A Phase 1/1b Study to Evaluate Safety, Tolerability and Pharmacokinetics of ZL-1503 in Healthy Volunteers and Participants With Moderate to Severe Atopic Dermatitis

Inclusion Criteria: * Part A: 1. Healthy male and female volunteers, 18-65 years of age 2. Body mass index (BMI) between ≥ 18.5 and \< 32.5 kg/m2 3. Negative pregnancy tests for women of childbearing potential. * Part B: 1. 18-65 years of age; 2. BMI between ≥18.5 and \<40.0 kg/m2 3. Have a diagnosis of AD at least 12 months prior to Day 1; 4. Moderate-to-severe AD at Screening and Baseline visit, defined as: 1. Eczema Area and Severity Index (EASI) score ≥ 16; 2. Affected Body Surface Area (BSA)≥ 10%; 3. vIGA-AD™ score ≥ 3 5. History of an inadequate response to treatment with topical medications 6. Average peak pruritus numeric rating scale (PP-NRS) score ≥4 in the 7 days before randomization. 7. Negative pregnancy tests for women of childbearing potential. Exclusion Criteria: * Part A and B: 1. Significant health issues, such as positive tests for human immunodeficiency virus (HIV), hepatitis C virus (HCV), hepatitis B surface antigen (HBsAg), active tuberculosis, immunodeficiencies or autoimmune diseases. 2. History of major metabolic, liver, kidney, hematologic or other significant disorders. 3. Abnormal Electrocardiogram (ECG) findings 4. Clinically relevant abnormal lab results, including low blood counts, or abnormal liver and kidney function. 5. History of drug abuse or addiction within 6 months prior to screening 6. Current smoker or use of any nicotine or tobacco containing products within the last 6 months prior to dosing. 7. Donated \>500mL blood within 2 months of dosing. * For Part B only: 1. Presence of dermatologic conditions and/or comorbidities that might confound the diagnosis of AD and/or might interfere with study assessments. 2. Uncontrolled chronic disease that might require bursts of oral corticosteroids. 3. Any other sound medical, psychiatric, and/or social reason as determined by the investigator.