Activation of blood coagulation by plaque rupture: mechanisms and prevention

Inhibition of thrombosis is proving to be an important treatment goal in many clinical situations, including coronary thrombolysis, angioplasty, and unstable angina. Heparin is a potent inhibitor of thrombin and thrombin generation, but its ability to accelerate thrombolysis, prevent acute reocclusion after vascular injury in angioplasty, and prevent myocardial infarction in unstable angina is relatively limited, possibly because clot-bound thrombin plays an important role in these clinical situations. Thus, when thrombin binds to fibrin, it remains enzymatically active and relatively impervious to inactivation by heparin or other fluid-phase inhibitors. However, direct thrombin inhibitors--such as D-Phe-L-Pro-L-Arg-CH2Cl (PPACK), hirudin, hirugen, and hirulog--inhibit free and clot-bound thrombin with equal efficacy, presumably because their sites of interaction are not masked when thrombin binds to fibrin. Advanced clinical trials suggest that the direct thrombin inhibitors and 7E3, an inhibitor of platelet glycoprotein IIb/IIIa, will soon be incorporated into the armamentarium against arterial thrombosis.