RS-5186, a novel thromboxane synthetase inhibitor with a potent and extended duration of action

RS-5186, sodium 6-[2-[1-(1H)-imidazolyl]methyl-4,5-dihydrobenzo[b]thiophene]- carboxylate, inhibited platelet thromboxane A2 (TXA2) synthetase with IC50 values of 6 nM and 13 nM for human and rabbit microsomes, respectively. It had a selectivity for TXA2 synthetase 10(5)-fold greater than that for cyclooxygenase, PGI2 synthetase, 5-lipoxygenase and phospholipase A2. When administered orally or intravenously to dogs at 1 mg/kg, RS-5186 suppressed serum TXB2 levels almost completely with sustained duration of action: the suppression during 0.5 hr to 8 hr after dosing was more than 90%, and was 70-80% at 24 hr. Similar suppression of serum TXB2 levels was observed in rats and rabbits. Such suppression by RS-5186 was more potent than that by OKY-046 and CV-4151. Serial administration of RS-5186 (0.1 mg/kg/day p.o.) to dogs for 7 days decreased the serum TXB2 levels constantly during the medication, and no rebound phenomenon was observed after the medication was stopped. In a thrombotic model induced by sodium arachidonate injection in rabbits, RS-5186 at 1 mg/kg p.o. completely protected against sudden death (ED50 = 0.12 mg/kg, 1 hr after dosing) and this protective effect extended over 8 hr. All these results show that RS-5186 is a potent and highly selective TXA2 synthetase inhibitor with a long duration of action, and suggest that the compound could be useful in diseases where TXA2 is involved.     

The role of thromboxane (TX) A2 in rabbit arterial thrombosis induced by endothelial damage

To clarify the role of thromboxane (TX) A2 in arterial thrombus formation, we examined the antithrombotic effects of both a TXA2 synthetase inhibitor (CV-4151) and a TXA2 receptor antagonist (AA-2414) on the rabbit common carotid artery thrombosis which was induced by injury of the endothelium by treatment with 0.25% pronase solution. CV-4151 (1,10 mg/kg, p.o.) and AA-2414 (10 mg/kg, p.o.) significantly inhibited thrombus formation. Furthermore, the combined use of CV-4151 and AA-2414 (0.1 mg/kg, p.o. each) significantly inhibited thrombus formation, though these drugs at the same doses had no effect when administered singly. The plasma level of 11-dehydro TXB2 increased significantly during thrombus formation, and CV-4151 (10 mg/kg) markedly inhibited this increase. There was a significant correlation between the in vivo antithrombotic effects of these drugs and their ex vivo inhibitory effects on arachidonic acid-induced platelet aggregation. The antithrombotic effect of CV-4151 also correlated significantly with its ability to inhibit the production of serum TXA2. These results show that TXA2 may play an important role in the thrombus formation in arterial thrombosis.     

Anti-thrombotic activity of PDR,a newly synthesized L-Arg derivative,on three thrombosis models in rats

The possibility of a newly synthesized L-arginine derivative, polyaspartoyl-L-arginine (PDR), as a novel anti-thrombotic agent and its mode of action were investigated. The anti-platelet effects of PDR in rats ex vivo, anti-thrombotic effects in three thrombosis models in rats and its effect on some autacoids (nitric oxide [NO], thromboxane [TXA2] and prostacyclin [PGI2]) were studied. PDR (i.g.) significantly inhibited ADP-, collagen- or thrombin-induced rat platelet aggregation. In arteriovenous shunt model and ferric chloride-induced arterial thrombosis model in rats, PDR (i.g.) significantly reduced the thrombus weight. In electrical stimulation-induced arterial thrombosis in rats, PDR (i.v.) dose-dependently prolonged the thrombus occlusion time (OT). PDR increased the concentration of NO in plasma. In contrast with aspirin (ASA), PDR did not influence on the TXA2 and PGI2 levels in plasma. In conclusion, PDR is provided with significant inhibitory effect on platelet aggregation and prevention effect on platelet related thrombosis, which is probably attributed to its inhibition on platelet function by L-arginine-NO pathway. The results demonstrate that PDR is a novel, oral and venous effective platelet aggregation inhibitor and has a possibility used as an anti-thrombotic agent.     

Selenium dependent glutathione peroxidase: a physiological regulatory system for platelet function

In human platelets the selenoenzyme glutathione peroxidase (GSH-Px) acts as a scavenger of the peroxides generated during the burst of arachidonic acid (AA) metabolism. Such a mechanism inhibits the biosynthesis of both thromboxane A2 (TXA2) and lipoxygenase products. The same mechanism is not effective on the prostacyclin (PGI2) biosynthesis from cultured endothelial cells. In order to evaluate this effect in vivo, besides in vitro, we activated the enzyme in eight normal volunteers by increasing their daily Se intake for 8 weeks, monitoring: platelet GSH-Px activity, platelet aggregation induced by AA and U 44069, and concurrent malondialdehyde (MDA) and thromboxane B2 (TXB2) production, urinary excretion of renal and systemic TXA2 and PGI2 metabolites, platelet enzyme activities of the hexose monophosphate pathway and glutathione content, platelet adenine nucleotides, bleeding time, plasma Se concentration. We found: a) progressive platelet GSH-Px activation by Se paralleling an enhancement of platelet aggregation threshold values for AA, but not for U 44069; b) concurrent inhibition of platelet biosynthesis of TXA2 both in vitro and in vivo while the biosynthesis of systemic prostacyclin was unaffected; c) a progressive increase in the bleeding time, unmodified by aspirin. In conclusion, we believe that Se-dependent GSH-Px represents a physiological mechanism regulating the biosynthesis of prostanoids with implications in platelet function and that a Se dietary supplement might be considered in the prevention of arterial thrombosis.     

The effect of a thromboxane synthetase inhibitor, OKY-046, on urinary excretion of immunoreactive thromboxane B2 and 6-keto-prostaglandin F1 alpha in patients with ischemic cerebrovascular disease

Thromboxane synthetase activity is selectively inhibited by (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY-046). A single dose of 100 mg OKY-046 was orally administered to patients with ischemic cerebrovascular disease and healthy volunteers. Platelet aggregation and thromboxane B2 (TXB2) generation of intact and homogenised platelets induced by 1.0 mM sodium arachidonate were measured before and at 1, 4, 6 and 8 h after dosing. OKY-046 inhibited arachidonate-induced aggregation in platelet rich plasma from some, but not all, individuals, whereas platelet TXB2 generation was almost completely inhibited by a single dose of 100 mg OKY-046, in all of the patients and healthy volunteers. Endogenous TXA2 and prostacyclin (PGI2) biosynthesis were assessed by measurement of urinary immunoreactive TXB2 (i-TXB2) and 6-keto-PGF1 alpha (i-6-keto-PGF1 alpha) before and at 0-3, 3-6, 6-9 h after dosing. OKY-046 increased the urinary i-6-keto-PGF1 alpha coincidently with a decrease of urinary i-TXB2, both in patients and healthy volunteers. These effects of a selective thromboxane synthetase inhibitor will improve a disturbed balance between TXA2 and PGI2, associated with the development of ischemic cerebrovascular disease.     

Prostaglandin H2 directly lowers human platelet cAMP levels

In the present study we investigated the ability of prostaglandin H2 (PGH2) to lower platelet cAMP levels independent of thromboxane A2 (TXA2) production. Human platelet-rich plasma was incubated (37 degrees C) with 13 nM prostacyclin (PGI2) to increase platelet cAMP levels. Addition of 500 microM arachidonic acid (AA) to PGI2-treated platelets resulted in a return of cAMP to control levels. Inhibition of both PGH2 and TXA2 production with indomethacin (20 microM) blocked the ability of AA to lower PGI2-stimulated increases in cAMP. On the other hand, selective inhibition of TXA2 production with the thromboxane synthetase blocker 7-(1 imidazoyl) heptanoic acid (7-IHA; 10 microM), did not prevent the cAMP lowering effect of added AA. These results provide evidence that PGH2 need not be converted to TXA2 in order to reduce platelet cAMP levels.     

Response of thromboxane B2, malondialdehyde and platelet sensitivity to 3 weeks low-dose aspirin (ASA) in healthy volunteers

To examine the effects of low-dose aspirin thromboxane B2 (TXB2), malondialdehyde (MDA) and platelet sensitivity to prostaglandin I2 (PGI2) have been measured in a total of 18 healthy volunteers. They were randomly assigned to 3 groups, 6 volunteers each, receiving either 1, 10 or 20 mg ASA orally a day for 3 weeks in a double-blind fashion. In order to assess the time course of ASA-induced changes, blood was drawn before, 1 hour and 2, 3, 5, 7, 9, 12, 14, 16 and 21 days after the first drug-intake. Serum-TXB2 was depressed time- and dose-dependently, after 1 mg daily to about 60%, after 10 mg to about 30%, after 20 mg to about 5% of controls. MDA-formation and conversion of exogenously added arachidonic acid (AA) to TXB2 also dropped significantly, (p less than 0.01), the extent depending on the ASA-dosage administered. The drop in MDA- and TXB2-values in the 3 groups correlated with r = 0.98, 0.94, 0.98, respectively. The platelet sensitivity during 20 and 10 mg ASA-administration did not change at all, whereas a significant increase (p less than 0.01) in platelet sensitivity during treatment with 1 mg ASA was observed.     

The sex-related differences in aspirin pharmacokinetics in rabbits and man and its relationship to antiplatelet effects

There are a number of reports which suggest that the antithrombotic effect of aspirin is limited to males. It is unclear whether this effect is due to sex-related differences in the effect of aspirin on platelets, the vessel wall, or the pharmacokinetics of aspirin. To test these possibilities we examined the sex-related differences in (1) vessel wall PGI2 release and its inhibition by and recovery from aspirin in rabbits; (2) the effects of aspirin on platelet aggregation, thromboxane B2 and beta-thromboglobulin (BTG) release in man, and (3) the pharmacokinetic characteristics of aspirin, in both rabbits and man. Vascular wall PGI2 measured as 6-keto-PGF1 alpha, was not different in male and females rabbits, and was inhibited to a similar extent by identical concentrations of aspirin. The duration of this inhibitory effect was also the same in males and females. The pattern of inhibition of collagen-induced platelet aggregation, and collagen-induced thromboxane B2 and BTG release by aspirin were not different in either sex. There was, however, a sex-related difference in a number of pharmacokinetic characteristics of aspirin both in rabbits and man. Thus, aspirin was absorbed more rapidly, distributed in larger apparent volume and was hydrolysed more rapidly in females. These observations suggest that the sex-related differences in the antithrombotic effects of aspirin seen in clinical studies are not due to differences in the effects of aspirin on the inhibition of platelet function mediated by the inhibition of cyclo-oxygenase in either the platelet or the vessel wall. An effect of aspirin on platelet function independent of the inhibition of cyclo-oxygenase has been described and it is possible that this effect may be influenced by sex-related differences in the pharmacokinetics of aspirin.     

Induction of prostacyclin/PGI2 synthase expression after cerebral ischemia-reperfusion.

Prostacyclin (PGI2), a potent vasodilator and inhibitor of platelet aggregation and leukocyte activation, is crucial in vascular diseases such as stroke. Prostacyclin synthase (PGIS) is the key enzyme for PGI2 synthesis. Although expression of PGIS was noted in the brain, its role in ischemic insult remains unclear. Here we reported the temporal and spatial expression of PGIS mRNA and protein after 60-min transient ischemia. Northern blot and in situ hybridization revealed a delayed increase of PGIS mRNA in the ischemic cortex at 24- to 72-h after ischemia; PGIS was detected mainly in the ipsilateral penumbra area, pyriform cortex, hippocampus, and leptomeninges. Western blot and immunohistochemical analysis revealed that PGIS proteins were expressed temporally and spatially similar to PGIS mRNA. PGIS was heavily colocalized with PECAM-1 to endothelial cells at the leptomeninges, large and small vessels, and localized to neuronal cells, largely at the penumbra area. A substantial amount of PGIS was also detected in the macrophage and glial cells. To evaluate its role against ischemic infarct, we overexpressed PGIS by adenoviral gene transfer. When infused 72 h before ischemia (- 72 h), Adv-PGIS reduced infarct volume by approximately 50%. However, it had no effect on infarct volume when infused immediately after ischemia (0 h). Eicosanoid analysis revealed selective elevation of PGI2 at - 72 h while PGI2 and TXB2 were both elevated at 0 h, altering the PGI2/thromboxane A2 (TXA2) ratio from 10 to 4. These findings indicate that PGIS protects the brain by enhancing PGI2 synthesis and creating a favorable PGI2/TXA2 ratio.     

血浆血栓素A2/前列腺素I2值与骨骼肌无复流现象

背景：在骨骼肌无复流现象中血栓素A2及前列腺素I2是重要参与因子。 目的：观察下肢动脉栓塞患者血浆血栓素A2/前列腺素I2值与骨骼肌发生无复流现象的关系。 方法：选择经影像学及临床表现确诊为下肢动脉栓塞患者36例,行下肢动脉取栓,根据患者取栓后缺血肢体是否发生无复流现象分为无复流组(n=10)和对照组(n=26)。 结果与结论：与对照组比较,无复流组取栓后0 h无复流组前列腺素I2明显减少(P < 0.01);取栓后24 h,无复流组血栓素A2明显升高、前列腺素I2明显下降(P < 0.01)。与对照组相比,无复流组术后血栓素A2/前列腺素I2值升高(P < 0.05)。与术前相比,对照组术后血栓素A2/前列腺素I2值升高后然后下降,无复流组取栓后均维持较高的水平。而取栓前后两组血小板数量及血浆纤维蛋白原的水平差异均无显著性意义。同时术后24 h点无复流组血栓素A2/前列腺素I2值与纤维蛋白原呈正相关(r=0.613, P=0.049);而与血小板数量无相关性(r=0.199,P=0.543)。提示血栓素A2/前列腺素I2值可以作为判断动脉栓塞患者缺血再灌注后骨骼肌发生无复流现象的指标之一。     

CV-4151--a potent, selective thromboxane A2 synthetase inhibitor

(E)-7-Phenyl-7-(3-pyridyl)-6-heptenoic acid (CV-4151) inhibited horse platelet microsomal thromboxane (TX) A2 synthetase with an IC50 of 2.6 X 10(-8) M, but even at a high concentration of 10(-4) M it had little effect on cyclooxygenase, PGI2 synthetase and 5-lipoxygenase in in vitro enzymatic assays. CV-4151 did not affect PGI2 release from rat and rabbit aortic tissues in in vitro (10(-4) M) and ex vivo (10 and 100 mg/kg, p.o.) experiments, whereas aspirin (10(-4) M or 10 and 100 mg/kg, p.o.) markedly inhibited PGI2 release in these preparations. When given orally to rats and dogs, CV-4151 markedly inhibited blood TXA2 synthetase activity: the ID50 values (mg/kg, 2 hr later) were 0.05 in rats and 0.17 in dogs. The inhibitory effects at an oral dose of 1 mg/kg lasted more than 24 hr in both species; the inhibition was 41% in rats and 32% in dogs 24 hr after the administration. When injected i.v. to rats and dogs, CV-4151 caused inhibitory effects on TXA2 synthetase equipotent to those observed with the oral administration. In both species, CV-4151 given orally increased concentration of serum immunoreactive 6-keto-PGF1 alpha concomitant with a decrease of serum TXB2-8 concentration. CV-4151 was equipotent to OKY-1580 (IC50: 2.3 X 10 M), a well documented TXA2 synthetase inhibitor, in an in vitro TXA2 synthetase assay. However, CV-4151, given orally or i.v. to rats and dogs, was much more potent and longer acting in inhibition of blood TXA2 production than OKY-1580. Dazoxiben was less potent than these compounds in vitro. In rats, serial oral administration of CV-4151 (10 mg/kg) once daily for 14 days produced a constant and marked reduction of serum TXB2 concentration with concomitant increase of serum immunoreactive 6-keto-PGF1 alpha concentration. No rebound phenomenon in inhibition of TXA2 synthetase was observed after the dosing was stopped. These findings indicate that CV-4151 is a potent and long acting selective inhibitor of TXA2 synthetase and may reorient the metabolism of PG endoperoxides to PGI2.     

Aspirin kinetics and inhibition of platelet thromboxane generation-relevance for a solution of the "aspirin dilemma"

Six healthy male volunteers received aspirin (ASA) in a compressed (320 mg) and an enteric-coated (800 mg) formulation as single oral doses ten days apart. Ten plasma samples were obtained from each volunteer between 5 and 120 min after compressed ASA, and seven between 10 and 240 min after enteric-coated ASA. ASA was undetectable (less than 100 ng/ml) in plasma from three subjects receiving compressed ASA and two receiving the enteric-coated preparation. Plasma levels and kinetic parameters of salicylate were the same in subjects with undetectable and detectable ASA plasma levels. More than 98% inhibition of pre-drug serum TXB2 was noted in all samples collected one and four hours after either ASA preparation. TXB2 generation recovered on average by 3.5% at 24 hr with both preparations. Thus inhibition of platelet TXB2 generation occurred independently of the amount of ASA reaching the peripheral circulation. If this is due to inhibition of platelet function in the enterohepatic circulation followed by extensive first-pass deacetylation of ASA, vascular PGI2 synthesis could be spared. A better knowledge of the kinetic parameters of ASA for each of the formulations used in thrombosis prevention trials might help in solving the "aspirin dilemma".     

Comparison of the inhibitory effects of the TXA2 receptor antagonist, vapiprost, and other antiplatelet drugs on arterial thrombosis in rats: possible role of TXA2.

The antithrombotic effect of the thromboxane A2 receptor antagonist, vapiprost, was compared with those of other antiplatelet drugs using an arterial thrombosis model which utilized photochemical reaction in the rat femoral artery. Vapiprost prolonged the time required to occlude the artery with thrombus and inhibited collagen-induced rat platelet aggregation in whole blood ex vivo, in a dose-dependent manner. The potency ranking of antithrombotic effect was vapiprost > ketanserin (serotonin 5-HT2 receptor antagonist) > ticlopidine (inhibitor of ADP-induced platelet aggregation) = dipyridamole (adenosine uptake inhibitor) > aspirin (cyclooxygenase inhibitor). On the other hand, the ranking of antiplatelet effect was ticlopidine > or = vapiprost > or = aspirin. Ketanserin and dipyridamole were ineffective. Relative to their antiplatelet effect, vapiprost and ketanserin had powerful antithrombotic effects. It is possible that the potent antithrombotic effects of vapiprost and ketanserin in vivo reflect the ability of these drugs to inhibit mediator-induced vascular contractions in addition to platelet aggregation. The results of the present study also suggest that TXA2 may play an important role in thrombogenesis in rats.     

Different effects of aspirin, dipyridamole and UD-CG 115 on platelet activation in a model of vascular injury: studies with extracellular matrix covered with endothelial cells

Cultured endothelial cells produce an extracellular matrix (ECM) which activates platelets, similarly to deendothelialized vascular segments. Platelet-rich plasma (PRP) was incubated with endothelial cells cultures seeded in various densities on ECM. The interaction of the platelets with this artificial intima was evaluated by phase microscopy and by thromboxane A2 (TXA2) and prostacyclin (PGI2) measurement. Large platelet aggregates were formed on exposed ECM. Platelets aggregation but not adhesion on the ECM was markedly inhibited by the presence of endothelial cells. Pretreatment of the endothelial cells with 0.1 mM aspirin reduced their PGI2 synthesis and was associated with platelet aggregation on the ECM. 10 microM dipyridamole markedly inhibited platelet activation by ECM when the drug was added to citrated whole blood before PRP preparation. UD-CG 115 which elevates cyclic AMP in cardiac muscle, inhibited platelet aggregation and TXA2 production induced by ECM, in the presence as well as in the absence of endothelial cells, without any effect on endothelial PGI2 production.     

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.     

Characterization of a new compound, S-145, as a specific TXA2 receptor antagonist in platelets

S-145, a newly synthesized thromboxane A2 (TXA2) receptor antagonist, was investigated for its effects on platelet TXA2 receptors. In rat washed platelets, S-145 (100 nM) completely suppressed both U46619-induced shape change and collagen-induced shape change and aggregation, whereas aggregation responses provoked by ADP and thrombin were unaffected by S-145. S-145 had no effect on the cAMP levels in rat platelets as well as no inhibitory activity for the bindings of 3H-PGE1, 3H-Iloprost, 3H-PGD2 and 3H-PGF2 alpha to rat platelet membranes. TXB2 formation was not affected by S-145 (10 microM) in rat washed platelets. S-145 antagonized not only the collagen-induced aggregation but also the binding of 3H-U46619 in rat washed platelets with stereospecificity and high potency, which exerted an IC50 of 4.7 nM or a Ki of 2.5 nM, respectively. The potent activity of S-145 in inhibition of 3H-U46619 binding to crude platelet membranes was seen in rat as well as rabbit and human. These results demonstrate that S-145 is a highly potent and selective antagonist for platelet TXA2 receptors.     

Incomplete thromboxane inhibition with 100 mg of intravenous acetylsalicylic acid in patients with acute ST elevation myocardial infarction: a placebo-controlled pilot trial.

BACKGROUND: Acetylsalicylic acid (ASA) is now a standard treatment of acute myocardial infarction (AMI). ASA inhibits thromboxane A(2) (TXA(2)) production by blocking the constitutive cyclooxygenase (COX)-1 enzyme, but only to a small degree the inducible COX-2. COX-2 is induced by increased concentrations of cytokines, which is related to an enhanced inflammatory response. Previously, we have found a complete inhibition of TXA(2) synthesis in healthy volunteers after intravenous administration of 50 mg of ASA. We measured in a randomized, placebo-controlled pilot trial the effect of 100 mg of ASA injected intravenously on TXA(2) synthesis in AMI patients treated with streptokinase. METHODS AND RESULTS: Nineteen patients with AMI treated with streptokinase were randomized to 100 mg of ASA or placebo injected intravenously. Se-TXB(2) and bleeding time were measured before and after drug administration. One hundred and eighty minutes after intravenous ASA administration, treatment with oral ASA was initiated. We found a significant decrease in serum concentrations of TXB(2) after 30, 60 and 180 min following ASA injection compared to placebo, but in none of the patients was complete inhibition of TXA(2) production achieved. No significant change in bleeding time could be demonstrated. CONCLUSION: Intravenous ASA in a dosage of 100 mg did not completely prevent TXA(2) production in AMI patients treated with streptokinase. This may be due to synthesis of TXA(2) by the inducible COX-2 enzyme and/or to a transcellular metabolism in platelets of prostanoids generated by endothelial cells.     

Differential inhibition of thromboxane A2 and prostacyclin synthesis by low dose acetylsalicylic acid in atherosclerotic patients

Differential inhibition of thromboxane A2 (TxA2) and prostacyclin (PGI2) biosynthesis has an antithrombotic potential, since it may change the TxA2/PGI2 formation ratio in a favourable direction. Very low doses of acetylsalicylic acid (ASA) have been demonstrated to elicit differential inhibition of TxA2 and PGI2 formation in healthy subjects; whether a similar effect can be obtained in patients with atherosclerosis is still an open question. We addressed this by analyzing the urinary excretion of the 2,3-dinor-metabolites of TxA2 (Tx-M) and PGI2 (PGI-M) in 10 patients with severe atherosclerosis during 10 consecutive days. The first three days were a basal period, under which no treatment was given. During the subsequent seven days a daily 50 mg oral dose of ASA was administered. In the basal state urinary Tx-M did not differ from that of PGI-M, the median excretion rates of the two eicosanoid metabolites being 526 (range 68-1490) and 562 (range 93-1970) pg/mg creatinine, respectively. During ASA treatment urinary Tx-M fell to a lower (p less than 0.001) level than PGI-M. Thus, during the last 5 days of ASA treatment the median excretion of Tx-M was depressed (p less than 0.001) to 148 (range 48-428) pg/mg creatinine, while that of PGI-M was decreased (p less than 0.01) to 313 (range 42-2658) pg/mg creatinine. These data indicate that a daily 50 mg dose of ASA inhibits cardiovascular formation of eicosanoids in patients with severe atherosclerosis and increased platelet TxA2 formation. Furthermore, this dose of ASA inhibits the formation of TxA2 more than that of PGI2.     

Aspirin inhibits platelet function independent of the acetylation of ciclo-oxygenase

Aspirin inhibits platelet function and prevents thrombosis in some clinical situations. This antithrombotic effect is attributed to the $\text{irreversible}$ inhibition of platelet thromboxane A₂ synthesis, an effect which is achieved by a low dose of aspirin. There is some evidence that higher doses of aspirin may have additional antithrombotic effects. To test this possibilitv, we measured the effect of high and low dose aspirin on hemostasis in vivo and platelet function ex vivo in the rabbit.Both carotid arteries were isolated. One was replaced with a 2 cm piece of polyethylene tubing and the other was left intact. The prosthetic and intact vessels were then punctured with a needle and the time take for bleeding from each to cease was measured. Aspirin (3 and 100mg/kg given 1 or 20 hours beforehand) had no effect on the bleeding from the intact vessel, but prolonged the bleeding time in the prosthetic vessel in a dose-related manner. Washed platelets obtained from the 100 mg/kg-treated rabbits were less responsive to collagen and thrombin than platelets obtained from the 3mg/kg-treated rabbits which in turn, were less responsive than control platelets. This additional effect of aspirin on platelet function was not due to the further inhibition of platelet thromboxane A₂ release nor to further inhibition of the platelet release phenomenon. It is suggested that the enhanced effect of high dose aspirin on haemostasis from the arterial prosthesis is related to the second platelet inhibiting effect of aspirin.     

Platelet aggregation and thromboxane B2 formation after ethanol abuse: Is there a relationship to stroke?

Formation of thromboxane B2 (TXB2), a metabolite of the potent platelet-aggregating and vasoconstrictor agent thromboxane A2 (TXA2), during ADP-induced platelet aggregation was studied in 10 healthy men and in 10 male alcoholics during the 2-week period of detoxification. None of the alcoholics had anemia or thrombo-embolic disease. The platelets of the alcoholics were more sensitive for ADP and synthesized as much as triple the amount of TXB2 compared to those of the nonalcoholic donors. The effect was most striking during the rebound thrombocytosis and suggests that it could possible contribute to the increased incidence of various thrombotic diseases in the alcoholic.