A Study to Determine if BHV-1400 is Effective and Safe in Adults With IgA Nephropathy

Key Inclusion Criteria: * Diagnosis of IgAN as confirmed by renal biopsy conducted within 10 years prior to Screening. * If a participant has a history of diabetes, the biopsy must have been conducted within 2 years prior to Screening with no evidence of diabetic nephropathy. * In all cases, if a historical biopsy report is not available, a biopsy may be performed prior to Screening. * UPCR ≥ 0.75 g/g or UPE ≥ 1.0 g/d determined via 24 hour collection. * eGFR ≥ 30 mL/min/1.73m2 (CKD-EPI equation). * Participants must have been on supportive care including a stable dose regimen of ACEi or ARB (at the locally approved maximal daily dose or the maximally tolerated dose per Investigators' judgment) for at least 90 days prior to Screening. Subjects who are not able to tolerate ACEi or ARB therapy may be eligible for participation in the trial if their overall management including blood pressure control is as per local applicable guidelines. This must be discussed with the medical monitor and documented by the Investigator. * Patients may be on a dual endothelin angiotensin receptor antagonist (DEARA) or endothelin receptor antagonist (ERA) but must be on a stable dose for at least 90 days prior to Screening and they must remain on a stable dose throughout the course of the study. Participants may be on a sodium-glucose cotransporter 2 (SGLT2) inhibitor, mineralocorticoid receptor antagonist (including Finerenone), but must be on a stable dose for 90 days prior to Screening and must remain on a stable dose throughout the course of the study. Key Exclusion Criteria: * Any secondary IgAN as defined by the Investigator; secondary IgAN can be associated with cirrhosis, celiac disease, HIV infection, herpetiformis, seronegative arthritis, small-cell carcinoma, lymphoma, disseminated tuberculosis, bronchiolitis obliterans, inflammatory bowel disease, familial Mediterranean fever, etc. NOTE: IgA Vasculitis excluded if patient has had any IgA Vasculitis related extrarenal signs or symptoms, or requirement for steroid or other immunosuppressive therapy in the past year. * Any cause of chronic kidney disease that is not diagnosed as IgAN or may be due to non-IgAN cause, such as diabetic nephropathy. If presence of other kidney disease or concurrent glomerulopathies felt to be non-dominant, consideration for inclusion must be discussed with and approved by the Sponsor Medical Monitor/Sponsor Designee. * Presence of rapidly progressive glomerulonephritis as defined by 50% decline in eGFR within 3 months prior to Screening. * Evidence of nephrotic syndrome, defined as 24-hour protein \> 3.5g with concurrent hypoalbuminemia (Albumin \< 3.0 g/dl), within 6 months of Screening * End-stage renal disease requiring dialysis or transplantation