A Study in Healthy Subjects to Investigate the Safety and Tolerability of ACT- 541478 as Well as What ACT-541478 Does to the Body and the Way the Body Takes up, Distributes, and Gets Rid of ACT-541478

Inclusion Criteria: Part A/B/C: * Signed informed consent in a language understandable to the subject prior to any study-mandated procedure. * BMI of 18.0 to 29.9 kg/m2 (inclusive) at Screening. * SBP 100-139 mmHg, DBP 50-89 mmHg, and pulse rate 50-90 bpm (inclusive), measured on either arm, after 5 min in the supine position at Screening and on Day -1 in the first period, if applicable. * Fertile male subjects (defined as physiologically capable of conceiving a child according to the investigator's judgment) must agree to refrain from fathering a child and: 1. be sexually abstinent with women of child-bearing potential or use condoms during sexual intercourse with women of child-bearing potential from (first) study treatment administration up to at least 90 days after (last) study treatment administration. Moreover, it is recommended that women of child-bearing potential partners of male subjects consistently and correctly use for the same period a highly effective method of contraception with a failure rate of \< 1% per year. 2. to not donate sperm from (first) study treatment administration up to at least 90 days after (last) study treatment administration. * Body temperature in the range of 35.5° to 37.5 °C at Screening and on Day -1 in the first period, if applicable. * 12-lead safety ECG: QTcF ≤ 450 ms, QRS ≤ 110 ms, PR ≤ 220 ms, and resting HR 50-90 bpm (inclusive) with no clinically relevant abnormalities on 12-lead safety ECG after at least 5 min in the supine position at Screening and on Day -1 in the first period, if applicable. * Normal renal function as confirmed by an estimated glomerular filtration rate ≥ 80 mL/min/1.73 m2 determined at Screening using the Chronic Kidney Disease Epidemiology Collaboration formula. Part A/B: \- Healthy male subjects aged between 18 and 55 years (inclusive) at Screening. Parts C1 to C3 (Adult subjects): \- Healthy male and female subjects aged between 18 and 55 years (inclusive) at Screening. Parts E1 and E2 (Elderly subjects): \- Healthy male and female subjects aged between 65 and 80 years (inclusive) at Screening. Parts C1 to C3, E1 and E2 (Female subjects): * Women of child-bearing 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 (from Screening, during the entire study, and for at least 30 days after last study treatment administration) a highly effective method of contraception with a failure rate of \< 1% per year, or be sexually inactive, or have a vasectomized partner. If a hormonal contraceptive is used, it must have been initiated at least 1 month before first study treatment administration. * Women of child-bearing potential (i.e., postmenopausal \[defined as 12 consecutive months with no menses without an alternative medical cause, confirmed by an FSH test \> 40 mIU/mL\], or with documented previous bilateral salpingectomy, bilateral salpingo oophorectomy or hysterectomy, or with premature ovarian failure \[confirmed by a specialist\], XY genotype, Turner syndrome, uterine agenesis). Exclusion Criteria: Part A/B/C * 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 (appendectomy and herniotomy allowed, cholecystectomy not allowed). * 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 presence of cardiac rhythm disorders (e.g., sinoatrial heart block, sick-sinus syndrome, 2nd or 3rd degree AV block, long QT syndrome, symptomatic bradycardia, atrial flutter, or atrial fibrillation). * Any illness with a potential to increase the risk of the subject based on medical history. * Clinically relevant findings in clinical laboratory tests (hematology, clinical chemistry, and urinalysis) at Screening (and on Day -1 in Part A and C) or any of the following laboratory parameters out of normal range: ALT, AST, total bilirubin, creatinine, TSH, and/or hemoglobin, at Screening and on Day -1 in the first period, if applicable. * Any signs or symptoms of active, ongoing infection judged to be clinically relevant by the investigator (special attention should be given to clinical signs and symptoms consistent with COVID-19). Part A (A3, effect of food) * Known lactose intolerance. * Known hereditary problems of galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption. * Inability or unwillingness to completely consume the required high-fat meal. Part C * Pregnant or lactating women. * Relevant history of a suicide attempt or suicidal behavior. Any recent suicidal ideation within the last 6 months (categories 4 or 5), or any suicidal behavior within the last 2 years, except for "Non-Suicidal Self-Injurious Behavior" (item also included in the Suicidal Behavior section), as judged by the investigator using the Columbia-Suicide Severity Rating Scale (C-SSRS) at Screening. * Result ≤ 27 in the Mini-Mental State Examination (MMSE®2™), assessed at Screening (E1 and E2 only).