Summary. The effect of recombinant human microplasmin was studied in ischemic stroke models in mice and in an extracorporeal loop thrombosis model in rabbits. Human microplasminogen (mPlg), which lacks the five 'kringle' domains of plasminogen was expressed with high yield in Pichia pastoris. It was purified, converted to microplasmin (mPli) and equilibrated with 5 mmol L À1 citrate, pH 3.1, yielding a stable preparation. In mice with middle cerebral artery (MCA) ligation, an intravenous (i.v.) bolus of 5.0 mg kg À1 mPli reduced infarct size at 24 h from 27 (26-30) to 25 (21-28) mm 3 (median and range, n ¼ 16 each, P ¼ 0.0001), whereas 4.0 mg kg À1 rt-PA and 40 mg kg À1 mPlg had no effect. Infarct reduction was observed with administration at 4 h after occlusion. In mice with MCA, infarct size at 24 h was reduced from 20 (14-30) to 9.1 (3.1-25) mm 3 with 5.0 mg kg À1 mPli (n ¼ 15 each, P <0.002) and to 11 (5.2-27) mm 3 with 4.0 mg kg À1 rt-PA (n ¼ 6; P ¼ 0.02). Infarct reduction was still observed at 10 h after occlusion with mPli but not with t-PA. In rabbits with radiolabeled clots in an extracorporeal arteriovenous loop, local infusion of 2.5 mg kg À1 mPli over 2 h, induced 51 AE 15% lysis (mean AE SD, n ¼ 11) vs. a control value of 23 AE 5.5%. mPli did not prolong template bleeding times, whereas equipotent doses of rt-PA were associated with extensive rebleeding. The potency of mPli in both models was similar to that of intact plasmin. These findings indicate that recombinant mPli may be useful for treatment of ischemic stroke and arterial thrombosis.

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