Introduction

Asthma is a common disease, affecting an estimated 334 million people worldwide, with considerable impact on quality of life and high associated costs.1–3 Asthma severity is assessed retrospectively from the level of treatment required to control symptoms and exacerbations. Approximately 5%–10% of patients with asthma are believed to suffer from severe disease.4 Patients with severe asthma typically require ongoing maintenance therapy with high-dose inhaled corticosteroid (ICS)/long-acting beta-agonist (LABA).2 Furthermore, systemic corticosteroids (SCS) are often required for potentially life-threatening exacerbations, but are associated with long-term risks of severe side effects.5

Patients with severe asthma which is uncontrolled despite optimal therapy, good compliance, trigger avoidance and management of comorbidities are classified as having refractory asthma (RA), associated with persistent symptoms despite maximal therapy and extensive re-evaluation of asthma management.2 6 7 Up to 3.6% of patients are estimated to suffer from severe RA despite high medication use.6 8

Eosinophils are instrumental in the pathogenesis of asthmatic airway inflammation: their numbers have been correlated to lung function impairment9 and increased risk of clinical asthma exacerbation (CAE).2 6 10 One phenotype of severe asthma is characterised by persistent airway inflammation with eosinophils.11 The eosinophil viability-enhancing factor, interleukin-5 (IL-5), controls their differentiation and maturation within the bone marrow and stimulates migration to sites of inflammation by acting on the …