Efficacy of pembrolizumab in MSI-high and BRCA-positive castration-resistant prostate cancer.

This report presents a rare case of metastatic castration-resistant prostate cancer (CRPC) in an adult patient characterized by dual molecular alterations: microsatellite instability-high (MSI-H) and a BRCA2 mutation. Despite initial treatment with castration, Abiraterone, and sequential chemotherapy with docetaxel and cabazitaxel, the patient progressed to CRPC. Genetic testing revealed MSI-H and a BRCA2 mutation, prompting pembrolizumab therapy. The treatment led to a dramatic prostate-specific antigen (PSA) reduction . This case underscores the importance of comprehensive genomic profiling for advanced prostate cancer. MSI-H tumors often respond to immune checkpoint inhibitors (ICIs) such as pembrolizumab, while BRCA2 mutations are associated with poly(ADP-ribose) polymerase inhibitors (PARPi) sensitivity. This dual alteration presents therapeutic challenges, as evidenced by pembrolizumab's remarkable efficacy in this patient, highlighting its potential as a treatment option for MSI-H and BRCA-positive CRPC. Moreover, next-generation sequencing (NGS) played a crucial role in identifying actionable biomarkers not detected by earlier BRCA analyses, emphasizing the necessity of thorough genetic testing. Further research is needed to optimize treatment strategies for cases with coexisting MSI-H and BRCA mutations, including exploring the synergistic effects of ICIs and PARP i. This case demonstrates the promise of pembrolizumab and advances the understanding of genetic testing's role in tailoring therapies for complex molecular profiles in prostate cancer.