Antiplatelet effects of prasugrel vs. double clopidogrel in patients on hemodialysis and with high on-treatment platelet reactivity

Authors


Dimitrios Alexopoulos, Department of Cardiology, Patras University Hospital, Rion 26500, Patras, Greece.
Tel./fax: +30 2610 992 941.
E-mail: dalex@med.upatras.gr

Abstract

Summary. Background: High on-treatment platelet reactivity (HTPR) is frequent in patients on hemodialysis (HD) receiving clopidrogel. Objectives: The primary aim of this study was to determine the antiplatelet effects of prasugrel vs. high-dose clopidogrel in patients on HD with HTPR. Patients/Methods: We performed a prospective, single-center, single-blind, investigator-initiated, randomized, crossover study to compare platelet inhibition by prasugrel 10 mg day−1 with that by high-dose 150 mg day−1 clopidogrel in 21 patients on chronic HD with HTPR. Platelet function was assessed with the VerifyNow assay, and genotyping was performed for CYP2C19*2 carriage. Results: The primary endpoint of platelet reactivity (PR, measured in P2Y12 reaction units [PRU]) was lower in patients receiving prasugrel (least squares [LS] estimate 156.6, 95% confidence interval [CI] 132.2–181.1) than in those receiving high-dose clopidogrel (LS 279.9, 95% CI 255.4–304.3), P < 0.001). The LS mean differences between the two treatments were − 113.4 PRU (95% CI − 152.9 to − 73.8, P < 0.001) and − 163.8 PRU (95% CI − 218.1 to − 109.2, P < 0.001) in non-carriers and carriers of at least one CYP2C19*2 allele, respectively. HTPR rates were lower for prasugrel than clopidogrel, in all patients (19% vs. 85.7%, P < 0.001) and in non-carriers (25.7% vs. 80%, P = 0.003). All carriers continued to show HTPR while receiving high-dose clopidogrel, but none showed it while receiving prasugrel. Conclusions: In HD patients exhibiting HTPR following standard clopidogrel treatment, prasugrel 10 mg day–1 is significantly more efficient than doubling the clopidogrel dosage in achieving adequate platelet inhibition. Neither effect seems to be influenced by carriage of the loss-of-function CYP2C19*2 allele.

Introduction

Patients with chronic kidney disease (CKD) present with accelerated atherosclerosis, have high cardiovascular morbidity and mortality, and are at significant risk of thrombotic complications, including stent thrombosis [1,2]. Specifically, patients with end-stage renal disease undergoing hemodialysis (HD) present with heightened platelet reactivity (PR), which is further increased by the exposure of their blood to the dialysis membrane [3]. Impaired renal function is associated with reduced clopidogrel-induced antiplatelet effects and a greater prevalence of high on-treatment platelet reactivity (HTPR) [4–6]. Furthermore, in CKD patients, post-percutaneous coronary intervention (PCI) cardiovascular mortality has been linked to inadequate P2Y12 platelet receptor inhibition by clopidogrel [7,8]. All the above factors underscore the need for more effective platelet-inhibiting strategies in CKD and particularly in patients on HD at risk for recurrent ischemic events.

In PCI patients with HTPR, doubling the standard 75-mg clopidogrel dose has been proposed as a strategy to overcome it [9,10], although it is rather ineffective in diabetic patients or those carrying the CYP2C19*2 loss-of-function allele [11,12]. Patients with moderate to severe CKD have been reported to exhibit a response to double clopidogrel similar to that in patients without renal disease or with mild CKD [13], although, in a small cohort of patients with more severe CKD (75% on HD), platelet responsiveness to clopidogrel did not improve with an increase in the clopidogrel dosage [4]. On the other hand, prasugrel achieves faster, more consistent and higher platelet inhibition than double clopidogrel, including in patients with type 2 diabetes mellitus and coronary artery disease, and in those with HTPR either post-PCI or while receiving chronic clopidogrel treatment [12,14–17]. The purpose of this study was to analyze the comparative antiplatelet action of prasugrel 10 mg day–1 vs. clopidogrel 150 mg day–1 in patients on HD who exhibited HTPR while receiving the standard 75 mg day–1 clopidogrel maintenance dose.

Materials and methods

Patient population and study protocol

We performed a prospective, single-center, single-blind, investigator-initiated, randomized, crossover study to compare platelet inhibition by prasugrel 10 mg day−1 with that by high-dose 150 mg day–1 clopidogrel in patients on chronic HD with HTPR. All patients on regular maintenance HD for > 6 months in our institution and receiving chronic (> 2 months) treatment with clopidogrel 75 mg day–1 were considered for PR assessment immediately prior to HD. Patients were excluded if they had a history of stroke/transient ischemic attack, bleeding diathesis, chronic oral anticoagulation treatment, contraindications to antiplatelet therapy, acute coronary syndrome, hemodynamic instability, PCI or coronary artery bypass grafting within the previous 3 months, platelet count of < 100 000 μL−1, and hematocrit of < 28%.

Patients with HTPR (as defined below) were randomized (day 0) in a 1 : 1 ratio, by the use of computerized random-number generation by an independent investigator, to clopidogrel 150 mg day–1, or prasugrel 10 mg day–1, until day 15 post-randomization. A day 15 ± 2 visit was performed for PR measurement and safety evaluation. Patient compliance with antiplatelet therapy was assessed by interview and meticulous tablet counting, followed by direct crossover to the alternative therapy for an additional 15 days without an intervening washout period. At day 30 ± 2, patients returned for the clinical and laboratory assessment as performed on the day 15 visit. Physicians and operators who performed platelet function testing were blinded as to the actual drug used, and an independent physician monitored bleeding and adverse event data. A flow chart diagram of the study is shown in Fig. 1. All studied patients were stable, with no change in their medication during the study, and were undergoing regular maintenance HD for approximately 4 h three times per week, with an appropriate HD dose as reflected by a Kt/Vurea of > 1.4. All HD characteristics were kept constant during the study period.

Figure 1.

 Study flow chart. PRU, P2Y12 reaction units.

Detailed informed consent was obtained in writing from all patients. The study protocol adhered to the Declaration of Helsinki and was approved by the ethics committee of the University Hospital of Patras, Greece.

Platelet function and genotyping assays

Peripheral venous blood samples were drawn in a fasting state with a loose tourniquet through a short venous catheter inserted into a forearm vein 12–16 h after drug ingestion and immediately before HD. The first 2–4 mL of blood was discarded to avoid spontaneous platelet activation, and blood was collected in 3.2% citrate (1.8-mL draw plastic Vacuette tubes; Greiner, Monroe, NC, USA). Platelet function testing was performed with the VerifyNow (Accumetrics, San Diego, CA, USA) point-of-care P2Y12 assay. Results are reported as P2Y12 reaction units (PRU), and a value ≥ 235 was considered to be an indication of HTPR [18].

Genotyping was performed for the single-nucleotide polymorphism CYP2C19*2 (G681A) with a previously described method [12].

Endpoints

Endpoints were prespecified in the study protocol and statistical analysis plan. The primary endpoint was PR assessed at the end of the two (pre-crossover and post-crossover) study periods. The HTPR rate at the end of the same periods was a secondary endpoint. Bleeding (major, minor or minimal according to the TIMI criteria) and major adverse cardiac events (cardiovascular death, myocardial infarction, and stroke) were also evaluated.

Statistical analysis

For sample size calculation, we hypothesized that prasugrel 10 mg would result in a PR absolute difference of 80 PRU relative to clopidogrel 150 mg (with the assumption that the within-patient standard deviation of the response variable is 70 PRU). With a power of 90% and a two-sided alpha level of 0.05, at least 19 patients in total were required to reach statistical significance, on the basis of the above assumptions.

Categorical data are presented as frequencies and group percentages, and continuous data with normal distribution as means ± standard deviation. Fisher’s exact test and two-sample t-tests were used for comparison of categorical and normally distributed continuous data respectively. The observed genotype frequencies were compared with those expected for a population in Hardy–Weinberg equilibrium by use of a chi-square test with one degree of freedom. The Mann–Whitney U-test was used for comparison of time on HD (presented as medians and ranges) between the two treatment groups. P-values of < 0.05 were considered to indicate statistical significance. Analyses were performed with spss for Windows (version 16.0; SPSS, Chicago, IL, USA). The primary study endpoint was analyzed with a hierarchical ancova (linear mixed-effects) model, with patient indicator as random effect, study period, treatment sequence (carryover effect) and treatment as fixed factors, and PR at baseline as a covariate. Least squares estimates – which assess parameters by minimizing the squared discrepancies between observed data and their expected values – of the mean difference are presented, with 95% confidence intervals (CIs) and a two-sided P-value for the treatment effect. Separate analyses of covariance (ancova) were conducted for the pre-crossover and post-crossover periods, with treatment as fixed effect and PR at baseline as a covariate. Additionally, to test for period effect, we compared the absolute PR mean difference the between pre-crossover and post-crossover periods within the two treatment sequences, with a two-sample t-test. To test for carryover effect, we compared the PR average in the pre-crossover and post-crossover periods between the two sequences with a two-sample t-test. The secondary study endpoint was analyzed with a Prescott test. Bleeding events and major adverse cardiac events are reported in a descriptive manner.

Results

From May to June 2010, of 25 HD patients who were receiving 75 mg of clopidogrel daily and had no exclusion criteria, we identified 21 patients (84.0%) who had HTPR, and these were randomized to the two treatment arms. The reason for clopidogrel administration was coronary artery disease (n = 8), peripheral artery disease (n = 4) or prophylactic therapy in patients at risk for vascular access problems (HD synthetic graft thrombosis and stent implantation, n = 11). The baseline characteristics of randomized patients are presented in Table 1. The primary endpoint of PR at the end of the two treatment periods was significantly lower in patients receiving prasugrel than in those receiving high-dose clopidogrel (Table 2). No period or carryover effect was found. Data for the pre-crossover and post-crossover periods are shown in Fig. 2. The secondary endpoint of HTPR rate at the end of the two treatment periods was lower for prasugrel than for clopidogrel (P < 0.001) (Table 3). Individual PR values according to treatment, with the HTPR threshold, are shown in Fig. 3.

Table 1.   Baseline characteristics of randomized patients
 Overall, N = 21Clopidogrel, N = 10Prasugrel, N = 11P
  1. ACE, angiotensin-converting enzyme; BMI, body mass index; CAD, coronary artery disease; CCB, calcium channel blocker; DM, diabetes mellitus; Hb, hemoglobin; HD, hemodialysis; Ht, hematocrit; PAD, peripheral artery disease; PPI, proton pump inhibitor; PR, platelet reactivity; PRU, P2Y12 reaction units. Values are expressed as means ± SD, medians (range) or n (%). P-value refers to comparison between patients initially randomized to clopidogrel and those to prasugrel arm.

Age (years)61.2 ± 12.058.2 ± 12.264.0 ± 11.60.3
Male gender14 (66.7)6 (60)8 (72.7)0.7
BMI26.2 ± 3.527.3 ± 4.325.2 ± 2.30.2
Time on HD (months)96 (6–204)80 (6–187)96 (11–204)0.9
Hyperlipidemia15 (71.4)7 (70.0)8 (72.7)1.0
Hypertension18 (85.7)10 (100)8 (72.7)0.2
DM10 (47.6)5 (50.0)5 (45.5)1.0
Smoking5 (23.8)3 (30.0)2 (18.2)0.6
PAD10 (47.6)5 (50.0)5 (45.5)1.0
History of CAD13 (61.9)5 (50.0)8 (72.7)0.4
Ht (%)33.3 ± 3.232.2 ± 2.834.4 ± 3.20.1
Hb (g dL−1)10.5 ± 1.210.1 ± 1.010.8 ± 1.30.1
Platelets (μL–1)226520 ± 67359247800 ± 64323207180 ± 669810.2
Albumin (mg dL−1)3.9 ± 0.33.8 ± 0.34.1 ± 0.10.03
Parathormone (pg mL−1)218.9 ± 171.9262 ± 228.4170.4 ± 53.10.3
Medication    
 Omega-3 fatty acids8 (38.1)3 (30.0)5 (45.5)0.7
 Statin9 (42.9)5 (50.0)4 (36.4)0.7
 PPIs16 (76.2)8 (80.0)8 (72.7)1.0
 β-Blocker13 (61.9)7 (70.0)6 (54.5)0.7
 Nitrates5 (23.8)2 (20.0)3 (27.3)1.0
 ACE inhibitors3 (14.3)2 (20.0)1 (9.1)0.6
 CCBs4 (19.0)3 (30.0)1 (9.1)0.3
 Aspirin10 (47.6)5 (50.0)5 (45.5)1.0
PR day 0 (PRU)335.1 ± 54.2342.5 ± 68.0328.5 ± 40.20.6
At least one CYP2C19*2 allele6 (28.6)2 (20)4 (36.4)0.6
Two CYP2C19*2 alleles2 (9.5)0 (0)2 (18.2)0.5
Table 2.   Platelet reactivity analysis
Platelet reactivityPrasugrel
LS estimates (95% CI)
Clopidogrel
LS estimates (95% CI)
LS mean difference (95% CI)P-value
  1. CI, confidence interval; LS, least squares. Maximum likelihood linear mixed model, with patient ID as random effect, period, treatment sequence and treatment as fixed effects, and baseline P2Y12 reaction units as a covariate.

Combined data (pre-crossover and post-crossover)N = 21
156.7 (132.2–181.1)
N = 21
279.9 (255.5–304.4)
− 123.2 (− 157.8 to − 88.7)< 0.001
Combined data (pre-crossover and post-crossover) for non-carriers of the CYP2C19*2 alleleN = 15
162.1 (134.1–190.0)
N = 15
275.5 (247.5–303.4)
− 113.4 (− 152.9 to − 73.8)< 0.001
Combined data (pre-crossover and post-crossover) for carriers of at least one CYP2C19*2 alleleN = 6
135.1 (97.6–172.6)
N = 6
298.9 (261.4–336.4)
− 163.8 (− 218.3 to − 109.2)< 0.001
Figure 2.

 Platelet reactivity PR (in P2Y12 reaction units [PRU]) by treatment sequence. PR is significantly lower in patients receiving prasugrel than in those receiving high-dose clopidogrel. Least squares estimates with 95% confidence intervals are presented.

Table 3.   High on-treatment platelet reactivity rates
 PrasugrelClopidogrelP-value
  1. Values are expressed as n (%).

Combined data (pre-crossover and post-crossover)N = 21
4 (19.0)
N = 21
18 (85.7)
< 0.001
Combined data (pre-crossover and post-crossover) for non-carriersN = 15
4 (26.7)
N = 15
12 (80.0)
0.003
Combined data (pre-crossover and post-crossover) for carriersN = 6
0 (0)
N = 6
6 (100)
0.07
Figure 3.

 Individual platelet reactivity responses according to treatment. Lines represent means, and error bars represent 95% confidence intervals. PRU, P2Y12 reaction units; HTPR, high on-treatment platelet reactivity.

Genotyping revealed carriage of at least one CYP2C19*2 allele in six (28.6%) patients (two homozygotes). No deviations from Hardy–Weinberg equilibrium were detected. PR was significantly lower for prasugrel than for clopidogrel 150 mg in non-carriers and carriers (P < 0.001 for both) (Table 2). Data for the pre-crossover and post-crossover periods are shown in Fig. 4. HTPR rates in non-carriers and carriers are shown in Table 3. Although our study was not designed and adequately powered for this analysis, comparison of PR in relation to CYP2C19*2 carriage separately for each treatment arm showed that clopidogrel 150 mg resulted in similar PR reductions in carriers and non-carriers (mean PRU difference − 11.9, 95% CI − 54.4 to 30.7, P = 0.6). Prasugrel also resulted in similar PR reductions in carriers and non-carriers (mean PRU difference 27.7, 95% CI −39.2 to 94.7, P = 0.4).

Figure 4.

 Platelet reactivity (PR) by treatment sequence in non-carriers and carriers of the CYP2C19*2 allele. Least squares estimates with 95% confidence intervals are presented. PR is significantly lower for prasugrel in both carriers and non-carriers. PRU, P2Y12 reaction units.

One patient during the pre-crossover period and another one during the post-crossover period experienced a minor bleeding event, both while receiving prasugrel.

Discussion

There are four main findings of the present study: (i) increasing the dosage of clopidogrel to 150 mg in HD patients with HTPR is highly ineffective at reducing PR; (ii) prasugrel 10 mg is much more effective in this population, although a sizeable proportion (19%) demonstrate ‘prasugrel resistance’; (iii) the well-known CYP2C19*2 allele may not have a role in determining HTPR in HD patients; and (iv) the HTPR rate may be particularly high in such patients, although its determination was not the primary aim of the study.

Doubling the standard 75-mg clopidogrel dose is rather ineffective in diabetic patients or those carrying the CYP2C19*2 allele [11,12], with response rates of 40% and 53%, respectively. In the Gauging Responsiveness with A VerifyNow assay – Impact on Thrombosis And Safety (GRAVITAS) trial, a clopidogrel double dose did not improve the outcome in stable HTPR patients post-PCI [19]. There is a paucity of data concerning the response of CKD and HD patients with HTPR to high-dose clopidogrel treatment. Patients with moderate to severe CKD exhibited a response to double clopidogrel similar to that of patients without renal disease or with mild CKD [13], although in this study the authors suggested that high-risk patients with CKD should receive higher clopidogrel doses. Park et al. [4] randomized 36 patients with severe CKD (75% on HD) to either 75 mg or 150 mg of clopidogrel, and found no difference in PR between the two arms, with 83.3% and 72.2% of patients, respectively, remaining hyporesponsive. We have shown that double clopidogrel has limited effectiveness in patients on HD demonstrating HTPR, with only 15% of them becoming responsive. This may be particularly important, as patients with CKD (with or without HD) and HTPR are at high risk for cardiovascular events [7,8], and are therefore most likely to benefit from therapeutic approaches such as adequate platelet inhibition, which have been shown to improve outcomes.

It has been hypothesized that high-risk patients with CKD undergoing PCI for acute coronary syndrome might be better candidates for new P2Y12 antagonists, such as prasugrel or ticagrelor [13,20]. Our study supports the idea that prasugrel 10 mg day–1 results in much higher platelet inhibition and a lower rate of hyporesponsiveness than double clopidogrel in patients on HD with HTPR. Although this was only a pharmacodynamic study in a selected population, our results are related to those of the TRial to Assess Improvement in Therapeutic Outcomes by Optimizing Platelet InhibitioN with Prasugrel–Thrombolysis In Myocardial Infarction 38 (TRITON-TIMI 38) in patients with creatinine clearance of < 60 mL min−1, where the clinical primary endpoint occurred in 15.1% in the prasugrel group and in 17.5% in the clopidogrel group, respectively (14% reduction in risk, albeit not significant) [21].

Carriers of the CYP2C19*2 allele post-PCI are at increased risk while receiving standard clopidogrel treatment, owing to reduced production of its active metabolite [22]. The CYP2C19*2 allele is the major known genetic determinant of clopidogrel action, although it accounts for only 4.5–12% of the overall variability in clopidogrel response [23,24].

In our HD population, CYP2C19*2 carriage had no significant effect on the superiority of prasugrel over double clopidogrel. Although the number of studied patients was relatively small, double clopidogrel was mostly ineffective in HD patients, with all carriers (100%) but also 80% of non-carriers remaining hyporesponsive to clopidogrel dose adjustment. Other, unspecified, factors are likely to be responsible for this weakened action of clopidogrel in HD patients, surpassing the efficient active metabolite production in non-carriers of the CYP2C19*2 allele. On the contrary, an effective prasugrel antiplatelet effect in patients on HD was present in both carriers and non-carriers of the CYP2C19*2 allele. A similar superiority of prasugrel over double clopidogrel in patients exhibiting HTPR post-PCI has been recently described in both carriers and non-carriers of the CYP2C19*2 allele [12]. In addition, in the outcome study of TRITON-TIMI 38, a beneficial effect of prasugrel was present irrespective of the carriage status [25].

Platelet function is impaired in patients on HD, even without thienopyridine treatment. Potential mechanisms for this may include an increase in the platelet turnover rate, poor bioavailability, coagulation disorders, extrinsic factors such as uremia and anemia, and an altered clopidogrel metabolism. In HD patients, one can speculate that platelet inhibition might occur at lower clopidogrel or prasugrel exposures than required for comparable platelet inhibition in healthy subjects. This was not confirmed in our study in HD patients with HTPR, as 19% of them remained hyporesponsive even to prasugrel. It is of note that cases of prasugrel resistance have been occasionally reported in non-HD patients as well [26,27]. Because of the increased bleeding tendency in HD patients, safety is of major concern with the use of antiplatelet agents. In the entire cohort of TRITON-TIMI 38 acute coronary syndrome patients (treated with either clopidogrel or prasugrel), creatinine clearance was independently associated with serious bleeding of any cause [28]. However, in the Platelet Inhibition and Patient Outcomes (PLATO) trial in acute coronary syndrome patients with CKD (excluding patients on HD), ticagrelor – a potent P2Y12 inhibitor – was not associated with a significant increase in major bleeding [20]. In a study of prasugrel pharmacokinetics and pharmacodynamics involving 28 patients on HD, despite a significantly lower exposure to prasugrel’s active metabolite, maximal platelet aggregation was similar to that of healthy volunteers, and there were no drug-related or bleeding-related adverse events [29]. In our study, lasting for 30 days in 21 patients on HD, we did not observe any major bleeding, and only two patients had minor bleeds while receiving prasugrel.

In a small study, it had been suggested that the extent of platelet inhibition by clopidogrel in HD patients was comparable to that in those with normal renal function, as reported in another study [30]. However, HTPR has been linked to impaired renal function in several reports [6,7,31], with a more prominent association being present in diabetic patients [5]. Htun et al. [8] recently described an increasing rate of low response to clopidogrel with worsening Kidney Disease Outcomes Quality Initiative index, although with no specific description of the rate of response to clopidogrel in the 31 HD patients. Clopidogrel metabolism may be altered in the setting of chronic HD, as suggested by Park et al. [4] These investigators reported an 83.3% prevalence of HTPR in 18 patients with severe CKD (13 on HD). In our exclusively chronic HD patient population, HTPR was found in most studied patients, raising the major question of whether clopidogrel administration might be pointless in such patients.

Clinical implications

HD patients represent a challenging, growing population, because they have a high incidence of acute myocardial infarction (AMI), a high mortality rate post-AMI, and marked underutilization of medications and other therapies used post-AMI, and the medication administered to them has been derived from randomized clinical trials that have historically excluded such patients. Our study implies that clopidogrel administration in HD patients requiring platelet inhibition, i.e. because of a history of coronary artery disease post-PCI or for prevention of thrombotic complications of the access site, may be inadequate, as even doubling of the clopidogrel dose does not suffice to overcome the particularly frequent HTPR observed in such patients. Our findings may also offer a potential explanation of why HD is an independent predictor of late and very late stent thrombosis [2]. Genotyping for CYP2C19*2 allele detection is most likely not helpful, as the ineffectiveness of clopidogrel in inhibiting HD patients’ platelets does not seem to be related to its presence. All of these factors are of outmost importance in cases where long-term platelet inhibition is mandatory, e.g. in patients with PCI using drug-eluting stents. If adequate platelet inhibition cannot be guaranteed, at least in most cases, the need for an alternative strategy is obvious. Prasugrel seems to be a more attractive solution, although it is not ideal, as a worrying level of prasugrel resistance was observed in 19% of the cases. The antithrombotic and safety balance of prasugrel in HD patients definitely needs further investigation, as does whether this is accompanied by an improved clinical outcome.

Limitations

VerifyNow testing was the only technique used for platelet function assessment, and our results need to be reproduced with other platelet function measurement assays. The levels of active metabolites of clopidogrel and prasugrel were not measured, and their metabolism might be altered in the setting of long-term dialysis. Only the CYP2C19*2 polymorphism was studied. However, this is well recognized as the main determinant of the clopidogrel variability caused by genetic factors. Any conclusions concerning the effect of CYP2C19*2 carriage on clopidogrel and prasugrel antiplatelet action should be concidered as hypothesis-generating only, as the study was not designed for this purpose. This study was not powered to detect clinical safety differences between the two treatment groups or to allow any meaningful conclusions to be drawn in this regard.

Conclusions

In CKD patients on HD exhibiting HTPR following standard clopidogrel treatment, prasugrel 10 mg day−1 is significantly more effective than a double maintenance dose of clopidogrel in achieving adequate platelet inhibition. These effects seem to be unrelated to the presence of the CYP2C19*2 loss-of-function allele.

Disclosure of Conflict of Interests

The authors state that they have no conflict of interest.

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