Capecitabine and Bevacizumab With or Without Atezolizumab in Treating Patients With Refractory Metastatic Colorectal Cancer

PRIMARY OBJECTIVE: I. To estimate the efficacy of capecitabine/bevacizumab + atezolizumab, as compared with capecitabine/bevacizumab + placebo in refractory metastatic colorectal cancer (mCRC) as measured by progression-free survival (defined as the time of randomization to the first occurrence of progression based on Response Evaluation Criteria in Solid Tumors version 1.1, clinical progression, or death from any cause on study as determined by the investigator). SECONDARY OBJECTIVES: I. To estimate the efficacy of capecitabine/bevacizumab + atezolizumab, as compared with capecitabine/bevacizumab + placebo in refractory mCRC as measured by objective response rate (defined as partial response plus complete response) as determined by the investigator using Response Evaluation Criteria in Solid Tumors version 1.1 and immune-related response criteria (irRC). II. To estimate the efficacy of capecitabine/bevacizumab + atezolizumab as compared with capecitabine/bevacizumab + placebo in refractory mCRC as measured by overall survival (defined as death from any cause from the time of randomization until study completion). III. To evaluate the safety and tolerability of atezolizumab in combination with bevacizumab and capecitabine in refractory mCRC as measured by the serious adverse events and adverse events \>= grade 3 according to National Cancer Institute Common Terminology Criteria for Adverse Events version 4.0. CORRELATIVE OBJECTIVE: I. To explore any correlation between tissue and blood based biomarkers and clinical outcomes. OUTLINE: Patients are randomized 2:1 to Arm I:Arm II. ARM I (ATEZOLIZUMAB, BEVACIZUMAB, CAPECITABINE): Patients receive atezolizumab intravenously (IV) over 30-60 minutes on day 1, bevacizumab IV over 30-90 minutes on day 1, and capecitabine orally (PO) twice daily (BID) on days 1-14. Courses repeat every 21 days in the absence of disease progression or unacceptable toxicity. ARM II (PLACEBO, BEVACIZUMAB, CAPECITABINE): Patients receive placebo IV over 30-60 minutes on day 1, bevacizumab IV over 30-90 minutes on day 1, and capecitabine PO BID on days 1-14. Courses repeat every 21 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up at 30 days, every 3 months until progressive disease, then every 6 months thereafter.