This is a non-confirmatory, randomized, subject and investigator blinded, placebo controlled study of QCC374 in PAH patients. The study will have 2 parts. In Part 1, an initial safety cohort, 8 subjects will be randomized in a 6:2 ratio and the starting dose will be 0.03 mg b.i.d.. In Part 2, ~ 30 subjects will be randomized in a 2:1 ratio, with a planned starting dose of 0.06 mg b.i.d.. In both Parts, subjects will be up-titrated during the first two weeks of the study to 0.12 mg b.i.d, or to their maximum tolerated dose (MTD) if their individual MTD is below 0.12 mg b.i.d. during the first two weeks of the study. The treatment duration is 16 weeks.