After screening (Visit 1), each subject will participate in the following consecutive treatment periods: Period 1(treatment with oral selexipag at Visit 2/Day 1), Period 2 (treatment with intravenous selexipag at Visit 2/ Day 2 and Day 3), Period 3 (treatment with oral selexipag starting in the evening of Visit 2/Day 3 and ending 7 to 11 days later at Visit 3). Then a safety follow-up period is planned up to end of study visit (EOS), which occurs between Day 33 and Day 40.