The purpose of this study is to determine whether the addition of sapropterin dihydrochloride (6R-BH4) to existing treatment has any effect in patients with pulmonary arterial hypertension (PAH). Patients with PAH have low levels of a substance called nitric oxide (NO). Tetrahydrobiopterin (BH4) is a substance produced by the body that is an essential requirement in the formation of NO. NO is thought to be helpful in keeping blood vessels in the lung healthy. 6R-BH4 is an experimental (unproven) medicine made in the lab that is very much like the BH4 that our own body makes. The researchers are investigating whether 6R-BH4 can be added safely to current treatment for PAH and whether there is any evidence of benefit from its use. The study will take approximately one year to complete from the time recruitment begins.