In this open-label , randomized , three way crossover study , 12 healthy male subjects received the following oral treatments : A ) a loading dose of 30mg macitentan on day 1 followed by 10mg once daily for 3 days , B ) sildenafil 20mg three times a day for 3 days and a single 20mg dose on day 4 and C ) both treatments A and B concomitantly .