P-274 Pembrolizumab for Patients with Previously Treated, Mismatch Repair–deficient Microsatellite Instability–high Advanced Colorectal Carcinoma: Phase 2 KEYNOTE-164 Study

Background: Approximately 5% of metastatic colorectal carcinomas (CRCs) are mismatch-repair (MMR) deficient, leading to high levels of microsatellite instability (MSI). CRCs with high MSI (MSI-H) contain abundant lymphocyte infiltrates and strong expression of immune checkpoints, including the programmed death 1 (PD-1) receptor and its ligand PD-L1. Pembrolizumab is a humanized monoclonal anti -PD-1 antibody designed to block the interaction between PD-1 and its ligands PD-L1 and PD-L2, thereby permitting activation of an antitumor immune response.