Inclusion Criteria:  Adult, age 18 years or older at screening. Diagnosis of chronic atopic dermatitis (AD). History of AD ≥1 year prior to baseline. Recent (within 6 months prior to baseline) documented history of inadequate response to topical AD treatments or subject for whom topical AD treatments are medically inadvisable. 7.1≤ Eczema Area and Severity Index (EASI) ≤50 at baseline. Validated Investigator Global Assessment Scale for Atopic Dermatitis (vIGA-AD) score ≥3 at baseline.  Exclusion Criteria:  Previous treatment with an oral histidine 4 receptor (H4R) antagonist (including LEO 152020) within 6 months prior to baseline. Previous treatment with 3 or more systemic AD treatments prior to screening. Women who are pregnant, intend to become pregnant, or are lactating.