This is a prospective, single-arm, multicenter Phase II study of patients with advanced HR+/HER2- breast cancer who are untreated or have failed previous first-line endocrine therapy。The primary objective of this study was to explore the efficacy and safety of the PD-L1 inhibitor adebrelimab in combination with the CDK4/6 inhibitor Dalpiciclib and standard endocrine therapy in advanced HR+/ HER2-breast cancer.

Inclusion Criteria:

1. Premenopausal/perimenopausal or postmenopausal women aged ≥18 years and ≤75 years;

2. Histologically confirmed HR+/HER2- invasive breast cancer (specific definition: ER >10% tumor cell positive is defined as ER positive, PR >10% tumor cell positive is defined as PR positive, ER and/or PR positive is defined as HR positive; HER2 0-1+ or HER2 ++ but negative by FISH test, no amplification, defined as HER2 negative);

3. Locally advanced breast cancer (radical local treatment is not possible) or recurrent metastatic breast cancer;

4. Did not receive any systemic anti-cancer therapy at the stage of recurrence and metastasis or failed to receive first-line endocrine therapy at the advanced stage;

5. Allowed to receive ≤1 line of chemotherapy

6. Have at least one measurable lesion according to RECIST version 1.1

7. Adequate hematology and organ function, including:

   hemoglobin > 9 g/dL without blood transfusion or erythropoietin in the past 14 days.

   ANC ≥ 1.5×109/L without using granulocyte colony stimulating factor in the past 14 days.

   PLT ≥ 75×109/L without blood transfusion in the past 14 days. TBIL ≤ 1.5 ×ULN (Gilbert syndrome allows ≤ 3 × ULN). ALT and AST ≤ 3 × ULN (if there is liver metastasis, ALT and AST ≤ 5 × ULN). Serum Cr ≤ 1.5 × ULN or endogenous creatinine clearance ≥ 50 mL/min (Cockcroft-Gault formula)

8. ECOG score 0 or 1, and life expectancy ≥3 months;

9. Fertile female subjects are required to use a medically approved contraceptive during the study treatment period and for at least 3 months after the last use of the study drug;

10. Subjects voluntarily joined the study, signed informed consent, had good compliance, and cooperated with follow-up.

Exclusion Criteria:

1. Previous use of CDK4/6 inhibitors or PD1/PD-L1 monoclonal antibody
2. Uncontrolled central nervous system metastasis (meaning symptoms or the use of glucocorticoids or mannitol to control symptoms);
3. A history of clinically significant or uncontrolled heart disease, including congestive heart failure, angina pectoris, myocardial infarction within the last 6 months, or ventricular arrhythmia;
4. Radiotherapy, surgery, or other targeted and immunotherapy for advanced HR+/HER2- breast cancer within 4 weeks prior to first administration of the study drug;
5. Pregnant or lactating patients;
6. Malignant tumors within the past three years (except for cured basal cell carcinoma of the skin and cervical carcinoma in situ);
7. Significant comorbidities, including mental illnesses that the investigator believes will adversely affect the patient's participation in the study;
8. Those who have received anti-tumor vaccine or have received live vaccine within 4 weeks before the first administration of the investigational drug;
9. Patients with known HBV or HCV infection active phase or HBV DNA≥500, or chronic phase with abnormal liver function;
10. History of active autoimmune disease (such as interstitial pneumonia, colitis, hepatitis, pituitaritis, vasculitis, nephritis, hyperthyroidism, hypothyroidism, including but not limited to these diseases or syndromes)
11. A history of immunodeficiency, including HIV testing positive, or other acquired, congenital immunodeficiency diseases, or a history of organ transplantation and allogeneic bone marrow transplantation; History of interstitial lung disease (except radiation pneumonia without hormone therapy) and non-infectious pneumonia;
12. Patients with active infection or who had been treated with systemic immune stimulating factors within 4 weeks prior to enrollment;
13. Allergic physique, or known allergic history of the drug components of this program; Or allergic to other monoclonal antibodies;
14. Previous thyroid dysfunction;
15. The investigator did not consider the patient suitable for participation in any other conditions of the study