Abstract A004: Immune response to sub-ablative radiation before surgery in pleural mesothelioma – Results from the SMARTER trial
Authors: de Perrot Marc , Zia Amin , Wu Licun , Boukhaled Giselle , Gadalla Ramy , Wang Ben , Brooks David , Man Jeffrey , Pugh Trevor , Pal Prodipto , Zaeimi Fatemeh , Chu Hebe , Tsao Ming-Sound , Bradbury Penelope , Cho John 
Published: December 2023

Abstract
Introduction. We developed a new treatment paradigm in pleural mesothelioma using a short course of sub-ablative hemithoracic radiation followed by surgery and observed encouraging results in epithelioid subtypes. We therefore performed a phase I trial to determine the safety and immunological benefit of 3 fractions of radiation before surgery.
Methods. The trial, Surgery for Mesothelioma After Radiation Therapy using Extensive pleural Resection (SMARTER), was designed as a 3+3 phase I trial with incremental hemithoracic radiation dose up to 18 Gy in 3 fractions (background dose) combined with a boost of at least 39 Gy to the gross disease (boost dose). Patients with histologically confirmed stage I-III pleural mesothelioma with tumor size of at least 2 cm in diameter on CT scan were eligible for the study. Surgery was performed 7-14 days after the last fraction of radiation. CyTOF and Luminex were performed on peripheral blood at baseline pre-radiation, post-radiation at the time of surgery, and during follow-up within 3 months from surgery. Tumor was collected at the time of surgery from the sites of low dose radiation (background, T1) and high dose radiation (boost, T2) and compared to pre-radiation (baseline, T0) using single cell RNA sequencing (scRNA seq). CD8 tumor infiltrating lymphocytes (TILs) was determined by immunostaining. The results were validated in a second cohort of patients treated off trial.
Results. A total of 12 patients were included in the trial with all patients safely proceeding to surgery after a background radiation dose of 0 Gy, 6 Gy, 12 Gy and 18 Gy. Total radiation boost dose ranged from 39 Gy to 54 Gy. The results were validated in a second cohort of 12 patients. Circulating IL-15, FLT-3L, and CXCL10 significantly increased at the time of surgery compared to baseline. CXCL10 continued to rise during follow-up and was associated with better survival at 1 year (86% vs 17% in the absence of CXCL10 upregulation, p=0.005). The peak of circulating IL-15 correlated with CD8 TILs in the resected tumor specimen. scRNA seq on tumor samples demonstrated that monocytes expressed IL-15 and IL-15Ra, and increased after radiation. Pseudotime trajectory analysis showed an evolution from classical to non-classical monocytes with a shift from M2-like to M1-like phenotype. This was associated with reinvigoration of cytotoxic CD8 TILs and expansion of GITR negative NK cells. CyTOF analysis on peripheral blood confirmed the upregulation of circulating monocytes followed by upregulation of CD8 effector memory T cells, which progressively decreased before tumor recurrence. Tumors with larger populations of IFN-TAM and CD8 ISG were characterized by high CXCL10 expression at baseline and a lack of CXCL10 upregulation after radiation associated with an absence of CD8 T cell reinvigoration and poor survival.
Conclusions. These results demonstrate the immunogenic benefit and clinical safety of sub-ablative radiation before surgery in pleural mesothelioma. The lack of benefit was observed in tumors expressing high CXCL10 at baseline.
Citation Format: Marc de Perrot, Amin Zia, Licun Wu, Giselle Boukhaled, Ramy Gadalla, Ben Wang, David Brooks, Jeffrey Man, Trevor Pugh, Prodipto Pal, Fatemeh Zaeimi, Hebe Chu, Ming-Sound Tsao, Penelope Bradbury, John Cho. Immune response to sub-ablative radiation before surgery in pleural mesothelioma – Results from the SMARTER trial [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2023 Oct 1-4; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Immunol Res 2023;11(12 Suppl):Abstract nr A004.