported in noncritically ill patients with normal renal function receiving once-daily fondaparinux. This regimen offers an alternative for patients with SRD when heparinoids must be avoided.

Protein Hydrolysates and Biopeptides: Production, Biological Activities, and Applications in Foods and Health Benefits. A Review

In recent years, a great deal of interest has been expressed regarding the production, characterization, and applications of protein hydrolysates and food-derived biopeptides due to their numerous beneficial health effects. In this regard, research is mainly focused on investigating the therapeutic potential of these natural compounds. Based on their amino acids composition, sequences, hydrophobicity, and length, peptides released from food proteins, beyond their nutritional properties, can exhibit various biological activities including antihypertensive, antioxidative, antithrombotic, hypoglycemic, hypocholesterolemic, and antibacterial activities among others. Protein hydrolysates are essentially produced by enzymatic hydrolysis of whole protein sources by appropriate proteolytic enzymes under controlled conditions, followed by posthydrolysis processing to isolate desired and potent bioactive peptides from a complex mixture of active and inactive peptides. Therefore, because of their human health potential and safety profiles, protein hydrolysates and biopeptides may be used as ingredients in functional foods and pharmaceuticals to improve human health and prevent diseases. In this review, we have focused on the major variables influencing the enzymatic process of protein hydrolysates production. The biological properties of protein hydrolysates will be described as well as their applications in foods and health benefits.

The safety of fondaparinux sodium for the treatment of venous thromboembolism

INTRODUCTION

Venous thromboembolism (VTE) is a common and potentially fatal disease. Fondaparinux is a synthetic agent able to act on single factors involved in the coagulation network, which could be administered at fixed doses and with a more predictable response.

AREAS COVERED

This review will focus on the efficacy and safety of fondaparinux in the treatment of major VTE (deep vein thrombosis and pulmonary embolism) and in the treatment of superficial vein thrombosis (SVT).

EXPERT OPINION

Results of high quality randomized controlled trials have clearly shown the efficacy and safety of fondaparinux in comparison to conventional treatment in patients with a major VTE. There are limited evidences on the safety and efficacy of different options in patients presenting with SVT. Fondaparinux has been evaluated in a large population of patients presenting with a SVT. Results of this high quality RCT provided the evidence on the efficacy and safety of fondaparinux 2.5 mg s.c./day for 45 days in this setting. Thus, considering the evidence of the literature and thanks to its pharmacokinetic and pharmacodynamic characteristics, fondaparinux represent a valid treatment option for both the acute management of patients with major VTE, and for the treatment of SVT.

Targeting the GPIbα binding site of thrombin to simultaneously induce dual anticoagulant and antiplatelet effects

Exosite 2 of human thrombin contributes to two opposing pathways, the anticoagulant pathway and the platelet aggregation pathway. We reasoned that an exosite 2 directed allosteric thrombin inhibitor should simultaneously induce anticoagulant and antiplatelet effects. To assess this, we synthesized SbO4L based on the sulfated tyrosine-containing sequence of GPIbα. SbO4L was synthesized in three simple steps in high yield and found to be a highly selective, direct inhibitor of thrombin. Michelis–Menten kinetic studies indicated a noncompetitive mechanism of inhibition. Competitive inhibition studies suggested ideal competition with heparin and glycoprotein Ibα, as predicted. Studies with site-directed mutants of thrombin indicated that SbO4L binds to Arg233, Lys235, and Lys236 of exosite 2. SbO4L prevented thrombin-mediated platelet activation and aggregation as expected on the basis of competition with GPIbα. SbO4L presents a novel paradigm of simultaneous dual anticoagulant and antiplatelet effects achieved through the GPIbα binding site of thrombin.

Commonly used drugs in hematologic disorders

The drugs used to treat hematologic disorders constitute an extraordinary array of agents and include therapy for anemias, bleeding diatheses, thromboembolism and hematologic malignancies. Within many of these categories, hematologic therapies represent molecular understanding of the disease pathophysiology and a treatment targeted precisely at the known aberration. In this chapter we discuss these treatments, with greater emphasis on the most commonly used agents and the latest advances in the specific field. The chapter includes discussion on old (cyclooxygenase inhibitors) and new antiplatelet agents (ADP receptor P2Y12 inhibitors), newly described oral thrombin inhibitors such as dabigatran, treatment targeted at the bcr-abl tyrosine kinase in chronic myeloid leukemia and demethylating agents in myelodysplasia. The discussion is also oriented towards neurologic implications, both therapeutic and toxic, for each therapy or therapeutic group.

Anticoagulant therapy: basic principles, classic approaches and recent developments

The standard multipotent anticoagulants (unfractionated and low molecular weight heparins, antagonists of vitamin K) are commonly used for treatment and/or prophylaxis of different thrombotic complications, such as deep vein thrombosis, thrombophilia, pulmonary embolism, myocardial infarction, stroke and so on. Advantages and shortcomings of these anticoagulants are considered. The modern tendencies to use small selective direct inhibitors of thrombin or factor Xa are surveyed. The search of the new targets in the coagulation cascade for development of new promising anticoagulants and improvement in antithrombotic therapy is discussed.

New anticoagulants for treatment of venous thromboembolism

Venous thromboembolism (VTE) is a major cause of morbidity and mortality and anticoagulation is the cornerstone of treatment. Although effective, traditional anticoagulants (i.e., heparins and vitamin K antagonists) have significant limitations and there are several unmet needs. Therefore, new drugs have been developed, including direct factor Xa inhibitors (e.g., rivaroxaban, apixaban and edoxaban) and thrombin inhibitors (e.g., dabigatran etexilate). The characteristics of these agents and the main results of published clinical studies dealing with VTE treatment are critically summarized in this article.

Rational design of potent, small, synthetic allosteric inhibitors of thrombin

Thrombin is a key enzyme targeted by the majority of current anticoagulants that are direct inhibitors. Allosteric inhibition of thrombin may offer a major advantage of finely tuned regulation. We present here sulfated benzofurans as the first examples of potent, small allosteric inhibitors of thrombin. A sulfated benzofuran library of 15 sulfated monomers and 13 sulfated dimers with different charged, polar, and hydrophobic substituents was studied in this work. Synthesis of the sulfated benzofurans was achieved through a multiple step, highly branched strategy, which culminated with microwave-assisted chemical sulfation. Of the 28 potential inhibitors, 11 exhibited reasonable inhibition of human α-thrombin at pH 7.4. Structure-activity relationship analysis indicated that sulfation at the 5-position of the benzofuran scaffold was essential for targeting thrombin. A tert-butyl 5-sulfated benzofuran derivative was found to be the most potent thrombin inhibitor with an IC(50) of 7.3 μM under physiologically relevant conditions. Michaelis-Menten studies showed an allosteric inhibition phenomenon. Plasma clotting assays indicate that the sulfated benzofurans prolong both the activated partial thromboplastin time and prothrombin time. Overall, this work puts forward sulfated benzofurans as the first small, synthetic molecules as powerful lead compounds for the design of a new class of allosteric inhibitors of thrombin.

Evaluating the impact of new anticoagulants in the hospital setting

The short-comings of current anticoagulants have led to the development of newer, albeit more expensive, oral alternatives.

OBJECTIVE

To explore the potential impact the new anticoagulants dabigatran and rivaroxaban in the local hospital setting, in terms of utilisation and subsequent costing.

METHOD

A preliminary costing analysis was performed based on a prospective 2-week clinical audit (29th June - 13th July 2009). Data regarding current anticoagulation management were extracted from the medical files of patients admitted to Ryde Hospital. To model potential costing implications of using the newer agents, the reported incidence of VTE/stroke and bleeding events were obtained from key clinical trials.

RESULTS

Data were collected for 67 patients treated with either warfarin (n=46) or enoxaparin (n=21) for prophylaxis of VTE/stroke. At least two-thirds of all patients were deemed suitable candidates for the use of newer oral anticoagulants (by current therapy: warfarin: 65.2% (AF), 34.8% (VTE); enoxaparin: 100%, (VTE)). The use of dabigatran in VTE/stroke prevention was found to be more cost- effective than warfarin and enoxaparin due to significantly lower costs of therapeutic monitoring and reduced administration costs. Rivaroxaban was more cost-effective than warfarin and enoxaparin for VTE/stroke prevention when supplier-rebates (33%) were factored into costing.

CONCLUSION

This study highlights the potential cost- effectiveness of newer anticoagulants, dabigatran and rivaroxaban, compared to warfarin and enoxaparin. These agents may offer economic advantages, as well as clinical benefits, in the hospital-based management of anticoagulated patients.

Old and new anticoagulant drugs: a minireview

Abstract The limits of traditional anticoagulants, such as heparin and warfarin, have prompted the search for new agents for prophylaxis and treatment of arterial and venous thromboembolism, including factor Xa and thrombin inhibitors. These agents can be given orally, and their most significant advantage is that no laboratory monitoring is needed. The anti-Xa inhibitor rivaroxaban and the direct thrombin inhibitor dabigatran etexilate are licensed for prophylaxis of venous thromboembolism (VTE) in high-risk orthopedic surgery. They are at least as safe and effective as heparins but much more expensive. Dabigatran, rivaroxaban, and other agents currently in the pipeline of clinical development have the potential to replace warfarin in the two most frequent indications for anticoagulation, i.e. secondary prophylaxis of VTE and atrial fibrillation. Prevention and treatment of coronary artery thrombosis in patients with ischemic heart disease is another area of investigation for the role of new anticoagulants. These drugs have the potential to meet some currently unmet needs of traditional anticoagulants, but available clinical data warrant confirmation and expansion. Lack of specific antidotes for anticoagulation reversal and the high cost are important limitations of their use.

Recent advances in oral anticoagulation for atrial fibrillation

Atrial fibrillation is the most common sustained rhythm disturbance. Thromboembolic events related to atrial fibrillation result in significant morbidity, mortality and increases in the cost of healthcare. Anticoagulants are pivotal agents for the prevention and treatment of thromboembolic disorders. The latest American College of Cardiology/American Heart Association guidelines recommend antithrombotic therapy to prevent thromboembolism for all patients with atrial fibrillation, except those with lone atrial fibrillation or contraindications. Vitamin K antagonists were first synthesized in 1948 and for the past six decades they have been the only agents used for long-term oral anticoagulant therapy. Although these drugs are effective, they have numerous limitations, which have led to the development of newer anticoagulant therapies. The emerging oral anticoagulant agents are target selective. They have predictable pharmacokinetic and pharmacodynamic parameters and do not require routine monitoring. They are not associated with significant food and drug interactions, and can be administered in simple fixed daily or twice daily doses. This article reviews the current literature on various targets for anticoagulant therapy and newer oral anticoagulants for atrial fibrillation.

A new era for anticoagulants

Selective inhibitors of specific coagulation factors represent a new class of antithrombotic agents, designed to overcome the limitations of traditional anticoagulants. Available clinical studies indicate that the most promising new anticoagulants are those selectively targeting factor Xa and thrombin. Typically, the standard steps for clinical evaluation of new anticoagulants are thromboprophylaxis in high risk orthopedic surgery, followed by treatment of established venous thromboembolism, nonvalvular atrial fibrillation and acute coronary syndromes. These agents - that have the potential to be more effective and easier to use than conventional drugs such as heparins and vitamin K antagonists - will greatly expand our armamentarium for the prevention and treatment of arterial and venous thromboembolism. The current knowledge on these antithrombotic agents is summarized in this review, particularly focusing on the early results of clinical trials.

Managing cancer-related venous thromboembolic disease: low-molecular-weight heparins and beyond

Venous thromboembolism is a major contributor to the morbidity and mortality of patients with cancer. For patients undergoing cancer surgery, several trials support the safety and efficacy of unfractionated heparin and of low-molecular-weight heparin for the prevention of venous thromboembolism, while data regarding the efficacy and safety of these agents in the setting of medical hospitalization is less definitive and must be extracted from trials including noncancer patients with different thrombotic risk factors. Randomized clinical studies confirm that patients with cancer who develop venous thromboembolism have superior outcomes when treated with long-term low-molecular-weight heparin as compared with warfarin. Novel anticoagulants that are orally bioavailable and function by directly inhibiting factor Xa or thrombin are entering the market. To date, data regarding the efficacy and safety of these novel anticoagulants as venous thromboembolism prophylaxis and treatment in cancer patients are not available and must be extracted from larger trials with heterogeneous patient populations.

Influence of aromatic and aliphatic moieties on thrombin inhibitors potency

Thrombin is a plasma serine protease that plays a key role in coagulation and hemostasis but also in thromboembolic diseases. Direct thrombin inhibitors could be beneficial for future anticoagulant therapy in the prophylaxis of venous and arterial thrombosis as well as myocardial infarction. To design the efficient thrombin inhibitors we have synthesized and studied peptide-based inhibitors resistant to enzymatic degradation. Compounds with general formula X-DArg-D-Phe-OMe, where X = residue of 3-[6-ethyl-7-hydroxy-3-(4-methyl-thiazol-2-yl)-4-oxo-4H-chromen-2-yl]-propionic acid (chromone) and lauric acid were synthesized by classic methods of peptides synthesis in solution. The comparative inhibitory analysis of prepared compounds in relation to thrombin was conducted. The analysis of the inhibition effect of the peptide with retro-D-sequence modified by residues of natural organic compounds (chromone or fatty acid moiety) has demonstrated that modification with the fatty acid residue appeared to be the most successful one. Introduction of lauric acid residue (Ki = 1,76 muM) maximally increased the inhibition effect. These findings establish an important role of fatty moiety in structure of inhibitors in preferential binding and inhibition of thrombin active side.

Advances in understanding pathogenic mechanisms of thrombophilic disorders

Venous thromboembolism is a major medical problem, annually affecting 1 in 1000 individuals. It is a typical multifactorial disease, involving both genetic and circumstantial risk factors that affect a delicate balance between procoagulant and anticoagulant forces. In the last 50 years, the molecular basis of blood coagulation and the anticoagulant systems that control it have been elucidated. This has laid the foundation for discoveries of both common and rare genetic traits that tip the natural balance in favor of coagulation, with a resulting lifelong increased risk of venous thrombosis. Multiple mutations in the genes for anticoagulant proteins such as antithrombin, protein C, and protein S have been identified and constitute important risk factors. Two single mutations in the genes for coagulation factor V (FV Leiden) and prothrombin (20210G>A), resulting from approximately 20,000-year-old mutations with subsequent founder effects, are common in the general population and constitute major genetic risk factors for thrombosis. In celebration of the 50-year anniversary of the American Society of Hematology, this invited review highlights discoveries that have contributed to our present understanding of the systems that control blood coagulation and the genetic factors that are involved in the pathogenesis of venous thrombosis.

Advancement in antithrombotics for stroke prevention in atrial fibrillation

The focus of this review is the evolving field of antithrombotic drug therapy for stroke prevention in patients with atrial fibrillation (AF). The current standard of therapy includes warfarin, acenocoumarol and phenprocoumon which have proven efficacy by reducing stroke by 68% against placebo. However, a narrow therapeutic index, wide variation in metabolism, and numerous food and drug interactions have limited their clinical application to only 50% of the indicated population. Newer agents such as direct thrombin inhibitors, factor Xa inhibitors, factor IX inhibitors, tissue factor inhibitors and a novel vitamin K antagonist are being developed to overcome the limitations of current agents. The direct thrombin inhibitor dabigatran is farthest along in development. Further clinical trial testing, and eventual incorporation into clinical practice will depend on safety, efficacy and cost. Development of a novel vitamin K antagonist with better INR control will challenge the newer mechanistic agents in their quest to replace the existing vitamin K antagonists. Till then, the large unfilled gap to replace conventional agents remains open. This review will assess all these agents, and compare their mechanism of action, stage of development and pharmacologic profile.

New oral Xa and IIa inhibitors: updates on clinical trial results

Heparins and VKAs have been the cornerstones of anticoagulation therapy for several decades and these agents have become most important drugs in the primary and secondary prevention of venous and arterial thromboembolic disease. Although effective, their use has been hampered by numerous limitations. In the search for new agents matching the 'ideal' anticoagulant profile, a number of different steps in the coagulation cascade have been targeted, including direct thrombin inhibition, and direct inhibition of Factor Xa. There are currently a host of promising new agents at various stages of development and clinical evaluation. With potential benefits including predictable efficacy, rapid onset of action, ability to bind clot-bound coagulation factors and no requirement for therapeutic monitoring, these new agents are set to improve the management of thromboembolic disorders.

Glucuronidation of monohydroxylated warfarin metabolites by human liver microsomes and human recombinant UDP-glucuronosyltransferases

Our understanding of human phase II metabolic pathways which facilitate detoxification and excretion of warfarin (Coumadin) is limited. The goal of this study was to test the hypothesis that there are specific human hepatic and extrahepatic UDP-glucuronosyltransferase (UGT) isozymes, which are responsible for conjugating warfarin and hydroxylated metabolites of warfarin. Glucuronidation activity of human liver microsomes (HLMs) and eight human recombinant UGTs toward (R)- and (S)-warfarin, racemic warfarin, and major cytochrome P450 metabolites of warfarin (4'-, 6-, 7-, 8-, and 10-hydroxywarfarin) has been assessed. HLMs, UGT1A1, 1A8, 1A9, and 1A10 showed glucuronidation activity toward 4'-, 6-, 7-, and/or 8-hydroxywarfarin with K(m) values ranging from 59 to 480 microM and V(max) values ranging from 0.03 to 0.78 microM/min/mg protein. Tandem mass spectrometry studies and structure comparisons suggested glucuronidation was occurring at the C4'-, C6-, C7-, and C8-positions. Of the hepatic UGT isozymes tested, UGT1A9 exclusively metabolized 8-hydroxywarfarin, whereas UGT1A1 metabolized 6-, 7-, and 8-hydroxywarfarin. Studies with extrahepatic UGT isoforms showed that UGT1A8 metabolized 7- and 8-hydroxywarfarin and that UGT1A10 glucuronidated 4'-, 6-, 7-, and 8-hydroxywarfarin. UGT1A4, 1A6, 1A7, and 2B7 did not have activity with any substrate, and none of the UGT isozymes evaluated catalyzed reactions with (R)- and (S)-warfarin, racemic warfarin, or 10-hydroxywarfarin. This is the first study identifying and characterizing specific human UGT isozymes, which glucuronidate major cytochrome P450 metabolites of warfarin with similar metabolic rates known to be associated with warfarin metabolism. Continued characterization of these pathways may enhance our ability to reduce life-threatening and costly complications associated with warfarin therapy.