AMB-320 (ARIES-1; NCT00423748) and AMB-321 (ARIES-2; NCT00423202) were 12-week, Phase 3, randomized, double-blind, placebo-controlled, multicenter, efficacy studies of AMB in subjects with PAH. The objectives of these studies were to determine the effect of three doses of AMB (2.5, 5.0, and 10.0 mg) on exercise capacity, as well as several clinical measures of PAH. The current study (NCT00578786) was unblinded (by design) prior to completion. The ARIES studies were identical except for the dose groups assessed and the geographic locations where the studies were conducted. Both studies evaluated placebo and 5.0-mg AMB dose groups; however, AMB-320 (NCT00423748) also examined an AMB dose of 10.0 mg, while AMB-321 (NCT00423202) included an AMB dose of 2.5 mg. AMB-320/321-E was an optional study for subjects who had participated in AMB-320 (NCT00423748) or AMB-321 (NCT00423202) that allowed continued long-term treatment with AMB.