Microsatellite Instability-High and High Tumor Mutation Burden Frequencies in Urologic Malignancies Off-Label for Immune Checkpoint Inhibitors in Japan: A Retrospective Single-Institutional Cohort.

Objectives Microsatellite instability-high (MSI-H) and high tumor mutation burden (TMB-high) frequencies were investigated to determine the efficacy and adverse events of pembrolizumab in patients with urologic malignancies (or their equivalents) for which immune checkpoint inhibitors (ICIs) are not covered by Japanese insurance. Methods Between February 2019 and April 2024, patients with urologic malignancies (or their equivalents) treated in our department for whom ICIs were not approved by Japanese insurance were screened with an MSI companion diagnostic kit or comprehensive genomic profiling (CGP). The efficacy of pembrolizumab therapy, presence of adverse events, and outcomes were evaluated retrospectively in patients with MSI-H or TMB-high. Results In total, 44 patients were tested, and the median age at testing was 70 years. Castration-resistant prostate cancer (CRPC) was the most common (n = 31). Overall, 49 tests were performed, including 22 MSI companion diagnostic kits and 27 CGP tests. Of the 49 tests, 1 detected MSI-H, 2 detected TMB-high, and 1 detected simultaneous MSI-H/TMB-high, with detection rates of 4.1% and 11.1% for MSI-H and TMB-high, respectively. A patient with MSI-H CRPC and neuroendocrine differentiation achieved a complete response and a prolonged duration of response to pembrolizumab without adverse events. The duration of response to pembrolizumab was shorter in a patient with TMB-high, and a CRPC patient with simultaneously detected MSI-H/TMB-high had to discontinue pembrolizumab early due to immune-related adverse events. Conclusions Despite the potential benefit of pembrolizumab, MSI-H or TMB-high was less frequently detected in urologic malignancies for which ICIs are not covered by Japanese insurance.