Written informed consent/assent from the subject and/or a legal representative prior to initiation of any study-mandated procedures.

Subjects who have completed Week 52 of the parent AC-055H301/RUBATO DB study (NCT03153137)

Women of childbearing potential must:

1. have a negative serum pregnancy test prior to first intake of OL study drug, and,

2. agree to perform monthly pregnancy tests up to the end of the safety follow up (S-FU) period, and,

3. use reliable methods of contraception from enrollment up to at least 30 days after study treatment discontinuation.

An Upcoming Clinical Study to Measure the Safety and Impact of a Drug Called Macitentan in Teenage and Adult Fontan Patients.