This is a multicenter, open-label, single -arm study with bosentan, initial dose of 62.5 mg b.i.d., with a target dose of 125 mg b.i.d. All patients will be assessed for eligibility during the baseline visit (same as Week 16, end of study BREATHE-5 visit), and will have the option to enter into the open-label extension study at a starting dose of 62.5 mg b.i.d., for safety reasons. Visit 2 is required as a safety visit to assess oxygen saturation (SpO2) after 1 week of bosentan treatment. Patients will return for Visit 4 after 4 weeks of treatment, and the dose will be up-titrated to 125 mg b.i.d. or maintained at 62.5 mg b.i.d. at the judgement of the investigator. Data will be collected for a total of 24 weeks or until the sponsor decides to stop the study. If the results from Study AC-052-403 or AC-052-405 demonstrate significant safety issues, this extension study may be terminated at the sponsor's request.