Impact of Formulation Change on Ovarian Suppression in Young Breast Cancer Patients.

Inclusion Criteria:

Step 1：iOFS detection phase

1. Female, aged ≥18 and ≤45 years;
2. Histologically confirmed hormone receptor-positive (HR+) (estrogen receptor \[ER\] and/or progesterone receptor \[PR\] ≥1%) and human epidermal growth factor receptor 2-negative (HER2-) early invasive breast cancer (stage I-III according to the American Joint Committee on Cancer \[AJCC\], version 8)；
3. Completed curative surgery, with prior (neo)adjuvant chemotherapy and radiotherapy completed if applicable;
4. Currently receiving adjuvant therapy with a 3-month GnRH agonist (GnRHa) plus aromatase inhibitor (AI) or tamoxifen (TAM), with or without CDK4/6 inhibitors (excluding abemaciclib due to its potential interference with estradiol monitoring), for ≥1 dose, and presenting with estradiol (E2) ≥30 pg/mL within 28 days prior to enrollment (measured by CLIA).
5. Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1; adequate bone marrow, hepatic, renal, and cardiac function.
6. Voluntarily sign a written informed consent form before the trial screening.

Step 2：Randomized treatment phase

1. iOFS confirmed using the SEMS assay (E2 ≥30 pg/mL);
2. Ongoing adjuvant therapy with a 3-month GnRHa plus AI or TAM, with or without CDK4/6 inhibitors.
3. No evidence of disease progression.

Exclusion Criteria:

Step 1：iOFS detection phase

1. Bilateral breast cancer, inflammatory breast cancer, or distant metastasis;
2. Use of GnRHa for ovarian function preservation;
3. History of ovarian resection or ablation; planned pregnancy or breastfeeding;
4. Concomitant use of hormonal agents other than estrogen, progesterone, selective estrogen receptor modulators (SERM), or selective estrogen receptor degraders (SERD);
5. Severe uncontrolled comorbidities;
6. Other malignancies within the past 5 years (except for curatively treated carcinoma in situ of the cervix or basal cell carcinoma);
7. Failure to comply with follow-up or psychiatric disorders.

Step 2：Randomized treatment phase

a)Prior conversion from a 3-month GnRHa to a 1-month GnRHa.