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%.