No abstract

1 The thromboxane A2 synthase (TXS) inhibitory activity and the thromboxane A2 (TP)-receptor blocking action of ZD1542 (4(Z)-6-[2S,4S,5R)-2-[1-methyl-1-(2-nitro-4-tolyloxy)ethyl]-4-(3-pyridyl)-1,3-dioxan-5-yl]hex-4-enoic acid) has been evaluated in vitro on platelets and whole blood from a range of species including man. Antagonist activity has also been investigated in vascular and pulmonary smooth muscle preparations in vitro. 2 ZD1542 caused concentration-dependent inhibition of human platelet microsomal thromboxane B2 (TXB2) production in vitro (IC50 = 0.0 16 JAM); this inhibition was associated with an increase in prostaglandin E2 (PGE2) and PGF2,, formation.3 ZD1542 also inhibited collagen-stimulated TXS in human, rat and dog whole blood giving IC50 values of 0.018, 0.009 and 0.049 JM respectively. The drug did not modify platelet cyclo-oxygenase activity as inhibition of TXB2 formation was associated with a concomitant increase in the levels of PGD2, PGE2 and PGF2.. ZD1542 had little if any effect against cultured human umbilical vein endothelial cell (HUVEC) cyclo-oxygenase (IC50> 100 JM) and prostacyclin (PGI2) synthase (IC50 = 18.0 ± 8.6 JM).4 ZD1542 caused concentration-dependent inhibition of U46619-induced aggregation responses of human, rat and, dog platelets yielding apparent pA2 values of 8.3, 8.5 and 9.1 respectively. The drug was selective as, at concentrations up to 100 JAM, it did not modify 5-hydroxytryptamine (5-HT) or the primary phases of adenosine diphosphate (ADP) and adrenaline-induced aggregation. Furthermore, ZD1 542 (100 JM) modified only weakly the platelet inhibitory effects of PGD2, PGE, and PGI2. 5 ZD1542 also caused concentration-dependent inhibition of U46619-mediated contractions of rat thoracic aorta, guinea-pig trachea and lung parenchyma preparations giving apparent pA2 values of 8.6, 8.3 and 8.5 respectively. At concentrations approaching three orders of magnitude greater than those required to block U46619-mediated contractions, the drug did not affect the actions of non-prostanoid agonists or exhibit agonist activity in any of the smooth muscle preparations employed; neither did it interact at EP-or FP-receptors. 6 In conclusion, the present study demonstrates that ZD1542 is a drug that exhibits both potent, selective TXS inhibition and TXA2 receptor antagonism.

The effects of simultaneous redirection of arachidonic acid metabolism, by inhibition of thromboxane A2 (TXA2) synthase and blockade of the platelet thromboxane A2 receptor (TP‐receptor), was examined on the rate of thrombus formation in a stenosed coronary artery with damaged endothelium in an anaesthetized dog. Redirection of arachidonic acid metabolism was achieved by intravenous doses of ICI D1542, a selective and potent inhibitor of TXA2 synthase and the TP‐receptor. Redirection of arachidonic acid metabolism was demonstrated in whole blood, stimulated ex vivo by collagen. The ED50 for inhibition of TXB2 production was 7.1 μg kg−1, i.v.; there were corresponding increases in the production of the eicosanoids prostaglandin D2 (PGD2), PGE2 and PGF2α. Thrombus formation was inhibited by D1542 (ED50 0.55 μg kg−1, i.v.), but could be restarted by an intravenous infusion of adrenaline (0.2–38 μg kg−1 min−1, i.v.). In the presence of the maximum effective dose of D1542 (1 mg kg−1, i.v.) a 190 fold increase in the infusion rate of adrenaline was required to restore thrombus formation. In the presence of D1542, removal of endoperoxide metabolites by inhibition of cyclo‐oxygenase with aspirin (5 mg kg−1, i.v.) caused thrombus formation to restart, indicating the ability of the endoperoxide metabolites to inhibit thrombus formation in vivo. These results indicate that, in the stenosed and damaged coronary artery of the dog, redirection of arachidonic acid metabolism by D1542 is more effective at preventing thrombus formation than inhibition of cyclo‐oxygenase by aspirin.

The effects of ICI 185,282 (5(Z)-7-([ 2,4,5-cis]-4-O-hydroxyphenyl-2-trifluoromethyl-1, 3-dioxan-5-yl)heptenoic acid) have been studied on guinea-pig platelets and pulmonary smooth muscle in-vitro and in-vivo. When tested on guinea-pig lung parenchyma in-vitro. ICI 185,282 (1 x 10(-7) M) produced a significant shift in U-46619 response curves (concentration ratio of 13:3); the antagonist (1 x 10(-5) M) did not modify histamine responses. When tested on guinea-pig trachea in-vitro ICI 185,282 (1 x 10(-7) M) caused significant inhibition of U-46619 and PGD2 responses (concentration ratios of 8.3 and 14.1, respectively); the antagonist (1 x 10(-5) M proved less effective against contractions of PGF2 alpha, LTD4 and histamine (concentration ratios of 7.0, 1.5 and 1.6). When added to guinea-pig platelet rich plasma in-vitro, ICI 185,282 (x 10(-6), 1 x 10(-5) M) caused concentration-dependent parallel shifts to the right of U-46619 aggregation curves, yielding concentration ratios of 13.6 and 141.9, respectively. In-vitro, addition of ICI 185,282 (x 10(-5) M) to indomethacin-treated pulmonary smooth muscle did not modify resting tone, neither did it induce aggregation or swelling in platelet-rich plasma preparations. When administered orally to guinea-pigs ICI 185,282 (0.1, 0.5 mg kg-1) caused a significant inhibition of U-46619-induced platelet aggregation ex-vivo which persisted greater than or equal to 8 h. In-vivo, a single oral dose of ICI 185,282 (1 mg kg-1) inhibited bronchospasm induced by U-46619, PGD2, PGF2 alpha, arachidonic acid, LTD4 and PAF; responses to histamine were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

ICI 180080 (5(Z)-7-[2,2-dimethyl-4-(2-hydroxyphenyl)-1,3-dioxan-cis-5-yl] heptenoic acid) potently inhibited contractions of rat and rabbit aortae and guinea-pig trachea elicited by 11,9-epoxymethano PGH2 (U-46619). This antagonism was selective because contractions of aortae to noradrenaline and 5-hydroxytryptamine and trachea to histamine were not antagonized by ICI 180080. Schild analysis of data obtained from experiments on rabbit aortae indicated that this thromboxane receptor antagonism was competitive (pA2 = 7.50, slope = 1.07). Addition of ICI 180080 to human platelet-rich plasma caused dose-related inhibition of U-46619-induced platelet aggregation. This modification of platelet aggregation was not associated with inhibition of thromboxane synthetase, cyclo-oxygenase or lipoxygenase. ICI 180080 did not modify the primary phase of ADP-induced aggregation of human platelets neither did it affect the platelet inhibitory activity of prostacyclin. When dosed orally to anaesthetized guinea-pigs, ICI 180080 (5-50 mg kg-1) caused dose-related inhibition of U-46619-evoked bronchoconstriction. We conclude that ICI 180080 is a potent, selective, competitive, orally active thromboxane antagonist.