Background: The long-term efficacy and safety of mepolizumab in patients with severe eosinophilic asthma has been evaluated in large-scale double-blind placebo-controlled trials. However, a prospective open-label trial of long-term subcutaneous administration of mepolizumab has not been performed in Japanese patients with severe eosinophilic asthma.
Methods: This study was a prospective, 48-week, open-label trial in 32 Japanese patients with severe eosinophilic asthma who received subcutaneous mepolizumab 100 mg every 4 weeks. Nine patients required oral corticosteroids daily despite receiving high-dose inhaled corticosteroids. Six patients had aspirin-exacerbated respiratory disease.
Results: All patients took mepolizumab throughout the study period. No patients experienced adverse events during the treatment. None of the patients experienced asthma exacerbations during the trial. In fact, forced expiratory volume in 1 second increased significantly at 24 weeks (P<.01) and at 48 weeks (P<.05). The peripheral blood eosinophil count in peripheral blood decreased after the first administration of mepolizumab in all patients and remained low until week 48. After starting mepolizumab, all oral corticosteroid–dependent asthmatics successfully withdrew corticosteroids without exacerbations and experienced a sustained reduction in peripheral blood eosinophil count. Blood levels of thymus and activation-regulated chemokine and IgE remained unchanged after 48 weeks of therapy with mepolizumab.
Conclusion: This first prospective open-label pilot study in Japan demonstrated the long-term efficacy and safety of mepolizumab in patients with severe eosinophilic asthma.

Key words: Anti–IL-5 antibody, Eosinophilic asthma, Mepolizumab, Prospective open-label study, Thymus and activation-regulated chemokine