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

• Authors: Deepak Ravindranathan, Emory University; Greta A Russler, Emory University; Lauran Yantorni, Emory University; Leylah M Drusbosky, Guardant Health, Inc; Mehmet Bilen, Emory University
• Journal Title: Case Reports in Oncology
• Volume: Volume 14, Number 1
• Publisher: S. Karger AG | 2021-01-01, Pages 190-196
• Type of Work: Article | Final Publisher PDF
• Publisher DOI: http://doi.org/10.1159/000512819
• Subjects: Health Sciences, Oncology
• Keywords: Immunotherapy; Liquid biopsy; Microsatellite instability; Prostate cancer

Abstract

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.