BACKGROUND:The POP Trial was a phase 1, open-label, rising-dose, randomised study that explored the safety and tolerability of calmangafodipir (superoxide dismutase mimetic) co-treatment with n-acetylcysteine (NAC) for paracetamol overdose. METHODS:Patients were recruited at the Royal Infirmary of Edinburgh (8th June 2017-10th May 2018). Inclusion criterion: adults within 24 h of a paracetamol overdose that required NAC. Within each of 3 sequential cohorts, participants were randomly assigned, with concealed allocation, to NAC and a single intravenous calmangafodipir dose (n = 6) or NAC alone (n = 2). Calmangafodipir doses were 2, 5, or 10 μmol/kg. Participants, study and clinical teams were not blinded. The primary outcome was safety and tolerability. Secondary outcomes were alanine transaminase (ALT), international normalised ratio (INR), keratin-18, caspase-cleaved keratin-18 (ccK18), microRNA-122, and glutamate dehydrogenase (GLDH). (Clinicaltrials.gov:NCT03177395). FINDINGS:All 24 participants received their allocated drug doses and were analysed. Primary endpoints: all participants experienced ≥1 adverse event (AE), most commonly gastrointestinal. Patients experiencing ≥1 serious adverse event (SAE): NAC alone, 2/6; NAC + calmangafodipir (2 μmol/kg), 4/6; NAC + calmangafodipir (5 μmol/kg), 2/6; NAC + calmangafodipir (10 μmol/kg), 3/6. No AEs or SAEs were probably or definitely calmangafodipir-related. Secondary safety outcomes demonstrated no differences between groups. With NAC alone, 2/6 had ALT > 100 U/L; with NAC + calmangafodipir, 0/18. No INR difference. Keratin-18 and ccK18 increased in the NAC alone group more than with calmangafodipir (baseline to 20 h fold change, NAC + calmangafodipir (5 μmol/kg) compared to NAC alone: 0.48 (95%CI 0.28-0.83)). microRNA-122 changes were similar to K18, GLDH was frequently undetected. INTERPRETATION:Calmangafodipir was tolerated when combined with NAC and may reduce biomarkers of paracetamol toxicity.