A Study of Mezagitamab in Adults With Kidney Condition Called IgA Nephropathy

Immunoglobulin A nephropathy (IgAN) is a kidney condition. It happens when the body's immune system creates groups of proteins (called immune complexes) that build-up in the kidneys causing swelling (inflammation). Over time, this inflammation may lead to kidney damage and cause the kidneys to no longer work properly. The main aim of this study is to check how well mezagitamab changes protein levels in the urine (proteinuria) compared to placebo in adults with primary IgAN. A placebo looks like medicine but doesn't have any active ingredients in it. Other aims are to check how safe mezagitamab is and how well participants with primary IgAN can tolerate it compared to placebo, and to find out if and how well mezagitamab continues to maintain kidney function over the long term compared to placebo. Participants will be placed in 1 of the 2 treatment groups; the main group and the open-label group. In the main group, participants will be placed by chance in either the mezagitamab or placebo treatment group at a 2:1 ratio. This means that out of 3 participants, 2 will receive mezagitamab and 1 will receive placebo. Participants can be in the study for 2 years (104 weeks). Participants will receive study treatment for about half a year (22 weeks) and then be observed for the remainder of the study (about 1.5 years). During observation, participants will continue to have check-ups about every month. In the open-label group, a small number of participants who either have lower levels of protein in their urine or have kidneys that do not filter the blood well, will receive mezagitamab treatment. This will include participants who have previously received mezagitamab in another study, TAK-079-1006. Every participant will receive mezagitamab in the same way as those in the main group receiving mezagitamab. During the study, participants will visit their study clinic several times.