DESIGN AND SYNTHESIS OF A KININOGEN-BASED SELECTIVE INHIBITOR OF THROMBIN-INDUCED PLATELET-AGGREGATION

Thrombin-induced platelet aggregation has been suggested to play an im portant role in reocclusion following thrombolytic therapy or angiopla sty for treatment of myocardial infarction. We previously demonstrated that thrombin-induced platelet aggregation is indirectly mediated by intracellularly activated calpain expressed on the platelet surface th rough the cleavage of aggregin, a putative ADP-receptor, and that high molecular weight kininogen (HK), a naturally occurring thiol protease inhibitor modulates thrombin-induced platelet aggregation. Considerin g the substrate specificity of calpain and the conserved sequence in H K, we studied selective inhibitors of thrombin-induced platelet aggreg ation by the affinity labeling approach with an S-3-nitro-2-pyridinesu lfenyl (Npys) group. H-Phe-Gln-Val-Val-Cys (Npys)-Gly-NH2, which combi nes chemical and structural features of calpain substrate specificity and the conserved sequence in HK, selectively inhibited thrombin-induc ed platelet aggregation. It did not inhibit the aggregatory effects of other platelet agonists, and did not inhibit amidolytic activity of t hrombin and thrombin-induced platelet shape change. The design and syn thesis of such inhibitors could lead to the development of a new class of inhibitors that selectively block thrombin-induced platelet aggreg ation.