<h3>Objective:</h3> To confirm dose selection and rationale of using the reduction of total immunoglobulin G (IgG) levels as a pharmacodynamic (PD) marker of noninferiority for subcutaneous (SC) efgartigimod PH20 (coformulated with recombinant human hyaluronidase PH20). <h3>Background:</h3> Efgartigimod intravenous (IV) was evaluated in the phase 3 ADAPT study, where it demonstrated clinical efficacy and was well tolerated in patients with generalized myasthenia gravis (gMG). Based on the observed association between reductions in total IgG levels (including pathogenic autoantibodies) and improvement in MG-ADL in the phase 2 study, a population PK/PD approach was utilized for dose selection of efgartigimod PH20 SC, that would result in a similar PD effect as efgartigimod IV (10 mg/kg). <h3>Design/Methods:</h3> PK and PD data from a phase 1 study where healthy participants (n=32) received a single injection of efgartigimod PH20 SC at various fixed doses or 10 mg/kg, were used for the PK/PD analysis. Based on the model, simulations were performed for a typical participant of 70 kg for a dose range of 750–1750 mg (25 mg increments). The selected dose was subsequently evaluated in healthy participants and patients with gMG (ADAPT-SC study), as treatment cycles of 4 weekly injections. <h3>Results:</h3> The 1000 mg dose of efgartigimod PH20 SC was predicted to result in comparable reduction in total IgG on day 29, one week after the fourth injection, and was selected for evaluation in the ADAPT-SC study. Patients ranging in weight from 42.0 to 150.2 kg (median=78.3) received one treatment cycle of efgartigimod PH20 SC 1000 mg and achieved similar total IgG reductions as those treated with efgartigimod IV. These reductions were also associated with improvements in MG-ADL clinical outcome measure. <h3>Conclusions:</h3> ADAPT-SC study demonstrated that the dose selection was appropriate as treatment with efgartigimod PH20 SC 1000 mg resulted in noninferior reduction in total IgG to efgartigimod IV at day 29. <b>Disclosure:</b> Dr. Li has nothing to disclose. Dr. Li has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Argenx. Dr. Li has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Catalyst. Dr. Li has received personal compensation in the range of $500-$4,999 for serving as a Consultant for UCB pharma. Dr. Li has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Immunovant. The institution of Dr. Li has received research support from Argenx. Dr. Vu has received personal compensation in the range of $500-$4,999 for serving as a Consultant for BPL. Dr. Vu has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Takeda. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for ARGENX. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion. Dr. Vu has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for UCB. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for Alexion. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for CSL Behring. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for AbbVie. Dr. Vu has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for Argenx. Dr. Vu has received personal compensation in the range of $5,000-$9,999 for serving on a Speakers Bureau for MedScape. The institution of Dr. Vu has received research support from CSL Behring. The institution of Dr. Vu has received research support from Alexion. The institution of Dr. Vu has received research support from RA Pharma/UCB. The institution of Dr. Vu has received research support from Mitsubishi Tanaka. The institution of Dr. Vu has received research support from Mass Gen Hospital/Healy Platform Study. The institution of Dr. Vu has received research support from Amylyx Pharma. The institution of Dr. Vu has received research support from ARGENX. The institution of Dr. Vu has received research support from Alector. The institution of Dr. Vu has received research support from Annexon. The institution of Dr. Vu has received research support from Apellis. The institution of Dr. Vu has received research support from Cytokinetics. The institution of Dr. Vu has received research support from Sanofi. The institution of Dr. Vu has received research support from Horizon. The institution of Dr. Vu has received research support from Woolsey Pharma. The institution of Dr. Vu has received research support from CSL Behring. The institution of Dr. Vu has received research support from Cartesian Therapeutics. The institution of Dr. Vu has received research support from Janssen/Momenta. Dr. Korobko has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Roche. Dr. Korobko has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Novartis. Dr. Korobko has received personal compensation in the range of $0-$499 for serving as a Consultant for Sanofi. Dr. Korobko has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Merck. Dr. Korobko has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Janssen (Johnson & Johnson company). Dr. Korobko has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck. Dr. Korobko has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Novartis. Dr. Korobko has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Janssen (Johnson & Johnson company). Dr. Smilowski has nothing to disclose. Dr. Liu has received personal compensation for serving as an employee of Argenx . Mrs. Steeland has nothing to disclose. Dr. Van Hoorick has received personal compensation for serving as an employee of argenx. Dr. Podhorna has received personal compensation for serving as an employee of argenx. Dr. Podhorna has stock in argenx. Jenna Casey has received personal compensation for serving as an employee of argenx. Mr. Noukens has nothing to disclose. Mrs. Van Bragt has nothing to disclose. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for argenx. Dr. Utsugisawa has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for UCB pharma. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Janssen pharma. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Viela Bio. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Chugai. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Mitsubisi Tanabe pharma. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck. Dr. Utsugisawa has received personal compensation in the range of $5,000-$9,999 for serving on a Speakers Bureau for argenx. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for Alexion phama. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for Japan Blood Products Organization. Dr. Utsugisawa has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for UCB. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Biogen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Bristol Myers Squibb. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for EMD Serono. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Novartis. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Roche. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Sanofi. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Immunovant. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Johnson&Johnson / Janssen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for UCB. Dr. Wiendl has received personal compensation in the range of $0-$499 for serving as a Consultant for Idorsia. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Actelion. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving as a Consultant for Argenx. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Gossamer Bio. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Immunic. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for Merck. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for NexGen. Dr. Wiendl has received personal compensation in the range of $5,000-$9,999 for serving as a Consultant for PSI CRO. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Worldwide Clinical Trials. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for BD. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Lundbeck. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Merck Serono. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Novartis. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Janssen. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion. Dr. Wiendl has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Argenx. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Bristol Myers Squibb. Dr. Wiendl has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sandoz. The institution of Dr. Wiendl has received research support from Biogen. The institution of Dr. Wiendl has received research support from Roche. The institution of Dr. Wiendl has received research support from Deutsche Forschungsgemeinschaft. The institution of Dr. Wiendl has received research support from European Union. The institution of Dr. Wiendl has received research support from Alexion. The institution of Dr. Wiendl has received research support from Argenx. The institution of Dr. Wiendl has received research support from CSL Behring. The institution of Dr. Wiendl has received research support from Merck. The institution of Dr. Wiendl has received research support from UCB. Dr. Wiendl has received publishing royalties from a publication relating to health care. Dr. Wiendl has received publishing royalties from a publication relating to health care. Dr. De Bleecker has received personal compensation in the range of $500-$4,999 for serving as a Consultant for ArgNX. Dr. De Bleecker has received personal compensation in the range of $0-$499 for serving as a Consultant for Biogen. Dr. De Bleecker has received personal compensation in the range of $0-$499 for serving on a Scientific Advisory or Data Safety Monitoring board for Sanofi. Dr. De Bleecker has received personal compensation in the range of $0-$499 for serving on a Scientific Advisory or Data Safety Monitoring board for Biogen. Dr. De Bleecker has received personal compensation in the range of $0-$499 for serving on a Scientific Advisory or Data Safety Monitoring board for CSL Behring. Dr. Mantegazza has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion. Dr. Mantegazza has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for UCB. Dr. Mantegazza has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for argenx. Dr. Mantegazza has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sanofi. Dr. Mantegazza has received personal compensation in the range of $500-$4,999 for serving on a Speakers Bureau for SERB. The institution of Dr. Mantegazza has received research support from Alpine Immune Sciences, Inc. Dr. Howard has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for arenx BVBA. Dr. Howard has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Regeneron Pharmaceuticals. Dr. Howard has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Alexion Pharmaceuticals. Dr. Howard has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Immunivant. Dr. Howard has received personal compensation in the range of $10,000-$49,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Ra Pharma (now UCB Biosciences). Dr. Howard has received personal compensation in the range of $5,000-$9,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Viela Bio (Horizon Therapeutics). Dr. Howard has received personal compensation in the range of $500-$4,999 for serving on a Scientific Advisory or Data Safety Monitoring board for Sanofi US. Dr. Howard has received personal compensation in the range of $10,000-$49,999 for serving on a Speakers Bureau for Academic CME. Dr. Howard has stock in General Electric dividends. Dr. Howard has stock in Johnson & Johnson dividends. Dr. Howard has stock in Pfizer dividends. An immediate family member of Dr. Howard has stock in GlaxoSmithKline dividends. The institution of Dr. Howard has received research support from Alexion Pharmaceuticals. The institution of Dr. Howard has received research support from argenx BVBA. The institution of Dr. Howard has received research support from Ra Pharma (now UCB Biosciences). The institution of Dr. Howard has received research support from NIH. The institution of Dr. Howard has received research support from Centers for Disease Control/Research Triangle Institute. The institution of Dr. Howard has received research support from Duke University (DCRI). The institution of Dr. Howard has received research support from Millenium Pharmaceuticals. The institution of Dr. Howard has received research support from Cartestian Therapeutics. Dr. Howard has a non-compensated relationship as a Scientific Advisiory Board member, Committee member with Myasthenia Gravis Foundation of America that is relevant to AAN interests or activities. Dr. Howard has a non-compensated relationship as a Committee member with American Assoc Neuromuscular and Electrodiagnostic Medicine that is relevant to AAN interests or activities. in collaboration with the ADAPT Investigator Study Group has nothing to disclose.