[Clinical Outcome of Microsatellite Instability-High Patients Treated with Pembrolizumab and Genetic Counseling]

Prospective studies have demonstrated the efficacy of pembrolizumab in patients with previously treated unresectable or metastatic microsatellite instability-high(MSI-H)cancers. Pembrolizumab has been covered by the Japanese health insurance system since December 2018. The frequency of MSI-H in patients is as low as approximately 2%. In addition, some patients with MSI-H cancers are diagnosed with Lynch syndrome. In the present study, we retrospectively investigated patients who received MSI testing at Kitasato University Hospital from April 2019 to June 2020. We also investigated the therapeutic effect of pembrolizumab for MSI-H cancers and patients who received genetic counseling for Lynch syndrome. Results identified that 5 out of 263 patients who underwent MSI testing(1.9%)had MSI-H. The therapeutic outcomes of pembrolizumab in those patients were as follows: 1(20%)complete response, 3(60%)partial response, and 1(20%) progressive disease. The positive-outcome rate of MSI-H treatment in our institution was comparable to that in the previous reports. The high response rate of pembrolizumab was confirmed in the present study. Four out of 5 patients received genetic counseling at the genetic clinic, and 1 patient underwent genetic testing for Lynch syndrome. No deleterious variant of Lynch syndrome was detected in the genetic testing.