AUTHOR=Yoshida Tsukasa , Yaegashi Hiroshi , Toriumi Ren , Kadomoto Suguru , Iwamoto Hiroaki , Izumi Kouji , Kadono Yoshifumi , Ikeda Hiroko , Mizokami Atsushi 

TITLE=Long response duration to pembrolizumab in metastatic, castration-resistant prostate cancer with microsatellite instability-high and neuroendocrine differentiation: A case report

JOURNAL=Frontiers in Oncology

VOLUME=Volume 12 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/oncology/articles/10.3389/fonc.2022.912490

DOI=10.3389/fonc.2022.912490

ISSN=2234-943X

ABSTRACT=Background: The detection of microsatellite instability in urologic cancers is rare, especially in metastatic, castration-resistant prostate cancer with neuroendocrine differentiation.
Case Presentation: This is a case of a 66-year-old Asian male with prostate adenocarcinoma who had metastases at initial presentation. Despite combined androgen deprivation therapy, his prostate-specific antigen (PSA) progressively increased, and prostate re-biopsy revealed small cell carcinoma. He was treated with platinum-based systemic chemotherapy, and his tumor markers, including PSA, remained negative; however, his local symptoms worsened. Subsequently, microsatellite instability-high was detected, and pembrolizumab was administered resulting in complete remission with resolution of symptoms and continued therapeutic effect for more than 14 months.
Conclusion: Microsatellite instability testing should be considered, despite its low detection rate, because response to pembrolizumab in metastatic, castration-resistant prostate cancer with detectable microsatellite instability is associated with a prolonged duration of response.