Inclusion Criteria:
* Part 1 only: Adult participants between 18 to 55 years of age, inclusive, at the time of signing the ICD
* Part 2 only: Adult participants, who at the time of screening, are between the ages of 18 and 70 years, inclusive.
* Part 1 only: Participants who are overtly healthy as determined by medical evaluation including medical history, physical examination, vital sign assessments, temperature, 12-lead ECGs, laboratory tests
* BMI of 17.5 to 40 kg/m2; and a total body weight \>50 kg (110 lbs)
* Part 2 only: Must meet the following AD criteria:
1. Have a clinical diagnosis of chronic AD (also known as atopic eczema) for at least 1 year prior to Day 1 and have the diagnosis of AD confirmed by photographs (at screening) and diagnostic criteria for AD.
2. Either an inadequate response to treatment with standard of care treatments (excluding systemic immunosuppressant treatments) consistent with AD treatment guidelines (for at least 4 consecutive weeks within 6 to 12 months (depending on time since initial diagnosis) of the first dose of the study intervention. OR Have a documented reason why topical treatments are considered medically inappropriate within the last year.
3. Have moderate to severe AD (defined as having an affected BSA (captured as part of EASI) ≥10%, IGA ≥3, and EASI ≥12 at both the screening and baseline visits).
4. Have an otherwise healthy medical evaluation (other than signs and symptoms of AD) including medical history, physical examination, vital sign assessments, temperature, 12-lead ECGs, laboratory tests.
Controlled comorbid diseases are acceptable so long as they do not require administration of prohibited medications. This includes participants with mild or moderate asthma that is well-controlled (not requiring high dose inhaled corticosteroids, systemic \[oral or parenteral\] corticosteroids, or biologic asthma treatments).
Exclusion Criteria:
* Have a history of systemic infection requiring hospitalization and parenteral antimicrobial therapy, any lymphoproliferative disorder, malignancies.
* Evidence or history of clinically significant hematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurological, immunological/rheumatological disorder.
* Have undergone significant trauma or major surgery within 1 month of the first dose of study intervention.
* Evidence of active, latent, or inadequately treated infection with Mycobacterium tuberculosis (TB) as defined by both of the following:
1. A positive QuantiFERON-TB Gold In-tube or equivalent test.
2. History of either untreated or inadequately treated latent or active TB infection, or current treatment for the same.
Part 2 Only
* Currently have active forms of other inflammatory skin diseases
* Have history of or current evidence of skin conditions at the time of Day 1 that would interfere with evaluation of atopic dermatitis or response to treatment. Have active chronic or acute skin infection requiring treatment with systemic antibiotics, antivirals, antiparasitics, antiprotozoals, or antifungals within 2 weeks prior to Day 1, or superficial skin infections within 1 week prior to Day 1.
* Score of ≥ 5 on the Fitzpatrick Skin Type Assessment.
* History of anaphylaxis with the following exceptions: participants with sensitivity and/or anaphylaxis only to a single, avoidable allergen (eg, aspirin, penicillin, sulfa drugs, nonsteroidal anti-inflammatory drugs \[NSAIDs\], peanuts) may be enrolled, if in the opinion of the investigator, the participant is aware of the hypersensitivity and avoids the problematic allergen. Participants must carry appropriate treatment for anaphylaxis and must know how to manage anaphylactic reactions.
* Any investigational or experimental therapy taken or procedure performed for AD, psoriasis, psoriatic arthritis, rheumatoid arthritis or other inflammatory diseases in the previous 1 year should be discussed with the Pfizer Medical Monitor (or designee).
