Study 200622: A Randomized, Double-blind, Placebo-controlled Study to Investigate the Efficacy and Safety of Mepolizumab in the Treatment of Adolescent and Adult Subjects With Severe Hypereosinophilic Syndrome

Mepolizumab is a humanized monoclonal antibody. In conditions where eosinophilia is
considered to play an important part in the pathology, including eosinophilic asthma, HES,
and eosinophilic granulomatosis with polyangiitis, a consistent reduction in blood eosinophil
counts is observed in association with mepolizumab administration, with concomitant clinical
improvement. This is a 32-week treatment period, randomized, double-blind,
placebo-controlled, parallel group, multicentre study of mepolizumab in adolescent and adult
subjects with severe HES receiving standard of care (SoC) therapy. This study will
demonstrate the efficacy of mepolizumab compared with placebo based on maintenance of control
of HES symptoms during the treatment period. The study will comprise of a screening period of
up to approximately 4 weeks followed by a 32-Week study treatment period (subjects will be
randomized 1:1 to placebo or mepolizumab) and up to 8-week additional follow-up period (12
weeks after the last dose of study treatment).