Thienopyridine therapy influences late outcome after coronary stent implantation

High on-treatment platelet reactivity – definition and measurement

In the last decade, several studies revealed inter-patient response variability to antiplatelet agents: patients who display negligible or no responses to these drugs are considered poor responders, or “resistant” to treatment. In order to identify poor responders to an antiplatelet drug, laboratory tests of platelet function that specifically explore the platelet activation pathway that is targeted by the drug should be utilised. In addition, they should be performed both at baseline and during treatment: however, most studies explored platelet function during antiplatelet treatment, in order to identify those patients with “high on-treatment platelet reactivity” (HPR), which exposes them to increased risk of major adverse cardiovascular events (MACE). Many tests of platelet function have been used, most of which are able to identify patients at risk of MACE. Unfortunately, universal cut-off values for HPR have not been clearly established yet. In addition, the concordance among different tests in the identification of patients at risk is very poor and the most effective and safe treatment for patients at risk is still unknown.

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

BACKGROUND

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

OBJECTIVES

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

DATA SOURCES

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

METHODS

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

RESULTS

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

LIMITATIONS

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

CONCLUSIONS

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

STUDY REGISTRATION

This study is registered as PROSPERO 2012:CRD42012002151.

FUNDING

The National Institute for Health Research Health Technology Assessment programme.

Justification of 150 mg clopidogrel in patients with high on-clopidogrel platelet reactivity

BACKGROUND

The GRAVITAS trial showed that 150 mg clopidogrel did not improve outcome in patients with high on-clopidogrel platelet reactivity (HPR) screened by the VerifyNow assay. We aimed to determine the impact of 150 mg clopidogrel in stable angina patients with HPR identified with conventional aggregometry (LTA).

MATERIALS AND METHODS

Clopidogrel-naive stable angina patients before ad hoc percutaneous coronary intervention were recruited into a randomized, double-blind, placebo-controlled trial (NCT00638326). Twelve to 24 h after the 600-mg loading dose of clopidogrel, ADP(5μM)-stimulated maximal (AGGmax), late platelet aggregation (AGGlate) and vasodilator-stimulated phosphoprotein phosphorylation (VASP-PRI) were evaluated. Patients with HPR (AGGmax ≥ 34%) were randomly allocated to 75 or 150 mg clopidogrel after 4 weeks. After control platelet function measurements at day 28, 75 mg clopidogrel was administered to all patients until 1 year.

RESULTS

The study was prematurely terminated at the stage of 200 enroled patients. Administration of 150 mg clopidogrel significantly reduced platelet aggregation (AGGmax: 45·0 ± 6·8 vs. 33·8 ± 15·1, P < 0·01; AGGlate: 27·1 ± 14·7 vs. 13·8 ± 18·0, P < 0·01) and VASP-PRI (57·5 ± 15·2 vs. 37·2 ± 17·1; P < 0·01), while platelet reactivity remained unchanged in patients with HPR receiving 75 mg clopidogrel. The higher maintenance dose of clopidogrel was associated with a significant reduction in cardiovascular (CV) death and myocardial infarction (MI) (0% vs. 11·4%, P = 0·04) and in CV death, MI or target vessel revascularization (24·6% vs. 3·1%; P = 0·01) during 1 year.

CONCLUSIONS

One-month administration of 150 mg maintenance dose of clopidogrel reduces platelet reactivity and might decrease the risk of thrombo-ischaemic complications in stable angina patients with HPR identified by LTA.

Impact of genetic variants on post-clopidogrel platelet reactivity in patients after elective percutaneous coronary intervention

Aim: To determine the effect of various SNPs on post-clopidogrel platelet reactivity and clinical outcome. Materials & methods: Cytochrome 2C19 (CYP2C19) loss-of-function (LOF; *2, *3) and gain-of-function (GOF; *17) allelic variants, together with ABCB1 (3435 C→T and 2677 G→T/A) and paraoxonase-1 (PON-1; 192 Q→R) SNPs were analyzed in 189 patients after elective stent implantation who participated in a randomized, placebo-controlled trial (NCT00638326). Platelet reactivity was determined with light transmission aggregometry and vasodilator stimulated phosphoprotein phosphorylation (VASP-PRI) 12–24 h after 600 mg clopidogrel. High on-treatment platelet reactivity (HTPR) was defined according to the consensus definition (ADP 5 µM >46%; VASP-PRI>50%). Results: In the case of CYP2C19 genotypes, a gene–dose effect was observed in ADP reactivity with the lowest values in GOF homozygotes and the highest degree in patients carrying two LOF alleles. The odds for HTPR also increased with the number of LOF alleles. There were no significant differences in platelet reactivity according to PON-1 or ABCB1 genotypes. In multivariate analysis, the presence of a CYP2C19 LOF allele turned out to be the independent determinant of HTPR. Although the study was not powered to clinical outcome (not LOF heterozygotes), only patients with two LOF alleles had a significantly higher risk for cardiovascular death, myocardial infarction or unplanned target vessel revascularization at 1 year compared with non-LOF carriers. Conclusion: Genetic variants in CYP2C19 have a gene–dose effect on post-clopidogrel platelet reactivity, with homozygote LOF carriers having the highest risk for HTPR and for adverse ischemic events. Neither ABCB1 nor PON-1 genotypes significantly influenced platelet reactivity or outcome.

Original submitted 28 February 2011; Revision submitted 5 May 2011

Comparison of conventional aggregometry with VASP for monitoring P2Y12-specific platelet inhibition

No consensus exists regarding the optimal estimate of light transmission aggregometry (LTA) to reflect P2Y12 ADP receptor inhibition in patients receiving thienopyridine therapy. Currently, the only completely P2Y12-receptor specific method is the measurement of vasodilator stimulated phosphoprotein (VASP) phosphorylation (PRI) with flow cytometry. In the current analysis, we aimed to test the superiority of the late platelet aggregation value over other estimates of light transmission aggregometry in determining P2Y12-receptor inhibition by comparing them to VASP-PRI. On-clopidogrel platelet reactivity was measured in 121 clopidogrel-naïve patients who underwent elective coronary stent implantation. Samples for LTA and VASP assessments were drawn at 12-18 hours after a 600-mg loading dose of clopidogrel and 25 days after the intervention. ADP 5 µM-induced maximal aggregation (AGGmax), 6-minute late aggregation (AGGlate), 6-minute disaggregation (disAGG) and area under the aggregation curve (AUC) were compared to VASP-PRI. Categorical agreement with VASP-defined normal and high platelet reactivity (HPR: PRI ≥ 50%) was calculated according to the optimal cutoff values obtained with receiver-operating characteristic (ROC) curves. The evaluation of 242 measurements showed significant, moderate-strength correlations between VASP-PRI and LTA estimates without the superiority of AGGlate over other estimates (AGGmax: ρ = 0.47; AGGlate: ρ = 0.45; disAGG: ρ = -0.44; AUC: ρ = 0.50). Notably, there were considerable intra-individual differences between VASP and LTA testing. LTA estimates were similar in classifying patients to VASP-defined normal or HPR categories (AGGmax: κ = 0.41; AGGlate: κ = 0.45; disAGG: κ = 0.44; AUC: κ = 0.44). When all estimates of LTA were compared in multivariable models, AUC proved to be the independent linear determinant of VASP-PRI and the best predictor of HPR. Estimates of LTA seem equal in determining the degree of P2Y12-receptor inhibition or in predicting VASP-defined HPR without the superiority of AGGlate over others. These results reject a commonly used hypothesis and might contribute to the standardization of the LTA assay.

Prognostic significance of high on-clopidogrel platelet reactivity after percutaneous coronary intervention: systematic review and meta-analysis

BACKGROUND

A growing number of observational studies suggest that high on-clopidogrel platelet reactivity (HPR) is associated with recurrent thrombotic events after percutaneous coronary intervention. We aimed to perform an updated systematic review and meta-analysis on the clinical relevance of HPR to summarize the available evidence and to define more precise effect estimates.

METHODS

Relevant observational studies published between January 2003 and February 2010 were searched that presented intent-to-treat analyses on the clinical relevance of HPR measured with an adenosine diphosphate (ADP)-specific platelet function assay. The main outcome measures were cardiovascular (CV) death, definite/probable stent thrombosis (ST), nonfatal myocardial infarction (MI), and a composite end point of reported ischemic events. The outcome parameters were pooled with the random-effect model via generic inverse variance weighting.

RESULTS

Twenty studies comprising a total number of 9,187 patients qualified. High on-clopidogrel platelet reactivity appeared to be a strong predictor of MI, ST, and the composite end point of reported ischemic events (odds ratios [95% CI]: 3.00 [2.26-3.99], 4.14 [2.74-6.25], and 4.95 [3.34-7.34], respectively; P < .00001 for all cases). According to the meta-analysis, patients with HPR had a 3.4-fold higher risk for CV death compared with patients with normal ADP reactivity (odds ratio 3.35, 95% CI 2.39-4.70, P < .00001). Although there were large differences in the methodology as well as in the definition of HPR between studies, the predicted risk for CV death, MI, or ST was not heterogeneous (I(2): 0%, 0%, and 12%, respectively; P = not significant for all cases).

CONCLUSIONS

High on-clopidogrel platelet reactivity, measured by an ADP-specific platelet function assay, is a strong predictor of CV death, MI, and ST in patients after percutaneous coronary intervention.