Abstract P6-05-10: CUDC-101 enhances estrogen-mediated anticancer effects in ERβ+ TNBC

Background: Epigenetic based therapies, such as histone deacetylase inhibitors, have proven successful in many cancer types including lymphomas and myelomas with ongoing phase III clinical trials in NSCLC. Epigenetic modulation in triple negative breast cancer (TNBC) has been increasingly explored in the past decade. A major issue preventing the uptake of these drugs in the clinic is the relatively high doses necessary to achieve therapeutic benefit. It is therefore of interest to identify combinatorial approaches that result in drug de-escalation and enhanced therapeutic benefit. We and others have demonstrated that estrogen receptor beta (ERβ) is expressed in approximately 20% of TNBC patients. ERβ targeted therapies, including estradiol and multiple ERβ selective agonists, result in suppression of cell proliferation, invasion, and migration in vitro and in vivo (Reese, PNAS 2018). For these reasons, we sought to determine if ERβ targeted therapies would synergize with epigenetic based therapies for the treatment of TNBC. Methods: Using multiple TNBC cell lines representative of ERβ+ and ERβ- disease, as well as a model of acquired resistance to ERβ targeted therapies, we performed an epigenetic drug screen assessing 145 compounds alone and in combination with estradiol treatment. The top hit from this screen was extensively profiled across multiple cell lines in the presence and absence of ERβ targeting agents and was compared to the efficacy observed in ERα+ models of breast cancer using cell proliferation assays, RT-PCR, and western blotting. Results: The most significant compound to elicit differential and therapeutically beneficial effects in ERβ+ TNBC was CUDC-101, a multitarget pan-HDACi, EGFRi, and HER2i that was developed based on the structures of Vorinostat, Erlotinib, and Lapatinib. CUDC-101 potently inhibits the growth of TNBC cells and synergizes with E2 stimulation in ERβ+ MDA-MB-231 TNBC cells, but not in ERβ- cells. CUDC-101 was also effective in ERβ-resistant MDA-MB-231 cells and restored the growth inhibitory effects of estrogen in this model. Treatment with Vorinostat (SAHA) alone elicited a similar, though far less potent, synergistic effect in the MDA-MB-231-ERβ+ cell line, suggesting that histone deacetylase inhibition is a critical, but not completely sufficient, component of the synergistic phenotype. CUDC-101, while effective at reducing the viability of MDA-MB-468-ERβ+ cell line, had no synergistic effect with E2 in this model suggesting differences in response between TNBC sub-types. Additionally, CUDC-101 was found to be ineffective in models of ERα+ disease (MCF7 and MCF7-ERβ) in both the presence and absence of E2 treatment. Conclusions: Therapeutic options remain extremely limited for TNBC patients and the development of novel treatment strategies for these patients continue to represent a major unmet clinical need. Here, we present evidence that CUDC-101 elicits potent anti-cancer effects in multiple models of TNBC and synergizes with ERβ-targeted therapies in some models. Our findings further strengthen the body of evidence suggesting ERβ as a valid therapeutic target and important biomarker in TNBC and further implicate HDACi/EGFRi as relevant therapeutic strategies within this patient population. Citation Format: Michael Justin Emch, David Benjamin, Elizabeth Bruinsma, Molly Nelson-Holte, Matthew Goetz, John Hawse. CUDC-101 enhances estrogen-mediated anticancer effects in ERβ+ TNBC [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-05-10.