New anticoagulants - promising and failed developments

New direct and indirect acting factor Xa (FXa) and thrombin inhibitors are being developed to overcome the downsides of the conventional anticoagulants - unfractionated and low molecular weight heparins and vitamin K antagonists. Ximelagatran and idraparinux failed to demonstrate an acceptable safety profile. Rivaroxaban and dabigatran are approved for the post-operative prevention of thromboembolic complications after elective hip or knee replacement surgery; dabigatran is approved for the prevention of embolism in patients with atrial fibrillation in an increasing number of countries. Several novel indirect antithrombin-dependent anticoagulants as well as antithrombin-independent oral direct FXa and thrombin inhibitors are investigated in multiple indications for the prophylaxis and treatment of venous thromboembolism and the prophylaxis of arterial thrombotic disorders. Quality-adjusted life years costs and incremental cost-effectiveness ratios are relatively high at present, but may decrease after approval of more new anticoagulants for additional indications.     

The therapeutic potential of novel anticoagulants

Conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin. These agents display liabilities prompting the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins(LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and, more recently, approval for their use in the USA. Paralleling the development of LMWHs has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors exert their effects independent of either antithrombin III (ATIII) or heparin cofactor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases)direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of the senatural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant peptide-5 (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness, and sometimes relative ineffectiveness,in arterial and venous thromboembolic disorders has been demonstrated. They are being tested for their potential as new antithrombotic agents that act via direct enzyme inhibition. Thus,the clinician should in future be able to target different thrombotic conditions with proven, specific anticoagulant interventions.     

Direct thrombin inhibitors for the prevention of venous thromboembolism after major orthopaedic surgery

Unfractionated heparin, low-molecular-weight heparins and vitamin K antagonists are established antithrombotic agents, but all have a number of limitations. Unfractionated heparin requires parenteral administration, has a short half-life and variable dose-response relationship. Low molecular weight heparins are more effective than low-fixed-dose unfractionated heparin in the prevention of postoperative venous thromboembolism in high-risk surgical patients but they still require subcutaneous administration. Vitamin K antagonists have a narrow therapeutic window and require a careful laboratory monitoring to minimize the risk of bleeding or thrombosis. Direct thrombin inhibitors are selective inhibitors of this key enzyme. Direct thrombin inhibitors inactivate thrombin without requiring any plasma cofactor, inhibit both free and fibrin-bound thrombin, and do not appreciably bind to plasma proteins. Ximelagatran, the first oral direct thrombin inhibitor, is rapidly absorbed and converted to its active form melagatran, which can itself be administered subcutaneously. Ximelagatran has been evaluated in the prevention of venous thromboembolism after major orthopaedic surgery, in the treatment of venous thromboembolism, in the prevention of stroke in patients with atrial fibrillation and in the prevention of recurrence after acute coronary syndromes. In the European studies in major orthopaedic surgery ximelagatran was administered orally after one or two days of melagatran, given subcutaneously. This review will report on the mechanism of action and clinical pharmacology of direct antithrombin agents and on the results of the clinical trials performed with direct thrombin inhibitors in the prevention of venous thromboembolism after major orthopaedic surgery. Taken together, these results indicate that direct thrombin inhibitors, ximelagatran in particular, have the potential to be a valid alternative to low molecular weight heparins and oral anticoagulants in the prevention of venous thromboembolism after major orthopaedic surgery.     

Discovery of the novel antithrombotic agent 5-chloro-N-({(5S)-2-oxo-3- [4-(3-oxomorpholin-4-yl)phenyl]-1,3-oxazolidin-5-yl}methyl)thiophene- 2-carboxamide (BAY 59-7939): an oral, direct factor Xa inhibitor

Despite recent progress in antithrombotic therapy, there is still an unmet medical need for safe and orally available anticoagulants. The coagulation enzyme Factor Xa (FXa) is a particularly promising target, and recent efforts in this field have focused on the identification of small-molecule inhibitors with good oral bioavailability. We identified oxazolidinone derivatives as a new class of potent FXa inhibitors. Lead optimization led to the discovery of BAY 59-7939 (5), a highly potent and selective, direct FXa inhibitor with excellent in vivo antithrombotic activity. The X-ray crystal structure of 5 in complex with human FXa clarified the binding mode and the stringent requirements for high affinity. The interaction of the neutral ligand chlorothiophene in the S1 subsite allows for the combination of good oral bioavailability and high potency for nonbasic 5. Compound 5 is currently under clinical development for the prevention and treatment of thromboembolic diseases.     

The therapeutic potential of novel anticoagulants

For several decades, conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin (coumadin). Although used widely and effectively, heparin and warfarin display liabilities which have prompted the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins (LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and more recently, approval for their use in the USA. Paralleling the development of LMWHs, but lagging behind in terms of clinical development, has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors, exert their effects independent of either antithrombin III (ATIII) or heparin co-factor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases), direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of these natural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant factor (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents have been and are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness (and sometimes relative ineffectiveness compared with conventional therapy) in arterial and venous thromboembolic disorders has been demonstrated. These novel anticoagulents are curently being tested or will for their validity and potential as new antithrombotic agents acting via direct enzyme inhibition. In doing so, it is hoped that the clinician will in future be able to turn to proven specific anticoagulant interventions, targeted at each respective thrombotic state.     

Direct thrombin inhibitor use during percutaneous coronary intervention

Direct thrombin inhibitors (DTI) are emerging as alternative anticoagulants to unfractionated heparin and indirect thrombin inhibitors in patients undergoing percutaneous coronary intervention (PCI). We review the pharmacological properties of these newer antithrombotic agents and evaluate the clinical data demonstrating their use in patients undergoing PCI.     

Selective factor Xa inhibition improves efficacy of venous thromboembolism prophylaxis in orthopedic surgery

Venous thromboembolism is a serious, frequent, and potentially fatal complication of major orthopedic surgery. Currently available pharmacologic agents for the prevention of venous thromboembolism in this high-risk population consist of the oral anticoagulants and the heparin family of antithrombotic agents (unfractionated heparin, low-molecular-weight heparin, heparinoids). These classes of agents interfere with the activity of both thrombin and factor Xa (or their respective zymogens) to varying degrees. Newer antithrombotic agents in various stages of development exert their antithrombotic effect through a more targeted mechanism of action. Direct factor Xa inhibitors and the newest class of antithrombotic agents, the indirect factor Xa inhibitors, the prototype of which is the synthetic pentasaccharide fondaparinux sodium, limit fibrin formation through their exclusive inactivation of factor Xa. Clinical data from venous thromboembolism prophylaxis trials in hip and knee replacement and hip fracture surgeries, including the recently completed fondaparinux phase II and phase III trials, indicate that selective antifactor Xa activity may improve the efficacy:safety ratio of antithrombotic therapies for the prevention of venous thromboembolism in high-risk major orthopedic surgery.     

Pharmacological strategies for inhibition of thrombin activity

For decades, the options for therapeutic anticoagulation were limited to unfractionated heparin (UFH) and vitamin K antagonists (VKA), and their well-known limitations had to be accepted. With the introduction of the various LMWHs, the short-term anticoagulation could be much improved. The heparins delivered the proof of concept that FXa and thrombin represent suitable targets for therapeutic anticoagulation. Consequently, the search for new anticoagulants focus on inhibitors of thrombin or FXa. Apart from the VKA, the anticoagulants presently available or in an advanced stage of development can thus be divided in two classes: One are the glyco-anticoagulants with the natural sulfated glycosaminoglycans (GAGs) (UFH, LMWHs, and danaparoid) and the synthetic oligosaccharides (OS) (fondaparinux, idraparinux, and SR123781A). The other class are the xenobiotic anticoagulants, i.e. proteins and synthetic chemical entities. Die glyco-anticoagulants act partially (GAGs) or exclusively (oligosaccharides) by catalysing antithrombin, whereas the xenobiotic anticoagulants are direct inhibitors of either thrombin or FXa. At present, three direct thrombin inhibitors (DTI) (lepirudin, argatroban, and bivalirudin) are clinically used for limited indications, whereas there is still no direct FXa inhibitor available. The DTI ximelagatran represented the first oral anticoagulant since the introduction of VKA, but was withdrawn due to safety concerns. Among numerous drug candidates in the clinical development, two orally active anticoagulants dabigatran etexilate, a DTI, and rivaroxaban, the direct FXa inhibitor, are in the most advanced stage of development and may allow a paradigm change in anticoagulation in the foreseeable future. This review describes the pharmacological profile of all these anticoagulants.     

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.     

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.     

Emerging anticoagulants

Warfarin, heparin and their derivatives have been the traditional anticoagulants used for prophylaxis and treatment of venous thromboembolism. While the modern clinician is familiar with the efficacy and pharmacokinetics of these agents, their adverse effects have provided the impetus for the development of newer anticoagulants with improved safety, ease of administration, more predictable pharmacodynamics and comparable efficacy. Research into haemostasis and the coagulation cascade has made the development of these newer anticoagulants possible. These drugs include the factor Xa inhibitors and IIa (thrombin) inhibitors. Direct and indirect factor Xa inhibitors are being developed with a relative rapid onset of action and stable pharmacokinetic profiles negating the need for close monitoring; this potentially makes them a more attractive option than heparin or warfarin. Examples of direct factor Xa inhibitors include apixaban, rivaroxaban, otamixaban, betrixaban and edoxaban. Examples of indirect factor Xa inhibitors include fondaparinux, idraparinux and idrabiotaparinux. Direct thrombin inhibitors (factor IIa inhibitors) were developed with the limitations of standard heparin and warfarin in mind. Examples include recombinant hirudin (lepirudin), bivalirudin, ximelagatran, argatroban, and dabigatran etexilate. This review will discuss emerging novel anticoagulants and their use for the prophylaxis and management of venous thromboembolism, for stroke prevention in nonvalvular atrial fibrillation and for coronary artery disease.     

New anticoagulants

The quest to develop new antithrombotic agents has been stimulated by clinical needs and by advances in biotechnology that have made it possible to produce drugs that target specific steps in thrombogenesis. Established anticoagulants such as unfractionated heparin and the coumarins are effective, but have two major limitations: narrow therapeutic windows and highly unpredictable dose-response relationships. Consequently, these drugs often cause complications such as serious bleeding that require close monitoring of their use by laboratory tests to balance safety and effect. These limitations provided the impetus for the development of new anticoagulants that inactivate thrombin, factor Xa, factor IXa or the factor VIIa/tissue factor complex. Similarly, agents that enhance the protein C anticoagulant pathway have also been developed. Of these, direct thrombin inhibitors, soluble thrombomodulin, protein C, and activated protein C have been evaluated clinically for parenteral administration. However, there is enormous interest in the development of safer and more effective oral anticoagulants. In the future, such orally active direct inhibitors of thrombin and factor Xa, if they can be given safely without the need for laboratory monitoring, may replace the coumarins for the long-term treatment of thromboembolic disorders. To achieve these goals, these compounds need high, consistent oral bioavailability.     

Contribution of in vivo models of thrombosis to the discovery and development of novel antithrombotic agents

Cardiovascular and cerebrovascular diseases continue to be the leading cause of death throughout the world. Over the past two decades, great advances have been made in the pharmacological treatment and prevention of thrombotic disorders (e.g., tissue plasminogen activators, platelet GPIIb/IIIa antagonists, ADP receptor antagonists such as clopidogrel, low-molecular weight heparins, and direct thrombin inhibitors). New research is leading to the next generation of antithrombotic compounds such as direct coagulation FVIIa inhibitors, tissue factor pathway inhibitors, gene therapy, and orally active direct thrombin inhibitors and coagulation Factor Xa (FXa) inhibitors. Animal models of thrombosis have played a crucial role in discovering and validiting novel drug targets, selecting new agents for clinical evaluation, and providing dosing and safety information for clinical trials. In addition, these models have provided valuable information regarding the mechanisms of these new agents and the interactions between antithrombotic agents that work by different mechanisms. This review briefly presents the pivitol preclinical studies that led to the development of drugs that have proven to be effective clinicallly. The role that animal models of thrombosis are playing in the discovery and development of novel antithrombotic agents is also described, with specific emphasis on FXa inhibitors. The major issues regarding the use of animal models of thrombosis, such as the use of positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species-specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models in thrombosis/hemostasis research and pharmacology is presented using gene-therapy for hemophilia as an example of how animal models have aided in the development of these therapies that are now being evaluated clinically. In summary, animal models have contributed greatly to the discovery of currently available antithrombotic agents and will play a primary role in the discovery and characterization of the novel antithrombotic agents that will provide safe and effective pharmacological treatment for life-threatening thrombotic diseases.     

Pharmacology and clinical potential of direct thrombin inhibitors

Direct thrombin inhibitors represent a new class of anticoagulants that bind directly to thrombin and block the enzyme's interactions with its substrates. These agents have been developed, at least in part, to overcome the limitations of heparin and vitamin K antagonists. This paper (a) reviews why thrombin is an ideal target for new anticoagulants, (b) describes the pharmacological profiles of the various direct thrombin inhibitors, (c) outlines the potential mechanistic advantages of parenteral direct thrombin inhibitors over heparin, (d) defines the potential benefits of ximelagatran, the first orally active direct thrombin inhibitor, over vitamin K antagonists, and (e) provides clinical perspective on the strengths and weaknesses of the various parenteral and oral direct thrombin inhibitors.     

Antithrombotic agents

The morbidity and mortality associated with thromboembolic diseases are a worldwide problem. Deep venous thrombosis (DVT) and pulmonary embolism (PE) were estimated, some years ago, to be responsible for between 300,000 and 600,000 hospitalisations annually in the US. Arterial thrombosis is a major cause of myocardial infarction (MI), cerebral infarction and peripheral arterial diseases. Currently available antithrombotic drugs can be divided into three classes: anticoagulants (including vitamin K antagonists, unfractionated heparin and low molecular weight heparins [LMWHs]), antiplatelet agents (platelet aggregation inhibitors such as aspirin, ticlopidine and dipyridamole) and thrombolytics (streptokinase, urokinase and tissue plasminogen activator [t-PA]). All these drugs have demonstrated efficacy in the clinic, but each has its limitations. The risk of haemorrhage persists (though to varying degrees), and efficacy in the prevention, in addition to the treatment, of thromboembolic diseases needs to be improved. In addition, reocclusion following thrombolytic therapy remains a far from negligible risk. Current research goals are focused on identifying more potent and specific inhibitors of targets playing a crucial role in the coagulation pathway, such as thrombin and Factor Xa. Long-term goals are to develop thrombin receptor antagonists or analogues of natural occurring anticoagulants (e.g., tissue factor pathway inhibitor [TFPI] and protein C). The search for more effective anti-aggregating agents has resulted in the ongoing development of clopidogrel (a chemical analogue of ticlopidine), and thromboxane synthase inhibitors and receptor blockers. However, the main efforts have been focused on developing antagonists of the platelet receptor glycoprotein, Gp IIb/IIIa.Much work has been performed to improve the fibrin specificity of thrombolytic agents. In all these areas, new promising chemical structures have been identified, and a number of new antithrombotic agents are in development. The potential development issues remain the risk of bleeding at therapeutic doses, the possible lack of efficacy in sub-groups of treated patients, as well as dose adjustment and treatment duration.The considerable clinical progress in interventional cardiology (e.g., percutaneous transluminal coronary angioplasty [PTCA] and stent implantation) has increased the need for antithrombotic drugs with high local efficacy at the site of arterial injury. Recent progress in the knowledge of plaque rupture-induced disorders, particularly the major role of tissue factor, is suggesting new concepts for antithrombotic therapy. The experience from clinical trials with antithrombotic agents, the validation of biological parameters predictive of a prethrombotic state, and the improvement in non-invasive methods to control drug efficacy are likely to contribute to significant therapeutic progress in the prevention and treatment of venous and arterial thrombosis.     

Direct thrombin inhibitors

Heparins and vitamin K antagonists have been the primary agents used for anticoagulation in certain cardiovascular and thromboembolic diseases for over 50 years. However, they can be difficult to administer and are fraught with limitations. In response to the need for new anticoagulants, direct thrombin inhibitors (DTIs) have been developed and investigated for their utility in prophylaxis and treatment of venous thromboembolism (VTE), heparin-induced thrombocytopenia (HIT), acute coronary syndromes (ACS), secondary prevention of coronary events after ACS, and nonvalvular atrial fibrillation. Currently, four parenteral direct inhibitors of thrombin activity are FDA-approved in North America: lepirudin, desirudin, bivalirudin and argatroban. Of the new oral DTIs, dabigatran etexilate is the most studied and promising of these agents. This review discusses the clinical indications and efficacy of these direct thrombin inhibitors as well as future directions in anticoagulant therapy.     

Reversing the Effect of Oral Anticoagulant Drugs: Established and Newer Options

The vitamin K antagonists (VKAs) have been the standard (and only) oral anticoagulants used for the long-term treatment or prevention of venous thromboembolism or stroke in patients with atrial fibrillation. The coagulopathy induced by VKAs can be reversed with vitamin K, and in urgent situations, the vitamin K-dependent coagulation factors can be replaced by transfusion. In the last decade, a new class of oral anticoagulants has been developed, direct oral anticoagulants that bind to a specific coagulation factor and neutralize it. These compounds were shown to be effective and safe compared with the VKAs and were licensed for specific indications, but without a specific reversal agent. The absence of a reversal agent is a barrier to more widespread use of these agents. Currently, for the management of major life-threatening bleeding with the direct oral anticoagulants, most authorities recommend the use of four factor prothrombin complex concentrates. There are now three reversal agents in development and poised to enter the market. Idarucizumab is a specific antidote targeted to reverse the direct thrombin inhibitor, dabigatran, which was recently approved for use in the USA. Andexanet alfa is an antidote targeted to reverse the oral direct factor Xa inhibitors as well as the indirect inhibitor enoxaparin. Ciraparantag is an antidote targeted to reverse the direct thrombin and factor Xa inhibitors as well as the indirect inhibitor enoxaparin.     

Antagonists of activated factor X and thrombin: innovative antithrombotic agents

Anticoagulant drugs are traditionally administered for the prevention and treatment of thrombosis. Besides multi-targeted, traditional anticoagulants, such as coumarins or heparins, the search for the optimal antithrombotic efficacy to bleeding risk ratio has prompted the development of a novel armamentarium of anticoagulant drugs, which is expected to be introduced in the market. These emerging drugs are mainly targeted to suppress the propagation of the coagulation cascade (thrombin burst), by direct thrombin inhibition or selective inhibition of activated factor X. Therefore, thrombin or activated factor X antagonists would produce an efficient anticoagulation while minimizing the risk of bleeding, the most common adverse events of conventional anticoagulants. No routine monitoring, favorable form of administration and better compliance are additional advantages of these innovative drugs, which are already in advanced development or already licensed for clinical use. The aim of this article is to provide an overview on the mechanisms of action, clinical applications, cost-effectiveness and side effects of these emerging anticoagulant strategies.     

Recent advances in serine protease inhibitors as anticoagulant agents

The drawbacks and limitations of existing anticoagulant therapy which may result in serious adverse effects and a high mortality rate, have given rise to many anticoagulant development programmes in the last decade, focusing mainly at development of thrombin and FXa low-molecular weight inhibitors. A detailed understanding of blood coagulation pathways, functioning of the serine proteases thrombin, FXa, FVIIa and FIXa and elucidation of their crystal structures resulted in many potent compounds, among which some have entered the clinical phase or have been approved for use in clinical practice. Recently, the focus of anticoagulant research turned to inhibition of the TF:FVIIa complex, with some promising clinical candidates on the horizon. This article provides an overview of the current development status of serine protease inhibitors as anticoagulants, including new trends such as dual coagulation factor inhibitors.     

Progress in the development of synthetic thrombin inhibitors as new orally active anticoagulants

The trypsin-like serine protease thrombin is a multifunctional key enzyme at the final step of the coagulation cascade and is involved in the regulation of hemostasis and thrombosis. An increased activation of coagulation can result in severe thromboembolic disorders, one of the major reasons responsible for mortality and morbidity in western world. Therefore, an effective, safe, and orally available thrombin inhibitor could be a useful anticoagulant drug for the daily prophylaxis of venous and arterial thrombosis and prevention of myocardial infarction for high-risk patients. Synthetic thrombin inhibitors have a long history; initial compounds were derived from electrophilic ketone- and aldehyde-analogs of arginine. First potent leads of non-covalent inhibitors were developed in the early eighties, which were further optimised in the nineties, after the X-ray structure of thrombin became available. In the meantime a huge number of highly active and selective inhibitors has been published, however, only a few of them have an appropriate pharmacokinetic and pharmacodynamic overall profile, which could justify their further development. Very recently, with Ximelagatran a first orally available thrombin inhibitor has been approved in France for the prevention of venous thromboembolic events in major orthopaedic surgery after successful clinical phase III. However, it still has to be awaited, whether the extensive clinical use of Ximelagatran can demonstrate for the first time that direct thrombin inhibitors offer a real benefit in terms of efficacy and safety over established antithrombotic therapies. This review summarizes the current status of synthetic thrombin inhibitors with a focus on more recently published and promising new compounds.