Clopidogrel Bioactivation Research Articles

Paraoxonase-1 Is Not a Major Determinant of Stent Thrombosis in a Taiwanese Population

BackgroundClopidogrel is a prodrug that undergoes in vivo bioactivation to show its antiplatelet effects. Recent studies have shown that cytochrome P450 (CYP), ATP-binding cassette transporters (ABCB1), and paraoxonase-1 (PON1) play crucial roles in clopidogrel bioactivation. Here, we aim to determine the effects of genetic polymorphisms of CYP (CYP 2C19*2, CYP 2C19*3, and CYP 2C19*17), ABCB1 (ABCB1 3435C>T, ABCB1 129T>C, and ABCB1 2677G>T/A), and PON1 (PON1 Q192R, PON1 L55M, and PON1 108C>T) on the development of stent thrombosis (ST) in patients receiving clopidogrel after percutaneous coronary intervention (PCI).Methods and ResultsWe evaluated the incidence of ST (0.64%) in 4964 patients who were recruited in the CAPTAIN registry (Cardiovascular Atherosclerosis and Percutaneous TrAnsluminal INterventions). The presence of genetic polymorphisms was assessed in 20 subjects who developed ST after aspirin and clopidogrel therapy and in 40 age- and sex-matched control subjects who did not develop ST, which was documented after 9 months of angiographic follow-up. ST was acute in 5 subjects, subacute in 7, late in 7, and very late in 1. The presence of CYP 2C19*2 allele was significantly associated with ST (adjusted odds ratio [ORadj]: 4.20, 95% confidence interval [CI], 1.263–9.544; P = 0.031). However, genetic variations in PON1 and ABCB1 showed no significant association with ST.ConclusionWe conclude that in a Taiwanese population, PON1 Q192R genotype is not associated with ST development after PCI. However, the presence of CYP 2C19*2 allele is a risk factor for ST development after PCI.

Cardiovascular Pharmacogenomics: Current Status and Future Directions—Report of a National Heart, Lung, and Blood Institute Working Group

The National Heart, Lung, and Blood Institute (NHLBI) convened a Working Group on January 7, 2011, at George Washington University in Washington, DC, to provide recommendations to the NHLBI that would guide informed decisions on research directions and priorities in the field of cardiovascular

Antithrombotics for secondary prevention of noncardioembolic ischaemic stroke

Antiplatelet therapy is more effective than anticoagulation for the prevention of noncardioembolic ischaemic stroke. The choice of antiplatelet regimen, however, remains contentious. Recent controversies regarding aspirin resistance and the optimal dosing of aspirin, as well as recognition of the variable bioactivation of clopidogrel, have added further confusion to the debate. The American Heart Association (AHA) and American Stroke Association (ASA) recently released their third joint guideline in the past 5 years on secondary stroke prevention. The European Stroke Organisation has published three guidelines on this issue since 2000. These frequent updates have been necessary because of rapidly accumulating data from clinical trials. Careful consideration of the sometimes confusing trial results reveals that the 2011 AHA-ASA guidelines are correct in no longer specifying a 'preferred' antiplatelet regimen from among the choices recently studied. This recommendation does not, however, mean that all antiplatelet regimens should be considered equal. This Review discusses the various antiplatelet regimens, and the trials that led to the rapid evolution of the guidelines for secondary prevention of ischaemic stroke.

Clarifying the importance of CYP2C19 and PON1 in the mechanism of clopidogrel bioactivation and in vivo antiplatelet response

It is thought that clopidogrel bioactivation and antiplatelet response are related to cytochrome P450 2C19 (CYP2C19). However, a recent study challenged this notion by proposing CYP2C19 as wholly irrelevant, while identifying paraoxonase-1 (PON1) and its Q192R polymorphism as the major driver of clopidogrel bioactivation and efficacy. The aim of this study was to systematically elucidate the mechanism and relative contribution of PON1 in comparison to CYP2C19 to clopidogrel bioactivation and antiplatelet response. First, the influence of CYP2C19 and PON1 polymorphisms and plasma paraoxonase activity on clopidogrel active metabolite (H4) levels and antiplatelet response was assessed in a cohort of healthy subjects (n = 21) after administration of a single 75 mg dose of clopidogrel. There was a remarkably good correlation between H4 AUC (0-8 h) and antiplatelet response (r2 = 0.78). Furthermore, CYP2C19 but not PON1 genotype was predictive of H4 levels and antiplatelet response. There was no correlation between plasma paraoxonase activity and H4 levels. Secondly, metabolic profiling of clopidogrel in vitro confirmed the role of CYP2C19 in bioactivating clopidogrel to H4. However, heterologous expression of PON1 in cell-based systems revealed that PON1 cannot generate H4, but mediates the formation of another thiol metabolite, termed Endo. Importantly, Endo plasma levels in humans are nearly 20-fold lower than H4 and was not associated with any antiplatelet response. Our results demonstrate that PON1 does not mediate clopidogrel active metabolite formation or antiplatelet action, while CYP2C19 activity and genotype remains a predictor of clopidogrel pharmacokinetics and antiplatelet response.

Abstract 3645: Omeprazole, A Potent Cyp 2c19 Inhibitor, Does Not Alter The Pharmacokinetics Or Platelet Aggregation Of Asa And Dipyridamole In Combination

Antiplatelet therapies such as aspirin (ASA), clopidogrel or a combination of ASA and extended-release dipyridamole (ASA/DP) are recommended therapeutic options for reducing the risk of stroke in patients with prior stroke or TIA. The clinical benefit of clopidogrel is highly dependent on the bioactivation of clopidogrel to its active metabolite by CYP 2C19. Omeprazole (OME), a frequently used gastric acid suppressant, is a potent inhibitor of CYP2C19, and has been identified as having a clinically significant interaction with clopidogrel. It was unclear whether ASA/DP would be subject to the same interaction risk when concomitantly administered with OME. This study formally tests whether multiple daily doses of OME 80 mg alters the pharmacokinetics of dipyridamole or the inhibition of platelet aggregation (IPA) of ASA/DP. This study was performed in 60 healthy volunteers as a multiple-dose, open-label, randomized, crossover trial with 4 treatments within 2 treatment sequences. The endpoints included the steady-state Cmax and AUC of DP as well as IPA at 4 and 12 hours postdose. The mean plasma DP concentration time profile for all treatments is displayed in Figure 1 . The 90% confidence interval for DP Cmax and AUC was fully contained within 80-125% bounds indicating no clinically significant interaction between ASA/DP and OME. Moreover, concomitant administration of ASA/DP with OME had no effect on the inhibition of arachidonic acid induced platelet aggregation over the normal dosing interval at steady-state ( Table 1 ). There was no difference detected regardless of whether subjects were pre-treated with OME alone or ASA/DP and OME. In conclusion, concurrent administration of OME with ASA/DP does not alter the steady-state pharmacokinetics of DP nor the inhibition of platelet aggregation of ASA/DP, regardless of the order of treatment administration. All treatments were well tolerated. [ Table 1 .] Table Notes: A: ASA/DP alone B: ASA/DP + Omeprazole following ASA/DP alone C: Omeprazole alone D: ASA/DP + Omeprazole following Omeprazole alone

Pregnane X receptor regulates drug metabolism and transport in the vasculature and protects from oxidative stress

AimsCirculating endogenous, dietary, and foreign chemicals can contribute to vascular dysfunction. The mechanism by which the vasculature protects itself from these chemicals is unknown. This study investigates whether the pregnane X receptor (PXR), the major transcriptional regulator of hepatic drug metabolism and transport that responds to such xenobiotics, mediates vascular protection by co-ordinating a defence gene programme in the vasculature.Methods and resultsPXR was detected in primary human and rat aortic endothelial and smooth muscle cells (SMC) and blood vessels including the human and rat aorta. Metabolic PXR target genes cytochrome P450 3A, 2B, 2C, and glutathione S-transferase mRNA and activity were induced by PXR ligands in rodent and human vascular cells and absent in the aortas from PXR-null mice stimulated in vivo or in rat aortic SMC expressing dominant-negative PXR. Activation of aortic PXR by classical agonists had several protective effects: increased xenobiotic metabolism demonstrated by bioactivation of the pro-drug clopidogrel, which reduced adenosine diphosphate-induced platelet aggregation; increased expression of multidrug resistance protein 1, mediating chemical efflux from the vasculature; and protection from reactive oxygen species-mediated cell death.ConclusionPXR co-ordinately up-regulates drug metabolism, transport, and antioxidant genes to protect the vasculature from endogenous and exogenous insults, thus representing a novel gatekeeper for vascular defence.

Cytochromes P450 Catalyze Both Steps of the Major Pathway of Clopidogrel Bioactivation, whereas Paraoxonase Catalyzes the Formation of a Minor Thiol Metabolite Isomer

The mechanism generally admitted for the bioactivation of the antithrombotic prodrug, clopidogrel, is its two-step enzymatic conversion into a biologically active thiol metabolite. The first step is a classical cytochrome P450 (P450)-dependent monooxygenation of its thiophene ring leading to 2-oxo-clopidogrel, a thiolactone metabolite. The second step was described as a P450-dependent oxidative opening of the thiolactone ring of 2-oxo-clopidogrel, with intermediate formation of a reactive sulfenic acid metabolite that is eventually reduced to the corresponding thiol 4b. A very recent paper published in Nat. Med. (Bouman et al., (2011) 17, 110) reported that the second step of clopidogrel bioactivation was not catalyzed by P450 enzymes but by paraoxonase-1(PON-1) and that PON-1 was a major determinant of clopidogrel efficacy. The results described in the present article show that there are two metabolic pathways for the opening of the thiolactone ring of 2-oxo-clopidogrel. The major one, that was previously described, results from a P450-dependent redox bioactivation of 2-oxo-clopidogrel and leads to 4b cis, two previously reported thiol diastereomers bearing an exocyclic double bond. The second, minor one, results from a hydrolysis of 2-oxo-clopidogrel, which seems to be dependent on PON-1, and leads to an isomer of 4b cis, 4b "endo", in which the double bond has migrated from an exocyclic to an endocyclic position in the piperidine ring. These results were obtained from a detailed study of the metabolism of 2-oxo-clopidogrel by human liver microsomes and human sera and analysis by HPLC-MS under conditions allowing a complete separation of the thiol metabolite isomers, either as such or after derivatization with 3'-methoxy phenacyl bromide or N-ethyl maleimide (NEM). These results also show that the major bioactive thiol isomer found in the plasma of clopidogrel-treated patients derives from 2-oxo-clopidogrel by the P450-dependent pathway. Finally, chemical experiments on 2-oxo-clopidogrel showed that this thiolactone is in equilibrium with its tautomer having a double bond inside the piperidine ring and that nucleophiles such as CH(3)O(-) preferentially react on the thioester function of this tautomer. This allowed us to understand why 4b cis has to be formed via an oxidative opening of 2-oxo-clopidogrel thiolactone, whereas a hydrolytic opening of this thiolactone ring leads to the "endo" thiol isomer 4b "endo".

Abstract 9551: Relative Contributions of Genetic Variants in PON 1, CYP2C19 And ABCB1 o n in-hospital and One Year Clinical Outcomes in Patients Treated with Clopidogrel After an Acute Myocardial Infarction

Background: Clopidogrel with aspirin reduces ischemic events in patients with acute myocardial infarction (AMI). Clopidogrel requires drug transport via P-glycoprotein, encoded by ABCB1 , and bioactivation by CYP450, mainly via CYP2C19 . The clinical response to clopidogrel is variable, depending in part on CYP2C19 and ABCB1 genetic polymorphisms. Recently however, paraoxonase-1 (PON1) was suggested as the crucial enzyme for clopidogrel bioactivation, with its Q192R polymorphism determining per se clopidogrel response on cardiovascular events. Methods We assessed the effect of PON1 (Q192R) polymorphisms by itself and alongside variants in CYP2C19 loss-of-function and ABCB1 on in-hospital and one-year cardiovascular events in clopidogrel-treated AMI patients participating in the Fast-MI study (n=2208), including the subset undergoing PCI (n=1538). Results : PON1 (Q192R) polymorphism was not associated with increased risk of in-hospital death and one year major adverse events: One-year adjusted hazard ratio [HR] for QQ versus RR individuals, 1.03; 95% CI, 0.66-1.61 in the whole population and HR, 0.77; 95% CI, 0.42-1.41 in PCI-treated patients. Adding ABCB1 and CYP2C19 polymorphisms had no impact on these risks. Presence of two CYP2C19 loss-of-function alleles was associated with the risk of in-hospital death and major clinical events at one year in the overall population (OR: 3.67; 95% CI, 1.05-12.80, and HR: 1.96, 95% CI, 1.08-3.54) and in PCI patients (OR, 6.87; 95% CI, 2.52-18.72, and HR: 3.06, 95% CI, 1.47-6.41). ABCB1 polymorphisms had an impact on the clinical response to clopidogrel in the overall population but not in PCI patients. Conclusion : PON1 polymorphism was not associated with an increased risk of events in clopidogrel-treated patients with AMI and those undergoing PCI. In contrast, CYP2C19 loss- -of-function polymorphism remained the major pharmacogenetic contributor of clinical response to clopidogrel in these populations.

Unraveling the Pharmacogenetics of Clopidogrel

Clopidogrel forms a cornerstone in the treatment of patients undergoing coronary stenting. Although the clinical effectiveness of clopidogrel has been shown repeatedly in large clinical trials, there appears to be a large interindividual response variability that influences the risk of atherothrombotic events.1,2 Studies3,4 have shown that differences in the plasma concentration of the active metabolite of clopidogrel are an important determinant of its antiplatelet effect. Although several environmental factors influence the formation of the active metabolite, most variation appears to be genetic.5 Numerous studies5–7 have shown that polymorphisms in genes encoding the cytochrome (CYP) p450 system, especially CYP2C19, influence bioactivation of clopidogrel and, hence, the risk of cardiovascular events, especially stent thrombosis. Still, polymorphic variation in CYP2C19 seems to explain only 5% to 11% of the response variability to clopidogrel, leaving variation in bioactivation of clopidogrel largely unexplained.5,8 Article see p 422 A new player in the pharmacogenetics of clopidogrel was introduced by Bouman et al,9 who identified paraoxonase-1 (PON1) as a major determinant of the bioactivation and clinical efficacy of clopidogrel in a series of elegant experiments and clinical studies among subjects from European descent. This hepatic esterase is associated with high-density lipoprotein and has antioxidative effects on low-density lipoprotein and macrophages. Bouman et al identified PON1 as the crucial enzyme for the bioactivation of clopidogrel, with its common Q192R polymorphism determining the rate of active thiol metabolite formation. In a case-cohort study among 41 cases with stent thrombosis and 71 control subjects, the Q allele was dose-dependently associated with lower PON1 activity and active metabolite concentrations in plasma, reduced platelet inhibition, and a higher risk of stent thrombosis. Q192R explained >70% of the response variability to clopidogrel. The authors corroborated their findings in a …

Current Evidence for Genetic Testing in Clopidogrel-Treated Patients Undergoing Coronary Stenting

A combined dual antiplatelet treatment consisting of aspirin and the P2Y12 receptor inhibitor clopidogrel is still considered the standard of care treatment in most of the patients undergoing percutaneous coronary intervention (PCI). Numerous research studies during the last decade, however, have highlighted possible shortcomings of the oral antiplatelet agent clopidogrel, namely its large response variability resulting in an unpredictable response for the individual patient,1,2 the association of both a low3 or enhanced response4,5 with a worse clinical outcome and the dependency of individual responsiveness on nongenetic and genetic variables.6 Response by Pare and Eikelboom on p 513 Clopidogrel is a prodrug that requires bioactivation into its active thiol metabolite before it targets the P2Y12 receptor on blood platelets. In vivo bioactivation of the drug is a 2-step process that is closely linked to the cytochrome P450 (CYP) system. Different isoenzymes are responsible for clopidogrel bioactivation and among them the isoenzyme CYP2C19 was found to play a key role in this setting by contributing to both clopidogrel bioactivation steps.7 In this context, common genetic variants within the CYP2C19 gene have been the subject of considerable attention and have stimulated numerous research projects in recent years.8–14 Beyond CYP2C19, other genes involved in clopidogrel absorption, bioactivation or interplay with the blood platelet and their receptors have been associated with drug responsiveness and clinical outcome as well. Indeed, a growing body of evidence suggests a possible role of genotyping in patients undergoing coronary stenting with a view on optimizing response to P2Y12 receptor inhibitors during and after the procedure. We review available evidence on the need of individualizing antiplatelet treatment regimens in everyday clinical practice. ### Genetic Determinants for Clopidogrel Response and Clinical Outcome In recent years, multiple genetic factors within different candidate genes being involved in clopidogrel absorption, bioactivation, and …

Paraoxonase-1 Q192R Polymorphism and Antiplatelet Effects of Clopidogrel in Patients Undergoing Elective Coronary Stent Placement

Recently published data indicate that the paraoxonase-1 (PON1) Q192R genotype-and not as previously shown activity of cytochrome P450 (CYP) 2C19-is the major determinant of metabolic bioactivation of clopidogrel and thereby variability of antiplatelet effect of clopidogrel. We sought to investigate whether the PON1 Q192R gene polymorphism affects platelet reactivity in patients undergoing elective coronary stent placement. The study included 760 consecutive patients undergoing elective coronary stent placement after loading with clopidogrel 600 mg. Platelet function was assessed by adenosine diphosphate-induced (ADP 5 and 20 μmol/L) platelet aggregation and by flow-cytometric analysis of platelet surface protein expression before clopidogrel, at the time of coronary stent placement, and before discharge after coronary stent placement. PON1 Q192R genotype [NM_000446.5:c.575A>G single nucleotide polymorphism (rs662)] was analyzed by TaqMan polymerase chain reaction. Residual platelet aggregation (ADP 5 μmol/L) at predischarge was 8.0% (3.0% to 17.0%) [median (interquartile range)] in PON1 QQ192 patients (n=384), 8.0% (3.0% to 15.0%) in PON1 QR192 (n=304), and 11.0% (3.0% to 18.0%) in PON1 RR192 (n=72; P=0.603). By multivariable linear regression, residual platelet aggregation was not associated with PON1 QQ192/QR192 (partial η(2)<0.001, P=0.728) but with CYP2C19*2 loss-of-function allele (partial η(2)=0.045, P<0.001) as well as any CYP2C19*17 gain-of-function allele (partial η(2)=0.012, P=0.004). All other platelet assays also showed no significant association between PON1 Q192R genotype and antiplatelet effect of clopidogrel. The 1-year incidence of death and myocardial infarction did not differ between PON1 Q192R genotypes. On-treatment platelet reactivity in patients undergoing coronary stent placement after loading with clopidogrel 600 mg was not associated with PON1 Q192R genotype. URL: http://www.clinicaltrials.gov. Unique identifier: NCT00457236.

Beyond CYP2C19 – A New Chapter in Clopidogrel Pharmacogenomics

Journal of Pharmacy Practice and ResearchVolume 41, Issue 1 p. 14-16 EditorialFree Access Beyond CYP2C19 – A New Chapter in Clopidogrel Pharmacogenomics Dr Thomas M Polasek BSc, BPharm (Hons), PhD, MSHP, Dr Thomas M Polasek BSc, BPharm (Hons), PhD, MSHP Lecturer in Clinical Pharmacology [email protected] Department of Clinical Pharmacology, Flinders University and Flinders Medical Centre, Adelaide, SA, 5042Search for more papers by this author Dr Thomas M Polasek BSc, BPharm (Hons), PhD, MSHP, Dr Thomas M Polasek BSc, BPharm (Hons), PhD, MSHP Lecturer in Clinical Pharmacology [email protected] Department of Clinical Pharmacology, Flinders University and Flinders Medical Centre, Adelaide, SA, 5042Search for more papers by this author First published: 13 April 2015 https://doi.org/10.1002/j.2055-2335.2011.tb00054.xCitations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References 1Winslow R. Plavix rival gains from studies. Wall St 30 August 2010. 2Sofi F, Marcucci R, Gori AM, Giusti B, Abbate R, Gensini GF. Clopidogrel non-responsiveness and risk of cardiovascular morbidity. An updated meta-analysis. Thromb Haemost 2010; 103: 841– 8. 3Hagihara K, Kazui M, Kurihara A, Yoshiike M, Honda K, Okazaki O, et al. A possible mechanism for the differences in efficiency and variability of active metabolite formation from thienopyridine antiplatelet agents, prasugrel and clopidogrel. Drug Metab Dispos 2009; 37: 2145– 52. 4Kazui M, Nishiya Y, Ishizuka T, Hagihara K, Farid NA, Okazaki O, et al. 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 2010; 38: 92– 9. 5Mega JL, Simon T, Collet JP, Anderson JL, Antman EM, Bliden K, et al. Reduced-function CYP2C19 genotype and risk of adverse clinical outcomes among patients treated with clopidogrel predominantly for PCI. JAMA 2010; 304: 1821– 30. 6Bouman HJ, Schömig E, van Werkum JW, Velder J, Hackeng CM, Hirschhäuser C, et al. Paraoxonase-1 is a major determinant of clopidogrel efficacy. Nat Med 2011; 17: 110– 16. Citing Literature Volume41, Issue1March 2011Pages 14-16 ReferencesRelatedInformation

Pondering the Significance of PON1

Another strong pharmacogenetic variant for clopidogrel response emerges.

Catapulting clopidogrel pharmacogenomics forward

The antiplatelet drug clopidogrel helps prevent stent-associated thrombosis, but the antiplatelet effects are quite variable and the clinical consequences can be serious. New findings show that the variability in clopidogrel efficacy is affected by the enzyme paraoxonase-1 (PON1), which is required for clopidogrel bioactivation (pages 110–116 ).

Paraoxonase-1 is a major determinant of clopidogrel efficacy

Clinical efficacy of the antiplatelet drug clopidogrel is hampered by its variable biotransformation into the active metabolite. The variability in the clinical response to clopidogrel treatment has been attributed to genetic factors, but the specific genes and mechanisms underlying clopidogrel bioactivation remain unclear. Using in vitro metabolomic profiling techniques, we identified paraoxonase-1 (PON1) as the crucial enzyme for clopidogrel bioactivation, with its common Q192R polymorphism determining the rate of active metabolite formation. We tested the clinical relevance of the PON1 Q192R genotype in a population of individuals with coronary artery disease who underwent stent implantation and received clopidogrel therapy. PON1 QQ192 homozygous individuals showed a considerably higher risk than RR192 homozygous individuals of stent thrombosis, lower PON1 plasma activity, lower plasma concentrations of active metabolite and lower platelet inhibition. Thus, we identified PON1 as a key factor for the bioactivation and clinical activity of clopidogrel. These findings have therapeutic implications and may be exploited to prospectively assess the clinical efficacy of clopidogrel.

Impact of Genetic Polymorphisms and Drug – Drug Interactions on Clopidogrel and Prasugrel Response Variability

Thienopyridine antiaggregating platelet agents (clopidogrel and prasugrel) act as irreversible P2Y12 receptor inhibitors. They are used with aspirin to prevent thrombotic complications after an acute coronary syndrome or percutaneous coronary intervention. A large interindividual variability in response to clopidogrel and to a lesser extent to prasugrel is observed and may be related to their metabolism. Clopidogrel and prasugrel are indeed prodrugs converted into their respective active metabolites by several cytochromes P450 (CYPs). Besides clopidogrel inactivation (85%) by esterases to the carboxylic acid, clopidogrel is metabolized by CYPs to 2-oxo-clopidogrel (15%) and further metabolized to an unstable but potent platelet-aggregating inhibitor. Prasugrel is more potent than clopidogrel with a better bioavailability and lower pharmacodynamic variability. Prasugrel is completely converted by esterases to an intermediate oxo-metabolite (R-95913) further bioactivated by CYPs. Numerous clinical studies have shown the influence of CYP2C19 polymorphism on clopidogrel antiplatelet activity. Moreover, unwanted drug-drug pharmacokinetic interactions influencing CYP2C19 activity and clopidogrel bioactivation such as with proton pump inhibitors remain a matter of intense controversy. Several studies have also demonstrated that CYP3A4/5 and CYP1A2 are important in clopidogrel bioactivation and should also be considered as potential targets for unwanted drug-drug interactions. Prasugrel bioactivation is mainly related to CYP3A4 and 2B6 activity and therefore the question of the effect of drug-drug interaction on its activity is open. The purpose of this review is to critically examine the current literature evaluating the influence of genetic and environmental factors such as unwanted drug-drug interaction affecting clopidogrel and prasugrel antiplatelet activity.

ACCF/AHA Clopidogrel Clinical Alert: Approaches to the FDA “Boxed Warning”

The recent US Food and Drug Administration (FDA) “boxed warning” on clopidogrel raises important questions for practitioners and patients. The warning addresses the need for pharmacogenomic testing to identify patients’ altered clopidogrel metabolism and thus their risk for a suboptimal clinical response to clopidogrel. Although there is an expanding database on genetic polymorphisms that may affect clopidogrel metabolism and thus clinical outcomes, there are no evidence-based data upon which todevelop specific recommendations on the role of genetic testing in routine care nor strategies proven to improve the safety/efficacy of specific pharmacologic approaches. To provide guidance on this issue, the American College of Cardiology Foundation (ACCF) and the American Heart Association (AHA) convened a writing committee. The ACCF and AHA adhere to a rigorous policy regarding relationships with industry and other entities (RWI) of authors and peer reviewers for clinical document development (see http://www.cardiosource.org/Science-And-Quality/Practice-Guidelines-and-Quality-Standards/Relationships-With-Industry-Policy.aspx). This policy requires that a majority of writing committee members have no relevant relationships with industry to this topic, a standard that has been achieved for this document as indicated in Appendix 1. In the spirit of full disclosure, comprehensive RWI (RWI not relevant to this document) for all authors is available online for public view. RWI …

Clopidogrel and Proton Pump Inhibitors: A New Drug Interaction?

Clopidogel is a prodrug requiring hepatic bioactivation via cytochrome P450 isozymes (CYP2C19,CYP3A4, CYP3A5) to its pharmacologically active form. Inhibition of cytochrome P450 may interfere with metabolicactivation of clopidogrel, reducing its antiplatelet activity andpotentially increasing the risk of thrombosis. More recently, thecytochrome P450 2C19 pathway has been identified as the keypathway in clopidogrel bioactivation.

Genotyping, Clopidogrel Metabolism, and the Search for the Therapeutic Window of Thienopyridines

It is somewhat remarkable to note that not until recently, nearly 12 years and tens of millions of treated patients after its initial approval, has the metabolism of clopidogrel, an inactive prodrug, begun to be more fully understood. Although this lack of knowledge may not be unique among common cardiovascular drugs, there are 4 characteristics of clopidogrel therapy that make this gap in understanding disconcerting: The potential for life-threatening complications (bleeding or thrombosis) at both ends of its therapeutic spectrum; the fact that 1 dose is supposed to fit all; the lack of a measureable, proven end point (such as blood pressure for antihypertensive drugs) that allows for titration to a target response; and the nearly mandatory need by many patients for clopidogrel, with no acceptable alternative until recently. Article see p 512 Historically, understanding the metabolism of a drug has been critical in identifying those patients most likely to benefit from and, more important, potentially be harmed by that drug. This is especially true for drugs with relatively narrow therapeutic windows, such as phenytoin, warfarin, and others. Although it seems safe to assume, on the basis of our extensive clinical experience, that the therapeutic window of clopidogrel is unlikely to be narrow, this certainly does not represent the depth of knowledge we would hope for after so much clinical experience and research. What we do know is that P2Y12 inhibition with thienopyridines is beneficial in preventing arterial thrombotic events, presumably requiring the achievement of some minimal level of receptor inhibition that may or may not vary on the basis of the patient and clinical scenario. We also know that it is associated with a significant increase in serious bleeding events, with higher levels of inhibition associated with greater bleeding. In the present and future practice environment, with a growing …

Identification of the Human Cytochrome P450 Enzymes Involved in the Two Oxidative Steps in the Bioactivation of Clopidogrel to Its Pharmacologically Active Metabolite

The aim of the current study is to identify the human cytochrome P450 (P450) isoforms involved in the two oxidative steps in the bioactivation of clopidogrel to its pharmacologically active metabolite. In the in vitro experiments using cDNA-expressed human P450 isoforms, clopidogrel was metabolized to 2-oxo-clopidogrel, the immediate precursor of its pharmacologically active metabolite. CYP1A2, CYP2B6, and CYP2C19 catalyzed this reaction. In the same system using 2-oxo-clopidogrel as the substrate, detection of the active metabolite of clopidogrel required the addition of glutathione to the system. CYP2B6, CYP2C9, CYP2C19, and CYP3A4 contributed to the production of the active metabolite. Secondly, the contribution of each P450 involved in both oxidative steps was estimated by using enzyme kinetic parameters. The contribution of CYP1A2, CYP2B6, and CYP2C19 to the formation of 2-oxo-clopidogrel was 35.8, 19.4, and 44.9%, respectively. The contribution of CYP2B6, CYP2C9, CYP2C19, and CYP3A4 to the formation of the active metabolite was 32.9, 6.76, 20.6, and 39.8%, respectively. In the inhibition studies with antibodies and selective chemical inhibitors to P450s, the outcomes obtained by inhibition studies were consistent with the results of P450 contributions in each oxidative step. These studies showed that CYP2C19 contributed substantially to both oxidative steps required in the formation of clopidogrel active metabolite and that CYP3A4 contributed substantially to the second oxidative step. These results help explain the role of genetic polymorphism of CYP2C19 and also the effect of potent CYP3A inhibitors on the pharmacokinetics and pharmacodynamics of clopidogrel in humans and on clinical outcomes.