This Phase Ⅱ study was designed to assess the efficacy and safety of the combination of tucidinostat, angiogenesis inhibitors and fulvestrant for advanced HR-positive, HER2-negative breast cancer patients after the failure of CDK4/6 inhibitor.

Inclusion Criteria:

• 1.Age≥18 years, ≤75, female; 2.Histologically confirmed HR positive and HER2 negative postmenopausal metastatic breast cancer patients [HER2 negative is defined as immunohistochemistry(IHC) 0 or IHC 1+, and if the score of IHC is 2+, fluorescence in situ hybridization technology (FISH) test must be negative, HR positve is defined as ER or PR ≥10%]; 3.Prior CDK4/6 inhibitor progression or intolerance to CDK4/6i before enrollment; 4.Premenopausal patients need OFS; 5.Recurrence and metastasis progressed more than 6 months after receiving endocrine therapy or more than 2 years after receiving adjuvant endocrine therapy; 6.ECOG performance status ≤ 1; 7.At least one measurable disease based on RECIST v1.1 8.Adequate organ function; 9.Life expectancy is more than 3 months; 10.Willing and able to provide written informed consent

Exclusion Criteria:

• 1.Prior exposed to histone deacetylase inhibitors; 2.Received TKI before enrollment; 3.Exist visceral crisis; 4.Received chemotherapy, targeted therapy, Chinese herbal medicine with anti-tumor indications, or immunomodulatory drugs (including thymosin, interferon, interleukin, etc.) within 4 weeks before enrollment, or still within 5 half-lives of such drugs; 5.Poorly controlled diabetes (FBG>10mmol/L); 6.Poorly controlled hypertension (SBP>150 mmHg, DBP>90 mmHg); 7.Urinalysis shows urine protein ≥ 2+ or 24-hour protein quantity test shows urinary protein ≥1 g; 8.PT>16s, APTT> 43s, TT>21s, FIB<2g/L; 9.Toxicities that did not recover to National Cancer Institute Common Adverse Event Terminology Version 5.0 (NCICTCAEv5.0) grade 0 or 1 toxicity from prior antineoplastic therapy prior to the first dose of study treatment(alopecia, grade 2 fatigue, grade 2 anemia, non-clinically critical and asymptomatic laboratory abnormalities can be enrolled); 10.Pregnant or lactating female. 11.Any other conditions deemed inappropriate by the investigator to participate in this study.