PD-1 blockade in neoadjuvant setting of DNA mismatch repair-deficient/microsatellite instability-high colorectal cancer.

84 Background: Although PD-1 blockade has significantly improved the survival of metastatic colorectal cancer with DNA Mismatch Repair-Deficient/Microsatellite Instability-High (MSI-H), the data on neoadjuvant setting is limited. Methods: In this retrospective study, we enrolled eight patients with advanced MSI-H colorectal cancer from three hospitals. four patients are locally advanced and four are metastatic. All the patients received at least two doses of PD-1 antibody with or without chemotherapy as neoadjuvant therapy. The aim of the present study was to evaluate the short‐term efficacy and toxicities of this strategy. Results: Neoadjuvant PD-1 Blockade is well tolerated with a few immune-related side-effect profiles. All the enrolled patients had a major response in imaging and/or pathological evaluation. Five of the seven resected patients were evaluated as pathological complete response. One patient without surgery has a cCR tumor response. For the patients with radical surgery, there is no surgery-related complication observed. Conclusions: Neoadjuvant PD-1 blockade induced tumor regression with a major clinical and pathological response in advanced dMMR/MSI-H colorectal cancer. Further studies are required to evaluate the long-term effect of this strategy.