Urinary rituximab loss and rate of treatment failure in membranous nephropathy.

Background: Rituximab is a first-line treatment for primary membranous nephropathy, but approximately one-third of patients fail to respond. Among the proposed mechanisms of rituximab resistance, urinary rituximab loss due to non-selective proteinuria could lead to drug underexposure and treatment failure. Although hypothesized, this has never been objectively measured with a practical and reproducible method.

Methods: We conducted a prospective, observational study of 25 patients with biopsy-proven primary membranous nephropathy treated with rituximab (two infusions of 1 g, two weeks apart) at Careggi University Hospital, Florence, Italy. Rituximab levels were measured in serum and urine using a commercially available ELISA adapted for urinary samples. Urinary rituximab concentrations were assessed in 60 individuals to identify a threshold below which rituximab is considered not present in urine. Eight non-responders with urinary rituximab ≥2 µg/mL (potentially underexposed) received a second treatment course to test whether a higher rituximab dose would provide benefits in terms of kidney outcomes.

Results: Urinary rituximab loss was associated with markers of disease severity, including lower eGFR (r= -0.50; p =0.01), higher proteinuria (r=0.46; p =0.02), and non-selective proteinuria (r=0.61; p =0.001). Patients with urinary rituximab ≥2 µg/mL had lower serum rituximab peak levels at 1 month (p=0.04), possibly reflecting drug underexposure, and significantly lower response rates (43% vs 100%, p=0.003). PLA2R antibody depletion was significantly more frequent in patients with lower urinary rituximab levels (80% vs 17%, p=0.008). In the re-treated subgroup, five of eight non-responders showing significant urinary rituximab loss and receiving a second course of treatment achieved clinical response.

Conclusions: In primary membranous nephropathy, urinary rituximab loss is associated with disease severity, drug underexposure, and treatment failure. Measuring urinary rituximab could help identify patients at risk of resistance who could benefit from additional rituximab dosing. This approach could improve outcome prediction and support personalized immunosuppressive strategies.