Estradiol Plus Olaparib for Breast Cancer (PHOEBE)

Inclusion Criteria: * Post-menopausal women with ER+/HER2- breast cancer. * Metastatic or locoregional recurrence not amenable to treatment with curative intent. * Received ≥1 prior line of endocrine-based therapy in the advanced/metastatic setting. Exclusion Criteria: * During study treatment, no concurrent anti-cancer therapies are allowed with the following exceptions: o Anti-resorptive bone therapies (e.g., bisphosphonates, denosumab) permitted. * Any investigational cancer therapy or systemic chemotherapy in the last 3 weeks. * Any radiation therapy in the last 2 weeks. * Known CNS disease, unless clinically stable for ≥ 3 months. * Concomitant use of known strong or moderate CYP3A inhibitors. * Persistent toxicities (≥CTCAE grade 2) caused by previous cancer therapy. * 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% * Severe renal impairment (creatinine clearance ≤ 30 mL/min).