TMB-High, MSI-High Castration-Resistant Prostate Cancer Treated With Pembrolizumab.

INTRODUCTION

The use of pembrolizumab in patients with microsatellite instability-high (MSI-high) and tumor mutation burden-high (TMB-high) prostate cancer in Japan is not widely reported. Here, we report the case of a patient with MSI-high and TMB-high prostate cancer who responded well to pembrolizumab after multiple systemic treatments.

CASE PRESENTATION

A 68-year-old Japanese man was diagnosed with cT4N1M1a prostate cancer. He was treated with several androgen receptor signaling inhibitors and chemotherapy. After intense systemic treatment, disease progression was confirmed, and genomic testing detected MSI-high and TMB-high. However, treatment with pembrolizumab resulted in marked prostate-specific antigen reduction and significant shrinkage of metastases.

CONCLUSION

Genomic tests should be considered for high-grade tumors. MSI-high and TMB-high prostate cancer responded well to pembrolizumab in this case, but patients should be carefully monitored for the development of side effects after administration of pembrolizumab.