Advances in Anticoagulation Therapy: The Role of Selective Inhibitors of Factor Xa and Thrombin in Thromboprophylaxis after Major Orthopedic Surgery

A new generation of antithrombotic agents that target a single enzyme within the procoagulant cascade is making its way into mainstream clinical practice. Borrowing selectively from the properties of their parent anticoagulant, unfractionated heparin, as well as from those of peptide anticoagulants from reptile or insect venoms, designers of the new drugs have created targeted inhibitors of thrombin, factor Xa, or other specific factors in the procoagulant pathways. These new agents promise efficacy and safety profiles far more favorable than those of conventional anticoagulants for thromboprophylaxis after major orthopedic surgery and require no laboratory monitoring of drug efficacy in this setting. Ximelagatran, the oral "prodrug" of the direct thrombin inhibitor melagatran, is in phase III of clinical development. Clinical trials using various dosages of ximelagatran, sometimes preceded by subcutaneous melagatran, as thromboprophylaxis after total hip or knee replacement surgery have suggested efficacy equal to or better than that of warfarin and enoxaparin. The best dosing regimen and optimal timing of first dose for melagatran and ximelagatran remain to be determined, as do the mechanism and impact of drug disturbance of hepatic function. Fondaparinux, a selective, synthetic inhibitor of factor Xa, has been shown in large clinical trials to be superior to low-molecular-weight heparins and is approved as a fixed once-daily subcutaneous 2.5-mg dose for thromboprophylaxis for hip or knee replacement surgery and after hip fracture repair. Fixed-dose fondaparinux 2.5 mg initiated 6 to 8 hours after surgery achieved superior efficacy and comparable safety in head-to-head comparisons with enoxaparin for the prevention of venous thromboembolism after major orthopedic surgery. Fondaparinux is the only agent approved for use in hip fracture patients in the United States at this time and has recently gained approval for extended prophylaxis in this patient population.