Eosinophils are major pro-inflammatory cells that make a major contribution to 
diseases that affect the upper and lower airways, skin and gastrointestinal 
tract. Interleukin (IL)-5 is central to their maturation and release from the 
bone marrow together with their subsequent accumulation and activation in the 
tissues. Mepolizumab is a humanized monoclonal antibody (mAb) with potent IL-5 
neutralizing effects that represents a potential treatment for eosinophilic 
diseases. Several clinical trials with mepolizumab reported that treatment of 
patients with mild to severe asthma resulted in a substantial reduction in blood 
and sputum eosinophil numbers. However, clinical outcomes were disappointing as 
there were no significant effects on airway hyper-reactivity or the late 
asthmatic reaction to inhaled allergen challenge. More recently two studies, one 
in in patients with refractory eosinophilic asthma with a history of recurrent 
severe exacerbations and the other in patients with persistent sputum 
eosinophilia and symptoms despite systemic treatment with prednisone treatment, 
reported that monthly intravenous mepolizumab reduced sputum/blood eosinophilia, 
asthma exacerbations together with improvments in quality of life. Mepolizumab 
also appears to be an effective therapy for hypereosinophilic syndrome while 
other trials have shown efficacy of mepolizumab therapy in eosinophilic 
esophagitis. This review will consider the current status of the clinical 
development of mepolizumab for diseases with a significant eosinophilic 
component to their pathology.