CHIR-265 is a potent selective inhibitor of c-Raf/B-Raf/mutB-Raf that effectively inhibits proliferation and survival of cancer cell lines with Ras/Raf pathway mutations

4855 The Ras/Raf/MEK/ERK signal transduction cascade is a very compelling pathway to disrupt in several cancer types because of its role as a vital mediator of cell proliferation, survival and other aspects of cellular behavior that contribute to the transformed phenotype. The Raf serine/threonine kinases are central to this signaling pathway and the fact that activating mutations often occur in B-Raf (melanoma, low-grade ovarian, papillary thyroid) underscores the importance of this target in cancer. CHIR-265 is an orally-bioavailable, selective, potent inhibitor of C-Raf, wild type B-Raf and mutant (V600E) B-Raf (IC 50 range from 3 to 60 nM). Concentrations of CHIR-265 that effectively block phosphorylation of Raf’s downstream substrates MEK and ERK in cells, also kill melanoma and colorectal cancer cell lines harboring B-Raf mutations independent of PTEN mutation status. EC 50 values for p-ERK inhibition range from 140 to 300 nM in SK-MEL-28 (human melanoma, mut B-Raf V600E , PTEN +, P53 -), Malme-3M (human melanoma, mut B-Raf V600E , PTEN +, P53 wt), and A375M (human melanoma, mut B-Raf V600E , PTEN -, P53 wt). Raf kinase inhibition by CHIR-265 in mutant B-Raf melanoma cell lines causes cell cycle arrest and induces apoptosis, mimicking the effect of Raf RNAi in these cells. Additionally, CHIR-265 inhibits vascular endothelial growth factor receptor type 2 (VEGFR-2). Potent inhibition of phosphorylation of VEGFR-2 and proliferation of VEGF-stimulated hMVEC (EC 50 = 30 &20 nM) indicate that CHIR-265 is a potent inhibitor of both Raf and the pro-angiogenic kinase, VEGFR. Consistent with its in vitro activity, CHIR-265 causes tumor regression and dose-dependent tumor growth inhibition in B-Raf-driven human melanoma xenograft model. The demonstration that CHIR-265 is very effective ( in vitro and in vivo ) in inhibiting Raf enzymes, key kinases for cancer cell growth and survival, suggests that this compound is an excellent clinical candidate to treat Raf driven cancer such as malignant melanoma.