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

The primary purpose of this study is to measure the efficacy and safety of efgartigimod intravenously (IV) compared to placebo in participants with Acetylcholine Receptor Binding Antibody (AChR-Ab) seronegative Generalized Myasthenia Gravis (gMG). Other objectives are to assess long-term efficacy, safety, and tolerability of efgartigimod.

Study will consist of:

* Screening
* Part A: participants will be randomized to receive either efgartigimod IV or placebo
* Part B: participants completing part A will receive open-label efgartigimod IV


Eligibility Criteria:
Inclusion 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.

Exclusion Criteria:

* 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 \&lt;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).

The complete list of exclusion criteria can be found in the protocol.