⁃ Subject must meet all of the following applicable inclusion criteria to participate in this study:

• Written informed consent (stage II/ main consent) and HIPAA authorization for release of personal health information obtained prior to performing any study-specific screening procedures. NOTE: HIPAA authorization may be included in the informed consent or obtained separately.

• Male or female age ≥ 18 years at the time of consent. NOTE: Both pre- and post-menopausal women are eligible. Post-menopausal status is defined as:

‣ Prior bilateral oophorectomy

⁃ Age ≥60

⁃ Age \<60 and amenorrhea for the last 12 or more months(in the absence of chemotherapy, tamoxifen, toremifen, or ovarian suppression) and FSH and estradiol in the postmenopausal range per local normal range.

• ECOG Performance Status of 0-1 within 28 days prior to registration.

• If patient is receiving tamoxifen or toremifene, a washout period of 5 half-lives (i.e. 35 days) prior to registration is required (during that period the participant can take AI).

• Patient has a histologically and/or cytologically confirmed diagnosis of estrogen-receptor positive and/or progesterone receptor positive breast cancer based on the most recently analyzed tissue sample and all tested by local laboratory.

• Patient has HER2-negative breast cancer defined as a negative in situ hybridization test or an IHC status of 0, 1+ or 2+. If IHC is 2+, a negative in situ hybridization (FISH, CISH, or SISH) test is required by local laboratory testing and based on the most recently analyzed tissue sample. If there is insufficient tissue from the most recently collected sample, earlier tissue may be used on a case-by-case basis if permission is granted by the sponsor investigator.

• Patients have had adequate local treatment for locoregional recurrence (LRR) of breast cancer.

‣ Locoregional recurrence is defined as recurrence in the ipsilateral breast, axilla, regional lymph nodes, or chest wall.

⁃ Local treatment is defined as either surgery, radiation therapy, or a combination of both if indicated.

⁃ Adequate local therapy is surgery with negative microscopic margins. Radiation therapy is mandated for patients with microscopically involved margins and recommended for all patients who had not received radiotherapy as part of their primary treatment.

⁃ Patients who have distant metastatic disease will not be eligible.

⁃ Prior treatment with neoadjuvant and adjuvant chemotherapy and ET is allowed.

• Patients must enroll within 6 months of the last local treatment, either local surgery or radiation; or systemic chemotherapy (if patient is receiving chemotherapy), whichever occurred last. Chemotherapy after local therapy is allowed. ET for recurrent disease is allowed for up to 12 months prior to enrollment.

• Patient has no contraindication to the adjuvant ET in the trial and is planned to be treated or continue treatment with ET.

• Demonstrate adequate organ function as defined below; all screening labs to be obtained within 28 days prior to registration.

‣ Hematological

• Absolute Neutrophil Count (ANC): ≥ 1.5 x 109/L

∙ Platelets: ≥ 100 x 109/L

∙ Hemoglobin (Hgb): ≥ 9.0 g/dL

⁃ Renal

∙ --Estimated glomerular filtration rate (eGFR): ≥ 30 mL/min/1.73m2 according to the Modification of Diet in Renal Disease (MDRD) formula

⁃ Hepatic

• Bilirubin: ≤ upper limit of normal (ULN) except for patients with Gilbert's syndrome who may only be included if the total bilirubin is ≤ 3.0 × ULN or direct bilirubin ≤ 1.5 × ULN

∙ Aspartate aminotransferase (AST): ≤ 2.5 × ULN except for patients with liver metastasis, who are only included if the AST is \< 5 × ULN

∙ Alanine aminotransferase (ALT): ≤ 2.5 × ULN except for patients with liver metastasis, who are only included if the ALT is \< 5 × ULN

⁃ Coagulation

∙ --International Normalized Ratio (INR) : ≤ 1.5 × ULN (unless is receiving anticoagulants and the INR is within the therapeutic range of intended use for that anticoagulant within 7 days prior to the first dose of study drug)

⁃ Electrolytes ---Potassium, Magnesium, and Total Calcium (corrected for serum albumin): Within normal limits or corrected to within normal limits with supplements.

• Standard 12-lead ECG values defined as

‣ QTcF interval at screening \< 450 msec (QT interval using Fridericia's correction)

⁃ Resting heart rate 50-90 bpm (determined from the ECG)

• Females of childbearing potential who are sexually active with a male able to father a child must have a negative pregnancy test (serum or urine) within 14 days prior to registration and must be willing to use a highly effective method of contraception that does not contain estrogen and/or progesterone. See the protocol for definition of childbearing potential.

• As determined by the enrolling physician or protocol designee, ability of the subject to understand and comply with study procedures for the entire length of the study.

• Ability to swallow and retain oral medication.