Abstract 258: The role of EGFR on RAF inhibitor resistance in B-RAFV600E melanoma and colon tumors

Abstract

Abstract Constitutive activation of the RAS, RAF, mitogen-activated protein kinase (MEK), and extracellular signal regulated kinase (ERK) pathway is a common finding in many human cancers. Activating mutations in K-RAS or B-RAF constitute over 30% of all mutations in human tumors. It has been demonstrated that RAF inhibitors selectively inhibit B-RAFV600E tumors and not RAS mutant tumors, despite functioning as one of the key effector enzymes downstream of RAS and upstream of MEK. Recent Phase I clinical data with RAF inhibitors has demonstrated a significant response rate (81% response rate) in metastatic melanoma patients with B-RAFV600E-positive tumors (N Engl J Med. 2010 363:809-19). In contrast, the response rates in B-RAFV600E-positive colon tumors have been relatively low. In this work we examine the molecular mechanism of EGFR pathway activation as a mechanism of resistance by comparing the effects of RAF inhibitors against a panel of B-RAFV600E, mutant K-RAS, and WT RAS/RAF tumor lines in the presence and absence of EGF stimulation. We demonstrate that in B-RAF WT lines, pathway induction is observed for RAF inhibitors as evidenced by increases in phospho-MEK and phospho-ERK levels. RAF-inhibitor induced pathway activation in this setting mimics the kinetics and mechanism of EGF-induced pathway stimulation where RAS-GTP levels, C-RAF kinase activity and pERK levels are elevated. Upon stimulation with EGF or serum in B-RAF WT or K-RAS mutant lines, RAF inhibitor induced pathway activation is attenuated. In contrast in B-RAFV600E tumor lines, RAF inhibitors effectively block the MAPK pathway under basal conditions but become ineffective in a subset of B-RAFV600E lines when cells are stimulated with EGF. Additional studies across a broader panel of colon and melanoma mutant B-RAF lines demonstrate that B-RAFV600E lines with high basal levels of EGFR tend to be more resistant to selective B-RAFV600E inhibitors. Immunohistochemistry of colorectal tumor patient samples with either wildtype B-RAF or B-RAFV600E illustrates that colorectal B-RAFV600E tumors are associated with a higher percentage of membrane EGFR. Taken together, these results provide insight into the therapeutic utility of MAPK pathway inhibitors, and emphasize not only the importance of targeting defined genetic backgrounds in cancer treatment but the value in assessing other key pathway markers (such as EGFR) in B-RAFV600E-positive tumor subsets as a factor in sensitivity or resistance to therapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 258. doi:10.1158/1538-7445.AM2011-258