Analysis of mismatch repair (MMR) proteins expression in a series of malignant pleural mesothelioma (MPM) patients

Background Promising results have been reported with immune checkpoint inhibitors (ICI) in a small proportion of MPM patients. MMR deficiency (dMMR) has been well described in several malignancies and was approved as a biomarker for anti-PD-1 inhibitors. Next generation sequencing (NGS) data demonstrated that 2% of MPM harbor microsatellite instability. The aim of this study is to characterize MMR by immunohistochemistry (IHC) in a series of MPM including a subset of patients treated with immunotherapy. Methods Tumors of 159 MPM p diagnosed between 2002 and 2017 were reviewed. Formalin-fixed, paraffin-embedded tissue was stained for MLH1, MSH2, MSH6 and PMS2 and tumors were classified as dMMR (MMR protein expression negative) and MMR intact (all MMR proteins positively expressed). We retrospectively collected clinical outcomes under standard chemotherapy and experimental immunotherapy in the entire cohort. Results MMR protein expression was analyzed in 158 samples with enough tissue and was positive in all of the cases. Twenty two patients received ICI with anti-CTLA4 or anti-PD-1 blockade in clinical trials, 58% had a response or stable disease for more than 6 m, with median progression-free survival (PFS) of 5.7 m (2.1–26.1 m). The median overall survival (mOS) in all population was 15 months (m) (13.5–18.8 m). In a multivariable model factors associated to improved mOS were PS 0, neutrophil–lymphocyte ratio (NLR) < 5 and epithelioid histology ( p < 0.001). Conclusions In our series we were unable to identify any MPM patient with dMMR by IHC. Further studies are needed to elucidate potential predictive biomarkers of ICI benefit in MPM.