Inclusion Criteria:

1. Male and female patients between the ages of 8 -18 years of age.
2. Diagnosis of idiopathic (or primary) pulmonary arterial hypertension according to the Venice Classification system (2003).54, 55
3. Functional classification of WHO class III - IV.
4. Pulmonary vascular resistance (PVR) >300 dynes / sec / cm5.
5. IPAH medications stable for at least 3 mo prior to baseline visit.
6. Female patients of child bearing potential who are sexually active must have negative pregnancy test within 7 days prior to initiation of study drug and use a double-barrier local contraception, i.e., intra-uterine device plus condom, or spermicidal gel plus condom up to the Study Completion visit.
7. Able to communicate well with the investigator, to understand and comply with the requirements of the study. Able to verbalize understanding and sign the written informed assent.
8. Parents, or legal guardians, must be able to communicate well with the investigator, to understand and comply with the requirements of the study. They must verbalize understanding and sign the written informed consent statement.

Exclusion Criteria:

1. Pre-existing lung disease including parasitic diseases affecting lungs, bronchial asthma, congenital abnormalities of the lungs, thorax and diaphragm.
2. Congenital heart disease, left ventricular failure, or left-sided obstructive lesion (pulmonary venous hypertension with pulmonary capillary wedge pressure > 12 mmHg) detected at right heart catheterization.
3. Chronic thromboembolic pulmonary hypertension, congenital or acquired deficiencies of blood coagulation, deficient thrombocyte function, thrombocytopenia < 40,000/μl, or Sickle Cell anemia.
4. Pregnancy, breast feeding, or lack of safe contraception (hormonal contraception, IUD, bilateral tubal ligation, hysterectomy) in premenopausal women.
5. Hepatic insufficiency with transaminase levels >4-fold the upper limit of normal, or a bilirubin > 2-fold the upper limit of normal.
6. Renal insufficiency (serum creatinine > 200 μmol/l).
7. History of clinically significant drug allergy or history of atopic allergy (asthma, urticaria, eczematous dermatitis). A known hypersensitivity to the study drug, or drugs similar to the study drug.
8. Previous therapeutic radiation of lungs or mediastinum.
9. Participation in any treatment studies within 3 months prior to dosing, or longer if required by local regulations, and for any other limitation of participation based on local regulations.
10. History of immunodeficiency diseases, including a positive HIV (ELISA and Western blot) test result, or a positive Hepatitis B surface antigen (HBsAg), or positive Hepatitis C test result.

11. Any surgical or medical condition which might significantly alter the absorption, distribution, metabolism or excretion of drugs, such as a history of inflammatory bowel syndrome, gastritis, ulcers, gastrointestinal or rectal bleeding, or a history of major gastrointestinal tract surgery such as gastrectomy, gastroenterostomy, or bowel resection.

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