Study of the Safety and Efficacy of OMS721 in Patients With Immunoglobulin A (IgA) Nephropathy

Inclusion Criteria: * Age 18 years or older at the onset of Screening * Biopsy confirmed diagnosis of IgAN within 8 years prior to Screening or Run-in Visit 1 * Documented history of proteinuria of \> 1 g/day within 6 months prior to Screening or uPCR \> 0.75 by spot urine at Screening * Mean of two proteinuria measurements \> 1 g/day at baseline * Estimated glomerular filtration rate of ≥ 30 mL/min/1.73 m² at Screening and baseline Exclusion Criteria: * Treatment with immunosuppressants (e.g., azathioprine or cyclophosphamide), Chinese traditional medicine with immunosuppressive function, cytotoxic drugs, or eculizumab within 8 weeks prior to Screening, unless such treatment is given for indications other than IgA. * Treatment with systemic corticosteroids within 8 weeks prior to Screening * Uncontrolled BP, a systolic BP of \> 150 mmHg and a diastolic BP of \> 100 mmHg at rest despite the combination of two or more anti-hypertensives including ACE inhibitors, ARBs, or direct renin inhibitors * Female patients who are pregnant, breast feeding, or planning to become pregnant up through 12 weeks after the last dose of study drug, including possible retreatments. Males who are planning to father children up through 12 weeks after the last dose of study drug, including possible retreatments * Clinical or biological evidence of Type 1 diabetes mellitus (DM), or poorly controlled DM with hemoglobin A1c \> 7.5 or with evidence of diabetic nephropathy on biopsy, systemic lupus erythematosus, IgA vasculitis (Henoch-Schonlein purpura), secondary IgAN, or other renal disease during Screening and Run-In * Presence of significant morbidity or other major illness or disease that may confound the interpretation of the clinical trial results or may result in death within 2 years of Screening * History of renal transplantation * Have a known hypersensitivity to any constituent of the investigational product * Rapidly progressive glomerulonephritis, defined as a fall in eGFR of \> 30 mL/min/1.73 m\^2 within 24 weeks or \> 15 mL/min/1.73 m\^2 within 12 weeks prior to Screening * Significant abnormalities in clinical laboratory values * History of human immunodeficiency virus (HIV), evidence of immune suppression, active hepatitis C virus (HCV) infection (patients with positive anti-HCV antibody but a non-detected HCV RNA PCR can enroll), hepatitis B virus (HBV) infection (patients with positive HBsAg are excluded; for patients with isolated positive anti-HBc antibody, HBV DNA test by PCR must be non-detectable to enroll). * Diagnosis of a malignancy except for adequately treated and cured basal or squamous cell skin cancer, curatively treated in situ disease, or other cancer from which the patient has been disease-free for ≥ 5 years * Have received any other investigational drug or device or experimental procedures within 30 days of the Screening Visit (SV) or within 5 times the plasma half-life of the administered experimental drug, whichever is longer * Initiation or change in dosing of sodium glucose co-transporter 2 inhibitors (SGLT2i) during Screening and Run-In Periods. However, a stable dose regimen established at least 8 weeks prior to screening is acceptable * Treatment with Tarpeyo™ (budesonide) or other approved treatments for IgAN within 6 months prior to screening. Treatment with Tarpeyo is not allowed during Screening and Run-In Periods * Treatment with Kerendia® (finerenone) within 6 months prior to screening. Treatment with Kerendia is not allowed during Screening and Run-In Periods * Initiation of treatment with Filspari™ (sparsentan), a dual Endothelin Angiotensin Receptor Antagonist (dEARA) or similar medication within three months prior to screening. A stable dose initiated at minimum 3 months before screening is acceptable and will take the place of ACEi/ARB as background therapy