Selective serotonin reuptake inhibitors and theophylline metabolism in human liver microsomes: potent inhibition by fluvoxamine

Potential impact of underlying diseases influencing ADME in nonclinical safety assessment

Nonclinical studies involve in vitro, in silico, and in vivo experiments to assess the toxicokinetics, toxicology, and safety pharmacology of drugs according to regulatory requirements by a national or international authority. In this review, we summarize the potential effects of various underlying diseases governing the absorption, distribution, metabolism, and excretion (ADME) of drugs to consider the use of animal models of diseases in nonclinical trials. Obesity models showed alterations in hepatic metabolizing enzymes, transporters, and renal pathophysiology, which increase the risk of drug-induced toxicity. Diabetes models displayed changes in hepatic metabolizing enzymes, transporters, and glomerular filtration rates (GFR), leading to variability in drug responses and susceptibility to toxicity. Animal models of advanced age exhibited impairment of drug metabolism and kidney function, thereby reducing the drug-metabolizing capacity and clearance. Along with changes in hepatic metabolic enzymes, animal models of metabolic syndrome-related hypertension showed renal dysfunction, resulting in a reduced GFR and urinary excretion of drugs. Taken together, underlying diseases can induce dysfunction of organs involved in the ADME of drugs, ultimately affecting toxicity. Therefore, the use of animal models of representative underlying diseases in nonclinical toxicity studies can be considered to improve the predictability of drug side effects before clinical trials.

CYP1A inhibitors: Recent progress, current challenges, and future perspectives

Mammalian cytochrome P450 1A (CYP1A) are key phase I xenobiotic-metabolizing enzymes that play a distinctive role in metabolic activation or metabolic clearance of a variety of procarcinogens, drugs, and endogenous substances. Human CYP1A subfamily contains two members (hCYP1A1 and hCYP1A2), which are known to catalyze the oxidative activation of some environmental procarcinogens into carcinogenic species. Increasing evidence has demonstrated that CYP1A inhibitor therapies are promising strategies for cancer chemoprevention or overcoming CYP1A-associated drug toxicity and resistance. Herein, we reviewed recent advances in the discovery and characterization of hCYP1A inhibitors, from the discovery approaches to structural features and biomedical applications of hCYP1A inhibitors. The inhibition potentials, inhibition modes, and inhibition constants of all reported hCYP1A inhibitors are comprehensively summarized. Meanwhile, the structural features and structure-activity relationships of different classes of hCYP1A1 and hCYP1A2 inhibitors are analyzed and discussed in depth. Furthermore, the major challenges and future directions for this field are presented and highlighted. Collectively, the information and knowledge presented here will strongly facilitate the researchers to discover and develop more efficacious CYP1A inhibitors for specific purposes, such as chemo-preventive agents or as tool molecules in hCYP1A-related fundamental studies.

Lower sertraline plasma concentration in patients co-medicated with clozapine-Implications for pharmacological augmentation strategies in schizophrenia

Augmentation of antipsychotic treatment with antidepressants represents a common and beneficial treatment strategy in patients suffering from schizophrenia. Combining clozapine and the selective serotonin reuptake inhibitor (SSRI) sertraline represents a clinically important strategy in patients with therapy-resistant schizophrenia, but there is limited knowledge about mutual pharmacokinetic interactions. In the present study, we assessed the impact of clozapine on sertraline plasma concentrations. Based on a therapeutic drug monitoring (TDM) database, sertraline plasma concentrations were compared between two groups: patients receiving a combined treatment with sertraline and clozapine (N = 15) and a matched control group receiving sertraline but no clozapine (N = 17). Group differences with respect to raw and dose-adjusted concentrations were assessed using nonparametric tests. Comedication with clozapine was associated with 67% lower median sertraline plasma concentrations (16 vs. 48 ng/mL; p = .022) and 28% lower median dose-adjusted plasma concentrations (C/D; 0.21 vs. 0.29 (ng/mL)/(mg/day); p = .049) as compared to the control group. Scatter plots revealed a complex relationship between the dosage of clozapine and dose-adjusted sertraline concentrations composed of an initial decrease at clozapine doses below 300 mg, an increase between 300 and 600 mg and a final decrease at 800 mg which was best modeled by a third order polynomial term. Cotreatment with clozapine may lead to reduced sertraline plasma concentrations which may be explained by clozapine-induced gastrointestinal hypo-mobility already present at low doses and cytochrome P450 3A4 inducing properties at high clozapine doses. For this drug combination, clinicians should consider TDM to confirm therapeutically effective plasma concentrations of sertraline.

Precision Medicine in Antidepressants Treatment

Precision medicine uses innovative approaches to improve disease prevention and treatment outcomes by taking into account people's genetic backgrounds, environments, and lifestyles. Treatment of depression is particularly challenging, given that 30-50% of patients do not respond adequately to antidepressants, while those who respond may experience unpleasant adverse drug reactions (ADRs) that decrease their quality of life and compliance. This chapter aims to present the available scientific data that focus on the impact of genetic variants on the efficacy and toxicity of antidepressants. We compiled data from candidate gene and genome-wide association studies that investigated associations between pharmacodynamic and pharmacokinetic genes and response to antidepressants regarding symptom improvement and ADRs. We also summarized the existing pharmacogenetic-based treatment guidelines for antidepressants, used to guide the selection of the right antidepressant and its dose based on the patient's genetic profile, aiming to achieve maximum efficacy and minimum toxicity. Finally, we reviewed the clinical implementation of pharmacogenomics studies focusing on patients on antidepressants. The available data demonstrate that precision medicine can increase the efficacy of antidepressants and reduce the occurrence of ADRs and ultimately improve patients' quality of life.

Population Pharmacokinetics of Clozapine and Norclozapine and Switchability Assessment between Brands in Uruguayan Patients with Schizophrenia

Clozapine (CZP) is an atypical antipsychotic agent commonly used in the treatment of schizophrenia. It is metabolized primarily by CYP1A2 enzyme, yielding a pharmacologically active metabolite, norclozapine (NCZP). Significant intra- and interindividual pharmacokinetic (PK) variability for CZP and NCZP has been observed in routine therapeutic drug monitoring. So the goal of this study was to evaluate the magnitude and variability of concentration exposure to CZP and its active metabolite NCZP on pharmacokinetic parameters in Uruguayan patients with schizophrenia with a focus on covariates such as cigarette smoking, age, sex, caffeine consumption, brands available of CZP, and comedication using population PK (PPK) modeling methodologies. Patients with a diagnosis of schizophrenia treated with brand-name CZP (Leponex®) for more than a year were included in the study. Then these patients were switched to the similar brand of CZP (Luverina®). Morning predose blood samples for determination of CZP and NCZP using a HPLC system equipped with a UV detector were withdrawn on both occasions at steady state and under the same comedication. Ninety-eight patients, 22 women and 76 men, took part in the study. Mean ± standard deviation for CZP and NCZP concentration was 421 ± 262 ng/mL and 275 ± 180 ng/mL, respectively. After covariate evaluation, only smoking status remained significant in CZP apparent clearance, inducing a mean increment of 32% but with no clinical impact. The results obtained with the two brands of CZP should ensure comparable efficacy and tolerability with the clinical use of either product. Smoking was significantly associated with a lower exposure to CZP due to higher clearance. The results obtained with the two brands commercialized in our country hint a bioequivalence scenario in the clinical setting.

Nanofractionation Platform with Parallel Mass Spectrometry for Identification of CYP1A2 Inhibitors in Metabolic Mixtures

With early assessment of inhibitory properties of drug candidates and their circulating metabolites toward cytochrome P450 enzymes, drug attrition, especially later in the drug development process, can be decreased. Here we describe the development and validation of an at-line nanofractionation platform, which was applied for screening of CYP1A2 inhibitors in Phase I metabolic mixtures. With this platform, a metabolic mixture is separated by liquid chromatography (LC), followed by parallel nanofractionation on a microtiter well plate and mass spectrometry (MS) analysis. After solvent evaporation, all metabolites present in the nanofractionated mixture are assayed utilizing a fluorescence CYP1A2 inhibition bioassay performed on the plate. Next, a bioactivity chromatogram is constructed from the bioassay results. By peak shape and retention time correlation of the bioactivity peaks with the obtained MS data, CYP1A2-bioactive inhibiting metabolites can be identified. The method correctly evaluated the potency of five CYP1A2 inhibitors. Mixtures comprising potent inhibitors of CYP1A2 or in vitro-generated metabolites of ellipticine were evaluated for their inhibitory bioactivities. In both cases, good LC separation of all compounds was achieved and bioactivity data could be accurately correlated with the parallel recorded MS data. Generation and evaluation of Phase II metabolites of hydroxylated ellipticine was also pursued.

Variation in the Response of Clozapine Biotransformation Pathways in Human Hepatic Microsomes to CYP1A2- and CYP3A4-selective Inhibitors

The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to conventional antipsychotic drugs. Cytochromes P450 (CYPs) 1A2 and 3A4 oxidize CLZ to norCLZ and CLZ N-oxide in human liver. Concurrent treatment with inducers and inhibitors of CYP1A2 modulates CLZ elimination that disrupts therapy. Drug-drug interactions involving CYP3A4 are also significant but less predictable. To further characterize the factors underlying these interactions, we used samples from a cohort of human livers to assess variation in CLZ oxidation pathways in relation to intrinsic CYP3A4 and CYP1A2 activities and the effects of the corresponding selective inhibitors ketoconazole (0.2 and 2 μM) and fluvoxamine (1 and 10 μM). The CYP3A4-selective inhibitor ketoconazole (2 μM) impaired CLZ N-oxide formation in all 14 of the livers used in inhibition studies (≥50% inhibition) while the CYP1A2-selective inhibitor fluvoxamine (10 μM) decreased norCLZ formation in nine. Ketoconazole effectively inhibited CLZ metabolism in five of seven livers that catalysed CYP3A4-dependent testosterone 6β-hydroxylation at or above the median rate and in four other livers with lower intrinsic CYP3A4 activity. Similarly, fluvoxamine (10 μM) readily inhibited CLZ oxidation in seven livers with high CYP1A2-mediated 7-ethoxyresorufin O-deethylation activity (at or above the median) and three livers with lower intrinsic CYP1A2 activity. In three livers, CLZ biotransformation was impaired by both ketoconazole and fluvoxamine, consistent with a major role for both CYPs. These findings suggest that the intrinsic activities of CYPs 1A2 and 3A4 are unrelated to the response to CYP-selective inhibitors and that assessment of the activities in vivo may not assist the prediction of drug-drug interactions.

The Impact of CYP1A2 and CYP2E1 Genes Polymorphism on Theophylline Response

Theophylline is a medicine with narrow therapeutic index. This implies that a small change in dosage would cause side effects. Theophylline is metabolized by CYP1A2 and CYP2E1. The aim of this review is to know the impact of CYP1A2 and CYP2E1 genes polymorphism on theophylline response. The review was done by searching literature in Pubmed and Science Direct databases with keywords 'polymorphism', 'pharmacogenetic', 'CYP1A2', 'CYP2E1' and 'theophylline'. There were 5 research articles from Pubmed and 65 articles (21 research articles, 23 review articles and 21 book chapters) from Science Direct. The exclusion criteria were - articles discussing about polymorphism but not CYP1A2 or CYP2E1, the ones with a mention of theophylline but not about its metabolism, articles on CYP1A2 and/or 2E1 polymorphism but not on the effect on theophylline. Thus, 33 articles were reviewed due to their suitability. The review discusses the influence of polymorphism of CYP1A2 and CYP2E1 genes on theophylline response.

Anxiolytic effects of fluoxetine and nicotine exposure on exploratory behavior in zebrafish

Zebrafish (Danio rerio) have emerged as a popular model for studying the pharmacology and behavior of anxiety. While there have been numerous studies documenting the anxiolytic and anxiogenic effects of common drugs in zebrafish, many do not report or test for behavioral differences between the sexes. Previous studies have indicated that males and females differ in their baseline level of anxiety. In this study, we test for a sex interaction with fluoxetine and nicotine. We exposed fish to system water (control), 10 mg/L fluoxetine, or 1 mg/L nicotine for three minutes prior to being subjected to four minutes in an open-field drop test. Video recordings were tracked using ProAnalyst. Fish from both drug treatments reduced swimming speed, increased vertical position, and increased use of the top half of the open field when compared with the control, though fluoxetine had a larger effect on depth related behaviors while nicotine mostly affected swimming speed. A significant sex effect was observed where females swam at a slower and more constant speed than males, however neither drug produced a sex-dependent response.

Theophylline Pharmacokinetics in Foetal Sheep: Maternal Metabolic Capacity is the Principal Driver

Understanding theophylline pharmacokinetics (PK) in the foetus is essential to prevent in utero toxicity and optimize prophylactic therapies. Previous studies in pregnancy have been obfuscated by maternal dosing and inadequate sampling in the foetus; both render modelling of foetal PK difficult. Six ewes carrying singleton foetuses received theophylline (60 mg) into the foetal jugular vein. Blood samples were drawn from the foetus and ewe over 36 hr. Serum concentrations were measured. Maternal and foetal pharmacokinetic parameters were estimated. Foetal non-compartmental pharmacokinetic parameters were as follows: half-life 7.37 ± 1.22 hr; volume of distribution 44.62 ± 11.45 L; area under the curve 14.82 ± 2.71 hr/(μg/mL); and clearance 4.15 ± 0.70 L/hr. Rapid theophylline distribution across the placenta was observed. Maternal non-compartmental pharmacokinetic parameters were as follows: half-life 6.54 ± 2.44 hr; volume of distribution 32.48 ± 9.99 L; area under the curve 16.28 ± 4.53 hr/(μg/mL); and clearance 3.69 ± 1.47 L/hr. Foetal and ewe serum concentration-time profiles were fit together into a 3-compartment population pharmacokinetic model, and parameters were as follows: central volume 1.38 ± 0.11 L; 2nd peripheral compartment volume 3.11 ± 0.29 L; 3rd peripheral compartment volume 60.14 ± 6.02 L; elimination clearance 9.89 ± 0.90 L/hr; distribution clearance between central and 2nd compartment 30.87 ± 2.31 L/hr; and distribution clearance between 2nd and 3rd compartments 13.89 ± 1.11 L/hr. Cytochrome P4501A expression was robust in maternal liver; negligible activities were observed in placenta, foetal liver and foetal kidney. In vitro protein binding of theophylline was 30% lower in foetal serum compared to maternal serum (29.7 ± 4.4 versus 42.0 ± 3.6%-bound). Free concentrations were lower in the foetus than in the ewe, suggesting active transport across placenta. In summary, foetal clearance of theophylline is attributable to rapid distribution into the maternal circulation across the placenta followed by greater maternal protein binding and metabolic activity.

Cytochrome P450 family 1 inhibitors and structure-activity relationships

With the widespread use of O-alkoxyresorufin dealkylation assays since the 1990s, thousands of inhibitors of cytochrome P450 family 1 enzymes (P450s 1A1, 1A2, and 1B1) have been identified and studied. Generally, planar polycyclic molecules such as polycyclic aromatic hydrocarbons, stilbenoids, and flavonoids are considered to potentially be effective inhibitors of these enzymes, however, the details of the structure-activity relationships and selectivity of these inhibitors are still ambiguous. In this review, we thoroughly discuss the selectivity of many representative P450 family 1 inhibitors reported in the past 20 years through a meta-analysis.

CYP1A2 polymorphism and theophylline clearance in Korean non-smoking asthmatics

Background

Theophylline is mainly metabolized by cytochrome P450 (CYP) 1A2 and CYP2E1 which show inter-individual variations. However, the underlying mechanism remains unknown in humans. We investigated the relationship between differences in theophylline clearance and genetic polymorphisms in the CYP1A2 and CYP2E1 gene in 89 Korean asthmatic patients.

Methods

Polymerase chain reaction (PCR) was performed on the 5'-flanking region of those genes. PCR products were directly sequenced and confirmed using the SNaP shot method. We determined whether the detected SNPs affected gene transcription using electrophoretic mobility shift assay (EMSA). Theophylline clearance (mL/kg/h) was assessed by using a Bayesian approach.

Results

Genetic polymorphisms were identified at 7 sites in the CYP1A2 gene and at 10 sites in the CYP2E1. Among them, subjects with genotypes (GA+AA) of the -3860G>A polymorphism were found to show higher theophylline clearance than those with genotypes GG (29.11 ± 0.91 mL/kg/h vs. 26.12 ± 0.80 mL/kg/h, p = 0.014). This polymorphic site was revealed to be a protein binding site by conducting EMSA on nuclear hepatocyte extracts.

Conclusion

In conclusion, increased theophylline clearance was significantly related to the -3860G>A polymorphism, which could be associated with increased CYP1A2 inducibility in Korean non-smoking asthmatics.

In vitro-to-in vivo predictions of drug-drug interactions involving multiple reversible inhibitors

INTRODUCTION

Predictions of drug-drug interactions (DDIs) are commonly performed for single inhibitors, but interactions involving multiple inhibitors also frequently occur. Predictions of such interactions involving stereoisomer pairs, parent/metabolite combinations and simultaneously administered multiple inhibitors are increasing in importance. This review provides the framework for predicting inhibitory DDIs of multiple inhibitors with any combination of reversible inhibition mechanism.

AREAS COVERED

The review provides an overview of the reliability of the in vitro determined reversible inhibition mechanism. Furthermore, the article provides a method to predict DDIs for multiple reversible inhibitors that allows substituting the inhibition constant (K(i)) with an inhibitor affinity (IC(50)) value determined at S << K(M).

EXPERT OPINION

A better understanding and the prediction methods of DDIs, resulting from multiple inhibitors, are important. The inhibition mechanism of a reversible inhibitor is often equivocal across studies and unreliable. Determination of the K(i) requires the assignment of reversible inhibition mechanism but in vitro-to-in vivo prediction of DDI risk can be achieved for multiple inhibitors from estimates of the inhibitor affinity (IC(50)) only, regardless of the inhibition mechanism.

Structure, function, regulation and polymorphism and the clinical significance of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a number of clinical drugs (e.g., clozapine, tacrine, tizanidine, and theophylline; n > 110), a number of procarcinogens (e.g., benzo[a]pyrene and aromatic amines), and several important endogenous compounds (e.g., steroids). CYP1A2 is subject to reversible and/or irreversible inhibition by a number of drugs, natural substances, and other compounds. The CYP1A gene cluster has been mapped on to chromosome 15q24.1, with close link between CYP1A1 and 1A2 sharing a common 5'-flanking region. The human CYP1A2 gene spans almost 7.8 kb comprising seven exons and six introns and codes a 515-residue protein with a molecular mass of 58,294 Da. The recently resolved CYP1A2 structure has a relatively compact, planar active site cavity that is highly adapted for the size and shape of its substrates. The architecture of the active site of 1A2 is characterized by multiple residues on helices F and I that constitutes two parallel substrate binding platforms on either side of the cavity. A large interindividual variability in the expression and activity of CYP1A2 has been observed, which is largely caused by genetic, epigenetic and environmental factors (e.g., smoking). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. Induction or inhibition of CYP1A2 may provide partial explanation for some clinical drug interactions. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of them have been associated with altered drug clearance and response and disease susceptibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Drug interactions

Drugs for allergy are often taken in combination with other drugs, either to treat allergy or other conditions. In common with many pharmaceuticals, most such drugs are subject to metabolism by P450 enzymes and to transmembrane transport. This gives rise to considerable potential for drug-drug interactions, to which must be added consideration of drug-diet interactions. The potential for metabolism-based drug interactions is increasingly being taken into account during drug development, using a variety of in silico and in vitro approaches. Prediction of transporter-based interactions is not as advanced. The clinical importance of a drug interaction will depend upon a number of factors, and it is important to address concerns quantitatively, taking into account the therapeutic index of the compound.

Effects of selective serotonin reuptake inhibitors on timolol metabolism in human liver microsomes and cryo-preserved hepatocytes

Timolol has been widely used in the treatment of glaucoma. Topically applied, timolol may cause adverse cardiovascular effects due to systemic absorption through the nasolacrimal duct. Timolol is mainly metabolized by cytochrome P450 2D6 (CYP2D6) in the liver. The aim of the present study was to characterize further the metabolism of timolol in vitro. Especially the effect of several drugs such as selective serotonin reuptake inhibitors on the metabolism of timolol was evaluated. In human liver microsomes, four timolol metabolites were identified, in cryo-preserved hepatocytes nine. In both in vitro experiments, the hydroxy metabolite M1 was the main metabolite. The in vivo half-life predicted for timolol was 3.7 hr. in cryo-preserved hepatocytes, corresponding to the half-life of timolol in humans in vivo. Fluoxetine, paroxetine, sertraline, citalopram and fluvoxamine inhibited the formation of M1 in microsomes with IC(50) values of 1.4, 2.0, 3.5, 21 and 20 microM, respectively. In human cryo-preserved hepatocytes, the IC(50) values for fluoxetine, paroxetine and fluvoxamine were 0.7, 0.5 and 5.9 microM, respectively. In conclusion, compounds known to be potent CYP2D6 inhibitors inhibited timolol metabolism in in vitro experiments. The present results strongly suggest that fluoxetine and paroxetine may significantly affect the metabolism of timolol also in vivo and may thus potentiate the adverse cardiovascular effects of topically administered timolol.

Insights into the substrate specificity, inhibitors, regulation, and polymorphisms and the clinical impact of human cytochrome P450 1A2

Human CYP1A2 is one of the major CYPs in human liver and metabolizes a variety of clinically important drugs (e.g., clozapine, tacrine, tizanidine, and theophylline), a number of procarcinogens (e.g. benzo[a]pyrene and aflatoxin B(1)), and several important endogenous compounds (e.g. steroids and arachidonic acids). Like many of other CYPs, CYP1A2 is subject to induction and inhibition by a number of compounds, which may provide an explanation for some drug interactions observed in clinical practice. A large interindividual variability in the expression and activity of CYP1A2 and elimination of drugs that are mainly metabolized by CYP1A2 has been observed, which is largely caused by genetic (e.g., SNPs) and epigenetic (e.g., DNA methylation) and environmental factors (e.g., smoking and comedication). CYP1A2 is primarily regulated by the aromatic hydrocarbon receptor (AhR) and CYP1A2 is induced through AhR-mediated transactivation following ligand binding and nuclear translocation. To date, more than 15 variant alleles and a series of subvariants of the CYP1A2 gene have been identified and some of they have been associated with altered drug clearance and response to drug therapy. For example, lack of response to clozapine therapy due to low plasma drug levels has been reported in smokers harboring the -163A/A genotype; there is an association between CYP1A2*1F (-163C>A) allele and the risk for leflunomide-induced host toxicity. The *1F allele is associated with increased enzyme inducibility whereas *1C causes reduced inducibility. Further studies are warranted to explore the clinical and toxicological significance of altered CYP1A2 expression and activity caused by genetic, epigenetic, and environmental factors.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

Variation in CYP1A2 activity and its clinical implications: influence of environmental factors and genetic polymorphisms

CYP1A2 is involved in the metabolism of several widely used drugs and endogenous compounds, and in the activation of procarcinogens. Both genetic and environmental factors influence the activity of this enzyme. The current knowledge regarding factors influencing the activity of CYP1A2 is summarized in this review. Substrates, inhibitors and inducers of CYP1A2 activity, as well as phenotyping probes, are discussed. The functional significance and clinical importance of CYP1A2 gene polymorphisms are reviewed and interethnic differences in the distribution of CYP1A2 variant alleles and haplotypes are summarized. Finally, future perspectives for the possible clinical applications of CYP1A2 genotyping are discussed.

Inhibition and induction of human cytochrome P450 enzymes: current status

Variability of drug metabolism, especially that of the most important phase I enzymes or cytochrome P450 (CYP) enzymes, is an important complicating factor in many areas of pharmacology and toxicology, in drug development, preclinical toxicity studies, clinical trials, drug therapy, environmental exposures and risk assessment. These frequently enormous consequences in mind, predictive and pre-emptying measures have been a top priority in both pharmacology and toxicology. This means the development of predictive in vitro approaches. The sound prediction is always based on the firm background of basic research on the phenomena of inhibition and induction and their underlying mechanisms; consequently the description of these aspects is the purpose of this review. We cover both inhibition and induction of CYP enzymes, always keeping in mind the basic mechanisms on which to build predictive and preventive in vitro approaches. Just because validation is an essential part of any in vitro-in vivo extrapolation scenario, we cover also necessary in vivo research and findings in order to provide a proper view to justify in vitro approaches and observations.

Factors affecting drug concentrations and QT interval during thioridazine therapy

The objective of this study was to investigate factors affecting steady-state plasma concentrations of thioridazine. A cross-sectional study of patients receiving chronic thioridazine was employed. Common allelic variants of CYP2D6 and CYP2C19, as well as thioridazine and metabolite concentrations and QTc intervals, were determined. In 97 patients, dose-corrected plasma concentrations (C/Ds) of thioridazine and metabolites were correlated with age but not sex or CYP2C19 genotype. Patients with no functional CYP2D6 alleles (n=9) had significantly higher C/D for thioridazine (P=0.017) and the ring sulfoxide metabolite and a significantly higher thioridazine/mesoridazine ratio compared with those with >/=1 functional CYP2D6 allele (n=82). Smokers had significantly lower C/D for thioridazine, mesoridazine, and sulforidazine and significantly lower thioridazine/ring sulfoxide ratios than non-smokers. QTc interval was not significantly affected by CYP2D6 or CYP2C19 genotypes. Plasma concentrations of thioridazine are influenced by age, smoking, and CYP2D6 genotype, but CYP2D6 genotype does not appear to influence on-treatment QTc interval.

Antidepressant-drug interactions are potentially but rarely clinically significant

The salient pharmacologic features of the selective serotonin reuptake inhibitors (SSRIs) discovered in the late 1980s included an in vitro ability to inhibit various cytochrome P450 enzymes (CYPs). Differences in potency among the SSRIs for CYP inhibition formed the basis of a marketing focus based largely on predictions of in vivo pharmacokinetic drug interactions from in vitro data, conclusions derived from case reports, and the extrapolation of the results of pharmacokinetic studies conducted in healthy volunteers to patients. Subsequently introduced antidepressants have undergone a similar post hoc scrutiny for potential drug-drug interactions. Concern for the untoward consequences of drug interactions led the FDA to publish guidance for the pharmaceutical industry in 1997 recommending that in vitro metabolic studies be conducted early in the drug development process to evaluate inhibitory properties toward the major CYPs. However, the prevalence of clinically significant enzyme inhibition interactions occurring during antidepressant treatment remains poorly defined despite millions of exposures. Although lack of evidence does not equate to evidence of absence, sparse epidemiological and post-marketing surveillance data do not substantiate a conclusion that widespread morbidity results from antidepressant-induced drug interactions. This commentary discusses points of uncertainty and controversy in the field of drug interactions, notes areas where inadequate data exist, and suggests explanations for a low prevalence of serious interactions. The conclusion is drawn that drug interactions from CYP inhibition caused by the newer antidepressants are potentially, but rarely, clinically significant.

Effect of Fluvoxamine and Erythromycin on the Pharmacokinetics of Oral Lidocaine

Lidocaine is metabolized by cytochrome P450 3A4 (CYP3A4) and CYP1A2 enzymes, but inhibitors of CYP3A4 have had only a minor effect on its pharmacokinetics. We studied the effect of co-administration of fluvoxamine (CYP1A2 inhibitor) and erythromycin (CYP3A4 inhibitor) on the pharmacokinetics of lidocaine in a double-blind, randomized, three-way cross-over study. Eight healthy volunteers ingested daily either 100 mg fluvoxamine and placebo, 100 mg fluvoxamine and 1500 mg erythromycin, or their corresponding placebos (control) for five days. On day 6, 1 mg/kg lidocaine was administered orally. Plasma concentrations of lidocaine, monoethylglycinexylidide (MEGX) and 3-hydroxylidocaine (3-OH-lidocaine) were measured for 10 hr. During the fluvoxamine phase the area under the plasma concentration-time curve (AUC) and peak concentration (Cmax) of oral lidocaine were 305% (P<0.001) and 220% (P<0.05) of the control values. During the combination of fluvoxamine and erythromycin, lidocaine AUC was 360% (P<0.001) and Cmax 250% (P<0.05) of those during placebo. Fluvoxamine alone had no statistically significant effect on the half-life of lidocaine (t1/2), but during the combination phase t1/2 (3.8 hr) was significantly longer than during the placebo phase (2.4 hr; P<0.01). Fluvoxamine alone and in the combination with erythromycin decreased MEGX peak concentrations by approximately 50% (P<0.001) and 30% (P<0.01), respectively. We conclude that inhibition of CYP1A2 by fluvoxamine considerably reduces the presystemic metabolism of oral lidocaine and may increase the risk of lidocaine toxicity if lidocaine is ingested. The concomitant use of both fluvoxamine and a CYP3A4 inhibitor like erythromycin may further increase plasma lidocaine concentrations.

Rifampicin is only a weak inducer of CYP1A2-mediated presystemic and systemic metabolism: studies with tizanidine and caffeine

OBJECTIVE

Rifampicin greatly reduces the plasma concentrations of many drugs. Our aim was to characterise the inducibility of cytochrome P450 (CYP) 1A2 by rifampicin, using tizanidine and caffeine as probe drugs for presystemic and systemic CYP1A2-mediated metabolism.

METHODS

In a randomised, 2-phase crossover study, ten healthy volunteers were given a 5-day pretreatment with 600 mg rifampicin or placebo once daily. On day 6, a single 4-mg dose of tizanidine was administered orally. Plasma and urine concentrations of parent tizanidine and several of its metabolites (M-3, M-4, M-5, M-9, M-10) and pharmacodynamic variables were measured up to 24 h. A caffeine test was performed in both phases.

RESULTS

Rifampicin moderately reduced the peak plasma concentration (by 51%; P = 0.002) and area under the plasma concentration-time curve [AUC(0-infinity)] (by 54%; P = 0.009) of parent tizanidine, and had no effect on its half-life. The tizanidine/M-3 and tizanidine/M-4 AUC(0-infinity) ratios were slightly (by 30%; P = 0.014; and by 38%; P = 0.007) decreased by rifampicin. Also, the excretion of metabolites M-3, M-4 and M-5 into urine was reduced (P < 0.005), but that of M-10 was increased (P = 0.008) by rifampicin. Rifampicin reduced the tizanidine/M-10 ratio (by 55%; P = 0.047) but had no significant effect on the other tizanidine/metabolite ratios in urine. The caffeine/paraxanthine ratio was reduced by 23% (P = 0.081) by rifampicin. The effect of rifampicin on the caffeine/paraxanthine ratio correlated significantly with the effect of rifampicin on, for example, the AUC(0-infinity) of tizanidine and the tizanidine/M-3 AUC(0-infinity) ratio. The pharmacodynamic effects of tizanidine were reduced by rifampicin.

CONCLUSIONS

Rifampicin moderately decreases the plasma concentrations of tizanidine. The strong inducing effects of rifampicin on other CYP enzymes, e.g. CYP3A4, may have contributed to the findings, and the inducibility of CYP1A2-mediated presystemic (tizanidine) and systemic (tizanidine, caffeine) metabolism by rifampicin is weak at the most. Compared to CYP3A4 substrate drugs, substrates of CYP1A2 are much less susceptible to drug interactions caused by enzyme inducers of the rifampicin type.

Artemisinin and thiabendazole are potent inhibitors of cytochrome P450 1A2 (CYP1A2) activity in humans

OBJECTIVE

To investigate the likelihood of artemisinin and thiabendazole causing pharmacokinetic interactions involving cytochrome P450 (CYP1A2) in humans given their potent inhibitory effects on the isoform in vitro.

METHODS

Ten healthy volunteers received caffeine (136.5 mg), and after a washout period of 48 h, the volunteers were given a caffeine tablet (136.5 mg) together with thiabendazole (500 mg). After an additional 14 days, the volunteers received caffeine together with artemisinin (500 mg). After each treatment, plasma was obtained up to 24 h post-dose. The plasma concentrations of the drugs were measured by HPLC with UV and MS detection.

RESULTS

Using the ratio of paraxanthine to caffeine after 4 h as an indicator of CYP1A2 activity, thiabendazole and artemisinin inhibited 92 and 66%, respectively, of the enzyme activity in vivo. In addition, the pharmacokinetics of caffeine were altered in the presence of the drugs; increases in AUC(0-24) of 1.6-fold (P < 0.01) and 1.3-fold of caffeine in the presence of thiabendazole and artemisinin respectively were measured. The use of in vitro data to predict the effects of thiabendazole on the formation of paraxanthine yielded good results and underestimated the effects of artemisinin when total plasma concentrations were used. Corrections for protein binding resulted in underestimation of inhibitory effects on CYP1A2.

CONCLUSIONS

Co-administration of thiabendazole or artemisinin with CYP1A2 substrates could result in clinically significant effects. Our results highlight the validity of in vitro data in predicting in vivo CYP inhibition. The formation of paraxanthine seems to be a better indicator of in vivo CYP1A2 activity than caffeine levels.

Theophylline pharmacokinetics: comparison of Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, humanized hCYP1A1_1A2 knock-in mice lacking either the mouse Cyp1a1 or Cyp1a2 gene, and Cyp1(+/+) wild-type mice

OBJECTIVES

Pharmacokinetics of theophylline was investigated in Cyp1(+/+) wild-type mice, Cyp1a1(-/-) and Cyp1a2(-/-) knockout mice, and humanized hCYP1A1_1A2 mice lacking either the mouse Cyp1a1 or Cyp1a2 gene.

METHODS AND RESULTS

Animals received a single dose of theophylline (8 mg/kg i.p.), either alone or pretreated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10 microg/kg i.p.) 24 h prior to theophylline. We found that mouse or human CYP1A2 is the predominant enzyme for theophylline metabolism, the contribution of mouse or human CYP1A1 to theophylline metabolism is negligible, and another TCDD-inducible enzyme plays a minor role in 1-methyluric acid and 1,3-dimethyluric acid formation as well as enhanced theophylline clearance from the body. The half-life of elimination from plasma was more than four times longer in Cyp1a2(-/-) than Cyp1(+/+) mice and more than 10 times different after TCDD pretreatment. In humanized hCYP1A1_1A2 mice lacking the mouse Cyp1a2 gene, the half-life of elimination from plasma was two to three times longer than that in Cyp1(+/+) mice and four to five times different after TCDD pretreatment.

CONCLUSION

Replacement of mouse Cyp1a2 with a functional human CYP1A2 gene restored the ability to metabolize theophylline, and the metabolism changed to a humanized profile (i.e. 3-methylxanthine formation, not seen in the wild-type mouse). TCDD-pretreated hCYP1A1_1A2 Cyp1a2(-/-) mice exhibited enhanced theophylline metabolism and clearance, due to induction of the human CYP1A2 enzyme. Comparing the hCYP1A1_1A2 Cyp1a2(-/-) and wild-type mice with published clinical studies, we found theophylline clearance to be about 5 times and 12 times, respectively, greater than that reported in humans.

Possible involvement of organic anion transporter 2 on the interaction of theophylline with erythromycin in the human liver

Organic anion transporter 2 (Oat2 [SLC22A7]) is a multispecific organic anion transporter. Although several substrates of human Oat2 (hOat2) have been elucidated, a possible involvement of hOat2 in drug interaction is less defined. The purpose of this study was to investigate the interaction of theophylline with erythromycin mediated by hOat2 using a Xenopus laevis oocyte expression system. When expressed in Xenopus oocytes, hOat2 mediated the transport of theophylline and erythromycin. The finding indicates that the two compounds are novel substrates for hOat2. The apparent K(m) values for the uptake of hOat2 that mediated the transport of theophylline and erythromycin were 12.6 muM and 18.5 muM, respectively. The hOat2-mediated uptake of [(14)C]theophylline and [(14)C]erythromycin was cis-inhibited by adding erythromycin and theophylline, respectively. Our present findings suggest that hOat2 may, at least in part, be involved in the theophylline-erythromycin interaction in the human liver.

Inhibition of Theophylline Metabolism by Suplatast and Its Metabolites in Rats

The inhibitory effect of suplatast (ST), an anti-allergic drug, on theophylline (TP) metabolism was investigated in rats in vivo and in vitro. Intravenous injection of aminophylline (AP) at 10 mg/kg of TP equivalent was performed with or without pretreatment by oral administration of 100 mg/kg of ST 2.5 h prior to AP. In the ST-pretreated group, plasma concentration (Cp), the area under Cp-time profile (AUC) and urinary excretion of TP increased significantly, and urinary excretion of TP metabolites, 1,3-dimethyluric acid (DMU) and 1-methyluric acid (1MU) decreased significantly. Metabolic clearance of DMU (CL(DMU)) and that of 1MU (CL(1MU)) were remarkably suppressed by ST pretreatment, however, renal clearance (CLr) of TP did not change. To compare the inhibitory effect of ST on TP metabolism with that of its main metabolite (M1) in vivo, a concomitant intravenous injection of AP (10 mg/kg of TP equivalent) with ST or M1 (40 mg/kg of ST equivalent) was carried out. In the M1 group, Cp and AUC of TP increased significantly, and the total body clearance of TP decreased significantly. In contrast, ST did not induce these changes. Then, the inhibitory effect of ST and M1 on TP metabolism in vitro was evaluated using rat-liver microsomes. ST and M1 suppressed DMU formation in a competitively inhibitory manner, and their equilibrium dissociation constants (Ki) were 822 and 731 microM, respectively. In conclusion, inhibition of TP metabolism by ST was demonstrated in vivo and in vitro, and the involvement of M1 and/or other metabolites in this drug interaction was suggested.

Dose-dependent alternations in the pharmacokinetics of olanzapine during coadministration of fluvoxamine in patients with schizophrenia

Olanzapine, an atypical antipsychotic agent, is a substrate of the cytochrome P4501A2 (CYP1A2) enzyme. Administration of a potent CYP1A2 inhibitor (eg, fluvoxamine) may alter the pharmacokinetics of olanzapine. This study investigated the pharmacokinetic interactions between olanzapine and fluvoxamine in patients with schizophrenia. Ten male smokers were administrated a single dose of olanzapine 10 mg at baseline, followed by 2 weeks of fluvoxamine 50 mg/day and another 2 weeks of fluvoxamine 100 mg/day. Olanzapine 10 mg was given at day 10 during each fluvoxamine treatment. After pretreatment with fluvoxamine, the area under the curve, maximal plasma concentration, and half-time of olanzapine were significantly increased by 30% to 55%, 12% to 64%, and 25% to 32%, respectively. Volume of distribution and apparent clearance were significantly reduced by 4% to 26% and 26% t O 38%, respectively, after administration of fluvoxamine. Increases in area under the plasma concentration-time curve from time 0 to infinity appear to be dose dependent. Presumably, altered olanzapine pharmacokinetics are attributed to the inhibition of CYP1A2. Patients treated with the combination of olanzapine and fluvoxamine should be monitored carefully.

Lack of pharmacologic interaction between paroxetine and alprazolam at steady state in healthy volunteers

This investigation aimed to provide evidence on the lack of pharmacokinetic interaction of paroxetine (20 mg/d) and alprazolam (1 mg/d) in combined therapy. In addition, the central effects of both drugs when administered alone and in combination were assessed to rule out any relevant synergistic depressant central effect. Twenty-five healthy young adult volunteers participated in a double-blind, double-dummy, placebo-controlled, repeated dose (15 days), 4-period crossover study. Each subject received each of 4 treatment sequences (ie, paroxetine-alprazolam placebo, alprazolam-paroxetine placebo, paroxetine-alprazolam, and paroxetine placebo-alprazolam placebo) in randomized order. The ratios for area under the curve within a dosing interval and maximum plasma concentration of the paroxetine plus alprazolam sequence to single agent paroxetine were 1.07 (90% confidence interval = 0.99 to 1.16) and 1.05 (90% confidence interval = 0.97 to 1.13), respectively, with no statistically significant differences between the 2 treatments. Similarly, for alprazolam, ratios for the combined to the single treatment sequence were 0.99 (90% confidence interval = 0.93 to 1.05) and 1.00 (90% confidence interval = 0.94 to 1.07) for area under the curve within a dosing interval and maximum plasma concentration, respectively, showing no evidence for interaction. Comparative pharmacodynamics on the combination was assessed using 6 Psychomotor Performance Tests and 5 Visual Analogue Scales focused on mood variables. Alprazolam and paroxetine plus alprazolam induced similar and significant performance impairment and sedation after both single and repeated dose administration, being less evident on day 15. After dosing, paroxetine plus alprazolam showed a lower recovery pattern than alprazolam alone, especially on day 15. No treatment sequence showed cumulative effects after repeated dose administration. Psychomotor Performance Tests and Visual Analogue Scales data suggested lack of pharmacodynamic interactions. Accordingly, study results showed no evidence for pharmacologic interactions between paroxetine and alprazolam at steady state. The most commonly reported adverse event was drowsiness, with a higher incidence under both single and combined alprazolam treatments.

Mechanism-based inhibition of human liver microsomal cytochrome P450 1A2 by zileuton, a 5-lipoxygenase inhibitor

Zileuton, a 5-lipoxygenase inhibitor, was evaluated as an inhibitor of cytochrome P450 activity in human liver microsomes. In the absence of preincubation, the racemate was found to be a weak inhibitor (IC50 > 100 microM) of phenacetin O-deethylation (POD) (CYP1A2), paclitaxel 6alpha-hydroxylation (CYP2C8), diclofenac 4'-hydroxylation (CYP2C9), (S)-mephenytoin 4'-hydroxylation (CYP2C19), bufuralol 1'-hydroxylation (CYP2D6), testosterone 6beta-hydroxylation (CYP3A4), chlorzoxazone 6-hydroxylation (CYP2E1), and bupropion hydroxylation (CYP2B6). When preincubated with NADPH-fortified human liver microsomes in the absence of substrate, zileuton (racemate) was shown to inhibit POD. The effect was NADPH-, time-, and concentration-dependent, and was characterized by a kinact (maximal rate of enzyme inactivation) and apparent KI(inhibitor concentration that supports half the maximal rate of inactivation) of 0.035 min(-1) and 117 microM, respectively (kinact/KIratio of 0.0003 min-1 microM(-1)). Preincubation-dependent inhibition of POD activity was also observed with the individual (S)-(-)- and (R)-(+)-enantiomers of zileuton [(S)-(-)-zileuton; kinact, 0.037 min(-1), KI, 98.2 microM, kinact/KIratio, 0.0004 min(-1) microM(-1); (R)-(+)-zileuton; kinact, 0.012 min(-1), KI, 66.6 microM, kinact/KIratio, 0.0002 min(-1) microM(-1)]. In addition, the inhibition of CYP1A2 was not reversed in the presence of reduced glutathione, catalase, and superoxide dismutase and was refractory to dialysis. Therefore, zileuton was characterized as a mechanism-based inhibitor of human liver microsomal CYP1A2. Mechanism-based inhibition of CYP1A2 may explain why zileuton decreases the oral clearance of antipyrine, propranolol, (R)-warfarin, and theophylline, at doses that have a minimal effect on the pharmacokinetics of (S)-warfarin, phenytoin, and terfenadine.

Comparison of in vitro and in vivo inhibition potencies of fluvoxamine toward CYP2C19

A previous study suggested that fluvoxamine inhibition potency toward CYP1A2 is 10 times greater in vivo than in vitro. The present study was designed to determine whether the same gap exists for CYP2C19, another isozyme inhibited by fluvoxamine. In vitro studies examined the effect of nonspecific binding on the determination of inhibition constant (K(i)) values of fluvoxamine toward CYP2C19 in human liver microsomes and in a cDNA-expressed microsomal (Supersomes) system using (S)-mephenytoin as a CYP2C19 probe. K(i) values based on total added fluvoxamine concentration (K(i,total)) and unbound fluvoxamine concentration (K(i,ub)) were calculated, and interindividual variability in K(i) values was examined in six nonfatty livers. K(i,total) values varied with microsomal protein concentration, whereas the corresponding K(i,ub) values were within a narrow range (70-80 nM). In vivo inhibition constants (K(i)iv) were obtained from a study of the disposition of a single oral dose (100 mg) of the CYP2C19 probe (S)-mephenytoin in 12 healthy volunteers receiving fluvoxamine at 0, 37.5, 62.6, and 87.5 mg/day to steady state. In this population, the ratio of (S)-4-hydroxy-mephenytoin formation clearances (uninhibited/inhibited) was positively correlated with fluvoxamine average steady-state concentration with an intercept of 0.85 (r(2) = 0.88, p < 0.001). The mean (+/-S.D.) values of K(i)iv based on total and unbound plasma concentrations were 13.5 +/- 5.6 and 1.9 +/- 1.1 nM, respectively. Comparison of in vitro and in vivo K(i) values, based on unbound fluvoxamine concentrations, suggests that fluvoxamine inhibition potency is roughly 40 times greater in vivo than in vitro.

Clinical pharmacokinetics and pharmacodynamics of repaglinide

Repaglinide is a novel, fast-acting prandial oral hypoglycaemic agent developed for the treatment of patients with type 2 diabetes whose disease cannot be controlled by diet and exercise alone. Although repaglinide binds to the sulphonylurea binding sites on pancreatic beta-cells and has a similar mechanism of action, repaglinide exhibits distinct pharmacological properties compared with these agents. Following administration, repaglinide is absorbed rapidly and has a fast onset of dose-dependent blood-glucose lowering effect. The drug is eliminated rapidly via the biliary route, without accumulation in the plasma after multiple doses. Repaglinide is well tolerated in patients with type 2 diabetes, including elderly patients and patients with hepatic or renal impairment. The pharmacokinetic profile of repaglinide and the improvements in post-prandial hyperglycaemia and overall glycaemic control make repaglinide suitable for administration preprandially, with the opportunity for flexible meal arrangements, including skipped meals, without the risk of hypoglycaemia.

The interindividual differences in the 3-demthylation of caffeine alias CYP1A2 is determined by both genetic and environmental factors

This study investigated the role of genetic factors (CYP1A2) in caffeine metabolism. The CYP1A2 activity was determined in 378 Danish twins following oral intake of a single dose of 200 mg caffeine and subsequent determination of the caffeine ratio (AFMU+1MU+1MX)/17DMU in a 6-h urine sample. The mean (+/- SD) caffeine ratio was 5.9 +/- 3.4. The caffeine ratio was statistically significantly higher in men compared to women, in smoking men and women compared to non-smoking persons of the same gender and in women not taking oral contraceptives compared with women on oral contraceptives. Thus, we confirmed that CYP1A2 is more active in men than in women, that it is induced by smoking and inhibited by oral contraceptives. In the subsequent analysis of heritability, we included 49 monozygotic twin pairs and 34 same gender dizygotic twin pairs concordant for non-smoking and non-use of oral contraceptives. The intraclass correlation coefficient was 0.798 (95% confidence interval, 0.696-0.900) and 0.394 (95% confidence interval, 0.109-0.680) in the monozygotic and dizygotic twins, respectively. The correlation was statistically significantly higher (P = 0.0015) in the former compared with the latter. A biometrical model for the caffeine ratio including only additive genetic factors and unique environmental factors was the overall best fitting model. Estimates based on this model gave a heritability estimate of 0.725 (95% confidence interval 0.577-0.822). Unique environmental effects seem to account for the remainder 0.275 (95% confidence interval, 0.178-0.423). Our study shows that the CYP1A2 activity is mainly governed by genetic factors.

The interaction of medications used in palliative care

Palliative care uses several classes of drugs, which are handled by the CYP P450 system. Interaction of drugs in this setting requires ongoing vigilance by the physician. Phenocopying may be more common than previously realized.

Spotlight on fluvoxamine in anxiety disorders in children and adolescents

Fluvoxamine is a selective serotonin reuptake inhibitor (SSRI) which may be used for the management of anxiety disorders in children and adolescents. Absorption of fluvoxamine was similar in adolescents to that in adults, which suggests that the maximum dosage of the drug for patients aged between 12 and 17 years can be as high as 300 mg/day. However, steady-state plasma fluvoxamine concentrations were 2 to 3 times higher in children (aged between 6 and 11 years) than in adolescents; thus, the maximum fluvoxamine dosage recommended for children is 200 mg/day. Fluvoxamine (50 to 300 mg/day) for 8 to 16 weeks significantly reduced symptoms of obsessive-compulsive disorder (OCD) [measured across multiple assessment scales] compared with placebo in a well controlled trial in paediatric patients (n = 120) or from baseline in noncomparative trials in adolescent (n = 20) or paediatric (n = 16) patients. Improvements with fluvoxamine (up to 200 mg/day) were observed for up to 1 year in 98 patients with OCD in a noncomparative trial. The drug (up to 250 or 300 mg/day) also improved symptoms of anxiety compared with placebo in an 8-week well controlled trial in 128 paediatric patients with social phobia, separation anxiety disorder or generalised anxiety disorder (GAD). Fluvoxamine (50 to 300 mg/day) appears to be well tolerated in paediatric patients, with most adverse events with the drug (except abdominal discomfort, which occurred more often in patients receiving fluvoxamine) occurring with a similar incidence to those with placebo. The most common adverse events involved the CNS or gastrointestinal system. Most adverse events reported by paediatric patients with OCD were similar to those reported by adults. In conclusion, fluvoxamine is generally well tolerated and has demonstrated short-term efficacy compared with placebo in the treatment of OCD, and social phobia, separation anxiety disorder or GAD in well controlled trials in paediatric patients. Reductions in symptoms of anxiety with fluvoxamine have been observed for up to 1 year in children and adolescents with OCD. However, there are currently no comparative trials of fluvoxamine with other pharmacological agents. In the absence of such trials, current consensus opinion recommends that when pharmacotherapy is indicated, fluvoxamine, like other SSRIs, can be used as first-line treatment for anxiety disorders, particularly OCD, in paediatric patients. However, direct comparisons are required to assess the relative efficacy and tolerability of pharmacological agents in order to make firm recommendations for the treatment of anxiety disorders in this patient group.

Fluvoxamine: a review of its therapeutic potential in the management of anxiety disorders in children and adolescents

Fluvoxamine is a selective serotonin reuptake inhibitor (SSRI) which may be used for the management of anxiety disorders in children and adolescents. Absorption of fluvoxamine was similar in adolescents to that in adults, which suggests that the maximum dosage of the drug for patients aged between 12 and 17 years can be as high as 300 mg/day. However, steady-state plasma fluvoxamine [corrected] concentrations were 2 to 3 times higher in children (aged between 6 and 11 years) than in adolescents; thus, the maximum fluvoxamine dosage recommended for children is 200 mg/day. Fluvoxamine (50 to 300 mg/day) for 8 to 16 weeks significantly reduced symptoms of obsessive-compulsive disorder (OCD) [measured across multiple assessment scales] compared with placebo in a well controlled trial in paediatric patients (n = 120) or from baseline in noncomparative trials in adolescent (n = 20) or paediatric (n = 16) patients. Improvements with fluvoxamine (up to 200 mg/day) were observed for up to 1 year in 98 patients with OCD in a noncomparative trial. The drug (up to 250 or 300 mg/day) also improved symptoms of anxiety compared with placebo in an 8-week well controlled trial in 128 paediatric patients with social phobia, separation anxiety disorder or generalised anxiety disorder (GAD). Fluvoxamine (50 to 300 mg/day) appears to be well tolerated in paediatric patients, with most adverse events with the drug (except abdominal discomfort, which occurred more often in patients receiving fluvoxamine) occurring with a similar incidence to those with placebo. The most common adverse events involved the central nervous system or gastrointestinal system. Most adverse events reported by paediatric patients with OCD were similar to those reported by adults. In conclusion, fluvoxamine is generally well tolerated and has demonstrated short-term efficacy compared with placebo in the treatment of OCD, and social phobia, separation anxiety disorder or GAD in well controlled trials in paediatric patients. Reductions in symptoms of anxiety with fluvoxamine have been observed for up to 1 year in children and adolescents with OCD. However, there are currently no comparative trials of fluvoxamine with other pharmacological agents. In the absence of such trials, current consensus opinion recommends that when pharmacotherapy is indicated, fluvoxamine, like other SSRIs, can be used as first-line treatment for anxiety disorders, particularly OCD, in paediatric patients. However, direct comparisons are required to assess the relative efficacy and tolerability of pharmacological agents in order to make firm recommendations for the treatment of anxiety disorders in this patient group.

The pharmacokinetics, metabolism and urinary detection time of etamiphylline in camels after intramuscular administration

The pharmacokinetics of etamiphylline were determined after an intramuscular (i.m.) dose of 3.5 mg/kg body weight in six healthy camels. Furthermore, the metabolites and drug detection time were evaluated. The data obtained median and (range) were as follows: the terminal elimination half-life (t(1/2 beta), h) was 3.04 (2.03-3.62); apparent total body clearance (Cl/F, L/h/kg) was 1.27 (0.74-2.99); the apparent volume of distribution at steady state (V(ss)/F, L/kg) was 4.94 (3.57-12.54); and renal clearance (Cl(r), L/h/kg) determined in two camels was 0.005 and 0.004, respectively. The detection time of etamiphylline in urine after an i.m. dose of 3.5 mg/kg body weight ranged between 12 and 13 days. Three etamiphylline metabolites were tentatively identified in camels urine: The first one desethyletamiphylline was the main metabolite and resulted from N-deethylation of etamiphylline had a molecular weight of 251, and was detected in urine for about 13-14 days. Theophylline (molecular weight 180) was the second metabolite and resulted from ring N-dealkylation of etamiphylline. It was present in small amounts and was detected for about 5 h after drug administration in urine. The third metabolite, possibly resulted from demethylation of etamiphylline, had a molecular weight of m/z 265, and was present in small amounts and was detected in urine for about 5 h after drug administration.

Pharmacology of antidepressants

Presently in the United States, 21 compounds have been approved by the Food and Drug Administration as antidepressants. Two additional drugs marketed outside the United States as antidepressants have been approved for obsessive-compulsive disorder. Nearly one half of all these compounds became available within the past 12 years, whereas the first antidepressant was available more than 40 years ago. After the clinical aspects of depression are introduced in this article, the pharmacology of the newer generation drugs is reviewed in relationship to the older compounds. The information in this review will help clinicians treat acute depression with pharmacological agents.

Olanzapine: an updated review of its use in the management of schizophrenia

Olanzapine, a thienobenzodiazepine derivative, is a second generation (atypical) antipsychotic agent which has proven efficacy against the positive and negative symptoms of schizophrenia. Compared with conventional antipsychotics, it has greater affinity for serotonin 5-HT2A than for dopamine D2 receptors. In large, well controlled trials in patients with schizophrenia or related psychoses, olanzapine 5 to 20 mg/day was significantly superior to haloperidol 5 to 20 mg/day in overall improvements in psychopathology rating scales and in the treatment of depressive and negative symptoms, and was comparable in effects on positive psychotic symptoms. The 1-year risk of relapse (rehospitalisation) was significantly lower with olanzapine than with haloperidol treatment. In the first double-blind comparative study (28-week) of olanzapine and risperidone, olanzapine 10 to 20 mg/day proved to be significantly more effective than risperidone 4 to 12 mg/day in the treatment of negative and depressive symptoms but not on overall psychopathology symptoms. In contrast, preliminary results from an 8-week controlled study suggested risperidone 2 to 6 mg/day was superior to olanzapine 5 to 20 mg/day against positive and anxiety/depressive symptoms (p < 0.05), although consistent with the first study, both agents demonstrated similar efficacy on measures of overall psychopathology. Improvements in general cognitive function seen with olanzapine treatment in a 1-year controlled study of patients with early-phase schizophrenia, were significantly greater than changes seen with either risperidone or haloperidol. However, preliminary results from an 8-week trial showed comparable cognitive enhancing effects of olanzapine and risperidone treatment in patients with schizophrenia or schizoaffective disorder. Several studies indicate that olanzapine has benefits against symptoms of aggression and agitation, while other studies strongly support the effectiveness of olanzapine in the treatment of depressive symptomatology. Olanzapine is associated with significantly fewer extrapyramidal symptoms than haloperidol and risperidone. In addition, olanzapine is not associated with a risk of agranulocytosis as seen with clozapine or clinically significant hyperprolactinaemia as seen with risperidone or prolongation of the QT interval. The most common adverse effects reported with olanzapine are bodyweight gain, somnolence, dizziness, anticholinergic effects (constipation and dry mouth) and transient asymptomatic liver enzyme elevations. In comparison with haloperidol, the adverse events reported significantly more frequently with olanzapine in > or = 3.5% of patients were dry mouth, bodyweight gain and increased appetite and compared with risperidone, only bodyweight gain occurred significantly more frequently with olanzapine. The high acquisition cost of olanzapine is offset by reductions in other treatment costs (inpatient and/or outpatient services) of schizophrenia. Pharmacoeconomic analyses indicate that olanzapine does not significantly increase, and may even decrease, the overall direct treatment costs of schizophrenia, compared with haloperidol. Compared with risperidone, olanzapine has also been reported to decrease overall treatment costs, despite the several-fold higher daily acquisition cost of the drug. Olanzapine treatment improves quality of life in patients with schizophrenia and related psychoses to a greater extent than haloperidol, and to broadly the same extent as risperidone.

CONCLUSIONS

Olanzapine demonstrated superior antipsychotic efficacy compared with haloperidol in the treatment of acute phase schizophrenia, and in the treatment of some patients with first-episode or treatment-resistant schizophrenia. The reduced risk of adverse events and therapeutic superiority compared with haloperidol and risperidone in the treatment of negative and depressive symptoms support the choice of olanzapine as a first-line option in the management of schizophrenia in the acute phase and for the maintenance of treatment response.

Inhibition of phenytoin hydroxylation in human liver microsomes by several selective serotonin re-uptake inhibitors

Several case reports have indicated that the selective serotonin re-uptake inhibitor (SSRI) fluoxetine increases phenytoin blood levels when given concurrently. The mechanism of this drug-drug interaction has been attributed to inhibition of CYP2C9-catalyzed hydroxylation of phenytoin to its major oxidative metabolite in humans, para-hydroxyphenyl phenyl hydantoin (HPPH). With a bank of human liver microsomes (HLM), four SSRIs (fluoxetine, norfluoxetine, sertraline, and paroxetine) were tested for inhibition of HPPH formation. Initially, the K(m) and V(max) values of phenytoin hydroxylation to HPPH were determined in the individual HLM samples. The average K(m) (n=8) was 9.7+/-2.9 microM. The V(max) varied fivefold, with an average value of 113+/-53 pmol HPPH/min/nmol CYP450. All of the SSRIs inhibited HPPH formation; resulting Ki values were 31.1+/-10.1 microM (fluoxetine) (n=5), 51.1+/-9.4 microM (norfluoxetine) (n=3), 52.2+/-21.5 microM (sertraline) (n=3), and 80.0+/-7.2 microM (paroxetine) (n=3). Sulfaphenazole (10 microM), utilized as a positive control for inhibition of HPPH formation, inhibited phenytoin hydroxylation (>95%) in all HLM samples. Diclofenac hydroxylation to 4'-OH diclofenac, a specific marker for CYP2C9 activity, was determined in HLM1-HLM6 and was highly correlated with HPPH formation in HLM1-HLM6, indicating that phenytoin hydroxylation in human liver microsomes is largely due to CYP2C9. This work presents direct evidence that the effect of fluoxetine on phenytoin blood levels may be explained by inhibition of CYP2C9-catalyzed phenytoin hydroxylation. In light of typical SSRI blood levels observed in patients, this study also suggests that the risk of a SSRI-phenytoin interaction is highest with fluoxetine and norfluoxetine, and less likely with sertraline and paroxetine.

Differential effects of fluvoxamine and other antidepressants on the biotransformation of melatonin

Melatonin, the predominant product of the pineal gland, is involved in the maintenance of diurnal rhythms. Nocturnal blood concentrations of melatonin have been shown to be enhanced by fluvoxamine, but not by other serotonin reuptake inhibitors. Because fluvoxamine is an inhibitor of several cytochrome P450 (CYP) enzymes, the authors studied the biotransformation of melatonin and the effects of fluvoxamine on the metabolism of melatonin in vitro using human liver microsomes and recombinant human CYP isoenzymes. Melatonin was found to be almost exclusively metabolized by CYP1A2 to 6-hydroxymelatonin and N-acetylserotonin with a minimal contribution of CYP2C19. Both reactions were potently inhibited by fluvoxamine, with a Ki of 0.02 microM for the formation of 6-hydroxymelatonin and 0.05 microM for the formation of N-acetylserotonin. Other than fluvoxamine, fluoxetine, paroxetine, citalopram, imipramine, and desipramine were also tested at 2 and 20 microM. Among the other antidepressants, only paroxetine was able to affect the metabolism of melatonin at supratherapeutic concentrations of 20 microM, which did not reach by far the magnitude of the inhibitory potency of fluvoxamine. The authors concluded that fluvoxamine is a potent inhibitor of melatonin degradation. Because this inhibitory action is also found in vivo, fluvoxamine might be used as an enhancer of melatonin, which might offer new therapeutic possibilities of fluvoxamine.

Phenytoin intoxication induced by fluvoxamine

A patient had phenytoin intoxication after administration of fluvoxamine, a selective serotonin reuptake inhibitor. The serum concentration of phenytoin increased dramatically from 16.6 to 49.1 microg/mL when fluvoxamine was coadministered, although the daily dosage of phenytoin and other drugs had not changed. During phenytoin and fluvoxamine treatment, ataxia, a typical side effect of phenytoin, was observed. The genotypes of CYP2C9 and 2C19, the enzymes responsible for phenytoin metabolism, were homozygous for the wild-type alleles (CYP2C9*1/*1 and 2C19*1/ *1). The interaction may be a result of inhibition of both CYP2C9 and 2C19 by fluvoxamine.