Peptidomimetic antagonist of the integrin α(v)β₃ inhibits leydig cell tumor growth and the development of hypercalcemia of malignancy

The integrin α(v)β₃ interacts with the arginine-glycine-aspartic acid (RGD) tripeptide recognition sequence of a variety of extracellular matrix proteins. Recent studies show that α(v)β₃ plays an important role in tumor-induced angiogenesis and tumor growth and that antagonists of α(v)β₃ inhibit angiogenesis processes that include endothelial cell adhesion and migration. Consequently, we reason that an RGD-based peptidomimetic antagonist of α(v)β₃ might inhibit tumor angiogenesis and tumor growth in vivo. An RGD-peptidomimetic library was screened to identify antagonists of vitronectin binding to α(v)β₃, and the compounds chosen were modified to produce selective and potent inhibitors of α(v)β₃. 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(v)β₃ and the closely related platelet receptor α(IIb)β₃, in a dose-responsive manner. SC-68448 inhibited vitronectin binding to α(v)β₃ (IC₅₀, 1 nM) and fibrinogen binding to the platelet receptor α(IIIb)β₃ (IC₅₀, >100 Nm), demonstrating that SC-68448 was 100-fold more potent as an inhibitor of α(v)β₃ versus α(IIb)β₃. In cell-based studies, SC-68448 inhibited α(v)β₃-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(v)β₃.