A Study of Efgartigimod in Patients With IgG4-Related Disease

Key Inclusion Criteria: * Have a clinical diagnosis of IgG4-related disease that requires treatment in the opinion of the investigator * Meet the 2019 ACR/EULAR Classification Criteria for IgG4-Related Disease * Have a serum IgG4 concentration greater than or equal to 2 times the upper limit of normal at Screening * Have involvement of the lacrimal gland(s), salivary gland(s), and/or pancreas * If lacrimal and/or salivary glands are involved, it must be symptomatic, including but not limited to discomfort, pain, dryness, headache, or vision changes * If the pancreas is involved, it must be asymptomatic, diffuse enlargement without signs or symptoms of obstruction or evidence of major organ dysfunction in the opinion of the investigator * Have a prior inadequate response to, or intolerance of, glucocorticoids, or who have experienced recurrent symptoms after previous treatment with glucocorticoids * Are not receiving current treatment with immunosuppressive medications * All women must test negative for pregnancy and agree to use a reliable method of birth control Key Exclusion Criteria: * Any exclusion criteria listed in the 2019 ACR/EULAR Classification Criteria for IgG4-Related Disease * Prior treatment with an FcRn inhibitor * Have conventional synthetic disease-modifying antirheumatic drug (csDMARD) or immunosuppressive use as follows: * Treatment with glucocorticoids within 28 days prior to Baseline or planned treatment during the study * Treatment with csDMARDs including but not limited to hydroxychloroquine, methotrexate, leflunomide, or sulfasalazine within 28 days prior to Baseline or planned treatment during the study * Treatment with cytotoxic or immunosuppressive drugs including but not limited to cyclophosphamide, mycophenolic acid, azathioprine, cyclosporine, sirolimus, or tacrolimus within 28 days prior to Baseline or planned treatment during the study * Treatment with a janus kinase (JAK) inhibitor including but not limited to tofacitinib, baricitinib, upadacitinib, or filgotinib within 28 days prior to Baseline or planned treatment during the study * Treatment with a Bruton's tyrosine kinase (BTK) inhibitor including but not limited to ibrutinib, zanubrutinib, acalabrutinib, pirtobrutinib, or rilzabrutinib within 28 days prior to Baseline or planned treatment during the study * Have biologic disease-modifying antirheumatic drug (bDMARD) use as follows: * Treatment with etanercept, adalimumab, or anakinra within 28 days before Baseline or planned treatment during the study * Treatment with infliximab, certolizumab pegol, golimumab, abatacept, or tocilizumab within 56 days before Baseline or planned treatment during the study * Treatment with a B cell depleting agent including but not limited to rituximab, ocrelizumab, obinutuzumab, ofatumumab, inebilizumab, ianalumab, or obexelimab ≤ 6 months prior to Baseline * Patients who received B-cell targeted therapy \> 6 and ≤ 12 months prior to Baseline must have a B-cell count that is within the laboratory reference range at Screening * Treatment with a BAFF antagonist including but not limited to belimumab or tabalumab within 6 months before Baseline or planned treatment during the study * Treatment with an IL-17 antagonist including but not limited to secukinumab, ixekizumab, or brodalumab within 6 months before Baseline or planned treatment during the study * Prior treatment with other bDMARDs may be allowed at the discretion of the investigator * A history of, or current, inflammatory or autoimmune disease (that could affect the interpretation of safety or efficacy outcomes) other than IgG4-related disease * Evidence of active tuberculosis, HIV, or hepatitis B or C infection * History of cancer except for skin basal or squamous cell carcinoma, cervical dysplasia or carcinoma in situ that has been treated and is considered cured \> 1 year prior to Baseline, prostate cancer considered cured for \> 5 years with a normal prostate specific antigen, or colon cancer considered cured \> 5 years