Ticlopidine inhibition of phenytoin metabolism mediated by potent inhibition of CYP2C19

PBPK Analysis to Study the Impact of Genetic Polymorphism of NAT2 on Drug-Drug Interaction Potential of Isoniazid

PURPOSE

Isoniazid (INH) is prescribed both for the prophylaxis as well as the treatment of tuberculosis. It is primarily metabolized through acetylation by a highly polymorphic enzyme, N-acetyl transferase 2 (NAT2), owing to which significant variable systemic drug levels have been reported among slow and rapid acetylators. Furthermore, many drugs, like phenytoin, diazepam, triazolam, etc., are known to show toxic manifestation when co-administered with INH and it happens prominently among slow acetylators. Additionally, it is revealed in in vitro inhibition studies that INH carries noteworthy potential to inhibit CYP2C19 and CYP3A4 enzymes. However, CYP inhibitory effect of INH gets masked by opposite enzyme-inducing effect of rifampicin, when used in combination. Thus, distinct objective of this study was to fill the knowledge gaps related to gene-drug-drug interactions (DDI) potential of INH when given alone for prophylactic purpose.

METHODS

Whole body-PBPK models of INH were developed and verified for both slow and fast acetylators. The same were then utilized to carry out prospective DDI studies with CYP2C19 and CYP3A4 substrates in both acetylator types.

RESULTS

The results highlighted likelihood of significant higher blood levels of CYP2C19 and CYP3A4 substrate drugs in subjects receiving INH pre-treatment. It was also re-established that interaction was more likely in slow acetylators, as compared to rapid acetylators.

CONCLUSION

The novel outcome of the present study is the indication that prescribers should give careful consideration while advising CYP2C19 and CYP3A4 substrate drugs to subjects who are on prophylaxis INH therapy, and are slow to metabolic acetylation.

Interactions of 7,8-Dihydroxyflavone with Serum Albumin as well as with CYP2C9, CYP2C19, CYP3A4, and Xanthine Oxidase Biotransformation Enzymes

7,8-dihydroxyflavone (DHF) is a flavone aglycone which has beneficial effects in several central nervous system diseases. Most of the pharmacokinetic properties of DHF have been characterized, while only limited information is available regarding its interactions with serum albumin and biotransformation enzymes. In this study, the interactions of DHF with albumin was examined employing fluorescence spectroscopy and ultrafiltration. Furthermore, the inhibitory effects of DHF on cytochrome P450 (CYP2C9, CYP2C19, and CYP3A4) and xanthine oxidase (XO) enzymes were also tested using in vitro models. Our results demonstrate that DHF forms a stable complex with albumin (K = 4.9 × 10⁵ L/mol) and that it is able to displace both Site I and Site II ligands. Moreover, DHF proved to be a potent inhibitor of each enzyme tested, showing similar or slightly weaker effects than the positive controls used. Considering the above-listed observations, the coadministration of DHF with drugs may interfere with the drug therapy due to the development of pharmacokinetic interactions.

Breast cancer screening utilization and understanding of current guidelines among rural U.S. women with private insurance

Women living in rural areas of the U.S. face disparities in screening mammography and breast cancer outcomes. We sought to evaluate utilization of mammography, awareness of screening guidelines, and attitudes towards screening among rural insured U.S. women. We conducted a cross-sectional self-administered anonymous survey among 2000 women aged 40-64 insured by the National Rural Electric Cooperative Association, a non-profit insurer for electrical utility workers in predominantly rural areas across the U.S. Outcomes included mammographic screening in the past year, screening interval, awareness of guidelines, and perceived barriers to screening. 1588 women responded to the survey (response rate 79.4 %). 74 % of respondents lived in a rural area. Among women aged 40-49, 66.5 % reported mammographic screening in the past year. 46 % received annual screening, 32 % biennial screening, and 22 % rare/no screening. Among women aged 50-64, 77.1 % reported screening in the past year. 63 % received annual screening, 25 % biennial screening, and 12 % rare/no screening. The majority of women (98 %) believed that the mammography can find breast cancer early and save lives. Less than 1 % of younger women, and only 14 % of women over age 50 identified the recommendations of the U.S. Preventative Services Screening Task Force as the current expert recommendations for screening. Screening practices tended to follow perceived guideline recommendations. When rural U.S. women over age 40 have insurance, most receive breast cancer screening. The screening guidelines of cancer advocacy groups and specialty societies appear more influential and widely recognized than those of the U.S. preventative services taskforce.

In vitro metabolism of piperaquine is primarily mediated by CYP3A4

Piperaquine (PQ) is part of a first-line treatment regimen for Plasmodium falciparum malaria recommended by the World Health Organization (WHO). We aimed to determine the major metabolic pathway(s) of PQ in vitro. A reliable, validated tandem mass spectrometry method was developed. Concentrations of PQ were measured after incubation with both human liver microsomes (HLMs) and expressed cytochrome P450 enzymes (P450s). In pooled HLMs, incubations with an initial PQ concentration of 0.3 µM resulted in a 34.8 ± 4.9% loss of substrate over 60 min, corresponding to a turnover rate of 0.009 min(-1) (r(2) = 0.9223). Miconazole, at nonspecific P450 inhibitory concentrations, resulted in almost complete inhibition of PQ metabolism. The greatest inhibition was demonstrated with selective CYP3A4 (100%) and CYP2C8 (66%) inhibitors. Using a mixture of recombinant P450 enzymes, turnover for PQ metabolism was estimated as 0.0099 min(-1); recombinant CYP3A4 had a higher metabolic rate (0.017 min(-1)) than recombinant CYP2C8 (p < .0001). Inhibition of CYP3A4-mediated PQ loss was greatest using the selective inhibitor ketoconazole (9.1 ± 3.5% loss with ketoconazole vs 60.7 ± 5.9% with no inhibitor, p < .0001). In summary, the extent of inhibition of in vitro metabolism with ketoconazole (83%) denotes that PQ appears to be primarily catalyzed by CYP3A4. Further studies to support these findings through the identification and characterization of PQ metabolites are planned.

Chemical inhibitors of cytochrome P450 isoforms in human liver microsomes: a re-evaluation of P450 isoform selectivity

The majority of marketed small-molecule drugs undergo metabolism by hepatic Cytochrome P450 (CYP) enzymes (Rendic 2002). Since these enzymes metabolize a structurally diverse number of drugs, metabolism-based drug-drug interactions (DDIs) can potentially occur when multiple drugs are coadministered to patients. Thus, a careful in vitro assessment of the contribution of various CYP isoforms to the total metabolism is important for predicting whether such DDIs might take place. One method of CYP phenotyping involves the use of potent and selective chemical inhibitors in human liver microsomal incubations in the presence of a test compound. The selectivity of such inhibitors plays a critical role in deciphering the involvement of specific CYP isoforms. Here, we review published data on the potency and selectivity of chemical inhibitors of the major human hepatic CYP isoforms. The most selective inhibitors available are furafylline (in co-incubation and pre-incubation conditions) for CYP1A2, 2-phenyl-2-(1-piperidinyl)propane (PPP) for CYP2B6, montelukast for CYP2C8, sulfaphenazole for CYP2C9, (-)-N-3-benzyl-phenobarbital for CYP2C19 and quinidine for CYP2D6. As for CYP2A6, tranylcypromine is the most widely used inhibitor, but on the basis of initial studies, either 3-(pyridin-3-yl)-1H-pyrazol-5-yl)methanamine (PPM) and 3-(2-methyl-1H-imidazol-1-yl)pyridine (MIP) can replace tranylcypromine as the most selective CYP2A6 inhibitor. For CYP3A4, ketoconazole is widely used in phenotyping studies, although azamulin is a far more selective CYP3A inhibitor. Most of the phenotyping studies do not include CYP2E1, mostly because of the limited number of new drug candidates that are metabolized by this enzyme. Among the inhibitors for this enzyme, 4-methylpyrazole appears to be selective.

Simple, direct, and informative method for the assessment of CYP2C19 enzyme inactivation kinetics

Many clinically relevant drug interactions involving cytochrome P450 inhibition are mediated by mechanism-based inactivation (MBI). Time-dependent inhibition is one of the major features distinguishing between reversible inhibition and MBI. It thus provides a useful screening approach for early drug interaction risk assessment. Accordingly, we developed an easy and informative fluorometric method for the assessment of CYP2C19 enzyme inactivation kinetics. Dibenzylfluorescein (DBF) is widely used as a profluorescent probe substrate for P450 activity and inhibition assays, but its use has been considered to be limited to traditional endpoint assays. We monitored CYP2C19-catalyzed metabolism of DBF using synthesized fluorescein benzyl ester and fluorescein benzyl ether along with commercially available fluorescein as intermediate standards. Furthermore, we demonstrated the use of DBF in a kinetic assay as a progress curve analysis for straightforward determination of whether a compound is a time-dependent inactivator of CYP2C19. The recombinant human CYP2C19 inactivation kinetics of isoniazid, ticlopidine, and tranylcypromine were evaluated, and their key kinetic parameters were measured from the same experiment. The known mechanism-based inactivators, isoniazid and ticlopidine, exhibited clear time-dependent inactivation with K(I) and k(inact) values of 250.5 ± 34 μM and 0.137 ± 0.006 min(-1) and 1.96 ± 0.5 μM and 0.135 ± 0.009 min(-1), respectively. Tranylcypromine did not display any time-dependent inhibition, which is consistent with its reported mechanism of competitive inhibition. In summary, DBF is suitable for use in the progress curve analysis approach and can be used as an initial screen to identify compounds that require more detailed investigations in drug interaction optimization.

Structures of cytochrome P450 2B4 complexed with the antiplatelet drugs ticlopidine and clopidogrel

Prior X-ray crystal structures of rabbit cytochrome P450 2B4 (2B4) in complexes with various imidazoles have demonstrated markedly different enzyme conformations depending on the size of the inhibitor occupying the active site. In this study, structures of 2B4 were determined with the antiplatelet drugs clopidogrel and ticlopidine, which were expected to have greater freedom of movement in the binding pocket. Ticlopidine could be modeled into the electron density maps in two distinct orientations, both of which are consistent with metabolic data gathered with other mammalian P450 enzymes. Results of ligand docking and heme-induced NMR relaxation of drug protons showed that ticlopidine was preferentially oriented with the chlorophenyl group closest to the heme. Because of its stereocenter, clopidogrel was easier to fit in the electron density and exhibited a single orientation, which points the chlorophenyl ring toward the heme. The C(α) traces of both complexes aligned very well with each other and revealed a compact, closed structure that resembles the conformation observed in two previously determined 2B4 structures with the small molecule inhibitors 4-(4-chlorophenyl)imidazole and 1-(4-chlorophenyl)imidazole. The 2B4 active site is able to accommodate small ligands by moving only a small number of side chains, suggesting that ligand reorientation is energetically favored over protein conformational changes for binding of these similarly sized molecules. Adjusting both protein conformation and ligand orientation in the active site gives 2B4 the flexibility to bind to the widest range of molecules, while also being energetically favorable.

Metabolism and disposition of the thienopyridine antiplatelet drugs ticlopidine, clopidogrel, and prasugrel in humans

Ticlopidine, clopidogrel, and prasugrel are thienopyridine prodrugs that inhibit adenosine-5'-diphosphate (ADP)-mediated platelet aggregation in vivo. These compounds are converted to thiol-containing active metabolites through a corresponding thiolactone. The 3 compounds differ in their metabolic pathways to their active metabolites in humans. Whereas ticlopidine and clopidogrel are metabolized to their thiolactones in the liver by cytochromes P450, prasugrel proceeds to its thiolactone following hydrolysis by carboxylesterase 2 during absorption, and a portion of prasugrel's active metabolite is also formed by intestinal CYP3A. Both ticlopidine and clopidogrel are subject to major competing metabolic pathways to inactive metabolites. Thus, varying efficiencies in the formation of active metabolites affect observed effects on the onset of action and extent of inhibition of platelet aggregation (IPA). Knowledge of the CYP-dependent formation of ticlopidine and clopidogrel thiolactones helps explain some of the observed drug-drug interactions with these molecules and, more important, the role of CYP2C19 genetic polymorphism on the pharmacokinetics of and pharmacodynamic response to clopidogrel. The lack of drug interaction potential and the absence of CYP2C19 genetic effect result in a predictable response to thienopyridine antiplatelet therapy with prasugrel. Current literature shows that greater ADP-mediated IPA is associated with significantly better clinical outcomes for patients with acute coronary syndrome.

Antiepileptic drug interactions

INTRODUCTION

Antiepileptics are drugs used in the long-term treatment of epilepsy and other conditions such as pain or psychiatric diseases. They are often administered as polytherapy or in combination with other treatments. It is therefore important to know their potential interactions (with each other and with other substances) in order to avoid altering their efficacy or potentiating their side effects.

OBJECTIVE

The purpose of this article is to review these aspects and stress the most important interactions in day-to-day clinical practice.

RESULTS

Older antiepileptic drugs (AEDs) such as phenytoin, carbamazepine, phenobarbital and valproic acid can significantly interfere not only with each other and other AEDs, but also with other treatments. Although newer AEDs have a more favourable pharmacokinetic profile, they are not entirely exempt from interactions and they are also commonly administered in combination with older AEDs. Another aspect that should be considered is the existence of any clinically important pharmacokinetic and pharmacodynamic interactions in patients requiring the continuous administration of other treatments.

CONCLUSION

We must be aware of the pharmacokinetic and pharmacodynamic interactions of AEDs. Because of a lack of significant interactions, drugs such as levetiracetam, gabapentin or pregabalin can be recommended in particular groups such as patients with cancer, transplants, anticoagulant treatments or HIV infection. In all cases, it is important to ensure AED efficacy and prevent serious complications.

Comparison of human cytochrome P450 inhibition by the thienopyridines prasugrel, clopidogrel, and ticlopidine

Differences in the inhibition of cytochrome P450 activities among thienopyridine antiplatelet agents, ticlopidine, clopidogrel, prasugrel, and the metabolites, 2-oxo-clopidogrel, clopidogrel acid metabolite, deacetylated metabolite of prasugrel (R-95913) and the pharmacologically active metabolites of clopidogrel and prasugrel, were examined using recombinant cytochromes P450 and fluorescent probe substrates. Ticlopidine and clopidogrel inhibited CYP2B6 with IC(50) values of 0.0517+/-0.0323 microM and 0.0182+/-0.0069 microM, respectively, and inhibited CYP2C19 with IC(50) values of 0.203+/-0.124 microM and 0.524+/-0.160 microM, respectively. Ticlopidine also inhibited CYP2D6 (IC(50) of 0.354+/-0.158 microM). In contrast, 2-oxo-clopidogrel, prasugrel and R-95913 were much weaker inhibitors of CYP2B6, CYP2C19 and CYP2D6. The inhibitory effects of all the compounds tested were much weaker on the isoforms other than those indicated above. The active metabolites of clopidogrel and prasugrel and clopidogrel acid metabolite also did not affect the activities of the P450s examined.

Dosing and therapeutic monitoring of phenytoin in young adults after neurotrauma: are current practices relevant?

Anticonvulsant drugs are commonly used to treat and prevent seizures after neurotrauma. However, many physiological changes occur in the neurotrauma patient, which alter the pharmacokinetics of drugs such as phenytoin. This raises concerns relating to the dosage and monitoring of phenytoin in these patients compared with its routine use in epileptic patients. Examples of pharmacokinetic alterations within the neurotrauma patient include changes in hepatic metabolism, protein binding alterations, and disruption of the blood-brain barrier. Drug interactions and genetic factors may also contribute to pharmacokinetic variations. Many studies have reported that neurotrauma patients often present with either subtherapeutic or highly variable phenytoin serum concentrations. When phenytoin doses recommended for the epileptic patient are used in the neurotrauma patient, efficacy is limited to early posttraumatic seizures, with no effect on morbidity, mortality, or the onset of late posttraumatic seizures. This review examines the effect of neurotrauma on the pharmacokinetics of phenytoin alongside clinical outcomes and questions the current dosing and therapeutic monitoring practices within this area.

Evaluation of 3-O-methylfluorescein as a selective fluorometric substrate for CYP2C19 in human liver microsomes

Cytochrome P450 (P450) fluorometric high-throughput inhibition assays have been widely used for drug-drug interaction screening particularly at the preclinical drug discovery stages. Many fluorometric substrates have been investigated for their selectivity, but most are found to be catalyzed by multiple P450 isozymes, limiting their utility. In this study, 3-O-methylfluorescein (OMF) was examined as a selective fluorescence substrate for CYP2C19 in human liver microsomes (HLMs). The kinetic studies of OMF O-demethylation in HLMs using a liquid chromatography/mass spectrometry method exhibited two-enzyme kinetics with apparent K(m) and V(max) values of 1.14 +/- 0.90 microM and 11.3 +/- 4.6 pmol/mg/min, respectively, for the high affinity component(s) and 57.0 +/- 6.4 microM and 258 +/- 6 pmol/mg/min, respectively, for the low affinity component(s). Studies utilizing cDNA-expressed individual P450 isoforms and P450-selective chemical inhibitors showed that OMF O-demethylation to fluorescein was selective for CYP2C19 at substrate concentrations < or =1 microM. At substrate concentrations > or =10 microM, other P450 isozymes were found to catalyze OMF O-demethylation. In HLMs, analysis of the two-enzyme kinetics in the presence of P450 isozyme-selective chemical inhibitors (ticlopidine for CYP2C19, sulfaphenazole for CYP2C9, and furafylline for CYP1A2) indicated that CYP2C19 was the high affinity component and CYP2C9 was the low affinity component. Based on these findings, a fluorometric assay was developed using 1 microM OMF and 2 microM sulfaphenazole for probing CYP2C19-mediated inhibition in HLMs. The IC(50) data of 13 substrates obtained from the fluorometric assay developed in this study correlated well with that reported in the literature using nonfluorescence assays.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Identification of Enzymes Responsible for Primary and Sequential Oxygenation Reactions of Capravirine in Human Liver Microsomes

Capravirine, a new non-nucleoside reverse transcriptase inhibitor, undergoes extensive oxygenation reactions, including N-oxidation, sulfoxidation, sulfonation, and hydroxylation in humans. Numerous primary (mono-oxygenated) and sequential (di-, tri-, and tetraoxygenated) metabolites of capravirine are formed via the individual or combined oxygenation pathways. In this study, cytochrome P450 enzymes responsible for the primary and sequential oxygenation reactions of capravirine in human liver microsomes were identified at the specific pathway level. The total oxygenation of capravirine is mediated predominantly (>90%) by CYP3A4 and marginally (<10%) by CYP2C8, 2C9, and 2C19 in humans. Specifically, each of the two major mono-oxygenated metabolites C23 (sulfoxide) and C26 (N-oxide), is mediated predominantly (>90%) by CYP3A4 and slightly (<10%) by CYP2C8, the minor tertiary hydroxylated metabolite C19 by CYP3A4, 2C8, and 2C19, and the minor primary hydroxylated metabolite C20 by CYP3A4, 2C8, and 2C9. However, all sequential oxygenation reactions are mediated exclusively by CYP3A4. Due to their relatively insignificant contributions of C19 and C20 to total capravirine metabolism, no attempt was made to determine relative contributions of cytochrome P450 enzymes to the formation of the two minor metabolites.

Interspecies pharmacokinetics and in vitro metabolism of SQ109

This study aimed at characterizing the interspecies absorption, distribution, metabolism and elimination (ADME) profile of N-geranyl-N'-(2-adamantyl)ethane-1,2-diamine (SQ109), a new diamine-based antitubercular drug. Single doses of SQ109 were administered (intravenously (i.v.) and per os (p.o.)) to rodents and dogs and blood samples were analyzed by liquid chromatography tandem mass spectrometry (LC/MS/MS). Based on i.v. equivalent body surface area dose, the terminal half-life (t1/2) of SQ109 in dogs was longer than that in rodents, reflected by a larger volume of distribution (Vss) and a higher clearance rate of SQ109 in dogs, compared to that in rodents. The oral bioavailability of SQ109 in dogs, rats and mice were 2.4-5, 12 and 3.8%, respectively. After oral administration of [14C]SQ109 to rats, the highest level of radioactivity was in the liver, followed by the lung, spleen and kidney. Tissue-to-blood ratios of [14C]SQ109 were greater than 1. Fecal elimination of [14C]SQ109 accounted for 22.2% of the total dose of [14C]SQ109, while urinary excretion accounted for only 5.6%. The binding of [14C]SQ109 (0.1-2.5 microg ml-1) to plasma proteins varied from 6 to 23% depending on the species (human, mouse, rat and dog). SQ109 was metabolized by rat, mouse, dog and human liver microsomes, resulting in 22.8, 48.4, 50.8 or 58.3%, respectively, of SQ109 remaining after a 10-min incubation at 37 degrees C. The predominant metabolites in the human liver microsomes gave intense ion signals at 195, 347 and 363m/z, suggesting the oxidation, epoxidation and N-dealkylation of SQ109. P450 reaction phenotyping using recombinant cDNA-expressed human CYPs in conjunction with specific CYP inhibitors indicated that CYP2D6 and CYP2C19 were the predominant CYPs involved in SQ109 metabolism.

Interaction magnitude, pharmacokinetics and pharmacodynamics of ticlopidine in relation to CYP2C19 genotypic status

OBJECTIVES

The aim of this study was to investigate the impact of CYP2C19 polymorphism on the extent of the interaction and on the pharmacokinetics and pharmacodynamics of ticlopidine.

METHODS

Homozygous (hmEMs) and heterozygous extensive metabolizers (htEMs), and poor metabolizers (PMs, n = 6 each) took an oral dose (20 mg) of omeprazole. After a 1-week washout period, each subject received ticlopidine (200 mg) for 8 days, and ticlopidine pharmacokinetics were studied on days 1 and 7. On day 8, omeprazole was given again and its kinetic disposition was compared with that in the first dose. ADP-induced platelet aggregation was measured as a pharmacodynamic index.

RESULTS

In contrast to the PMs, whose mean kinetic parameters were not altered by the repeated dosings of ticlopidine, an eight- to 10-fold increase in the mean AUC ratio of omeprazole to 5-hydroxyomeprazole was observed in both the EM groups. No significant intergenotypic differences in the pharmacokinetic parameters of ticlopidine were observed, although the accumulation ratio tended to be greater in hmEMs than in PMs (2.4 +/- 0.2 versus 1.7 +/- 0.2). A significantly positive correlation (P = 0.031) was observed between the individual percent inhibition of platelet aggregation and AUC0-24 of ticlopidine regardless of the CYP2C19 polymorphism.

CONCLUSIONS

Ticlopidine is a potent inhibitor for CYP2C19 and may be associated with the phenocopy when CYPC19 substrates are co-administered to EMs. Whether and to what extent CYP2C19 would be involved in the metabolism of ticlopidine remain unanswered from the present in-vivo study.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Biosynthesis of epoxyeicosatrienoic acids varies between polymorphic CYP2C enzymes

Arachidonic acid is oxidized by cytochromes P450 2C (CYP2C) to epoxyeicosatrienoic acids (EETs), possessing vasoactive properties, with 11,12-EET as the endothelium derived hyperpolarization factor. Genetic variants of CYP2C enzymes have altered drug metabolizing capacity. Our primary aim was to determine whether EET biosynthesis differed in human liver microsomes with known CYP2C genotypes. Human liver microsomes (n = 25) of different CYP2C genotypes or yeast-expressed CYP2C enzymes were used. Analysis of metabolites was performed by liquid chromatography/mass spectrometry. Samples genotyped as CYP2C8*3/*3/CYP2C9*2/*2 exhibited a 34% (p < 0.05) decreased EET biosynthesis, compared to other CYP2C8/CYP2C9 haplotypes. Inhibition experiments suggested CYP2C8 and CYP2C9 to be the predominant catalysts of EETs. We found no differences between the three recombinantly expressed CYP2C9 variants, but CYP2C8.1 had lower K(m) than these isoforms. In conclusion, there are genetic differences in the CYP2C-dependent oxidation of arachidonic acid to vasoactive metabolites, of which the relevance to cardiovascular pathophysiology is still unclear.

In vitro sulfoxidation of thioether compounds by human cytochrome P450 and flavin-containing monooxygenase isoforms with particular reference to the CYP2C subfamily

Cytochrome P450 (P450) and flavin-containing monooxygenase (FMO) enzymes are major catalysts involved in the metabolism of xenobiotics. The sulfoxidation of the thioether pesticides, phorate, disulfoton, sulprofos, and methiocarb, was investigated. Using pooled human liver microsomes (HLMs), thioether compounds displayed similar affinities; however, phorate and disulfoton displayed higher intrinsic clearance rates than either sulprofos or methiocarb. The sulfoxidation of thioethers by HLMs was found to be predominantly P450-driven (85-90%) compared with FMO (10-15%). Among 16 cDNA-expressed human P450 isoforms and 3 human FMO isoforms examined, the following isoforms and their polymorphisms had the highest rates for sulfoxidation, as follows: phorate, CYP1A2, 3A4, 2B6, 2C9*1, 2C18, 2C19, 2D6*1, and FMO1; disulfoton, CYP1A2, 3A4, 2B6, 2C9*1, 2C9*2, 2C18, 2C19, 2D6*1, and FMO1; sulprofos, CYP1A1, 1A2, 3A4, 2C9*1, 2C9*2, 2C9*3, 2C18, 2C19, 2D6*1, and FMO1; methiocarb, CYP1A1, 1A2, 3A4, 2B6, 2C9*1, 2C19, 2D6*1, and FMO1. Among these isoforms, members of the CYP2C subfamily often had the highest affinities and clearance rates. Moreover, sulfaphenazole, a CYP2C9 competitive inhibitor, inhibited disulfoton sulfoxidation by CYP2C9 (IC50 0.84 microM) as well as in HLMs. Ticlopidine, a CYP2C19 mechanism-based inhibitor, inhibited disulfoton sulfoxidation by CYP2C19 (IC50 after coincubation, 43.5 microM; IC50 after preincubation, 4.3 microM) and also in HLMs. Our results indicate that current models of the substrate binding site of the CYP2C subfamily would not effectively predict thioether pesticide metabolism. Thus, the substrate specificity of CYP2Cs is more extensive than is currently believed, and some reevaluation of structure-activity relationships may be required.

Overview of the Clinical Pharmacokinetics of Oxcarbazepine

Oxcarbazepine (GP 47680, 10,11-dihydro-10-oxo-5H-dibenz[b,f]azepine- 5-carboxamide) is an antiepileptic drug registered worldwide by Novartis under the trade name Trileptal((R)). Trileptal((R))is approved as adjunctive therapy or monotherapy for the treatment of partial seizures in adults and in children. In the US, Trileptal((R)) is approved as adjunctive therapy in adults and in children >/=4 years of age and as monotherapy in adults and in children.Trileptal((R))is currently marketed as 150, 300 and 600mg film-coated tablets for oral administration. A 60 mg/mL (6%) oral suspension formulation has also been registered worldwide.Oxcarbazepine and its pharmacologically active metabolite, 10-monohydroxy derivative (MHD; 10,11-dihydro-10-hydro-carbamazepine; GP 47779) show potent antiepileptic activity in animal models comparable to that of carbamazepine (Tegretol((R))) and phenytoin. Oxcarbazepine and MHD have been shown to exert antiepileptic activity by blockade of voltage-dependent sodium channels in the brain.Oxcarbazepine is rapidly reduced by cytosolic enzymes in the liver to MHD, which is responsible for the pharmacological effect of the drug. This step is mediated by cytosolic arylketone reductases. MHD is eliminated by conjugation with glucuronic acid. Minor amounts (4% of the dose) are oxidised to the pharmacologically inactive dihydroxy derivative (DHD). The absorption of oxcarbazepine is complete. In plasma after a single oral administration of oxcarbazepine the mean apparent elimination half-life (t((1/2))) of MHD in adults was 8-9h. Food has no effect on the bioavailability of the highest strength of the final market image tablet (600mg). At steady state MHD displays predictable linear pharmacokinetics at doses ranging from 300 to 2400mg. In children with normal renal function, renal clearance of MHD is higher than in adults, with a corresponding reduction in the terminal t((1/2)) of MHD. Consequently, although no special dose recommendation is needed, an increase in the dose of oxcarbazepine may be necessary to achieve similar plasma levels to those in adults. In patients with moderate to severe renal impairment (creatinine clearance <30 mL/min), the elimination t((1/2)) of MHD is prolonged with a corresponding 2-fold increase in area under the concentration-time curve. Therefore, a dose reduction of at least 50% and a prolongation of the titration period is necessary in these patients. Mild-to-moderate hepatic impairment does not affect the pharmacokinetics of MHD. Based on in vitro and in vivo findings and compared with antiepileptic drugs such as carbamazepine, phenytoin and phenobarbital, oxcarbazepine has a low propensity for drug-drug interactions. In vitro, MHD inhibits the cytochrome P450 (CYP) 2C19 (ki [inhibition constant] = 88 micromol/L). At oxcarbazepine doses above 1.2g, a 40% increase in the concentration of phenytoin and a 15% increase in phenobarbital levels were observed. Oxcarbazepine/MHD at high doses may slightly increase phenobarbital and phenytoin plasma concentrations. Therefore, when using high doses of oxcarbazepine an adjustment in the dose of phenytoin may be required. In vitro, MHD is only a weak inducer of uridine diphospate (UDP)-glucuronyltransferase (UDPGT) and therefore is unlikely to have an effect on drugs that are mainly eliminated by conjugation through the UDPGT enzymes (e.g. valproic acid and lamotrigine). Weak interactions between MHD and antiepileptic drugs that are strong inducers of CYP enzymes have been identified. Carbamazepine, phenobarbital and phenytoin have been shown to reduce MHD levels by 30-40% when coadministered with oxcarbazepine, with no decrease in efficacy. Oxcarbazepine decreases the plasma hormone levels (ethinylestradiol and levonorgestrel) of oral contraceptives and may therefore have the potential to cause oral contraception failure.

Potent Mechanism-Based Inhibition of Human CYP2B6 by Clopidogrel and Ticlopidine

The thienopyridine derivatives ticlopidine and clopidogrel are inhibitors of ADP-induced platelet aggregation. Pharmacological activity of these prodrugs depends on cytochrome P450 (P450)-dependent oxidation to the active antithrombotic agent. In this study, we investigated the interaction potential of clopidogrel and ticlopidine by using human liver microsomes and recombinantly expressed P450 isoforms. Both clopidogrel and ticlopidine inhibited CYP2B6 with highest potency and CYP2C19 with lower potency. Clopidogrel also inhibited CYP2C9, and ticlopidine also inhibited CYP1A2, with lower potency. Inhibition of CYP2B6 was time- and concentration-dependent, and as shown by dialysis experiments, it was irreversible and dependent on NADPH, suggesting a mechanism-based mode of action. Inactivation was of nonpseudo-firstorder type with maximal rates of inactivation (K(inact)) for clopidogrel and ticlopidine in microsomes (recombinant CYP2B6) of 0.35 (1.5 min(-1)) and 0.5 min(-1) (0.8 min(-1)), respectively, and half-maximal inactivator concentrations (KI) were 0.5 microM (1.1 microM) for clopidogrel and 0.2 microM (0.8 microM) for ticlopidine. Inhibition was attenuated by the presence of alternative active site ligands but not by nucleophilic trapping agents or reactive oxygen scavengers, further supporting mechanism-based action. A chemical mechanism is discussed based on the known metabolic activation of clopidogrel and on the finding that hemoprotein integrity of recombinant CYP2B6 was not affected by irreversible inhibition. These results suggest the possibility of drug interactions between thienopyridine derivates and drug substrates of CYP2B6 and CYP2C19.

Clinically important drug interactions in epilepsy: interactions between antiepileptic drugs and other drugs

Antiepileptic drugs (AEDs) are commonly prescribed for long periods, up to a lifetime, and many patients will require treatment with other agents for the management of concomitant or intercurrent conditions. When two or more drugs are prescribed together, clinically important interactions can occur. Among old-generation AEDs, carbamazepine, phenytoin, phenobarbital, and primidone are potent inducers of hepatic enzymes, and decrease the plasma concentration of many psychotropic, immunosuppressant, antineoplastic, antimicrobial, and cardiovascular drugs, as well as oral contraceptive steroids. Most new generation AEDs do not have clinically important enzyme inducing effects. Other drugs can affect the pharmacokinetics of AEDs; examples include the stimulation of lamotrigine metabolism by oral contraceptive steroids and the inhibition of carbamazepine metabolism by certain macrolide antibiotics, antifungals, verapamil, diltiazem, and isoniazid. Careful monitoring of clinical response is recommended whenever a drug is added or removed from a patient's AED regimen.

The thienopyridine derivatives (platelet adenosine diphosphate receptor antagonists), pharmacology and clinical developments

The thienopyridines, ticlopidine and clopidogrel, are antiplatelet drugs. They are prodrugs and are metabolised in the liver to active metabolites that are non-competitive antagonists of the platelet adenosine diphosphate receptor, P2Y12. Inhibition of platelet aggregation by these drugs is delayed until 24-48 h after administration, with maximal inhibition achieved after 3-5 days. Recovery of platelet function after drug withdrawal is slow (7-14 days). Ticlopidine and clopidogrel are effective in preventing atherothrombotic events in cardiovascular, cerebrovascular and peripheral vascular disease. Gastrointestinal side effects and skin rashes are common. However, neutropenia and thrombotic thrombocytopenic purpura are significant and sometimes fatal adverse effects of ticlopidine. Clopidogrel appears to offer several advantages over ticlopidine: a more rapid onset of action and a lower incidence of neutropenia and thrombotic thrombocytopenic purpura.A combination of clopidogrel and aspirin has become standard for antithrombotic therapy in cardiovascular disease. The anaesthetic considerations of patients taking the thienopyridine compounds are discussed.

Carbamazepine and oxcarbazepine decrease phenytoin metabolism through inhibition of CYP2C19

Multiple studies suggest that phenytoin concentrations increase with CBZ co-medication. This study evaluated the hypothesis that CBZ and/or its major metabolite (CBZE) inhibit CYP2C19-mediated phenytoin metabolism using human liver microsomes and cDNA-expressed CYP2C19. Oxcarbazepine (OXC), and its 10-monohydroxy metabolite (MHD) were also evaluated. CBZ and MHD inhibited CYP2C19-mediated phenytoin metabolism at therapeutic concentrations. Thus, administration of CBZ and OXC with CYP2C19 substrates with narrow therapeutic ranges should be done cautiously.

Clinical significance of the cytochrome P450 2C19 genetic polymorphism

Cytochrome P450 2C19 (CYP2C19) is the main (or partial) cause for large differences in the pharmacokinetics of a number of clinically important drugs. On the basis of their ability to metabolise (S)-mephenytoin or other CYP2C19 substrates, individuals can be classified as extensive metabolisers (EMs) or poor metabolisers (PMs). Eight variant alleles (CYP2C19*2 to CYP2C19*8) that predict PMs have been identified. The distribution of EM and PM genotypes and phenotypes shows wide interethnic differences. Nongenetic factors such as enzyme inhibition and induction, old age and liver cirrhosis can also modulate CYP2C19 activity. In EMs, approximately 80% of doses of the proton pump inhibitors (PPIs) omeprazole, lansoprazole and pantoprazole seem to be cleared by CYP2C19, whereas CYP3A is more important in PMs. Five-fold higher exposure to these drugs is observed in PMs than in EMs of CYP2C19, and further increases occur during inhibition of CYP3A-catalysed alternative metabolic pathways in PMs. As a result, PMs of CYP2C19 experience more effective acid suppression and better healing of duodenal and gastric ulcers during treatment with omeprazole and lansoprazole compared with EMs. The pharmacoeconomic value of CYP2C19 genotyping remains unclear. Our calculations suggest that genotyping for CYP2C19 could save approximately 5000 US dollars for every 100 Asians tested, but none for Caucasian patients. Nevertheless, genotyping for the common alleles of CYP2C19 before initiating PPIs for the treatment of reflux disease and H. pylori infection is a cost effective tool to determine appropriate duration of treatment and dosage regimens. Altered CYP2C19 activity does not seem to increase the risk for adverse drug reactions/interactions of PPIs. Phenytoin plasma concentrations and toxicity have been shown to increase in patients taking inhibitors of CYP2C19 or who have variant alleles and, because of its narrow therapeutic range, genotyping of CYP2C19 in addition to CYP2C9 may be needed to optimise the dosage of phenytoin. Increased risk of toxicity of tricyclic antidepressants is likely in patients whose CYP2C19 and/or CYP2D6 activities are diminished. CYP2C19 is a major enzyme in proguanil activation to cycloguanil, but there are no clinical data that suggest that PMs of CYP2C19 are at a greater risk for failure of malaria prophylaxis or treatment. Diazepam clearance is clearly diminished in PMs or when inhibitors of CYP2C19 are coprescribed, but the clinical consequences are generally minimal. Finally, many studies have attempted to identify relationships between CYP2C19 genotype and phenotype and susceptibility to xenobiotic-induced disease, but none of these are compelling.

Pharmacokinetic interaction between albendazole sulfoxide enantiomers and antiepileptic drugs in patients with neurocysticercosis

The aim of the present investigation was to determine the interaction between the antiepileptic drugs (AEDs) phenytoin, carbamazepine, and phenobarbital and the enantioselective metabolism of albendazole. Thirty-two adults with a diagnosis of the active form of intraparenchymatous neurocysticercosis and treated with albendazole at the dose of 7.5 mg/kg every 12 hours for 8 days were studied. The patients were divided into four groups based on the combined use of AEDs or not: control group (n = 9), phenytoin group (n = 9 patients treated with 3-4 mg/kg/d sodium phenytoin), carbamazepine group (n = 9 patients treated with 10-20 mg/kg/d carbamazepine), and phenobarbital group (n = 5 patients treated with 1.5-4.5 mg/kg/d phenobarbital). Serial blood collections were carried out on day 8 of albendazole treatment during the last 12-hour dose interval. Plasma concentrations of the (+)- and (-)-albendazole sulfoxide (ASOX) and albendazole sulfone (ASON) metabolites were determined by high-performance liquid chromatography using a chiral phase column and fluorescence detection. The pharmacokinetic parameters were analyzed by analysis of variance followed by the Tukey-Kramer test. The results are reported as means. The following differences (P < 0.05) were observed between the control and the phenytoin, carbamazepine, and phenobarbital groups, respectively: (+)-ASOX area under the concentration-time curve for 0 to 12 hours after treatment (AUC(0-12)) 6.1, 2.1, 3.1, 2.4 microg/h/mL; (+)-ASOX maximum plasma concentration (C(max)) 0.8, 0.3, 0.4, 0.3 microg/mL; (+)-ASOX half-life (t1/2) 8.0, 3.8, 4.1, 4.9 h; (-)-ASOX AUC(0-12) 1.8, 0.4, 0.6, 0.5 microg/h/mL; (-)-ASOX C(max) 0.2, 0.06, 0.1, 0.1 microg/mL; (-)-ASOX (t(1/2)) 4.3, 1.9, 2.2, 2.1 h; ASON AUC(0-12) 0.5, 0.2 microg/h/mL; ASON C(max) 0.8, 0.3, 0.4, 0.3 microg/mL; ASON (t(1/2)) 8.0, 3.8, 4.1 h. The results show that phenytoin, carbamazepine, and phenobarbital induce to approximately the same extent the oxidative metabolism of albendazole in a nonenantioselective manner. Notably, a significant reduction in the plasma concentration of the active ASOX metabolite was observed in patients with neurocysticercosis treated with these AEDs.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Pharmacokinetic differences between lansoprazole enantiomers and contribution of cytochrome P450 isoforms to enantioselective metabolism of lansoprazole in dogs

The purpose of this study was to evaluate the pharmacokinetics of lansoprazole enantiomers and contribution of cytochrome P450 enzymes to enantioselective metabolism in dogs. The mean Cmax and area under the curve (AUC) values of (+)-lansoprazole were 4-5 times greater than those of (-)-lansoprazole following oral administration of 30-mg racemic lansoprazole to dogs. The CLtot/F values of (+)-lansoprazole were significantly smaller than those of (-)-lansoprazole (p<0.05). The mean unbound fraction of (-)-lansoprazole was significantly greater than that of the (+)-lansoprazole. The amount of (+)-lansoprazole remaining was significantly greater than that of the (-)-lansoprazole after incubation of racemic lansoprazole in dog liver microsomes. When the effects of ticlopidine or ketoconazole on the metabolism of lansoprazole were studied using dog liver microsomes, ticlopidine significantly inhibited the formation of 5-hydroxylansoprazole, but not another metabolite, lansoprazole sulfone; however ketoconazole significantly inhibited formation of both metabolites. When the amount of (+)- and (-)-enantiomers remaining was measured in the presence and absence of ticlopidine, the amount of (+)-lansoprazole was significantly greater than that of the (-)-lansoprazole. On the other hand, there was no significant difference between the amount of (+)- and (-)-enantiomers remaining in combination with ketoconazole. These results suggest that the enantioselective pharmacokinetics of lansoprazole enantiomers are probably ascribable to their enantioselective protein binding and/or metabolism, and among the cytochrome P450 enzymes, CYP3A contributed to the enantioselective metabolism of lansoprazole.

Inhibition of cytochrome P450 2C9 activity in vitro by 5-hydroxytryptamine and adrenaline

In the present study, the occurrence of a modulatory effect of 14 neurotransmitters, precursors and metabolites on the cytochrome P450 2C9 (CYP2C9) enzyme activity, as determined by diclofenac 4-hydroxylation, was studied in human liver microsomes. Two indoleamines, 5-hydroxytryptamine (5-HT) and adrenaline, showed a non-competitive-type inhibitory effect of approximately 90% of the diclofenac 4-hydroxylase activity, with Ki values of 63.5 (0.7 and 156 (89.3 microM, respectively. The rest of substances analysed were weak inhibitors or had no inhibitory effect. CYP2C subfamily is present in human brain, although CYP2C9 isozyme has not yet been identified in this tissue, and CYP2C9 is involved in the metabolism of psychoactive drugs. Therefore, the fact that endogenous compounds could modulate the CYP2C9 activity, suggests that an hypothetical local activity of brain CYP2C9 might be susceptible to regulatory mechanisms. The possible clinical implications of this modulation are discussed.

Inhibition of cytochrome P450 (CYP450) isoforms by isoniazid: potent inhibition of CYP2C19 and CYP3A

Isoniazid (INH) remains the most safe and cost-effective drug for the treatment and prophylaxis of tuberculosis. The use of INH has increased over the past years, largely as a result of the coepidemic of human immunodeficiency virus infection. It is frequently given chronically to critically ill patients who are coprescribed multiple medications. The ability of INH to elevate the concentrations in plasma and/or toxicity of coadministered drugs, including those of narrow therapeutic range (e.g., phenytoin), has been documented in humans, but the mechanisms involved are not well understood. Using human liver microsomes (HLMs), we tested the inhibitory effect of INH on the activity of common drug-metabolizing human cytochrome P450 (CYP450) isoforms using isoform-specific substrate probe reactions. Incubation experiments were performed at a single concentration of each substrate probe at its K(m) value with a range of INH concentrations. CYP2C19 and CYP3A were inhibited potently by INH in a concentration-dependent manner. At 50 microM INH (approximately 6.86 microg/ml), the activities of these isoforms decreased by approximately 40%. INH did not show significant inhibition (<10% at 50 microM) of other isoforms (CYP2C9, CYP1A2, and CYP2D6). To accurately estimate the inhibition constants (K(i) values) for each isoform, four concentrations of INH were incubated across a range of five concentrations of specific substrate probes. The mean K(i) values (+/- standard deviation) for the inhibition of CYP2C19 by INH in HLMs and recombinant human CYP2C19 were 25.4 +/- 6.2 and 13 +/- 2.4 microM, respectively. INH showed potent noncompetitive inhibition of CYP3A (K(i) = 51.8 +/- 2.5 to 75.9 +/- 7.8 microM, depending on the substrate used). INH was a weak noncompetitive inhibitor of CYP2E1 (K(i) = 110 +/- 33 microM) and a competitive inhibitor of CYP2D6 (K(i) = 126 +/- 23 microM), but the mean K(i) values for the inhibition of CYP2C9 and CYP1A2 were above 500 microM. Inhibition of one or both CYP2C19 and CYP3A isoforms is the likely mechanism by which INH slows the elimination of coadministered drugs, including phenytoin, carbamazepine, diazepam, triazolam, and primidone. Slow acetylators of INH may be at greater risk for adverse drug interactions, as the degree of inhibition was concentration dependent. These data provide a rational basis for understanding drug interaction with INH and predict that other drugs metabolized by these two enzymes may also interact.

Selection of drugs to treat gastro-oesophageal reflux disease: the role of drug interactions

Gastro-oesophageal reflux disease is probably the most common acid-peptic disease in Western countries, and the successful treatment of mild to moderate disease with pharmacotherapy has become commonplace. A large number of effective drugs are now available, and so the decision-making process for physicians increasingly relies on considerations other than pure efficacy. Cost, adverse effects and drug interactions have therefore become important, particularly in the most vulnerable patients - children, the elderly and patients who are ill and are taking medications that may influence the efficacy of antireflux therapy. Important drug interactions with antacids include the prevention of the absorption of antibacterials such as tetracycline, azithromycin and quinolones. H2 antagonists, proton pump inhibitors and prokinetic agents undergo metabolism by the cytochrome P450 (CYP) system present in the liver and gastrointestinal tract. Cimetidine is an inhibitor of CYP3A and it may cause significant interactions with drugs of narrow therapeutic range and low bioavailability that are metabolised by these enzymes. The gastroparietal proton pump inhibitors lansoprazole, omeprazole and pantoprazole are all primarily metabolised by a genetically polymorphic enzyme, CYP2C19, that is absent from approximately 3% of Caucasians and 20% of Asians. These drugs may also interact with CYP3A, but to a lesser extent. Interactions with prokinetic agents carry the greatest potential for harm. Metoclopramide is a dopamine antagonist that may cause extrapyramidal effects when administered alone at high concentrations, or when coadministered with antipsychotic agents such as haloperidol or phenothiazines. Cisapride is clearly able to prolong the electrocardiographic QT interval and cause lethal ventricular arrhythmias when its metabolism is slowed by interaction with inhibitors of CYP3A, such as erythromycin, ketoconazole or itraconazole.

In vitro inhibition of the cytochrome P450 (CYP450) system by the antiplatelet drug ticlopidine: potent effect on CYP2C19 and CYP2D6

AIMS

To examine the potency of ticlopidine (TCL) as an inhibitor of cytochrome P450s (CYP450s) in vitro using human liver microsomes (HLMs) and recombinant human CYP450s.

METHODS

Isoform-specific substrate probes of CYP1A2, 2C19, 2C9, 2D6, 2E1 and 3A4 were incubated in HLMs or recombinant CYPs with or without TCL. Preliminary data were generated to simulate an appropriate range of substrate and inhibitor concentrations to construct Dixon plots. In order to estimate accurately inhibition constants (Ki values) of TCL and determine the type of inhibition, data from experiments with three different HLMs for each isoform were fitted to relevant nonlinear regression enzyme inhibition models by WinNonlin.

RESULTS

TCL was a potent, competitive inhibitor of CYP2C19 (Ki = 1.2 +/- 0.5 microM) and of CYP2D6 (Ki = 3.4 +/- 0.3 microM). These Ki values fell within the therapeutic steady-state plasma concentrations of TCL (1-3 microM). TCL was also a moderate inhibitor of CYP1A2 (Ki = 49 +/- 19 microM) and a weak inhibitor of CYP2C9 (Ki > 75 microM), but its effect on the activities of CYP2E1 (Ki = 584 +/- 48 microM) and CYP3A (> 1000 microM) was marginal.

CONCLUSIONS

TCL appears to be a broad-spectrum inhibitor of the CYP isoforms, but clinically significant adverse drug interactions are most likely with drugs that are substrates of CYP2C19 or CYP2D6.

Genetic polymorphism of the CYP2C subfamily and excessive serum phenytoin concentration with central nervous system intoxication

The authors report on a Japanese adult male patient with a long history of partial seizures that were poorly controlled by conventional doses of phenytoin and other drugs. His treatment was complicated by toxic symptoms and an excessive serum phenytoin concentration, 32.6 microg/mL at a dose of 187.5 mg/day. Polymerase chain reaction-restriction fragment length polymorphism analysis disclosed heterozygosity involving cytochrome P450 subfamilies 2C9 (*1/*3) and 2C19 (*1/*3). Currently, it is generally accepted that the former mutation is responsible for the CYP2C9 poor metabolizer phenotype. Pharmacokinetic parameters were estimated by a kinetic analysis, MULTI, using 17 observed dose-concentration data sets: a lower Vmax (5.6 mg/kg/day) and a higher Km (11.5 microg/mL) were observed. Although phenytoin is metabolized predominantly by CYP2C9 with a minor contribution of CYP2C19, patients with the Leu359 variant should be monitored closely when treated with a moderate to high daily dose of phenytoin.

Ticlopidine and clopidogrel

The thienopyridines ticlopidine and clopidogrel are inhibitors of platelet function in vivo. Their mode of action has not been defined, but it appears that they require conversion to as yet unidentified metabolites that are noncompetitive antagonists of the platelet ADP receptor. Inhibition of platelet aggregation with these compounds is delayed until 24 to 48 hours after administration. Maximum inhibition occurs after 3 to 5 days, and recovery is slow after drug withdrawal. Ticlopidine is effective in preventing cardiovascular events in cerebrovascular, cardiovascular, and peripheral vascular disease, with an efficacy that is similar to aspirin. However, its use is associated with significant and sometimes fatal adverse reactions, specifically neutropenia and bone marrow aplasia. Gastrointestinal side effects and skin rashes are common and result in discontinuation of therapy in up to 10% of patients. Clopidogrel is at least as effective as aspirin in preventing cardiovascular events in patients with a history of vascular disease. It appears to be safer than ticlopidine, although its efficacy in acute coronary syndromes or post-coronary-stent insertion has not been reported. Important outstanding issues are whether clopidogrel adds to the benefit of aspirin and whether the combination of these agents is safe. If so, this combination may become the standard for antithrombotic therapy in cardiovascular disease.

Pharmacokinetics and hemodynamic effects of single oral doses of thalidomide in asymptomatic human immunodeficiency virus-infected subjects

Thalidomide (alpha-N-phthalimidoglutarimide), a potent inhibitor of tumor necrosis factor alpha (TNF-alpha), is proving to be a promising drug in the treatment of a number of inflammatory, autoimmune, and HIV-associated disorders. The pharmacokinetics and hemodynamic effects of two single oral doses of thalidomide (100 and 200 mg) were investigated, using a randomized, two-period crossover design, in a group of asymptomatic, male HIV-seropositive subjects. Thalidomide pharmacokinetics were linear at the doses studied, and were best described by a one-compartment model with first-order absorption and elimination processes. The drug was rapidly absorbed, with a mean absorption half-life of 0.95 hr (range, 0.16-2.49 hr) and 1.19 hr (range, 0.33-3.53 hr) after 100- and 200-mg doses, respectively. The corresponding mean Cmax values were 1.15+/-0.24 microg/ml (100 mg) and 1.92+/-0.47 microg/ml (200 mg; p<0.001), which were achieved (Tmax) at 2.5+/-1.5 h and 3.3+/-1.4 hr, respectively. Plasma concentrations of thalidomide declined thereafter, in a log-linear manner, with elimination half-lives of 4.6+/-1.2 hr (100 mg) and 5.3+/-2.2 hr (200 mg). The apparent volumes of distribution (Vdss/F) were 69.9+/-15.6 liters (100 mg) and 82.7+/-34.9 liters (200 mg) while total body clearances (Cl/F) were 10.4+/-2.1 and 10.8+/-1.7 liters/hr, respectively. Significant dose-dependent decreases in supine systolic and diastolic blood pressures were seen for up to 2 hr postdosing; somnolence, headache, dizziness, and confusion were also reported more frequently at the higher dose of thalidomide.

Analysis of antiplatelet effect of ticlopidine in humans: modeling based on irreversible inhibition of platelet precursors in bone marrow

The relationship between plasma concentration of ticlopidine and its inhibitory effect on platelet aggregation in human was analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model. The data of plasma concentration and inhibitory effect on platelet aggregation were taken from the literature. A two-compartment open model was fitted to plasma ticlopidine concentrations. Assuming that ticlopidine acts on platelet precursors in the bone marrow, the apparent reaction rate constant of ticlopidine and platelet precursors (K), apparent transformation rate constant of platelet precursors (kr) and apparent elimination rate constant of platelets (ke) were estimated. The estimated values +/- S.D. were 1.01 +/- 1.08 ml micrograms-1 hr-1 for K, 0.265 +/- 0.259 hr-1 for kr and 0.0747 +/- 0.0112 hr-1 for ke. The antiaggregation effects of ticlopidine on platelets after administration of 100, 200, and 300 mg (bid for 8 days) were simulated using the PD parameters of K, kr, and ke. While the antiaggregation effect reached steady state within 3-4 days without dose dependency of the interval, the maximum effect increased with dose. Furthermore, changing the elimination rate constant of ticlopidine from the central compartment in the model significantly changed the duration of inhibitory effect of ticlopidine on platelet aggregation. Therefore, the reported long duration of antiplatelet effect after discontinuation of ticlopidine, which is believed to be irreversible binding to the platelet, might have been partially caused by the delayed plasma elimination after a long therapy of ticlopidine. On the other hand, the mean life-span of platelets in the blood estimated by 1/ke after administration of ticlopidine was 14 hr, far below the life-span of platelets in the blood. For a more detailed analysis of the antiplatelet effect of ticlopidine, the possible contribution of reversible binding of the drug to glycoprotein IIb/IIIa should be considered in future PK/PD models.

Ticlopidine decreases the in vivo activity of CYP2C19 as measured by omeprazole metabolism

AIMS

To examine the effect of ticlopidine administration on the activities CYP2C19 and CYP3 A in vivo using omeprazole as a model substrate.

METHODS

A single dose of 40 mg omeprazole was administered orally with or without ticlopidine (300 mg daily for 6 days) to six Japanese extensive metabolisers with respect to CYP2C19. Blood samples were taken for the measurement of plasma concentrations of omeprazole, 5-hydroxyomeprazole and omeprazole sulphone.

RESULTS

Ticlopidine administration increased omeprazole Cmax (1978+/-859/ 3442+/-569 (control phase/ticlopidine phase, nm )) and decreased the oral clearance of omeprazole (CL/F; 25.70+/-16. 17/10.76+/-1.16 (control phase/ticlopidine phase, l h-1 )) significantly. The 5-hydroxyomeprazole to omeprazole AUC ratio (0. 817+/-0.448/0.236+/-0.053 (control phase/ticlopidine phase)) and the 5-hydroxyomeprazole to omeprazole sulphone AUC ratio (1.114+/-0. 782/0.256+/-0.051 (control phase/ticlopidine phase)) were decreased significantly after ticlopidine administration. The decrease in omeprazole CL/F and the 5-hydroxyomeprazole to omeprazole AUC ratio correlated significantly with their respective absolute values when the drug was given alone. The decrease in CL/F following ticlopidine administration correlated with that in the 5-hydroxyomeprazole to omeprazole AUC ratio.

CONCLUSIONS

These findings suggest that ticlopidine inhibited the in vivo activity of CYP2C19, but not, or to a lesser extent CYP3 A4, and that the magnitude of inhibition by ticlopidine is related to the in vivo activity of CYP2C19 before inhibition.

Rash in patients receiving ticlopidine after intracoronary stent placement

As many as half of the approximately 400,000 patients who undergo nonsurgical coronary artery procedures every year receive an intracoronary stent. To prevent thrombus formation within the stent, antiplatelet treatment with ticlopidine and aspirin is administered. Ticlopidine is known to cause cutaneous adverse reactions in up to 5.1% of patients, with a 3.4% discontinuation rate. However, published studies of patients receiving the drug to prevent subacute thrombosis after intracoronary stent placement report a frequency of rash ranging from 0.8-1.6%. We hypothesized that the frequency of rash in this patient population is underreported, and conducted a retrospective chart review, collecting data on frequency and severity of rash, treatment required, patient demographics, and concomitant drugs that may predispose patients to rash. The frequency of rash was approximately 7% in this group of patients.

Ticlopidine-induced phenytoin toxicity

OBJECTIVE

To report a probable case of ticlopidine-induced phenytoin toxicity.

CASE SUMMARY

A 72-year-old white man suddenly developed combative behavior, refused to leave his room, stopped eating, and began falling to the floor 6 weeks after being given ticlopidine. The total phenytoin concentration was measured at 43.6 micrograms/mL; the dosage of phenytoin was decreased and the symptoms later resolved. After ticlopidine was discontinued, the patient was rechallenged with the same dose of phenytoin without symptoms of toxicity.

DISCUSSION

Possible mechanisms of the drug interaction are discussed with emphasis on cytochrome P450 metabolism.

CONCLUSIONS

Clinicians should be aware of this potentially serious drug interaction and either avoid the phenytoin-ticlopidine combination, or monitor closely for phenytoin toxicity.