Introduction
Dual antiplatelet therapy with aspirin and the thienopyridine clopidogrel is the therapy of choice in patients undergoing percutaneous coronary intervention (PCI) with stent implantation [
1,
2]. However, despite this treatment ischaemic events still occur, and multiple studies have clearly demonstrated a relationship between the magnitude of on-treatment platelet reactivity and the occurrence of atherothrombotic events [
3‐
8]. Therefore, novel antiplatelet agents with more consistent response rates among patients have been introduced. One of these is the thienopyridine prasugrel which, similar to clopidogrel, is a specific, irreversible adenosine diphosphate (ADP)-receptor antagonist, but it is faster acting and a more potent platelet inhibitor. The Trial to Assess Improvement in Therapeutic Outcomes by Optimising Platelet Inhibition with Prasugrel–Thrombolysis in Myocardial Infarction 38 (TRITON-TIMI 38) demonstrated a significant risk reduction for the occurrence of thrombotic events in patients with an acute coronary syndrome (ACS) undergoing PCI with prasugrel as compared with clopidogrel [
9]. This reduction, however, was counterbalanced by a 30% increased risk of bleeding, suggesting a link between platelet reactivity inhibition and bleeding [
10,
11].
Three subgroups of patients were identified as having less clinical efficacy from prasugrel and greater absolute risk of bleeding than the overall cohort. These included (1) patients with a prior history of stroke or transient ischaemic attack (TIA), (2) the elderly (≥75 years of age), and (3) patients with a body weight <60 kg [
9]. However, the underlying pathobiology leading to this increased bleeding risk remains to be elucidated.
It has been hypothesised that the higher bleeding rate may be due to a stronger prasugrel-induced inhibition of ADP-induced platelet aggregation in these subgroups. Since prasugrel has been introduced only recently, few pharmacodynamic data are available. However, in the POPular study (The Do Platelet Function Assays Predict Clinical Outcomes in Clopidogrel Pretreated Patients Undergoing Elective PCI study), the influence of the other thienopyridine clopidogrel on the inhibition of platelet reactivity was determined in patients undergoing elective coronary stent implantation. The aim of the present sub-analysis study was to establish whether the on-clopidogrel platelet reactivity is lower in the three subpopulations at risk for bleeding as compared with in other patients in a large cohort of patients on clopidogrel undergoing elective coronary stenting.
Methods
Study population
The POPular study was a prospective, observational study that included consecutive patients with established coronary artery disease scheduled for elective PCI with stent implantation. The entry and exclusion criteria were described in the original publication [
3]. All patients had been on dual antiplatelet therapy with clopidogrel and low-dose aspirin of 80–100 mg daily for at least 10 days, unless they were on long-term treatment with coumarin derivates. This study complied with the Declaration of Helsinki and was approved by the local institutional review board. Written informed consent was obtained from every patient prior to elective PCI.
Blood sampling and platelet function testing
Prior to heparinisation, whole blood was drawn from the femoral or radial artery sheath. After discarding the first 10 ml of blood, samples were collected into citrated tubes (3.2% for light transmittance aggregometry (LTA) and the VerifyNow® system and 3.8% for PFA). The magnitude of platelet reactivity was measured using three platelet function tests in parallel; the platelet function analyser (PFA-100) and ‘classical’ LTA. All methods were performed between 30 min and 2 h after blood collection.
Light transmittance aggregometry
LTA was quantified in non-adjusted platelet-rich plasma on a four-channel APACT 4004 aggregometer (LABiTec, Arensburg, Germany). Platelet-poor plasma was set as 100% aggregation, and maximal (peak) platelet aggregation (%) was measured spontaneously and after stimulation by ADP in final concentrations of 5 and 20 μmol/L.
The VerifyNow® P2Y12 assay
The VerifyNow® P2Y12 assay (Accumetrics, Inc, San Diego, USA) is an automated whole blood, cartridge-based method to determine the magnitude of platelet agglutination as induced by ADP/prostaglandin E
1 [
12]. The results are reported in P2Y12 Reaction Units.
PFA-100
The PFA-100 System (Siemens Healthcare Diagnostics Products GmbH, Germany) measures platelet function, in particular adhesion and aggregation, in whole blood under high shear conditions (5000 s
−1) [
13]. The time needed to form a platelet plug occluding the aperture cut into a membrane coated with collagen/ADP, an agonist, was determined and reported as closure time in seconds, which is inversely related to platelet reactivity. A closure time of >300 s was referred to as ‘non-closure’.
Statistical analysis
Continuous variables are presented as mean (SD). Categorical data are reported as frequencies (percentages). Categorical variables were compared using the χ2 test or Fisher’s exact test when frequencies were <5. The distribution of variables was determined by the Kolmogorov–Smirnov goodness-of-fit test. Normally distributed continuous variables were compared with a two-sided unpaired t test.
Logistic regression modelling was performed to identify independent correlates of the magnitude of platelet reactivity and to adjust for potential confounders. Being part of a high-risk group was entered as a dichotomous variable. All univariate variables with a p value <0.10 were included in multivariable analysis (binary logistic regression).
Discussion
Whereas TRITON-TIMI 38 demonstrated that prasugrel, a thienopyridine resulting in lower on-treatment platelet reactivity as compared with clopidogrel, was associated with less recurrent atherothrombotic events in ACS patients undergoing PCI, an increased risk of bleeding was observed in patients treated with prasugrel [
9]. The presence of a therapeutic window was already acknowledged by Paracelsus, who stated as early as in the fifteenth century that ‘
All drugs are poisons, the benefit depends on the dosage’ [
14]. There is currently a growing body of evidence supporting the association between bleeding and adverse outcomes, including myocardial infarction, stroke and death [
15‐
17], and several studies have suggested a link between the inhibition of platelet reactivity and the occurrence of bleeding [
10,
18‐
20]. Thus, the identification of a window of platelet inhibition that on the one hand prevents atherothrombotic events and on the other hand does not lead to an increase in bleeding events, is of utmost importance [
21].
The TRITON-TIMI 38 study has identified three subgroups of patients with a higher risk of bleeding during treatment with prasugrel: (1) patients with a prior history of stroke or TIA, (2) the elderly (>75 years of age), and (3) patients with a body weight less <60 kg [
9]. However, the underlying pathobiology leading to this increased bleeding risk remains to be elucidated. It has been hypothesised that the higher bleeding rate might be the consequence of a stronger prasugrel-induced inhibition of ADP-induced platelet aggregation in these subgroups. Since prasugrel was introduced only recently, few pharmacodynamic data are available. The present study, with the aim to determine whether on-clopidogrel platelet reactivity is lower in these risk subgroups as compared with other patients, demonstrated that in the two high-risk subgroups for bleeding, patients >75 years of age and patients with previous stroke, on-treatment platelet reactivity is contrarily increased. However, in the third high-risk subgroup for bleeding, patients with a low body weight, on-treatment platelet reactivity is indeed decreased. When these data are applied to the hypothesis that prasugrel leads to a stronger platelet inhibition, it seems that in TRITON-TIMI 38, a stronger response to prasugrel might have only led to more bleeds in patients with low body weight.
These observations are in line with results from a recent analysis of 16 phase-I clinical pharmacological studies performed in healthy patients. In this analysis, no effect of advanced age on the availability of the active metabolite of prasugrel was perceived [
22]. On the contrary, in the TRITON-TIMI 38, patients ≥ 75 years had 19% higher exposure to the active metabolite as compared with those <75 years and even 25% higher exposure as compared with patients <60 years of age [
23]. However, in the latter the concentration of the active metabolite was not measured, but estimated from its inactive metabolite. In contrast, body weight had the greatest influence on exposure to the active metabolite of prasugrel in both clinical pharmacology studies and the TRITON-TIMI 38, with an increase in exposure as body weight decreased. Exposure was 40% higher in individuals <60 kg as compared with those ≥60 kg [
22,
23]. Modelling data suggest that decreasing the maintenance dose of prasugrel to 5 mg in these subjects would reduce exposure to the active metabolite to levels consistent with those <75 years and ≥60 kg [
22]. Both European and American regulatory agents therefore recommend a daily dose of 5 mg in patients <60 kg. For patients ≥75 years, the US Food and Drug Administration advises that prasugrel is generally not recommended, but might be considered in patients at high risk of recurrent atherothrombotic events at a maintenance dose of 10 mg in those ≥60 kg [
23]. On the contrary, the European Medicines Agency recommends that prasugrel is avoided in the elderly, but if used, the dose should be halved to 5 mg [
23]. Taking the findings from the present study into account and the fact that elderly patients have a higher risk of bleeding as compared with younger individuals, even when not on thienopyridine therapy [
24], we do not consider prasugrel to result in an excess in platelet inhibition, thereby accounting for the increased bleeding risk in this population.
In conclusion, the results from the present analysis confirmed previous studies and identified body weight as the most influential covariate on the magnitude of ADP-induced platelet reactivity, which might have implications for prasugrel maintenance dose in daily clinical practice.