Effects of Two Dosing Regimens of Bosentan in Children With Pulmonary Arterial Hypertension

Inclusion Criteria: 1. PAH diagnosis confirmed with right heart catheterization (RHC): * Idiopathic or heritable PAH, or * Associated PAH persisting after complete repair of a congenital heart defect (PAH has to be persistent for at least 6 months after surgery) or * PAH-Congenital Heart Disease (PAH-CHD) associated with systemic-to-pulmonary shunts (after global amendment dated 09 May 2012) 2. World Health Organization functional Class (WHO FC) I, II or III 3. Male or female ≥ 3 months and \< 12 years of age (maximum age at randomization is 11.5 years) 4. Body weight ≥ 3.5 kg 5. Peripheral oxygen saturation (SpO2) ≥ 88% (at rest, on room air) 6. Baseline PAH-therapy (Calcium channel blocker, bosentan, prostanoid, phosphodiesterase type-5 inhibitor) if present, has to be stable for at least 3 months prior to screening. During the study, all background treatments should remain stable 7. Signed informed consent by the parents or legal representatives Exclusion Criteria: 1. PAH etiologies other than listed above 2. Non-stable disease status 3. Need or plan to wean patient from intravenous epoprostenol or intravenous or inhaled iloprost 4. Systolic blood pressure \< 80% of the lower limit of normal range 5. Aspartate aminotransferase and/or alanine aminotransferase values \> 1.5 times the upper limit of normal range. 6. Moderate to severe hepatic impairment, i.e., Child-Pugh Class B or C 7. Hemoglobin and/or hematocrit levels \< 75% of the lower limit of normal range. 8. Known intolerance or hypersensitivity to bosentan or any of the excipients of the dispersible Tracleer tablet 9. Treatment with forbidden medication within 2 weeks or at least 5 times the half-life prior to randomization, whichever is the longest: * Glibenclamide (glyburide) * Cyclosporin A * Sirolimus * Tacrolimus * Fluconazole * Rifampicin (rifampin) * Ritonavir * Co-administration of CYP2C9 inhibitors (e.g., amiodarone, voriconazole) and moderate/strong CYP3A4 inhibitors (e.g., amprenavir, erythromycin, ketoconazole, diltiazem, itraconazole) * Endothelin receptor antagonists (ERAs) other than bosentan 10. Treatment with another investigational drug within 1 month prior to randomization or planned treatment