The study terminated for safety reasons.

Part 1: To investigate the safety and tolerability of intravenous (IV) or subcutaneous (SC) administration of a single dose of KHK4323 to Japanese or Caucasian healthy adult males in a double-blind, placebo-controlled study.

Part 2: To investigate the safety and tolerability of intravenous (IV) administration of repeated doses of KHK4323 to atopic dermatitis patients in a double-blind, placebo-controlled study.

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