Molecular and immunological features of a prolonged exceptional responder with malignant pleural mesothelioma treated initially and rechallenged with pembrolizumab

BACKGROUND: This case represents an exceptional response to pembrolizumab in a patient with epithelioid mesothelioma with a further response on rechallenge. CASE PRESENTATION: A 77-year-old woman with advanced epithelioid mesothelioma extensively pretreated with chemotherapy demonstrated a prolonged response of 45 months to 52 cycles of pembrolizumab. On rechallenge with pembrolizumab, further disease stability was achieved. Serial biopsies and analysis by immunohistochemistry and immunofluorescence demonstrated marked immune infiltration and documented the emergency of markers of immune exhaustion. Whole exome sequencing demonstrated a reduction in tumor mutational burden consistent with subclone elimination by immune checkpoint inhibitor (CPI) therapy. The relapse biopsy had missense mutation in BTN2A1. CONCLUSION: This case supports rechallenge of programme death receptor 1 inhibitor in cases of previous CPI sensitivity and gives molecular insights.