Abstract 3485: Estrogen receptor ligands and their responses in de novo and tamoxifen resistant cell models

Abstract

Abstract Approximately 75% of breast cancer incidences are Estrogen Receptor positive (ER+). Tamoxifen, a selective estrogen receptor modulator (SERM), is the standard of care for many of these ER+ breast cancer patients. Unfortunately, tamoxifen resistance occurs in almost 50% of patients within 5 years of treatment, and endocrine-independence accompanying resistance negates the effects of aromatase inhibitors. Paradoxically, estradiol (E2) has shown clinical efficacy in patients with resistant breast cancer. Understanding the antiproliferative role of E2 and ER signaling in resistant ER+ cell lines is essential to gain a better understanding of paradoxical clinical efficacy and for the appropriate biomarker-assisted selection of endocrine therapy for various stages of ER+ breast cancer. We have discovered ER ligands, based upon a single chemical scaffold with a diverse set of pharmacological responses, which can be used to better understand the role of ER signaling in resistance and therapy: selective ER modulators (SERMs), selective ER downregulators (SERDs), selective estrogen mimics (SEMs), and selective human ER partial agonists (ShERPAs). These compounds were initially classified using an ERE luciferase reporter assay and affinity for ER confirmed by biochemical assays. The effects of these novel ER-directed chemical probes on cell viability were further examined in multiple 2D and 3D spheroid models of tamoxifen resistance. Finally, ERα localization upon administration of these ligands in tamoxifen-sensitive and tamoxifen-resistant cells was studied. While SERDs and SEMs showed growth inhibition in tamoxifen resistant cell lines, both molecules had different responses and mechanisms of growth inhibition. ERα was localized to extranuclear sites upon administration of E2, SEMs, and ShERPAs, an observation specific to the resistant phenotype and mechanistically associated with spheroid disintegration. SERDs inhibited the antiproliferative actions of E2, but were antiproliferative in resistant cell lines. Further dissection of the role of ER in resistance and survival is needed to define the appropriate ER-directed, endocrine therapy in ER+ breast cancer. Citation Format: Lauren M. Gutgesell, Gregory R. J. Thatcher, Hitisha Patel, Rui Xiong. Estrogen receptor ligands and their responses in de novo and tamoxifen resistant cell models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3485.