Non-bacterial Cystitis With Increased Expression of Programmed Cell Death Ligand 1 in the Urothelium: An Unusual Immune-Related Adverse Event After Atezolizumab Administration for Metastatic Breast Cancer

We report a case of non-bacterial cystitis that occurred after administration of atezolizumab, an antibody against programmed cell death ligand 1 (PD-L1). This cystitis was considered an immune-related adverse event (irAE). A 67-year-old woman with advanced breast cancer (cT4bN1M1, cStage IV) was treated with atezolizumab and nanoparticle albumin-bound (nab) paclitaxel. She consulted a physician for urethral pain and frequent urination during the fourth cycle of treatment. Cystitis symptoms were not relieved by antibiotic treatment and worsened. The results of her urine culture and cytology were negative for malignancy. Cystoscopy showed diffuse redness of the bladder mucosa. A bladder biopsy revealed no evidence of malignancy. Since the patient’s symptoms resolved with steroid therapy, urethral pain and frequent urination associated with atezolizumab were considered to be irAE by the diagnosis of exclusion. After immunostaining of the bladder biopsy sections, high PD-L1 expression was detected in the urothelium, which could explain the cause of irAE.