Cytochromes P450 catalyze both steps of the major pathway of clopidogrel bioactivation, whereas paraoxonase catalyzes the formation of a minor thiol metabolite isomer

Paraoxonases-1, -2 and -3: What are their functions?

Paraoxonase-1 (PON1), an esterase/lactonase primarily associated with plasma high-density lipoprotein (HDL), was the first member of this family of enzymes to be characterized. Its name was derived from its ability to hydrolyze paraoxon, the toxic metabolite of the insecticide parathion. Related enzymes PON2 and PON3 were named from their evolutionary relationship with PON1. Mice with each PON gene knocked out were generated at UCLA and have been key for elucidating their roles in organophosphorus (OP) metabolism, cardiovascular disease, innate immunity, obesity, and cancer. PON1 status, determined with two-substrate analyses, reveals an individual's functional Q192R genotype and activity levels. The three-dimensional structure for a chimeric PON1 has been useful for understanding the structural properties of PON1 and for engineering PON1 as a catalytic scavenger of OP compounds. All three PONs hydrolyze microbial N-acyl homoserine lactone quorum sensing factors, quenching Pseudomonas aeruginosa's pathogenesis. All three PONs modulate oxidative stress and inflammation. PON2 is localized in the mitochondria and endoplasmic reticulum. PON2 has potent antioxidant properties and is found at 3- to 4-fold higher levels in females than males, providing increased protection against oxidative stress, as observed in primary cultures of neurons and astrocytes from female mice compared with male mice. The higher levels of PON2 in females may explain the lower frequency of neurological and cardiovascular diseases in females and the ability to identify males but not females with Parkinson's disease using a special PON1 status assay. Less is known about PON3; however, recent experiments with PON3 knockout mice show them to be susceptible to obesity, gallstone formation and atherosclerosis. Like PONs 1 and 2, PON3 also appears to modulate oxidative stress. It is localized in the endoplasmic reticulum, mitochondria and on HDL. Both PON2 and PON3 are upregulated in cancer, favoring tumor progression through mitochondrial protection against oxidative stress and apoptosis.

Development of a sensitive and fast UHPLC-MS/MS method for determination of clopidogrel, clopidogrel acid and clopidogrel active metabolite H4 in human plasma

BACKGROUND

Quantitation of CAM H4 in biological matrix has been a challenge due to low concentrations, instability and the existence of four CAM diastereomers. Historically, either inactive clopidogrel acid or CAM diastereomers without separation were measured for exposure and PK parameters.

RESULTS

This study presents a sensitive and fast UHPLC-MS/MS method for simultaneous quantitation of clopidogrel, clopidogrel acid and active metabolite H4 in human plasma. The method demonstrated the separation of H4 from other isomers and yet retained clopidogrel acid. Matrix stabilities, accuracy and precision, recovery and matrix effect were evaluated during development.

CONCLUSION

With LLOQ of 0.05 ng/ml for clopidogrel and H4, and 5.5 min LC gradient, it is the most sensitive and fastest method to our knowledge.

Toxicity Originating from Thiophene Containing Drugs: Exploring the Mechanism using Quantum Chemical Methods

Drug metabolism of thiophene containing substrates by cytochrome P450s (CYP450) leads to toxic side effects, for example, nephrotoxicity (suprofen, ticlopidine), hepatotoxicity (tienilic acid), thrombotic thrombocytopenic purpura (clopidogrel), and aplastic anemia (ticlopidine). The origin of toxicity in these cases has been attributed to two different CYP450 mediated metabolic reactions: S-oxidation and epoxidation. In this work, the molecular level details of the bioinorganic chemistry associated with the generation of these competitive reactions are reported. Density functional theory was utilized (i) to explore the molecular mechanism for S-oxidation and epoxidation using the radical cationic center Cpd I [(iron(IV)-oxo-heme porphine system with SH(-) as the axial ligand, to mimic CYP450s] as the model oxidant, (ii) to establish the 3D structures of the reactants, transition states, and products on both the metabolic pathways, and (iii) to examine the potential energy (PE) profile for both the pathways to determine the energetically preferred toxic metabolite formation. The energy barrier required for S-oxidation was observed to be 14.75 kcal/mol as compared to that of the epoxidation reaction (13.23 kcal/mol) on the doublet PE surface of Cpd I. The formation of the epoxide metabolite was found to be highly exothermic (-23.24 kcal/mol), as compared to S-oxidation (-8.08 kcal/mol). Hence, on a relative scale the epoxidation process was observed to be thermodynamically and kinetically more favorable. The energy profiles associated with the reactions of the S-oxide and epoxide toxic metabolites were also explored. This study helps in understanding the CYP450-catalyzed toxic reactions of drugs containing the thiophene ring at the atomic level.

Human Liver Cytochrome P450 Enzymes and Microsomal Thiol Methyltransferase Are Involved in the Stereoselective Formation and Methylation of the Pharmacologically Active Metabolite of Clopidogrel

Clopidogrel, a thienopyridine antiplatelet prodrug, is metabolized by oxidation to 2-oxo-clopidogrel, followed by conversion to its pharmacologically active thiol metabolite. After oral administration of clopidogrel to humans, two thiol isomers (H3 and H4) are observed in plasma, with similar concentrations, and only H4 is active in humans. In this work, the mechanism of stereoselectivity in the formation and S-methylation of H3 and H4 was investigated in vitro. The two diastereomers of 2-oxo-clopidogrel were epimerized rapidly at physiologic pH. The intrinsic clearance (CLint) for H3 formation from 2-oxo-clopidogrel in human liver microsomes (HLMs) was 3.1-fold higher than that for H4 formation, indicating stereoselective metabolism. Kinetic studies using expressed enzymes demonstrated that the contributions of CYP2B6, CYP2C19, and CYP3A4 to the formation of H4 from 2-oxo-clopidogrel were 18.5%, 26.1%, and 53.5%, respectively. The CLint ratios of H3 formation to H4 formation from 2-oxo-clopidogrel by CYP2B6, CYP2C19, and CYP3A4 were 2.2, 1.0, and 1.7, respectively. In HLMs, H3 and H4 were further S-methylated, and the S-methylation was inhibited by 2,3-dichloromethyl benzylamine, indicating the involvement of thiol S-methyltransferase. The CLint value for the S-methylation of H3 in HLMs was 98.1-fold higher than that for H4. The stereoselective formation of H3 from 2-oxo-clopidogrel and the stereoselective S-methylation of H3 may lead to the similar exposure levels of H3 and H4 previously reported in humans. The epimerization of 2-oxo-clopidogrel and the variations of thiol S-methyltransferase may affect the exposure to H4 in humans.

Synthesis of Biologically Active Piperidine Metabolites of Clopidogrel: Determination of Structure and Analyte Development

Clopidogrel is a prodrug anticoagulant with active metabolites that irreversibly inhibit the platelet surface GPCR P2Y12 and thus inhibit platelet activation. However, gaining an understanding of patient response has been limited due to imprecise understanding of metabolite activity and stereochemistry, and a lack of acceptable analytes for quantifying in vivo metabolite formation. Methods for the production of all bioactive metabolites of clopidogrel, their stereochemical assignment, and the development of stable analytes via three conceptually orthogonal routes are disclosed.

Bioactivation of clopidogrel and prasugrel: factors determining the stereochemistry of the thiol metabolite double bond

The antithrombotics of the tetrahydrothienopyridine series, clopidogrel and prasugrel, are prodrugs that must be metabolized in two steps to become pharmacologically active. The first step is the formation of a thiolactone metabolite. The second step is a further oxidation with the formation of a thiolactone sulfoxide whose hydrolytic opening leads to a sulfenic acid that is eventually reduced into the corresponding active cis thiol. Very few data were available on the formation of the isomer of the active cis thiol having a trans configuration of the double bond, the most striking result in that regard being that both cis and trans thiols were formed upon the metabolism of clopidogrel by human liver microsomes in the presence of glutathione (GSH), whereas only the cis thiol was detected in the sera of patients treated with this drug. This article shows that trans thiols are also formed upon the microsomal metabolism of prasugrel or its thiolactone metabolite in the presence of GSH and that metabolites having the trans configuration of the double bond are only formed when microsomal incubations are done in the presence of thiols, such as GSH, N-acetyl-cysteine, and mercaptoethanol. Intermediate formation of thioesters resulting from the reaction of GSH with the thiolactone sulfoxide metabolite appears to be responsible for trans thiol formation. Addition of human liver cytosol to the microsomal incubations led to a dramatic decrease of the formation of the trans thiol metabolites. These data suggest that cytosolic esterases would accelerate the hydrolytic opening of thiolactone sulfoxide intermediates and disfavor the formation of thioesters resulting from the reaction of these intermediates with GSH that is responsible for trans isomer formation. This would explain why trans thiols have not been detected in the sera of patients treated with clopidogrel.

Human paraoxonase-1 (PON1): Gene structure and expression, promiscuous activities and multiple physiological roles

Human PON1 is a HDL-associated lipolactonase capable of preventing LDL and cell membrane oxidation and is therefore considered to be atheroprotective. PON1 contributes to the antioxidative function of HDL and reductions in HDL-PON1 activity, prevalent in a wide variety of diseases with an inflammatory component, are believed to lead to dysfunctional HDL which can promote inflammation and atherosclerosis. However, PON1 is multifunctional and may contribute to other HDL functions such as in innate immunity, preventing infection by quorum sensing gram negative bacteria by destroying acyl lactone mediators of quorum sensing, and putative new roles in cancer development and the promotion of healthy ageing. In this review we explore the physiological roles of PON1 in disease development, as well as PON1 gene and protein structure, promiscuous activities and the roles of SNPs and ethnicity in determining PON1 activity.

The effects of CES1A2 A(-816)C and CYP2C19 loss-of-function polymorphisms on clopidogrel response variability among Chinese patients with coronary heart disease

OBJECTIVE

Carboxylesterase 1 hydrolyzes the majority of clopidogrel to the inactive metabolite. The aim of this study was to assess the effects of the CES1A2 A(-816)C polymorphism and other genetic and clinical factors on clopidogrel response variability. An additional aim was to investigate the relationship between genetic variations and development of stent thrombosis (ST).

METHODS

We recruited 162 coronary heart disease patients treated with aspirin and clopidogrel, and we genotyped them for the CES1A2 A(-816)C, CYP2C19 *2/*3, PON1 Q192R, and ABCB1 C3435T polymorphisms. Platelet reactivity was analyzed using the VASP-PRI assay. We also carried out a case-control study in which 22 patients undergoing stent implantation who had ST were matched with 86 ST-free controls.

RESULTS

The VASP-PRI values were significantly higher in the carriers of the CES1A2 -816C allele (P=0.014) and CYP2C19 loss of function (LOF) alleles (P=0.004). Furthermore, the patients with CYP2C19 LOF alleles showed an increased risk of ST (ORadj=4.28, P=0.033). However, there was no significant association between the CES1A2 -816C allele and the development of ST. The CYP2C19 and CES1A2 genotypes alone could explain 6.1 and 3.7% of the interindividual variability in the VASP-PRI results, respectively. The value increased to 12.5% when clinical factors (e.g. BMI and triglycerides) were also considered. The PON1 Q192R and ABCB1 C3435T genetic variations produced no significant impact.

CONCLUSION

The CES1A2 -816C and the CYP2C19 LOF alleles were associated with attenuated platelet reactivity to clopidogrel. CYP2C19 LOF was also predictive of ST; however, the association between the CES1A2 -816C allele and development of ST requires further study.

Bioactivation potential of thiophene-containing drugs

Thiophene is a five-membered, sulfur-containing heteroaromatic ring commonly used as a building block in drugs. It is considered to be a structural alert, as its metabolism can lead to the formation of reactive metabolites. Thiophene S-oxides and thiophene epoxides are highly reactive electrophilic thiophene metabolites whose formation is cytochrome P450-dependent. These reactive thiophene-based metabolites are quite often responsible for drug-induced hepatotoxicity. Tienilic acid is an example of a thiophene-based drug that was withdrawn from the market after only a few months of use, due to severe cases of immune hepatitis. However, inclusion of the thiophene moiety in drugs does not necessarily result in toxic effects. The presence of other, less toxic metabolic pathways, as well as an effective detoxification system in our body, protects us from the bioactivation potential of the thiophene ring. Thus, the presence of a structural alert itself is insufficient to predict a compound's toxicity. The question therefore arises as to which factors significantly influence the toxicity of thiophene-containing drugs. There is no easy way to answer this question. However, the findings presented here indicate that, for a number of reasons, daily dose and alternative metabolic pathways are important factors when predicting toxicity and will therefore be discussed together with examples.

Identification of alcohol-dependent clopidogrel metabolites using conventional liquid chromatography/triple quadrupole mass spectrometry

RATIONALE

Clopidogrel (CLO) is a prodrug used to prevent ischemic events in patients undergoing percutaneous coronary intervention or with myocardial infarction. A previous study found ethyl clopidogrel (ECLO) is formed by transesterification of CLO when incubated with alcohol in human liver microsomes. We hypothesize that ECLO will be subject to further metabolism and developed an assay to identify its metabolites.

METHODS

A liquid chromatography/triple quadrupole mass spectrometry (LC/MS/MS) method was developed to identify metabolites of ECLO. According to the predicted metabolic pathway of ECLO, precursor-product ion pairs were used to screen the possible metabolites of ECLO in human liver S9 fractions. Subsequently, the detected metabolites were characterized by the results of product ion scan.

RESULTS

In the presence of alcohol, CLO was tranesterified to ECLO, which was further oxidized to form ethylated 2-oxo-clopidogrel and several ethylated thiol metabolites including the ethylated form of the H4 active metabolite.

CONCLUSIONS

The ECLO formed by transesterification with alcohol is subject to metabolism by CYP450 enzymes producing ethylated forms of 2-oxo-clopidogrel and the active H4 thiol metabolite.

Effects of simvastatin/ezetimibe on microparticles, endothelial progenitor cells and platelet aggregation in subjects with coronary heart disease under antiplatelet therapy

It is not known whether the addition of ezetimibe to statins adds cardiovascular protection beyond the expected changes in lipid levels. Subjects with coronary heart disease were treated with four consecutive 1-week courses of therapy (T) and evaluations. The courses were: T1, 100 mg aspirin alone; T2, 100 mg aspirin and 40 mg simvastatin/10 mg ezetimibe; T3, 40 mg simvastatin/10 mg ezetimibe, and 75 mg clopidogrel (300 mg initial loading dose); T4, 75 mg clopidogrel alone. Platelet aggregation was examined in whole blood. Endothelial microparticles (CD51), platelet microparticles (CD42/CD31), and endothelial progenitor cells (CD34/CD133; CDKDR/CD133, or CD34/KDR) were quantified by flow cytometry. Endothelial function was examined by flow-mediated dilation. Comparisons between therapies revealed differences in lipids (T2 and T3<T1 and T4 for total cholesterol, LDL-C, and triglycerides; P<0.002 for all), as well as for endothelial function (T2>T1 and T4, P=0.001). Decreased platelet aggregation was observed after aspirin (arachidonic acid, T1<T3 and T4, P=0.034) and clopidogrel (adenosine, T3 and T4<T1 and T2, P<0.0001) therapy. Simvastatin/ezetimibe diphosphate did not change platelet aggregation, the amount of circulating endothelial and platelet microparticles, or endothelial progenitor cells. Cardiovascular protection following therapy with simvastatin/ezetimibe seems restricted to lipid changes and improvement of endothelial function not affecting the release of microparticles, mobilization of endothelial progenitor cells or decreased platelet aggregation.

Pharmacogenetics of paraoxonase activity: elucidating the role of high-density lipoprotein in disease

PON1 is a key component of high-density lipoproteins (HDLs) and is at least partially responsible for HDL's antioxidant/atheroprotective properties. PON1 is also associated with numerous human diseases, including cardiovascular disease, Parkinson's disease and cancer. In addition, PON1 metabolizes a broad variety of substrates, including toxic organophosphorous compounds, statin adducts, glucocorticoids, the likely atherogenic L-homocysteine thiolactone and the quorum-sensing factor of Pseudomonas aeruginosa. Numerous cardiovascular and antidiabetic pharmacologic agents, dietary macronutrients, lifestyle factors and antioxidant supplements affect PON1 expression and enzyme activity levels. Owing to the importance of PON1 to HDL function and its individual association with diverse human diseases, pharmacogenomic interactions between PON1 and the various factors that alter its expression and activity may represent an important therapeutic target for future investigation.

Hepatocellular toxicity of clopidogrel: mechanisms and risk factors

Clopidogrel is a prodrug used widely as a platelet aggregation inhibitor. After intestinal absorption, approximately 90% is converted to inactive clopidogrel carboxylate and 10% via a two-step procedure to the active metabolite containing a mercapto group. Hepatotoxicity is a rare but potentially serious adverse reaction associated with clopidogrel. The aim of this study was to find out the mechanisms and susceptibility factors for clopidogrel-associated hepatotoxicity. In primary human hepatocytes, clopidogrel (10 and 100 μM) was cytotoxic only after cytochrome P450 (CYP) induction by rifampicin. Clopidogrel (10 and 100 μM) was also toxic for HepG2 cells expressing human CYP3A4 (HepG2/CYP3A4) and HepG2 cells co-incubated with CYP3A4 supersomes (HepG2/CYP3A4 supersome), but not for wild-type HepG2 cells (HepG2/wt). Clopidogrel (100 μM) decreased the cellular glutathione content in HepG2/CYP3A4 supersome and triggered an oxidative stress reaction (10 and 100 µM) in HepG2/CYP3A4, but not in HepG2/wt. Glutathione depletion significantly increased the cytotoxicity of clopidogrel (10 and 100 µM) in HepG2/CYP3A4 supersome. Co-incubation with 1 μM ketoconazole or 10mM glutathione almost completely prevented the cytotoxic effect of clopidogrel in HepG2/CYP3A4 and HepG2/CYP3A4 supersome. HepG2/CYP3A4 incubated with 100 μM clopidogrel showed mitochondrial damage and cytochrome c release, eventually promoting apoptosis and/or necrosis. In contrast to clopidogrel, clopidogrel carboxylate was not toxic for HepG2/wt or HepG2/CYP3A4 up to 100 µM. In conclusion, clopidogrel incubated with CYP3A4 is associated with the formation of metabolites that are toxic for hepatocytes and can be trapped by glutathione. High CYP3A4 activity and low cellular glutathione stores may be risk factors for clopidogrel-associated hepatocellular toxicity.

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.

PON1 Q192R genotype influences clopidogrel responsiveness by relative platelet inhibition instead of on-treatment platelet reactivity

BACKGROUND

paraoxonase-1 (PON1) was recently identified as the crucial enzyme for clopidogrel bioactivation, with PON1 Q192R (rs662) polymorphism determining the clopidogel antiplatelet efficacy. However, subsequent studies showed controversies over the findings. This study aimed to evaluate the impact of PON1 Q192R in parallel to that of CYP2C19*2 (rs4244285) on clopidogrel responsiveness in a cohort of Chinese patients with unstable angina pectoris.

MATERIAL AND METHODS

One hundred and eighty Chinese-Han patients diagnosed with unstable angina pectoris and treated with clopidogrel were consecutively recruited. Clopidogrel responsiveness, measured by relative platelet inhibition {RI=[(pretreatment aggregation-posttreatment aggregation at 5days)/(pretreatment aggregation)] x100%}, was assessed in relation to PON1 Q192R and CYP2C19*2 genotypes. RI values were stratified into four quartiles, with patients in quartile 1 defined as individuals of clopidogrel non-responsiveness. The contributions of PON1 Q192R and CYP2C19*2 to on-treatment platelet reactivity (OTPR) at 5days maintenance dose of clopidogrel were also evaluated.

RESULTS

For PON1 Q192R genotypes, RI values were significantly lower in patients with QR and RR alleles than in patients with QQ alleles (p=0.01). OTPR values at 5days maintenance dose of clopidogrel were similar across all the PON1 Q192R genotypes (p=0.41). PON1 192 QR and RR conferred increased risks for clopidogrel non-responsiveness [OR 3.64; 95% CI (1.21-10.92), p=0.02]. For CYP2C19*2 genotypes, compared to CYP2C19*1/*1 wild type carriers, CYP2C19*2 carriers showed a significantly higher OTPR (p=0.009), and a trend for lower RI values (p=0.06). An increased risk for clopidogrel non-responsiveness was found in patients with CYP2C19*2 genotype [OR 2.02; 95% CI (1.03-3.96), p=0.04].

CONCLUSIONS

Both PON1 Q192R and CYP2C19*2 genotypes influence clopidogrel responsiveness, with the impact of PON1 Q192R mainly on relative platelet inhibition instead of OTPR of clopidogrel.

Advances in molecular modeling of human cytochrome P450 polymorphism

Cytochrome P450 (CYP) is a supergene family of metabolizing enzymes involved in the phase I metabolism of drugs and endogenous compounds. CYP oxidation often leads to inactive drug metabolites or to highly toxic or carcinogenic metabolites involved in adverse drug reactions (ADR). During the last decade, the impact of CYP polymorphism in various drug responses and ADR has been demonstrated. Of the drugs involved in ADR, 56% are metabolized by polymorphic phase I metabolizing enzymes, 86% among them being CYP. Here, we review the major CYP polymorphic forms, their impact for drug response and current advances in molecular modeling of CYP polymorphism. We focus on recent studies exploring CYP polymorphism performed by the use of sequence-based and/or protein-structure-based computational approaches. The importance of understanding the molecular mechanisms related to CYP polymorphism and drug response at the atomic level is outlined.

Genetic variation of platelet function and pharmacology: an update of current knowledge

Platelets are critically involved in atherosclerosis and acute thrombosis. The platelet phenotype shows a wide variability documented by the inherited difference of platelet reactivity, platelet volume and count and function of platelet surface receptors. Several candidate genes have been put into focus and investigated for their functional and prognostic role in healthy individuals and patients with cardiovascular (CV) disease treated with antiplatelet agents. In addition to genetic variation, other clinical, disease-related and demographic factors are important so-called non-genetic factors. Due to the small effect sizes of single nucleotide polymorphisms (SNP) in candidate genes and due to the low allele frequencies of functional relevant candidate SNPs, the identification of genetic risk factors with high predictive values generally depends on the sample size of study cohorts. In the post-genome era new array and bioinformatic technologies facilitate high throughput genome-wide association studies (GWAS) for the identification of novel candidate genes in large cardiovascular cohorts. One of the crucial aspects of platelet genomic studies is the precise definition of a specific clinical phenotype (e.g. stent thrombosis) as this will impact importantly the findings of genomic studies like GWAS. Here, we provide an update on genetic variation of platelet receptors and drug metabolising enzymes under specific consideration of data derived by GWAS. The potential impact of this information and the role in personalised therapeutic concepts will be discussed.

[Metabolism of xenobiotics: beneficial and adverse effects]

The systems developed by living organisms for the metabolism of xenobiotics play a key role in the adaptation of living species to their chemical environment. Recent data about mammalian cytochrome P450 structures have led to a better understanding of the molecular basis for the adaptability of these enzymes to xenobiotics exhibiting highly variable structures. The action of these enzymes on xenobiotics leads to other beneficial effects such as the bioactivation of some drugs, but also to adverse effects with the formation of aggressive metabolites for the cell that are responsible for the appearance of many toxicities.

The functional G143E variant of carboxylesterase 1 is associated with increased clopidogrel active metabolite levels and greater clopidogrel response

INTRODUCTION

Carboxylesterase 1 (CES1) is the primary enzyme responsible for converting clopidogrel into biologically inactive carboxylic acid metabolites.

METHODS

We genotyped a functional variant in CES1, G143E, in participants of the Pharmacogenomics of Anti-Platelet Intervention (PAPI) study (n=566) and in 350 patients with coronary heart disease treated with clopidogrel, and carried out an association analysis of bioactive metabolite levels, on-clopidogrel ADP-stimulated platelet aggregation, and cardiovascular outcomes.

RESULTS

The levels of clopidogrel active metabolite were significantly greater in CES1 143E-allele carriers (P=0.001). Consistent with these findings, individuals who carried the CES1 143E-allele showed a better clopidogrel response as measured by ADP-stimulated platelet aggregation in both participants of the PAPI study (P=0.003) and clopidogrel-treated coronary heart disease patients (P=0.03). No association was found between this single nucleotide polymorphism and baseline measures of platelet aggregation in either cohort.

CONCLUSION

Taken together, these findings suggest, for the first time, that genetic variation in CES1 may be an important determinant of the efficacy of clopidogrel.

Paraoxonases: ancient substrate hunters and their evolving role in ischemic heart disease

Interest in the role of paraoxonases (PON) in cardiovascular research has increased substantially over the past two decades. These multifaceted and pleiotropic enzymes are encoded by three highly conserved genes (PON1, PON2, and PON3) located on chromosome 7q21.3-22.1. Phylogenetic analysis suggests that PON2 is the ancient gene from which PON1 and PON3 arose via gene duplication. Although PON are primarily lactonases with overlapping, but distinct specificities, their physiologic substrates remain poorly characterized. The most interesting characteristic of PON, however, is their multifunctional roles in various biochemical pathways. These include protection against oxidative damage and lipid peroxidation, contribution to innate immunity, detoxification of reactive molecules, bioactivation of drugs, modulation of endoplasmic reticulum stress, and regulation of cell proliferation/apoptosis. In general, PON appear as "hunters" of old and new substrates often involved in athero- and thrombogenesis. Although reduced PON activity appears associated with increased cardiovascular risk, the correlation between PON genotype and ischemic heart disease remains controversial. In this review, we examine the biochemical pathways impacted by these unique enzymes and investigate the potential use of PON as diagnostic tools and their impact on development of future therapeutic strategies.

Thiolactone sulfoxides as new reactive metabolites acting as bis-electrophiles: implication in clopidogrel and prasugrel bioactivation

The antithrombotics of the tetrahydrothienopyridine series, clopidogrel and prasugrel, are prodrugs that must be metabolized in two steps to become pharmacologically active. The first step is the formation of a thiolactone metabolite. The second step is a cytochrome P450 (P450)-dependent oxidation of this thiolactone resulting in the formation of a sulfenic acid that is eventually reduced into the corresponding active thiol. It has been postulated that the sulfenic acid metabolite resulted from a nucleophilic attack of water on a highly reactive thiolactone sulfoxide derived from P450-dependent oxidation of the thiolactone primary metabolite. The data described in the present article are in complete agreement with this proposition as they show that it was possible to trap these thiolactone sulfoxides by a series of nucleophiles such as amines, thiols, or cyclopentane-1,3-dione (CPDH), an equivalent of dimedone that is used as a sulfenic acid trapping agent. HPLC-MS studies showed that various bis-adducts having incorporated two nucleophile molecules were formed in these reactions. One of them that resulted from the oxidation of 2-oxo-prasugrel by human liver microsomes in the presence of ethanolamine and CPDH was isolated and completely characterized by (1)H and (13)C NMR spectroscopy in addition to MS and MS(2) spectrometry. All metabolites derived from an attack of H2O or an amine at the CO carbon of the intermediate thiolactone sulfoxide existed as a mixture of two diastereomers having a cis configuration of the double bond, whereas those formed in the presence of thiols appeared as a mixture of four diastereomers with a cis or trans configuration of the double bond. This led us to propose tentative mechanisms for the previously reported formation of trans isomers of the active thiol metabolite of clopidogrel upon microsomal metabolism of this antithrombotic in the presence of thiols. The results described in this article showed that thiolactone sulfoxides are formed as reactive metabolites during the metabolism of clopidogrel and prasugrel and are able to react as bis-electrophiles with a variety of nucleophiles. The possible implications of the formation of these reactive metabolites in the pharmacological and/or secondary toxic effects of these drugs remain to be studied.

Deuterated clopidogrel analogues as a new generation of antiplatelet agents

Clopidogrel (CPG) is an antithrombotic prodrug that needs hepatic cytochrome P450 (CYP) enzymes for its bioactivation. The clinical effects of CPG have been associated with high intersubject variability and a certain level of resistance. Recently, comprehensive biotransformation studies of CPG support that the observed clinical uncertainty stems from the low bioactivation efficiency, which is attributed to extensive attritional metabolism (e.g., hydrolysis of the methyl ester functionality and oxidation of the piperidine moiety). With the goal of potentiating the desired thiophene 2-oxidation through minimal structural modification, we have adopted the strategy of targeted metabolism shift and have designed and synthesized deuterated piperidine analogues of CPG. In vitro studies showed that the prodrug activation percentages have been significantly increased for the deuterated analogues as a result of stability enhancement of the piperidine moiety. In a pharmacological study with a rat model, oral administration of the deuterated analogues also demonstrated higher inhibitory activity than that of CPG against adenosine diphosphate (ADP) induced platelet aggregation. These deuterated analogues represent a new generation of antiplatelet agents with the potential to overcome the major clinical drawbacks of CPG.

Cytochrome P450-activated prodrugs

A prodrug is a compound that has negligible, or lower, activity against a specified pharmacological target than one of its major metabolites. Prodrugs can be used to improve drug delivery or pharmacokinetics, to decrease toxicity, or to target the drug to specific cells or tissues. Ester and phosphate hydrolysis are widely used in prodrug design because of their simplicity, but such approaches are relatively ineffective for targeting drugs to specific sites. The activation of prodrugs by the cytochrome P450 system provides a highly versatile approach to prodrug design that is particularly adaptable for targeting drug activation to the liver, to tumors or to hypoxic tissues.

Formation, reactivity, and antiplatelet activity of mixed disulfide conjugates of clopidogrel

In this work, we investigated the formation, reactivity, and antiplatelet activity of various mixed disulfide conjugates of clopidogrel. Our results showed that the production of the active metabolite (AM) from 2-oxoclopidogrel by human liver microsomes (HLMs) is greatly affected by the thiol reductants used. Among the 10 thiol compounds tested, glutathione (GSH) is most efficient in producing the AM at a rate of 167 pmoles AM/min/mg HLM. Interestingly, no AM but only the mixed disulfide conjugates were formed in the presence of 6-chloropyridazine-3-thiol (CPT), 2,5-dimethylfuran-3-thiol, and 3-nitropyridine-2-thiol (NPT). The mass spectrometry (MS) and MS(2) spectra of the conjugates of these thiol compounds confirmed the presence of a mixed disulfide bond linkage between the AM and the thiol reductants. Kinetic studies revealed that the mixed disulfide conjugates were capable of exchanging thiols with GSH to release the AM with second order rate constants ranging from 1.2 to 28 M(-1)s(-1). The mixed disulfide conjugates of CPT and NPT showed potent inhibition of platelet aggregation after pretreatment with 1 mM GSH, confirming that the AM is responsible for the antiplatelet activity of clopidogrel. Collectively, our results provide strong support for a cytochrome P450 (P450)-mediated bioactivation mechanism involving the initial formation of a glutathionyl conjugate, followed by thiol-disulfide exchange with another GSH molecule to release the AM. Furthermore, the stable mixed disulfide conjugates identified in this study provide a platform to quantitatively generate the therapeutic AM without the need for P450-mediated bioactivation. This property can be further explored to overcome the interindividual variability in clopidogrel therapy.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Quantitative determination of clopidogrel and its metabolites in biological samples: a mini-review

Clopidogrel has been applied in antiplatelet therapy since 1998 and is the thienopyridine with the largest clinical experience. By 2011, clopidogrel (Plavix(®)) was the second top-selling drug in the world. Following complete patent expiry in 2012/2013 its use is expected to grow even further from generics entering the market. Prefaced by a brief description of clopidogrel metabolism, this review analyzes analytical methods addressing the quantification of clopidogrel and its metabolites in biological samples. Techniques that have been applied to analyze human plasma or serum are predominantly LC-MS and LC-MS/MS. The lowest level of clopidogrel quantification that has been achieved is 5pg/mL, the shortest runtime is 1.5min and almost 100% recovery has been reported using solid-phase extraction for sample preparation.

Structural characterization of human cytochrome P450 2C19: active site differences between P450s 2C8, 2C9, and 2C19

To identify the structural features underlying the distinct substrate and inhibitor profiles of P450 2C19 relative to the closely related human enzymes, P450s 2C8 and 2C9, the atomic structure (Protein Data Bank code 4GQS) of cytochrome P450 2C19 complexed with the inhibitor (2-methyl-1-benzofuran-3-yl)-(4-hydroxy-3,5-dimethylphenyl)methanone (Protein Data Bank chemical component 0XV) was determined to 2.87 Å resolution by x-ray crystallography. The conformation of the peptide backbone of P450 2C19 is most similar to that of P450 2C8, but the substrate-binding cavity of P450 2C8 is much larger than that of P450 2C19 due to differences in the amino acid residues that form the substrate-binding cavities of the two enzymes. In contrast, the substrate-binding cavity of P450 2C19 is much more similar in size to that of the structure of the P450 2C9 flurbiprofen complex than to that of a modified P450 2C9 or that of P450 2C8. The cavities of the P450 2C19 0XV complex and the P450 2C9 flurbiprofen complex differ, however, because the helix B-C loops of the two enzymes are dissimilar. These conformational differences reflect the effects of adjacent structural elements that interact with the B-C loops and that differ between the two enzymes. The availability of a structure for 2C19 will facilitate computational approaches for predictions of substrate and inhibitor binding to this enzyme.

Identification of the Significant Involvement and Mechanistic Role of CYP3A4/5 in Clopidogrel Bioactivation

The clinical response to the antiplatelet prodrug clopidogrel is associated with high intersubject variability and a certain level of therapeutic resistance. Previous studies have suggested that genetic polymorphism of CYP2C19 might be one determinant of clopidogrel efficacy and led to the CYP2C19 genotype-tailored antithrombotic therapy. However, evidence against the role of CYP2C19 from multiple studies implied the involvement of other factors. Here, we report that prodrug activation of the thiophene motif in clopidogrel is attenuated by heavy metabolic attrition of the piperidine motif. CYP3A4/5 was identified to be the enzyme metabolizing the piperidine motif. Inhibiting CYP3A4/5-mediated attrition was shown to potentiate active metabolite formation, which was found to be catalyzed by multiple CYP enzymes. Identifying the significant involvement of CYP3A4/5 and characterizing its mechanistic role in clopidogrel bioactivation might assist future pharmacogenomic studies in exploring the full mechanism underlying clopidogrel efficacy.

Pharmacokinetics and pharmacodynamics of ticagrelor in patients with stable coronary artery disease: results from the ONSET-OFFSET and RESPOND studies

BACKGROUND AND OBJECTIVES

Ticagrelor, the first reversibly binding oral P2Y(12) receptor antagonist, improves outcomes in patients with acute coronary syndromes (ACS) compared with clopidogrel. In the ONSET-OFFSET study (parallel group trial) and the RESPOND study (crossover trial), the pharmacodynamic effects of ticagrelor were compared with clopidogrel in patients with coronary artery disease (CAD). We now report the pharmacokinetic analyses of ticagrelor, and the exposure-inhibition of platelet aggregation (IPA) relationships from these studies.

PATIENTS AND METHODS

Patients were treated with ticagrelor (180 mg loading dose, 90 mg twice daily maintenance dose) or clopidogrel (600 mg loading dose, 75 mg once daily maintenance dose) in addition to aspirin (acetylsalicylic acid) [75-100 mg once daily]. Ticagrelor administration was for 6 weeks in ONSET-OFFSET. In RESPOND, ticagrelor was given for 14 days before or after 2 weeks of clopidogrel in patients classified as clopidogrel responders or non-responders. Pharmacokinetics and IPA were evaluated following the loading and last maintenance doses. Exposure-IPA relationships were evaluated using a sigmoid maximum effect (E(max)) model.

OUTCOME MEASURES

The outcome measures were ticagrelor and AR-C124910XX (active metabolite) pharmacokinetics and exposure-IPA relationships in both trials, including the effect of prior clopidogrel exposure, and effects in clopidogrel responders and non-responders in RESPOND.

RESULTS

In ONSET-OFFSET, maximum (peak) plasma concentration (C(max)), time to C(max) (t(max)) and area under the plasma concentration-time curve from time 0 to 8 hours (AUC(8)) for ticagrelor were 733 ng/mL, 2.0 hours and 4130 ng · h/mL, respectively; and for AR-C124910XX were 210 ng/mL, 2.1 hours and 1325 ng · h/mL, respectively. E(max) estimates were IPA >97%. Trough plasma ticagrelor (305 ng/mL) and AR-C124910XX (121 ng/mL) concentrations were 5.2 and 7.7 times higher than respective concentration producing 50% of maximum effect (EC(50)) estimates. In RESPOND, ticagrelor mean C(max) and AUC(8) following 2-week dosing were comparable between clopidogrel responders (724 ng/mL and 3983 ng · h/mL, respectively) and non-responders (764 ng/mL and 3986 ng · h/mL, respectively). Pharmacokinetics of ticagrelor were unaffected by prior clopidogrel dosing. E(max) estimates were IPA >96% for both responders and non-responders. Trough plasma concentrations were sufficient to achieve high IPA.

CONCLUSIONS

Ticagrelor pharmacokinetics in stable CAD patients were comparable to previous findings in stable atherosclerotic and ACS patients, and were not affected by prior clopidogrel exposure or clopidogrel responsiveness. Ticagrelor effectively inhibited platelet aggregation, and trough plasma concentrations of ticagrelor and AR-C124910XX were sufficient to result in high IPA in stable CAD patients.

Influence of the paraoxonase-1 Q192R genetic variant on clopidogrel responsiveness and recurrent cardiovascular events: a systematic review and meta-analysis

BACKGROUND

A poor biological response to clopidogrel is associated with an increased risk of major cardiovascular ischemic events (MACE). Paraoxonase 1 (PON1) enzyme activity is modulated by the PON1-Q192R variant (rs662) and was recently suggested to be strongly involved in clopidogrel bioactivation, but the influence of the PON1-Q192R variant on the risk of MACE in clopidogrel-treated patients is controversial.

OBJECTIVES

To determine whether the PON1-Q192R variant influences clopidogrel biological responsiveness and the risk of MACE in patients treated with clopidogrel.

METHODS

Systematic review and meta-analysis of studies of the association between the PON1-Q192R polymorphism and the biological response to clopidogrel and/or the risk of MACE during clopidogrel administration.

RESULTS

Seventeen studies were included. In the 12 studies of the biological response to clopidogrel (n = 5302 patients), there was no significant difference between 192QQ and 192QR + 192RR subjects, whatever the laboratory method used (global mean standardized difference = 0.10 [-0.06; 0.25], P = 0.22). Eleven studies assessed the risk of MACE, four using a case-control design (n = 2739 patients) and seven a prospective design (n = 5353 patients). Overall, MACE occurred in 19% of patients in case-control studies and in 6% of patients in prospective cohort studies, with no significant difference between 192QQ and 192QR + 192RR patients (OR = 1.28 [0.97; 1.68], P = 0.08). Similar results were obtained when study design was taken into account. Heterogeneity was mainly driven by one publication.

CONCLUSIONS

This meta-analysis suggests that the PON1-Q192R polymorphism has no major impact on the risk of MACE and does not alter the biological response to clopidogrel in clopidogrel-treated patients.

Paraoxonase 1 and homocysteine metabolism

Paraoxonase 1 (PON1), a component of high-density lipoprotein (HDL), is a calcium-dependent multifunctional enzyme that connects metabolisms of lipoproteins and homocysteine (Hcy). Both PON1 and Hcy have been implicated in human diseases, including atherosclerosis and neurodegeneration. The involvement of Hcy in disease could be mediated through its interactions with PON1. Due to its ability to reduce oxidative stress, PON1 contributes to atheroprotective functions of HDL in mice and humans. Although PON1 has the ability to hydrolyze a variety of substrates, only one of them-Hcy-thiolactone-is known to occur naturally. In humans and mice, Hcy-thiolactonase activity of PON1 protects against N-homocysteinylation, which is detrimental to protein structure and function. PON1 also protects against neurotoxicity associated with hyperhomocysteinemia in mouse models. The links between PON1 and Hcy in relation to pathological states such as coronary artery disease, stroke, diabetic mellitus, kidney failure and Alzheimer's disease that emerge from recent studies are the topics of this review.

Formation of the thiol conjugates and active metabolite of clopidogrel by human liver microsomes

We reported previously the formation of a glutathionyl conjugate of the active metabolite (AM) of clopidogrel and the covalent modification of a cysteinyl residue of human cytochrome P450 2B6 in a reconstituted system (Mol Pharmacol 80:839-847, 2011). In this work, we extended our studies of the metabolism of clopidogrel to human liver microsomes in the presence of four reductants, namely, GSH, l-Cys, N-acetyl-l-cysteine (NAC), and ascorbic acid. Our results demonstrated that formation of the AM was greatly affected by the reductant used and the relative amounts of the AM formed were increased in the following order: NAC (17%) < l-Cys (53%) < ascorbic acid (61%) < GSH (100%). AM-thiol conjugates were observed in the presence of NAC, l-Cys, and GSH. In the case of GSH, the formation of both the AM and the glutathionyl conjugate was dependent on the GSH concentrations, with similar K(m) values of ~0.5 mM, which indicates that formation of the thiol conjugates constitutes an integral part of the bioactivation processes for clopidogrel. It was observed that the AM was slowly converted to the thiol conjugate, with a half-life of ~10 h. Addition of dithiothreitol to the reaction mixture reversed the conversion, which resulted in a decrease in AM-thiol conjugate levels and a concomitant increase in AM levels, whereas addition of NAC led to the formation of AM-NAC and a concomitant decrease in AM-GSH levels. These results not only confirm that the AM is formed through oxidative opening of the thiolactone ring but also suggest the existence of an equilibrium between the AM, the thiol conjugates, and the reductants. These factors may affect the effective concentrations of the AM in vivo.

Metabolic activation of prasugrel: nature of the two competitive pathways resulting in the opening of its thiophene ring

The mechanism generally admitted for the bioactivation of the antithrombotic prodrug, prasugrel, 1c, is its two-step enzymatic conversion into a biologically active thiol metabolite. The first step is an esterase-catalyzed hydrolysis of its acetate function leading to a thiolactone metabolite 2c. The second step was described as a cytochrome P450 (P450)-dependent oxidative opening of the thiolactone ring of 2c, with intermediate formation of a reactive sulfenic acid metabolite that is eventually reduced to the corresponding active thiol 3c. This article describes a detailed study of the metabolism of 1c by human liver microsomes and human sera, with an analysis by HPLC-MS under conditions allowing a complete separation of the thiol metabolite isomers, after derivatization with 3'-methoxy phenacyl bromide. It shows that there are two competing metabolic pathways for the opening of the 2c thiolactone ring. The major one, which was previously described, results from a P450- and NADPH-dependent redox bioactivation of 2c and leads to 3c, two previously reported thiol diastereomers bearing an exocyclic double bond. It occurs with NADPH-supplemented human liver microsomes but not with human sera. The second one results from a hydrolysis of 2c and leads to an isomer of 3c, 3c endo, in which the double bond has migrated from an exocyclic to an endocyclic position in the piperidine ring. It occurs both with human liver microsomes and human sera, and does not require NADPH. However, it requires Ca(2+) and is inhibited by paraoxon, which suggests that it is catalyzed by a thioesterase such as PON-1. Chemical experiments have confirmed that hydrolytic opening of thiolactone 2c exclusively leads to derivatives of the endo thiol isomer 3c endo.