The goal of this clinical trial is to assess the safety, tolerability, and pharmacokinetics of ATTO-3712 in healthy adults and patients with atopic dermatitis.

The main questions it aims to answer are:

What medical problems do participants have when taking ATTO-3712? How long does ATTO-3712 stay in the body after dosing? Researchers will compare ATTO-3712 to a placebo (a look-alike substance that contains no drug).

Participants will be dosed with ATTO-3712 or a placebo, visit the clinic for checkups and tests, and keep a diary of their symptoms.

This is a 3-part study. Parts 1 and 2 will be a single and multiple ascending dose design, respectively, assessing the safety, tolerability, and PK of ATTO-3712 in healthy adult volunteers. Part 3 will consist of multiple doses in adult patients with atopic dermatitis to assess safety, tolerability, PK, and PD based on biomarkers in the blood.

Parts 1&2 (Healthy Volunteers) Key Inclusion Criteria:

• Any sex or gender who is 18 to 65 years old, inclusive, at Screening.
• Body weight of 50 to 125 kg and body mass index (BMI) between 18.5 and 35 kg/m2
• Considered in good general health based on medical history, physical exam, 12-lead ECG, screening clinical laboratory findings, and vital signs
• Negative pregnancy test for participants of child-bearing potential.

Part 3 (Participants with Atopic Dermatitis) Key Inclusion Criteria:

• Any sex or gender who is 18 to 65 years old
• Body weight of 50 to 125 kg and BMI between 18.5 and 40 kg/m2
• Clinically confirmed diagnosis of active AD
• History of inadequate response to treatment with topical medications
• Baseline weekly mean of daily PP-NRS ≥ 4 at Day 1.
• EASI score of ≥ 16 at Screening and Day 1
• vIGA-AD score of ≥ 3 at Screening and Day 1
• ≥10% of body surface area (BSA) affected by AD at Screening and Day 1
• Use of topical bland emollient (moisturizer) at least once daily for at least 5 of the 7 days immediately before Day 1 and agrees to continue using that same emollient at the same frequency throughout the study
• Negative pregnancy test for participants of childbearing potential

Parts 1 & 2 (Healthy Volunteers) Key Exclusion Criteria

• Any clinically significant underlying illness
• History of malignancy within 5 years of Screening
• History of major surgery within 8 weeks prior to Day 1 or has a major surgery planned during the study
• History of uncontrolled asthma requiring rescue treatment with a bronchodilator for an increase in symptoms more than twice per week
• History of hypersensitivity (including anaphylaxis) to a biologic medication, vaccine, an immunoglobulin product (plasma-derived or recombinant, eg, monoclonal antibody), or to any of the IP excipients (sucrose, polysorbate 80, or histidine)
• Active hepatitis B virus (HBV) or hepatitis C virus (HCV) or is positive for HIV
• Active or latent tuberculosis infection
• Smoking more than 20 cigarettes (or cigars, cigarillos, or e-cigarettes equivalent) per day
• History of drug or alcohol abuse
• Laboratory values outside of the normal range

Part 3 (Participants with Atopic Dermatitis) Key Exclusion Criteria:

• Any clinically significant underlying illness
• Presence of skin comorbidities that may interfere with study assessments
• Has taken prescription medication for the treatment of AD or other prohibited medication within the restricted time limits (defined in the protocol)
• Has applied topical corticosteroids within 2 weeks prior to dosing.
• History of malignancy within 5 years of Screening
• History of major surgery within 8 weeks prior to Day 1 or has a major surgery planned during the study
• History of uncontrolled asthma requiring rescue treatment with a bronchodilator for an increase in symptoms more than twice per week
• History of recurrent eczema herpeticum
• History of known primary immunodeficiency, is considered immunocompromised, history of untreated latent tuberculosis infection, has been treated for active tuberculosis in the past year, or has been treated for a parasitic infection in the past 6 months
• History of major depression
• History of hypersensitivity (including anaphylaxis) to a biologic medication, vaccine, an immunoglobulin product (plasma-derived or recombinant, eg, monoclonal antibody), or to any of the IP excipients (sucrose, polysorbate 80, or histidine)
• Participant has experienced significant flare(s) in AD in the 2 weeks prior to Screening or during the Screening period
• EASI score for the participant has more than doubled between Screening and Day 1
• Active HBV or HCV or is positive for HIV
• Participant is smoking more than 20 cigarettes (or cigars, cigarillos, or e-cigarettes equivalent to approximately 40 mg nicotine) per day
• Participant has an ECG with a QTcF > 450 msec for males or > 470 msec for females at Screening
• History of drug or alcohol abuse
• Laboratory values outside of the normal range