The integrin α_vβ₃ interacts with the arginine-glycine-aspartic acid (RGD) tripeptide recognition sequence of a variety of extracellular matrix proteins. Recent studies show that α_vβ₃ plays an important role in tumor-induced angiogenesis and tumor growth and that antagonists of α_vβ₃ inhibit angiogenic processes that include endothelial cell adhesion and migration. Consequently, we reasoned that an RGD-based peptidomimetic antagonist of α_vβ₃ might inhibit tumor angiogenesis and tumor growth in vivo. An RGD-peptidomimetic library was screened to identify antagonists of vitronectin binding to α_vβ₃, and the compounds chosen were modified to produce selective and potent inhibitors of α_vβ₃. One of these compounds, β-[[2-2-[[[3-[(aminoiminomethyl)amino]-phenyl]carbonyl]amino]acetyl]amino]-

3,5-dichlorobenzenepropanoic acid (SC-68448), inhibited vitronectin binding to both α_vβ₃ and the closely related platelet receptor, α_IIbβ₃, in a dose-responsive manner. SC-68448 inhibited vitronectin binding to α_vβ₃ (IC₅₀, 1 nm) and fibrinogen binding to the platelet receptor α_IIbβ₃ (IC₅₀, >100 nm), demonstrating that SC-68448 was 100-fold more potent as an inhibitor of α_vβ₃ versus α_IIbβ₃. In cell-based studies, SC-68448 inhibited α_vβ₃-mediated endothelial cell proliferation in a dose-dependent manner but did not inhibit tumor cell proliferation, suggesting that effects on endothelial cell proliferation were not due to SC-68448-induced cytotoxicity. In accord with these results, SC-68448 inhibited angiogenesis in vivo in a basic fibroblast growth factor-induced rat corneal neovascularization model. A xenogeneic severe combined immune deficiency mouse/rat Leydig cell tumor model was developed for testing SC-68448 as an inhibitor of tumor growth in vivo. Rat Leydig cell tumors grew rapidly in severe combined immune deficiency mice and produced humoral hypercalcemia of malignancy. SC-68448 inhibited the growth of the tumors in mice by up to 80% and completely blocked the development of hypercalcemia. Together, these results demonstrate the feasibility of antitumor therapies based upon the development of nontoxic small molecule pharmacological antagonists of integrin α_vβ₃.