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What is already known about this topic? A phase III study demonstrated that compared with placebo, 4-weekly add-on mepolizumab (300 mg subcutaneously) reduced flares in patients with uncontrolled FIP1-like-1-platelet-derived growth factor receptor a (FIP1L1-PDGFRA)-negative hypereosinophilic syndrome (HES), with a positive benefiterisk profile.What does this article add to our knowledge? This open-label extension study found no new safety signals with mepolizumab treatment in patients with FIP1L1-PDGFRA-negative HES. Moreover, mepolizumab continued to control flares and blood eosinophil counts after 52 weeks of continuous treatment.How does this study impact current management guidelines? Findings from this open-label extension study provide further evidence that patients with FIP1L1-PDGFRA-negative HES are likely to benefit from treatment with mepolizumab and may be able to reduce oral corticosteroid use.BACKGROUND: A double-blind, placebo-controlled, phase III study (200622) showed that mepolizumab reduces disease flares for patients with uncontrolled FIP1-like-1-platelet-derived growth factor receptor aenegative hypereosinophilic syndrome (HES) and two or more flares in the previous year. OBJECTIVE: To further characterize the safety, clinical benefit, and pharmacodynamics of mepolizumab. METHODS: Eligible patients from both treatment arms of the double-blind study could enter an open-label extension study (205203; NCT03306043) to receive 4-weekly mepolizumab (300 mg subcutaneously) plus background therapy for 20 weeks. Primary end points were safety-based; other end points included flare rates and changes from baseline in mean daily oral corticosteroid (OCS) dose and blood eosinophil count. RESULTS: Of 104 patients who completed the double-blind study, 98% (previous placebo, n [ 52; previous mepolizumab, n [ 50) enrolled in the open-label extension. Overall, 66 of patients reported adverse events (AEs) (65%), 15 reported

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