Abstract 1930: Targeting the c-Cbl tumor suppressor protein as a novel therapeutic approach in overcoming acquired tamoxifen resistance in luminal breast cancer

Background and aims: Treatment of luminal breast cancer (LBC) with tamoxifen (TMX) is associated with the emergence of resistance in ~40% of patients. Such resistance is associated with emergence of tumor initiating cells (TICs, also known as cancer stem cells), resistance to other therapeutic agents and multiple other obstacles to successful treatment. This study aims to investigate the role of tumor suppressor gene c-Cbl in TMX resistance and identify novel therapeutic approaches in treatment of acquired TMX resistant LBC that address unmet clinical needs. Methods: TMX resistant MCF7 and T47D cell lines were generated by chronic exposure to TMX in vitro. Unbiased drug screening using the NINDS-II library revealed previously approved drugs with unexpected activities to restore c-Cbl activity. Assays, including dose-response studies, western blot, co-immunoprecipitation analyses and tumorsphere assays, were used to determine the mechanisms of action and utility of our lead agent (CRA-1). Finally, response to treatment was verified in xenograft mouse models. Findings: In TMX-resistant LBC cells, c-Cbl was inhibited due to complex formation with the Cool-1/spix (C1sp) protein. Restoration of c-Cbl function by C1sp knockdown or by treatment with our lead c-Cbl restorative agent (CRA-1, which is approved for other purposes and is suitable for prolonged use in patients) restored sensitivity to TMX and decreased TIC function. Treatment with CRA-1+TMX increased c-Cbl activation via the non-canonical redox/Fyn/c-Cbl (RFC) pathway by harnessing TMX9s estrogen receptor-independent pro-oxidative activity. This treatment also decreased levels of the c-Cbl/C1sp inhibitory complex, and of levels and/or activity of multiple direct and indirect c-Cbl targets important in TMX resistance (e.g. EGFR, HER2, Notch-1, sox2, s-catenin). Treatment with CRA-1+TMX also increased sensitivity to cyclophosphamide (CPP), one of the standard chemotherapeutic agents to treat TMX resistant LBCs. In vivo, treatment with MPT+TMX + ultra-low-dose of CPP completely suppressed tumor growth in a xenograft animal model for 20 weeks without any obvious toxicity. Tumors thus far have not recurred (15 weeks) after treatment was stopped. Conclusions: Our study offers a new approach to overcoming acquired TMX resistance that also: (i) has the potential for rapid clinical translation due to the use of drugs already approved for other purposes; (ii) eliminates TICs; (iii) suppresses tumor growth in vivo with only a small fraction of clinically relevant CPP dosages; and (iv) exhibits greatly reduced toxicity effects and applicability for extended treatment periods. Citation Format: Xiaowen Wang, Hsing-Yu Chen, Jennifer Stripay, Mark Noble. Targeting the c-Cbl tumor suppressor protein as a novel therapeutic approach in overcoming acquired tamoxifen resistance in luminal breast cancer [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 1930.