Possible role of cyclic AMP phosphodiesterases in the actions of ibudilast on eosinophil thromboxane generation and airways smooth muscle tone

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Abstract
1 The possible role of cyclic AMP phosphodiesterase (PDE) in the inhibitory actions of ibudilast on tracheal smooth muscle contractility and eosinophil thromboxane generation was investigated. 2 Ibudilast was a non-selective inhibitor of partially purified cyclic nucleotide PDE isoenzymes from pig aorta and bovine tracheal smooth muscle, exhibiting only moderate potency against bovine tracheal PDE IV (IC50 = 12 ± 4 μm, n = 3). Similar or slightly lower potencies were displayed against PDEs I, II, III and V. In contrast, rolipram exhibited selectivity for PDE IV (3 ± 0.5 μm, n = 3). 3 Ibudilast (IC50 = 0.87 ± 0.37 μm, n = 3), like rolipram (IC50 = 0.20 ± 0.04 μm, n = 3), was a more potent inhibitor of membrane-bound PDE IV from guinea-pig eosinophils than of partially purified PDE IV from bovine tracheal smooth muscle. The potency of ibudilast increased when the eosinophil enzyme was solubilised with deoxycholate and NaCl (IC50 = 0.11 ± 0.05 μm, n = 3) or exposed to vanadate/glutathione complex (V/GSH) (IC50 = 0.11 ± 0.02 μm, n = 3). The potency of rolipram was also increased by solubilization (IC50 = 0.012 ± 0.003, n = 3) or V/GSH (IC50 = 0.012 ± 0.003, n = 3). 4 In intact eosinophils, ibudilast (0.032 μm − 20 μm) potentiated isoprenaline-induced cyclic AMP accumulation in a concentration-dependent manner, being approximately 20 fold less potent than rolipram. Little or no effect on basal cyclic AMP levels was observed with either compound. The cyclic AMP-dependent protein kinase activity ratio was significantly increased following incubation of eosinophils with either ibudilast (20 μm) or rolipram (20 μm) in the absence or presence of isoprenaline. 5 Leukotriene B4 (300 nm)-induced thromboxane generation from guinea-pig eosinophils was inhibited by ibudilast (IC50 = 11.3 ± 3.7 μm, n = 5) and rolipram (IC50 = 0.280 ± 0.067 μm, n = 5) in a concentration-dependent manner. 6 Ibudilast (10 nm − 1 μm), whilst generally less potent than rolipram (1 nm − 1 μm), produced concentration-dependent relaxation of spasmogen (methacholine, histamine, LTD4)-induced tone in the guinea-pig isolated tracheal strip. Ibudilast was less potent in reversing the methacholine (IC30 = 1.95 ± 0.40 μm, n = 6)-induced contraction than those of histamine (IC50 = 0.18 ± 0.70 μm, n = 6) or leukotriene D4 (LTD4, IC50 = 0.12 ± 0.05 μm, n = 6). Rolipram also exhibited a similar pattern of activity, although the difference in potency against methacholine (IC50 = 0.1 ± 0.01 μm, n = 6) compared with the other two spasmogens, histamine (IC50 = 0.034 ± 0.017 μm, n = 7) and LTD4 (IC50 = 0.026 ± 0.008 μm, n = 7), was not as great. 7 These results demonstrate that ibudilast, like rolipram, has several biological actions on the eosinophil and airways smooth muscle which may be attributed to inhibition of cyclic AMP PDE. These actions may account, at least in part, for the recently reported anti-asthma effects of ibudilast.

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