Abstract SY10-01: Strategies to overcome resistance to MAPK inhibition.
Authors: Corcoran Ryan B. 
Published: August 2014

Abstract
Activating mutations in BRAF and KRAS, key signaling components of the MAPK pathway, are among the most common oncogenic mutations in human cancer, occurring in ~7% and ~20% of all cancers, respectively. Consequently, inhibitors of MAPK signaling, including selective RAF and MEK inhibitors, have been under active clinical development. Yet, key resistance signals have limited the clinical effectiveness of these inhibitors.
Although selective RAF inhibitors have proven remarkably effective in BRAF mutant melanoma (~60-80% response rates), efficacy has been disappointing in the ~10-15% of colorectal cancers (CRCs) that also harbor BRAF mutations (~5% response rate). Compared to BRAF mutant melanoma cells, BRAF mutant CRC cells were less sensitive to the RAF inhibitor vemurafenib (PLX4032), and phospho-ERK (P-ERK) suppression was not sustained in response to treatment. Although transient inhibition of P-ERK by vemurafenib was observed in BRAF mutant CRC, rapid ERK reactivation occurred through EGFR-mediated activation of RAS and CRAF. BRAF mutant CRCs expressed higher levels of phospho-EGFR than BRAF mutant melanomas, suggesting that BRAF mutant CRCs are specifically poised to exhibit EGFR-mediated resistance. Combined RAF and EGFR inhibition blocked reactivation of MAPK signaling in BRAF mutant CRC cells and markedly improved efficacy in vitro and in vivo, leading to tumor regressions in BRAF mutant CRC xenograft models. These findings support evaluation of combined RAF and EGFR inhibition in BRAF mutant CRC patients, and clinical trials evaluating this strategy are currently underway.
Although KRAS is the most commonly mutated oncogene in human cancer, KRAS has proven difficult to target pharmacologically, and no effective therapies exist for KRAS mutant cancers. Recently, there has been evidence that targeted therapy combinations inhibiting multiple downstream effectors of KRAS may be a promising approach for KRAS mutant cancers. We developed a pooled shRNA-drug screen strategy to identify genes that, when inhibited, cooperate with MEK inhibitors to kill KRAS mutant cancer cells. The anti-apoptotic BH3 family gene BCL-XL emerged as a top hit through this approach. ABT-263 (navitoclax), a chemical inhibitor that blocks the ability of BCL-XL to bind and inhibit pro-apoptotic proteins, in combination with a MEK inhibitor led to dramatic apoptosis in the vast majority of KRAS mutant cell lines tested from different tissue types. Mechanistic studies revealed that MEK inhibition led to marked induction of the pro-apoptotic protein BIM in KRAS mutant cancer cells, but that BIM remained bound and inhibited by BCL-XL. Pharmacologic inhibition of BCL-XL with ABT-263 disrupted this inhibitory complex, allowing BIM to trigger apoptosis. Epithelial differentiation and E-cadherin expression correlated with increased sensitivity to this inhibitor combination across a panel of 30 KRAS mutant cell lines, while epithelial-to-mesenchymal transition (EMT) correlated with resistance. This combination also caused marked in vivo tumor regressions in three independent KRAS mutant xenografts and in established lung tumors in two genetically-engineered KRAS driven lung cancer mouse models. These data support combined BCL-XL/MEK inhibition as a promising therapeutic approach for evaluation in future clinical trials for patients with KRAS mutant cancers.
Citation Format: Ryan B. Corcoran. Strategies to overcome resistance to MAPK inhibition. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr SY10-01. doi:10.1158/1538-7445.AM2013-SY10-01