Mycophenolate mofetil for corticosteroid-resistant immune-related cystitis associated with tislelizumab: a case report.

Immune-related cystitis is an uncommon immune-related adverse event (irAE) linked to immune checkpoint inhibitors (ICIs), and managing cases refractory to corticosteroids remains clinically challenging due to the lack of standardized treatment guidelines. We describe a 56-year-old male with extensive-stage small-cell lung cancer who developed corticosteroid-refractory immune-related cystitis following fourth-line therapy with Tislelizumab combined with irinotecan and anlotinib. The patient presented with frequent urination, urgent urination, and dysuria. Urine cultures were negative, and antibiotic treatment failed to alleviate symptoms, leading to a diagnosis of immune-related cystitis (CTCAE grade 3). Initial treatment with methylprednisolone 80 mg/day provided symptom relief, but symptoms recurred when the dose was tapered. Even after escalating the dose to 40 mg/day, symptoms persisted, confirming corticosteroid refractoriness. Combination therapy with prednisone 30 mg/day and mycophenolate mofetil (MMF) 1 g once daily resulted in symptom resolution within 1 week, with normalization of urine routine findings by 8 weeks. Corticosteroids were successfully tapered off without recurrence. This is the first reported case of successful MMF use for ICI-related cystitis, offering a new therapeutic option for corticosteroid-refractory cases. We also discuss considerations for ICI reinitiation to provide a clinical reference.