Exceptional and durable response with pembrolizumab in a patient with microsatellite stable metastatic castrate-resistant prostate cancer: High tumor mutational burden and circulating tumor DNA clearance as predictive biomarkers

Emerging biomarkers, such as tumor mutational burden and circulating tumor DNA (ctDNA), offer promising tools for identifying patients with prostate cancer who may benefit from immune checkpoint inhibitor therapy. This report describes an exceptional response to pembrolizumab in a patient with a microsatellite stability metastatic castration-resistant prostate cancer and ultrahigh tumor mutational burden. Additionally, this case further emphasizes the utility of ctDNA for monitoring molecular residual disease in patients whose disease burden is not adequately reflected by prostate-specific antigen levels and supports the use of ctDNA as a biomarker for personalized treatment monitoring and guiding treatment de-escalation.