- Subjects currently being treated for PH with prostnoids, PDE-5 inhibitors and or endothelin receptor antagonist. The use of PDE-5 inhibitors "as needed" for erectile dysfunction is acceptable as long as the subject does not take the medication within 72 hours of an efficacy assessment. Other concomitant medications are allowed provided the subject has been on a stable dose regimen for at least 4 weeks prior to enrollment and the regimen continues and remains stable during the treatment and follow-up periods.