Abstract 4031: Tissue Plasminogen Activator Exerts Neurotoxic Effects Despite Successful Thrombus Dissolution in Ischemic Stroke

Abstract

Introduction: Ischemic stroke, caused by thrombotic arterial occlusion, is a leading cause of death and disability. Tissue plasminogen activator (TPA) reduces neurological disability post-stroke in humans but its therapeutic effect is limited by ischemic time and hemorrhagic risk. Of concern, mechanical occlusion models of stroke suggest that TPA exerts direct neurotoxic effects, but the relevance of these non-thrombotic models to human stroke is controversial. Methods: We examined the integrated effects of TPA dose and ischemic time in a thromboembolic model of middle cerebral artery (MCA) stroke with translational relevance to human stroke. In anesthetized mice, blood flow was assessed by laser Doppler, neuronal cell death was quantified by TTC staining and thrombus dissolution was measured with 125I-fibrin. Results: MCA thromboembolism reduced ipsilateral hemispheric blood flow by ∼80%. Control, placebo-treated, mice had significant infarcts (27.5±2.8%) and limited thromboembolus dissolution (20.6 ± 2.5%). Standard dose TPA given after 15 min. of ischemia, dissolved the thromboembolus (63.3±3.6 %) and reduced neuronal cell death (1.3± 0.7 %) without increasing hemorrhage (0.02 ± 0.02 %). TPA given after 1 hr. or 2.5 hrs. of ischemia also extensively dissolved the MCA thromboembolus (69.5 ± 8.1% and 64.6 ± 1.5%, respectively, p< 0.001). However, after 1 hr. TPA failed to reduce neuronal cell death and, after 2.5 hrs. of ischemia, TPA significantly increased stroke infarct size (p<0.01). Similarly, after 15 min. or 1 hr. TPA did not cause hemorrhage, but after 2.5 hrs of ischemia TPA caused significant hemorrhage (p<0.01). Lower dose TPA (5-fold) caused less hemorrhage (p<0.01) and less fibrinolysis (21.6 ± 2.2 vs. 64.6 ± 1.5, p<0.001) than standard dose TPA after 2.5 hrs., but it also significantly increased neuronal damage (p<0.01) vs. controls. Conclusions: TPA dissolved MCA thromboemboli independent of duration of ischemia. However, TPA only reduced stroke size within a narrow time window (15 min.); after 2.5 hrs., TPA markedly enhanced neuronal cell death and brain hemorrhage. Thus, in a model of thromboembolic stroke with translational relevance, despite successful dissolution of thromboemboli, TPA exerts neurotoxic effects related to dose and duration of ischemia.