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.