A 3-part Study of SYX-5219 in Healthy Volunteers and Participants With Atopic Dermatitis

Inclusion Criteria: Parts 1 \& 2 * Healthy male and female participant, between ≥ 18 to ≤ 65 years of age, inclusive, with a BMI of body mass index (BMI) of 18-32 kg/m2. * Female participant of non-childbearing potential or female of childbearing potential that is sexually abstinent. * No clinically significant abnormalities in laboratory, vital signs or ECG measurements. Part 3 * Male and female participants with clinically confirmed diagnosis of active AD, between ≥ 18 to ≤ 65 years of age, inclusive, with a BMI of body mass index (BMI) of ≤40 kg/m2. * Meet minimum AD entry criteria; * AD covering ≥10% of the body surface area (BSA) at screening and baseline. * Eczema Area and Severity Index (EASI) score ≥16 at screening and baseline. * Validated Investigator's Global Assessment (vIGA) score of ≥ 3 (moderate) at screening and baseline. * Peak Pruritus NRS score of ≥ 4 at screening and baseline. Exclusion Criteria: Parts 1 \& 2 • Use of prescription or non-prescription drugs, including vitamins, herbal and dietary supplements within 35 days or 5 half-lives (whichever is longer) prior to the first dose of IMP. Part 3 * Any clinically significant medical condition or physical/laboratory/ECG/vital signs abnormality that would, in the opinion of the Investigator, put the participant at undue risk. * Has medical history as stated in the main study exclusion criteria. * Received treatment(s) as stated in the main study exclusion criteria.