Abstract MIP-056: CONSTITUTIVELY ACTIVE ESTROGEN RECEPTOR–ALPHA LIGAND BINDING DOMAIN (ERA–LBD) MUTATIONS IN OVARIAN CARCINOMA

OBJECTIVES: Mutations in ESR1 that are acquired following treatment with aromatase inhibitors commonly underlie resistance to endocrine therapy in breast cancer. ERa-LBD mutations, especially at ESR1 Y537 and D538, confer constitutive activity and partial resistance to selective estrogen receptor modulators (SERMs), such as tamoxifen, and degraders (SERDs) such as fulvestrant in transcription assays. Some ovarian tumors also respond to anti-hormone therapy, and a subset which initially respond subsequently become resistant. The mechanisms of resistance to hormone therapy in ovarian cancer have not been previously described. Comprehensive genomic profiling (CGP) of ovarian tumors was used to analyze ESR1 and correlate responsiveness to aromatase inhibitors in a subset of these patients. METHODS: DNA was extracted from tumor specimens and sequenced using hybridization-capture (FoundationOne) targeting up to 315 genes, plus select introns frequently rearranged in cancer. Demographic and clinicopathologic data will be presented for a subset of patients whose tumors harbor ERa-LBD mutations associated with activation and resistance.. RESULTS: CGP of 3,641 ovarian and peritoneal carcinomas revealed 31 (0.9%) samples with amplification of ESR1 and 16 (0.4%) samples with acquired or de novo mutations affecting the ERa-LBD: Y537S (10), Y537N (1), D638G (4), and S341L (1). Within the 16 samples, ovarian serous carcinoma (10), ovarian endometrioid adenocarcinoma (3), ovarian epithelial carcinoma NOS (2), and peritoneal serous carcinoma (1) were represented. In cases of acquired mutations following prior hormone therapy, there was evidence of clinical benefit to SERM/SERDs. CONCLUSIONS: Acquired activating mutations in ESR1, previously described in breast cancer, can occur in ovarian serous carcinomas and endometrioid carcinomas initially responsive to hormone therapy and suggest a similar mechanism for acquired aromatase inhibitor resistance. However, ERa mutations can occur in gynecologic malignancies even in the absence of prior endocrine therapy. Tumors harboring these mutations may remain responsive to SERM/SERDs. Citation Format: J. A. Elvin, L. Gay, G. Colon-Otero, M. Jorgensen, L. Havrilesky, D. Zajchowski, L Shawver, F. A. Valea, S Aithal, J. S. Ross, M. Markman, S. Gaillard. CONSTITUTIVELY ACTIVE ESTROGEN RECEPTOR–ALPHA LIGAND BINDING DOMAIN (ERA–LBD) MUTATIONS IN OVARIAN CARCINOMA [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-056.