Inhibition of mutant BRAF splice variant signaling by next‐generation, selective RAF inhibitors

Summary Vemurafenib and dabrafenib block MEK ‐ ERK 1/2 signaling and cause tumor regression in the majority of advanced‐stage BRAF V 600E melanoma patients; however, acquired resistance and paradoxical signaling have driven efforts for more potent and selective RAF inhibitors. Next‐generation RAF inhibitors, such as PLX 7904 ( PB 04), effectively inhibit RAF signaling in BRAF V 600E melanoma cells without paradoxical effects in wild‐type cells. Furthermore, PLX 7904 blocks the growth of vemurafenib‐resistant BRAF V 600E cells that express mutant NRAS . Acquired resistance to vemurafenib and dabrafenib is also frequently driven by expression of mutation BRAF splice variants; thus, we tested the effects of PLX 7904 and its clinical analog, PLX 8394 ( PB 03), in BRAF V 600E splice variant‐mediated vemurafenib‐resistant cells. We show that paradox‐breaker RAF inhibitors potently block MEK ‐ ERK 1/2 signaling, G1/S cell cycle events, survival and growth of vemurafenib/ PLX 4720‐resistant cells harboring distinct BRAF V 600E splice variants. These data support the further investigation of paradox‐breaker RAF inhibitors as a second‐line treatment option for patients failing on vemurafenib or dabrafenib.