Towards safer thrombolytic therapy

Plasminogen activators (PA) are unique agents that are currently applied as thrombolytic therapy to achieve rapid vascular reperfusion. Regimens of PA plus anticoagulants and antiplatelet drugs have attained a high degree of sophistication and predictable rates of positive clinical outcomes for acute myocardial infarction (MI), ischemic stroke, pulmonary embolism (PE), deep vein thrombosis (DVT), and thrombosed catheters. Included in the repertoire are newly approved mutants of tissue plasminogen activator (TPA), which have biochemical advantages that allow for bolus administration. Yet, despite tremendous effort devoted to enormous trials to establish the clinical efficacy of these agents in acute MI, mortality results are not superior to those with native TPA or streptokinase (SK). Furthermore, all PAs have the potential for hemorrhagic complication, most critically intracranial hemorrhage (ICH), occurring in 0.9% of patients treated with native or mutant TPA. It is possible that a limit of clinical effectiveness has been reached, beyond which more potent PAs do not achieve greater benefit without a serious increase in risk of bleeding. A breakthrough is possible, however, if the risk of ICH could be avoided. One solution is the application of the direct-acting thrombolytic enzyme, plasmin. While intravenous plasmin is not effective when administered systemically, regional infusion to a thrombus induces local thrombolysis. Unlike the PAs, plasmin treatment should not cause hemorrhage from vascular trauma sites, as it is neutralized by antiplasmin in the blood. Animal studies are fully consistent with this approach, which offers potential for achieving a truly regional thrombolytic treatment.