BACKGROUND: IL-5 is a cytokine critically involved in regulating several aspects 
of eosinophils including their production, activation, and tissue recruitment. 
As such, IL-5 may be involved in the pathogenesis of hypereosinophilic 
syndromes, a group of poorly treated diverse disorders characterized by 
sustained peripheral blood and/or tissue eosinophilia.
OBJECTIVE: We aimed to assess the safety and efficacy of a humanized blocking 
monoclonal antibody against IL-5 (mepolizumab) in patients with several forms of 
hyper-eosinophilic syndromes.
METHODS: We performed an open-label trial of anti-IL-5 in which 3 intravenous 
doses (10 mg/kg, maximum 750 mg) were administered at 4-week intervals to 4 
patients with hypereosinophilic syndromes (defined by peripheral blood and/or 
tissue eosinophilia). The effects of treatment on safety, eosinophil levels (in 
peripheral blood and/or diseased tissue), pulmonary function, and quality of 
life were measured over a 28-week period.
RESULTS: Anti-IL-5 was well tolerated in all patients and lowered peripheral 
blood eosinophil counts despite ongoing systemic glucocorticoid therapy. The 
decline in circulating eosinophil counts was sustained for at least 12 weeks 
after the last dose of anti-IL-5. In addition, anti-IL-5 improved clinical and 
quality of life measurements. In one patient with striking tissue eosinophilia 
(eosinophilic esophagitis), anti-IL-5 resulted in a 10-fold reduction in tissue 
eosinophil levels.
CONCLUSIONS: These results suggest that anti-IL-5 is safe, effective in lowering 
eosinophil levels, and has potential glucocorticoid-sparing effects in patients 
with a variety of hyper-eosinophilic syndromes. As such, anti-IL-5 may have 
significant therapeutic potential for hypereosinophilic syndromes.