The goal of this interventional study is to learn if TRIV-509 works to treat moderate to severe atopic dermatitis in adults. It will also evaluate the safety of TRIV-509. Participants will receive 4 doses of the study intervention that they are randomized to (TRIV-509 or placebo), and at Week 16 will cross over and receive 4 doses of the other study intervention. The study duration for individual participants is up to 57 weeks, including a Screening period of up to 30 days.