1. Participants with dMMR /MSI-H colorectal cancer.
2. CYP2D6 ultra-rapid metabolizers (URMs).
3. Active, known or suspected autoimmune diseases.
4. The patients is taking a CYP2D6 inhibitor and/or concomitantly with a strong or moderate CYP3A inhibitor.
5. Subjects are being treated with either corticosteroids (>10 mg daily prednisone equivalent) or other immunosuppressive medications within 14 days of enrollment.
6. History of severe hypersensitive reactions to other monoclonal antibodies.
7. History of allergy or intolerance to study drug components.
8. Known brain metastases or active central nervous system (CNS). Subjects with CNS metastases who were treated with radiotherapy for at least 3 months prior to enrollment, have no central nervous symptoms and are off corticosteroids, are eligible for enrollment, but require a brain MRI screening.
9. Uncontrolled intercurrent illness, including ongoing or active systemic infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia (excluding insignificant sinus bradycardia and sinus tachycardia) or psychiatric illness/social situations and any other illness that would limit compliance with study requirements and jeopardize the safety of the patient.
10. Known positive test result for human immunodeficiency virus (HIV) or acquired immune deficiency syndrome (AIDS).
11. Previous or concurrent cancer within 3 years prior to treatment start EXCEPT for curatively treated cervical cancer in situ, non-melanoma skin cancer, superficial bladder tumors [Ta (non-invasive tumor), Tis (carcinoma in situ) and T1 (tumor invades lamina propria)].
12. Major surgery or trauma occurred within 28 days prior to enrollment, or major side effects have not been recovered.
13. Vaccination within 30 days of study enrollment.
14. Active bleeding or known hemorrhagic tendency.Any life Threatening bleeding within 3 months prior to the enrollment.
15. Uncontrolled hypertension (systolic pressure >150 mm Hg or diastolic pressure > 100 mm Hg on repeated measurement) despite optimal medical management.
16. Pregnant or breast-feeding. Women of childbearing potential must have a pregnancy test performed within 7 days before the enrollment, and a negative result must be documented
17. Being participating any other trials or withdraw within 4 weeks.
18. Researchers believe that other reasons are not suitable for clinical trials.