INTRODUCTION: Five percent of asthmatics have severe symptoms despite high doses 
of inhaled (ICS) or additional oral corticosteroids (OCS): these patients have 
high morbidity, risk for asthma death, and account for half of asthma healthcare 
spending. A subgroup (20 - 40%) of these has persistent airway eosinophilia and 
frequent exacerbations. Mepolizumab is a humanized monoclonal antibody that 
blocks binding of the key cytokine implicated specifically in eosinophil 
maturation and survival, interleukin-5, to its receptor.
AREAS COVERED: Pharmacology, Phase I/IIa and Phase II/III studies of mepolizumab 
for asthma. Mepolizumab depleted blood and sputum eosinophils and partially 
reduced airway and bone marrow eosinophils. It also reduced airway remodeling. 
In unselected patients with moderate/severe asthma there was no clinically 
significant effect on lung function, but a trend to reduced exacerbation rates. 
When patients were selected for persistent sputum eosinophilia despite high-dose 
ICS/OCS, and frequent exacerbations, mepolizumab reduced exacerbations by 50%.
EXPERT OPINION: Mepolizumab can reduce exacerbation rates in the severe asthma 
cohort who have eosinophilic airway inflammation despite corticosteroid 
treatment. This may be 30% of severe asthmatics and represents a new and 
important treatment option. Further studies need to confirm efficacy and 
indications for asthma (and other eosinophilic airway disease), and to examine 
clinical consequences of reducing remodeling.