A Study to Evaluate Exemestane Tablets Combined With Ovarian Function Suppression/Ablation in Treatment of Premenopausal Breast Cancer Patients With CYP2D6*10 Mutations （STEP）

The target population for this study is early stage premenopausal estrogen-receptor positive breast cancer patients with CYP2D6\*10 mutations. All the potential subjects must provide the informed consent. The subjects who provide the informed consent form (ICF) will enter the screening period, and will be evaluated for the eligibility. This study expects to enroll 300 subjects, eligible subjects will be randomized at a ratio of 1:1 into treatment group (Exemestane Tablets combined with ovarian function suppression/ablation) or control group (Tamoxifen Tablets combined with ovarian function suppression/ablation). The subjects who are assigned to treatment group will receive Exemestane Tablet combined with ovarian function suppression/ablation, Exemestane Tablets orally, once a day, one tablet each time (25 mg) and to be taken within 8 weeks from receiving ovarian function suppression treatment or after bilateral ovariectomy, and continue for 5 years or until endpoint event occurs. If the patients are not treated for 5 years because of adverse events (AE) or other reasons, the patient should be followed up until 5 years. The subjects who are assigned to the control group will receive Tamoxifen Tablets combined with ovarian function suppression/ablation, Tamoxifen Tablets orally, twice a day, one tablet each time (10mg) and to be taken within 8 weeks from receiving ovarian function suppression treatment or after bilateral ovariectomy, and continue for 5 years or until endpoint event occurs. If the patients are not treated for 5 years because of adverse events (AE) or other reasons, the patient should be followed up until 5 years. The treatment regimen of ovarian function suppression/ablation is as follows: Gonadotropin-releasing hormone analogue Goserelin Injection 3.6mg or Leuprorelin Injection 3.75 mg, a subcutaneous injection will be done every 28±2 days, or bilateral ovariectomy. According to clinical practice of adjuvant endocrine therapy for Chinese patients with breast cancer, safety and effectiveness of all patients will be evaluated once every 3 months within 2 years after receiving the study treatment, every 6 months in 3 and 4 years, every 12 months in 5 years and 30 days after treatment discontinuation. After randomization, all patients should be regularly checked for signs, symptoms, and evidence of disease recurrence by collecting their medical history, physical examination, and local examination of the breast (specialized physical examination and/or imaging examination). Patients are advised to take a bone mineral density test once a year within five years from 6 months after randomization, and receive calcium or phosphate correspondingly based on the condition. Throughout the study, the adverse events need to be closely monitored, and the frequency and severity of adverse events are recorded as well. The study results will be statistically analyzed after the study to compare the efficacy and safety variables between the two groups, thus, the superiority of Exemestane Tablets combined with ovarian function suppression/ablation to Tamoxifen Tablets combined with ovarian function suppression/ablation in disease-free survival (DFS) will be demonstrated. Stratified analyses according to lymph-node status and age.