Association of cyclooxygenase-1-dependent and -independent platelet function assays with adverse clinical outcomes in aspirin-treated patients presenting for cardiac catheterization

Cardiovascular events occur independently of high on-aspirin platelet reactivity and residual COX-1 activity in stable cardiovascular patients

Several studies have indicated that approximately 25 % of patients treated with aspirin exhibit high on-treatment platelet reactivity (HTPR), which is potentially associated with cardiovascular events (CVEs). However, this association is still controversial, since the mechanisms by which HTPR contributes to CVEs remain unclear and a no standardised definition of HTPR has been established. To determine whether HTPR is associated with CVE recurrence and what type of assay would best predict CVE recurrence, we conducted a multicentre prospective cohort study of 592 stable cardiovascular outpatients treated with aspirin monotherapy for secondary prevention. Their HTPR was determined by arachidonic acid- or collagen-induced aggregation assays using two different agonist concentrations. Residual cyclooxygenase (COX)-1 activity was assessed by measuring serum thromboxane (TX)B2 or urinary 11-dehydro TXB2. Shear-induced platelet thrombus formation was also examined. We followed all patients for two years to evaluate how these seven indexes were related to the recurrence of CVEs (cerebral infarction, transient ischaemic attack, myocardial infarction, unstable angina, revascularisation, other arterial thrombosis, or cardiovascular death). Of 583 patients eligible for the analysis, CVEs occurred in 69 (11.8 %). A Cox regression model identified several classical risk factors associated with CVEs. However, neither HTPR nor high residual COX-1 activity was significantly associated with CVEs, even by applying cut-off values suggested in previous reports or a receiver-operating characteristic analysis. In conclusion, recurrence of CVEs occurred independently of HTPR and residual COX-1 activity. Thus, our findings do not support the use of platelet or COX-1 functional testing for predicting clinical outcomes in stable cardiovascular patients.

MRP4 (ABCC4) as a potential pharmacologic target for cardiovascular disease

This review focuses on multidrug resistance protein 4 (MRP4 or ABCC4) that has recently been shown to play a role in cAMP homeostasis, a key-pathway in vascular biology and in platelet functions. In vascular system, recent data provide evidence that inhibition of MRP4 prevents human coronary artery smooth muscle cell proliferation in vitro and in vivo, as well as human pulmonary artery smooth muscle cell proliferation in vitro and pulmonary hypertension in mice in vivo. In the heart, MRP4 silencing in adult rat ventricular myocytes results in an increase in intracellular cAMP levels leading to enhanced cardiomyocyte contractility. However, a prolonged inhibition of MRP4 can promote cardiac hypertrophy. In addition, secreted cAMP, through its metabolite adenosine, prevents adrenergically induced cardiac hypertrophy and fibrosis. Finally, MRP4 inhibition in platelets induces a moderate thrombopathy. The localization of MRP4 underlines the emerging concept of cAMP compartmentalization in platelets, which is a major regulatory mechanism in other cells. cAMP storage in platelet dense granules might limit the cAMP cytosolic concentration upon adenylate cyclase activation, a necessary step to induce platelet activation. In this review, we discuss the therapeutic potential of direct pharmacological inhibition of MRP4 in atherothrombotic disease, via its vasodilating and antiplatelet effects.

Correlation Between SNPs in Candidate Genes and VerifyNow-Detected Platelet Responsiveness to Aspirin and Clopidogrel Treatment

PURPOSE

Patients with high on-treatment platelet reactivity (HPR) against aspirin or clopidogrel are at increased risk for adverse cardiovascular events. In this study, we explored the predictive value of common SNPs for the on-treatment platelet reactivity (OPR) against aspirin and clopidogrel assessed by VerifyNow assays.

METHODS

This study recruited 286 Han Chinese individuals undergoing antiplatelet treatment, including 159 cases with aspirin only (100 mg/day) and 127 cases with dual therapy (aspirin 100 mg/day plus clopidogrel 75 mg/day) for at least 2 weeks. The OPR against aspirin and clopidogrel were assessed by VerifyNow Aspirin (ARU) and P2Y12 assays (PRU), respectively. Genotyping for the selected 25 SNPs within 11 genes and 2 GWAS loci was carried out by ABI multiplex SNaPshot method.

RESULTS

The results indicated that rs4244285 (CYP2C19) and rs342293 (7q22.3) were significantly associated with PRU value (both P < 0.01). As for the OPR to aspirin, a weak statistical significance was observed in rs5445 (GNB3) (P = 0.049) and rs5758 (TBXA2R) (P = 0.045). After adjusting for the covariates including gender, age and smoking, carriers of allele A of rs4244285 remained as a strong predictor for HPR against clopidogrel.

CONCLUSION

The current study suggests that common SNPs may predict OPR against clopidogrel as assessed by VerifyNow P2Y12, but are less likely to respond against aspirin as assessed by VerifyNow Aspirin.

The prognostic utility of tests of platelet function for the detection of 'aspirin resistance' in patients with established cardiovascular or cerebrovascular disease: a systematic review and economic evaluation

BACKGROUND

The use of aspirin is well established for secondary prevention of cardiovascular disease. However, a proportion of patients suffer repeat cardiovascular events despite being prescribed aspirin treatment. It is uncertain whether or not this is due to an inherent inability of aspirin to sufficiently modify platelet activity. This report aims to investigate whether or not insufficient platelet function inhibition by aspirin ('aspirin resistance'), as defined using platelet function tests (PFTs), is linked to the occurrence of adverse clinical outcomes, and further, whether or not patients at risk of future adverse clinical events can be identified through PFTs.

OBJECTIVES

To review systematically the clinical effectiveness and cost-effectiveness evidence regarding the association between PFT designation of 'aspirin resistance' and the risk of adverse clinical outcome(s) in patients prescribed aspirin therapy. To undertake exploratory model-based cost-effectiveness analysis on the use of PFTs.

DATA SOURCES

Bibliographic databases (e.g. MEDLINE from inception and EMBASE from 1980), conference proceedings and ongoing trial registries up to April 2012.

METHODS

Standard systematic review methods were used for identifying clinical and cost studies. A risk-of-bias assessment tool was adapted from checklists for prognostic and diagnostic studies. (Un)adjusted odds and hazard ratios for the association between 'aspirin resistance', for different PFTs, and clinical outcomes are presented; however, heterogeneity between studies precluded pooling of results. A speculative economic model of a PFT and change of therapy strategy was developed.

RESULTS

One hundred and eight relevant studies using a variety of PFTs, 58 in patients on aspirin monotherapy, were analysed in detail. Results indicated that some PFTs may have some prognostic utility, i.e. a trend for more clinical events to be associated with groups classified as 'aspirin resistant'. Methodological and clinical heterogeneity prevented a quantitative summary of prognostic effect. Study-level effect sizes were generally small and absolute outcome risk was not substantially different between 'aspirin resistant' and 'aspirin sensitive' designations. No studies on the cost-effectiveness of PFTs for 'aspirin resistance' were identified. Based on assumptions of PFTs being able to accurately identify patients at high risk of clinical events and such patients benefiting from treatment modification, the economic model found that a test-treat strategy was likely to be cost-effective. However, neither assumption is currently evidence based.

LIMITATIONS

Poor or incomplete reporting of studies suggests a potentially large volume of inaccessible data. Analyses were confined to studies on patients prescribed aspirin as sole antiplatelet therapy at the time of PFT. Clinical and methodological heterogeneity across studies precluded meta-analysis. Given the lack of robust data the economic modelling was speculative.

CONCLUSIONS

Although evidence indicates that some PFTs may have some prognostic value, methodological and clinical heterogeneity between studies and different approaches to analyses create confusion and inconsistency in prognostic results, and prevented a quantitative summary of their prognostic effect. Protocol-driven and adequately powered primary studies are needed, using standardised methods of measurements to evaluate the prognostic ability of each test in the same population(s), and ideally presenting individual patient data. For any PFT to inform individual risk prediction, it will likely need to be considered in combination with other prognostic factors, within a prognostic model.

STUDY REGISTRATION

This study is registered as PROSPERO 2012:CRD42012002151.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

Serum Brain-Derived Neurotrophic Factor is Related to Platelet Reactivity but not to Genetic Polymorphisms within BDNF Encoding Gene in Patients with Type 2 Diabetes

Background

The aim of this study was to investigate the association between serum concentrations of the brain-derived neurotrophic factor (BDNF), platelet reactivity and inflammatory markers, as well as its association with BDNF encoding gene variants in type 2 diabetic patients (T2DM) during acetylsalicylic acid (ASA) therapy.

Material/Methods

This retrospective, open-label study enrolled 91 patients. Serum BDNF, genotype variants, hematological, biochemical, and inflammatory markers were measured. Blood samples were taken in the morning 2–3 h after the last ASA dose. The BDNF genotypes for selected variants were analyzed by use of the iPLEX Sequenom assay.

Results

In multivariate linear regression analysis, CADP-CT >74 sec (p<0.001) and sP-selectin concentration (p=0.03) were predictive of high serum BDNF. In multivariate logistic regression analysis, CADP-CT >74 sec (p=0.02) and IL-6 concentration (p=0.03) were risk factors for serum BDNF above the median. Non-significant differences were observed between intronic SNP rs925946, missense SNP rs6265, and intronic SNP rs4923463 allelic groups and BDNF concentrations in the investigated cohort.

Conclusions

Chronic inflammatory condition and enhanced immune system are associated with the production of BDNF, which may be why the serum BDNF level in T2DM patients with high platelet reactivity was higher compared to subjects with normal platelet reactivity in this study.

In Vivo and protease-activated receptor-1-mediated platelet activation in patients presenting for cardiac catheterization

Pathways of platelet activation that are not targeted by current antithrombotic therapy may be crucial for the development of ischemic events in patients undergoing coronary angiography. We therefore investigated whether in vivo and thrombin receptor activating peptide (TRAP)-stimulated platelet activation and monocyte-platelet aggregate (MPA) levels can serve as independent risk markers for adverse outcomes in aspirin-treated patients presenting for cardiac catheterization. In vivo and TRAP-stimulated platelet surface P-selectin, activated glycoprotein IIb/IIIa (GPIIb/IIIa) and MPA levels were determined in 682 consecutive patients undergoing cardiac catheterization and in 47 healthy controls. Two-year follow-up data were obtained from 562 patients. In vivo platelet surface P-selectin, activated GPIIb/IIIa and MPA levels were significantly higher in patients with angiographically-proven coronary artery disease than in healthy controls (all p≤0.02). Patients with an acute coronary syndrome (ACS; n=125) had significantly higher levels of in vivo MPA than patients without ACS (n=437; p=0.01). In the overall study population (n=562) the surface expression of P-selectin and activated GPIIb/IIIa, and the levels of MPA in vivo and in response to TRAP were similar in patients without and with subsequent ischemic events (all p>0.05). Similar results were obtained when only patients with angiographically-proven coronary artery disease (n=459), stent implantation (n=205) or ACS (n=125) were analyzed. Receiver-operating characteristic curve analyses did not reveal cut-off values for P-selectin, activated GPIIb/IIIa, and MPA levels for the prediction of ischemic events. In conclusion, in vivo and TRAP-stimulated platelet activation and MPA levels did not predict adverse ischemic outcomes in aspirin-treated patients presenting for cardiac catheterization.

Oxylipid Profile of Low-Dose Aspirin Exposure: A Pharmacometabolomics Study

Background

While aspirin is a well‐established and generally effective anti‐platelet agent, considerable inter‐individual variation in drug response exists, for which mechanisms are not completely understood. Metabolomics allows for extensive measurement of small molecules in biological samples, enabling detailed mapping of pathways involved in drug response.

Methods and Results

We used a mass‐spectrometry‐based metabolomics platform to investigate the changes in the serum oxylipid metabolome induced by an aspirin intervention (14 days, 81 mg/day) in healthy subjects (n=156). We observed a global decrease in serum oxylipids in response to aspirin (25 metabolites decreased out of 30 measured) regardless of sex. This decrease was concomitant with a significant decrease in serum linoleic acid levels (−19%, P=1.3×10⁻⁵), one of the main precursors for oxylipid synthesis. Interestingly, several linoleic acid‐derived oxylipids were not significantly associated with arachidonic‐induced ex vivo platelet aggregation, a widely accepted marker of aspirin response, but were significantly correlated with platelet reactivity in response to collagen.

Conclusions

Together, these results suggest that linoleic acid‐derived oxylipids may contribute to the non‐COX1 mediated variability in response to aspirin. Pharmacometabolomics allowed for more comprehensive interrogation of mechanisms of action of low dose aspirin and of variation in aspirin response.

Aspirin influences megakaryocytic gene expression leading to up-regulation of multidrug resistance protein-4 in human platelets

Aim

The aim of the study was to investigate whether human megakaryocytic cells have an adaptive response to aspirin treatment, leading to an enhancement of multidrug resistance protein-4 (MRP4) expression in circulating platelets responsible for a reduced aspirin action. We recently found that platelet MRP4 overexpression has a role in reducing aspirin action in patients after by-pass surgery. Aspirin enhances MRP4-mRNA levels in rat liver and drug administration transcriptionally regulates MRP4 gene expression through peroxisome proliferator-activated receptor-α (PPARα).

Methods

The effects induced by aspirin or PPARα agonist (WY14643) on MRP4 modulation were evaluated in vitro in a human megakaryoblastic DAMI cell line, in megakaryocytes (MKs) and in platelets obtained from human haematopoietic progenitor cell (HPC) cultures, and in vivo platelets obtained from aspirin treated healthy volunteers (HV).

Results

In DAMI cells, aspirin and WY14643 treatment induced a significant increase in MRP4 and PPARα expression. In human MKs grown in the presence of either aspirin or WY14643, MRP4 and PPARα-mRNA were higher than in control cultures and derived platelets showed an enhancement in MRP4 protein expression. The ability of aspirin to modulate MRP4 expression in MKs and to transfer it to platelets was also confirmed in vivo. In fact, we found the highest MRP4 mRNA and protein expression in platelets obtained from HV after 15 days' aspirin treatment.

Conclusions

The present study provides evidence, for the first time, that aspirin treatment affects the platelet protein pattern through MK genomic modulation. This work represents an innovative and attractive approach, useful both to identify patients less sensitive to aspirin and to improve pharmacological treatment in cardiovascular high-risk patients.

Aspirin response: Differences in serum thromboxane B2 levels between clinical studies

Serum thromboxane B2 (TxB2) is a specific marker of platelet inhibition by aspirin. Yet, TxB2 levels differ by up to 10-fold between some aspirin-treated patient cohorts. This study aimed to identify factors responsible for differences in serum TxB2 between cohorts in the ADRIE study (n = 657) and the BOSTON study (n = 678) of aspirin-treated cardiovascular patients originally tested with different ELISA assays. TxB2 levels were assessed in representative subgroups of the two cohorts (34 samples in BOSTON and 39 in ADRIE) by both ELISAs, as well as liquid chromatography and tandem mass spectroscopy (MS). A multivariate analysis was performed on the whole cohort database to identify determinants of the difference of TxB2 levels between cohorts. There was no systematic bias between the original ELISA TxB2 values and the MS values and the median difference was small, 0.12 ng/ml, thus not explaining the difference between median TxB2 levels in the two study populations (7 and 0.6 ng/ml in the ADRIE and BOSTON studies, respectively). In the combined dataset of the ADRIE and BOSTON cohorts (n = 1342), body mass index, age, gender, aspirin dose, time from aspirin intake to blood draw, NSAID intake, platelet count and C-reactive protein were significantly associated with TxB2 levels. After adjustment for patient characteristics, the difference between cohorts did not decrease. Unexplained differences in serum TxB2 levels in different populations of aspirin-treated cardiovascular patients suggest that further studies are needed to confirm the role of serum TxB2 level as a prognostic factor or rather as a marker of therapeutic observance.

Current antiplatelet treatment strategy in patients with diabetes mellitus

Patients with diabetes mellitus (DM) have accelerated atherosclerosis with an increased risk for atherothrombotic cardiovascular complications. A state of high platelet reactivity and activation, hypercoagulability (prothrombotic state) and a subdued response to standard antiplatelet agents may explain high rate of adverse cardiovascular events in patients with DM. Several antithrombotic treatment strategies have been developed to control the prothrombotic state in patients with DM: dose modification of commonly used agents; use of potent agents; and addition of a third antithrombotic drug (triple therapy) to commonly prescribed dual antiplatelet therapy of aspirin and a P2Y12 inhibitor. The present review aims to provide an overview of the current knowledge on platelet abnormalities in patients with DM, focusing on the challenges and perspectives of antiplatelet treatment strategies in this population.

Increased platelet expression of glycoprotein IIIa following aspirin treatment in aspirin-resistant but not aspirin-sensitive subjects

AIMS

Aspirin is widely used as an anti-platelet agent for cardiovascular prophylaxis. Despite aspirin treatment, many patients experience recurrent thrombotic events, and aspirin resistance may contribute to this. We examined the prevalence of aspirin resistance in a healthy population, and investigated whether the platelet proteome differed in aspirin-resistant subjects.

METHODS

Ninety-three healthy subjects received aspirin 300 mg daily for 28 days. Before and at the end of treatment, urine was taken to determine 11-dehydrothromboxane B2 , and blood was taken to measure arachidonic acid (AA)-induced aggregation of platelet-rich plasma and to interrogate the platelet proteome by mass spectrometric analysis with further confirmation of findings using Western blotting.

RESULTS

In two of the 93 subjects, neither AA-induced aggregation nor urinary 11-dehydrothromboxane B2 was effectively suppressed by aspirin, despite measurable plasma salicylate concentrations, suggesting the presence of true aspirin resistance. Despite no detectable differences in the platelet proteome at baseline, following aspirin a marked increase was seen in platelet glycoprotein IIIa expression in the aspirin-resistant but not aspirin-sensitive subjects. An increase in platelet glycoprotein IIIa expression with aspirin resistance was confirmed in a separate cohort of 17 patients with stable coronary artery disease on long term aspirin treatment, four of whom exhibited aspirin resistance.

CONCLUSIONS

In a healthy population, true aspirin resistance is uncommon but exists. Resistance is associated with an increase in platelet glycoprotein IIIa expression in response to aspirin. These data shed new light on the mechanism of aspirin resistance, and provide the potential to identify aspirin-resistant subjects using a novel biomarker.

Oxidative stress-related mechanisms affecting response to aspirin in diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a major cardiovascular risk factor. Persistent platelet activation plays a key role in atherothrombosis in T2DM. However, current antiplatelet treatments appear less effective in T2DM patients vs nondiabetics at similar risk. A large body of evidence supports the contention that oxidative stress, which characterizes DM, may be responsible, at least in part, for less-than-expected response to aspirin, with multiple mechanisms acting at several levels. This review discusses the pathophysiological mechanisms related to oxidative stress and contributing to suboptimal aspirin action or responsiveness. These include: (1) mechanisms counteracting the antiplatelet effect of aspirin, such as reduced platelet sensitivity to the antiaggregating effects of NO, due to high-glucose-mediated oxidative stress; (2) mechanisms interfering with COX acetylation especially at the platelet level, e.g., lipid hydroperoxide-dependent impaired acetylating effects of aspirin; (3) mechanisms favoring platelet priming (lipid hydroperoxides) or activation (F2-isoprostanes, acting as partial agonists of thromboxane receptor), or aldose-reductase pathway-mediated oxidative stress, leading to enhanced platelet thromboxane A2 generation or thromboxane receptor activation; (4) mechanisms favoring platelet recruitment, such as aspirin-induced platelet isoprostane formation; (5) modulation of megakaryocyte generation and thrombopoiesis by oxidative HO-1 inhibition; and (6) aspirin-iron interactions, eventually resulting in impaired pharmacological activity of aspirin, lipoperoxide burden, and enhanced generation of hydroxyl radicals capable of promoting protein kinase C activation and platelet aggregation. Acknowledgment of oxidative stress as a major contributor, not only of vascular complications, but also of suboptimal response to antiplatelet agents in T2DM, may open the way to designing and testing novel antithrombotic strategies, specifically targeting oxidative stress-mediated mechanisms of less-than-expected response to aspirin.

Personalized antiplatelet and anticoagulation therapy: applications and significance of pharmacogenomics

In recent years, substantial effort has been made to better understand the influence of genetic factors on the efficacy and safety of numerous medications. These investigations suggest that the use of pharmacogenetic data to inform physician decision-making has great potential to enhance patient care by reducing on-treatment clinical events, adverse drug reactions, and health care-related costs. In fact, integration of such information into the clinical setting may be particularly applicable for antiplatelet and anticoagulation therapeutics, given the increasing body of evidence implicating genetic variation in variable drug response. In this review, we summarize currently available pharmacogenetic information for the most commonly used antiplatelet (ie, clopidogrel and aspirin) and anticoagulation (ie, warfarin) medications. Furthermore, we highlight the currently known role of genetic variability in response to next-generation antiplatelet (prasugrel and ticagrelor) and anticoagulant (dabigatran) agents. While compelling evidence suggests that genetic variants are important determinants of antiplatelet and anticoagulation therapy response, significant barriers to clinical implementation of pharmacogenetic testing exist and are described herein. In addition, we briefly discuss development of new diagnostic targets and therapeutic strategies as well as implications for enhanced patient care. In conclusion, pharmacogenetic testing can provide important information to assist clinicians with prescribing the most personalized and effective antiplatelet and anticoagulation therapy. However, several factors may limit its usefulness and should be considered.

Why does aspirin decrease the risk of venous thromboembolism? On old and novel antithrombotic effects of acetyl salicylic acid

It is well established that aspirin, an irreversible inhibitor of platelet cyclooxygenase activity, is effective in secondary prevention of arterial thromboembolic events. The pooled results of the recent randomized, multicenter WARFASA and ASPIRE aspirin trials showed a 32% reduction in the rate of recurrence of venous thromboembolism (VTE) in patients receiving aspirin following VTE. These clinical data support evidence that platelets contribute to the initiation and progression of venous thrombosis and aspirin inhibits thrombin formation and thrombin-mediated coagulant reactions. In addition to the known acetylation of serine 529 residue in platelet cyclooxygenase-1, the postulated mechanisms of aspirin-induced antithrombotic actions also involve the acetylation of other proteins in blood coagulation, including fibrinogen, resulting in more efficient fibrinolysis. This review summarizes current knowledge on the aspirin-induced antithrombotic effects that potentially explain clinical studies showing reduced rates of VTE events in aspirin-treated subjects.

The PlA1/A2 polymorphism of glycoprotein IIIa in relation to efficacy of antiplatelet drugs: a systematic review and meta-analysis

AIM

The PlA1/A2 polymorphism of glycoprotein IIIa (GPIIIa) has been associated with both antiplatelet drug resistance and increased cardiovascular events. The aim of this study was to conduct the first meta-analysis investigating the association between carriage of the PlA2 allele and resistance to currently licensed antiplatelet drugs.

METHODS

Electronic databases (MEDLINE and EMBASE) were searched for all articles evaluating genetic polymorphisms of GPIIIa. For studies where antiplatelet resistance was measured using validated techniques, pooled odds ratios (ORs) were calculated using fixed effects and random effects models.

RESULTS

Sixteen studies were eligible for statistical analysis and included 1650 PlA1 homozygous subjects and 668 carriers of the PlA2 allele. For carriers of the PlA2 allele, OR 0.924 (n = 2318; 95% CI 0.743, 1.151; P = 0.481) was observed for resistance to any antiplatelet drug, OR 0.862 (n = 2085; 95% CI 0.685, 1.086; P = 0.208) for resistance to aspirin and OR 1.429 (n = 233; 95% CI 0.791, 2.582; P = 0.237) for resistance to clopidogrel. In the aspirin cohort, sub-group analysis revealed no statistical association in either healthy subjects or those with cardiovascular disease. PlA2 carriage was marginally associated with aspirin sensitivity using the fixed effects model when identified by the PFA-100 assay (n = 1151; OR 0.743, 95% CI 0.558, 0.989; P = 0.041) but with significant heterogeneity (I(2) = 55%; P = 0.002). Significance was lost with analysis using a random effects model.

CONCLUSIONS

The totality of published data does not support an association between carriage of the PlA2 allele and antiplatelet drug resistance. Significant heterogeneity indicates the need for larger studies using validated and standardized assays.

Younger age, higher body mass index and lower adiponectin concentration predict higher serum thromboxane B2 level in aspirin-treated patients with type 2 diabetes: an observational study

Background

Evidence from the literature suggests diminished acetylsalicylic acid (ASA) treatment efficacy in type 2 diabetes (DM2). High on-aspirin platelet reactivity (HAPR) in DM2 has been linked to poor glycemic and lipid control. However, there are no consistent data on the association between HAPR and insulin resistance or adipose tissue metabolic activity. The aim of this study was to assess the relationship between laboratory response to ASA and metabolic control, insulin resistance and adipokines in DM2.

Methods

A total of 186 DM2 patients treated with oral antidiabetic drugs and receiving 75 mg ASA daily were included in the analysis. Response to ASA was assessed by measuring serum thromboxane B₂ (TXB₂) concentration and expressed as quartiles of TXB₂ level. The achievement of treatment targets in terms of glycemic and lipid control, insulin resistance parameters (including Homeostatic Model Assessment-Insulin Resistance, HOMA-IR, index), and serum concentrations of high-molecular weight (HMW) adiponectin, leptin and resistin, were evaluated in all patients. Univariate and multivariate logistic regression analyses were performed to determine the predictive factors of serum TXB₂ concentration above the upper quartile and above the median.

Results

Significant trends in age, body mass index (BMI), HOMA-IR, HMW adiponectin concentration, C-reactive protein concentration and the frequency of achieving target triglyceride levels were observed across increasing quartiles of TXB₂. In a multivariate analysis, only younger age and higher BMI were independent predictors of TXB₂ concentration above the upper quartile, while younger age and lower HMW adiponectin concentration were predictors of TXB₂ concentration above the median.

Conclusions

These results suggest that in DM2, the most important predictor of HAPR is younger age. Younger DM2 patients may therefore require total daily ASA doses higher than 75 mg, preferably as a twice-daily regimen, to achieve full therapeutic effect. Higher BMI and lower HMW adiponectin concentration were also associated with less potent ASA effect. This is the first study to demonstrate an association of lower adiponectin concentration with higher serum TXB₂ level in patients treated with ASA.

Reappraisal of the clinical pharmacology of low-dose aspirin by comparing novel direct and traditional indirect biomarkers of drug action

BACKGROUND

Even though the acetylation of platelet cyclooxygenase (COX)-1 at serine-529 is the direct mechanism of action of low-dose aspirin, its antiplatelet effect has been characterized using indirect indexes of COX-1 activity.

OBJECTIVES

We performed a clinical study with enteric-coated low-dose aspirin (EC-aspirin), in healthy subjects, to evaluate the effects on the extent and duration of platelet COX-1 acetylation, using a novel proteomic strategy for absolute protein quantification (termed AQUA), as compared with traditional pharmacokinetic and pharmacodynamic parameters.

SUBJECTS AND METHODS

In a phase I, single-arm, open-label study of EC aspirin (100 mg day(-1) ) administered to 24 healthy subjects, we compared, over a 24 h-period on day 1 and 7, % platelet acetylated COX-1 (AceCOX-1) with traditional pharmacokinetic and pharmacodynamics [i.e. serum thromboxane (TX) B2 , platelet function by monitoring CEPI(collagen/epinephrine) closure time (CT) using whole-blood PFA-100 and urinary excretion of 11-dehydro-TXB2 ] parameters.

RESULTS

Acetylation of platelet COX-1 was measurable before detection of aspirin levels in the systemic circulation and increased in a cumulative fashion upon repeated dosing. After the last dose of EC-aspirin, %AceCOX-1, serum TXB2 and CEPI-CT values were maximally and persistently modified throughout 24 h; they averaged 76 ± 2%, 99.0 ± 0.4% and 271 ± 5 s, respectively. EC-aspirin caused 75% reduction in urinary 11-dehydro-TXB2 excretion. After chronic dosing with aspirin, the pharmacokinetics of acetylsalicylic acid was completely dissociated from pharmacodynamics.

CONCLUSIONS

The demonstrated feasibility of quantifying the extent and duration of platelet COX-1 acetylation will allow characterizing the genetic, pharmacokinetic and pharmacodynamic determinants of the inter-individual variability in the antiplatelet response to low-dose aspirin as well as identifying extra-platelet sites of drug action.

Low angle light scattering analysis: a novel quantitative method for functional characterization of human and murine platelet receptors

BACKGROUND

Determinations of platelet receptor functions are indispensable diagnostic indicators of cardiovascular and hemostatic diseases including hereditary and acquired receptor defects and receptor responses to drugs. However, presently available techniques for assessing platelet function have some disadvantages, such as low sensitivity and the requirement of large sample sizes and unphysiologically high agonist concentrations. Our goal was to develop and initially characterize a new technique designed to quantitatively analyze platelet receptor activation and platelet function on the basis of measuring changes in low angle light scattering.

METHODS

We developed a novel technique based on low angle light scattering registering changes in light scattering at a range of different angles in platelet suspensions during activation.

RESULTS

The method proved to be highly sensitive for simultaneous real time detection of changes in size and shape of platelets during activation. Unlike commonly-used methods, the light scattering method could detect platelet shape change and aggregation in response to nanomolar concentrations of extracellular nucleotides. Furthermore, our results demonstrate that the advantages of the light scattering method make it a choice method for platelet receptor monitoring and for investigation of both murine and human platelets in disease models.

CONCLUSIONS

Our data demonstrate the suitability and superiority of this new low angle light scattering method for comprehensive analyses of platelet receptors and functions. This highly sensitive, quantitative, and online detection of essential physiological, pathophysiological and pharmacological-response properties of human and mouse platelets is a significant improvement over conventional techniques.

Pharmacometabolomics reveals that serotonin is implicated in aspirin response variability

While aspirin is generally effective for prevention of cardiovascular disease, considerable variation in drug response exists, resulting in some individuals displaying high on-treatment platelet reactivity. We used pharmacometabolomics to define pathways implicated in variation of response to treatment. We profiled serum samples from healthy subjects pre- and postaspirin (14 days, 81 mg/day) using mass spectrometry. We established a strong signature of aspirin exposure independent of response (15/34 metabolites changed). In our discovery (N = 80) and replication (N = 125) cohorts, higher serotonin levels pre- and postaspirin correlated with high, postaspirin, collagen-induced platelet aggregation. In a third cohort, platelets from subjects with the highest levels of serotonin preaspirin retained higher reactivity after incubation with aspirin than platelets from subjects with the lowest serotonin levels preaspirin (72 ± 8 vs. 61 ± 11%, P = 0.02, N = 20). Finally, ex vivo, serotonin strongly increased platelet reactivity after platelet incubation with aspirin (+20%, P = 4.9 × 10⁻⁴, N = 12). These results suggest that serotonin is implicated in aspirin response variability.

24-hour antiplatelet effect of aspirin in patients with previous definite stent thrombosis

OBJECTIVE

Once-daily aspirin is standard treatment, but recent studies point towards increased platelet function at the end of the dosing interval. Stent thrombosis (ST) has been linked with reduced antiplatelet effect of aspirin, so we investigated if platelet inhibition by aspirin declines through 24 h in patients with previous definite ST. Furthermore, we explored whether increased levels of immature platelets and thrombopoietin are associated with a particularly rapid recovery of platelet function.

METHODS

This case-control study included 50 patients with previous definite ST matched with 100 patients with stable coronary artery disease and 50 healthy volunteers. All participants were on aspirin 75 mg/day mono antiplatelet therapy. Platelet aggregation was measured 1 and 24 h after aspirin intake using platelet aggregometry (Multiplate® Analyzer). Cyclooxygenase-1 activity, platelet activation, immature platelets, and thrombopoietin were measured.

RESULTS

Platelet aggregation increased by 109±150 (arachidonic acid) and 47±155 (collagen) aggregation units per minute from 1 to 24 h after aspirin intake (p-values <0.0001) with corresponding increases in thromboxane B2 (5.6±5.1 ng/ml, p<0.0001) and soluble P-selectin (6.2±15.5 ng/ml, p<0.0001). Platelet aggregation increased equally in all groups, but patients with previous ST displayed the highest levels of platelet aggregation at 24 h (p-values≤0.05) and the highest levels of immature platelets (p<0.01) and thrombopoietin (p<0.0001).

CONCLUSIONS

Platelet inhibition declined significantly during the 24-hour dosing interval in aspirin-treated patients with previous definite ST or stable coronary artery disease and in healthy individuals. Increased levels of immature platelets and thrombopoietin were observed in patients with previous definite ST.

Suboptimal inhibition of platelet cyclooxygenase 1 by aspirin in systemic lupus erythematosus: association with metabolic syndrome

OBJECTIVE

Low-dose aspirin prevents platelet aggregation by suppressing thromboxane A2 (TXA2 ) synthesis. However, in some individuals TXA2 suppression by aspirin is impaired, indicating suboptimal inhibition of platelet cyclooxygenase 1 (COX-1) by aspirin. Because patients with systemic lupus erythematosus (SLE) have increased risk of thrombotic events, many receive aspirin; however, the efficacy of aspirin in SLE has not been determined. We examined the hypothesis that aspirin response is impaired in SLE.

METHODS

We assessed the effect of aspirin by measuring concentrations of the stable metabolite of TXA2 , serum thromboxane B2 (sTXB2 ), before and after treatment with daily aspirin (81 mg) for 7 days in 34 patients with SLE and 36 control subjects. The inability to suppress sTXB2 synthesis to <10 ng/ml represents suboptimal inhibition of platelet COX-1 by aspirin.

RESULTS

Aspirin almost completely suppressed sTXB2 in control subjects to median 1.5 ng/ml (interquartile range [IQR] 0.8-2.7) but had less effect in patients with SLE (median 3.1 ng/ml [IQR 2.2-5.3]) (P = 0.002). A suboptimal effect of aspirin was present in 15% (5 of 34) of the patients with SLE but not in control subjects (0 of 36) (P = 0.023). Incomplete responders were more likely to have metabolic syndrome (P = 0.048), obesity (P = 0.048), and higher concentrations of C-reactive protein (CRP) (P = 0.018).

CONCLUSION

The pharmacologic effect of aspirin is suboptimal in 15% of patients with SLE but in none of the control subjects, and the suboptimal response was associated with metabolic syndrome, obesity, and higher CRP concentrations.

Screening and Primary prevention of Colorectal Cancer: a Review of sex-specific and site-specific differences

Background

Colorectal cancer (CRC) is the second commonest cancer in England. Incidence rates for colorectal adenomas and advanced colorectal neoplasia are higher in men than in women of all age groups. The male-to-female ratio for CRC incidence rates differs for different parts of the large bowel.

Objective

To summarize the current evidence on colorectal screening and prevention, focussing on potential differences in benefits between sexes and colorectal sites.

Methods

(i), We reviewed the evidence from randomized controlled trials (RCTs) of the impact of different screening approaches on CRC incidence and mortality, overall, for each sex separately, and for subsites of the large bowel. (ii) We reviewed studies comparing detection parameters for faecal immunochemical tests for haemoglobin (FIT) with guaiac FOBt (gFOBt). (iii) The role of aspirin in CRC prevention in the general population was reviewed using evidence from RCTs, with specific emphasis on the differences observed between sexes and lesion site.

Results

(i) Our intention-to-treat random-effects meta-analysis showed that once-only flexible sigmoidoscopy (FS) screening performed on average-risk individuals aged 55 + decreased CRC incidence by 18% and mortality by 28%, but sex-specific results were lacking. (ii) Modern quantitative FIT were superior to qualitative gFOBt in average-risk population screening in their ability to discriminate between individuals with and without colorectal neoplasia. Some recent FIT studies suggest varying operating characteristics in men and women. (iii) Evidence of an effect of aspirin on the incidence of CRC (in particular, proximal disease) in both sexes aged 40 and over at average-risk of CRC is emerging.

Conclusions

We encourage researchers of CRC screening and prevention to publish their results by sex where possible. Pilot studies should be undertaken before implementation of quantitative FIT in a national screening programme to establish the appropriate threshold. Finally, individual risk assessment for CRC and non-CRC events, will be necessary to make an informed decision on whether a patient should receive aspirin chemoprevention.

PlaCor PRT measurement of shear-activated platelet aggregate formation in stable patients treated with single and dual antiplatelet therapy

Shear forces play a key role in thrombus formation and shear-based tests may better reflect physiological conditions in vivo compared with agonist-based tests. We evaluated the PlaCor PRT®, a novel platelet reactivity test based on shear-induced platelet aggregation, in patients with stable coronary artery disease (CAD) treated with single (SAPT) and dual antiplatelet therapy (DAPT). We examined 100 patients with multiple risk factors for CAD and/or documented stable CAD: 38 treated with SAPT, aspirin 100 mg qd, 62 treated with DAPT, aspirin 100 mg + clopidogrel 75 mg qd, compared with age- and sex-matched healthy volunteers without antiplatelet therapy (HV, n = 35). Measures of shear-induced platelet aggregation were performed with the PlaCor PRT®. In 25 patients in SAPT, the PlaCor test was also performed before and after a 12-hour-loading dose of clopidogrel 600 mg. The mean ± SD PRT time (seconds) in HV was 78 ± 13 and was significantly lower compared with SAPT (118 ± 16, p = 0.030) and to DAPT patients (242 ± 11, p < 0.0001). A statistically significant difference was also reported between SAPT and DAPT patients (p < 0.0001). After a loading dose of clopidogrel, the PRT time of SAPT patients increased significantly from 112 ± 20 to 254 ± 17, p < 0.0001. 2.7 and 26% of patients were considered as "poor responders" to single and dual antiplatelet therapy, respectively. This study shows that in patients with multiple risk factors for CAD and/or documented stable CAD, SAPT and DAPT play an important role in reducing platelet aggregation mediated by shear forces as evaluated with the novel PlaCor PRT®. Further studies will be required to confirm and assess the extent of these findings in patients with acute coronary syndromes.

Microfluidic assay of platelet deposition on collagen by perfusion of whole blood from healthy individuals taking aspirin

BACKGROUND

Microfluidic devices can create hemodynamic conditions for platelet assays. We validated an 8-channel device in a study of interdonor response to acetylsalicylic acid (ASA, aspirin) with whole blood from 28 healthy individuals.

METHODS

Platelet deposition was assessed before treatment or 24 h after ingestion of 325 mg ASA. Whole blood (plus 100 μmol/L H-d-Phe-Pro-Arg-chloromethylketone to inhibit thrombin) was further treated ex vivo with ASA (0-500 μmol/L) and perfused over fibrillar collagen for 300 s at a venous wall shear rate (200 s(-1)).

RESULTS

Ex vivo ASA addition to blood drawn before aspirin ingestion caused a reduction in platelet deposition [half-maximal inhibitory concentration (IC50) approximately 10-20 μmol/L], especially between 150 and 300 s of perfusion, when secondary aggregation mediated by thromboxane was expected. Twenty-seven of 28 individuals displayed smaller deposits (45% mean reduction; range 10%-90%; P < 0.001) from blood obtained 24 h after ASA ingestion (no ASA added ex vivo). In replicate tests, an R value to score secondary aggregation [deposition rate from 150 to 300 s normalized by rate from 60 to 150 s] showed R < 1 in only 2 of 28 individuals without ASA ingestion, with R > 1 in only 3 of 28 individuals after 500 μmol/L ASA addition ex vivo. At 24 h after ASA ingestion, 21 of 28 individuals displayed poor secondary aggregation (R < 1) without ex vivo ASA addition, whereas the 7 individuals with residual secondary aggregation (R > 1) displayed insensitivity to ex vivo ASA addition. Platelet deposition was not correlated with platelet count. Ex vivo ASA addition caused similar inhibition at venous and arterial wall shear rates.

CONCLUSIONS

Microfluidic devices quantified platelet deposition after ingestion or ex vivo addition of aspirin.

Aspirin exposure reveals novel genes associated with platelet function and cardiovascular events

OBJECTIVES

The aim of this study was to develop ribonucleic acid (RNA) profiles that could serve as novel biomarkers for the response to aspirin.

BACKGROUND

Aspirin reduces death and myocardial infarction (MI), suggesting that aspirin interacts with biological pathways that may underlie these events.

METHODS

Aspirin was administered, followed by whole-blood RNA microarray profiling, in a discovery cohort of healthy volunteers (HV1) (n = 50) and 2 validation cohorts of healthy volunteers (HV2) (n = 53) and outpatient cardiology patients (OPC) (n = 25). Platelet function was assessed using the platelet function score (PFS) in HV1 and HV2 and the VerifyNow Aspirin Test (Accumetrics, Inc., San Diego, California) in OPC. Bayesian sparse factor analysis identified sets of coexpressed transcripts, which were examined for associations with PFS in HV1 and validated in HV2 and OPC. Proteomic analysis confirmed the association of validated transcripts in platelet proteins. Validated gene sets were tested for association with death or MI in 2 patient cohorts (n = 587 total) from RNA samples collected at cardiac catheterization.

RESULTS

A set of 60 coexpressed genes named the "aspirin response signature" (ARS) was associated with PFS in HV1 (r = -0.31, p = 0.03), HV2 (r = -0.34, Bonferroni p = 0.03), and OPC (p = 0.046). Corresponding proteins for the 17 ARS genes were identified in the platelet proteome, of which 6 were associated with PFS. The ARS was associated with death or MI in both patient cohorts (odds ratio: 1.2 [p = 0.01]; hazard ratio: 1.5 [p = 0.001]), independent of cardiovascular risk factors. Compared with traditional risk factors, reclassification (net reclassification index = 31% to 37%, p ≤ 0.0002) was improved by including the ARS or 1 of its genes, ITGA2B.

CONCLUSIONS

RNA profiles of platelet-specific genes are novel biomarkers for identifying patients who do not respond adequately to aspirin and who are at risk for death or MI.

Aspirin Half Maximal Inhibitory Concentration Value on Platelet Cyclooxygenase1 in Severe Type-2 Diabetes Mellitus is not Significantly Different from that of Healthy Individuals

It is implicated that diabetic patients are more resistant to aspirin therapy than patients with other diseases or healthy individuals. We evaluated the inhibitory effects of aspirin on aggregation and the cyclooxygenase activity of platelets of 10 patients with severe type-2 diabetes mellitis (DM) and compared the results with those of healthy individuals. Although platelet aggregation had a tendency to be more resistant to aspirin with the DM group, there was no significant difference in half maximal inhibitory concentration 50 values of aspirin on the cyclooxygenase activity between the patients with DM and healthy individuals. Thus, the residual platelet aggregability uninhibited by aspirin appears to be independent of the cyclooxygenase activity. Since adenosine diphosphate (ADP) receptor blocking almost completely inhibited the residual platelet aggregability, it is suggested that hyperreactivity to ADP is more prevalent in patients with DM.

Platelet activation and inhibition in polycythemia vera and essential thrombocythemia

Persistently enhanced platelet activation has been characterized in polycythemia vera (PV) and essential thrombocythemia (ET) and shown to contribute to a higher risk of both arterial and venous thrombotic complications. The incidence of major bleeding complications is also somewhat higher in PV and ET than in the general population. Although its efficacy and safety was assessed in just 1 relatively small trial in PV, low-dose aspirin is currently recommended in practically all PV and ET patients. Although for most patients with a thrombosis history the benefit/risk profile of antiplatelet therapy is likely to be favorable, in those with no such history this balance will depend critically on the level of thrombotic and hemorrhagic risks of the individual patient. Recent evidence for a chemopreventive effect of low-dose aspirin may tilt the balance of benefits and harm in favor of using aspirin more broadly, but the potential for additional benefits needs regulatory scrutiny and novel treatment guidelines. A clear pharmacodynamic rationale and analytical tools are available for a personalized approach to antiplatelet therapy in ET, and an improved regimen of low-dose aspirin therapy should be tested in a properly sized randomized trial.

Reduction of platelet cytosolic phospholipase A2 activity by atorvastatin and simvastatin: biochemical regulatory mechanisms

Statins have demonstrated effects beyond reducing cholesterol level that may contribute to their clinical benefit, including effects on platelet biochemistry and function.

OBJECTIVES

To explore and compare the antiplatelet effect of two lipophilic statins (atorvastatin and simvastatin) and one hydrophilic statin (pravastatin) concerning: a) collagen-induced platelet aggregation and thromboxane A2 (TXA2) synthesis; b) the additive effect of statins on TXA2 synthesis in platelets treated with a submaximally effective concentration of aspirin and c) the biochemical mechanisms involved.

METHODS AND RESULTS

Washed human platelets were incubated with statins (1-20μM), and stimulated with collagen (1μg/ml) or arachidonic acid (AA) (200μM) and TXB2 was quantified by ELISA. Incubation with simvastatin or atorvastatin reduced (36.2% and 31.0%, respectively) collagen-induced TXB2 synthesis (p<0.05) and platelet aggregation (p<0.001), whereas pravastatin had no effects. Simultaneous incubation with a submaximally effective concentration of aspirin (1μM) and atorvastatin or simvastatin significantly increased the inhibition of TXB2 synthesis by aspirin by 4.4- and 4.1-fold, respectively. Statins did not affect AA-induced TXB2 synthesis, excluding an effect on COX-1/TXA2 synthase activities. Atorvastatin and simvastatin concentration-dependently inhibited the collagen-induced increase in cytosolic calcium and the kinetics of cPLA2 phosphorylation. Lipophilic statins reduced phosphorylation of both ERK1/2 and p38 MAPK, which regulate cPLA2 phosphorylation and calcium movement.

CONCLUSION

We report for the first time a direct downregulation by atorvastatin and simvastatin of platelet cPLA2 activity through effects on calcium and MAPK, which reduce collagen-induced TXA2 synthesis. These mechanisms might contribute to their beneficial effects, even in aspirin-treated patients.

Platelet flow cytometry

Flow cytometry is a powerful and versatile tool which can be used to provide substantial phenotypic data on platelets by yielding quantitative information of their physical and antigenic properties. This includes surface expression of functional receptors, bound ligands, expression of granule components, interaction of platelets with other platelets via aggregation, or interaction with other blood components, such as leukocytes or the plasma coagulation system. Quantitative assessment of these parameters may facilitate the diagnosis of inherited or acquired platelet disorders, assist in the diagnosis of diseases associated with platelet activation, or assist in the monitoring of safety and efficacy of antiplatelet therapy.

Towards personalized medicine based on platelet function testing for stent thrombosis patients

Stent thrombosis (ST) is a severe and feared complication of coronary stenting. Patients who have suffered from ST are usually treated according to the “one-size-fits-all” dosing regimen of aspirin and clopidogrel. Many ST patients show high on-treatment platelet reactivity (HPR) despite this antiplatelet therapy (APT). It has been shown that HPR is a risk factor for major adverse cardiac events. Therefore, ST patients with HPR are at a high risk for recurrent atherothrombotic events. New insights into the variable response to clopidogrel and the advent of stronger P2Y12 inhibitors prasugrel and ticagrelor have changed the attention from a fixed APT treatment strategy towards “personalized APT strategies.” Strategies can be based on platelet function testing, which gives insight into the overall response of a patient to APT. At our outpatient ST clinic, we practice personalized APT based on platelet function testing to guide the cardiologist to a presumed optimal antiplatelet treatment of ST patients. Beside results of platelet function testing, comedication, clinical characteristics, and genetics have to be considered to decide on personalized APT. Ongoing studies have yet to reveal the optimal personalized APT strategy for cardiologists to prevent their patients from atherothrombotic and bleeding events.

Persistent high on-treatment platelet reactivity in acute coronary syndrome

Persistent high on-treatment platelet reactivity in acute coronary syndrome (ACS) patients managed with appropriate antiplatelet therapy has been correlated with increased risk of cardiovascular events; however, the evolution of this phenomenon overtime is not well known. We investigated platelet activity at a three month follow-up after initial presentation with an ACS. We enrolled a total of 124 patients in the study, 65 were diagnosed with ACS and 59 controls who presented with non-cardiac chest pain for baseline comparisons. Of the enrolled patients, we had 25 ACS patients return, in stable condition, three months after their initial presentation for repeat platelet functional testing. Epinephrine (EPI), adenosine diphosphate (ADP), and arachidonic acid induced platelet aggregation were monitored at baseline with repeat measurement of EPI- and ADP-stimulated aggregation at follow-up. In addition, P-selectin and PAC-1 expression were monitored at presentation and at a three month follow-up period. ACS patients were maintained on aspirin therapy during the intervening period. At the three month follow-up visit, ACS patients initiated on aspirin had no significant percentage change in aggregation to submaximal concentrations of EPI and ADP. They also had no significant percentage change in PAC-1 or P-selectin expression. This study demonstrates persistent high on-treatment platelet reactivity in ACS patients at a three month follow-up, which may place these patients at increased risk of recurrent cardiovascular events.

Time-dependent changes in non-COX-1-dependent platelet function with daily aspirin therapy

To develop an integrated metric of non-COX-1-dependent platelet function (NCDPF) to measure the temporal response to aspirin in healthy volunteers and diabetics. NCDPF on aspirin demonstrates wide variability, despite suppression of COX-1. Although a variety of NCDPF assays are available, no standard exists and their reproducibility is not established. We administered 325 mg/day aspirin to two cohorts of volunteers (HV1, n = 52, and HV2, n = 96) and diabetics (DM, n = 74) and measured NCDPF using epinephrine, collagen, and ADP aggregometry and PFA100 (collagen/epi) before (Pre), after one dose (Post), and after several weeks (Final). COX-1 activity was assessed with arachidonic acid aggregometry (AAA). The primary outcome of the study, the platelet function score (PFS), was derived from a principal components analysis of NCDPF measures. The PFS strongly correlated with each measure of NCDPF in each cohort. After 2 or 4 weeks of daily aspirin the Final PFS strongly correlated (r > 0.7, P < 0.0001) and was higher (P < 0.01) than the Post PFS. The magnitude and direction of the change in PFS (Final–Post) in an individual subject was moderately inversely proportional to the Post PFS in HV1 (r = -0.45), HV2 (r = -0.54), DM (r = -0.68), P < 0.0001 for all. AAA remained suppressed during aspirin therapy. The PFS summarizes multiple measures of NCDPF. Despite suppression of COX-1 activity, NCDPF during aspirin therapy is predictably dynamic: those with heightened NCDPF continue to decline whereas those with low/normal NCDPF return to pre-aspirin levels over time.

A contemporary viewpoint on 'aspirin resistance'

Aspirin is an irreversible inhibitor of platelet prostaglandin synthase activity, and is the most widely prescribed drug for the secondary prevention of cardiovascular disease. In recent years, clinical and laboratory evidence has shown significant individual variability in the response to aspirin and its link to clinical outcome. The term 'aspirin resistance' has been introduced to describe situations when clinical or ex-vivo effects of aspirin are less than expected. The accumulating evidence of increased risk of major adverse clinical events (MACE) associated with 'aspirin resistance' in the settings of acute coronary syndrome (ACS), stroke, and peripheral arterial disease has stimulated the search for ways of overcoming aspirin resistance. Existence of the link between high on-treatment platelet reactivity and atherothrombotic events suggests the common mechanisms for atherosclerosis progression and thrombotic complications with the platelets, being a key cellular interface between coagulation and inflammation. This review article provides a contemporary view on 'aspirin resistance' and discusses its definition, clinical importance, and possible mechanisms in light of recent data on the role of platelets in atherothrombosis.

Decreased cyclooxygenase inhibition by aspirin in polymorphic variants of human prostaglandin H synthase-1

OBJECTIVES

Aspirin (ASA), a major antiplatelet and cancer-preventing drug, irreversibly blocks the cyclooxygenase (COX) activity of prostaglandin H synthase-1 (PGHS-1). Considerable differences in ASA effectiveness are observed between individuals, and some of this variability may be due to PGHS-1 protein variants. Our overall aim is to determine which, if any, of the known variants in the mature PGHS-1 protein lead to functional alterations in COX catalysis or inhibition by ASA. The present study targeted four PGHS-1 variants: R53H, R108Q, L237M, and V481I.

METHODS

Wild-type human PGHS-1 and the four polymorphic variants were expressed as histidine-tagged, homodimeric proteins in insect cells using baculovirus vectors, solubilized with a detergent, and purified by affinity chromatography. The purified proteins were characterized in vitro to evaluate COX and peroxidase (POX) catalytic parameters and the kinetics of COX inhibition by ASA and NS-398.

RESULTS

Compared with the wild type, several variants showed a higher COX/POX ratio (up to 1.5-fold, for R108Q), an elevated arachidonate Km (up to 1.9-fold, for R108Q), and/or a lower ASA reactivity (up to 60% less, for R108Q). The decreased ASA reactivity in R108Q reflected both a 70% increase in the Ki for ASA and a 30% decrease in the rate constant for acetyl group transfer to the protein. Computational modeling of the brief ASA pulses experienced by PGHS-1 in circulating platelets during daily ASA dosing predicted that the 60% lower ASA reactivity in R108Q yields a 15-fold increase in surviving COX activity; smaller, approximately two-fold increases in surviving COX activity were predicted for L237M and V481I. NS-398 competitively inhibited COX catalysis of the wild type (Ki=6 µmol/l) and inhibited COX inactivation by 1.0 mmol/l ASA in both the wild type (IC50=0.8 µmol/l) and R108Q (IC50=2.1 µmol/l).

CONCLUSION

Of the four PGHS-1 variants examined, R108Q exerts the largest functional effects, with evidence for impaired interactions with a COX substrate and inhibitors. As Arg108 is located on the protein surface and not in the active site, the effects of R108Q suggest a novel, unsuspected mechanism for the modulation of the PGHS-1 active site structure. The lower intrinsic ASA reactivity of R108Q, V481I, and L237M, combined with the rapid hydrolysis of ASA in the blood, suggests that these variants decrease the antiplatelet effectiveness of the drug. These PGHS-1 variants are uncommon but ASA is used widely; hence, a considerable number of individuals could be affected. Further examination of these and other PGHS-1 variants will be needed to determine whether PGHS-1 genotyping can be used to personalize anti-COX therapy.

Impact of platelet reactivity on long-term clinical outcomes and bleeding events in Japanese patients receiving antiplatelet therapy with aspirin

AIM

Aspirin is an antiplatelet drug widely used for the prevention of cardiovascular disease; however, it is known to increase bleeding events. A low response to aspirin was reported to correlate with poor prognosis in patients undergoing antiplatelet therapy with aspirin. The aim of this study was to evaluate the impact of the antiplatelet activity of aspirin on cardiovascular and bleeding events in Japanese patients.

METHODS

We analyzed the clinical course of 239 Japanese patients undergoing antiplatelet therapy with aspirin for a median of 64 months in this study. Their residual platelet reactivity was examined at enrollment and after 2 years. The co-primary endpoints were the occurrence of major adverse cardiac and cerebrovascular events (MACCEs) and bleeding events.

RESULTS

The annual incidence of MACCEs and major bleeding events was 3.7% and 0.48%, respectively. With defined criteria, 67 patients (28%) were classified as low responders based on the platelet aggregability measured at enrollment. Low response to aspirin was not associated with increased MACCEs, while it clearly increased MACCEs in patients less than 70 years old (low responders 36.9% vs. responders 14.8%, log rank p=0.008). Five major types of bleeding occurred in the responders, but not in low responders, although the difference was not statistically significant (p= 0.07).

CONCLUSION

Low response to aspirin was not associated with the increase of long-term MACCEs, while it increased MACCEs in patients less than 70 years old; however, it tended to decrease major bleeding events in Japanese patients.