To present interim results of ADAPT JR (NCT04833894), a Phase 2/3 trial assessing pharmacokinetics, pharmacodynamics, safety, and activity of intravenous (IV) efgartigimod in participants with acetylcholine receptor antibody–positive (AChR-Ab+) juvenile generalized myasthenia gravis (gMG) to confirm an age-appropriate dose in this population.

Efgartigimod, a human immunoglobulin G1 antibody Fc fragment, blocks the neonatal Fc receptor. Phase 3 ADAPT/ADAPT+ trials demonstrated that efgartigimod IV is efficacious and well tolerated in adults with gMG. There is an unmet need for safe and effective treatments in juvenile gMG, which is rarer than adult-onset gMG (incidence: 1-5 cases vs 30 cases per 1,000,000 people per year).

Staggered enrollment in ADAPT JR began with an adolescent cohort (aged 12-17 years) and is continuing with a child cohort (aged 2-11 years). During dose-confirmatory Part A (8 weeks), participants received 1 efgartigimod infusion; pharmacokinetic, pharmacodynamic, and safety endpoints were monitored. During treatment response–confirmatory Part B (18 weeks), participants received 1 or 2 cycles of 4 once-weekly efgartigimod infusions; additional endpoints, including MG-ADL scores, were assessed.

In the adolescent cohort, 6 participants enrolled in Part A and continued to Part B, and 5 participants enrolled directly in Part B (N=11; median [range] age, 15.0 [12-17] years). Efgartigimod treatment resulted in IgG and AChR-Ab decreases similar to those observed in previous studies of adults with gMG. Safety profile and MG-ADL improvements were also similar to previous observations.

Pharmacodynamics, safety, and efficacy of efgartigimod in adolescent participants were similar to previous studies in adults. ADAPT JR child cohort enrollment is ongoing.