Study of KHK 4323 in Healthy Volunteers and Subjects With Atopic Dermatitis

Inclusion Criteria: Part 1: * Japanese or Caucasian male aged 20 to under 45 years old at the time consent was obtained * BMI ≥ 18.5 to \< 30.0 at time of screening tests Part 2: * Men and women aged 18 years or older at the time of consent * Patients with EASI ≥ 16 in pre-administration testing * Patients with IGA of "3: Moderate" or higher in pre-administration testing * Patients with BSA ≥ 10% at screening in pre-administration testing "Exclusion Criteria: Part 1: * Persons with existing respiratory disease, heart disease, gastrointestinal disease, kidney disease, or liver disease * Persons confirmed to have a bacterial, viral, fungal, or parasitic infection within 28 days prior to obtainment of consent * Persons who have contracted an infectious disease requiring hospitalization or IV administration of an antibiotic within 6 months prior to obtainment of consent * Persons who have been treated with a biological preparation (antibody, etc.) or have been administered an investigational drug within 6 months prior to the obtainment of consent * Persons who have used a medication (including over-the-counter drugs, topical agents, vitamins, and herbal medicines) within 2 weeks prior to obtainment of consent (for an immunosuppressant drug, within 60 days) * Persons who routinely smoke an average of more than 10 cigarettes a day (to be confirmed in interview at time of screening tests) or cannot follow the rules regarding smoking during the clinical trial period Part 2: * Patients with severe complications judged to affect the implementation and evaluation of the study in the opinion of the investigator or sub-investigator. Includes but is not limited to the following. Severe cardiovascular disease (e.g., class III or IV according to New York Heart Association functional classification), poorly controlled diabetes mellitus (HbA1c \> 8.5%), poorly controlled hypertension, liver disease with severity of moderate or higher (e.g., class B or C according to Child-Pugh classification), kidney disease, respiratory disease, gastrointestinal disease, blood dyscrasia, central nervous system disease, psychiatric disease, autoimmune disease, etc. * Patients observed to have one of the following laboratory test abnormalities in screening tests * Neutrophil count: \< 1500/μL * Serum creatinine: \> 1.5 mg/dL * AST or ALT: \> 2.5-fold the upper limit of the reference range * Other laboratory test abnormalities that the investigator or sub-investigator thinks could affect the completion or evaluation of the clinical trial