Evaluation of thrombolytic agents

The mechanism of action of currently available thrombolytic agents, such as streptokinase, urokinase, alteplase (recombinant tissue-type plasminogen activator; rt-PA) and anistreplase (anisoylated plasminogen streptokinase activator complex; APSAC), involves conversion of inactive plasminogen to plasmin, a potent fibrinolytic. However, the relatively weak substrate specificity of first generation agents (streptokinase and urokinase) can result in a state of systemic fibrinolysis and associated bleeding complications. The second generation drugs such as alteplase were developed in an attempt to enhance fibrin specificity, so that only enzymatic conversion of fibrin-complexed plasminogen would take place, thus avoiding systemic fibrinolysis. Results from large clinical trials have failed to consistently show any significant differences between first and second generation thrombolytic agents in the incidence of bleeding. In the clinical setting, thrombolytic agents have been evaluated primarily in patients with acute myocardial infarction and have been shown to significantly reduce morbidity and mortality compared with conservative treatment. The focus of current and future research is to investigate these agents in patients with other vaso-occlusive or ischaemic conditions (e.g. stroke), and also to evaluate different drug administration regimens and the use of adjunctive therapies such as aspirin (acetylsalicylic acid) and heparin.