Durvalumab for MSI-H or POLE Mutated Metastatic Colorectal Cancer

Later-line therapies after failure of standard treatments for metastatic colorectal cancer patients are limited; regorafenib and TAS-102 have shown clinical activity for these patients, however, efficacy outcomes seemed not to be sufficient although there have been rather higher frequencies of adverse events. Mismatch repair (MMR) deficiency or microsatellite instability-high (MSI-H) played a role of negative predictive factor for adjuvant fluorouracil-based chemotherapy in patients with resected colorectal cancer. In the metastatic setting, deficient MMR or MSI-H represented poor prognosis; however, their predictive role has been documented after the pembrolizumab trial was reported. The results of the pembrolizumab trial demonstrated that the PD-1 blockade with pembrolizumab monotherapy showed 40% of confirmed immune-related objective response rates in patients with MMR deficient metastatic colorectal cancers; hence there was no objective response in those with MMR proficient tumors. The progression-free rates at 20 weeks were 78% versus 11%, respectively, also favouring those with MMR deficient tumors. However, the MMR deficiency of MSI-H is found in only about 5% in patients with metastatic colorectal cancer, which is too small to expand potential candidate of immunotherapy. One of the proposed mechanism of promising efficacy from pembrolizumab for MMR deficient colorectal cancer is that MMR deficient or MSI-H colorectal cancers harbour higher somatic mutation loads than MMR proficient colorectal cancer (a mean of 1782 somatic mutations per tumor in the MMR deficient tumors versus 73 in the MMR proficient tumors in the results of pembrolizumab trial); somatic mutations have the potential to encode non-self immunogenic antigens; therefore, immunotherapy enhancing immune surveillance produced promising treatment efficacy in the MMR deficient tumors. The POLE gene encodes the catalytic subunit of DNA polymerase epsilon, and it involves DNA repair and chromosomal replication. The POLE mutations are located in the exonuclease domain, and their presence has already been reported in the various cancers including colorectal and endometrial cancer.