Abstract LB-293: Targeting the PI3K pathway to intercept cetuximab tolerance in metastatic colorectal cancer

In the precision medicine era, multi-parametric interrogation of tumors in cancer patients has enabled rationally refined clinical trials and considerably improved response rates. Nevertheless, incomplete mass obliteration and late relapse invariably emerge. This also relates to metastatic colorectal cancer (mCRC), as massive tumor regression upon anti-EGFR therapy with cetuximab is relatively infrequent. Clinical evidence suggests that tumor recurrence may be endorsed by a population of lingering cancer cells that survive therapy and fuel residual disease through reversible, non-mutational cues, thus installing a “poised” state of drug tolerance that anticipates tumor relapse. The mechanisms underlying this resilience, and the strategies to tackle it, remain to be explored. In this study, we capitalized on different exploratory resources, including a proprietary collection of patient-derived xenografts, as well as CRC cell lines and primary cultures, to explore the mechanisms of cetuximab tolerance. Biochemical characterization of cetuximab persisters evidenced a dichotomous output, with full MAPK neutralization but persistent PI3K/AKT activity, suggesting that PI3K is engaged as a competitor route. Consistently, concomitant EGFR/PI3K blockade specifically unleashed apoptosis in cell lines, as opposed to the cytostatic activity of cetuximab alone, and prevented colosphere growth recovery after drug washout. In vivo , evaluation of residual cellularity confirmed the superior efficacy of dual EGFR/PI3K deactivation. Hence, both the number and fitness of tolerant cells were thwarted by EGFR/PI3K interception. Integrative morphologic analysis, immunohistochemistry, gene-expression profiling and reporter assays in vitro put forward increased beta-catenin activity and Paneth cell-like reprogramming as distinctive traits of residual cells surviving cetuximab treatment. These features are likely related to cetuximab-induced biochemical rewiring, as relieve of ERK-mediated beta-catenin repression in conjunction with persistent PI3K-AKT activity may converge on Wnt-dependent Paneth cell-like differentiation. Importantly, PI3K inhibition dampened cetuximab-triggered Paneth cell specification, thus intercepting a putative mechanism of drug-tolerance. Future studies will extricate the relative contribution of PI3K, MAPK, and Wnt-dependent networks to sustaining the reservoir of persisters, and how Paneth-like traits may foster residual disease. Citation Format: Barbara Lupo, Paolo Gagliardi, Francesco Sassi, Eugenia Rosalinda Zanella, Francesca Cottino, Andrea Bertotti, Livio Trusolino. Targeting the PI3K pathway to intercept cetuximab tolerance in metastatic colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-293. doi:10.1158/1538-7445.AM2017-LB-293