This is a study to evaluate the relative pharmacokinetic properties and the tolerability and safety of ACT-128800 in Japanese and Caucasian healthy male and female subjects after single-dose administration.

10 Japanese and 10 Caucasian healthy male and female subjects in a one to one male to female ratio will be included in the study. Japanese and Caucasian subjects will be matched for body weight.

Inclusion Criteria:

• Signed informed consent prior to any study-mandated procedure.
• Japanese or Caucasian. (Japanese subjects: both parents of the subject are Japanese [born in Japan]. Caucasian subjects: both parents of the subject are Caucasian).
• Body mass index between 18 and 28 kg/m^2, inclusive.
• Women not of childbearing potential:
• Women of childbearing potential must have a negative serum pregnancy test at screening and a negative urine pregnancy test prior to drug intake on Day 1; use a reliable method of contraception and continue this contraception for the duration of the study and for at least 2 months after study drug intake. In addition, her partner must use a condom.
• Systolic blood pressure 100-145 mmHg, diastolic blood pressure 50-90 mmHg, and heart rate (HR) 50-95 beats per minute (inclusive).
• 12-lead electrocardiogram without clinically relevant abnormalities at screening.
• Hematology and clinical chemistry results not deviating from the normal range to a clinically relevant extent at screening.
• Negative results from urine drug screen at screening.
• Ability to communicate well with the Investigator (if necessary with the help of an interpreter) and to understand and comply with the requirements of the study.

Exclusion Criteria:

• Electrocardiograph PQ/PR interval (time interval from the beginning of the P wave to the beginning of the QRS complex) > 200 milliseconds at screening.
• Nursing woman.
• History of asthma or chronic obstructive pulmonary disease.
• Known hypersensitivity to any excipients of the drug formulation.
• Treatment with another investigational drug within 3 months prior to screening.
• Excessive caffeine consumption, defined as > 800 mg per day at screening.
• History or clinical evidence of any disease and/or existence of any surgical or medical condition that might interfere with the absorption, distribution, metabolism or excretion of the study drug.
• Any cardiac condition or illness, including ECG abnormalities, with a potential to increase the cardiac risk of the subject.
• Smoking within the last month prior to screening.
• Any immunosuppressive treatment within 6 weeks before study drug administration.
• Previous treatment with any prescribed or over-the-counter medications within 2 weeks prior to screening.
• Loss of 250 mL or more of blood within 3 months prior to screening.
• Lymphopenia (< 1,000 lymphocytes/μL).
• Viral, fungal, bacterial or protozoal infection within 4 weeks before study drug administration.
• Positive results from the hepatitis serology, except for vaccinated subjects, at screening.
• Positive results from human immunodeficiency vrus serology at screening.
• Legal incapacity or limited legal capacity at screening.
• History of alcoholism or drug abuse.
• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol.