Paradox breakers inhibit mutant BRAF without inducing MAPK in cells with upstream pathway activation.

  • Major finding: Paradox breakers inhibit mutant BRAF without inducing MAPK in cells with upstream pathway activation.

  • Concept: These drugs may limit paradox-driven skin tumors and overcome vemurafenib resistance mechanisms.

  • Impact: These agents may provide enhanced safety and efficacy over first-generation and pan-RAF inhibitors.

First-generation RAF inhibitors such as vemurafenib and dabrafenib selectively inhibit mutant BRAFV600E and show clinical efficacy in patients with BRAFV600E-driven metastatic melanoma. However, these drugs induce paradoxical MAPK activation in cells with elevated upstream signaling, occasionally resulting in the formation of cutaneous squamous cell carcinomas (cuSCC) and keratoacanthomas, and, less commonly, progression of other RAS-driven cancers. Combined treatment with RAF and MEK inhibitors has been proposed to overcome paradoxical MAPK activation, but results in increased toxicity and does not eliminate resistance. As an alternative approach, Zhang and colleagues screened vemurafenib analogues to develop next-generation RAF inhibitors, termed paradox breakers, that suppressed ERK phosphorylation in BRAF-mutant melanoma cells but did not stimulate ERK signaling in RAS-mutant cells. The paradox breakers PLX7904 and its analogue PLX8394 maintained selective inhibition of mutant BRAFV600E and suppressed the growth of BRAFV600E-mutant melanoma and colorectal cancer cells both in vitro and in vivo. However, in contrast to vemurafenib, PLX7904 did not promote the growth of HRASQ61L-mutant cuSCC cells in soft agar and did not accelerate cuSCC xenograft tumor growth. Furthermore, unlike vemurafenib, PLX7904 did not induce the expression of EGFR ligands that have been implicated in cuSCC development and vemurafenib resistance. Analysis of the crystal structure of PLX7904 in complex with BRAFV600E revealed a stronger interaction of Leu505 in the regulatory αC helix of BRAF with PLX7904 compared with vemurafenib, suggesting an important role for this residue in mediating the effect of paradox breakers. PLX7904 did not enhance BRAF–CRAF heterodimer formation in RAS-mutant cells, and vemurafenib-resistant cells, including cells in which resistance was driven by dimerization of a truncated BRAFV600E variant, remained sensitive to treatment with PLX7904 or PLX8394. These findings support the notion that the uncoupling of MAPK inhibition and paradoxical activation may result in increased safety and clinical efficacy compared with first-generation and pan-RAF inhibitors.

Zhang C, Spevak W, Zhang Y, Burton EA, MA Y, Habets G, et al. RAF inhibitors that evade paradoxical MAPK pathway activation. Nature 2015 Oct 14 [Epub ahead of print].