Can a cell-cycle blocker boost hormone therapy and stop breast cancer from returning?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: 1. Women aged 18-70 years 2. ECOG score of 0-1 3. Histologically confirmed unilateral invasive cancer (regardless of pathology type) 4. No macroscopic or microscopic tumor residue after surgical resection 5. Early-stage breast cancer, pathologically confirmed as ER-positive and HER2-negative (ER-positive defined as immunohistochemical detection of ER \>10% of tumor cells positive; HER2 0-1+ or HER2 2+ but FISH negative, with no amplification, defined as HER2-negative) 6. Postoperative pathological staging of pT1c or above, N0M0, and meeting one of the following criteria: (1) G3; (2) G2 and meeting one of the following: i. Ki-67 ≥20%; ii. 21-gene recurrence score ≥26 or high genomic risk profile; iii. Age ≤40 years and vascular invasion positive 7. No prior neoadjuvant treatment 8. Time from surgery or completion of chemotherapy or radiotherapy (whichever occurs later) to randomization does not exceed 8 weeks 9. Good postoperative recovery, at least 1 week after surgery 10. Normal function of major organs, meeting the following criteria: (1) Hematological criteria: HB ≥90 g/L (no blood transfusion in the last 14 days); ANC ≥1.5 × 10\^9 /L; PLT ≥100 × 10\^9 /L; (2) Biochemical criteria: TBIL ≤1.5 × ULN (upper limit of normal); ALT and AST ≤3 × ULN; serum Cr ≤1.5 × ULN 11. Women of childbearing age must use contraception during treatment 12. Subjects voluntarily join this study, sign informed consent, demonstrate good compliance, and cooperate with follow-up Exclusion Criteria: 1. Bilateral breast cancer or ductal/lobular carcinoma in situ (DCIS/LCIS) 2. Received treatment for advanced disease 3. Metastasis at any site 4. Any tumor \> T4a (with skin involvement, fixation, inflammatory breast cancer) 5. Clinically or radiologically suspected malignancy in the contralateral breast not confirmed and requiring biopsy 6. Received neoadjuvant therapy, including chemotherapy, radiotherapy, and endocrine therapy 7. History of malignant tumors within the past 5 years (excluding basal cell carcinoma of the skin and cervical carcinoma in situ), including contralateral breast cancer 8. Patient has been enrolled in other clinical trials 9. Patient has severe systemic diseases and/or uncontrolled infections that prevent participation in the study 10. Severe cardiovascular or cerebrovascular diseases within 6 months prior to randomization (e.g., unstable angina, chronic heart failure, uncontrolled hypertension \>150/90 mmHg, myocardial infarction, or stroke) 11. Known allergy to relevant medications 12. Women of childbearing age who refuse contraception during treatment and for 8 weeks after treatment completion 13. Pregnant or breastfeeding women 14. Positive pregnancy test before drug administration after joining the trial 15. Individuals with mental illness or cognitive impairment who cannot understand the trial protocol and its side effects, unable to comply with the protocol and follow-up (systematic assessment required prior to enrollment) 16. Individuals without personal freedom and independent civil capacity