Macitentan, sold under the brand name Opsumit, is an endothelin receptor antagonist developed by Actelion and approved for the treatment of pulmonary arterial hypertension (PAH).

[3] Endothelin (ET) is an extremely potent blood vessel constricting substance that is secreted by endothelial cells.

[9] Upon release, ET-1 can bind to the ET receptors that are expressed on arterial smooth muscle cells and fibroblasts in the lungs.

The rise in intracellular calcium leads to contraction of the arterial smooth muscle, as well as vascular remodelling due to cell proliferation.

[11] Although aprocitentan has a lower affinity for the ET receptors than its parent compound,[6] It maintains higher plasma concentrations than macitentan.

[10] Co-administration of ciclosporin has only a slight effect on the concentrations of macitentan and its active metabolite, while rifampicin decreases the area under the curve (AUC) of the drug's blood plasma concentration by 79%, and ketoconazole approximately doubles it.