Discovery of a Novel, Potent, and Specific Family of Factor Xa Inhibitors via Combinatorial Chemistry

ABSTRACT: A series of low molecular weight peptide inhibitors of factor Xa, unrelated to any previously described, was identified by screening a combinatorial peptide library composed of L-amino acids. The minimal inhibitory sequence is a tripeptide, L-tyrosinyl-L-isoleucyl-L-arginyl, which competitively inhibits the hydrolysis of small chromogenic substrates by factor Xa but binds in an orientation which prevents a productive nucleophilic attack by serine 195 of the catalytic triad on the carbonyl carbon of the carboxy-terminal arginine. The initial leads identified in an octamer combinatorial peptide library ranged in potency from 4 to 15 íM. These peptides were modified into peptide mimetics with a greater than 1000-fold increase in potency while retaining unusual selectivity for factor Xa over the related serine proteases thrombin, factor VIIa/tissue factor, plasmin, activated protein C, kallikrein, and trypsin. One of the most potent analogues, SEL 2711, with a Ki of 0.003 íM for factor Xa and 40 íM for thrombin, is active in in Vitro and ex ViVo coagulation assays, suggesting the potential application of these inhibitors in anticoagulant therapy. Hemostasis limits blood loss by the complex interaction of two interdependent processes. The primary hemostatic mechanism, platelet activation and formation of a platele