Biological and molecular properties of a new αxβ3/αvβ5 integrin antagonist

The aim of the present study was to identify specific α v β 3 /α v β 5 integrin antagonists active on tumor-induced angiogenesis. To this purpose, in vitro integrin-binding assays were used to screen a library of conformationally constrained bicyclic lactam Arg-Gly-Asp-containing pseudopeptides. The results identified ST1646 as a high-affinity specific ligand for α v β 3 and α v β 5 integrins with negligible interacting with α 5 β 1 integrin. In all the assays, ST1646 was equipotent to or more potent than the well-characterized integrin antagonists c(RGDfV) and cyclo(Arg-Gly-Asp-D-Phe-[NMe]Val) (EMD121974). In the chorioallantoic membrane assay, topical administration of ST1646 was able to prevent the angiogenic responses elicited by recombinant fibroblast growth factor-2 or vascular endothelial growth factor. In addition, systemic administration of ST1646 in mice exerted a significant antiangiogenic activity on neovascularization triggered by mammary carcinoma MDA-MB435 cells implanted s.c. in a dorsal air sac via a (Millipore Filter Corporation, Bedford, MA) chamber. Moreover, ST1646 delivery via an osmotic pump inhibited the growth and vascularization of tumor xenografts originating from the injection of α v β 3 /α v β 5 -expressing human ovarian carcinoma cells in nude mice. In agreement with the biochemical and pharmacologic studies, Monte Carlo/Stochastic Dynamics simulation showed that the bicyclic scaffold in ST1646 forced the compound to assume a preferred conformation superimposable to the X-ray conformation of α v β 3 -bound EMD121974. Accordingly, computer-docking studies indicated that the ST1646-α v β 3 integrin complex maintains the ligand-receptor distances and interactions observed in the crystalline EMD121974-α v β 3 integrin complex. Taken together, these observations indicate that ST1646 represents a dual α v β 3 /α v β 5 integrin antagonist with interesting biochemical and biological features to be tested in cancer therapy.