Pathological Complete Response to Liver Metastasis With Pembrolizumab in a Previously Treated Patient With Microsatellite Instability-high Colorectal Cancer.

Background/aim: Immune checkpoint inhibitors are effective in treating microsatellite instability-high (MSI-H) metastatic colorectal cancer (CRC). Pathological complete response to immune checkpoint inhibitors for MSI-H metastatic CRC have been described in several reports. Liver metastasis is known to predict resistance to ani-programmed death 1 (PD-1)/PD-1 ligand 1 (PD-L1) therapy in several cancers, including CRC.

Case report: Herein, we report the case of a 23-year-old man with MSI-H colorectal liver metastasis who exhibited a pathological complete response to pembrolizumab following systemic chemotherapies. Pathological examination of the primary lesion revealed strong HLA-class I and HLA-DR expression in cancer cells. Elevated PD-L1 expression was observed in areas of increased CD8-postive T cell infiltration. Additional pathological study of regional lymph nodes showed increased PD-L1 and CD169 expression.

Conclusion: A detailed pathological examination revealed PD-L1 expression not only in the primary CRC lesion but also in regional lymph nodes. Recent studies have highlighted the significance of regional lymph nodes in anti-cancer immune responses. Therefore, pathological studies using resected lymph nodes might be beneficial for predicting the response of anti-PD-1/PD-L1 therapy.