Abstract
While checkpoint inhibitor therapy has revolutionized the treatment landscape of some solid tumors, it has shown limited efficacy in metastatic castration-resistant prostate cancers (mCRPC). A small (~3-5%) but clinically distinct subset of mCRPC tumors have a DNA mismatch repair deficiency (dMMR) and develop a hypermutation phenotype with elevated tumor mutational burden and high microsatellite instability (MSI-H). Retrospective analyses have shown dMMR/MSI-H status to be a predictive biomarker for response to pembrolizumab in prostate tumors. Here, in this report, we present a case of a patient with mCRPC harboring a somatic dMMR who had progressed on pembrolizumab after an initial response. He enrolled on a clinical trial with JNJ-081, a prostate-specific membrane antigen-CD3 bispecific T-cell engager antibody and experienced a partial response with course complicated by cytokine release syndrome. On progression, he was reinitiated on pembrolizumab and experienced an exceptional second response, with his prostate-specific antigen falling from a high of 20.01 to undetectable after 6 weeks and remaining undetectable for >11 months. To our knowledge, this represents the first reported case of bispecific T-cell engager-mediated re-sensitization to checkpoint inhibitor therapy in any cancer.