Antithrombotic strategies targeting thrombin activities, thrombin receptors and thrombin generation

Thrombin mediates acute vascular thrombosis and subsequent vascular lesion formation following mechanical denuding injury or spontaneous atherosclerotic plaque rupture. In the process of generating thrombin Factor VII/VIIa binds avidly with tissue factor (TF) exposed on cellular membranes, and coagulation serine proteases are sequentially cleaved via macromolecular catalytic complexes on phospholipid surfaces. Thrombin activates platelets, blood leukocytes, endothelium and vascular smooth muscle cells (SMCs) by cleaving G protein-coupled thrombin receptors (TRs), leading to SMC intimal proliferation and synthesis of extracellular matrix in the local formation of stenosing neointimal vascular lesions. Therapeutic strategies include inactivation of bound thrombin, inhibition of TR activation by thrombin, and interruption of thrombin generation. In patients having orthopedic surgery, inactivating bound thrombin with direct antithrombins markedly reduces venous thromboembolic events, compared with heparin or its derivatives, without significant impairment of hemostasis. However, acute coronary syndrome patients are not benefitted when given systemic direct antithrombins at safe levels, because interrupting TR-dependent platelet thrombosis demands systemic levels of direct antithrombins that concurrently compromise hemostatic function. Local drug delivery strategies have yet to be explored. In preclinical studies: a) enhancing the formation of endogenous activated Protein C (APC) by Protein C-selective thrombin mutants produces antithrombotic levels of APC; b) inhibiting thrombin activation of TRs abolishes platelet recruitment in arterial thrombogenesis in nonhuman primates, while sparing fibrin formation in hemostatic plugs; and c) preventing thrombin generation by inhibiting precursor serine protease function interrupts the formation of both acute thrombosis and chronic stenotic lesions after denuding vascular damage without significant hemostatic compromise. TF antagonists appear to have a highly favorable efficacy:safety therapeutic relationship for preventing the formation of thrombosis and vascular lesions.