Biologic Agents in Idiopathic Hypereosinophilic Syndrome

<b>Background:</b> Hypereosinophilic syndrome (HES) is a heterogeneous group of rare disorders defined by the presence of marked eosinophilia resulting in end organ damage. The diagnostic approach is multidisciplinary and treatment goals include reductions in flares and eosinophils with minimal drug-related side effects. <b>Results:</b> Eleven patients (<i>n</i> = 11) with a diagnosis of idiopathic HES were included in the study [M/F: 6/5, median age: 54 (95% CI: 38.2 to 68.5), smokers/never smokers: 5/6]. Asthma was present in the majority of them (<i>n</i> = 8, 72.7%); four patients (<i>n</i> = 4, 36.4%) presented with eosinophilic pleural effusions, two patients (<i>n</i> = 2, 18.2%) with cardiac arrhythmias, and one with bilateral eyelid angioedema. Eight patients (72.7%) were treated with mepolizumab (300 mg/month) and three (27.3%) with benralizumab (30 mg/4 weeks). The median values of eosinophils at baseline and 12 months after initiation of biologic agent were 3000 (95% CI: 2172 to 11,365) K/μL and 50 (95% CI: 3 to 190) K/μL, respectively, <i>p</i> = 0.0002. All patients with concomitant asthma (<i>n</i> = 8) experienced elimination of asthma flares, asthma control (ACQ < 0.75), functional improvement (mean ΔFEV1: 857 ± 594 mL), and an 82% reduction in oral corticosteroids, <i>p</i> = 0.0001. <b>Materials and Methods:</b> Patients with highly characterized idiopathic HES treated with anti-eosinophilic agents between 1 October 2019 and 1 October 2023 were retrospectively included in the study. The aim of this study was to present clinical, laboratory, and functional features and outcomes in patients with thoroughly investigated idiopathic HES treated with biologic agents targeting eosinophils. <b>Conclusions:</b> Biologic agents in patients with idiopathic HES-following thorough diagnostic investigation-are both safe and effective, sparing the toxicity of immunosuppressive agents. Real-life data from larger registries are greatly anticipated.