Abstract LB-350: EGFR-mediated re-activation of MAPK signaling contributes to insensitivity of BRAF mutant colorectal cancers to RAF inhibition by vemurafenib

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Oncogenic BRAF mutations occur in 10-15% of colorectal cancers (CRCs) and confer adverse outcome. While RAF inhibitors such as vemurafenib (PLX4032) have produced dramatic response rates in BRAF mutant melanoma (∼60-80%), they are surprisingly ineffective in BRAF mutant CRCs (∼5% response rate), and the reason for this disparity remains unclear. Compared to BRAF mutant melanoma cells, BRAF mutant CRC cells were less sensitive to vemurafenib, and P-ERK suppression was not sustained in response to treatment. Although transient inhibition of phospho-ERK by vemurafenib was observed in CRC, rapid ERK re-activation occurred through EGFR-mediated activation of RAS and CRAF. BRAF mutant CRC cell lines and patient tumor specimens expressed higher levels of phospho-EGFR than BRAF mutant melanoma cell lines and primary tumor specimens, suggesting that CRCs are specifically poised for EGFR-mediated resistance. Combined RAF and EGFR inhibition blocked reactivation of MAPK signaling in BRAF mutant CRC cells, markedly improved efficacy in vitro, and led to robust tumor regressions in vivo in BRAF mutant xenografts models. These findings support combined RAF and EGFR inhibition as a novel and promising therapeutic strategy for evaluation in clinical trials in patients with BRAF mutant CRC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-350. doi:1538-7445.AM2012-LB-350