RAF Inhibitors

In the last decade, RAF kinases have emerged as valuable druggable targets, in particular after the discovery of the prevalence of oncogenic BRAF mutations in a number of solid tumours, including cutaneous melanoma, colorectal and papillary thyroid cancers. The BRAF V600E aminoacid substitution accounts for the majority of alterations and confers constitutive kinase activity. Among RAF inhibitors undergoing preclinical or clinical development, some display some preferential selectivity towards the mutant isoform (GSK2118436, vemurafenib), while others have similar activity on both mutated and wild type BRAF or other RAF isoforms. The first targeted drug expected to reach clinical use is vemurafenib (known also as PLX4032, RO5185426, RG7204) that has demonstrated remarkable efficacy in melanoma patients carrying BRAF V600E mutated tumours. Initial enthusiasm has been dampened by the relatively short duration of clinical response. Indeed, secondary resistance to this drug emerges in patients who relapse after a median time of 6−7 months. Acquired resistance does not seem to be mediated by additional ‘gatekeeper’ mutations in the BRAF target gene itself. Rather, a number of studies on cell lines and melanoma samples indicate that tumours become resistant by modulating the expression or activity of other genes (such as NRAS, CRAF, COT, MEK1, PDGFR, IGF1R, PTEN) that compensate BRAF inhibition or obviate for the need of BRAF signaling. Consequently, combinatorial strategies with MEK or PI3K pathway targeted drugs or single molecule dual RAF/MEK inhibitors (such as RO5126766) are being evaluated to delay the onset of secondary resistance and improve patient survival. Interestingly, not all BRAF V600E mutated tumours are sensitive to vemurafenib, with 15−20% melanoma cases and the vast majority of BRAF mutant colorectal cancer patients displaying intrinsic resistance to treatment with this inhibitor. The molecular bases of these partial failures represent an area of active research investigations. Finally, newer compounds such as PLX-PB-4 and ARQ680 (and its prodrug ARQ736) are being developed to overcome the limitations of firstgeneration drugs, and, in particular, their ability to promote dimerization of RAF family members and paradoxical activation of MAPK pathway in treated cells bearing oncogenic or normally activated RAS. It is likely that this could be achieved by designing inhibitors with increased potency against CRAF and/or with the ability to block RAF dimerization.