American Heart Journal
Volume 157, Issue 3 , Pages 562.e1-562.e9, March 2009

Assessment of P2Y12 inhibition with the point-of-care device VerifyNow P2Y12 in patients treated with prasugrel or clopidogrel coadministered with aspirin

  • Christoph Varenhorst, MD

      Affiliations

    • Uppsala Clinical Research Center and Department Medical Sciences and Cardiology, Uppsala University, Sweden
    • Corresponding Author InformationReprint requests: Christoph Varenhorst, MD, Uppsala Clinical Research Center and Department Medical Sciences, Uppsala University Hospital, 751 85 Uppsala, Sweden.
    • ,
  • Stefan James, MD, PhD

      Affiliations

    • Uppsala Clinical Research Center and Department Medical Sciences and Cardiology, Uppsala University, Sweden
    • ,
  • David Erlinge, MD, PhD

      Affiliations

    • Department of Cardiology, Lund University, Lund, Sweden
    • ,
  • Oscar Ö. Braun, MD, PhD

      Affiliations

    • Department of Cardiology, Lund University, Lund, Sweden
    • ,
  • John T. Brandt, MD

      Affiliations

    • Eli Lilly and Company, Indianapolis, IN
    • ,
  • Kenneth J. Winters, MD

      Affiliations

    • Eli Lilly and Company, Indianapolis, IN
    • ,
  • Joseph A. Jakubowski, PhD

      Affiliations

    • Eli Lilly and Company, Indianapolis, IN
    • ,
  • Sylvia Olofsson, MSci

      Affiliations

    • Uppsala Clinical Research Center and Department Medical Sciences and Cardiology, Uppsala University, Sweden
    • ,
  • Lars Wallentin, MD, PhD

      Affiliations

    • Uppsala Clinical Research Center and Department Medical Sciences and Cardiology, Uppsala University, Sweden
    • ,
  • Agneta Siegbahn, MD, PhD

      Affiliations

    • Coagulation Laboratory, Department Medical Sciences, Uppsala, Sweden

Received 20 May 2008; accepted 26 November 2008. published online 09 February 2009.

Article Outline

Background

Variability in response to thienopyridines has led to the development of point-of-care devices to assess adenosine diphosphate (ADP)-induced platelet aggregation. These tests need to be evaluated in comparison to reference measurements of P2Y12 function during different thienopyridine treatments.

Methods

After a run-in on 75 mg aspirin, 110 subjects were randomized to double-blind treatment with clopidogrel 600 mg loading dose (LD)/75 mg maintenance dose (MD) or prasugrel 60 mg LD/10 mg MD. Antiplatelet effects were evaluated by VerifyNow P2Y12 (VN-P2Y12) device (Accumetrics, San Diego, CA), vasodilator-stimulated phosphoprotein (VASP) phosphorylation assay, and light transmission aggregometry (LTA). Prasugrel's and clopidogrel's active metabolite concentration were also determined.

Results

Dose- and time-dependent inhibition of P2Y12 was evident with VN-P2Y12. There was strong correlation with VN-P2Y12 and VASP or LTA for all treatments through a wide range of P2Y12 function. At high levels of P2Y12 inhibition, platelet function measured by VN-P2Y12 was maximally inhibited and could not reflect further changes seen with VASP or LTA methods. Correlation was also observed between exposure to clopidogrel's active metabolite and VN-P2Y12 during MD and LD, whereas it was observed only with prasugrel MD.

Conclusion

The VN-P2Y12 correlated strongly with inhibition of P2Y12 function, as measured with either VASP or LTA. VN-P2Y12 also correlated to exposure to the active metabolite of prasugrel and clopidogrel up to levels associated with assumed saturation of the P2Y12 receptor.

 

Platelet inhibition with clopidogrel and aspirin is recommended for patients with acute coronary syndromes and patients undergoing stent implantation.1, 2, 3 There is, however, a substantial response variability to clopidogrel with a 5% to 44% incidence of poor responders.4 Small-scale studies also suggest that poor responsiveness to clopidogrel is linked to a higher risk of cardiovascular events.5, 6, 7, 8, 9, 10

Prasugrel (CS-747, LY640315) is a novel thienopyridine adenosine diphosphate (ADP)-receptor antagonist. Like clopidogrel, it is an orally administered prodrug that, after absorption, is converted to an active metabolite with potent inhibition of platelet aggregation via antagonism of the P2Y12 receptor.11, 12 Recently, we reported greater and faster P2Y12 receptor-mediated platelet inhibition, as measured by the P2Y12-specific flow cytometric assessment of vasodilator-stimulated phosphoprotein (VASP) phosphorylation, and maximal platelet aggregation (MPA) by traditional light transmission aggregometry (LTA), with prasugrel 60 mg loading dose (LD)/10 mg maintenance dose (MD) compared with clopidogrel 600 mg LD/75 mg MD due to a more efficient generation of prasugrel's active metabolite in patients with stable coronary disease.13 The same prasugrel regimen was recently shown to significantly reduce the rate of ischemic events, including stent thrombosis, in patients with acute coronary syndrome undergoing PCI, but at the cost of an increased rate of bleeding events compared with clopidogrel in the large phase 3 Trial to Assess Improvement in Therapeutic Outcomes by Optimizing Platelet Inhibition with Prasugrel-Thrombolysis in Myocardial Infarction (TRITON-TIMI) 38 trial.14

To monitor and potentially tailor thienopyidine treatment to the individual patient, point-of-care (POC) devices to assess platelet P2Y12 inhibition in the clinical setting have been developed.15 The aim of this prespecified substudy was to evaluate the utility and reliability of a POC instrument with current gold standard reference methods for measurement of P2Y12 inhibition during treatments with different types and doses of thienopyridines. Therefore, we compared the measurement of platelet function by the Accumetrics (San Diego, CA) VerifyNow P2Y12 (VN-P2Y12) POC device with P2Y12 function estimated by the VASP and LTA assays in 110 aspirin-treated patients with stable coronary artery disease randomized to prasugrel or clopidogrel.13

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Methods 

Patients and study design 

The present study was a prespecified part of a randomized, double-blind, double-dummy, 2-arm, parallel-group study comparing platelet inhibition with prasugrel 60 mg LD/10 mg MD versus clopidogrel 600 mg LD/75 mg MD conducted in adult male and female patients with stable coronary artery disease.13 Subjects visited the research units on 5 different occasions. Blood samples for pharmacodynamic assessment were taken by direct puncture of an antecubital vein at predefined time points and collected in 3.8% sodium citrate.

All study subjects received aspirin 75 mg once daily for a run-in period of 5 to 21 days before randomization and continued throughout the study. After aspirin run-in, all patients were assigned blinded treatment and administered a LD of either prasugrel 60 mg or clopidogrel 600 mg on day 1 followed by either prasugrel 10 mg or clopidogrel 75 mg as a once-daily MD for 29 ± 3 days.

Assessment of platelet activity 

VerifyNow P2Y12 Assay 

The VerifyNow P2Y12 assay (Accumetrics) is a whole-blood, POC, light transmission–based optical detection assay that measures platelet aggregation in a cartridge containing fibrinogen-coated beads.16 The assay was performed according to the manufacturer's directions within 10 to 15 minutes of venipuncture. In addition to ADP, prostaglandin E1 is incorporated into the VN-P2Y12 assay. Prostaglandin E1 suppresses intracellular free-calcium levels and thereby reportedly reduces the contribution of activation by ADP binding to P2Y1 receptors (Accumetrics). In a separate channel, where iso-TRAP is used as the agonist, a baseline value (BASE) for platelet function is obtained, claimed by the manufacturer to be independent of level of P2Y12 blockade. Results from the device are reported as P2Y12 reaction units (PRU), percent inhibition, and BASE. The percent inhibition is calculated as [(1 − PRU/BASE) × 100]. For comparison with the device-reported percent inhibition, we used PRU baseline values to calculate percent inhibition as [(PRUADP0 − PRUt)/PRUADP0 × 100], where PRUADP0 is the baseline PRU value before administration of prasugrel or clopidogrel, and PRUt is the value for each subject at selected time points. The VN-P2Y12 assay was performed on day 1 at baseline (predose), 2 hours, and 24 hours ± 4 hours post-LD and predose during the MD period on day 14 ± 3 and day 29 ± 3.

Vasodilator-stimulated phosphoprotein phosphorylation assessed with flow cytometry 

The VASP phosphorylation assay17 was performed within 2 hours of venipuncture using a commercially available method according to the manufacturer's specifications (Platelet VASP kit, BioCytex, Marseille, France). The platelet reactivity index (PRI, %) was calculated from the corrected mean fluorescence intensity (cMFI) after incubation of the platelets with either prostaglandin E1 alone or prostaglandin E1 + ADP as follows:

The VASP assay was performed on samples obtained before the LD and at 2 hours and 24 hours ± 4 hours after the LD and before the daily MD at day 14 ± 3 and day 29 ± 3. Samples from Uppsala were analyzed on an Epics XL from Beckman Coulter (Fullurton, CA), and samples from Lund were analyzed on a FACScan from Becton Dickinson (Franklin Lakes, NJ). Synchronization between the flow cytometers was performed.

Light transmission aggregometry 

ADP-induced platelet aggregation was measured in platelet-rich plasma by LTA on day 1 at baseline (predose), 2 hours, 24 hours ± 4 hours post-LD, and predose during the MD period on day 14 ± 3 and day 29 ± 3. Light transmission aggregometry was performed within 180 minutes of venipuncture on a BioData PAP-4 optical aggregometer, with temperature maintained at 37°C and using each subject's platelet-poor plasma to set 100% light transmission. Platelet aggregation was allowed to proceed for approximately 7 minutes after addition of 20 μmol/L ADP. Maximal platelet aggregation was recorded as the highest value achieved during this observation period, whereas residual platelet aggregation (RPA) was the percent aggregation value measured at 6 minutes after the addition of ADP during the LTA-monitoring period in which MPA was also determined. Inhibition of platelet aggregation (IPA at time t = IPAt) was calculated using the formula IPAt = [1 − (MPAt/MPA0)] × 100%, where MPA0 is the MPA at baseline.

Concentration of active metabolite 

Plasma concentrations of prasugrel active metabolite (R-138727) and clopidogrel active metabolite were analyzed in samples obtained at 30 minutes, 1 hour, 2 hours, 4 hours, and 6 hours post-LD and during the MD period on day 2, day 14, and day 29 at 30 minutes, 1 hour, 2 hours, and 4 hours post-MD. The area under the plasma time-concentration curve (AUC) of prasugrel's and clopidogrel's respective active metabolites in individual patients were estimated using an established population pharmacokinetic model.18 Further details on sampling and laboratory analyses were previously described.13

Statistical analyses 

A repeated-measure linear mixed-effect model was used to evaluate VN-P2Y12 PRU, with PRU at each time as responses, treatment and study site as fixed factors, and the baseline value of PRU as a covariate. The differences between treatments at each time point were calculated with the corresponding 90% CI and P values.

Pearson correlation was used to assess relationship between methods. Cohen κ statistics were calculated to evaluate the extremes of inhibition using VASP and LTA methods and to assess the correlation with VN-P2Y12. The first and last quartiles were used as cutoffs. Lin's concordance correlation was used to assess agreement between VN-P2Y12 device–reported percent inhibition and “calculated percent inhibition.” The calculated percent inhibition was derived as the percentage of decrease from baseline in PRU. The relationship between the logarithmically transformed active metabolite AUC and PRU was also evaluated with Pearson correlation.

All P values reported are 2-sided and regarded as statistically significant if <.05. No adjustments for multiplicity were made because the results are to be considered exploratory. The software used for statistical analyses was SAS Version 9.1 (SAS Institute Inc, Cary, NC).

This study was funded by Daiichi Sankyo Company, Limited (Tokyo, Japan), and Eli Lilly and Company.

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Results 

Patients 

The 2 treatment groups were not significantly different with respect to demographic and clinical characteristics except a small difference in age (<3 years). Details of the baseline characteristics and clinical events were reported previously.13

Platelet inhibition assessed by the POC device VerifyNow P2Y12 

Administration of prasugrel or clopidogrel was associated with a time-dependent inhibition of platelet function, as measured by the VN-P2Y12 device (Figure 1, Table I). As previously reported for LTA and VASP, the 60 mg LD/10 mg MD regimen of prasugrel resulted in more rapid and greater inhibition of platelet function, as measured by VN-P2Y12, than the 600 mg LD/75 mg MD clopidogrel regimen.13 Consequently, this study provided a wide range of P2Y12 function for comparison of VN-P2Y12 to LTA and VASP measures of P2Y12 function.

Table I. Summary of VerifyNow P2Y12 reported percent inhibition after prasugrel and clopidogrel loading and maintenance doses
DayTime (h)Least squares mean percent inhibition (90% CI)Percent inhibition difference (prasugrel – clopidogrel)
Prasugrel (60 mg LD/10 mg MD) (n = 55)Clopidogrel (600 mg LD/75 mg MD) (n = 55)Difference (90% CI)P
12⁎93.8 (91.2-96.5)43.9 (37.3-50.5)49.9 (42.8-57.1)<.001
24 ± 4⁎92.5 (90.2-94.8)48.7 (42.3-55.1)43.8 (37.0-50.6)<.001
14 ± 3Predose†76.0 (72.0-80.0)40.7 (35.1-46.3)35.3 (28.5-42.2)<.001
29 ± 3Predose†73.4 (68.7-78.1)42.8 (37.0-48.5)30.7 (23.2-38.1)<.001

⁎LD.

†MD.

Treatment with either prasugrel or clopidogrel resulted in a reduction in BASE values reported by the VN-P2Y12 at all time points, although the change was most pronounced for prasugrel LD with a maximal reduction from baseline at 2 hours of 24% compared with 13% for clopidogrel (P < .0001, data not shown). To explore whether a reduction in BASE values by thienopyridines leads to underestimation of the P2Y12 inhibition, we compared device-reported percent inhibition with calculated percent inhibition using drug-free baseline PRU. The scatter plot illustrated in Figure 2 coupled with Lin's concordance correlation coefficient of 0.97 (P < .0001) suggests a strong agreement between the 2 methods of calculating the percent inhibition values. Of note, most measurements fall just above the line of unity, implying only a modest underestimation by the device-reported percent inhibition level.

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  • Figure 2. 

    Association of percent inhibition reported by VerifyNow P2Y12 and calculated percent inhibition with PRU at baseline (predose). Correlation coefficient (r) was calculated by Lin's concordance method. ■ represents prasugrel; ▲, clopidogrel; filled line, unity line.

Comparison between VN-P2Y12 and VASP 

The correlation between PRI (VASP) and PRU reported by the VN-P2Y12 device was r = 0.86 during LD phase and r = 0.81 during MD phase (Pearson, both P < .0001). The VN-P2Y12 assay was unable to differentiate low PRI levels at very high levels of P2Y12 inhibition as measured by the VASP assay, particularly after the prasugrel LD (Figure 3, A and B). When comparing the LD results, the results with prasugrel clustered in the lower left-hand corner (low PRU levels) suggesting almost near maximal P2Y12 receptor inhibition with the prasugrel 60 mg LD compared with submaximal inhibition with the clopidogrel 600 mg LD (Figure 3, A). During the MD phase, with a lower level of P2Y12 receptor blockade, there was a more even distribution of the results for both treatments through the linear response range of PRU (Figure 3, B).

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  • Figure 3. 

    Association between PRU from VN-P2Y12 and percent PRI from VASP after prasugrel and clopidogrel LD (A) and MD (B) regimens. Symbols represent individual simultaneous measurements at baseline, 2 hours, and 24 hours after LD and at predose at days 14 and 29. Correlation coefficients (r) were calculated by the Pearson method. ■ represents prasugrel; ▲, clopidogrel.

Comparison between VN-P2Y12 and RPA 

The correlation between VN-P2Y12 PRU and RPA showed a similar pattern of platelet inhibition to that observed with VN-P2Y12 PRU and PRI (VASP). The correlation between RPA to 20 μmol/L ADP and PRU reported by the VN-P2Y12 device was 0.88, P < .0001 (Pearson) during LD (baseline up to 24 hours post-LD) and 0.79 (P < .0001) during MD (Figure 4, A and B).

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  • Figure 4. 

    Association between PRU from VN-P2Y12 and RPA from LTA induced with 20 μmol/L ADP after prasugrel and clopidogrel LD (A) and MD (B) regimens. Symbols represent individual simultaneous measurements at baseline, 2 hours, and 24 hours after LD and at predose at days 14 and 29. Correlation coefficients (r) were calculated by the Pearson method. ■ represents prasugrel; ▲, clopidogrel.

Measurements showing maximal inhibition with VN-P2Y12 generally corresponded to RPA levels of submaximal inhibition, especially after prasugrel LD. Accordingly, at very high levels of P2Y12 inhibition as seen after prasugrel LD, platelet activation measured by VN-P2Y12 was maximally inhibited and could not distinguish between different levels of RPA (Figure 4, A). Also, IPA based on MPA to 20 μmol/L ADP correlated with percent inhibition as reported by the VN-P2Y12 device (0.76, P < .001) (data not shown). Similarly to RPA versus PRU, VN-P2Y12 device–reported percent inhibition reached a ceiling at higher IPA levels (data not shown).

Relationship between active metabolite and VN-P2Y12 

Prasugrel 60 mg LD achieved considerably higher AUC (μmol/L · h) of the active metabolite compared with clopidogrel 600 mg LD.13 These high AUC values corresponded to very low PRU levels with small intersubject variability in PRU for the prasugrel 60 mg LD in contrast to the clopidogrel 600 mg LD. An AUC of active metabolite above ∼0.5 μmol/L · h after the LD seemed to be associated with maximal inhibition with the VN-P2Y12 assay (Figure 5, A). During MD, prasugrel was again associated with a higher AUC of active metabolite and a lower PRU compared with clopidogrel (Figure 5, B). For subjects treated with prasugrel, there was no correlation between exposure to the active metabolite, as measured by AUC and PRU after LD (Pearson r = −0.17, P = .23), reflecting the maximum inhibition of the VN-P2Y12 device. For clopidogrel-treated subjects, the correlation between AUC and PRU was −0.6 after LD (Pearson, P < .0001) (Figure 5, A). Under MD conditions, there was a strong correlation between the AUC and PRU (Pearson r = −0.76, P < .0001) when results for prasugrel and clopidogrel were analyzed together (Figure 5, B) or when the response to each thienopyridine was analyzed individually (data not shown). In each treatment group for LD and MD analyses, one measurement, defined as an outlier, was removed.

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  • Figure 5. 

    Scatter plot of PRU from VN-P2Y12 versus AUC (μmol/L · h) of active metabolite of prasugrel and clopidogrel. A, During prasugrel and clopidogrel LD regimens with AUC determined over the 24 hours post-LD. B, During prasugrel and clopidogrel MD. Correlation coefficients (r) were calculated by the Pearson method. ■ represents prasugrel; ▲, clopidogrel.

Comparison between VN-P2Y12 and VASP or VN-P2Y12 and RPA at the extremes of platelet inhibition 

The κ values for agreement between VN-P2Y12 (PRU) and VASP (PRI, %) or VN-P2Y12 and RPA ranged from 0.35 to 0.55 under conditions with high levels of platelet inhibition (defined as values in the first quartile) (Table II). Corresponding κ values during conditions with low levels of platelet inhibition (defined as values in the fourth quartile) ranged from 0.66 to 0.79 (Table II). According to Landis and Koch,19 a κ statistic between 0.81 and 1.00 reflects almost perfect agreement, 0.61 and 0.80 reflects substantial agreement and 0.41 and 0.60 reflects moderate agreement. Our results indicate a lower level of agreement during conditions with high levels of platelet inhibition compared with low levels of platelet inhibition.

Table II. Stratification of platelet activity into quartiles and agreement of different assays (VN-P2Y12 vs VASP or VN-P2Y12 vs LTA) at the extremes of platelet inhibition during maintenance and loading dosing
Level of platelet inhibition Simple κ (95% CI)
High platelet inhibition
Quartile 1 vs quartiles 2-4
% PRI vs PRULD0.35 (0.20-0.49)
MD0.55 (0.42-0.68)
% RPA (20 μmol/L ADP) vs PRULD0.54 (0.41-0.67)
MD0.52 (0.39-0.66)
Low platelet inhibition
Quartile 4 vs quartiles 1-3
% PRI vs PRULD0.79 (0.69-0.89)
MD0.66 (0.54-0.78)
% RPA (20 μmol/L ADP) vs PRULD0.75 (0.64-0.85)
MD0.66 (0.54-0.77)

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Discussion 

This study verifies that the POC device, VerifyNow P2Y12, reliably reflects the substantially faster and greater P2Y12 receptor–mediated IPA achieved with prasugrel as compared with clopidogrel in aspirin-treated patients with stable coronary artery disease. As early as 2 hours post-LD, the VN-P2Y12 mean percent inhibition was near maximal and was twice as high with prasugrel 60 mg LD compared with clopidogrel 600 mg LD. During MD, prasugrel 10 mg showed a greater platelet inhibition than clopidogrel 75 mg with an absolute difference of approximately 30%. These results are concordant with previous results comparing prasugrel and clopidogrel LD from other phase 1 trials both in healthy subjects and in patients with stable coronary artery disease.20, 21, 22, 23 Thus, the VN-P2Y12 device would appear to be a reliable bedside tool to assess the effects of both prasugrel and clopidogrel in aspirin-treated patients with coronary artery disease both early after an LD and under maintenance treatment.

Light transmission aggregometry is commonly used when assessing thienopyridine antiplatelet effects and is traditionally considered to be the gold standard. In LTA, platelet-rich plasma is stimulated with various agonists resulting in platelet aggregation. Light transmission aggregometry using ADP as the agonist for assessing P2Y12 receptor activity is confounded because ADP-receptor subtypes other than P2Y12 can be activated and contribute to platelet aggregation.24 Other disadvantages of using LTA to monitor thienopyridine efficacy include the expense, length of sample preparation and assay time, high sample volume, and poor reproducibility.25 Therefore, other ex vivo methods specifically assessing P2Y12 activity, such as the flow cytometry VASP phosphorylation assay, have been developed to more accurately reflect the functionality of the P2Y12 receptor.17 However, this assay is still complex, time-consuming, and requires special laboratory facilities.25 The VN-P2Y12 POC device avoids the practical problems with the LTA and VASP techniques and seems to offer a convenient method for assessment of the platelet-inhibitory effect of thienopyridine treatment in daily practice. Paniccia et al26 compared VN-P2Y12 and the Platelet Function Analyzer-100 (PFA-100) with LTA and VASP in 1,267 patients undergoing percutaneous coronary intervention. In this study the VN-P2Y12 significantly correlated with both LTA and VASP, but the PFA-100 did not correlate to either LTA or VN-P2Y12 confirming that the current PFA-100 system is not suitable for monitoring the platelet response to thienopyridines.

The results in the current study demonstrate strong correlations between estimates of the effect of thienopyridines on platelet functions obtained by the VN-P2Y12 and VASP phosphorylation assay or VN-P2Y12 and the LTA method. Similar to VN-P2Y12, the VASP-phosphorylation assay provides a more specific measurement of P2Y12 activity than LTA,17 and our data illustrate a strong correlation between these 2 methods. There was also good agreement between the VN-P2Y12 assay and various parameters obtained by the LTA method, including RPA and IPA.

The correlation between PRU and RPA to 20 μmol/L ADP obtained in this study was similar to earlier comparisons through a narrower dose-response range27, 28 and across a wider dose-response range in healthy subjects randomized to clopidogrel or prasugrel.29 Furthermore, our data also illustrate that IPA in response to 20 μmol/L ADP correlated to both PRU and percent inhibition reported by the VN-P2Y12 assay (data not shown). Previous results suggest that PRU correlates to LTA irrespective of the LTA parameter used27; however, in the present study, the agreement between VN-P2Y12 and RPA-derived data was higher than IPA-derived data (data not shown). Consistent with our findings, Husted et al30 suggested that RPA is a more specific indicator of P2Y12 function than MPA. The discrepancy between these studies might be related to methodological differences.

In the clinical setting, it might be an advantage to rapidly obtain an estimate of thienopyridine-induced inhibition of platelet function.31 The VN-P2Y12 offers the advantage of determining percent inhibition without the need of a baseline measurement over LTA method, made possible by the presence of TRAP channel, included in the assay cartridge. To examine the accuracy of this parameter, we determined the thienopyridine effect on BASE levels and calculated VN-P2Y12 percent inhibition using individual baseline (drug-free) PRU levels. We also compared the calculated VN-P2Y12 percent inhibition with those of device-reported percent inhibition. The strong agreement between the instrument-reported and calculated percent inhibition suggests that small changes in BASE levels with iso-TRAP as the agonist during thienopyridine treatment did not affect device-reported percent inhibition to a clinically meaningful extent. Thus, this convenient POC approach can be used to estimate the effect of thienopyridine treatment on platelet activity in patients already on treatment.

The VN-P2Y12 device correlated well to both VASP and LTA at submaximal levels of platelet inhibition, such as those observed during MD with either prasugrel or clopidogrel. The device, however, did not differentiate among the very high degrees of platelet inhibition achieved after LD with prasugrel. Accordingly, the κ values comparing the agreement between methods at the extremes a platelet inhibition suggest only a fair to moderate agreement at high levels of platelet inhibition. Similarly, when comparing AUC of the active metabolite with PRU after the LD of prasugrel, the interindividual differences of PRU values were very small, which suggests near maximal levels of the P2Y12-receptor blockage at high plasma levels of prasugrel active metabolite. Thus, the VN-P2Y12 device might have some limitations in discriminating between very high degrees of P2Y12 inhibition after prasugrel LD. This might be a potential weakness of the current version of the device, especially when newer and more potent antiplatelet drugs acting on the P2Y12 receptor, such as prasugrel, cangrelor, and AZD6140, are evaluated in relation to risk of bleeding.

The optimal therapeutic range for thienopyridine treatment using the VN-P2Y12 device needs further evaluation in prospective clinical trials. Several smaller clinical studies have linked poor clopidogrel responsiveness to ischemic events.32, 33, 34, 35 Using VN-P2Y12 device, Price et al36 recently reported that a high PRU level measured by VN-P2Y12 device in patients on clopidogrel undergoing PCI is associated with increased rates of the combined end point of cardiovascular death, nonfatal myocardial infarction, and stent thrombosis. On the other hand, P2Y12 inhibition by thienopyridines increases the risk of bleeding.37, 38 The VN-P2Y12 device has also been used to monitor platelet aggregation after thienopyridine discontinuation to identify the recovery time for platelet function.39 Platelet function assessed with LTA has been shown to identify patients at highest risk for perioperative bleeding and transfusions.40 Using P2Y12-inhibition testing to identify patients at greater risk for bleeding complications must be further investigated.

The present study evaluated a small, stable, coronary artery disease population under a limited period in an experimental setting, restricting the results to the trial's pharmacokinetic and pharmacodynamic objectives. Although bleeding and bruising events were more common in patients treated with prasugrel, the incidence of spontaneous, not procedure-related, bleeding was very low in both groups. Therefore, the study does not allow any reliable conclusions on predicted efficacy or bleeding risk in relation to degree of platelet inhibition with any method.

Nonetheless, testing of P2Y12 inhibition to identify patients with high or low levels of platelet inhibition by thienopyridines could facilitate decisions to tailor antiplatelet treatment for patients and potentially improve both efficacy and safety of dual antiplatelet treatment. In this setting, VN-P2Y12 device offers a more accessible and convenient alternative to LTA and VASP assays for POC assessment of platelet inhibition by thienopyridines. However, the clinical impact of this approach needs further evaluation in prospective trials.

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Conclusion 

The VN-P2Y12 correlated strongly with inhibition of P2Y12 function, as measured with either VASP or LTA, across a broad range of inhibition levels. However, at very high levels of P2Y12 receptor blockade, VN-P2Y12 did not reflect further changes seen with VASP or LTA. VN-P2Y12 also correlated with exposure to the active metabolite of prasugrel and clopidogrel. Overall, the VN-P2Y12 device, designed to evaluate clopidogrel induced platelet inhibition, might not discriminate among subjects with very high levels of platelet inhibition as achieved after the prasugrel LD or in those individuals who respond well to clopidogrel.

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Disclosures 

Drs Varenhorst, James, Erlinge, Braun, Wallentin, Siegbahn, and Ms Olofsson received an institutional research grant from Daiichi Sankyo Company, Limited, and Eli Lilly and Company to conduct this research. Drs Jakubowski, Brandt, and Winters are employees and stockholders of Eli Lilly and Company. The following employees of Eli Lilly and Company contributed to the paper without co-authorship: Timothy Costigan, PhD (statistical analysis assistance); Vivian Thieu, PhD (writing and administrative assistance); and Julie Sherman, AAS (editorial assistance).

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PII: S0002-8703(08)01044-2

doi:10.1016/j.ahj.2008.11.021

American Heart Journal
Volume 157, Issue 3 , Pages 562.e1-562.e9, March 2009

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