An Evaluation of LEO 138559 in Adults With Moderate to Severe Atopic Dermatitis.

Inclusion Criteria: * 18-64 years old (both included) at screening. * Diagnosis of atopic dermatitis (AD) (as defined by the American Academy of Dermatology Consensus Criteria) that has been present for ≥1 year prior to screening. * Subjects who have a recent history (within 6 months before screening) of inadequate response to treatment with topical medication, or for whom topical treatments are otherwise medically inadvisable. * Eczema Area and Severity Index (EASI) score ≥12 at screening and ≥16 at baseline. * Validated Investigator Global Assessment Scale for Atopic Dermatitis (vIGA-AD) score ≥3 at screening and baseline. * Body surface area (BSA) of AD involvement ≥10% at screening and baseline. * Worst Daily Pruritus Numeric Rating Scale (NRS, weekly average) of ≥3 points at baseline. Exclusion Criteria: * Treatment with systemic immunosuppressive/immunomodulating medication, immunoglobulin/blood products, or phototherapy within 4 weeks or 5 half-lives prior to randomization, whichever is longer. * Treatment with biologics within 5 half-lives (if known) or 16 weeks prior to randomization, whichever is longer. * Treatment with topical corticosteroid(s) (TCS), topical calcineurin inhibitor(s) (TCI), topical phosphodiesterase-4 (PDE-4) inhibitor, or other topical prescription treatments within 1 week prior to randomization. * Treatment with a live (attenuated) vaccine within 12 weeks prior to randomization. * Clinically significant active chronic or acute infection requiring systemic treatment within 4 weeks prior to randomization that may compromise the safety of the subject. * Skin infection within 1 week prior to the baseline visit. * Presence of hepatitis B or C infection at screening. * History of human immunodeficiency virus (HIV) infection or positive HIV serology at screening. * Participant has a positive or indeterminate test for tuberculosis at screening. * Participant is pregnant or lactating.