ETHAN - ET for Male BC

Inclusion Criteria:

• Men aged 18 years or older, with diagnosis of invasive breast cancer who have notundergone surgical resection of the primary tumor and axillary nodes.

• Stage I, II, or III per American Joint Committee on Cancer (AJCC) staging 8thedition (112).

• Breast cancer must be hormone receptor-positive and HER2-negative according todefinition below assessed by local pathology.

• Hormone receptor-positive is defined as: positivity for at least one of thehormone receptors (estrogen receptor [ER] or progesterone receptor [PR]) byIHC. ER and PR assays are considered positive if there are > 1% positive tumornuclei in the samples.

• HER2-negative is defined per the current American Society of ClinicalOncology/College of American Pathologists Clinical Practice Guideline (113).

• Patients with multifocal or multicentric disease are eligible if the treatinginvestigator has determined the patient should be treated as ER-positive andHER2-negative.

• Bilateral breast cancers are allowed if the treating investigator has determined thepatient 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 highlyeffective form of non-hormonal contraception or two effective forms of non-hormonalcontraception by the patient and/or partner and continue its use for the duration ofthe 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-completedquestionnaires.

• 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 therapyfor the current breast cancer diagnosis.

• Prior endocrine therapy, systemic therapy, radiation therapy, or investigationaltherapy 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 HeartAssociation (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 (forexample, interstitial lung disease, severe dyspnea at rest or requiring oxygentherapy, severe renal impairment [e.g. estimated creatinine clearance <30ml/min],history of major surgical resection involving the stomach or small bowel, orpreexisting Crohn's disease or ulcerative colitis or a preexisting chronic conditionresulting 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 detectableviral infection (such as known human immunodeficiency virus positivity or with knownactive 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 ofcardiovascular etiology, ventricular arrhythmia of pathological origin (including,but not limited to, ventricular tachycardia and ventricular fibrillation), or suddencardiac arrest.