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The term ‘aspirin resistance’ has been used to describe a number of different phenomena, including the inability of aspirin to: (i) protect individuals from thrombotic complications; (ii) cause a prolongation of the bleeding time; (iii) inhibit thromboxane (TX) biosynthesis; or (iv) produce an anticipated effect on one or more in vitro tests of platelet function [1]. The fact that some patients may experience recurrent vascular events despite long-term aspirin therapy should be properly labeled as ‘treatment failure’ rather than aspirin resistance. This is a common phenomenon occurring with any drug (e.g. lipid-lowering or antihypertensive drugs). Given the multifactorial nature of atherothrombosis, it is not surprising that only a fraction (usually one-quarter to one-third) of all vascular complications can be prevented by any single preventive strategy (Fig. 1).

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Figure 1. The risk of vascular complications is the major determinant of the absolute benefit of preventive strategies. Data are plotted from placebo-controlled trials of aspirin (○), statins (□) and antihypertensive drugs (▵) in different clinical settings. For each category of patients, the abscissa denotes the absolute risk of experiencing a major vascular event as recorded in the placebo arm of the trial(s). The absolute benefit of the preventive treatment is reported on the ordinate as the number of subjects in whom an important vascular event (non-fatal myocardial infarction, non-fatal stroke, or vascular death) is actually prevented by treating 1000 subjects for 1 year. MI, myocardial infarction; CHD, coronary heart disease; HC, hypercholesterolemic subjects; EH, essential hypertensives. Modified from [1].

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Platelet aggregation studies

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

A variable proportion (up to one-quarter) of patients with cerebrovascular disease achieve only partial inhibition of platelet aggregation at initial testing, and some (up to one-third) seem to develop ‘resistance’ to aspirin over time, even with increasing doses [2–4]. The results of these long-term studies carried out by Helgason et al. are at variance with those of a short-term study of Weksler et al. [5] showing that 40 mg aspirin daily inhibited platelet aggregation and thromboxane A2 (TXA2) formation as effectively as higher doses of aspirin in patients who had recent cerebral ischemia [5]. Variable platelet responses to aspirin have also been described in patients with peripheral arterial disease and with ischemic heart disease. In the study of Buchanan and Brister [6], aspirin ‘non-responders’ were identified on the basis of bleeding time measurements. Approximately 40% of patients undergoing elective coronary artery bypass grafting showed no prolongation of bleeding time in response to aspirin. This was associated with increased platelet adhesion and 12-HETE synthesis. In contrast, repeated measurements of platelet aggregation carried out over 24 months of placebo-controlled treatment by Berglund and Wallentin [7] demonstrated that 100 patients with unstable coronary artery disease randomized to receive 75 mg aspirin daily in the RISC study had consistently reduced platelet aggregation without attenuation during long-term treatment. Based on measurements of platelet aggregation in response to arachidonate and ADP, 5 and 24% of patients with stable cardiovascular disease who were receiving aspirin (325 mg day−1 for at least 7 days) were defined as ‘resistant’ and ‘semiresponders’, respectively [8]. Similar results have been reported using the platelet function analyzer (PFA)-100 system, with approximately 35% of patients with a recent myocardial infarction [9] or ischemic cerebrovascular accident [10] being defined ‘non-responders’ to aspirin doses in the range of 75–160 mg daily. Platelets from aspirin-resistant patients were found to be more sensitive to the aggregating effect of ADP [11]. Moreover, platelet responsiveness to aspirin was found to be reduced in patients with hyperlipidemia [12]. Using a variety of techniques, including conventional aggregometry, shear stress-induced activation, and the expression of platelet surface receptors, Sane et al. [13] have recently reported that 57% of a group of 88 patients with documented heart failure who had been treated with aspirin 325 mg day−1 for at least 1 month showed ‘aspirin-non-responsiveness’.

The lack of appropriate controls (e.g. patients treated with another antiplatelet agent) in these studies precludes unequivocal interpretation of their findings. Only a properly controlled, randomized study would allow examining the influence of intrasubject variability in platelet aggregation over time, compliance with study medication and potential drug interactions on the repeated measurements of platelet function. ‘Resistance’ to thienopyridines has been reported recently [14,15].

Several relatively small studies (n = 39–180) in stroke patients have suggested that aspirin ‘resistance’ may contribute to ‘treatment failure’, i.e. recurrent ischemic events while on antiplatelet therapy, and that doses higher than 500 mg may be more effective than lower doses in limiting this phenomenon [16–18]. The uncontrolled nature and small sample size of these studies makes it difficult to interpret the results. A much larger data-base failed to substantiate a dose-dependent effect of aspirin in stroke prevention [19], an effect that one would theoretically expect if aspirin ‘resistance’ could be overcome, at least in part, by increasing the daily dose of the drug.

The apparent discrepancy between the theoretical predictions originating from studies of aspirin ‘resistance’ based on platelet aggregation measurements ex vivo and the actual findings of over 70 randomized clinical trials of aspirin prophylaxis in high-risk patients (Table 1) can be reconciled by acknowledging the limitations of platelet function studies. Thus, platelet aggregation as measured by conventional methods ex vivo has less than ideal intra- and intersubject variability and displays limited sensitivity to the effect of aspirin, often considered a ‘weak’ antiplatelet agent based on such measurements. In addition to technical variables, platelet aggregation responses among normal persons can vary with mental stress, age, gender, race, diet, and hematocrit level, and a person may have different responses on repeated determinations [20]. Moreover, the relevance of changes in this index of capacity to the actual occurrence of platelet activation and inhibition in vivo is largely unknown. Similarly, the bleeding time has serious problems of methodological standardization and is of limited value in predicting hemostatic competence.

Table 1.  Indirect comparison of aspirin doses reducing vascular events in high risk patients *
Aspirin dose (mg day−1)Number of trialsNumber of patientsOdds reduction (%)
  1. * Data from [19].

500–1500342245119 ± 3
160–325192651326 ± 3
75–15012677632 ± 6

Interindividual variability in the response to antiplatelet agents

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

As with any drug, it is not surprising that there is some interindividual variability in the response to low-dose aspirin. However, reliable assessment of its occurrence and prevalence would require a long-term controlled study comparing the degree and persistence of the antiplatelet effects of aspirin vis-à-vis another antiplatelet agent in a sizable group of patients requiring antiplatelet therapy. Clopidogrel would be an ideal comparator, because of similarities in the mechanism of action (permanent inactivation of a platelet protein), pharmacokinetics (short half-life of the active moiety) and once-daily regimen as with aspirin [1]. It should be noted, however, that while aspirin is currently used at doses that represent a 2.5- to 10-fold excess over the dose of 30 mg necessary and sufficient to fully inactivate platelet cyclooxygenase (COX)-1 activity upon repeated daily dosing [21], clopidogrel is used routinely at 1 × the dose that appears necessary and sufficient to fully inactivate platelet P2Y12 upon repeated daily dosing [1]. Thus, the main determinants of the interindividual variability in the antiplatelet effects of the two drugs are substantially different (Table 2).

Table 2.  Main determinants of the interindividual variability in the antiplatelet effects of aspirin and clopidogrel
DeterminantAspirinClopidogrel
  1. Reproduced from [1].

Dependence on systemic bioavailabilityNoYes
Dependence on liver metabolism to active moietyNoYes
Recommended dose: minimum effective dose for full pharmacodynamic effect2–31
Relevance of pharmacodynamic interactions at the target siteYes?
Relevance of extra-platelet sources of the platelet agonistYesNo

Aspirin-resistant thromboxane biosynthesis

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

At least three potential mechanisms may underlie the occurrence of aspirin-resistant TXA2 biosynthesis. The transient expression of COX-2 in newly formed platelets [22] in clinical settings of enhanced platelet turn-over is a potentially important mechanism that deserves further investigation. Extra-platelet sources of TXA2 (e.g. monocyte/macrophage COX-2) may contribute to aspirin-insensitive TXA2 biosynthesis in acute coronary syndromes [23]. Furthermore, Catella-Lawson et al. [24] have recently reported that concomitant administration of non-steroidal anti-inflammatory drugs (NSAIDs) (e.g. ibuprofen) may interfere with the irreversible inactivation of platelet COX-1 by aspirin. This is due to competition for a common docking site within the cyclooxygenase channel (Arg120), which aspirin binds to with weak affinity prior to irreversible acetylation of Ser529 [1]. This pharmacodynamic interaction does not occur with rofecoxib, or diclofenac, drugs endowed with variable COX-2 selectivity [25]. Thus, concomitant treatment with readily available over-the-counter NSAIDs may limit the cardioprotective effects of aspirin and contribute to aspirin ‘resistance’. Investigative tools are readily available to evaluate potential sources of aspirin-resistant TXA2 biosynthesis (Table 3).

Table 3.  Analytical tools for assessing potential sources of aspirin-resistant thromboxane
Source of resistanceAnalytical tool
  1. NSAID, nonsteroidal antiinflammatory drug; TXM, thromboxane metabolite (e.g., 11-dehydro-TXB2)

Non-complianceSerum TXB2 (– aspirin in vitro)
Interaction with NSAIDSerum TXB2 (2 & 24 hr sampling)
Extra-platelet sources of TXA2Urinary TXM (– selective COX-2 inhibitor in vivo)

Clinical read-outs of aspirin resistance

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

Several recent observational studies suggest that concomitant use of ibuprofen and low-dose aspirin may impair the antithrombotic efficacy of the latter [25,26], consistently with the pharmacodynamic interaction described by Catella-Lawson et al. [24]. In addition, the clinical relevance of aspirin-resistant TXA2 biosynthesis has been explored by Eikelboom et al. [27] who performed a nested case-control study of baseline urinary thromboxane metabolite excretion in relation to the occurrence of major vascular events in aspirin-treated high-risk patients enrolled in the HOPE trial. After adjustment for baseline differences, the odds for the composite outcome of myocardial infarction, stroke, or cardiovascular death increased with each increasing quartile of 11-dehydro-TXB2 excretion, with patients in the upper quartile having a 1.8-times higher risk than those in the lower quartile (Fig. 2). However, both observational and case-control studies suffer from potential confounding and cannot reliably establish a cause-effect relationship between aspirin-resistant TXA2 biosynthesis and enhanced risk of vascular events.

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Figure 2. Association between quartiles of urinary 11-dehydro-TXB2 excretion and composite of myocardial infarction, stroke or cardiovascular death, after adjustment for baseline differences between cases and control high-risk, aspirin-treated patients recruited in the HOPE trial. The P-value is for trend of association. MI, myocardial infarction. Reproduced from [27] with permission from the American Heart Association, Inc.

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How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

These interesting findings provide a rationale for testing the efficacy and safety of additional treatments that more effectively block in vivo TXA2 biosynthesis or action in a subset of high-risk patients displaying aspirin-resistant TXA2 biosynthesis.

Highly selective COX-2 inhibitors (e.g. rofecoxib, etoricoxib or lumiracoxib: see [25]) could be used in phase II dose-finding studies with a primary biochemical end-point (i.e. urinary 11-dehydro-TXB2 excretion) in the appropriate clinical setting. Although concerns have been expressed on their cardiovascular safety because of inhibition of COX-2-dependent PGI2 production [28], it should be recognized that several endothelial mediators of thromboresistance and vasoregulation exist, including COX-1-dependent PGI2. In addition, any hazard deriving from COX-2 inhibition on top of low-dose aspirin would be shared by any NSAID, regardless of COX-2 selectivity [29].

Thromboxane receptor (TP) antagonists would offer the advantage vis-à-vis coxibs of not interfering with PGI2 production. In addition, they might antagonize the platelet and vascular effects of biologically active F2-isoprostanes acting as incidental ligands of TP receptors [30] (Fig. 3). A draw-back in performing dose-finding studies with a TP antagonist would be the lack of a suitable biochemical end-point related to aspirin-resistant TXA2 biosynthesis.

image

Figure 3. Antagonism of aspirin-insensitive thromboxane receptor (TP) agonists. The scheme illustrates enzymatic as well as non-enzymatic pathways of arachidonic acid metabolism generating agonists of the platelet and vascular TP receptors that are largely insensitive to inhibition by low-dose aspirin. These include TXA2 derived from COX-2 expressing cells and F2-isoprostanes, e.g. 8-iso-PGF.PG, prostaglandin; COX, cyclooxygenase; TX, thromboxane; TP, thromboxane A2 receptor.

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Conclusions

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

Early reports of aspirin ‘resistance’ based on platelet aggregation measurements represent descriptive phenomenology of difficult interpretation because of the uncontrolled nature of the observations and lack of a mechanistic insight. It is perhaps important to remember that platelet aggregation measurements have not been particularly useful in describing the human pharmacology of aspirin. In fact, the development of low-dose aspirin as an antiplatelet agent was largely based on measurements of serum TXB2, i.e. a mechanism-based biochemical end-point [31].

On the other hand, recent progress has been made in characterizing aspirin-insensitive or aspirin-resistant TXA2 biosynthesis in terms of potential mechanisms, biochemical and clinical read-outs. Thus, there may be clinical circumstances under which the mechanism(s) and cellular source(s) of TXA2 biosynthesis are inadequately blocked by conventional antiplatelet doses of aspirin. The clinical relevance of this phenomenon deserves further investigation.

If we go back to the best characterized example of drug resistance, i.e. resistance to antimicrobial agents, we know that this may be related to: (i) the drug not reaching the target; (ii) the drug being inactivated; and/or (iii) the drug target being altered. If we apply this reading frame to aspirin, we can think of the first possibility concerning the pharmacodynamic interaction with ibuprofen at Arg120 preventing aspirin to access Ser529; the second might apply to aspirin being extensively hydrolyzed by esterases in the gastrointestinal mucosa before it gets a chance of seeing platelets in the portal blood, but we have little evidence for this phenomenon; as far as the third mechanism is concerned, we only have a published example of COX-1 polymorphism that enhances not diminishes aspirin-induced COX-1 inhibition [32].

Physicians should be aware of potential interactions with concurrently (self-)prescribed drugs that may limit the desired pharmacodynamic effect of aspirin. Finally, given the multifactorial nature of atherothrombosis, it is not surprising that vascular events can occur while on aspirin therapy (in fact the opposite would be surprising) as they occur while on clopidogrel or on statin therapy. These are treatment failures most likely reflecting the variable importance of any particular mechanism being targeted by that particular drug in a population of patients apparently affected by the same disease.

Acknowledgements

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References

This work was supported by a grant from the Italian Ministry of Research and Education (MIUR) to the Center of Excellence on Aging of the University of Chieti ‘G. D’Annunzio'. I am indebted to Barry Coller, Garret FitzGerald, Jack Hirsh and Gerald Roth for many stimulating discussions on this topic. The expert editorial assistance of Daniela Basilico is gratefully acknowledged.

References

  1. Top of page
  2. Platelet aggregation studies
  3. Interindividual variability in the response to antiplatelet agents
  4. Aspirin-resistant thromboxane biosynthesis
  5. Clinical read-outs of aspirin resistance
  6. How to test the clinical relevance of aspirin-resistant thromboxane biosynthesis?
  7. Conclusions
  8. Acknowledgements
  9. References
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