Safety and Efficacy of Fruquintinib Plus Chidamide and Sintilimab in the Third and Later Line Treatment of MSS/pMMR Metastatic Colorectal Cancer

Inclusion Criteria: 1. Fully understand this study and voluntarily sign the informed consent form; 2. Age between 18-75 years inclusive; 3. Patients with histologically confirmed unresectable locally advanced, recurrent, or metastatic colorectal adenocarcinoma; 4. Failure of standard second-line systemic treatment with measurable lesions; 5. Tumor tissue tested for microsatellite stability (MSS) or low microsatellite instability (MSI-L) by PCR, or confirmed pMMR by immunohistochemistry for DNA mismatch repair (MMR) protein (including MLH1, MSH2, MSH6, and PMS2 protein expression); 6. ECOG performance status of 0-2, with no deterioration within 7 days; 7. BMI≥18; 8. Expected survival ≥3 months; 9. Major organ functions meet the following requirements (no use of any blood components and growth factors within 14 days before enrollment): * Absolute neutrophil count ≥1.5×109/L, white blood cells ≥4.0×109/L; * Platelets ≥100×109/L; * Hemoglobin ≥90g/L; * Total bilirubin TBIL ≤1.5 times ULN; * ALT and AST ≤5 times ULN; * Urea/BUN and creatinine (Cr) ≤1.5×ULN (and creatinine clearance (CCr) ≥ 50mL/min); * Left ventricular ejection fraction (LVEF) ≥50%; * Corrected QT interval by Fridericia's formula (QTcF) \<470 milliseconds. * INR ≤1.5×ULN, APTT ≤1.5×ULN. 10. Women of childbearing age must use effective contraception; 11. Good compliance and cooperation with follow-up. Exclusion Criteria: 1. Unable to comply with the study protocol or procedures; 2. Pregnant or breastfeeding women; 3. Concurrent with any of the following conditions: uncontrolled hypertension, coronary artery disease, arrhythmias, and heart failure; 4. Previous treatment with small molecule tyrosine kinase inhibitors for metastatic disease; 5. Previous treatment with romidepsin; 6. Previous treatment with immune checkpoint inhibitors for metastatic disease; 7. Uncontrollable severe concurrent infections; 8. Acute myocardial infarction, acute coronary syndrome, or CABG within 3 months before the first treatment; 9. Subjects allergic to the study medication or any of its excipients; 10. Known human immunodeficiency virus (HIV) infection. Known clinically significant liver disease history, including viral hepatitis \[known carriers of hepatitis B virus (HBV) must exclude active HBV infection, i.e., HBV DNA positive (\>1×10\^4 copies/mL or \>2000 IU/mL); known hepatitis C virus (HCV) infection and HCV RNA positive (\>1×10\^3 copies/mL)\]; 12\. Patients whom the investigator deems inappropriate for inclusion in this study.