This is a two-part study that will evaluate the safety and efficacy of APG777 in participants with moderate-to-severe Atopic Dermatitis (AD). Part A (Proof-of-concept) and Part B (Dose-regimen Finding) will evaluate the safety and efficacy of various induction and maintenance dose regimens of APG777 compared to placebo. The study duration for any individual participant will be up to 106 weeks which includes: screening, induction, maintenance, and post-treatment follow-up periods. Participants randomized in Part A are not permitted to participate in Part B.