Abstract 4039: Deletion of peroxiredoxin 3 (PRX3) impairs mitochondrial bioenergetics and tumor growth in mesothelioma, supporting the first in human clinical testing of the PRX3 inhibitor RSO-021

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 upregulate antioxidant expression and activity, including the mitochondrial hydrogen peroxide (H2O2) scavenging enzyme peroxiredoxin 3 (PRX3). PRX3 is overexpressed in many cancers and promotes proliferation, survival and chemoresistance, highlighting it as a promising therapeutic target in oncology. Here we demonstrate that genetic deletion of PRX3 via CRISPR/Cas9 in the H-MESO-1 human mesothelioma cell line impairs mitochondrial bioenergetics and suppress mesothelioma tumor growth. PRX3 knockout (PRX3 KO) cells displayed significantly reduced oxygen consumption rates, extracellular acidification rates, and elevated mitochondrial ROS (mROS), indicating PRX3 deficient cells exist in an energetically compromised state. PRX3 deletion also decreased proliferation, colony formation and 3D spheroid growth in Matrigel. In vivo PRX3 KO cells failed to form tumors in SCID mice, whereas control cells generated substantial tumor burden. RNAseq-based gene set enrichment analysis of PRX3 KO cells supported the observed phenotypes including downregulation of oxidative phosphorylation, glycolysis and E2F and MYC targets. Re-expression of PRX3 in PRX3 KO cells partially rescued these phenotypes, reducing mROS, improving mitochondrial function, and enhancing colony formation. In a first-in-human phase 1 trial treating patients via weekly local intrapleural administration with RSO-021, the clinical formulation of the covalent PRX3 inhibitor thiostrepton (TS), at 90 mgs was well tolerated leading to disease control in 67% of patients at 12 weeks and was associated with tumor reductions (NCT05278975). Functional genomic screening identified SLC7A11, a cystine/glutamate transporter, as a mediator of resistance to PRX3 inhibition. 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. Phase 2 testing of RSO-021 is ongoing. Citation Format: Victoria Gibson, Joanna Dzialo, Charlotte Poile, Jan Rogel, Aida Habibovic, Kelly Butnor, Sean Duloo, James Spicer, Dean Anthony Fennell, Brian Cunniff. Deletion of peroxiredoxin 3 (PRX3) impairs mitochondrial bioenergetics and tumor growth in mesothelioma, supporting the first in human clinical testing of the PRX3 inhibitor RSO-021 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 4039.