Inclusion Criteria:
1. Signed Informed Consent Form (ICF);
2. Age ≥ 18 years at time of signing ICF;
3. Ability to comply with the study protocol;
4. Histological or cytological confirmed colorectal cancer;
5. Immunohistochemically confirmation of microsatellite stable (MSS) phenotype;
6. Disease progression during treatment with standard of care;
7. Measurable disease per Response Evaluation Criteria in Solid Tumours version 1.1 (RECIST v1.1). Previously irradiated lesions can be considered as measurable disease only if progressive disease has been unequivocally documented at that site since radiation;
8. Able and willing to undergo blood sampling and tumour biopsies at baseline, if no adequate archival material is available, and during therapy;
9. Availability of representative tumor specimen for exploratory biomarker research;
10. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1;
11. Life expectancy of at least 3 months;
12. Negative HIV test at screening. Patients with a positive HIV test at screening are eligible provided they are stable on anti-retroviral therapy, have a CD4 count ≥ 200/µL, and have an undetectable viral load;
13. Negative hepatitis B test at screening;
14. Negative hepatitis C virus test at screening;
15. Adequate hematologic and end-organ function as defined by:
* Absolute neutrophil (segmented and bands) count ≥1.0×109/L
* Lymphocyte count ≥0.5×109/L
* Platelets≥100×109/L
* Hemoglobin ≥5.6 mmol/L
* AST≤2.5×ULN
* ALT≤2.5×ULN
* AP ≤2.5×ULN
* Bilirubin ≤1.5×ULN
* Estimated glomerular filtration rate ≥50 mL/min by CKD-EPI
* Albumin ≥25 g/L
* INR ≤1.5×ULN
* aPTT ≤1.5×ULN
16. Negative pregnancy test (urine or serum) for female patients with childbearing potential.
Exclusion Criteria:
1. Unable to follow study procedures;
2. Patients using prohibited medication;
3. Any unresolved grade ≥ 2 toxicities related to prior treatments (excluding alopecia) according to CTCAE version 5.0;
4. Symptomatic or actively progressing central nervous system (CNS) metastases. Asymptomatic patients with treated or untreated CNS lesions are eligible, provided that all of the following criteria are met:
* Measurable disease, per RECIST v1.1, must be present outside the CNS;
* The patient has no history of intracranial haemorrhage or spinal cord haemorrhage;
* The patient has not undergone stereotactic radiotherapy within 7 days prior to initiation of study treatment, whole-brain radiotherapy within 14 days prior to initiation of study treatment, or neurosurgical resection within 28 days prior to initiation of study treatment;
* The patient has no ongoing requirement for corticosteroids as therapy for CNS disease;
* If the patient is receiving anti-convulsant therapy, the dose is considered stable;
5. History of leptomeningeal disease;
6. Uncontrolled tumor-related pain. Patients requiring pain medication must be on a sta-ble regimen at study entry;
7. Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures. Patients with indwelling catheters are allowed;
8. Uncontrolled symptomatic hypercalcemia (ionized calcium \> 1.5 mmol/L, calcium \>12 mg/dL, or corrected calcium greater than ULN);
9. Active or history of auto-immune disease or immune deficiency;
10. History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest CT scan. History of radiation pneumonitis in the radiation field is permitted;
11. Active tuberculosis;
12. Significant cardiovascular disease (such as New York Heart Association Class II or greater cardiac disease, myocardial infarction, or cerebrovascular accident) within 3 months prior to initiation of study treatment, unstable arrhythmia, or unstable angina;
13. Major surgical procedure, other than for diagnosis, within 4 weeks prior to initiation of study treatment, or anticipation of need for a major surgical procedure during the study;
14. History of malignancy within 2 years prior to initiation of study treatment, with the exception of the cancer under investigation in this study and malignancies with a negligible risk of metastasis or death (e.g., 5-year OS rate \>90%);
15. Severe infection within 4 weeks prior to initiation of study treatment;
16. Treatment with therapeutic oral or IV antibiotics within 2 weeks prior to initiation of study treatment. Patients receiving prophylactic antibiotics are eligible for the study;
17. Prior allogeneic stem cell or solid organ transplantation;
18. Any other disease, metabolic dysfunction, physical examination finding, or clinical laboratory finding that contraindicates the use of an investigational drug, may affect the interpretation of the results, or may render the patient at high risk from treatment complications;
19. Treatment with a live, attenuated vaccine within 4 weeks prior to initiation of study treatment, or anticipation of need for such a vaccine during atezolizumab treatment or within 5 months after the final dose of atezolizumab;
20. Current treatment with anti-viral therapy for HBV;
21. Treatment with investigational therapy within 28 days prior to initiation of study treat-ment;
22. Prior treatment with CD137 agonists or immune checkpoint blockade therapies;
23. Treatment with systemic immunostimulatory agents within 4 weeks or 5 drug-elimination half-lives prior to initiation of study treatment;
24. Treatment with systemic immunosuppressive medication within 2 weeks prior to initiation of study treatment, or anticipation of need for systemic immunosuppressive medication during study treatment;
25. History of severe allergic anaphylactic reactions to chimeric or humanized antibodies or fusion proteins;
26. Known hypersensitivity to Chinese hamster ovary cell products or to any component of the Atezolizumab formulation;
27. Pregnancy or breastfeeding, or intention of becoming pregnant during study treatment or within 5 months after the final dose of study treatment.
