GSL Synthetase Inhibitor Plus Immune Checkpoint Inhibitor and/or Regorafenib in Previously Treated pMMR/MSS CRC.

Inclusion Criteria: 1. Age ≥18 years old and ≤75 years old. 2. Histologically confirmed diagnosis of unresectable locally advanced, recurrent or metastatic colorectal cance have failed at least two lines of prior treatment. 3. Tumor tissues were identified as pMMR by immunohistochemistry (IHC) method or MSS by polymerase chain reaction (PCR). 4. CYP2D6 extensive metabolizers (EMs), intermediate metabolizers (IMs), or poor metabolizers (PMs). 5. Eastern Cooperative Oncology Group (ECOG) performance status score≤2 and Estimated life expectancy of more than 3 months. 6. At least one measurable lesion at baseline according to RECIST version 1.1. 7. Fresh solid tumor samples or formalin-fixed paraffin embedded tumor archival samples within 6 months are necessary; Fresh tumor samples are preferred. Subjects are willing to accept tumor rebiopsy in the process of this study. 8. Have adequate organ function as assessed by the laboratory required by protocol, which should be confirmed within 2 weeks prior to the first dose of study drugs. 9. Previous treatment must be completed for more than 4 weeks prior to the enrollment of this study, and subjects have recovered to \<= grade 1 toxicity. 10. Previous treatment with anti-PD-1/PD-L1 antibodies or cytotoxic T lymphocyte associated antigen 4 (CTLA-4) inhibitors are allowed. 11. Pregnancy tests for women of childbearing age shall be negative; Both men and women agreed to use effective contraception during treatment and during the subsequent 1 year. 12. Ability to understand and sign a written informed consent document. Exclusion Criteria: 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.