Comparing Chidamide+Sintilimab+Bev With Standard Second-line FOLFIRI+Bev in Advanced MSS/pMMR mCRC

Inclusion Criteria: 1. Locally advanced unresectable or metastatic colorectal adenocarcinoma (excluding mixed adenosquamous carcinoma and other pathological types) confirmed by pathological histology or cytology. 2. Diagnosis of pMMR confirmed by PCR for microsatellite stability (MSS) or low microsatellite instability (MSI-L), or by immunohistochemistry for DNA mismatch repair (MMR) proteins, including MLH1, MSH2, MSH6 and PMS2 protein expression, which result in no protein deletion. 3. Patients who have failed first-line oxaliplatin-containing standard therapy and have imaging evidence (e.g., CT scan) or clinical evidence (e.g., cytology report of new ascites or pleural effusion) of disease progression during or after treatment; patients whose intolerance of toxicity has led to discontinuation of first-line oxaliplatin-containing standard therapy may be enrolled; relapse within 180 days after the last dose of oxaliplatin-containing adjuvant therapy. 4. There is at least one measurable lesion according to RECIST v1.1. 5. ECOG PS score is in the range of 0\~1. 6. Subjects who have signed a written informed consent form and who are able to comply with the visits and related procedures specified in the protocol. 7. Aged ≥ 18 years old and ≤ 75 years old. 8. Expected survival time ≥ 18 weeks. 9. Female subjects of childbearing age or male subjects whose sexual partners are at childbearing age are required to take effective contraception measures throughout the treatment period and for 6 months after the treatment (see section 4.3 of the protocol). 10. Subjects having adequate organ and bone marrow functions with laboratory test values within 7 days prior to enrollment meeting the following requirements (no blood components, cell growth factors, albumin, and other corrective therapy drugs are allowed to be given within the first 14 days of obtaining laboratory tests), as follows: 1\) Blood routine: absolute neutrophil count (ANC) ≥ 1.5×109/L; platelet count (PLT) ≥ 100×109/L; hemoglobin level (HGB) ≥ 9.0 g/dL. 2\) Liver function: serum total bilirubin (TBIL) ≤ 1.5 × upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3.0 × ULN in subjects without liver metastases, and ALT and AST ≤ 5.0 × ULN in subjects with liver metastases; serum albumin ≥ 25 g/L. 3\) Renal function: serum creatinine (Cr) ≤ 1.5 x ULN. 4) Patients with routine urine results showing urine protein \<2+ or routine urine testing showing urine protein ≥ 2+ at baseline should undergo 24-hour urine collection and 24-hour urine protein quantification \< 1 g. 5\) Coagulation function: international normalized ratio (INR) ≤ 1.5×ULN and activated partial thromboplastin time (APTT) ≤ 1.5×ULN. Exclusion Criteria: 1. Prior exposure to any anti-PD-1 antibody, anti-PD-L1 antibody, HDAC inhibitor, or irinotecan. 2. Receiving any investigational drug within 4 weeks prior to the first dose of the study drug. 3. Concurrent participation in another interventional clinical study, except in an observational (non-interventional) clinical study or in the follow-up phase of an interventional study. 4. Receiving the last dose of antitumor therapy (chemotherapy, targeted therapy, tumor immunotherapy, or tumor embolization) within 3 weeks prior to the first dose. 5. Receiving radiotherapy within 4 weeks prior to the first dose. 6. Patients who have received radiotherapy more than 4 weeks prior to the first dose with any radiotherapy-related toxic reactions, such as radiation pneumonia, radiation hepatitis, radiation enteritis, including clinical symptoms only, or requiring glucocorticoid therapy. 7. Use of immunosuppressive drugs within 4 weeks prior to the first dose, excluding topical glucocorticoids by nasal spray, inhalation or other routes or physiological doses of systemic glucocorticoids (i.e., no more than 10 mg/day prednisone or equivalent doses of other glucocorticoids). 8. Receiving live attenuated vaccine within 4 weeks prior to the first dose or planning to receive during the study period. 9. Major surgical procedure (craniotomy, thoracotomy or open heart surgery) or unhealed wound, ulcer or fracture within 4 weeks prior to the first dose. 10. Presence of toxicity (excluding alopecia, non-clinically significant and asymptomatic laboratory abnormalities) from prior antineoplastic therapy not recovered to ≤ grade 1 by NCI common terminology criteriafor adverse events (CTCAE) Version 5.0 (NCI CTCAE Version 5.0) prior to the first dose. 11. Known presence of symptomatic CNS metastases and/or carcinomatous meningitis. Subjects with prior treatment for brain metastases may participate in the study provided that the brain metastases have remained stable for at least 4 weeks prior to the first dose of study treatment; and that neurological symptoms have recovered to ≤ grade 1 by NCI CTCAE version 5.0. 12. Active autoimmune disease requiring systemic therapy (e.g., use of disease-relieving drugs, corticosteroids, or immunosuppressants) within 2 years prior to the first dose. Alternative therapies (e.g., thyroxine, insulin, or physiologic corticosteroids for adrenal or pituitary insufficiency) are allowed. A known history of primary immunodeficiency. For patients with only positive autoimmune antibodies, the presence of autoimmune diseases should be confirmed at the discretion of the investigator. 13. Patients who are known to have active tuberculosis and are receiving anti-tuberculosis treatment or have received anti-tuberculosis treatment within 1 year prior to the first dose. 14. Known to have interstitial lung disease requiring steroid hormone therapy. 15. Known to have acute or chronic active hepatitis B (HBsAg positive and HBV DNA ≥ 200 IU/mL or ≥ 103 copies/mL) or acute or chronic active hepatitis C (HCV antibody positive and HCV RNA positive). 16. Infected with human immunodeficiency virus (HIV) (HIV 1/2 antibody positive), known to be infected with syphilis. 17. Severe infections that are