Dirk Taubert

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 P-glycoprotein on clopidogrel absorption.

The antiplatelet activity of clopidogrel is characterized by considerable interindividual differences. Variable intestinal absorption is suggested to contribute to the inconsistencies in response. We tested the hypothesis that the intestinal efflux transporter P‐glycoprotein (P‐gp) limits the oral bioavailability of clopidogrel and that variance in the MDR1 gene encoding P‐gp predicts absorption variability.

Pharmacokinetics of clopidogrel after administration of a high loading dose

The adenosine diphosphate (ADP) receptor P2Y12 blocking agent clopidogrel is clinically proven to be efficient in preventing thrombotic events. However, its therapeutic value is limited by an, as yet poorly explained, interindividual heterogeneity in platelet inhibition. To evaluate possible pharmacokinetic determinants of this response variability, we developed a sensitive and specific liquid chromatography tandem mass spectrometry (LC-MS/MS) assay for quantification of unmodified inactive clopidogrel, its inactive carboxyl metabolite, and its active thiol metabolite in plasma. Analyte concentrations and platelet aggregation were assessed in ten healthy volunteers receiving an oral load of 600 mg clopidogrel. Subjects showed marked inter-individual differences in maximal platelet inhibition and in plasma pharmacokinetics. Univariate regression revealed linear correlations between maximal antiplatelet effect and peak plasma concentrations (cmax) of unchanged clopidogrel (r=0.76; p=0.01), of the carboxyl metabolite (r=0.70; p=0.03), and of the thiol metabolite (r=0.73; p=0.02), as well as linear correlations between cmax values of clopidogrel and its metabolites. This indicates that the response variability is predominantly caused by individual differences in clopidogrel absorption and that other factors, such as ADP receptor reactivity or differences in bioactivation of clopidogrel, do not play a major role.

Impact of CYP2C19 variant genotypes on clinical efficacy of antiplatelet treatment with clopidogrel: systematic review and meta-analysis

Objective To evaluate the accumulated information from genetic association studies investigating the impact of variants of the cytochrome P450 (CYP) 2C19 genotype on the clinical efficacy of clopidogrel. Design Systematic review and meta-analysis with a structured search algorithm and prespecified eligibility criteria for retrieval of relevant studies; dominant genetic model assumptions and quantitative methods for calculating summary effect estimates from study level odds ratios; systematic assessment of bias within and between studies; and grading of the cumulative evidence by consensus criteria. Data sources Medline, Embase, the Cochrane Library, online databases, contents pages and bibliographies of general medical, cardiovascular, pharmacological, and genetic journals. Eligibility criteria for selecting studies Original full length reports assessing the cumulative incidence of major adverse cardiovascular events or stent thrombosis over a follow-up period of at least a month in association with carrier status for the loss of function or gain of function CYP2C19 allele in adult patients with coronary artery disease and a clinical presentation of acute coronary syndrome or stable angina pectoris who were taking clopidogrel. Results 15 studies met the inclusion criteria. The random effects summary odds ratio for stent thrombosis in carriers of at least one CYP2C19 loss of function allele versus non-carriers combining nine studies was 1.77 (95% confidence interval 1.31 to 2.40; P<0.001). This nominally significant odds ratio was subject to considerable bias across the studies (small study effect bias and replication diversity). The adjustment for these quality modifiers tended to abolish the association. The corresponding random effects summary odds ratio of major adverse cardiovascular events for 12 studies combined was 1.11 (0.89 to 1.39; P=0.36). The random effects summary odds ratio of stent thrombosis in carriers versus non-carriers of at least one CYP2C19*17 gain of function allele for three studies combined was 0.99 (0.60 to 1.62; P=0.96), and the corresponding odds ratio of major adverse cardiovascular events in five studies was 0.93 (0.75 to 1.14; P=0.48). The overall quality of epidemiological evidence was graded as low, which excludes reliable clinical assessments. Conclusions Accumulated information from genetic association studies does not indicate a substantial or consistent influence of CYP2C19 gene polymorphisms on the clinical efficacy of clopidogrel. The current evidence does not support the use of individualised antiplatelet regimens guided by CYP2C19 genotype.

Impaired bioavailability of clopidogrel in patients with a ST-segment elevation myocardial infarction.

a Department of Cardiology, St Antonius Hospital, Nieuwegein, the Netherlands b Department of Pharmacology, University Hospital, University of Cologne, Cologne, Germany c Department of Clinical Chemistry, St Antonius Hospital Nieuwegein, the Netherlands d Department of Cardiology, Deutsches Herzzentrum and 1. Medizinische Klinik rechts der Isar, Technische Universitat Munchen, Munich, Germany e Department of Cardiology, UMC Nijmegen, The Netherlands

Relevance of the Organic Cation Transporters 1 and 2 for Antiretroviral Drug Therapy in Human Immunodeficiency Virus Infection

Carrier-mediated transport across cell membranes is an important determinant of activity, resistance, and toxicity of chemotherapeutic agents including antiretroviral (ARV) drugs (ARDs). The organic cation transporters (OCTs) 1 and 2 have been implicated in the translocation of different cationic drugs but so far were insufficiently tested for interactions with ARDs. Here, we assessed among cationic drugs commonly used in human immunodeficiency virus (HIV) therapy inhibitors and substrates of OCTs, and analyzed the tissue distribution of OCTs and their expression in lymph nodes (LNs), the primary intracellular target of HIV and ARDs. Inhibitors were identified by measuring the attenuated uptake of the radiolabeled model substrate 1-methyl-4-phenylpyridinium into OCT-transfected human embryonic kidney-293 cells in the presence of ARDs. Substrates were identified by measuring OCT-specific intracellular accumulation using liquid chromatography/tandem mass spectrometry. Inhibitory drugs were (in order of increasing potency): nelfinavir < ritonavir < saquinavir < indinavir < trimethoprim < pentamidine, with consistently lower IC50 values determined for OCT1. Substrates with highest transport efficacy (Vmax/Km) were lamivudine (OCT1, 8 μl/mg protein/min; OCT2, 4.4 μl/mg protein/min) and zalcitabine (OCT1, 4.1 μl/mg protein/min; OCT2, 2.6 μl/mg protein/min). Using quantitative real-time polymerase chain reaction, a marked expression level of OCT1 was detected in human samples of liver, ovary, prostate, and testis, and of OCT2 in kidney, colon, heart, skeletal muscle, and testis. Expression of OCTs in LNs was low in HIV-negative control individuals but dramatically increased in HIV-infected persons. These data suggest that drug interactions about the OCTs may be relevant for the ARV therapy, in particular by influencing drug accession to infected tissues and hepatic or renal elimination.

Aspirin induces nitric oxide release from vascular endothelium: A novel mechanism of action

The study was designed to test the hypothesis that aspirin may stimulate nitric oxide (NO) release from vascular endothelium, a pivotal factor for maintenance of vascular homeostasis. Clinical evidence suggests that low‐dose aspirin may improve vascular endothelial function. Since other cyclooxygenase (COX) inhibitors showed no beneficial vascular effects, aspirin may exhibit a vasculoprotective, COX‐independent mechanism. Luminal NO release was monitored in real time on dissected porcine coronary arteries (PCA) by an amperometric, NO‐selective sensor. Additionally, endothelial NO synthase (eNOS) activity was measured in EA.hy 926 cell homogenates by an L‐[3H]citrulline/L‐[3H]arginine conversion assay. Superoxide scavenging capacity was assessed by lucigenin‐enhanced luminescence. Aspirin induced an immediate concentration‐dependent NO release from PCA with an EC50 of 50 nM and potentiated the NO stimulation by the receptor‐dependent agonist substance P. These effects were independent of an increase in intracellular calcium and could be mimicked by stimulation with acetylating aspirin derivatives. The aspirin metabolite salicylic acid or the reversible cyclooxygenase inhibitor indomethacin failed to modulate NO release. Incubation of soluble eNOS for 15 min with 100 μM aspirin or acetylating aspirin analogues increased the L‐[3H]citrulline yield by 40–80%, while salicylic acid had no effect. Aspirin and salicylic acid showed a similar, but only modest, magnitude and velocity of superoxide scavenging. Our findings demonstrate that therapeutically relevant concentrations of aspirin elicit NO release from vascular endothelium. This effect appears to be due to a direct acetylation of the eNOS protein, but is independent of COX inhibition or inhibition of superoxide‐mediated NO degradation.

Which platelet function test is suitable to monitor clopidogrel responsiveness? A pharmacokinetic analysis on the active metabolite of clopidogrel

Summary.  Background: Multiple platelet function tests claim to be P2Y12‐pathway specific and capable of capturing the biological activity of clopidogrel. Objectives: The aim of the present study was to determine which platelet function test provides the best reflection of the in vivo plasma levels of the active metabolite of clopidogrel (AMC). Patients/methods: Clopidogrel‐naive patients scheduled for elective percutaneous coronary intervention (PCI) received a 600 mg loading dose of clopidogrel and 100 mg of aspirin. For pharmacokinetic analysis, blood was drawn at 0, 20, 40, 60, 90, 120, 180, 240 and 360 min after clopidogrel loading and peak plasma concentrations (Cmax) of the AMC were quantified with liquid chromatography‐tandem mass spectrometry (LC‐MS/MS). Platelet function testing was performed at baseline and 360 min after the clopidogrel loading. Results: The VASP‐assay, the VerifyNow P2Y12‐assay and 20 μmol L−1 adenosine diphosphate (ADP)‐induced light transmittance aggregometry (LTA) showed strong correlations with Cmax of the AMC (VASP: R2 = 0.56, P < 0.001; VerifyNow platelet reactivity units (PRU): R2 = 0.48, P < 0.001; VerifyNow %inhibition: R2 = 0.59, P < 0.001; 20 μmol L−1 ADP‐induced LTA: R2 = 0.47, P < 0.001). Agreement with Cmax of the AMC was less evident for 5 μmol L−1 ADP‐induced LTA or whole blood aggregometry (WBA), whereas the IMPACT‐R ADP test did not show any correlation with plasmalevels of the AMC. Conclusion: The flow cytometric VASP‐assay, the VerifyNow P2Y12 assay and, although to a lesser extent, 20 μmol L−1 ADP‐induced LTA correlate best with the maximal plasma level of the AMC, suggesting these may be the preferred platelet function tests for monitoring the responsiveness to clopidogrel.

Toxicokinetics of Acrylamide in Humans after Ingestion of a Defined Dose in a Test Meal to Improve Risk Assessment for Acrylamide Carcinogenicity

High amounts of acrylamide in some foods result in an estimated daily mean intake of 50 μg for a western style diet. Animal studies have shown the carcinogenicity of acrylamide upon oral exposure. However, only sparse human toxicokinetic data is available for acrylamide, which is needed for the extrapolation of human cancer risk from animal data. We evaluated the toxicokinetics of acrylamide in six young healthy volunteers after the consumption of a meal containing 0.94 mg of acrylamide. Urine was collected up to 72 hours thereafter. Unchanged acrylamide, its mercapturic acid metabolite N-acetyl-S-(2-carbamoylethyl)cysteine (AAMA), its epoxy derivative glycidamide, and the respective metabolite of glycidamide, N-acetyl-S-(2-hydroxy-2-carbamoylethyl)cysteine (GAMA), were quantified in the urine by liquid chromatography-mass spectrometry. Toxicokinetic variables were obtained by noncompartmental methods. Overall, 60.3 ± 11.2% of the dose was recovered in the urine. Although no glycidamide was found, unchanged acrylamide, AAMA, and GAMA accounted for urinary excretion of (mean ± SD) 4.4 ± 1.5%, 50.0 ± 9.4%, and 5.9 ± 1.2% of the dose, respectively. Apparent terminal elimination half-lives for the substances were 2.4 ± 0.4, 17.4 ± 3.9, and 25.1 ± 6.4 hours. The ratio of GAMA/AAMA amounts excreted was 0.12 ± 0.02. In conclusion, most of the acrylamide ingested with food is absorbed in humans. Conjugation with glutathione exceeds the formation of the reactive metabolite glycidamide. The data suggests an at least 2-fold and 4-fold lower relative internal exposure for glycidamide from dietary acrylamide in humans compared with rats or mice, respectively. This should be considered for quantitative cancer risk assessment. (Cancer Epidemiol Biomarkers Prev 2006;15(2):266–71)

Nitric Oxide Formation and Corresponding Relaxation of Porcine Coronary Arteries Induced by Plant Phenols: Essential Structural Features

The high intake of polyphenols is thought to contribute to the beneficial cardiovascular effects of plant-centered diets. A putative mechanism underlying the cardioprotective activity is thought to be a plant phenol–induced increase of nitric oxide formation by the constitutive endothelial nitric oxide synthase. Twenty-eight phenols of different classes commonly occurring in plant foods were examined for their capability of enhancing the endothelial nitric oxide release of isolated porcine coronary arteries by direct real-time measurement of the luminal surface nitric oxide concentration with an amperometric microsensor. Additionally, the relaxing activity of the phenols was measured on porcine coronary rings. Quercetin, myricetin, leucocyanidol, and oligomeric proanthocyanidins induced the highest increases in nitric oxide release (&Dgr;[NO] > 8.5 n M); caffeic acid, fisetin, hyperosid, and isoquercitrin were moderately active (5 n M < &Dgr;[NO] < 8.5 n M); the other phenolic compounds caused only marginal increases of the nitric oxide levels (&Dgr;[NO] < 5 n M). The nitric oxide–stimulating activity of the phenols was uniformly positively correlated with their vasorelaxing activity. However, endothelium-dependent vasorelaxations were limited to phenols inducing nitric oxide elevations > 5 n M (= Km value of the soluble guanylate cyclase). Analysis of structure-activity relations revealed that a high nitric oxide activity was confined to a flavan-moiety with free hydroxyl-residues at C3, C3′, C4′, C5, and C7 and a hydroxyl-, oxo-, or phenolic substituent at C4, whereas the caffeic acid scaffolding emerged as the minimally essential motif for the nitric oxide–dependent vasorelaxation.