Registration (Step 1) – Inclusion Criteria:

• Age ≥18 years and ˂50 years.

• Have histologically or cytologically confirmed, localized or locally advanced hormone positive breast cancer Stage I-III (Defined as ER Immunohistochemistry [IHC] >1%) having completed curative intent therapy and clinically in remission.

• Currently receiving ovarian function suppression (OFS) with use of goserelin on a monthly basis in the abdomen and either aromatase inhibitor or tamoxifen for at least 6 months prior to study enrollment for treatment of hormone receptor positive breast cancer with plan to continue medical OFS for at least the next 12 months.

• ECOG Performance Status (PS) 0, 1 or 2 (Appendix I). Or Karnofsky Performance Status of ≥60% (Appendix II).

• Provide written informed consent.

• Ability to complete questionnaire(s) by themselves or with assistance.

• Willingness to provide mandatory blood specimens for correlative research (see Section 14.0).

• Willing to return to enrolling institution for follow-up (during the Active Monitoring Phase of the study)

• Negative serum pregnancy test ≤14 days prior to registration, and a negative urine pregnancy test ≤7 days prior to randomization.

• Sexually active patients and their partners must use an effective method of contraception associated with a low failure rate prior to study entry and for the duration of study participation and for at least 3 months after the last dose of study drug.

  Note: The following are considered effective contraceptives: oral contraceptive pill; condom plus spermicide; diaphragm plus spermicide; patient or partner surgically sterile; patient or partner more than 12 months postmenopausal; or injectable or implantable agent/device. Male patients should refrain from sperm donation and female patients should refrain from breastfeeding throughout this period.

Registration (Step 1) – Exclusion Criteria

• Any of the following prior therapies:

  • Chemotherapy ≤6 months prior to registration. NOTE: concurrent receipt of HER2 directed antibodies or antibody-drug conjugates, endocrine therapy, or CDK 4/6 inhibitor is permitted.

• Receiving any estrogen or progestin containing medications, including topical estrogens.

• Planning to temporarily or permanently discontinue medical OFS in the next 12 months.

Randomization (Step 2) – Inclusion Criteria

• Completion of the lead-in treatment of 6 cycles of ovarian function suppression (OFS) with use of goserelin, with estradiol E2 value of ˂20 pg/mL after Cycle 6 blood draw.

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

Eligibility last updated 11/7/2025. Questions regarding updates should be directed to the study team contact.