Iloprost is a second generation structural analog of prostacyclin (PGI) with about ten-fold greater potency than the first generation stable analogs, typified by carbaprostacyclin. Iloprost binds with equal affinity to the human recombinant IP and EP1 receptors with a Ki of 11 nM. Iloprost constricts the isolated guinea pig ilium and fundus circular smooth muscle (an EP1 receptor preparation) as strongly as PGE2 itself. Iloprost inhibits the ADP, thrombin, and collagen-induced aggregation of human platelets with an ED50 of about 13 nM. In whole animals, iloprost acts as a vasodilator, hypotensive, antidiuretic, and prolongs bleeding time. It has been evaluated in several human clinical studies as a treatment for idiopathic pulmonary hypertension. In these studies, an aerosolized dose of 30 µg/day was effective, and doses as high as 150 µg/day for up to a year were well tolerated.