The Case for Universal Testing of Colorectal Tumors for Microsatellite Instability: A Coming Mismatch Between Clinical and Laboratory Testing

The Potential Value of Immunotherapy in Colorectal Cancers: Review of the Evidence for Programmed Death-1 Inhibitor Therapy

Colorectal cancers (CRCs) have been identified as potential targets for immunotherapy with programmed cell death (PD)-1 inhibitors. English-language publications from MedLine and Embase that evaluated PD-1/PD ligand 1 (PD-L1) in the CRC tumor microenvironment and clinical trials that assessed PD-1 inhibitors were included. Sixteen abstracts were screened. Fifteen met the inclusion criteria. After review of the full texts, this resulted in a final reference list of 8 studies eligible for review. Five studies that assessed PD-1/PD-L1 in CRC and 3 trials that assessed PD-1 inhibitors were included. PD-1-positive (PD-1⁺) tumor-infiltrating lymphocytes and PD-L1⁺ cancer cells featured more prominently in high-level microsatellite instability (MSI-H) CRCs compared with microsatellite stable (MSS) CRCs, except in 1 study in which PD-L1 expression was higher in MSS CRCs. In the 3 trials that assessed PD-1 inhibitor, all 3 studies recruited patients with metastatic CRC (mCRC). One study also included patients with recurrent CRC. The objective response according to the Response Evaluation Criteria in Solid Tumors criteria was 0% (19 CRC patients with unknown microsatellite instability status) in the nivolumab study. In the pembrolizumab study, the objective response to PD-1 inhibitor was 40% and 0% in patients with MSI-H and MSS mCRC, respectively (10 patients in the MSI-H group, 18 patients in the MSS group). Seventy-eight percent of the patients in the MSI-H mCRC group compared with 11% in the MSS mCRC group (P < .005) showed no further disease progression at 12 weeks. In the nivolumab with or without ipilimumab study, objective partial response at 12 weeks to PD-1 inhibitor with or without cytotoxic T-lymphocyte-associated protein 4 inhibitor was 25.5% to 33.3% and 5% in the MSI-H and MSS groups, respectively (100 patients in the MSI-H group, 20 patients in the MSS group). Clinical trials that assessed PD-1 inhibitor immunotherapy in patients with CRC have recruited only small cohorts of patients with mCRC. Studies on the tumor microenvironment have been on the basis of archival specimens with different antibody PD-1 and PD-L1 preparations for immunohistochemistry, independent from immunotherapy trials. Immunotherapy with PD-1 therapy has potential benefit for immunogenic MSI-H CRCs whereas there is no evidence to date to suggest immunotherapy benefit in MSS CRCs. The available data are limited, and there is no information on non-mCRCs. Future trials are under way to determine its benefits.