No influence of mild-to-moderate hepatic impairment on the pharmacokinetics and pharmacodynamics of ximelagatran, an oral direct thrombin inhibitor

Apixaban Pharmacokinetics and Pharmacodynamics in Subjects with Mild or Moderate Hepatic Impairment

Background

Hepatic impairment can impact apixaban pharmacokinetics and pharmacodynamics by decreasing cytochrome P450-mediated metabolism and factor X production.

Objective

This study evaluated the effect of mild or moderate (Child–Pugh A and B) hepatic impairment on apixaban pharmacokinetics, pharmacodynamics, and safety.

Methods

This open-label, parallel-group, single-dose study included eight mildly and eight moderately hepatically impaired subjects, and 16 healthy subjects. Subjects received a single oral apixaban 5-mg dose (day 1). Pharmacokinetic, pharmacodynamic, and safety assessments were completed at prespecified time points. Apixaban maximum plasma concentration and area under the concentration–time curve to infinity were compared between subjects with hepatic impairment and healthy subjects.

Results

Apixaban area under the concentration–time curve to infinity point estimates and 90% confidence intervals were 1.03 (0.80–1.32) and 1.09 (0.85–1.41) for subjects with mild and moderate hepatic impairment vs healthy subjects. Maximum plasma concentration results were similar. Mean (standard deviation) apixaban unbound fraction was 6.8% (1.4), 7.9% (1.8), and 7.1% (1.3) in subjects with mild or moderate hepatic impairment and in healthy subjects. Mean change from baseline in international normalized ratio (3 h post-dose) was 14.7%, 12.7%, and 10.7% for subjects with mild or moderate hepatic impairment and healthy subjects, respectively. A direct relationship was observed between apixaban anti-factor Xa activity and plasma concentration across groups. No serious adverse events or discontinuations due to adverse events occurred.

Conclusions

Mild or moderate hepatic impairment had no clinically relevant impact on apixaban pharmacokinetic or pharmacodynamic measures, suggesting that dose adjustment may not be required.

Synthesis, Crystal Structural, and Spectral Characterisation of Dabigatran Etexilate Tetrahydrate

Dabigatran etexilate tetrahydrate, C34H49N7O9, has been crystallised at ambient conditions. The colourless crystal was investigated using X-ray crystallography with single crystals and powder techniques, and was characterised by thermogravimetric-differential thermal analysis (TG-DTA) and infrared spectroscopy (IR). The compound was shown to be a tetrahydrate. A dabigatran etexilate molecule and four water molecules form a large ring structure, and intra-molecular hydrogen bonds contribute to the formation of a stable molecule in the unit cell.

Ximelagatran for stroke prevention in atrial fibrillation

Atrial fibrillation is the most common sustained cardiac arrhythmia and the most frequently encountered cause of embolic stroke. Vitamin K antagonists (such as warfarin) have represented the cornerstone of anticoagulation practice for the last 60 years. Although highly effective in preventing thromboembolic events among patients with atrial fibrillation, warfarin therapy is limited by a multitude of potential problems. Hence, warfarin is significantly underused in clinical practice, with only half of warfarin-treated patients actually achieving therapeutic anticoagulation in routine clinical practice. Consequently, there is an overwhelming need for an alternative oral anticoagulant for patients with atrial fibrillation that is safer, more practical and effective. Ximelagatran (Exanta, AstraZeneca) is a novel oral direct thrombin inhibitor that is rapidly converted to the active compound melagatran after oral absorption. It has a low potential for drug interactions, anticoagulation monitoring is not required, and it is administered at a fixed twice-daily dose. The Stroke Prevention using the ORal Thrombin Inhibitor in patients with nonvalvular atrial Fibrillation (SPORTIF) III and V trials have together demonstrated the noninferiority of ximelagatran relative to warfarin for the prevention of stroke and embolic events in atrial fibrillation. Unfortunately, initial optimism has been tempered by serious concerns over its safety data in view of its propensity to cause elevation in liver enzymes.

A Preliminary Assessment of the Critical Differences Between Novel Oral Anticoagulants Currently in Development

Chronic anticoagulation represents a clinical conundrum for the health care community that balances unquestionable morbidity and mortality benefits against interindividual variability, leading to drug interactions, adverse events, and thromoembolic events related to underdosing. Despite the growing data regarding the appropriate use and dosing of agents used for chronic anticoagulation, use in clinical practice remains low, thus leading to a theoretical reduction in the risk-to-benefit ratio in the clinical setting relative to that reported in the literature. Oral anticoagulants currently in development represent a heterogeneous group of compounds that are specific for the final common pathway in the coagulation cascade and show indications toward a reduced drug interaction profile, reduced interpatient variation in pharmacokinetic parameters, and morbidity and mortality benefits that might be similar to currently available treatment modalities. This review highlights the critical differences among oral anticoagulants in development and places their role in the context of the benefits and limitations of currently available anticoagulants.

New Anticoagulant Agents: Direct Thrombin Inhibitors

Decades of research have been devoted to developing effective, safe, and convenient anticoagulant agents. In recent years, much emphasis has been placed on the development of direct thrombin inhibitors (DTIs) that offer benefits over agents like heparin and warfarin including the inhibition of both circulating and clot-bound thrombin; a more predictable anticoagulant response, because they do not bind to plasma proteins and are not neutralized by platelet factor 4; lack of required cofactors, such as antithrombin or heparin cofactor II; inhibiting thrombin-induced platelet aggregation; and absence of induction of immune-mediated thrombocytopenia. Various injectable DTIs are currently available and used for many indications. In addition, research is now focusing on oral DTIs that seem promising and offer various advantages, such as oral administration, predictable pharmacokinetics and pharmacodynamics, a broad therapeutic window, no routine monitoring, no significant drug interactions, and fixed-dose administration.

Ximelagatran: a clinical perspective

Ximelagatran is a novel oral anticoagulant belonging to a class of drugs known as direct thrombin inhibitors. Numerous recent large-scale, randomised controlled clinical trials have given the drug a large clinical platform. These include data on the thromboprophylaxis of venous thromboembolism following major orthopaedic surgery and knee arthroscopy, as well as in the treatment of deep vein thrombosis and prevention of stroke with nonvalvular atrial fibrillation. One phase II study has also shown the efficacy and safety of ximelagatran in secondary prevention post-myocardial infarction. Unfortunately, approximately 6% of patients develop usually self-limiting derangement of liver dysfunction, and frequent monitoring of liver function is likely to be recommended for the first 6 months of treatment. Unlike the vitamin K antagonists, ximelagatran has a wide therapeutic interval with few food, alcohol or drug interactions, and it does not require anticoagulant monitoring. The aim of this overview is to review the clinical trials pertaining to this new drug, which is the first new oral anticoagulant for over 60 years, and one that is likely to influence our management of thrombosis-related disorders.

Ximelagatran: An orally active direct thrombin inhibitor

PURPOSE

The chemistry, pharmacology, pharmacokinetics, clinical efficacy, dosage and administration, contraindications, and adverse effects of ximelagatran are reviewed.

SUMMARY

Ximelagatran is the first orally active direct thrombin inhibitor to be tested in Phase III clinical trials. After oral administration, ximelagatran is rapidly converted to its active metabolite, melagatran. Melagatran (after oral ximelagatran administration) predictably inhibits thrombin function without need for routine anticoagulation monitoring. Melagatran effectively inhibits both free and clot-bound thrombin-a potential pharmacodynamic advantage over heparin products. Melagatran has a half-life of 2.4-4.6 hours, necessitating twice-daily administration. Melagatran is primarily eliminated by the kidneys and has not been studied clinically in patients with severe renal failure. Ximelagatran has undergone 10 Phase III trials (6 for prophylaxis of venous thromboembolism [VTE] due to orthopedic surgery, 1 for initial treatment of VTE, 1 for long-term prevention of VTE recurrence, and 2 for stroke prophylaxis due to atrial fibrillation). Results were generally positive. AstraZeneca applied in December 2003 for marketing approval of ximelagatran for prevention of VTE after total knee replacement surgery, long-term prevention of VTE recurrence after standard therapy, and stroke prevention due to atrial fibrillation. FDA denied approval of ximelagatran for all indications, mainly because of increased rates of coronary artery disease events in ximelagatran recipients in some studies and the possibility of hepatic failure when the medication is used for long-term therapy.

CONCLUSION

Ximelagatran has shown promise as a possible alternative to warfarin and other anticoagulants but will require further study to ensure its safety.

Ximelagatran: the first oral direct thrombin inhibitor

Oral anticoagulants are often prescribed for long-term prevention and treatment of venous or arterial thromboembolism. The only orally active anticoagulants currently available are the vitamin K antagonists. Although effective, they have a narrow therapeutic window and require routine coagulation monitoring to ensure that a therapeutic level has been achieved. Furthermore, genetic differences in metabolism and multiple food and drug interactions affect the anticoagulant response to vitamin K antagonists. These factors add to the need for routine coagulation monitoring, which is problematic for patients and physicians and costly for the healthcare system. Ximelagatran, the first oral direct thrombin inhibitor, was designed to overcome many of the drawbacks of vitamin K antagonists. Since it produces a predictable anticoagulant response, ximelagatran does not require coagulation monitoring. Phase III clinical trials have evaluated the efficacy and safety of ximelagatran for the prevention and treatment of venous thromboembolism and for the prevention of thromboembolic events in patients with atrial fibrillation. Focusing on ximelagatran, this review will discuss the appropriateness of thrombin as a target for new anticoagulants, compare and contrast direct and indirect thrombin inhibitors and describe the theoretical advantages of direct thrombin inhibitors. It will also review the pharmacology of ximelagatran, discuss the clinical trial results with ximelagatran and provide perspective on the advantages and potential limitations of ximelagatran.

Novel therapies for the prevention of stroke

The health and economic burden of stroke to society is enormous. Pharmacological therapies remain the primary stroke prevention strategy for the vast majority. Several existing and newer pharmacological agents aimed at the treatment of hypertension and lowering cholesterol are proving to be effective. For example, the antiplatelet agent clopidogrel has reduced end points in the secondary prevention of stroke, as have combinations of aspirin with traditional therapies, including dipyramidole. The direct oral thrombin inhibitor ximelagatran is a novel oral anticoagulant that has shown significant potential as a possible replacement to warfarin therapy, for the prevention of stroke for patients with non-valvular atrial fibrillation. Additional novel agents with hypothetical, although not yet proven, benefits in stroke prevention include fish oils, homocysteine-lowering therapy and anti-inflammatory agents. Finally, a controversial novel polypill, which would include fixed combinations of several pharmacological agents, may yet become a realistic and promising stroke prevention option.

New blood thinner offers first potential alternative in 50 years: ximelagatran

Traditional anticoagulants employed in the treatment of thrombosis include the injectable heparins and oral warfarin. Though effective, these traditional agents are fraught with limitations in their ease of use in the clinical setting. Warfarin, for example, has many pharmacokinetic properties and food-and-drug interactions that result in unpredictable patient response and the need for expensive and time-consuming monitoring of coagulation status. Ximelagatran is a novel, promising, orally active, direct thrombin inhibitor currently in development that, for the first time in 50 years, offers a potential alternative to the mainstay oral agent "warfarin." Advantages of ximelagatran over warfarin include predictable pharmacokinetics and pharmacodynamics, a broad therapeutic window, no routine anticoagulant monitoring, no clinically significant drug interactions, and fixed-dose administration. Ximelagatran has been evaluated for thromboprophylaxis following orthopedic surgery, acute treatment and secondary prevention of venous thrombosis, stroke prevention in atrial fibrillation, and acute coronary syndromes. Results of clinical trials suggest that ximelagatran is equally or more efficacious than warfarin and/or low-molecular-weight heparin therapy without increasing rates of minor or major bleeding. Although postmarketing surveillance will provide the final test of this drug, the future looks promising for addition of a new anticoagulant with the potential to provide excellent efficacy, predictable response, and reduced adverse effects. Pending regulatory approval, ximelagatran may help overcome barriers to appropriate anticoagulant therapy, thereby decreasing morbidity and mortality associated with thrombotic diseases.

The Direct Thrombin Inhibitors: Their Role and Use for Rational Anticoagulation

Major clinical advantages are achieved when direct thrombin inhibitors are used in venous thromboembolism. These agents provide more reliable anticoagulant response patterns because they are not significantly bound to plasma proteins and few, if any, drug-drug interactions are seen. The studies to date confirm that not all direct thrombin inhibitors are the same. The new reversible, short-acting catalytic site-specific drugs provide an excellent safety profile and high degree of efficacy for the prophylaxis and treatment of venous thromboembolism and pulmonary embolic states. The availability of the oral prodrug ximelagatran allows reproducible, effective, and safe direct thrombin inhibition without the requirement for coagulation laboratory monitoring; it appears destined to be the oral anticoagulant of the future.

Comparative Pharmacokinetics of Vitamin K Antagonists

Vitamin K antagonists belong to the group of most frequently used drugs worldwide. They are used for long-term anticoagulation therapy, and exhibit their anticoagulant effect by inhibition of vitamin K epoxide reductase. Each drug exists in two different enantiomeric forms and is administered orally as a racemate. The use of vitamin K antagonists is complicated by a narrow therapeutic index and an unpredictable dose-response relationship, giving rise to frequent bleeding complications or insufficient anticoagulation. These large dose response variations are markedly influenced by pharmacokinetic aspects that are determined by genetic, environmental and possibly other yet unknown factors. Previous knowledge in this regard principally referred to warfarin. Cytochrome P450 (CYP) 2C9 has clearly been established as the predominant catalyst responsible for the metabolism of its more potent S-enantiomer. More recently, CYP2C9 has also been reported to catalyse the hydroxylation of phenprocoumon and acenocoumarol. However, the relative importance of CYP2C9 for the clearance of each anticoagulant substantially differs. Overall, the CYP2C9 isoenzyme appears to be most important for the clearance of warfarin, followed by acenocoumarol and, lastly, phenprocoumon. The less important role of CYP2C9 for the clearance of phenprocoumon is due to the involvement of CYP3A4 as an additional catalyst of phenprocoumon hydroxylation and significant excretion of unchanged drug in bile and urine, while the elimination of warfarin and acenocoumarol is almost completely by metabolism. Consequently, the effects of CYP2C9 polymorphisms on the pharmacokinetics and anticoagulant response are also least pronounced in the case of phenprocoumon; this drug seems preferable for therapeutic anticoagulation in poor metabolisers of CYP2C9. In addition to these vitamin K antagonists, oral thrombin inhibitors are currently under clinical development for the prevention and treatment of thromboembolism. Of these, ximelagatran has recently gained marketing authorisation in Europe. These novel drugs all feature some major advantages over traditional anticoagulants, including a wide therapeutic interval, the lack of anticoagulant effect monitoring and a low drug-drug interaction potential. However, they are also characterised by some pitfalls. Amendments of traditional anticoagulant therapy, including self-monitoring of international normalised ratio values or prospective genotyping for individual dose-tailoring may contribute to the continuous use of warfarin, phenprocoumon and acenocoumarol in the future.

Oral direct thrombin inhibition: an effective and novel approach for venous thromboembolism

Venous thromboembolism is a serious illness that affects patient morbidity and mortality and presents a significant management challenge to healthcare providers world-wide. Despite major achievements in the significant reduction of thromboembolic complications, the most common therapies currently used for prevention and treatment of venous thromboembolism--heparins and vitamin K antagonists such as warfarin--have several limitations. In particular, unfractionated heparin and warfarin show significant inter-patient variability in pharmacokinetics and pharmacodynamics, which makes regular coagulation monitoring necessary. Furthermore, only warfarin is suitable for long-term use, as it is administered orally. A new class of anticoagulants has been developed that directly target thrombin, a key enzyme in the blood coagulation cascade. Unlike warfarin and heparin, these direct thrombin inhibitors are able to inhibit fibrin-bound thrombin and so produce more effective inhibition of coagulation. Importantly, some members of this class of drugs have been developed for oral administration. Ximelagatran, which is converted to its active form melagatran, has predictable pharmacokinetics and pharmacodynamics. Therefore, ximelagatran can be administered in fixed doses with no need for coagulation monitoring. Its efficacy and safety profile have been demonstrated in preclinical and clinical studies. As the first oral agent in the new class, direct thrombin inhibitors, ximelagatran has significant potential for improving the prevention and treatment of venous thromboembolism.

New anticoagulants for treatment of venous thromboembolism

Treatment of venous thromboembolism (VTE) usually starts with concomitant administration of heparin or low-molecular-weight heparin (LMWH) and a vitamin K antagonist. The parenteral anticoagulant, which is given for at least 5 days, is stopped once the vitamin K antagonist produces a therapeutic level of anticoagulation. Although the introduction of LMWH has simplified the initial treatment of VTE, problems remain. LMWH must be given by daily subcutaneous (SC) injection and vitamin K antagonists require routine coagulation monitoring, which is inconvenient for patients and physicians. Recently, 3 new anticoagulants have been introduced in an attempt to overcome these limitations. These include fondaparinux and idraparinux, synthetic analogs of the pentasaccharide sequence that mediates the interaction of heparin and LMWH with antithrombin, and ximelagatran, an orally active inhibitor of thrombin. These agents produce a predictable anticoagulant response; thus, routine coagulation monitoring is unnecessary. Because they do not bind to platelets or platelet factor 4, fondaparinux and idraparinux do not cause heparin-induced thrombocytopenia (HIT). Unlike vitamin K antagonists, ximelagatran has a rapid onset of action, thereby obviating the need for concomitant administration of a parenteral anticoagulant when starting treatment. The lack of an antidote for these new agents is a drawback, particularly for idraparinux, which has a long half-life.

Ximelagatran: Pharmacology, Pharmacokinetics, and Pharmacodynamics

Thromboembolism is the largest cause of morbidity and mortality in the western world, yet oral anticoagulation is currently available only with vitamin K antagonists--most often, warfarin. Warfarin has been used for treatment of thrombotic disease for about 50 years. However, despite its widespread use, it is associated with several limitations, such as varied patient response, a narrow therapeutic window, numerous drug and food interactions, and need for frequent therapeutic monitoring. In addition, its full anticoagulant effect usually takes at least 4-5 days after the start of therapy or any dosage change, and it has a slow offset of therapy. A new oral anticoagulant, ximelagatran, has considerable advantages compared with warfarin. The agent requires no therapeutic monitoring, has a wide therapeutic window, and is not known to interact with food or drugs. The advantages ximelagatran brings to clinical practice should be a welcome addition to the options for management of thrombotic disease.

Ximelagatran: a new antithrombotic option in atrial fibrillation

Treatment strategies in patients with atrial fibrillation typically involve pharmacologic or interventional invasive therapies to suppress the rhythm, control ventricular contraction rates, or prevent thromboembolic complications. Current therapies used for rhythm conversion in atrial fibrillation may have undesirable risks or side effects that limit this approach. Lifelong anticoagulation may be necessary to prevent the formation of thrombus in the left atrial chamber that can travel into the cerebral circulation to cause a stroke. Currently, warfarin is the most commonly prescribed anticoagulant for this purpose. Unfortunately, many patients with atrial fibrillation may not receive warfarin because of the difficulties in dosing and maintaining desirable target goals. The oral direct thrombin inhibitor ximelagatran has several pharmacologic properties that provide a unique and potentially desirable treatment option. Clinical studies have demonstrated that ximelagatran, administered in twice-daily doses of 36 mg, is non-inferior to warfarin for thromboprophylaxis against stroke or systemic embolism in atrial fibrillation. The pharmacology of ximelagatran and clinical trials with its use in atrial fibrillation is reviewed.

Ximelagatran: an oral direct thrombin inhibitor

OBJECTIVE

To present the chemistry, pharmacology, and pharmacokinetics of ximelagatran, an oral direct thrombin inhibitor (DTI), and to review available comparative clinical trial data evaluating its efficacy and safety relative to other antithrombotic agents in the prevention and treatment of thromboembolism.

DATA SOURCES

A search of the PubMed and Cochrane databases (1995-August 2004), supplemented by a manual search of article bibliographies, conference abstracts, and data on file from the manufacturer, was conducted. Key search terms were ximelagatran, melagatran, H376/95, and direct thrombin inhibitors.

STUDY SELECTION AND DATA EXTRACTION

Pertinent information from available clinical trials, including study design, patient demographics, dosing regimens, anticoagulant comparators, methods for evaluating effectiveness, treatment outcomes, adverse events, and pharmacokinetic and pharmacodynamic evaluations, was extracted.

DATA SYNTHESIS

Ximelagatran is an orally administered DTI under development for use in the treatment of venous thromboembolism (VTE), long-term prevention of a second VTE event, stroke secondary to atrial fibrillation, prevention of VTE after orthopedic procedures, and recurrent ischemic events after acute myocardial infarction.

CONCLUSIONS

Ximelagatran, in twice-daily doses of 24 or 36 mg, is an alternative to low-molecular-weight heparins or warfarin in thromboprophylaxis following orthopedic knee replacement, atrial fibrillation, or initial treatment of VTE. Improved outcomes versus placebo were seen in the long-term prevention of VTE in patients who completed an initial 6 months of treatment. Liver-related effects need further clarification.

Ximelagatran, a new oral direct thrombin inhibitor, for the prevention of venous thromboembolic events in major elective orthopaedic surgery. Efficacy, safety and anaesthetic considerations

The oral direct thrombin inhibitor ximelagatran shows great promise for prevention of venous thromboembolic events following major elective orthopaedic surgery. Its consistent and predictable pharmacokinetics and pharmacodynamics across a wide range of patient populations allow administration with fixed dosing and with no coagulation monitoring. In orthopaedic surgery clinical trials, ximelagatran was effective and well tolerated compared with standard therapy, with dose and timing relative to surgery important factors in determining its optimal profile. In European trials, an initial 3-mg postoperative dose of subcutaneous melagatran, the active form of ximelagatran, followed by oral ximelagatran 24 mg twice daily achieved similar efficacy and safety to enoxaparin. Although the risk of spinal haematoma following neuraxial anaesthesia is rare, it is increased by the concomitant use of anticoagulants. In orthopaedic surgery trials with ximelagatran to date, complications such as spinal haematoma have not been reported. The pharmacokinetic profile of ximelagatran suggests that concurrent use with neuraxial anaesthesia should require no further precautions than those currently necessary with low-molecular-weight heparin.

New approaches to anticoagulation in atrial fibrillation

Oral direct thrombin inhibitors (DTIs) are a potential alternative to vitamin K antagonists, such as warfarin, for anticoagulant therapy. The oral DTI at the most advanced stage of clinical development is ximelagatran, which is rapidly absorbed and bioconverted to the active form melagatran. Oral ximelagatran has been evaluated in randomized, controlled trials for several indications, including stroke prevention in atrial fibrillation (AF). Recently, two pivotal phase III trials demonstrated that fixed-dose oral ximelagatran, 36 mg twice daily without coagulation monitoring, prevents stroke and systemic embolic events in patients with nonvalvular AF as effectively as well-controlled, adjusted-dose warfarin. Oral ximelagatran was generally well tolerated and caused less total (major plus minor) bleeding than warfarin. In a minority of ximelagatran-treated patients, elevated serum alanine aminotransferase levels were reported, but were typically not associated with specific symptoms, and returned toward the pretreatment baseline whether treatment was continued or discontinued. In AF, oral ximelagatran promises a better benefit to risk ratio than warfarin.

Anticoagulation for non-valvular atrial aibrillation - towards a new beginning with ximelagatran

OBJECTIVES: Ximelagatran is a novel oral direct thrombin inhibitor. It has favorable pharmacodynamic properties, with a broad therapeutic range without the need for anticoagulation monitoring. We aimed to discover whether ximelagatran offers a genuine future replacement to warfarin for patients in persistent atrial fibrillation (AF). MATERIALS AND METHODS: We provide an evidence-based review of the relative merits and disadvantages of warfarin and aspirin. We subsequently present an overview of the evidence for the utility of ximelagatran in the treatment of AF. RESULTS: Adjusted dose warfarin is recommended over aspirin for patients in AF at high risk of future stroke. Some of this benefit is partially offset by the higher bleeding risks associated with warfarin therapy. The SPORTIF III and V studies have shown that ximelagatran is not inferior to warfarin in the prevention of all strokes in patients with AF (both persistent and paroxysmal). This benefit was partially offset by the finding of a significant elevation of liver transaminases (>3 x normal) in 6% of patients. CONCLUSIONS: Current data would suggest that ximelagatran might represent a future alternative to warfarin. The lack of need for anticoagulant monitoring has been partially offset by a need for regular monitoring of liver function. Further data from randomized clinical trials is clearly needed.

Prevention of Venous Thromboembolism Following Orthopaedic Surgery

Patients undergoing total hip or total knee replacement are at high risk of venous thromboembolism (VTE), and are therefore considered to be populations well suited for the evaluation and dose optimisation of new anticoagulants. Deep vein thrombosis may lead to life-threatening pulmonary embolism, disabling morbidity in the form of the post-thrombotic syndrome, and risk of recurrent thrombotic events. There is increasing evidence that anticoagulant treatment for the prevention of VTE should be extended from 1 to at least 4 weeks after surgery. Anticoagulation with vitamin K antagonists (such as warfarin), low molecular weight heparin or unfractionated heparin effectively lowers the risk of VTE, but these anticoagulants have limitations such as the need for coagulation monitoring and subsequent dose adjustment (vitamin K antagonists), difficulty of continuing prophylaxis out of hospital because of the requirement for parenteral administration, and risk of heparin-induced thrombocytopenia. The development of new anticoagulants has been pursued with the aim of finding more effective, safer and/or more convenient therapies. Thrombin is a central regulator in the coagulation and inflammation process and several direct thrombin inhibitors (DTIs) with distinct pharmacological profiles, as well as pharmacological differences from the conventional anticoagulants, are currently in clinical use for certain indications or are under development. Clinical experience with parenterally administered DTIs has accumulated since the mid 1990s, although only desirudin (a recombinant hirudin) is currently approved for use in patients undergoing orthopaedic surgery. Two oral DTIs, ximelagatran and dabigatran etexilate, are in clinical development. Dabigatran etexilate has recently been evaluated in phase II clinical trials in patients undergoing total hip replacement. Several large phase III trials have now demonstrated the efficacy and safety of ximelagatran in the prevention of VTE following total hip or knee replacement. Ximelagatran can be used with an oral fixed dose without the need for coagulation monitoring or dose adjustment. Hence, it offers significant potential to facilitate the management of anticoagulation in or out of hospital.

Ximelagatran

Although there have been many significant advances over the last 50 years with regards to anticoagulant therapy, warfarin remains the definitive standard for the long-term prevention of thromboembolic events in many patients at risk for these complications. Although effective, warfarin has a narrow therapeutic window, necessitating frequent laboratory monitoring for anticoagulant effect. Ximelagatran is an investigational anticoagulant that directly inhibits thrombin, unlike heparin or warfarin, which are indirect inhibitors. Although indirect thrombin inhibitors are mainly only effective at inhibiting circulating thrombin, direct thrombin inhibitors are able to inhibit both free and clot-bound thrombin, thereby producing more effective anticoagulation. Ximelagatran is the first orally available direct thrombin inhibitor to reach phase 3 clinical trials. Ximelagatran is a prodrug for the active metabolite melagatran, and has been demonstrated to have a relatively wide therapeutic window in terms of bleeding and antithrombotic effect compared with warfarin. Clinical studies have demonstrated ximelagatran to be comparable in efficacy to warfarin and low-molecular-weight heparins (LMWH) for prophylaxis of venous thromboembolism, comparable to warfarin for stroke prevention in the setting of atrial fibrillation, and, when combined with aspirin, possible more effective than aspirin alone at preventing major adverse cardiovascular events in patients with a recent myocardial infarction. Adverse effects with ximelagatran primarily involve bleeding complications, which are more frequent than with placebo, but appear comparable to those occurring with standard anticoagulant treatment (ie, warfarin and LMWH). Ximelagatran has also been demonstrated to cause transient increases in liver enzymes, the significance of which will need to be addressed in ongoing phase 3 studies. Should ongoing trials prove ximelagatran to have at least similar therapeutic efficacy and safety as warfarin, ximelagatran may become a first-line anticoagulant due to its ease of administration and lack of a need for drug monitoring. The results of these trials are eagerly awaited in helping to defining the place in therapy for this promising new agent.