The study is designed to explore the clinical benefit following treatment with tucidinosta in combination with metronomic capecitabine and endocrine therapy in patients with hormone receptor-positive, Her2-negative advanced breast cancer who have received CDK4/6 Inhibitor treatment.

Inclusion Criteria:

1. ≥18 years old, and ≤75 years old, 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. After the failure of previous CDK4/6 inhibitor therapy; 4.Receive one line of chemotherapy at most; 5.Premenopausal women need to take effective measures to suppress ovarian function, such as drug suppression, oophorectomy; 6.ECOG score 0-1; 7. According to RECIST1.1 criteria, at least there is one measurable lesion or simple bone metastasis; 8. The main organ and bone marrow function levels meet the following requirements:

1. Blood routine: neutrophil (ANC) ≥ 1.5×109/L; platelet count (PLT) ≥ 90× 109/L; hemoglobin (Hb) ≥ 90g/L; It is required that no blood products (including red blood cells and platelet products, etc.) have been transfused and no growth factors (including colony-stimulating factor, interleukin, and erythropoietin, etc.) have been used for supportive treatment within 2 weeks before the examination.
2. Liver function: serum total bilirubin (TBIL) ≤ 1.5 × upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 × ULN (with liver Requirements for metastatic patients: ALT and AST≤5×ULN);
3. Renal function: serum creatinine (Cr)≤1.5×ULN or creatinine clearance rate>60mL/min;

9. Expected survival time ≥ 3 months; 10. Voluntary participation study, signed written informed consent.

Exclusion Criteria:

1. Known hypersensitivity to any formulation component of the study drug;
2. patients with previous severe, unexpected reactions to fluoropyrimidines or known hypersensitivity to fluoropyrimidines;
3. Patients with complete deficiency of dihydropyrimidine dehydrogenase (DPD) activity;
4. Visceral crisis;
5. Previously received treatment with histone deacetylase inhibitors or fluoropyrimidine drugs such as capecitabine;
6. 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;
7. Received palliative radiotherapy for local lesions within 4 weeks before enrollment;
8. Patients with gastrointestinal perforation, fistula, abdominal abscess, gastrointestinal ulcer or active diverticulosis before enrollment;
9. Significant malnutrition (weight loss > 5% in the past 1 month or > 15% in the past 3 months, or food intake decreased by 1/2 or more in the past week), or still need to rely on intravenous nutritional support during the screening period;
10. 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);
11. Patients with currently symptomatic brain or meningeal metastasis;
12. Received immunosuppressive drugs within 4 weeks before enrollment, excluding nasal spray, inhalation or other local glucocorticoids or physiological doses systemic glucocorticoids (no more than 10 mg/day of prednisone or other glucocorticoids at an equivalent dose), or use of glucocorticoids for the prevention of contrast medium allergy;
13. The patient has any active autoimmune disease or has history of immune disorders (including but not limited to: interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism, hypothyroidism; patients with vitiligo or complete remission of asthma in childhood without any intervention in adulthood can be included; those with asthma that require medical intervention with bronchodilators are not included);
14. Patients with active pulmonary tuberculosis who are receiving anti-tuberculosis treatment or have received anti-tuberculosis treatment within 1 year before screening;
15. Complications requiring long-term use of immunosuppressive drugs, or systemic or topical use of corticosteroids with immunosuppressive doses;
16. Received any anti-infection vaccine (such as influenza vaccine, chickenpox vaccine, etc.) within 4 weeks before enrollment;
17. Received major surgery (craniotomy, thoracotomy or laparotomy) within 4 weeks before enrollment or is expected to undergo major surgery during the study treatment period; received exploratory laparoscopic surgery within 2 weeks prior to enrollment;received central venous catheterization within 7 days prior to enrollment;
18. Concurrent participation in another interventional clinical study, unless participating in an observational (non-interventional) clinical study or in the safety follow-up of an interventional study Stage;
19. Known acute or chronic active hepatitis B (HBsAg positive and HBV DNA virus Load ≥ULN or ≥10^3 copies/mL) or acute or chronic active hepatitis C (HCV antibody positive and HCV RNA positive);
20. Severe heart disease or discomfort, including but not limited to the following diseases: 1) Diagnosed history of heart failure or systolic dysfunction (LVEF<50%); 2) arrhythmias requiring medical treatment or clinically significant; 3) high-risk uncontrolled arrhythmias, such as atrial tachycardia, resting heart rate > 100 bpm, significant ventricular arrhythmia (such as ventricular tachycardia) or higher-grade AV block (ie Mobitz II second-degree AV block or third-degree AV block); 4) Angina pectoris; 5 ) Clinically significant valvular heart disease; 6) ECG shows transmural myocardial infarction; 7) Any other heart disease judged by the investigator to be inappropriate to participate in this trial, etc.;
21. Inability to swallow, bowel obstruction, or other factors that interfere with drug taking and absorption;
22. A history of immunodeficiency, including HIV test positive, or other acquired or congenital immunodeficiency diseases, or a history of organ transplantation;
23. Pregnant, lactating female patients, female patients of childbearing potential with a positive baseline pregnancy test, or patients of childbearing age who are unwilling to use effective contraception throughout the trial and within 6 months after the last study drug;
24. Serious concomitant disease or other comorbidities that would interfere with planned treatment;
25. Any other conditions deemed inappropriate by the investigator to participate in this study.