Expression of PD-L1 in Patients With Malignant Peritoneal Mesothelioma: A Pilot Study

Frequency of PD-L1 expression and the role of immunotherapy in malignant peritoneal mesothelioma (MPM) have not been well characterized. The purpose of this study was to determine PD-L1 expression in patients with MPM and perform an exploratory analysis for associations between PD-L1 and its biological behavior in MPM.Tumor samples were collected from patients undergoing surgical interventions between January 2018 and June 2020. Specimens were stained with anti-PD-L1 antibodies (Dako 22c3) and positivity was determined by tumor proportion score (TPS) or combined positive score (CPS) being ≥1%.Twenty one samples were obtained from 21 patients. Sixteen of 21 (76%) samples were CPS positive and 9 of 21 (43%) were TPS positive. Three samples had more aggressive biphasic/sarcomatoid histology and a high CPS and TPS (CPS: 3, 75, 95%; TPS: 2, 60, 90%). On an exploratory analysis, as the CPS or TPS threshold increased, there was a trend towards worse survival.MPM has a high frequency of PD-L1 expression, which may be associated with more aggressive tumor biology. These data provide the foundation for continued evaluation of checkpoint inhibition in patients with MPM.