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

Abstract

e20062 Background: The increasing incidence and poor outcome associated with MPM demand identification of effective treatment options. Promising results have been reported with immunotherapy (IO) in a small proportion of MPM patients (p). MMR deficiency (dMMR) has been well described in several malignancies and was recently approved as a tumor biomarker for IO with anti-PD-1 checkpoint inhibitor. 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 p. Methods: Tumors of 159 MPM p from Vall d´Hebron University Hospital and October 12th University Hospital 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 when any MMR protein expression was negative and MMR intact when all MMR proteins were positively expressed. Associations between clinical variables and outcome were assessed with Cox regression models and survival data were calculated by the Kaplan-Meier method. Results: P characteristics: median age: 69 years (29-88 years), males: 71%, performance status (PS) 1:69%, asbestos exposure: 52%, stage III at diagnosis: 42%, epithelial subtype: 65%, systemic treatment 81% (57% chemotherapy with cisplatin plus pemetrexed in first line), 50% received second line and 28% third line. MMR protein expression was analyzed in 158 samples with enough tissue and was positive in all of the cases. The median overall survival (mOS) in all population was 15 months (m) (13.5-18.8m). In a multivariate model factors associated to improved mOS were PS 0 vs PS2 (13 v 2 m, HR 12.8, p < 0.01), neutrophil-lymphocyte ratio (NLR) < 5 (18 v 9 m in NLR ≥5,HR 1.5, p < 0.05) and epitheliod vs sarcomatoid histology (18 vs 4 m HR 4.7, p < 0.01). Thirteen p received IO 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 (2.1-26.1m). Conclusions: In our series we were unable to identify any MPM patient with dMMR by IHC. Further studies are needed to elucidate novel predictive biomarkers benefit from IO in MPM.