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

Salicylate measurement: clinical usefulness and methodology

The salicylates are the most commonly used analgesic, antipyretic, and anti-inflammatory drugs. They are available in hundreds of preparations, many of which are over-the-counter medications. The easy access to large quantities of the drug and the widespread perception that the drug is harmless have contributed to salicylate intoxication becoming a serious and common problem, particularly among the pediatric and geriatric populations. Salicylate is still the major drug for the treatment of rheumatic diseases. The use of salicylate in high doses for the management of these patients requires close monitoring of serum salicylate levels because of the large interindividual variation in dose-serum level relationships and the narrowness of the therapeutic range. Thus, both for the management of patients intoxicated with salicylate and patients who are on high-dose salicylate therapy, the measurement of serum salicylate levels is an important clinical laboratory service. Recent research on the inhibitory effect of aspirin on platelet aggregation has led to the prophylactic use of aspirin in low doses as an antithrombotic drug. This new therapeutic use of aspirin can be aided by monitoring low serum levels of salicylate and perhaps aspirin itself. This article reviews the current state of the knowledge of the pharmacokinetics and clinical toxicology of salicylate, the clinical usefulness of salicylate measurement by the clinical laboratory, and recent development in the analytical technology for salicylate analysis.

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.

Dose-dependent inhibition of phosphoinositide metabolism in human platelets by aspirin in vitro and in vivo

The effects of aspirin on the metabolism of phosphoinositides in human platelets were studied in vitro and in vivo. Eight volunteers received, at two-weekly intervals, a single dose of 10, 30, 100 or 600 mg aspirin. Before the first dose platelets were taken and incubated in vitro with a range of concentrations (10 nM-100 microM) of aspirin. Formation of inositol phosphates (IP) was measured in [3H]-inositol labelled platelets after incubation with collagen and thrombin for 30 min, a time at which a maximal increase in [3H]-IP was observed. The in vitro IC50 for inhibition of the response to collagen by aspirin was approximately 1 microM; the in vivo ID50 was 40-50 mg. Aspirin did not fully inhibit the collagen stimulated IP formation either in vitro or in vivo, and the response to thrombin was unaffected. The ID50 and IC50 of aspirin is thus in accord with the doses of aspirin associated with inhibition of platelet aggregation and thromboxane production in other studies. The possible relevance of these data to the clinical uses of aspirin is discussed.

Platelet activity in cluster headache

Platelets are known to be activated in common or classic migraine both during the attack and in headache-free periods. Platelet behavior is less well known in cluster headache. We have investigated beta-thromboglobulin (beta-TG) and platelet factor four (PF4) plasma levels, markers of in vivo platelet activation, in patients during remission and during bouts of cluster headache with and without pain. The results indicated that statistically significantly higher levels of beta-TG and PF4 occur in the patients during the remission period when compared with the control subjects. Such high levels seemed to persist between paroxysmal episodes in cluster periods. However, during the attacks of cluster headache beta-TG and PF4 plasma levels decreased by 42% and 50%, respectively, in comparison with plasma concentrations measured outside of attack. Thus, although platelet activation also occurs in patients with cluster headache, the attack as such seems to be characterized by a marked reduction in platelet activation.

Extrahepatic metabolism and distribution of aspirin in vascular beds of sheep

The dispositions of aspirin, its metabolite, salicylic acid, and its subsequent metabolite, salicyluric acid, were studied in eight anesthetized sheep infused with aspirin (61 and 485 microgram X min-1 X kg-1) for 75 min. Plasma samples were withdrawn from the portal vein, hepatic vein, pulmonary artery, left ventricle, left femoral vein, and left femoral artery. Significant extraction of aspirin occurred across the liver and hind leg, with mean availabilities of 0.75, 0.98, and 0.82 observed across the liver, lung, and hind leg, respectively. The extraction of aspirin was not affected by coadministration of sodium salicylate (30-1200 mg equiv salicylic acid). This extraction reflected hydrolysis of aspirin to salicylic acid; the metabolism of aspirin in the hind leg, lung, and liver being confirmed with tissue homogenate studies. The metabolism of aspirin in the extrahepatic tissues is significant in relation to the proposed selective presystemic acetylation of platelet cyclooxygenase by aspirin and the use of low-dose aspirin for thrombotic indications.

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

In rats intravenous aspirin was only slightly more effective an inhibitor of platelet thromboxane B2 (TxB2) than of aorta 6-keto-prostaglandin (PGF)1 alpha generation (1.9 versus 2.1 mg/kg). In contrast, oral aspirin was about five times more effective on platelet than on aorta cyclooxygenase activity. The "biochemical selectivity" of aspirin as an inhibitor of platelet and vascular cyclooxygenase thus was not apparent after intravenous administration of the drug. However, this could be achieved by relatively low doses of oral (or intraduodenal) aspirin, on account of "presystemic" acetylation of platelet cyclooxygenase. Even in this condition, though, aspirin selectivity was relative to "systemic" peripheral vessels but not to the vessels of the enterohepatic circulation. Indeed after an oral or intraduodenal dose of 5 mg/kg aspirin, generation of portal vein 6-keto-PGF1 alpha was inhibited to much the same extent as platelet TxB2, while inferior vena cava 6-keto-PGF1 alpha formation was spared.

Metabolism of aspirin and procaine in mice pretreated with O-4-nitrophenyl methyl(phenyl)phosphinate or O-4-nitrophenyl diphenylphosphinate

Concentrations of [carboxyl-14C]procaine in blood of mice were increased threefold for 27 min by exposure to O-4-nitrophenyl diphenylphosphinate 2 hr prior to [carboxyl-14C]procaine injection ip, while there was no effect of O-4-nitrophenyl methyl(phenyl)phosphinate pretreatment. There was no effect of either organophosphinate on the primary hydrolysis of [acetyl-l-14C]aspirin when assessed by the expiration of [14C]carbon dioxide; however, O-4-nitrophenyl diphenylphosphinate pretreatment produced transient increases in blood concentrations of both [carboxyl-14C]aspirin and [carboxyl-14C]salicylic acid following administration of [carboxyl-14C]aspirin. Liver carboxylesterase activity in O-4-nitrophenyl diphenylphosphinate pretreated mice was 11% of control activity. These results indicate the potential for drug interaction with O-4-nitrophenyl diphenylphosphinate but not with O-4-nitrophenyl methyl(phenyl)phosphinate. It appears that liver carboxylesterase activity has a minor role in hydrolysis of aspirin in vivo, but may be more important in procaine metabolism.

In vivo indexes of platelet and vascular function during fish-oil administration in patients with atherosclerosis

Populations that consume a diet rich in marine lipids may have a lower risk of atherosclerotic disease. Fish oil contains the N-3 polyunsaturated fatty acid eicosapentaenoate, and the biosynthesis of thromboxanes and prostacyclins from eicosapentaenoate (thromboxane A3 and prostaglandin I3), rather than from the usual precursor arachidonate (thromboxane A2 and prostaglandin I2), may help to reduce the risk. To examine this hypothesis, we studied the effect of eicosapentaenoate supplementation (10 g per day) for one month on the synthesis of thromboxanes and prostacyclins, as assessed by urinary metabolite excretion, in six patients with peripheral vascular disease and seven normal controls. Supplementation markedly increased the eicosapentaenoate content of phospholipids from red cells and platelets. Synthesis of the platelet agonist thromboxane A2, which was elevated in the patients at base line, declined by 58 percent during supplementation but did not reach normal values. The decline in thromboxane A2, which is synthesized from arachidonate, coincided with the formation of the inactive thromboxane A3, which is synthesized from eicosapentaenoate. A lower dose of eicosapentaenoate (1 g per day) was not sufficient to maintain the changes in thromboxane A2 synthesis. Platelet function was only moderately inhibited during eicosapentaenoate supplementation, consistent with incomplete suppression of thromboxane A2 synthesis. These studies show that a high dose of eicosapentaenoate alters the pattern of synthesis of thromboxanes and prostacyclins. However, effects comparable to those of aspirin require long-term administration in high doses. Whether other properties of fish oil might render it a more attractive antithrombotic therapy remains to be determined.

The biochemical pharmacology of thromboxane synthase inhibition in man

Selective inhibitors of thromboxane synthase have two theoretical advantages over inhibitors of the cyclooxygenase enzyme as potential antithrombotic compounds. First, they do not prevent formation of prostacyclin, a platelet-inhibitory, vasodilator compound, coincident with inhibiting thromboxane biosynthesis. Second, the prostaglandin endoperoxide substrate that accumulates in the platelet in the presence of thromboxane synthase inhibition may be donated to endothelial prostacyclin synthase at the site of platelet-vascular interactions (endoperoxide "steal"). Selective inhibition of thromboxane biosynthesis coincident with enhanced prostacyclin formation in vivo has been observed after administration of these compounds to man. Despite these attractive features and the efficacy of these compounds in diverse short-term animal preparations of thrombosis, investigations of their efficacy in human disease have proven disappointing. This may reflect on the importance of thromboxane A2 in the diseases that have been investigated. Alternatively, the lack of drug efficacy may have resulted from either incomplete suppression of thromboxane biosynthesis and/or substitution for the biological effects of thromboxane A2 by prostaglandin endoperoxides during long-term dosing studies. Given that selective inhibition of thromboxane formation can be approached with aspirin, the particular value of these compounds is dependent on enhancing prostacyclin formation. Aspirin inhibits thromboxane-dependent platelet activation, but many platelet agonists are likely to act in concert in vivo and prostacyclin inhibits platelet aggregation induced by both thromboxane-dependent and thromboxane-independent mechanisms. To test the hypothesis that thromboxane synthase inhibitors are efficacious in human disease, compounds of longer duration of action are required. Combination with antagonists of the prostaglandin/thromboxane A2 receptor may be necessary to reveal their full beneficial action.

Clinical pharmacology of platelet cyclooxygenase inhibition

Nonsteroidal anti-inflammatory drugs and sulfinpyrazone compete dose-dependently with arachidonate for binding to platelet cyclooxygenase. Such a process closely follows systemic plasma drug concentrations and is reversible as a function of drug elimination. Peak inhibition and extent of its reversibility at 24 hr varies consistently with individual pharmacokinetic profile. Inhibition of platelet cyclooxygenase activity by these agents is associated with variable effects on prostaglandin (PG) synthesis in the gastric mucosa and the kidney. Aspirin acetylates platelet cyclooxygenase and permanently inhibits thromboxane (TX) A2 production in a dose-dependent fashion when single doses of 0.1 to 2.0 mg/kg are given. Acetylation of the enzyme by low-dose aspirin is cumulative on repeated dosing. The fractional dose of aspirin necessary to achieve a given level of acetylation by virtue of cumulative effects approximately equals the fractional daily platelet turnover. Serum TXB2 measurements obtained during long-term dosing with 0.11, 0.22, and 0.44 mg/kg aspirin in four healthy subjects could be fitted by a theoretical model assuming identical acetylation of platelet (irreversible) and megakaryocyte (reversible) cyclooxygenase. For a given dose within this range, both the rate at which cumulative acetylation occurs and its maximal extent largely depend upon the rate of platelet turnover. Continuous administration of low-dose aspirin (20 to 40 mg/day) has no statistically significant effect on urinary excretion of either 6-keto-PGF1 alpha or 2,3-dinor-6-keto-PGF1 alpha, i.e., indexes of renal and extrarenal PGI2 biosynthesis in vivo. Whether a selective sparing of extraplatelet cyclooxygenase activity by low-dose aspirin will result in increased antithrombotic efficacy, fewer toxic reactions, or both remains to be established in prospective clinical trials.

Modes of action of aspirin-like drugs

Current dogma holds that nonsteroidal anti-inflammatory drugs (NSAIDs) act by inhibition of the synthesis and release of prostaglandins. However, NSAIDs also inhibit the activation of neutrophils, which provoke inflammation by releasing products other than prostaglandins. We now report that NSAIDs (e.g., indomethacin, piroxicam) inhibit activation of neutrophils by inflammatory stimuli, such as C5-derived peptides and leukotriene B4, even when cyclooxygenase products generated in suspensions of stimulated neutrophils (prostaglandin E and thromboxanes) are present. Sodium salicylate (3 mM) greatly inhibited aggregation of neutrophils but had no effect on aggregation of platelets or production of thromboxane induced by arachidonate. Sodium salicylate and other NSAIDs also inhibit calcium movements (45Ca uptake, changes in fluorescence of chlortetracycline and quin-2). Aspirin, sodium salicylate, indomethacin, and piroxicam also enhanced the poststimulation increase in intracellular cyclic AMP. NSAIDs therefore inhibit early steps in neutrophil activation as reflected by their capacity to inhibit movements of Ca and to enhance intracellular levels of cyclic AMP.

CSM update: adverse drug reactions and the liver

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Idiopathic hypopituitarism in the elderly

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The low dose aspirin controversy solved at last?

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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 4.5% of the initial values, respectively, whereas in the mixed blood sample aggregation was not impaired. Inhibition of thromboxane formation was constantly 73% 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 microgram/ml; n = 5). After the 1500 mg ASA dose platelet function and thromboxane formation decreased to 29.8 and 2.0% of the initial 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 micrograms/ml. IC50 values were 1.00 +/- 0.36 and 0.30 +/- 0.05 microgram/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.

Thromboembolism in patients with aortic porcine bioprostheses

Thromboembolism was prospectively studied in 215 patients who survived aortic valve replacement with porcine bioprostheses. All patients were anticoagulated with warfarin sodium during the first 3 postoperative months. Thereafter, 80 patients received aspirin (325 or 650 mg per day) and 135 received no antiplatelet or anticoagulant drugs. The two groups of patients were similar. Thromboembolic complications were carefully searched for during the follow-up interviews. Patients were removed from the study after a thromboembolic event unless there was no change in their management. The follow-up ranged from 6 to 80 months (mean, 36 months). The linearized thromboembolic rate in patients on a regimen of aspirin was 1.3% per patient-year and in patients not taking aspirin, 5.2% per patient-year (p less than 0.02). Replacement of the ascending aorta and patch enlargement of the aortic annulus with a Teflon graft were identified as significant risk factors for thromboembolism in patients with aortic porcine bioprostheses. These findings indicate that patients with aortic porcine bioprostheses should receive aspirin, especially if they also had replacement of the ascending aorta or patch enlargement of the aortic annulus with a Teflon graft.

Endogenous biosynthesis of prostacyclin during cardiac catheterization and angiography in man

The potent platelet inhibitory and vasodilator properties of prostacyclin suggest that levels of this substance may be of relevance to drug action and pathologic processes in the coronary vascular bed. Attempts to estimate the coronary secretion rate of prostacyclin have relied on measurements of metabolites obtained via cardiac catheter, usually as an adjunct to coronary angiography. To test the hypothesis that such procedures might themselves perturb endogenous biosynthesis of prostacyclin we used mass spectrometry to measure plasma levels of 6-keto-prostaglandin (PG) F1 alpha across the coronary vascular bed, as well as to assess the excretion of a major urinary metabolite, 2,3-dinor-6-keto-PGF1 alpha (PGI-M), in patients undergoing cardiac catheterization. PGI-M excretion increased variably from a median 100 to 205 pg/mg creatinine (p less than .01) during catheterization with angiography and remained elevated 2 to 4 hr after initiation of the procedure. However, cardiac catheterization without angiography also stimulated metabolite excretion, perhaps reflecting catheter-induced vascular trauma. The direct effect of radiocontrast media on vascular release of prostacyclin was indicated by increased PGI-M excretion in healthy volunteers administered intravenous radiocontrast and by studies of the canine coronary artery and jugular vein in vitro. Measurement of plasma 6-keto-PGF1 alpha after left heart catheterization showed that levels in aortic (21 +/- 8 pg/ml) and coronary sinus (14 +/- 2 pg/ml) blood were increased compared with peripheral venous levels (less than or equal to 4 + 1 pg/ml) determined before this procedure.(ABSTRACT TRUNCATED AT 250 WORDS)