A study to examine the safety, tolerability, and pharmacokinetics of single- and multiple-ascending doses of ACT-777991 in healthy subjects.

General Inclusion Criteria:

• Signed informed consent in a language understandable to the subject prior to any study-mandated procedure.
• Healthy male (Part A and B) and female subjects (Part B) aged between 18 and 55 years (inclusive) at Screening.
• Healthy on the basis of medical history, physical examination, cardiovascular assessments, and clinical laboratory tests.
• Male subjects with a partner who might become pregnant must either be vasectomized or agree to practice adequate contraception from admission to the study site until 3 months after dosing, or the partner must consistently and correctly use a highly effective method of contraception.

Inclusion Criteria for Part B:

• Women of childbearing potential must have a negative serum pregnancy test at Screening and a negative urine pregnancy test on Day -1. They must consistently and correctly use a highly effective method of contraception with a failure rate of less than 1% per year, be sexually inactive, or have a vasectomized partner.
• Women of non-childbearing potential must have a negative serum pregnancy test at Screening and a negative urine pregnancy test on Day -1.

General Exclusion Criteria:

• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol.
• History or clinical evidence of any disease and/or existence of any surgical or medical condition, which, in the opinion of the investigator, are likely to interfere with the absorption, distribution, metabolism, or excretion of the study treatment.

Exclusion Criteria for the absorption,distribution, metabolism and excretion (ADME) evaluation:

• Radiation exposure, excluding background radiation but including diagnostic X-rays and other medical exposures, exceeding 5 milli sievert (mSv) in the last 12 months or 10 mSv in the last 5 years. Occupationally exposed workers, as defined in the relevant Ionising Radiation Regulations, must not participate in the study.
• Participation in any study involving administration of any Carbon-14 (14C) radiolabeled compound within the 12 months prior to Screening.

Exclusion Criteria for Part B:

• Pregnant or lactating women.