How to overcome resistance to immune checkpoint inhibitors in colorectal cancer: From mechanisms to translation

Immunotherapy, especially with immune checkpoint inhibitors (ICIs), has shown advantages in cancer treatment and is a new hope for patients who have failed multiline therapy. However, in colorectal cancer (CRC), the benefit is limited to a small subset of patients with microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) metastatic CRC (mCRC). In addition, 45% to 60% of dMMR/MSI-H mCRC patients showed primary or acquired resistance to ICIs. This means that these patients may have potential unknown pathways mediating immune escape. Almost all mismatch repair-proficient (pMMR) or microsatellite-stable (MSS) mCRC patients do not benefit from ICIs. In this review, we discuss the mechanisms of action of ICIs and their current status in CRC. We then discuss the mechanisms of primary and acquired resistance to ICIs in CRC. Finally, we discuss promising therapeutic strategies to overcome resistance to ICIs in the clinic.