• Female or male participants between the ages of 18 and 55 years inclusive (ages 18 and 45 years for Part C).

• BMI of 18 to 32 kg/m\^2; and a total body weight \> 50 kg (110 lbs).

• Evidence of a personally signed and dated informed consent document indicating that the participant has been informed of all pertinent aspects of the study.

• Participants who are willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.

• Nonsterile male participants must be willing to use a highly effective method of birth control and refrain from sperm donation for the duration of the study and for 90 days after the last dose of study drug.

• Female participants of childbearing potential can only join Part F and must be willing to use a highly effective method of birth control and refrain from egg donation for the duration of the study and for 90 days after the last dose of study drug. They must also have a negative urine pregnancy test at baseline before first dose of study drug.

• Female or male participants between the ages of 18 to 75 years of age.

• Female participants of childbearing potential must be willing to use a highly effective method of birth control and refrain from egg donation for the duration of the study and for 90 days after the last dose of study drug. They must also have a negative urine pregnancy test at baseline before first dose of study drug.

• AD Cohort E1:

‣ Chronic AD diagnosed by the Eichenfield revised criteria of Hannifin and Rajka that has been present for at least 1 year before the Screening Visit.

⁃ Prior to baseline assessment, participants with AD must have used only nonmedicated topical emollients twice daily for at least 7 days, without any active ingredients or additives that could impact AD treatment (such as hyaluronic acid, urea, ceramide, or filaggrin degradation products). Participant's response to treatment must have remained inadequate at baseline. Additionally, the participant must be willing and able to adhere to standardized background topical therapy as outlined in the protocol throughout the remainder of the study.

• PN Cohort E2:

‣ Diagnosed as PN by a dermatologist for at least 3 months before the Screening Visit with prurigo lesions on upper limbs with or without lesions on the trunk or lower limbs.

⁃ Minimum of 20 PN lesions in total on either of the following: both legs, both arms, and/or the trunk at the Screening Visit and on Day 1.

∙ 1\. Must agree to avoid prolonged exposure to the sun and not to use tanning booths, sun lamps, or other ultraviolet light sources during the study.