Inclusion Criteria:

- Male and female patients ≥ 18 years of age .

- Biopsy-confirmed IgAN. (Patients with an eGFR of 30 to 45 mL/min/1.73m^2 must have had a kidney biopsy performed within 36 months of the screening visit).

- Stable and maximally tolerated dose of ACEI and/or ARB for at least 3 months prior to screening. Patients who are on a stable  dose of SGLT2i may participate if treatment was initiated ≥3 months prior to screening. Patients who are unable to take an ACEI or ARB may participate if their overall management conforms with standards of care and
other protocol requirements.

- Subject has screening urine protein/creatinine ratio (uPCR) ≥ 0.75 g/g or urine protein ≥ 1.0 g/day, as measured from 24-hour urine samples (geometric mean of the two 24-hour urine samples collected up to 2 weeks apart at screening) and when the subject is considered to be in steady-state.

- Subject has eGFR ≥ 30 mL/min/1.73 m^2, calculated using the 2021 CKD-EPI equation. Subjects with an eGFR of 30 to 44 mL/min/1.73m^2 are required to have had a kidney biopsy performed within 36 months of the screening visit. For the exploratory cohort only: subject has eGFR of 20 to 29 mL/min/1.73 m^2, calculated using the 2021 CKD-EPI equation. Serum creatinine assessments for the determination of eGFR should be taken when the subject is considered to be in a steady-state without recent changes in volume status, medications that could impact the result (e.g., nonsteroidal anti-inflammatory drugs [NSAIDs], aminoglycosides, trimethoprim, cimetidine, pyrimethamine, fenofibrate, or creatine supplements), or changes in dietary protein intake.

- Ability to provide written informed consent prior to initiation of any trial-specific procedures, and ability, in the opinion of the principal investigator, to comply with all the requirements of the trial.

Exclusion Criteria:

- Secondary forms of IgAN or IgA vasculitis.

- Coexisting chronic kidney disease other than IgAN.

- Kidney biopsy findings in addition to IgAN including those of diabetic nephropathy, membranous nephropathy, or lupus nephritis. Hypertensive vascular changes are acceptable.

- Kidney biopsy MEST or MEST-C score of T2 or C2 (Oxford IgAN classification). If MEST-scoring was not performed, the presence of > 50% tubulo-interstitial fibrosis, or crescents in > 25% of glomeruli is exclusionary. This does not apply to the exploratory cohort.

- Biological male subjects who do not agree to avoid donation of sperm from the time of consent through the end of the subject’s participation in the trial and an additional 90 days thereafter.

- Nephrotic syndrome.

- Serum IgG < 600 mg/dL at screening.

- Chronic systemic immunosuppression, including glucocorticoids, within 16 weeks of randomization.

- Participation in another interventional clinical trial and receipt of another investigational drug within 30 days prior to the administration of IMP or 5 half-lives from last investigational drug administration, whichever is longer.

- Chronic infectious disease, or acute infectious disease at time of screening.

- Type 1 diabetes, or poorly controlled Type 2 diabetes.

- Uncontrolled hypertension.

- Subject has kidney biopsy with a MEST or MEST-C score of T2 or C2 from the Oxford IgAN classification schema. If MEST-scoring was not performed, the presence of > 50% tubulo-interstitial fibrosis, or crescents in > 25% of glomeruli is exclusionary.

Note: this criterion does not apply to the exploratory cohort.

Note: The most recent kidney biopsy will take precedence for purposes of eligibility.

- History of a previous hypersensitivity or severe allergic reaction with generalized urticaria, angioedema, or anaphylaxis to any of the ingredients of the sibeprenlimab SC injection formulation.

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 1/8/24. Questions regarding updates should be directed to the study team contact.