PHARMACOPHORE REFINEMENT OF GPIIB IIIA ANTAGONISTS BASED ON COMPARATIVE-STUDIES OF ANTIADHESIVE CYCLIC AND ACYCLIC RGD PEPTIDES/

Structurally guided design approaches to low-molecular-weight platelet aggregation antagonists addressing the platelet-associated heterodime ric cell surface receptor gpIIb/IIIa rely on comparative studies of an ensemble of conformationally and biologically characterized compounds , since no high-resolution structure of the receptor system is availab le. We report a classical indirect and comparative pharmacophore refin ement approach based on a series of small cyclic Arg-Gly-Asp (RGD) pep tides as gpIIb/IIIa-fibrinogen interaction antagonists. These peptides have previously been investigated as potent and selective tumor cell adhesion inhibitors. The definition of geometrical descriptors classif ying the RGD peptide conformations and their subsequent analysis over selected RGD- and RXD-containing protein structures allows for a corre lation of distinct structural features for platelet aggregation inhibi tion. An almost parallel alignment of the Arg and Asp side chains was identified by a vector analysis as being present in all active cyclic hexa- and pentapeptides. This orientation is induced mainly by the con straint of backbone cyclization and is not of any covalent tripeptide- inherent origin, which was rationalized by a 500 ps high-energy MD sim ulation of a sequentially related linear model peptide. The incorporat ion of the recognition tripeptide Arg-Gly-Asp into the cyclic peptide templates acted as a filter mechanism, restricting the otherwise free torsional relation of both side chains to a parallel orientation. Base d on the derived results, several detailed features of the receptor bi nding site could be deduced in terms of receptor complementarity. Thes e findings should govern the design of next-generation compounds with enhanced activities. Furthermore, the complementary stereochemical cha racteristics of the substrate can be used as boundary conditions for p seudoreceptor modelling studies that are capable of reconstructing a h ypothetical binding pocket, qualitatively resembling the steric and el ectronic demands of gpIIb/IIIa. It is interesting to note that these f eatures provide clear differentiation to requirements far inhibition o f alpha(nu)beta(3), substrate binding. This can account for the extrem ely high selectivity and activity of some of our constrained peptides for either the alpha(IIb)beta(3) or the alpha(nu)beta(3) receptor.