Therapeutic strategies for targeting BRAF in human cancer

Constitutive ERK activation is a common finding in human cancer and is often the result of activating mutations of BRAF and RAS. BRAF missense mutations occur in approximately 8% of human tumors, most frequently in melanoma, papillary thyroid cancer and colon cancer. Mutations in BRAF have been found predominantly in tumors in which RAS is commonly mutated but concurrent mutations of both BRAF and RAS are extremely rare. Though over 40 different kinase domain mutations in BRAF have been identified, a single base-pair substitution in exon 15 at codon 600 (V600E) is found in over 80% of cases. These mutations cluster in the glycine-rich loop and activation segments of the kinase and are predicted to induce kinase activation by disrupting the inhibitory glycine-rich loop/activation segment interaction which characterizes the inactive conformation. The majority of mutations identified cause constitutive kinase activation with the V600E mutation demonstrating approximately 500-fold greater kinase activity than wild-type BRAF. Supporting its classification as an oncogene, V600E BRAF stimulates ERK signaling, induces proliferation and is capable in model systems of promoting transformation. However, BRAF mutations are common in nevi and colon polyps suggesting that BRAF mutation alone is insufficient for tumorigenesis and additional mutations are required for cancer development. Though such data suggest that BRAF mutation is likely an early initiating event in tumors such as melanoma and colon cancer, preclinical studies suggest that tumors with V600E BRAF mutation remain dependent upon BRAF for proliferation and survival. Given its frequent occurrence in human cancer and the continued requirement for BRAF activity in tumors with BRAF mutation, efforts are underway to develop targeted inhibitors of BRAF and its downstream effectors. The first generation of RAF inhibitors, including sorafenib, were notable for their lack of specificity and potency for RAF and these agents have shown limited efficacy in tumors with a high incidence of BRAF mutation such as melanoma. Novel inhibitors of the pathway with greater selectivity for BRAF and MEK are now in Phase 1 and 2 clinical trials with promising early results. To maximize the likelihood of success with these agents, clinical trials enriched with patients whose tumors possess BRAF and RAS mutations have been proposed.