Design of potent and specific integrin antagonists. Peptide antagonists with high specificity for glycoprotein IIb-IIIa.

Members of the snake venon-derived, peptide family containing the Arg-Gly-Asp (RGD) amino acid sequence are among the most potent inhibitors of the binding of adhesive proteins to platelet glycoprotein (GP) IIb-IIIa. However, GPIIb-IIIa antagonists containing the RGD sequence are not integrin specific and inhibit the adhesive functions of many other RGD-dependent integrins. The single disintegrin peptide, barbourin, containing a conservative amino acid substitution of Lys (K) for Arg (R) in the RGD sequence, is however, highly specific for GPIIb-IIIa. Using this information we have tested the hypothesis that both structural and conformational elements of barbourin are important for its high affinity and selectivity for platelet GPIIb-IIIa by synthesizing a series of conformationally constrained, disulfide-bridged peptides containing the KGD amino acid sequence. Incorporation of the KGD sequence into a cyclic peptide template, followed by systematic optimization of the cyclic ring size, optimization of secondary hydrophobic binding site interactions, and the derivatization of the lysyl side chain functionality of the KGD sequence has resulted in peptide analogs which display inhibitory potency and GPIIb-IIIa selectivity comparable to that of barbourin. This study demonstrates that the specificity and potency of the disintegrin family of antagonists, in particular barbourin, can be mimicked by small, conformationally restrained peptides.