Abstract 3106: The first-in-class PRX3 inhibitor RSO-021 modulates the protein composition of malignant pleural effusions by increasing immunomodulating phenotypes and decreasing epithelial to mesenchymal protein expression

Malignant pleural effusion (MPE) is the buildup of fluid, cellular material, cytokines, and excreted proteins in the pleural space due to tumor burden. Approximately 90% of pleural mesothelioma (PM) patients and 20% of all advanced cancer patients will present with an MPE. The composition of MPE has been shown to be predicative of tumor burden and response to therapy. In the ongoing MITOPE phase 1/2 clinical trial, RSO-021 is being tested for efficacy in patients with MPE arising from mesothelioma or other metastatic tumors to the lung (NCT05278975). Results from the Phase 1 trial of RSO-021 showed promise that this approach may extend survival of PM patients (J Clin Oncol. Vol 42, 16). RSO-021 is delivered via an intrapleural catheter, limiting systemic exposure. RSO-021 is an inhibitor of the mitochondrial antioxidant enzyme peroxiredoxin 3 (PRX3) which protects tumor cells from oxidative stress induced apoptosis. Covalent inhibition of PRX3 with RSO-021 has shown cytotoxic activity in cell, mouse models, and human clinical trials, giving promise to a new therapy for aggressive cancers. To evaluate patient responses to RSO-021, we conducted mass spectrometry (MS) of MPE from patients pre and post RSO-021 treatment and identified a composition of proteins with differential abundance related to RSO-021 treatment. These data correlated with cytokine ELISA assays conducted with the same fluid. In addition, RNA sequencing of tumor explants identified common immunologic responses, stress response pathways and downregulation of pro-metastatic markers. Using Metascape Gene Annotation software we determined enrichment pathways for differential levels of abundant peptides within the PE fluid collected pre and post treatment. Pathways increased by RSO-021 include neutrophil degranulation, cell killing, hydrogen peroxide metabolic processes, aerobic glycolysis, and apoptosis. These findings support our current data for the proposed MOA of RSO-021. Increased levels of the cytokine resistin were identified in treated patient MPE by proteomics and ELISA assays. Resistin has been shown to activate tumor associated macrophages to increase anti-tumor activity in mesothelioma. Lastly, across cell lines, tumor explants and humans treated with RSO-021 we find significant downregulation of proteins related to the epithelial to mesenchymal transition (EMT) including TGFBI and periostin. Studies are underway correlating patient responses to MPE composition to identify biomarkers of sensitivity and resistance to RSO-021. Together, these data show the positive effects of RSO-021 on anti-tumor protein signatures related to tumor immunophenotypes and EMT and support the continued investigation of RSO-021 in MPE associated solid tumors. Citation Format: Terri Messier, Bradley Cech, Joanna Dzialo, Charlotte Poile, Apostolos Nakas, Kudzayi Kutywayo, Aleksandra Bzura, Jan Rogel, Victoria Gibson, Ying Wai Lam, George Naumov, Dean Fennell, Christopher C. Landry, Brian Cunniff. The first-in-class PRX3 inhibitor RSO-021 modulates the protein composition of malignant pleural effusions by increasing immunomodulating phenotypes and decreasing epithelial to mesenchymal protein expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3106.