Pembrolizumab for a patient with metastatic castration‐resistant prostate cancer with microsatellite instability‐high

INTRODUCTION: We report the case of a patient with metastatic castration‐resistant prostate cancer with microsatellite instability‐high who was treated with pembrolizumab after cabazitaxel administration. CASE PRESENTATION: A 58‐year‐old patient with heavily pretreated metastatic castration‐resistant prostate cancer, whose prostate surgical specimen was disclosed as microsatellite instability‐high, underwent pembrolizumab therapy. After initiation of pembrolizumab, his prostate‐specific antigen level decreased, imaging findings showed good response with lymph node shrinkage, and his walking difficulty decreased dramatically. CONCLUSION: The rarity of microsatellite instability‐high tumor in castration‐resistant prostate cancer may hamper pembrolizumab administration. This potentially active agent should be considered as part of a treatment regimen for patients with microsatellite instability‐high castration‐resistant prostate cancer. To the best of our knowledge, this is the first report of a Japanese castration‐resistant prostate cancer patient who demonstrated clinical benefit from pembrolizumab treatment.