The drug being tested in this study is called mezagitamab. Mezagitamab is being tested for the first time in this patient population and might help to treat people who have Primary Immunoglobulin (IgA) Nephropathy. This study will evaluate the safety, tolerability, pharmacokinetics, and efficacy of mezagitamab in combination with stable background therapy.~The study will enroll approximately 16 participants. The study will consist of 2 key components: a main study and a long-term extension (LTE) study, which includes an observation period and a retreatment period. The observation period of the LTE study is a non-interventional study segment and the retreatment period of the LTE study consists of a redosing period in which participants will be administered mezagitamab at the same dose level as in the main study. Only participants who have a positive outcome during the main study will enter LTE study.~Participants will be enrolled to the following cohort:~• Mezagitamab~This multi-center trial will be conducted in the United States, Europe, and Asia Pacific. The overall time to participate in this study is approximately 154 weeks.
This study will have two parts. The main aims are to:~check the side effects from mezagitamab.~check for long-term side effects from mezagitamab.~Before starting the study, participants will be asked to provide a 24-hour urine sample. A few weeks later, if enrolled they will begin receiving a subcutaneous injection (under the skin) of mezagitamab once a week for 8 weeks then once every 2 weeks for 16 weeks. When treatment has ended, there will be a 24-week follow-up period.~Participants who receive benefit from the treatment may continue in the second part of the study where they will be monitored for up to 96 weeks and possibly retreated for another 24 weeks.