Molecular mechanisms of fibrinolysis and their application to fibrin‐specific thrombolytic therapy

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Abstract
The fibrinolytic system comprises a proenzyme, plasminogen, which can be converted to the active enzyme, plasmin, which degrades fibrin. Plasminogen activation is mediated by plasminogen activators, which are classified as either tissue‐type plasminogen activators (t‐PA) or urokinase‐type plasminogen activators (u‐PA). Inhibition of the fibrinolytic system may occur at the level of the activators or at the level of generated plasmin. Plasmin has a low substrate specificity, and when circulating freely in the blood it degrades several proteins including fibrinogen, factor V, and factor VIII. Plasma does, however, contain a fast‐acting plasmin inhibitor, α2‐antiplasmin, which inhibits free plasmin extremely rapidly but which reacts much slower with plasmin bound to fibrin. A “systemic fibrinolytic state” may, however, occur by extensive activation of plasminogen and depletion of α2‐antiplasmin. Clot‐specific thrombolysis therefore requires plasminogen activation restricted to the vicinity of the fibrin. Two physiological plasminogen activators, t‐PA and single‐chain u‐PA (scu‐PA) induce clot‐specific thrombolysis, via entirely different mechanisms, however. t‐PA is relatively inactive in the absence of fibrin, but fibrin strikingly enhances the activation rate of plasminogen by t‐PA. This is explained by an increased affinity of fibrin‐bound t‐PA for plasminogen and not by alteration of the catalytic rate constant of the enzyme. The high affinity of t‐PA for plasminogen in the presence of fibrin thus allows efficient activation on the fibrin clot, while no significant plasminogen activation by t‐PA occurs in plasma. scu‐PA has a high affinity for plasminogen (Km = 0.3 μM) but a low catalytic rate constant (kcat = 0.02 sec−1). However, scu‐PA does not activate plasminogen in plasma in the absence of a fibrin clot, owing to the presence of (a) competitive inhibitor(s). Fibrin‐specific thrombolysis appears to be due to the fact that fibrin reverses the competitive inhibition. The thrombolytic efficacy and fibrin specificity of natural and recombinant t‐PA has been demonstrated in animal models of pulmonary embolism, venous thrombosis, and coronary artery thrombosis. In all these studies intravenous infusion of t‐PA at sufficiently high rates caused efficient thromblysis in the absence of systemic fibrinolytic activation. The efficacy and relative fibrinogen‐sparing effect of t‐PA was recently confirmed in three multicenter clinical trials in patients with acute myocardial infarction. Intravenous infusion of 0.5–1 mg of t‐PA per kg body weight over 1–3 hr resulted in coronary reperfusion in approximately 70% of patients. It raised the plasma level about 1,000‐fold but was associated with an average decrease of the plasma fibrinogen level by 30%. Specific thrombolysis by scu‐PA has also been demonstrated in animal models of pulmonary embolism, venous thrombosis, and coronary thrombosis, Again, intravenous infusion of scu‐PA at sufficiently high rates caused thrombolysis in the absence of systemic fibrinolytic activation. We have treated six patients with acute myocardial infarction with scu‐PA and have obtained coronary reperfusion during intravenous infusion of 40 mg scu‐PA over 60 min in four of the patients and during subsequent intracoronary infusion in one additional patient. A decrease of fibrinogen to 25% of the preinfusion value was observed in one patient.