Inclusion Criteria for the Extension Cohort:
1\. Participants who have completed the treatment phase of the qualifying parent study (age 2 to <12 years old).
• No contraception methods are required for male participants. Female participants must not be pregnant or breastfeeding and, if the participant is of child-bearing potential, must use a highly effective form of contraception (i.e., abstinence) during the study intervention period and for at least 28 days after the last dose of study intervention.
Inclusion Criteria for the De Novo Cohort:
Age
1. Children aged 6 to <12 years at the time of informed consent/assent.
• No contraception methods are required for male participants.
Disease Characteristics:
2. Participants who meet all of the following AD criteria:
- A documented diagnosis of chronic AD for at least 6 months prior to screening and confirmed at screening and baseline visits according to the Hanifin and Rajka criteria; and
- A diagnosis of moderate-to-severe AD at the baseline visit (must fulfill all of the following criteria: BSA ≥10%, vIGA ≥3, EASI ≥16, and WI-NRS ≥4); and
- Documented history (within 6 months of the screening visit) of inadequate response to treatment with topical medical therapy for AD (eg, TCS and TCI), for at least 4 weeks and are candidates for systemic therapy.
Other Inclusion Criteria:
3. Body weight ≥15 kg
Exclusion Criteria for the Extension Cohort:
Medical Conditions:
1. Any medical or psychiatric condition including any active suicidal ideation in the past year or suicidal behavior in the past 5 years or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
If the participant has SDQ total score ≥17, the investigator should exclude the child or refer them to a pediatric MHP to determine if it is safe to participate in the study. A copy or summary of the evaluation should be placed in the site source documents.
Prior/Concomitant Therapy:
2. Required use of any prohibited concomitant treatments outlined in Section 6.9.3 and Appendix 9 of study protocol.
3. Required vaccination with live attenuated vaccines during study treatment and for 6 weeks after discontinuing study treatment.
Diagnostic Assessments:
4. Ongoing adverse event in the parent studies which in the opinion of the investigator, or sponsor, is an ongoing safety concern OR the participant is currently triggering safety monitoring criteria.
5. Discontinued from treatment early in the parent studies OR triggered a discontinuation criterion at any point during the parent studies OR meets exclusion criteria from the parent studies which in the opinion of the investigator, or sponsor, is an ongoing safety concern.
Exclusion Criteria for the De Novo Cohort
Medical Conditions:
1. Any medical or psychiatric condition including any active suicidal ideation in the past year or suicidal behavior in the past 5 years or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.
If the participant has SDQ total score ≥17, the investigator should exclude them or refer the child to a pediatric MHP to determine if it is safe to participate in the study. A copy or summary of the evaluation should be placed in the site source documents.
2. Have any of the following medical conditions:
- Skin infections that require treatment with systemic antimicrobials within 2 weeks prior to Day 1 (baseline) or have superficial skin infections within 1 week of Day 1.
- History of systemic infection requiring hospitalization or parenteral antimicrobial therapy or as otherwise judged clinically significant by the investigator within 1 month prior to Day 1.
- Have a history (single episode) of disseminated herpes zoster or disseminated herpes simplex, or a recurrent localized, dermatomal herpes zoster.
- Infection with HIV, hepatitis B, and/or hepatitis C
- Evidence of active TB or inadequately treated latent TB.
- Skin Conditions:
\- Including but not limited to psoriasis, seborrheic dermatitis or lupus on Day 1 that would interfere with evaluation of AD or response to treatment.
- Documented history of skeletal dysplasia.
- Documented history of retinal detachment.
- History of or conditions associated with thrombocytopenia, coagulopathy or platelet dysfunction.
- Prior history of leukemia, lymphoma, sarcoma or any other malignancy.
- Immunodeficiency disorder or a first-degree relative with a hereditary immunodeficiency.
- Any other medical conditions that in the investigator's judgment make the participant inappropriate for the study.
Prior/Concomitant Therapy:
3. Prior treatment with a systemic JAK inhibitor for AD.
4. Live attenuated vaccination within 6 weeks prior to Day 1 or require vaccination with live attenuated vaccines during treatment or within 6 weeks after the last dose of study intervention.
5. Concomitant use of strong inhibitors and inducers of CYP2C19 enzymes and strong inducers of CYP2C9 enzymes is not allowed in the study.
Prior/Concurrent Clinical Study Experience:
6. Previous administration of an investigational drug within 30 days or 5 half lives, whichever is longer, of Day 1.
Diagnostic Assessments:
7. Hepatic and/or renal and/or hematological abnormalities defined as:
- AST >2 x ULN
- Hemoglobin <10 g/dL
- ALT >2 x ULN
- ANC <1000/mm3
- Total bilirubin ≥1.5 x ULN
- ALC <500/mm3
- eGFR <60 mL/min/1.73 m2
- Platelets <150,000 /mm3
Other Exclusion Criteria:
8. Investigator site staff directly involved in the conduct of the study and their family members, site staff otherwise supervised by the investigator, and sponsor and sponsor delegate employees directly involved in the conduct of the study and their family members.