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/m2.

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