This is a Phase III clinical trial designed to evaluate the efficacy and safety of XZP-3287 in combination with Letrozole/Anastrozole versus placebo in combination with Letrozole/Anastrozole in patients with HR-positive and HER2-negative recurrent/metastatic breast cancer who have not received systemic anticancer therapy.

Inclusion Criteria:

1. Female patients aged ≥18 years and ≤75 years old;
2. Patient is in the menopausal state；
3. Histologically or cytologically confirmed locally advanced, recurrent or metastatic breast cancer, which was pathologically confirmed to be HR-positive and HER2-negative;
4. Evidence of locally advanced, recurrent, or metastatic disease that is not amenable to curative surgical resection or radiation therapy and is not clinically amenable to chemotherapy;
5. No prior systemic anticancer therapy for locally advanced, recurrent, or metastatic disease;
6. At least one measurable lesion (based on RECIST v1.1) or only bone metastases;
7. Patient has an Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1;
8. Adequate organ and marrow function;
9. Patient of childbearing age must undergo a serum pregnancy test within 7 days before randomization, and the result is negative; patient is willing to use a medically approved high-efficiency contraceptive method during the study period and within 6 months after the last study drug treatment;
10. Patient with acute toxic reactions caused by previous anti-tumor treatments or surgical operations were alleviated to grade 0 to 1 (NCI-CTCAE v5.0), or to the level specified by the enrollment criteria;
11. Patient has signed informed consent before any trial related activities.

Exclusion Criteria:

1. Patients with disease recurrence or metastasis at or within 12 months after previous neoadjuvant or adjuvant therapy with endocrine drugs;
2. Patient with visceral crisis or brain metastases, except for patient with stable brain metastases;
3. Patient had clinically significant pleural effusions, ascites effusions, or pericardial effusions in the 4 weeks before enrollment;
4. Patient who received prior treatment with mTOR inhibitors or CDK4/6 inhibitors ;
5. Participation in a prior treatment of major surgery, chemotherapy, radiotherapy, and any anti-tumor treatment within 14 days before enrollment；
6. Patient who participated in other clinical trials within 14 days before enrollment or within 5 half-lives of the trial drug, whichever is longer;
7. Patient used CYP3A4 potent inhibitors or potent inducers within 14 days before enrollment or within 5 half-lives of the drug, whichever is longer;
8. Initiation of bisphosphonates or RANKL inhibitors within 7 days before enrollment, and subjects who have initiated treatment > 7 days before enrollment should not change the method of use;
9. Any other malignant tumor has been diagnosed within 5 years before randomization;
10. Patient is in the active stage of HBV, HCV or co-infected with HBV, HCV, or Patient with positive HIV antibody;
11. Patient with severe infection within 4 weeks before enrollment, or unexplained fever> 38.3℃ during screening/before enrollment;
12. Patient with heart function impaired or clinically significant heart disease within 6 months before enrollment;
13. Cerebrovascular accident occurred within 6 months before enrollment, including history of transient ischemic attack or stroke; pulmonary embolism;
14. Inability to swallow, intestinal obstruction or other factors that affect the taking and absorption of the drug;
15. Patient with a known hypersensitivity to any of the excipients in this study;
16. A prior history of autologous or allogeneic hematopoietic stem cell transplantation;
17. A prior history of psychotropic drug abuse or drug use;
18. Nursing Mothers;
19. The researchers considered that there were some cases that were not suitable for inclusion.

The data supporting the findings of this trial, including source data, cannot be made available openly owing to their proprietary nature and that the BRIGHT-3 study is still ongoing and blinded.