A Study of Sotatercept (MK-7962) in Japanese Children With Pulmonary Arterial Hypertension (PAH) (MK-7962-032)

• Sotatercept (Biological), also known as MK-7962, ACE-011, ActRIIA-IgG1Fc, WINREVAIR®: Sotatercept subcutaneous injection every 3 weeks

Primary outcomes

• Number of Participants With One or More Adverse Events [Up to approximately 24 weeks]: An adverse event is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Number of Participants Who Discontinue Study Intervention Due to an Adverse Event [Up to approximately 24 weeks]: An adverse event is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Laboratory Parameter (Hematology): Concentration of Hemoglobin [Up to approximately 24 weeks]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The concentration of hemoglobin will be presented.
• Laboratory Parameter (Hematology): Hematocrit [Up to approximately 24 weeks]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The hematocrit will be presented.
• Laboratory Parameter (Hematology): Red Blood Cell (RBC) Count [Up to approximately 24 weeks]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The RBC count will be presented.
• Laboratory Parameter (Hematology): Reticulocyte Count [Up to approximately 24 weeks]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The reticulocyte count will be presented.
• Laboratory Parameter (Hematology): Platelet Count [Up to approximately 24 weeks]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The platelet count will be presented.
• Blood Pressure (BP) [Up to approximately 24 weeks]: BP will be assessed while the participant was seated after a period of rest in a quiet setting with no distractions (eg, television and cell phones).
• Number of Participants with Detectable Anti-Drug Antibodies (ADAs) [Up to approximately 24 weeks]: Blood samples will be collected to assess antidrug antibodies to sotatercept. The number of participants with detectable ADAs at any time during the study will be presented.
• Number of Participants with Detectable Neutralizing Antibodies to Sotatercept [Up to approximately 24 weeks]: Neutralizing antibody analysis will be performed on blood samples for participants who are confirmed as antidrug antibody-positive. The number of participants with detectable neutralizing antibodies at any time during the study will be presented.
• Serum Trough Concentration (Ctrough) of Sotatercept [At designated time points and up to approximately 24 weeks]: Serum samples collected predose will be used to determine serum trough concentration (Ctrough) of sotatercept.
• Change From Baseline in Pulmonary Vascular Resistance Index (PVRI) [Baseline and Week 24]: The change from baseline in PVRI will be presented.

Secondary outcomes

• Mean Change from Baseline in 6-Minute Walk Distance (6MWD) [Baseline and Week 24]: 6MWD will be assessed using the 6-minute walk test (6MWT).
• Mean Change from Baseline in N-terminal Prohormone B-type Natriuretic Peptide (NT-proBNP) [Baseline and Week 24]: The change from baseline in plasma NT-proBNP levels will be reported.
• Percentage of Participants Who Either Improved or Maintained Their World Health Organization Functional Class (WHO FC) [Baseline and Week 24]: The severity of an individual's PAH symptoms will be graded using the WHO FC system. WHO functional classification for PAH range from Class I (no limitation in physical activity, no dyspnea with normal activity), Class II (slight limitation of physical activity), Class III (marked limitation of physical activity) and Class IV (cannot perform a physical activity without any symptoms, dyspnea at rest). The change from baseline in WHO FC will be classified into "Improved", "No change" and "Worsened". Improvement = reduction in FC, worsened = increase in FC and no change = no change in FC.
• Number of Participants With One or More Adverse Events [From Week 24 up to approximately 58 months]: An adverse event is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Number of Participants Who Discontinue Study Intervention Due to an Adverse Event [From Week 24 up to approximately 58 months]: An adverse event is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Laboratory Parameter (Hematology): Concentration of Hemoglobin [From Week 24 up to approximately 58 months]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The concentration of hemoglobin will be presented.
• Laboratory Parameter (Hematology): Hematocrit [From Week 24 up to approximately 58 months]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The hematocrit will be presented.
• Laboratory Parameter (Hematology): Red Blood Cell (RBC) Count [From Week 24 up to approximately 58 months]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The RBC count will be presented.
• Laboratory Parameter (Hematology): Reticulocyte Count [From Week 24 up to approximately 58 months]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The reticulocyte count will be presented.
• Laboratory Parameter (Hematology): Platelet Count [From Week 24 up to approximately 58 months]: Hematological parameters will be investigated in blood samples from participants by means of clinical laboratory assays and evaluated by the investigator. The platelet count will be presented.
• Blood Pressure (BP) [From Week 24 up to approximately 58 months]: BP will be assessed while the participant was seated after a period of rest in a quiet setting with no distractions (eg, television and cell phones).
• Number of Participants with Detectable ADAs [From Week 24 up to approximately 58 months]: Blood samples will be collected to assess antidrug antibodies to sotatercept. The number of participants with detectable ADAs at any time during the study will be presented.
• Number of Participants with Detectable Neutralizing Antibodies to Sotatercept [From Week 24 up to approximately 58 months]: Neutralizing antibody analysis will be performed on blood samples for participants who are confirmed as ADA-positive. The number of participants with detectable neutralizing antibodies at any time during the study will be presented.

Sex: All, ages: 1 Year to 17 Years

Inclusion Criteria:

The main inclusion criteria include but are not limited to the following:

* Has pulmonary arterial hypertension (PAH, World Health Organization Group 1) confirmed by a prior right heart catheterization
* Has idiopathic, heritable, drug- or toxin-induced PAH, connective tissue disease-associated PAH, repaired congenital heart disease-associated PAH, or PAH with coincidental shunt
* Has PAH classified as World Health Organization Functional Class I, or symptomatic World Health Organization Functional Class II to IV
* Has been receiving stable standard-of-care background therapy for PAH for at least 90 days
* Is Japanese

Exclusion Criteria:

The main exclusion criteria include but are not limited to the following:

* History of left-sided heart disease
* Has severe congenital or developmental abnormalities of the lung, thorax, and/or diaphragm
* History of Eisenmenger syndrome, Potts shunt, or recent atrial septostomy within 180 days
* Has unrepaired or residual cardiac shunt with Qp/Qs >1.5
* Has pulmonary veno-occlusive disease, pulmonary capillary hemangiomatosis, or overt signs of pulmonary capillary and/or venous involvement
* PAH associated with portal hypertension
* Known visceral arteriovenous malformations
* History of full or partial pneumonectomy
* Untreated more than mild obstructive sleep apnea
* History of known pericardial constriction
* Family history of sudden cardiac death or long QT syndrome
* History of symptomatic coronary disease within 6 months or cerebrovascular accident within 3 months
* Prior treatment with sotatercept or luspatercept

• Medical Director, Study director, Merck Sharp & Dohme LLC