Bayer has enrolled the first patient in a global Phase IV study assessing the clinical effects of riociguat in patients with pulmonary arterial hypertension (PAH). All enrolled patients were being treated with a Phosphodiesterase-5 inhibitors (PDE-5 inhibitors), either as monotherapy or in combination with an endothelin receptor antagonist (ERA), and who did not reach their therapeutic goal. The study, which is a part of the collaboration between Bayer and Merck, is seeking to enroll patients at 26 sites in the United States with a total of 100 study sites worldwide.