Inclusion Criteria:
* Men aged 18 years or older, with diagnosis of invasive breast cancer who have not undergone surgical resection of the primary tumor and axillary nodes.
* Stage I, II, or III per American Joint Committee on Cancer (AJCC) staging 8th edition (112).
* Breast cancer must be hormone receptor-positive and HER2-negative according to definition below assessed by local pathology.
* Hormone receptor-positive is defined as: positivity for at least one of the hormone receptors (estrogen receptor \[ER\] or progesterone receptor \[PR\]) by IHC. ER and PR assays are considered positive if there are \> 1% positive tumor nuclei in the samples.
* HER2-negative is defined per the current American Society of Clinical Oncology/College of American Pathologists Clinical Practice Guideline (113).
* Patients with multifocal or multicentric disease are eligible if the treating investigator has determined the patient should be treated as ER-positive and HER2-negative.
* Bilateral breast cancers are allowed if the treating investigator has determined the patient should be treated as ER-positive and HER2-negative.
* Patients with a history of ipsilateral or contralateral DCIS or LCIS are eligible.
* ECOG performance status ≤ 2.
* Required laboratory values demonstrating adequate organ function:
* ANC ≥ 1000/mm3
* Hemoglobin ≥ 8 g/dl
* Platelets ≥ 50,000/mm3
* Serum creatinine ≤ 3.0 x ULN (institutional)
* Total bilirubin ≤ 2.0 x ULN (institutional).
* AST and ALT ≤ 5.0 x ULN (institutional)
* Men with partners of childbearing potential must be willing to use one highly effective form of non-hormonal contraception or two effective forms of non-hormonal contraception by the patient and/or partner and continue its use for the duration of the study treatment and for 6 months after the last dose of study treatment.
* Non-English-speaking patients are eligible but will be exempt from patient-completed questionnaires.
* Willing and able to sign informed consent.
* Willing to undergo breast biopsy after completion of window phase.
* Patient is able to swallow oral medications.
Exclusion Criteria:
* Prior endocrine therapy, chemotherapy, radiation therapy, or investigational therapy for the current breast cancer diagnosis.
* Prior endocrine therapy, systemic therapy, radiation therapy, or investigational therapy for any other malignancy within the past 12 months.
* Diagnosis of inflammatory breast cancer (T4d).
* Other concurrent serious diseases that may interfere with planned treatment, including severe cardiac disease, congestive heart failure (CHF) of New York Heart Association (NYHA) Class III or higher, severe pulmonary conditions/illness, uncontrolled infections.
* The patient has serious and/or uncontrolled preexisting medical condition(s) that, in the judgment of the investigator, would preclude participation in this study (for example, interstitial lung disease, severe dyspnea at rest or requiring oxygen therapy, severe renal impairment \[e.g. estimated creatinine clearance \<30ml/min\], history of major surgical resection involving the stomach or small bowel, or preexisting Crohn's disease or ulcerative colitis or a preexisting chronic condition resulting in baseline Grade 2 or higher diarrhea).
* The patient has active systemic bacterial infection (requiring intravenous \[IV\] antibiotics at time of initiating study treatment), fungal infection, or detectable viral infection (such as known human immunodeficiency virus positivity or with known active hepatitis B or C \[for example, hepatitis B surface antigen positive\]. Screening is not required for enrollment.
* The patient has a personal history of any of the following conditions: syncope of cardiovascular etiology, ventricular arrhythmia of pathological origin (including, but not limited to, ventricular tachycardia and ventricular fibrillation), or sudden cardiac arrest.
