A Study of EDP1815 in Healthy Participants and Participants With Mild to Moderate Psoriasis and Atopic Dermatitis

Inclusion Criteria: General: Participant has a body mass index of ≥ 18 kg/m2 to ≤ 35 kg/m2 at Screening. Healthy Volunteers: Participants who are overtly healthy as determined by medical evaluation including medical history, physical examination, laboratory tests, and cardiac monitoring. Mild to moderate psoriasis: 1. Participant has had a confirmed diagnosis of mild to moderate plaque-type psoriasis for at least 6 months involving ≤ 10% of body surface area (BSA) (excluding the scalp). 2. Participant has a minimum of 2 psoriatic lesions with at least 1 plaque in a site suitable for biopsy. Mild to moderate atopic dermatitis: 1. Mild to moderate atopic dermatitis with a minimum of 3% to a maximum of 15% BSA involvement. 2. Participant has had a confirmed diagnosis of mild to moderate atopic dermatitis for at least 6 months (IGA score of 2 or 3). 3. Participant has a minimum of 2 atopic dermatitis lesions with at least 1 in a site suitable for biopsy. Exclusion Criteria: 1. Female participant who is pregnant, or plans to become pregnant during the study, or breastfeeding, or sexually active with childbearing potential who is not using a medically accepted birth control method. 2. Participant has received live attenuated vaccination within 6 weeks prior to Screening or intends to have such a vaccination during the course of the study. 3. Participant has received any investigational drug or experimental procedure within 90 days or 5 half-lives, whichever is longer, prior to study intervention administration. 4. Participant requires treatment with an anti-inflammatory drug during the study period. Paracetamol will be permitted for use as an antipyretic and/or analgesic (maximum of 4 grams/day in any 24 hour period). 5. Participant has an active infection (e.g. sepsis, pneumonia, abscess) or has had an infection requiring antibiotic treatment within 6 weeks prior to Investigational Medicinal Product (IMP) administration. When in doubt, the investigator should confer with the Sponsor study physician. 6. Participant has renal or liver impairment, defined as: 1. For healthy volunteers: i. For women, serum creatinine level ≥ 125 μmol/L; for men, ≥ 135 μmol/L, or ii. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≥ 1.5 x upper limit of normal (ULN), or iii. Alkaline phosphatase (ALP) and/or bilirubin \> 1.5 x ULN 2. For participants with mild to moderate atopic dermatitis or psoriasis: i. For women, serum creatinine level ≥ 125 μmol/L; for men, ≥ 135 μmol/L, or ii. ALT or AST \> 2 x ULN and/or bilirubin \> 1.5 x ULN