Prevalence and biologic profile of aspirin resistance in patients with angiographically proven coronary artery disease

BACKGROUND: Aspirin protects from cardiovascular events. However, a number of patients who take this drug suffer events, probably due to aspirin resistance. The role of certain biologic variables that may affect resistance is still uncertain. AIM: To determine the prevalence of aspirin resistance in patients taking this drug and to test if resistance is related to haemostatic, inflammatory and lipidic variables. METHODS: Platelet function measured with PFA-100 was studied in 268 patients (185 men) with stable coronary disease who took aspirin (100 to 300 mg/day). Aspirin resistance was defined when epinephrine closure time <174 s. Results of lipoprotein(a) are expressed in median (interquartile range). RESULTS: Aspirin resistance was found in 16% of cases. Patients with aspirin resistance had higher levels of Apolipoprotein B (109.27+/-27.65 vs 100.92+/-23.77 mg/dl; p<0.05), lipoprotein(a) [20.37 (4.83-36.72) vs 10.02 (1.88-25.41); p<0.01], Platelet Count (241.42+/-75.35 vs 213.94+/-56.74 mm(3); p<0.05) and fibrinogen (388.93+/-107.27 vs 354.33+/-89.35 mg/dl; p<0.05). We used the logistic regression analysis to detect the independent predictors of aspirin resistance. Lipoprotein(a) was found to be the only independent risk factor to identify aspirin resistance (p<0.05; OR: 1.302; CI 95%: 1.003-1.688). CONCLUSIONS: Although the potential mechanisms of aspirin resistance still remains uncertain, we found that platelet responsiveness to aspirin is reduced in patients with high levels of Apolipoprotein B and lipoprotein(a). Our work demonstrate that lipoprotein(a) is an independent risk factor for aspirin resistance possibly due to the interaction of Apolipoprotein(a) with human platelets.     

Using the Platelet Function Analyzer-100 for monitoring aspirin therapy

INTRODUCTION: The aim of the study was to evaluate the test characteristics of the Platelet Function Analyzer-100 (PFA-100) in patients treated with aspirin. METHODS AND RESULTS: The study consisted of two sub-studies. In study 1, 10 patients with ischemic heart disease (IHD) and 10 controls had platelet function assessed by optical platelet aggregation and the PFA-100 method in two 5-week periods. Patients with IHD were treated with aspirin 150 mg/day (first 5-week period), and 300 mg/day (second 5-week period), whereas the controls only received aspirin (150 mg/day) during the second 5-week period. From the results of study 1, we found that a cut-off value for the PFA-100 collagen/epinephrine cartridge <165 s identified patients not taking aspirin (sensitivity 0.91, specificity 1.00). A good agreement between the PFA-100 method and optical platelet aggregation was found. Within-subject variation for the PFA-100 collagen/epinephrine cartridge was +/-28%, as compared to +/-17% for the optical platelet aggregation. Study 2 included 298 aspirin treated patients who were admitted with symptoms suggestive of an acute myocardial infarction. Platelet function was assessed in duplicate by the PFA-100 collagen/epinephrine cartridge, and a 95% Limit of Agreement interval at [-65%, 65%] indicated a limited precision. CONCLUSION: We defined a cut-off value below which patients not taking aspirin can be identified. However, due to imprecision of the PFA-100 method repeated duplicate assessment of the collagen/epinephrine Closure Time is recommended.     

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.     

Measurement of aspirin concentrations in portal and systemic blood in pigs: effect on platelet aggregation, thromboxane and prostacyclin production

Low doses of enteric-coated aspirin were administered orally to pigs. Plasma aspirin concentrations measured in blood obtained simultaneously from permanent catheters in a systemic artery and portal vein for 6 hours after dosage showed a large variation in the plasma aspirin concentration: time profile between pigs. After 50 mg single dose the ratio of the arterial: portal area under the plasma concentration versus time curve (AUC) was 0.63 +/- 0.08 (mean +/- SE, n = 6). In three pigs which received all three dosage regimens, the arterial: portal AUC ratios were 0.48 +/- 0.05 after 50 mg single dose, 0.52 +/- 0.02 after 100 mg single dose and 0.47 +/- 0.02 after 100 mg daily for 1 week. Platelet aggregation in response to sodium arachidonate (1.65 mM) was completely abolished after chronic aspirin administration of 100 mg daily. Thromboxane production (pg/10(6) platelets) induced by this stimulus decreased from 536 +/- 117 before aspirin to 57 +/- 14 after aspirin (mean +/- SE, n = 4; p = 0.03). Aortic prostacyclin synthesis, measured as 6-keto PGF1 alpha (ng/disc after 10 min incubation), was 1.66 +/- 0.28 (mean +/- SE, n = 4) in untreated pigs and 0.95 +/- 0.25 (n = 5) in treated pigs (p = 0.07). Results from this study support the idea that a difference between aspirin concentrations in the portal and systemic circulations can be achieved. Whether this can be translated into a clinically useful differential effect on the vessel wall compared to the platelet remains to be determined.     

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.     

脑梗死患者阿司匹林抵抗与血小板环氧合酶2相关性的研究

目的探讨脑梗死患者阿司匹林抵抗与血小板环氧合酶2(cyclooxygenase-2,COX-2)的关系。方法 95例脑梗死患者服用阿司匹林(100mg/d)14d后,用二磷酸腺苷(adenosine phosphate,ADP)和花生四烯酸(arachidonic acid,AA)作诱导剂,测定血小板最大聚集率,并通过测定其尿11-脱氢-血栓素B2(11-dehydrothromboxane B2,11-dh-TXB2)的水平观察体内阿司匹林对血小板功能的抑制程度(11-dh-TXB2≤1500pg/mg判定为阳性),同时用蛋白质印迹技术检测了血小板COX-2的表达水平。结果脑梗死患者阿司匹林抵抗和半抵抗的发生率为27.4%,阿司匹林抵抗组尿11-dh-TXB2水平比敏感组显著升高(P<0.001),而阳性率显著降低(P<0.001),血小板COX-2表达明显增多。结论血小板COX-2可能参与了脑梗死患者阿司匹林抵抗的发生机制。     

Assessing aspirin-induced attenuation of platelet reactivity by flow cytometry

INTRODUCTION: Certain patient subpopulations do not respond to antithrombotic effects of aspirin and different approaches have been proposed to detect and define this so-called aspirin resistance. In this study, a methodological and clinical evaluation of a flow cytometric method for the detection of aspirin-induced inhibition of platelet cyclooxygenase (COX) is presented. MATERIALS AND METHODS: Platelet CD62p-antigen (P-selectin) expression was determined by flow cytometry after incubating diluted platelet rich plasma (PRP) with arachidonic acid (ARA). After establishing the method's technical characteristics, it was used to investigate 114 individuals (70 patients with atherosclerotic vascular disease under long-term medication of 100 mg aspirin daily, 29 age-matched patients with vascular disease without anti-platelet medication and 15 healthy volunteers). Data were compared to those obtained by the PFA-100 platelet function analyzer. RESULTS: Imprecision was between 3.3% and 13%. Sample storage at room temperature increased baseline activity of platelets already after 2 h. After ARA stimulation, the proportion of CD62p-positive platelets was considerably lower in aspirin-treated patients than in controls (median [lower-upper quartile]: 4% [3-6] vs. 50% [29-68], p<0.001). Only one aspirin-treated patient (1.5%) showed normal reactivity to ARA. In contrast to flow cytometry, PFA-100 analysis yielded normal results in 17% of aspirin-treated patients. CONCLUSIONS: The presented flow cytometric method can be used to monitor aspirin-induced inhibition of platelet COX. Aspirin resistance defined as failure to inhibit platelet COX is a rare phenomenon suggesting that most cases of aspirin resistance detected using the PFA-100 are caused by COX-independent mechanisms.     

Monitoring of aspirin (ASA) pharmacodynamics with the platelet function analyzer PFA-100

BACKGROUND: Anti-platelet drug therapy is currently performed without monitoring, because the established method of platelet aggregometry is cumbersome. The recently developed platelet function analyzer PFA-100 measures shear stress dependent, collagen epinephrine (CEPI) and collagen adenosine diphosphate (CADP) induced platelet plug formation. As the PFA-100 provides a valuable tool to detect patients with platelet dysfunction more efficiently and cost-effectively than aggregometry, we investigated its potential to monitor the efficacy of aspirin treatment. METHODS: All healthy volunteers (n = 10) received a fractionated infusion of L-aspirin to establish individual dose-response curves. Further, in a randomized, double-blind, placebo controlled two-way cross over study the same volunteers received either 50 or 100 mg aspirin/day p.o. for a period of 11 days to determine the day-to-day variability CEPI induced closure time (CT) under constant intake of low dose aspirin, and to compare the efficacy of those two doses. RESULTS: Intra- and intersubject variability of CEPI-CT averaged 9% and 22%, respectively. Seven volunteers exceeded the maximum of CEPI-CT (>300 s) already after infusion of 100 mg L-aspirin. Intake of 100 mg of aspirin elicited a more rapid onset of effect than 50 mg, which was only significant on days 3 and 4 of aspirin intake. The aspirin induced CEPI-CT prolongation correlated positively with basal CEPI-CT values (r = 0.86; p = 0.001) and were strongly dependent on von Willebrand Factor levels (r = -0.9; p = 0.001). CONCLUSION: Thus, the PFA-100 system appears suitable to demonstrate an aspirin-induced platelet effect in a longitudinal study, and may be adequate to monitor a patient's compliance. However, prospective trials have to be conducted to demonstrate whether the EPI-CT achieved under ASA-intake has predictive value for cardiovascular outcome.     

Relationship between changes in platelet reactivity and changes in platelet receptor expression induced by physical exercise

INTRODUCTION: In previous studies we have consistently shown a significant increase of platelet reactivity after exercise in patients with obstructive coronary artery disease (CAD). We also observed a significant individual variability in the response to exercise of platelet reactivity in these patients. Whether exercise-induced changes in platelet reactivity correlate with changes in platelet membrane receptors in patients with CAD is unknown. METHODS: We studied 26 patients with stable CAD and 10 matched healthy controls who underwent a symptom-limited treadmill exercise stress test. Venous blood samples were collected at rest and within 5 min of peak exercise. Platelet reactivity was measured by the PFA-100 method as time to occlude (closure time, CT) a ring coated with collagen/adenosine diphosphate (C/ADP). Platelet expression of glycoprotein (GP) IIb/IIIa, in both global (CD41) and active form (PAC-1), and P-selectin (CD62P) and formation of leukocyte-platelet aggregates were assessed by flow cytometry. RESULTS: After exercise CT did not change in controls (85.4+/-12 to 84.0+/-9 s, p=0.37), whereas it decreased in CAD patients (98.8+/-24 to 91.4+/-25 s, p<0.001). After exercise, CD41 and PAC-1 platelet expression increased significantly in CAD patients (p=0.04 for both), but not in controls (p=0.39 and p=0.98, respectively). To evaluate the relationship between the response to exercise of platelet reactivity and of platelet receptor expression, CAD patients were divided into two groups: CAD group 1 (16 patients, decrease in CT >5 s after exercise) and CAD group 2 (10 patients no increase in platelet reactivity after exercise). CD41 and PAC-1 expression increased in CAD group 1 (p=0.008 and p=0.026, respectively) but not in CAD group 2 (p=0.39 and p=0.50, respectively). No significant differences were observed between the 2 groups for changes in CD62P and leukocyte-platelet aggregates. CONCLUSIONS: Our data show that, in patients with stable CAD, an increased platelet reactivity to C/ADP stimulation after exercise, as assessed by the PFA-100 method, is specifically associated with an increased expression of platelet GP IIb/IIIa receptor.     

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.     

Reduced sensitivity of platelets from type 2 diabetic patients to acetylsalicylic acid (aspirin)-its relation to metabolic control

Aspirin (acetylsalicylic acid, ASA), which is recommended for primary and secondary prevention in diabetes mellitus (DM), has been shown to have a lower antiplatelet activity in diabetic patients. We conducted a crossover designed observational study to evaluate whether there is an association between the parameters relevant to metabolic control of diabetes and platelet sensitivity to aspirin in type 2 diabetic patients. Platelets' ability to adhere and aggregate was monitored with the use of platelet function analyser (PFA-100 collagen/epinephrine closure time, CT(CEPI) or collagen/ADP closure time, CT(CADP)), classical turbidimetric aggregometry and whole blood electrical aggregometry (WBEA), using collagen (WBEA(coll)), ADP (WBEA(ADP)) and arachidonic acid (WBEA(AA)) as platelet agonists, in 48 control healthy volunteers (mean age+/-S.D., 49+/-9 years) and 31 type 2 DM patients (50+/-9 years; HbA(1c) 9.4+/-1.6%). In majority of control subjects (69%) and minority of diabetic patients (29%, p=0.0006), the use of 150 mg aspirin daily for 1 week significantly reduced platelet adhesiveness and reactivity (by 14.1% in diabetes vs. 78.6% in control, p(np)=0.0035, as expressed by the relative changes in CT(CEPI)). Aspirin reduced WBEA(coll) and WBEA(AA) to a lesser extent in diabetic patients (by 2.1% vs. 8.3% in controls, p(np)=0.0397, and by 97.3+/-12.8% vs. 100% in controls, p(np)=0.0383, respectively), which corresponded to ASA-mediated decreased aggregation in platelet-rich plasma (PRP, r(S)=0.45 and r(S)=0.78 for collagen- or arachidonate-agonized platelets, p<0.01 or lower). The maximal inhibition of platelet aggregation was lower and IC(50) higher in diabetic compared to control subjects, both in the presence of arachidonic acid (71% vs. 39%, p(np)0.0001; 0.5 microg/ml vs. 1.3 microg/ml, p<0.0001) and collagen (52% vs. 35%, p<0.0004; 1.6 microg/ml vs. 2.1 microg/ml, p<0.01). The reduced response of platelets from diabetic subjects to aspirin was associated with a higher level of HbA(1c), lower concentration of HDL-cholesterol and a higher total cholesterol concentration. Overall, there is evidence that reduced platelets response to aspirin may occur more often in diabetic patients. Poor metabolic control may play a role in the reduced platelet sensitivity to aspirin in DM patients. Thus, our findings strongly support the requirements for an excellent near-normal metabolic control and may suggest a need for alternative ASA dosing schedules in DM patients.     

Influence of tiapride on platelet counts in healthy volunteers and patients with movement disorders

BACKGROUND: The selective D2 antagonist tiapride is administered in various movement disorders. Furthermore, there are indications that tiapride increases platelet counts. AIM: To characterize tiapride's potential to increase platelet counts in healthy subjects and patients with movement disorders. METHODS: In Part A, 10 healthy volunteers received tiapride (300 mg/day) for 21 days in a longitudinal, prospective, open trial. One hundred healthy subjects served as controls. Part B was a retrospective analysis of 15 patients with movement disorders on tiapride [Huntington's disease (n=6), Morbus Little (n=3), hyperkinetic syndromes of undetermined etiology (n=3), blepharospasm (n=1), cervical dystonia (n=1), perioral dyskinesia (n=1)] and 15 age- and sex-matched controls. RESULTS: Part A: Although serum prolactin levels increased by 526+/-14%, confirming good drug compliance, tiapride elicited only minor changes in platelet counts. Part B: Platelet counts correlated positively with the dose of tiapride (100-800 mg/day; r=.67; P=.007). Platelet counts were significantly higher in patients on tiapride compared to healthy age-matched controls (P<.001). Four patients responded to an increase in the tiapride dosage with an increase in platelet count by 97-173 cells/nl. CONCLUSION: Three weeks of treatment with tiapride (300 mg/day) is insufficient to elevate platelet counts to a clinically relevant extent in young healthy volunteers. However, in elderly patients with movement disorders tiapride treatment is associated with markedly increased platelet counts.     

Antiplatelet effects of combination therapy with low-dose aspirin and ticlopidine in cerebral ischemia

Antiplatelet effects of combination therapy with aspirin and ticlopidine were investigated in comparison with single aspirin or ticlopidine therapy in 62 patients with cerebral thrombosis or transient ischemic attack. The 14, 21 and 27 patients were given orally daily aspirin 300mg, ticlopidine 200mg and aspirin 81mg with ticlopidine 100mg, respectively. Various platelet function tests were performed before and a week after medication. They included platelet aggregation (PA) to adenosine diphosphate (ADP), arachidonic acid (AA) and platelet activating factor (PAF) with turbidimetry, plasma beta-thromboglobulin (beta TG), platelet factor 4(PF4), thromboxane B2(TXB2) and 6keto-prostaglandin-F1 alpha(6keto PGF1 alpha) with radioimmunoassay, bleeding time with Simplate device, and platelet survival and lysis with Indium-111-tropolone-labelled platelets. Aspirin inhibited PA to ADP and AA but not to PAF, while ticlopidine inhibited PA to ADP and PAF but not to AA. In contrast, aspirin with ticlopidine inhibited PA to all of these agonists despite their smaller doses used. Aspirin reduced plasma TXB2 but not beta TG or PF4, while ticlopidine reduced beta TG and PF4 but not TXB2. On the contrary, aspirin with ticlopidine reduced TXB2 as well as beta TG and PF4. 6keto PGF1 alpha tended to be reduced by aspirin 300mg alone but not by ticlopidine with or without aspirin 81mg. Bleeding time was significantly prolonged by aspirin or ticlopidine alone, although most prolongation was produced by combination of aspirin and ticlopidine. Platelet survival and lysis remained unaltered in 4 patients treated with aspirin or ticlopidine alone, whereas platelet survival was prolonged and platelet lysis was reduced in 4 patients treated with both aspirin and ticlopidine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional impact of high clopidogrel maintenance dosing in patients undergoing elective percutaneous coronary interventions. Results of a randomized study

The currently recommended maintenance dose of clopidogrel is often associated with inadequate platelet inhibition, suggesting the need for a higher dose. The aim of this pilot study was to assess the functional impact of a high (150 mg/day) maintenance dose of clopidogrel in patients undergoing elective percutaneous coronary intervention (PCI). This is a prospective, randomized, platelet function study which was performed in elective PCI patients assigned to treatment with either a 75 mg (n = 20) or 150 mg (n = 20) daily maintenance dose of clopidogrel for 30 days; afterwards, all patients resumed standard dosing. Platelet aggregation was performed using light transmittance aggregometry following 20 microM and 5 microM adenosine diphosphate (ADP) stimuli 30 days after randomization and 30 days after resuming standard dosing. Patients treated with 150 mg/day clopidogrel had lower 20 microM ADP-induced platelet aggregation compared to patients on 75 mg/day (52.1 +/- 9% vs. 64.0 +/- 8%; p < 0.001; primary endpoint). The dose-dependent effect was confirmed by the absolute and relative increase in platelet aggregation after resuming standard dosing (p < 0.001). No changes were observed in patients randomized to standard dosing. Parallel findings were observed following 5 microM ADP stimuli for all assessments. A broad variability in clopidogrel-induced antiplatelet effects was observed irrespective of dosing. In conclusion, a 150 mg/day maintenance dose regimen of clopidogrel is associated with reduced platelet reactivity and enhanced platelet inhibition compared to that achieved with the currently recommended 75 mg/day in patients undergoing elective PCI.     

Delayed thrombin-induced platelet-fibrin clot generation by clopidogrel: a new dose-related effect demonstrated by thrombelastography in patients undergoing coronary artery stenting

BACKGROUND: We have previously demonstrated that clopidogrel reduces platelet activation and aggregation in patients undergoing stenting. However, the effect of the clopidogrel loading dose on the rate of thrombin-induced platelet-fibrin clot formation is unknown in this patient population. METHODS AND RESULTS: Using thrombelastography (TEG) we measured the time to platelet-fibrin clot formation (R), a marker of the speed of thrombin generation, in 120 patients undergoing elective coronary artery stenting treated with standard and high loading doses of clopidogrel. Platelet reactivity to adenosine diphosphate (ADP) by light transmittance aggregometry (LTA) was determined simultaneously. Measurements were made immediately before and at 24 h after clopidogrel treatment. Clopidogrel produced a prolongation in R (4.4+/-1.4 min pre vs. 5.4+/-1.7 min post, p<0.001) that directly correlated with the change in platelet aggregation (r=0.65, p<0.0001). Prolongation in R was greatest in patients treated with a high loading dose (p=0.004). CONCLUSIONS: Delayed thrombin-induced platelet-fibrin clot formation as measured by TEG is a newly reported dose-related effect of clopidogrel that may contribute to the overall antithrombotic properties of the drug in patients undergoing stenting. This effect was more marked in patients loaded with 600 mg, lending further mechanistic support for this dose of clopidogrel as a more effective antithrombotic regimen than the standard 300 mg dose. Measurement of R may serve as a new indicator of clopidogrel responsiveness.     

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.     

The use of monoamine oxidase inhibition to estimate megakaryocyte-platelet regeneration time (MPRT)

The megakaryocyte-platelet regeneration time (MPRT) was measured in 16 normal subjects after irreversible monoamine oxidase (MAO) inhibition. When ten subjects were given a single 20 mg dose of Parnate (tranylcypromine) the mean MPRT was 242 +/- 20 hours (linear model) or 212 +/- 22 hours (weighted mean model) with a mean %-linearity of 75 +/- 22%. The renewal of platelet MAO activity was initially retarded for about 2 days, suggesting that inhibition of megakaryocyte MAO had occurred. Six subjects who had taken MAO inhibitors daily for 1-6 months gave longer MPRT values (274 +/- 57 hours (lin.) or 246 +/- 67 hours (w. m.)), but the differences between the groups were not statistically significant. The single-dose MPRT results agree well with those obtained using the irreversible inhibition of cyclo-oxygenase by aspirin, but exceed the platelet life-spans estimated using radiochemical methods by about 24-36 hours. Platelet serotonin levels did not change significantly after a single dose of Parnate, but supine systolic blood pressure rose by 9 +/- 7 mm Hg (p less than 0.01) 24 hours after the dose. Estimation of MPRT by MAO inhibition is a useful alternative to the aspirin method but screening of subjects and clinical supervision is needed to avoid hypertensive complications.     

Research on the critical dose for antithrombotic effectiveness of aspirin in the Chinese

We studied 345 cases taking aspirin due to increased platelet aggregation. 220 were male and 125 were female. They were divided into five groups (0, 30 mg, 50 mg, 300 mg, 500 mg, daily) with double-blind method. Platelet aggregation tests were performed before and after medication. The result shows that the response in male is good. For female, the benefit in menopausal women and contraceptive taker is more prominent than that in adolescent and middle aged women. From our clinical trial we suggest 50 mg of aspirin daily as the optimal dosage.     

Time and dose dependent augmentation of inhibitory effects of abciximab by aspirin

Aspirin and abciximab independently decrease the incidence of cardiac events. To identify potential interactions, antiplatelet effects of abciximab were characterized in blood from healthy subjects given aspirin. Platelet activation was determined in whole blood with and without abciximab (2 microg/ml) added in vitro. Flow cytometry was used to quantify fibrinogen binding (glycoprotein IIb-IIIa activation). Binding of fluorochrome-labeled and 125I-labeled abciximab was determined before and after exposure to aspirin. In blood from subjects given aspirin for 5 days, abciximab-induced inhibition of the capacity to bind fibrinogen in response to 1 microM ADP was greater when the daily dose had been 325 mg compared with 81 mg (% inhibition: no aspirin 53 +/- 6; 81 mg daily 62 +/- 5; 325 mg daily 69 +/- 6). The effect of 5 daily doses of aspirin was greater than that of one. Larger single doses elicited larger effects (% inhibition 2 h after 325 mg 59 +/- 6; 2 h after 650 mg 78 +/- 5). Neither salicylsalicylic acid nor naproxen sodium potentiated the effect of abciximab. Exposure of platelets to 14C-acetylsalicylic acid led to acetylation of glycoprotein IIb and IIIa. Binding of 125I-abciximab to platelets was increased after 30 and 60 min. Acetylation of glycoprotein IIb-IIIa by aspirin augments inhibitory effects of abciximab in a dose- and time-dependent manner by increasing binding of abciximab to platelets.     

Twice daily dosing of aspirin improves platelet inhibition in whole blood in patients with type 2 diabetes mellitus and micro- or macrovascular complications

The efficacy of low-dose aspirin in type 2 diabetes mellitus (T2DM) has been questioned. We tested if twice daily dosing of aspirin would be more effective in T2DM, possibly due to increased platelet turnover. A randomised cross-over study compared 75 mg aspirin OD, 75 mg BID and 320 mg OD (≥ 2 week treatment periods) in 25 patients with T2DM and micro- or macrovascular complications. Platelet responses were examined by impedance aggregometry (WBA) and the IMPACT-R aspirin test in whole blood, light transmittance aggregometry in platelet-rich plasma (LTA), and urinary 11-dehydro-thromboxane B2 (TxM). Aspirin 75 mg BID decreased arachidonic acid (AA)-induced WBA compared to 75 mg OD (9.7 ± 4.5 vs. 12.6 ± 3.5 ohm; p = 0.003) or to 320 mg OD (11.5 ± 4.2 Ohms; p = 0.049). WBA responses to collagen were similarly attenuated by BID or high dosing (by 12-14%; p = 0.02 for both). The IMPACT-R showed a better response to 75 mg BID compared to 75 mg OD (p = 0.049), but not to 320 mg OD. AA-induced aggregation by LTA was <6.5% on all occasions, with no differences between aspirin dosages. TxM was reduced after 320 mg OD (p = 0.002), but not 75 mg BID (p = 0.07). Reticulated platelets were highly correlated with mean platelet volume (MPV; r2 = 0.74, p<0.0001). Both markers for platelet turnover were correlated with AA-induced WBA, but neither identified patients who benefited from BID dosing dependably. In conclusion, twice daily dosing improved laboratory responses to aspirin in high risk T2DM patients. Studies of whether BID dosing of aspirin can improve clinical outcomes in such patients are of interest.