Fluvoxamine inhibits the CYP2C19-catalyzed bioactivation of chloroguanide

Triazole antifungal drug interactions-practical considerations for excellent prescribing

Systemic antifungal therapy is critical for reducing the mortality from many invasive and chronic fungal infections. Triazole antifungals are the most frequently prescribed antifungals but require attention to dosing and drug interactions. Nearly 600 severe drug-drug interactions and over 1100 moderate interactions requiring dose modifications are described or anticipated with systemic antifungal agents (see https://www.aspergillus.org.uk/antifungal-drug-interactions/). In this article, we address the common and less common, but serious, drug interactions observed in clinical practice with triazole antifungals, including a group of drugs that cannot be prescribed with all or most triazole antifungals (ivabradine, ranolazine, eplerenone, fentanyl, apomorphine, quetiapine, bedaquiline, rifampicin, rifabutin, sirolimus, phenytoin and carbamazepine). We highlight interactions with drugs used in children and new agents introduced for the treatment of haematological malignancies or graft versus host disease (midostaurin, ibrutinib, ruxolitinib and venetoclax). We also summarize the multiple interactions between oral and inhaled corticosteroids and triazole antifungals, and the strategies needed to optimize the therapeutic benefits of triazole antifungal therapy while minimizing potential harm to patients.

Pharmacogenomic landscape of COVID-19 therapies from Indian population genomes

Aim: Numerous drugs are being widely prescribed for COVID-19 treatment without any direct evidence for the drug safety/efficacy in patients across diverse ethnic populations. Materials & methods: We analyzed whole genomes of 1029 Indian individuals (IndiGen) to understand the extent of drug–gene (pharmacogenetic), drug–drug and drug–drug–gene interactions associated with COVID-19 therapy in the Indian population. Results: We identified 30 clinically significant pharmacogenetic variants and 73 predicted deleterious pharmacogenetic variants. COVID-19-associated pharmacogenes were substantially overlapped with those of metabolic disorder therapeutics. CYP3A4, ABCB1 and ALB are the most shared pharmacogenes. Fifteen COVID-19 therapeutics were predicted as likely drug–drug interaction candidates when used with four CYP inhibitor drugs. Conclusion: Our findings provide actionable insights for future validation studies and improved clinical decisions for COVID-19 therapy in Indians.

Pharmacogenetics of drug-drug interaction and drug-drug-gene interaction: a systematic review on CYP2C9, CYP2C19 and CYP2D6

Currently, most guidelines on drug-drug interaction (DDI) neither consider the potential effect of genetic polymorphism in the strength of the interaction nor do they account for the complex interaction caused by the combination of DDI and drug-gene interaction (DGI) where there are multiple biotransformation pathways, which is referred to as drug-drug-gene interaction (DDGI). In this systematic review, we report the impact of pharmacogenetics on DDI and DDGI in which three major drug-metabolizing enzymes - CYP2C9, CYP2C19 and CYP2D6 - are central. We observed that several DDI and DDGI are highly gene-dependent, leading to a different magnitude of interaction. Precision drug therapy should take pharmacogenetics into account when drug interactions in clinical practice are expected.

Pharmacogenetics of drug oxidation via cytochrome P450 (CYP) in the populations of Denmark, Faroe Islands and Greenland

Denmark, the Faroe Islands and Greenland are three population-wise small countries on the northern part of the Northern Hemisphere, and studies carried out here on the genetic control over drug metabolism via cytochrome P450 have led to several important discoveries. Thus, CYP2D6 catalyzes the 2-hydroxylation, and CYP2C19 in part catalyzes the N-demethylation of imipramine. The phenomenon of phenocopy with regard to CYP2D6 was first described when Danish patients changed phenotype from extensive to poor metabolizers during treatment with quinidine. It was a Danish extensive metabolizer patient that became a poor metabolizer during paroxetine treatment, and this was due to the potent inhibition of CYP2D6 by paroxetine, which is also is metabolized by this enzyme. Fluoxetine and norfluoxetine are also potent inhibitors of CYP2D6, and fluvoxamine is a potent inhibitor of both CYP1A2 and CYP2C19. The bioactivation of proguanil to cycloguanil is impaired in CYP2C19 poor metabolizers. The O-demethylation of codeine and tramadol to their respective my-opioid active metabolites, morphine and (+)-O-desmethyltramadol was markedly impaired in CYP2D6 poor metabolizers compared to extensive metabolizers, and this impairs the hypoalgesic effect of the two drugs in the poor metabolizers. The frequency of CYP2D6 poor metabolizers is 2%-3% in Greenlanders and nearly 15% in the Faroese population. The frequency of CYP2C19 poor metabolizers in East Greenlanders is approximately 10%. A study in Danish mono and dizygotic twins showed that the non-polymorphic 3-N-demethylation of caffeine catalyzed by CYP1A2 is subject to approximately 70% genetic control.

CYP2C19*17 increases clopidogrel-mediated platelet inhibition but does not alter the pharmacokinetics of the active metabolite of clopidogrel

The aim of the present study was to determine the impact of CYP2C19*17 on the pharmacokinetics and pharmacodynamics of the active metabolite of clopidogrel and the pharmacokinetics of proguanil. Thus, we conducted an open-label two-phase cross-over study in 31 healthy male volunteers (11 CYP2C19*1/*1, 11 CYP2C19*1/*17 and nine CYP2C19*17/*17). In Phase A, the pharmacokinetics of the derivatized active metabolite of clopidogrel (CAMD) and platelet function were determined after administration of a single oral dose of 600 mg clopidogrel (Plavix; Sanofi-Avensis, Horsholm, Denmark). In Phase B, the pharmacokinetics of proguanil and its metabolites cycloguanil and 4-chlorphenylbiguanide (4-CPB) were determined in 29 of 31 subjects after a single oral dose of 200 mg proguanil given as the combination drug Malarone (GlaxoSmithKline Pharma, Brondby, Denmark). Significant correlations were found between the area under the time-concentration curve (AUC0-∞ ) of CAMD and both the absolute ADP-induced P2Y12 receptor-activated platelet aggregation (r = -0.60, P = 0.0007) and the percentage inhibition of aggregation (r = 0.59, P = 0.0009). In addition, the CYP2C19*17/*17 and CYP2C19*1/*17 genotype groups had significantly higher percentage inhibition of platelet aggregation compared with the CYP2C19*1/*1 subjects (geometric mean percentage inhibition of 84%, 73% and 63%, respectively; P = 0.014). Neither the absolute ADP-induced P2Y12 receptor-activated platelet aggregation, exposure to CAMD nor the pharmacokinetic parameters of proguanil, cycloguanil and 4-CPB exhibited any significant differences among the genotype groups. In conclusion, carriers of CYP2C19*17 exhibit higher percentage inhibition of platelet aggregation, but do not have significantly lower absolute P2Y12 receptor-activated platelet aggregation or higher exposure to the active metabolite after a single oral administration of 600 mg clopidogrel.

Clinical pharmacokinetics and pharmacodynamics of solifenacin

The succinate salt of solifenacin, a tertiary amine with anticholinergic properties, is used for symptomatic treatment of overactive bladder. Solifenacin peak plasma concentrations of 24.0 and 40.6 ng/mL are reached 3-8 hours after long-term oral administration of a 5 or 10 mg solifenacin dose, respectively. Studies in healthy adults have shown that the drug has high absolute bioavailability of about 90%, which does not decrease with concomitant food intake. Solifenacin has an apparent volume of distribution of 600 L, is 93-96% plasma protein bound, and probably crosses the blood-brain barrier. Solifenacin is eliminated mainly through hepatic metabolism via cytochrome P450 (CYP) 3A4, with about only 7% (3-13%) of the dose being excreted unchanged in the urine. Solifenacin metabolites are unlikely to contribute to clinical solifenacin effects. In healthy adults, total clearance of solifenacin amounts to 7-14 L/h. The terminal elimination half-life ranges from 33 to 85 hours, permitting once-daily administration. Urinary excretion plays a minor role in the elimination of solifenacin, resulting in renal clearance of 0.67-1.51 L/h. Solifenacin does not influence the activity of CYP1A1/2, 2C9, 2D6 and 3A4, and shows a weak inhibitory potential for CYP2C19 and P-glycoprotein in vitro; however, clinical drug-drug interactions with CYP2C19 and P-glycoprotein substrates are very unlikely. Exposure to solifenacin is increased about 1.2-fold in elderly subjects and about 2-fold in subjects with moderate hepatic and severe renal impairment, as well as by coadministration of the potent CYP3A4 inhibitor ketoconazole 200 mg/day. The full therapeutic effects of solifenacin occur after 2-4 weeks of treatment and are maintained upon long-term therapy. Although solifenacin pharmacokinetics display linearity at doses of 5-40 mg, no obvious dose dependency was observed in efficacy and tolerability studies. The efficacy of solifenacin (5 or 10 mg/day) is at least equal to that of extended-release (ER) tolterodine (4 mg/day) in reducing the mean number of micturitions per 24 hours and urgency episodes, and in increasing the volume voided per micturition. Solifenacin (5 mg/day) appears to be superior to ER tolterodine (4 mg/day) in reducing incontinence episodes (mean -1.30 vs -0.90, p = 0.018) and is superior to propiverine (20 mg/day) at the dose of 10 mg/day in reducing urgency (-2.30 vs -2.78, p = 0.012) and nocturia episodes. Based on withdrawal rates due to adverse effects during the 52-week treatment period, solifenacin appears to have better tolerability than immediate-release (IR) oxybutynin 10-15 mg/day and IR tolterodine 4 mg/day. With regard to the pharmacokinetics of solifenacin, and for safety reasons, doses exceeding 5 mg/day are not recommended for patients with moderate hepatic impairment (Child-Pugh score 7-9), patients with severe renal impairment (creatinine clearance <30 mL/min) and subjects undergoing concomitant therapy with CYP3A4 inhibitors.

The effects of human CYP2C8 genotype and fluvoxamine on the pharmacokinetics of rosiglitazone in healthy subjects

AIMS

To determine the effect of CYP2C8 genotype and of fluvoxamine on the pharmacokinetics of rosiglitazone.

METHODS

Twenty-three healthy subjects with the following genotypes were included in a two-phase, open-label, cross-over trial: CYP2C8*3/ *3 (n = 3), CYP2C8*1/ *3 (n = 10) and CYP2C8*1/ *1 (n = 10). In Phase A, the subjects were given 4 mg rosiglitazone as a single oral dose. In Phase B, the subjects were treated with multiple oral doses of 50 mg fluvoxamine maleate for 3 days prior to the single oral administration of 4 mg rosiglitazone. Plasma concentrations of rosiglitazone and relative amounts of N-desmethylrosiglitazone were measured in both phases for 24 h after drug administration.

RESULTS

The pharmacokinetics of rosiglitazone and N-desmethylrosiglitazone were not significantly different between the CYP2C8 genotypic groups. Fluvoxamine caused a statistically significant (P = 0.0066) increase in the AUC(0-infinity) of rosiglitazone, with a geometric mean ratio of 1.21 [95% confidence interval (CI) 1.06-1.39]. The elimination half-life (t(1/2)) was also significantly higher (P = 0.0203) with a geometric mean ratio of 1.38 [95% CI 1.06-1.79]. The coadministration of fluvoxamine had no influence on the pharmacokinetics of N-desmethylrosiglitazone.

CONCLUSION

The importance of the CYP2C8*3 mutation in the in vivo metabolism of rosiglitazone could not be confirmed. Fluvoxamine increased the AUC(0-infinity) and t(1/2) of rosiglitazone moderately and hence may be a weak inhibitor of CYP2C8.

Papel del polimorfismo genético CYP2C19 en los efectos adversos a fármacos y en el riesgo para diversas enfermedades

There are a great number of polymorphic genes in the human genome. Many of them codify enzymes that metabolizes drugs and xenobiotic agents, including carcinogens. Among the better known of them, there are a number of isozymes of the microsomal oxidative system (CYP3A4, CYP2C9, CYP2C19 y CYP2D6). This article reviews the following issues: a) frequency of presentation of the "poor metabolizer" genotype and/or phenotype for substrates of CYP2C19; b) role of CYP2C19 polymorphism on the metabolism of some drugs (mephenytoine and other antiepileptic drugs, proton pump inhibitors, several antidepressants and anxyolitics, the antimalaria aggent proguanyl, and propranolol, among others, use this metabolic pathway), and c) possible role of CYP2C19 polymorphism in the risk for development of neoplasia and other diseases (systemic lupus erythematosus, psoriasis, hip osteonecrosis, Alzheimer's disease, amyotrophic lateral sclerosis, essential tremor).

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.

Consumption of Charcoal-Broiled Meat as an Experimental Tool for Discerning CYP1A2-Mediated Drug Metabolism in vivo

Cytochrome P450 1A2 (CYP1A2) is a major drug-metabolising enzyme. Polycyclic aromatic hydrocarbons, present in high concentrations in tobacco smoke and charcoal-broiled meat, are known to induce CYP1A2. The purpose of the present study was to validate enzyme induction by consumption of charcoal-broiled meat as an experimental tool for discerning CYP1A2-mediated drug metabolism in vivo. Twenty-four healthy, non-smoking men, all extensive metabolisers of sparteine (CYP2D6), participated in the study. All participants were genotyped for a putative CYP1A2-inducibility genotype. In the study diet period charcoal-broiled meat was served for lunch and dinner for five consecutive days. All participants were tested with probe reactions for CYP1A2 (caffeine) and CYP2C19 (proguanil) before and after consuming the study diet. Further, in three subgroups, they were tested with either the CYP1A2-substrate tacrine or probe reactions for CYP3A4 (quinidine) or CYP2C9 (tolbutamide). Neither probe reactions for CYP1A2, CYP2C9, CYP2C19 or CYP3A4 were affected by consumption of charcoal-broiled meat as practised in this study. No modifying role of the CYP1A2-inducibility genotype was evident. A number of experimental limitations are discussed, among them the lack of standardisation of exposure, the timing of phenotyping, and the choice of probe reactions. In conclusion, consumption of charcoal-broiled meat as practised in the present study appears not to be a useful experimental tool for discerning CYP1A2-mediated metabolism in vivo.

Different inhibitory effect of fluvoxamine on omeprazole metabolism between CYP2C19 genotypes

AIMS

Omeprazole is mainly metabolized by the polymorphic cytochrome P450 (CYP) 2C19. The inhibitory effect of fluvoxamine, an inhibitor of CYP2C19 as well as CYP1A2, on the metabolism of omeprazole was compared between different genotypes for CYP2C19.

METHODS

Eighteen volunteers, of whom six were homozygous extensive metabolizers (EMs), six were heterozygous EMs and six were poor metabolizers (PMs) for CYP2C19, participated in the study. A randomized double-blind, placebo-controlled crossover study was performed. All subjects received two six-day courses of either daily 50 mg fluvoxamine or placebo in a randomized fashion with a single oral 40 mg dose of omeprazole on day six in both cases. Plasma concentrations of omeprazole and its metabolites, 5-hydroxyomeprazole, omeprazole sulphone, and fluvoxamine were monitored up to 8 h after the dosing.

RESULTS

During placebo administration, geometric means of peak concentration (C(max)), under the plasma concentration-time curve from 0 to 8 h (AUC(0,8 h)) and elimination half-life (t(1/2)) of omeprazole were 900 ng ml(-1), 1481 ng ml(-1) h, and 0.6 h in homozygous EMs, 1648 ng ml(-1), 4225 ng ml(-1) h, and 1.1 h in heterozygous EMs, and 2991 ng ml(-1), 11537 ng ml(-1) h, and 2.8 h in PMs, respectively. Fluvoxamine treatment increased C(max) of omeprazole by 3.7-fold (95%CI, 2.4, 5.0-fold, P < 0.01) and 2.0-fold (1.4, 2.6-fold, P < 0.01), AUC(0,8 h) by 6.0-fold (3.3, 8.7-fold, P < 0.001) and 2.4-fold (1.7, 3.2-fold, P < 0.01), AUC(0, infinity ) by 6.2-fold (3.0, 9.3-fold, P < 0.01) and 2.5-fold (1.6, 3.4-fold, P < 0.001) and prolonged t((1/2)) by 2.6-fold (1.9, 3.4-fold, P < 0.001) and 1.4-fold (1.02, 1.7-fold, P < 0.05), respectively. However, no pharmacokinetic parameters were changed in PMs. The AUC(0,8 h) ratios of 5-hydroxyomeprazole to omeprazole were decreased with fluvoxamine in homozygous EMs (P < 0.05) and heterozygous EMs (P < 0.01).

CONCLUSIONS

Even a low dose of fluvoxamine increased omeprazole exposure in EMs, but did not increase omeprazole exposure in PMs after a single oral dose of omeprazole. These findings confirm a potent inhibitory effect of fluvoxamine on CYP2C19 activity. The bioavailability of omeprazole might, to some extent, be increased through inhibition of P-glycoprotein during fluvoxamine treatment.

Some Aspects of Genetic Polymorphism in the Biotransformation of Antidepressants

Tricyclic antidepressants are all hydroxylated by cytochrome P450 (CYP) 2D6, but the tertiary amines, amitriptyline, clomipramine and imipramine, are also N-demethylated to the active metabolites, nortriptyline, N-desmethylclomipramine and desipramine, by several CYPs, including the polymorphic CYP2C19, CYP1A2 and CYP3A4. The five selective serotonin reuptake inhibitors, citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline are also oxidised by the CYP enzyme system. Thus, fluoxetine, fluvoxamine and paroxetine are partially metabolised by CYP2D6, citalopram by CYP2C19 and sertraline by at least five different CYPs. Paroxetine and fluoxetine are very potent inhibitors of CYP2D6 while citalopram, fluvoxamine and sertraline are moderate inhibitors of this enzyme. Fluvoxamine is a potent inhibitor of CYP1A2 and CYP2C19 and a moderate inhibitor of CYP2C9. Since the termination of the human genome project, there is no longer a technical hindrance to the identification of all of the genes involved in the clinical response to antidepressants. Research in the future will involve modern technologies and new scientific disciplines, including DNA-micro-array technology and bioinformatics. The research ultimately aims at developing better and safer antidepressants and/or better and safer use of currently available antidepressants.

Pharmacokinetic Factors in the Adverse Cardiovascular Effects of Antipsychotic Drugs

Antipsychotics may cause serious adverse cardiovascular effects, including prolonged QT interval and sudden death. This review considers antipsychotic-induced cardiovascular events from three perspectives: high-risk drugs, high-risk individuals and high-risk drug interactions. Pharmacokinetic drug interactions involving the cytochrome P450 (CYP) enzymatic pathway and pharmacodynamic interactions leading to direct cardiotoxic effects are discussed. Original reports on antipsychotic-induced drug interactions are reviewed, with consideration of management guidelines. The literature was reviewed from 1 January 1966 to 1 February 2002. The literature search revealed only 12 original articles published on antipsychotic drug interactions leading to cardiovascular adverse events. Only 4 of the 12 reports were prospective studies; the remainder were either retrospective or anecdotal.Although poor study designs preclude a definitive statement, it appears that pharmacokinetic interactions primarily involved the CYP2D6 and CYP3A4 enzymatic pathways. Those involving the CYP2D6 isozyme included interactions with tricyclic antidepressants, selective serotonergic reuptake inhibitors and beta-blockers. Among these drug interactions, tricyclic antidepressants were most likely to reach clinical significance because of their limited therapeutic index. Drug interactions related to the CYP3A4 pathway were generally less severe, and involved high-potency antipsychotics coadministered with inhibitors such as clarithromycin. Strategies are discussed for the management of adverse cardiovascular events related to antipsychotic drug interactions, including the use of an algorithm. Large, randomised, placebo-controlled studies with strict inclusion criteria are needed to determine the role that antipsychotics play in QT prolongation and sudden death.

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.

Polymorphic CYP2C19 and N-acetylation: human variability in kinetics and pathway-related uncertainty factors

CYP2C19-mediated oxidation and N-acetylation constitute major phase I and phase II polymorphic pathways of xenobiotic metabolism in humans. Analysis of human variability in kinetics for these pathways has been carried out for compounds metabolised extensively (>60%) by these routes. Data for minor substrates for CYP2C19 metabolism (10-60%) have also been analysed. Published pharmacokinetic studies (after oral and intravenous dosing) in CYP2C19 non-phenotyped healthy adults (NPs), and phenotyped extensive (EMs), slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve) and primarily to acute exposure (peak concentration). Similar analyses were performed for the N-acetylation pathway using data for fast acetylators (FA) and slow acetylators (SA). Interindividual variability in the kinetics of CYP2C19 substrates after oral dosage was greater in EMs than in NPs (60 vs 43% for clearances and 54 vs 45% for Cmax). Lower variability was found for N-acetylation for both phenotypes (32 and 22% for FA and SA, respectively). The internal dose of CYP2C19 substrates in PM subjects would be 31-fold higher than in EMs, while for N-acetylated substrates there was a three-fold difference between SA and FA subjects. Pathway-related uncertainty factors were above the default safety factor of 3.16 for most subgroups and values of 52 and 5.2 would be necessary to cover to the 99th centile of the poor metaboliser phenotype for CYP2C19 and N-acetylation, respectively. An exponential relationship (R(2)=0.86) was found between the extent of CYP2C19 metabolism and the difference in internal dose between EMs and PMs. The kinetic default factor (3.16) would cover PMs for substrates for which CYP2C19 was responsible for up to 20-30% of the metabolism in EMs.

Enzyme kinetics for the formation of 3-hydroxyquinine and three new metabolites of quinine in vitro; 3-hydroxylation by CYP3A4 is indeed the major metabolic pathway

The formation kinetics of 3-hydroxyquinine, 2'-quininone, (10S)-11-dihydroxydihydroquinine, and (10R)-11-dihydroxydihydroquinine were investigated in human liver microsomes and in human recombinant-expressed CYP3A4. The inhibition profile was studied by the use of different concentrations of ketoconazole, troleandomycin, and fluvoxamine. In addition, formation rates of the metabolites were correlated to different enzyme probe activities of CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4 in microsomes from 20 human livers. Formation of 3-hydroxyquinine had the highest intrinsic clearance in human liver microsomes (mean +/- S.D.) of 11.0 +/- 4.6 micro l/min/mg. A markedly lower intrinsic clearance, 1.4 +/- 0.7, 0.5 +/- 0.1, and 1.1 +/- 0.2 micro l/min/mg was measured for 2'-quininone, (10R)-11-dihydroxydihydroquinine and (10S)-11-dihydroxydihydroquinine, respectively. Incubation with human recombinant CYP3A4 resulted in a 20-fold higher intrinsic clearance for 3-hydroxyquinine compared with 2'-quininone formation whereas no other metabolites were detected. The formation rate of 3-hydroxyquinine was completely inhibited by ketoconazole (1 micro M) and troleandomycin (80 micro M). Strong inhibition was observed on the formation of 2'-quininone whereas the formation of (10S)-11-dihydroxydihydroquinine was partly inhibited by these two inhibitors. No inhibition on the formation of (10R)-11-dihydroxydihydroquinine was observed. There was a significant correlation between the formation rates of quinine metabolites and activities of the CYP3A4 selected marker probes. This in vitro study demonstrates that 3-hydroxyquinine is the principal metabolite of quinine and CYP3A4 is the major enzyme involved in this metabolic pathway.

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.

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.

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.

Analysis of variation in plasma concentrations of nelfinavir and its active metabolite M8 in HIV-positive patients

OBJECTIVE

To characterize sources of variation in plasma concentrations of nelfinavir and its active metabolite M8 and to evaluate the use of therapeutic drug monitoring for nelfinavir treatment.

METHODS

Plasma samples and patient's characteristics were obtained from outpatient clinic. Differences between groups of patients were studied by comparing the observed plasma concentrations with the corresponding concentration on a pharmacokinetic population curve based on median plasma levels.

RESULTS

Plasma samples (618) were available from 355 patients taking 1250 mg nelfinavir twice daily. The median ratio between M8 and nelfinavir concentrations was 0.29. This ratio appeared to be independent of the time after ingestion. Statistically significantly lower M8 concentrations were found in Black and Asian patients, or when comedication with CYP3A4 inducers was used. Coadministration of CYP2C19 inhibitors, such as omeprazole, decreased the median M8/nelfinavir ratio. Nevertheless, nelfinavir concentrations and summed concentrations of nelfinavir and M8 were only marginally affected in these patients. Diarrhoea was identified as a cause for lower nelfinavir concentrations, without changing the M8/nelfinavir ratio. In a number of patients with suspected therapy failure or intoxication, abnormal nelfinavir plasma concentrations were found. Dose adjustments based on nelfinavir plasma levels were helpful in a number of patients.

CONCLUSION

This study shows that the total concentration of nelfinavir and M8 together is not significantly influenced when variation in M8 levels occurs. Consequently, measuring M8 concentrations in addition to nelfinavir concentrations is not required for the purpose of therapeutic drug monitoring for this drug.

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.

Fluvoxamine as an adjunctive agent in schizophrenia

Schizophrenia is a common mental disorder that has an early onset and rates high as a cause of medical disability. Antipsychotic agents are the mainstay of treatment but response is often inadequate. Negative symptoms (disturbances in volition, social interaction and affective functions) are particularly difficult to treat and form a major obstacle to rehabilitation. A promising approach to improve response of negative symptoms has been to add a selective serotonin reuptake inhibitor (SSRI) antidepressant to antipsychotic treatment. This review examines evidence pertaining to the efficacy, tolerability, and safety of the SSRI fluvoxamine, combined with antipsychotic agents, in the treatment of negative symptoms in schizophrenia. Important methodological issues, such as differentiating primary and secondary negative symptoms, are discussed. The balance of available evidence indicates that fluvoxamine can improve primary negative symptoms in chronic schizophrenia patients treated with typical antipsychotics and suggests that it may also do so in some patients treated with clozapine. This combination is generally safe and well tolerated although, as antipsychotic drug concentrations may be elevated, attention to dose and drug monitoring should be considered appropriately. Combination with clozapine may require particular caution because of potential toxicity if serum clozapine levels rise steeply. The fluvoxamine doses effective in augmentation are lower than those usually used to treat depression. Evidence regarding the use of fluvoxamine augmentation to treat phenomena, such as obsessions and aggression, which may be associated with schizophrenia, is also examined. An important goal of future studies will be to define which patient groups can benefit from combined treatment.

Selecting a Selective Serotonin Reuptake Inhibitor: Clinically Important Distinguishing Features

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed to treat depression. Although these drugs presumably have the same mechanism of action, they vary in several clinically important ways, including how long they remain in the body and the extent to which they interfere with the metabolism of other medications. This article reviews the pharmacologic differences among SSRIs and how these differences may affect various aspects of treatment, such as dosing, administration, and discontinuation. Understanding the distinct properties of SSRIs may help primary care physicians to design the most appropriate therapeutic plan for individual patients.

Effect of selective serotonin reuptake inhibitors on the oxidative metabolism of propafenone: in vitro studies using human liver microsomes

Propafenone is mainly metabolized by CYP2D6 to form 5-hydroxypropafenone (5-OHP) and to a minor extent by CYP1A2 and CYP3A4 to form N-depropylpropafenone (N-DPP). The in vitro inhibitory effect of selective serotonin reuptake inhibitors (SSRIs) on the formation of both metabolites was studied, using human liver microsomes. The 5-OHP formation from racemic propafenone and from its individual enantiomers followed one-enzyme Michaelis-Menten kinetics. Incubation with the racemate yielded a mean Vmax of 64 pmol x min(-1) x mg(-1) and a mean Km of 0.12 microM (N = 3). Stereoselectivity in Vmax and Km values was observed, with (S)-propafenone displaying higher Km and Vmax values. N-DPP formation from racemic propafenone followed one-enzyme Michaelis-Menten kinetics and yielded a mean Vmax of 403 pmol x min(-1) x mg(-1) and a mean Km of 116 microM (N = 3). No stereoselectivity in propafenone N-dealkylation was observed. The influence of SSRIs and quinidine, a prototypical CYP2D6 inhbitor, on propafenone 5-hydroxylation was investigated. Quinidine was the most potent inhibitor, followed by fluoxetine, norfluoxetine, and paroxetine. Sertraline, desmethylsertraline, and fluvoxamine had only a moderate inhibitory effect, whereas citalopram displayed slight or no inhibition when racemic propafenone was used as substrate. Mean Ki values of quinidine, fluoxetine, norfluoxetine, and paroxetine were 0.13, 0.33, 0.55, and 0.54 microM, respectively (N = 3). Quinidine and paroxetine were also tested as inhibitors using the individual enantiomers, but no stereoselectivity was observed. Among the SSRIs tested, only fluvoxamine substantially inhbited propafenone N-dealkylation with a mean IC50 of 7.0 microM (N = 3). There was a more pronounced inhibitory effect of fluvoxamine on (R)-propafenone than on (S)-propafenone N-dealkylation. In conclusion, these in vitro data suggest that an in vivo interaction between propafenone and the SSRIs, fluoxetine and paroxetine, can be expected, which can lead to clinically relevant beta-blockade and an increased risk of side effects in the central nervous system. An interaction with fluvoxamine may be of importance in poor metabolizers for CYP2D6.

Is therapeutic drug monitoring a case for optimizing clinical outcome and avoiding interactions of the selective serotonin reuptake inhibitors?

The selective serotonin reuptake inhibitors (SSRIs) comprise citalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline and they differ from each other in chemical structure, by pharmacokinetic properties and, most importantly, with respect to enzyme-specific metabolism and interactions. Citalopram is administered as a racemic mixture. The drug is oxidated to desmethylcitalopram in the liver, partially by CYP2C19 and partially by CYP3A4. Fluoxetine is administered as a racemate of R- and S-fluoxetine. Both R- and S-fluoxetine are metabolized by CYP2D6 to the active metabolites R- and S-norfluoxetine. Fluvoxamine is metabolized to inactive metabolites by CYP1A2 and CYP2D6. Paroxetine is metabolized to inactive metabolites partially by CYP2D6, and accordingly the metabolism of paroxetine is dependent on the genetic polymorphism of CYP2D6. Sertraline is metabolized to desmethylsertraline, probably by CYP3A4. Several analytical methods have been described for all SSRIs. Most assays are based on separation by high-performance liquid chromatography or gas chromatography. Stereoselective methods for the analysis of racemic citalopram and fluoxetine have been published. The SSRIs are generally well tolerated and their therapeutic indices are large. In several studies there has not been found a clear relationship between clinical efficacy and plasma concentration, nor any threshold that defines toxic concentrations. The available data do not suggest that any benefit be obtained from routine monitoring of SSRI plasma levels. Therefore therapeutic drug monitoring (TDM) of the SSRIs may be useful mainly in situations where poor compliance is suspected and when therapeutic failure or toxic events are experienced at clinically relevant dosages. Further, in special populations, such as in elderly patients, poor metabolizers of sparteine (CYP2D6) or mephenytoin (CYP2C19), and patients with liver impairment, the measurement of plasma concentrations may be useful.

Pharmacokinetic interaction of fluvoxamine and thioridazine in schizophrenic patients

This study investigated to what extent fluvoxamine affects the pharmacokinetics of thioridazine (THD) in schizophrenic patients under steady-state conditions. Concentrations of THD, mesoridazine, and sulforidazine were measured in plasma samples obtained from 10 male inpatients, aged 36 to 78 years, at three different time points: A, during habitual monotherapy with THD at 88 +/-54 mg/day; B, after addition of a low dosage of fluvoxamine (25 mg twice a day) for 1 week; and C, 2 weeks after fluvoxamine discontinuation. After the addition of fluvoxamine, THD concentrations relative to time point A significantly increased approximately threefold from 0.40 to 1.21 micromol/L (225%) (p < 0.002), mesoridazine concentrations increased from 0.65 to 2.0 micromol/L (219%) (p < 0.004), and sulforidazine levels increased from 0.21 to 0.56 micromol/L (258%) (p < 0.004). The THD-mesoridazine and THD-sulforidazine ratios remained unchanged during the study. Mean plasma THD, mesoridazine, and sulforidazine levels decreased at time point C, but despite fluvoxamine discontinuation for 2 weeks, three patients continued to exhibit elevated concentrations of THD and its metabolites. In conclusion, fluvoxamine markedly interferes with the metabolism of THD, probably at the CYP2C19 and/or CYP1A2 enzyme level. Therefore, clinicians should be aware of the potential for a clinical drug interaction between both compounds, and careful monitoring of THD levels is valuable to prevent the accumulation of the drug and resulting toxicity.

Pharmacokinetics of haloperidol: an update

Haloperidol is commonly used in the therapy of patients with acute and chronic schizophrenia. The enzymes involved in the biotransformation of haloperidol include cytochrome P450 (CYP), carbonyl reductase and uridine diphosphoglucose glucuronosyltransferase. The greatest proportion of the intrinsic hepatic clearance of haloperidol is by glucuronidation, followed by the reduction of haloperidol to reduced haloperidol and by CYP-mediated oxidation. In studies of CYP-mediated disposition in vitro, CYP3A4 appears to be the major isoform responsible for the metabolism of haloperidol in humans. The intrinsic clearances of the back-oxidation of reduced haloperidol to the parent compound, oxidative N-dealkylation and pyridinium formation are of the same order of magnitude, suggesting that the same enzyme system is responsible for the 3 reactions. Large variation in the catalytic activity was observed in the CYP-mediated reactions, whereas there appeared to be only small variations in the glucuronidation and carbonyl reduction pathways. Haloperidol is a substrate of CYP3A4 and an inhibitor, as well as a stimulator, of CYP2D6. Reduced haloperidol is also a substrate of CYP3A4 and inhibitor of CYP2D6. Pharmacokinetic interactions occur between haloperidol and various drugs given concomitantly, for example, carbamazepine, phenytoin, phenobarbital, fluoxetine, fluvoxamine, nefazodone, venlafaxine, buspirone, alprazolam, rifampicin (rifampin), quinidine and carteolol. Overall, drug interaction studies have suggested that CYP3A4 is involved in the biotransformation of haloperidol in humans. Interactions of haloperidol with most drugs lead to only small changes in plasma haloperidol concentrations, suggesting that the interactions have little clinical significance. On the other hand, the coadministration of carbamazepine, phenytoin, phenobarbital, rifampicin or quinidine affects the pharmacokinetics of haloperidol to an extent that alterations in clinical consequences would be expected. In vivo pharmacogenetic studies have indicated that the metabolism and disposition of haloperidol may be regulated by genetically determined polymorphic CYP2D6 activity. However, these findings appear to contradict those from studies in vitro with human liver microsomes and from studies of drug interactions in vivo. Interethnic and pharmacogenetic differences in haloperidol metabolism may explain these observations.

Human cytochromes and some newer antidepressants: kinetics, metabolism, and drug interactions

The appearance of selective serotonin reuptake inhibitor antidepressants in the mid-1980s caused the discipline of clinical psychopharmacology to refocus attention to the topics of drug metabolism and drug interactions. This article reviews the metabolic profiles of some newer antidepressants, the clinical implications of metabolic properties, and research methodology that can be applied in determining which specific human cytochromes P450 (CYP) mediate metabolic pathways. Also reviewed are the relative activities of various new antidepressants as inhibitors of CYPs, and the benefits and drawbacks of in vivo and in vitro methodologies for identification and quantitation of drug interactions.

Genetic polymorphism of (S)-mephenytoin 4'-hydroxylation in populations of African descent

AIMS

The frequency of CYP2C19 poor metabolizers (PMs) in populations of African descent has been reported to range from 1.0% to 35.4%. In order to determine with greater certainty the frequency of CYP2C19 PMs in such black populations we have performed a meta-analysis of the studies.

METHODS

Relevant data on the frequency of both the PM phenotype of probe drugs (mephenytoin, omeprazole, and proguanil), and the distribution frequencies of CYP2C19 alleles and genotypes in black populations were summarized and reanalysed using a meta-analytical approach.

RESULTS

Of nine reported studies two were excluded because of significant heterogeneity (chi2=115, P<0.0001). The combined data from the remaining seven studies showed that the frequency of the PM phenotype in 922 healthy unrelated black Africans and black Americans ranged from 1.0% to 7.5% (n=7 for combined data) with an overall frequency being 3.9% (36 of 922; 95%CI: 2.7%-5.2%). The frequency of the PM genotypes in blacks was 3.7% (36 of 966; 95%CI: 2.5%-4.9%), in agreement with the frequency of the PM phenotype. In the extensive metabolizers (EMs) 29% (271 of 930) were heterozygotes (wt/m ). The observed frequencies of the three Mendelian genotypes were 0.68 for wt/wt, 0.28 for wt/m, and 0.04 for m/m. The allelic distribution was appropriate at 82.3% (95%CI: 80.5%-83.9%) for CYP2C19*1, 17.3% (95%CI:15.7%-19.0%) for CYP2C19*2 (m1 ), and 0.4% (95%CI: 0.1%-0.7%) for CYP2C19*3 (m2 ) in these populations.

CONCLUSIONS

We conclude that subjects of African ancestry have a low frequency of the CYP2C19 PM phenotype and genotype; that the defective CYP2C19 alleles are uncommon, and that a small proportion of heterozygotes exists in the EM subpopulation.

Drug metabolism and atypical antipsychotics

The introduction of the atypical antipsychotics clozapine, risperidone, olanzapine, quetiapine and sertindole for the treatment of schizophrenia has coincided with an increased awareness of the potential of drug-drug interactions, particularly involving the cytochrome P450 (CYP) enzymes. The current literature describing the pharmacokinetics of the metabolism of these agents, including their potential to influence the metabolism of other medications, is reviewed. Clozapine appears to be metabolized primarily by CYP1A2 and CYP3A4, with additional contributions by CYP2C19 and CYP2D6. In addition, clozapine may inhibit the activity of CYP2C9 and CYP2C19, and induce CYP1A, CYP2B and CYP3A. Risperidone is metabolized by CYP2D6, and possibly CYP3A4. In vitro data indicate that olanzapine is metabolized by CYP1A2 and CYP2D6. Quetiapine is metabolised by CYP3A4 and sertindole by CYP2D6. There is, however, a general paucity of in vivo data regarding the metabolism of the atypical antipsychotics, indicating a need for further research in this area.

Fluvoxamine is a potent inhibitor of the metabolism of caffeine in vitro

The selective serotonin re-uptake inhibitor, fluvoxamine, is a very potent inhibitor of CYP1A2, and accordingly causes pharmacokinetic interactions with drugs metabolised by CYP1A2, such as caffeine, theophylline, imipramine, tacrine and clozapine. Interaction between caffeine and fluvoxamine has been described in vivo, leading to lowering of total clearance of caffeine by 80% during fluvoxamine intake. The main purpose of the present study was to evaluate this interaction in vitro in human liver microsomes. A high-performance liquid chromatography method was developed in order to assay 1,3-dimethylxanthine, 1,7-dimethylxanthine, 3,7-dimethylxanthine and 1,3,7-trimethyluric acid formed from caffeine by human liver microsomes. The limit of detection was 0.06 nmol.mg protein-1.hr-1. As expected, fluvoxamine was a very potent inhibitor of the formation of the N-demethylated caffeine metabolites, displaying Ki values of 0.08-0.28 microM. The formation of 1,7-dimethylxanthine was virtually abolished by 10 microM of fluvoxamine, indicating that the N3-demethylation of caffeine is almost exclusively catalysed by CYP1A2. The CYP3A4 inhibitors, ketoconazole and bromocriptine, inhibited 1,3,7-trimethyluric acid formation with Kis of 0.75 microM and 5 microM, respectively, thus further supporting the involvement of CYP3A4 in the 8-hydroxylation of caffeine. The study shows that fluvoxamine, as expected, is a potent inhibitor of the metabolism of caffeine in vitro.

Metabolism of ropivacaine in humans is mediated by CYP1A2 and to a minor extent by CYP3A4: an interaction study with fluvoxamine and ketoconazole as in vivo inhibitors

BACKGROUND

Potential drug-drug interactions can be identified in vitro by exploring the importance of specific cytochrome P450 (CYP) isozymes for drug metabolism. The metabolism of the local anesthetic ropivacaine to 3-hydroxyropivacaine and (S)-2',6'-pipecoloxylidide was shown in vitro to be dependent on CYP1A2 and 3A4, respectively. In this in vivo model study we quantitated the role of these 2 isozymes for the metabolism of ropivacaine.

METHODS

In a randomized, 3-way crossover study, 12 healthy subjects received a single dose of 40 mg ropivacaine intravenously alone or combined either with 25 mg fluvoxamine as a CYP1A2 inhibitor or with 100 mg ketoconazole as a CYP3A4 inhibitor twice daily for 2 days. Venous plasma and urine samples were collected over 10 hours and 24 hours, respectively. The samples were analyzed for ropivacaine base, 3-hydroxyropivacaine, and (S)-2',6'-pipecoloxylidide.

RESULTS

Coadministration with fluvoxamine decreased the mean total plasma clearance of ropivacaine from 354 to 112 mL/min (68%), whereas ketoconazole decreased plasma clearance to 302 mL/min (15%). The relative changes in unbound plasma clearance were similar to the changes in total plasma clearance. The ropivacaine half-life (t1/2) of 1.9 hours was almost doubled during fluvoxamine administration and the plasma concentration at the end of infusion increased slightly, whereas the corresponding parameters after ketoconazole administration remained unchanged. Coadministration with ketoconazole almost abolished the (S)-2',6'-pipecoloxylidide concentrations in plasma, whereas fluvoxamine administration increased the (S)-2',6'-pipecoloxylidide levels. The fraction of dose excreted as 3-hydroxyropivacaine in urine decreased during fluvoxamine administration from 39% to 13%.

CONCLUSIONS

CYP1A2 is the most important isozyme for the metabolism of ropivacaine. Drug-drug interactions with strong inhibitors of this isozyme could be of clinical relevance during repeated administration. A potent inhibitor of CYP3A4 causes a minor decrease in clearance, which should be of no clinical relevance.

Determinants of interindividual variability and extent of CYP2D6 and CYP1A2 inhibition by paroxetine and fluvoxamine in vivo

Major depression may require antidepressant treatment for several years. This necessitates consideration of the long-term effects of antidepressants on multiple clinical endpoints. The antidepressants paroxetine and fluvoxamine are potent in vitro inhibitors of CYP2D6 and CYP1A2 isozymes, respectively. CYP2D6 and CYP1A2 are important for the clearance of 30 or more frequently used medications. Moreover, CYP1A2 also contributes to metabolism of 17beta-estradiol and metabolic activation of environmental procarcinogens (e.g., arylamines in cigarette smoke). The aim of this study was to assess the determinants of interindividual variability and extent of CYP2D6 and CYP1A2 inhibition during paroxetine and fluvoxamine treatment. Healthy volunteers and patients received caffeine (100 mg) and dextromethorphan (30 mg) at baseline and at steady state of paroxetine (10-20 mg/day, 5-74 days, N = 13) or fluvoxamine (50-100 mg/day, 5-43 days, N = 8). The caffeine metabolic ratio (CMR) and the log O-demethylation ratio (ODMR) of dextromethorphan in overnight urine were used as in vivo indices of the CYP1A2 and CYP2D6 isozyme activities, respectively. All subjects had an extensive metabolizer phenotype for CYP2D6. After fluvoxamine treatment, baseline CMR 5.1 +/- 1.4 (mean +/- SD) decreased to 2.7 +/- 1.1 (p < 0.01). Paroxetine did not have a significant effect on CMR (p > 0.05). In seven of eight subjects in the fluvoxamine group, posttreatment CMR was comparable with the minimum CMR value (2.0) attainable in nonsmoking healthy volunteers. After paroxetine treatment, log ODMR changed from a baseline value of -2.28 +/- 0.37 to -1.13 +/- 0.44, indicating significant inhibition of CYP2D6 (p < 0.001). Subjects' CYP2D6 phenotype did not change after paroxetine treatment. Fluvoxamine had no significant effect on log ODMR (p > 0.05). The extent of inhibition of CYP2D6 and CYP1A2 by paroxetine and fluvoxamine, respectively, displayed a positive correlation with baseline enzyme activity (p < 0.05). In addition, a negative association was found between the plasma paroxetine concentration and the CYP2D6 activity after paroxetine treatment (r = -0.47, p < 0.05). These data indicate that paroxetine and fluvoxamine treatment with minimum clinically effective doses significantly inhibit CYP2D6 and CYP1A2, respectively. The extent of inhibition of CYP2D6 by paroxetine and of CYP1A2 by fluvoxamine is dependent in part on the baseline enzyme activity. The interindividual variability in CYP2D6 inhibition by paroxetine can also be explained by variability in plasma paroxetine concentration. Most patients treated with fluvoxamine (50-100 mg/day) will reach population minimums for CYP1A2 activity. These results have potential implications for interindividual variability in the risk for drug-drug interactions mediated by CYP2D6 and CYP1A2 as well as for the disposition of 17beta-estradiol and environmental procarcinogens.