A Study to Evaluate the Safety, PD, PK and Efficacy of ARGX-113 in Patients With Pemphigus

Inclusion Criteria: 1. Male or female patients aged ≥ 18 years. 2. Clinical diagnosis of PV or PF, that has been confirmed by positive direct immunofluorescence and by positive indirect immunofluorescence and/or ELISA. 3. Mild to moderate disease severity (PDAI \< 45). 4. Newly diagnosed patients or relapsing patients off therapy with or without a first course of prednisone of maximum 4 weeks, and for whom an initial period of ARGX-113 monotherapy is judged clinically acceptable; or newly diagnosed patients or relapsing patients off therapy on a first course of oral prednisone at stable dose for at least 2 weeks and for whom ARGX-113 monotherapy is considered not clinically acceptable; or patients who relapse despite oral prednisone at tapered dose +/- a conventional immunosuppressant (e.g. azathioprine, mycophenolate mofetil). 5. Identified serum levels of autoantibodies directed against Dsg 3 and/or Dsg-1 antigens at screening, using indirect immunofluorescence or ELISA. 6. Ability to understand the requirements of the study, provide written informed consent (including consent for the use and disclosure of research-related health information), and comply with the study protocol procedures (including required study visits). Exclusion Criteria: 1. Pregnant and lactating women, and those intending to become pregnant during the study or within 90 days after the last dosing. Women of childbearing potential should have a negative serum pregnancy test at Screening and a negative urine pregnancy test at Baseline, prior to administration of IMP. 2. Male patients who are sexually active that do not intend to use effective methods of contraception during the study or within 90 days after the last dosing. 3. Confirmed diagnosis of paraneoplastic pemphigus, drug-induced pemphigus or any other non-PV/non-PF autoimmune blistering disease. 4. History of refractory disease to active third line therapy (e.g. intravenous polyvalent human immunoglobulins \[IVIg\], rituximab, plasma exchange/ immunoadsorption). 5. Use of therapies other than oral prednisone and conventional immunosuppressants, that can interfere in the clinical course of the disease (e.g. intravenous \[IV\] prednisolone bolus, dapsone, sulfasalazine, tetracyclines, nicotinamide, plasmapheresis/ plasma exchange, immunoadsorption and IVIg) within 2 months prior to Baseline visit. 6. Use of rituximab and other CD20 target biologics within 6 months prior to Baseline visit. 7. History of anaphylactic reaction, or a known hypersensitivity reaction to one of the components of the IMP. 8. History of vaccination within the last 4 weeks prior to Baseline visit, or with a planned vaccination during the study, with the exception of seasonal vaccination (e.g. influenza vaccine). 9. Recent serious infection (i.e., requiring injectable antimicrobial therapy or hospitalization) within the 8 weeks prior to Baseline visit. 10. Known active or chronic viral infection with hepatitis B virus (HBV); refer to the Centers for Disease Control and Prevention (CDC) guidelines. 11. Known seropositive or active infection with hepatitis C virus (HCV). 12. Known history of or known viral infection with human immunodeficiency virus (HIV 1 and 2 antibodies). 13. Body Mass Index (BMI) at Screening \> 35,0 kg/m2. 14. Clinical evidence of significant active, unstable or uncontrolled concomitant disease (e.g. cardiovascular, pulmonary, hematologic, gastrointestinal, endocrinological and metabolic, hepatic, renal, neurologic, malignancy, infectious diseases, coagulopathies, other autoimmune disease) or condition (lack of peripheral venous access, recent major surgery, etc), which, in the opinion of the investigator, puts the patient at undue risk or may affect the interpretation of the results. 15. Patients in general health condition not allowing study participation (Karnofsky index \< 60%; see Appendix 14.2). 16. At Screening, have clinically significant laboratory abnormalities as below: 1. Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) \> 3 × upper limit of normal (ULN) 2. Total serum bilirubin of \> 1.5 x ULN (except for Grade 1 hyperbilirubinemia as defined by the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI-CTCAE), due solely to a documented medical diagnosis of Gilbert's syndrome) 3. Serum creatinine of \> 1.5 mg/dL or creatinine clearance \< 50 mL/min (using the Chronic Kidney Disease Epidemiology Creatinine formula) 4. Hemoglobin (Hb) ≤ 9 g/dL 5. International normalized ratio (INR) \> 1.5 or activated partial thromboplastin time (aPTT) \> 1.5 x ULN 6. Total immunoglobulin G (IgG) level \< 6 g/L 7. Presence of \> 1 + proteinuria dipstick 17. Patient having participated in another interventional study within the last 3 months prior to Baseline visit.