Targeting integrins: insights into structure and activity of cyclic RGD pentapeptide mimics containing azabicycloalkane amino acids

A small library of cyclic RGD pentapeptide mimics incorporating stereoisomeric 5,6- and 5,7-fused bicyclic lactams was synthesized. This library Was found to contain high-affinity ligands for the alpha(v)beta(3) integrin. The aim of this study was to investigate activity, selectivity, and structure of these ligands in order to identify new specific alpha(v)-integrin antagonists that could be evaluated as tumor angiogenesis inhibitors. In vitro screening, including receptor-binding assays to purified alpha(v)beta(3), alpha(v)beta(5), and alpha(5)beta(1) integrins, and platelet aggregation assay, revealed ST1646 as a potent, highly selective alpha(v)beta(3)/alpha(v)beta(5) integrin antagonist. Structure determination of the cyclic RGD pentapeptide mimics performed by a combination of NMR spectroscopy, and molecular mechanics and dynamics calculations showed a strong dependence of the RGD cyclopeptide conformation on lactain ring size and stereochemistry. ST1646 revealed the highest ability within the library to adopt the proper RGD orientation required for binding to the alpha(v)beta(3) integrin, as deduced from the recently solved crystal structure of the extracellular segment of integrin alpha(v)beta(3) in complex with a cyclic pentapeptide ligand.