Equal Antiplatelet Effects of Aspirin 50 or 324 mg/Day in Patients After Acute Myocardial Infarction

Low-dose aspirin for the prevention of atherosclerotic cardiovascular disease

During the past 30 years, several developments have occurred in the antiplatelet field, including the role of aspirin in primary prevention of atherosclerotic cardiovascular disease. There have been several attempts to develop antiplatelet drugs more effective and safer than aspirin and a shift in emphasis from efficacy to safety, advocating aspirin-free antiplatelet regimens after percutaneous coronary intervention. Evidence supporting a chemopreventive effect of low-dose aspirin against colorectal (and other digestive tract) cancer has also strengthened. The aim of this article is to revisit the role of aspirin in the prevention of atherothrombosis across the cardiovascular risk continuum, in view of developments in the antiplatelet field. The review will offer a clinical perspective on aspirin's mechanism of action, pharmacokinetics, and pharmacodynamics. This will be followed by a detailed discussion of its clinical efficacy and safety.

Comparison of Aspirin and Rivaroxaban Plus Aspirin in the Management of Stable Coronary Artery Disease or Peripheral Artery Disease: A Systematic Review of Randomized Controlled Trials

Introduction

Low-dose aspirin or clopidogrel, statins, renin–angiotensin system inhibitors, and beta blockers are the cornerstone therapy for cardiovascular prevention in patients with coronary heart disease. Using only single-antiplatelet therapy for secondary prevention in patients with stable coronary artery disease (SCAD) and/or peripheral artery disease (PAD) has a significant risk of recurrent thrombotic complications.

Objective

This systematic review aimed to compare aspirin alone and its combination with rivaroxaban for secondary cardiovascular prevention in patients with SCAD and/or PAD.

Methods

The literature search was conducted on PubMed, ClinicalTrials.gov, Cochrane Library, and Google Scholar for articles published from November 2011 to September 2021. An advanced search strategy was used to retrieve relevant studies related to aspirin and/or rivaroxaban use for secondary cardiovascular prevention in patients with SCAD and/or PAD. Records identified from the databases were extracted using a data-abstraction format prepared in Microsoft Excel. Studies’ methodological quality was assessed using the Cochrane risk-of-bias tool for randomized trials. This systematic review is registered in PROSPERO (CRD42022306598) and was prepared based on the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines.

Results

A total of five randomized controlled trials (RCTs) with 33,959 participants were included for final analysis. These studies showed that rivaroxaban with aspirin was more effective than the standard therapy of aspirin alone in the prevention of secondary cardiovascular events (major adverse cardiovascular events (MACEs) and/or major adverse limb events (MALEs), but the combination increased major bleeding.

Conclusion

The combination of rivaroxaban with aspirin is more effective than aspirin alone in the prevention of both MACEs and MALEs in patients with stable CAD and/or PAD. However, the combination treatment is associated with increased of major bleeding. Therefore, the combination of rivaroxaban and aspirin is superior to monotherapy in the management of patients with a high risk of developing MACEs and MALEs.

Control and Treatment of Hemostasis in Cardiovascular Surgery

The hemostasis protocol applied at the Cardiovascular Surgery Dept. of La Pitié Hospital has greatly reduced thromboembolic accidents and excessive bleeding, with consequent benefits for patients as well as cost reduction. Protocol also has been adopted for patients implanted with a circulatory assist device or a TAH. This paper presents our criteria on supervision and treatment of coagulation with such patients, who reflect all the acquired pathology in clinical hemostasis. From 04/86 to 07/94, 82 patients underwent TAH as a bridge to transplantation. Mean age: 38. Overall duration of mechanical support: 1930 days (mean: 23), of which 137 and 603 for 2 patients. Average duration of CPB: 150 min. Systematic approach to complex TAH-blood interaction and pre-operative multiple organ dysfunction used to control bleeding and/or thromboembolism after CPB. In addition to routine tests, specific regular testing was carried out at least once a day for platelet functions, for thrombin formation and its regulatory pathways, and for the fibrinolytic system. Patients were treated with small doses of Heparin, large doses of Dypyridamole, small doses of Aspirin, modulated doses of Aprotinin, Ticlopidine, Pentoxifylline, FFP, as well as Fibrinogen and AT III concentrates. Dosage was adapted to patient's clinical profile as well as to test interpretation criteria to provide personalized treatment. DIC, widely present in its different phases, was thus diagnosed and treated. All DIC bleeding was controlled, making it possible to detect other causes of post-operatory bleeding and use blood derivates rationally. There were no thromboembolic complications and no iatrogenic bleeding. TAH explantation shows no evidence of macroscopic clots in high risk sites, confirmed by microscopic analysis.

Effect of Vorapaxar Alone and in Combination with Aspirin on Bleeding Time and Platelet Aggregation in Healthy Adult Subjects

The effect of the protease‐activated receptor‐1 (PAR‐1) antagonist vorapaxar on human bleeding time is not known. This was a randomized, two‐period, open‐label trial in healthy men (n = 31) and women (n = 5). In period 1, subjects received 81 mg aspirin q.d. or a vorapaxar regimen achieving steady‐state plasma concentrations equivalent to chronic 2.5 mg q.d. doses, for 7 days. In period 2, each group added 7 days of the therapy alternate to that of period 1 without washout. Bleeding time and platelet aggregation using arachidonic acid, ADP, and TRAP agonists were assessed. Bleeding time geometric mean ratio (90% CI) for vorapaxar/baseline was 1.01 (0.88–1.15), aspirin/baseline was 1.32 (1.15–1.51), vorapaxar + aspirin/vorapaxar was 1.47 (1.26–1.70), and vorapaxar + aspirin/aspirin was 1.12 (0.96–1.30). Unlike aspirin, vorapaxar did not prolong bleeding time compared with baseline. Bleeding time following administration of vorapaxar with aspirin was similar to that following aspirin alone.

Fast platelet suppression by lysine acetylsalicylate in chronic stable coronary patients. Potential clinical impact over regular aspirin for coronary syndromes

BACKGROUND

The rapid utilization of fibrinolytics following Q-wave myocardial infarction has clearly modified the evolution of this disease. However, it is still not known whether the immediate inhibition of platelet aggregation (PA) during the coronary event improves outcomes.

HYPOTHESIS

The present study was designed to test, in patients with known coronary artery disease (chronic stable angina), whether the particular kinetic pattern of lysine acetylsalicylate (LA) compared with aspirin may affect the time to onset of inhibition of platelet aggregation.

METHODS

Ten patients suffering from chronic stable angina participated in this study to compare the efficacy and speed of the inhibition of PA with 320 mg of LA versus 320 mg of aspirin. All patients discontinued the use of aspirin and any other anti-inflammatory agents for 15 days prior to the beginning of the study. They were randomly assigned to LA or aspirin. Blood specimens were obtained to measure the PA at admission, and 5, 10, 20, 30, and 60 min after ingestion. Patients continued to take the assigned drug once a day for the following 4 days. On Day 5, a new blood sample was taken. After this, patients underwent a 15-day wash-out period, and then crossed over to the opposite drug. The samples were analyzed immediately using platelet-rich plasma stimulated with adenosine diphosphate (ADP) 2 mumol/l, collagen 1 microgram/ml, epinephrine 20 mumol/l, and sodium arachidonate acid 0.75 mm/l.

RESULTS

The same level of PA inhibition after 30 and 60 min of aspirin administration can be obtained with LA 5 min following ingestion (sodium arachidonate acid: LA: 16.3 +/- 25.9 vs. aspirin 57.6 +/- 8.2; p = 0.00014; collagen: LA 18.9 +/- 20.1 vs. aspirin 47.2 +/- 10.5; p = 0.00092; ADP: LA 27.3 +/- 18.4 vs. aspirin 39.7 +/- 21.8, p = 0.18; epinephrine: LA 22.0 +/- 9.9 vs. aspirin 55.4 +/- 10.9, p = 0.00002.

CONCLUSIONS

Platelet aggregation inhibition immediately following LA may have significant clinical implications for the treatment of coronary syndromes.

Pharmacokinetics, Pharmacodynamics, and Safety of a Prostaglandin D2 Receptor Antagonist

Laropiprant is a selective antagonist of the prostaglandin D(2) (PGD(2)) receptor subtype 1 (DP1). Three double-blind, randomized, placebo-controlled studies evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of single and multiple oral doses of laropiprant in healthy male volunteers. Single doses up to 900 mg and multiple doses up to 450 mg were generally well tolerated. Laropiprant exhibited dose-proportional pharmacokinetics. Oral absorption is rapid (T(max)=0.8-2.0 h) and the terminal half-life is approximately 12-18 h. The pharmacokinetics of laropiprant was not affected by food. Single doses of 6 mg and higher were effective in suppressing PGD(2)-induced cyclic AMP accumulation in platelets, demonstrating laropiprant target engagement with DP1. Laropiprant has detectable off-target antagonist effects at the thromboxane A(2) receptor but no clinically significant effect on collagen-induced platelet aggregation or bleeding times with multiple doses up to 200 mg.

Aspirin resistance and adverse clinical events in patients with coronary artery disease

PURPOSE

We sought to determine the clinical significance of aspirin resistance measured by a point-of-care assay in stable patients with coronary artery disease (CAD).

METHODS

We used the VerifyNow Aspirin (Accumetrics Inc, San Diego, Calif) to determine aspirin responsiveness of 468 stable CAD patients on aspirin 80 to 325 mg daily for > or =4 weeks. Aspirin resistance was defined as an Aspirin Reaction Unit > or =550. The primary outcome was the composite of cardiovascular death, myocardial infarction (MI), unstable angina requiring hospitalization, stroke, and transient ischemic attack.

RESULTS

Aspirin resistance was noted in 128 (27.4%) patients. After a mean follow-up of 379+/-200 days, patients with aspirin resistance were at increased risk of the composite outcome compared to patients who were aspirin-sensitive (15.6% vs 5.3%, hazard ratio [HR] 3.12, 95% confidence intervals [CI], 1.65-5.91, P < .001). Cox proportional hazard regression modeling identified aspirin resistance, diabetes, prior MI, and a low hemoglobin to be independently associated with major adverse long-term outcomes (HR for aspirin resistance 2.46, 95% CI, 1.27-4.76, P = .007).

CONCLUSIONS

Aspirin resistance, defined by an aggregation-based rapid platelet function assay, is associated with an increased risk of adverse clinical outcomes in stable patients with CAD.

Novel synthetic approach to N-aryl-4-(3-pyridyl)thiazol-2-amine and analogues using HMCM-41 as catalyst, and their biological evaluation as human platelet aggregation inhibitors

A novel synthetic approach to N-aryl-4-(3-pyridyl)thiazol-2-amine and analogues using HMCM-41, a mesoporous aluminosilicate catalyst and their in vitro ADP-induced platelet aggregation inhibitory activity on human blood platelets is described. Among the test compounds N-(2'-flourophenyl)-4-(3-pyridyl)thiazol-2-amine (9e) was found to be the most potent, IC(50)=4.84x10(-7)M.

Aspirin resistance

Aspirin resistance refers to less than expected suppression of thromboxane A(2) production by aspirin and has been reported to be independently associated with an increased risk of adverse cardiovascular events. Possible causes of aspirin resistance include poor compliance, drug interaction, inadequate aspirin dose, increase turnover of platelets, genetic polymorphisms of cyclo-oxygenase-1, and upregulation of alternate (non-platelet) pathways of thromboxane production. Laboratory methods used to detect aspirin resistance include those that measure thromboxane A(2) production and thromboxane A(2)-dependent platelet function. However, since there is currently no standardised approach to the diagnosis and there are no proven effective treatments for aspirin resistance that improve outcome, patients with cardiovascular disease receiving aspirin should not be routinely tested for aspirin resistance. Instead physicians should be aware of the factors that may impair aspirin function, ensure that they use an appropriate dose of aspirin and optimise compliance with aspirin therapy. Further research exploring the mechanisms of aspirin resistance is needed in order to better define and develop a standardised test for aspirin resistance that is specific, reliable, can be readily applied in routine laboratories and correlate with an increased risk of cardiovascular events.

Aspirin resistance

Aspirin resistance is the inability of aspirin to reduce platelet production of thromboxane A2 and thereby platelet activation and aggregation. Increasing degrees of aspirin resistance may correlate independently with increasing risk of cardiovascular events. Aspirin resistance can be detected by laboratory tests of platelet thromboxane A2 production or platelet function that depend on platelet thromboxane production. Potential causes of aspirin resistance include inadequate dose, drug interactions, genetic polymorphisms of COX-1 and other genes involved in thromboxane biosynthesis, upregulation of non-platelet sources of thromboxane biosynthesis, and increased platelet turnover. Aspirin resistance can be overcome by treating the cause or causes, and reduced by minimising thromboxane production and activity, and blocking other pathways of platelet activation. Future research is aimed at defining aspirin resistance, developing reliable tests for it, and establishing the risk of associated cardiovascular events. Potential mechanisms of aspirin resistance can then be explored and treatments assessed.

Does Ginkgo biloba Special Extract EGb??761?? Provide Additional Effects on Coagulation and Bleeding when Added to Acetylsalicylic Acid 500mg Daily?

OBJECTIVE

The aim of this study was to determine whether Ginkgo biloba special extract EGb 761 amplifies the known effects of acetylsalicylic acid (ASA) on platelet aggregation, bleeding time or other coagulation parameters in healthy subjects.

METHODS

In a double-blind, double-dummy procedure, 50 healthy male subjects (20-44 years) were randomly allocated in equal numbers to one of two possible treatment sequences, i.e. ASA followed by ASA + EGb 761 or ASA + EGb 761 followed by ASA. Each treatment lasted 7 days; the washout period between treatments was 3 weeks. Study medication was taken twice daily (ASA group: ASA 500 mg tablet + placebo-coated tablet in the morning and placebo tablet + placebo-coated tablet in the evening; ASA + EGb 761 group: ASA 500 mg tablet + EGb 761 120 mg-coated tablet in the morning and placebo tablet + EGb 761 120 mg-coated tablet in the evening), resulting in a daily dose of ASA 500 mg in the ASA group and 500 mg ASA + 240 mg EGb 761 in the ASA + EGb 761 group. Bleeding time, coagulation parameters and platelet activity in response to various agonists were determined. In addition, adverse events, laboratory variables and vital signs were measured. The primary variable bleeding time was assessed in confirmatory analysis, all other variables were evaluated descriptively. The coagulation variables were analysed by ANOVA under the crossover model.

RESULTS

ASA given alone clearly prolonged bleeding time. ASA and the combination of ASA + EGb 761 exerted quite similar effects on all coagulation parameters measured, including bleeding time (ASA alone: 4.1 min before therapy, 6.2 min after therapy; ASA + EGb 761: 4.2 min before therapy, 6.3 min after therapy; ratio of means: 1.01, 90% CI 0.86, 1.19) and agonist-induced platelet aggregation (collagen-induced platelet aggregation - ASA: 84.5% before therapy, 81.0% after therapy; ASA + EGb 761: 86.6% before therapy, 81.0% after therapy; ratio of means: 1.00, 90% CI 0.95, 1.05; adenosine diphosphate-induced platelet aggregation - ASA: 72.6% before therapy, 47.2% after therapy; ASA + EGb 761: 71.7% before therapy, 44.8% after therapy; ratio of means: 0.95, 90% CI 0.85, 1.06). Both treatments were well tolerated, and both the number and nature of adverse events in the two groups were similar.

CONCLUSIONS

Our findings suggest that co-administration of ASA and EGb 761 does not constitute a safety risk, including in an elderly patient population undergoing treatment with EGb 761.

Resistance to antiplatelet drugs: current status and future research

Platelet reactivity and activation are important factors during the development of atherothrombotic processes and subsequent ischaemic complications. Pharmacological agents that suppress platelet function are proved to be the most efficient in the prevention and treatment of thrombotic complications. As the activation of platelets during thrombus generation involves many complex and redundant pathways, simultaneous use of different antiplatelet drugs that are directed against different targets have been effective in reducing adverse clinical events. The main antiplatelet drugs are aspirin (which inhibits thromboxane synthesis), thienopyridines (which block P2Y12 receptors) and glycoprotein IIb/IIIa antagonists (which block glycoprotein IIb/IIIa receptors). In recent years, resistance or nonresponsiveness to antiplatelet therapy has been reported and, more importantly, are linked to the occurrence of adverse cardiovascular events. New treatment strategies to overcome nonresponsiveness are being sought. A focus on the development of simple, reproducible and user friendly point-of-care methods to determine aspirin/clopidogrel responsiveness should be undertaken to assist clinicians in tailoring antiplatelet therapy to the individual patient.

Short-term prevention of thromboembolic complications in patients with atrial fibrillation with aspirin plus clopidogrel: the Clopidogrel-Aspirin Atrial Fibrillation (CLAAF) pilot study

BACKGROUND

We evaluated the short-term safety and efficacy of aspirin-plus-clopidogrel as antithrombotic therapy in nonvalvular atrial fibrillation (AF).

METHODS AND RESULTS

Thirty patients (11 women, 45 to 75 years of age) with non-high-risk permanent (n = 12) or persistent AF awaiting cardioversion (n = 18) underwent transesophageal echocardiography to exclude left heart thrombi and were then randomly assigned to receive warfarin (international normalized ratio, 2 to 3 for 3 weeks) or aspirin (100 mg/d alone for 1 week)-plus-clopidogrel (75 mg/d added to aspirin for 3 weeks). Bleeding time and serum thromboxane B2 were measured at entry and at 3 weeks. Bleeding time, not affected by warfarin, was prolonged by 71% by aspirin (P <.05) and further, by 144%, by adding clopidogrel (P <.01 vs aspirin alone; +319%, P <.01, vs baseline). Thromboxane B2, not affected by warfarin, was reduced by aspirin (-98%, P <.01) but not further by clopidogrel. No thrombi or dense spontaneous echo-contrast were found at the 3-week transesophageal echocardiography. Seven of 9 patients receiving warfarin and 7 of 9 patients receiving aspirin-plus-clopidogrel, undergoing electrical cardioversion, achieved sinus rhythm. No thromboembolic or hemorrhagic events occurred in both arms throughout the 3-week treatment and a further 3-month follow-up.

CONCLUSIONS

Aspirin-plus-clopidogrel and warfarin were equally safe and effective in preventing thromboembolism in this small group of patients with non-high-risk AF.

Effect of different aspirin doses on platelet aggregation in patients with stable coronary artery disease

BACKGROUND

Aspirin is widely used as an antiplatelet agent in the primary and secondary prevention of cardiovascular disease. In order to spare prostacyclin formation and reduce gastrointestinal side-effects, very low doses of aspirin have been introduced. However, it remains unclear whether these low doses are equally effective with respect to inhibition of platelet aggregation.

AIMS

In a randomized, controlled study in 60 patients with stable coronary artery disease, the effects on platelet aggregation of five doses (50, 80, 100, 162.5 and 325 mg) of aspirin, which are widely used in clinical practice, given for 70 days, were investigated. Two reagents, adenosine diphosphate (ADP) and epinephrine, were used to induce platelet aggregation in platelet-rich plasma. An age- and sex-matched group of people without coronary artery disease served as the control.

RESULTS

ADP- and epinephrine-induced platelet aggregation was 78.2 +/- 12.8% and 76.7 +/- 15.5% of maximum aggregation in the control group. Aspirin inhibited platelet aggregation in a dose-dependent manner. Minimum platelet aggregation was observed at a dose of 325 mg aspirin (27.5 +/- 17.4% with ADP). Doses of 50 and 80 mg aspirin were much less effective in inhibiting platelet aggregation (59.1 +/- 11.4% and 50.3 +/- 12.1% with ADP, respectively). Doses of 100 and 162.5 mg aspirin produced significantly greater inhibition of platelet aggregation than lower doses (36.2 +/- 11.7% and 38.5 +/- 19.8% platelet aggregation with ADP, respectively).

CONCLUSION

Our results demonstrate that doses of aspirin less than 100 mg are not as effective at inhibiting platelet aggregation as doses greater than 100 mg.

Effects of enteric-coated, low-dose aspirin on parameters of platelet function

BACKGROUND

Aspirin is widely used as an anti-thrombotic drug; however, it has been suggested that enteric-coated formulations of aspirin may be less bioavailable and less effective as anti-thrombotic agents.

AIM

To assess the effect of a formulation of enteric-coated, low-dose (81 mg) aspirin on serum generated thromboxane B2 and platelet aggregation in healthy subjects.

METHODS

Twenty-four subjects participated in a double-blind, randomized, placebo-controlled, parallel-group, multiple-dose study. Twelve subjects in each of two groups received a daily oral dose of enteric-coated aspirin (81 mg) or matching placebo for 7 days. Serum thromboxane B2 and platelet aggregation (using 1 mm arachidonic acid and 1 microg/mL collagen as agonists) were measured 1-3 days prior to day 1, on day 1 (prior to therapy) and 4 h after the last dose on day 7.

RESULTS

After seven daily doses of enteric-coated aspirin, the mean percentage inhibition from baseline of ex vivo generated serum thromboxane B2 was 97.4%, compared with a 7.8% increase after placebo treatment. The mean percentage inhibition of arachidonic acid- and collagen-induced platelet aggregation was 97.9% and 70.9%, respectively, following enteric-coated aspirin, compared with - 1.0% and 2.7%, respectively, after placebo.

CONCLUSIONS

The anti-platelet effects of multiple, daily, low-dose aspirin (as assessed by inhibition of serum thromboxane B2 and platelet aggregation) are not adversely affected by enteric coating.

The effect of different antithrombotic regimens on platelet aggregation after myocardial infarction

Platelet aggregate ratio (PAR) was measured according to the method of Wu & Hoak in 143 patients after acute myocardial infarction (AMI) and in 54 controls. A PAR < 1 expresses the presence of platelet aggregates. The patients were randomized to aspirin 160 mg/d, or warfarin, or aspirin 75 mg/d + warfarin. In patients on aspirin, PAR was measured 24 h after aspirin intake, and in 76 patients also 2 h after aspirin. The median PAR in patients on warfarin was 0.85, on warfarin + aspirin 0.91 and on aspirin alone 0.94, all significantly lower than the median PAR of 0.97 in the controls. In 14 patients on aspirin the PARs were below a cut-off point of 0.82 (secondary aspirin non-responders). PAR increase significantly 2 h after aspirin intake. In two patients, however, PAR remained low (primary aspirin non-responders). It is concluded that some patients do not seem to respond to aspirin, the clinical implication of which has yet to be determined.

Effective long-term inhibition of thromboxane production but not of serotonin release in patients with coronary heart disease by 30 mg/d acetylsalicylic acid dosage

Efficacy of aspirin (Acetylsalicylic acid, ASA) antiaggregatory prevention was demonstrated in a series of clinical trials. The recommended ASA doses decreased gradually and doses 50-30 mg ASA/d are intensively studied at the present time. A group of 42 patients with coronary heart disease was evaluated: (1) Basal TXB2 production during spontaneous blood clotting was 360 +/- 37.6 ng/ml; (2) Two initial doses were tested: while 200 mg ASA inhibited, during spontaneous blood clotting, median TXB2 production by 99.9% (serum TXB2 concentration 1.35 ng/ml), 30 mg ASA median inhibition was just 42.0% (serum TXB2 151 ng/ml); (3) 30 mg ASA/d maintenance dose was evaluated for 3 months. The median TXB2 production inhibition was 98.5% (serum TXB2 3.75 ng/ml, first month) and 94.0% (serum TXB2 14.2 ng/ml, third month); (4) Four patients did not respond sufficiently, because of noncompliance verified by the determination of salicyluric acid urinary excretion, the lower limit of excretion being <3 micromol/2 h; (5) Both initial and maintenance ASA dose decreased metabolic TXA2 endproducts in urine; (6) 5HT platelet release did not decrease; (7) Potential changes of 5HT metabolic elimination were excluded by the simultaneous determination of 5-hydroxyindoleacetic acid (5HIAA). In conclusion, 200 mg initial dose and 30 mg ASA/d maintenance dose are suggested to be maximally inhibitory for TXB2 production without influence on 5HT release.

Effect of aspirin dosage and enteric coating on platelet reactivity

Although aspirin is effective in the prevention and treatment of cardiovascular diseases, the optimal dose remains uncertain. The purpose of this study was to compare the platelet inhibitory and prostacyclin-sparing effects of 2 doses (81 and 325 mg) and forms (enteric-coated and regular) of aspirin. Since platelet reactivity has been reported to increase after strenuous exercise, a known trigger of myocardial infarction, subjects were studied following maximal treadmill exercise as well as at rest. Forty male healthy subjects were evaluated using a randomized, double-blind, parallel study design. Blood samples were obtained before and after maximal treadmill exercise at baseline and after 7 days on aspirin therapy. Both enteric and regular aspirin in 81- and 325-mg dosages markedly inhibited adenosine diphosphate and epinephrine-induced aggregation at rest and after exercise. Aspirin also inhibited the platelet response to collagen as assessed by a longer lag time to aggregation. The prolongation of lag time was greater for 325 mg than for 81 mg (100 +/- 7 vs 91 +/- 7; p = 0.04, after exercise). There were no significant dose-related differences in plasma 6-keto-prostaglandin F1alpha level; however, enteric-coated aspirin inhibited the exercise-induced increase in 6-keto-prostaglandin F1alpha to a lesser extent than regular aspirin. Although both doses (81 and 325 mg) and types (regular and enteric-coated) of aspirin inhibited adenosine diphosphate and epinephrine-induced aggregation equally, the 325-mg dose inhibited collagen-induced aggregation to a greater extent than 81 mg. The greater platelet inhibition observed with 325 mg may be clinically relevant in acute coronary syndromes characterized by plaque rupture with extensive collagen exposure and platelet activation.

Aspirin therapy: optimized platelet inhibition with different loading and maintenance doses

Inhibition of cyclooxygenase by aspirin has been shown to be beneficial in clinical situations such as acute myocardial infarction or unstable angina. The precise effect of various doses of aspirin on acute and long-term inhibition of platelet aggregation and thromboxane synthesis remains unclear. In this study we evaluated the effect of oral aspirin (0, 40, 100, 300, or 500 mg) as the initial loading dose in combination with different maintenance doses of aspirin (0, 40, or 100 mg/day) for 14 days on platelet function in healthy men. Bleeding time 2 or 24 hours after the first aspirin administration was significantly increased for 300 and 500 mg aspirin (p < 0.01). Two hours after the first administration of 100, 300, and 500 mg aspirin, a significant inhibition of collagen-induced platelet aggregation (ED50 collagen: from 3 +/- 1 to 17 +/- 2, 24 +/- 3, 22 +/- 3 micrograms/ml, respectively) was seen. At the same time serum thromboxane B2 synthesis was inhibited by more than 99% with 300 and 500 mg aspirin. At the end of the 14-day observation period, bleeding time was significantly prolonged for the different combinations of aspirin doses compared with initial values (p < 0.01). Collagen-induced platelet aggregation and serum thromboxane B2 synthesis were significantly inhibited for all aspirin combinations tested at 14 days (p < 0.05). The 40/40 mg aspirin combination was less effective, because it reached its maximal effect very late at day 7 of the observation period compared with the other combinations.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of low-dose aspirin on in vitro platelet aggregation in the early minutes after ingestion in normal subjects

Aspirin interferes with platelet aggregation by inhibiting the metabolism of arachidonic acid to thromboxane A2. Although both high- and low-dose aspirin therapies are effective for secondary prophylaxis in patients with atherosclerotic vascular disease, the acute response to low-dose aspirin therapy is controversial. Eighteen volunteer subjects ingested 81, 162, or 324 mg of aspirin in a longitudinal crossover study design. Initial doses were randomly assigned and dosing intervals were separated by 2 weeks. Platelet aggregation in response to 0.9 mM arachidonic acid was measured at baseline, 15, 30, 60, and 90 minutes after ingestion. Thromboxane B2 production was assayed on simultaneously obtained samples after stimulation with arachidonic acid. The median inhibition of aggregation was 97%, 97%, and 97% 15 minutes after ingestion of 81, 162, and 324 mg, respectively. Four subjects had < 20% inhibition 15 minutes after ingesting 81 mg, but all 4 had > 90% inhibition after 30 minutes. Thromboxane B2 production declined by > 93% in all subjects at each dose. There was no difference between doses in inhibition of thromboxane B2 production.

Inter-individual variations of the effect of low dose aspirin regime on platelet cyclooxygenase activity

Thirteen healthy men (age range 24-59 years) received three single doses (30, 75, and 150 mg/day) of aspirin for seven days, followed by a wash-out period of three weeks, in a randomized order. The arachidonic acid metabolite 12-L-5,8,10-heptadecatrienoic acid (12-HHT) was taken as a measure of platelet cyclooxygenase activity. There was a large inter-individual variation in 12-HHT production prior to and during aspirin treatment. After one week of treatment the mean reduction was 69, 72 and 83% for the doses 30, 75 and 150 mg/day respectively. When the degree of cyclooxygenase inhibition was expressed per microgram aspirin administered per kg bw, a positive correlation was established to the activity before medication. It was found that doses exceeding 1500 micrograms per kg bw is required to achieve a predictable reduction in cyclooxygenase activity. Thus, by determining the pre-treatment cyclooxygenase activity in an individual it should be possible to adjust the enzyme activity to any desired level below 40% of its initial value. 150 mg aspirin/day for one week had a stimulating effect on the platelet basal production of 12-HHT when measured three weeks after the cessation of treatment. This rebound phenomenon was also observed up to six weeks after a single dose of 600 or 1200 mg of aspirin.

In vitro and ex vivo effects of aspirin in patients on a low-dose aspirin therapy

In 19 patients on a low-dose aspirin therapy with 100 mg/d, an insufficient effect of aspirin was observed in five patients (aggregations induced by arachidonic acid and collagen, thromboxane B2-formation in serum and after collagen). Aspirin added in vitro increased the inhibition to a degree comparable to that seen in the other 14 patients, i.e. the insufficient effect could be due to a lack of compliance or to a reduced availability of the drug. In another 20 patients there was a good inhibitory effect of aspirin; additional aspirin did not increase the inhibition of arachidonic acid-induced aggregation and serum-thromboxane B2, but slightly increased collagen-induced aggregation and thromboxane B2 formation. The effect was the same, whether the aspirin was given in vivo or added in vitro.

Single-dose effect of enteric-coated aspirin on platelet function and thromboxane generation in middle-aged men

OBJECTIVE

To evaluate the effect of a single dose of enteric-coated aspirin (ECA) in three different dosages on platelet function and thromboxane generation in middle-aged men.

DESIGN AND METHODS

In a nonblind, nonplacebo-controlled, crossover study, a single dose of ECA (50, 250, or 1000 mg) was given in a tablet form to a group of healthy, middle-aged men. Ten subjects, aged 50-67 years, volunteered to participate in this study. Platelet functions including bleeding time, platelet aggregation, adenine nucleotides, beta-thromboglobulin, platelet factor 4, thromboxane generation, and aspirin measurement were determined.

RESULTS

Before ECA ingestion, the intracellular adenine nucleotides (adenosine triphosphate, adenosine diphosphate) were decreased, and both beta-thromboglobulin and platelet factor 4 were increased. These observations suggested that platelets were activated in vivo in middle-aged men. These findings returned to normal within 8 hours after the ingestion of ECA, and maintained normal for at least two days. Bleeding time was significantly prolonged at 8 and 24 hours compared with that before ingestion of ECA 1000 mg (p < 0.05). The generation of platelet thromboxane was maximally inhibited by approximately 40 percent in the samples 8 hours after ECA ingestion. Abnormal values of adenine nucleotides, beta-thromboglobulin, and platelet factor 4 returned to normal within 8 hours. Arachidonic acid-induced platelet aggregation was inhibited compared with that before treatment (p < 0.01) and the inhibitory effect was maintained for at least three days. Adenosine diphosphate- and epinephrine-induced aggregations were less inhibited than those induced by arachidonic acid. Inhibitory effects of ECA on platelet aggregation were dose dependent.

CONCLUSIONS

Our study indicates that platelets are activated in middle-aged men and that a single dose of ECA 50 mg is safe and can inhibit thromboxane synthesis and platelet aggregation. These results suggest that a daily dose of ECA 50 mg may be useful for blocking platelet activation and preventing thrombosis.

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.

Equivalent inhibition of in vivo platelet function by low dose and high dose aspirin treatment

In vitro platelet function was inhibited in healthy volunteers by two different doses of aspirin, as confirmed by measurement of maximum serum production of thromboxane B2 (TXB2) by platelets. 75 mg aspirin did not fully inhibit serum TXB2 production after 24 hours, whereas 300 mg aspirin did. Inhibition of platelet function in vitro was maintained by both 75 mg/day aspirin or 300 mg/alternate day aspirin. In contrast, in vivo production of TXB2, measured as urinary levels of the 11-keto-TXB2 metabolite, was inhibited similarly by both doses of aspirin throughout the study. These findings suggest that 75 mg/day aspirin may be sufficient adequately to inhibit platelet aggregation in vivo.

Thromboxane synthesis inhibitors and receptor antagonists

The biochemical, functional and clinical consequences of thromboxane (TX) A2 synthesis inhibition and receptor antagonism are reviewed, with emphasis on human studies in health and disease. Both platelet and glomerular TXA2 synthesis and action are discussed as potential targets for cyclooxygenase inhibitors, TX-synthase inhibitors and TXA2-receptor antagonists. While Aspirin remains the reference standard for new agents aimed at suppressing TXA2-dependent platelet activation, both synthase inhibitors and receptor antagonists might have a unique therapeutic potential in affecting TXA2-dependent loss of renal function.

Aspirin and human platelets: from clinical trials to acetylation of cyclooxygenase and back

Aspirin has been convincingly shown to reduce the incidence of vascular occlusive events in a wide range of patients at risk of thrombotic complications. These beneficial effects are currently linked to suppression of thromboxane A2-dependent platelet aggregation. This in turn reflects permanent loss of the cyclooxygenase activity of platelet prostaglandin G/H synthase, through acetylation of Ser530. Progress in our understanding of the molecular mechanism of action of aspirin and definition of the clinical pharmacology of its platelet effects has been associated with a downward trend in its daily dosage. This has been reduced by a factor of ten over the last decade, substantially reducing gastrointestinal toxicity, while leaving antithrombotic efficacy virtually unchanged. Carlo Patrono reviews the biochemical, pharmacological and clinical data that form the basis of the present consensus and provide a rationale for clinical trials of low-dose aspirin.

Effect of low-dose aspirin on fetal and maternal generation of thromboxane by platelets in women at risk for pregnancy-induced hypertension

There is evidence that aspirin in low doses favorably influences the course of pregnancy-induced hypertension, but the mechanism, although assumed to involve suppression of the production of thromboxane by platelets, has not been established. We performed a randomized study of the effect of the long-term daily administration of 60 mg of aspirin (n = 17) or placebo (n = 16) on platelet thromboxane A2 and vascular prostacyclin in women at risk for pregnancy-induced hypertension. Low doses of aspirin were associated with a longer pregnancy and increased weight of newborns. Serum levels of thromboxane B2, a stable product of thromboxane A2, were almost completely (greater than 90 percent) inhibited by low doses of aspirin. The urinary excretion of immunoreactive thromboxane B2 was significantly reduced without changes in the level of 6-keto-prostaglandin F1 alpha, a product of prostacyclin. Mass spectrometric analysis showed that aspirin reduced the excretion of the 2,3-dinor-thromboxane B2 metabolite--mainly of platelet origin--by 81 percent and of thromboxane B2, probably chiefly of renal origin, by 59 percent. The urinary excretion of 6-keto-prostaglandin F1 alpha and of its metabolite 2,3-dinor-6-keto-prostaglandin F1 alpha was not affected. Low doses of aspirin only partially (63 percent) reduced neonatal serum thromboxane B2. No hemorrhagic complications were observed in the newborns. Thus, in women at risk for pregnancy-induced hypertension, low doses of aspirin selectively suppressed maternal platelet thromboxane B2 while sparing vascular prostacyclin, but only partially suppressed neonatal platelet thromboxane B2, allowing hemostatic competence in the fetus and newborn.

Improvement of renal function with selective thromboxane antagonism in lupus nephritis

To test the hypothesis that the vasoconstrictor thromboxane A2 may affect renal hemodynamics in lupus nephritis, we examined the short-term effects of a selective thromboxane-receptor antagonist, BM 13,177, and of low-dose aspirin. In a randomized, double-blind, crossover study, 10 patients with biopsy-proved lupus nephritis were given a 48-hour continuous infusion of BM 13,177 or placebo. At base line, seven patients had markedly elevated urinary levels of thromboxane B2, the breakdown product of thromboxane A2. During the infusion of BM 13,177, the inulin clearance rate, which was 68 ml per minute per 1.73 m2 of body-surface area at base line, increased by an average of 24 percent (range, 12 to 47 percent; P less than 0.01). Para-aminohippurate clearance was increased to the same extent, with no change in the filtration fraction. The bleeding time doubled, indicating an occupancy of platelet thromboxane receptors of more than 95 percent. The hemodynamic changes were associated with a significant increase in sodium excretion from 76 to 118 mmol per day (P less than 0.01) but with no change in arterial blood pressure. In another study, 10 additional patients with lupus nephritis were randomly assigned to receive either placebo or 20 mg of aspirin twice daily for four weeks. The aspirin regimen produced a selective, cumulative inhibition of platelet cyclooxygenase activity and a doubling of bleeding time. However, there was no change in the inulin clearance rate and no change in urinary levels of thromboxane B2 or 6-keto-prostaglandin F1 alpha, which are indicators of renal synthesis of thromboxane A2 and prostacyclin, respectively. We conclude that in lupus nephritis, impairment of renal function is at least in part mediated hemodynamically and is reversible with a thromboxane antagonist. Platelets, however, are not a major source of thromboxane A2 synthesis and action within the kidney.

Effects of low doses of aspirin, 10 mg and 30 mg daily, on bleeding time, thromboxane production and 6-keto-PGF1 alpha excretion in healthy subjects

To compare the long term effects of two low doses of aspirin taken daily, we performed a placebo-controlled cross-over study in 19 normal subjects. Aspirin 10 mg daily for 3 weeks caused a significant inhibition by 61 +/- 12 percent of platelet thromboxane B2 generation but had no effect on the Simplate bleeding time. Aspirin 30 mg daily for 3 weeks reduced thromboxane B2 production by 94 +/- 5 percent and caused a significant prolongation of the bleeding time, 1.6 times the control value. No cumulative inhibitory effects were observed after the first week of treatment. Both doses did not effect the urinary excretion of 6-keto-PGF1 alpha.

Effects of dipyridamole and low-dose aspirin therapy on platelet adhesion to vascular subendothelium

The effect on platelet function of low-dose aspirin (ASA) and dipyridamole alone or in combination was evaluated after repeated dosing in 5 healthy volunteers. The subjects were treated according to a randomized, single-blind, crossover design with 150 mg of dipyridamole, 25 mg of ASA, the 2 drugs together or placebo twice a day for 3 days. Platelet adhesin was evaluated using an experimental model of adhesion to rat aorta subendothelium under controlled hemodynamic conditions in the presence of red blood cells. Dipyridamole significantly reduced platelet adhesion both alone and in combination with ASA. ASA by itself did not significantly modify platelet adhesion, but completely blocked serum thromboxane production and platelet aggregation by arachidonic acid. Thus, low-dose ASA and dipyridamole may have a complementary action, modifying at the same time 2 platelet functions, adhesion and aggregation, both relevant in the pathogenesis of thrombosis.

Effects of oral diltiazem on platelet function: alone and in combination with "low dose" aspirin

The effects of 3 days of oral diltiazem, "low dose" aspirin (40 mg/day), and their combination on platelet function was studied in 5 normal subjects. Both drugs inhibited platelet aggregation and ATP release induced by collagen, epinephrine and threshold concentrations of ADP. Aspirin and diltiazem decreased thromboxane A2 generation during ADP induced aggregation by 94 percent and 53 percent respectively, however both agents inhibited aggregation similarly, which suggests that diltiazem's anti-platelet effect was due to mechanisms other than inhibition of thromboxane metabolism alone. Combination therapy resulted in a partially additive inhibitory effect on ADP induced aggregation and thromboxane A2 generation. Two subjects had bleeding times over 15 minutes after receiving combination therapy.

Platelet aggregation and thromboxane release induced by arachidonic acid, collagen, ADP and platelet-activating factor following low dose acetylsalicylic acid in man

The present study was undertaken in order to characterize the dose-dependent nature of acetylsalicylic acid (ASA) on platelet aggregation and plasma thromboxane B2 (TXB2) release in healthy volunteers. Volunteers received either 25, 50, 100 or 500 mg daily for five consecutive days. At the end of the five day period, all dosages of ASA were capable of completely suppressing TXB2 production and arachidonic acid-induced platelet aggregation. At that time, the second phase of ADP-induced aggregation was also blocked. However, while the inhibition following 500 mg ASA was complete after 24 hours, total inhibition with 100, 50 and 25 mg was attained only after two, three and four days, respectively, indicating the cumulative effect of ASA on platelets. Aggregation induced by collagen was also inhibited dose-dependently- yet slower and at no time complete. ASA had no inhibitory effect on aggregation by platelet-activating factor (PAF). It is concluded that a daily dose of 50 mg ASA would suffice in blocking platelet TXA2 production and aggregation induced by most physiological agents.