Bioassay of thromboxane A2 by contraction of guinea-pig lung parenchymal strip

Fenflumizole: interactions with the arachidonic acid cascade

Fenflumizole (2-(2,4-difluorophenyl)-4,5-bis(4-methoxyphenyl)imidazole), a new non-steroidal anti-inflammatory agent, was investigated for interference with cyclo-oxygenase activity in vivo, ex vivo and in vitro in comparison with indomethacin (and aspirin). Fenflumizole was comparable to indomethacin ex vivo in inhibition of thromboxane (TX)A2 production in rabbit platelets and inhibition of prostaglandin (PG)I2 (approximately prostacyclin) generation in rabbit mesenteric arteries and in vivo as an inhibitor of PGE2 formation in inflammatory exudates in rats. Fenflumizole was 18 times less active than indomethacin in inhibition of PGE2 synthesis in vitro and 170 times weaker as an inhibitor of PGI2 generation in the rat stomach mucosa ex vivo. Fenflumizole was 20-50 times more potent than indomethacin in vivo in inhibition of arachidonic acid induced bronchoconstriction in guinea-pigs, in inhibition of platelet aggregation on tendons superfused with blood from rabbits and in vitro in inhibition of aggregation of human and rabbit platelets. Neither fenflumizole nor indomethacin inhibited TXA2-synthetase in vitro. Aspirin-when tested-was less potent than fenflumizole and indomethacin. It is concluded that fenflumizole is a potent cyclo-oxygenase inhibitor. The very potent activity of fenflumizole against platelet aggregation and bronchoconstriction suggests a selectivity in the mode of action. The weak inhibition of gastric PGI2 generation may account for the previously observed weak gastro-ulcerogenicity of fenflumizole.

Evidence for a mediator role of thromboxane A2 in the myotropic action of leukotriene B4 (LTB4) on the guinea-pig lung

The mechanism of action of LTB4 has been investigated on the guinea-pig lung parenchymal strip. Mepacrine (20 microgram/ml), an inhibitor of phospholipase A2, abolished the action of LTB4 on parenchymal strips. Eicosatetraynoic acid (10 microgram/ml) and BW755C (40 microgram/ml) which are inhibitors of cyclooxygenase and lipoxygenase pathways, produced a marked inhibition of the lung strip contraction to LTB4. Similarly, aspirin (30 micrograms/ml) and flufenamate (1 microgram/ml) showed a strong inhibition of the contraction of parenchymal strips to LTB4; these results suggested that cyclooxygenase products mediate the action of LTB4. The response to LTB4 was unaffected by 15-hydroperoxyeicosatatraenoic acid (15-HPETE; 1 microgram/ml) while L8027 (25 ng/ml) reduced the contraction by 50%, suggesting that thromboxane A2 rather than prostacyclin was involved. Since parenchymal strips do not appear to be very sensitive to PGF2 alpha, PGE2 and the endoperoxides, and since effluents from LTB4-treated lungs produced contractions of lung strip and rabbit aorta which were reduced after 5 min. at 25 degrees, thromboxane A2 was postulated to mediate the lung effect of LTB4. The release of thromboxane B2 (TxB2) from lungs stimulated with LTB4 was confirmed by gas-chromatography-mass spectrometric (GC-MS) analyses.