First-in-Human Trial to Assess REGN20423 in Healthy Adult Participants and Adult Participants With Atopic Dermatitis

Part A: Enrolls healthy participants Part B: Enrolls participants with atopic dermatitis Key Inclusion Criteria: Part A: 1. Is judged by the investigator to be in good health based on medical history, physical examination, vital signs, Electrocardiograms (ECGs), and laboratory safety testing at screening and prior to initial dose of study intervention 2. Has a BMI within 18 to 31 kg/m² (inclusive) at the screening visit Part B: 3. Has a history of AD for at least 6 months at screening 4. Has EASI score ≥5 at both the screening and baseline visits 5. Has IGA score ≥2 at both the screening and baseline visits Key Exclusion Criteria Part A: 1. Hospitalized (\>24 hours) for any reason within 30 days prior to the screening visit 2. Hypersensitivity to the study treatment or any of its excipients 3. History of clinical parasite infection, except treated trichomoniasis 4. Received a live attenuated vaccine within 1 month prior to the first screening visit or anticipates need for a live attenuated vaccine during the study 5. History of alcohol or drug abuse as determined by the investigator Part B: 6. Known or suspected history of immunosuppression 7. Presence of skin comorbidities at screening that may interfere with study assessments 8. Inability to discontinue medications and treatments prior to baseline and during the study per the Excluded Medications and Treatments list 9. Uncontrolled chronic disease that might require bursts of oral corticosteroids during the study Note: Other protocol defined Inclusion/ Exclusion criteria apply