Efficacy and Safety of Mepolizumab in Hypereosinophilic Syndrome: A Phase III, Randomized, Placebo-Controlled Trial

Abstract Background Anti-interleukin-5 therapy is a potential treatment for patients with hypereosinophilic syndrome (HES), although its clinical efficacy is unclear.background Objective To investigate the clinical efficacy and safety of mepolizumab versus placebo in patients with HES. Methods This randomized, multicenter, double-blind, placebo-controlled, Phase III trial was conducted across 39 centers in 13 countries. Eligible patients had FIP1L1-PDGFRA-negative HES, experienced ≥2 flares (worsening of HES-related symptoms or blood eosinophil count requiring therapeutic escalation) in the previous 12 months and had a screening blood eosinophil count ≥1000 cells/μL. Patients were randomized (1:1) to subcutaneous mepolizumab (300 mg) or placebo every 4 weeks for 32 weeks, plus existing HES therapy. The primary outcome was the proportion of patients with ≥1 flare (worsening of HES-related symptoms necessitating therapy escalation or ≥2 courses of blinded rescue oral corticosteroids) during the study; in addition, patients who withdrew early from the study were counted as having a flare. Safety endpoints were also assessed. Results The proportion of patients experiencing ≥1 flare/withdrawing from the study was 50% lower with mepolizumab versus placebo (15/54 [28%] vs 30/54 [56%]; p=0.002). Logistic regression analysis was consistent with the primary analysis (odds ratio: 0.28; 95% confidence interval: 0.12, 0.64; p=0.003). Similar proportions of patients in the mepolizumab and placebo groups experienced on-treatment adverse events (48/54 [89%] vs 47/54 [87%]).results Conclusion Compared with placebo, mepolizumab significantly reduced the occurrence of flares in patients with HES, with no new safety signals identified.conclusion