This was a single-center, randomized, double-blind, placebo-controlled, up-titration Phase 1 study. Sixteen subjects in two groups (at least 40% of subjects of either male or female sex), with 12 subjects in the active treatment group with an up-titration scheme from 10 to 100 mg, and 4 subjects in the placebo treatment group. Subjects were administered ascending doses of ACT-128800/placebo once daily for 3 days at each dose level: 10 mg, 20 mg, 40 mg, 60 mg, 80 mg, and 100 mg.