Tucidinostat and Fulvestrant in Hormone-receptor Positive Advanced Breast Cancer

Inclusion Criteria: 1. Female patients aged 18-75 years (including cutoff value); 2. The disease condition is inoperable, recurrent breast cancer, or metastatic breast cancer; 3. Histological or cytological confirmation of hormone receptor-positive \[estrogen receptor (ER) positive and progesterone receptors (PgR) positive or negative\] breast cancer; 4. At least one measurable lesion according to RECIST 1.1; 5. Prior treatment: have not received systemic chemotherapy for recurrent or metastatic breast cancer; 6. Eastern Cooperative Oncology Group Performance Status of 0-1; 7. Adequate function of major organs meets the following requirements): Absolute Neutrophils count≥ 1.5×10\^9/L; Platelets count≥ 90×10\^9/L; Hemoglobin ≥ 90g/L; Total bilirubin≤ 1.5 × the upper limit of normal (ULN); ALT and AST ≤ 2.5 × ULN; BUN and Cr ≤ 1.5 × ULN; Left ventricular ejection fraction (LVEF) ≥ 50%; QTcF(Fridericia correction) ≤ 470 ms; International normalized ratio(INR)≤1.5 × ULN; activated partial thromboplastin time(APTT) ≤ 1.5 × ULN; 8. Life expectancy ≥ 3 months; 9. Have signed informed consent. Exclusion Criteria: 1. Patients have untreated central nervous system (CNS) metastases; 2. Patients with no measurable lesion according to RECIST 1.1; 3. Patients with bilateral breast cancer; 4. Patients with human epidermal growth factor receptor-2 (Her-2) positive; 5. Recurrent or metastatic disease occurs within 2 years during adjuvant endocrine therapy; 6. Patients previously received systemic chemotherapy for recurrent or metastatic breast cancer; 7. Patients previously received any HDAC inhibitor or fulvestrant treatment; 8. There are ascites, pleural effusion, pericardial effusion with clinical symptoms at baseline, those who need drainage, or those who have undergone drainage of serous effusion within 4 weeks before the first dose; 9. Inability to swallow, intestinal obstruction or other factors affecting the administration and absorption of the drug; 10. Patients with other invasive malignancies within 5 years or at the same time, with the exception of curatively-treated basal cell or squamous cell carcinoma of the skin or cervical carcinoma in situ; 11. Patients with a history of allergies to the drug components of this regimen; 12. Patients with active HBV and HCV infection; stable hepatitis B after drug treatment (HBV virus copy number is higher than the upper limit of reference value) and cured hepatitis C patients (HCV virus copy number exceeds the lower limit of detection method) can be included; 13. Patients with a history of immunodeficiency, including HIV positive, or other acquired or congenital immunodeficiency disease, history of organ transplantation; 14. Patients have uncontrolled or significant cardiovascular disease, including: Myocardial infarction (\< the last 12 months); Uncontrolled angina (\< the last 6 months); Congestive heart failure (\< the last 6 months), or Left Ventricular Ejection Fraction (LVEF) \< 50% prior to study entry; 15. Any mental or cognitive disorder, that would interfere the ability to understand the informed consent document or the operation and compliance of study; 16. Any other condition which is inappropriate for the study in the opinion of the investigators.