Prostacyclin (PGI 2 ) and its analogs are useful for the treatment of various vascular disorders, but their half-lives are too short for widespread clinical application. To overcome this drawback, we have synthesized a novel diphenylpyrazine derivative, 2-{4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N-(methylsulfonyl)acetamide (NS-304), a prodrug of the active form {4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}ac-etic acid (MRE-269). NS-304 is an orally available and potent agonist for the PGI 2 receptor (IP receptor). The inhibition constant (K i ) of MRE-269 for the human IP receptor was 20 nM; in contrast, the K i values for other prostanoid receptors were Ͼ2.6 M. MRE-269 was therefore a highly selective agonist for the IP receptor. The plasma concentrations of MRE-269 remained near peak levels for more than 8 h after oral administration of NS-304 to rats and dogs, and NS-304 increased femoral skin blood flow in rats in a long-lasting manner without affecting the hemodynamics. These findings indicate that NS-304 acts as a long-acting IP receptor agonist in vivo. The continuous vasodilation evoked by NS-304 was not attenuated by repeated treatment, indicating that NS-304 is unlikely to cause severe desensitization of the IP receptor in rats. Moreover, a microdose pharmacokinetic study in which NS-304 was orally administered to healthy male volunteers showed conversion of NS-304 to MRE-269 and a long plasma elimination half-life for MRE-269 (7.9 h). In conclusion, NS-304 is an orally available and long-acting IP receptor agonist prodrug, and its active form, MRE-269, is highly selective for the IP receptor. Therefore, NS-304 is a promising drug candidate for various vascular diseases, especially pulmonary arterial hypertension and arteriosclerosis obliterans.Five prostanoids, prostaglandin (PG) D 2 , PGE 2 , PGF 2␣ , prostacyclin (PGI 2 ), and thromboxane A 2 (TXA 2 ), are cyclooxygenase metabolites of arachidonic acid. These lipid mediators interact with specific G protein-coupled receptors (DP, EP 1 , EP 2 , EP 3 , EP 4 , FP, IP, and TP) and activate the corresponding intracellular signaling cascade (Narumiya et al., 1999;Breyer et al., 2001). PGI 2 , which is mainly produced in vascular endothelial cells (Moncada et al., 1976;Moncada and Vane, 1978), acts via the PGI 2 receptor (IP receptor) as a potent vasodilator and inhibitor of platelet aggregation (Namba et al., 1994), and it counteracts the actions of TXA 2 , a potent vasoconstrictor and initiator of platelet aggregation. Activation of the IP receptor leads to stimulation of adenylate cyclase through coupling to G s protein, with a resulting increase in intracellular cAMP levels (Adie et al., 1992), and to activation of phospholipase C through coupling to G q protein leading to elevation of cytosolic Ca 2ϩ concentrations (Vassaux et al., 1992).The expression of IP receptor mRNA has been observed in various organs, including the heart, lung, thymus, spleen, and kidney (Oida et al., 1995

2-{4-[(5,6-Diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N-(methylsulfonyl)acetamide (NS-304) is an orally available, longacting nonprostanoid prostacyclin receptor (IP receptor) agonist prodrug. In a rat model of pulmonary hypertension induced by monocrotaline (MCT), NS-304 ameliorated vascular endothelial dysfunction, pulmonary arterial wall hypertrophy, and right ventricular hypertrophy, and it elevated right ventricular systolic pressure and improved survival. {4-[(5,6-Diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}acetic acid (MRE-269), the active form of NS-304, is much more selective for the IP receptor than are the prostacyclin analogs beraprost and iloprost, which also have high affinity for the EP 3 receptor. To investigate the effect of receptor selectivity on vasodilation of the pulmonary artery, we assessed the relaxant response to these IP agonists in rats. MRE-269 induced vasodilation equally in large pulmonary arteries (LPA) and small pulmonary arteries (SPA), whereas beraprost and iloprost induced less vasodilation in SPA than in LPA. An EP 3 agonist, sulprostone, induced SPA and LPA vasoconstriction, and an EP 3 antagonist attenuated the vasoconstriction. Beraprost showed EP 3 agonism and induced LPA and SPA vasoconstriction, whereas the EP 3 antagonist inhibited this vasoconstriction and enhanced beraprost-and iloprost-induced SPA vasodilation. These findings suggest that the EP 3 agonism of beraprost and iloprost interfered with the SPA vasodilation resulting from their IP receptor agonism. Endothelium removal markedly attenuated the vasodilation induced by beraprost, but not that induced by MRE-269 or iloprost. Moreover, the vasodilation induced by beraprost and iloprost, but not that induced by MRE-269, was more strongly attenuated in LPA from MCT-treated rats than from normal rats. NS-304 is a promising alternative medication for pulmonary arterial hypertension with prospects for good patient compliance.Pulmonary arterial hypertension (PAH) is a life-threatening disorder characterized by a progressive increase in pulmonary arterial pressure and pulmonary vascular resistance, leading ultimately to right ventricular failure and death (Rich et al., 1987;Farber and Loscalzo, 2004). The pathogenesis of PAH includes at least three processes occurring in small pulmonary arteries (SPA): vasoconstriction, vascular remodeling, and thrombosis (Rubin, 1997;Runo and Loyd, 2003).Prostacyclin (PGI 2 ) is a potent endogenous vasodilator and inhibitor of platelet aggregation that acts at the PGI 2 receptor (IP receptor) (Namba et al., 1994). PGI 2 contributes to the maintenance of homeostasis and has various other physiological effects in many organ systems (Narumiya et al., 1999 ABBREVIATIONS: PAH, pulmonary arterial hypertension; SPA, small pulmonary arteries; PGI 2 , prostacyclin; IP receptor, PGI 2 receptor; PH, pulmonary hypertension; MCT, monocrotaline; NS-304, 2-{4-[(5,6-diphenylpyrazin-2-yl)(isopropyl)amino]butoxy}-N-(methylsulfonyl)acetamide; MRE-269, {4-[(5,6-diphenylpyrazin-2-yl)(isopropy...

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