A First-in-human Study to Investigate Single Doses of DCY636 in Healthy Volunteers and Multiple Doses in Participants With Moderate to Severe Atopic Dermatitis

Key Inclusion Criteria: Healthy Participants (Part 1) • Healthy male and non-childbearing potential female participants 18 to 55 years of age inclusive. Participants with moderate to severe atopic dermatitis (Part 2) * Males and non-pregnant females age 18 years or older * Diagnosis of atopic dermatitis for at least 1 year not adequately controlled by topicals * Moderate to severe atopic dermatitis as defined by all of the following: * EASI score ≥12 at screening visit and ≥16 at baseline (BL) visit * IGA score ≥3 at screening visit and baseline visit * Total Body surface area (BSA) affected by AD ≥ 10 % at screening visit and baseline visit * Peak Pruritus NRS score ≥4 at baseline visit, based on weekly average of daily assessment in the week prior to baseline visit Key Exclusion Criteria: All Participants (Part 1, Part 2) * Use of other investigational drugs within the last 30 days or 5 half-lives of the other drugs prior to initial dosing, whichever is longer. * Meet any of the prohibited medication use criteria at baseline visit. * A positive syphilis test result during screening period. * Evidence of active or latent TB infection, as determined by T-Spot test during screening period. * History of immunodeficiency diseases, or a positive human immunodeficiency virus (HIV) test result. * Recent (within last half year) or ongoing helminth infection. * History of hepatitis B or hepatitis C or serologic evidence for viral hepatitis. A positive Hepatitis B virus surface antigen (HBsAg), Hepatitis B virus core antibody (HBcAb) and/or Hepatitis B surface antibody (HBsAb) test during screening period excludes a participant. A positive test for HBsAb can be included if the test for HBsAg and HBcAb are negative and the history of hepatitis B vaccination is known. Participants with a positive Hepatitis C virus (HCV) antibody test should be excluded. Healthy Participants (Part 1) * Women of childbearing potential * Smokers Participants with moderate to severe atopic dermatitis (Part 2) * Regular use (more than 2 visits per week) of a tanning booth/parlor or extended sun exposure (per investigator judgement) within 4 weeks prior to baseline visit * Have any chronic, uncontrolled medical condition, which would put the participant at increased risk during study participation, such as uncontrolled: diabetes, hypertension, morbid obesity, thyroid, adrenal, cardiovascular, pulmonary, hepatic, renal, neurologic or psychiatric disease, or other disease of concern, as per investigator judgment * Women of childbearing potential (WOCBP) are excluded unless they are using highly effective methods of contraception (failure rate \< 1% per year) while taking study treatment and for 202 days (= 5 times the terminal half-life) of study treatment after stopping study treatment. Other protocol-defined inclusion/exclusion criteria may apply.