Analysis of pulmonary vasodilator responses to SB-772077-B [4-(7-((3-amino-1-pyrrolidinyl)carbonyl)-1-ethyl-1H-imidazo(4,5-c)pyridin-2-yl)-1,2,5-oxadiazol-3-amine], a novel aminofurazan-based Rho kinase inhibitor

Abstract

The effects of SB-772077-B [4-(7-((3-amino-1-pyrrolidinyl)carbonyl)-1-ethyl-1H-imidazo(4,5-c)pyridin-2-yl)-1,2,5-oxadiazol-3-amine], an aminofurazan-based Rho kinase inhibitor, on the pulmonary vascular bed and on monocrotaline-induced pulmonary hypertension were investigated in the rat. The intravenous injections of SB-772077-B decreased pulmonary and systemic arterial pressures and increased cardiac output. The decreases in pulmonary arterial pressure were enhanced when pulmonary vascular resistance was increased by U46619 [9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha)], hypoxia, or N(omega)-nitro-L-arginine methyl ester. SB-772077-B was more potent than Y-27632 [trans-4-[(1R)-1-aminoethyl]-N-4-pyridinyl-cyclohexanecarboxamide dihydrochloride] or fasudil [5-(1,4-diazepane-1-sulfonyl)isoquinoline] in decreasing pulmonary and systemic arterial pressures. The results with SB-772077-B, fasudil, and Y-27632 suggest that Rho kinase is constitutively active and is involved in the regulation of baseline tone and vasoconstrictor responses. Chronic treatment with SB-772077-B attenuated the increase in pulmonary arterial pressure induced by monocrotaline. The intravenous injection of SB-772077-B decreased pulmonary and systemic arterial pressures in rats with monocrotaline-induced pulmonary hypertension. The decreases in pulmonary arterial pressure in response to SB-772077-B in monocrotaline-treated rats were smaller than responses in U46619-infused animals, and the analysis of responses suggests that approximately 60% of the pulmonary hypertensive response is mediated by a Rho kinase-sensitive mechanism. The observation that Rho kinase inhibitors decrease pulmonary arterial pressure when pulmonary vascular resistance is increased by interventions such as hypoxia, U46619, angiotensin II, nitric-oxide synthase inhibition, and Bay K 8644 [S-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester] suggest that the vasodilatation is independent of the mechanisms used to increase intracellular calcium and promote vasoconstriction. The present results suggest that SB-772077-B would be beneficial in the treatment of pulmonary hypertensive disorders.

Fig. 1.

Bar graphs comparing changes in pulmonary and systemic arterial pressures and cardiac output in response to intravenous injections of SB-772077-B (10–300 μg/kg) under control baseline conditions (A) and during continuous intravenous infusion of the TP receptor agonist U46619 to increase pulmonary arterial pressure to approximately 30 mm Hg (B). n, number of experiments. *, p < 0.05 compared with baseline value.

Fig. 2.

Dose-response curves comparing relative potency of SB-772077-B, Y-27632, and fasudil in decreasing pulmonary and systemic arterial pressures in U-44619-infused animals. n, number of experiments.

Fig. 3.

Bar graphs comparing decreases in pulmonary and systemic arterial pressure and increases in cardiac output in response to intravenous injections of SB-772077-B in l-NAME-treated animals. The intravenous injections of l-NAME in doses of 5 to 25 mg/kg i.v. produced a significant increase in pulmonary and systemic arterial pressures. n, number of experiments. *, significantly different from baseline.

Fig. 4.

Bar graphs showing the decreases in pulmonary arterial pressure in response to SB-772077-B when pulmonary arterial pressure was increased by ventilation with the 10% O₂/90% N₂ gas mixture. The Rho kinase inhibitor was injected when the increase in pulmonary arterial pressure reached a peak in response to ventilation with the hypoxic gas mixture (A). The bar graph in B shows that the hypoxic pulmonary vasoconstrictor response is completely reversed by the intravenous injection of SB-772077-B (300 μg/kg) injected at the peak of the response to hypoxia. Bar graph in C shows that the hypoxic pulmonary vasoconstrictor response is prevented by intravenous injections of SB-772077-B (300 μg/kg) 5 min before the hypoxic response was initiated. n, number of experiments. *, response is significantly different from control.

Fig. 5.

Effect of SB-772077-B on increases in pulmonary arterial pressure in response to bolus intravenous injections of angiotensin II, U46619, and Bay K 8644. The vasoconstrictor agonists were injected before and 5 min after intravenous injection of SB-772077-B (300 μg/kg). n, number of experiments. *, significantly different from control.

Fig. 6.

Effect of intravenous injections of SB-772077-B on pulmonary and systemic arterial pressures and cardiac output in monocrotaline-treated rats. The responses to SB-772077-B were measured on day 29 after treatment with the plant alkaloid in a dose of 60 mg/kg i.v.

Fig. 7.

Comparison of decreases in pulmonary arterial pressure in response to intravenous injection of SB-772077-B (300 μg/kg) in control animals and in animals with monocrotaline-induced pulmonary hypertension. The decrease in pulmonary arterial pressure in response to the Rho kinase inhibitor provides an estimate of the amount of vasoconstrictor tone in the pulmonary vascular bed in control animals under baseline conditions. The decrease in pulmonary arterial pressure in response to intravenous injection of SB-772077-B in monocrotaline-treated animals provides an estimate of the amount of SB-772077-B reversible vasoconstrictor tone in rats with monocrotaline-induced pulmonary hypertension. The difference between the baseline value of tone or nadir in the control animals and in the monocrotaline-treated animals provides an estimate of the amount of SB-772077-B nonreversible (or fixed) tone. These data suggest that 60% of the vasoconstrictor tone in the monocrotaline-treated animals is reversible tone, and 40% of the tone is fixed due to structural alterations in the pulmonary vascular bed. n, number of animals. *, p < 0.05 compared with initial value.