An expanding role for immunotherapy in colorectal cancer

Colorectal cancer (CRC) is a leading cause of cancer-related mortality in the United States. Response rates to second- and third-line therapy for metastatic CRC (mCRC) remain low, and immunotherapy is an attractive strategy for treatment in these patients given generally better tolerability than conventional chemotherapy and the potential for long-lasting durable responses. In particular, the novel checkpoint inhibitors (CPIs) have demonstrated unprecedented clinical activity in a wide range of cancers. The observation of clinical activity in microsatellite instability-high (MSI-H) mCRC was the first indication of a potential for CRC to respond to these agents, and has led to a breakthrough designation by the FDA for CPI use in this subset. Despite this, a proportion of MSI-H and nearly all microsatellite stable disease will not respond to single-agent checkpoint inhibition, and clinical trials are ongoing to increase responses to immunotherapy in mCRC through both better patient selection and novel combinations of immunotherapeutic agents. This review will provide a focused update on the most compelling clinical results of immunotherapy in CRC to date, as well as a summary of current strategies being tested in clinical trials in increase responses to immunotherapy in CRC.