The aim of the study is to demonstrate that the exposure to bosentan in children with idiopathic pulmonary arterial hypertension (PAH) or familial pulmonary arterial hypertension, using a pediatric formulation, is similar to that in adults with PAH and to evaluate the tolerability and safety of a pediatric formulation of bosentan in this patient population.

Inclusion Criteria:

• Signed informed consent by the parents or the legal representatives.

• Males or females >= 2 and < 12 years of age.

• Idiopathic PAH or familial PAH diagnosed by right heart catheterization (Clinical classification of pulmonary hypertension, Venice 2003).

• World Health Organization (WHO) functional class II or III.

• Oxygen saturation (SpO2) >= 88% (at rest, on room air).

• PAH treatment-naïve patients or patients already treated with either:

  • Bosentan monotherapy
  • Intravenous epoprostenol monotherapy
  • Intravenous or inhaled iloprost monotherapy
  • Combination of bosentan and intravenous epoprostenol
  • Combination of bosentan and intravenous or inhaled iloprost.

• All patients should start the study drug (bosentan pediatric formulation) at 2 mg/kg twice daily (b.i.d.), whether or not they were previously treated with bosentan.

• PAH therapy stable for at least 3 months prior to Screening.

• Stable treatment with calcium channel blockers, if any, for at least 3 months prior to Screening.

• Patient's PAH condition stable for at least 3 months prior to Screening.

Exclusion Criteria:

• PAH associated with conditions other than idiopathic or familial PAH.
• Non-stable patients, e.g., history (in the last 3 months prior to Screening) of recurrent syncope, or signs and symptoms of non-compensated right heart failure.
• Need or plan to wean patients from intravenous epoprostenol, or intravenous, or inhaled iloprost.
• Body weight < 4 kg.
• Systolic blood pressure < 80%, the lower limit of normal range, according to age and gender.
• AST and/or ALT values > 3 times the upper limit of normal ranges.
• Moderate to severe hepatic impairment, i.e., Child-Pugh Class B or C.
• Hemoglobin and/or hematocrit levels < 75% of the lower limit of normal ranges.
• Pregnancy.
• Known intolerance or hypersensitivity to bosentan or any of the excipients.