Biologic agents in idiopathic hypereosinophilic syndrome: a 4-year-follow up in a case series

<bold>Introduction:</bold> Hypereosinophilic syndrome (HES) is a heterogeneous group of rare disorders defined by the presence of marked eosinophilia resulting in end organ damage. Treatment goals include reduction of flares and eosinophils with minimal drug related side-effects. <bold>Aim:</bold> To present clinical, laboratory, functional features and outcomes in patients with idiopathic HES treated with biologic agents targeting eosinophils. <bold>Methods:</bold> Patients with idiopathic HES treated with anti-eosinophilic agent between 01/10/2019 and 01/10/2023 were retrospectively included in the study. <bold>Results:</bold> Eleven patients (n=11) with 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 majority of them (n=8, 72.7%,), four patients (n=4, 36.4%) presented with eosinophilic pleural effusions, two patients (n=2, 18.2%) with cardiac arrhythmias and one with bilateral eyelid angioedema. Eight patients (72.7%) were treated with mepolizumab (300 mg/month) and 3 (27.3%) with benralizumab (30 mg/4 weeks). Median values of eosinophils at baseline and 12 months after initiation of biologic agent were 3000 (95% CI: 2172 to 11365) K/μl and 50 (95% CI: 3 to 190) K/μl, respectively. All patients with concomitant asthma (n=8) experienced elimination of asthma flares, asthma control (ACQ<0.75), functional improvement (mean ΔFEV1: 857 ± 594 ml) and 82% reduction in oral corticosteroids. <bold>Conclusion:</bold> Biologic agents in patients with idiopathic HES are both safe and effective sparing toxicity of immunosuppressive agents. Real-life data from larger registries are greatly anticipated.