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

[PURPOSE] 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 <10 without tMSI-H group. In intrapatient assessments, patients with tTMB ≥10 had more favorable TTNT with subsequent ICI versus prior taxane. Detection of bMSI-H was associated with more favorable TTNT on ICI (HR, 0.34; 95% CI, 0.14-0.83) and OS (HR, 0.21; 95% CI, 0.06-0.75) when tumor fraction ≥1%.

[CONCLUSIONS] These findings add support for tTMB and tMSI in predicting ICI monotherapy benefit in mCRPC and provide evidence supporting bMSI testing when tissue is unavailable.