Why are some individuals resistant to the cardioprotective effects of aspirin? Could it be thromboxane A2?

Discovery of Safe COX-2 Inhibitors: Achieving Reduced Colitis Side Effects through Balanced COX Inhibition

The severe adverse effects associated with imbalanced cyclooxygenase-2 (COX-2) inhibition continue to pose significant challenges in the development of contemporary anti-inflammatory drugs. In recent years, the approach to COX-2 inhibitor drug development has shifted from a focus on highly selective inhibition of COX-2 to a strategy that emphasizes more moderate selectivity. The amino acid sequence and structural similarities between inducible COX-2 and constitutive cyclooxygenase-1 (COX-1) isoforms present both substantial opportunities and challenges for the design of next generation of balanced COX-2 inhibitors. As part of our ongoing research into the discovering novel and safer COX-2 inhibitors, we reported herein a highly potent and balanced COX-2 inhibitor 21 d (IC50 value=1.35 μM, selectivity profile (IC50 (COX-1)/IC50 (COX-2)=22.34)). In vivo assays demonstrated that 21 d significantly alleviated histological damage and provided robust protection against dextran sulfate sodium (DSS)-induced acute colitis.

Structural modification of natural axinelline A: Achieving reduced colitis side effects through balanced COX inhibition

Marine natural products continue to hold great promise as potential candidates for the discovery of anti-inflammatory drug. In our previous investigation, we successfully synthesized axinelline A, a naturally occurring cyclooxygenase-2 (COX-2) inhibitor, as a promising anti-inflammatory lead compound. This study was to discover novel COX inhibitors with balanced inhibition, aiming to mitigate the severe adverse effects through further structural modification of axinelline A. Of the synthetic derivatives, compound 5e showed highest COX-2 inhibitory activity and balanced COX inhibition (IC50 = 1.74 µM; selectivity ((IC50 (COX-1)/IC50(COX-2) = 16.32). The in vitro anti-inflammatory results indicated that 5e effectively suppressed the expression of pro-inflammatory mediators via the NF-κB signaling pathway rather than the MAPK signaling pathway. The in vivo ulcerative colitis assay demonstrated 5e significantly ameliorated the histological damages and showed strong protection against DSS-induced acute colitis. Therefore, our findings suggest that compound 5e exhibits potential as a promising anti-inflammatory agent with attenuated colitis-related adverse effects.

Structural modification based on the diclofenac scaffold: Achieving reduced colitis side effects through COX-2/NLRP3 selective inhibition

COX-2/NLPR3-targeted therapy might be beneficial for the inflammation diseases. To discover novel anti-inflammatory compounds with favorable safety profiles, three new series of non-carboxylic diclofenac analogues bearing various ring systems, such as oxadiazoles 4a-4w, triazoles 6a-6m, and cyclic imides 7a and 7b, were synthesized. The synthesized analogues were evaluated for their inhibitory activity against COX-2 enzyme. Among them, compound 6k exhibited potent selective COX-2 inhibition (IC50 = 1.53 μM; selectivity ((IC50 (COX-1)/IC50(COX-2) = 17.19). Treatment with compound 6k effectively suppressed the NF-κB/NLRP3 signaling pathway, resulting in reduced expression of pro-inflammatory factors. The in vivo ulcerative colitis assay demonstrated that compound 6k significantly ameliorated histological damages and showed strong protection against DSS-induced acute colitis. The collected results indicated that compound 6k displays anti-inflammatory activity through COX-2/NLRP3 inhibition. Therefore, compound 6k represents a promising candidate for further development as a new lead compound with reduced colitis side effects.

Synthesis and anti-inflammatory activity of novel firocoxib analogues with balanced COX inhibition

The adverse effects caused by nonselective and selective cyclooxygenase-2 (COX-2) inhibitors remain a challenge for current anti-inflammatory medications. A balanced inhibition of COX-1/-2 represents a promising strategy for the development of novel COX-2 inhibitors. In this study, we present the design and synthesis of a novel series of firocoxib analogues incorporating an amide bond to facilitate essential hydrogen bonding with amino residues in COX-2. The synthesized analogs were evaluated for their inhibitory activity against both COX-1 and COX-2 enzymes. Among them, compound 9d demonstrated potent and balanced inhibition. Inhibition of COX enzymes by 9d in lipopolysaccharide (LPS)-stimulated murine RAW264.7 macrophages resulted in the suppression of the NF-κB signaling pathway to reduced expression of pro-inflammatory factors such as inducible nitric oxide synthase (iNOS), COX-2, nitric oxide (NO), and reactive oxygen species (ROS). The remarkable in vitro anti-inflammatory activity exhibited by 9d positions it as a promising candidate for further development as a novel lead compound for inflammation treatment.

Design of Balanced Cyclooxygenase Inhibitors Based on Natural Anti-inflammatory Ascidian Metabolites and Celecoxib

The serious adverse effects caused by non-selective and selective cyclooxygenase-2 (COX-2) inhibitors remain significant concerns for current anti-inflammatory drugs. In this study, we present the design and synthesis of a novel series of celecoxib analogs incorporating a hydrazone linker, which were subjected to in silico analysis to compare their binding poses with those of clinically used nonsteroidal anti-inflammatory drugs (NSAIDs) against COX-1 and COX-2. The synthesized analogs were evaluated for their inhibitory activity against both COX enzymes, and compound 6 m, exhibiting potent balanced inhibition, was selected for subsequent in vitro anti-inflammatory assays. Treatment with 6 m effectively suppressed the NF-κB signaling pathway in lipopolysaccharide (LPS)-stimulated murine RAW264.7 macrophages, resulting in reduced expression of pro-inflammatory factors such as inducible nitric oxide synthase (iNOS), COX-2, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), IL-1β, as well as decreased production of prostaglandin E2 (PGE2 ), nitric oxide (NO), and reactive oxygen species (ROS). However, 6 m has no effect on the MAPK signaling pathway. Therefore, due to its potent in vitro anti-inflammatory activity coupled with lack of cytotoxicity, 6 m represents a promising candidate for further development as a new lead compound targeting inflammation.

Urinary 11-dehydro-thromboxane B2 levels are associated with vascular inflammation and prognosis in atherosclerotic cardiovascular disease

Cyclooxygenase-derived thromboxane (TxA2) and prostacyclin (PGI2) regulate atherogenesis in preclinical models. However, the relationship between TxA2 and PGI2 biosynthesis, vascular inflammation, and atherosclerotic cardiovascular disease (ASCVD) progression in humans remains unclear. The association between stable urine metabolites of thromboxane (TxA2-M) and prostacyclin (PGI2-M), circulating levels of cellular adhesion molecules (CAMs: E-selectin, P-selectin), chemokines and C-reactive protein, and the incidence of major adverse cardiovascular events (MACE) were evaluated in 120 patients with stable ASCVD on aspirin therapy. Urinary TxA2-M levels were significantly correlated with circulating P-selectin (r=0.319, p<0.001) and E-selectin (r=0.245, p=0.007) levels, and associated with higher risk of MACE (p=0.043). In contrast, PGI2-M levels were not significantly associated with CAM levels or MACE. These results provide insight into the contribution of TxA2 biosynthesis to ASCVD progression in humans, and suggest that patients with elevated TxA2-M levels may be predisposed to advanced platelet and endothelial activation and higher risk of adverse cardiovascular outcomes.

Establishment of reference ranges and evaluation of in vitro concentration-dependent platelet inhibition by acetylsalicylic acid for multiple electrode impedance aggregometry in healthy dogs

Acetylsalicylic acid (ASA, aspirin) is an antiplatelet medication used for prevention of thromboembolism. Effects of ASA appear to vary widely between dogs, but the underlying mechanisms are not understood. The Multiplate analyzer is a newer form of whole-blood impedance aggregometry recently validated for use in healthy dogs. A method utilizing this instrument to measure ASA effects on platelet function has not been established. The goals of this study were to establish reference ranges for the Multiplate in healthy dogs and secondly, to develop a technique to determine the in vitro concentration of ASA needed to cause 50% inhibition of platelet aggregation (IC50). Reference ranges established from 40 dogs at multiple test times for three agonists were consistent with previously published values. In vitro IC50 values were calculated using the sigmoid E_max model in 20 healthy dogs on two occasions to determine individual repeatability. Calculated in vitro IC50 demonstrated four ASA response groups: responder (n = 16), poor responder (n = 1), variable responder (n = 2), and nonresponder (n = 1). Multiplate within-assay variability was  <10% for area under the curve (AUC), and between-assay baseline AUC variability was  <15%. The described technique allowed for determination of an in vitro IC50 for ASA in dogs using a multiple electrode impedance aggregometer.

Urinary 11-Dehydro-Thromboxane B2 and Mortality in Patients With Stable Coronary Artery Disease

Antiplatelet therapy with aspirin has been shown to reduce adverse outcomes in patients with coronary artery disease (CAD). Aspirin irreversibly inhibits platelet cyclooxygenase-1 and attenuates thromboxane A₂ (TXA₂)-mediated platelet aggregation, but there is variable suppression of cyclooxygenase-1. From a cohort of patients with stable CAD, we performed blinded, detailed chart abstraction, and measured urinary 11-dehydro-thromboxane B₂ (11dhTxB₂), an inactive metabolite of TxA₂ from frozen samples. There were 327 men (73%) and 122 women (27%) with a mean age (±SD) of 67 ± 10 and 65 ± 10 years, respectively. A positive linear trend for age was observed among tertiles of 11dhTxB₂ (p trend = 0.01). Higher proportions of women (p = 0.001), chronic obstructive pulmonary disease (p trend = 0.0003), and heart failure (p trend = 0.003) were observed in the upper tertile of 11dhTxB₂. Sixty-seven patients (14.9%) died over a median follow-up of 1,149 days and 87.5% of the deaths were due to cardiovascular causes. Twenty-six nonsurvivors (38.8%) were treated with P2Y12 receptor antagonists versus 161 survivors (42.2%; p = 0.61). By stepwise Cox proportional hazards analysis, we identified that patients in the middle (hazard ratio 7.14; 95% CI 2.46 to 20.68) and upper tertiles (hazard ratio 9.91; 95% CI 3.45 to 28.50) had higher risks for mortality after adjusting for age and co-morbidities. In conclusion, urinary concentration of 11dhTxB₂ was a strong independent risk factor for all-cause mortality among patients with stable CAD on aspirin therapy and may be a marker for patients with CAD who require more intensive secondary prevention measures.

The Benefit and Safety of Aspirin for Primary Prevention of Ischemic Stroke: A Meta-Analysis of Randomized Trials

Background: Although aspirin is effective in the secondary prevention of stroke among men and women, its use in primary prevention remains controversial. We conducted a meta-analysis of randomized trials to evaluate the benefit and safety of aspirin for the primary prevention of ischemic stroke. Methods: We searched three electronic databases (Medline, the Cochrane Central Register of Controlled Trials, and Embase) for articles published before August 1st, 2016. Randomized trials reporting the effect of aspirin on the primary prevention of ischemic stroke and its side effects (hemorrhagic stroke and severe gastrointestinal bleeding) were included. We used a fixed-effect model to quantify the effect of aspirin on the primary prevention of stroke when the heterogeneity was low, or else applied the random-effect model. Results: Fourteen randomized trials were included. Overall, aspirin use was associated with a decreased risk of ischemic stroke compared with non-aspirin use (OR: 0.83, 95% CI: 0.74-0.93, P = 0.45). In subgroup analyses, the effect of aspirin on ischemic stroke in apparently healthy adults remained significant (OR: 0.83, 95% confidence interval: 0.74-0.94, I² = 22%, P = 0.28); while in patients with cardiovascular diseases there was no difference in the risk of ischemic stroke between aspirin and non-aspirin groups (OR: 0.75, 95% confidence interval: 0.44-1.29, P = 0.46). As for adverse effects, the prophylactic use of aspirin potentially increased the risk of serious bleeding events in a population of apparently healthy individuals and in patients with previous cardiovascular diseases. Conclusion: This meta-analysis of randomized trials indicated that both the apparently healthy adults and patients with cardiovascular diseases will derive little protective benefit from aspirin considering the increased risk of severe bleeding events.

The lack of aspirin resistance in patients with coronary artery disease

Background

Aspirin resistance established by different laboratory methods is still a debated problem. Using COX1 specific methods no aspirin resistance was detected among healthy volunteers. Here we tested the effect of chronic aspirin treatment on platelets from patients with stable coronary artery disease. The expression of COX2 mRNA in platelets and its influences on the effect of aspirin was also investigated.

Methods

One hundred and forty four patients were enrolled in the study. The direct measurement of COX1 acetylation was carried out by monoclonal antibodies specific to acetylated and non-acetylated COX1 (acCOX1 and nacCOX1) using Western blotting technique. Arachidonic acid (AA) induced TXB₂ production by platelets was measured by competitive immunoassay. AA induced platelet aggregation, ATP secretion and VerifyNow Aspirin Assay were also performed. COX2 and COX1 mRNA expression in platelets were measured in 56 patients by RT-qPCR.

Results

In 138 patients only acCOX1 was detected, in the remaining six patients nacCOX1 disappeared after a compliance period. AA induced TXB₂ production by platelets was very low in all patients including the 6 patients after compliance. AA induced platelet aggregation, secretion and with a few exceptions the VerifyNow Assay also demonstrated the effect of aspirin. Smoking, diabetes mellitus and inflammatory conditions did not influence the results. The very low amount of COX2 mRNA detected in 39 % of the investigated platelets did not influence the effect of aspirin.

Conclusions

No aspirin resistance was detected among patients with stable coronary artery disease. COX2 expression in platelets did not influence the effect of aspirin.

Cardiac thromboxane A2 receptor activation does not directly induce cardiomyocyte hypertrophy but does cause cell death that is prevented with gentamicin and 2-APB

Background

We have previously shown that the thromboxane (TXA2) receptor agonist, U46619, can directly induce ventricular arrhythmias that were associated with increases in intracellular calcium in cardiomyocytes. Since TXA2 is an inflammatory mediator and induces direct calcium changes in cardiomyocytes, we hypothesized that TXA2 released during ischemia or inflammation could also cause cardiac remodeling.

Methods

U46619 (0.1-10 μM) was applied to isolated adult mouse ventricular primary cardiomyocytes, mouse ventricular cardiac muscle strips, and cultured HL-1 cardiomyocytes and markers of hypertrophy and cell death were measured.

Results

We found that TXA2 receptors were expressed in ventricular cardiomyocytes and were functional via calcium imaging. U46619 treatment for 24 h did not increase expression of pathological hypertrophy genes (atrial natriuretic peptide, β-myosin heavy chain, skeletal muscle α-actin) and it did not increase protein synthesis. There was also no increase in cardiomyocyte size after 48 h treatment with U46619 as measured by flow cytometry. However, U46619 (0.1-10 μM) caused a concentration-dependent increase in cardiomyocyte death (trypan blue, MTT assays, visual cell counts and TUNEL stain) after 24 h. Treatment of cells with the TXA2 receptor antagonist SQ29548 and inhibitors of the IP3 pathway, gentamicin and 2-APB, eliminated the increase in cell death induced by U46619.

Conclusions

Our data suggests that TXA2 does not induce cardiac hypertrophy, but does induce cell death that is mediated in part by IP3 signaling pathways. These findings may provide important therapeutic targets for inflammatory-induced cardiac apoptosis that can lead to heart failure.

Evaluation of laboratory methods routinely used to detect the effect of aspirin against new reference methods

BACKGROUND

Aspirin, a commonly used antiplatelet agent, blocks platelet thromboxane A₂ (TXA₂) formation from arachidonic acid (AA) by acetylating platelet cyclooxygenase-1 (COX-1). Laboratory methods currently used to detect this antiplatelet effect of aspirin provide variable results. We have reported three methods that assess platelet COX-1 acetylation (inactivation) by aspirin and its direct consequences. The first and second assays use monoclonal anti-human-COX-1 antibodies that only detect acetylated (inactivated) COX-1 and active (non-acetylated) COX-1, respectively. The third method measures platelet production of TXB₂ (the stable metabolite of TXA₂) in vitro in response to AA. We compared the results of these three reference methods with other routinely used methods for assessing the functional consequences aspirin treatment.

METHODS

108 healthy volunteers were treated with low-dose aspirin for 7 days. On day 7 following aspirin treatment COX-1 in the platelets was fully acetylated whereas only non-acetylated COX-1 was present in the day 0 platelets. Further, TXB2 production by day 7 platelets was completely blocked. The following tests were performed on the samples obtained from study participants before and after seven days of aspirin treatment: PFA-100 closure time with collagen/epinephrine cartridge, VerifyNow (VN) Aspirin Assay, platelet aggregation and ATP secretion using AA, ADP, epinephrine and collagen as agonists.

RESULTS

Comparing the pre-treatment and day 7 values, methods that use AA as platelet agonist (AA-induced platelet aggregation/secretion and VN Aspirin Assay) showed high discriminative power. In contrast, results of the other tests showed considerable overlap between day 7 and day 0 values.

CONCLUSIONS

Only assays that clearly distinguish between acetylated and non-acetylated platelet COX-1 are useful for establishing the antiplatelet effect of aspirin. The other tests are not suitable for this purpose.

Therapy with acetylsalicylic acid does not interfere with oral surgery

The acetylsalicylic acid (ASA) treatment is widespread therapeutic strategy in cardiology clinics. On the other hand, patients with heart diseases represent a significant group of cases in dental clinics. Accordingly, we studied the local hemostatic thrombus formation after dental tooth extractions (n=47) and other oral surgery treatment (n=13) in 60 patients with heart disease being on ASA therapy without drug discontinuation. In the control group free of ASA therapy it was: (n=24) and (n=6), respectively. In all studied patients, the aggregative activity of blood platelets by PFA-100 analyzer was assessed. It was found that 61.7% patients treated with ASA presented inhibition of platelets aggregation. Unexpectedly, in 35% of such patients, platelet aggregation function remained unchanged. In the control group, normal platelet aggregation was found in all subjects. It has been shown that ASA therapy has neutral effects on both thrombus formation and pain complications in patients with heart disease underwent tooth extraction and other oral surgery. Thus discontinuation of ASA therapy before surgery seems to be weakly validated.

Aspirin hydrolysis in human and experimental animal plasma and the effect of metal cations on hydrolase activities

The hydrolyzing properties of plasma esterases for aspirin were investigated in human plasma and plasma from experimental animals. The observed rates of aspirin hydrolysis were in the following order: rabbit > human > monkey > rat > mouse > dog > minipig. In human, monkey, and dog plasma, aspirin was hydrolyzed by their major hydrolases, paraoxonase (PON), butyrylcholinesterase (BChE), and albumin. In rabbit, mouse, and rat plasma, carboxylesterase (CES) was determined to be the enzyme responsible for aspirin hydrolysis, and in mouse and rat plasma, especially the latter, hydrolase activity was increased by the addition of ethopropazine, a specific inhibitor of BChE. Interestingly, divalent cations affected the plasma activity by enhancing or inhibiting the hydrolase activity of plasma BChE. The addition of 2 mM calcium increased the hydrolysis of aspirin in human, monkey, and dog plasma by 2.7-, 1.9-, and 2.3-fold, respectively. Magnesium showed a similar but lesser effect. Increasing concentrations of calcium and magnesium resulted in a two-phase stimulatory effect on aspirin hydrolysis in human plasma. In contrast, the addition of zinc had an inhibitory effect on plasma BChE activity. It is postulated that calcium and magnesium bind to BChE and thereby change the conformation of the enzyme to a more appropriate position for aspirin hydrolysis.

Polymorphisms and high on-aspirin platelet reactivity after off-pump coronary artery bypass grafting

OBJECTIVES

High on-aspirin residual platelet reactivity (RPR) after coronary artery bypass grafting (CABG) is a transient phenomenon with important implications for graft patency. This study was designed to determine the role of polymorphisms [TBXA2R (T924C), GPIIIa (Pl(A1/A2)), P2Y1 (A1622G), and GP1Bα (C1018T)] on RPR in Chinese patients undergoing off-pump CABG (OPCAB).

METHODS

Of 420 patients recruited to this study, 210 patients underwent primary OPCAB and 210 controls with ischemic heart disease received optimal medical therapy. Arachidonic acid-induced platelet aggregation and urinary 11-dehydro thromboxane B2 were measured at baseline and following aspirin administration on days 1, 4, 10, and on 6th month. Four polymorphisms were identified [TBXA2R (T924C), P2Y1 (A1622G), Pl(A1/A2) and GP1Bα (C1018T)].

RESULTS

On the first post-operative day, 62 patients (29.5%) were with high RPR and 148 (70.5%) were with low RPR. Of the former, 33 (15.7%), 10 (4.6%), and 0 (0%) patients remained with high RPR on days 4, 10, and on 6 month, respectively. No individuals with high RPR was found in controls. Logistic regression identified TBXA2R-924TT (OR = 4.5; 95% CI, 1.8-11.1) and body mass index > 27 kg/m(2) (OR = 2.73; 95% CI, 1.1-7.0) as independent risk factors for high on-aspirin RPR.

CONCLUSIONS

High on-aspirin RPR after OPCAB is associated with genetic polymorphism TBXA2R-924TT and obesity.

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.

Statins but not aspirin reduce thrombotic risk assessed by thrombin generation in diabetic patients without cardiovascular events: the RATIONAL trial

BACKGROUND

The systematic use of aspirin and statins in patients with diabetes and no previous cardiovascular events is controversial. We sought to assess the effects of aspirin and statins on the thrombotic risk assessed by thrombin generation (TG) among patients with type II diabetes mellitus and no previous cardiovascular events.

METHODOLOGY/PRINCIPAL FINDINGS

Prospective, randomized, open, blinded to events evaluation, controlled, 2×2 factorial clinical trial including 30 patients randomly allocated to aspirin 100 mg/d, atorvastatin 40 mg/d, both or none. Outcome measurements included changes in TG levels after treatment (8 to 10 weeks), assessed by a calibrated automated thrombogram. At baseline all groups had similar clinical and biochemical profiles, including TG levels. There was no interaction between aspirin and atorvastatin. Atorvastatin significantly reduced TG measured as peak TG with saline (85.09±55.34 nmol vs 153.26±75.55 nmol for atorvastatin and control groups, respectively; p = 0.018). On the other hand, aspirin had no effect on TG (121.51±81.83 nmol vs 116.85±67.66 nmol, for aspirin and control groups, respectively; p = 0.716). The effects of treatments on measurements of TG using other agonists were consistent.

CONCLUSIONS/SIGNIFICANCE

While waiting for data from ongoing large clinical randomized trials to definitively outline the role of aspirin in primary prevention, our study shows that among diabetic patients without previous vascular events, statins but not aspirin reduce thrombotic risk assessed by TG.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00793754.

Daily administration of the TP receptor antagonist terutroban improved endothelial function in high-cardiovascular-risk patients with atherosclerosis

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Terutroban is a selective TP receptor antagonist, i.e. a specific antagonist of the thromboxane A(2) and prostaglandin endoperoxide receptors, shown to improve endothelial function after a single administration in patients with coronary artery disease.

WHAT THIS STUDY ADDS

• This randomized, double-blind, placebo-controlled trial demonstrates that repeated-dose terutroban for 15 days improves endothelial function and inhibits thromboxane A(2) -induced platelet aggregation in high-cardiovascular-risk patients taking 300 mg of aspirin per day. Terutroban may prove useful for preventing cardiovascular events in such patients.

AIMS

The specific TP receptor antagonist terutroban improves endothelial function after a single dose in patients with coronary artery disease. Our aim was to evaluate the effects and dose dependency of repeated-dose terutroban on endothelial function and platelet aggregation in high-cardiovascular-risk patients with carotid atherosclerosis.

METHODS

We randomly allocated 48 patients taking 300 mg aspirin per day to placebo or to one of three terutroban dosages (2.5, 5 or 10 mg) for 15 days in a double-blind study. Flow-mediated vasodilatation was evaluated before and 2 h after the first oral dose on day 0 and 2 h after the last oral dose on day 14.

RESULTS

On day 0 and day 14, all three terutroban dosages improved flow-mediated vasodilatation and abolished platelet aggregation induced by the TP receptor agonist U46619, without changing the aggregation response to ADP or collagen.

CONCLUSION

Terutroban, by chronically improving endothelium-dependent vasodilatation and inhibiting platelet aggregation, may prove useful for preventing cardiovascular events in high-risk patients.

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".

Terutroban, a thromboxane/prostaglandin endoperoxide receptor antagonist, prevents hypertensive vascular hypertrophy and fibrosis

Thromboxane A2 and other eicosanoids such as isoprostanes contribute to vascular proliferation and atherosclerosis by binding to the thromboxane/prostaglandin endoperoxide receptors. The effects of terutroban, a thromboxane/prostaglandin endoperoxide receptor antagonist, on aorta remodeling were evaluated in spontaneously hypertensive stroke-prone rats (SHRSPs), a model of severe hypertension, endothelial dysfunction, vascular inflammation, and cerebrovascular diseases. Male SHRSPs were allocated to three groups receiving a standard diet ( n = 5) or a high-sodium permissive diet plus vehicle ( n = 6) or plus terutroban (30 mg·kg−1·day−1; n = 6). After 6 wk of dietary treatment, all of the animals were injected with bromodeoxyuridine and simultaneously euthanized for aorta collection. The aortic media thickness-to-lumen ratio significantly ( P < 0.0001) increased in the salt-loaded rats compared with the rats fed a standard diet, whereas terutroban treatment completely prevented media thickening ( P < 0.001). When compared with vehicle, terutroban was also effective in preventing cell proliferation in the media, as indicated by the reduced number of bromodeoxyuridine-positive ( P < 0.0001) and proliferating cell nuclear antigen-positive cells ( P < 0.0001). Severe fibrosis characterized by a significant accumulation of collagen and fibronectin in the vascular wall was observed in the vehicle-treated rats ( P < 0.01) but was completely prevented by terutroban ( P < 0.001). The latter also inhibited heat shock protein-47 ( P < 0.01) and TGF-1β expression ( P < 0.001), which were significantly increased by the high-salt diet. In conclusion, terutroban prevents the development of aorta hyperplasia and has beneficial effects on fibrotic processes by affecting TGF-β and heat shock protein-47 expression in SHRSPs. These findings provide mechanistic data supporting the beneficial effects of terutroban in preventing or retarding atherogenesis.

Antiplatelet drug interactions

Both laboratory studies in healthy volunteers and clinical studies have suggested adverse interactions between antiplatelet drugs and other commonly used medications. Interactions described include those between aspirin and ibuprofen, aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), and the thienopyridine, clopidogrel, and drugs inhibiting CYP2C19, notably the proton pump inhibitors (PPI) omeprazole and esomeprazole. Other interactions between thienopyridines and CYP3A4/5 have also been reported for statins and calcium channel blockers. The ibuprofen/aspirin interaction is thought to be caused by ibuprofen blocking the access of aspirin to platelet cyclo-oxygenase. The thienopyridine interactions are caused by inhibition of microsomal enzymes that metabolize these pro-drugs to their active metabolites. We review the evidence for these interactions, assess their clinical importance and suggest strategies of how to deal with them in clinical practice. We conclude that ibuprofen is likely to interact with aspirin and reduce its anti-platelet action particularly in those patients who take ibuprofen chronically. This interaction is of greater relevance to those patients at high cardiovascular risk. A sensible strategy is to advise users of aspirin to avoid chronic ibuprofen or to ingest aspirin at least 2 h prior to ibuprofen. Clearly the use of NSAIDs that do not interact in this way is preferred. For the clopidogrel CYP2C19 and CYP3A4/5 interactions, there is good evidence that these interactions occur. However, there is less good evidence to support the clinical importance of these interactions. Again, a reasonable strategy is to avoid the chronic use of drugs that inhibit CYP2C19, notably PPIs, in subjects taking clopidogrel and use high dose H2 antagonists instead. Finally, anti-platelet agents probably interact with other drugs that affect platelet function such as selective serotonin reuptake inhibitors, and clinicians should probably judge patients taking such combination therapies as at high risk for bleeding.

Thromboxane Antagonism with terutroban in Peripheral Arterial Disease: the TAIPAD study

BACKGROUND

Terutroban is a selective prostaglandin endoperoxide (TP) receptor antagonist with antithrombotic, antivasoconstrictive and antiatherosclerotic properties and is currently in development for long-term cardiovascular secondary prevention.

OBJECTIVES

TAIPAD is an international, double-blind, randomized controlled study comparing the effects of five dosages of oral terutroban vs. aspirin and placebo on platelet aggregation in peripheral arterial disease (PAD) patients.

PATIENTS/METHODS

After 10 day's placebo run-in, included patients (n = 435; ankle-brachial pressure index, 0.7 ± 0.1) were randomly allocated to aspirin 75 mg day(-1), terutroban 1, 2.5, 5, 10 or 30 mg day(-1) or placebo. On day 5, the placebo group was reallocated to one of the terutroban groups for the rest of the study (day 83). Ex vivo platelet aggregation induced by the thromboxane analog U46619 (7 μm) was measured 24 h after dosing, as well as platelet aggregation induced by arachidonic acid (AA), collagen and ADP.

RESULTS

Terutroban dose-dependently inhibited U46619-induced platelet aggregation at days 5 and 83. At day 5, the inhibition was significant vs. placebo for all terutroban dosages (P < 0.001). Terutroban (5, 10 and 30 mg day(-1)) was at least as effective as aspirin in inhibiting platelet aggregation induced by arachidonic acid (AA), collagen and adenosine diphosphate (ADP). Terutroban was well tolerated, with a safety profile similar to aspirin.

CONCLUSIONS

In PAD patients, terutroban dose-dependently inhibited platelet aggregation 24 h after dosing, and was at least as effective as aspirin at 5, 10 and 30 mg day(-1). Terutroban was well tolerated.

Aspirin resistance: Fact or fiction? A point of view

Aspirin is a wonder drug that has been used for well over 100 years for its analgesic and antipyretic effects. For the past three decades, it has increasingly been used for the prevention of primary and secondary cardiovascular events. Lately, it has been suggested that a significant number of individuals taking aspirin have become resistant to this drug. The phenomenon of "aspirin resistance" is based on the observation of clinical events in some patients taking aspirin, and/or a diminished platelet aggregation inhibitory response to aspirin therapy. Unfortunately, laboratory assays used to monitor the efficacy of aspirin are far from accurate and the results are not reproducible. Furthermore, results of different platelet function tests are often not congruent. In addition, platelet aggregation studies show marked inter-individual and intra-individual variability. Patients with coronary heart disease take many drugs that interfere with the effect of aspirin on platelet aggregation. Besides inhibiting formation of thromboxane A(2) from arachidonic acid, aspirin has a host of platelet-independent effects that complement its platelet inhibitory effects. Laboratory assays designed to measure platelet function do not take into account these pleiotropic effects of aspirin. In our view, use of the term "aspirin resistance" based on inadequate knowledge of imperfect laboratory tests does a disservice to physicians and patients.

Assessment, mechanisms, and clinical implication of variability in platelet response to aspirin and clopidogrel therapy

Antiplatelet therapy is the mainstay of treatment for patients with cardiovascular disease. However, some patients experience adverse cardiac events despite treatment with single- or dual-antiplatelet (aspirin and clopidogrel) therapy. Some of those events could be caused by low responsiveness to aspirin or clopidogrel. The frequency of this phenomenon has been reported to range from 1% to 45% for the 2 drugs. This wide range arises from the lack of a "gold-standard" definition to assess antiplatelet drug response and differences in assays, agonist concentrations, and cut-off points. Regardless of the variability in the incidence of aspirin or clopidogrel low responsiveness, several studies have indicated a clear relation between clopidogrel or aspirin low responsiveness and cardiovascular events. The evidence for an association between adverse clinical events and the results of ex vivo platelet function tests is stronger for clopidogrel than for aspirin. Currently, there is no established therapeutic approach for managing low response to aspirin or clopidogrel that has been shown in large trials to have clinical benefit. This review focuses on laboratory testing of antiplatelet response to aspirin and clopidogrel, the prevalence of low response, potential mechanisms, clinical significance, and prognostic value of this phenomenon and alternative approaches to optimize treatment in patients with low response to the drugs.

Aspirin resistance in coronary artery disease is correlated to elevated markers for oxidative stress but not to the expression of cyclooxygenase (COX) 1/2, a novel COX-1 polymorphism or the PlA1/2polymorphism

Aspirin resistance (AR) is estimated to be present in 5-75% of patients and is related to increased cardiovascular mortality. However, the underlying mechanisms are mostly unknown. In the present study, AR was detected in 14 out of 55 patients (25%) with coronary artery disease. The presence of concomitant anti-inflammatory drugs did not affect AR. Plasma levels of thromboxane B(2) as well as the markers for oxidative stress and known platelet activators 8-isoprostane and lipid peroxidation products were significantly higher in aspirin-resistant individuals (349.3 pg/ml, 53.9 pg/ml, and 538 micromol/l) compared to controls (113.7 pg/ml, 10.3 pg/ml, and 32.2 micromol/l; P < 0.05, respectively). Platelet cyclooxygenase-1 (COX-1) and COX-2 mRNA and protein expression were without significant differences between the two groups. DNA sequencing detected a novel platelet COX-1 single nucleotide polymorphism (SNP) resulting in amino acid exchange at position 8 (Arg8/Trp8). The wild-type as well as the heterozygous and homozygous SNP were present in both patient groups without significant differences. The aspirin binding (Arg120) and acetylation site (Ser529) were unaffected in the samples tested. Neither was AR related to the platelet integrin PlA(1)/A(2) polymorphism. In conclusion, AR appears to be unrelated to differences in platelet COX-1 and COX-2 expression or to a novel platelet COX-1 SNP and the PlA(1)/A(2) SNP. However, a correlation exists to elevated eicosanoids generated by oxidative stress indicating COX-1-independent pathways for the generation of platelet activating molecules represent a potential cause for AR.

Aspirin resistance

The development of adverse cardiovascular events despite aspirin use has established an interest in a possible resistance to the drug. Several definitions have been set and various laboratory testing modalities are available. This has led to a wide range of prevalence reports in different clinical entities. The etiologic mechanism has been related to clinical, genetic, and other miscellaneous factors. The clinical implications of this phenomenon are significant and warrant concern. Management strategies are currently limited to dosing alteration and introduction of other anitplatelet agents. However, these measures have not met the expected efficacy or safety.

Augmentation of U46619 induced human platelet aggregation by aspirin

Aspirin is known to suppress platelet function markedly. However, aspirin at concentrations higher than 1 mM was observed to augment 1.3 microM U46619 (a stable thromboxane receptor (TP receptor) agonist) induced human platelet aggregation in this study. Moreover, at a concentration as low as 250 microM aspirin increased the aggregation induced by U46619 in 13% of normal and healthy individuals. The degree of platelet aggregation and the amount of ATP release were enhanced in U46619 stimulated platelet rich plasma by the addition of aspirin (>250 microM). U46619 was previously reported to inhibit forskolin-stimulated adenyl cyclase and to reduce the cAMP formation. Both of the augmentation effects of aspirin on U46619-induced aggregation and ATP release were blocked by MeSAMP, a P2Y(12) receptor antagonist. U46619 induced aggregation was suppressed by the addition of ADP scavenger (CP/CPK) with no significant change on ATP measured and the effect of CP/CPK could not be reversed by aspirin. In addition, aspirin augmented the inhibitory effect of U46619 on the cAMP production. Our present results suggested that the potentiation effect of aspirin on U46619 induced aggregation was related with the secreted ADP and the subsequent P2Y(12)/Gi related signaling.

Assessment of aspirin resistance varies on a temporal basis in patients with ischaemic heart disease

Objective:

Laboratory tests including optical platelet aggregometry (OPA), platelet function analyser (PFA-100), and thromboxane B₂ (TXB₂) metabolite levels have been used to define aspirin resistance. This study characterised the prevalence of aspirin resistance in patients with ischaemic heart disease (IHD) and investigated the concordance and repeatability of these tests.

Design, setting and patients:

Consecutive outpatients with stable IHD were enrolled. They were commenced on 150 mg aspirin daily (day 0) and had platelet function assessment (OPA and PFA-100) and quantitative analysis of serum/urine TXB₂ at day ⩾7 and then at a second visit approximately 2 weeks later.

Main outcome measures:

We assessed the prevalence of aspirin resistance by each method, concordance between methods of measuring response to aspirin and association between time points to assess the predictability of response over time.

Results:

172 patients (62.7 (SD 8.7) years, 83.1% male) were recruited. At visits 1 and 2, respectively, 1.7% and 4.7% were aspirin resistant by OPA, whereas 22.1% and 20.3% were aspirin resistant by PFA-100. There were poor associations between PFA-100 and OPA, and between TXB₂ metabolites and platelet function tests. OPA and PFA-100 results were poorly associated between visits (κ = 0.16 and κ = 0.42, respectively) as were TXB₂ metabolites, suggesting that aspirin resistance is not predictable over time.

Conclusions:

The prevalence of aspirin resistance is dependent on the method of testing. Response varies on a temporal basis, indicating that testing on a single occasion is inadequate to diagnose resistance or guide therapy in a clinical setting.

Aspirin therapy and thromboxane biosynthesis in systemic lupus erythematosus

Incomplete suppression of thromboxane biosynthesis during aspirin therapy is associated with increased cardiovascular risk. Since systemic lupus erythematosus (SLE) is associated with platelet activation and increased cardiovascular mortality, we compared thromboxane and prostacyclin biosynthesis in patients with SLE and control subjects, and measured inhibition of thromboxane excretion in aspirin-treated subjects. We measured the urinary excretion of 11-dehydro thromboxane B 2 (TXB2) and 2,3-dinor 6-ketoPGF1α (PGI-M), the stable metabolites of thromboxane A2 and prostacyclin, respectively, in 74 patients with SLE and 70 controls. In subjects who were not receiving aspirin, TXB2 excretion was higher in patients with SLE [0.40 ng/mg creatinine (0.26—0.64), median (interquartile range)] than controls [0.31 ng/mg creatinine (0.23—0.44)] ( P = 0.04), and in these patients, TXB2 excretion correlated with disease activity (rho = 0.28, P = 0.03) and tumor necrosis factor alpha (rho = 0.48, P < 0.001). Aspirin therapy resulted in significantly lower TXB2 excretion in controls ( P = 0.01), but not in patients with SLE ( P = 0.10), compared with subjects not receiving aspirin. Prostacyclin biosynthesis did not differ among patients and controls, and was not affected by aspirin ( P all >0.35). Thromboxane biosynthesis is increased in SLE and is associated with disease activity. Additionally, response to aspirin may be attenuated in some patients with SLE. Lupus (2007) 16, 981—986.

Coagulation management in pediatric mechanical circulatory support

The anticoagulation treatment of children on pulsatile assist devices was studied. The results of 40 children (group A) on mechanical circulatory support with the formerly used anticoagulation treatment were compared with 32 children (group B) on assist devices treated with a new anticoagulation regime. In groups A and B, respectively, 19 (47.5%) and 12 (38%) had bleeding complications; 14 (35%) and 8 (25%) had chest re-exploration; 2 (5%) and 1 (3%) had cerebral hemorrhage; 9 (23%) and 7 patients (22%) had thromboembolic events; and 6 (15%) and 2 (6%) had strokes. In groups A and B, respectively, mean time of support was 19 days (range, 0 to 111 days) and 57 days (range, 1 to 420 days), with long-term survival of 33% and 78%. The differences in survival were significant (alpha = 0.003). Embolic and bleeding complications were seen in both groups, but the incidence was lower in group B. Since, however, thrombogenicity of the systems continues to be their main problem, further improvement of treatment is required.

Aspirin and Simvastatin Combination for Cardiovascular Events Prevention Trial in Diabetes (ACCEPT-D): design of a randomized study of the efficacy of low-dose aspirin in the prevention of cardiovascular events in subjects with diabetes mellitus treated with statins

Background

Despite the high cardiovascular risk, evidence of efficacy of preventive strategies in individuals with diabetes is scant. In particular, recommendations on the use of aspirin in patients with diabetes mostly reflect an extrapolation from data deriving from other high risk populations. Furthermore, the putative additive effects of aspirin and statins in diabetes remain to be investigated. This aspect is of particular interest in the light of the existing debate regarding the need of multiple interventions to reduce total cardiovascular risk, which has also led to the proposal of a polypill. Aim of the study is to evaluate the efficacy of aspirin in the primary prevention of major cardiovascular events in diabetic patients candidate for treatment with statins. These preventive strategies will be evaluated on the top of the other strategies aimed at optimizing the care of diabetic patients in terms of metabolic control and control of the other cardiovascular risk factors.

Methods/Design

The ACCEPT-D is an open-label trial assessing whether 100 mg/daily of aspirin prevent cardiovascular events in patients without clinically manifest vascular disease and treated with simvastatin (starting dose 20 mg/die). Eligible patients will be randomly assigned to receive aspirin + simvastatin or simvastatin alone. Eligibility criteria: male and female individuals aged >=50 years with diagnosis of type 1 or type 2 diabetes, already on treatment with statins or candidate to start the treatment (LDL-cholesterol >=100 mg/dL persisting after 3 months of dietary advise). The primary combined end-point will include cardiovascular death, non-fatal myocardial infarction, non-fatal stroke, and hospital admission for cardiovascular causes (acute coronary syndrome, transient ischemic attack, not planned revascularization procedures, peripheral vascular disease). A total of 515 first events are needed to detect a reduction in the risk of major cardiovascular events of 25% (alpha = 0.05; 1-beta = 0.90). Overall, 5170 patients will be enrolled. The study will be conducted by diabetes specialists and general practitioners.

Discussion

The study will provide important information regarding the preventive role of aspirin in diabetes when used on the top of the other strategies aimed to control cardiovascular risk factors.

Trial registration

Current Controlled Trials ISRCTN48110081.

Frequency of genetic polymorphisms of COX1, GPIIIa and P2Y1 in a Chinese population and association with attenuated response to aspirin

Background: Aspirin is a frequently prescribed drug for primary and secondary prevention of myocardial infarction, stroke and cardiovascular death. However, aspirin resistance may affect up to 45% of the population. Little is known on the role of genetic factors that contribute to resistance or augmented response to aspirin in different human populations. Methods: In a large sample of nonsmoker, medication-free healthy volunteers from mainland China (n = 323; age: 22.1 ± 2.0 years) (mean ± standard deviation), we determined the frequency of polymorphisms in cyclooxygenase 1 (COX1) (A-842G and C50T), glycoprotein IIIa (GPIIIa) (PLA1/A2) and purinergic receptor P2Y (P2Y1) (C893T and A1622G) genes. These candidate genes were chosen on the basis of their impact on platelet physiology and aspirin mode of action. A four panel P2Y1 genotype-stratified sample of healthy volunteers (n = 24 in total), identified from the large study sample above, prospectively received a 100 mg daily oral dose of aspirin for 7 days. We measured changes in platelet aggregation before and after aspirin treatment. As a comparison reference group, 6 out of 24 subjects in the prospective aspirin trial had the P2Y1 CT893/AG1622 genotype that displays a low frequency (<7%) in the Chinese population. Results: COX1 A-842G, C50T and GPIIIa PLA1/A2 genetic polymorphisms were not observed in our sample from mainland China. Allele frequencies of P2Y1 893T and 1622G were 3.5 and 30.6%, respectively. The heterozygosity for the P2Y1 A1622G polymorphism observed in the present study was different to Caucasians; Chinese displayed a higher allele frequency for the 1622G allele. After aspirin treatment, the net decrease in arachidonic acid-induced platelet aggregation was significantly larger in the P2Y1 CT893/AG1622 genotype panel (83.4 ± 3.7%, net reduction by aspirin expressed as percentage of baseline) compared with CC893/GG1622 (68.2 ± 13.5%), CC893/AG1622 (68.9  ± 9.6%) and CC893/AA1622 (65.1 ± 9.1%) genotypic groups (p = 0.012, 0.025 and 0.004, respectively; statistical power = 77%). There was no significant difference in antiplatelet effect of aspirin among the CC893/GG1622, CC893/AG1622 and CC893/AA1622 genotypes (p > 0.05). Conclusions: The COX1 A-842G, C50T and GPIIIa PLA1/A2 polymorphisms are rare in Chinese. In contrast to previous studies in Caucasian populations, these candidate functional polymorphisms are unlikely to be significant contributors to aspirin pharmacodynamics in Chinese persons. Importantly, the presence of the P2Y1 893CC genotype appears to confer an attenuated antiplatelet effect during aspirin treatment in healthy Chinese volunteers. These data collectively underscore the importance of population-to-population variability in clinical pharmacogenetics research and provide a basis for further long-term studies of aspirin response and P2Y1 genetic variation in patients with cardiovascular risk.

Coronary stent thrombosis related to aspirin resistance: what are the underlying mechanisms?

Acute coronary stent thrombosis represents a serious complication for which aspirin resistance could be a potential cause. The actual case is a myocardial infarction treated with immediate stenting of the right coronary artery followed by elective stenting of the left anterior descending artery 4 days later. Despite standard antiplatelet therapy, stents in both arteries occluded 2 days after the last procedure. The patient's blood samples were analyzed and revealed the presence of aspirin resistance as well as elevated thromboxane B(2) plasma levels. No thrombophilic disorder was detected. Today the mechanism of aspirin resistance is mostly unknown. Further research as well as recommendations for laboratory screening and for alternative pharmacological treatment options are required.

Aspirin resistance or variable response or both?

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

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.

Residual arachidonic acid-induced platelet activation via an adenosine diphosphate-dependent but cyclooxygenase-1- and cyclooxygenase-2-independent pathway: a 700-patient study of aspirin resistance

BACKGROUND

Thrombotic events still occur in aspirin-treated patients with coronary artery disease.

METHODS AND RESULTS

To better understand aspirin "resistance," serum thromboxane B2 (TXB2) and flow cytometric measures of arachidonic acid-induced platelet activation (before and after the ex vivo addition of aspirin and indomethacin) were analyzed in 700 consecutive aspirin-treated patients undergoing cardiac catheterization. In 680 of 682 evaluable patients, serum TXB2 concentrations were reduced compared with nonaspirinated healthy donors. Twelve patients had serum TXB2 that was lower than nonaspirinated healthy donors but >10 ng/mL. Arachidonic acid stimulated greater platelet activation in patients with high serum TXB2 (>10 ng/mL) than in patients with low serum TXB2. Addition of ex vivo aspirin reduced arachidonic acid-induced platelet activation to similar levels regardless of serum TXB2 concentrations, which suggests that patients with high residual serum TXB2 concentrations were either noncompliant or underdosed with aspirin. Among the remaining 98% of patients, ex vivo administration of either aspirin or indomethacin failed to prevent platelet activation across all degrees of arachidonic acid-induced platelet activation and aspirin doses. Although the patients were not randomized with respect to clopidogrel treatment, multivariate analysis showed that arachidonic acid-induced platelet activation was less in patients receiving clopidogrel.

CONCLUSIONS

There is a residual arachidonic acid-induced platelet activation in aspirin-treated patients that (1) is caused by underdosing and/or noncompliance in only approximately 2% of patients and (2) in the remaining patients, occurs via a cyclooxygenase-1 and cyclooxygenase-2 independent pathway, in direct proportion to the degree of baseline platelet activation, and is mediated in part by adenosine diphosphate-induced platelet activation.

Aspirin resistance detected with aggregometry cannot be explained by cyclooxygenase activity: involvement of other signaling pathway(s) in cardiovascular events of aspirin-treated patients

OBJECTIVES

Although the concept of aspirin resistance is extensively reported in medical literature, its precise mechanisms and clinical outcomes are largely unknown. In this study, we examined individual thromboxane biosynthesis and platelet aggregation in aspirin-treated patients, and whether the results of a platelet aggregation test influenced clinical outcomes.

RESULTS

Subjects taking 81 mg of aspirin (n = 50) and controls (n = 38) were evaluated for platelet aggregation and platelet cyclooxygenase-1 (COX-1) activity by measuring collagen-induced thromboxane B2 production. For aggregometry, both light transmission (LT) and laser-light scattering methods were employed to quantitatively evaluate aggregate sizes and numbers. Aspirin treatment resulted in the inhibition of collagen-induced platelet aggregation, particularly the transition from small to large platelet aggregates. Although platelet COX-1 activity seemed to be uniformly inhibited in all patients, platelet aggregation studies showed great inter-individual differences; variation in platelet COX-1 activity only accounted for 6-20% of the individual aggregations. Factor analysis revealed the existence of a common factor (other than platelet COX-1) that explained 48.4% of the variations in platelet aggregation induced by collagen, adenosine diphosphate (ADP), and collagen-related peptide. We then prospectively enrolled 136 aspirin-treated patients in our study, and we found that being in the upper quartile level of LT, or with large aggregate formation induced by collagen, was an independent risk factor for developing cardiovascular events within 12 months [hazard ratio (HR) = 7.98, P = 0.008 for LT; HR = 7.76, P = 0.007 for large aggregates]. On the other hand, the existence of diabetes mellitus was an independent risk factor for overall outcomes (HR 1.30-11.9, P = 0.015-0.033).

CONCLUSIONS

Aspirin resistance expressed as unsuppressed platelet COX-1 activity is a rare condition in an out-patient population. Other factor(s) affecting collagen-induced platelet aggregation may influence early outcomes in aspirin-treated patients.

Is there an interaction between the cardiovascular protective effects of low-dose aspirin and ibuprofen?

Laboratory studies and studies in normal volunteers have suggested a possible adverse interaction between aspirin and ibuprofen. It is thought that ibuprofen prevents aspirin from gaining access to platelet cyclooxygenase. Following the publication of these studies, a number of observational and other studies have been published. We review these studies identified by structured searching of Medline and Pubmed databases. We conclude that on the balance of probabilities that ibuprofen is more likely to interact with aspirin than not. This interaction is more likely in those who take ibuprofen chronically and is more important in those at high cardiovascular risk. Thus, a reasonable strategy might be to advise aspirin takers to avoid chronic ibuprofen.