Inclusion Criteria:

1. Patients stable with PAH upon completion of the BREATHE-5 randomized, double-blind, placebo- controlled 16-week study.
2. For female patients, only non-pregnant women who are surgically sterile, postmenopausal or have documented infertility (over 50 years of age and amenrrheic for at least 1 year), or those of childbearing potential using one of the following methods of contraception: Barrier-type devices (e.g., condom, diaphragm) used ONLY in combination with a spermicide. A double-barrier method is recommended; intrauterine devices (IUDs); oral or implanted contraceptives, if used in combination with a barrier method.
3. Patients providing written informed consent.

Exclusion Criteria:

1. Patients who withdrew prematurely from BREATHE-5, AC-052-405.
2. Patients who are pregnant or nursing
3. Patients who are receiving epoprostenol or prostacyclin analogues, or are expected to receive any of these drugs during the study.
4. Patients with AST and/or ALT values greater than 3 times the upper limit of normal.
5. Patients with hemoglobin or hematocrit that is more than 30% below the normal range (patients with secondary polycythemia are permitted in the study.
6. Patients with systolic blood pressure < 85 mm Hg
7. Patients taking phosphodiesterase inhibitors, other endothelin receptor antagonists or are receiving another investigational product
8. Patients active on organ transplant list
9. Patients who are receiving or expected to receive glyburide, cyclosporin A or tacrolimus
10. Patients not able to comply with the protocol or adhere to therapy