The goal of this clinical trial is to verify the effectiveness and safety of Lactobacillus johnsonii in combination with CapeOX and Pembrolizumab for the treatment of MSS/pMMR metastatic colorectal cancer (CRC). The main questions it aims to answer are:

① To verify the effectiveness of Lactobacillus johnsonii in the treatment of MSS/pMMR type metastatic colorectal cancer (mCRC) who have failed the standard regimen of chemotherapy;

② To explore the safety of Lactobacillus johnsonii in the treatment of MSS/pMMR type metastatic colorectal cancer (mCRC) who have failed the standard regimen of chemotherapy.

Participants who meet all the inclusion criteria will be enrolled in the study and randomly assigned in a 1:1 ratio to either the CapeOX + Pembrolizumab + placebo group or the CapeOX + Pembrolizumab + Lactobacillus johnsonii group.

Inclusion Criteria

• Ages between 18 and 75 years old;

  • Pathologically confirmed MSS or pMMR colorectal adenocarcinoma (PCR method, with 0 or 1 locus unstable; or immunohistochemical method, with expression of MLH1/MSH2/MSH6/PMS2 proteins);

    • Patients who have failed standard first- and second-line treatments (including CapeOX ± targeted therapy, FOLFOX ± targeted therapy, 5-FU/LV ± targeted therapy); ④ Radiological imaging indicates distant metastases throughout the body, considered as unresectable lesions; ⑤ Willingness to comply with the study visit schedule, as well as the prohibitions and restrictions stipulated in this agreement.

Exclusion Criteria

• Use of antibiotics within 4 weeks prior to treatment;

  • Concurrent immunodeficiency diseases (such as HIV or post-transplantation status) or receipt of high-dose systemic corticosteroid therapy or any other form of immunosuppressive therapy within the last 4 weeks;

    • Concurrent malignancies in other organs; ④ Concurrent active autoimmune diseases (i.e., requiring corticosteroids or immunosuppressant medications);

      • Concurrent active infections requiring systemic treatment; ⑥ Concurrent severe systemic diseases, diseases of vital organs such as the heart, lungs, and brain, liver and renal insufficiency, among others;

        • Participation in other drug studies within 3 months prior to treatment or inability of the patient to correctly express their chief complaints and cooperate with this study; ⑧ Any severe or uncontrolled medical condition that, in the investigator's opinion, may increase the risks associated with study participation or study drug administration, impair the subject's ability to receive the protocol-specified treatment, or interfere with the interpretation of study results.