BAP1 Loss Might Be a Predictive Biomarker for Immunotherapy Response in Pleural Mesothelioma

INTRODUCTION: Reliable predictive biomarkers for immune checkpoint inhibitors (ICIs) in pleural mesothelioma (PM) are lacking. Loss of BRCA1-associated protein 1 (BAP1) is a frequent molecular alteration in PM and may influence the tumor immune microenvironment. We evaluated whether BAP1 loss is associated with clinical outcomes following immunotherapy. METHODS: We retrospectively analyzed 14 patients with PM who were treated with ICIs between April 2014 and May 2024. BAP1 status was assessed by immunohistochemical staining of resected tumor specimens. Patients were categorized into BAP1-loss and BAP1-positive groups. Progression-free survival (PFS), overall survival (OS), response rate (RR), and disease control rate (DCR) were compared. RESULTS: BAP1 loss was observed in nine patients (64%). Median PFS was significantly longer in the BAP1-loss group compared with the BAP1-positive group (5.3 vs. 2.1 months; log-rank p = 0.03). Median OS was also prolonged in the BAP1-loss group (8.6 vs. 2.1 months; log-rank p = 0.03). RR and DCR were numerically higher among patients with BAP1 loss. CONCLUSION: BAP1 loss was associated with improved clinical outcomes following immunotherapy in PM. These findings support that a molecular profile that includes BAP1 deletion may serve as a biomarker for predicting response to immunotherapy in PM.