This is a phase III clinical trial to evaluate the efficacy and safety of XZP-3287 in combination with Fulvestrant versus placebo combined with Fulvestrant in Patients who have HR positive and Her2 negative recurrent/metastatic breast cancer and have received prior endocrine therapy are eligible for study.

Inclusion Criteria:

1. Female patients aged ≥18 years and ≤75 years old;
2. Patient is in the menopausal state；
3. Pathologically-confirmed HR positive and Her2 negative Breast Cancer;
4. Locally advanced stage, recurrence or metastasis breast cancer; 4.1 Disease progression after previous endocrine therapy； 4.2 One previous line of chemotherapy for advanced/metastatic disease is allowed in addition to endocrine therapy;
5. At least one measurable lesion (based on RECIST v1.1) or only bone metastases;
6. Patient has an Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1;
7. Adequate organ and marrow function;
8. Patient of childbearing age must undergo a serum pregnancy test within 14 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;
9. 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;
10. Patient has signed informed consent before any trial related activities.

Exclusion Criteria:

1. Patient with visceral crisis, inflammatory breast cancer, or brain metastases, except for patient with stable brain metastases;
2. Patient had clinically significant pleural effusions, ascites effusions, or pericardial effusions in the 4 weeks before enrollment;
3. Patient who received prior treatment with mTOR inhibitors, CDK4/6 inhibitors or fulvestrant;
4. Participation in a prior treatment of major surgery, chemotherapy, radiotherapy, and any anti-tumor treatment within 14 days before enrollment；
5. Patient who participated in other clinical trials within 14 days before enrollment or within 5 half-lives of the trial drug, whichever is longer;
6. Patient used CYP3A4 potent inhibitors or potent inducers within 14 days before enrollment or within 5 half-lives of the drug, whichever is longer;
7. Patient who used bisphosphonates or RANKL inhibitors within 7 days before enrollment, patient who have started treatment during the study should not change the method of use;
8. Any other malignant tumor has been diagnosed within 3 years before randomization;
9. Patient is in the active stage of HBV, HCV or co-infected with HBV, HCV, or Patient with positive HIV antibody;
10. Patient with severe infection within 4 weeks before enrollment, or unexplained fever> 38.5℃ during screening/before enrollment;
11. Patient with heart function impaired or clinically significant heart disease within 6 months before enrollment;
12. Cerebrovascular accident occurred within 6 months before enrollment, including history of transient ischemic attack or stroke; symptomatic pulmonary embolism;
13. Inability to swallow, intestinal obstruction or other factors that affect the taking and absorption of the drug;
14. Patient with a known hypersensitivity to any of the excipients in this study;
15. A prior history of autologous or allogeneic hematopoietic stem cell transplantation;
16. A prior history of psychotropic drug abuse or drug use;
17. Pregnant or breastfeeding;
18. The researchers considered that there were some cases that were not suitable for inclusion.