Coronary artery disease (CAD), specifically acute coronary syndrome (ACS), is still the leading cause of disability and mortality worldwide (Fox et al., 2010; Jernberg et al., 2015). Oral dual antiplatelet therapy with aspirin and a P2Y₁₂ receptor inhibitor is the standard postoperative maintenance strategy for patients undergoing percutaneous coronary intervention (PCI) for ACS (Levine et al., 2011; Levine et al., 2016). Clopidogrel, the most widely used P2Y₁₂ inhibitor, was previously shown to improve the prognosis of ACS patients. However, evidence from pharmacogenomics has gradually raised concerns about the clinical efficacy of clopidogrel. As a prodrug, clopidogrel needs to be converted into active substances by cytochrome (CYP2C19) enzymes. Patients with a CYP2C19 loss-of-function (LOF) allele were shown to have lower conversion rates of clopidogrel by their CYP2C19 enzymes (Mega et al., 2009; Shuldiner et al., 2009). The population frequency of a LOF allele is approximately 60% in East Asian countries and approximately 30% in the rest of the world (Klein et al., 2019). Our previous study identified possession of a CYP2C19 LOF allele is an independent risk factor for clopidogrel-related high platelet responsiveness (Peng et al., 2019), which reduces the effectiveness of the drug (Mega et al., 2009; Shuldiner et al., 2009; Mao et al., 2013; Xi et al., 2019).

Ticagrelor is a P2Y₁₂ inhibitor that acts directly on platelets, and its efficacy is hardly affected by CYP2C19 gene polymorphisms (Wallentin et al., 2010; Li et al., 2017). The PLATO trial confirmed that ticagrelor is superior to clopidogrel in reducing adverse cardiovascular events in patients with ACS (Wallentin et al., 2009). Furthermore, current guidelines recommend the use of ticagrelor or prasugrel in preference to clopidogrel in ACS patients (Levine et al., 2011; Levine et al., 2016). However, the high incidences of bleeding and discontinuation due to dyspnea, as well as the high price, have limited the widespread clinical use of ticagrelor (Wallentin et al., 2009; Bonaca et al., 2015).

Emerging evidence from observational studies and randomized controlled trials has confirmed the feasibility of implementing CYP2C19 gene-guided pharmacotherapy (Cavallari et al., 2018b; Lee et al., 2018; Notarangelo et al., 2018; Claassens et al., 2019; Tuteja et al., 2020). In addition, increasing numbers of medical institutions are beginning to incorporate CYP2C19 genotyping into clinical practice. However, the PCI guidelines state that CYP2C19 pharmacogenetic testing should only be considered for high-risk patients (Class IIb; Level of Evidence: C) (Levine et al., 2011; Levine et al., 2016). In the absence of strong support from current guidelines, there are limited real-world data in East Asian populations to validate the implementation of genotype-guided antiplatelet therapy in clinical practice. Furthermore, the impact of genetic testing results on P2Y₁₂ inhibitor selection by doctors in China remains unclear.

Therefore, we performed a single-center retrospective observational study in Chinese patients with ACS after PCI, with the following objectives: 1) to determine the effects of returned genotype results on the prescription behavior of cardiologists and 2) to understand the relationships of the identified genotypes with P2Y₁₂ receptor inhibitor selection and clinical outcomes.

Baseline demographic and clinical factors were assessed using descriptive statistics. Frequency (percentage) was used to report discrete variables. Mean ± standard deviation was used to report continuous variables. Unadjusted comparisons between groups were performed using Fisher’s exact test, chi-square test, or Student’s t-test, as appropriate. Demographic and clinical factors related to P2Y₁₂ inhibitor selection were determined by univariate and multivariate logistic regression. Patients with no events within 1 year after PCI were censored at the time of the most recent follow-up. The 1-year cumulative event rates of MACCE, MACCE plus unstable angina, and clinically significant bleeding events were plotted using the Kaplan-Meier method and evaluated by the log-rank test. Survival analysis was implemented using Cox proportional hazards models. Three pairs of comparisons were constructed separately, comprising LOF-clopidogrel vs. LOF-ticagrelor, non-LOF-clopidogrel vs. LOF- ticagrelor, and non-LOF-ticagrelor vs. LOF- ticagrelor. Inverse probability treatment weighting (IPTW) was performed to avoid bias in comparisons. Differences between groups were examined with standardized differences using a 10% threshold to indicate significant meaningful imbalances in covariates (Austin, 2015). Three sensitivity analyses and secondary analyses were conducted.

Kaplan–Meier curves were drawn using Prism 7.0 (GraphPad Software, La Jolla, CA). Statistical analyses were performed using R software version 3.4.3 (https://www.r-project.org/). Values of p < 0.05 were considered statistically significant.

The present study included 1,361 patients who underwent PCI for ACS (Figure 1). The mean age was 60.16 ± 9.73 years, 24.32% were female, 98.02% were Han nationality, and 92.29% had an implanted stent. Comorbidities such as hypertension (63.56%), hyperlipidemia (52.31%), and diabetes mellitus (34.53%) were common. Before admission, 26.97% of patients were on P2Y₁₂ inhibitors.

The baseline demographic and clinical factors in the LOF-clopidogrel group, non-LOF-clopidogrel group, and non-LOF-ticagrelor group were compared with those in the LOF-ticagrelor group (Table 1). As shown in Table 2, after the values were well-balanced by IPTW adjustment, the differences between the LOF-clopidogrel group and LOF-ticagrelor group were anticoagulant agent use and end-stage kidney disease. The imbalances between the non-LOF-clopidogrel group and LOF-ticagrelor group were ticagrelor use before admission, anticoagulant agent use, and end-stage kidney disease. End-stage kidney disease differed between the non-LOF-ticagrelor group and LOF-ticagrelor group.

In this large single-center observational cohort study, we investigated the effect of CYP2C19 genotype-guided antiplatelet therapy on P2Y₁₂ inhibitor selection and clinical outcomes. The significant findings were as follows: 1) efficient returned CYP2C19 genotype results did, to a certain extent, guide cardiologists to prescribe ticagrelor for ACS in IM/PM patients; 2) for IM/PM patients, use of clopidogrel was associated with a higher risk of developing MACCE than ticagrelor; and 3) drug selection based on CYP2C19 genotype guidance did not increase the risk of significantly clinical bleeding events within 1 year in patients.

The frequency of CYP2C19 LOF alleles in the present study was 60.7%, which was similar to previous data in other East Asian populations (Xie et al., 2013; Shen et al., 2016; Wang et al., 2016) and higher than data in other populations (approximately 30% in African American and Caucasian populations) (Klein et al., 2019). Because clopidogrel remains the most widely used P2Y₁₂ inhibitor after PCI, it is vital to verify the effects of CYP2C19 genotype-guided antiplatelet therapy, especially for patients in East Asia. However, there are limited data in East Asia that have focused on CYP2C19 genotype-guided antiplatelet therapy in patients with coronary heart disease. A randomized controlled trial (Xie et al., 2013) and a prospective study (Shen et al., 2016) from China confirmed that personalized antiplatelet therapy based on CYP2C19 genotype after PCI could decrease the rates of cardiovascular adverse events with no difference in bleeding in Chinese populations. A small sample-size prospective study in Japan reported that fewer MACCE and hemorrhagic events were observed compared with conventional treatments after genotype-guided antiplatelet therapy was implemented in patients with ACS (Ozawa et al., 2018). Our real-world data support the above-mentioned studies, and demonstrate clinical benefits of genotype-guided antiplatelet therapy.

Our conclusions are also consistent with previous studies conducted in other populations. A multicenter study conducted by the IGNITE network in the United States examined the outcomes of genotype-guided P2Y₁₂-receptor inhibitor therapy after PCI, and prasugrel/ticagrelor were recommended for IM/PM patients. These retrospective data proved that clopidogrel, as a maintenance treatment, had a higher MACCE risk for IM/PM patients than prasugrel or ticagrelor, especially for those with ACS indications (Cavallari et al., 2018b). Meanwhile, a single-center observational study from the United States, including 1,193 post-PCI patients demonstrated that IM/PM patients receiving prasugrel/ticagrelor had a lower risk of developing cardiovascular adverse events than those receiving clopidogrel, and no increased risk of clinically significant bleeding events was observed (Lee et al., 2018). Recently, the Patient Outcome after Primary PCI (POPular Genetics) trial in the The Netherlands involving 2,751 patients with STEMI who underwent PCI showed that genotype-guided therapy was not inferior to standard treatment with prasugrel or ticagrelor in terms of major cardiovascular adverse events and had a lower incidence of bleeding risk events (Claassens et al., 2019). The Pharmacogenetics of Clopidogrel in Acute Coronary Syndromes (PHARMCLO) trial in Italy also demonstrated that composite endpoints of ischemic events and major bleeding events were reduced in the genotype-guided group compared with the conventional treatment group (Notarangelo et al., 2018).

In this non-intervention study, 36.6% of patients with a LOF allele were prescribed ticagrelor, which was lower that the corresponding proportions of 60.5% in the IGNITE study (Cavallari et al., 2018b) and 53% in another study (Tuteja et al., 2020). Although important, the CYP2C19 genotype is not the only factor considered when prescribing P2Y₁₂ inhibitors. Various clinical factors, especially risk factors for bleeding, are related to the use of clopidogrel. Some of these potential factors also make cardiologists very cautious about prescribing ticagrelor. First, previous reports indicated that Asian ACS patients had a higher risk of drug-related bleeding and a lower risk of ischemia than Caucasian ACS patients (Mak et al., 2009; Kumar et al., 2013; Levine et al., 2014). Second, ticagrelor had higher discontinuation rates and costs than clopidogrel (Wallentin et al., 2009; Bonaca et al., 2015). These concerns may stop clinicians from prescribing ticagrelor even after receiving an unoptimistic pharmacogenomics result. However, given the higher risk of ischemia associated with clopidogrel use in patients with a LOF allele, and the clear cardiovascular benefits of using ticagrelor, it is recommended that clinicians should increase the weight of genetics when choosing P2Y₁₂ inhibitors.

We observed an imbalance between IMs (44.0%) and PMs (34.7%) when physicians implemented genotype-guided drug selection. Such inequalities also existed in other previous studies (Cavallari et al., 2018b; Tuteja et al., 2020). Controversy about the value of IM status in genotype-guided antiplatelet therapy has always existed, despite sufficient evidence that IMs are associated with reduced clinical efficacy of clopidogrel (Xi et al., 2019). Previously, the CPIC recommended an alternative therapy for IM patients (Scott et al., 2013). However, in the clopidogrel drug manual, the black box warning from the FDA only recommends that PMs should receive P2Y₁₂ receptor inhibitors other than clopidogrel, and does not mention the risk for IMs (Holmes et al., 2010). Our real-world data prove that an increased risk of cardiovascular adverse events exists in both PMs and IMs. This conclusion not only validates the results of the IGNITE study (Cavallari et al., 2018b) and earlier studies (Xie et al., 2013; Shen et al., 2016; Gross et al., 2018; Lee et al., 2018; Notarangelo et al., 2018; Claassens et al., 2019; Khan et al., 2019), but also supports recommendations for prescribing alternative therapies to IMs and PMs from the CPIC (Scott et al., 2013). Therefore, we recommend that cardiologists should pay attention to the risk of cardiovascular adverse events in IMs, and increase the weight of IMs when making drug selection decisions, similar to the case for PMs.

Improving the adoption of genetic testing results is very urgent and challenging. In our study, although all of the genotype results were available before the patients were discharged, our data and those in previous studies (Bagai et al., 2014; Lee et al., 2018; Tuteja et al., 2020) suggested that clinicians were reluctant to switch existing treatments regardless of genetic outcomes. It is necessary to provide feedback on the results of pharmacogenomics tests as quickly as possible, because this can help physicians to adopt CYP2C19 genotype results. We agree that delayed return of genetic test reports may result in lower acceptance of the genetic results (Cavallari et al., 2018a; Tuteja et al., 2020). Regular conduct of clinician education is also essential, because it allows cardiologists to continue to pay attention to pharmacogenomics reports (Bell et al., 2014). Besides, the participation of clinical pharmacists can support the clinical decision-making of cardiologists, which is very important (Owusu-Obeng et al., 2014).

Based on our real-world data, clopidogrel was similar to ticagrelor in reducing cardiovascular clinical events in patients without a LOF allele. Because prescription of clopidogrel has better safety and economic profiles, it is worth encouraging the prescription of clopidogrel for NM/RM/UM patients. For patients prescribed ticagrelor after PCI, early de-escalation to clopidogrel may be a strategy worth considering. The results of The TROPICAL-ACS genotyping substudy (Gross et al., 2018) and a recent observational study (Martin et al., 2020) confirmed that de-escalation to clopidogrel in UM/RM/NM patients did not significantly increase the risk of MACCE compared with continuation of ticagrelor/prasugrel. However, there is very little clinical evidence from the East Asian population to verify this strategy, and this will be the focus of our future studies.

Our study has several limitations. First, due to the inherent limitations of observational studies, the implementation of genotype-guided drug therapy in this study was non-randomized. Therefore, whether to detect genotypes and whether to obey genotype guidance were dependent on the decisions of individual doctors. Although we tried to balance the covariates at baseline by IPTW adjustment, we were still unable to rule out residual confounding. Second, the proportions of STEMI and non-STEMI were relatively low in real-world ACS patients who received genotype testing, which is inconsistent with epidemiological data (Wang et al., 2020). The reason may be that some patients with myocardial infarction were prescribed ticagrelor upon admission, and their genetic testing was unnecessary. However, in the Supplementary Material, we have demonstrated the clinical benefits of gene-guided drug selection in both STEMI/non-STEMI and UA. Therefore, the conclusions of this study are robust. Third, patients who did not have CYP2C19 genotype results were not included in the study. Therefore, we did not explore the factors that influence clinicians’ indications and frequencies of CYP2C19 genotype testing. Fourth, since prasugrel is not licensed in China, ticagrelor was the only P2Y₁₂ receptor inhibitor used for alternative therapy. However, the results of a previous meta-analysis suggested that ticagrelor and prasugrel showed no difference in preventing adverse cardiovascular events and bleeding events in patients with ACS (Khan et al., 2019). Fifth, the conclusions of the study were based on a registration database from a single medical center in China, and thus the results may not be widely applicable to other settings or populations.

In conclusion, this observational study in a real-world setting confirmed that CYP2C19 genomics test results could guide cardiologists to prescribe ticagrelor for IM/PM patients, but this is not universal. In patients with a LOF allele, the incidence of MACCE with clopidogrel was significantly higher than that with ticagrelor. The findings of the present study support the implementation of CYP2C19 gene-guided antiplatelet therapy in clinical practice. However, our conclusions still need to be proven by large multicenter randomized controlled trials.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.

The studies involving human participants were reviewed and approved by the Ethics Committee of the Clinical Research Center at Beijing Anzhen Hospital. The patients/participants provided their written informed consent to participate in this study.

Conception and design: YZ and YL. Administrative support: YL and X-JS. Determination of clinical events: S-WC, NN, and YZ. Collection and upload of data: YZ, W-XP, J-LH, B-DL, RZ, Y-NZ, J-LY, J-JW, Y-FW, and Z-WF. Data analysis and interpretation: YZ, J-LH, and B-DL. Manuscript writing: All authors. Final approval of manuscript: All authors.

This study was supported by the National Major Scientific and Technological Special Project for “Significant New Drugs Development” during the Thirteenth Five-year Plan Period (2017ZX09304017) and Beijing Municipal Administration of Hospitals Clinical Medicine Development of Special Funding Support (Grant Number ZYLX201805).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.