Study of BEBT-209 in Combination With Fulvestrant Versus Placebo in Combination With Fulvestrant in Patients With HR+/HER2- Locally Advanced or Metastatic Breast Cancer Who Have Progressed After Prior Endocrine Therapy

Inclusion Criteria: 1. Women aged 18 years or older who are postmenopausal or premenopausal/perimenopausal, and meet one of the following criteria: previous bilateral oophorectomy, or age ≥60 years; or age \<60 years with natural menopause (defined as spontaneous cessation of regular menstruation for at least 12 consecutive months without other pathological or physiological causes), and estradiol (E2) and follicle-stimulating hormone (FSH) levels in the postmenopausal range; or premenopausal or perimenopausal women willing to receive luteinizing hormone-releasing hormone (LHRH) agonist therapy during the study period. 2. Patients with histologically confirmed HR+/HER2- breast cancer (based on the most recent test results from either recurrent/metastatic lesion tissue samples or prior primary lesion tissue samples): a) ER+ and/or PR+ is defined as ≥10% of tumor cells showing positive staining for ER/PR; b) HER2- is defined as an immunohistochemistry(IHC) score of 0 or 1+, or a fluorescence in situ hybridization(FISH) ratio of HER2/CEP17 less than 2.0, or HER2 gene copy number less than 4. 3. Evidence of focal recurrence or metastasis, not suitable for curative surgery or radiotherapy, and without clinical indications necessitating chemotherapy. 4. Previous endocrine therapy must meet one of the following criteria: a) progression during or within 12 months after discontinuation of adjuvant endocrine therapy (with an aromatase inhibitor \[aromatase inhibitor or selective estrogen receptor modulator such as tamoxifen, toremifene, etc.); b) progression during or within 1 month after discontinuation of endocrine therapy for first recurrence or metastasis. 5. Patients are allowed to have received no more than one line of chemotherapy during the recurrence or metastatic phase. 6. Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1. 7. Presence of measurable lesions according to response evaluation criteria in solid tumors (RECIST) 1.1 criteria or bone metastases only (including lytic or mixed lesions). 8. Adequate organ and marrow function, defined as follows: absolute neutrophil count (ANC) ≥1.5×10⁹/L (without use of growth factors within 14 days); platelets ≥100×10⁹/L (without corrective treatment within 7 days); hemoglobin ≥90 g/L (without corrective treatment within 7 days); serum creatinine ≤1.5 times the upper limit of normal (ULN) or estimated creatinine clearance ≥60 mL/min; aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 times the ULN (≤5 times the ULN for patients with liver metastases); total serum bilirubin (TBIL) ≤2.5 times the ULN; 12-lead electrocardiogram: Fridericia-corrected QT interval (QTcF) \< 470 msec (for females). 9. Women of childbearing potential must have a negative serum pregnancy test within 7 days prior to the first dose of study drug and must be willing to use a medically accepted and highly effective method of contraception from the time of signing the informed consent form, throughout the study period, and for 1 year after the last administration of the study drug. 10. All acute toxicities from prior anti-tumor therapy must have resolved to grade 0-1 (according to national cancer institute(NCI) common terminology criteria for adverse events(CTCAE) version 5.0) or to the levels specified in the inclusion/exclusion criteria. Exceptions include alopecia and other toxicities deemed by the investigator to pose no safety risk to the patient. 11. The patient has provided written informed consent and is willing and able to comply with the planned visits, study treatment schedule, laboratory tests, and other study procedures. Exclusion Criteria: 1. Patients who are deemed by the investigator to be unsuitable for endocrine therapy, including those with symptomatic, visceral metastatic disease, or those at risk of life-threatening complications in the short term (including patients with uncontrollable effusions \[pleural, pericardial, peritoneal\] despite drainage or other measures, pulmonary lymphangitis, or more than 50% liver involvement). 2. Known uncontrolled or symptomatic active central nervous system (CNS) metastases, characterized by clinical symptoms, brain edema, spinal cord compression, carcinomatous meningitis, leptomeningeal disease, and/or progressive growth; patients with a history of CNS metastases or spinal cord compression may be eligible if they have received definitive treatment and have been clinically stable for 4 weeks after discontinuation of anticonvulsants and corticosteroids prior to the first dose of study drug. 3. Previous treatment with fulvestrant, everolimus, or CDK4/6 inhibitors. 4. From the end of the last treatment to the first dose: underwent surgical or radiation therapy within 14 days prior to the first dose and has not fully recovered from the treatment; received chemotherapy, any investigational drug, or other anti-cancer therapy within 14 days or within five half-lives of the therapeutic agent prior to the first dose (whichever is longer is acceptable for enrollment). 5. A diagnosis of any other malignancy within 3 years prior to randomization, with the exception of non-melanoma skin cancer (basal cell or squamous cell carcinoma) or cervical carcinoma in situ that has been treated with curative intent. 6. Infection with human immunodeficiency virus (HIV) or known acquired immune deficiency syndrome (AIDS); active hepatitis B (patients positive for hepatitis B surface antigen \[HBsAg\] and with hepatitis B virus DNA \[HBV-DNA\] levels above the ULN of the local laboratory). Patients with HBV DNA levels above the ULN of the local laboratory are permitted to receive antiviral therapy prior to screening, and may be enrolled once viral copies are reduced below the ULN, but must continue antiviral therapy for hepatitis B during the study; hepatitis C (positive for hepatitis C virus antibody and with HCV RNA levels above the ULN of the local laboratory); or co-infection with both hepatitis