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

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.

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