Atacicept in Subjects With IgA Nephropathy

Key Inclusion Criteria: * Must have the ability to understand and sign a written informed consent form * Male or female of ≥18 years of age * Total urine protein excretion ≥1.0 g per 24-hour or urine protein to creatinine ratio (UPCR) ≥1.0 mg/mg based on a 24-hour urine sample during the Screening Period * Diagnosis of IgAN as demonstrated by renal biopsy conducted within 10 years * eGFR ≥ 30 mL/min/1.73 m2, as per the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. * On a stable prescribed regimen of RAASi for at least 12 weeks that is at the maximum labeled or tolerated dose at Screening * Systolic blood pressure ≤150 mmHg and diastolic blood pressure ≤90 mmHg Key Exclusion Criteria: * IgAN secondary to another condition (e.g., liver cirrhosis), or other causes of mesangial IgA deposition including IgA vasculitis (i.e., Henoch-Schonlein purpura), systemic lupus erythematosus (SLE), dermatitis herpetiformis, ankylosing spondylitis * Total urine protein excretion ≥ 5g per 24-hour or urine protein to creatinine ratio (UPCR) ≥ 5 mg/mg based on a 24-hour urine sample during the Screening Period * 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 \<30g/L in association with UPCR \>3.5 mg/mg * Renal or other organ transplantation prior to, or expected during the study * Concomitant chronic renal disease in addition to IgAN * Uncontrolled diabetes, defined as hemoglobin-A1c (HbA1c) \>7.5% at screening * History of tuberculosis (TB), untreated latent TB infection (LTBI), or evidence of active TB determined by a positive Quantiferon test * Participation in the Phase 2b (Parts A and B) study or any previous treatment with atacicept