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.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.

Eligibility last updated 4/19/23. Questions regarding updates should be directed to the study team contact.