Differential Impacts of CYP2C19 Gene Polymorphisms on the Antiplatelet Effects of Clopidogrel and Ticlopidine

Antiplatelet Use in Ischemic Stroke

Objective:

A literature review of antiplatelet agents for primary and secondary stroke prevention, including mechanism of action, cost, and reasons for lack of benefit.

Data sources:

Articles were gathered from MEDLINE, Cochrane Reviews, and PubMed databases (1980-2021). Abstracts from scientific meetings were considered. Search terms included ischemic stroke, aspirin, clopidogrel, dipyridamole, ticagrelor, cilostazol, prasugrel, glycoprotein IIb/IIIa inhibitors.

Study selection and data extraction:

English-language original and review articles were evaluated. Guidelines from multiple countries were reviewed. Articles were evaluated independently by 2 authors.

Data synthesis:

An abundance of evidence supports aspirin and clopidogrel use for secondary stroke prevention. In the acute phase (first 21 days postinitial stroke), these medications have higher efficacy for preventing further stroke when combined, but long-term combination therapy is associated with higher hemorrhage rates. Antiplatelet treatment failure is influenced by poor adherence and genetic polymorphisms. Antiplatelet agents such as cilostazol may provide extra benefit over clopidogrel and aspirin, in certain racial groups, but further research in more diverse ethnic populations is needed.

Relevance to patient care and clinical practice:

This review presents the data available on the use of different antiplatelet agents poststroke. Dual therapy, recurrence after initiation of secondary preventative therapy, and areas for future research are discussed.

Conclusions:

Although good evidence exists for the use of certain antiplatelet agents postischemic stroke, there are considerable opportunities for future research to investigate personalized therapies. These include screening patients for platelet polymorphisms that confer antiplatelet resistance and for randomized trials including more racially diverse populations.

Clinical Utility of CYP2C19 Genotype-Guided Antiplatelet Therapy in Patients at Risk of Adverse Cardiovascular and Cerebrovascular Events: A Review of Emerging Evidence

In patients undergoing percutaneous coronary intervention (PCI), the standard of care is dual antiplatelet therapy with a P2Y₁₂ inhibitor (clopidogrel, prasugrel, or ticagrelor) and aspirin. Current clinical practice guidelines now recommend more potent P2Y₁₂ inhibitors (prasugrel or ticagrelor) over clopidogrel in acute coronary syndrome (ACS). However, clopidogrel remains the most commonly prescribed P2Y₁₂ inhibitor in the setting of PCI and is also the preferred agent in the treatment and secondary prevention of stroke. Clopidogrel is a prodrug that requires bioactivation by the CYP2C19 enzyme. It has been shown that clopidogrel use in patients who are CYP2C19 no function allele carriers are associated with impaired antiplatelet inhibition and a higher risk of major adverse cardiovascular and cerebrovascular events. Compared to clopidogrel, prasugrel and ticagrelor clinical response is not impacted by CYP2C19 genotype. Even with a demonstrated increased risk of adverse outcomes in CYP2C19 no function allele carriers treated with clopidogrel, routine implementation of CYP2C19 genotyping to guide antiplatelet therapy selection has remained controversial and has not been widely adopted. Recent results from multiple prospective randomized and nonrandomized clinical trials investigating the use of CYP2C19 genotype-guided antiplatelet therapy following PCI have advanced the evidence base demonstrating the clinical utility of this strategy. Multiple recent studies have examined the effects of CYP2C19 genotype on clopidogrel outcomes in the setting of stroke and neurointerventional procedures. In this review, we discern the clinical utility of using CYP2C19 genotype testing to guide antiplatelet therapy prescribing by evaluating the impact of CYP2C19 genotype-guided selection of antiplatelet therapy on clinical outcomes, summarizing emerging data from cardiovascular and neurology clinical studies, and discussing implications for clinical practice guidelines, remaining knowledge gaps and future research directions.

Incidence of neutropenia in patients with ticlopidine/Ginkgo biloba extract combination drug for vascular events: A post-marketing cohort study

BACKGROUND AND PURPOSE

The ticlopidine/Ginkgo biloba ext. combination drug (Yuclid) is used as an antiplatelet agent for prevention of vascular events since its approval in 2008. The purpose of this study is to explore the safety of ticlopidine/Ginkgo biloba combination, mainly regarding the incidence of neutropenia, through a post-marketing surveillance study.

METHODS

From March 2009 to October 2015, a total of 4839 subjects had been enrolled in this study. The enrollments were conducted by 152 doctors of 89 hospitals according to the regulations for post-marketing surveillance programs in Korea. If a subject was administered the drug once, he/she was included in the safety analysis set for any adverse events and bleedings, and the primary safety evaluation regarding neutropenia was conducted in subjects who completed 3-month blood test follow-up. We predefined that 1% reduction in neutropenia incidence by ticlopidine/Ginkgo biloba ext. combination from the previously reported incidence of ticlopidine of 2.3% was clinically meaningful.

RESULTS

Among the safety analysis set of 4831 patients (99.8% of the enrolled subjects), 3150 (65.1%) completed evaluation for neutropenia at 3 months which is the primary safety endpoint. The major causes of dropout were no follow-up visit at 3 months (n = 1016) and violation of the follow-up period (n = 503). Nine patients experienced neutropenia (Absolute neutrophil count [ANC] ≤ 1200mm3) and the estimated cumulative incidence at 3 months is 0.29% (95% confidence interval, 0.13%- 0.54%). Severe neutropenia (ANC ≤ 450mm3) did not occur in any patients.

CONCLUSIONS

The incidence of neutropenia with addition of Ginkgo biloba ext. to ticlopidine may be lower than the previously reported incidence of neutropenia with ticlopidine, which needs to be confirmed in randomized controlled trials.

Comparison of Clopidogrel and Ticlopidine/Ginkgo Biloba in Patients With Clopidogrel Resistance and Carotid Stenting

Background and Purpose: Patients undergoing carotid artery stenting (CAS) who show low responsiveness to clopidogrel may have a higher risk of peri-procedural embolic events. This study aimed to compare the effectiveness and safety of clopidogrel and ticlopidine plus Ginkgo biloba in clopidogrel-resistant patients undergoing CAS.

Methods: In this multi-center, randomized, controlled trial, we used platelet reactivity test to select patients undergoing CAS who showed clopidogrel resistance, and compared treatments using clopidogrel and ticlopidine plus ginkgo. The primary outcome was the incidence of new ischemic lesion in the ipsilateral hemisphere of CAS. Detection of microembolic signal on transcranial Doppler was the secondary outcome. The clinical outcomes were also monitored.

Results: This trial was discontinued after 42 patients were randomized after preplanned interim sample size re-estimation indicated an impractical sample size. The primary endpoint occurred in 12/22 patients (54.5%) in the clopidogrel group and 13/20 patients (65.0%) in the ticlopidine–ginkgo group (P = 0.610). No significant differences in the presence of microembolic signal (15.0 vs. 11.8%, P = 0.580), clinical outcomes (ischemic stroke or transient ischemic attack, 0.0 vs. 5.5%; acute myocardial infarction 0.0 vs. 0.0%; all-cause death, 4.5 vs. 0.0%), or incidence of adverse events were found in the two groups. In terms of resistance to clopidogrel, treatment with ticlopidine–ginkgo significantly increased the P2Y12 Reaction Units (difference, 0.0 [−0.3–3.0] vs. 21.0 [6.0–35.0], P < 0.001).

Conclusions: In patients who showed clopidogrel resistance, ticlopidine–ginkgo treatment was safe and increased P2Y12 Reaction Units; however, compared to clopidogrel, it failed to improve surrogate and clinical endpoints in patients undergoing CAS. This multimodal biomarker-based clinical trial is feasible in neurointerventional research.

Clinical Trial Registration:http://www.clinicaltrials.gov. Unique identifier: NCT02133989.

Comparison of antiplatelet treatment in patients with clopidogrel nonresponders with or without carriage of CYP2C19 polymorphism

BACKGROUND/AIMS

Several interventions exist for overcoming high platelet reactivity (HPR) on clopidogrel therapy. The goal of this study was to identify strategies that improve inhibition of platelet reactivity in clopidogrel nonresponders with or without loss of function CYP2C19 genotypes, resulting in platelet reactivity similar to that in responders.

METHODS

A total of 376 patients with stenting for coronary artery disease underwent platelet function testing in three centers. Blinded platelet function tests were performed after 75 mg daily clopidogrel treatment for 28 days. In total, 183 nonresponders were genotyped, were randomized to four treatment groups with each treatment lasting approximately 28 days, and underwent repeated measurements of platelet reactivity after treatment.

RESULTS

With 75 mg of daily clopidogrel, nonresponders had significantly higher HPR than did responders (multiple electrode aggregometry [MEA, arbitrary platelet aggregation unit]: mean, 71.4; 95% confidence intervals [CI], 68.6 to 74.3; and mean, 27.5; 95% CI, 26.0 to 28.9, respectively; p < 0.001). Ticagrelor or ticlopidine treatment in nonresponders resulted in platelet reactivity similar to that in responders in intermediate metabolizers (mean, 24.0; 95% CI, 19.6 to 28.4; p > 0.05; and mean, 30.0; 95% CI, 24.7 to 37.5; p > 0.05, respectively) and poor metabolizers (mean, 23.2; 95% CI, 18.0 to 28.3; p > 0.05; and mean, 30.3; 95% CI, 24.5 to 6.0; p > 0.05, respectively). However, in extensive metabolizers, only ticagrelor treatment showed platelet reactivity similar to that in responders (mean, 26.1; 95% CI, 24.1 to 28.0; p > 0.05).

CONCLUSIONS

Among clopidogrel nonresponders with cardiovascular disease on 75 mg daily clopidogrel, ticagrelor resulted in a comparable degree of platelet inhibition in all nonresponders compared with 150 mg daily clopidogrel or triple therapy with clopidogrel and cilostazol, irrespective of phenotype.

Use of Pharmacogenetic Information in the Treatment of Cardiovascular Disease

BACKGROUND

In 1964, Robert A. O'Reilly's research group identified members of a family who required remarkably high warfarin doses (up to 145 mg/day, 20 times the average dose) to achieve appropriate anticoagulation. Since this time, pharmacogenetics has become a mainstay of cardiovascular science, and genetic variants have been implicated in several fundamental classes of medications used in cardiovascular medicine.

CONTENT

In this review, we discuss genetic variants that affect drug response to 3 classes of cardiovascular drugs: statins, platelet P2Y12 inhibitors, and anticoagulants. These genetic variations have pharmacodynamic and pharmacokinetic effects and have been shown to explain differences in drug response such as lipid lowering, prevention of cardiovascular disease, and prevention of stroke, as well as incidence of adverse events such as musculoskeletal side effects and bleeding. Several groups have begun to implement pharmacogenetics testing as part of routine clinical care with the goal of improving health outcomes. Such strategies identify both patients at increased risk of adverse outcomes and alternative strategies to mitigate this risk as well as patients with “normal” genotypes, who, armed with this information, may have increased confidence and adherence to prescribed medications. While much is known about the genetic variants that underlie these effects, translation of this knowledge into clinical practice has been hampered by difficulty in implementing cost-effective, point-of-care tools to improve physician decision-making as well as a lack of data, as of yet, demonstrating the efficacy of using genetic information to improve health.

SUMMARY

Many genetic variants that affect individual responses to drugs used in cardiovascular disease prevention and treatment have been described. Further study of these variants is needed before successful implementation into clinical practice.

Population pharmacokinetic analysis of axitinib in healthy volunteers

AIMS

Axitinib is a potent and selective second generation inhibitor of vascular endothelial growth factor receptors 1, 2 and 3 approved for second line treatment of advanced renal cell carcinoma. The objectives of this analysis were to assess plasma pharmacokinetics and identify covariates that may explain variability in axitinib disposition following single dose administration in healthy volunteers.

METHODS

Plasma concentration-time data from 337 healthy volunteers in 10 phase I studies were analyzed, using non-linear mixed effects modelling (nonmem) to estimate population pharmacokinetic parameters and evaluate relationships between parameters and food, formulation, demographic factors, measures of renal and hepatic function and metabolic genotypes (UGT1A1*28 and CYP2C19).

RESULTS

A two compartment structural model with first order absorption and lag time best described axitinib pharmacokinetics. Population estimates for systemic clearance (CL), central volume of distribution (Vc ), absorption rate constant (ka ) and absolute bioavailability (F) were 17.0 l h(-1) , 45.3 l, 0.523 h(-1) and 46.5%, respectively. With axitinib Form IV, ka and F increased in the fasted state by 207% and 33.8%, respectively. For Form XLI (marketed formulation), F was 15% lower compared with Form IV. CL was not significantly influenced by any of the covariates studied. Body weight significantly affected Vc , but the effect was within the estimated interindividual variability for Vc .

CONCLUSIONS

The analysis established a model that adequately characterizes axitinib pharmacokinetics in healthy volunteers. Vc was found to increase with body weight. However, no change in plasma exposures is expected with change in body weight; hence no dose adjustment is warranted.

Genetic determinations of variable responsiveness to clopidogrel and implications for neurointerventional procedures

Endovascular intervention is emerging as a substitute for open surgical procedures for the treatment of cerebrovascular disease. However, up to 9% of patients undergoing neurointerventional procedures develop thromboembolic complications. Strategies to reduce periprocedural thromboembolic events are dominated by the use of dual antiplatelet therapy (DAT) which has been validated based on studies of peripheral vascular and coronary intervention. Of note, DAT decreases adverse vascular outcomes by 75-80% in patients undergoing percutaneous coronary intervention (PCI). It follows that similar treatment effects would be observed in neurointerventional populations. However, a growing body of evidence demonstrates that a subgroup of patients respond suboptimally to DAT, and in particular to clopidogrel (termed clopidogrel hyporesponders). These patients may be at an increased risk of thromboembolic complications such as in-stent thrombosis following neurointerventional procedures. Previous studies report 5-30% suboptimal response to clopidogrel in the cardiovascular population, while a higher prevalence is seen in populations undergoing neurointerventional procedures, i.e. as much as 66%. Knowledge of the mechanism leading to clopidogrel hyporesponsiveness is accumulating. A number of genetic polymorphisms, in particular CYP 2C19*2, have been associated with clopidogrel hyporesponsiveness and clinical outcomes. In addition, there are significant differences in the prevalence of CYP 2C19*2 across racial groups. Approximately 50% of Asians and 25% of Caucasians harbor the CYP 2C19*2 allele. While no prospective randomized trials currently exist to demonstrate improved clinical outcomes with genotype-based treatment for carriers of the CYP 2C19*2 polymorphism, a number of studies show that an increased dose of clopidogrel improves platelet inhibition in hyporesponders. The aim of the review is to examine the current understanding of the genetic basis of clopidogrel hyporesponsiveness in patients undergoing neurointerventional procedures and to explore current efforts using genotype and phenotype testing as well as alternative strategies to overcome the clopidogrel hyporesponsiveness.

Functional characterization of 21 CYP2C19 allelic variants for clopidogrel 2-oxidation

Genetic variations in cytochrome P450 2C19 (CYP2C19) contribute to interindividual variability in the metabolism of therapeutic agents such as clopidogrel. Polymorphisms in CYP2C19 are associated with large interindividual variations in the therapeutic efficacy of clopidogrel. This study evaluated the in vitro oxidation of clopidogrel by 21 CYP2C19 variants harboring amino acid substitutions. These CYP2C19 variants were heterologously expressed in COS-7 cells, and the kinetic parameters of clopidogrel 2-oxidation were estimated. Among the 21 CYP2C19 variants, 12 (that is, CYP2C19.5A, CYP2C19.5B, CYP2C19.6, CYP2C19.8, CYP2C19.9, CYP2C19.10, CYP2C19.14, CYP2C19.16, CYP2C19.19, CYP2C19.22, CYP2C19.24 and CYP2C19.25) showed no or markedly low activity compared with the wild-type protein CYP2C19.1B. This comprehensive in vitro assessment provided insights into the specific metabolic activities of CYP2C19 proteins encoded by variant alleles, and this may to be valuable when interpreting the results of in vivo studies.

Polymorphisms in the organic anion transporting polypeptide genes influence liver parenchymal enhancement in gadoxetic acid-enhanced MRI

AIM

This study aimed to investigate whether genetic polymorphisms of the organic anion transporting polypeptides influence hepatic enhancement in gadoxetic acid-enhanced MRI.

PATIENTS & METHODS

We analyzed the genotypes of SLCO1B1 388A>G, SLCO1B1 521T>C, SLCO1B3 334T>G and NR1H4 -1G>T and calculated the mean quantitative liver-spleen contrast ratio, as an index of liver parenchymal enhancement, in 226 patients with liver disease.

RESULTS

Multiple linear regression analysis using the mean quantitative liver-spleen contrast ratio as the dependent variable revealed that not only Child-Pugh score, but also SLCO1B1*1b haplotype (β = 0.12; p = 0.04), were significant predictors of liver parenchymal enhancement. In addition, SLCO1B3 334T>G (β = -0.18; p = 0.03) was a significant predictor when the data were analyzed in a subgroup of 117 patients, excluding the carriers of NR1H4 -1G>T, who reportedly exhibit reduced transcriptional activity of SLCO1B3.

CONCLUSION

These genetic variants, as well as hepatic function, may contribute to individual differences in hepatic enhancement with gadoxetic acid.

Clinical pharmacology of axitinib

Axitinib is a potent and selective second-generation inhibitor of vascular endothelial growth factor receptors 1, 2, and 3 that is approved in the US and several other countries for treatment of patients with advanced renal cell carcinoma after failure of one prior systemic therapy. The recommended clinical starting dose of axitinib is 5 mg twice daily, taken with or without food. Dose increase (up to a maximum of 10 mg twice daily) or reduction is permitted based on individual tolerability. Axitinib pharmacokinetics are dose-proportional within 1-20 mg twice daily, which includes the clinical dose range. Axitinib has a short effective plasma half-life (range 2.5-6.1 h), and the plasma accumulation of axitinib is in agreement with what is expected based on the plasma half-life of the drug. Axitinib is absorbed relatively rapidly, reaching maximum observed plasma concentrations (C max) within 4 h of oral administration. The mean absolute bioavailability of axitinib is 58 %. Axitinib is highly (>99 %) bound to human plasma proteins with preferential binding to albumin and moderate binding to α1-acid glycoprotein. In patients with advanced renal cell carcinoma, at the 5-mg twice-daily dose in the fed state, the geometric mean (% coefficient of variation) C max and area under the plasma concentration-time curve (AUC) from time 0-24 h (AUC24) were 27.8 ng/mL (79 %) and 265 ng·h/mL (77 %), respectively. Axitinib is metabolized primarily in the liver by cytochrome P450 (CYP) 3A4/5 and, to a lesser extent (<10 % each), by CYP1A2, CYP2C19, and uridine diphosphate glucuronosyltransferase (UGT) 1A1. The two major human plasma metabolites, M12 (sulfoxide product) and M7 (glucuronide product), are considered pharmacologically inactive. Axitinib is eliminated via hepatobiliary excretion with negligible urinary excretion. Although mild hepatic impairment does not affect axitinib plasma exposures compared with subjects with normal hepatic function, there was a 2-fold increase in AUC from time zero to infinity (AUC∞) following a single 5-mg dose in subjects with moderate hepatic impairment. In the presence of ketoconazole, a strong CYP3A4/5 inhibitor, axitinib C max and AUC∞ increased by 1.5- and 2-fold, respectively, whereas co-administration of rifampin, a strong CYP3A4/5 inducer, resulted in a 71 and 79 % decrease in the C max and AUC∞, respectively. Axitinib does not inhibit CYP3A4/5, CYP1A2, CYP2C8, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, or UGT1A1 at concentrations obtained with the clinical doses and is not expected to have major interactions with drugs that are metabolized by these enzymes. Axitinib is an inhibitor of the efflux transporter P-glycoprotein (P-gp) in vitro, but is not expected to inhibit P-gp at therapeutic plasma concentrations. A two-compartment population pharmacokinetic model with first-order absorption and lag time was used to describe axitinib pharmacokinetics. No clinically relevant effects of age, sex, body weight, race, renal function, UGT1A1 genotype, or CYP2C19 inferred phenotype on the clearance of axitinib were identified.

Multiple cytochrome P450 isoforms are involved in the generation of a pharmacologically active thiol metabolite, whereas paraoxonase 1 and carboxylesterase 1 catalyze the formation of a thiol metabolite isomer from ticlopidine

Ticlopidine is a first-generation thienopyridine antiplatelet drug that prevents adenosine 5'-diphosphate (ADP)-induced platelet aggregation. We identified the enzymes responsible for the two-step metabolic bioactivation of ticlopidine in human liver microsomes and plasma. Formation of 2-oxo-ticlopidine, an intermediate metabolite, was NADPH dependent and cytochrome P450 (CYP) 1A2, 2B6, 2C19, and 2D6 were involved in this reaction. Conversion of 2-oxo-ticlopidine to thiol metabolites was observed in both microsomes (M1 and M2) and plasma (M1). These two metabolites were considered as isomers, and mass spectral analysis suggested that M2 was a thiol metabolite bearing an exocyclic double bond, whereas M1 was an isomer in which the double bond was migrated to an endocyclic position in the piperidine ring. The conversion of 2-oxo-ticlopidine to M1 in plasma was significantly increased by the addition of 1 mM CaCl2. In contrast, the activity in microsomes was not changed in the presence of CaCl2. M1 formation in plasma was inhibited by EDTA but not by other esterase inhibitors, whereas this activity in microsomes was substantially inhibited by carboxylesterase (CES) inhibitors such as bis-(p-nitrophenyl)phosphate (BNPP), diisopropylphosphorofluoride (DFP), and clopidogrel. The conversion of 2-oxo-ticlopidine to M1 was further confirmed with recombinant paraoxonase 1 (PON1) and CES1. However, M2 was detected only in NADPH-dependent microsomal incubation, and multiple CYP isoforms were involved in M2 formation with highest contribution of CYP2B6. In vitro platelet aggregation assay demonstrated that M2 was pharmacologically active. These results collectively indicated that the formation of M2 was mediated by CYP isoforms whereas M1, an isomer of M2, was generated either by human PON1 in plasma or by CES1 in the human liver.

Clopidogrel and warfarin pharmacogenetic tests: what is the evidence for use in clinical practice?

PURPOSE OF REVIEW

To review the most promising genetic markers associated with the variability in the safety or efficacy of warfarin and clopidogrel and highlight the verification and validation initiatives for translating clopidogrel and warfarin pharmacogenetic tests to clinical practice.

RECENT FINDINGS

Rapid advances in pharmacogenetics, continuous decrease in genotyping cost, development of point-of-care devices and the newly established clinical genotyping programs at several institutions hold the promise of individualizing clopidogrel and warfarin based on genotype. Guidelines have been established to assist clinicians in prescribing clopidogrel or warfarin dose based on genotype. However, the clinical utility of clopidogrel and warfarin is still limited. Accordingly, large randomized clinical trials are underway to define the role of clopidogrel and warfarin pharmacogenetics in clinical practice.

SUMMARY

Pharmacogenetics has offered compelling evidence toward the individualization of clopidogrel and warfarin therapies. The rapid advances in technology make the clinical implementation of clopidogrel and warfarin pharmacogenetics possible. The clinical genotyping programs and the ongoing clinical trials will help in overcoming some of the barriers facing the clinical implementation of clopidogrel and warfarin pharmacogenetics.

Role of CYP450 in Antiplatelet Therapy: Considerations for Patients at Risk for Further Cardiovascular or Cerebrovascular Problems

Objective:

To determine how interactions with cytochrome P450 ( CYP450) isoenzyme pathways may influence the selection of antiplatelet therapy in patients with cardiovascular and cerebrovascular disease.

Data Sources:

A MEDLINE search (1966-September 2012) was performed using the following MeSH search terms: clopidogrel, prasugrel, ticagrelor, aspirin, dipyridamole, drug interactions, polymorphism, cytochrome P450, platelets, platelet reactivity testing, and platelet function. In addition, reference citations were reviewed.

Study Selection and Data Extraction:

Selected studies analyzed the effects of CYP450 isoenzymes on the efficacy of antiplatelet therapy and potential drug-drug interactions.

Data Synthesis:

Antiplatelet therapy is pivotal in reducing the risk of acute coronary syndromes. Thienopyridines are commonly prescribed for antiplatelet drug therapy; however, a multitude of factors can influence their efficacy, given that thienopyridines require activation through CYP450 isoenzymes. Genetic polymorphisms in CYP450 isoenzymes and the susceptibility of drug-drug interactions mediated through the CYP450 system have led to variability in antiplatelet response. Two factors in particular have been prime targets in elucidating the role of CYP450 isoenzymes on thienopyridine activity and bioavailability: CYP2C19*2 polymorphisms and drug interactions with proton pump inhibitors. Additionally, many studies have surfaced in an attempt to quantify a patient's response to antiplatelet therapy but, to date, none has determined whether platelet reactivity testing is mandatory prior to initiation of therapy or whether it correlates with clinical efficacy. This review spotlights the pharmacokinetic and pharmacodynamic influences that CYP450 isoenzymes exert on antiplatelet therapies and attempts to categorize the role of platelet reactivity testing in determining the clinical significance of drug interactions.

Conclusions:

Suboptimal responses to antiplatelet therapy may occur because of interactions with CYP450 isoenzyme pathways. Further studies are warranted to clarify the impact of genetic polymorphisms and drug-drug interactions on clinical outcomes in patients with cardiovascular/cerebrovascular disease.

P2Y12 Receptor Inhibitors in Acute Coronary Syndromes: What Is New on the Horizon?

Dual antiplatelet therapy with aspirin and a P2Y₁₂ receptor inhibitor represents the cornerstone therapy for patients with acute coronary syndromes or undergoing percutaneous interventions, leading to a reduction of subsequent ischemic events. Variable response to clopidogrel has received close attention, and pharmacokinetic, pharmacodynamic, and pharmacogenomic factors have been identified as culprits. This led to the introduction of newer, potentially safer, and more effective antiplatelet agents (prasugrel and ticagrelor). Additionally, several point-of-care assays of platelet function have been developed in recent years to rapidly screen individuals on antiplatelet therapy. While the routine use of platelet function testing is uncertain and not currently recommended, it may be useful in instances when the degree of platelet inhibition may be uncertain such as high-risk patients undergoing percutaneous coronary intervention or when there may be a suspected pharmacodynamic interaction with other drugs. The current paper focuses on the P2Y₁₂ receptor inhibitors and their pharmacogenetics and indications in patients with acute coronary syndromes or receiving percutaneous coronary interventions as well as the applicability of platelet function testing in this clinical context.

Genetic polymorphisms and drug interactions leading to clopidogrel resistance: why the Asian population requires special attention

Ischemic heart disease and stroke are the two leading causes of death worldwide. Antiplatelet therapy plays the most significant role in the management of these cardiovascular and cerebrovascular occlusive events to prevent recurrent ischemic attack. Clopidogrel, an antiplatelet drug, is widely prescribed either alone or in combination with aspirin as dual antiplatelet therapy for the prevention of vascular occlusive events. The antiplatelet response to clopidogrel varies widely. Hyporesponders and nonresponders are likely to have adverse cardiovascular events during follow-up. Some drugs, such as proton pump inhibitors (omeprazole), calcium channel blockers, selective serotonin reuptake inhibitors (nefazadone), coumarin derivatives (phenprocoumon), benzodiazepines, sulfonylurea, erythromycin, and itraconazole, decrease the antiplatelet effect of clopidogrel when administered concomitantly. Decreased response to clopidogrel is common among Asians due to genetic polymorphisms associated with clopidogrel resistance, and it is nearly 70% in some of the Asian communities. It is necessary to study Asian populations, because there are a large number of Asians throughout the world due to increased migration. Current guidelines do not make genetic testing or platelet response testing mandatory prior to clopidogrel prescription. Therefore, it is important for clinicians treating Asian patients to keep in mind the interindividual variability in response to clopidogrel when prescribing the drug.

Clinical Pharmacogenomics of Warfarin and Clopidogrel

Genetic polymorphisms significantly influence responses to warfarin and clopidogrel. Polymorphisms in the cytochrome P450 (CYP) 2C9 and vitamin K epoxide reductase genes change warfarin pharmacokinetics and pharmacodynamics, respectively. Because these polymorphisms influence warfarin dose requirements, they may primarily help determine therapeutic warfarin doses in patients who newly start on the drug. To assist in estimating therapeutic warfarin doses, the warfarin label provides a pharmacogenomic dosing table and various warfarin pharmacogenomic dosing algorithms are available. On the other hand, polymorphisms in the CYP2C19 gene affect clopidogrel pharmacokinetics. These polymorphisms may be useful to identify clopidogrel nonresponders who may benefit from taking an alternative antiplatelet agent such as prasugrel and ticagrelor. Although both drugs have pharmacogenomic tests available for clinical use, their clinical utilities have not been established and are currently being actively studied. In this review, clinical application of warfarin and clopidogrel pharmacogenomics will be focused. With the current level of evidence, potential patients who may get benefit from warfarin and clopidogrel pharmacogenomic testing will be discussed. In addition, the interpretation of the warfarin and clopidogrel test results and the current barriers to widespread use of warfarin and clopidogrel pharmacogenomic testing will be discussed.

Genetic determinants of platelet response to clopidogrel

Antiplatelet agents are the mainstay treatment in the prevention and management of atherothrombotic complications. However, a substantial interpatient variability in response to clopidogrel has been reported. Furthermore, patients with coronary artery disease and lesser platelet inhibition in response to clopidogrel are at increased risk for cardiovascular events. Clopidogrel after absorption requires two-step oxidation by the hepatic cytochrome P450 to generate its active metabolite. Polymorphisms of genes encoding the cytochrome enzymes and P-glycoprotein involved in clopidogrel absorption are regarded as major determinants of the interindividual variability in the clopidogrel-induced platelet inhibition. In our review we discuss the prevalence and clinical significance of various alleles of the genes: CYP2C19 and ABCB1 in the setting of coronary artery disease. Allele CYP2C19*2 is associated with excess of ischaemic events including myocardial infarction and stent thrombosis. On the other hand, CYP2C19*17 allele poses a serious threat of bleeding. Data concerning the prognostic value of genetic variant 3435C→T of ABCB1 remain inconclusive.

Antiplatelet resistance and thromboembolic complications in neurointerventional procedures

Antiplatelet resistance is emerging as a significant factor in effective secondary stroke prevention. Prevalence of aspirin and clopidogrel resistance is dependent upon laboratory test and remains contentious. Large studies in cardiovascular disease populations have demonstrated worse ischemic outcomes in patients with antiplatelet resistance, particularly in patients with coronary stents. Thromboembolism is a complication of neurointerventional procedures that leads to stroke. Stroke rates related to aneurysm coiling range from 2 to 10% and may be higher when considering silent ischemia. Stroke associated with carotid stenting is a major cause of morbidity. Antiplatelet use in the periprocedure setting varies among different centers. No guidelines exist for use of antiplatelet regimens in neurointerventional procedures. Incidence of stroke in patients post procedure may be partly explained by resistance to antiplatelet agents. Further research is required to establish the incidence of stroke in patients with antiplatelet resistance undergoing neurointerventional procedures.

Genetic polymorphisms and the impact of a higher clopidogrel dose regimen on active metabolite exposure and antiplatelet response in healthy subjects

A double-blind crossover study was conducted in four CYP2C19 genotype-defined metabolizer groups to assess whether increase in clopidogrel dosing can overcome reduced pharmacodynamic response in CYP2C19 poor metabolizers (PMs). Ten healthy subjects in each of four metabolizer groups were randomized to a clopidogrel regimen of a 300-mg loading dose (LD) and a 75-mg/day maintenance dose (MD) for 4 days followed by 600-mg LD and 150 mg/day MD, or vice versa. The exposure levels of clopidogrel's active metabolite H4 (clopi-H4) in PMs were 71% lower on the 75-mg/day regimen and 64% lower on the 150-mg/day regimen than the corresponding exposure levels in extensive metabolizers (EMs). In PMs, the maximal platelet aggregation (MPA) induced by adenosine diphosphate (ADP) 5 µmol/l was 10.5% lower on the 75-mg/day regimen and 7.9% lower on the 150-mg/day regimen than the corresponding values in EMs. PMs who were on the clopidogrel regimen of 600-mg LD/150 mg/day MD showed clopi-H4 exposure and MPA levels similar to those in EMs who were on the regimen of 300-mg LD/75 mg/day MD. In a pooled analysis evaluating CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP3A5, CYP2D6, ABCB1, and P2RY12 polymorphisms (N = 396 healthy subjects), only CYP2C19 had a significant impact on antiplatelet response. In healthy CYP2C19 PMs, a clopidogrel regimen of 600-mg LD/150 mg/day MD largely overcomes diminished clopi-H4 exposure and antiplatelet response, as assessed by MPA levels.