Incidence of microsatellite instability-high hepatocellular carcinoma among Japanese patients and response to pembrolizumab

Aim Pembrolizumab has been quickly approved in many countries for the treatment of patients with unresectable or metastatic, microsatellite instability-high (MSI-H) solid tumors, which have progressed following previous treatment and who have no satisfactory alternative treatment options. We aimed to determine the incidence of MSI-H tumors in Japanese patients with advanced hepatocellular carcinoma (HCC). Methods We investigated the incidence of MSI-H tumors in 82 consecutive Japanese patients with unresectable HCC that had progressed after standard of care treatment. Using a companion diagnostic sequencing kit (polymerase chain reaction analysis of five microsatellite markers: BAT25, BAT26, NR21, NR24 and MONO27), we analyzed 49 biopsy specimens and 33 resection specimens. Responses to pembrolizumab were assessed with the modified Response Evaluation Criteria in Solid Tumors. Results MSI-H tumors were found in only two patients (2.4%), in whom all five markers showed slight shortening. One patient had a complete response to pembrolizumab for over 10 months, and the other was a non-responder. Conclusions MSI-H tumor status was found in only two of 82 (2.4%) Japanese patients with advanced HCC, one of whom had a complete response to pembrolizumab. Thus, MSI status should be assessed in patients with HCC who progress after standard of care treatment.