Relationship of incomplete inhibition of PI3K pathway signaling and efficacy of cetuximab in KRAS wild-type colorectal cancers.

Abstract

462 Background: Therapies targeting receptor tyrosine kinases (RTKs) are typically effective only when they lead to simultaneous downregulation of PI3K-AKT and MEK-ERK signaling. Cetuximab is a monoclonal antibody directed against EGFR that has been approved for the treatment of metastatic colorectal cancer, and several studies have suggested that its benefit is restricted to patients with KRAS wildtype cancers. However, even in KRAS wildtype colorectal cancers, the response rate to single-agent cetuximab is low, and the mechanistic basis for this is not well-understood. Methods: The ability of cetuximab to downregulate PI3K-AKT and MEK-ERK signaling in a panel of KRAS wildtype colorectal cancer cell lines was assessed by immunoblotting. We preformed immunoprecipitations of the regulatory subunit of PI3K on lysates derived from cell lines and primary colorectal tumor specimens to identify the RTKs involved in PI3K activation. Cetuximab-based therapeutic combinations were evaluated in KRAS wildtype cell lines and tumor xenograft models. Results: Cetuximab led to effective inhibition of MEK-ERK signaling (>75% reduction in phospho-ERK), but led to incomplete inhibition of PI3K-AKT signaling (0-50% reduction in phospho-AKT) in all KRAS wildtype colorectal cell lines tested. PI3K regulatory subunit immunoprecipitations revealed multiple RTK inputs to PI3K in cell lines and primary tumors, most notably from IGFIR. Combined treatment with cetuximab and an IGFIR inhibitor or a PI3K inhibitor led to improved efficacy in vitro and in tumor xenograft models. Conclusions: Cetuximab fails to completely downregulate PI3K-AKT signaling in KRAS wildtype colorectal cancer cell lines. Biochemical analysis of cell lines and primary colorectal tumors reveals that additional RTKs other than EGFR play a role in PI3K activation in these cancers. Therefore, combining cetuximab with targeted therapies directed against the PI3K pathway may lead to improved efficacy in KRAS wildtype colorectal cancers.