Clopidogrel versus ticagrelor in the treatment of Chinese patients undergoing percutaneous coronary intervention: effects on platelet function assessed by platelet function tests and mean platelet volume

It is known that dual antiplatelet therapy (DAPT) which consists of aspirin and one of the P2Y12 receptor antagonists has been used for secondary prevention of thrombotic events, particularly in acute coronary syndromes (ACS) and percutaneous coronary intervention (PCI) with stenting [1, 2]. Clopidogrel, a popular P2Y12 receptor inhibitor, is most widely used. However, the wide inter- and intra-individual variability in clopidogrel response represents a significant clinical limitation [3, 4], and high on-treatment platelet reactivity (HPR) to adenosine diphosphate (ADP) is now regarded as a well-established marker reflecting the thrombotic recurrence risk [5, 6]. In comparison with clopidogrel (a thienopyridine P2Y12R antagonist), which can bind irreversibly to the ADP receptor, ticagrelor, a novel non-thienopyridine ADP antagonist, reversibly inhibits the ADP P2Y12 receptor located on platelets, preventing platelet activation and aggregation [7]. To date, most clinical studies have demonstrated that ticagrelor can provide a more rapid effect on platelet inhibition and a more favorable pharmacodynamic profile when compared with clopidogrel [8, 9]. In addition, it has been recommended in the current guideline that new DAPT with aspirin and ticagrelor can be administered to patients with ACS after stenting (IIA) [10].

It has been suggested that ethnic differences in the response to P2Y12 inhibitors could influence the pharmacodynamic effects in different races. East Asian patients are at a higher bleeding risk under DAPT, compared with non-East Asians [11]. In Korean and Japanese patients with ACS, ticagrelor presented a higher incidence of significant bleeding, compared with clopidogrel [12, 13]. Unfortunately, few Chinese patients were included in the studies that investigated the responses to clopidogrel and ticagrelor. Therefore, exploring the effects of the two popular antiplatelet drugs on platelet function in Asian/Chinese patients is increasingly urgent and will provide more potent clinical and experimental evidence to guide the use of these antiplatelet drugs among East Asian populations.

Platelet activation by ADP is central to the development of atherothrombosis. Platelet function tests (PFT) play an important role in evaluating individual antiplatelet drug responses and the therapeutic effects of different treatments. Therefore, PFT are recommended to guide the clinical treatment of patients with high risk factors for ischemia and those undergoing PCI or who have poor drug compliance [14]. In recent years, platelet reactivity has been measured by various systems in PFT [15]. Modern laboratory techniques, including the platelet function analyzer (PFA), the VerifyNow P2Y12 assay, light-transmission aggregometry (LTA), multiple electrode platelet aggregometry (MEA), thromboelastography (TEG) and vasodilator-stimulated phosphoprotein (VASP) assay, are all applied to measure different properties associated with platelet reactivity, with different detection principles. LTA, invented by Born [16, 17] and O’Brien [18], is the oldest available method for assessment of platelet activation and was regarded as the “gold standard” [19]. The VASP assay was based on flow cytometric measurement of the VASP phosphorylation level [20]. Furthermore, mean platelet volume (MPV), which is a common index indicating platelet size, has been recommended as a marker of platelet activity [21]. However, the role of MPV in evaluating HPR rates in patients with stents is still debated, because contrasting results have been reported so far on the relationship between platelet size and aggregation [22‐24].

Therefore, our present study aimed to evaluate the pharmacodynamic effects of clopidogrel and ticagrelor on Chinese patients undergoing PCI, with LTA and VASP assay used to assess platelet function. We also investigated the relationship between MPV and HPR among patients receiving DAPT.

Our study, for the first time, characterized the effects of clopidogrel and ticagrelor on platelet function, using LTA and VASP assay, in a Chinese population undergoing PCI. This study provides evidence that MPV is independently associated with HPR at VASP assay and could also be used to evaluate the platelet reactivity of patients receiving clopidogrel or ticagrelor. The three major findings in this study are as follows. (1) The effect of ticagrelor on platelet reactivity was significantly greater than that of clopidogrel, with a more potent inhibition of platelet activity measured by LTA and VASP assay. (2) Patients with ACS undergoing PCI who were received standard-of-care treatment with ticagrelor, on a basis of aspirin, had a lower prevalence of HPR when compared with those given clopidogrel. (3) MPV can independently indicate HPR in patients, measured by VASP assay, and was much higher in patients using clopidogrel, which potentially reflects a higher prevalence of HPR.

Many studies have previously assessed the pharmacodynamic effects of clopidogrel and ticagrelor in patients with ST-segment elevation myocardial infarction (STEMI) undergoing early PCI using different PFTs, and have reported that ticagrelor provided more potent and prompt platelet inhibition than clopidogrel [30]. In particular, several clinical trials demonstrated that the primary efficacy end point and clinical benefits favored ticagrelor compared with clopidogrel in patients with ACS; the former markedly reduced the mortality due to stroke, vascular causes, and myocardial infarction [31, 32]. In this study, the effect on platelet function of ticagrelor and clopidogrel in Chinese patients agreed with the findings reported in Western populations. Although there have been several studies investigating the pharmacology and bleeding risk associated with two anti-platelet drugs among Asian populations [33‐36], few studies have concentrated on their different effects on platelet function in such populations, using different PFTs including LTA and VASP test, even though only Verify Now was applied to evaluate the platelet inhibition with ticagrelor versus clopidogrel in diabetic patients after PCI in the study of Zhenyu Liu et al. [37].

Various kinds of PFT have been applied to monitor platelet activity in the setting of DAPT (aspirin and clopidogrel or ticagrelor) in large clinical trials. Verify Now, LTA, MEA and flow cytometry are most intensively used among those techniques. LTA, as a traditional technique, has always been acknowledged as the most classical method. VASP phosphorylation measures activation-dependent platelet signaling. This assay requires small sample volumes and whole blood, maintaining high stability, and is dependent on the P2Y12 receptor, the site of action for clopidogrel and ticagrelor [38]. Therefore, the VASP assay has been used in many clinical trials on the background of the above characteristics. Meanwhile, it has shown a relatively good correlation with LTA results, which was also seen in our results in Fig. S2. In our study, we used LTA and VASP assay and found evidence for a significant effect of ticagrelor on platelet inhibition. In addition, use of LTA and VASP testing may identify patients who are at high risk of thrombotic events such as cardiac death and stent thrombosis during follow up [39]. However, the difference in HPR prevalence with the two tests was relatively large. According to studies reported, the results of platelet aggregation and HPR remained difference with various PFTs. It was considered that the definition standard of HPR influenced our results, especially HPR in the VASP assay which was referenced with the definition of France but not Asian population. Therefore, the definition of HPR at VASP assay might be adjusted with the data of Asians in further investigations. The association between the HPR with the two tests and the ending of patients should be followed up and the study population need to be enlarged as recommended in consensus [40, 41].

Furthermore, a higher HPR rate in patients receiving clopidogrel was observed in our study. This finding indicated that clopidogrel was more prone to induce drug resistance in Chinese individuals. This could guide a more efficient tailored therapy for those patients who were identified as at very high risk. On the genetic level, CYP2C19 polymorphism have been identified as the most prominent effector on platelet activity after clopidogrel treatment [42]. According to literature reported [43, 44], as compared with Africans and whites, East Asian population has a high prevalence of the CYP2C19 loss-of-function (LOF) genotype with CYP2C19*3 variant which was in accordance with previous results found in our laboratory center with Chinese population [45]. Additionally, the prevalence of the LOF mutations among different Asian populations also presented difference, and ticagrelor could become a substitute for clopidogrel in those with LOF mutations [46].Therefore, it could be implied that our results in Chinese population may provide additional information to Asian population. Of note, patients in this study were not assigned by randomization to clopidogrel or ticagrelor because the choice of anti-platelet drugs should obey the clinical guidelines and should be evaluated by clinicians according to patients’ symptoms. The data in Table S1 demonstrate that the results of HPR might not be influenced by differences in hsCRP, transaminases, MPV and other variables between the two groups.

Further, we provided evidence that MPV, which has been proposed as a cheap and easy-to-obtain marker of platelet size, could indicate platelet reactivity and the level of HPR in patients receiving DAPT. The inverse relationship between MPV and platelet count in our results proved the reliability of this study, which was in accordance with reported findings. A close relationship has been demonstrated between MPV and cardiovascular risk factors including obesity, diabetes mellitus, hypertension, hypercholesterolemia and other factors [47, 48]. However, data relating MPV with acute coronary and cerebrovascular events are still contrasting. Lippi et al. [49] demonstrated that there was a significant increase of MPV levels in ACS patients when compared with non-ACS patients. In addition, platelet size could predict impaired angiographic reperfusion and the death rate in patients with STEMI undergoing PCI [50]. In contrast, Tavil et al. demonstrated that MPV was related to central obesity, hypertension and hypercholesterolemia, but not to coronary artery disease (CAD), in patients referred for coronary angiography [51]. The role of MPV in indicating the response to antiplatelet drugs has also raised great debate. Asher et al. [52] documented a higher rate of HPR with larger platelets after clopidogrel loading dose received by patients with acute myocardial infarction. Kubica et al. [53] reported similar findings in patients undergoing PCI. In addition, larger-sized platelets could independently predict the risk of high residual platelet reactivity for treatment with aspirin + clopidogrel, also in patients treated with PCI [54]. However, Monica et al. [24] found no impact of larger platelet volume on the majority of platelet function tests, and in particular on ADP-mediated aggregation or the response to clopidogrel or ticagrelor. Our present study evaluated the association between MPV and platelet function in patients with DAPT and confirmed the well-established strict association of MPV with other platelet function parameters, including ADP-induced aggregation conducted by LTA and PRI in the VASP assay. Regarding the variation in the role of MPV in indicating platelet function, it has been considered that this may be related to the various PFTs used in different studies. We found that MPV was related to HPR only at the VASP test but not at the LTA assay. Similar results for the association between MPV and HPR could be found at MEA test, according to Kim et al. [54]. Of note, there was a very low correlation between the results of the MEA test and platelet volume indices, although there was a strong correlation between VASP test parameters and MPV. It could be noted that there was a significant difference in the baseline value of MPV in the two treatment arms. To avoid drawing questionable conclusions on MPV, the dual antiplatelet therapy was included in the multivariate regression analysis to assess the independence of MPV and MPV tertiles in predicting HPR. Therefore, the results obtained are rational and the conclusion we drew is also reasonable. In this study, the present results of multivariable logistic regression analysis showed that MPV as a continuous variable could independently indicate HPR at VASP assay and the results of MPV tertile 3 were marginally significant. This might because the sample size of the study is not large enough to ensure enough number of samples in MPV tertiles. It could be inferred that MPV tertiles would be significantly and independently associated with HPR if the sample size were enlarged. Therefore, the sample size should be enlarged to verify the results and investigate the role of MPV tertiles in indicating HPR in the further study. Based on the results of ROC analysis, a threshold could be found (MPV = 11.65 fL) for which a switch from clopidogrel to ticagrelor should be considered. This provided additional information and evidence regarding to the role of MPV in guiding clinical practice and anti-platelet drugs using. Of note, the swelling effect of EDTA on blood cells especially on platelet volume over the first two hours should be addressed. A significant increase in MPV could be observed in blood samples anticoagulated with EDTA over time [55]. In our study, it was recommended that MPV test should be conducted within 2 h after blood draw. Therefore, time elapsed between blood collection and testing is quite important for the accuracy of testing results which should be paid more attention [55].

It has been demonstrated that ticagrelor had superiority over clopidogrel in suppressing platelet function in patients with ACS, with a more pronounced antiplatelet effect during the initial treatment phase and during maintenance therapy [56]. Our present study is consistent with previous research. Recently, the EROSION study [57] reported that DAPT with aspirin and ticagrelor without stenting may be an option for patients with ACS caused by plaque erosion. Therefore, the potential clinical application of ticagrelor in anti-thrombotic therapy for ACS patients should not be ignored.

In this study, several limitations of the design should be stated. First, the absence of long-term follow-up of our patients should be considered, and, therefore, we cannot evaluate the impact of the two different types of DAPT on clinical outcome. Furthermore, we need to enlarge the dataset in order to improve the representativeness and reliability of the results. In addition, the impact of genetic factors, such as CYP2C19 polymorphism, on anti-platelet drug responsiveness was not evaluated in this study; the response to clopidogrel is closely associated with the polymorphism of CYP2C19. The results of this study may therefore have been confounded by the prevalence of CYP2C19 polymorphism.

Our study investigated the antiplatelet effect of clopidogrel and ticagrelor in Chinese patients undergoing PCI, using two platelet function tests, LTA and VASP assay. Ticagrelor has markedly greater antiplatelet effect than clopidogrel, with a superiority in inhibiting platelet activity and a lower HPR rate. In addition, an independent association between MPV and high HPR prevalence in the VASP assay was found. Clinicians should be aware that MPV could be another potential marker to reflect the platelet reactivity in response to anti-platelet drugs and take it into consideration during antiplatelet therapy.