Inhibition Of Thromboxane Formation Research Articles

Thromboxane Formation Assay to Identify High On-Treatment Platelet Reactivity to Aspirin

Platelet inhibition by aspirin is indispensable in the secondary prevention of cardiovascular events. Nevertheless, impaired aspirin antiplatelet effects (high on-treatment platelet reactivity [HTPR]) are frequent. This is associated with an enhanced risk of cardiovascular events. The current gold standard to evaluate platelet hyper-reactivity despite aspirin intake is the light-transmittance aggregometry (LTA). However, pharmacologically, the most specific test is the measurement of arachidonic acid (AA)-induced thromboxane (TX) B2 formation. Currently, the optimal cut-off to define HTPR to aspirin by inhibition of TX formation is not known. Therefore, in this pilot study, we aimed to calculate a TX formation cut-off value to detect HTPR defined by the current gold standard LTA. We measured platelet function in 2,507 samples. AA-induced TX formation by ELISA and AA-induced LTA were used to measure aspirin antiplatelet effects. TX formation correlated nonlinearly with the maximum of aggregation in the AA-induced LTA (Spearman's rho R = 0.7396; 95% CI 0.7208-0.7573, p < 0.0001). Receiver operating characteristic analysis and Youden's J statistics revealed 209.8 ng/mL as the optimal cut-off value to detect HTPR to aspirin with the TX ELISA (area under the curve: 0.92, p < 0.0001, sensitivity of 82.7%, specificity of 90.3%). In summary, TX formation ELISA is reliable in detecting HTPR to aspirin. The calculated cut-off level needs to be tested in trials with clinical end points.

Abstract 15635: Consumption of Aspirin and Naproxen by Platelet Donors and the Quality of Platelet Transfusion

Over-the-counter pain killers are amongst the most frequently consumed drugs world-wide. Individuals who self-report ingesting aspirin or aspirin containing drugs within the preceding 48-72 hours - depending on regional practice - are excluded from platelet donation, given the assumed platelet functional inhibition due to irreversible acetylation of platelet cyclooxygenase-1 (COX-1) by aspirin and the consequent suppression of thromboxane formation. Other nonsteroidal anti-inflammatory drugs, which bind COX-1 tightly enough to cause long acting platelet inhibition, such as naproxen, may also impact the quality of platelet preparations for transfusion but are not typically exclusionary. The prevalence of functionally relevant exposure of platelet donors to aspirin and naproxen is unknown. We used sensitive and specific mass spectrometry methods to quantitate the acetylation of platelet COX-1 by aspirin and platelet protein binding of naproxen in platelet transfusion units as indices of donor exposure to these drugs. Platelet thromboxane formation was assessed as a functional parameter. We screened single-donor apheresis platelet units transfused at the Hospital of the University of Pennsylvania. Over 30 % (71 out of 235) of the analyzed platelet units showed evidence of aspirin exposure ranging from residual COX-1 acetylation after an appropriate washout period to complete acetylation suggestive of recent aspirin exposure. Approximately 7% (16 out of 235) of the platelet units showed complete COX-1 acetylation and inhibition of thromboxane formation. More than 15% (22 out of 143) of analyzed platelet units showed high naproxen concentrations that were associated with depression of thromboxane formation. The prevalence of aspirin and naproxen consumption by platelet donors is as high as would be expected in the general population and results in transfusion of inhibited platelets with suppressed thromboxane formation.

Platelets in vascular disease

Platelets play a key role in primary hemostasis and in the pathogenesis of atherosclerosis and atherothrombotic events such as stroke and myocardial infarction. When a plaque ruptures, platelets adhere to the underlying collagen matrix, become activated and aggregate, which may lead to vascular occlusions. Hemorheological aspects are intimately involved in this process. The assessment of this platelet function in vitro is difficult and has not reached the stage of routine use. Inhibition of platelet aggregation is the corner stone of any treatment of vascular disease. It is achieved mainly by to mechanisms, inhibition of thromboxane formation by acetylsalicylic acid, and with ADP receptor antagonists such as clopidogrel. Newer agents are being developed with the difficult mission to inhibit platelet aggregation more efficiently, and simultaneously reduce the risk of bleeding.

The effect of erythropoietin on platelet and endothelial activation markers: A prospective trial in healthy volunteers

Erythropoietin (EPO) enhances formation of red blood cells and also affects thrombopoiesis and platelet function. We hypothesized that the effect of EPO may be reflected by changes in thromboxane B2 (TXB2) and endothelial cell function. Six male and six female subjects received recombinant human epoetin alpha (Erypo®) intravenously (300 U/kg). Biomarker levels were assessed at baseline and 4, 24, 48 and 72 hours after infusion. Epoetin alpha increased TXB2 levels by 140%, which reached significance at 48 hours (6.6 ± 5 ng/ml vs. 15 ± 9 ng/ml; p = 0.044) and remained at that level at 72 hours. In line, epoetin alpha increased E-selectin levels by 25% already at 24 hours (39 ± 21 ng/ml vs. 49 ± 26 ng/ml; p < 0.001) which stayed at this level until 72 hours (p < 0.001). The raise in platelet activation markers corresponded to an 88% increase in reticulocyte count (43 ± 10 × 109/l vs. 81 ± 17 × 109/l; p < 0.001) and a 9% increase in platelet count at 72 hours (224 ± 45 × 109/l vs. 244 ± 52 × 109/l; p = 0.005). Thrombomodulin and von Willebrand factor concentrations were not significantly altered by epoetin alpha. Interestingly, gender differences in the baseline levels of E-selectin and thrombomodulin were observed. E-selectin and thrombomodulin levels were doubled in men compared to women (51 ± 24 and 28 ± 10 ng/ml; p = 0.025 and 30 ± 5 ng/ml vs. 16 ± 5 ng/ml; p = 0.002, respectively). EPO increases TXB2 serum levels and soluble E-selectin. Further studies are needed to investigate whether these markers might be useful for estimation of thromboembolic risk during EPO-therapy and whether inhibition of thromboxane formation may lower thrombotic complications during EPO treatment: NCT01392612.

Inhibition of platelet thromboxane formation and phosphoinositides breakdown by diisoeugenol.

Diisoeugenol inhibited the platelet aggregation and ATP release of rabbit platelets caused by ADP, arachidonic acid, platelet-activating factor (PAF), collagen and thrombin. Prolongation of the incubation time of platelets with diisoeugenol did not cause further inhibition and the aggregability of platelets could not be restored after washing. In human platelet-rich plasma, diisoeugenol inhibited the biphasic aggregation and ATP release induced by adrenaline and ADP in a concentration-dependent manner. Thromboxane B2 formation caused by arachidonic acid, collagen and thrombin was markedly inhibited by diisoeugenol in a concentration-dependent manner. Diisoeugenol also inhibited the formation of inositol monophosphate caused by collagen, PAF and thrombin. The cAMP level of washed platelets was not changed by diisoeugenol. It is concluded that the antiplatelet effect of diisoeugenol is due to the inhibition of thromboxane formation and phosphoinositides breakdown.

Mechanism of action of p-chlorobiphenyl on the inhibition of platelet aggregation.

p-Chlorobiphenyl (1-50 microM) concentration-dependently inhibited the aggregation and release reaction of rabbit washed platelets induced by arachidonic acid and collagen, but not those induced by platelet-activating factor (PAF), U46619 and thrombin. The IC50 values of p-chlorobiphenyl on the arachidonic acid and collagen-induced platelet aggregation were 2.9 +/- 0.5 and 12.8 +/- 2.3 microM, respectively. The formation of both platelet thromboxane B2 and prostaglandin D2 caused by arachidonic acid was inhibited by p-chlorobiphenyl concentration-dependently. In myo-[3H]inositol-labeled and fura-2-loaded platelets. [3H]inositol monophosphate generation and the rise in intracellular Ca2- stimulated by arachidonic acid were inhibited by p-chlorobiphenyl. In human platelet-rich plasma, p-chlorobiphenyl and indomethacin prevented the secondary aggregation and blocked ATP release from platelets induced by adenosine 5'-diphosphate and adrenaline without affecting the primary aggregation. It is concluded that p-chlorobiphenyl may be a cyclo-oxygenase inhibitor and its antiplatelet action is mainly due to the inhibition of thromboxane formation.

Release of sphingosine‐1‐phosphate from human platelets is dependent on thromboxane formation

Platelets release the immune-modulating lipid sphingosine-1-phosphate (S1P). However, the mechanisms of platelet S1P secretion are not fully understood. The present study investigates the function of thromboxane (TX) for platelet S1P secretion during platelet activation and the consequences for monocyte chemotaxis. S1P was detected using thin-layer chromatography in [(3)H]sphingosine-labeled platelets and by mass spectrometry. Monocyte migration was measured in modified Boyden chamber chemotaxis assays. Release of S1P from platelets was stimulated with protease-activated receptor-1-activating peptide (PAR-1-AP, 100 μM). Acetylsalicylic acid (ASA) and two structurally unrelated reversible cyclooxygenase inhibitors diclofenac and ibuprofen suppressed S1P release. Oral ASA (500-mg single dose or 100 mg over 3 days) attenuated S1P release from platelets in healthy human volunteers ex vivo. This was paralleled by inhibition of TX formation. S1P release was increased by the TX receptor (TP) agonist U-46619, and inhibited by the TP antagonist ramatroban and by inhibitors of ABC-transport. Furthermore, thrombin-induced release of S1P was attenuated in platelets from TP-deficient mice. Supernatants from PAR-1-AP-stimulated human platelets increased the chemotactic capacity of human peripheral monocytes in a S1P-dependent manner via S1P receptors-1 and -3. These effects were inhibited by ASA-pretreatment of platelets. TX synthesis and TP activation mediate S1P release after thrombin receptor activation. Inhibition of this pathway may contribute to the anti-inflammatory actions of ASA, for example by affecting activity of monocytes at sites of vascular injury.

Towards a definition of aspirin resistance: a typological approach

'Aspirin resistance' is a poorly defined term to describe the inability of aspirin to protect individuals from thrombotic complications and there are conflicting reports on incidence rates and clinical relevance of this phenomenon. Using collagen (1 w g/ml)-induced platelet aggregation and thromboxane formation (measured as thromboxane B 2 ) in citrated platelet-rich plasma, this study demonstrates that aspirin resistance can be classified into three distinct types. In aspirin responders, both, collagen-induced platelet aggregation and thromboxane formation was completely (>95%) inhibited by oral aspirin treatment (100 mg/day). In type I resistance (pharmacokinetic type), oral treatment with aspirin was ineffective but addition of aspirin (100 w M) in vitro resulted in a complete inhibition of collagen-induced platelet aggregation and thromboxane formation. In type II resistance (pharmacodynamic type), neither oral treatment with aspirin nor addition of aspirin in vitro inhibited collagen-induced platelet aggregation and thromboxane formation. In type III resistance (pseudo-resistance), platelet aggregation was induced by a low concentration of collagen (1 w g/ml) despite of a complete inhibition of thromboxane formation by oral aspirin treatment. This typology of aspirin resistance should help to clarify the mechanisms, the actual rate, and the possible clinical consequences of this phenomenon.

Inhibitory effect of curcumin, a food spice from turmeric, on platelet-activating factor- and arachidonic acid-mediated platelet aggregation through inhibition of thromboxane formation and Ca 2+ signaling

Curcumin, a dietary spice from turmeric, is known to be anti-inflammatory, anticarcinogenic, and antithrombotic. Here, we studied the mechanism of the antiplatelet action of curcumin. We show that curcumin inhibited platelet aggregation mediated by the platelet agonists epinephrine (200 μM), ADP (4 μM), platelet-activating factor (PAF; 800 nM), collagen (20 μg/mL), and arachidonic acid (AA: 0.75 mM). Curcumin preferentially inhibited PAF- and AA-induced aggregation ( ic 50; 25–20 μM), whereas much higher concentrations of curcumin were required to inhibit aggregation induced by other platelet agonists. Pretreatment of platelets with curcumin resulted in inhibition of platelet aggregation induced by calcium ionophore A-23187 ( ic 50; 100 μM), but curcumin up to 250 μM had no inhibitory effect on aggregation induced by the protein kinase C (PKC) activator phorbol myrsitate acetate (1 μM). Curcumin (100 μM) inhibited the A-23187-induced mobilization of intracellular Ca 2+ as determined by using fura-2 acetoxymethyl ester. Curcumin also inhibited the formation of thromboxane A 2 (TXA 2) by platelets ( ic 50; 70 μM). These results suggest that the curcumin-mediated preferential inhibition of PAF- and AA-induced platelet aggregation involves inhibitory effects on TXA 2 synthesis and Ca 2+ signaling, but without the involvement of PKC.

Effects of apocynin, a drug isolated from the roots of Picrorhiza kurroa, on arachidonic acid metabolism

Apocynin is a constituent of root extracts of the medicinal herb Picrorhiza kurroa and has been shown to possess anti-inflammatory properties. We investigated the effects of apocynin on the production of arachidonic acid-derived inflammatory mediators by guinea pig pulmonary macrophages. Apocynin concentration-dependently inhibited the formation of thromboxane A 2, whereas the release of prostaglandins E 2 and F 2α was stimulated. Apocynin potently inhibited arachidonic acid-induced aggregation of bovine platelets, possibly through inhibition of thromboxane formation. The present results suggest that apocynin might, beside its therapeutic effects in inflammatory conditions when given in a root extract of P. kurroa, also be a valuable tool in the development of new anti-inflammatory or anti-thrombic drugs.

A new role for nicotine: selective inhibition of thromboxane formation by direct interaction with thromboxane synthase in human promyelocytic leukaemia cells differentiating into macrophages.

Thromboxane, one of the major oxygenated arachidonic acid metabolites of human macrophages, is the most potent vasoconstricting and proaggregatory molecule known. In addition, thromboxane has been shown to be related to host defence mechanisms. We studied the effects of nicotine and its major metabolites on thromboxane formation using cultured macrophage-like cells (HL-60), microsomal assays and purified thromboxane synthase. In intact cells, nicotine, cotinine and methylnicotine at submicromolar concentrations inhibited the rate of conversion of both arachidonic acid and the unstable endoperoxide prostaglandin H2 into thromboxane but not into other eicosanoids. This indicates that nicotine selectively inhibits thromboxane synthase at concentrations that are readily observed in the circulation of smokers. Microsomal assays revealed that nicotine decreased the maximal velocity of thromboxane synthase without affecting the apparent affinity of the enzyme for its substrate. In contrast, no effect of nicotine on kinetic parameters of prostaglandin H synthase or prostacyclin synthase could be observed. Difference spectra, using purified thromboxane synthase, revealed that nicotine directly interacts with the enzyme, presumably by binding the nitrogen of the nicotine ring structure to the iron of the cytochrome P-450 component of thromboxane synthase.

Inhibition of platelet aggregation by some flavonoids

The inhibitory effects of five flavonoids on the aggregation and secretion of platelets were studied. These flavonoids inhibited markedly platelet aggregation and ATP release of rabbit platelets induced by arachidonic acid or collagen, and slightly those by platelet-activating factor. ADP-induced platelet aggregation was also suppressed by myricetin, fisetin and quercetin. The IC 50 on arachidonic acid-induced platelet aggregation was: fisetin, 22 μM; kaempferol, 20 μM; quercetin, 13 μM; morin, 150 μM<IC 50<300 μM. The thromboxane B 2 formations were also inhibited by flavonoids in platelets challenged with arachidonic acid. Fisetin, kaempferol, morin and quercetin antagonized the aggregation of washed platelets induced by U46619, a thromboxane A 2/prostaglandin endoperoxides mimetic receptor agonist. In human platelet-rich plasma, quercetin prevented the secondary aggregation and blocked ATP release from platelets induced by epinephrine or ADP. These results demonstrate that the major antiplatelet effect of flavonoids tested may be due to both the inhibition of thromboxane formation and thromboxane receptor antagonism.

The antiplatelet effect of daily low dose enteric-coated aspirin in man: A time course of onset and recovery

We have studied the onset and recovery of inhibition of platelet function by low dose aspirin. Enteric-coated aspirin 50mg daily was administered to five human volunteers for five weeks and then 100mg daily was given for a further five weeks. We studied platelet aggregation and thromboxane formation in response to a range of stimuli: ADP, adrenaline, arachidonate and collagen, and also measured thromboxane formation after coagulation of whole blood (serum thromboxane). The onset of inhibition of platelet aggregation was progressive over several days for each of the four platelet stimuli, and was synchronous with the inhibition of thromboxane formation. Maximum inhibition occurred by day three for the weak stimuli ADP and adrenaline, by day five for the stronger stimuli arachidonate and collagen, but did not occur until day eight for serum thromboxane. Further inhibitory effects on both aggregation and thromboxane generation were observed after 100mg daily. Two weeks after the cessation of aspirin the responses to collagen and arachidonate and serum thromboxane had returned to normal. Platelet aggregation in response to the weaker stimuli, ADP and adrenaline, still showed detectable inhibition two weeks after cessation of aspirin, but had returned to normal by four weeks. These experiments provided no evidence for an effect of aspirin on platelets separate to its effect on cyclooxygenase. The onset and recovery of inhibition of platelet function by low dose aspirin was dependent on the strength of the stimulus studied.

Inhibition of Platelet Thromboxane Formation and Phosphoinositides Breakdown by Osthole from Angelica pubescens

Osthole, isolated from Chinese herb Angelica pubescens, inhibited platelet aggregation and ATP release induced by ADP, arachidonic acid, PAF, collagen, ionophore A23187 and thrombin in washed rabbit platelets. It showed a weak activity in platelet-rich plasma. Osthole inhibited the thromboxane B2 formation caused by arachidonic acid, collagen, ionophore A23187 and thrombin in washed platelets, and also the thromboxane B2 formation caused by the incubation of lysed platelet homogenate with arachidonic acid. The generation of inositol phosphates in washed platelets caused by collagen, PAF and thrombin was suppressed by osthole. These data indicate that the inhibitory effect of osthole on platelet aggregation and release reaction was due to the inhibition of thromboxane formation and phosphoinositides breakdown.

Modulation of Platelet Function by SIN-1A, a Metabolite of Molsidomine

Molsidomine or its active metabolite SIN-1A have antithrombotic properties in experimental models. This effect seems to be related, at least in part, to their antiplatelet activities. SIN-1A inhibited platelet adhesion to collagen, platelet aggregation, and the release reaction in in vitro experiments. SIN-1A acts as an early step of platelet activation by inhibiting calcium influx and phospholipase activity, which leads to inhibition of thromboxane formation and fibrinogen binding. Antiplatelet properties were observed after oral absorption in healthy volunteers and in patients with myocardial infarction after intravenous infusion.

Effect of single oral administrations of non steroidal antiinflammatory drugs to healthy volunteers on arachidonic acid metabolism in peripheral polymorphonuclear and mononuclear leukocytes

The effects of a single oral administration of acetylsalicylic acid (500 mg), indomethacin (50 mg) and piroxicam (40 mg) to healthy volunteers on functional and biochemical parameters of platelets, polymorphonuclear (PMN) and mononuclear (MNL) leukocytes were evaluated. Blood was collected before and two hours after the drug intake and blood cells separated according to conventional techniques. The considered drugs almost completely suppressed the aggregation of platelets, whereas they did not affect either PMN and MNL aggregation. Superoxide anion generation by leukocytes was significantly increased after aspirin intake, whilst a significant reduction (PMN), or no effect (MNL) was observed after piroxicam and indomethacin respectively. The formation of arachidonate metabolites via the 5-lipoxygenase pathway by PMN and MNL challenged with 10 μM A23187 was unchanged following aspirin and indomethacin. In this respect a selective increase of 5-HETE and LTC 4 synthesis by MNL only was detected after piroxicam administration. The three drugs similarly reduced TXB 2 synthesis by platelets and PMN (−80% for aspirin and indomethacin, and −40% for piroxicam). As far as MNL is concerned, aspirin inhibited this metabolite by 80%, while indomethacin reduced it by 40% only. In contrast piroxicam increased TXB 2 synthesis by stimulated MNL. It can be concluded that the considered antiinflammatory drugs 1) differently affect the cyclooxygenase enzyme in platelets and leukocytes; 2) at variance with the situation in platelets, the inhibition of thromboxane formation by leukocytes is not related to modifications of cellular function; 3) the formation of arachidonate metabolites via the 5-lipoxygenase pathway is affected by piroxicam only.

Plasma concentrations and anti-platelet effects after low dose acetylsalicylic acid

The present study has investigated whether low-dose acetylsalicylic acid (ASA) can inhibit platelet aggregation locally at its site of gastrointestinal absorption without concentrations in the systemic circulation high enough for inhibition of cyclooxygenase. For this purpose platelet aggregation, thromboxane formation as well as ASA plasma concentrations were measured in 8 volunteers before oral intake of 100 mg ASA as well as 20 to 300 minutes thereafter. At each time 5 ml of blood were mixed with 5 ml of blood obtained from a second, untreated volunteer. Aggregation and thromboxane formation were also determined in these mixed blood samples. The same protocol was performed with 4 volunteers after administration of 1500 mg ASA as well as after no drug intake. In a separate experiment the concentration-effect-relationship of ASA was assessed in vitro. One hundred and forty minutes after administration of 100 mg ASA aggregation and thromboxane formation were significantly decreased to 49.4 and 4.596 of the initial values, respectively, whereas In the mixed blood sample aggregation was not impaired. Inhibition of thromboxane formation was constantly 7396 of the Inhibition observed in the unmixed sample throughout the study period and thus most probably was caused by dilution of the platelets of the untreated volunteer by the inactivated platelets of the ASA-treated volunteer. These data suggest the absence of pharmacologically active drug concentrations in the peripheral blood. ASA plasma concentration was highest after 40 minutes (2.2 ± 1.6 μg/ml; n=5). After the 1500 mg ASA dose platelet function and thromboxane formation decreased to 29.8 and 2.0% of the inital values, respectively. Furthermore, aggregation and thromboxane formation in the mixed blood sample were markedly reduced. Thus, after the high dose of ASA effective plasma concentrations were present in the peripheral circulation. Highest ASA plasma concentrations were 21.1 ± 8.9 μg/ml. IC 50 values were 1.00 ± 0.36 and 0.30 ± 0.05 μg/ml for aggregation and thromboxane formation in vitro, respectively. It is concluded that low dose ASA can effectively inhibit platelet function without producing pharmacologically active concentrations in the peripheral circulation.

Dose-related kinetics of aspirin. Presystemic acetylation of platelet cyclooxygenase.

When aspirin is administered by mouth in low doses, poor systemic bioavailability may contribute to its apparent dose-related "selective inhibition" of thromboxane A2 formation. Systemic bioavailability of orally administered aspirin is necessary to inhibit prostacyclin synthesis by systemic vascular endothelium, whereas cumulative inhibition of thromboxane A2 formation by platelets may occur in the presystemic (portal) circulation. We simultaneously administered unlabeled aspirin orally and deuterium-labeled aspirin intravenously in five healthy volunteers. This permitted an estimation of the bioavailability of an oral dose from the ratio of plasma drug concentration-time curves for the labeled and the unlabeled species. Systemic bioavailability ranged from 46 to 51 per cent of single oral doses of 20, 40, 325, and 1300 mg of aspirin. Bioavailability was similar after single-dose and long-term oral administration of 325 mg. Thromboxane B2 formation in serum ex vivo after oral administration of 20 mg of unlabeled aspirin was reduced 39 per cent before aspirin was detected in the systemic circulation. Furthermore, incubation of simulated peak plasma aspirin concentrations in whole blood in vitro underestimated the inhibition of thromboxane B2 ex vivo after oral administration of 20 or 40 mg of unlabeled aspirin. These data are consistent with presystemic inhibition of platelets by aspirin and suggest that biochemical "selectivity" might be enhanced by slow administration of very low doses of aspirin, thereby optimizing conditions for cumulative, presystemic acetylation of platelet cyclooxygenase and inhibition of thromboxane formation.

Inhibition of ADP-induced platelet aggregation by reduced factor Xa.

Treatment of blood coagulation factor Xa with insolubilized hexyl-agarose derivative of prostaglandin E1 (PGE1) results in the generation of two sulfhydryl groups in the protein molecule. The reduced factor Xa was found to be a potent inhibitor of platelet aggregation and thromboxane A2 synthesis induced by ADP. In contrast to the inhibition of thromboxane formation, the reduced factor Xa had no effect on the formation of PGE2 indicating that thromboxane synthetase might be selectively inhibited by the reduced factor Xa. Incubation with oxidized glutathione reversed the inhibitory activity of factor Xa previously exposed to the insolubilized hormone. Soluble PGE1 also reduces factor Xa, but more slowly than the insolubilized PGE1. PGE1 also exhibits reducing ability as tested with redox dyes. Reduction of factor Xa by dithiothreitol also transformed the coagulation factor into an inhibitor of platelet aggregation and thromboxane A2 formation. These experiments indicate that reduction of factor Xa leads to a reversible alteration of the molecule which inhibits platelet aggregation induced by ADP. This effect of reduced factor Xa is probably mediated through the inhibition of thromboxane A2 synthesis.

Sodium salicylate interferes with the inhibitory effects of aspirin and indomethacin on human platelets

The interference of sodium salicylate with the effects of acetylsalicylic acid (ASA, aspirin) and indomethacin on arachidonic acid-induced platelet aggregation and thromboxane formation was studied in human platelet rich plasma. ASA and indomethacin suppressed both aggregation and the concomitant formation of thromboxane B whereas sodium salicylate alone had no significant effect on these parameters of platelet function. It did, however, partially prevent the inhibitory effects of ASA and indomethacin on platelet aggregation when it was added to platelet rich plasma before ASA or indomethacin. The inhibition of thromboxane formation by indomethacin was also prevented by sodium salicylate. When sodium salicylate was added to platelet rich plasma after ASA or indomethacin it did not modify the effects of these drugs. The present study indicates that sodium salicylate interferes with the effects of ASA and indomethacin on human platelet cyclo-oxygenase in vitro.