Hypertension, Pulmonary

Inclusion Criteria

-  Male or female patients (18 -75 years of age) with idiopathic, familial, drug/toxin induced and associated PAH due to congenital heart disease (Group I / Dana Point Classification of PH) demonstrating insufficient response to treatment with PDE-5i for at least 3 months
 - Patients with and without endothelin receptor antagonist (ERA) therapy
 - World Health Organization Functional Class (WHO FC) III at screening
 - 6-minute walking distance (6MWD) of 165-440 m
 - Cardiac index <3.0 L/min/m*2.

Exclusion Criteria

- All types of PH except subtypes of Dana Point Group I specified in the inclusion criteria
 - Evidence of clinically significant restrictive or obstructive parenchymal lung diseases
 - Diffusing capacity of the lung for carbon monoxide (DLCO) <30% predicted
 - History or active state of serious hemoptysis / pulmonary hemorrhage including those managed by bronchial artery embolization
 - Patients unable to perform a valid 6MWD test
 - Pregnant women (i.e. positive pregnancy test or other signs of pregnancy), or breast feeding women, or women with childbearing potential not using a combination of 2 effective methods of birth control, for example a combination of condoms with a safe and highly effective contraception method (prescription oral contraceptives, contraceptive injections, contraceptive patch, intrauterine device) or a double barrier method is used throughout the study.