Trial of Sibeprenlimab in Children and Adolescents With Immunoglobulin A Nephropathy (IgAN)

This Phase 3, open-label study will evaluate the efficacy, safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and immunogenicity of sibeprenlimab administered subcutaneously every 4 weeks in pediatric participants aged 2 to 17 years with IgA nephropathy (IgAN). Sibeprenlimab will be administered as an add-on to standard of care therapy consisting of angiotensin converting enzyme inhibitors (ACEIs) and/or angiotensin receptor blockers (ARBs).

1. Source-verified, biopsy-confirmed IgA nephropathy (IgAN). 2. Receiving a stable and maximally tolerated dose of authorized angiotensin converting enzyme inhibitors (ACEIs) and/or angiotensin receptor blockers (ARBs) for at least 12 weeks. 3. Participants receiving a stable dose of sodium-glucose cotransporter-2 inhibitor (SGLT2i) therapy for IgAN, if available, in addition to ACEIs and/or ARBs, must have initiated SGLT2i treatment for at least 12 weeks. 4. Participants must have a urine protein/creatinine ratio (uPCR) ≥ 0.75 grams per gram (g/g), as measured from the geometric mean of 3 first void spot urine samples (collected within the screening window prior to Dose 1). 5. Estimated glomerular filtration rate (eGFR) ≥ 30 milliliters per minute per 1.73 square meters (mL/min/1.73 m\^2). 6. No evidence of coexisting chronic kidney disease other than IgAN. 7. Serum immunoglobulin G (IgG) ≥ 600 mg/dL at screening. 8. Kidney biopsy findings not demonstrating additional pathological features inconsistent with IgAN.