Defective DNA mismatch repair genes can lead to
microsatellite instability (MSI)-high status in
prostate cancer (PC). Accumulation of replication errors in
DNA leads to the production of abundant neoantigens, which could be targets for
immune checkpoint inhibitors (CPIs). However, the incidence of MSI-high PC is low, and not all patients show a satisfactory therapeutic response to CPIs. Here, we present the case of a patient with MSI-high
castration-resistant PC who showed a remarkable and durable response to
pembrolizumab. The patient was resistant to
abiraterone,
docetaxel, and
cabazitaxel and was suffering from multiple
tumor-associated or treatment-related complications, such as
urinary tract infection,
infective endocarditis, and uncontrollable prostatic
hemorrhage. Soon after the start of
pembrolizumab therapy, the patient showed a dramatic decrease in
prostate-specific antigen from 35.67 ng/mL to an undetectable level and a remarkable reduction in the size of a massive prostate mass and
lymph node metastases, with an absence of treatment-related complications. Specimens from the
transurethral resection of prostate cancer during
cabazitaxel treatment for control of prostate
bleeding and also that from the prostate biopsy at initial diagnosis revealed MSI-high status. Immunohistochemistry showed loss of MSH2 and MSH6, and whole-exome sequencing revealed an approximate
tumor mutation burden of 61 mutations/Mb as well as biallelic loss of MSH2
Pembrolizumab could show a significant effect even in a heavily treated patient with MSI-high advanced PC. Accumulation of detailed clinical and genomic information of cases of MSI-high PC treated with
pembrolizumab is necessary for optimal patient selection.