Sintilimab and Chidamide in Combination With or Without IBI305 in Advanced or Metastatic pMMR/MSS Colorectal Carcinoma

Inclusion Criteria: 1. Histologically confirmed diagnosis of unresectable locally advanced, recurrent or metastatic colorectal adenocarcinoma. 2. Tumor tissues were identified as mismatch repair-proficient (pMMR) by immunohistochemistry (IHC) method or microsatellite stability (MSS) by polymerase chain reaction (PCR). 3. Subjects must have failed at least two lines of prior treatment. 4. Subjects must have one measurable lesion according to RECIST v1.1 at least. 5. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. 6. 18-75 years old. 7. Life expectancy of at least 12 weeks. 8. Adequate bone marrow, liver, renal and coagulation function as assessed by the laboratory required by protocol Exclusion Criteria: 1. Previously received anti-programmed death-1 (PD-1) or its ligand (PD-L1) antibody or histone deacetylase (HDAC) inhibitor. 2. Received last dose of anti-tumor therapy (chemotherapy, targeted therapy, tumor immunotherapy or arterial embolization) within 3 weeks of the first dose of study medication. 3. Received radiotherapy with 4 weeks of the first dose of study medication. 4. Underwent major operation within 4 weeks of the first dose of study medication or open wound, ulcer or fracture. 5. Known symptomatic central nervous system (CNS) metastasis and/or carcinomatous meningitis. Subjects received prior treatment and have stable disease more than 4 weeks from first dose of study medication are permitted to enroll. 6. Active, known or suspected autoimmune disease or has a history of the disease within the last 2 years. 7. Interstitial lung disease requiring corticosteroids. 8. Active or poorly controlled serious infections. 9. Significant malnutrition. 10. Symptomatic congestive heart failure (NYHA Class II-IV) or symptomatic or poorly controlled arrhythmia. 11. Uncontrolled hypertension (systolic blood pressure ≥ 150 mmHg or diastolic blood pressure ≥ 100 mmHg) despite standard treatment. 12. Within 6 months prior to the enrollment, history of gastrointestinal perforation and/or fistula, gastrointestinal ulcer, bowel obstruction, extensive bowel resection, Crohn's disease, or ulcerative colitis, intra-abdominal abscesses, or long-term chronic diarrhea. 13. History or evidence of inherited bleeding diathesis or coagulopathy or thrombus 14. Any life-threatening bleeding within 3 months prior to the enrollment. 15. High risk of bleeding.