Long-Term Clinical Outcomes of High-Dose Mepolizumab Treatment for Hypereosinophilic Syndrome

Background: Conventional therapies for hypereosinophilic syndromes (HES) have variable efficacy and carry significant long-term toxicities. Anti-IL-5 (mepolizumab) therapy has a glucocorticoid (GC)-sparing effect in GC-sensitive HES, but the efficacy of mepolizumab in treatment-refractory HES patients with severe disease has not been examined to date. Objective: To identify predictors of response to mepolizumab in subjects with severe treatment-refractory HES and compare long-term outcomes in these subjects with HES subjects treated with conventional therapies. Methods: Retrospective analysis of clinical and laboratory data from 35 HES subjects treated with mepolizumab and 55 HES subjects on conventional therapy, all followed at a single center, was performed. Results: Peak eosinophilia, GC sensitivity, pulmonary involvement, HES clinical subtype, and pretreatment serum IL-5 were correlated with mepolizumab response. Despite evidence of more severe disease at baseline, mepolizumab-treated subjects had comparable long-term clinical outcomes to HES subjects treated with conventional therapies and reported improvements in therapy-related comorbidities. Subjects managed with mepolizumab monotherapy had fewer disease flares than HES subjects on conventional therapies or mepolizumab-treated HES subjects requiring additional HES therapies. Conclusions: This study confirms that mepolizumab is an effective and well-tolerated therapy for HES, but suggests that response is more likely in GC-responsive subjects with idiopathic or overlap forms of HES. A primary benefit of treatment is the reduction of comorbidity due to discontinuation or the reduction of conventional HES therapies. Although subjects who completely discontinued GC had the most benefit, high-dose mepolizumab was a safe and effective salvage therapy for severe, treatment-refractory HES.