This clinical trial is studying IgA nephropathy, a kidney disease in which the immune system protein called immunoglobulin A (IgA) builds up in the kidneys and can lead to damage over time. The treatment used in the study is zigakibart, also known as Bion 1301/FUB523, given as a subcutaneous injection under the skin. The purpose of the study is to find out the long-term safety and tolerability of zigakibart in adults who have already taken part in earlier zigakibart studies.

People in the study receive zigakibart during the extension period, which means treatment continues after the earlier trial has ended. The study follows participants over time to see how they do with ongoing treatment and to collect information about side effects, blood and urine test results, and other health checks. It also looks at how the body handles the medicine and whether the immune system makes proteins that react to it.

Because this is a long-term extension study, the main focus is continued treatment and follow-up rather than changing between different medicines. The study is designed to gather more safety information while participants remain on zigakibart.