Phase 1: intrapleural RSO-021 (thiostrepton) controlled disease in 67% of relapsed pleural mesothelioma patients at 12 weeks

In the phase 1 MITOPE trial (NCT05278975), weekly intrapleural RSO-021 at 90 mg achieved disease control in 67% of patients with relapsed pleural mesothelioma and malignant pleural effusion, assessed at 12 weeks; the regimen was well tolerated and survival exceeded expectations for this heavily pretreated group. RSO-021 is a formulation of thiostrepton, which covalently inhibits mitochondrial peroxiredoxin 3 (PRX3) — an antioxidant enzyme the tumour depends on to survive oxidative stress — so the drug disables the cancer's own redox defence rather than damaging DNA. This is an early-phase, single-arm result delivered directly into the pleural cavity, not a randomised comparison, and it does not yet establish a survival benefit. Preclinical characterisation and the phase 1 data were published in Nature Communications (2026).