Clopidogrel enhancement of rt-PA thrombolysis in a thrombo-embolic model of cerebral ischemia in rats

Objective: The effects of postembolic treatment with clopidogrel-a thienopyridine derivative and inhibitor of platelet aggregation-alone and in combination with rt-PA were investigated in a thromboembolic model of monofocal cerebral ischemia in rats. Methods: After injection of a single in vitro prefabricated fibrin-rich autologous macroclot (400 mu m x 4 mm) into the internal carotid artery of Sprague Dawley rats, the animals were assigned to one of four intravenous treatment groups: (1) continuous saline infusion over 45 min starting 60 min after embolization (control); (2) rt-PA as a continuous infusion at 6 mg/kg for 45 min starting 60 min after embolization; (3) clopidogrel at 20 mg/kg immediately after embolization followed by a continuous saline infusion for 45 min starting 60 min after injection; (4) rt-PA as in group 2 plus clopidogrel as in group 3. Results: Embolization produced occlusions of the ipsilateral middle cerebral artery or the internal carotid artery at the origin of the middle cerebral artery. Occlusions were stable in the control group. Administration of rt-PA at a thrombolytic threshold dose together with clopidogrel resulted in a recanalization of the middle cerebral artery as demonstrated by angiographic control immediately after treatment. Recanalization was associated with a decrease in the neurological deficit (30% vs control) and in the subcortical and total infarct volumes (18 and 25%) at 24 h albeit the differences did not attain significance. No gross intracranial hemorrhages were observed following clopidogrel/rt-PA coadministration. Postembolic treatment with clopidogrel alone tended to reduce infarct volume and to improve neurological outcome. Tail transection bleeding time was prolonged in the clopidogrel group (4.7-fold) and in both rt-PA-treated groups (> g-fold). Ex vivo platelet aggregation was inhibited by clopidogrel treatment but not by rt-PA infusion. Conclusion: These data provide evidence for the first time that rt-PA thrombolysis after ischemic stroke can be enhanced by adjunctive postembolic antiplatelet therapy. This effect was not accompanied by gross intracranial hemorrhages.