A Study to Evaluate the Pharmacokinetics, Pharmacodynamics, Safety and Tolerability of Inebilizumab in Children With Generalized Myasthenia Gravis (gMG)

Inclusion Criteria

* Participant's legally authorized representative has provided informed consent when the participant is legally too young to provide informed consent and the participant has provided written assent based on local regulations and/or guidelines before any study-specific activities/procedures being initiated.
* Age ≥ 2 to \< 18 years of age on the day of enrollment.
* Diagnosis of gMG defined as:

  * Positive serologic test for anti-acetylcholine receptor (AChR) or anti-muscle-specific tyrosine kinase (MuSK) antibody (Ab) titers as confirmed at screening (1 retest allowed), and
  * At least 1 of the following:

    * History of abnormal neuromuscular transmission test results demonstrated by single-fiber electromyography or repetitive nerve stimulation; or
    * History of positive anticholinesterase test (eg, edrophonium chloride test); or
    * Participant demonstrated improvement in gMG signs on oral cholinesterase inhibitors, as assessed by the treating physician; or
    * Clinical syndrome consistent with a diagnosis of gMG, and not otherwise explained by another condition.
* Myasthenia Gravis Foundation of America Clinical Classification Class II, III, or IV at the time of screening.
* Participants must be on:

  * Corticosteroids only, with no dose increase within 4 weeks prior to screening, or
  * One allowed non-steroidal immunosuppressive therapies (IST), (azathioprine, mycophenolate mofetil, or mycophenolic acid) with continuous use for at least 6 months prior to screening and no dose increase within 4 months prior to screening, or
  * Combination of (1) corticosteroids with no dose increase within 4 weeks prior to screening and (2) one allowed non-steroidal IST with continuous use for at least 6 months prior to screening and no dose increase within 4 months prior to screening.

Note:

The maximum allowed dose of prednisone at the time of enrollment will be 40 mg/day or 80 mg every other day, or equivalent corticosteroid dose.

Tacrolimus is allowed in Japan only, with continued use for ≥ 6 months prior to screening and no dose increase within 4 months prior to screening.

* Participants may enter the study on a stable dose of acetylcholinesterase inhibitors (pyridostigmine dose). The acetylcholinesterase inhibitor dose must have been stable for at least 2 weeks prior to enrollment.
* Vital signs and laboratory parameters within the normal ranges at screening, or, if outside normal ranges, deemed not clinically significant by the investigator.

Exclusion Criteria

* Employees of the Sponsor, contract research organization (CRO), site staff, and their family members.
* Thymectomy within 12 months prior to baseline (Day 1) visit or planned thymectomy during the duration of the treatment period.
* Unresected thymoma- Participants with benign thymoma resected \> 12 months prior to screening may enroll.
* History of recurrent significant infections.
* Known immunodeficiency disorder, including current infection or positive test for human immunodeficiency virus (HIV).
* Positive test for chronic hepatitis B infection at screening.
* History of untreated hepatitis C infection, or positive antibody test for hepatitis C virus (HCV).
* Active tuberculosis (TB); latent TB with no documented history of adequate treatment per local standard of care; or a positive QuantiFERON®-TB test at screening, unless treatment for TB was completed per local guidelines.
* History of progressive multifocal leukoencephalopathy.
* Participants diagnosed with congenital myasthenic syndromes.
* Receipt of any biologic B-cell-depleting therapy (eg, rituximab, ocrelizumab, obinutuzumab, ofatumumab, inebilizumab) or any experimental B-cell-depleting agent in the 6 months prior to screening.
* Receipt of any other monoclonal antibody (mAb) or large molecule biologic, including but not limited to FcRn inhibitors, anti-TNF mAbs, anti-janus kinase (JAK) Stat mAbs, and complement inhibitors within 6 months prior to screening.
* Receipt of the following medications or treatments at any time prior to enrollment: alemtuzumab, total lymphoid irradiation, bone marrow transplant, T-cell vaccination therapy, natalizumab.
* Participants who are pregnant or breastfeeding or planning to get pregnant.
* Receipt of intravenous immunoglobulin (IVIg) or subcutaneous immunoglobulin (SCIg) within 4 weeks prior to enrollment.