This is a multicenter, double-blind, Long-Term Extension (LTE) study to evaluate the long-term safety and efficacy of APG777 in patients with moderate-to-severe AD who have completed treatment in an APG777 Parent Study (NCT06395948).

The LTE study will consist of 3 periods: 1) Screening Visit will coincide with the last visit of the Maintenance Period in the Parent Study 2) Extended Treatment Period 3) Post-treatment Follow-up Period.

This study will be conducted in participants with atopic dermatitis (AD) who completed the Treatment Period in a prior APG777 study and who, in the opinion of the Investigator, would benefit from long-term treatment with APG777.

Inclusion Criteria:

• Participants who have completed the Treatment Period in a prior APG777 study and were, in the Investigator's opinion, compliant with the study protocol
• Participants who, in the Investigator's opinion, would benefit from long-term treatment with APG777
• Use the same non-prescription non-medicated emollient/moisturizer of their choice from the last day of the Parent Study and throughout the LTE study

Exclusion Criteria:

• Participants who have developed an AE while participating in the Parent Study, which, in the opinion of the Investigator or of the Medical Monitor, could indicate that continued treatment with APG777 may present an unreasonable risk for the patient
• Participants who terminated early from the Parent Study or permanently discontinued the study drug during the Parent Study
• Use of any of the prohibited medications from Screening Visit (Visit 1) of the LTE study
• Presence of dermatologic conditions and/or comorbidities that might confound the diagnosis of AD and/or interfere with study assessments

Note: Additional protocol defined Inclusion/Exclusion criteria may apply