A Clinical Trial to Investigate the Safety and Tolerability, Efficacy, Pharmacokinetics, Pharmacodynamics and Immunogenicity of 2 Dose Regimens of ARGX-117 in Adults with Multifocal Motor Neuropathy

Inclusion Criteria: 1. Capable of giving signed informed consent form (ICF) 2. Male/female at least 18 years of age at the time the informed consent form (ICF) is signed 3. Probable or definite MMN according to the European Federation of Neurological Societies (EFNS)/Peripheral Nerve Society (PNS) (EFNS/PNS) 2010 guidelines at screening confirmed by the MMN Confirmation Committee (MCC) 4. Receiving a stable IVIg regimen for at least 3 months before screening or recently initiated IVIg treatment 5. IVIg treatment dependency confirmation by the MMN Confirmation Committee (MCC) 6. Immunization with the first meningococcal vaccine and pneumococcal vaccine, and the single Haemophilus influenza type B vaccine must be performed at least 14 days before IMP administration at V1 according to local country-specific immunization schedules. A documented history of vaccination against Neisseria meningitides, Haemophilus influenza type B, and streptococcus pneumonia will be permitted 7. Contraceptive use by men and women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies Exclusion Criteria: 1. Any coexisting condition which may interfere with the outcome assessments 2. Clinical signs or symptoms suggestive for neuropathies other than MMN such as motor neuron disease or other inflammatory neuropathies 3. Severe psychiatric disorder, history of suicide attempt, or current suicidal ideation that in the opinion of the investigator could create undue risk to the participant or could affect adherence with the trial protocol. 4. Clinically significant uncontrolled active or chronic bacterial, viral, or fungal infection during the screening and/or IVIg monitoring period (IVMP). 5. Any other known autoimmune disease that, in the opinion of the investigator, would interfere with an accurate assessment of clinical symptoms of MMN or put the participant at undue risk (eg, SLE). 6. History of malignancy unless resolved by adequate treatment with no evidence of recurrence for ≥3 years before the first administration of the IMP. Participants with the following carcinomas will be eligible: 1. Adequately treated basal cell or squamous cell skin cancer 2. Carcinoma in situ of the cervix 3. Carcinoma in situ of the breast 4. Incidental histological finding of prostate cancer 7. Clinical evidence of other significant serious diseases, have had a recent major surgery (including a splenectomy at any time), or who have any other condition in the opinion of the investigator, that could confound the results of the trial or put the participant at undue risk 8. Prior/concomitant therapy 1. Cyclophosphamide and/or rituximab and/or eculizumab and/or mycophenolate mofetil within 3 months prior to screening 2. Use of an investigational product within 3 months or 5 half-lives (whichever is longer) before the first dose of the IMP. 9. Positive serum test at screening for an active viral infection with any of the following conditions: 1. Hepatitis B virus (HBV) that is indicative of an acute or chronic infection 2. Hepatitis C virus (HCV) based on HCV antibody assay 3. HIV based on test results that are associated with an AIDS-defining condition 10. Current or history of (ie, within 12 months of screening) alcohol, drug, or medication abuse 11. Known hypersensitivity reaction to 1 of the components of the IMP or any of its excipients 12. Female participants with a positive serum or urine pregnancy test, lactating females, and those who intend to become pregnant during the trial or within 15 months after last dose of the IMP 13. ALT or AST ≥2 × upper limit of normal and total bilirubin ≥1.5 × upper limit of normal of the central laboratory reference range 14. An estimated glomerular filtration rate of ≤60 mL/min/1.73m2