The purpose of this trial is to investigate the PK, PD, safety, and activity of efgartigimod IV in children and adolescents aged from 2 to less than 18 years of age with gMG.

Trial details include:

• The maximum trial duration for each individual participant will be approximately 28 weeks
• The treatment duration will be 8 weeks for the dose-confirmatory part (Part A) and 18 weeks for the treatment response-confirmatory part (Part B)

Inclusion Criteria:

1. Ability of the participant and/or his/her legally authorized representative to understand the requirements of the trial and provide written informed consent/assent, if applicable (including consent/assent for the use and disclosure of research-related health information), willingness and ability to comply with the trial protocol procedures (including attending the required trial visits).

2. Male or female participants between 2 to less than 18 years of age at the time of providing informed consent/assent. Age groups are enrolled in a staggered fashion respectively: 6 participants in the 12 to less than 18 years of age group followed by 6 participants in the 2 to less than 12 years of age group at the time of providing informed consent/assent.

3. Diagnosed with Generalized Myasthenia Gravis (gMG) with confirmed documentation

4. Meeting the clinical criteria as defined by the Myasthenia Gravis Foundation of America (MGFA) class II, III, and IVa.

5. Eligible participants should have an unsatisfactory response (efficacy and/or safety) to immunosuppressants, steroids or acetylcholinesterase (AChE) inhibitors and should be on stable concomitant gMG therapy of adequate duration before screening.

6. Positive serologic test for acetylcholine receptor (anti-AChR) antibodies at screening (for younger participants (<15kg) historical values can be used).

7. Contraceptive use should be consistent with local regulations regarding the methods of contraception for those participating in clinical trials. A subject is of childbearing potential if, in the opinion of the investigator, he/she is biologically capable of having children and is sexually active.

   1. Male participants: Male participants must agree to not donate sperm from of providing informed consent/assent until they have completed the trial.
   2. Female participants: Female adolescents of childbearing potential must have a negative serum pregnancy test at screening and a negative urine pregnancy test at baseline before investigational medicinal product (IMP) can be administered.

Exclusion Criteria:

1. Participants with MGFA class I, IVb, and V.

2. Female adolescents of childbearing potential: Pregnancy or lactation, or the participant intends to become pregnant during the trial or within 90 days after the last dose of IMP.

3. Has any of the following medical conditions:

   1. Clinically significant uncontrolled active or chronic bacterial, viral, or fungal infection at screening.
   2. Any other known autoimmune disease that, in the opinion of the investigator, would interfere with an accurate assessment of clinical symptoms of myasthenia gravis or put the participant at undue risk.
   3. History of malignancy unless deemed cured by adequate treatment with no evidence of recurrence for ≥3 years before the first administration of IMP. Participants with the following cancers can be included at any time: Adequately treated 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
   4. Clinical evidence of other significant serious diseases, or have had a recent major surgery, or who have any other condition that, in the opinion of the investigator, could confound the results of the trial or put the participant at undue risk

4. Worsening muscle weakness secondary to concurrent infections or medications (aminoglycosides, fluoro-quinolones, beta-blockers, etc).

5. A documented lack of clinical response to plasma exchange (PLEX).

6. Received a live or live-attenuated vaccine fewer than 28 days before screening. Receiving an inactivated, subunit, polysaccharide, or conjugate vaccine any time before screening is not exclusionary.

7. Received a thymectomy <3 months before screening or 1 is planned to be performed during the trial period.

8. The following results from these diagnostic assessments will be considered exclusionary:

   a. Positive serum test at screening for an active viral infection with any of the following conditions: Hepatitis B virus (HBV) that is indicative of an acute or chronic infection; Hepatitis C virus (HCV) based on HCV antibody assay; Positive HIV serology at screening; Positive nasopharyngeal swab polymerase chain reaction (PCR) test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at screening.

9. Using the following prior or concomitant therapies: Use of an investigational product within 3 months or 5 half-lives (whichever is longer) before the first dose of IMP, Use of any monoclonal antibody within the 6 months before the first dose of IMP, Use of intravenous immunoglobulin (IVIg), administered subcutaneously or intramuscularly, or PLEX within 4 weeks before screening.

10. Total immunoglobulin (IgG) levels <6 g/L below the lower limit of normal (LLN) according to the reference ranges of the central laboratory for participant by sex and age at screening.

11. A known hypersensitivity reaction to efgartigimod or any of its excipients.

12. Current participation in another interventional clinical trial or previous participation in an efgartigimod trial with at least 1 dose of IMP received.

13. History (within 12 months of screening) of current alcohol, drug, or medication abuse as assessed by the investigator.