Challenges and promises of developing thrombin receptor antagonists.

Despite the availability of dual antiplatelet therapy comprised of aspirin and clopidogrel, there is still significant unmet medical need for treating and preventing arterial thrombotic diseases. To achieve further reduction of cardiovascular events without exceeding bleeding tolerability and safety limits, novel antiplatelet strategies might need to trade in antiplatelet efficacy by partial inhibition of an important platelet activation pathway or by differentially targeting pathological versus physiological thrombogenesis pathways. Thrombin, the central enzyme in coagulation and the most potent platelet agonist tested in vitro, is one of the key factors driving the formation of occlusive thrombi. Platelet thrombin receptors, namely protease-activated receptor 1 (PAR-1) and protease-activated receptor 4 (PAR-4), act in concert to elicit robust platelet responses to thrombin. PAR-1 is the high affinity thrombin receptor and represents a novel antithrombotic target. PAR-4 is a low affinity thrombin receptor with less understood function. This review discusses the genetic and pharmacological evidence for PAR-1 target validation and highlights the progresses and challenges in developing oral PAR-1 antagonists, especially SCH 530348 from Merck/Schering-Plough and E-5555 from Eisai Co. Recent patents disclosing several novel chemical series of PAR-1 antagonists from Sanofi-Aventis and Pierre Fabre are also presented.