Phase 2 study to evaluate the novel mitochondrial PRX3 inhibitor, RSO-021, as an intrapleural monotherapy and in combination with IV paclitaxel in patients with malignant pleural effusion due to mesothelioma or another advanced solid tumor.
Authors: Dulloo Sean , Lord Simon , Fennell Dean Anthony , Thistlethwaite Fiona , Maskell Nick , Popat Sanjay , Blyth Kevin G , Szlosarek Peter Wojciech , Aujayeb Avinash , Javed Sohail Rooman , Herrero Colomina Julio , Cunniff Brian , Duncan Jarrett , Graham Philippa , Bexon Alice Susannah , Naumov George N , Spicer James F. 
Published: June 2024

TPS8124 Background: Tumor cells generate elevated levels of reactive oxygen species (ROS), leading to increased expression and activity of critical ROS scavenging pathways. Peroxiredoxin 3 (PRX3) is the principal peroxide scavenging enzyme in mitochondria. RSO–021 is a novel small molecule PRX3 inhibitor that inactivates PRX3 through direct covalent adduction of active site cysteine residues, in turn inducing oxidative stress that is incompatible with tumor cell survival. Preclinical studies of RSO-021 show that it selectively inhibits mitochondrial PRX3 in vitro and is highly effective in a variety of tumor models, including mesothelioma and many solid tumors. RSO-021 has completed phase 1 evaluation in patients with advanced mesothelioma or solid tumors with predominant pleural disease and malignant pleural effusions. The phase 1 data supported weekly intrapleural dosing of RSO-021 at the 90 mg dose level, with acceptable safety and early signals of efficacy. Some patients were able to receive >30 weeks of treatment. Pre- and post-dose primary prophylaxis for local inflammatory reactions to intrapleural injection was instituted during dose escalation, using corticosteroids, NSAIDS and paracetamol. It also became clear that RSO-021 should not be administered to patients with a dry pleural tap, due to risk of extravasation. Methods: The objectives of this open-label, multi-center, 4-arm phase 2 study (MITOPE) are to confirm the efficacy, safety, tolerability, and pharmacology of weekly intrapleural RSO-021 administration. Exploratory endpoints include target engagement as well as biomarkers of response and inflammation. The study explores 2 different RSO-021 doses (90 mg/wk and 45 mg/wk) per FDA Project Optimus, both as monotherapy and in combination with IV paclitaxel 175 mg/m2 every 3 weeks at until progression or limiting toxicity. Each arm at each dose has a 2-stage Simon design (12-21 patients) requiring 1 response to trigger stage 2 (null/target response rate 3%/20% and type I/type II error 0.05/0.8) for a maximum of 168 patients in the trial. Three of the 4 study arms are investigating the antitumor activity of RSO-021 monotherapy (arms 1-3) and the fourth is assessing the paclitaxel combination. Eligibility (by arm) requires: newly diagnosed treatment-naïve mesothelioma with pleural effusion; mesothelioma with pleural effusion that progressed on ≥1 standard of care (SoC) regimen; solid tumor with predominant pleural/lung disease and effusion that progressed on ≥1 SoC regimen; and breast, epithelial ovarian or non-small cell lung cancer with predominant pleural/lung disease and effusion that progressed on ≥1 SoC regimen. The first patient was treated in 4Q2023 and the trial is actively recruiting to all 4 arms at 9 UK sites. Additional sites are planned. Clinical trial information: NCT05278975 .