A Study of Belimumab in Idiopathic Membranous Glomerulonephropathy

This is a phase II, open label, experimental medicine study to evaluate the efficacy, safety and mechanism of action of belimumab in subjects with antiphospholipase A2 receptor (PLA2R) autoantibody positive idiopathic membranous glomerulonephropathy (IMGN), and to profile the relationship between biomarkers, autoantibody status and clinical response. 10 milligrams per kilogram (mg/kg) belimumab intravenous (IV) will be administered at weeks 0, 2, and then every 4 weeks, over a 24-week treatment period in subjects with anti-PLA2R antibody positive IMGN followed by a further long term treatment period until subjects reach remission of proteinuria, up to a maximum of 2 years total treatment. All subjects will receive background supportive therapy throughout the study. The dosing frequency will be adjusted to every 2 weeks if the subject\'s proteinuria as assessed by urinary protein creatinine ratio (PCR) is greater than 1000 milligrams per millimole (mg/mmol) \[greater than 10 grams(g)/24 hours (h)\], to compensate for loss of belimumab in the urine. Effects on mechanistic markers will be measured by the level of proteinuria, levels of anti-PLA2R antibodies, and various other measures of kidney function. These will be compared to historical data. The pharmacokinetics of belimumab will be measured to confirm dosing in heavily proteinuric subjects. Pharmacodynamic (PD) markers, biomarkers and Quality of Life(QoL) in IMGN subjects will also be investigated. Safety will be assessed by adverse events (AE), clinical laboratory evaluations, and vital signs.