Phase I/II RAF kinase inhibitors in cancer therapy

Introduction: Aberrant activation of RAF signalling is a frequent finding in human cancers. BRAF is the only RAF family member that is commonly mutated, whilst CRAF and ARAF play important roles in the signal transduction from mutant RAS. BRAF-specific inhibitors have been more effective in the treatment of BRAF-mutant melanoma than BRAF-mutant thyroid and colorectal cancers. Areas covered: The review summarises the experience with RAF kinase inhibitors, including efficacy, modes of acquired resistance, and the mechanism behind the progression of pre-malignant RAS-mutant lesions observed with RAF kinase inhibitors. The authors review all the completed and ongoing Phase I or II clinical trials of RAF kinase inhibitors and discuss in detail the rationale behind the combinatorial approaches. Expert opinion: The success of RAF kinase inhibitors has demonstrated the necessity of genotype-driven treatment selection for cancer patients. The spectrum of responses in different tumour types is explained by feedback events that are determined by cell lineage. Dissection of these events and the mechanisms of acquired resistance will determine the appropriate combination therapies. Ongoing characterisation of RAS-MAPK regulation in malignant cells may aid the development of novel agents that have greater potency for the inhibition of activated RAF kinase, and lesser propensity for promotion of RAS-mutant tumours.