Study in healthy subjects to examine the safety and tolerability of ACT-1004-1239 given as multiple, gradually increasing doses and to examine the effects of ACT-1004-1239 on the body and the way the body takes up, distributes, and gets rid of ACT-1004-1239

Inclusion Criteria:

• Signed informed consent prior to any study-mandated procedure.
• Healthy male or female subject aged between 18 and 55 years (inclusive) at Screening.
• A female subject of childbearing potential must have a negative serum pregnancy test at Screening and a negative urine pregnancy test on Day - 1 and must agree to consistently and correctly use a highly effective method of contraception with a failure rate of ≤1% per year, be sexually inactive, or have a vasectomized partner.
• A female subject of non-childbearing potential must be postmenopausal, or must have a medical history of previous bilateral salpingectomy, bilateral salpingo-oophorectomy, hysterectomy, premature ovarian failure, XY genotype, Turner syndrome, or uterine agenesis.
• A male subject must use adequate contraception from first study treatment administration up to at least 90 days after last administration unless he is vasectomized.
• A male subject must agree to refrain from donation of semen from first study treatment administration up to at least 90 days after last administration.

Exclusion Criteria:

• Previous exposure to ACT-1004-1239.
• Known hypersensitivity to ACT-1004-1239, or any of its excipients.
• History of major medical or surgical disorders which, in the opinion of the investigator, are likely to interfere with the absorption, distribution, metabolism, or excretion of the study treatment.
• Acute, ongoing, recurrent, or chronic systemic disease able to interfere with the evaluation of the study.
• Previous treatment with any prescribed medications (including vaccines) or over-the-counter (OTC) medications (including herbal medicines such as St John's Wort, homeopathic preparations, vitamins, and minerals) within 3 weeks prior to first study treatment administration.
• Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol.
• Pregnant or lactating woman.
• Any cardiac condition or illness that may jeopardize the safety of the study subject per investigator judgment based on medical history or 12-lead ECG measured at Screening.
• Any immunosuppressive treatment within 6 weeks or 5x terminal half-life (t½), whichever is longer, before study treatment administration.