This study will test a study medicine called REGN20423 to see how effective it is in healthy participants (in Part A) and participants with atopic dermatitis (eczema) (in Part B). REGN20423 is a study medicine that is only used in clinical studies. REGN20423 has not previously been studied in humans.

The study is looking at:

• What side effects REGN20423 might cause
• How much REGN20423 is in the blood at different times
• How well REGN20423 works in adults with atopic dermatitis
• Whether the body makes antibodies against REGN20423
• How the body changes after having REGN20423, which can help researchers understand why REGN20423 works better in some people than others
• What the best dose of REGN20423 is

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

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf