Safety, Tolerability and PK of ATTO-3712 in Healthy Volunteers and Patients With Atopic Dermatitis

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