New perspectives for targeting RAF kinase in human cancer

Key Points The oncoprotein BRAF is frequently activated due to genetic alterations in tumours promoting deregulation of RAF–MEK–ERK signalling. Targeting BRAF with inhibitors is a validated therapeutic strategy for a substantial proportion of cancer patients. RAF inhibitors alone or in combination with MEK inhibitors have elicited dramatic responses and prolonged the survival of patients with melanoma whose tumours harbour mutationally activated BRAF-V600. However, their effectiveness is limited by the development of drug resistance, frequently by mechanisms that promote reactivation of RAF–ERK signalling in the presence of the drug. In BRAF-V600 tumours other than melanoma, or in tumours carrying BRAF alterations other than the BRAF-V600 mutation, current clinical RAF inhibitors have shown modest effectiveness. RAF inhibitors have unique biochemical properties that account for their wide therapeutic window, on-target toxicities and major mechanisms of resistance. RAF dimerization is a common mechanism of both intrinsic and acquired resistance to current clinical RAF inhibitors vemurafenib and dabrafenib, which stabilize the αC-helix of RAF kinase in the OUT position. These compounds effectively inhibit monomeric mutant BRAF but fail to inhibit dimeric RAF. Structurally, inhibitor resistance due to RAF dimerization is the result of negative allostery for inhibitor binding to the second protomer of the RAF dimer, once the first is bound to an inhibitor. Next-generation RAF inhibitors that stabilize the αC-helix of RAF kinase in the active (IN) position will inhibit RAF monomers and dimers, but they are predicted to have a narrow therapeutic window due to inhibition of wild-type BRAF in normal cells. Thus, combinatorial approaches with current clinical inhibitors may be beneficial. Paradoxical pathway activation (allosteric priming) is a critical adverse event observed with most RAF inhibitors in the presence of RAS. Its effect on downstream signalling is currently ameliorated with the combined use of MEK inhibitors. Several structurally diverse, next-generation RAF inhibitors are under preclinical or clinical development and may be effective in BRAF-mutant tumours that are resistant to current clinical RAF inhibitors. Several types of human tumour are dependent on mutations in BRAF. This led to the development of RAF inhibitors, which prolong patient survival but are limited by resistance. This Review discusses the recent advances in our understanding of BR