Otamixaban is a synthetically derived parenteral fXa inhibitor currently in late stage clinical development at Sanofi-Aventis for the management of acute coronary syndrome. Otamixaban is a potent (Ki = 0.5 nM), selective, rapid acting, competitive and reversible fXa inhibitor that effectively inhibits both free and prothrombinase-bound fXa. Factor Xa (fXa) is a critical serine protease situated at the confluence of the intrinsic and extrinsic pathways of the blood coagulation cascade. FXa catalyzes the conversion of prothrombin to thrombin via the prothrombinase complex. Its singular role in thrombin generation, coupled with its potentiating effects on clot formation render it an attractive target for therapeutic intervention. In vivo experiments have demonstrated that Otamixaban is highly efficacious in rodent, canine and porcine models of thrombosis. In addition, recent clinical findings indicate that Otamixaban is efficacious, safe and well tolerated in humans and therefore has considerable potential for the treatment of acute coronary syndrome. Following the results of the Treatment of non-ST elevation Acute coronary syndrome with otamixaban, Sanofi has decided to discontinue the investigational programme with otamixaban, due to efficacy lower than expected. Otamixaban did not show superior benefit/risk to the combination of unfractionated heparin.