Abstract PR02: Identification of ERK1/2 mutations that confer resistance to MAPK pathway inhibitors

Recent treatment advances in metastatic BRAF mutant melanoma using targeted therapy have increased progression free survival by many months. However, resistance to these targeted agents is a common occurrence, with relapse occurring in almost every case. Many of the alterations that confer clinical resistance to RAF and MEK inhibitors in BRAF mutant melanoma reactivate the MAPK pathway, but remain sensitive to ERK inhibition in vitro. Prospectively determining mechanisms of resistance to MAPK pathway inhibitors, such as an ERK inhibitor, before clinical trials begin may inform which patients will respond to therapy, potential markers of resistance, or best therapy combinations. Previous studies have shown that in vitro random mutagenesis screens can select specific alleles seen in resistant patient populations. Since ERK inhibitors are entering clinical trials, we wanted to determine mutations in ERK1 and ERK2 that could confer resistance to ERK or RAF/MEK inhibition. Therefore, we expressed dox-inducible ERK1 or ERK2 mutant libraries in A375 cells, which were then treated with MAPK inhibitor(s) and doxycycline until resistant cells emerged. Exogenous ERK1 or ERK2 was analyzed in resistant cells by massively parallel DNA sequencing. We validated 15 alleles that conferred resistance to the ERK inhibitor, VX-11e, and 11 alleles that conferred resistance to RAF, MEK, or RAF+MEK inhibition (dabrafenib and trametinib). Some resistance alleles were analogous between ERK1 and ERK2, while others were specific to ERK1 or ERK2. ERK inhibitor resistant alleles were localized to areas surrounding the ATP/drug binding site, including the glycine-rich loop, c-helix, and activation loop. These alleles were also cross-resistant to another ERK inhibitor, SCH-772984, albeit some only at lower concentrations. The RAF/MEK inhibitor resistant residues were located in the c-helix, activation loop, and common docking domain. Only a single ERK1 allele was found to confer resistance to RAF + MEK and ERK inhibition, suggesting that resistance mutations in ERK might be suppressed if RAF, MEK, and ERK inhibitors are given in combination. In addition, an ERK mutation arising in a patient treated with an ERK inhibitor would still be sensitive to RAF/MEK inhibition and vice versa. Screening for these alleles before administering therapy or after relapse may help inform patient treatment. Citation Format: Eva M. Goetz, Mahmoud Ghandi, Daniel Treacy, Nikhil Wagle, Levi A. Garraway. Identification of ERK1/2 mutations that confer resistance to MAPK pathway inhibitors. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr PR02.