Peripheral venous plasma aspirin concentrations and platelet aggregation inhibition produced by enteric-coated aspirin formulations

In a random cross-over design, six healthy consenting adult volunteers were given on separate occasions single doses of 300-650 mg of 3 different formulations of enteric-coated aspirin. Over various intervals for 48-54 h following dosage, plasma aspirin and salicylate concentrations were measured together with percentage inhibition of platelet aggregation activated by threshold concentrations of sodium arachidonate alone and combined with ADP and collagen. In all subjects each formulation delivered measurable quantities of aspirin to the peripheral circulation, the unchanged drug being detected at various times up to and including 28 h after dosage. Moreover, low aspirin concentrations were found to co-exist with unimpaired platelet aggregation. All 3 formulations yielded statistically significant (P less than 0.01) inhibition of platelet aggregation activated both by arachidonate and by the combination of aggregants when tested 24-29 and 48-54 h after dosage; there were no significant differences (P greater than 0.05) between the 3 formulations in this regard. Two different patterns of delivery of unchanged aspirin to the systemic circulation from these enteric-coated formulations were apparent. These patterns may be important when considering which aspirin formulation might be most appropriate in chronic use for an antiplatelet effect. None of the enteric-coated formulations used in this study may be optimal in this regard.     

Potent low dose platelet inhibitory effects of clopidogrel and aspirin on coronary thrombus formation in an animal model of acute unstable angina

Application of clopidogrel before percutaneous coronary intervention in patients with acute coronary syndrome reduces the risk of cardiac events. Clopidogrel administration before surgery increases bleeding complications after CABG.Therefore,the antithrombotic effect of the low-dose combination of clopidogrel and aspirin was investigated in an in vivo pig model of coronary artery thrombus formation with cyclic flow reductions.The platelet inhibitory effect was determined by platelet aggregation and CFR, according to the methodology described by Folts. CFR were initiated by endothelial damage and placement of a constrictor around the LAD. 30 min after CFR were established, clopidogrel (0. I mg/kg or 5 mg/kg), aspirin (I mg/kg or 7 mg/kg) or LDC (0. I mg/kg clopidogrel and I mg/kg aspirin) were administered orally. CFR-frequency was determined for further 240 min.CFR-frequency (CFR/30 min) was significantly reduced at 60 min in response to aspirin (7 mg/kg, -48%, p<0.05), and at 120 min in response to clopidogrel (5 mg/kg,-65%, p<0.05) but not at low doses of either compound. In contrast, LDC of clopidogrel (0. I mg/kg) plus aspirin (I mg/kg) resulted in a complete and rapid abrogation of CFR at 90 min (-70%, p<0.05 y. Furthermore, LDC led to reduction of platelet aggregation when CFR-frequency was already significantly decreased. In contrast, high dose groups presented a significant reduction of platelet aggregation prior to CFR-frequency decrease. Low dose combination of clopidogrel plus aspirin demonstrates a potent over additive anti-thrombotic effect in vivo with a significant reduction in thrombus formation early after drug application.The effect occurs before inhibition of platelet aggregation is detectable.     

Effect of increasing doses of aspirin on platelet aggregation among stroke patients

BACKGROUND AND PURPOSE: Aspirin has been shown to reduce the risk of myocardial infarction and stroke. Some investigators believe that low-dose aspirin inhibits platelet aggregation to the same degree as high-dose aspirin. Our study aimed to assess the effect of increasing doses of aspirin on the degree of platelet aggregation induced by collagen and adenosine diphosphate (ADP) among stroke patients. METHODS: Sixteen poststroke patients were prescribed aspirin at daily doses of 40, 80, 160, 325, 650, and 1,300 mg, each dose to be taken for 14 days (total duration 12 weeks). Platelet aggregation studies using 2 microgram/ml collagen and 2 microM ADP were performed on platelet-rich plasma at baseline and on the 14th day of each dose. RESULTS: Platelet aggregation studies using 2 microgram/ml collagen at the start of treatment and at the 14th day of each dose revealed dose-dependent inhibition by aspirin starting at 40 mg/day, but was optimal at 80- 160 mg/day. ADP-induced platelet aggregation inhibition appears to be dose dependent up to 1,300 mg/day. CONCLUSION: Inhibition of collagen-induced platelet aggregation by aspirin appears to be optimal at 80-160 mg/day, while ADP-induced platelet aggregation inhibition by aspirin appears to be dose dependent up to 1,300 mg/day in our poststroke patients, albeit to a less remarkable degree at higher doses.     

The effects of ligustrazine, aspirin and beta-histine on platelet aggregation in patients with acute ischemic stroke

With a auto-balanced terbidimeter, the effects of ligustrazine, aspirin and betahistine on platelet aggregation were studied in 45 patients with acute ischemic stroke. The adopted parameters were: 1) the maximal aggregation (Amax), 2) the maximal aggregation velocity (Vmax), 3) the effective disaggregation rate in 5 minutes (DA 5), 4) the inhibition rate of aggregation given by drugs (IR), and 5) the concentration causing 50% inhibition of aggregation (IC 50). The results indicated that these three drugs could inhibit platelet aggregation both in vivo and in vitro. Aspirin could promote the aggregated platelets disaggregation in vivo and ligustrazine, in vitro. Regarding the effects on inhibiting platelet aggregation in vitro, ligustrazine was the most noticeable among the 3 drugs and aspirin was more effective than betahistine. The IC50 of ligustrazine, aspirin and betahistine were 0.568 mg/ml, 1,286 mg/ml and 1.722 mg/ml respectively. The authors considered that all three drugs possessed anti-platelet effects but they showed some differences among them.     

The effect of aspirin on in vivo platelet function in humans

Both in vivo platelet aggregation time and disaggregation time are significantly increased 90 minutes after ingesting 10 grains of acetylsalicylic acid. The significant prolongation of both aggregation and disaggregation times were evident even 24 hours after the ingestion of the acetylsalicylic acid. The increased aggregation time positively correlated with blood salicylate levels while the increased disaggregation times correlated with linoleate and salicylate levels.     

Aspirin treatment influences platelet-related inflammatory biomarkers in healthy individuals but not in acute stroke patients

Objectives

Platelet-leukocyte aggregation is believed to contribute to acute thrombotic events. While the effect of aspirin on platelet-to-platelet aggregation is well established, the impact of the drug on pro-inflammatory platelet function remains equivocal. Thus we investigated the effect of aspirin on selected platelet-related inflammatory biomarkers in both acute ischaemic stroke patients and healthy volunteers.

Methods

Using five-colour flow cytometry the platelet surface expression of CD62P and CD40L and subpopulations of leukocyte-platelet aggregates were assessed in 63 acute stroke patients and 40 healthy volunteers at baseline and after a 10-day period of aspirin intake at a daily dose of 150 mg. Simultaneously the plasma levels of soluble CD62P and CD40L, serum level of TxB₂, and whole blood impedance platelet aggregation under arachidonic acid (AA) stimulation were investigated.

Results

No differences in values of studied platelet-related inflammatory biomarkers in both resting platelets and those activated with TRAP after 10-day treatment with aspirin were confirmed in stroke subjects. In healthy individuals the resting platelet expression of CD62P, plasma level of soluble CD62P and percentage of circulating monocyte-platelet aggregates were lower after the aspirin intake period (P = 0.009; P = 0.04; P = 0.004, respectively). In both studied groups serum level of TxB₂ and platelet aggregation under AA stimulation were lower than before treatment (P < 0.001).

Conclusion

Despite effective inhibition of COX-1-dependent platelet aggregation, aspirin does not influence the platelet α-granule-derived inflammatory mediators and monocyte-platelet aggregation in acute stroke subjects, although it does in healthy individuals.     

24-hour time-dependent aspirin efficacy in patients with stable coronary artery disease

Aspirin-induced cyclooxygenase (COX)-1 acetylation is irreversible and it is assumed that the platelet thromboxane-A2 aggregation pathway is inhibited for at least 24 hours (h) after aspirin ingestion. However, time course of biological efficacy of daily low-dose aspirin has rarely been assessed in patients with coronary artery disease (CAD). We aimed to assess the 24-h biological efficacy of daily low-dose aspirin in CAD patients. The peak and trough (2 h-24 h) effect of a chronic treatment with once daily dose aspirin were studied in 150 consecutive stable CAD patients. The main outcome measure was light transmission aggregometry (LTA) triggered with 0.5 mg/ml arachidonic acid (AA). In the last 47 consecutive patients, additional tests were conducted at 6, 12, 16, 20 h after last aspirin administration. 4.7% of the patients had significant aggregation (>20% maximal intensity LTA-AA) 2 h after aspirin ingestion and 24.7% at 24 h (p<0.0001). The more precise assessments in the last 47 patients showed that significant platelet aggregation progressively reappeared with time after aspirin intake (2 h--4% of patients, 6 h-- 4%, 12 h--11%, 16 h--16%, 20 h--19% and 24 h--28%). Concordant results were observed using production of thromboxane-B2 and other techniques evaluating AA-induced platelet aggregation/activation. No significant differences were found between lower (75-100 mg/day) and higher (>100 mg/day) dose aspirin. Such aspirin ?resistance? at 24 h after ingestion was related to biological inflammatory markers, current smoking and diabetes. In conclusion, once daily aspirin does not provide stable 24-h antiplatelet protection in a significant proportion of CAD patients. Any biological assessment of aspirin efficacy should take time since last aspirin intake into consideration.     

Combination therapy with low-dose aspirin and ticlopidine in cerebral ischemia

We compared combination therapy with low-dose aspirin plus ticlopidine to therapy with aspirin alone or ticlopidine alone in patients suffering transient ischemic attack or cerebral infarction. In 17, 24, and 23 patients, respectively, 300 mg/day aspirin, 200 mg/day ticlopidine, and 81 mg/day aspirin plus 100 mg/day ticlopidine were administered orally. Aspirin alone markedly inhibited platelet aggregation induced by arachidonic acid, partially inhibited platelet aggregation induced by adenosine diphosphate, and did not inhibit platelet aggregation induced by platelet activating factor. Ticlopidine alone inhibited platelet aggregation induced by adenosine diphosphate and platelet activating factor, but did not inhibit platelet aggregation induced by arachidonic acid. Combination therapy with aspirin plus ticlopidine markedly inhibited platelet aggregation induced by all three agonists. Plasma concentrations of beta-thromboglobulin and platelet factor 4 remained unchanged by aspirin alone, were slightly reduced by ticlopidine alone, and were markedly reduced by aspirin plus ticlopidine. Plasma concentration of thromboxane B2 was reduced by aspirin alone or with ticlopidine, but not by ticlopidine alone. The level of 6-ketoprostaglandin F1 alpha was reduced only by aspirin alone. Bleeding time was significantly prolonged by aspirin alone and by ticlopidine alone, although the greatest prolongation was produced by aspirin plus ticlopidine. Our results indicate that the combination of aspirin plus ticlopidine is a potent antiplatelet strategy, although the clinical importance of the changes observed need to be determined by a properly designed and controlled prospective study.     

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

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

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

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

Action of some salicylate derivatives on in vitro platelet aggregation. Inhibitory and inhibition antagonistic effects

Twenty salicylate derivatives were tested for their antagonistic activity on the inhibitory effect of aspirin on platelet aggregation. The blocking effect was not limited to the salicylate but also characterised some of its substituted compounds. The substituant influence did not seem to be related to electronic or size parameters. This antagonistic activity of these derivatives decreased as concentrations increased, owing to the emergence of their own inhibitory activity: several salicylate derivatives showed dual inhibitory and inhibition antagonistic activity, with both properties present at the same concentration. A mechanism involving dissociated activities on the two enzymatic sites of cyclooxygenase is proposed.     

Effect of 1-methyl-2-mercapto-5-(3-pyridyl)-imidazole (KC-6141) on rabbit platelet aggregation in vitro and rat platelet retention

Effect of 1-methyl-2-mercapto-5-(3-pyridyl)-imidazole (KC-6141) on rabbit platelet aggregation in vitro and rat platelet retention investigated. In the in vitro study, KC-6141 inhibited ADP-induced aggregation by 27% at 5 X 10(-4)M, being more active than dipyridamole but much less than adenosine. Inhibition of arachidonic acid- and collagen-induced aggregation by KC-6141 was more effective than that of ADP-induced one and its ED50 was 2.1 X 10(-5) and 8 X 10(-5)M, respectively. KC-6141 was 10 and 4 times more potent than aspirin in arachiodonic acid- and collagen-induced aggregation, respectively. The dose-response curve of KC-6141 was parallel to that of aspirin, suggesting it is an aspirin-like compound. In the platelet retenion study, a method for determining platelet retintion in rats was devised so that platelet retention can be measured with a volume of blood as small as possible. By use of the method, effects of KC-6141, aspirin and dipyridamole were compared. When deministered intraperitoneally at 100 mg/kg, KC-6141 indicated 54.8% inhibition of platelet retention, whereas aspirin and dipyridamole showed only 23.5 and 5.2% inhibition, respectively. On the oral administration at 200 mg/kg KC-6141 inhibited by 60.8% and its ED50 was 125 mg/kg. The activity lasted over 32 hr. The above results demonstrated that KC-6141 is a compound with more potent action on the platelet aggregation, as well as on the platelet retention than aspirin and dipyridamole-a known antithrombotic drug.     

Dipyridamole inhibits platelet aggregation in whole blood

Dipyridamole possesses antithrombotic properties in the animal and in man but it does not inhibit platelet aggregation in plasma. We evaluated the effect of dipyridamole ex vivo and in vitro on platelet aggregation induced by collagen and adenosine-5'-diphosphate (ADP) in human whole blood with an impedance aggregometer. Two hundred mg dipyridamole induced a significant inhibition of both ADP- and collagen-induced aggregation in human blood samples taken 2 hr after oral drug intake. Administration of the drug for four days, 400 mg/day, further increased the antiplatelet effect. A significant negative correlation was found between collagen-induced platelet aggregation in whole blood and dipyridamole levels in plasma (p less than 0.001). A statistically significant inhibition of both collagen (p less than 0.0025) and ADP-induced (p less than 0.005) platelet aggregation was also obtained by incubating whole blood in vitro for 2 min at 37 degrees C with dipyridamole (3.9 microM). No such effects were seen in platelet-rich plasma, even after enrichment with leukocytes. Low-dose adenosine enhanced in vitro inhibition in whole blood. Our results demonstrate that dipyridamole impedes platelet aggregation in whole blood by an interaction with red blood cells, probably involving adenosine.     

The active metabolite of prasugrel inhibits ADP-stimulated thrombo-inflammatory markers of platelet activation: Influence of other blood cells, calcium, and aspirin

The novel thienopyridine prodrug prasugrel, a platelet P2Y(12) ADP receptor antagonist, requires in vivo metabolism for activity. Although pharmacological data have been collected on the effects of prasugrel on platelet aggregation, there are few data on the direct effects of the prasugrel's active metabolite, R-138727, on other aspects of platelet function. Here we examined the effects of R-138727 on thrombo-inflammatory markers of platelet activation, and the possible modulatory effects of other blood cells, calcium, and aspirin. Blood (PPACK or citrate anticoagulated) from healthy donors pre- and post-aspirin was incubated with R-138727 and the response to ADP assessed in whole blood or platelet-rich plasma (PRP) by aggregometry and flow cytometric analysis of leukocyte-platelet aggregates, platelet surface P-selectin, and GPIIb-IIIa activation. Low-micromolar concentrations of R-138727 resulted in a rapid and consistent inhibition of these ADP-stimulated thrombo-inflammatory markers. These rapid kinetics required physiological calcium levels, but were largely unaffected by aspirin. Lower IC(50) values in whole blood relative to PRP suggested that other blood cells affect ADP-induced platelet activation and hence the net inhibition by R-138727. R-138727 did not inhibit P2Y(12)-mediated ADP-induced shape change, even at concentrations that completely inhibited platelet aggregation, confirming the specificity of R-138727 for P2Y(12). In conclusion, R-138727, the active metabolite of prasugrel, results in rapid, potent, consistent, and selective inhibition of P2Y(12)-mediated up-regulation of thrombo-inflammatory markers of platelet activation. This inhibition is enhanced in the presence other blood cells and calcium, but not aspirin.     

Coagulation, fibrinolytic and platelet function in patients on long-term therapy with aspirin 300 mg or 1,200 mg daily compared with placebo

Aspirin has been shown to be beneficial in the prophylaxis of arterial thromboembolic disease. The rationale for its use as an antithrombotic drug lies in its inhibition of thromboxane A2-dependent platelet function. However, the effect of aspirin on coagulation and fibrinolysis during chronic therapy has not been studied. We have measured a range of haemostatic and platelet functions in 49 patients with transient ischaemic attacks randomly allocated to aspirin 300 mg a day, aspirin 1,200 mg a day or placebo. All had been taking their allocated treatment for between 9 months and 4 years prior to investigation. Bleeding time was prolonged, serum thromboxane diminished and platelet aggregation to arachidonic acid but not ADP was abolished by both 300 mg and 1,200 mg aspirin, in a non-dose dependent fashion. Serum salicylate increased with the dose of aspirin ingested. No effect was seen with either dose of aspirin on urinary thromboxane and 6-keto-PGF1 alpha excretion, or on coagulation. Patients taking 1,200 mg aspirin a day had a lower haemoglobin and packed cell volume, lower resting fibrinopeptide A concentration and lower basal plasminogen activator activity than those on placebo. Response to venous occlusion was normal in all groups. The results suggest 300 mg and 1,200 mg aspirin have an equivalent platelet inhibitory effect but 1,200 mg aspirin causes greater gastro-intestinal blood loss.     

Increased dipyridamole plasma concentrations associated with salicylate administration. Relationship to effects on platelet aggregation in vivo

Dipyridamole, in relatively low doses, when used in combination with aspirin was shown to be effective in normalizing reduced platelet survival seen in patients with prosthetic heart valves, whereas either drug, when tested alone, was ineffective. To determine whether this combined drug effect was due to a synergism of the two drugs or due to an alteration in the pharmacokinetics of one drug in the presence of the other, we examined the effects of aspirin and sodium salicylate on 1) the effects of dipyridamole on platelet function and 2) the concentration of dipyridamole in the plasma. Both aspirin and salicylate altered the plasma concentration of dipyridamole and prolonged the effect of dipyridamole on collagen-induced platelet aggregation in the rabbit. This effect appeared to be due to saturation of the glucuronide pathway(s) which is the major route of clearance of dipyridamole from the circulation.     

Comparison of turbidimetric aggregation and in vitro bleeding time (PFA-100) for monitoring the platelet inhibitory profile of antiplatelet agents in patients undergoing stent implantation

The present study compared classical ADP-induced platelet aggregation vs. PFA-100 closure times using collagen/ADP membrane cartridges to monitor the degree of platelet-inhibiting effect of three drug regimens: ticlopidin, abciximab/ticlopidin and loading dose clopidogrel, each on top of aspirin (acetylsalicylic acid, ASA) during and after elective stent placement (intervention) in a total of 31 patients with acute coronary syndrome. Ticlopidin was started directly after stent implantation, abciximab was started before coronary intervention and given intravenously for 12 h, and a clopidogrel loading dose was given before intervention. The 10 patients treated with ticlopidin (500 mg daily) showed no significant prolongation of PFA closure times and a slight increase of ADP-induced platelet aggregation shortly after intervention. In 11 patients treated with abciximab/ticlopidin, the PFA closure times were significantly prolonged, and ADP-induced platelet aggregation was reduced by more than 80% during the 12-h abciximab infusion after intervention. The 10 patients pretreated with loading dose clopidogrel (450 mg followed by 75 mg daily) showed an intermediate but significant prolongation of PFA closure times and reduction of ADP-induced platelet aggregation at levels between the ticlopidin/aspirin- and the abciximab/ticlopidin/aspirin-treated groups. At 20 h after intervention, a similar degree of PFA closure time prolongation and inhibition of ADP-induced aggregation was observed in the abciximab/ticlopidin/aspirin- and the clopidogrel/aspirin-treated patient groups. Both measurement of PFA-100 closure times and inhibition of ADP-induced platelet aggregation showed a similar degree of platelet inhibition, but had rather broad SD ranges, which limit their precision for the follow-up of individual patients. In conclusion, abciximab on top of ticlopidin/aspirin showed a stronger antiplatelet effect for only less than 20 h, as compared to loading dose clopidogrel/aspirin in acute coronary syndrome patients undergoing stent implantation. Whether such a short-term superiority of abciximab, as compared to loading dose clopidogrel, translates into an overall clinical benefit of thombotic and bleeding complications remains to be established in a randomized clinical trial.     

Effect of increasing doses of aspirin on platelet function as measured by PFA-100 in patients with diabetes

INTRODUCTION: Platelets of diabetic patients have been reported to be less sensitive to aspirin. The aim of this study is to compare a medium (300 mg) and low (100 mg) dose of aspirin on platelet function in diabetic patients. METHODS: We have included one hundred and two patients with type 2 diabetes mellitus. Platelet function was measured as closure time (CT) with the Platelet Function Analyzer (PFA)-100 before the administration of aspirin. Initially the patients were given 100 mg aspirin once daily for seven days, and then the measurements were repeated. If the CT exceeded the upper limit of 300 s, the study was terminated. If not, the patients continued the aspirin therapy with a dose of 300 mg daily for another seven days, and the CTs were measured again. RESULTS: After taking 100 mg aspirin, the CT significantly increased from 126+/-29 s to 256+/-66 s (p<0.001). In 68 of 102 (67%) patients, the CT increased to 300 s. In the remaining 34 patients, the baseline CT was 113+/-29, and increased to 170+/-45 s after 100 mg aspirin (p<0.001). In these patients, there was a further increase in the CT from 170+/-45 to 229+/-75 s following 300 mg aspirin (p<0.001). On average, the CT was increased by 60% and 39% following ingestion of 100 and 300 mg aspirin, respectively. CT>300 s were obtained in 15 (44%) of 34 patients after 300 mg aspirin. CONCLUSIONS: Although, a daily dose of 100 mg aspirin effectively inhibited platelet function in a majority of diabetics, a considerable proportion of patients showed a greater platelet inhibition with the use of 300 mg aspirin. The PFA-100 closure time may be used to separate those patients who require a higher dose of aspirin to achieve desired antiplatelet effect.     

Dose- and time-dependent antiplatelet effects of aspirin

Aspirin is widely used, but dosages in different clinical situations and the possible importance of "aspirin resistance" are debated. We performed an open cross-over study comparing no treatment (baseline) with three aspirin dosage regimens--37.5 mg/day for 10 days, 320 mg/day for 7 days, and, finally, a single 640 mg dose (cumulative dose 960 mg)--in 15 healthy male volunteers. Platelet aggregability was assessed in whole blood (WB) and platelet rich plasma (PRP). The urinary excretions of stable thromboxane (TxM) and prostacyclin (PGI-M) metabolites, and bleeding time were also measured. Platelet COX inhibition was nearly complete already at 37.5 mg aspirin daily, as evidenced by >98% suppression of serum thromboxane B2 and almost abolished arachidonic acid (AA) induced aggregation in PRP 2-6 h after dosing. Bleeding time was similarly prolonged by all dosages of aspirin. Once daily dosing was associated with considerable recovery of AA induced platelet aggregation in WB after 24 hours, even after 960 mg aspirin. Collagen induced aggregation in WB with normal extracellular calcium levels (hirudin anticoagulated) was inhibited <40% at all dosages. TxM excretion was incompletely suppressed, and increased <24 hours after the cumulative 960 mg dose. Aspirin treatment reduced PGI-M already at the lowest dosage (by approximately 25%), but PGI-M excretion and platelet aggregability were not correlated. Antiplatelet effects of aspirin are limited in WB with normal calcium levels. Since recovery of COX-dependent platelet aggregation occurred within 24 hours, once daily dosing of aspirin might be insufficient in patients with increased platelet turnover.