Idiopathic hypereosinophilic syndrome in remission with benralizumab treatment after relapse with mepolizumab.

We report a patient with idiopathic hypereosinophilic syndrome (I-HES) who achieved remission with benralizumab after relapsing on mepolizumab. An 83-year-old man was admitted to Showa General Hospital after presenting with hypoxaemia and multiple erythematous lesions. He showed a marked increase in blood eosinophil count. Skin biopsy revealed an invasion of eosinophils in the dermis. He was diagnosed with I-HES. He was commenced on prednisolone 40 mg/day with a plan to wean this over time after pulse steroid therapy for three days. Mepolizumab was added when the prednisolone dose was 25 mg/day. Unfortunately, at a prednisolone dose of 5 mg/day, there was evidence of disease progression and the patient was switched to benralizumab. Prednisolone was tapered again and, finally, the patient was in remission. Benralizumab targets interleukin (IL)-5R and induces antibody-dependent cell-mediated cytotoxicity, thereby reducing the eosinophil counts in the tissue. This can be attributed to the therapeutic efficacy against I-HES. We believe this report may help develop novel therapeutic strategies for I-HES.