Data from Additive Clinical Utility of Microsatellite Instability and Tumor Mutational Burden to Predict Immune Checkpoint Inhibitor Effectiveness in Metastatic Castration-Resistant Prostate Cancer

AbstractPurpose: Immune checkpoint inhibitors (ICI) have shown limited efficacy in unselected patients with metastatic castration-resistant prostate cancer (mCRPC). However, ICIs are approved for biomarker-defined subsets: microsatellite instability–high (MSI-H) and/or high tumor mutational burden (TMB-H). The efficacy of ICIs in TMB-H but not MSI-H disease remains unclear, and limited data exist evaluating ICI outcomes associated with blood-based MSI (bMSI) in mCRPC. Experimental Design: This study used the United States–based deidentified Flatiron Health–Foundation Medicine prostate cancer Clinico-Genomic Database. Patients with tissue-assessed MSI (tMSI) and TMB (tTMB) status by an algorithm supporting an FDA-approved CDx for pembrolizumab were included if treated with single-agent ICI. Separately, outcomes on ICI associated with bMSI were assessed, including if treated with single-agent ICI or taxane. Results: Among 2,965 patients with mCRPC, tMSI-H (3.2%) was nearly always also tTMB ≥10 mut/Mb (4.7%). In 84 ICI-treated patients, time to next treatment (TTNT) and overall survival (OS) were more favorable in tMSI-H with any TMB [TTNT HR, 0.18; 95% confidence interval (CI), 0.09–0.37 and OS HR, 0.32; 95% CI, 0.15–0.66] and tTMB ≥10 without tMSI-H (TTNT HR, 0.18; 95% CI, 0.04–0.48 and OS HR, 0.20; 95% CI, 0.05–0.77) compared with tTMB