Treatment of Platinum Nonresponsive Metastatic Malignant Peritoneal Mesothelioma With Combination Chemoimmunotherapy.

Malignant peritoneal mesothelioma is a rare cancer associated with minimal durable disease control with chemotherapy and poor overall survival. The efficacy of combined cytotoxic chemotherapy and immune checkpoint inhibitors (ICIs) in malignant peritoneal mesothelioma has not previously been studied. We describe the clinical course of 2 patients with metastatic peritoneal mesothelioma who both relapsed with platinum nonresponsive disease after initial cytoreductive surgery and chemotherapy. In both cases, addition of pembrolizumab to platinum and pemetrexed treatment resulted in a substantial partial and a near complete disease response. Notably, both patients possessed tumors without validated biomarkers of ICI response, including low tumor mutational burden and negative programmed death ligand-1. The unique genomic landscape of each patient may have enabled increased tumor immunorecognition and ICI efficacy. In addition, chemotherapy priming of the tumor microenvironment may have improved ICI response. This report supports future research to characterize the benefit of combination chemotherapy and ICI in peritoneal mesothelioma.