Effect of aspirin treatment in patients with peripheral arterial disease monitored with the platelet function analyzer PFA-100

Frequency of development of aspirin resistance in the early postoperative period and inadequate inhibition of thromboxane A2 production after coronary artery bypass surgery

Background

This study aims to investigate the frequency of the development of aspirin resistance, whether or not this resistance was reversible, and to evaluate the efficiency of the mechanism of incomplete inhibition of thromboxane A2 in development of aspirin resistance in the early postoperative period in patients who had undergone coronary artery bypass grafting.

Methods

Eighty patients (55 males, 25 females; mean age 63.1±9.2 years; range 51 to 75 years) who underwent coronary artery bypass grafting between February 2009 and March 2010 at our clinic were prospectively evaluated. Venous blood samples were collected from all patients and evaluated by a platelet function analyzer in the preoperative period and on postoperative days 7 and 15. Aspirin resistance diagnosis was defined as collagen-epinephrine closure time less than 186 seconds. The urine levels of 11-dehidro thromboxane B2 were also measured on postoperative day one.

Results

Aspirin resistance was found in 23 patients (28.75%) in the preoperative period, in 31 patients (38.75%) on the postoperative seventh day and in 25 patients (31.25%) on the postoperative 15th day. The urine levels of 11-dehidro thromboxane B2 in patients with aspirin resistance on the postoperative seventh day were significantly higher than those in patients without aspirin resistance (p<0.001). The mean aortic cross-clamping time (p=0.003) and cardiopulmonary bypass time (p=0.029) in the patients with aspirin resistance on the postoperative seventh day were significantly higher than those in patients without aspirin resistance.

Conclusion

The results of this study suggest that aspirin resistance develops within the first seven days after coronary artery bypass grafting and is highly reversible, and that the mechanism of inadequate inhibition of thromboxane A2 by aspirin has a role in the development of aspirin resistance in the early postoperative period.

The Relationship Between Aspirin Resistance and Carotid Imaging in Young Patients With ST-Segment Elevated Myocardial Infarction: A Cross-Sectional Study

The presence of carotid atherosclerosis accompanied by coronary artery disease is associated with poor prognosis. A subset of patients who take aspirin continue to have recurrent cardiovascular events, which may be due to aspirin resistance (AR). Also, carotid plaques may cause turbulent flow which in turn may lead to platelet activation and poor antiplatelet response. In our study, we aimed to show the prevalence of AR and its relationship between high-risk carotid images in young patients with ST-segment elevated myocardial infarction (STEMI). In our study, we included 112 patients younger than 45 years with STEMI. Aspirin response test was evaluated 1 hour after aspirin intake using multiplate platelet function analyzer, and carotid ultrasonography has been performed to determine carotid intima–media thickness (CIMT) and the presence of carotid plaque. We identified 30.3% AR in young patients with STEMI. Carotid intima–media thickness (P = .002), carotid plaque (P = .012), and high-risk carotid image (P = .015) values are significantly high in patients who have AR. Independent of other risk factors, the presence of carotid plaque and being in the high-risk carotid group were associated with 3.7 times and 3.2 times increased odds for AR, respectively. In young patients with STEMI, physicians should be careful about AR, especially in patients who have carotid plaque and thicker CIMT.

Searching for factors associated with resistance to acetylsalicylic acid used for secondary prevention of stroke

INTRODUCTION

The aim of the study was to evaluate the prevalence of resistance to acetylsalicylic acid (ASA), used for secondary prevention of stroke, including the assessment of risk factors associated with the lack of ASA anti-aggregatory action.

MATERIAL AND METHODS

Patients after a transient ischaemic attack (TIA) or ischaemic stroke in the acute (n = 111) and chronic phase (n = 87) were enrolled in the study. The assessment of platelet function was performed by whole blood impedance aggregometry using a multi-channel platelet function analyser (Multiplate).

RESULTS

A proper response to ASA was found in 121 patients (61.1%) (ASA responders), a partial response to ASA in 59 patients (29.8%) (ASA partial responders), and ASA resistance in 18 patients (9.1%) (ASA non-responders). Acetylsalicylic acid resistance was observed more frequently in the chronic phase. The mean low-density lipoprotein (LDL) concentration was higher in ASA non-responders (p = 0.02). The mean heart rate (p = 0.03) and the mean haematocrit (p = 0.03) were higher in the group of ASA partial responders and ASA non-responders. Angiotensin II receptor antagonists were more often used in the group of ASA partial responders and ASA non-responders (p = 0.04). Diuretics were more rarely used by ASA non-responders, whereas fibrates were more rarely used by ASA partial responders.

CONCLUSIONS

The method enabled the detection of ASA resistance in some patients with cerebrovascular disease. The study revealed some possible risk factors of ASA resistance: long ASA therapy, increased heart rate, higher LDL concentration, and higher haematocrit value. The relationship between the effect of ASA and other medications (angiotensin II receptor blockers, fibrates, diuretics) requires further study. Platelet function monitoring should be considered in patients at a greater risk of ASA resistance.

Resistance to acetylsalicylic acid in patients with type 2 diabetes mellitus is associated with lipid disorders and history of current smoking

BACKGROUND

Diabetes mellitus (DM) is an important risk factor for stroke. Acetylsalicylic acid (ASA) is the most frequently used medication for prevention of cardio-cerebral vascular diseases. However, some patients experience ischaemic vascular events despite the use of ASA. This phenomenon is known as "aspirin resistance" (AR). The aim of this study was to assess the prevalence of AR in diabetic patients and search for factors associated with it.

MATERIALS AND METHODS

The examined group consisted of 96 subjects with diagnosed type 2 DM. Platelet function test was performed by the method of whole blood impedance aggregometry.

RESULTS

Among examined subjects, 51 patients (53.1 %) were sensitive to ASA action (ASA responders) and 45 patients (46.9 %) were resistant to ASA action (ASA non-responders). No association was found between platelet aggregation and gender, age, dose of ASA, known duration of diabetes, BMI, heart rate, mean systolic and diastolic blood pressure, and risk factors except for current smoking (p = 0.030). ASA non-responders were treated shorter with ASA than ASA responders (p = 0.010). The mean total cholesterol (p = 0.020), LDL concentration (p = 0.005), HCT (p = 0.010), WBC (p = 0.030), and PLT (p = 0.050) were significantly higher in ASA non-responders. No association was found between AR and results of other laboratory tests and medications. Multiple logistic regression analysis revealed factors associated with AR: current smoking and LDL concentration higher than 3.5 mmol/l.

CONCLUSIONS

Results of our study did not confirm the association between poor glycaemic control in the diabetic patients and AR. Resistance to ASA in diabetic patients is associated with lipid disorders and history of current smoking.

Clinical implications of aspirin resistance

Aspirin (acetylsalicylic acid) is one of the main therapeutic medications used in the prevention of thromboembolic vascular events. Aspirin exhibits its antiplatelet action by irreversibly inhibiting platelet cyclooxygenase-1 enzyme, thus preventing the production of thromboxane A2 (TXA2). Aspirin resistance, as measured in vitro, is the inability of aspirin to reduce platelet activation and aggregation by failure to suppress the platelet production of TXA2. Laboratory tests of platelet TXA2 production or platelet function dependent on TXA2 can detect aspirin resistance in vitro. The clinical implication of this laboratory definition has not yet been elucidated via prospective trials that have controlled for confounders, such as hypertension, diabetes and dyslipidemia. Large meta-analyses have found low-dose aspirin to be as effective as high-dose aspirin in preventing vascular events, making a dose-dependent improvement in laboratory response clinically irrelevant. Possible causes of aspirin resistance include poor compliance, inadequate dose, drug interactions, genetic polymorphisms of cyclooxygenase-1, increased platelet turnover and upregulation of non-platelet pathways of thromboxane production. However, there is currently no standardized approach to the diagnosis and no proven effective treatment for aspirin resistance. Further research exploring the mechanisms of aspirin resistance is needed in order to better define aspirin resistance, as well as to develop a standardized laboratory test that is specific and reliable, and can correlate with the clinical risk of vascular events. The intent of this paper is to review the literature discussing possible mechanisms, diagnostic testing and clinical trials of aspirin resistance and to discuss its clinical relevance as it pertains to cerebrovascular and cardiovascular disease.

Relationship between endothelial functions and acetylsalicylic acid resistance in newly diagnosed hypertensive patients

BACKGROUND

We aimed to investigate the effects and dose dependency of aspirin on endothelial functions and prevalence of aspirin resistance in newly diagnosed hypertensive patients without previous drug therapy and development of cardiac complications.

HYPOTHESIS

Acetylsalicyclic acid improves endothelial function.

METHODS

Fifty-eight hypertensive patients and 61 healthy subjects in the control group were included in the study. Endothelial functions of the patient and control groups were evaluated with brachial artery examination. Patient and control groups were divided into 2 groups. A total of 100 mg and 300 mg of aspirin were given to the separate groups for 1 week. After 1 week, endothelial functions were reevaluated and aspirin resistance examined with a platelet function analyzer (PFA-100; Dade Behring, Marbourg, Germany).

RESULTS

Baseline flow-mediated dilatation (FMD) change percent in hypertensive patients was 9.8%, and it was significantly lower than in the control group (12%) (P < 0.001). Frequency of acetylsalicylic acid (ASA) resistance was 20% and 26% in control and hypertensive patient groups, respectively (P = not significant). ASA resistance was 28% and 24% in 100 mg and 300 mg in hypertensive patients, respectively (P = not significant). FMD change percent increased both in the control and hypertensive groups after ASA treatment from 12.4% to 13.3% and 9.8 % to 11.9 %, respectively. FMD percentage change was significantly increased in hypertensive patients irrespective of ASA resistance (P = 0.02, for ASA resistance [+]; P < 0.012, for ASA resistance [-]).

CONCLUSIONS

Endothelial functions were impaired more in hypertensive patients compared to the control group. Endothelial functions were improved with all ASA doses in hypertensive patients irrespective of ASA resistance.

Plasma salicylate level and aspirin resistance in survivors of myocardial infarction

To investigate the effect of aspirin on the platelets of survivors of myocardial infarction we correlated plasma salicylate level with platelet reactivity in ten patients and ten normal controls. The patients and controls were tested at the end of 2 week periods on 75, 150 and 300 mg aspirin daily by mouth. Platelet reactivity was measured, under high shear stress conditions, using cartridges containing adrenaline and adenosine diphosphate in a PFA-100 platelet function analyser. The time taken by the developing platelet aggregate to close an aperture in the collagen membrane of the cartridge, the closure time, was taken as an index of platelet reactivity. There was no difference in baseline haematocrit, platelet count or plasma vWF antigen level between the groups. There was a dose-dependent increase in closure time of the adrenaline containing cartridge in the controls (P < 0.001), but not in the patients (P = 0.08), compatible with a reduced anti-platelet effect of aspirin in the patients. Furthermore, plasma salicylate level was higher in the patient group (P < 0.05).

The impact of intravenous aspirin administration on platelet aspirin resistance after on-pump coronary artery bypass surgery

Aspirin resistance continues to be a major challenge in patients after coronary artery bypass grafting (CABG). We investigated the impact of intravenous aspirin on platelet function in this clinical setting. Forty-two patients received 100 mg of oral aspirin once daily, beginning on day 1 after the operation. Between day 6 and 8 post operation one oral dose was replaced by an intravenous dose of 300 mg. Platelet function analyzer (PFA-100) closure times (CT), turbidimetric platelet aggregation (TPA) and impedance platelet aggregation (IPA) induced by arachidonic acid (AA), collagen and ADP were measured prior to and 1 h and 24 h after intravenous aspirin. Results obtained prior to the intravenous aspirin were compared with respective values from 120 healthy individuals. Despite the postoperative oral aspirin that was given once daily, ADP-induced TPA (ADPTPA) and IPA values induced by AA, ADP or collagen were significantly greater in patients than in controls, while PFA-100 CT were significantly shorter. Intravenous aspirin induced a significant reduction of platelet aggregability as measured by collagen/epinephrine (CEPI) CT, TPA and IPA induced by AA and collagen 1 h and 24 h after administration. Intravenous aspirin was not found to influence collagen/ADP (CADP) CT and IPA induced by ADP. Concomitantly, the number of patients with laboratory aspirin resistance as measured by CEPI-CT and TPA but not by IPA induced by AA or collagen dropped significantly. Agreement in the detection of aspirin responders and non-responders among platelet function tests was poor. Our findings indicate that the intravenous aspirin appears to be a promising approach for reducing laboratory aspirin resistance during the postoperative phase of CABG.

Resistance to low-dose aspirin therapy among patients with acute coronary syndrome in relation to associated risk factors

BACKGROUND

A substantial proportion of patients have recurrence of vascular events despite daily intake of low-dose aspirin therapy. Therefore, different patients may require different aspirin dosages to achieve complete inhibition of platelet function.

OBJECTIVE

The aim of this work was to measure the response to low-dose aspirin therapy (150 mg/day) among patients with unstable angina or non-ST-segment elevation myocardial infarction and to find out whether titrating aspirin dosage to 300 mg/day, would provide a better therapeutic response in the resistant cases. Moreover, we also aimed to study any association between aspirin non-responsiveness and atherothrombotic risk factors.

METHODS

The antiplatelet effect of 150 mg/day aspirin was studied prospectively in 50 consecutive patients with unstable angina or non-ST-segment elevation myocardial infarction. Platelet aggregation was measured using optical platelet aggregometry and serum thromboxane B(2) level. Aspirin resistance was defined as collagen (1 μg/mL) and adenosine diphosphate (ADP) (5 μmol/L)-induced platelet aggregation of ≥ 40% when compared with control values. Twenty healthy age- and sex-matched individuals were taken as a control group. All patients were subjected to complete medical history (risk factors, medications), thorough clinical examination, ECG, coronary angiography and laboratory investigations including: complete haemogram, coagulation, kidney, liver and lipid profiles, fasting blood glucose and glycated haemoglobin (HbA(1C) ).

RESULTS

Eleven of 50 patients (22%) were found to be aspirin resistant. A highly significant difference was found between the mean values of ADP, collagen-induced platelet aggregation percentage and thromboxane B(2) level after aspirin 150 mg/day when compared with the corresponding mean values after aspirin 300 mg/day among the resistant patients (66 ± 7.01%, 62 ± 4.34% and 620 ± 64.58 pg/mL, respectively, vs. 26.87 ± 2.85%, 16.5 ± 3.8% and 77 ± 11.3 pg/mL) indicating enhanced response to aspirin after escalating the dose. The presence of atherothrombotic risk factors (hypertension, smoking, family history of ischaemic heart disease and previous MI) were not statistically different between aspirin-resistant and aspirin-sensitive patients. However, there was a highly significant difference between the aspirin sensitive and the resistant patients regarding the other risk factors (diabetes mellitus and dyslipidaemia) (P < 0.01).

CONCLUSION

There is inter-individual variability in response to the antiplatelet effect of standard doses of aspirin (150, 300 mg/day). The response to aspirin 300 mg/day is enhanced in resistant patients when compared to 150 mg/day. There was a significant association between aspirin resistance and atherothrombotic risk factors (diabetes, hyperlipidaemia and obesity).

The surgical application of point-of-care haemostasis and platelet function testing

Background

Disordered coagulation complicates many diseases and their treatments, often predisposing to haemorrhage. Conversely, patients with cardiovascular disease who demonstrate antiplatelet resistance may be at increased thromboembolic risk. Prompt identification of these patients facilitates optimization of haemostatic dysfunction. Point-of-care (POC) tests are performed ‘near patient’ to provide a rapid assessment of haemostasis and platelet function.

Methods

This article reviews situations in which POC tests may guide surgical practice. Their limitations and potential developments are discussed. The paper is based on a Medline and PubMed search for English language articles on POC haemostasis and platelet function testing in surgical practice.

Results

POC tests identifying perioperative bleeding tendency are already widely used in cardiovascular and hepatic surgery. They are associated with reduced blood loss and transfusion requirements. POC tests to identify thrombotic predisposition are able to determine antiplatelet resistance, predicting thromboembolic risk. So far, however, these tests remain research tools.

Conclusion

POC haemostasis testing is a growing field in surgical practice. Such testing can be correlated with improved clinical outcome.

Aspirin resistance: biological and clinical implications

Cardiovascular events are the leading causes of mortality and morbidity in the United States. This development has prompted the rise of aspirin therapy in the prevention of atherothrombotic events. However, not all patients benefit to the same extent from aspirin therapy and many continue experiencing atherothrombotic complications. Researchers have labeled this phenomenon aspirin resistance, and despite drawing much attention from both researchers and lay people the cause remains unknown. Much needs to be clarified and standardized regarding the phenomenon of aspirin resistance, including the prevalence, definition, appropriate measurement methods, mechanisms, and, most important, linking low response to aspirin with worsened clinical outcomes.

Laboratory Evaluation of Platelet Function

Platelets play a central role in hemostasis. Consequently, they lie at the heart of many inherited and acquired bleeding disorders and thrombotic events. The diagnosis of these disorders and monitoring of antiplatelet therapy require a thorough understanding of tests that measure platelet quantity and function. This article outlines basic concepts of platelet physiology and describes the tests that are commonly used in the clinical assessment of platelet function.

Residual platelet reactivity is associated with clinical and laboratory characteristics in patients with ischemic heart disease undergoing PCI on dual antiplatelet therapy

A residual platelet reactivity (RPR) on antiplatelet therapy in patients with ischemic heart disease (IHD) has been reported to be associated with adverse clinical events by some Authors. However, scarce data are present on the clinical parameters associated with this phenomenon. No study, at our knowledge, was designed with the specific aim to examine the relationship between clinical characteristics and RPR. We sought to evaluate the clinical and laboratory characteristics associated with RPR in patients with IHD undergoing coronary revascularization on dual (aspirin plus clopidogrel) antiplatelet therapy. We included in the study 868 patients undergoing a coronary angiography: 386 with acute coronary syndromes undergoing a primary coronary revascularization and 482 IHD patients scheduled to undergo an elective coronary angiography. We measured platelet function by both platelet aggregation with two agonists [0.5 mg/mL arachidonic acid (AA); 2 and 10 microM adenosine 5'-diphosphate (ADP)] and a point-of-care assay (PFA-100) on venous blood samples collected within 24 h from the end of the procedure. In patients with acute coronary syndromes and elective PCI diabetes is independently associated with RPR [group A: OR=2.9 (1.5-5.7) by 10 microM ADP, OR=5.3 (1.1-27.8) by PFA-100; group B: OR=4.0 (1.6-10.0) by 10 microM ADP]; reduced left ventricular systolic function [OR=3.7 (2.2-6.5) by AA-PA, OR=2.7 (1.6-4.6) by PFA-100], chronic use of aspirin [OR=0.2 (0.1-0.4) by AA-PA, OR=0.3 (0.2-0.5) by PFA-100] and loading dose of clopidogrel [OR=0.2 (0.06-0.5) by 10 microM ADP] were independent variables significantly associated with RPR in patients undergoing elective PCI. In addition, inflammatory status was found to be significantly associated with RPR in both groups of patients. These results provide indications for the selection of patients for whom the evaluation of platelet reactivity could be useful.

The frequency of aspirin resistance and its risk factors in patients with metabolic syndrome

BACKGROUND

Although different populations were examined for the incidence of aspirin resistance, the frequency and related risk factors for aspirin resistance in patients with metabolic syndrome have not been reported yet. This study aimed to determine the frequency of aspirin resistance and its risk factors in patients with metabolic syndrome.

METHODS

We performed a cross-sectional study in 110 patients with metabolic syndrome. After one week of 100 mg/day aspirin, blood samples were obtained. Platelet function analyzer (PFA-100) was used to determine the frequency of aspirin resistance. Endothelial functions, carotid intima media thickness, and the presence of plaques in the carotid arteries were evaluated for subclinical atherosclerosis and the levels of inflammatory markers were assessed as risk factors for aspirin resistance. The presence of subclinical atherosclerosis was defined as a maximum carotid intima media thickness of > or = 0.9 mm and/or the presence of carotid atheroma.

RESULTS

Aspirin resistance was detected in 21.9% of the patients. In the multivariate analysis, hs-CRP levels (odds ratio [95% CI]=2.8 [1.3-5.9], p=0.009), diastolic blood pressure, (0.9 [0.8-1.0], p=0.007), and the presence of subclinical atherosclerosis (4.1 [1.4-12.2], p=0.012) were statistically significant risk factors for aspirin resistance.

CONCLUSIONS

We concluded that the frequency of aspirin resistance confirmed in this cohort of patients with metabolic syndrome was higher in patients with a lower diastolic blood pressure, higher hs-CRP levels and atherosclerotic changes in their carotid arteries.

A review of aspirin resistance; definition, possible mechanisms, detection with platelet function tests, and its clinical outcomes

Aspirin (acetylsalicylic acid) is one of the main therapeutics in prevention of thrombo-embolic vascular events. Its efficiency is proved in the prevention of cardiovascular events. However, antiplatelet effect of aspirin is not absolute in all patients and some patients experience thrombo-embolic events despite aspirin. These patients are clinically called as aspirin resistant or aspirin non-responders. Globally, a lot of people are affected by aspirin resistance according to the high prevalence of athero-thrombotic vascular diseases. A prevalence of 5.5-45% in patients with various cardiovascular disease by different laboratory methods has been reported for aspirin resistance. Clinical outcome of aspirin resistance has been demonstrated in patients with different vascular diseases. Detection of platelet function in patients treated with aspirin may be necessary in the prediction of clinical outcome. Point of care methods, which have correlated results with the standard light transmittance aggregometry may be appropriate choices in the detection of platelets' response to antiplatelet therapy. Adequate additional therapies may reduce atherothrombotic risks and major cardiovascular events rate in aspirin resistant subjects. None of the current researches advised the cessation of aspirin therapy. There is need to investigate the efficacy of additional adenosine diphosphate receptor antagonists or newer antiplatelet agents in aspirin resistant subjects. The intent of this paper is to review the literature discussing possible mechanisms, determination techniques, and clinical effects of aspirin resistance.

Aspirin resistance or variable response or both?

Numerous clinical trials have demonstrated that aspirin is effective in secondary prevention and in high-risk primary prevention of adverse cardiovascular events. However, a constellation of clinical and laboratory evidence exists that demonstrates diminished or absent response to aspirin in some patients. This has led to the concept of "aspirin resistance," which is a poorly defined, somewhat misleading term. The mechanism for aspirin resistance has not been fully established, but it is almost certainly due to a combination of clinical, biological, and genetic properties affecting platelet function. There are no criteria for distinguishing true resistance from treatment failure, and there is no consensus on whether the definition of aspirin resistance should be based on clinical outcomes, laboratory evidence, or both. Studies in large populations are needed to define antiplatelet resistance using consistent and reproducible assays and correlate the measurements with clinical outcomes. One such prospective randomized trial is completed, and 2 others are under way: the Clopidogrel for High Atherothrombotic Risk and Ischemic Stabilization, Management, and Avoidance (CHARISMA) trial compared clopidogrel and aspirin with placebo and aspirin for high-risk primary or secondary prevention, and the Aspirin Nonresponsiveness and Clopidogrel Endpoint Trial (ASCET) is evaluating whether switching to clopidogrel will be superior to continued aspirin therapy in improving clinical outcomes in aspirin-resistant patients with angiographically documented coronary artery disease. The Research Evaluation to Study Individuals Who Show Thromboxane or P2Y(12) Receptor Resistance (RESISTOR) trial is investigating whether modifying antiplatelet regimens could prevent myonecrosis after percutaneous coronary intervention in patients with aspirin and clopidogrel resistance.

POINT OF CARE TESTING OF ASPIRIN RESISTANCE IN PATIENTS WITH VASCULAR DISEASE

INTRODUCTION

The reported range in rates of aspirin resistance (5.5-60%) may reflect difficulties in studying platelet function and the variety of tests used. This study used a platelet function analyzer (PFA-100) to prospectively document aspirin resistance in a cohort of patients with arterial disease.

METHODS

Patients with internal carotid artery (ICA) stenosis or intermittent claudication (IC) were recruited. Exclusion criteria were contraindications to aspirin, prescription of other medication with known antiplatelet effects or known platelet abnormalities. After prescription of 100 mg aspirin/day for 2 weeks an uncuffed venous blood sample was taken and analysed with the PFA-100. Aspirin resistance was defined as closure time (CT) less than the upper limit of normal (158 s collagen/epinephrine agonist; 118 s collagen/adenosine diphosphate (ADP) agonist).

RESULTS

Thirty-three patients with IC and 12 patients with ICA stenosis were recruited (n = 45). Median (range) age was 74 years (49-85) and the male to female ratio was 1.5:1. The median (range) CT was >300 (85 to >300) s with collagen/epinephrine and 100 (52 to >300) s with collagen/ADP agonist. Twelve patients (27%) in the collagen/epinephrine group had normal CT despite treatment with 100 mg aspirin, indicating resistance. Of the 33 patients with collagen/epinephrine CT prolonged by aspirin, 10 patients also had prolonged collagen/ADP CT, suggesting excessive platelet inhibition.

CONCLUSION

A significant proportion of patients taking aspirin do not show laboratory evidence of platelet inhibition and may not be protected from atherothrombotic events. The PFA-100 appears to be a useful tool to screen for both aspirin resistance and excessive aspirin mediated platelet inhibition.

Aspirin resistance measured by cationic propyl gallate platelet aggregometry and recurrent cardiovascular events during 4 years of follow-up

BACKGROUND

Aspirin resistance appears to be an important prognostic factor in patients with coronary artery disease, yet there is no standardized method to measure it and limited data on its correlation to clinical outcomes.

METHODS

In a prospective study we followed 103 patients (mean age 64 years) with acute coronary syndrome (ACS) without ST segment elevation who were treated with 100 mg of aspirin (ASA) daily. Optical platelet aggregometry using cationic propyl gallate (CPG) as an inductor was measured at ACS onset and after 3, 12, 24, 36, and 48 months. ASA responsiveness was defined both by the slope of the aggregation curve (<53%/min) and by spontaneous aggregation (<5%). The primary outcomes were the recurrence of ACS or stroke.

RESULTS

Patients with ACS exhibited a greater prevalence of ASA resistance (55%) than healthy volunteers (4%; p<0.01). ASA resistance occurred more often in patients with type 2 diabetes, hypertriacylglycerolemia, and decreased HDL levels, and in smokers (p<0.05). A single assessment of platelet aggregometry was sufficient to identify ASA-resistant patients. During the 4-year follow-up, the patients with ASA resistance had an 88% incidence of recurrent cardiovascular events versus 46% for the patients without ASA resistance (p<0.01). In the subgroup with recurrent cardiovascular (CV) events, significantly more patients were ASA-resistant than in the subgroup without recurrent CV events (72% vs. 8%, p<0.01).

CONCLUSION

ASA resistance measured by CPG-induced platelet aggregometry is more common among patients with ACS and some metabolic risk factors, and ASA-resistant patients have a significantly higher recurrence of cardiovascular events.

Biological effects of aspirin and clopidogrel in a randomized cross-over study in 96 healthy volunteers

BACKGROUND

Some data suggest that biological 'resistance' to aspirin or clopidogrel may influence clinical outcome.

OBJECTIVE

The aim of this study was to evaluate the relationship between aspirin and clopidogrel responsiveness in healthy subjects.

METHODS

Ninety-six healthy subjects were randomly assigned to receive a 1-week course of aspirin 100 mg day(-1) followed by a 1-week course of clopidogrel (300 mg on day 1, then 75 mg day(-1)), or the reverse sequence, separated by a 2-week wash-out period. The drug effects were assessed by means of serum TxB2 assay, platelet aggregation tests, and the PFA -100 and Ultegra RPFA -Verify Now methods.

RESULTS

Only one subject had true aspirin resistance, defined as a serum TxB2 level > 80 pg microL(-1) at the end of aspirin administration and confirmed by platelet incubation with aspirin. PFA-100 values were normal in 29% of the subjects after aspirin intake, despite a drastic reduction in TxB2 production; these subjects were considered to have aspirin pseudo-resistance. Clopidogrel responsiveness was not related to aspirin pseudo-resistance. Selected polymorphisms of platelet receptor genes were not associated with either aspirin or clopidogrel responsiveness.

CONCLUSIONS

In healthy subjects, true aspirin resistance is rare and aspirin pseudo-resistance is not related to clopidogrel responsiveness.

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.

Platelet function analyzer (PFA)-100 closure time in the evaluation of platelet disorders and platelet function

BACKGROUND

Closure time (CT), measured by platelet function analyzer (PFA-100) device, is now available to the clinical laboratory as a possible alternative or supplement to the bleeding time test.

AIM

On behalf of the Platelet Physiology Subcommittee of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH-SSC), a working Group was formed to review and make recommendations on the use of the PFA-100 CT in the evaluation of platelet function within the clinical laboratory.

METHODS

The Medline database was searched to review the published information on the PFA-100 CT in the evaluation of platelet disorders and platelet function. This information, and expert opinion, was used to prepare a report and generate consensus recommendations.

RESULTS

Although the PFA-100 CT is abnormal in some forms of platelet disorders, the test does not have sufficient sensitivity or specificity to be used as a screening tool for platelet disorders. A role of the PFA-100 CT in therapeutic monitoring of platelet function remains to be established.

CONCLUSIONS

The PFA-100 closure time should be considered optional in the evaluation of platelet disorders and function, and its use in therapeutic monitoring of platelet function is currently best restricted to research studies and prospective clinical trials.

Aspirin Resistance: An Evaluation of Current Evidence and Measurement Methods

Aspirin resistance is a poorly characterized phenomenon, whereby certain patients do not benefit from the antithrombotic effect of aspirin. The frequency of aspirin resistance is unknown, but estimates range from 5-60%. The mechanism of aspirin resistance also is unknown; proposed mechanisms are poor patient compliance, poor aspirin absorption, increased isoprostane activity, platelet hypersensitivity to agonists, increased cyclooxygenase-2 activity, a cyclooxygenase-1 polymorphism, and the platelet alloantigen 2 polymorphism of platelet glycoprotein IIIa. Aspirin resistance appears to be dose related in some patients and therefore may be overcome with high doses. Evidence indicates that aspirin resistance is a dynamic state, with significant intrapatient variability in aspirin sensitivity with time. To date, a sensitive and specific assay of aspirin effect that reliably predicts treatment failure has not emerged. However, several commercially available products are being marketed for this purpose without convincing clinical data. Despite a wealth of literature on the topic, aspirin resistance remains an enigma. Further investigation is needed regarding strategies to identify and treat patients resistant to aspirin.

Resistance to antiplatelet therapy

Cardiovascular mortality continues to be high and events continue to occur in patients taking antiplatelet medications. Aspirin and clopidogrel have become integral parts of management in patients with coronary artery disease and after percutaneous angioplasty. However, the platelet responses to aspirin and clopidogrel are not uniform. Diminished or lack of response to these agents has been termed aspirin resistance and clopidogrel resistance. These phenomena have tremendous clinical significance as together they may occur in more than 50% of all patients on chronic therapy with aspirin or clopidogrel. Postulated mechanisms of aspirin and clopidogrel resistance include alterations in genetic, pharmacokinetic, and platelet properties. There is a dearth of information in regard to their clinical significance, methods to test them, and strategies to treat them. Further research is necessary in these areas to identify these patients and treat them appropriately.

Limited usefulness of the PFA-100 for the monitoring of ADP receptor antagonists--in vitro experience

We have evaluated the usefulness of the PFA-100 system (collagen/ADP and collagen/epinephrine cartridges) to assess the in vitro effects of a few platelet function inhibitors: Aspisol (60 microg/ml), 4-[4-[4-(aminoiminomethyl]-1-piperazinyl]-1-piperidineactetic acid, hydrochloride trihydrate (GR144053F, fibrinogen receptor antagonist, 100 nM), adenosine-3',5'-diphosphate (A3P5P, P2Y1 ADP receptor antagonist, 500 microM) and Bis[(adenosine-5'-O-phosphorodithioyl)methylene]-phosphinic acid (APTMPA, P2Y12 ADP receptor antagonist, 500 microM) on platelet function, as compared with the other commonly used diagnostic technique, a whole blood electrical aggregometry (20 microM ADP or 0.5 mM arachidonic acid). The in vitro studies were carried out on a group of 38 subjects. Whereas all the examined platelet antagonists and inhibitors almost completely blocked the 20 microM ADP- or 0.5 mM arachidonic acid-induced (in the case of acetylsalicylic acid) whole blood aggregation, only two inhibitors (Aspisol and GR144053F) remained effective in a significant prolongation of the PFA-100 occlusion time. Otherwise, using the PFA-100 system we were not able to detect the inhibitory actions of ADP receptor antagonists- P2Y1 and P2Y12. Our findings point to a limited usefulness of the PFA-100 system for the monitoring of the effectiveness of ADP receptor antagonists. The outcomes of this study show that platelet aggregometry in whole blood is characterised by the highest sensitivity in the monitoring of the investigated blood platelet inhibitors.

Aspirin and clopidogrel: efficacy, safety, and the issue of drug resistance

Aspirin and the thienopyridines ticlopidine and clopidogrel are antiplatelet agents that display good antithrombotic activity. In the past few years, the concept of aspirin resistance has been largely emphasized in the medical literature, although its definition is still uncertain. I suggest that "aspirin-resistant" should be considered as a description for those individuals in whom aspirin fails to inhibit thromboxane A2 production, irrespective of the results of unspecific tests of platelet function, such as the bleeding time, platelet aggregation, or the PFA-100 system. Less well known than aspirin resistance, but certainly better characterized, is the issue of "clopidogrel resistance," which is probably mostly caused by inefficient metabolism of the prodrug clopidogrel to its active metabolite. At present, aspirin and clopidogrel resistance should not be looked for in the clinical setting, because there is no definite demonstration of an association with clinical events conditioning cost-effective changes in patient management.

In vitro aspirin resistance detected by PFA-100 closure time: pivotal role of plasma von Willebrand factor

The in vitro closure time (CT), determined by the Platelet Function Analyzer (PFA-100), is used to monitor patients treated with aspirin. A relatively high percentage of in vitro aspirin resistance was reported despite an adequate inhibition of platelet response to arachidonic acid and we investigated whether high plasma levels of von Willebrand factor ristocetin cofactor activity (vWF:RCo) may contribute to this profile. Platelet aggregation test, CT [collagen adrenaline (CEPI-CT) and collagen adenosine 5'-diphosphate (ADP) (CADP-CT)], and vWF:RCo levels were evaluated in 55 consecutive patients receiving aspirin (75-250 mg/d) versus 32 untreated control subjects. All the aspirin-treated patients showed platelet aggregation responses that reflected the aspirin intake. However, CT data analysis enabled aspirin good-responder (GR) and aspirin bad-responder (BR) patients to be identified. All GR group subjects (n = 27), had a CEPI-CT and a CADP-CT longer than 300 s and 96 s respectively. The BR group (n = 28) had CEPI-CT values below 200 s and all CADP-CT were in the normal range (77 +/- 19 s). Interestingly, the BR plasma vWF:RCo levels were significantly higher (159 +/- 43%) than those of the GR group (121 +/- 34%) (P < 0.01), which were similar to control values (114 +/- 31%). A negative correlation between vWF:RCo and CT values was established. We demonstrate that in vitro aspirin-resistance, revealed by PFA-100 CT prolongation failure, is correlated to increased plasmatic vWF:RCo levels, reinforcing its particular importance in PFA-100 cartridges performance.

Failure of aspirin to prevent atherothrombosis: potential mechanisms and implications for clinical practice

Aspirin (acetylsalicylic acid) reduces the odds of serious atherothrombotic vascular events and death in a broad category of high risk patients by about one-quarter. The mechanism is believed to be inhibition of thromboxane biosynthesis by inactivation of platelet cyclo-oxygenase-1 enzyme. However, aspirin is not that effective; it still fails to prevent the majority of serious vascular events. Mechanisms that may account for the failure of aspirin to prevent vascular events include non-atherothrombotic causes of vascular disease, non-adherence to aspirin therapy, an inadequate dosage, alternative "upstream" pathways of platelet activation (e.g. via stimulation of the ADP, collagen or thrombin receptors on platelets), aspirin-insensitive thromboxane biosynthesis (e.g. via monocyte cyclo-oxygenase-2), or drugs that interfere with the antiplatelet effects of aspirin. Genetic or acquired factors may further modify the inhibitory effects of aspirin on platelets (e.g. polymorphisms involving platelet-associated proteins, increased platelet turnover states). Identification and treatment of the potential causes of aspirin failure could prevent at least another 20% of serious vascular events (i.e. over and above those that are currently prevented by aspirin). There is currently no role for routine laboratory testing to measure the antiplatelet effects of aspirin. Clinicians should ensure that patients at high risk of atherothrombosis (>3% risk over 5 years) are compliant with aspirin therapy and are taking the correct dosage (75-150 mg/day). Patients who cannot tolerate aspirin, are allergic to aspirin, or have experienced recurrent serious atherothrombotic events whilst taking aspirin, should be treated with clopidogrel, and patients with acute coronary syndromes benefit from the combination of clopidogrel plus aspirin. Future research is required to standardize and validate laboratory testing of the antiplatelet effects of aspirin and to identify treatments that can both improve these laboratory measures and reduce the risk of future atherothrombotic events.

Evaluation and medical treatment of peripheral arterial disease

PURPOSE OF REVIEW

This review reports various advances in the evaluation and medical management of patients with peripheral arterial disease (PAD) in the last 1 to 2 years.

RECENT FINDINGS

Several community surveys have clearly highlighted the fact that despite being a very highly prevalent disease, physicians underdiagnose and undertreat PAD. This led to the Executive Committee of the Prevention of Atherothrombotic Disease Network to issue a "call to action," citing critical issues in PAD detection and management.

SUMMARY

PAD affects more than 27 million people in North America and Europe, and the prevalence of this disease continues to increase as the population ages. This disease has significant adverse effects on the quality of life and survival, with mortality as high as 30% in 5 years and 50% in 10 years. Although surgical, endovascular, and medical therapies for atherosclerosis in general, and PAD specifically, continue to be developed, there appears to be considerable room for improvement in physician adoption of proven effective therapies, such as cholesterol-lowering therapies and blood pressure management. Additionally, new therapies, such as gene transfer and cell therapy, are under development for this population.

PFA-100 and flow cytometry: can they challenge aggregometry to assess antiplatelet agents, other than GPIIbIIIa blockers, in coronary angioplasty?

INTRODUCTION

Platelet response to inhibitors varies widely, leading to a higher risk of abrupt closure events in insufficiently treated-coronary heart disease patients. The aim of this study was to compare, in patients under various antiplatelet regimens, three platelet function assays: aggregometry, PFA-100 and flow cytometry. These assays stand for available tests, as "ready-to-use" device (PFA-100) and sophisticated assay (cytometry). We chose the setting of percutaneous coronary intervention as a standardized procedure to determine which test was appropriate to detect the effect of (1) an aspirin bolus in patients under long-term aspirin treatment, and (2) ticlopidin in case of stent implantation.

METHODS

Fifty patients under oral aspirin treatment were randomized to receive a bolus of 500 mg aspirin before angioplasty (n=25). Ticlopidin was given at a 500 mg loading dose in the case of stent implantation (n=38). Platelet function was assessed before, at 2 and 24 h after angioplasty.

RESULTS

Considering aspirin antiplatelet effect, the following was observed: (1) a lack of further inhibition after the bolus whatever assay was used and (2) a disagreement between aggregometry and PFA-100 to classify patients as being poor or good aspirin responders (kappa were 0.11 and 0.28 between ADP 4 or 6 microM aggregation, respectively, and PFA-100). Another finding was the good performance of flow cytometry, which evaluated GPIIbIIIa activation, and aggregometry, to detect ticlopidin the day after the loading dose. In contrast, PFA-100 was insensitive to ticlopidin.

CONCLUSION

Current assays are not interchangeable to monitor antiplatelet treatment in daily practice.