CLINICAL TRIAL / NCT03941730

Inclusion Criteria: - PRE-SCREENING CRITERIA (STEP 0): Women of age >= 18 years - PRE-SCREENING CRITERIA (STEP 0): History of locally advanced or metastatic breast cancer that is ERalpha negative or low (< 1% nuclear staining) and HER2 negative. - Note: HER2 negative disease per 2018 American Society of Clinical Oncology/College of American of Pathologists (ASCO/CAP) guidelines, one of the following must apply: - 0 or 1+ by immunohistochemistry (IHC) and not amplified by in situ hybridization (ISH); - 0 or 1+ by IHC and ISH not done; - 2+ by IHC and ISH results are: < 6.0 HER2 signals/cell with HER2/CEP17 ratio < 2.0; - IHC not done and not amplified by ISH. - PRE-SCREENING CRITERIA (STEP 0): =< 3 prior chemotherapy regimens for treatment of metastatic breast cancer. - Note: Prior use of monoclonal antibodies targeting PD1, PDL1 is allowed (if administered as monotherapy it is not counted as a chemotherapy regimen). - PRE-SCREENING CRITERIA (STEP 0): Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 - PRE-SCREENING CRITERIA (STEP 0): Willing to submit a biopsy specimen from locally recurrent or metastatic site (or primary if metastatic site not available) of breast cancer for ERbeta staining to Mayo Clinic Anatomic Pathology. - PRE-SCREENING CRITERIA (STEP 0): No prior history of metastatic ERalpha positive breast cancer (>= 1%) - PRE-REGISTRATION CRITERIA (STEP 1): Presence of moderate or strong nuclear ERbeta staining in > 25% of cells in specimen submitted during Pre-Screening Step. - PRE-REGISTRATION CRITERIA (STEP 1): For patients who did not have a biopsy or lacking ERalpha, progesterone receptor (PR), and HER2 results from a locally advanced or metastatic site performed =< 12 months prior to Pre-Registration: Willing to undergo a standard of care biopsy of locally recurrent or metastatic breast cancer for ERalpha, PR, and HER2 as well as additional research cores. - PRE-REGISTRATION CRITERIA (STEP 1): Measurable or non-measurable disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST) criteria that will be assessed using imaging-based evaluations. - Note: The tumor lesion biopsied during the pre-registration period is not considered measurable disease nor a target lesion. - PRE-REGISTRATION CRITERIA (STEP 1): If history of brain metastases must meet the following criteria: - Patients with a history of brain metastases are eligible only if they are asymptomatic and have stable disease for >= 3 months, including < 28 days of prior to pre-registration. - Not receiving steroids for brain metastases. - PRE-REGISTRATION CRITERIA (STEP 1): ECOG performance status 0 or 1. - PRE-REGISTRATION CRITERIA (STEP 1): =< 3 prior chemotherapy regimens for treatment of metastatic breast cancer. - NOTE: Prior use of monoclonal antibodies targeting PD1, PDL1 is allowed. - PRE-REGISTRATION CRITERIA (STEP 1): Women must be postmenopausal. - NOTE: Postmenopausal status is verified by: - Prior bilateral surgical oophorectomy, or - Age >= 60 years, or - Age < 60 years with no menses for > 1 year with estradiol levels within postmenopausal range, according to institutional standard. - PRE-REGISTRATION CRITERIA (STEP 1): Able to swallow oral medications. - PRE-REGISTRATION CRITERIA (STEP 1): Willingness to stop use of strong inducers or inhibitors of CYP3A4 prior to registration. - NOTE: Use of strong inducers or inhibitors is allowed during pre-registration as long as patient will complete course prior to registration. - REGISTRATION CRITERIA (STEP 2): For patents who had a biopsy taken from a metastatic site =< 12 months prior to Pre-Registration: Confirmation from the local lab that the tumor from this biopsy was ERalpha negative (< 1% nuclear staining) and HER2 negative - REGISTRATION CRITERIA (STEP 2): For patients who underwent a pre-registration biopsy: Histologic confirmation from local lab that tumor is ERalpha negative (< 1% nuclear staining), and HER2 negative - REGISTRATION CRITERIA (STEP 2): Hemoglobin >= 8 g/dL (=< 14 days prior to registration). - REGISTRATION CRITERIA (STEP 2): Platelet count >= 75,000/mm^3 (=< 14 days prior to registration). - REGISTRATION CRITERIA (STEP 2): Creatinine =< 1.5 x upper limit of normal (ULN) (=< 14 days prior to registration). - REGISTRATION CRITERIA (STEP 2): Total bilirubin =< 1.5 x ULN (=< 14 days prior to registration). - REGISTRATION CRITERIA (STEP 2): Aspartate aminotransferase/serum glutamic-oxaloacetic transaminase (AST/SGOT) =< 2.5 x ULN (=< 14 days prior to registration). - For patients with liver metastasis =< 5 x ULN. Exclusion Criteria: - PRE-REGISTRATION CRITERIA: Uncontrolled intercurrent illness including, but not limited to: - Ongoing or active infection. - Symptomatic congestive heart failure. - Unstable angina pectoris. - Uncontrolled symptomatic cardiac arrhythmia. - Uncontrolled hypertension (defined as blood pressure > 160/90). - PRE-REGISTRATION CRITERIA: Deep vein thrombosis / pulmonary embolism (DVT/PE) =< 12 months prior to pre-registration. - Note: Patients who are on anticoagulant therapy for maintenance are eligible as long as the DVT and/or PE occurred > 6 months prior to pre-registration, and there is no evidence for active thrombosis (either DVT or PE). - PRE-REGISTRATION CRITERIA: Stroke =< 6 months prior to pre-registration. - PRE-REGISTRATION CRITERIA: Two or more episodes of DVT and/or PE =< 5 years prior to pre-registration. - PRE-REGISTRATION CRITERIA: Abnormal uterine bleeding =< 6 months prior to pre-registration - PRE-REGISTRATION CRITERIA: History of coagulopathy. - PRE-REGISTRATION CRITERIA: Other active second malignancy other than non-melanoma skin cancers within 3 years prior to pre-registration. - NOTE: A second malignancy is not considered active if all treatment for that malignancy is completed and the patient has been disease-free for >= 3 years prior to pre-registration. - REGISTRATION CRITERIA: None of the following therapies are allowed =< 14 days prior to registration. - Chemotherapy. - Immunotherapy. - Biologic therapy. - Hormonal therapy. - Monoclonal antibodies. - Anti-HER2 or other "targeted" (e.g. mTOR) therapy. - Note: Any adverse events derived from these therapies must be =< grade 2 prior to starting study therapy (exceptions for alopecia).