5831 Tumor survival, growth, and metastasis depend on tumor cell proliferation and tumor angiogenesis. The novel bi-aryl urea, BAY 43-9006, is a dual RAF Kinase and VEGFR inhibitor that also possesses significant activity against several receptor tyrosine kinases involved in neovascularization and tumor progression, including, VEGFR-2, VEGFR-3, and PDGFR-β. This report describes the in vivo anti-tumor activity of BAY 43-9006 administered orally against subcutaneous (s.c., ectopic) and sub-renal capsule (SRC, orthotopic) models of a murine renal adenocarcinoma (Renca) and explores the mechanism of action for this novel agent. Female athymic mice (NCr-nu/nu) were implanted s.c. with Renca cells or in the SRC with 0.5 to 1 mm3 Renca tumor fragments, respectively. In a separate experiment, tumor fragments of a differing histological type, HCT-116 human colon adenocarcinoma, were also implanted in the SRC to assess the effect of the local micro-environment. At the end of treatment (14 days), kidneys were removed and gross surface photomicrographs taken. To determine the effect of BAY 43-9006 on tumor vasculature, immunohistochemical analyses using anti-murine CD-31 and anti-α-smooth muscle actin (αSMA) were performed and quantified using histomorphomometery (Soft Image System). Once daily oral dosing of BAY 43-9006 produced a dose-dependent tumor growth inhibition (TGI) against s.c.-implanted Renca tumors ranging from 30% at a dose of 7.5 mg/kg to 84% at a dose of 60 mg/kg. BAY 43-9006-treated tumors implanted in the SRC were noticeably smaller in size, opaque in color and displayed little vasculature relative to control tumors, which were vibrant in color, highly vascularized and grew much larger, virtually overwhelming the kidney. Immunohistochemical staining with anti-CD-31 or anti-αSMA antibodies confirmed the decrease in tumor vasculature following BAY 43-9006 treatment. A decrease in vasculature was observed in both ectopically- and orthopically-implanted Renca tumors. Similar inhibition of tumor neovascularization (CD-31 and αSMA) were observed when fragments of human colon tumor HCT-116 were implanted in the sub-renal capsule. No inhibition of phospho-histone H3, an indicator of tumor cell proliferation, was observed in BAY 43-9006-treated tumors. These results demonstrate that BAY 43-9006 potently inhibits the growth of both ectopically- and orthotopically-implanted Renca tumors and against HCT-116 tumor fragments implanted in the SRC, and indicate that a mechanism of action of BAY 43-9006 in these tumor models is via inhibition of tumor angiogenesis.