• Females aged 18-75 years (inclusive);

• ECOG performance status (PS) score of 0-1;

• Patients with histopathologically confirmed metastatic or unresectable locally advanced breast cancer, histopathologically confirmed ER-positive or PR-positive;

• Menopausal status:

∙ Having had bilateral oophorectomy, or aged ≥ 60 years old; or

‣ Aged \< 60, natural menopause with E2 and FSH at postmenopausal levels; or

‣ Premenopausal or perimenopausal patients, but they should receive LHRH agonists during the study and the treatment should be initiated prior to study treatment.

• Disease progression evidenced by imaging during or after the last systemic anti-tumor treatment prior to the first dose (limited to the efficacy expansion stage);

• With at least one extracranial measurable target lesion at baseline per RECIST v1.1;

• Life expectancy of \> 3 months;

• The functional level of organs must meet the following requirements :

• Absolute neutrophil count ≥ 1.5 × 109/L; Platelet count ≥ 90 × 109/L; Hemoglobin ≥ 10 g/dL; Normal blood creatinine or creatinine clearance ≥ 50 mL/min (calculated by standard Cockcroft-Gault formula); Serum albumin ≥ 3.0 g/dL; Total bilirubin ≤ 1.5 × upper limit of normal (ULN); Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 × ULN, or ≤ 5.0 × ULN for patients with liver metastasis; Prothrombin time (PT) and partial thromboplastin time (APTT) ≤ 1.5 × ULN; Urine protein \< 2+ or 24-h urine protein \< 1 g; Left ventricular ejection fraction (LVEF) ≥ 50%; QTcF ≤ 470 msec.

• Female subjects of childbearing potential should agree to adopt effective contraceptive measures during the study period and within 6 months after the end of the study treatment; female subjects of childbearing potential must have a negative serum HCG test result within 7 days before enrollment in the study and must not be in the lactation;

⁃ Voluntarily participate in this clinical study, be willing and able to comply with procedures related to clinical visits and study, and understand and have signed written informed consent.