The AC-055B201/MUSIC study is a Phase II study, comparing one dose of ACT-064922 (macitentan) 10 mg with placebo in patients with idiopathic pulmonary fibrosis (IPF). The main study objective is to demonstrate that macitentan positively affects the forced vital capacity (FVC) in comparison with placebo in patients with idiopathic pulmonary fibrosis (IPF).