This is a Phase I/II Study of MH004 in Healthy Adult Volunteers, participants with Mild to Moderate Atopic Dermatitis.

The study includes 2 parts: The first part (Phase I) is the first-in-human trial of MH004, consisting of a single ascending dose (SAD) study and a multiple ascending dose (MAD) study in healthy volunteers. The second part is a multi-center, randomized, double-Blind, vehicle-controlled phase II study to evaluate the safety, tolerability and PK of MH004 Ointment for AD participants, using defined concentrations of the topical cream based on the PK and safety data in healthy volunteers in Phase I.

Inclusion Criteria:

1. Phase I: Adults, age 18 - 70 years old, inclusive. Phase II: Adolescents aged ≥ 12 to 17 years, inclusive. Adults, age 18 - 70 years old, inclusive, at time of screening.

2. Phase II: clinical diagnosis of mild to moderate atopic dermatitis (AD) for at least 6 months prior to Day 1.

3. Phase II: IGA 2 to 3, affected BSA 3% to 20% (excluding scalp) at the baseline visit.

4. Participants who agree to discontinue all agents used to treat AD from screening through the final follow-up visit.

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Exclusion Criteria:

1. Unstable course of AD (spontaneously improving or rapidly deteriorating) as determined by the investigator in the 4 weeks prior to baseline.

2. Concurrent conditions and history of other diseases:

   • Immunocompromised.
   • Chronic or acute infection requiring treatment with systemic antibiotics, antivirals, antiparasitics, antiprotozoals, or antifungals within 2 weeks before Baseline.
   • Active acute bacterial, fungal, or viral skin infection within 1 week before Baseline.
   • Any other concomitant skin disorder, pigmentation, or extensive scarring that, in the opinion of the investigator, may interfere with the evaluation of AD lesions or compromise participant safety.
   • Presence of AD lesions only on the hands or feet without prior history of involvement of other classical areas of involvement such as the face or the folds.
   • Other types of eczema.

3. Clinically significant cardiac disease; new cerebral infarction within 6 months from dosing; malignancies within 5 years from dosing; low hemoglobin; severe renal disease on dialysis; liver disease.

4. Systemic corticosteroids treated within 4 weeks before dosing; immunizations or sedating antihistamines treated within 4 weeks before dosing; other topical treatments for AD within 1 week before dosing.

5. Previously received Janus kinase (JAK) inhibitors, systemic or topical.

6. Ultraviolet light therapy or prolonged exposure to UV radiation within 2 weeks before dosing.

7. Liver function tests: AST or ALT ≥ 2 × ULN; alkaline phosphatase and/or bilirubin > 1.5 × ULN.

8. Pregnant or lactating participants, or those considering pregnancy.

9. Alcohol or drug abuse. -

Double-blind