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).