A Case of Eosinophilic Granulomatosis with Polyangiitis Emerging During Benralizumab Therapy: Successful Management Through a Switch to Mepolizumab Therapy.

Background: Eosinophilic granulomatosis with polyangiitis (EGPA) is a rare systemic vasculitis characterized by eosinophilic inflammation and involvement of small- to medium-sized vessels. Given its heterogeneous clinical presentation, early diagnosis and appropriate management are critical. Although systemic corticosteroids remain the mainstay of therapy, concerns regarding their long-term adverse effects underscore the growing need for alternative treatment strategies. Case Presentation: A 70-year-old man with a medical history of sinusitis, eosinophilic pneumonia, and bronchial asthma was referred for assessment of pleural effusion, which resolved spontaneously at the time of presentation. The patient was observed without intervention, while continuing asthma management. Approximately 6 months later, the patient experienced exacerbation of asthma and recurrence of pleural effusion. Thoracentesis was performed for symptom relief and benralizumab (30 mg) was initiated, resulting in temporary clinical improvement. However, after the third dose, the patient developed abdominal pain, peripheral neuropathy, and a skin rash. Laboratory studies revealed peripheral eosinophilia and elevated levels of rheumatoid factor (RF), myeloperoxidase-anti-neutrophil cytoplasmic antibody (MPO-ANCA), and total immunoglobulin E (IgE). A second skin biopsy revealed granulomatous inflammation, confirming the diagnosis of EGPA. Mepolizumab treatment (300 mg) was initiated, resulting in clinical resolution. Conclusion: This case highlights that EGPA may develop during anti-IL-5 therapy. Close monitoring of clinical signs, along with biomarkers such as eosinophil count, RF, MPO-ANCA, and IgE, is essential for timely diagnosis. Furthermore, successful disease control by switching to mepolizumab monotherapy suggests the potential for managing EGPA using appropriate biologic regimens to minimize corticosteroid-related adverse effects. Further accumulation of similar cases and larger studies is warranted to inform future clinical practice.