Abstract 309: Targeting anti-apoptotic reprogramming to counteract drug tolerance in EGFR-inhibited colorectal tumors

Treatment with EGFR monoclonal antibodies, such as cetuximab, has improved the outcome of patients with metastatic colorectal cancer (mCRC). However, EGFR inhibition - even in cases that respond with tumor shrinkage - is cytostatic rather than cytotoxic, with persistence of drug-tolerant cells that appear to be less prone to undergo apoptosis and poised to foster residual disease, which preludes tumor relapse. Our preliminary data, obtained in mCRC patient-derived xenografts (PDXs), indicate that CRC residual cells surviving EGFR blockade display increased expression of the anti-apoptotic proteins BCL-2 and BCL-XL. Inhibition of such proteins together with EGFR prompts apoptosis of PDX-derived organoids, suggesting a pro-survival adaptation triggered by cetuximab and neutralized by pro-apoptotic drugs. On these premises, we plan to investigate how the entire apoptotic machinery is modulated in response to cetuximab in mCRC, with the final goals to: (i) identify and validate new mechanisms sustaining the persistence of drug-tolerant cells, and (ii) find new therapeutic strategies to eradicate residual disease. We first performed a gene expression analysis of the main BCL-2 family members in cetuximab-sensitive PDXs after prolonged EGFR inhibition and found a pronounced reprogramming whereby anti-apoptotic outcomes prevail over cell death signals and apoptosis gets aborted. In order to generate a list of candidates potentially involved in sustaining this anti-apoptotic adaptation, we plan to combine gene expression analysis with “Dynamic BH3 profiling” (DBP), a novel functional approach that enables assessing (i) how cetuximab treatment changes CRC cell propensity to undergo apoptosis and (ii) which anti-apoptotic proteins prevent such priming from switching into overt apoptosis. We performed DBP in two PDX-derived organoid lines and found that exposure to cetuximab induced a strong dependence on BCL-XL in one model. We next examined the effect of combining cetuximab with a specific BCL-XL inhibitor in both lines and found that while either drug alone was insufficient to cause cell death, the combination therapy induced massive apoptosis only in the case featuring cetuximab-induced BCL-XL dependence, as predicted by DBP. This analysis will be extended to a larger number of PDX-derived organoids; then, therapeutic strategies with significant activity in vitro will be validated in vivo in matched PDXs and the underlying mechanistic underpinnings will be further investigated. These preliminary results illustrate how perturbations of the apoptotic machinery can influence the response to EGFR blockade in mCRCs. This information will guide the identification of critical functional nodes involved in sustaining the drug-tolerant state, thus providing new hints to design rational ways to convert the cytostatic effect of cetuximab into a cytotoxic, fully apoptotic outcome. Citation Format: Simonetta M. Leto, Irene Catalano, Valentina Vurchio, Francesca Cottino, Giorgia Migliardi, Livio Trusolino, Andrea Bertotti. Targeting anti-apoptotic reprogramming to counteract drug tolerance in EGFR-inhibited colorectal tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 309.