Abstract 7780: The first-in-class covalent peroxiredoxin 3 (PRX3) inhibitor RSO-021 modulates immune phenotypes in mesothelioma

Abstract RSO-021 is the clinical formulation of the first-in-class covalent peroxiredoxin 3 (PRX3) inhibitor thiostrepton (TS). TS covalently inactivates PRX3 active site cysteines leading to increased oxidative stress and apoptotic tumor cell death. RSO-021 was safe and showed early anti-tumor activity in a phase 1 trial in patients with malignant pleural effusion (MPE) arising from mesothelioma or metastatic disease to the lung (Fennell et al. J Clin Oncol 42, 3019-3019(2024)). The phase 2 trial is currently ongoing (NCT05278975). In this study we profiled immune signatures of human mesothelioma cell lines and patient derived explants treated with TS, and malignant pleural effusions from patients treated with RSO-021. Treatment of human mesothelioma cell lines showed TS down-regulated expression of PD-L1 and increased MHC class 1 expression. RNA-sequencing of human mesothelioma cells and patient-derived mesothelioma explants treated with TS showed modulation of transforming growth factor beta (TGFβ), tumor necrosis factor (TNFα), interferon alpha, and interferon gamma gene signatures. We performed multiplex ELISA analysis of cytokines in MPE from patients treated with RSO-021 via an intrapleural catheter to the pleural space. The levels of Resistin, a cytokine related to macrophage signaling in mesothelioma, was significantly increased 24 hours and 7-days post RSO-021 treatment. Levels of IL-6 and IL-8 were increased 24 hours after RSO-021 treatment and returned to baseline at 7 days. Lastly, in a syngeneic mouse model of mesothelioma end stage tumor burden was assessed in mice treated with TS, anti-PD1/anti-CTLA4 antibodies, or a combination of TS and anti-PD1/anti-CTLA4 antibodies. TS treated mice showed 30% tumor reduction. Anti-PD1/anti-CTLA4 antibody treated animals showed partial responses with 2 of 6 mice having complete tumor reduction. The combo of TS plus anti-PD1/anti-CTLA4 antibody showed complete tumor reduction in 6 of 6 mice. Together, these data highlight the immunomodulatory activity in cells, mouse models, and human tissue with RSO-021 and support further clinical development of RSO-021 in combination with immune-oncology therapies. Citation Format: Victoria Gibson, Joanna Dzialo, Aida Habibovic, Christopher Landry, Dean Fennell, Brian Cunniff. The first-in-class covalent peroxiredoxin 3 (PRX3) inhibitor RSO-021 modulates immune phenotypes in mesothelioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7780.