A new strategy for immunotherapy of microsatellite‐stable (MSS)‐type advanced colorectal cancer: Multi‐pathway combination therapy with PD‐1/PD‐L1 inhibitors

Abstract Colorectal cancer (CRC) is a frequent gastrointestinal malignancy with high rates of morbidity and mortality; 85% of these tumours are proficient mismatch repair (pMMR)‐microsatellite instability‐low (MSI‐L)/microsatellite stable (MSS) CRC known as ‘cold’ tumours that are resistant to immunosuppressive drugs. Monotherapy with programmed death 1 (PD‐1)/programmed death ligand 1 (PD‐L1) inhibitors is ineffective for treating MSS CRC, making immunotherapy for MSS CRC a bottleneck. Recent studies have found that the multi‐pathway regimens combined with PD‐1/PD‐L1 inhibitors can enhance the efficacy of anti‐PD‐1/PD‐L1 in MSS CRC by increasing the number of CD8+ T cells, upregulating PD‐L1 expression and improving the tumour microenvironment. This paper reviews the research progress of PD‐1/PD‐L1 inhibitors in combination with cytotoxic T‐lymphocyte–associated antigen 4 (CTLA‐4) inhibitors, oncolytic virus, intestinal flora, antiangiogenic agents, chemotherapy, radiotherapy and epigenetic drugs for the treatment of pMMR‐MSI‐L/MSS CRC.