Detection of Microsatellite Instability via Circulating Tumor DNA and Response to Immunotherapy in Metastatic Castration-Resistant Prostate Cancer: A Case Series

Pembrolizumab has been approved by the US Food and Drug Administration for the treatment of metastatic or unresectable solid tumors that are microsatellite instability-high (MSI-H) or mismatch repair deficient. Blood-based circulating tumor DNA (ctDNA) assays have been validated to identify tumors with MSI-H status without the need for tissue biopsy. We report 2 patients with metastatic castration-resistant prostate cancer (mCRPC) who had prior treatment with multiple lines of therapy and underwent ctDNA testing, which detected MSI-H status. Both patients were treated with pembrolizumab, resulting in an excellent clinical response measured with liquid biopsies before and after initiation of therapy, which demonstrated a significant reduction in somatic-variant allele frequency in addition to a decrease in prostate serum antigen levels.