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

Vemurafenib and dabrafenib block MEK-ERK1/2 signaling and cause tumor regression in the majority of advanced-stage BRaf(V600E) melanoma patients; however, acquired resistance and paradoxical signaling have driven efforts for more potent and selective Raf inhibitors. Next-generation Raf inhibitors, such as PLX7904 (PB04), effectively inhibit Raf signaling in BRaf(V600E) melanoma cells without paradoxical effects in wild-type cells. Furthermore, PLX7904 blocks the growth of vemurafenib-resistant BRaf(V600E) 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 PLX7904 and its clinical analog, PLX8394 (PB03), in BRaf(V600E) splice variant-mediated vemurafenib-resistant cells. We show that paradox-breaker Raf inhibitors potently block MEK-ERK1/2 signaling, G1/S cell cycle events, survival and growth of vemurafenib/PLX4720-resistant cells harboring distinct BRaf(V600E) 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.