Ridogrel inhibits systemic and renal formation of thromboxane A2 and antagonizes platelet thromboxane A2/prostaglandin endoperoxide receptors upon chronic administration to man.

The effects of ridogrel, a dual thromboxane A2 (TXA2) synthase inhibitor and TXA2/prostaglandin (PG) endoperoxide receptor antagonist, on systemic and renal production of prostaglandins and on platelet TXA2/PG endoperoxide receptors was evaluated upon chronic administration (300 mg b.i.d. orally, for 8 and 29 days) to man. Such a medication with ridogrel inhibits the systemic as well as the renal production of TXA2 as measured by the urinary excretion of 2,3-dinor-TXB2 and TXB2 respectively without inducing significant changes in systemic or renal PGI2 production. Simultaneously with the latter effects, the production of TXB2 by spontaneously coagulated whole blood ex vivo is inhibited (greater than 99%) while that of PGE2 and PGF2 alpha is largely increased. Administration of ridogrel causes a three- to five-fold shift to the right of concentration-response curves for U46619 in eliciting platelet aggregation; no tachyphylaxis is observed after 29 days of treatment in this respect. Apart from a reduction of serum uric acid levels with a concomitant increase in urinary uric acid excretion during the first days of treatment, no clinically significant changes in hematological, biochemical, hemodynamic and coagulation parameters occur during the 8 days or 29 days study. The study demonstrates that ridogrel is a potent inhibitor of the systemic as well as renal TXA2 synthase and an antagonist of platelet TXA2/PG endoperoxide receptor in man, covering full activity during 24 h at steady-state plasma level conditions without tachyphylaxis during 29 days of medication. The compound is well tolerated, at least during 1 month of administration.

Vitamin C increases the formation of prostacyclin by aortic rings from various species and neutralizes the inhibitory effect of 15-hydroperoxy-arachidonic acid.

Aortic rings from rats, rabbits and guinea-pigs produce different amounts of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha), the stable breakdown product of prostacyclin, i.e. 2760 +/- 195, 160 +/- 10 and 87 +/- 17 pg 6-oxo-PGF1 alpha per mg wet weight in 30 min. Vitamin C enhances the production of 6-oxo-PGF1 alpha by the aortic tissue of these three species, independent of their basal release. This increase was only significant if vitamin C was present in the preincubation as well as in the incubation fluid. 15-Hydroperoxy-arachidonic acid inhibits the production of 6-oxo-PGF1 alpha (IC50:6 microM) and this inhibitory effect was completely neutralized by vitamin C. The increased production of 6-oxo-PGF1 alpha is not due to an increased release of the substrate arachidonic acid. It is suggested that vitamin C enhances the formation of 6-oxo-PGF1 alpha by protecting the cyclo-oxygenase and PGI-synthase.

Ketoconazole inhibits the biosynthesis of leukotrienes in vitro and in vivo.

Ketoconazole inhibits in vitro (IC50:2.6 X 10(-5) M) the formation of 5-HETE and LTB4 by isolated, carrageenin-elicited rat peritoneal PMN leukocytes, challenged with the Ca2+-ionophore A23187 in the presence of [14C]-arachidonic acid ([14C]-AA). The relative potency of various compounds tested in this respect is NDGA greater than nafazatrom greater than phenidone greater than ketoconazole greater than BW 755C. In contrast to the other compounds studies, ketoconazole in vitro, up to 1 X 10(-4) M, has no effect on the fatty acid cyclo-oxygenase or the 12-lipoxygenase-mediated metabolism of [14C]-AA by isolated human platelets; however, it stimulates the 15-lipoxygenase activity in phenylhydrazine-induced rabbit reticulocytes. After oral administration (10-40 mg/kg, -2 hr), ketoconazole inhibits in a dose-dependent way, the leukotriene-mediated anaphylactic bronchoconstriction in guinea pigs. This study demonstrates that ketoconazole is a comparatively specific and orally active inhibitor of the 5-lipoxygenase activity bearing on the production of leukotrienes derived from arachidonic acid.

Suppression of PAF-induced bronchoconstriction in the guinea-pig by oxatomide: mechanism of action.

Oxatomide potently (ED50 0.9 mg/kg orally, -2 h) attenuates the reduction of pulmonary tidal volume elicited by PAF (250 ng/kg i.v.) in anaesthetized, ventilated and propranolol-treated guinea-pigs. The increase of the pulmonary inflation pressure elicited by PAF (40 ng/kg i.v.) in such animals, ventilated at a fixed tidal volume, is also significantly reduced by the compound, but substantially higher doses (5 mg/kg i.v., -15 min) are required. The potency of oxatomide in the latter respect (50.4% reduction) is equivalent to that of ketotifen at 5 mg/kg i.v. (55% reduction). In spontaneously breathing, anaesthetized guinea-pigs, oxatomide (5 mg/kg i.p., -1 h) significantly reduces the increase in pulmonary resistance, but not the reduction in dynamic compliance, elicited by PAF (30, 60, 90 ng/kg i.v.), suggesting a pharmacological interference mainly with PAF-induced processes in the larger airways. Changes in arterial blood pressure, haemoconcentration, thrombocytopenia and leukopenia induced by PAF in vivo, contraction of guinea-pig lung parenchymal strips, production of superoxide anion by alveolar macrophages, aggregation and release of ATP by platelets challenged with PAF in vitro are not affected by the compound. These observations suggest that oxatomide attenuates the PAF-induced pulmonary reactions by inhibiting the release and/or the effect of allergic mediators elicited by the phospholipid rather than by a direct antagonism at the PAF receptors.

Production of thromboxane and prostaglandins in human blood in the presence of thromboxane synthase inhibitors: a comparison of RIA and GC/MS determinations.

The radioimmunological determination (RIA) of primary prostaglandins (PGs) in serum and plasma was evaluated with gas chromatography/mass spectrometry (GC/MS). Human blood was stimulated in vitro in the presence or absence of the specific thromboxane synthase inhibitor ridogrel. TxB2, 6-keto-PGF1 alpha, PGE2, PGD2 and PGF2 alpha were determined with RIA and GC/MS on the same samples. For GC/MS analysis, prostanoids were extracted and derivatized to their methoximepentafluorobenzyl-esters-trimethylsilyl-ethers. An excellent correlation was observed between levels of all eicosanoids determined by RIA or GC/MS (r = 0.996-0.999) when plasma was spiked with known amounts of PGs and TxB2 (2-500 ng/ml). In stimulated blood, with or without inhibition of thromboxane synthase, the correlation between RIA and GC/MS values remained good, except for 6-keto-PGF1 alpha. RIA largely overestimated the levels of 6-keto-PGF1 alpha and no significant correlation was found with levels detected by GC/MS. The results demonstrate the importance of corroborating the reliability of RIA with GC/MS.

Influence of vitamin C on the metabolism of arachidonic acid and the development of aortic lesions during experimental atherosclerosis in rabbits.

Feeding rabbits a cholesterol-rich diet (0.3%) resulted in morphological changes and a decrease in the prostacyclin production by the aortic endothelium. Addition of vitamin C (0.1%) to the cholesterol-rich diet resulted in a decreased lipid infiltration and intimal thickening. Although there was a tendency to restore the prostacyclin output, vitamin C, in the amount administered, was unable to completely normalise the endothelial PGI2 production.

Vitamin C increases the prostacyclin production and decreases the vascular lesions in experimental atherosclerosis in rabbits.

Feeding a cholesterol-rich diet (0.3%) to rabbits resulted in an intimal thickening and lipid infiltration of the aorta. The prostacyclin production by the vascular endothelium was significantly decreased, after a transient increase after 2 weeks of diet. The arachidonic acid metabolism in platelets was hardly changed. Addition of a low dose vitamin C (150 mg/day) to the cholesterol rich diet resulted in decreased lipid infiltration and intimal thickening and the transient increase of the prostacyclin production was postponed to the 4th week. Although this dose of vitamin C could not restore the decreased prostacyclin production observed after 6 weeks diet, a higher dose of vitamin C (600 mg/day), besides its beneficial effect on the lipid infiltration and the intimal thickening in the thoracic aorta, kept the intimal prostacyclin production at normal levels for at least 8 weeks.

Biphasic response of intimal prostacyclin production during the development of experimental atherosclerosis.

Feeding a cholesterol rich diet (0.3%) to rabbits for up to 10 weeks resulted in morphological changes of the vascular wall. Microscopic evaluation of the aorta revealed a lipid infiltration and an intimal thickening containing foam cells, which both became more pronounced as the cholesterol feeding was more prolonged. The intimal prostacyclin production showed a transient increase after 2 weeks, but was significantly decreased after 6 weeks of diet and remained at this low level during the rest of the experiment. No significant changes in formation of thromboxane B2 by the platelets could be observed, whereas the production of 12-HETE was enhanced.

Stimulation of prostacyclin production by vitamin C in ram seminal vesicle microsomes: possible mode of action.

The enhancing effect of vitamin C on the conversion of arachidonic acid, endoperoxide G2 and endoperoxide H2 to 6-keto-PGF 1 alpha, the stable metabolite of prostacyclin, by ram seminal vesicle microsomes was further investigated. From the incubations of these substrates with 1-tryptophan, catalase, superoxide dismutase and 15-HPETE it became clear that vitamin C apparently acts mainly through neutralization of the oxidative species formed during the reduction of endoperoxide G2 to endoperoxide H2. Although it has also a more direct stimulating activity on the prostacyclin synthase, a possible interference with hydroperoxy derivatives of arachidonic acid cannot be completely ruled out.