A phase I/Ib study of regorafenib and nivolumab in mismatch repair proficient advanced refractory colorectal cancer

•Mismatch repair proficient (pMMR) colon cancer (CRC) is unresponsive to immunotherapy.•Regorafenib (REGO) may enhance the antitumor activity of nivolumab (NIVO).•The dose-limiting toxicity of REGO/NIVO was rash.•REGO/NIVO showed limited anticancer activity in pMMR CRC.•REGO/NIVO showed anticancer activity in non-liver or lung metastatic pMMR CRC.