The purpose of this study is to evaluate whether the addition of selexipag to standard of care treatment delays disease progression in children with Pulmonary Arterial Hypertension (PAH) in comparison to placebo.

Pediatric PAH is a rare and progressive disorder associated with considerable morbidity and mortality. Given the significant medical need to develop treatments in children with PAH, further clinical studies in the pediatric population are therefore needed to provide more data for the management of PAH in children. Selexipag (JNJ-67896049) is an orally available, selective, and long-acting non-prostanoid agonist of the prostacyclin receptor approved and commercially available for the treatment of adult participants with PAH. Selexipag and its metabolite possess anti-fibrotic, anti-proliferative, and anti-thrombotic properties. Currently, no medicines targeting prostacyclin pathway are approved for pediatric use in PAH. An effective and orally available therapy acting on the prostacyclin receptor such as selexipag introduced at medically appropriate stage of PAH disease, and primarily in combination with current first-line oral PAH-specific medicines in participants in need of additional therapy because of insufficient disease control would represents a major advance to the therapeutic management of PAH pediatric participants. This study consists of a screening period of up to 6 weeks and a double-blind treatment period, including up-titration and maintenance periods, followed by a 3-year open-label extension period (OLEP) and a 30-day safety follow-up period that occurs after the last dose of study intervention (either double-blind or open-label). Safety, pharmacokinetic and efficacy assessments will be performed during the study. An Independent Data Monitoring Committee (IDMC) will be established to monitor data on an ongoing basis, to review interim data, and to ensure the continuing safety of the participants enrolled in this study. The approximate duration of the study is 8 years.

Inclusion Criteria:

• Participants between greater than or equal to (>=) 2 and less than (<) 18 years of age weighing >=9 kilogram (kg) at randomization
• Pulmonary arterial hypertension (PAH) diagnosis confirmed by documented historical right heart catheterization (RHC) performed at any time before participant's screening
• PAH (World Health Organization [WHO] Group 1), including participants with Down syndrome, of the following etiologies: Idiopathic PAH (IPAH); Heritable PAH (HPAH); PAH associated with congenital heart disease (PAH-associated with congenital heart disease [aCHD]) (PAH with coincidental CHD [that is, a small atrial septal defect, ventricular septal defect, or patent ductus arteriosus that does not itself account for the development of elevated PVR] and if approved by the BCAC) and Post-operative PAH (persisting / recurring/ developing >=6 months after repair of CHD); Drug or toxin-induced; PAH associated with Human immunodeficiency virus (HIV)
• WHO functional class (FC) II and III
• Participants treated with at least 1 PAH-specific treatment, example, an Endothelin receptor antagonist (ERA) and/or a Phosphodiesterase type-5 (PDE-5) inhibitor/soluble guanylate cyclase stimulator, provided that the treatment dose(s) has been stable for at least 3 months prior to first dose of study intervention

Exclusion Criteria:

• PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease, and/or pulmonary capillary hemangiomatosis
• PAH associated with Eisenmenger syndrome
• Previous exposure to Uptravi (selexipag)
• Known concomitant life-threatening disease with a life expectancy <12 months
• Pregnant, planning to become pregnant, or lactating
• Known allergies, hypersensitivity, or intolerance to selexipag or its excipients

The data sharing policy of the Janssen Pharmaceutical Companies of Johnson & Johnson is available at www.janssen.com/clinical-trials/transparency. As noted on this site, requests for access to the study data can be submitted through Yale open Data Access (YODA) Project site at yoda.yale.edu