ObjectivesTo investigate the efficacy of mepolizumab in patients with eosinophilic granulomatosis with polyangiitis (EGPA) and factors contributing to glucocorticoid (GC) discontinuation.MethodsWe retrospectively studied EGPA patients treated with mepolizumab who were on GC at the time of induction of mepolizumab, at Japanese single center as of January 2023.Patients were classified into those who were able to discontinue GC at the time of the investigation (GC-free group) and those who continued (GC-continue group). Patient characteristics at the time of EGPA diagnosis (age, gender, absolute eosinophil counts, serum CRP level, serum IgE level, Rheumatoid factor (RF) / anti-neutrophil cytoplasmic antibody (ANCA) positivity, presence of asthma, affected organ, Five factor score (FFS), Birmingham Vasculitis Activity Score (BVAS) and characteristics at the time of mepolizumab induction (daily prednisolone dose, concomitant immunosuppressive maintenance therapy at the mepolizumab induction, prior history of GC pulse therapy, concomitant immunosuppressive therapy for remission induction,), history of relapse before mepolizumab induction and the duration of mepolizumab treatment were compared.We also followed the clinical indicators (absolute eosinophil counts, CRP and IgE levels, BVAS, Vascular Damage Index (VDI)) and daily prednisolone dosage at the EGPA diagnosis, at the mepolizumab induction and at the survey.ResultsTwenty-seven patients were included in the study. At the time of the study, patients had received mepolizumab for median 31 months (IQR, 26 to 40), the daily prednisolone dose was median 1 mg (IQR, 0 to 1.8) and GC-free was achieved in 13 patients (48%). Among clinical indicators that have improved by conventional therapy before the induction of mepolizumab, eosinophil counts, GC doses and BVAS have successively shown significant reductions throughout the observation period both GC-free and GC-continue. Of the GC-free patients, 7 were ANCA positive and 12 had FFS1 or more. Univariate analysis showed that the absolute eosinophil counts at diagnosis was significantly higher in the GC-free group (median 8165/µl (IQR, 5138 to 13,409) vs. 4360/µl (IQR, 151 to 8380),P= 0.037) and significantly fewer patients presented with gastrointestinal lesions (2 (15%) vs. 8 (57%),P= 0.025), while multivariate analysis showed no significant differences. Mepolizumab treatment significantly improved VDI in the GC-continue group (P= 0.004).ConclusionsAfter three years of treatment with mepolizumab, approximately 50% of patients with EGPA achieved GC-free status. GC could be discontinued even in severe cases and ANCA-positive cases. Although multivariate analysis did not extract any significant factors contributing to achieving GC-free, we found that improvement in eosinophil counts and BVAS led to GC reduction, resulted in protection of organ damages in both the GC-free and continuation groups. The significance of achieving GC-free remission in EGPA patients was demonstrated.