A Long-term, Single-Arm, Open-label, Multicenter, Follow-on Trial of ARGX-113-2006 to Evaluate Safety of Efgartigimod Administered Intravenously in Children With Generalized Myasthenia Gravis

gMG is a rare, chronic, neuromuscular autoimmune disease caused by pathogenic
IgGs targeting the neuromuscular junction, producing reduced neuromuscular
transmission and debilitating and potentially life-threatening muscle weakness
and chronic fatigue. Generalized muscle weakness results in difficulties in
mobility, speech, swallowing, vision, and respiration.

Efgartigimod is a human immunoglobulin (Ig) G1 (IgG1)-derived Fc fragment that
binds with nanomolar affinity to human neonatal Fc receptor (FcRn) that is
being developed for the treatment of generalized myasthenia gravis. Overall,
efgartigimod IV has been well tolerated in healthy adult participants and in
participants with gMG. No major safety findings have arisen in ongoing and
completed studies with efgartigimod. Further nformation can be found in the
IB.

This trial is a follow-on of the ARGX-113-2006 trial (NL78028.058.21), the
first clinical trial administering efgartigimod intravenously in a pediatric
population.
Only participants of ARGX-113-2006 who have completed the trial will be
enrolled in the ARGX-113-2008 trial.

This study has been transitioned to CTIS with ID 2023-507379-23-00 check the CTIS register for the current data.

The aim of this trial is to investigate the long-term safety, tolerability, and
immunogenicity of efgartigimod administered intravenously (IV) in pediatric
participants rolling over from the ARGX-113-2006 trial and to ensure access to
the drug before commercial availability or until another option to access
efgartigimod is available.

ARGX-113-2008 is an open-label, multicenter, uncontrolled clinical trial in
prediatric participants with gMG who have completed the ARGX-113-2006 trial.
The trial consists of Treatment periods and Treatment-Free periods. A treatment
period lasts 4 weeks with a weekly visit and infusion with study medication.
The amount of Treatment periods per patient depends on the course of the
disease in that patient.
The Treatment Free period will last for at least 28 days and after that the
duration is depending on when a new treatment period is considered needed.
Patients are seen 1 week after the last dose, and as long as no retreatment is
required, patients are followed up by a phone call every 3 monts and seen at
the site every 6 months.
Participants will start with either a Treatment Period or Treatment Free period
based on their personal status as evaluated by the investigator and the number
of days since the last day of the last treatment.

The trial end when commercial access to the drug or until another option to
access efgartigimod is available.

Intravenous infusion with efgartigimod. This consists of 4 weekly infusions per
Treatment period.

In previous clinical studies with efgartigimod in adults, some people
experienced side effects. Some side effects could be related to efgartigimod.
Many of these side effects were mild, lasted a short time, and required little
or no treatment.
The most commonly reported side effects in studies where healthy adults
received efgartigimod or placebo were:
• Headache
• decreases in white blood cell counts
• increases in levels of a blood test marker for inflammation (C-reactive
protein)
• fatigue
• common cold
• mouth/throat discomfort
• feeling cold
• injection site redness
• injection site bruise
• back pain

Some of the side effects occurred in healthy participants who received the
placebo, and some occurred in healthy participants who
received efgartigimod at higher doses than the dose that will be used in this
study.

The most common reported side effects in other studies in adults with gMG and
thrombocytopenia (a blood problem) who received
efgartigimod were:
• headache
• common cold
• diarrhea
• upper respiratory tract infection
• nausea
• urinary tract infection
• muscle pain
• mouth/throat discomfort
Most of the side effects mentioned above were of mild to moderate intensity,
resolved quickly, and assessed as not related to
efgartigimod treatment. The most common side effects that were related to the
study drug included inflammation of the airway
passages, upper respiratory tract infection, urinary tract infection, headache,
and muscle pain.

The study will be executed in line with the Code of conduct for resisting
minors.