Tislelizumab With Fruquintinib, Metronidazole, in Mismatch Repair-proficient or Microsatellite Stability, Advanced Colorectal Cancer: a Multicenter, Single Arm, Clinical Trial

1. No gender limit, age ≥ 18 years old and ≤ 80 years old 2. Colorectal cancer (AJCC stage IV) diagnosed by histological or cytological biopsy as metastatic or unresectable, with immunohistochemical or genetic testing indicating MSS or MSH-L type 3. Treatment requires third line or above, has previously received standard first and second line treatment, and the disease has progressed after treatment 4. Before accepting tirelizumab, qPCR should be used to detect the Fn CT value \< 35 in fecal samples, and Fn is confirmed positive by agarose gel electrophoresis or Sanger sequencing 5. ECOG score: 0-1 points 6. Baseline blood routine and biochemical indicators meet the following criteria (no blood transfusion or use of colony stimulating factors 2 weeks prior to screening) Blood routine: hemoglobin ≥ 90g/L, absolute neutrophil count (ANC) ≥ 1.5 × 109/L, platelet ≥ 100 × 109/L Liver function: ALT and AST ≤ 2.5 x ULN, serum total bilirubin ≤ 1.5 x ULN (if there is liver metastasis, ALT and AST ≤ 5 x ULN, serum total bilirubin ≤ 3 x ULN), serum albumin ≥ 30g/L Renal function: serum creatinine ≤ 1.5 x upper limit of normal (ULN), or creatinine clearance rate ≥ 50 mL/min (calculated according to Cockcroft Gault formula). 7. Those who voluntarily participate in the trial and sign an informed consent form.