Severe eosinophilic asthma treated with mepolizumab stratified by baseline eosinophil thresholds: a secondary analysis of the DREAM and MENSA studies

Background Findings from previous studies showed that mepolizumab significantly reduces the rate of exacerbations in patients with severe eosinophilic asthma. To assess the relationship between baseline blood eosinophil counts and efficacy of mepolizumab we did a secondary analysis of data from two studies, stratifying patients by different baseline blood eosinophil thresholds. Methods We did a post-hoc analysis of data, which was completed on Sept 25, 2015, from two randomised, double-blind, placebo-controlled studies of at least 32 weeks duration (NCT01000506 [DREAM] and NCT01691521 [MENSA]) done between 2009 and 2014. In these studies, mepolizumab (DREAM: 75 mg, 250 mg, or 750 mg intravenously; MENSA: 75 mg intravenously or 100 mg subcutaneously) versus placebo was given at 4-week intervals in addition to standard care (high-dose inhaled corticosteroids plus >= 1 additional controller with or without daily oral corticosteroids) to patients aged 12 years or older with a clinical diagnosis of asthma, a history of at least two exacerbations in the previous year that required systemic corticosteroid treatment, and evidence of eosinophilic airway inflammation. The primary endpoint in both studies was the annual rate of clinically significant exacerbations (defined as worsening of asthma that required the use of systemic corticosteroids, or admission to hospital, or an emergency-room visit, or a combination of these occurrences). In our analysis, the primary outcome was the annualised rate of exacerbations in patients stratified by baseline eosinophil counts (>= 150 cells per mu L, >= 300 cells per mu L, >= 400 cells per mu L, and >= 500 cells per mu L) and baseline blood eosinophil ranges (< 150 cells per mu L, >= 150 cells per mu L to < 300 cells per mu L, >= 300 cells per mu L to < 500 cells per mu L, and >= 500 cells per mu L). We based our analysis on the intention-to-treat populations of the two original studies, and all mepolizumab doses were combined for analysis. Findings Of 1192 patients, 846 received mepolizumab and 346 received placebo. The overall rate of mean exacerbations per person per year was reduced from 1.91 with placebo to 1.01 with mepolizumab (47% reduction; rate ratio [RR] 0.53, 95% CI 0.44-0.62; p<0.0001). The exacerbation rate reduction with mepolizumab versus placebo increased progressively from 52% (0.48, 0.39-0.58) in patients with a baseline blood eosinophil count of at least 150 cells per mu L to 70% (0.30, 0.23-0.40) in patients with a baseline count of at least 500 cells per mu L. At a baseline count less than 150 cells per mu L, predicted efficacy of mepolizumab was reduced. Interpretation Our analysis has shown a close relationship between baseline blood eosinophil count and clinical efficacy of mepolizumab in patients with severe eosinophilic asthma and a history of exacerbations. We noted clinically relevant reductions in exacerbation frequency in patients with a count of 150 cells per mu L or more at baseline. The use of this baseline biomarker will help to select patients who are likely to achieve important asthma outcomes with mepolizumab.