Clinical activity of pembrolizumab in metastatic prostate cancer with microsatellite instability high (MSI-H) detected by circulating tumor DNA.

To report a multi-institutional case series of patients with advanced microsatellite instability high (MSI-H) prostate adenocarcinoma identified with clinical cell-free DNA (cfDNA) next-generation sequencing (NGS) testing and treated with immune checkpoint inhibitors. Retrospective analysis of patients with metastatic castration-resistant prostate cancer (mCRPC) and MSI-H tumor detected by a commercially available cfDNA NGS assay Guardant360 (G360, Guardant Health) at eight different Academic Institutions in the USA, from September 2018 to April 2020. From a total of 14 MSI-H metastatic prostate cancer patients at participating centers, nine patients with mCRPC with 56% bone, 33% nodal, 11% liver and 11% soft-tissue metastases and a median PSA of 29.3 ng/dL, were treated with pembrolizumab after 2 lines of therapy for CRPC. The estimated median time on pembrolizumab was 9.9 (95% CI 1.0 to 18.8) months. Four patients (44%) achieved PSA50 after a median of 4 (3-12) weeks after treatment initiation including three patients with >99% PSA decline. Among the patients evaluable for radiographic response (n=5), the response rate was 60% with one complete response and two partial responses. Best response was observed after a median of 3.3 (1.4-7.6) months. At time of cut-off, four patients were still on pembrolizumab while four patients discontinued therapy due to progressive disease and one due to COVID-19 infection. Half of the patients with PSA50 had both MSI-H and pathogenic alterations in BRCA1 and BRCA2 in their G360 assays. The use of liquid biopsy to identify metastatic prostate cancer patients with MSI-H is feasible in clinical practice and may overcome some of the obstacles associated with prostate cancer tumor tissue testing. The robust activity of pembrolizumab in selected patients supports the generalized testing for MSI-H.

Journal for immunotherapy of cancer. 2020 Aug [Epub]

Pedro Barata, Neeraj Agarwal, Roberto Nussenzveig, Benjamin Gerendash, Ellen Jaeger, Whitley Hatton, Elisa Ledet, Brian Lewis, Jodi Layton, Hani Babiker, Alan Bryce, Rohan Garje, Cy Stein, Lesli Kiedrowski, Philip Saylor, Oliver Sartor

Deming Department of Medicine, Tulane University, New Orleans, Louisiana, USA ., Department of Medical Oncology, University of Utah, Salt Lake City, Utah, USA., Department of Medical Oncology and Experimental Therapeutics, City of Hope Comprehensive Cancer Center, Duarte, California, USA., Deming Department of Medicine, Tulane University, New Orleans, Louisiana, USA., Department of Medicine, University of Arizona Arizona Cancer Center, Tucson, Arizona, USA., Department of Oncology, Mayo Clinic, Scottsdale, Arizona, USA., Division of Hematology, Oncology, and Blood and Marrow Transplant, University of Iowa, Iowa City, Iowa, USA., Guardant Health Inc, Redwood City, California, USA., Department of Oncology, Massachusetts General Hospital, Boston, Massachusetts, USA.