Prospective, multi-center, open-label, single-arm, Phase 3b extension study of macitentan in patients with PAH.

To assess the long-term safety of macitentan in patients with pulmonary arterial hypertension (PAH) beyond treatment in the AC-055-310 study.

To assess the long-term safety of macitentan in patients with pulmonary arterial hypertension (PAH) beyond treatment in the AC-055-310 study.

Inclusion Criteria:

1. Signed informed consent prior to any study-mandated procedure.

2. Patients with PAH who completed the ORCHESTRA study (AC-055-310) as scheduled

3. Women of childbearing potential (as defined below) must:

   • Have a negative serum pregnancy test at Visit 1 (i.e., Visit 4 of study AC 055 310) and agree to perform monthly serum pregnancy tests.

   • Agree to use two reliable methods of contraception in parallel, from Visit 1 until 1 month after study drug discontinuation (see details below).

     • A female is considered to have childbearing potential unless she meets at least one of the following criteria:

       • Previous bilateral salpingo and/or oophorectomy, or hysterectomy.
       • Premature ovarian failure confirmed by a specialist.
       • Pre-pubescence, XY genotype, Turner syndrome, uterine agenesis.
       • Postmenopausal, defined as 12 consecutive months with no menses without an alternative medical cause.

     • Of the two contraceptive methods that must be used, one must be from Group 1, and one must be from Group 2, defined as follows:

       • Group 1: Oral, implantable, transdermal or injectable hormonal contraceptives, intrauterine devices, female sterilization (tubal ligation or non-surgical sterilization, e.g., permanent contraception with Essure procedure), or partner's sterilization (vasectomy). If a hormonal contraceptive is chosen from this group, it must be taken for at least one month prior to enrollment. Alternatively, if the Essure procedure is chosen as a contraceptive method, a hysterosalpingogram must have been performed to confirm correct location of the microinserts and tubal occlusion (as per manufacturer's recommendations).
       • Group 2: Female or male condoms, diaphragm or cervical cap, any of them in combination with a spermicide.

     • Sexual abstinence, rhythm methods, or contraception by the partner alone are not considered as acceptable methods of contraception for this study.

Exclusion Criteria:

1. Patients who prematurely discontinued study drug in study AC-055-310.
2. Females who are lactating or pregnant (positive Visit 1 pregnancy test) or plan to become pregnant during the study.
3. AST and/or ALT more than 3 X ULN.