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Circulation, Vol 86, 548-552, Copyright © 1992 by American Heart Association

ARTICLES

A recombinant, chimeric enzyme with a novel mechanism of action leading to greater potency and selectivity than tissue-type plasminogen activator

JH Robinson, MJ Browne, JE Carey, PD Chamberlain, CG Chapman, DW Cronk, I Dodd, C Entwisle, AF Esmail and SB Kalindjian
Department of Biotechnology, SmithKline Beecham Pharmaceuticals, Surrey, England, UK.

BACKGROUND. Early intervention with thrombolytic agents has been shown unequivocally to reduce mortality after acute myocardial infarction. Presently used agents have disadvantages such as short half-life, immunogenicity, hypotension, and bleeding complications. Therefore, there is a need to develop improved thrombolytic drugs with novel mechanisms of action leading to improved properties. METHODS AND RESULTS. Hybrid plasminogen/tissue-type plasminogen activator (t-PA) complementary DNA was constructed and expressed in Chinese hamster ovary cells. The chimeric protein, comprising the fibrin-binding domains of plasminogen covalently linked to the catalytic domain of t- PA, was purified and evaluated in vitro and in vivo. The hybrid was inhibited rapidly in human and animal plasmas. The mediator of this rapid inhibition was shown to be alpha 2-antiplasmin. The active center of the hybrid could be protected by reversible active center acylation with a novel inverse acylating agent, 4'-amidinophenyl-4- chloroanthranilic acid (AP-CLAN). An acylated (CLAN-) hybrid was cleared from the bloodstream of guinea pigs at 0.35 +/- 0.02 ml/min.kg- 1 compared with a clearance rate of 36 +/- 4 ml/min.kg-1 for t-PA. The CLAN-plasminogen/t-PA hybrid was evaluated in a quantitative, "humanized" guinea pig pulmonary embolism model and shown to be approximately threefold more potent when given by bolus than an infusion of t-PA. Furthermore, the acylated hybrid was more fibrin selective than t-PA as determined by the relation between clot lysis and fibrinogen degradation. CONCLUSIONS. An acylated, recombinant plasminogen/t-PA hybrid has sufficiently slow clearance to be administered by bolus and is more potent and fibrin selective than t-PA in vivo.