ep-045 and dazoxiben

The immunomodulatory effects of thromboxane A2 and prostaglandin E2 on peripheral blood mononuclear leucocytes stimulated with PHA in vitro, and the relationship of this to the time-course of their synthesis in culture, were investigated using prostaglandin E2, a thromboxane A2 synthesis inhibitor (UK37248), a thromboxane A2 mimic (U46619) and a thromboxane A2 receptor blocker (EP045). The inhibitory effect of prostaglandin E2 on PHA-induced human peripheral blood mononuclear leucocyte proliferation diminishes if the addition of PGE2 is delayed. If added 4 hr after a maximum concentration of PHA (5 micrograms/ml), the effect of PGE2 was reduced by 60%. If a submaximal concentration of PHA (1 microgram/ml) was used, the effect of PGE2 was not reduced if added 4 hr later but fell by about 60% after 16 hr. UK37248 moderately inhibited PHA-induced activation while substantially inhibiting thromboxane A2 synthesis and simultaneously enhancing PGE2 synthesis. The enhanced accumulation of PGE2 occurs while sensitivity to PGE2 is dropping. U46619, exogenously applied as a thromboxane A2 mimic, inhibited PHA-induced activation at concentrations that did not significantly alter PGE2 synthesis. EP045, which may modulate the effects of endogenous thromboxane A2 by blocking receptors, did not alter PHA-induced activation. We conclude that thromboxane A2 may have a role in inhibiting PHA-induced activation on the basis of the effect of U46619. However, this study highlights difficulties in utilizing prostaglandin and thromboxane receptor and synthesis inhibitors to examine their endogenous role in the modulation of mitogen-induced activation in vitro. If sensitivity to the purported endogenous substance is limited to the early stages of culture and if only low levels are synthesized at this early stage, then blocking drugs would have little effect.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Adult; Dinoprostone; Humans; Imidazoles; Kinetics; Leukocytes; Lymphocyte Activation; Male; Phytohemagglutinins; Prostaglandin Endoperoxides, Synthetic; Prostaglandins E; Prostaglandins, Synthetic; Thromboxane A2

The effects of indomethacin, dazoxiben and EPO45 on collagen-induced platelet aggregation in vivo were studied in guinea-pigs and rats to determine the involvement of the prostaglandin endoperoxide/thromboxane A2 pathway in the aggregatory response. Indomethacin and EPO45 (a thromboxane receptor antagonist) partially inhibited platelet aggregation in rats. It was concluded that only one third of the aggregatory response to collagen was mediated by the products of cyclo-oxygenase conversion of arachidonic acid. In rats, dazoxiben was inactive although the conversion of the prostaglandin endoperoxides to thromboxane A2 was inhibited (measured as thromboxane B2). 6-keto PGF1 alpha was detected in plasma after collagen was injected into dazoxiben-treated rats. In this species therefore, the endoperoxides have significant aggregatory activity whilst the apparent increase in the level of prostacyclin was not sufficient to have any anti-aggregatory effect. All three drugs were active in the guinea-pig. About 60% of the aggregatory response to collagen was due to the products of the cyclo-oxygenase pathway, the main mediator being thromboxane A2. In guinea-pigs, dazoxiben also elevated 6-keto PGF1 alpha in the plasma after an injection of collagen. However, this apparent increase in prostacyclin production did not contribute to the anti-aggregatory effect.

Topics: 6-Ketoprostaglandin F1 alpha; Animals; Collagen; Female; Guinea Pigs; Imidazoles; In Vitro Techniques; Indomethacin; Male; Platelet Aggregation; Prostaglandin Endoperoxides; Prostaglandin Endoperoxides, Synthetic; Prostaglandin H2; Prostaglandins H; Prostaglandins, Synthetic; Rats; Rats, Inbred Strains; Receptors, Prostaglandin; Receptors, Thromboxane; Species Specificity; Thromboxane A2; Thromboxane B2; Thromboxanes

The effects of a cyclooxygenase inhibitor (indomethacin), a thromboxane synthetase inhibitor (dazoxiben), and a thromboxane antagonist (EPO45) on rabbit platelet aggregation induced by collagen were studied and compared with effects on platelet uptake both by damaged rabbit aorta and by collagen-coated glass. Platelet aggregation and associated release of serotonin were inhibited to a similar extent both by indomethacin and EPO45. Dazoxiben had a minimal inhibitory effect on aggregation but reduced the release of serotonin by about 40% compared with control. Platelet uptake onto collagen-coated glass was markedly reduced both by indomethacin and EPO45 but not by dazoxiben. In contrast, EPO45 and dazoxiben were equally effective in reducing platelet adhesion to damaged rabbit aorta. At the concentrations used for adhesion studies the formation of thromboxane B2 was reduced both by dazoxiben and by indomethacin (both greater than 95% inhibition compared with control) and to a lesser extent by EPO45 (less than 40% inhibition). The results indicate that thromboxane A2 (and cyclic endoperoxide) released by adherent platelets may enhance thromboxane synthesis and promote platelet uptake both onto collagen-coated glass and onto damaged rabbit aorta. In the presence of vascular tissue, cyclic endoperoxides are readily metabolized and thereby removed. The potential antithrombotic activity of TXA2 synthetase inhibitors could be impaired in situations in which endoperoxide clearance is limited (e.g., accompanying platelet uptake onto artificial surfaces) but not at the damaged vessel wall. Thus, both inhibitors and antagonists are likely to have similar potency as antithrombotic agents in vivo.

Topics: 6-Ketoprostaglandin F1 alpha; Animals; Collagen; Imidazoles; In Vitro Techniques; Indomethacin; Male; Muscle, Smooth, Vascular; Platelet Adhesiveness; Platelet Aggregation; Prostaglandins, Synthetic; Rabbits; Serotonin; Thromboxane B2