Efficacy and Safety Study of Intravenous Belimumab Versus Placebo in Subjects With Idiopathic Membranous Nephropathy

The main clinical study will be a randomized, double-blind, placebo-controlled, long term study involving a 100 week treatment period. The purpose of this study is to test for superiority of treatment with belimumab 10 mg/kg plus supportive therapy compared to placebo plus supportive therapy in idiopathic membranous nephropathy (IMN). The purpose of this study is also to investigate the effect of initiating earlier treatment with belimumab compared to delayed treatment with current immunosuppressive treatment regimens. The study will also determine the pharmacokinetic (PK) profile of belimumab and further explore the mechanism of action of Belimumab as well as effects on quality of life. All subjects (on either active treatment or placebo) will receive background supportive therapy throughout the main clinical study, which includes angiotensin-converting enzyme inhibitors (ACEi) and/or angiotensin receptor blockers (ARBs) unless contraindicated and may include statins, diuretics, dietary salt restriction but excludes immunosuppressants (except low dose corticosteroids). Screening will be done within 5 to 2 weeks before the first scheduled dose of study treatment. A total of 94 evaluable subjects will be randomized in a 1:1 ratio such that 47 subjects receive intravenous belimumab 10 mg/kg and 47 receive intravenous placebo. Subjects will be dosed on Days 0, 14, 28 and then every 4 weeks through to, and including, Week 100, resulting in a total of 27 doses (giving 104 weeks of drug exposure). 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 1000mg/mmol (greater than 10 g/24 h), to compensate for loss of belimumab in the urine. Subjects who are withdrawn from study treatment at any time during the study, eg for rescue therapy, will participate in follow-up visits every 12 weeks up to week 104. A subject will be regarded as having completed the main clinical study i