Fibrotic Chronic Eosinophilic Pneumonia Treated with an Anti-IL-5 Monoclonal Antibody: A Case Report

Background: Chronic eosinophilic pneumonia (CEP) is a rare inflammatory lung disease typically responsive to glucocorticoids, but is prone to relapse and, in some cases, progressive deterioration. A subset of patients develops fibrosing CEP, a distinct phenotype characterized by irreversible parenchymal remodeling and declining lung function, for which no standard treatment exists. Although biologic therapies targeting interleukin-5 (IL-5) are effective in relapsing CEP, their role in fibrosing forms remains unclear. Case Presentation: We report the case of a 43-year-old man with idiopathic CEP initially treated with systemic glucocorticoids, which were discontinued due to severe adverse effects. Despite subsequent therapy with inhaled steroids and azathioprine, the disease relapsed and progressed to a fibrosing phenotype, as confirmed by radiologic and functional assessments. An off-label treatment with subcutaneous mepolizumab, 100 mg, every 4 weeks was started. After eight months of therapy, the patient achieved clinical stability, improved lung function, and the radiologic stabilization of fibrotic changes, without the need for any further treatment with a corticosteroid. Conclusions: This is, to the best of our knowledge, the first documented case of fibrosing CEP treated with an anti-IL-5 monoclonal antibody, highlighting its potential role as a steroid-sparing agent and immunomodulator even in the fibrotic phase of disease. Further research is warranted to define the place of biologics in the management of CEP with a fibrosing evolution and possible combinations with antifibrotic drugs.