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 (IC₅₀ 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. EC₅₀ 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 (EC₅₀= 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.