Introduction In ADAPT, efgartigimod, a human IgG1 antibody Fc-fragment blocking neonatal Fc receptor, resulted in clinically meaningful improvement (CMI) in myasthenia gravis (MG)-specific measures. Patients completing ADAPT were eligible to enrol in ADAPT+ (open-label, 3-year extension study).

Methods Efgartigimod, 10 mg/kg intravenous infusion administered once-weekly for 4 weeks; subsequent cycles initiated based on predefined criteria. Myasthenia Gravis Activities of Daily Living (MG-ADL) and Quantitative MG (QMG) scales were completed every cycle.

Results From ADAPT 91% patients (151/167) entered ADAPT+. As of February 2021, 106 AChR-Ab+ and 33 AChR-Ab– had received ≥1 dose of efgartigimod (including 66 ADAPT placebo patients). Mean(SD) study duration: 363(114) days. The most common adverse events (AEs) in the efgartigimod-efgartigimod and placebo-efgartigimod arms: headache (15.1%/30.3%), nasopharyngitis (8.2%/13.6%), diarrhoea (6.8%/10.6%). Five deaths occurred, none considered efgartigimod-related by investigators. AEs were predominantly mild or moderate. CMI was observed in AChR-Ab+ patients during each cycle (up to 10 cycles), comparable to improvements at week 3, cycle 1 (mean[SE]: MG-ADL, –5.1[0.34]; QMG, –4.7[0.41]). Clinical improvements mirrored maximal reductions in total IgG and AChR-Abs across all cycles. Similar results observed in AChR-Ab– patients.

Conclusions This suggests long-term efgartigimod was well-tolerated and efficacious, regardless of antibody status. No new safety signals were identified.