PTGIR and pulmonary arterial hypertension: Each drug represents the three main therapeutic pathways in PAH: bosentan is an endothelin receptor antagonist that targets the endothelin pathway, MRE-269 is a prostacyclin receptor agonist that targets the prostacylcin pathway, and tadalafil is a phosphodiesterase 5 inhibitor that increases the intracellular concentration of cyclic GMP which is a signaling messenger molecule downstream of the nitric oxide signaling pathway (Humbert et al., 2004).