A Study to Assess Whether Macitentan Delays Disease Progression in Children With Pulmonary Arterial Hypertension (PAH)

Key Inclusion Criteria:

Signed informed consent by the parent(s) or legally designated representative and assent from developmentally capable children prior to initiation of any study-mandated procedure
Males or females between greater than or equal to (>=) 1 month and less than (<) 18 years of age
Participants with body weight >= 3.5 kilograms (kg) at randomization
Pulmonary arterial hypertension (PAH) diagnosis confirmed by historical RHC (mPAP greater than or equal to [>=] 25 millimeters of mercury [mmHg], and Pulmonary artery wedge pressure [PAWP] less than or equal to [<=] 15 mmHg, and Pulmonary vascular resistance index [PVRi] greater than [>] 3 WU × m2), where in the absence of pulmonary vein obstruction and/or significant lung disease PAWP can be replaced by Left atrium pressure [LAP] or Left ventricular end diastolic pressure [LVEDP] (in absence of mitral stenosis) assessed by heart catheterization
PAH belonging to the Nice 2013 Updated Classification Group 1 (including participants with Down Syndrome) and of following etiologies: idiopathic PAH; heritable PAH; PAH associated with congenital heart disease (CHD); Drug or toxin induced PAH; PAH associated with HIV; PAH associated with connective tissue diseases (PAH-aCTD); and World health organization (WHO) Functional class I to III
Females of childbearing potential must have a negative pregnancy test at Screening and at Baseline, and must agree to undertake monthly pregnancy tests, and to use a reliable method of contraception (if sexually active) up to the end of study (EOS)

Key Exclusion Criteria:

Participants with PAH due to portal hypertension, schistosomiasis, or with pulmonary veno-occlusive disease and/or pulmonary capillary hemangiomatosis, and persistent pulmonary hypertension of the newborn
Participants with PAH associated with Eisenmenger syndrome, or with moderate to large left-to-right shunts
Participants receiving a combination of > 2 PAH-specific treatments at randomization.
Treatment with intravenous (IV) or subcutaneous (SC) prostanoids within 4 weeks before randomization, unless given for vasoreactivity testing
Hemoglobin or hematocrit <75 percent (%) of the lower limit of normal range
Serum Aspartate aminotransferase (AST) and/or Alanine aminotransferase (ALT) greater than (>) 3 times the upper limit of normal range
Pregnancy (including family planning) or breastfeeding.
Any circumstances or conditions, which, in the opinion of the investigator, may affect full participation in the study or compliance with the protocol
Severe hepatic impairment, for example Child-Pugh Class C
Clinical signs of hypotension which in the investigator's judgment would preclude initiation of a PAH-specific therapy
Severe renal insufficiency (estimated creatinine clearance <30 mL/min or serum creatinine >221 micro-moles per liter [micro-mol/L])
Participants with known diagnosis of bronchopulmonary dysplasia

For more information on this trial, visit clinicaltrials.gov.