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This study compared the pharmacokinetic (PK) profile of a new liquid formulation of mepolizumab with the established lyophilized formulation. In this open‐label, parallel‐group, single‐dose study (NCT03014674; GSK ID: 204958), healthy participants were randomized (1:1:1) to receive a single mepolizumab dose (100 mg) administered subcutaneously as liquid in a single‐use prefilled syringe or single‐use prefilled autoinjector, or as a lyophilized formulation. Maximum plasma concentration, area under the plasma concentration–time curve from time zero (predose) to time of last quantifiable concentration (AUC0–t), and AUC from time zero to infinity (AUC0–∞) as well as additional PK parameters, safety assessments, and blood eosinophil count were evaluated. In total, 244 participants received study drug. All PK parameters were similar across the 3 groups; 90% confidence intervals for maximum plasma concentration, AUC0–t, and AUC0–∞ treatment ratios (liquid prefilled syringe or autoinjector vs lyophilized formulation) were within conventional bioequivalence bounds (0.80‐1.25), demonstrating statistical PK comparability. On‐treatment adverse event incidence was 29% to 38%. Mepolizumab liquid formulation administered via prefilled syringe or autoinjector had similar PK properties to the lyophilized formulation, with no safety concerns identified.

Interleukin 5 (IL‐5) and eosinophils are thought to play an important role in the pathology of asthma. This study evaluated the pharmacokinetics, pharmacodynamics, safety, and tolerability of mepolizumab, a humanized anti‐IL5 IgG1 monoclonal antibody, in development for the treatment of severe eosinophilic asthma. This single‐blind study randomized 35 healthy Japanese male subjects (3:1) to receive either a single mepolizumab intravenous dose (10, 75, 250, or 750 mg) or placebo. Subjects were observed for up to 151 days postdose, depending on the dose administered. Blood samples were collected to measure mepolizumab concentrations, blood eosinophils, IL‐5, and antibodies to mepolizumab. Mepolizumab exhibited dose‐proportional pharmacokinetics. The terminal phase half‐life was 19.7–34.6 days, independent of dose. Higher mepolizumab plasma concentrations were associated with lower blood eosinophil counts. Mepolizumab 75–750 mg reduced blood eosinophils for ≥3 months postdose. Mepolizumab demonstrated a favorable safety profile: of 41 reported adverse events, most were mild in severity and none were serious. No neutralizing antibodies to mepolizumab were detected. Sustained reduction in blood eosinophils after single intravenous mepolizumab doses ≥ 75 mg, along with mepolizumab pharmacokinetics and a favorable tolerability profile in healthy Japanese subjects, provides a solid foundation for future studies with mepolizumab in Japanese patients with asthma.