Plasminogen mutants activated by thrombin. Potential thrombus-selective thrombolytic agents

Plasmin, the enzyme responsible for degradation of fibrin in blood clots and thus thrombolysis, is normally formed when its zymogen plasminogen is activated by cleavage of the Arg561-Val562 bond by specific plasminogen activators. We have altered the activation characteristics of plasminogen by substituting the P3, P2, and P1' cleavage site residues with sequences from thrombin-cleavable proteins to produce a novel thrombolytic agent which instead is activated by the blood clotting system. Plasminogen variants with thrombin cleavage sites from fibrinogen, the thrombin receptor, factor XIII, and factor XI were cleaved by thrombin with times to 50% cleavage of 28 h, 2.5 h, 5.7 min, and 3 min, respectively. In vitro clot lysis studies have shown that a variant in which the P3-P1' residues of plasminogen were substituted by the P7-P1' residues (Thr363-Ile370) from factor XI (T51) was sufficiently rapidly cleaved by thrombin to be activated by the endogenous thrombin produced by the coagulation cascade, resulting in rapid clot dissolution. Thrombin-activatable plasminogen therefore has the capacity to short circuit the physiological hemostatic mechanisms and produce fibrinolytic activity localized to the site of thrombin formation, that is, at the thrombus itself. The novel activation mechanism combined with the natural long circulating half-life of plasminogen gives this type of thrombolytic agent the potential for thrombus-selective plasmin generation and an extended duration of action.