Type II RAF inhibition predicts superior ERK suppression compared to type I RAF inhibition in different BRAF mutant types recurrently found in lung cancer.

Abstract

e23154 Background: Somatic driver BRAF mutations account for 6-8% of lung cancers. As opposed to melanoma in which V600E mutant BRAF predominates, the majority of lung cancer-derived BRAF mutations are non-V600. Yet, the efficacy of RAF-inhibitors and the possible resistance mechanisms in non-V600 BRAF mutant cells remain to be uncovered. Recently, we have shown that non-V600 BRAF mutations recurrently found in lung cancer predict sensitivity to the combination of type I RAF inhibitor Dabrafenib and a MEK inhibitor Trametinib. As a single agent, Dabrafenib shows only weak suppression of mutant BRAF-induced ERK signaling; moreover it can induce ERK paradoxical activation in CRAF overexpressing cells. Methods: Several recombinant BRAF expression vectors were generated by performing site-directed mutagenesis. We compared the effects of Dabrafenib and a type II RAF inhibitor (AZD-628) at clinically relevant dose as single agents or in combination with MEK inhibitor Trametinib on ERK activity in HEK293T cells expressing several tumor-derived BRAF mutants and a non-V600 BRAF mutant lung cancer cell line (H1666). Viability and caspase3/7 activation assays were performed using the H1666 cell line model. Results: In contrast to Dabrafenib, AZD-628 does not induce paradoxical ERK activation in CRAF expressing cells. Increased CRAF expression desensitizes BRAF-mutant expressing cells to Dabrafenib but not to AZD-628. Notably, AZD-628 has superior ERK-inhibitory effect in HEK293T cells co-expressing several different BRAF-mutants with CRAF and in H1666 cells. Combination of Trametinib and AZD-628 has superior MEK-inhibitory and pro-apoptotic effect in H1666 cells compared to combined Trametinib/Dabrafenib. Moreover, upon down titration of the RAF inhibitors with a steady dose of Trametinib, AZD-628 resulted in overall stronger effect on viability compared to Dabrafenib. Conclusions: In our in vitro model, we obtained strong indications that at conventional doses, type II RAF-inhibitor AZD628 is superior to type I RAF-inhibitor Dabrafenib in combination with MEK inhibitor Trametinib for the treatment of non-V600 BRAF mutant lung cancer.