Is There a Role for Programmed Death Ligand-1 Testing and Immunotherapy in Colorectal Cancer With Microsatellite Instability? Part II—The Challenge of Programmed Death Ligand-1 Testing and Its Role in Microsatellite Instability-High Colorectal Cancer

Abstract

Context.— The world of oncology has changed dramatically in the past few years with the introduction of checkpoint inhibitors and immunotherapy. The promising findings of a small, phase 2 clinical trial that led to the US Food and Drug Administration breakthrough designation and approval of the anti–programmed death receptor-1 (PD-1) drug pembrolizumab (Keytruda, Merck, Kenilworth, New Jersey) to treat metastatic/refractory microsatellite instability–high colorectal cancer (CRC) has significantly boosted interest in immunomodulatory therapies in microsatellite instability–high CRC. Objectives.— To review the immune response to cancer and the role of immune checkpoints, focusing on the technical and interpretation challenges of PD-1/programmed death ligand-1 (PD-L1) testing by pathologists and the clinical implications of the test and the therapeutic potential of treating CRC with checkpoint inhibitors. Data Sources.— A PubMed review was performed of articles pertaining to CRC, microsatellite instability and mismatch repair systems, molecular classification, immune response, PD-1/PD-L1, and immunotherapy. Conclusions.— Exciting success with anti–PD-1/PD-L1 and anticytotoxic T-lymphocyte–associated protein 4 (CTLA4) checkpoint inhibitors has already been reported in melanoma and in lung and renal carcinomas. Recently, microsatellite instability–high CRCs, expressing PD-L1 by immunohistochemistry, regardless of the level of that PD-L1 expression, appeared to respond to checkpoint blockades with anti–PD-1 or anti–PD-L1 agents, whereas microsatellite-stable tumors were much less responsive. With microsatellite instability routinely tested by most centers, studies that include larger cohorts are required to study the predictive role of PD-1/PD-L1 expression in microsatellite instability–high CRC, to assess which immunohistochemistry antibodies to use, to refine the scoring criteria, and to critically analyze the interpretation pitfalls.