This is a Phase 2, open-label, multicenter, randomized study comprising parallel groups of participants receiving weekly and/or monthly doses of atacicept

This Phase 2 study is to explore the effectiveness, safety and tolerability of different dosing regimens of atacicept. Regimens include monthly and weekly dosing.

Inclusion Criteria:

• Must have the ability to understand and sign a written informed consent form, which must be obtained prior to initiation of study assessments
• Adult male or female of ≥18 years of age, or as per country specific legally or nationally recognized adult age, who provides written informed consent prior to performing any study assessments
• Diagnosis of IgAN as demonstrated by renal biopsy conducted within 10 years
• Total urine protein excretion ≥0.75 g per 24-hour or urine protein-to-creatinine ratio (UPCR) ≥0.75 mg/mg based on a 24-hour urine sample
• eGFR ≥30 mL/min/1.73 m2 at screening, as per the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)
• On a stable prescribed regimen of RASi (angiotensin converting enzyme inhibitor or angiotensin II receptor blocker) for at least 8 weeks that is at the maximum labeled or tolerated dose at screening and from screening to study Day 1

Exclusion Criteria:

• IgAN secondary to another condition (eg, liver cirrhosis), or other causes of mesangial IgA deposition including IgA vasculitis (ie, Henoch-Schonlein purpura), systemic lupus erythematosus, dermatitis herpetiformis, ankylosing spondylitis
• Evidence of rapidly progressive glomerulonephritis (loss of ≥50% of eGFR within 3 months of screening)
• Evidence of nephrotic syndrome within 6 months of screening (serum albumin <3.0 g/dL in association with UPCR >3.5 mg/mg)
• Renal or other organ transplantation prior to or expected during the study, with the exception of corneal transplants
• Concomitant chronic renal disease in addition to IgAN (eg, diabetic nephropathy, primary focal segmental glomerulosclerosis, membranous nephropathy, C3 glomerulopathy, lupus nephritis)