Abstract 1832: Combined EGFR and MEK inhibition prevents the emergence of drug resistance in EGFR mutant non-small cell lung cancer (NSCLC)
Authors: Tricker Erin M. , Xu Chunxiao , Ercan Dalia , Ogino Atsuko , Wong Kwok-kin , Janne Pasi 
Published: May 2015

Abstract
Background: EGFR inhibitors are effective clinical therapies for EGFR mutant NSCLC, but efficacy is limited by the development of acquired drug resistance. The mechanisms of resistance include secondary mutations in EGFR (T790M) and activation of compensatory signaling pathways (MET, IGFR and AXL). Mutant selective EGFR inhibitors (WZ4002, CO-1686 and AZD9291) are effective in preclinical and clinical models of EGFR T790M. Our lab has demonstrated that reactivation of ERK signaling is a mechanism of resistance to WZ4002 (WZ) which could be reversed or prevented by using a combination of WZ and MEK inhibitors. Given the broad array of possible EGFR inhibitor resistance mechanisms, we evaluated whether WZ combined with the MEK inhibitor trametinib (TRA) can prevent the emergence of resistance in vitro and in vivo.
Methods and Results: We developed a plate based resistance assay in which 350 cells/well are plated in 96 well plates and treated weekly. This system allows for evaluation of drug efficacy over weeks of treatment and can be used to measure both time to development and frequency of drug resistance. Given that ERK reactivation can cause resistance to WZ, we evaluated WZ, TRA or the combination thereof (WZ/TRA) using PC9 cells. Single agents led to 100% resistance in 2-3 weeks, versus 5% with WZ/TRA in 24 weeks. We then tested WZ/TRA in 5 additional EGFR TKI sensitive cell lines, including several known to develop acquired resistance to gefitinib (HCC827, MET amplification; HCC4006, EMT or FGFR activation; HCC2279; FGFR activation; H3255, T790M; HCC2935). In all cases, WZ/TRA significantly reduced the emergence of drug resistant clones compared to each single agent. In models with established drug resistance mechanisms, WZ/TRA was effective in 3/3 models with T790M mutations, but not (0/5) in models with established non-T790M resistance mechanisms. In models where WZ/TRA was effective, the combination led to increased apoptosis compared to single agents and effective inhibition of ERK signaling. We also evaluated the effectiveness of the WZ/TRA combination in vivo using EGFR L858R/T790M genetically engineered mouse model and PC9GR4 (T790M+) xenografts. Combination WZ/TRA treatment prevented tumor outgrowth for 24 weeks in 5 EGFR L858R/T790M GEMM mice whereas individual treatments did not. Additionally, we found that WZ/TRA combination treatment, but not single agents, could cure 7/15 xenograft tumors as assessed by lack of tumor regrowth after treatment cessation.
Conclusion: Our studies suggest that the combination of a mutant selective EGFR inhibitor and a MEK inhibitor can prevent the emergence of both T790M and non-T790M mediated drug resistance mechanisms. This strategy is more effective at preventing (6/6) than treating (3/8) cancers with established drug resistance mechanisms and should be evaluated in clinical trials in EGFR TKI naïve EGFR mutant NSCLC patients.
Citation Format: Erin M. Tricker, Chunxiao Xu, Dalia Ercan, Atsuko Ogino, Kwok-kin Wong, Pasi Janne. Combined EGFR and MEK inhibition prevents the emergence of drug resistance in EGFR mutant non-small cell lung cancer (NSCLC). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1832. doi:10.1158/1538-7445.AM2014-1832