Intrinsic and acquired resistance to BRAF inhibitors is a persistent clinical problem in the treatment of BRAF-mutant melanoma. In many cases, resistance to BRAF inhibitors results from acquisition of secondary mutations in NRAS, upstream from another member of the RAF family, CRAF. Moreover, although BRAF inhibitors block MEK and ERK signalling in BRAF-mutant cells, they activate this pathway in RAS-mutant cells in what is called paradoxical activation of the MEK–ERK pathway, a mechanism responsible for the development of secondary malignancies in patients on treatment with BRAF inhibitors. The obvious strategy to overcome both resistance and paradoxical activation of the MEK–ERK pathway is to combine BRAF inhibitors with MEK inhibitors. However, although such combinations have improved clinical responses, patients still develop resistance.

Credit: Lara Crow / NPG

Because resistance to combination of BRAF and MEK inhibitors can be driven by CRAF, as well as by receptor tyrosine kinases through SRC, Girotti et al. designed, synthesized and characterized two inhibitors of the inactive conformation of BRAF-V600E, CCT196969 and CCT241161, which also inhibited CRAF and tyrosine kinases SRC and LCK. Both compounds inhibited MEK and ERK in BRAF-mutant and NRAS-mutant melanoma but not in BRAF and NRAS wild-type cells. They also blocked BRAF-mutant melanoma cell growth in vitro more potently than PLX4720, an analogue of the BRAF-selective inhibitor vemurafenib.

The authors then tested whetherthe compounds were active in melanoma cells that were resistant to BRAF inhibitors. Cells that haddeveloped resistance to PLX4720 after continuous exposure to the drug were sensitive to both CCT196969 and CCT241161, and both compounds inhibited the growth of PLX4720-resistant A375 xenografts in mice through inhibition of ERK and SRC. Importantly, both compounds were effective when administered orally at therapeutic doses and were well tolerated by the mice. Finally, the authors tested the compounds in vemurafenib-resistant patient-derived xenografts (PDXs) of advanced-stage BRAF-mutant melanomas. Despite being resistant to PLX4720, the PDX tumours remained sensitive to CCT196969 and CCT241161 after prolonged exposure. The compounds were also effective in PDXs from patients who had intrinsic resistance to vemurafenib.

the inhibitors characterized by Girotti et al. could be effective in both first- and second-line treatment of patients with melanoma

Moriceau et al. were interested in the genetic mechanisms underlying acquired resistance to combinatorial inhibition of BRAF and MEK. The authors analysed tissues from patients with melanoma who had developed resistance to combination treatment with BRAF and MEK inhibitors as first-line treatment and as second-line treatment after an initial response and subsequent resistance to BRAF inhibition alone. They searched for alterations in the exons of those genes in the core pathways involved in resistance to BRAF inhibition and found that resistant melanomas showed dramatic genomic alterations — BRAFV600E ultra-amplification or NRASQ61K amplification — singly or in combination with mutations that altered protein–protein interactions involving CRAF and ERK.

The authors also uncovered that melanoma cells with acquired resistance to the combination treatment were much more sensitive to drug withdrawal than those with acquired resistance to BRAF inhibition alone. This suggested that intermittent therapy with the combination of BRAF and MEK inhibitors may forestall resistance.

These studies offer new possibilities to fight BRAF inhibition resistance. Whereas the results from Moriceau et al. highlight intermittent therapy or disruption of the BRAF–CRAF–MEK signalling complex to delay acquired resistance, the inhibitors characterized by Girotti et al. could be effective in both first- and second-line treatment of patients with melanoma. Phase I clinical trials to test these inhibitors are set to open in 2015, and a Phase II trial to test intermittent BRAF and MEK inhibition is already ongoing.