Preclinical efficacy of a RAF inhibitor that evades paradoxical MAPK pathway activation in protein kinaseBRAF-mutant lung cancer

Abstract

Oncogenic activation of protein kinaseBRAFdrives tumor growth by promoting mitogen-activated protein kinase (MAPK) pathway signaling. Because oncogenic mutations inBRAFoccur in ∼2–7% of lung adenocarcinoma (LA),BRAF-mutant LA is the most frequent cause ofBRAF-mutant cancer mortality worldwide. Whereas most tumor types harbor predominantly theBRAFV600E-mutant allele, the spectrum ofBRAFmutations in LA includesBRAFV600E(∼60% of cases) and non-V600E mutant alleles (∼40% of cases) such asBRAFG469AandBRAFG466V. The presence ofBRAFV600Ein LA has prompted clinical trials testing selective BRAF inhibitors such as vemurafenib inBRAFV600E-mutant patients. Despite promising clinical efficacy, both innate and acquired resistance often result from reactivation of MAPK pathway signaling, thus limiting durable responses to the current BRAF inhibitors. Further, the optimal therapeutic strategy to block non-V600EBRAF-mutant LA remains unclear. Here, we report the efficacy of the Raf proto-oncogene serine/threonine protein kinase (RAF) inhibitor, PLX8394, that evades MAPK pathway reactivation inBRAF-mutant LA models. We show that PLX8394 treatment is effective in bothBRAFV600Eand certain non-V600 LA models, in vitro and in vivo. PLX8394 was effective against treatment-naiveBRAF-mutant LAs and those with acquired vemurafenib resistance caused by an alternatively spliced, truncatedBRAFV600Ethat promotes vemurafenib-insensitive MAPK pathway signaling. We further show that acquired PLX8394 resistance occurs via EGFR-mediated RAS-mTOR signaling and is prevented by upfront combination therapy with PLX8394 and either an EGFR or mTOR inhibitor. Our study provides a biological rationale and potential polytherapy strategy to aid the deployment of PLX8394 in lung cancer patients.