Abstract B041: Mitochondrial antioxidant peroxiredoxin 3 is a tractable therapeutic target in mesothelioma

Abstract Aggressive tumors, including mesothelioma, generate high levels of reactive oxygen species (ROS) to support rapid growth and proliferation. To survive under increased oxidative stress, tumor cells must upregulate antioxidant expression and activity, including the mitochondrial hydrogen peroxide (H2O2) scavenging enzyme peroxiredoxin 3 (PRX3). PRX3 is overexpressed in various cancers and is linked to increased tumor proliferation, and resistance to apoptosis and chemotherapy. PRX3 has emerged as a promising therapeutic target due to its critical role in maintaining redox balance and mitochondrial function. Mesothelioma, an aggressive pleural malignancy associated with asbestos exposure, has limited therapeutic options, particularly in relapsed or refractory settings highlighting a need for therapeutic advances. Here we demonstrate that genetic deletion of PRX3 through CRISPR/Cas9 and pharmacological inhibition using RSO-021, the clinical formulation of the covalent PRX3 inhibitor thiostrepton, resulted in impaired mitochondrial bioenergetics, elevated mitochondrial ROS, and suppression of mesothelioma tumor growth. PRX3 knockout (PRX3 KO) mesothelioma cell lines displayed significantly reduced proliferation, migration, and basal/maximal oxygen consumption, along with increased mitochondrial ROS levels, as measured by Seahorse XF96 Mito Stress Test and HyPer7, a H2O2 responsive biosensor. In vivo studies demonstrated that PRX3 KO cells failed to form tumors in SCID mice, unlike control cells which generated substantial tumor burden. RNA-sequencing of PRX3 KO cells revealed downregulation of epithelial-to-mesenchymal transition and TGF-β signaling genes, key drivers of tumor progression and metastasis. This transcriptional signature was also observed in patient-derived mesothelioma explants treated with RSO-021, confirming its biological relevance. In a first-in-human phase 1 clinical trial, RSO-021 was administered intrapleurally to patients with malignant pleural effusion, including those with mesothelioma. Treatment was well tolerated at a dose of 90 mg/week, demonstrated early signs of antitumor efficacy, including tumor reduction, reduced pleural effusion, and a 67% disease control rate at 12 weeks per mRECIST 1.1. Notably, all patients were alive at 1-year overall survival follow-up, warranting transition to the ongoing phase 2 portion of the MITOPE trial (NCT05278975). Functional genomic screening identified SLC7A11, a cystine/glutamate transporter, as a mediator of resistance to PRX3 inhibition where knockdown in mesothelioma cells restored RSO-021 sensitivity. Targeting PRX3, especially in combination with SLC7A11 inhibition, represents a promising approach for treating aggressive and treatment-resistant forms of cancer. Collectively, these findings identify PRX3 as a key regulator of redox metabolism, mitochondrial function, and mesothelioma tumorigenesis, and establish RSO-021 as a novel first-in-class PRX3 inhibitor with significant therapeutic potential in oncology. Citation Format: Victoria Gibson, Joanna Dzialo, Terri Messier, Aleksandra Bzura, Charlotte Poile, Jan Rogel, Jens Hahne, Aida Habibovic, Stephanie Stead, Alexis Saaman, Kevin G. Blyth, Peter W. Szlosarek, Simon Lord, Min Zhang, Apostolos Nakas, Peter Wells-Jordan, Kudzayi Kutywayo, Kelly Butnor, Nicholas H. Heintz, George N. Naumov, Sean Dulloo, Burak Aktas, James Spicer, Dean A. Fennell, Brian Cunniff. Mitochondrial antioxidant peroxiredoxin 3 is a tractable therapeutic target in mesothelioma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr B041.