Complete Response to Pembrolizumab in Prostate Cancer With Microsatellite Instability–High, Deficient Mismatch Repair, Tumor Mutational Burden–High, and Programmed Death–Ligand 1 Positivity

A 72-year-old man initially presented with prostate cancer and a metastatic right rib lesion. He developed rapid progression on androgen deprivation therapy in 7 months and subsequent treatment with enzalutamide in 3 months. Biomarker studies after progression to castration-resistant prostate cancer revealed deficient mismatch repair, microsatellite instability–high, tumor mutational burden of 21 mut/Mb, and programmed death–ligand 1–positive disease. Based on the results of the KEYNOTE 158 study, immunotherapy with pembrolizumab was initiated. The patient had a complete clinical, radiologic, and biochemical response with an undetectable prostate-specific antigen level within 3 months and continued pembrolizumab 8 years after diagnosis.