Rituximab for the treatment of membranous nephropathy in a patient with ankylosing spondylitis: a case report.

Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton. Renal involvement is uncommon in AS, occurring in only 5-13% of patients, with membranous nephropathy (MN) being particularly rare, with only 10 cases reported in PubMed to date. We report the case of a 37-year-old man with an 11-year history of AS who developed nephrotic syndrome during the stable phase of his disease. Renal biopsy confirmed MN, revealing IgG and C3 deposition along the glomerular capillary walls and subepithelial electron-dense deposits. After excluding secondary causes, rituximab was selected as the initial immunosuppressive therapy. Six months after treatment, the patient's 24-h urinary protein excretion decreased from 12.9 g to 836 mg, and serum albumin increased from 21.4 g/L to 35.4 g/L, achieving partial remission. This case provides additional clinical evidence of a potential association between AS and MN and demonstrates the efficacy and safety of rituximab in this rare clinical scenario, offering insight into potential treatment strategies.