STUDY02001740;22SCH740: Estradiol For ER+ Advanced Breast Cancer (ESTHER)

Inclusion Criteria: * Post-menopausal women with ER+ breast cancer. * Metastatic or locoregional recurrence not amenable to treatment with curative * intent. * Received ≥1 prior line of endocrine-based therapy (e.g., including tamoxifen, aromatase inhibitors, fulvestrant, or combinations) in the advanced/metastatic setting Exclusion Criteria: * During the study Treatment Phase with 17b-estradiol, no concurrent anti-cancer therapies are allowed with the following exceptions: * Exception: Trastuzumab is allowed for the treatment of subjects with a history of HER2+ disease, and will be used at the physician's discretion. * Exception: Anti-resorptive bone therapies (e.g., bisphosphonates, denosumab) are permitted. * Any investigational cancer therapy in the last 3 weeks. * Known CNS disease, unless clinically stable for ≥ 3 months. * History of any of the following: * Deep venous thrombosis. * Pulmonary embolism. * Stroke. * Acute myocardial infarction. * Congestive heart failure. * Previous malignancy not treated with curative intent, or with an estimated recurrence risk ≥30%.