[From heparin to synthetic antithrombotic drugs]

Low molecular weight heparins (LMWH) have become the reference drugs for the prevention and treatment of venous thromboembolism. Among the newer antithrombotic agents, the pentasaccharide is the most advanced and promising alternative of low molecular weight heparins. The pentasaccharide is a synthetic and specific inhibitor of activated coagulation factor X (factor Xa). The pentasaccharide is identical to the active sequence of heparin which is present in about a third of commercially available heparin mixtures. The pentasaccharide binds specifically to antithrombin III (ATIII) thus potentiating its inhibitory activity on factor Xa. As a result, thrombin generation and the formation of fibrin and thus of thrombi is inhibited. After a large phase II dose finding clinical trial, four large clinical trials with pentasaccharide in the prevention of venous thromboembolism in hip or knee surgery have been conducted in Europe and North America. The results of these trials show that pentasaccharide prevents venous thromboembolism to a larger extent than enoxaparine with an overall risk reduction of 50% and no significant increase in bleeding. Among newer antithrombotic drugs, an oral, direct thrombin inhibitor, ximelagatran, has been developed and evaluated in the prophylaxis of venous thromboembolism in hip or knee surgery in comparison with enoxaparine with promising RESULTS. Oral direct thrombin inhibitors have the advantage of predictable dose response with the possibility of oral administration without laboratory monitoring.