A Phase 3 Study to Evaluate the Efficacy and Safety of Efgartigimod IV in Patients with Acetylcholine Receptor Binding Antibody Seronegative Generalized Myasthenia Gravis

Eligibility Criteria

• * The participant is at least 18 years of age and the local legal age of consent for clinical studies when signing the ICF.
• * The participant is capable of providing signed informed consent and following with protocol requirements.
• * The participant agrees to use contraceptive measures consistent with local regulations and the women of child-bearing potential (WOCBP) must have a negative serum pregnancy test result at screening and a negative urine pregnancy test result at baseline before receiving the study drug.
• * The participant has no known weakness in infancy and later develop fatigable weakness after aged 16 years and diagnosed with acquired gMG of both of the following:
• 1. History of abnormal neuromuscular transmission demonstrated by single fiber electromyography or repetitive nerve stimulation (RNS) or is anti-muscle-specific kinase antibodies (MuSK-Ab) seropositive
• 2. Either a history of positive edrophonium chloride test OR a demonstrated improvement in MG signs with treatments such as oral acetylcholinesterase (AChE) inhibitors, plasma exchange (PLEX), immunoabsorption, or intravenous immunoglobulin (IVIg)/ subcutaneous immunoglobulin (SCIg) treatment
• * The participant is receiving a stable dose of MG therapy before screening that includes acetylcholinesterase (AChE) inhibitors, steroids, or nonsteroidal immunosuppressive therapies (NSISTs) in combination or alone.
• * Known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of gMG or puts the participant at undue risk
• * History of malignancy, cancer, unless considered cured by adequate treatment with no evidence of recurrence for ≥3 years. Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast, Incidental histological findings of prostate cancer
• * Clinically significant active infection that is not sufficiently resolved in the investigator's opinion or positive serum test at screening for active infection with any of the following: Hepatitis B virus (HBV), Hepatitis C virus (HCV), HIV
• * Current participation in another interventional clinical study or previous participation in an efgartigimod clinical study and received at least 1 dose of the study drug
• * Known hypersensitivity to study drug or one of its excipients (inactive ingredients)
• * History of or current alcohol, drug, or medication abuse as assessed by the investigator
• * Pregnant or lactating state or intention to become pregnant during the study
• * Live or live-attenuated vaccine received \<4 weeks before screening
• * Worsening muscle weakness secondary to concurrent infections or medications
• * Received a thymectomy less than 3 months before screening or thymectomy is planned during the study
• * Use of some medications before screening (more information is found in the protocol).