Background Endocrine therapy (ET) targeting ER signaling is the mainstay of care for ER+ metastatic BC. There remains an unmet need in patients whose tumors become resistant to currently available ET. Selective ER degraders (SERDs) were developed to overcome resistance to existing ER-directed therapies by both competitively antagonizing and degrading ERs, while exploiting continued dependence of the tumor on ER signaling. SAR439859 is a potent SERD with robust preclinical ER degrading activity. In AMEERA-1, a Phase 1 dose escalation trial, SAR439859, had a favorable safety profile with no dose-limiting toxicities across all doses (20-600 mg once per day; QD). ER occupancy generally exceeded > 87% with plasma concentrations > 100 ng/mL. Overall response rate was 6.3% and the recommended Phase 2 monotherapy dose was 400 mg QD. Methods AMEERA-3 is an international, prospective, open-label, randomized Phase 2 study (NCT04059484; ACT16105) designed to assess safety and efficacy of SAR439859 in pts with ER+ (>1%)/HER2 metastatic or locally advanced BC progressing on ≥ 6 months of continuous ET (0-2 lines in the metastatic setting). Prior cyclin-dependent kinase (CDK) inhibitors are allowed. Exclusion criteria include: Eastern Cooperative Oncology Group performance status (ECOG PS) ≥ 2, life expectancy < 3 months, > 1 chemotherapy or targeted therapy in the metastatic setting, concomitant illness and factors potentially affecting SAR439859 absorption. Patients are randomized 1:1 to SAR439859 400 mg QD orally or physician’s choice of endocrine monotherapy (fulvestrant, tamoxifen, aromatase inhibitor). Patients receive 28-day cycles until unacceptable toxicity, progression, death, investigator decision, or patient request. Stratification factors include visceral metastases, prior CDK4/6 inhibitors, and ECOG PS. The primary endpoint is progression-free survival (Response Evaluation Criteria in Solid Tumors v1.1). Secondary endpoints include overall survival, response rate, duration of response, clinical benefit, pharmacokinetics, quality of life and safety. Target enrollment: n = 282; current enrollment: n = 9. Funding: Sanofi. © 2020 American Society of Clinical Oncology, Inc. Reused with permission. This abstract was accepted and previously presented at the 2020 ASCO Annual Meeting. All rights reserved.

Citation Format: Sara Tolaney, Irfan Cicin, Rafael Betancourt, Arlene Chan, Diego Kaen, Peter Kaufman, Suzette Delaloge, Qianying Liu, Sylvaine Cartot-Cotton, Amele Amrate, Vasiliki Pelekanou, Katharine Cuff. AMEERA-3, a phase 2 trial of SAR439859 vs endocrine monotherapy in pre- and post-menopausal, estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2-negative (her2−), locally advanced or metastatic breast cancer (BC) with prior exposure to hormonal therapies [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr OT-09-09.