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 andassent from developmentally capable children prior to initiation of anystudy-mandated procedure

• Males or females between greater than or equal to (>=) 1 month and less than (<) 18years of age

• Participants with body weight >= 3.5 kilograms (kg) at randomization

• Pulmonary arterial hypertension (PAH) diagnosis confirmed by historical RHC (mPAPgreater than or equal to [>=] 25 millimeters of mercury [mmHg], and Pulmonary arterywedge pressure [PAWP] less than or equal to [<=] 15 mmHg, and Pulmonary vascularresistance index [PVRi] greater than [>] 3 WU × m2), where in the absence ofpulmonary vein obstruction and/or significant lung disease PAWP can be replaced byLeft atrium pressure [LAP] or Left ventricular end diastolic pressure [LVEDP] (inabsence of mitral stenosis) assessed by heart catheterization

• PAH belonging to the Nice 2013 Updated Classification Group 1 (includingparticipants with Down Syndrome) and of following etiologies: idiopathic PAH;heritable PAH; PAH associated with congenital heart disease (CHD); Drug or toxininduced 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 Screeningand at Baseline, and must agree to undertake monthly pregnancy tests, and to use areliable 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 pulmonaryveno-occlusive disease and/or pulmonary capillary hemangiomatosis, and persistentpulmonary hypertension of the newborn

• Participants with PAH associated with Eisenmenger syndrome, or with moderate tolarge left-to-right shunts

• Participants receiving a combination of > 2 PAH-specific treatments atrandomization.

• Treatment with intravenous (IV) or subcutaneous (SC) prostanoids within 4 weeksbefore 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) greaterthan (>) 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, mayaffect 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 precludeinitiation of a PAH-specific therapy

• Severe renal insufficiency (estimated creatinine clearance <30 mL/min or serumcreatinine >221 micro-moles per liter [micro-mol/L])

• Participants with known diagnosis of bronchopulmonary dysplasia