Development Of Direct Thrombin Inhibitors Research Articles

Anaesthetic management of a patient with thrombocytopenia

New insights into the role of platelets in haemostasis have enabled a better understanding of the pathophysiology of conditions associated with thrombocytopenia. The development of new drugs and the advances in blood transfusion technology have improved the clinical management of patients with thrombocytopenia. The aim of this article is to provide a concise review of major advances in the perioperative management of patients with thrombocytopenia. Thrombocytopenia is a symptom, and the underlying cause must be evaluated. Immunotherapy, corticosteroids, immunosuppressive drugs and thrombopoietic growth factors can increase the number of platelets in thrombocytopenic patients. Further, a better understanding of the pathophysiology of heparin-induced thrombocytopenia type II and the development of direct thrombin inhibitors have improved the management of these patients, especially in the perioperative period. The lack of clinical methods for predicting which type of patients with thrombocytopenia are at risk of bleeding and the effectiveness of various platelet preparations call for research initiatives to provide better guidelines for transfusion practice. Controlled clinical trials are required to evaluate strategies for the prophylactic use of platelets and thrombopoietic factors in idiopathic thrombocytopenic purpura and the use of direct thrombin inhibitors in patients with heparin-induced thrombocytopenia.

Direct thrombin inhibitors - a survey of recent developments.

Thrombin is a plasma serine protease that plays a key role in coagulation and hemostasis but also in thromboembolic diseases. Direct thrombin inhibitors could, therefore, be beneficial for future anticoagulant therapy in the prophylaxis of venous and arterial thrombosis as well as myocardial infarction. However, development of direct thrombin inhibitors has brought researchers more heartache than success. The most recent setback came this year when AstraZeneca withdrew Ximelagatran, the first orally bioavailable direct thrombin inhibitor that had received regulatory approval (France, 2003), after reports of serious hepatoxicity in a fraction of patients. This review describes the status of direct thrombin inhibitors, focusing on drug candidates that are at present in clinical trials. In addition, some more recent research strategies in the design of novel direct thrombin inhibitors are discussed, which may very well contribute to future developments of potent anticoagulants.

Role of Thrombin Exosites in Protection from Inhibition by Antithrombin in the Presence of Heparin and Fibrin.

The γ-chain of fibrinogen exists in two forms, γA and γ′, such that circulating fibrinogen consists of two populations, γA/γA (90%) and γA/γ′ (10%). The 16 amino acid extension at the COOH-terminus of the γ′ chain contains numerous negatively-charged residues. This alteration endows γA/γ′-fibrin (Fn) with a greater capacity to bind thrombin (IIa), a feature that may render thrombi prothrombotic. The purpose of this study was to explore how the various mechanisms by which IIa binds to Fn impact on IIa protection from inactivation by antithrombin/heparin. IIa binds weakly to γA/γA-Fn utilizing exosite 1. IIa binds with higher affinity to γA/γ′-Fn due to the additional exosite 2/γ′ interaction. In the presence of heparin, IIa can bind to γA/γA-Fn with high affinity by forming a ternary complex wherein heparin bridges IIa to Fn via exosite 2, thereby heightening the interaction of IIa with Fn via exosite 1. Consequently, the amount of IIa bound to γA/γA-Fn clots increases in the presence of heparin. Formation of the γA/γA-Fn/heparin/IIa ternary complex reduces the second order rate constant of IIa inhibition by antithrombin (AT) 11-fold compared with the heparin-catalyzed rate of inhibition of free IIa (1.1x108 M−1 min−1). This reduction reflects the inability of AT-bound heparin to access exosite 2. When γA/γ′-Fn is used in place of γA/γA-Fn, the heparin-catalyzed rate is reduced 55-fold. The enhanced protection with γA/γ′-Fn is due to the exosite 2-mediated interaction of IIa with the γ′-chain because addition of an antibody against the γ′ sequence that blocks this interaction reduces the protection from AT to the level seen with γA/γA-Fn. Thus, both γA/γ′-Fn/IIa and γA/γA-Fn/heparin/IIa complexes restrict the access of heparin to exosite 2, thereby impairing inhibition by heparin/AT. Heparin cofactor II (HCII) also utilizes heparin to bridge to IIa, but, unlike AT, HCII must also directly engage exosite 1 to effect IIa inhibition. Consequently, the heparin-catalyzed rate of IIa inhibition by HCII is reduced 27-fold in the presence of γA/γA-Fn compared with the 11-fold reduction with AT. To examine the contribution of the heparin-Fn interaction to this phenomenon, dermatan sulfate (DS) was used to catalyze HCII because, unlike heparin, DS does not bind Fn. γA/γA-Fn produced only a 5-fold reduction in the DS-catalyzed rate of IIa inhibition by HCII. This suggests that in the absence of heparin, occupation of exosite 1 by γA/γA-Fn only modestly impairs inhibition by HCII. In contrast, γA/γ′-Fn produced a 28-fold reduction in the DS-catalyzed rate of IIa inhibition by HCII. This protection is abolished by addition of the antibody that blocks binding of IIa to the γ′ chain. Therefore, although exosite 1 mediates IIa binding to both γA/γA- and γA/γ′-Fn, interaction of exosite 2 with the γ′-chain in γA/γ′-Fn heightens exosite 1-mediated binding. These findings provide independent confirmation that ligation of both exosites on IIa accentuates the affinity of the individual exosite 1 and 2 interactions. Thus, reactants that require access to either exosite will be restricted when IIa binding is mediated by both exosites as occurs with γA/γA-Fn/heparin or with γA/γ′-Fn. These results confirm that Fn can serve as a reservoir of active IIa and that bound IIa is protected from inhibition by circulating inhibitors. Our data also highlight the limitations of physiological inhibitors of IIa and validate the need for development of direct thrombin inhibitors.

The re-emergence of anticoagulation in coronary disease

The underlying pathophysiological process in acute coronary syndromes (ACS) is intracoronary thrombus formation, leading to varying degrees of occlusion of the culprit artery and the resulting clinical manifestations of ST-elevation myocardial infarction (complete occlusion) or unstable angina/non-ST-elevation myocardial infarction (partial occlusion). Given the central role that thrombin plays in the pathogenesis of ACS, antithrombin agents are an important component of treatment. Current antithrombotics, however, have important limitations. Antiplatelet agents such as aspirin or clopidogrel, alone or in combination, fail to suppress activation of the coagulation cascade and thrombin generation in patients with ACS, highlighting the need for more intensive antithrombin therapy. Use of indirect thrombin inhibitors such as the heparins (unfractionated heparin (UFH) or low-molecular weight heparins) are restricted by the need for parenteral administration, risk of thrombocytopenia, inability to inhibit fibrin-bound thrombin, and the potential for rebound activation of the coagulation cascade and further ischaemic events after discontinuation of therapy. Direct thrombin inhibitors are potent inhibitors of both free and fibrin-bound thrombin. Trials of the intravenous direct thrombin inhibitors have shown these agents to be more efficacious than UFH for the prevention of recurrent myocardial infarction with short-term (7 days) use, supporting the rationale for the further development of the direct thrombin inhibitors. The oral anticoagulants, the vitamin K antagonists (VKAs), in combination with low-dose aspirin have demonstrated superior clinical benefits compared with aspirin treatment alone when moderate-intensity anticoagulation (international normalized ratio 2–3) is achieved. Combination antithrombotic therapy with VKAs is, however, associated with an increased risk of bleeding in a relatively high proportion of patients who subsequently have to discontinue therapy, and the complexity of treatment is a significant drawback. The limitations of existing antithrombotic therapies have provided the stimulus for the development of the oral direct thrombin inhibitors. Ximelagatran, the first in this class of agents, has the potential to overcome the limitations of existing antithrombotic therapies and is currently undergoing extensive clinical evaluation in a range of indications, including ACS.

Direct thrombin inhibitors in acute coronary syndromes: principal results of a meta-analysis based on individual patients' data

To obtain more reliable and precise estimates of the effect of direct thrombin inhibitors in the management of acute coronary syndromes, including patients undergoing percutaneous coronary intervention, we undertook a meta-analysis based on individual patients' data from randomised trials comparing a direct thrombin inhibitor (hirudin, bivalirudin, argatroban, efegatran, or inogatran) with heparin. We included trials that involved at least 200 patients. The primary efficacy outcome was death or myocardial infarction, and the primary safety outcome was major bleeding. Data from individual trials were combined by use of a modified Mantel-Haenszel method. In 11 randomised trials, 35,970 patients were assigned up to 7 days' treatment with a direct thrombin inhibitor or heparin and followed up for at least 30 days. Compared with heparin, direct thrombin inhibitors were associated with a lower risk of death or myocardial infarction at the end of treatment (4.3% vs 5.1%; odds ratio 0.85 [95% CI 0.77-0.94]; p=0.001) and at 30 days (7.4% vs 8.2%; 0.91 [0.84-0.99]; p=0.02). This was due primarily to a reduction in myocardial infarctions (2.8% vs 3.5%; 0.80 [0.71-0.90]; p<0.001) with no apparent effect on deaths (1.9% vs 2.0%; 0.97 [0.83-1.13]; p=0.69). Subgroup analyses suggested a benefit of direct thrombin inhibitors on death or myocardial infarction in trials of both acute coronary syndromes and percutaneous coronary interventions. A reduction in death or myocardial infarction was seen with hirudin and bivalirudin but not with univalent agents. Compared with heparin, there was an increased risk of major bleeding with hirudin, but a reduction with bivalirudin. There was no excess in intracranial haemorrhage with direct thrombin inhibitors. Direct thrombin inhibitors are superior to heparin for the prevention of death or myocardial infarction in patients with acute coronary syndromes. This information should prompt further clinical development of direct thrombin inhibitors for the management of arterial thrombosis.

Development of Direct Thrombin Inhibitors in Comparison with Glycosaminoglycans

The progress in molecular biology has stimulated interest in the structure and function of thrombin. It has improved the understanding of its central role in thrombogenesis and has clarified the molecular events of inhibitor binding. This development has resulted in the production of recombinant hirudins and the design of hirudin analogues. It has also allowed the molecular design of synthetic antithrombins and encouraged the development of these products for clinical use. All pharmacological aspects speak in favor of the use of the direct thrombin inhibitors as antithrombotic agents, especially in the potential indications in which thrombin plays a crucial role in the pathogenesis. If their apparent advantages in comparison to glycosaminoglycans can be shown effectively, the direct thrombin inhibitors may become the drug of choice for certain indications.