This study is looking to see whether the combination of Tucidinostat and nab-paclitaxel is safe and effective in participants with advanced HR+/HER2- breast cancer.

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 ≥1%];
3. Primary endocrine resistance or at least experienced one line of endocrine therapy for recurrent or metastatic disease;
4. No more than 1 prior line of chemothrapy for recurrent or metastatic disease;
5. Patients who have been exposed to taxanes in neoadjuvant or adjuvant setting will be allowed to enroll, if they have obtained benifits from neoadjuvant treatment or have progressed ≥ 12 months from completion of adjuvant treatment;
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, or received taxanes for metastatic disease;
2. Known hypersensitivity to any formulation component of the study drug;
3. 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;
4. 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);
5. Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis.
6. Pregnant or lactating female.
7. Any other conditions deemed inappropriate by the investigator to participate in this study.