Fibrinolysis: a Misunderstood Natural Defense Whose Therapeutic Potential Is Unknown

Abstract

Ever since tissue plasminogen activator (tPA) was approved for therapeutic fibrinolysis in 1987, it has been the fibrinolytic of choice. At the same time, it is also recognized that tPA never lived up to its clinical expectations and has more recently been replaced by percutaneous coronary intervention (PCI) as the treatment of choice for acute myocardial infarction (AMI). For other occlusive vascular diseases and for patients in remote areas, tPA remains an essential option. In view of the continued importance of fibrinolysis, it is disappointing that fibrinolysis never evolved beyond what it was when tPA replaced streptokinase (SK) 32 years ago. The endovascular procedure replacement for AMI treatment suffers from being technically demanding, time-consuming, and costly. An untested alternative fibrinolytic paradigm is that of the endogenous, physiological system, which is initiated by tPA but then is followed by the other natural plasminogen activator, urokinase plasminogen activator (uPA). In this combination, it is uPA rather than tPA that has the dominant function. This is also evident from gene knockout studies where deletion of uPA that it has the dominant effect. The fibrinolytic properties of tPA and uPA are complementary so that their combined effect is synergistic, especially when they are administered sequentially starting with tPA. Endogenous fibrinolysis functions without bleeding side effects and is ongoing. This is evidenced by the invariable presence in blood of the fibrin degradation product, D-dimer (normal concentration 110-250 ng/ml). This activator combination, consisting of a mini bolus of tPA followed by a 90-min proUK infusion, was once used to treat 101 AMI patients. Compared with the best of the tPA mega trials, this regimen resulted in an almost a doubling of the infarct artery patency rate and reduced mortality sixfold. To date, a second trial has not yet been done.

Keywords: Fibrinolysis; Thrombosis; Tissue plasminogen activator; Urokinase plasminogen activator.