The purpose of this observational study is to understand the effect of gut microbiota on the efficacy of immunotherapy in patients with metastatic colorectal cancer and to explore the specific mechanisms of this process. In this way, it provides new ideas for the clinical treatment of colorectal cancer.

This study is a single-center, prospective, observational study. This study plans to enroll 50 patients with mCRC who received immunotherapy. Stool and blood samples were collected from patients with mCRC before receiving immunotherapy (baseline) and after one cycle of immunotherapy and stored immediately in a -80°C freezer. Six months after treatment with a PD-1 inhibitor in combination with fruquintinib, patients with mCRC were evaluated radiographically according to the modified RECIST1.1 criteria for immunotherapy (iRECIST) and were divided into responsive and non-responsive groups.

In this study, we intend to use metagenomic sequencing to screen the key intestinal microbiota that affect the efficacy of PD-1 inhibitors and predict their possible functional pathways in patients with metastatic colorectal cancer receiving anti-PD-1 immunotherapy. Then, proteomics and metabolomics methods were used to screen differential proteins and metabolites, and the correlation analysis with key intestinal microbiota was carried out, and the efficacy of immunotherapy in patients with mCRC was evaluated. Finally, the above findings were verified in animal models, and then the specific mechanism of intestinal microbiota affecting the efficacy of PD-1 inhibitors was explained by taking the changes in body metabolism and immunity as the starting point.

Inclusion Criteria:

• Clinical and pathological diagnosis of metastatic colorectal cancer
• 35-75 years old (both ends inclusive)
• complete clinical information
• signed informed consent

Exclusion Criteria:

• combined with severe respiratory and circulatory diseases
• combined with other malignant tumors
• recent severe active bleeding, uncontrolled active infection or active peptic ulcer
• moderate to severe renal insufficiency
• other circumstances that are judged by the investigator to be unsuitable for participating in this study

A total of 50 patients with metastatic colorectal cancer who received immunotherapy from the First Hospital of Jilin University from 2024 to 2026 were included