Biochemical selectivity of oral versus intravenous aspirin in rats. Inhibition by oral aspirin of cyclooxygenase activity in platelets and presystemic but not systemic vessels

Pharmacokinetics and pharmacodynamics of acetylsalicylic acid after intravenous and oral administration to healthy volunteers

Background

The pharmacology of single doses of acetylsalicylic acid (ASA) administered intravenously (250 or 500 mg) or orally (100, 300, or 500 mg) was evaluated in a randomized, placebo-controlled, crossover study.

Methods

Blood and urine samples were collected before and up to 24 hours after administration of ASA in 22 healthy volunteers. Pharmacokinetic parameters and measurements of platelet aggregation were determined using validated techniques.

Results

A comparison between administration routes showed that the geometric mean dose-corrected peak concentrations (C_max/D) and the geometric mean dose-corrected area under the curve (AUC_0–∞/D) were higher following intravenous administration of ASA 500 mg compared with oral administration (estimated ratios were 11.23 and 2.03, respectively). Complete inhibition of platelet aggregation was achieved within 5 minutes with both intravenous ASA doses, reflecting a rapid onset of inhibition that was not observed with oral dosing. At 5 minutes after administration, the mean reduction in arachidonic acid-induced thromboxane B₂ synthesis ex vivo was 99.3% with ASA 250 mg intravenously and 99.7% with ASA 500 mg intravenously. In exploratory analyses, thromboxane B₂ synthesis was significantly lower after intravenous versus oral ASA 500 mg (P<0.0001) at each observed time point up to the first hour after administration. Concentrations of 6-keto-prostaglandin_1α at 5 and 20 minutes after dosing were also significantly lower with ASA 500 mg intravenously than with ASA 500 mg orally.

Conclusion

This study demonstrates that intravenous ASA provides more rapid and consistent platelet inhibition than oral ASA within the first hour after dosing.

Usefulness of whole blood aggregometry and its comparison with thromboxane generation assay in monitoring acetylsalicylic acid effectiveness--a multiparametric study in rats

BACKGROUND

There is a need for consensus concerning universal methodological criteria for detection of suboptimal response to acetylsalicylic acid (ASA) therapy. Therefore, animal models to test for ASA effectiveness remain of interest. Our objective was to verify the usefulness of multiparametric whole-blood impedance aggregometry and thromboxane A(2) generation, which are the most popular techniques used for monitoring of ASA treatment effectiveness.

METHODS

Using multiparametric analysis of whole-blood impedance aggregometry, we examined which parameters of platelet aggregation or disaggregation allow for the best discrimination between ASA-treated (4 or 40 mg/kg for 60 days) and non-treated male rats. The effectiveness of ASA-mediated inhibition of platelet cyclooxygenase-1 was verified by determination of plasma thromboxane B(2) and urine 11-dehydro-thromboxane B(2), accepted as reference assays for monitoring of ASA-mediated platelet cyclooxygenase-1 inhibition.

RESULTS

Two of the platelet agonists used, collagen (1 mg/L) and arachidonic acid (0.5 mmol/L), allowed discrimination of control and ASA-treated animals, whereas adenosine diphosphate (5 micromol/L) was not effective. It is noteworthy that only ASA-mediated changes in duration of the rising phase for platelet aggregation and the area under the curve for collagen-induced aggregation allowed significant discrimination between low and high ASA dose and remained correlated with the reference parameter, plasma thromboxane B(2).

CONCLUSIONS

Analysis of aggregation curves, routinely based only on the amplitude and rate of platelet aggregation, may not be enough discriminative to distinguish between varying ASA doses and treatment schedules.

The lipoxygenase-cyclooxygenase inhibitor licofelone prevents thromboxane A2-mediated cardiovascular derangement triggered by the inflammatory peptide fMLP in the rabbit

Licofelone is an analogue of arachidonic acid that inhibits 5-lipoxygenase (LOX), cyclooxygenase (COX)-1 and COX-2. We investigated the effects of licofelone on cardiovascular derangements and production of thromboxane (Tx)A(2) induced by the inflammatory agonist n-formyl-methionyl-leucyl-phenylalanine (fMLP) in the rabbit, in comparison with those of aspirin or rofecoxib, inhibitors of COX-1 and COX-2, respectively. In control rabbits, injection of fMLP (30 nmol/kg) in the jugular vein evokes ischemic electrocardiographic (ECG) changes in the first 1-5 min, i.e. a profound depression of the ST segment and inversion of the T wave. Simultaneously, fMLP induces bradycardia and hypotension and increases TxB(2) blood levels. All changes are transient. Licofelone (60 mg/kg/5 days, p.os) prevented fMLP-induced ECG ischemic changes in all treated animals, reverted bradycardia and hypotension, and significantly reduced TxB(2). Aspirin (10 mg/kg/5 days, p.os) prevented ischemic ECG alterations in 2 out of 5 treated animals and did not modify either bradycardia or hypotension. One rabbit died two min after fMLP. In 2 rabbits, aspirin reduced TxB(2) levels by more than 80% respect to mean control values; the remaining two rabbits produced an amount of TxB(2) similar to controls. These two rabbits also showed ischemic ECG changes. Rofecoxib (10 mg/kg/5 days, p.os) did not prevent fMLP-induced ischemic ECG alteration, bradycardia and hypotension, and did not significantly modify the increase of TxB(2). These results indicate that the capacity of licofelone to efficiently suppress TxA(2) production, is responsible for the protection from the cardiovascular derangement triggered by an inflammatory stimulus.

Selective inhibition of cyclooxygenase-2 enhances platelet adhesion in hamster arterioles in vivo

BACKGROUND

Selective inhibitors of cyclooxygenase-2 (Cox-2) are reported to cause cardiovascular side effects in patients at risk. However, direct proof of prothrombotic effects of these drugs is lacking. We investigated in the microcirculation in vivo whether selective inhibition of Cox-2 induces platelet activation.

METHODS AND RESULTS

The behavior of fluorescence-labeled human platelets was studied in hamster arterioles (dorsal skinfold chamber) by intravital microscopy. Transient platelet-vessel wall interactions (PVWIs), firm platelet adhesion to the vessel wall, and vessel occlusion after FeCl3-induced wall injury were analyzed as platelet activation parameters. In vitro experiments in human umbilical vein endothelial cells (HUVECs) were performed to assess specific effects of Cox-2 inhibition on platelet adhesion under shear stress (16 dyn/cm2) and on endothelial release of 6-ketoprostaglandin (PG) F(1alpha). Selective inhibition of Cox-2 (NS-398, 0.5 mg/kg) increased platelet adhesion to the vessel wall in vivo (11.9+/-3.9 platelets/mm2; controls, 1.4+/-1.4 platelets/mm2, P<0.05) and platelet adhesion after ADP stimulation in vitro. PVWIs were significantly enhanced in NS-398-treated animals, which were reduced by platelet pretreatment with aspirin (5 mg/kg) or iloprost (1 nmol/L). Inhibition of Cox-2 reduced levels of 6-keto-PGF1alpha in vivo and in HUVEC supernatants. Time to occlusion after vessel wall injury was significantly shortened by NS-398 (125.4+/-13.6 seconds in NS-398-treated animals versus 270.8+/-46 seconds in controls; P<0.01).

CONCLUSIONS

Selective inhibition of Cox-2 reduces 6-keto-PGF(1alpha) endothelial release, increases PVWIs, and increases firm platelet adhesion in hamster arterioles. Moreover, it leads to faster occlusion of damaged microvessels. Thus, selective inhibition of Cox-2 may trigger thrombotic events by diminishing the antiplatelet properties of the endothelium.

Antiplatelet and antithrombotic effects of a novel selective phosphodiesterase 3 inhibitor, NSP-513, in mice and rats

We investigated the effects of NSP-513, (R)-4,5-dihydro-5-methyl-6-[4-(2-propyl-3-oxo-1-cyclohexenyl)amino] phenyl-3(2H)-pyridazinone, on phosphodiesterase (PDE) isozyme activities, in vitro platelet aggregation and in vivo thrombus formation. NSP-513 selectively inhibited human platelet PDE 3 isozyme with an IC50 value of 0.039 microM. In an in vitro human platelet aggregation assay, the IC50 values (microM) of NSP-513 for platelet aggregation induced by collagen, U-46619, arachidonic acid, adenosine diphosphate (ADP), epinephrine and thrombin were 0.31, 0.25, 0.082, 0.66, 0.23 and 0.73, respectively. In a mouse pulmonary thromboembolism model, orally administered NSP-513 showed in vivo antithrombotic effects that were 320 to 470 times more potent than those of cilostazol. In a rat carotid arterial thrombosis model, intraduodenally administered NSP-513 (0.1 mg/kg), cilostazol (30 mg/kg) and aspirin (30 mg/kg) reduced thrombus formation by 75%, 66% and 48%, respectively. However, intravenously administered dipyridamole (10 mg/kg) did not significantly prevent thrombus formation. These results demonstrate that NSP-513 has the potential to prevent not only in vitro platelet aggregation but also in vivo thrombus formation and indicate that the highly selective PDE 3 inhibitory effect of NSP-513 may make this compound useful for assessing the physiological role of PDE 3.

Modulation of haemostatic function and prevention of experimental thrombosis by red wine in rats: a role for increased nitric oxide production

1. The effects of ethyl alcohol and wine (red and white) on haemostatic parameters and experimental thrombosis were studied in rats; NO was evaluated as a possible mediator of these effects. 2. We found that red wine (12% alcohol) supplementation (8.4 +/- 0.4 ml d-1 in drinking water, for 10 days) induced a marked prolongation of 'template' bleeding time (BT) (258 +/- 13 vs 132 +/- 13 s in controls; P < 0.001), a decrease in platelet adhesion to fibrillar collagen (11.6 +/- 1.0 vs 32.2 +/- 1.3%; P < 0.01) and a reduction in thrombus weight (1.45 +/- 0.33 vs 3.27 +/- 0.39 mg; P < 0.01). 3. Alcohol-free red wine showed an effect similar to red wine. In contrast, neither ethyl alcohol (12%) nor white wine (12% alcohol) affected these systems. 4. All these effects were also observed after red wine i.v. injection (1 ml kg-1 of 1:4 dilution) 15 min before the experiments. 5. The effects of red wine were prevented by the NO inhibitor, N omega nitro-L-arginine-methyl ester (L-NAME). L-arginine, not D-arginine, reversed the effect of L-NAME on red wine infusion. 6. Red wine injection induced a 3 fold increase in total radical-trapping antioxidant parameter values of rat plasma with respect to controls, while white wine and alcohol did not show any effect. 7. Our study provides evidence that red wine modulates primary haemostasis and prevents experimental thrombosis in rats, independently of its alcohol content, by a NO-mediated mechanism.

Acetaminophen effects on behavioral thermoregulation in albino rats

Acetaminophen (N-Acetyl-p-aminophenol) was administered intraperitoneally to 15 Sprague-Dawley rats partitioned into 3 studies (5 rats per study) using a within subjects, repeated-measures reversal design. Behavioral thermoregulation was assessed in a cold Skinner Box using 5-sec. exposures of microwave radiation [Specific Absorption Rate = 0.34 Watts/kg/(mW/cm2)] as reinforcing stimuli under a fixed-interval 2-min. schedule of positive reinforcement. Doses of 10, 20, 30, 40, and 50 mg/kg (in solutions of 1%, 2%, 3%, 4%, and 5%) acetaminophen showed stable rates of operant responding for heat compared with significant changes in rates for comparable doses of aspirin in a 1993 study by Vitulli, et al. Weight reductions and temperature increases varied significantly with before-session and after-session measures, respectively. 1994-95 biochemical data of Murphy, et al. from humans following aspirin or acetaminophen ingestion which affect thermoregulation and sleep patterns are discussed in conjunction with behavioral data from rats.

Intravenous aspirin causes a paradoxical attenuation of cerebrovascular thrombolysis

BACKGROUND AND PURPOSE

Aspirin treatment is recognized as an advantageous adjunct to thrombolytic agents in myocardial infarct patients. In this study we examined the effects of aspirin on the rate of clot lysis and on the frequency and extent of hemorrhagic transformations in rabbit models of embolic stroke.

METHODS

Rabbit models of ex vivo platelet aggregation and cutaneous template bleeding times were used to show the anticoagulant effects of aspirin in our experimental paradigm. We monitored tissue-type plasminogen activator (TPA)-induced clot lysis in two rabbit models of embolic stroke by (1) scintigraphically following the dissolution of a 99mTc-tagged clot or (2) using roentgenography to follow the disappearance of an Sn-tagged clot.

RESULTS

In animals pretreated (18 hours) with a single administration of aspirin (1, 5, or 20 mg/kg IV) or 1 mg/kg per day for 3 days, the aggregation response of platelets to collagen (3.3 micrograms/mL) or arachidonic acid (0.5 mmol/L) was attenuated. High-dose aspirin also increased ear template bleeding time from 1.6 to 2.6 minutes. When aspirin (20 mg/kg) was administered 18 hours before embolism and subsequent lysis with TPA (0.3 mg/kg bolus; 3 mg/kg per hour IV), the pretreatment significantly antagonized the rate and extent of TPA-induced clot lysis by up to 70%. This was confirmed in a second embolic stroke model. The suppression of TPA-induced lysis was reversed by administration of the prostacyclin analogue iloprost (10 micrograms/kg per hour) directly into the cerebral circulation.

CONCLUSIONS

We conclude that aspirin reduces the effects of TPA in embolic stroke models. This effect may be the result of a loss of endothelial prostacyclin production since the effect is reversed by iloprost.

Aspirin (acetylsalicylic acid) effects on behavioral thermoregulation with microwave radiation

Aspirin is a widely used over-the-counter drug in our society which has wide therapeutic value, yet not all of the behavioral side effects have been studied. Different doses of aspirin solutions were administered (ip) prior to fixed-interval 2-min. schedules of microwave reinforcement in rats tested in a cold environment. Four Sprague-Dawley rats were conditioned to regulate their thermal environment with 5-sec. exposures of MW reinforcement. Friedman's nonparametric test showed significant differences among aspirin and saline-control doses. Post hoc sign tests showed that a moderate dose of aspirin increased operant behavior reinforced by MW radiation, yet lower and higher doses decreased and then increased the rate of responding which resulted in an inverted U-shaped trend. Possible multiple effects of aspirin in terms of its thermoregulatory as well as its pain-tolerance properties, and implications for hypothalamic "set point" are discussed.

Intracellular Ca2+ rise in human platelets induced by polymorphonuclear-leucocyte-derived cathepsin G

Cathepsin G, a serine protease released by polymorphonuclear-leucocyte azurophilic granules upon stimulation, activates human platelets, inducing an increase in intra-platelet Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner (50-200 nM). The [Ca2+]i rises elicited by low (50-80 nM) cathepsin G concentrations in fura-2-loaded platelets showed a biphasic mode, with a first small peak followed by a greater and more prolonged Ca2+ transient. Higher (100-200 nM) cathepsin G concentrations induced a monophasic increase in intracellular Ca2+. Acetylsalicylic acid, nordihydroguaiaretic acid and ketanserin did not affect platelet activation by cathepsin G, whereas the ADP-scavenger system phosphocreatine/creatine kinase significantly decreased Ca2+ mobilization, platelet aggregation and 5-hydroxytryptamine secretion by cathepsin G. Preventing cathepsin G-induced platelet aggregation with the synthetic peptide RGDSP (Arg-Gly-Asp-Ser-Pro) did not significantly affect cathepsin G-induced Ca2+ transients. Ni2+ (4 mM), a bivalent-cation-channel inhibitor, decreased the cathepsin G-induced fluorescence rise by more than 90%. This effect was reversed by either decreasing Ni2+ or increasing cathepsin G concentration. Preventing Ca2+ influx across the plasma membrane with 4 mM-EGTA totally abolished Ca2+ transients. However, EGTA also strongly decreased catalytic activity of cathepsin G, which is essential for platelet activation. Evidence of a rapid and sustained bivalent-cation channel opening in the platelet membrane was obtained by adding Mn2+ to the platelet suspension 30 s or 3 min after cathepsin G. No accumulation of InsP3 could be detected when platelets were stimulated with cathepsin G. All these data indicate that cathepsin G induces a [Ca2+]i increase mainly through an influx across the plasma membrane. This massive Ca2+ entry is probably due to opening of receptor-operated channels and is amplified by endogenous ADP release.

Urinary excretion of thromboxane and prostacyclin metabolites during chronic low-dose aspirin: evidence for an extrarenal origin of urinary thromboxane B2 and 6-keto-prostaglandin F1 alpha in healthy subjects

In vivo biosynthesis of thromboxane and prostacyclin is currently evaluated by measuring urinary excretion of selected metabolites. Urinary thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) (non-enzymatic hydrolysis products of thromboxane and prostacyclin) are thought to derive from renal biosynthesis of the parent compounds, while enzymatic metabolites such as 2,3-dinor-TXB2 and 2,3-dinor-6-keto-PGF1 alpha appear to be mainly derived from systemic (platelet) thromboxane and (vascular) prostacyclin, respectively. Using immunoaffinity extraction and high-resolution gas chromatography-negative ion chemical ionization mass spectrometry (HRGC-NICIMS), we measured the paired excretion of non-enzymatic and enzymatic metabolites of thromboxane and prostacyclin in healthy subjects before, during and after an eight-day schedule of oral low-dose aspirin (30 mg/day), a treatment known to inhibit platelet and perhaps vascular but not renal cyclooxygenase. Low-dose aspirin cumulatively reduced urinary excretion of TXB2 and 2,3-dinor-TXB2 (about 80% inhibition on day 8 of aspirin treatment, P less than 0.01), as well as 6-keto-PGF1 alpha and 2,3-dinor-6-keto-PGF1 alpha (about 45% inhibition on day 8 of aspirin treatment, P less than 0.01). Excretion of all metabolites recovered slowly after aspirin withdrawal. Urinary PGE2, taken as an index of renal cyclooxygenase activity, was not inhibited by aspirin. A highly significant correlation was found between paired excretion values of non-enzymatic vs. enzymatic metabolites of thromboxane and prostacyclin in all individuals studied (TXB2 vs. 2,3-dinor-TXB2 (r = 0.91 +/- 0.03); 6-keto-PGF1 alpha vs. 2,3-dinor-6-keto-PGF1 alpha (r = 0.92 +/- 0.06], irrespective of aspirin treatment. TXB2/2,3-dinor-TXB2 and 6-keto-PGF1 alpha/2,3-dinor-6-keto-PGF1 alpha mean ratios remained unchanged throughout the experiment. These data do not support the view that urinary TXB2 and 6-keto-PGF1 alpha derive mainly from renal biosynthesis in healthy subjects, but rather suggest that they may represent a fraction of systemic (platelet) thromboxane and (vascular) prostacyclin escaping metabolism. These data also suggest that chronic low-dose aspirin may partly inhibit vascular prostacyclin in addition to platelet thromboxane biosynthesis.

Thromboxane synthase inhibitors and receptor antagonists

Platelet activation plays a major role in myocardial infarction and in reocclusion following successful thrombolysis, as corroborated by several clinical studies using aspirin. However, the overall reduction of new vascular complications in patients with symptomatic arterial disease by aspirin was only around 25%. Therefore, there is great interest in finding new means to inhibit platelet activation more efficiently. One line of research has focused on ways to interfere with the action of thromboxane A2 in a more selective way than aspirin does. As such, the development of thromboxane synthase inhibitors, followed by thromboxane receptor antagonists, raised hopes for a better treatment. However, both classes of drugs have some drawbacks, which could be overcome by combining them. This aim has led to the development of compounds that intrinsically possess both activities. Ongoing research indicates that such a dual inhibitor may indeed be more powerful than either aspirin or drugs with the single actions.

Suppression of thromboxane A2 but not of systemic prostacyclin by controlled-release aspirin

BACKGROUND

The antithrombotic efficacy of aspirin is attributed to its inhibition of the enzyme prostaglandin G/H synthase, which is necessary for the formation of thromboxane A2 in platelets. Thromboxane A2 is a potent vasoconstrictor and platelet agonist. However, the formation of prostacyclin by vascular endothelium also requires prostaglandin G/H synthase, and prostacyclin exerts opposite effects on platelet function and vascular tone. We wanted to see whether controlled-release aspirin would affect the formation of thromboxane A2 but not prostacyclin by reducing the aspirin concentration that reaches the posthepatic circulation.

METHODS

A controlled-release formulation containing 75 mg of aspirin, designed to release 10 mg per hour, was developed to inhibit prostaglandin G/H synthase in platelets in the prehepatic circulation. The effects of the controlled-release preparation on plasma levels of aspirin and salicylate, serum levels of thromboxane B2, and urinary dinor metabolites of prostacyclin and thromboxane B2 (measured by gas chromatography-mass spectrometry) were compared with those of conventional immediate-release aspirin in normal volunteers. Prostacyclin release was stimulated by intravenous bradykinin.

RESULTS

Steady-state inhibition of serum thromboxane B2 required two to four days and appeared slower with 75 mg of controlled-release than with the same amount of immediate-release aspirin. Maximal inhibition was achieved rapidly by adding a single loading dose of 162.5 mg of immediate-release aspirin to the regimen. Over a 28-day period, suppression of thromboxane A2 with this regimen was comparable to that with immediate-release aspirin taken either as 162.5 mg daily or as 325 mg on alternate days, despite the minimal systemic bioavailability of controlled-release aspirin. Bleeding time was prolonged to a similar degree with each of the three regimens. The five- to sixfold increase in the prostacyclin metabolite induced by bradykinin was depressed by pretreatment for four days with 75 mg of immediate-release aspirin, but not by 75 mg of controlled-release aspirin.

CONCLUSIONS

Maximal inhibition of platelet thromboxane A2 production was sustained during long-term dosing with controlled-release aspirin, whereas basal prostacyclin biosynthesis fell only slightly and systemic synthesis of prostacyclin stimulated by bradykinin was preserved. Controlled-release aspirin may facilitate determination of the clinical importance of preserving prostacyclin during platelet inhibition in humans.

Acute carotid artery occlusive thrombosis and its pharmacological prevention in the rabbit

A simple and reproducible method to induce an occlusive thrombus in rabbit carotid artery is reported. Rabbits were anesthetized and prepared to record arterial pressure, heart rate, and carotid blood flow. A critical stenosis of a damaged carotid artery was obtained using an external plastic cylinder. Complete occlusion occurred within 6 to 12 minutes, as measured by the decrease in blood flow. Both stenosis of the vessel and deliberate damage (clamping by surgical forceps) were found essential to occlusion. Occlusion was prevented by administration of heparin (200 IU/kg), tissue plasminogen activator (300 micrograms/kg), iloprost (10 micrograms/kg) or the synthetic thrombin inhibitor, FPRCH2Cl (0.5 mg/kg), while ASA (100 mg/kg) was uneffective. The procedure permits an easy and rapid evaluation of thrombus formation and of anti-thrombotic drugs affecting the hemostatic process.

The (+)-enantiomer is responsible for the antiplatelet and anti-inflammatory activity of (+/-)-indobufen

The racemic compound indobufen and its (+)- and (-)-enantiomers have been compared for their effects on blood platelet function and rat carrageenan pleurisy. The antiplatelet properties were studied in-vitro in human platelets by measuring the inhibition of platelet aggregation and generation of serum thromboxane (Tx) B2. In-vivo, the antiplatelet and anti-inflammatory properties were studied in rats by measuring the inhibition of serum TxB2, the amount of 6-keto-PGF1 alpha in pleural exudate and pleural exudate volume. In all tests the (+)-enantiomer was slightly more potent than the racemate, while the (-)-enantiomer was far less potent. In the same rats, treatment with the lowest doses of the compounds giving 90% inhibition of serum thromboxane B2 generation was associated with occasional macroscopic lesions of the gastric mucosa.

Differential effect of aspirin on thromboxane and prostaglandin biosynthesis in man

1. Effects of a single intravenous dose of aspirin (600 mg) on bradykinin-stimulated prostaglandin (PG) and on thromboxane (TX) biosynthesis were determined in nine healthy male volunteers. Plasma concentrations of 6-oxo-PGF1 alpha and 13,14-dihydro-15-oxo-PGF2 alpha were measured in samples obtained during repeated 10 min intravenous infusions of bradykinin before and up to 6 h after the dose of aspirin. TXB2 was measured in serum from blood allowed to clot at 37 degrees C. 2. Aspirin inhibited bradykinin stimulated PG and platelet TX biosynthesis 0.5 h after the dose. Serum TXB2 remained low, whereas PG synthesis recovered within 6 h. 3. Effects of intravenous sodium salicylate (600 mg) were studied identically in eight subjects. Prostanoid biosynthesis was not inhibited. 4. Biosynthesis of prostacyclin and TXA2 under basal conditions was studied in eight subjects by measuring 2,3-dinor-6-oxo-PGF1 alpha and 2,3-dinor-TXB2 in hourly urine samples obtained during and after intravenous infusion of aspirin and, on a separate occasion, of vehicle. 5. Aspirin infusion reduced urinary excretion of both metabolites greater than 90%, but excretion of 2,3-dinor-6-oxo-PGF1 alpha recovered more rapidly than did that of 2,3-dinor-TXB2. 6. We conclude that cyclo-oxygenase is rapidly synthesised in bradykinin-responsive tissues in vivo and that this reflects similarly rapid enzyme biosynthesis in tissues that produce PGI2 under basal conditions.

Stenosis and vascular damage as a cause of thrombosis in the dog femoral artery

We describe here an experimental model of peripheral arterial thrombosis and the effect of several drugs which are known to affect vessel and platelet biological functions. A similar method has been previously applied by us and others on dog coronary arteries. Male Beagle dogs, under pentobarbital anesthesia, were instrumented to measure arterial pressure, heart rate, ECG, femoral blood flow and expired CO2. A segment of the femoral artery was squeezed with forceps to damage the endothelium, and a plastic cylinder was placed around the vessel in the area of the damage. The cylinders had a length of 2 mm and an internal diameter of 1.6-1.8 mm. Under these circumstances blood flow in the stenosed artery was reduced by about 60-70% from control value and showed cyclic blood flow variations (CBFV). CBFV eventually led either to a total occlusion of the vessel (documented by blood flow measurement and by angiographic analysis), or to a spontaneous partial restoration of flow, followed by another decrease, in a repetitive fashion. Drug effect was monitored by observing the changes in frequency and amplitude of CBFV. Ketanserin (0.25 mg/kg), dazmegrel (0.5 mg/kg), and chlorpromazine (0.5 mg/kg), abolished or greatly reduced CBFV in all the experiments, while acetylsalycilic acid (ASA, 10 mg/kg) reduced or abolished CBFV in 60% of the treated dogs. Heparin (50 I.U./kg), dipyridamole (1.0 mg/kg) and prazosin (0.1 mg/kg) did not change CBFV. These results emphasize the importance of serotonin and thromboxane as mediators of vascular occlusion in this particular experimental model. This approach provides a reproducible in vivo preparation to study the pharmacological control of peripheral arterial thrombosis.

Effects of current therapy of Kawasaki disease on eicosanoid metabolism

Coronary artery inflammation in Kawasaki disease is accompanied by thrombocytosis and platelet activation. It was hypothesized that abnormal metabolism of bioactive eicosanoids could result from or contribute to these events. Circulating plasma thromboxane B2, 6-keto-prostaglandin F1 alpha and prostaglandin E were measured by double antibody radioimmunoassay in patients with Kawasaki disease before and after aspirin alone or aspirin and intravenous gamma globulin therapy. Plasma prostaglandin E concentrations were normal in all patient groups. Pretreatment thromboxane B2 was elevated compared with age-matched controls, fell moderately with high-dose aspirin (60 to 100 mg/kg/day) and marginally increased with low-dose aspirin (3 to 5 mg/kg/day) 6 to 8 weeks after treatment. Plasma 6-keto-prostaglandin F1 alpha was not detected in 12 of 16 patients before therapy and remained low in all but 1 patients by 6 to 8 weeks. Thromboxane B2 correlated weakly with serum salicylate concentration but had no relation to platelet mass. The results in these patients with Kawasaki disease indicate only partial thromboxane suppression and depressed prostacyclin generation regardless of therapy. This balance favors coronary vasoconstriction and platelet aggregation capable of potentiating myocardial ischemia or infarction. The results justify consideration of higher or more frequent aspirin doses for longer duration and thromboxane receptor blockade in this disease.

Impairment of primary haemostasis by low molecular weight heparins in rats

Different low molecular weight (LMW) heparins were tested on primary haemostasis in rats. Four preparations were studied; one was devoid of any effect on the bleeding time, while the other three prolonged the bleeding time to varying extents. As a consequence we studied the effect of these heparins on platelet aggregation. The fractions which prolonged the bleeding time, also inhibited the ex vivo and in vitro platelet aggregation, whereas the one devoid of any effect on the bleeding time did not affect platelet aggregation. Similar results were obtained using both platelet-rich plasma (PRP) and gel-filtered platelets. The in vitro response of platelets to aggregating agents may offer a parameter to detect the presence of 'bleeding factor(s)' in some LMW heparin preparations.

Low dose aspirin treatment in late pregnancy differentially inhibits cyclo-oxygenase in maternal platelets

Eighteen pregnant women were treated with aspirin, 37.5 mg once daily by mouth. Treatment was started two weeks before the expected date of delivery, and continued until delivery. Seventeen untreated women were studied concurrently. Platelet thromboxane (TX) production was determined by radioimmunoassay of TXB2 in serum from blood incubated for one hour with thrombin at 37 degrees C. Maternal blood was studied before treatment and at delivery. Fetal blood, from the cord, was studied at delivery. Prostacyclin (PGI2) production by rings of umbilical artery incubated in Hanks' solution at 37 degrees C for one hour was determined by radio-immunoassay of its hydrolysis product, 6-oxo-prostaglandin (PG) F1 alpha. Maternal and fetal blood from untreated women produced similar amounts of TXB2. Aspirin, in the dose regimen used, significantly inhibited TXB2 production in maternal but not in fetal blood, and did not impair PGI2 synthesis by umbilical artery rings. This differential effect on the cyclo-oxygenase of maternal platelets is probably due to the unusual kinetic properties of aspirin, and may prove therapeutically useful.

Effects of 1 gram oral or intravenous aspirin on urinary excretion of thromboxane B2 and 6-keto-PGF1 alpha in healthy subjects

Aspirin inhibits cyclo-oxygenase, thus preventing prostanoids formation. After oral administration aspirin is hydrolysed to inactive salicylate partly within the gastrointestinal tract, partly during first pass in the liver, partly in the circulation by plasma esterases. Intravenous aspirin, in contrast, mainly undergoes plasma esterase-catalysed deacetylation. Six healthy male subjects were given 1 g aspirin orally and intravenously two weeks apart according to a cross-over randomized design. Whereas serum TxB2 generation reflecting platelet cyclo-oxygenase activity was suppressed by aspirin by both routes, urinary excretion of TxB2 and 6-keto-PGF1 alpha was not affected by oral aspirin, but was partially though significantly reduced by the i.v. drug. Drug disposition seems therefore to be essential in determining the "biochemical selectivity" of aspirin as related to platelet and renal prostanoids generation.