This Phase 2, randomized, double-blind, placebo-controlled, multicenter, dose-ranging study will compare the efficacy, safety, and tolerability of cicletanine hydrochloride (HCl) to placebo in subjects with PAH. Study drug will be administered alone, or on the background of stable PAH therapy. The study will consist of 3 periods: a screening period, a 12-week placebo-controlled treatment period, and a long-term, blinded extension period.