In vitro bleeding test with PFA-100-aspects of controlling individual acetylsalicylic acid induced platelet inhibition in patients with cardiovascular disease

Frequency of Aspirin Resistance in Ischemic Stroke Patients and Healthy Controls from Colombia

Objective

To evaluate the aspirin resistance prevalence in patients with previous ischemic cerebrovascular disease undergoing aspirin therapy for secondary prevention.

Materials and Methods

Three hundred fifty patients presenting ischemic strokes and 100 healthy controls under aspirin treatment were evaluated using the optic platelet aggregation test.

Results

Aspirin resistance was found in 7.4% of the patients with ischemic stroke and 4% of controls. Aspirin resistance was associated with stroke recurrence in univariate analysis (p = 0.004). Aspirin resistance was not associated with smoking, diabetes, or hypercholesterolemia.

Conclusion

Aspirin resistance is present in Colombian patients with ischemic stroke as well as in healthy controls.

The in vitro effect of the new antithrombotic drug candidate ALX-0081 on blood samples of patients undergoing percutaneous coronary intervention

Compound ALX-0081 is a bivalent humanised Nanobody® that binds the A1-domain of von Willebrand factor (VWF) with high affinity. Consequently, it can block the interaction between VWF and its platelet-receptor- glycoprotein Ib, which leads inevitably to formation of arterial thrombi. It was the objective of this study to assess the in vitro effects of ALX-0081 on platelet adhesion and aggregation in coronary artery disease (CAD) patients to determine the optimal concentration of ALX-0081 and the effect of co-medication. We included nine CAD patients, who were scheduled for elective percutaneous coronary intervention (PCI), and 11 healthy volunteers. At admission all patients received aspirin, clopidogrel and heparin. Blood was drawn 24 hours (h) before and 1 h after start of the PCI procedure and was subsequently spiked with different concentrations of ALX-0081 or buffer. The efficacy of ALX-0081 was assessed by in vitro experiments: flow chamber experiments, ristocetin-induced platelet aggregation (RIPA), and the platelet function analyser (PFA-100TM). VWF levels in CAD patients were significantly higher than in healthy controls. During PCI VWF levels did not rise. In all in vitro experiments, ALX-0081 led to complete inhibition of platelet adhesion and aggregation. However, the required effective concentration was higher in patients than in controls and was related to plasma VWF levels. In conclusion, ALX-0081 is able to completely inhibit in vitro platelet adhesion and aggregation in CAD patients scheduled for elective PCI. The efficacy of ALX-0081 is not influenced by PCI or co-medication. However, due to higher VWF levels in CAD patients a higher effective concentration of ALX-0081 was required than in healthy individuals.

Platelets as Predictors of Vascular Risk: Is There a Practical Index of Platelet Activity?

Activated platelets play a role in the pathogenesis of coronary heart disease (CHD). Following activation, platelets change shape, aggregate, and release several bioactive substances. The aim of this review is to identify if there is a simple and cost-effective method that indicates platelet activation and predicts the risk of CHD and vascular events. The rationale for identifying high-risk patients is to reduce their risk of vascular events by administering appropriate and effective antiplatelet treatment, like aspirin, clopidogrel, or combination regimens. Many laboratory tests estimating platelet activity have been described. Some are relatively simple, such as spontaneous or agonist-induced platelet aggregation. Other tests include measuring the mean platelet volume (MPV) or plasma soluble P-selectin levels. Some more complex tests include flow cytometry to determine platelet GP Ilb/Illa receptors, platelet surface P-selectin, plateletmonocyte aggregates, and microparticles. Only few prospective studies assessed the predictive value of platelet activation in healthy individuals. Although the MPV seems an 'easy method, there are insufficient data supporting its ability to predict the risk of a vascular event in healthy adults. Platelet aggregation, in whole blood or in platelet-rich plasma was not consistently predictive of vascular risk. Soluble P-selectin measurement is a promising method but it needs further evaluation. Flow cytometry methods are costly, time-consuming, and need specialized equipment. Thus, they are unlikely to be useful in estimating the risk in large numbers of patients. There is as yet no ideal test for the detection of platelet activation. Each currently available test has merits and disadvantages. Simple methods such as the MPV and the determination of platelet release products need further evaluation.

[Acetylsalicylic acid non-responders after ischemic insult in geriatric patients]

BACKGROUND

The cardiovascular and cerebral ischemic risk is defined as the risk of suffering a thromboembolic event. The common secondary prophylaxis is still the use of acetylsalicylic acid (ASA). The usual daily dose is 100 mg. Efficacy concerning platelet aggregation is not routinely checked.

MATERIAL AND METHODS

In this study, patients taking 100 mg aspirin daily (orally) were examined after admittance to the stroke unit due to a stroke or stroke recurrence. Platelet aggregation was performed using a Platelet Function Analyser (PFA 100).

RESULTS

A total of 71 patients were examined, 53(73%) had experienced a primary ischemic insult, and 18(25.4%) stroke recurrence. Patients with prolonged closure time in the collagen/epinephrine cell (normal range 85-165 s) were classified as responders to ASA, while 14 (19.7%) were classified as non-responders.

CONCLUSION

It remains open whether the secondary prophylaxis or a more effective inhibition of platelet aggregation results in the improved protection against a future event.

Genetics of platelet reactivity in normal, healthy individuals

BACKGROUND

The Platelet Function Analyzer-100 (PFA-100) is widely used to measure platelet reactivity in whole blood under high shear.

OBJECTIVE

To characterize the genetic component of platelet reactivity among normal individuals, using the PFA-100.

METHODS

We compared baseline platelet reactivity with sex, age, platelet count, hematocrit, plasma von Willebrand factor antigen (VWF:Ag), and alleles of seven candidate genes: integrin subunits alpha2 (ITGA2) and beta3 (ITGB3), platelet glycoproteins GPIbalpha (GP1BA) and GPVI (GP6), purinogenic receptors (P2RY1 and P2RY12) and cyclooxygenase-1 (COX1).

RESULTS

Based on linear and logistic regression models, we report an inverse correlation between baseline closure time (CT) initiated by collagen plus epinephrine (CEPI) and plasma VWF:Ag level, ITGA2 807T and P2RY1 893C, and an inverse correlation between baseline CT initiated by collagen plus adenosine diphosphate (CADP) and P2RY1 893C or GP1BA -5C.

CONCLUSIONS

These results indicate that genetic polymorphisms in ITGA2 and P2RY1 combine with plasma VWF:Ag levels to modulate baseline platelet reactivity in response to collagen plus EPI, while genetic differences in P2RY1 and GP1BA significantly effect platelet responses to collagen plus ADP. Our results demonstrate that the PFA-100 can be used to evaluate the effects of genetic predictors of platelet function.

Prevalence of Platelet Dysfunction and Abnormal Coagulation: Results of a Population-Based Study

The prevalence of impairments in the hemostatic process is unknown in acutely ill people. Data on hemostasis (PFA 100®) and the coagulation cascade of 1015 people are presented here, establishing a cohort of unselected emergency patients in a population-based approach. A high prevalence of reduced platelet function (38%) was found, which was more frequent than expected. In contrast, there was a lower prevalence (20%) of abnormal plasmatic coagulation, which was almost always explained by medication, whereas medication could not predict abnormal platelet function. Moreover, a history of disproportionate bleeding did not correlate well with abnormal platelet or coagulation factor function and could not substitute for a screening in this setting. The effect of acetylsalicylic acid (ASA) on PFA-closure time was frequently missing (34%), indicating a considerable prevalence of ASA nonresponse among the study population. These data should be applicable in similar settings. The high prevalence of unexpectedly abnormal platelet function in acute illness as well as the high prevalence of possible ASA nonresponders suggests a functional platelet assay to be effective in screening certain subpopulations of emergency patients.

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.

Measuring antiplatelet drug effects in the laboratory

This review discusses the advantages and disadvantages of currently available tests for the monitoring of antiplatelet therapy (especially aspirin and clopidogrel). Many tests of platelet function are now available for clinical use, and some of these tests have been shown to predict clinical outcomes after antiplatelet therapy. However, in most of these studies, the number of major adverse clinical events was low. No published studies address the clinical effectiveness of altering therapy based on the results of monitoring antiplatelet therapy. Therefore, the correct treatment, if any, of "resistance" to antiplatelet therapy is unknown and, other than in research trials, monitoring of antiplatelet therapy in patients is not generally recommended. A clinically meaningful definition of "resistance" to antiplatelet drugs needs to be developed, based on data linking drug-dependent laboratory tests to clinical outcomes in patients.

Heparin therapy leads to platelet activation and prolongation of PFA-100 closure time

BACKGROUND

Heparin is used in the treatment of venous and arterial thromboembolic events, including unstable angina. Once unfractionated heparin is discontinued during the acute phase of unstable angina, it has been demonstrated that the disease process may be reactivated within hours. It is hypothesized that this reactivation may be a result of direct platelet activation by heparin that can linger even after the drug itself has been stopped. Prior studies have shown that heparin can either increase or decrease platelet activation. More recent studies have also shown conflicting effects of unfractionated heparin on PFA-100 testing.

METHODS

We studied the in-vitro effects of unfractionated heparin on platelet function and PFA-100 testing. Unfractionated heparin was incubated with whole blood taken from 18 healthy volunteers. Platelet activation and aggregation was assessed with and without the presence of heparin.

RESULTS

Platelet aggregation and activation were increased in the presence of heparin. Unfractionated heparin also significantly prolonged collagen/adenosine diphosphate closure time but did not affect collagen/epinephrine closure time.

CONCLUSIONS

Unfractionated heparin leads to direct platelet activation and increases platelet aggregation in vitro. Unfractionated heparin causes prolongation of the collagen/adenosine diphosphate closure time in PFA-100 testing, possibly as a result of direct binding to von Willebrand factor in solution and interference with von Willebrand factor-glycoprotein Ib binding.

Aspirin resistance in secondary stroke prevention

BACKGROUND

We investigated the platelet function in stroke patients treated with aspirin [acetylsalicylic acid (ASA)] for secondary stroke prevention during a follow-up period of 1 year.

METHODS

In this prospective study 291 patients with first initiated aspirin therapy (300 mg/day) for secondary stroke prevention were included. Platelet aggregation measurements were performed 24 h, 3, 6, and 12 months after starting medication.

RESULTS

Twenty-one of 291 patients (7.2%) were identified as primary ASA-non-responders (initial insufficient platelet inhibition) and 4.1% as secondary ASA-non-responders (insufficient platelet inhibition during follow-up). There were no significant differences between ASA-responders and ASA-non-responders concerning age, gender, risk factors, and stroke characteristics.

CONCLUSION

Aspirin resistance in stroke patients is not uncommon. The clinical usefulness of routine platelet function tests needs to be proved by further trials.

Assessment of a platelet function analyser in horses: reference range and influence of a platelet aggregation inhibitor

The objective of this study was to assess whether a new human platelet function analyser (the PFA-100) could be used to evaluate platelet function in horses and detect acetylsalicylic acid (ASA)-induced platelet dysfunctions. Citrated blood samples from 40 healthy horses were processed to obtain reference values for closure time (CT) using cartridges with collagen-ADP (CT-ADP) and collagen-epinephrine (CT-EPI) as platelet agonists. In addition, CT-ADP and CT-EPI were also measured before and 24 h after oral ASA administration in another 12 healthy horses. The sensitivity and specificity of the test were also determined. In normal horses, means+/-SD value for CT-ADP was 85.1+/-13.1 s (median, 82 s), and CT-EPI ranged from 158 to >300 s (median 291 s). Calculated reference ranges were 60.5-115.9 s and 158.5->300 s for CT-ADP and CT-EPI, respectively. Administration of ASA significantly (P<0.001) prolonged CT-ADP values from 91.0+/-13 to 113.5+/-14.4 s, and CT-EPI values were also significantly (P<0.008) prolonged after ASA administration. Sensitivity and specificity results for ADP cartridges showed that a prolonged CT value would be highly suggestive of a platelet aggregation inhibition. In conclusion, ADP cartridges can be used in horses to assess primary haemostasis and may be a valuable test for the detection of platelet aggregation inhibition.

A Critical Appraisal of the Phenomenon of Aspirin Resistance

Aspirin is the mainstay antiplatelet treatment in patients with high risk of cardiovascular atherothrombotic events, and its beneficial effect is documented in several clinical trials. Nevertheless, the effectiveness of aspirin has been questioned by the emergence of the concept of 'aspirin resistance' (AR). This phenomenon, although lacking a precise definition, covers the fact that some patients do not exhibit the expected platelet inhibition by use of various techniques for measuring platelet function. In this critical review, we evaluate the methods used for measuring AR. We will discuss the available data regarding the prevalence and the clinical importance of the phenomenon. Finally, the potential mechanisms underlying AR are considered.

Prevalence of aspirin resistance measured by PFA-100

BACKGROUND

Aspirin protects from cardiovascular events. However, a number of patients who take this drug suffer events, probably due to aspirin resistance. Our objective was to determine the prevalence of aspirin resistance in patients taking this drug and to test if resistance is related to different variables.

METHODS

Platelet function was studied in 113 patients (90 men) aged 63+/-9 (80 with stable ischaemic heart disease) who took aspirin (100 to 300 mg/day). By a platelet function analyzer, called PFA-100, the epinephrine closure time was studied. We also analysed the possible relationship between epinephrine closure time and the following variables: total cholesterol, LDL, HDL cholesterol, total/HDL cholesterol, triglycerides, lipoprotein(a), and C reactive protein. The possible association between aspirin resistance (epinephrine closure time <161 s) and different variables was also analyzed with the SPSS statistical package. Results are expressed in median (interquartile range).

RESULTS

Aspirin resistance was found in 32% of cases. Ischaemic heart disease, smoking habit, and treatment with statins were associated with a significantly greater percent of resistance (p=0.049, 0.009, and 0.043, respectively). Patients with aspirin resistance had higher levels of total/HDL cholesterol: 4.46 (3.76-5.55) vs. 3.97 (3.20-4.75) (p = 0.023); and lipoprotein(a): 57.2 (24.8-85.0) mg/dl vs. 13.1 (3.7-38.0) mg/dl (p = 0.007).

CONCLUSIONS

Aspirin resistance is frequent and easily detected by PFA-100. It occurs more frequently in smokers. A mild association is found with ischaemic heart disease, some lipids, and treatment with statins. Our results support the applicability of this method to clinical practice.

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.