POSSIBLE ROLE OF CYCLIC-AMP PHOSPHODIESTERASES IN THE ACTIONS OF IBUDILAST ON EOSINOPHIL THROMBOXANE GENERATION AND AIRWAYS SMOOTH-MUSCLE TONE

1 The possible role of cyclic AMP phosphodiesterase (PDE) in the inhib itory 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 i soenzymes from pig aorta and bovine tracheal smooth muscle, exhibiting only moderate potency against bovine tracheal PDE IV (IC50 = 12 +/- 4 muM, n = 3). Similar or slightly lower potencies were displayed again st PDEs I, II, III and V. In contrast, rolipram exhibited selectivity for PDE IV (3 +/- 0.5 muM, n = 3). 3 Ibudilast (IC50 = 0.87 +/- 0.37 m uM, n = 3), like rolipram (IC50 = 0.20 +/- 0.04 muM, n = 3), was a mor e potent inhibitor of membrane-bound PDE IV from guinea-pig eosinophil s than of partially purified PDE IV from bovine tracheal smooth muscle . The potency of ibudilast increased when the eosinophil enzyme was so lubilised with deoxycholate and NaCl (IC50 = 0.11 +/- 0.05 muM, n = 3) or exposed to vanadate/glutathione complex (V/GSH) (IC50 = 0.11 +/- 0 .02 muM, n = 3). The potency of rolipram was also increased by solubil ization (IC50 = 0.012 +/- 0.003, n = 3) or V/GSH (IC50 = 0.012 +/- 0.0 03, n = 3). 4 In intact eosinophils, ibudilast (0.032 muM-20 muM) pote ntiated isoprenaline-induced cyclic AMP accumulation in a concentratio n-dependent manner, being approximately 20 fold less potent than rolip ram. Little or no effect on basal cyclic AMP levels was observed with either compound. The cyclic AMP-dependent protein kinase activity rati o was significantly increased following incubation of eosinophils with either ibudilast (20 muM) or rolipram (20 muM) in the absence or pres ence of isoprenaline. 5 Leukotriene B4 (300 nM)-induced thromboxane ge neration from guinea-pig eosinophils was inhibited by ibudilast (IC50 = 11.3 +/- 3.7 muM, n = 5) and rolipram (IC50 = 0.280 +/- 0.067 muM, n = 5) in a concentration-dependent manner. 6 Ibudilast (10 nm - 1 muM) , whilst generally less potent than rolipram (1 nm - 1 muM), produced concentration-dependent relaxation of spasmogen (methacholine, histami ne, LTD4)-induced tone in the guinea-pig isolated tracheal strip. Ibud ilast was less potent in reversing the methacholine (IC50 = 1.95 +/- 0 .40 muM, n = 6)-induced contraction than those of histamine (IC50 = 0. 18 +/- 0.70 muM, n = 6) or leukotriene D4 (LTD4, IC50 = 0.12 +/- 0.05 muM, n = 6). Rolipram also exhibited a similar pattern of activity, al though the difference in potency against methacholine (IC50 = 0.1 +/- 0.01 muM, n = 6) compared with the other two spasmogens, histamine (IC 50 = 0.034 +/- 0.017 muM, n = 7) and LTD4 (IC50 = 0.026 +/- 0.008 muM, n = 7), was not as great. 7 These results demonstrate that ibudilast, like rolipram, has several biological actions on the eosinophil and a irways smooth muscle which may be attributed to inhibition of cyclic A MP PDE. These action may account, at lest in part, for the recently re ported anti-asthma effects of ibudilast.