Aspirin resistance: a revival of platelet aggregation tests?

A brief review on high on-aspirin residual platelet reactivity

Although aspirin is effective in secondary prevention in coronary heart disease, new thromboembolic events in patients on aspirin are frequently seen. In trials on aspirin-treated patients, platelet function tests have revealed large variability in platelet aggregation. This phenomenon has been named aspirin resistance, aspirin non-responsiveness or high-on-aspirin residual platelet reactivity. The mechanism of aspirin antiplatelet effect is due to the inhibition of cyclooxygenase-1 enzyme in platelets. In some trials, almost all patients on aspirin have a very low level of serum thromboxane B2, indicating that the measured platelet reactivity in aspirin-treated patients might be due to platelet activation via other pathways, such as ADP or thrombin. The prevalence of real aspirin resistance seems to be very low, and probably the term "high-on-aspirin residual platelet reactivity" should be preferred to describe this phenomenon.

The clinical utility of Multiplate analyser measurement in platelet function testing following stroke and transient ischaemic attack

BACKGROUND

Platelet responsiveness to aspirin in people with cerebrovascular disease is poorly understood.

OBJECTIVES

To determine: (i) normal reference range, imprecision and reproducibility of the Multiplate instrument in healthy volunteers naive to aspirin; (ii) imprecision and reproducibility of the Multiplate instrument in acute stroke and transient ischaemic attack (TIA); (iii) the relationship between aspirin responsiveness and clinical outcome.

MATERIALS AND METHODS

We evaluated platelet function response to three agonists [Adenosine Diphosphate (ADP), Arachidonic Acid (AA), Collagen (Col)] using the Mulitplate platelet function analyser in a two-phase pilot study. In phase 1, we recruited healthy volunteers to determine the normal reference range and imprecision of the Multiplate instrument. In phase 2, we assessed platelet function in acute stroke or TIA patients presenting to hospital. These patients were bled within 24 h of presentation and between 12 and 24 h after ≥75 mg dose of Aspirin. Patients were followed up to 1 yr to assess mortality and recurrent cardiovascular event.

RESULTS

Overall, 29 healthy volunteers and 81 stroke/TIA patients were recruited. On assessing components of variance, Multiplate testing is reproducible and precise in volunteers and stroke/TIA patients. In stroke patients receiving aspirin, Bland-Altman plots show initial day 1 measurement provided a reliable measure of continuing response to aspirin at day 3. We defined one-third of patients as aspirin resistant [31.8% (95% CI: 22.1%-42.8%)] using cut off mean aggregation of ≥23.08% for AA and mean aggregation of ≥80.76% for ADP.

CONCLUSION

The Multiplate device gives reproducible, precise results in volunteers and stroke/TIA patients.

Antiplatelet therapy: aspirin resistance and all that jazz!

Platelets play a crucial role in the pathogenesis of atherosclerosis, thrombosis, and stroke. Aspirin used alone or in combination with other antiplatelet drugs has been shown to offer significant benefit to patients at high risk of vascular events. Resistance to the action of aspirin may decrease this benefit. Aspirin resistance has been defined by clinical and/or laboratory criteria; however, detection by laboratory methods prior to experiencing a clinical event will likely provide the greatest opportunity for intervention. Numerous laboratory methods with different cutoff points have been used to evaluate the resistance. Noncompliance with aspirin treatment has also confounded studies. A single assay is currently insufficient to establish resistance. Combinations of results to confirm compliance and platelet inhibition may identify "at-risk" individuals who truly have aspirin resistance. The most effective strategy for managing patients with aspirin resistance is unknown; however, studies are currently underway to address this issue.

Aspirin prophylaxis for the prevention of thrombosis: expectations and limitations

Platelets play a very important role in the pathogenesis of acute vascular events leading to thrombosis of the coronary and cerebral arteries. Blockage of these arteries leading to regional ischemia of heart and brain tissues precipitate heart attacks and stroke. Acetyl salicylic acid (Aspirin) has been the drug of choice for over half a century for the primary and secondary prophylaxis of thrombotic events. In spite of its extensive use as an antiplatelet drug for the prevention of vascular thrombosis, there is considerable concern about the degree of protection it offers, to patients under aspirin therapy. In this paper, we explain the phenomenon of aspirin resistance, discuss the limitations of aspirin therapy, and suggest methods to monitor "at-risk" individuals. Ability to monitor and determine at risk patients will provide opportunities for the clinicians to customize antiplatelet therapies.

Variability of platelet indices and function: acquired and genetic factors

Each individual has an inherent variable risk of bleeding linked to genetic or acquired abnormal platelet number or platelet dysfunction. In contrast, it is less obvious that the variability of platelet phenotypes (number, mean platelet volume, function) may contribute to the variable individual risk of thrombosis. Interindividual variability of platelet indices or function may be either due to acquired factors, such as age, sex, metabolic variables, smoke, dietary habits, and ongoing inflammation, or due to genetic factors. Acquired variables explain a small portion of the heterogeneity of platelet parameters. Genetic factors, instead, appear to play a major role, although a consistent portion of such a genetic variance has not yet been attributed to any specific genetic factor, possibly due to the high number of DNA loci potentially involved and to the limited effect size of each individual SNP. A portion of variance remains thus unexplained, also due to variability of test performance. A major contradiction in present platelet knowledge is, indeed, the difficulty to reconcile the universally accepted importance of platelet indices or function and the lack of reliable platelet parameters in cardiovascular risk prediction models. Trials on antiplatelet drugs were generally designed to select a homogeneous sample, whose results could be applied to an "average subject," tending to exclude the deviation/extreme values. As the current indications for antiplatelet treatment in primary or secondary prevention of ischemic vascular disease still derive from the results of such clinical trials where platelet function and its variability was not investigated, we cannot at present rely upon any current platelet test to either initiate, or monitor, or modify or stop treatment with any antiplatelet drug. Evidence is, however, increasing that traditional platelet aggregometry and other more recently developed platelet function assays could be useful to optimize antiplatelet therapy and to predict major adverse cardiac events.The observation of interindividual differences in platelet response to antiplatelet drugs has enlarged the spectrum and the possible clinical relevance of the variability of platelet indices or function. The development of "personalized medicine" will benefit from the concepts discussed in this chapter.

Aspirin resistance and platelet turnover: a 25-year old issue

The evidence of an incomplete inhibition of platelet function by aspirin, despite therapeutic doses of the drug proved to be clinically effective are employed, was first reported in the '80s, in the frame of studies devoted to platelet turnover. Because inhibition of platelet aggregation by aspirin is irreversible, the return after an interval of time of the ability to form thromboxane by platelets in circulating blood should reflect the entry into the circulation of platelets whose cyclooxygenase activity has not been affected by aspirin. Based on this concept, the possibility of monitoring the entry of newly formed platelets into the circulation after aspirin ingestion was documented by measuring the return of thromboxane biosynthesis by platelets challenged in vitro by pairs of aggregating agents. The data obtained showed that platelets with intact cyclooxygenase activity could be detected into the circulation of control individuals as early as 4-6 h after aspirin ingestion, and at shorter time intervals in diabetic angiopathy. In the latter setting,the data allowed to conclude that "schedules of aspirin which may suffice in normals are not effective in patients with diabetic angiopathy, presumably because these patients have a high rate of entry of new platelets into the circulation".

Functional testing methods for the antiplatelet effects of aspirin

At antiplatelet doses of 75–325 mg/day, aspirin irreversibly inhibits the platelet cyclooxygenase (COX)-1-dependent thromboxane A2 (TXA2) formation. This is the pharmacological mode of action of aspirin, and it can be predicted that if aspirin does not inhibit COX-1 sufficiently, patients will not benefit from its antiplatelet effects. A pharmacodynamic failure of aspirin occurs in 1–2% of patients. The vast majority of atherothrombotic events in patients treated with aspirin result from mechanisms that are dependent on residual (non-COX-1-dependent) platelet reactivity. Global tests of platelet activation in vitro may identify patients with high residual platelet reactivity but are not sufficiently specific to test the pharmacological effect of aspirin. A further problem is the absence of standardized normal ranges for many assays and the fact that different equipment measures different signals, which are also influenced by the agonist and the anticoagulant used. Similar considerations apply for the determination of platelet-derived biomarkers such as circulating P-selectin, soluble CD40 ligand and others. The direct measurement of inhibition of thromboxane-forming capacity is the most specific pharmacological assay for aspirin. However, there is no linear correlation between inhibition of TXA2 formation and inhibition of platelet function. Measurement of urinary levels of the TXB2 metabolite, 11-dehydro-thromboxane B2, represents an index of TXA2 biosynthesis in vivo, but is also sensitive to other cellular sources of TXA2. One general problem of all assays is the relationship with clinical outcome, which is still unclear. Monitoring aspirin treatment by testing platelet function or measuring biomarkers in clinical practice should not be recommended until a clear relationship for the predictive value of these assays for clinical outcome has been established.

Antiplatelet therapy in percutaneous coronary intervention: a critical review of the 2007 AHA/ACC/SCAI guidelines and beyond

Antiplatelet therapy is a mainstay in the treatment of patients who have undergone percutaneous coronary intervention (PCI). Although the 2007 PCI treatment guidelines were published by the American College of Cardiology, the American Heart Association, and the Society for Cardiovascular Angiography and Interventions, new clinical evidence has emerged, expanding our understanding of antiplatelet use and potentially affecting the treatment guidelines. For example, clinical trial results prompted a Science Advisory to recommend that dual therapy with aspirin and clopidogrel be used for longer periods-up to 1 year in patients who receive bare metal stents and at least 1 year in patients receiving drug-eluting stents. New trial results have also emerged regarding the use of glycoprotein IIb/IIIa antagonists such as abciximab, eptifibatide, and tirofiban. This article reviews the current recommendations for antiplatelet therapy in PCI patients, recent trial results, newly developed agents, ongoing clinical trials, and the future direction of antiplatelet therapy in patients who undergo PCI.

Clopidogrel resistance — The cardiologist's perspective

Platelet activation and aggregation play a major role in the pathogenesis of acute coronary syndromes. While clopidogrel has convincingly been shown to reduce atherothrombotic events in patients with acute coronary syndromes, and following percutaneous coronary interventions (PCI), a significant portion of patients continue to suffer cardiovascular events. A growing body of literature suggests that at least part of this treatment failure can be attributed to resistance to anti-thrombotic treatment in these patients. The purpose of this review is to clarify the current knowledge regarding clopidogrel resistance.

Antithrombotic properties of aspirin and resistance to aspirin: beyond strictly antiplatelet actions

Aspirin is effective in the prevention of cardiovascular events in high-risk patients. The primary established effect of aspirin on hemostasis is to impair platelet aggregation via inhibition of platelet thromboxane A(2) synthesis, thus reducing thrombus formation on the surface of the damaged arterial wall. Growing evidence also indicates that aspirin exerts additional antithrombotic effects, which appear to some extent unrelated to platelet thromboxane A(2) production. Aspirin can reduce thrombin generation with the subsequent attenuation of thrombin-mediated coagulant reactions such as factor XIII activation. Aspirin also acetylates lysine residues in fibrinogen resulting in increased fibrin clot permeability and enhanced clot lysis as well as directly promoting fibrinolysis with high-dose aspirin. The variable effectiveness of aspirin in terms of clinical outcomes and laboratory findings, which has been termed aspirin resistance, may be related to these additional antithrombotic effects that are altered when associated with common genetic polymorphisms such as the Leu33Pro beta(3)-integrin or Val34Leu factor XIII mutations. However, the clinical relevance of these observations is still unclear. Elucidation of the actual impacts of aspirin other than antiaggregation effects could be important in view of the widespread use of this drug in the prevention of thrombotic manifestations of atherosclerosis.

Resveratrol inhibits aggregation of platelets from high-risk cardiac patients with aspirin resistance

Up to 20% of serious vascular events in high-risk vascular patients is attributable to a failure of aspirin (ASA) to suppress platelet aggregation. Resveratrol is a cardioprotective phytoestrogen that can inhibit platelet aggregation in animal models. We hypothesized that resveratrol can also inhibit aggregation of platelets from ASA-resistant (ASA-R) patients. Thus, platelet-rich plasma was isolated from ASA-sensitive (ASA-S) and ASA-R patients (aspirin resistance was defined as higher-than-expected aggregation to collagen and epinephrine [>/=40%] after oral treatment with 100 mg/d ASA). Aggregation to adenosine diphosphate (ADP; 5 and 10 mumol/L), collagen (2 mug/mL), and epinephrine (10 mumol/L) in the absence and presence of resveratrol (10 mol/L) was measured by optical aggregometry. Maximal aggregation to 5 mumol/L ADP was only slightly affected by resveratrol. Similar results were obtained using 10 mumol/L ADP. Maximal aggregation of ASA-R platelets to collagen was significantly decreased by resveratrol, whereas resveratrol had only marginal effects in ASA-S platelets. Similar results were obtained with epinephrine as well. Collectively, resveratrol effectively inhibited collagen- and epinephrine-induced aggregation of platelets from ASA-R patients, which may contribute to its cardioprotective effects in high-risk cardiac patients.

Aspirin resistance, an emerging, often overlooked, factor in the management of patients with coronary artery disease

Aspirin is the most widely used medication in patients with cardiovascular disease. It has had a greater effect on patients with cardiovascular disease than any other drug. With the importance of aspirin now known for decades, it is recently becoming clearer that some patients do not derive as great a benefit from this "wonder drug" secondary to their resistance to its effects. Aspirin resistance, its prevalence, its identification, and how to overcome or avert it with other medications then becomes a central topic of discussion as important, if not more so, than the importance of aspirin itself as a cornerstone in the treatment of patients with cardiovascular disease. This review explores the current understanding of the mechanism of aspirin resistance with regard to its prevalence and the magnitude of its clinical significance. It also examines the therapeutic implications of a diagnosis of aspirin resistance.

Aspirin resistance – does it clinically matter?

A variable responsiveness to acetylsalicylic acid(ASA) is a clinical reality that does not principally differ from variable responses to other kinds of drug treatment in other therapeutic fields. Two questions arise: (i) is any resulting "treatment failure"due to a pharmacological failure of the drug to act and (ii) is any reduced antiplatelet activity to ASA related to the clinical outcome oft he patient?Two major laboratory techniques are available to quantify platelet variability to ASA ex vivo: Measurement of platelet function and measurement of thromboxane formation. Both methods have limitations and did not yet result in a generally accepted definition of a pharmacological ASA "resistance".A "true" pharmacological resistance to ASA exists in selected groups of patients. However, unless more information is available,results from in vitro assays of platelet function should not be over-interpreted. More data from prospective trials are required,predominantly by measuring serum thromboxane formation which is a platelet-specific, ASA sensitive reaction. At this time,there is no reason to change there commended daily maintenance dose of about 100 mg ASA without particular requirements in patients who need coronary protection.

Aspirin "resistance"

A variable responsiveness to antiplatelet drugs is a clinical phenomenon that does not principally differ from other drug treatments in other therapeutic fields. The pharmacological part is to clarify whether a "true" resistance exists in pharmacological terms, i.e., a reduced potency of the compound to work as suggested and to find out the underlying cellular mechanism(s). Two principally different methods of laboratory control for platelet sensitivity to aspirin (ASA) are available: measurement of platelet function (ex vivo) or measurement of inhibition of thromboxane formation. Both methods have limitations and did not yet result in a generally accepted definition of a pharmacological ASA "resistance". The new typological approach of Weber et al. [A.A. Weber, B. Przytulski, A. Schanz, et al., Towards a definition of aspirin resistance: a typological approach. Platelets 13 (2002) 37.] helps to identify different subtypes of ASA resistance in pharmacological terms by combining in vitro aggregometry with thromboxane measurement. Using this method, a "true" pharmacological resistance, associated with a reduced antiplatelet response to ASA and reduced inhibition of thromboxane formation, was found in patients undergoing coronary artery bypass surgery. Platelets of these patients expressed a hitherto unknown isoform of COX-2-COX-2a which might generate a different gene product. In this context, it is interesting that CABG patients express transiently an immunoreactive COX-2 protein with lower molecular weight. Studies on the significance of this finding for ASA resistance are in progress.

Aspirin and clopidogrel resistance: an emerging clinical entity

Antiplatelet therapy is a cornerstone of cardiovascular medicine. Aspirin and clopidogrel have emerged as critical therapies in the treatment of cardiovascular disease. Despite their efficacy, patients on these medications continue to suffer complications. Millions of patients are currently on low-dose antiplatelet therapy but it is unknown how many of these patients are under-treated or on the wrong medication. Aspirin and clopidogrel resistance are emerging clinical entities with potentially severe consequences such as recurrent myocardial infarction, stroke, or death. The mechanism of resistance remains incompletely defined, but there are specific clinical, cellular, and genetic factors that influence therapeutic failure. These factors range from physicians who fail to prescribe these medications despite appropriate indications to polymorphisms of platelet membrane glycoproteins. Rapid and accurate diagnosis of antiplatelet resistance also remains an issue as new bedside tests are developed. By understanding the mechanism of therapeutic failure and by improving the diagnosis of this clinical entity, a new era of individualized antiplatelet therapy may arise with routine measurements of platelet activity in the same way that cholesterol, blood pressure, and blood sugar are followed, thus improving the care for millions of people.

A new point-of-care method for monitoring anti-platelet therapy: application of the cone and plate(let) analyzer

Recent studies suggest that anti-platelet agents are not equally effective in all individuals. We have developed a new method to evaluate the effect of anti-platelet drugs using the cone and plate(let) analyzer (CPA) test. The method is based on the ability of activators to reduce platelet adhesion under flow conditions. Treatment of a blood sample with arachidonic acid (AA) or ADP in vitro significantly decreased platelet deposition to a surface coverage (SC) of 2.1+/-0.4 and 1.3+/-0.6%, respectively, compared with the basic SC of 12.3+/-6.8%. The effect of AA was prevented by aspirin (SC 8.1+/-3.8%) and that of ADP was reduced by 2-methylthio-AMP, a P2Y12 ADP receptor inhibitor (SC 4.8+/-2.0%). Pre-incubation with AA of whole blood samples from untreated healthy volunteers resulted in a marked decline of SC (from SC 9.8+/-2.2 to 0.6+/-0.3%). In contrast, in volunteers treated with 100, 300, and 500 mg aspirin per day, AA (but not ADP) decreased SC only to 3.5+/-1.3, 4.4+/-1.7, and 4.1+/-2.0%, respectively (P<0.001 versus SC with AA before treatment). A good correlation was observed between the modified CPA and aggregometry (R2=0.55). In conclusion, the modified CPA test is a useful tool to evaluate the efficacy of anti-platelet therapy.

Aspirin resistance

Treatment failures occur with any drug and aspirin is no exception. Evidence is growing to indicate that there are subpopulations that do not respond to antithrombotic action of aspirin. The term 'aspirin resistance' has been used to describe a number of different phenomena, including inability of aspirin to: (i) protect against cardiovascular events despite its regular intake; (ii) to affect various laboratory tests, reflecting platelet activity. Research on aspirin resistance yielded interesting results in clinical pharmacology and pharmacogenetics. Future studies will show whether genotyping for polymorphisms might be of value in everyday clinical use of aspirin. Present data indicate that in survivors of recent myocardial infarction or unstable angina, patients receiving coronary artery bypass grafts, as well as in subjects with hypercholesterolemia, aspirin resistance has to be considered when implementing antithrombotic therapy. However, in individual patients the available laboratory tests are of no particular use to predict reliably the clinical outcome or to guide in making therapeutic decision. Prospective clinical trials seem necessary to reach such conclusions.

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.

Unstable angina, stroke, myocardial infarction and death in aspirin non-responders. A prospective, randomized trial. The ASCET (ASpirin non-responsiveness and Clopidogrel Endpoint Trial) design

BACKGROUND

Aspirin is widely used as an antiplatelet drug in patients with coronary heart disease. Despite documented clinical benefit, many patients on aspirin still experience severe cardiovascular events. Several laboratory reports have shown lack of platelet inhibition in 5-40% of aspirin-treated patients, and the term aspirin resistance has been introduced. The clinical relevance of these laboratory findings is, however, still unknown. New antiplatelet drugs have been developed, and the adenosin diphosphate (ADP) receptor inhibitor clopidogrel has at least the same efficacy as aspirin with an acceptable safety profile. Laboratory methods for determination of platelet reactivity and treatment efficacy have been complicated and time consuming. New methodologies, like the PFA-100 system, have made such analyses more suitable for clinical use.

DESIGN

In the ASCET study, 1000 patients with documented coronary heart disease will be randomized to either continued treatment with aspirin 160 mg/d or change to clopidogrel 75 mg/d after initial determination of their platelet reactivity while on aspirin treatment. Clinical endpoints will be recorded for at least 2 years and related to the initial aspirin response.

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.

Pharmacokinetic and pharmacodynamic differences between two low dosages of aspirin may affect therapeutic outcomes

BACKGROUND

Meta-analyses of the prevention of major vascular events by aspirin suggest therapeutic equivalence of all dosages. However, the optimal dosage still remains problematic, and a recent trial found aspirin 160 mg/day to be more effective than 80 mg/day for secondary prevention of ischaemic stroke.

OBJECTIVE

To evaluate two low dosages of aspirin in terms of pharmacokinetics and pharmacodynamics (inhibition of platelet thromboxane generation and urinary excretion of thromboxane and prostacyclin metabolites).

DESIGN AND PARTICIPANTS

A randomised cross-over study was performed in 16 healthy volunteers (9 women and 7 men, 33.8 +/- 5.1 years old) given enteric-coated aspirin 80 or 160 mg/day for 7 days.

METHODS

Plasma concentrations of salicylate and aspirin were measured by high-performance liquid chromatography (HPLC) after both the first and the last dose (days 1 and 7). The usual pharmacokinetic parameters were then derived. Serum thromboxane B2 (TxB2) was measured by radioimmunoassay. The urinary excretion of 11-dehydro-TxB2 and 2,3-dinor-6-keto-prostaglandin F1alpha were measured on 8-hour urine samples by immunoassay after extraction and HPLC separation, both before and after 7 days of drug administration.

RESULTS

With the 160 mg dosage, but not with the 80 mg dosage, higher concentrations of aspirin were found at day 7 compared with day 1. For aspirin 80 mg/day, 24-hour area under the concentration-time curve (AUC24) was similar on days 1 and 7 (569 +/- 339 vs 605 +/- 377 microg. h/L), but increased from 904 +/- 356 microg. h/L on day 1 to 1355 +/- 883 microg. h/L on day 7 with the higher dosage. Similarly, the AUC24 for salicylate was similar on days 1 and 7 with the lower dosage, but significantly increased from day 1 to day 7 after the higher dosage. This paralleled inhibition of serum TxB2 levels (99% vs 95% average inhibition by 160 and 80 mg/day) and of urinary excretion of thromboxane metabolite (77% vs 61% average inhibition by 160 and 80 mg/day), without altering the excretion of prostacyclin metabolite.

CONCLUSIONS

Inhibition of serum TxB2 generation and of thromboxane metabolite urinary excretion by the lower dosage of aspirin, although substantial, still appeared incomplete. The small but significant further increase of serum TxB2 inhibition by the higher dosage was accompanied by an even greater inhibition of urinary excretion. We suggest that in some instances this difference would translate into a greater clinical benefit with the higher aspirin dosage. Our findings may also contribute to better definition of the recent concept of 'aspirin resistance'.

An in vitro model for the detection of reduced platelet sensitivity to acetylsalicylic acid

A discovery of 'aspirin resistance' has prompted the search for fast and reliable methods for the monitoring of antiplatelet efficacy of acetylsalicylic acid (ASA). Our aims were: (1) to evaluate the in vitro model-based method for detecting a reduced platelets' sensitivity to ASA using a point-of-care platelet function analyser PFA-100; and (2) to propose a simple method of data analysis that might be successfully employed to discriminate between 'good' and 'poor' responders to aspirin. Whole blood platelets from healthy volunteers were incubated in vitro with 30 microg/ml ASA (the in vitro method under evaluation) or analysed following a 10-day intake of an average 150 mg ASA (Aspirin Protect) daily (the reference ex vivo method). According to polynomial regression analysis of the bimodally distributed data, the donors with lower ('ASA poor responders') or higher platelet sensitivity to ASA ('ASA good responders') were discriminated at 58.6% of platelet function inhibition. Despite the similar proportions of 'ASA poor responders' (44 versus 41% using the ex vivo and in vitro tests, respectively), 30% of discordant classifications point to a rather unsatisfactory convergence between both methods. Due to a considerable discordance between the in vitro and ex vivo tests of ASA efficacy performed with the use of the PFA-100 system, the former cannot be reliably and interchangeably used for the monitoring of aspirin therapy and the selection of an effective therapeutic ASA dose. A novel approach to data analysis of the distribution of platelet inhibition rates facilitates an evaluation of cut-off points required to discriminate between 'poor responders' and 'good responders' to aspirin.

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.

Mutations within the cyclooxygenase-1 gene in aspirin non-responders with recurrence of stroke

INTRODUCTION

Aspirin is a common antiplatelet drug used in the prevention of ischemic stroke due to its inhibitory effect on platelet cyclooxygenase-1 (Cox-1). Patients can be categorized as either aspirin 'responders' or 'non-responders' depending on whether they are protected against a secondary stroke event or not. In this study, we have searched for variants of the Cox-1 gene that could possibly result in an unblocked and thus, aspirin-resistant Cox-1 enzyme and phenotype.

MATERIALS AND METHODS

The Cox-1 gene was sequenced in 68 patients with recurrent ischemic stroke despite taking aspirin. The genotype distribution of identified variants was determined and compared with healthy control subjects. Mutations that involved amino acid substitutions of the mature Cox-1 molecule were analysed by molecular modelling and functional analysis using whole blood aggregometry.

RESULTS

Fourteen variants of the Cox-1 gene were identified. Seven of the variants involved amino acid substitutions of the Cox-1 molecule. None of the mutations were located near the catalytic site as judged from a three-dimensional model of the human Cox-1. Carriers and non-carriers of one of the mutations behaved similarly when aggregation and granule content release function were studied using collagen, ADP and arachidonic acid as agonists.

CONCLUSION

The results do not support the hypothesis that common variants of the Cox-1 gene results in unblocked Cox-1 molecules in aspirin non-responders.