INHIBITION OF TUMOR-GROWTH BY TARGETING TUMOR ENDOTHELIUM USING A SOLUBLE VASCULAR ENDOTHELIAL GROWTH-FACTOR RECEPTOR

Vascular endothelial growth factor (VEGF) is a leading candidate for a n endogenous mediator of tumor angiogenesis, Recently, two endothelial cell surface receptors, flk-1 and flt-1, have been shown to mediate t he angiogenic activities of VEGF, In this study, we have evaluated whe ther a soluble VEGF receptor could suppress tumor angiogenesis and the reby inhibit tumor growth. A soluble VEGF receptor was constructed by fusing the entire extracellular domain of murine flk-1 to a six-histid ine tag at the COOH terminus (ExFlk.6His). In vitro, recombinant ExFlk .6His protein bound VEGF with high affinity (K-d, 16 nM) and blocked r eceptor activation in a dose-dependent manner and inhibited VEGF-induc ed endothelial cell proliferation and migration, ExFlk.6His bound to e ndothelia( cells only in the presence of VEGF, and cell surface cross- linking yielded a high molecular weight complex consistent with the VE GF-mediated formation of a heterodimer between ExFlk.6His and the endo genous VEGF receptor, In vivo, ExFlk.6His potently inhibited corneal n eovascularization induced by conditioned media from a rat mammary carc inoma cell line (R3230AC), Moreover, when ExFlk.6His protein was admin istered into a cutaneous tumor window chamber concomitantly with R3230 AC carcinoma transplants, tumor growth was inhibited by 75% (P < 0.005 ) and vascular density was reduced by 50% (P < 0.002) compared with co ntrol-treated tumors, These results demonstrate the potential of ExFlk .6His to inhibit VEGF action by a potent ''dominant-negative'' mechani sm and suggest that targeting VEGF action using a soluble receptor may be an effective antiangiogenic therapy for cancer and other ''angioge nic'' diseases.