Phase 1a/1b Study of TRB-061 in Healthy Participants & Atopic Dermatitis Patients

Inclusion Criteria: 1. Male or female participants aged 18 to 70 years, inclusive, at the time of informed consent. 2. Body weight ≥50 kg and body mass index (BMI) between 18.5 and 35.0 kg/m², inclusive. 3. Participant is in good general health as determined by the investigator, based on medical history, physical examination, and clinical laboratory tests. 4. For healthy participants (Parts 1 and 2): no clinically significant abnormalities in laboratory results, ECGs, or physical exams at screening. 5. For participants in Part 3: Confirmed diagnosis of AD with onset of symptoms at least 1 year prior to Screening. 6. Must be nonpregnant and nonlactating with negative pregnancy tests at screening and prior to dosing. 7. Must be a non-smoker or ≤5 cigarettes per week for the past 6 months (SAD/MAD only). 8. Moderate-to-severe AD at Screening and at Day 1 visit as defined by: 1. EASI score ≥16, 2. BSA affected ≥10%, 3. vIGA-AD score ≥3, and 4. Pruritus NRS score ≥3. Exclusion Criteria: 1. History of any clinically significant disease or disorder which, in the opinion of the investigator, may put the participant at risk or interfere with study results. 2. History or presence of any condition requiring systemic immunosuppressive or immunomodulatory therapy within a defined period before screening. 3. Use of any biologic agent (e.g., monoclonal antibodies) within 3 months or 5 half-lives (whichever is longer) before Day 1. 4. Participation in another investigational drug trial within 30 days or 5 half-lives of the investigational product (whichever is longer) before Day 1. 5. History of hypersensitivity or allergic reaction to any component of the study drug or placebo formulation. 6. Known active or latent tuberculosis (TB) infection or history of incomplete TB treatment. 7. Positive test at screening for hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) RNA, or human immunodeficiency virus (HIV). 8. Active infection or history of serious infections within 4 weeks prior to Day 1. 9. History of malignancy (except treated and cured non-melanoma skin cancers or cervical carcinoma in situ). 10. Pregnant or breastfeeding women, or women planning to become pregnant during the study or within a specified time after the last dose. 11. Clinically significant ECG abnormalities (e.g., QTcF \>470 ms) or other cardiac risk factors. 12. Abnormal laboratory values at screening. 13. Use of live vaccines within 4 weeks before Day 1. 14. Use of systemic corticosteroids, immunosuppressants, or immunomodulatory drugs within protocol-defined washout periods (Part 3 Phase 1b only). 15. Active COVID-19 infection or symptoms, or positive COVID-19 test at screening. 16. History of alcohol or drug abuse within the past year. 17. Use of tobacco/nicotine products beyond protocol-allowed limits. 18. Positive cotinine test at check-in (SAD/MAD only). 19. Any other reason, in the opinion of the investigator or sponsor, that would make the participant unsuitable for participation in the study. 20. Any medical or psychiatric condition that, in the opinion of the Investigator or Sponsor's medical monitor, would place the participant at risk, interfere with study participation, or interfere with the interpretation of study results. 21. Surgery within the past 90 days prior to dosing as determined by the Investigator or Sponsor's medical monitor to be clinically relevant or planned surgery to be performed during the study and 30 days after the last dose of study drug.