ObjectiveDipeptidase-1 (DPEP-1) is a recently discovered leucocyte adhesion receptor for neutrophils and monocytes in the lungs and kidneys and serves as a potential therapeutic target to attenuate inflammation in moderate-to-severe COVID-19. We aimed to evaluate the safety and efficacy of the DPEP-1 inhibitor, LSALT peptide, to prevent specific organ dysfunction in patients hospitalised with COVID-19.DesignPhase 2a randomised, placebo-controlled, double-blinded, trial.SettingHospitals in Canada, Turkey and the USA.ParticipantsA total of 61 subjects with moderate-to-severe COVID-19.InterventionsRandomisation to LSALT peptide 5 mg intravenously daily or placebo for up to 14 days.Primary and secondary outcome measuresThe primary endpoint was the proportion of subjects alive and free of respiratory failure and/or the need for renal replacement therapy (RRT). Numerous secondary and exploratory endpoints were assessed including ventilation-free days, and changes in kidney function or serum biomarkers.ResultsAt 28 days, 27 (90.3%) and 28 (93.3%) of subjects in the placebo and LSALT groups were free of respiratory failure and the need for RRT (p=0.86). On days 14 and 28, the number of patients still requiring more intensive respiratory support (O2 ≥6 L/minute, non-invasive or invasive mechanical ventilation or extracorporeal membrane oxygenation) was 6 (19.4%) and 3 (9.7%) in the placebo group versus 2 (6.7%) and 2 (6.7%) in the LSALT group, respectively (p=0.14; p=0.67). Unadjusted analysis of ventilation-free days demonstrated 22.8 days for the LSALT group compared with 20.9 in the placebo group (p=0.4). LSALT-treated subjects had a significant reduction in the fold expression from baseline to end of treatment of serum CXCL10 compared with placebo (p=0.02). Treatment-emergent adverse events were similar between groups.ConclusionIn a Phase 2 study, LSALT peptide was demonstrated to be safe and tolerated in patients hospitalised with moderate-to-severe COVID-19.Trial registration numberNCT04402957.