The primary objective of the study was to evaluate the safety and pharmacokinetic of 611 in Chinese children and adolescents with moderate to severe atopic dermatitis.

Inclusion Criteria:

• Subjects or their legal guardians must be able to understand and comply with the study procedures. and must participate voluntarily and sign the written informed consent.
• When signing the informed consent form (ICF), Part 1: 12 years old ≤ age < 18 years old, weight ≥ 30kg; Part 2: 6 years old ≤ age < 12 years old, weight ≥ 15kg.
• AD (according to Hanifin-Rajka Criteria) that had been present for at least 1 years before the screening visit.
• (Investigator's Global Assessment) IGA ≥ 3 at screening and baseline visits.
• (Eczema Area and Severity Index) EASI ≥16 at screening and baseline visits.
• Participants with >=10 percent (%) body surface area (BSA) of AD involvement at screening and baseline visits.
• Baseline peak pruritus Numerical Rating Scale (NRS) average score for maximum itch intensity ≥4.
• With documented recent history (within 1 year before the baseline visit) of inadequate response to topical AD medication(s) or for whom topical treatments is medically inadvisable.
• Willing apply a stable dose of topical emollient (moisturizer) twice daily for at least the 7 consecutive days immediately before the baseline visit and continue for the duration of the study.
• Potentially fertile subjects (such as women who have had their first period or men who have had sperm implantation) must use highly effective contraception throughout the study period and for at least 3 months after the last dose.
• Patient, either alone or with help of parents/legal guardians, as appropriate, must be able to understand and complete study-related questionnaires.

Exclusion Criteria:

• Presence of skin comorbidities that may interfere with study assessments.
• Presence of active endoparasitic infections; or suspected endoparasitic.
• Any history of vernal keratoconjunctivitis (VKC) and atopic keratoconjunctivitis (AKC).
• History of malignancy within 5 years before the baseline visit.
• Participants who had a severe infection.
• Known or suspected history of immunosuppression.
• Active tuberculosis, unless that was well documented that the participants had adequately treated.
• Any medical condition that, in the opinion of the investigator, is serious or unstable and may affect the subject's safety and/or prevent the subject from completing the study.
• Treatment with topical drugs within 2 weeks before baseline; or systemic corticosteroids or immunosuppressive drugs within 4 weeks or 5 half-lives (whichever is longer) before baseline.
• Have been vaccinated with live (attenuated) vaccine within 4 weeks before baseline.
• The lab abnormalities at screening or baseline and not suitable for inclusion in the study judged by investigator.
• Patients who have active Hepatitis B, Hepatitis C or HIV infections as determined by positive results at Screening.
• History of alcohol or drug abuse within 6 months before baseline.
• History of hypersensitivity to 611 or their excipients.
• Have been participated in other clinical trials and used any experimental drugs within 8 weeks before baseline.
• Planned major surgical procedure during the patient's participation in this study.
• Patient is female who is pregnant, breastfeeding, or planning to become pregnant or breastfeed during the study.
• Any reason which, in the opinion of investigator, would prevent the subject from participating in the study.