Recent insights on biochemical and molecular basis for developing antihaemostatic agents: A review

The normal coagulation process is initiated by disruption and exposure of the subendothelial components of blood vessels. Platelets adhere to subendothelium-bound von Willebrand factor via glycoprotein (GP) Ib complex. This initial interaction per se and the release of platelet agonists transduce signals that leads to the rise in intracellular Ca2+ which induces shape change, prostaglandin synthesis, release of granular contents and conformational changes in platelet Gp IIb-IIIa. Gp IIb-IIIa in activated platelets binds fibrinogen and other adhesive proteins and mediates platelet cohesion the primary haemostatic plug. Furthermore, the activated platelets due to aggregation, result in the formation of fibrin (secondary hemostasis). Normally the haemostatic process plays a delicate balance between keeping the blood in the fluid state to maintain flow and rapidly forming an occluding plug following vessel injury. Thrombosis occurs because of alteration in this delicate balance. Arterial thrombosis occurs in the setting of previous vessel wall injury mostly because of atherosclerosis, while venous thrombosis occurs in areas of stasis. The recent advances in understanding of the haemostatic process have led to a better understanding of the mechanism of action of many antithrombotic drugs and identification of new targets for drug development. The molecular target of the ticlopidine has been identified. Large numbers of IIb-IIIa inhibitors have been developed. The mechanism of action of heparin has been defined at the molecular level. As a result, a synthetic pentasaccharide, based on antithrombin-binding domain of heparin, has been developed and tested successfully in clinical trials. New generation direct thrombin inhibitors are being developed. Factor Xa has a critical position at the convergence of intrinsic and extrinsic pathway. The clinical tolerability and the efficacy of low molecular weight heparins has established that inhibition of further thrombin generation, by blocking factor Xa alone can be an effective way of preventing thrombus growth without inactivating thrombin. A large number of specific factor Xa inhibitors are under development. Some of these are in preliminary clinical trials and appear to be promising. Future clinical trials will determine whether these new drugs will provide better risk-benefit ratio in treatment of thrombotic disorders. Similarly role of thrombolytics has been clearly established in many diseases including coronary artery disease.