The Influence of Omeprazole on Platelet Inhibition of Clopidogrel in Various CYP2C19 Mutant Alleles

Abstract

Currently, concerns of clopidogrel and proton pump inhibitors (especially omeprazole) interaction are raised, because they are both metabolized by CYP2C19. What is more, omeprazole can also inhibit the activity of CYP2C19. The study was to compare the influence of omeprazole on platelet inhibition of clopidogrel in various CYP2C19 mutant alleles. One hundred forty-two consecutive patients undergoing elective coronary stenting received aspirin and clopidogrel, and were randomized to omeprazole or the placebo. Enrolled patients were analyzed for adenosine diphosphate-induced platelet aggregation (ADP-Ag), and CYP2C19*2 and CYP2C19*3 were identified by polymerase chain reaction-restriction fragment length polymorphism. Of the patients included, 47 (33.1%) belonged to homozygous extensive metabolizers (homEMs) (CYP2C19*1/*1), 70 (49.3%) belonged to heterozygous extensive metabolizers (hetEMs) (*1/*2 or *1/*3), and 25 (17.6%) belonged to poor metabolizers (PMs) (*2/*3 or *2/*2). ADP-Ag had a significant difference among the three genotypic groups (p<0.01). Moreover, the present study revealed that the degree of the interaction between clopidogrel and omeprazole was not homogeneous within the various genotypes of CYP2C19. The difference of ADP-Ag between the patients with and without omeprazole was significantly largest in homEMs (45.7%±14.2% vs. 35.5%±16.0%, p<0.05). However, any significant difference of ADP-Ag between the patients with and without omeprazole was not observed in other two genotypic groups (hetEMs and PMs, p>0.05). In conclusion, concomitant therapy with omeprazole appears to reduce the antiplatelet effect of clopidogrel most significantly in homEMs of CYP2C19.

Introduction

Clopidogrel is a thienopyridine that inhibits platelet aggregation through an irreversible blockage of the platelet adenosine diphosphate (ADP) P2Y12 receptor, and is widely used in clinical situations of high risk for atherothrombotic events in combination with aspirin (Gerschutz and Bkatt, 2001). However, several studies have shown a broad variability of biological response to clopidogrel (Matetzky et al., 2004; Cuisset et al., 2006; Angiolillo et al., 2007). Mechanisms underlying this variability of response remain controversial, and multiple factors are involved, including genetic factors, metabolic parameters, or interaction with other medications (Buonamici et al., 2007). Clopidogrel, an inactive prodrug, must be metabolized by CYP450 enzymes to produce the active metabolite that irreversibly inhibits platelet aggregation. Accordingly, clopidogrel response has been related to the level of activation of the CYP450 (Lau et al., 2004). Recent work suggests that the genetic polymorphism of CYP2C19 is the most important determinant of the pharmacokinetic and pharmacodynamic response to clopidogrel (Hulot et al., 2006; Frere et al., 2008; Mega et al., 2009; Kazui et al., 2010). If there is a loss of function of CYP2C19, it follows that less clopidogrel can be converted to active drug, and the clinical outcome could be adversely affected. In addition, medications metabolized by CYP450, such as proton pump inhibitors (PPIs), have been shown to influence the clopidogrel effect (Lau et al., 2003). As a substrate and an inhibitor of CYP2C19 (Ko et al., 1997; Ishizaki and Horai, 1999; Dojo et al., 2001), omeprazole has been associated with a lower efficacy of clopidogrel, as assessed by the platelet reactivity index vasoactive stimulated phosphoprotein (Gilard et al., 2008). However, the question is whether the degree of the interaction between clopidogrel and omeprazole could be homogeneous within the various genotypes of CYP2C19. Because omeprazole is only an inhibitor of CYP2C19, the reported negative omeprazole-clopidogrel drug interaction may not be caused if there is a loss of function of CYP2C19. We therefore designed a randomized study to compare the influence of omeprazole on the antiplatelet effect of clopidogrel by measurement of the ex vivo platelet aggregation among patients undergoing coronary stent with CYP2C19 wild-type and mutant alleles (CYP2C19*2 and CYP2C19*3).

Methods

Patients

Patients >18-years old who underwent successful percutaneous coronary intervention with drug-eluting stents were prospectively eligible for inclusion. Inclusion criteria were symptomatic coronary artery disease or documented by revascularization or myocardial infarction. Exclusion criteria were history of thrombocytopenia (platelet count <100×10⁹/L) or bleeding disorder, serious liver disease, gastrointestinal ulcer, use of glycoprotein (GP) IIb/IIIa inhibitors before the procedure, and previous allergy to or prior use of clopidogrel or PPIs. Patients received a 300-mg loading dose of clopidogrel and a 300-mg loading dose of aspirin at least 12 h before stenting. Stents were deployed according to standard techniques. Anticoagulation was obtained with low-weight-molecular heparin when possible, or unfractionated heparin in patients older than 75 years of age or with renal failure. Use of a GP IIb/IIIa antagonist was allowed at the operator's discretion during the procedure. Maintenance doses were clopidogrel 75 mg and aspirin 100 mg daily. At the start of clopidogrel treatment, patients were randomized to omeprazole 20 mg/day or placebo with randomization by sealed envelopes. Enrolled patients were analyzed for post-treatment platelet reactivity with adenosine diphosphate-induced platelet aggregation (ADP-Ag), and given genetic analysis.

Patients were documented for atherosclerosis risk factors, such as diabetes mellitus, arterial hypertension, hypercholesterolemia, and smoking status. Concomitant cardiovascular medications (β-blockers, angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers, calcium channel blockers, and statins) at inclusion were also recorded. The ethics committee of our institution approved the study protocol.

Platelet reactivity

Blood samples for testing platelet reactivity were drawn at least 12 h after clopidogrel and omeprazole co-therapy, and before administration of GP IIb/IIIa antagonist if needed. The blood-citrate mixture was centrifuged at 800 rpm for 10 min. The resulting platelet-rich plasma was kept at room temperature for use within 1 h. Platelet-rich plasma obtained by further centrifugation of samples at 4000 rpm for 10 min was used to adjust the platelet count to 2.5×10⁸/mL. Platelets were stimulated with ADP (5 μM), and aggregation was assessed with a PACKS-4 Aggregometer. Aggregation was expressed as the percentage change in light transmittance from baseline with platelet poor plasma as a reference. Here we analyzed data on the maximal intensity of ADP-Ag. An ADP-Ag >50% was defined as high post-treatment platelet reactivity (HTPR) as previously described (Barragan et al., 2003; Cuisset et al., 2008).

DNA extraction

Genomic DNA was isolated from whole blood by BioSpin Whole-Blood Genomic DNA Extraction Kit.

Genotyping

The CYP2C19 wild-type gene and two mutated alleles, CYP2C19*2 in exon 5 and CYP2C19*3 in exon 4, were identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). For the analysis of the CYP2C19*2 allele, the forward and reverse primers were 5′-AATTACAACCAGAGCTTGGC-3′ and 5′-TATCACTTTCCATAAAAGCAAG-3′. The final 50 μL of PCR mixture contained 34 μL of PCR-grade water, 5.0 μL of 10×PCR buffer with 15 mM MgCl₂, 4.0 μL of deoxyribonucleoside triphosphates (2.5 mM each), 1.0 μL of primers (10 mM each), 1.0 μL of Taq DNA polymerase (5 U/μL; BioFlus), and 4.0 μL of genomic DNA sample. Temperature cycling proceeded as follows: initial denaturation for 5 min at 94°C, followed by 35 cycles of 30 s at 94°C, 30 s at 55°C, 40 s at 72°C, and a terminal extension for 7 min at 72°C. The forward and reverse primers used for detection of the CYP2C19*3 allele were 5′-AAATTGTTTCCAATCATTTAGCT-3′ and 5′-ACTTCAGGGCTTGGTCAATA-3′. The reaction system and amplification conditions were similar to those of the CYP2C19*2, except that the annealing temperature was 53°C. The 169- and 271-bp gene fragments were amplified by PCR, respectively, and their products were digested with the restriction endonucleases (SmaI [Fermentas] for CYP2C19*2 and BamHI [Fermentas] for CYP2C19*3). The digested products were analyzed on 3% agarose gels and stained with ethidium bromide.

Statistics

The SPSS statistical package Version 14.0. was used for statistical analysis. Continuous variables were expressed as mean±standard deviation. Categorical variables were expressed as frequencies and percentages. Groups were compared with the chi-square or Fisher exact test for categorical variables. Continuous variables among groups were tested via Student's t-tests and one-way analysis of variance or nonparametric statistical testing (Mann-Whitney) when there was no variance homogeneity.

Results

A total of 142 consecutive patients were prospectively included in the study, and given a genetic analysis. Baseline characteristics of the patients are summarized in Table 1. Demographic indicators and concomitant medications were well balanced among the treatment groups at baseline.

Table 1.

Baseline Characteristics of Patients and Concomitant Medications at Study Entry

		The various CYP2C19 genotypes				According to omeprazole
	Overall	homEMs	hetEMs	PMs	p Value	Not PPI	Omeprazole	p Value
Patients' characteristics	142	47 (33.1%)	70 (49.3%)	25 (17.6%)		59 (40.8%)	83 (59.2%)
Male gender	99 (69.7%)	34 (72.3%)	50 (71.4%)	15 (60.0%)	0.504	43 (72.9%)	56 (67.5%)	0.579
Age (years)	59.9±9.8	59.2±10.4	61.1±8.6	61.1±8.6	0.819	58.8±9.2	60.7±10.2	0.283
Previous or current smoker	71 (50%)	27 (57.4%)	34 (48.6%)	11 (44.0%)	0.489	25 (43.1%)	47 (56.6%)	0.114
Hypertension	71 (50%)	24 (51.1%)	32 (45.7%)	15 (60.0%)	0.464	33 (55.9%)	38 (45.8%)	0.233
Diabetes mellitus	38 (26.8%)	13 (27.7%)	15 (21.4%)	10 (40.0%)	0.195	19 (32.8%)	19 (22.9%)	0.194
Hypercholesterolemia	87 (61.2%)	27 (57.4%)	46 (65.7%)	14 (56.0%)	0.559	32 (55.2%)	56 (67.5%)	0.138
Concomitant cardiovascular medication
β-blocker	127 (89.4%)	44 (93.6%)	62 (88.6%)	21 (84.0%)	0.426	53 (89.8%)	74 (89.2%)	0.898
ACEI or ARB	122 (85.9%)	37 (78.7%)	61 (92.2%)	24 (96.0%)	0.123	53 (89.8%)	69 (83.1%)	0.258
Calcium-channel blocker	53 (37.3%)	19 (40.4%)	24 (35.7%)	9 (36.0%)	0.865	24 (40.7%)	29 (34.9%)	0.486
Statin	109 (76.8%)	39 (83.0%)	51 (72.9%)	19 (76.0%)	0.444	43 (72.9%)	66 (79.5%)	0.356

homEMs, homozygous extensive metabolizers; hetEMs, heterozygous extensive metabolizers; PMs, poor metabolizers; PPI, proton pump inhibitors; ACEI, angiotensin-converting enzyme inhibitors; ARB, angiotensin II receptor blockers.

Of the patients, 47 (33.1%) belonged to homozygous extensive metabolizers (homEMs; *1/*1), 70 (49.3%) belonged to heterozygous extensive metabolizers (hetEMs; *1/*2 or *1/*3), and 25 (17.6%) belonged to poor metabolizers (PMs; *2/*3 or *2/*2). However, homozygotes for CYP2C19*3 were not found in the study. The allele frequencies of the CYP2C19*2 and CYP2C19*3 mutations in the subjects were 0.38 and 0.05 (data not shown), respectively. ADP-Ag was significantly different among the three genotypic groups: 42.0%±15.5% in homEMs, 44.2%±13.9% in hetEMs, and 53.3%±14.1% in PMs (p<0.01). What is more, the prevalence of HTPR in EM genotypic groups was 35.0% (31.9% in homEMs and 37.1% in hetEMs), significantly lower than PM genotypic groups (p<0.01, Table 2). The prevalence of HTPR in PMs was 68.0%.

Table 2.

Comparison of Response to Clopidogrel According to Genetic Polymorphisms of CYP2C19

	All patients	Good responders	HTPR
Variable	N=142	N=84	N=58	p Value
homEMs	47 (33.1%)	32 (68.1%)	15 (31.9%)	0.008
hetEMs	70 (49.3%)	44 (62.9%)	26 (37.1%)
PMs	25 (17.6%)	8 (32.0%)	17 (68.0%)

HTPR, high post-treatment platelet reactivity.

The patients were randomized to receive omeprazole (n=83, 59.2%). Patients without PPIs had a better platelet response to clopidogrel significantly assessed by ADP-Ag: 41.4%±15.5% versus 47.8%±14.0% (p<0.05). The difference of ADP-Ag between the patients with and without omeprazole was significantly largest in homEMs (45.7%±14.2% vs. 35.5%±16.0%, p<0.05, Fig. 1), indicating the effect of omeprazole on attenuated platelet inhibition of clopidogrel in homEMs. We identified more clopidogrel nonresponders in the omeprazole group than in the placebo group: 38.7% versus 18.8% (p<0.05, Fig. 2). However, any significant difference of ADP-Ag between the patients with and without omeprazole was not observed in other two genotypic groups (hetEMs and PMs, p>0.05, Fig. 1).

FIG. 1.

Mean values of ADP-Ag between the patients with and without omeprazole among the various CYP2C19 genotypic groups. The difference of ADP-Ag values between the patients with and without omeprazole was significantly largest in homEMs: 45.7%±14.2% versus 35.5%±16.0%, p=0.028. However, ADP-Ag between the patients with and without omeprazole was 47.2%±13.1% and 41.4%±14.4% in hetEMs, respectively (p=0.085), and patients with or without omeprazole had similar ADP-Ag values in PMs (53.9±13.0 vs. 52.9±15.0, p=0.864). ADP-Ag, adenosine diphosphate-induced platelet aggregation; homEMs, homozygous extensive metabolizers; hetEMs, heterozygous extensive metabolizers; PMs, poor metabolizers.

FIG. 2.

Differential prevalence of HTPR between the patients with and without omeprazole among the various CYP2C19 genotypic groups. They were not homogeneous within the various genotypic groups: 38.7% versus 18.8% in homEMs (p=0.049), 38.9% versus 32.4% in hetEMs (p=0.568), and 68.9% versus 66.7% in PMs (p=1.000). HTPR, high post-treatment platelet reactivity.

Discussion

CYP2C19 plays an important role in the bioactivation of clopidogrel. Once absorbed, only ∼15% of clopidogrel is bioactivated by two sequential steps dependent on several CYP450 isoenzymes (Kazui et al., 2010), with contributions from the isoenzymes CYP2C19, CYP3A4 or CYP3A5, CYP2C9, CYP1A2, and CYP2B6. Of these, CYP2C19 is responsible for ∼45% of the first step (the formation of 2-oxo-clopidogrel) and ∼20% of the final step to generate the pharmacologically active thiol metabolite. CYP2C19 is a polymorphically expressed enzyme (Goldstein, 2001), and the CYP2C19 gene is located on chromosome 10p, which has at least 25 variant mutations (Ingelman-Sundberg et al., 2009). CYP2C19*2 and *3, as the major mutant alleles of CYP2C19, are known to be associated with the genetically deficient metabolic activity, and have the highest occurrence among Asian populations (Desta et al., 2002). A number of studies have shown a considerable interethnic difference in the frequency of PMs of CYP2C19 19-23% among the Japanese population, 15% among the Chinese, and 13% among Koreans (Kubota et al., 1996; Shu and Zhou, 2000). This is much higher than that reported in Caucasian populations, which is 3-5% (Jacqz et al., 1988). The activity of CYP2C19 may influence the antiplatelet effect of clopidogrel in both healthy volunteers and patients (Chen et al., 2008; Simon et al., 2009), and our study also confirmed that the loss-of-function CYP2C19*2 and CYP2C19*3 alleles were associated with a poor response to clopidogrel. Other genetic polymorphisms (CYP2C19*3, *4, *5, and *8) associated with impaired CYP2C19 activity and possibly adverse clinical events are much less common in Asians, and thus were not investigated in the study.

This was the first time to investigate the influence of omeprazole on platelet inhibition of clopidogrel in various CYP2C19 mutant alleles. Concerns of PPIs and clopidogrel interaction were raised when omeprazole was found to inhibit the antiplatelet effect of clopidogrel in an in vivo study of 104 patients (Cuisset et al., 2009), because omeprazole is both a substrate and an inhibitor of CYP2C19. Ko et al. (1997) showed that omeprazole, but not other PPIs (esomeprazole, rabeprazole, and pantoprazole), appeared to inhibit CYP2C19 activity. What is more, the affinity of omeprazole for CYP2C19 may result in omeprazole being a more potent inhibitor of CYP2C19 isoenzyme. Therefore, metabolism of clopidogrel to its active form may be reduced in patients receiving omeprazole concomitantly (Siller-Matula et al., 2009). Indeed, Juurlink et al. (2009) suggested that among patients receiving clopidogrel after an acute myocardial infarction, a loss of the beneficial effects of clopidogrel was associated with concomitant therapy with omeprazole. However, the present study found that the degree of the interaction between clopidogrel and omeprazole was not homogeneous within the various genotypic groups. It confirmed that the use of omeprazole significantly reduced the antiplatelet activity of clopidogrel in homEMs, whereas omeprazole had no significant effect on platelet reactivity assessed by ADP-Ag in other genotypes (hetEMs and PMs), which has not been found in several other studies. The authors analyzed the potential causes: metabolic pathways of clopidogrel are dependent on several CYP450 isoenzymes, as previously mentioned. If the activity of the isoenzyme CYP2C19 is reduced, mainly caused by the loss-of-function alleles, its contribution to the metabolism of clopidogrel may be also minimized. Instead, other CYP450 isoenzymes may make more contribution to the metabolism of clopidogrel. However, omeprazole can just inhibit CYP2C19 activity, and thus it is less associated with a poor response to clopidogrel. It is suggested that among patients receiving clopidogrel after an acute myocardial infarction in homEMs, concomitant therapy with omeprazole appears to influence clopidogrel effect. However, the combination of clopidogrel and omeprazole can be continued to prevent patients in hetEMs and PMs from gastrointestinal bleeding. This is of great importance for daily practice, because omeprazole is now a standard medicine of care for patients receiving dual-antiplatelet therapy to prevent the risk of gastrointestinal bleeding (Bhatt et al., 2008).

Besides, studies examining the effect of various PPIs on the pharmacodynamics of clopidogrel have shown that omeprazole decreased the effect of clopidogrel, whereas lansoprazole, pantoprazole, and esomeprazole had no influence on the antiplatelet effect of clopidogrel (Gilard et al., 2008; Cuisset et al., 2009; Siller-Matula et al., 2009). A possible explanation is that metabolism of omeprazole mainly occurs through this pathway, while esomeprazole, pantoprazole, and lansoprazole are progressively less metabolized via this pathway. Additional large prospective clinical studies are required to confirm the interaction of clopidogrel and other PPIs for the patient with the loss-of-function alleles.

In conclusion, our study shows that the degree of the interaction between clopidogrel and omeprazole was not homogeneous within the various genotypes of CYP2C19. Concomitant therapy with omeprazole appears to reduce the clopidogrel effect significantly in homEMs, whereas omeprazole had no significant effect on the antiplatelet activity of clopidogrel in other genotypes (hetEMs and PMs).

Footnotes

Acknowledgments

The authors thanked their doctors, nurses, team, and technicians for assistance in executing this study.

Author Disclosure Statement

No competing financial interests exist.

References

Angiolillo

, Fernandez-Ortiz

, Bernardo

et al. 2007. Variability in individual responsiveness to clopidogrel clinical implications, management, and future perspectives. J Am Coll Cardiol, 49:1505-1516.

Barragan

, Bouvier

, Roquebert

et al. 2003. Resistance to thienopyridines: clinical detection of coronary stent thrombosis by monitoring of vasodilator-stimulated phosphoprotein phosphorylation. Catheter Cardiovasc Interv, 59:295-302.

Bhatt

, Scheiman

, Abraham

et al. 2008. ACCF/ACG/AHA 2008 expert consensus document on reducing the gastrointestinal risks of antiplatelet therapy and NSAID Use: a report of the American College of Cardiology Foundation Task Force on Clinical Expert Consensus Documents. J Am Coll Cardiol, 52:1502-1517.

Buonamici

, Marcucci

, Migliorini

et al. 2007. Impact of platelet reactivity after clopidogrel administration on drug-eluting stent thrombosis. J Am Coll Cardiol, 49:2312-2317.

Chen

, Zhang

, Li

et al. 2008. Inhibition of ADP-induced platelet aggregation by clopidogrel is related to CYP2C19 genetic polymorphisms. Clin Exp Pharmacol Physiol, 35:904-908.

Cuisset

, Frere

, Quilici

et al. 2006. High post-treatment platelet reactivity identified low-responders to dual antiplatelet therapy at increased risk of recurrent cardiovascular events after stenting for acute coronary syndrome. J Thromb Haemost, 4:542-549.

Cuisset

, Frere

, Quilici

et al. 2009. Comparison of omeprazole and pantoprazole influence on a high 150-mg clopidogrel maintenance dose. The PACA (proton pump inhibitors and clopidogrel association) prospective randomized study. J Am Coll Cardiol, 54:1149-1153.

Cuisset

, Hamilos

, Sarma

et al. 2008. Relation of low response to clopidogrel assessed with point-of-care assay to periprocedural myonecrosis in patients undergoing elective coronary stenting for stable angina pectoris. Am J Cardiol, 101:1700-1703.

Desta

, Zhao

, Shin

et al. 2002. Clinical significance of the cytochrome P450 2C19 genetic polymorphism. Clin Pharmacokinet, 41:913-958.

10.

Dojo

, Azuma

, Saito

et al. 2001. Effects of CYP2C19 gene polymorphism on cure rates for Helicobacter pylori infection by triple therapy with proton pump inhibitor (omeprazole or rabeprazole), amoxycillin and clarithromycin in Japan. Digest Liver Dis, 33:671-675.

11.

Frere

, Cuisset

, Morange

et al. 2008. Effect of cytochrome P450 polymorphisms on platelet reactivity after treatment with clopidogrel in acute coronary syndrome. Am J Cardiol, 101:1088-1093.

12.

Gerschutz

, Bkatt

. 2001. The clopidogrel in unstable angina to prevent recurrent events trial (CURE) study: effects of clopidogrel in addition to aspirin in patients with acutecoronary syndromes without ST-segment elevation. N Engl J Med, 345:494-502.

13.

Gilard

, Arnaud

, Cornily

et al. 2008. Influence of omeprazole on the antiplatelet action of clopidogrel associated with aspirin: the randomized, double-blind OCLA (omeprazole clopidogrel aspirin) study. J Am Coll Cardiol, 51:256-260.

14.

Goldstein

. 2001. Clinical relevance of genetic polymorphisms in the human CYP2C subfamily. Br J Clin Pharmacol, 52:349-355.

15.

Hulot

, Bura

, Villard

et al. 2006. Cytochrome P450 2C19 loss-of-function polymorphism is a major determinant of clopidogrel responsiveness in healthy subjects. Blood, 108:2244-2247.

16.

Ingelman-Sundberg

, Daly

, Nebert

. 2009. Human cytochrome P450 allele nomenclature committee. www.cypalleles.ki.se/. 2009 Aug 25.

17.

Ishizaki

, Horai

. 1999. Review article: cytochrome P450 and the metabolism of proton pump inhibitors—emphasis on rabeprazole. Aliment Pharmacol Ther, 13,Suppl 3:27-36.

18.

Jacqz

, Dulac

, Mathieu

. 1988. Phenotyping polymorphic drug metabolism in the French Caucasian population. Eur J Clin Pharmacol, 35:167-171.

19.

Juurlink

, Gomes

, Ko

et al. 2009. A population-based study of the drug interaction between proton pump inhibitors and clopidogrel. CMAJ, 180:713-718.

20.

Kazui

, Nishiya

, Ishizuka

et al. 2010. Identification of the human cytochrome P450 enzymes involved in the two oxidative steps in the bioactivation of clopidogrel to its pharmacologically active metabolite. Drug Metab Dispos, 38:92-99.

21.

, Sukhova

, Thacker

et al. 1997. Evaluation of omeprazole and lansoprazole as inhibitors of cytochrome P450 isoforms. Drug Metab Dispos, 25:853-862.

22.

Kubota

, Chiba

, Ishizaki

. 1996. Genotyping of S-mephenytoin 4-hydroxylation in an extended Japanese population. Clin Pharmacol Ther, 60:661-666.

23.

Lau

, Gurbel

, Watkins

et al. 2004. Contribution of hepatic cytochrome P450 3A4 metabolic activity to the phenomenon of clopidogrel resistance. Circulation, 109:166-171.

24.

Lau

, Waskell

, Watkins

et al. 2003. Atorvastatin reduces the ability of clopidogrel to inhibit platelet aggregation: a new drug-drug interaction. Circulation, 107:32-37.

25.

Matetzky

, Shenkman

, Guetta

et al. 2004. Clopidogrel resistance is associated with increased risk of recurrent atherothrombotic events in patients with acute myocardial infarction. Circulation, 109:3171-3175.

26.

Mega

, Close

, Wiviott

et al. 2009. Cytochrome P-450 polymorphisms and response to clopidogrel. N Engl J Med, 360:354-364.

27.

Shu

, Zhou

. 2000. Individual and ethnic differences in CYP2C19 activity in Chinese populations. Acta Pharmacol Sin, 21:193-199.

28.

Siller-Matula

, Spiel

, Lang

et al. 2009. Effects of pantoprazole and esomeprazole on platelet inhibition by clopidogrel. Am Heart J, 157:148.e1-148.e5.

29.

Simon

, Verstuyft

, Mary-Krause

et al. 2009. Genetic determinants of response to clopidogrel and cardiovascular events. N Eng J Med, 360:411-413.