Macitentan is an orally active potent endothelin receptor antagonist, active on both ETA and ETB receptors and approximately 100-fold more selective for ETA as compared to ETB in vitro. Macitentan displays high affinity and sustained occupancy of the ET receptors in human pulmonary arterial smooth muscle cells. This prevents endothelin-mediated activation of second messenger systems that result in vasoconstriction and smooth muscle cell proliferation.