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.

The primary objective of the study is to assess the efficacy of felzartamab compared to tacrolimus in participants with PMN in achieving complete remission (CR) of proteinuria. The secondary objectives of the study are to evaluate the efficacy of felzartamab through additional clinical endpoints and timepoints, to assess the impact of felzartamab on serum anti-phospholipase A2 receptor (PLA2R) antibodies and patient-reported outcomes, and to assess the safety, pharmacokinetics (PK) and immunogenicity of felzartamab.

Key Inclusion Criteria:

• Diagnosed with PMN in need of IST according to the Investigator's clinical judgment. The diagnosis of PMN must be documented with the presence of nephrotic syndrome, and hypoalbuminemia, and confirmed with a kidney biopsy either during Screening or within 5 years of signing the informed consent form (ICF) [see kidney biopsy exception below for participants positive for anti-PLA2R antibodies]. For these participants, the biopsy report with redacted protected health information must be available to be reviewed by the Sponsor or an independent nephropathologist. If the participant requires a kidney biopsy during Screening, medical monitor approval must be obtained and all other eligibility criteria should be reviewed to ensure that the participant is otherwise eligible prior to performing the kidney biopsy.

  a. Kidney biopsy exception for anti-PLA2R antibody positive participants: Participants who are positive for anti-PLA2R antibodies and have not had a kidney biopsy performed within 5 years of signing the ICF, may be eligible for the study without undergoing a kidney biopsy based on medical monitor review confirming normal estimated glomerular filtration rate (eGFR), presence of nephrotic syndrome, hypoalbuminemia, positive anti-PLA2R antibody test (defined as an anti-PLA2R antibody titer > 20 RU/mL), and documentation provided by the Investigator that the work-up for secondary causes of membranous nephropathy (MN) was negative with no identifiable secondary causes.

• Meets one of the following:

  1. Newly diagnosed PMN, defined as having never received IST for PMN in the past.
  2. Relapsed PMN, defined as documented achievement of CR or partial remission (PR) after treatment with an IST for PMN followed by reappearance of nephrotic range proteinuria (urine protein to creatinine ratio [UPCR] ≥ 3.0 gram per gram [g/g] from a 24-hour urine collection or proteinuria ≥ 3.5 gram per 24 hour [g/24 h]).

• Participants must be on the maximally approved dose or maximally tolerated dose of angiotensin-converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB) for at least 3 months prior to Screening. Participants not on the maximally approved dose of renin-angiotensin-aldosterone system (RAAS) inhibition may be enrolled provided there is documented intolerance to maximal RAAS inhibition (e.g., angioedema, development of postural hypotension, lightheadedness, hyperkalemia, etc).

• A UPCR of ≥ 3.0 g/g (as determined by a 24-hour urine collection) or total proteinuria ≥ 3.5 g/24 h (as determined by a 24-hour urine collection) at Screening after best supportive care for at least 3 months prior to signing the ICF.

Key Exclusion Criteria:

• Secondary cause of MN (e.g., malignancies, medications, systemic lupus erythematosus [SLE], hepatitis B, hepatitis C, etc).
• Severe renal impairment defined as an eGFR ≤ 30 mL/min/1.73m^2 at Screening or including the need for dialysis or renal replacement therapy.

Note: Other protocol-defined Inclusion/Exclusion criteria may apply.

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/