An Open-Label, Multicenter, Randomized Phase 3 Study Evaluating the Efficacy and Safety of Felzartamab in Participants With Primary Membranous Nephropathy (PMN) [PROMINENT]

In this study, researchers will learn more about the use of felzartamab in participants with primary membranous nephropathy, also known as PMN. In people with PMN, autoantibodies build up in the glomeruli of the kidney. Antibodies are proteins that help the body fight off infection. An autoantibody is a type of antibody that mistakenly targets and attacks the body's own tissues. Glomeruli are the filters of the kidney that remove waste and extra fluid from the body. In PMN, the build-up of autoantibodies in the glomeruli causes damage to the kidneys. Kidney damage can lead to too much protein and blood leaking into the urine. High levels of protein in the urine, called proteinuria, are common in people with PMN. Symptoms of PMN can include swelling in the legs and body, tiredness, and high blood pressure. If left untreated, PMN can eventually lead to kidney failure. In this study, researchers will learn more about how a study drug called felzartamab affects people with PMN. Felzartamab is a monoclonal antibody, which means it is an antibody made in a laboratory. Felzartamab can target immune cells that produce autoantibodies, helping to lower their buildup in the kidneys. The main goal of this study is to compare how felzartamab works compared to a drug called tacrolimus. Tacrolimus is another drug given to people with PMN and kidney disease. The main question that researchers want to answer is: * How many participants achieve a complete response after 104 weeks of treatment? * A complete response means that their urine protein levels decrease to a low level and their kidney function remains stable. Researchers will also learn about: * How long it takes before the participants' disease gets worse * How long the participants' urine protein levels stay low * How many participants develop antibodies against felzartamab in the blood? * How many participants achieve a complete response after 76 weeks of treatment * How many participants have medical problems during the study * How felzartamab is processed by the body * How felzartamab affects participants' tiredness and overall physical health The study will be done as follows: * Participants will be screened to check if they can join the study. This may take up to 42 days. * Participants will be randomized to receive either felzartamab as intravenous (IV) infusions or tacrolimus, taken orally as tablets. * If participants have worsening kidney function or worsening proteinuria, or if their PMN relapses, or if they show no signs of improvement in their PMN, they will have a chance to receive rescue treatment. * If a participant stops treatment early, there will be follow-up visits every 12 weeks until they reach Week 104. * In total, participants will have up to 23 study visits. Participants who do not need rescue treatment will stay in the study for up to 104 weeks. Participants who need rescue treatment will stay in the study for up to 156 weeks.