Abstract C027: RSO-021, a first-in-class covalent inhibitor of mitochondrial PRX3: From bench to bedside
Authors: Fennell Dean A. , Dulloo Sean , Szlosarek Peter , Thistlethwaite Fiona , Lord Simon , Rahman Najib M , Cunniff Brian , Dzialo Joanna , Poile Charlotte , Panchal Rakesh , Duncan Jarrett , Graham Pip , Bexon Alice , Naumov George N. , Spicer James 
Published: December 2023

Abstract
RSO-021 is a first-in-class mitochondrial peroxiredoxin 3 (PRX3) covalent small molecule inhibitor. The safety and tolerability of RSO-021 are currently being tested in the phase 1-2 MITOPE clinical trial in patients with malignant pleural effusion (MPE) arising from malignant mesothelioma or metastatic cancer to the lungs (NCT05278975). Altered redox status, resulting in elevated cellular hydrogen peroxide (H2O2) levels, drives the growth and proliferation of mesothelioma and other solid tumors. PRX3, the primary mitochondrial H2O2 scavenging enzyme, supports tumor cell escape from oxidative stress and aggressive tumor growth. RSO-021 attacks active site cysteine residues in PRX3 covalently to inactivate the enzyme, resulting in elevated H2O2 levels in the mitochondria incompatible with tumor cell survival. RSO-021 showed potent PRX3 inactivation and anti-tumor activity in pre-clinical cell and mouse models of mesothelioma with good tolerability supporting its clinical development. The MITOPE trial (NCT05278975) is an open-label, non-randomized, multicenter, translational phase 1-2 dose-escalation and expansion study designed to determine the safety, tolerability, dose-limiting toxicity (DLT), pharmacokinetics (PK), pharmacodynamics (PD), and preliminary anti-tumor activity of RSO-021 after intrapleural administration after complete drainage in patients with MPE due to mesothelioma or other metastatic disease. RSO-021 is administered locally as a pleural infusion through an indwelling catheter as a single dose on Day 1, 8 and 15 of each 3-week cycle. Safety is assessed at the end of the 21-day DLT period and efficacy every 6 weeks. PK and PD are assessed on pleural fluid, biopsies and with systemic sampling. RSO-021 is administered until disease progression, unacceptable toxicity, withdrawal of consent or study termination. Currently, MITOPE is actively recruiting patients in 4 open UK sites, with additional 9 sites on-boarding shortly. The dose escalation phase of the trial has 13 patients dosed at either 90, 120 or 180 mg flat dose levels and is anticipated to complete recruitment by the end of 2023. The trial will then continue to recruit patients as part of the dose expansion phase of the trial, which is exploring specific indications, including newly-diagnosed and previously-treated MM with MPE, as well as combination with systemic therapy in patients with refractory solid tumors and MPE.
Citation Format: Dean A. Fennell, Sean Dulloo, Peter Szlosarek, Fiona Thistlethwaite, Simon Lord, Najib M Rahman, Brian Cunniff, Joanna Dzialo, Charlotte Poile, Rakesh Panchal, Jarrett Duncan, Pip Graham, Alice Bexon, George N. Naumov, James Spicer. RSO-021, a first-in-class covalent inhibitor of mitochondrial PRX3: From bench to bedside [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr C027.