This clinical trial is designed to be a multi-center prospective, parallel-controlled Phase III clinical study. In this study, the efficacy of tamoxifen versus toremifene shall be compared in CYP2D6 intermediate/poor metabolizers of premenopausal patients with estrogen receptor-positive early breast cancer.

STUDY BACKGROUND Breast cancer is a serious disease that threatens human health and life. Especially in China, the incidence rate is increasing year by year. According to WHO data, the incidence of breast cancer in China in 2020 will reach 214,000. Selective estrogen receptor modulators (SERMs) are a classic form of endocrine therapy for early breast cancers, but not all hormone receptor positive breast cancers benefit from specific SERMs. Numerous studies have shown that CYP2D6 variant carriers (around 50% CYP2D6 variant carriers in Chinese population) will not benefit a lot from tamoxifen, and combined use of CYP2D6 inhibitors will further affect the efficacy of tamoxifen. However, studies on another SERM drug - toremifene have shown that its metabolism and pharmacological effects are not influenced by CYP2D6 genotype or enzyme activity. Therefore, in the principle of individualized medicine, it is necessary to compare the efficacy of using tamoxifen and toremifene in CYP2D6 variant carriers in China so as to provide more guidance for clinical use.

OBJECTIVES:

1. The main purpose of this study is to compare 5-year disease-free survival rate of adjuvant endocrine therapy with tamoxifen and toremifene in premenopausal women with estrogen receptor-positive early breast cancer who are CYP2D6 intermediate/poor metabolizers.

2. The secondary purpose of this study includes:

   1. To compare 5-year overall survival (OS) and safety of adjuvant endocrine therapy with tamoxifen and toremifene in premenopausal patients with estrogen receptor-positive early breast cancer who are CYP2D6 intermediate/poor metabolizers.
   2. To compare the changes of plasma concentration of the parent drugs and metabolites of tamoxifen and toremifene in premenopausal patients with estrogen receptor-positive early breast cancer who are CYP2D6 intermediate/poor metabolizers.
   3. To assess the pharmacokinetics of tamoxifen and toremifene in premenopausal patients with estrogen receptor-positive early breast cancer who are CYP2D6 intermediate/poor metabolizers.

OUTLINE:

First, CYP2D6 genotype screening shall be conducted in premenopausal patients with estrogen receptor-positive early breast cancer in order to determine the frequency of different alleles. Then, patients who are CYP2D6 intermediate/poor metabolizers (with *4, *5, *10, *14, *17, *41 alleles) shall be stratified and randomized at the ratio of 1:1 ratio: allele status of CYP2D6 CYP2D6 intermediate/poor metabolizer (Heterozygous or homozygous), lymph node metastasis (with vs. without), prior chemotherapy (with vs. without), and HER2 status (positive vs. negative). Included patients shall be divided into two groups. One group will be given Tamoxifen (10mg Bid) for 5 years and the other group will be given toremifene (60mg qd) for 5 years. Then 5-year disease-free rate and overall survival and safety will be compared between these two groups. At Month 6, pharmacokinetic study on tamoxifen, toremifene and their metabolites will be conducted on patients.

Inclusion Criteria:

1. Premenopausal women aged 18-50 years;
2. ECOG PS: 0-2 points;
3. Invasive breast cancer confirmed by histology with ER ≥ 10% (all test results should be reviewed and confirmed by Department of Pathology of the participant institution);
4. Participants have completed the standard local radical treatment (modified or conservative radical mastectomy) with or without neo-adjuvant/adjuvant chemotherapy or radiotherapy;
5. Participants must be able to understand this study and are willing to participate, agree to genotype screening and sign informed consent form with good compliance and cooperation in follow-ups;
6. Polymorphism analysis showed that patients are CYP2D6 * 4, * 5, * 10, * 14, * 17, * 41 allele carriers;
7. Hemoglobin ≥ 90g/L, neutrophils ≥ 1.5 × 109/L, platelets ≥ 75 × 109/L, AST and ALT ≤ 2.5 times the upper limit of normal (ULN), serum creatinine and urea nitrogen ≤ ULN.

Exclusion Criteria:

1. Patients have previously received neoadjuvant endocrine therapy or have started adjuvant endocrine therapy;
2. There are any comorbidities that may increase the level of sex hormones: such as pituitary adenomas, ovarian tumors, thymic carcinomas, etc.;
3. There are any comorbidities that may reduce the level of sex hormones such as hyperthyroidism, hypothyroidism, cirrhosis, severe malnutrition, Turner syndrome, lack of sex hormone synthetase, intracranial tumors, pituitary atrophy etc.;
4. Patients have undergone or planned to conduct ovariectomy or ovarian function inhibition;
5. Patients needs to take other medicines which can influence the activity of CYP2D6 (such as fluoxetine, paroxetine, quinidine, bupropion), CYP3A4 (such as erythromycin, acetylspiramycin, ritonavir, ketoconazole, nicardipine);
6. Patients have been treated with other trial medications in the past 2 weeks;
7. Pregnant or lactating women (women of childbearing age must have a negative pregnancy test within 14 days of the first dosing, and if pregnant, Patients are required for ultrasound examination to exclude pregnancy);
8. Women of childbearing age who are not willing to take effective contraception during treatment;
9. There are serious non-malignant tumor comorbidities that may affect long-term follow-up;
10. Patients have family history of endometrial, ovarian or other gynecologic malignancies;
11. Transvaginal ultrasound suggested more serious ovarian abnormalities or endometrial thickening;
12. Patients have had thrombotic events such as cerebrovascular accident (including transient ischemic attack), deep venous thrombosis, and pulmonary embolism within 6 months prior to study initiation;
13. Serious liver insufficiency with Child-Pugh C grade;
14. Serious cardiac insufficiency with New York Heart Association (NYHA) grade ≥III;
15. Patients are known severely allergic to study drug;
16. Patients have history of other malignancies in the past five years, except for cutaneous basal cell carcinoma and cervical carcinoma in situ which have been cured;
17. In other cases, the researchers don't think the subjects are suitable for participate in the study.