Purpose

In this study, participants with previously-treated locally-advanced unresectable or metastatic mismatched repair (MMR) deficient or microsatellite instability-high (MSI-H) colorectal carcinoma (CRC) will be treated with pembrolizumab (MK-3475, KEYTRUDA®) monotherapy. There will be two cohorts in this study: Cohort A and Cohort B. For Cohort A, participants are required to have been previously treated with standard therapies, which must include fluoropyrimidine, oxaliplatin, and irinotecan. Enrollment into Cohort A has been completed. For Cohort B, participants are required to have been previously treated with at least one line of systemic standard of care therapy: fluoropyrimidine + oxaliplatin or fluoropyrimidine + irinotecan +/ - anti-vascular endothelial growth factor (VEGF)/ epidermal growth factor regulator (EGFR) monoclonal antibody. The primary hypothesis is that Objective Response Rate (ORR) based on Response Evaluation Criteria in Solid Tumors v 1.1 (RECIST 1.1) assessed by central imaging vendor in participants with locally advanced unresectable or metastatic MMR deficient or MSI high CRC is greater than 15%.