Tissue-type plasminogen activator mutants. Theoretical and clinical considerations.

W'ITithin a short time after the first report on ~ the successful cloning and expression of TV tissue-type plasminogen activator (t-PA),l several laboratories began working on creating new t-PA derivatives through site-directed mutagenesis.2-4 t-PA has unique properties that make it an attractive choice for thrombolytic therapy. t-PA is "fibrin directed"; that is, it preferentially activates the fibrinolytic proenzyme plasminogen into plasmin on fibrin thrombi. In contrast, streptokinase and urokinase, the older-used thrombolytic agents, indiscriminately activate thrombus as well as plasma plasminogen.5 Nevertheless, work on creating new t-PA mutants continued because the following shortcomings of the native enzyme were realized: 1) t-PA is not a very efficient plasminogen activator even on a fibrin thrombus; 2) although t-PA was anticipated to cause minimal bleeding complications, this prediction was not confirmed in major clinical trials in which t-PA was given in the doses necessary for effective coronary thrombolysis (100 mg per treatment); it proved as likely to cause minor and major bleeding complications as streptokinase and in some patients caused profound fibrinogen depletion and a serious coagulation defect6-8; 3) t-PA, even when given concomitantly with or followed by heparin, carried a high risk of reocclusion (10-20%) once reperfusion has been established; 4) the activity of infused t-PA is attenuated because it is neutralized by a number of plasma protease inhibitors, most notably plasminogen activator inhibitor 1 (PAI-1)9; and 5) t-PA possesses a very short biologic half-life (<5 minutes) in humans and, therefore, must be administered in high doses as a constant intravenous infusion. The study by C.E.L. Lucore, S. Fujii and