This is one of two studies of mepolizumab published together early online in the New England Journal of Medicine.
An accompanying editorial notes that anti-IL5 therapy may offer an important advance in the care for patients with severe eosinophilic asthma, particularly as a method of decreasing exacerbations in patients who are dependent on daily use of oral glucocorticoids. The author notes however that not all patients with uncontrolled asthma and peripheral blood eosinophilia will benefit from this therapy. For example in this study, the patient group had a high baseline rate of exacerbation (mean 3.6 per year), suggesting that their doses of inhaled corticosteroid and long-acting bronchodilators may not have been optimised. Additionally there was a high placebo response (50% reduction in exacerbation rate), and the authors postulate that most patients in this study might have had improvement in symptoms without mepolizumab simply by the institution of good clinical practice, as recommended by current international guidelines.
Although persistent blood eosinophilia may be sufficient to identify patients who are likely to have a response to this treatment, whether this biomarker is sufficient or is as effective as airway eosinophilia in monitoring the response to treatment remains to be seen. Airway eosinophils were not monitored in this study; it is therefore not possible to know whether better outcomes could have been achieved by adjusting the mepolizumab dose to suppress these levels. In addition, further evaluation is required to determine the most effective dose and the most effective route and frequency of administration of mepolizumab in these patients, particularly in those who require daily oral glucocorticoids.