Article

Platelet Function Testing in Clinical Practice - Experience and Views from Europe and the US

Register or Login to View PDF Permissions
Permissions× For commercial reprint enquiries please contact Springer Healthcare: ReprintsWarehouse@springernature.com.

For permissions and non-commercial reprint enquiries, please visit Copyright.com to start a request.

For author reprints, please email rob.barclay@radcliffe-group.com.

  • Views: 219

  • Likes: 0

  • Citations: 1

Average (ratings)
No ratings
Your rating

Abstract

A 68-year-old male is admitted to our hospital because of a 50-minute episode of chest pain occurring during the night. On admission he is symptom-free. His electrocardiogram shows new negative T waves in the leads V1–V3. Troponin T, measured twice, with a six-hour interval, is negative. His previous history mentions diabetes mellitus type II, hypercholesterolaemia and obesity (110 kg/176 cm/body mass index 35). He is treated with aspirin (300 mg loading dose, then 80 mg/day), clopidogrel (600 mg loading dose, then 75 mg/day), fondaparinux (2.5 mg subcutaneously/day) and metoprolol (100 mg/day). He is scheduled to undergo coronary angiography the next day. The question is: should we measure platelet function and/or genetics in this patient?

Keywords

Coronary angiography, percutaneous coronary intervention, platelet function testing, dual antiplatelet therapy,

Disclosure:The authors have no conflicts of interest to declare.

Received:

Accepted:

DOI:https://doi.org/10.15420/ecr.2011.7.3.203

Support:The publication of this article was funded by Accumetrics.

Correspondence Details:Jurrien M ten Berg, Department of Cardiology, St Antonius Hospital, PO Box 2500, 3435 CM Nieuwegein, The Netherlands. E: jurtenberg@gmail.com

Copyright Statement:

The copyright in this work belongs to Radcliffe Medical Media. Only articles clearly marked with the CC BY-NC logo are published with the Creative Commons by Attribution Licence. The CC BY-NC option was not available for Radcliffe journals before 1 January 2019. Articles marked ‘Open Access’ but not marked ‘CC BY-NC’ are made freely accessible at the time of publication but are subject to standard copyright law regarding reproduction and distribution. Permission is required for reuse of this content.

Coronary angiography and subsequent percutaneous coronary intervention (PCI) is the appropriate treatment in many patients with moderate- and high-risk acute coronary syndromes (ACS). As ACS and complications after PCI are primarily platelet-driven, adequate antiplatelet therapy is of utmost importance in patients with ACS and after coronary stent implantation. The main antiplatelet agents currently used are salicylates (aspirin) and thienopyridines (clopidogrel, prasugrel). Both aspirin and clopidogrel inhibit platelet function irreversibly. Aspirin non-selectively acetylates the enzyme cyclooxygenase-1 (COX-1), thereby permanently inhibiting the ability of the platelet to synthesise the pro-thrombotic eicosanoid thromboxane A2. Clopidogrel and prasugrel inhibit the P2Y12 adenosine diphosphate (ADP) receptor, thereby preventing ADP-mediated platelet activation. After ACS and coronary stent deployment, the currently recommended treatment strategy is the use of dual antiplatelet therapy (DAPT) for at least one year, although the optimal duration of DAPT is subject to intensive investigation. Beyond this 12-month period, aspirin monotherapy is the currently recommended antiplatelet therapy.

Based on the superior efficacy associated with DAPT in various randomised clinical trials, an earlier unselective ‘one-size-fits-all’ strategy of DAPT was overwhelmingly adapted in clinical practice. However, continued occurrence of ischaemic events, particularly stent thrombosis (ST) and myocardial infarction (MI), despite DAPT at recommended doses, raised doubts about the appropriateness of a one-size-fits-all strategy. Given the incidence of ST events (2–4 %), approximately 100,000 patients worldwide develop an ST event annually, resulting in substantial mortality (in up to 40 % of cases).1,2 Numerous pharmacodynamic studies using multiple ex vivo methods indicating the ADP-induced P2Y12 receptor reactivity demonstrated the antiplatelet response variability to clopidogrel and the association of antiplatelet resistance/high on-treatment platelet reactivity to ADP to increased occurrence of ischaemic events, particularly in patients treated with PCI and stenting.3 Thus, persistent occurrence of ischaemic events and the irrefutable demonstration of clopidogrel response variability are two potent arguments against the widely practised non-selective or one-size-fits- all strategy of administering clopidogrel therapy and provided a strong rationale for monitoring platelet function during clopidogrel therapy.

Clopidogrel Response Variability

Järemo et al. reported inter-individual variability in antiplatelet response by measuring ADP-induced fibrinogen binding.4 In a subsequent important study, clopidogrel response variability was demonstrated by ADP-induced platelet aggregation and P-selectin and activated glycoprotein (GP) IIb/IIIa expression serially assessed at baseline and for 30 days following stenting (300 mg load and 75 mg/day maintenance dose); some patients had no demonstrable antiplatelet effect. The immediate concern at that time was that these latter patients were least protected from thrombotic event occurrence. In these potentially unprotected patients, the absolute difference between pre- and post-treatment platelet aggregation was ≤10 %. These patients were regarded as ‘resistant’.5 In this study, ~30 % of patients were resistant at days 1 and 5 post-stenting, and 15 % were resistant at day 30.

Since then numerous studies, using various laboratory methods to assess ADP-induced platelet function, such as turbidimetric aggregation, flow cytometry, to measure P-selectin and activated GP IIb/IIIa expression and vasodilator-stimulated phosphoprotein (VASP) phosphorylation levels, and point-of-care methods – VerifyNow P2Y12 assay, platelet mapping with thrombelastography and the Multiplate analyser – have been used to demonstrate clopidogrel response variability and resistance. Clopidogrel response variability and resistance are now accepted pharmacodynamic phenomena.3

Linking Clinical Outcomes to Clopidogrel Resistance/High On-treatment Platelet Reactivity

The pivotal properties of the P2Y12 receptor provide the rationale for ex vivo quantification of the intensity of the ADP-P2Y12 interaction as a means of identifying patients at increased thrombotic risk who require adjustment in antiplatelet therapy. To support the latter hypothesis, subsequent translational research studies aimed to link clopidogrel resistance or high on-treatment platelet reactivity (HPR) during clopidogrel treatment to ischaemic event occurrence in patients treated with PCI. Barragan et al. first demonstrated an association between a platelet reactivity index (PRI) >50 %, measured by VASP phosphorylation, and the occurrence of thrombotic events in a case-control study.6

At the same time, Matetzky et al., using aggregometry, observed that patients undergoing primary PCI for ST-segment elevation MI who were in the lowest quartile of clopidogrel responsiveness had the highest rates of ischaemic events during follow-up.7 Subsequently, it was suggested that the level of on-treatment platelet reactivity might be a superior risk predictor compared with clopidogrel responsiveness, because platelet reactivity to ADP was variable before clopidogrel treatment in patients on aspirin therapy.8 The important relationship between HPR, as measured by turbidimetric aggregometry, and the occurrence of ischaemic events in patients treated with stents was first prospectively demonstrated in the Platelet reactivity in patients and recurrent events post-stenting (PREPARE POST-STENTING) study (upper quartile, odds ratio [OR]: 2.6).9 In the Clopidogrel loading with eptifibatide to arrest the reactivity of platelets (CLEAR-PLATELETS) study, ADP-induced aggregation was measured serially over 18–24 hours in patients undergoing stenting.10 Patients who experienced peri-procedural MI had significantly greater mean aggregation compared with patients without MI; a threshold of 50 % mean platelet aggregation was associated with MI and was proposed as a therapeutic target. Multiple subsequent studies have consistently demonstrated that HPR is an important independent risk factor for the occurrence of thrombotic/ischaemic events after PCI (see Table 1).6,7,9–34 A recent consensus document has indicated that several platelet function tests are capable of identifying patients exhibiting a high on-treatment platelet reactivity status.3

Evidence to Support the Therapeutic Window Concept

In the PREPARE POST-STENTING study, a threshold of ~50 % maximal peri-procedural aggregation (20 μM ADP) was associated with six-month ischaemic event occurrence.9 Similarly, in the Clopidogrel effect on platelet reactivity in patients with stent thrombosis (CREST) study, ~40 % aggregation (20 μM ADP) was associated with stent thrombosis occurrence.22 In a third study, ~40 % pre-procedural platelet aggregation (5 μM ADP) among patients receiving long-term clopidogrel and aspirin therapy before stenting was associated with 12-month ischaemic event occurrence.12 These data suggest that adequate protection against ischaemic events with aspirin and clopidogrel therapy might be achieved by overall low to moderate levels of post-treatment platelet reactivity in the majority of patients. These findings have important implications for bleeding risks that may accompany markedly low levels of post-treatment platelet reactivity. There is some evidence from the recent studies to support the latter concept.

An increased responsiveness to clopidogrel measured by ADP-induced platelet aggregation using multiple electrode aggregometry (MEA) was associated with a 3.5-fold increased risk of procedure-related major bleeding in patients (n=2,533) undergoing PCI.35 In this study, more bleeding events were observed in patients with <188 aggregation unit (AU)-minutes, whereas the same investigators demonstrated a significant association of ischaemic events in patients with >468 AU-minutes. In a recent study by Campo et al., it was demonstrated that ≤86 P2Y12 reaction units (PRu) as measured by VerifyNow P2Y12 assay was significantly associated with one-month bleeding events, whereas ≥239 PRu was associated with one-month ischaemic event occurrence in 507 patients undergoing PCI.36 In another study by Gurbel et al., using the Thrombelastography Platelet Mapping assay in 225 patients undergoing stenting and treated with aspirin and clopidogrel, receiver-operating characteristic (ROC) curve analysis indicated that ADP-induced platelet-fibrin clot strength (MAADP) of >47 was best associated with three-year ischaemic event occurrence, whereas ≤31 MAADP was associated with bleeding events.37 Since new P2Y12 receptor blockers, such as prasugrel and ticagrelor, are associated with greater platelet inhibition in pharmacodynamic studies and more bleeding events in clinical trials, the concept of a therapeutic window based on platelet function measurement assumes significant importance in avoiding excessive bleeding risk.

High On-treatment Platelet Reactivity Defined by Receiver-operating Characteristic Curve Analysis

Importantly, studies have emerged that have used ROC curve analysis to define a threshold of on-treatment platelet reactivity associated with the optimal combination of sensitivity and specificity to identify thrombotic risk. It should be noted that such cut-off points may depend on the subset of patients studied. In fact, to date, cut-off values have been mainly investigated in patients undergoing PCI and there may be different targets, depending on the clinical setting and baseline risk profile.3 Most importantly, the observed cut-off values for platelet reactivity had a very high negative predictive value for thrombotic/ischaemic event occurrence, an observation of great potential clinical importance. However, the positive predictive value was fairly low for all assays. The latter observations are consistent with the fact that although a major determinant of thrombotic events, high on-treatment platelet reactivity is not the sole factor responsible for these events.

Several – some preventable – mechanisms may play a role in the occurrence of a heightened on-treatment platelet reactivity status, e.g. accelerated platelet turnover, genetic factors playing a role in the metabolisation of clopidogrel, a heightened baseline platelet reactivity, poor compliance, underdosing and drug–drug interactions.38

Platelet Function Testing in the Catheterisation Laboratory

Many studies have demonstrated that platelet function testing identifies patients with high on-treatment platelet reactivity and a higher risk of thrombotic events. It has also been shown that platelet reactivity decreases with higher doses of clopidogrel, switching to prasugrel or ticagrelor or with adding GP IIb/IIIa therapy. In addition, in small studies, tailoring antiplatelet therapy based on HPR was able to improve outcome after PCI.39–41 So it might be useful to test for HPR in these patients. However, very recently, the outcome of the Gauging responsiveness with a VerifyNow assay – impact on thrombosis and safety (GRAVITAS) trial was presented. In GRAVITAS, doubling the clopidogrel maintenance dose based on platelet function results did not reduce the incidence of thrombotic events after successful PCI.42 However, the GRAVITAS trial has some serious drawbacks: platelet function testing was done after successful PCI excluding the chance of preventing peri-procedural MI; the higher maintenance dose of clopidogrel was unable to prevent HPR in 40 % of the patients; the population included was mainly stable with a lower than anticipated event rate of “only 2.3 %” after six months of follow-up dose, making the trial severely underpowered (see Table 2). The proposed remedy of doubling the maintenance dosing of clopidogrel to 150 mg has been associated with only a moderate reduction in the prevalence of high on-treatment platelet reactivity and the use of more potent P2Y12 inhibitors, such as prasugrel or ticagrelor, might therefore be a better option.43–45

Thus the issue of tailoring antiplatelet therapy based on platelet function results prior to coronary stent implantation is certainly not a closed case and more work is urgently needed, preferably with potent platelet inhibitors. Probably the patients of interest are the higher-risk population, e.g. with multi-vessel stenting, left main stenting, ACS (including unstable angina, non-ST-elevation MI [NSTEMI] and ST-elevation MI [STEMI]), and patients with important risk-contributing clinical risk factors, such as diabetes mellitus, renal failure and reduced left ventricular ejection fraction, who have more to gain from stronger antiplatelet drugs.

On the other hand, it has been demonstrated that the majority of our patients undergoing PCI do not have HPR and that these patients have a low risk of thrombotic events. Applying stronger antiplatelet drugs to those patients will probably not reduce thrombotic risk but increase the risk of bleeding. Platelet function testing may therefore be used to reduce the thrombotic risk of our high-risk patients.

Genetics – The CYP2C19 Gene and Individualised Treatment with Clopidogrel

The conversion of clopidogrel to its active metabolite depends on the CYP2C19 gene. Carriers of at least one loss-of-function CYP2C19 allele (mostly the loss-of-function *2 variant) have reduced exposure to clopidogrel’s active metabolite, and consequently reduced platelet aggregation response compared with non-carriers.46

Recently, a meta-analysis in patients treated with clopidogrel (nine studies, 9,685 subjects) demonstrated that patients carrying one or two loss-of-function allele(s) had a higher risk of major adverse cardiovascular events (MACE) with hazard ratios (HR) of 1.55 and 1.76, respectively (see Figure 1).47 Furthermore, the HR for stent thrombosis for carriers of one loss-of-function allele was 2.67, and 3.97 for carriers of two loss-of-function alleles, compared with non-carriers (see Figure 2). The genetic post hoc analysis of the Platelet inhibition and patient outcomes (PLATO),48 Clopidogrel in unstable angina to prevent recurrent events (CURE)49 and Atrial fibrillation clopidogrel trial with irbesartan for prevention of vascular events (ACTIVE)-A49 trials and one case-control study50 were not included in this meta-analysis. Representing a population of which 64 % underwent PCI, PLATO showed a CYP2C19 effect only at 30 days (HR 1.37).48 A CYP2C19–clopidogrel interaction was not found and may have been missed in the ACTIVE-A and CURE trials because of the inclusion of a lower-risk population and a low proportion (<20 %) of patients that underwent PCI.49 In addition, a case-control study showed that stent thrombosis was significantly associated with the CYP2C19*2 allele (OR 1.70), corroborating the findings of the meta-analysis (see Table 3).50

In contrast to the *2 variant, the *17 variant is associated with increased CYP2C19 enzyme activity. Subsequently, the *17 variant has been shown to decrease both platelet reactivity51 and may reduce the risk of MACE52 in response to clopidogrel. However, patients with the CYP2C19*2/*17 genotype were found to exhibit increased platelet reactivity in response to clopidogrel, as compared with patients with the CYP2C19*1/*1 genotype.51 The heightening effect of CYP2C19*2 was only partly diminished by the concomitant presence of the CYP2C19*17 allele. This might be explained by the fact that carriage of the *2 allele leads to a complete loss of enzyme function, while CYP2C19*17 only enhances existing enzyme activity.

The ABCB1 gene is important because it encodes an intestinal efflux transporter also affecting clopidogrel. Four studies investigated the association between the ABCB1 C3435T polymorphism and adverse cardiovascular events in clopidogrel-treated patients.50,52–54 In the French registry of acute STEMI and NSTEMI (FAST-MI) cohort, TT genotype carriers showed a 72 % increased risk of death, non-fatal MI or stroke at one year compared with the CC genotype (see Figure 3).54 Also, the Trial to assess improvement in therapeutic outcomes by optimising platelet inhibition with prasugrel – Thrombolysis in myocardial infarction (TRITON–TIMI) 38 showed a 72 % increased risk of cardiovascular death, MI or stroke at 15 months compared with CC or CT patients (see Figure 4).53 No clopidogrel ABCB1 C3435T interaction was found in two other studies.50,52

Response to clopidogrel is most likely multigenic given its complex mode of action and hepatic activation. In fact, patients with two CYP2C19 loss-of-function alleles and at least one ABCB1 C3435T variant allele were shown to be at the highest risk of thrombotic events (HR 5.31).53,54

Recently, the common functional PON1 Q192R gene polymorphism was shown to determine pharmacokinetics, platelet response and clinical antithrombotic efficiency of clopidogrel among patients undergoing PCI. However, confirmation in other studies is awaited before PON1 polymorphism may be clinically relevant.

Tailor-made Medicine

The question of whether CYP2C19 genotypes and/or platelet function testing should assist clinical decision making is heavily debated. The US Food and Drug Administration recently approved a new label for clopidogrel that includes a boxed warning stating that there is diminished effectiveness of clopidogrel in homozygous CYP2C19 loss-of-function allele carriers and alternative treatment should be considered in those patients. Following the new label, a high-quality report (American College of Cardiology Foundation [ACCF]/American Heart Association [AHA] clinical alert) was released providing nuanced guidance for clinicians on how to deal with the warning. It is recommended that genetic testing of CYP2C19*2 should be considered before initiating clopidogrel treatment in patients at moderate/high risk including patients undergoing elective high-risk PCI procedures, with other therapies such as prasugrel as a proposed alternative.55 Taking into account the knowledge about CYP2C19 loss-of-function variants and clopidogrel response, several strategies for antiplatelet treatment can be proposed. First, the decreased responsiveness to clopidogrel in loss-of-function allele carriers may be overcome by doubling the dose. There are indications that this may be a good strategy because a double dose for carriers of the *2 variant was shown to counterbalance the reduced platelet aggregation response to clopidogrel in a study of 60 patients undergoing elective PCI.56 Whether higher dosing for loss-of-function allele carriers translates into better outcomes remains to be determined.39 As previously stated in GRAVITAS, doubling the clopidogrel maintenance dose based on platelet function results did not reduce the incidence of thrombotic events after successful PCI.42 A second strategy could be treating all patients with prasugrel, of which the efficacy is independent of a patient’s CYP2C19 genotype. However, this would imply an enormous increase in drug cost, now generic clopidogrel is available. In addition, prasugrel is associated with an increased risk of bleeding compared with clopidogrel and is contraindicated for patients with a history of stroke or transient ischaemic attack (TIA). Finally, a genotype-guided antiplatelet strategy could be proposed in which patients are genotyped and those without a CYP2C19 loss-of-function allele receive clopidogrel, whereas prasugrel treatment would be initiated for patients (without history of stroke/TIA) carrying one or two loss-of-function alleles. Similarly, the allocation of prasugrel could be based on platelet function testing. By using modern techniques, genotype results can be obtained within a couple of hours.

It may be an option to use genotyping in clopidogrel-naive patients (STEMI, NSTEMI within four hours of a clopidogrel loading dose) and platelet function testing in those on adequate clopidogrel (75 mg for at least a week, 300 mg loading >24 hours or 600 mg loading >4 hours).

A cost-effectiveness study will soon be started at our department to evaluate this strategy. In addition, various trials are currently ongoing to investigate treatment strategies assisted by genetic testing (see www.clinicaltrials.gov NCT01134380, NCT00995514 and NCT01177592). It is highly questionable whether we should await the results from these studies to warrant genotyping-guided antiplatelet treatment. After all, there are data available showing there is a clopidogrel safety issue for patients carrying a CYP2C19 loss-of-function allele that can be solved using genotyping.

Future Directions

Obviously, the most important issue at the present time is the need to establish clinical utility (clinical implications with clear treatment decisions) to support the value of point-of-care platelet function testing in daily practice. The crucial question is whether dose or treatment adjustments on the basis of the results of platelet function testing improve clinical outcomes. Moreover, the magnitude of benefit and its clinical relevance depend highly on the absolute risk profile of the studied patient population. Crucial components in this clinical evaluation include an assessment of the absolute risk of the patient (multi-vessel coronary disease, left main intervention, STEMI, NSTEMI and important contributing clinical risk factors, such as diabetes mellitus, renal failure and reduced left ventricular ejection fraction).

No matter what level of evidence is required, it will be necessary to develop simple clinical algorithms to aid physicians in their interpretation and use of platelet function testing. The issue of personalised antiplatelet therapy on the basis of platelet function testing is important and worthy of effort and further study, as the cardiology community eagerly awaits the results of ongoing clinical trials.

Conclusions

DAPT is the cornerstone of treatment for patients with ACS and for those undergoing PCI. However, a wide inter-individual variability in the response to antiplatelet agents (and, in particular, clopidogrel) exists, resulting in a clinically relevant high on-treatment platelet reactivity state in a substantial subset of patients. At the present time, the routine measurement of platelet reactivity in the catheterisation laboratory has not been widely implemented because the management of patients who exhibit a high on-treatment platelet reactivity state is unknown. Moreover, the absolute risk profile of the studied patient population appears to be of crucial importance. Therefore, simple clinical algorithms to aid physicians in their interpretation and use of platelet function testing should be developed before the routine use of platelet function measurements in the routine care of patients with cardiovascular disease is recommended.

References

  1. Cutlip DE, Baim DS, Ho KK, et al., Stent thrombosis in the modern era: a pooled analysis of multicenter coronary stent clinical trials, Circulation, 2001;103:1967–71.
    Crossref | PubMed
  2. Iakovou I, Schmidt T, Bonizzoni E, et al., Incidence, predictors, and outcome of thrombosis after successful implantation of drug-eluting stents, JAMA, 2005;293:2126–30.
    Crossref | PubMed
  3. Bonello L, Tantry US, Marcucci R, et al., Consensus and future directions on the definition of high on-treatment platelet reactivity to adenosine diphosphate, J Am Coll Cardiol, 2010;56:919–33.
    Crossref | PubMed
  4. Järemo P, Lindahl TL, Fransson SG, Richter A, Individual variations of platelet inhibition after loading doses of clopidogrel, J Intern Med, 2002;252:233–8.
    Crossref | PubMed
  5. Gurbel PA, Bliden KP, Hiatt BL, O’Connor CM, Clopidogrel for coronary stenting: response variability, drug resistance, and the effect of pretreatment platelet reactivity, Circulation, 2003;107:2908–13.
    Crossref | PubMed
  6. Barragan P, Bouvier JL, Roquebert PO, et al., Resistance to thienopyridines: clinical detection of coronary stent thrombosis by monitoring of vasodilator-stimulated phosphoprotein phosphorylation, Catheter Cardiovasc Interv, 2003;59:295–302.
    Crossref | PubMed
  7. Matetzky S, Shenkman B, Guetta V, et al., Clopidogrel resistance is associated with increased risk of recurrent atherothrombotic events in patients with acute myocardial infarction, Circulation, 2004;109:3171–5.
    Crossref | PubMed
  8. Samara WM, Bliden KP, Tantry US, Gurbel PA, The difference between clopidogrel responsiveness and posttreatment platelet reactivity, Thromb Res, 2005;115:89–94.
    Crossref | PubMed
  9. Gurbel PA, Bliden KP, Guyer K, et al., Platelet reactivity in patients and recurrent events post-stenting: results of the PREPARE POSTSTENTING Study, J Am Coll Cardiol, 2005;46:1820–6.
    Crossref | PubMed
  10. Gurbel PA, Bliden KP, Zaman KA, et al., Clopidogrel loading with eptifibatide to arrest the reactivity of platelets: results of the Clopidogrel Loading With Eptifibatide to Arrest the Reactivity of Platelets (CLEAR PLATELETS) study, Circulation, 2005;111:1153–9.
    Crossref | PubMed
  11. Gurbel PA, Bliden KP, Saucedo JF, et al., Bivalirudin and clopidogrel with and without eptifibatide for elective stenting: Effects on platelet function, thrombelastographic indices and their relation to periprocedural infarction: Results of the CLEAR PLATELETS-2 study, J Am Coll Cardiol, 2009;53:648–57.
    Crossref
  12. Bliden KP, DiChiara J, Tantry US, et al., Increased risk in patients with high platelet aggregation receiving chronic clopidogrel therapy undergoing percutaneous coronary intervention: Is the current antiplatelet therapy adequate?, J Am Coll Cardiol, 2007;49:657–66.
    Crossref | PubMed
  13. Lev EI, Patel RT, Maresh KJ, et al., Aspirin and clopidogrel drug response in patients undergoing percutaneous coronary intervention: The role of dual drug resistance, J Am Coll Cardiol, 2006;47:27–33.
    Crossref | PubMed
  14. Blindt R, Stellbrink K, de Taeye A, et al., The significance of vasodilator-stimulated phosphoprotein for risk stratification of stent thrombosis, Thromb Haemost, 2007;98:1329–34.
    PubMed
  15. Cuisset T, Frere C, Quilici J, et al., High post-treatment platelet reactivity is associated with a high incidence of myonecrosis after stenting for non-ST elevation acute coronary syndromes, Thromb Haemost, 2007;97:282–7.
    PubMed
  16. Frere C, Cuisset T, Quilici J, et al., ADP-induced platelet aggregation and platelet reactivity index VASP are good predictive markers for clinical outcomes in non-ST elevation acute coronary syndrome, Thromb Haemost, 2007;98:838–43.
    PubMed
  17. Geisler T, Langer H, Wydymus M, et al., Low response to clopidogrel is associated with cardiovascular outcome after coronary stent implantation, Eur Heart J, 2006;27:2420–5.
    Crossref | PubMed
  18. Geisler T, Grass D, Bigalke B, et al., The Residual Platelet Aggregation after Deployment of Intracoronary Stent (PREDICT) score, J Thromb Haemost, 2008;6:54–61.
    Crossref | PubMed
  19. Hochholzer W, Trenk D, Bestehorn HP, et al., Impact of the degree of peri-interventional platelet inhibition after loading with clopidogrel on early clinical outcome of elective coronary stent placement, J Am Coll Cardiol, 2006;48:1742–50.
    Crossref | PubMed
  20. Price MJ, Endemann S, Gollapudi RR, et al., Prognostic significance of post-clopidogrel platelet reactivity assessed by a point-of-care assay on thrombotic events after drugeluting stent implantation, Eur Heart J, 2008;29:992–1000.
    Crossref | PubMed
  21. Gurbel PA, Antonino MJ, Bliden KP, et al., Platelet reactivity to adenosine diphosphate and long-term ischemic event occurrence following percutaneous coronary intervention: A potential antiplatelet therapeutic target, Platelets, 2008;19:595–604.
    Crossref | PubMed
  22. Gurbel PA, Bliden KP, Samara W, et al., The clopidogrel Resistance and Stent Thrombosis (CREST) study, J Am Coll Cardiol, 2005;46:1827–32.
    Crossref | PubMed
  23. Buonamici P, Marcucci R, Miglironi A, et al., Impact of platelet reactivity after clopidogrel administration on drugeluting stent thrombosis, J Am Coll Cardiol, 2007;49:2312–7.
    Crossref | PubMed
  24. Bonello L, Paganelli F, Arpin-Bornet M, et al., Vasodilatorstimulated phosphoprotein phosphorylation analysis prior to percutaneous coronary intervention for exclusion of postprocedural major adverse cardiovascular events, J Thromb Haemost, 2007;5:1630–6.
    Crossref | PubMed
  25. Cuisset T, Hamilos M, Sarma J, et al., Relation of low response to clopidogrel assessed with point-of-care assay to periprocedural myonecrosis in patients undergoing elective coronary stenting for stable angina pectoris, Am J Cardiol, 2008;101:1700–3.
    Crossref | PubMed
  26. Migliorini A, Valenti R, Marcucci R, et al., High residual platelet reactivity after clopidogrel loading and long-term clinical outcome after drug-eluting stenting for unprotected left main coronary disease, Circulation, 2009;120:2214–21.
    Crossref | PubMed
  27. Marcucci R, Gori AM, Paniccia R, et al., Cardiovascular death and nonfatal myocardial infarction in acute coronary syndrome patients receiving coronary stenting are predicted by residual platelet reactivity to ADP detected by a point-ofcare assay: A 12-month follow-up, Circulation, 2009;119:237–42.
    Crossref | PubMed
  28. Bonello L, Camoin-Jau L, Arques S, et al., Adjusted clopidogrel loading doses according to vasodilatorstimulated phosphoprotein phosphorylation index decrease rate of major adverse cardiovascular events in patients with clopidogrel resistance: A multicenter randomized prospective study, J Am Coll Cardiol, 2008;51:1404–11.
    Crossref | PubMed
  29. Bonello L, Camoin-Jau L, Armero S, et al., Tailored clopidogrel loading dose according to platelet reactivity monitoring to prevent acute and subacute stent thrombosis, Am J Cardiol, 2009;103:5–10.
    Crossref | PubMed
  30. Valgimigli M, Campo G, de Cesare N, et al., Tailoring Treatment With Tirofiban in Patients Showing Resistance to Aspirin and/or Resistance to Clopidogrel (3T/2R) Investigators. Intensifying platelet inhibition with tirofiban in poor responders to aspirin, clopidogrel, or both agents undergoing elective coronary intervention: results from the double-blind, prospective, randomized Tailoring Treatment with Tirofiban in Patients Showing Resistance to Aspirin and/or Resistance to Clopidogrel study, Circulation, 2009;119:3215–22.
    Crossref | PubMed
  31. Patti G, Nusca A, Mangiacapra F, et al., Point-of-care measurement of clopidogrel responsiveness predicts clinical outcome in patients undergoing percutaneous coronary intervention results of the ARMYDA-PRO (Antiplatelet therapy for Reduction of MYocardial Damage during Angioplasty-Platelet Reactivity Predicts Outcome) study, J Am Coll Cardiol, 2008;52:1128–33.
    Crossref | PubMed
  32. Sibbing D, Braun S, Morath T, et al., Platelet reactivity after clopidogrel treatment assessed with point-of-care analysis and early drug-eluting stent thrombosis, J Am Coll Cardiol, 2009;53:849–56.
    Crossref | PubMed
  33. Cuisset T, Frere C, Quilici J, et al., Predictive values of posttreatment adenosine diphosphate-induced aggregation and vasodilator-stimulated phosphoprotein index for stent thrombosis after acute coronary syndrome in clopidogreltreated patients, Am J Cardiol, 2009;104:1078–82.
    Crossref
  34. Breet NJ, Van Werkum JW, Bouman HJ, et al., Comparison of platelet function tests in predicting clinical outcome in patients undergoing coronary stent implantation, JAMA, 2010;303:754–62.
    Crossref | PubMed
  35. Sibbing D, Schulz S, Braun S, et al., Antiplatelet effects of clopidogrel and bleeding in patients undergoing coronary stent placement, J Thromb Haemost, 2010;8:250–6.
    Crossref | PubMed
  36. Campo G, Parrinello G, Ferraresi P, et al., Prospective evaluation of on-clopidogrel platelet reactivity over time in patients treated with percutaneous coronary intervention relationship with gene polymorphisms and clinical outcome, J Am Coll Cardiol, 2011;57:2474–83.
    Crossref | PubMed
  37. Gurbel PA, Bliden KP, Navickas IA, et al., Adenosine diphosphate-induced platelet-fibrin clot strength: a new thrombelastographic indicator of long-term poststenting ischemic events, Am Heart J, 2010;160:346–54.
    Crossref | PubMed
  38. Elsenberg EH, van Werkum JW, van de Wal RM, et al., The influence of clinical characteristics, laboratory and inflammatory markers on 'high on-treatment platelet reactivity' as measured with different platelet function tests, Thromb Haemost, 2009;102:719–27.
    Crossref | PubMed
  39. Bonello L, Camoin-Jau L, Arques S, et al., Adjusted clopidogrel loading doses according to vasodilatorstimulated phosphoprotein phosphorylation index decrease rate of major adverse cardiovascular events in patients with clopidogrel resistance: a multicenter randomized prospective study, J Am Coll Cardiol, 2008;51:1404–11.
    Crossref | PubMed
  40. Pena A, Collet JP, Hulot JS, et al., Can we override clopidogrel resistance?, Circulation, 2009;119:2854–7.
    Crossref | PubMed
  41. Valgimigli M, Campo G, de Cesare N, et al., Intensifying platelet inhibition with tirofiban in poor responders to aspirin, clopidogrel, or both agents undergoing elective coronary intervention: results from the double-blind, prospective, randomized Tailoring Treatment with Tirofiban in Patients Showing Resistance to Aspirin and/or Resistance to Clopidogrel study, Circulation, 2009;119:3215–22.
    Crossref | PubMed
  42. Price MJ, Berger PB, Teirstein PS, et al., Standard- vs highdose clopidogrel based on platelet function testing after percutaneous coronary intervention: the GRAVITAS randomized trial, JAMA, 2011;305:1097–1105.
    Crossref | PubMed
  43. Angiolillo DJ, Shoemaker SB, Desai B, et al., Randomized comparison of a high clopidogrel maintenance dose in patients with diabetes mellitus and coronary artery disease: results of the Optimizing Antiplatelet Therapy in Diabetes Mellitus (OPTIMUS) study, Circulation, 2007;115:708–16.
    Crossref | PubMed
  44. Barker CM, Murray SS, Teirstein PS, et al., Pilot study of the antiplatelet effect of increased clopidogrel maintenance dosing and its relationship to CYP2C19 genotype in patients with high on-treatment reactivity, JACC Cardiovasc Interv, 2010;3:1001–7.
    Crossref | PubMed
  45. von Beckerath N, Kastrati A, Wieczorek A, et al., A doubleblind, randomized study on platelet aggregation in patients treated with a daily dose of 150 or 75 mg of clopidogrel for 30 days, Eur Heart J, 2007;28:1814–9.
    Crossref | PubMed
  46. Mega JL, Close SL, Wiviott SD, et al., Cytochrome p-450 polymorphisms and response to clopidogrel, N Engl J Med, 2009;360:354–62.
    Crossref | PubMed
  47. Mega JL, Simon T, Collet JP, et al., Reduced-function CYP2C19 genotype and risk of adverse clinical outcomes among patients treated with clopidogrel predominantly for PCI: a meta-analysis, JAMA, 2010;304:1821–30.
    Crossref | PubMed
  48. Wallentin L, James S, Storey RF, et al., Effect of CYP2C19 and ABCB1 single nucleotide polymorphisms on outcomes of treatment with ticagrelor versus clopidogrel for acute coronary syndromes: a genetic substudy of the PLATO trial, Lancet, 2010;376:1320–8.
    Crossref | PubMed
  49. Pare G, Mehta SR, Yusuf S, et al., Effects of CYP2C19 genotype on outcomes of clopidogrel treatment, N Engl J Med, 2010;363:1704–14.
    Crossref | PubMed
  50. Harmsze AM, van Werkum JW, Ten Berg JM, et al., CYP2C19*2 and CYP2C9*3 alleles are associated with stent thrombosis: a case-control study, Eur Heart J, 2010;31:3046–53.
    Crossref | PubMed
  51. Sibbing D, Gebhard D, Koch W, et al., Isolated and interactive impact of common CYP2C19 genetic variants on the antiplatelet effect of chronic clopidogrel therapy, J Thromb Haemost, 2010;8:1685–93.
    Crossref | PubMed
  52. Tiroch KA, Sibbing D, Koch W, et al., Protective effect of the CYP2C19 *17 polymorphism with increased activation of clopidogrel on cardiovascular events, Am Heart J, 2010;160:506–12.
    Crossref | PubMed
  53. Mega JL, Close SL, Wiviott SD, et al., Genetic variants in ABCB1 and CYP2C19 and cardiovascular outcomes after treatment with clopidogrel and prasugrel in the TRITON-TIMI 38 trial: a pharmacogenetic analysis, Lancet, 2010;376:1312–9.
    Crossref | PubMed
  54. Simon T, Verstuyft C, Mary-Krause M, et al., Genetic determinants of response to clopidogrel and cardiovascular events, N Engl J Med, 2009;360:363–75.
    Crossref | PubMed
  55. Holmes DR Jr, Dehmer GJ, Kaul S, et al., ACCF/AHA Clopidogrel clinical alert: approaches to the FDA "boxed warning": a report of the American College of Cardiology Foundation Task Force on Clinical Expert Consensus Documents and the American Heart Association, Circulation, 2010;122:537–57.
    Crossref | PubMed
  56. Gladding P, Webster M, Zeng I, et al., The pharmacogenetics and pharmacodynamics of clopidogrel response: an analysis from the PRINC (Plavix Response in Coronary Intervention) trial, JACC Cardiovasc Interv, 2008;1:620–7.
    Crossref | PubMed
  57. Marcucci R, Gori AM, Paniccia R, et al., Cardiovascular death and nonfatal myocardial infarction in acute coronary syndrome patients receiving coronary stenting are predicted by residual platelet reactivity to ADP detected by a point-ofcare assay: a 12-month follow-up, Circulation, 2009;119:237–42.
    Crossref | PubMed
  58. Price MJ, Endemann S, Gollapudi RR, et al., Prognostic significance of post-clopidogrel platelet reactivity assessed by a point-of-care assay on thrombotic events after drugeluting stent implantation, Eur Heart J, 2008;29:992–1000.
    Crossref | PubMed
Previous Volume Article Next Volume Article