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.