Mepolizumab as a Possible Treatment for Eosinophilic Ureteritis.

Eosinophilic ureteritis (EU) is a rare inflammatory condition characterized by eosinophilic infiltration of the ureter, often presenting with flank pain, hematuria, or obstructive uropathy. Fewer than 30 cases have been reported in the literature, and no standardized treatment strategy currently exists. We report the case of a 54-year-old woman with a history of eosinophilic cystitis who developed progressive left-sided flank pain and hydronephrosis following radical cystectomy. Ureteroscopy with biopsy confirmed EU. A six-week course of corticosteroids resulted in partial radiographic improvement but persistent eosinophilic infiltration on repeat biopsy. Given ongoing biopsy-proven eosinophilic inflammation despite corticosteroid therapy, a multidisciplinary discussion between urology and rheumatology led to initiation of mepolizumab (300 mg administered as three 100-mg subcutaneous injections every four weeks) to target IL-5-mediated eosinophilic inflammation. Following initiation of therapy, the patient experienced progressive clinical improvement, with symptom relief beginning approximately three months after treatment initiation. Repeat imaging demonstrated decreased ureteral inflammation, and follow-up ureteral biopsy showed complete absence of eosinophils, consistent with remission of active disease. The patient has remained clinically stable for approximately 18 months of follow-up without the need for ureteral stenting or additional surgical intervention. This case suggests a potential therapeutic role for IL-5 pathway inhibition with mepolizumab in EU and highlights the biologic rationale for targeting eosinophil-mediated inflammation in this condition. However, because this report describes a single patient and off-label biologic therapy, these findings should be interpreted cautiously. Further investigation through additional case series, prospective studies, and rare-disease registries will be necessary to better define the role of IL-5-targeted therapies in EU, with potential for future clinical trials if sufficient evidence emerges.