This phase 2b study is designed to evaluate the safety and efficacy of KT-621 in adult and adolescent participants with moderate-to-severe atopic dermatitis (AD), a common form of eczema. The main goals of this study are to learn how effective KT-621 is at reducing the severity and extent of AD, the safety and tolerability of KT-621, how KT-621 behaves in the body, and how the body responds to KT-621. This is a 16-week double-blind, placebo-controlled study with a 52-week open-label period.