Enantioselective amitriptyline metabolism in patients phenotyped for two cytochrome P450 isozymes

Impact of COVID- 19 pandemic on antidepressants consumptions by wastewater analysis in Turkey

The COVID-19 pandemic has been a major challenge worldwide, forcing countries to take restrictive measures beyond conventional methods in their fight against the spread of the disease. Followingly, many studies have been conducted on the effects of these measures on mental health. Wastewater-based epidemiology (WBE) was used in this study to monitor and estimate changes in antidepressant use under normal conditions (2019) and COVID-19 pandemic conditions (2020). Likewise, this study utilized wastewater-based epidemiology (WBE) to monitor and assess changing trends from the pre-pandemic period (2019) to COVID-19 pandemic conditions in antidepressant use (2020). Wastewater samples were collected from 11 cities in Turkey throughout six sampling periods covering the pre-pandemic and during-pandemic periods (June 2019-December 2020). Then, samples were analyzed via LC-MS/MS method. As a result, we observed that venlafaxine was the drug with the highest concentration (mean ± SD: 103.6 ± 112.1 mg/1000p/day). Moreover, city number 6 presented the highest venlafaxine use and the most dramatic increase during the pandemic period. Finally, this study revealed the potential of WBE to estimate the changing trends in mental health during the ongoing pandemic.

Genetic Polymorphisms of Cytochrome P450 Enzymes and the Effect on Interindividual, Pharmacokinetic Variability in Extensive Metabolizers

Genetic polymorphisms of cytochrome P450 (CYP) enzymes are one of the factors that contribute to the pharmacokinetic (PK) variability of drugs. PK variability is observed in the bimodal distribution between extensive metabolizers (EMs) and poor metabolizers (PMs). PK variability may also exist between individuals genotyped as homozygous EMs and heterozygous EMs. This may carry implications for drug dosing and drug response (e.g., risk of therapeutic failure or drug toxicity). Studies have reported significant PK differences between homozygous and heterozygous EMs. Some literature suggests that this distinction may be of clinical relevance. Due to study design limitations and data that are either sparse or conflicting, generalizations regarding the potential impact of the CYP genotype, within EMs, are difficult. Optimally designed clinical trials are needed. This review evaluates the potential impact of CYP genetic polymorphisms on interindividual PK variability of drugs within an EM population.

Persistence of the tricyclic antidepressant drugs amitriptyline and nortriptyline in agriculture soils

Amitriptyline and nortriptyline are widely used tricyclic antidepressant drugs. They have been detected in wastewater, surface runoff, and effluents from sewage treatment plants. As such, they could potentially reach agriculture land through the application of municipal biosolids or reclaimed water. In the absence of data on their fate in the environment, the persistence and dissipation pathways of radiolabeled amitriptyline were determined in three agriculture soils varying widely in texture and chemical properties (loam soil, clay loam soil, and sandy loam soil). Tritiated amitriptyline was added to laboratory microcosms containing soils, and the metabolism of the extractable (3) H was monitored during incubation at 30°C. The total solvent extractable radioactivity decreased in all three soils with times to dissipate 50% of material (DT50) ranging from 34.1 ± 3.2 (loam soil) to 85.3 ± 3.2 d (sandy soil). Nortriptyline (N-desmethyl amitriptyline) and amitriptyline-N-oxide were identified as major transformation products in all three soils by high performance liquid chromatography with photodiode array detector and time-of-flight mass spectrometry (HPLC-TOF-MS/UV). The addition of liquid municipal biosolids to the loam soil had no effect on the dissipation of amitriptyline. The persistence of nortriptyline was evaluated in the loam soil. The DT50 of nortriptyline was 40.5 ± 3.2 d estimated with HPLC-TOF-MS/UV. Approximately 10% of added nortriptyline was converted to hydroxylated products after 50 d of incubation. In summary, amitriptyline persisted in agricultural soils with major dissipation mechanisms, including forming nonextractable residues and producing various transformation products including the psychoactive drug nortriptyline.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

The CYP2D6 polymorphism in relation to the metabolism of amitriptyline and nortriptyline in the Faroese population

AIM

To determine the frequency of CYP2D6 poor metabolizers (PMs) in a Faroese patient group medicated with amitriptyline (AT) and to investigate plasma concentrations of AT and metabolites in relation to CYP2D6.

METHODS

CYP2D6 phenotype and genotype were determined in 23 Faroese patients treated with AT. Plasma concentrations of AT and metabolites were determined by high-performance liquid chromatography and investigated in relation to CYP2D6 activity.

RESULTS

Of the 23 patients phenotyped and genotyped, five (22%) (95% confidence interval 7.5, 43.7) were CYP2D6 PMs. No difference was found in AT daily dosage between PMs (median 25 mg day(-1); range 5-80) and extensive metabolizers (EMs) (median 27.5 mg day(-1); range 10-100). The (E)-10-OH-nortriptyline (NT)/dose concentrations were higher in EMs than in PMs and the NT/(E)-10-OH-NT and AT/(E)-10-OH-AT ratios were higher in PMs compared with EMs. The log sparteine metabolic ratio correlated positively with the NT/(E)-10-OH-NT ratio (r(s) = 0.821; P < 0.0005) and the AT/(E)-10-OH-AT ratio (r(s) = 0.605; P < 0.006).

CONCLUSION

A high proportion of CYP2D6 PMs was found in a Faroese patient group medicated with AT. However, similar doses of AT and concentrations of AT and NT were noted in EMs and PMs, probably due to varying doses and indications for AT treatment.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

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

The impact of the CYP2D6-polymorphism on dose recommendations for current antidepressants

Cytochrome P450 CYP2D6 represents an extensively characterized polymorphic drug-metabolizing enzyme. The CYP2D6-gene is highly polymorphic and more than 70 different alleles are known currently. The activity of the enzyme markedly varies among individuals from poor to intermediate and extensive up to ultrarapid metabolism on the basis of the polymorphism of the CYP2D6 gene. Association studies provide growing evidence for the clinical importance of the CYP2D6 polymorphism investigating whether the CYP2D6 genotype distribution differs from that of the normal population either in patients with marked adverse effects or in nonresponders during treatment with CYP2D6 substrates. However, these scientifically important studies present less information for dose adjustments necessary to individualize pharmacotherapy in a given clinical case. With respect to psychopharmacological drug metabolism several antidepressants were characterized as being CYP2D6 substrates. Thus, this review summarizes dose recommendations of current antidepressants.

Lopinavir/ritonavir induces the hepatic activity of cytochrome P450 enzymes CYP2C9, CYP2C19, and CYP1A2 but inhibits the hepatic and intestinal activity of CYP3A as measured by a phenotyping drug cocktail in healthy volunteers

OBJECTIVE

The effect of lopinavir/ritonavir (LPV/r) administration on cytochrome P450 (CYP) enzyme activity was quantified using a phenotyping biomarker cocktail. Changes in CYP2C9, CYP2C19, CYP3A, CYP1A2, N-acetyltransferase-2 (NAT-2), and xanthine oxidase (XO) activities were evaluated using warfarin (WARF) + vitamin K, omeprazole (OMP), intravenous (IV) and oral (PO) midazolam (MDZ), and caffeine (CAF).

DESIGN

: Open-label, multiple-dose, pharmacokinetic study in healthy volunteers.

METHODS

Subjects (n = 14) simultaneously received PO WARF 10 mg, vitamin K 10 mg, OMP 40 mg, CAF 2 mg/kg, and IV MDZ 0.025 mg/kg on days (D) 1 and 14, and PO MDZ 5 mg on D2 and D15. LPV/r (400/100 mg twice daily) was administered on D4-17. CYP2C9 and CYP2C19 activities were quantified by S-WARF AUC0-inf and OMP/5-hydroxy OMP ratio, respectively. CYP1A2, NAT-2, and XO activities were quantified by urinary CAF metabolite ratios. Hepatic and intestinal + hepatic CYP3A activities were quantified by IV (CL) and PO (CL/F) MDZ clearance, respectively.

RESULTS

After LPV/r therapy, CYP2C9, CYP2C19, and CYP1A2 activity increased by 29%, 100%, and 43% (P = 0.001, 0.046, and 0.001), respectively. No changes were seen in NAT-2 or XO activity. Hepatic and intestinal + hepatic CYP3A activity decreased by 77% (P < 0.001) and 92% (P = 0.001), respectively.

CONCLUSION

LPV/r therapy results in modest induction of CYP1A2 and CYP2C9 and potent induction of CYP2C19 activity. Increasing doses of concomitant medications metabolized by these enzymes may be necessary. LPV/r inhibited intestinal CYP3A to a greater extent than hepatic CYP3A activity. Doses of concomitant CYP3A substrates should be reduced when combined with LPV/r, although intravenously administered compounds may require less of a relative dose reduction than orally administered compounds.

Maribavir pharmacokinetics and the effects of multiple-dose maribavir on cytochrome P450 (CYP) 1A2, CYP 2C9, CYP 2C19, CYP 2D6, CYP 3A, N-acetyltransferase-2, and xanthine oxidase activities in healthy adults

Maribavir (1263W94, VP-41263) is an oral anticytomegalovirus agent under clinical development. The pharmacokinetics and safety of maribavir and the effects of maribavir on the activities of cytochrome P450 (CYP) 1A2, CYP 2C9, CYP 2C19, CYP 2D6, CYP 3A, N-acetyltransferase-2 (NAT-2), and xanthine oxidase (XO) were evaluated in a randomized, double-blind, placebo-controlled study. Twenty healthy subjects received a five-drug phenotyping cocktail of caffeine (CYP 1A2, NAT-2, XO), warfarin plus vitamin K (CYP 2C9), omeprazole (CYP 2C19), dextromethorphan (CYP 2D6), and midazolam (CYP 3A) 4 days before and after 7 days of treatment with maribavir at 400 mg twice daily (16 subjects) or placebo (4 subjects) for 10 days. Maribavir did not affect the CYP 1A2, CYP 2C9, CYP 3A, NAT-2, or XO activities. Bioequivalence was not demonstrated for CYP 2C19 and CYP 2D6, suggesting a decrease or inhibition of CYP 2C19 and CYP 2D6 activities. The pharmacokinetics of maribavir following a single dose and after 10 days of treatment were similar, with minimal accumulation at steady state. Maribavir was safe and well tolerated. Taste disturbance was the most frequently reported adverse event. These results will further guide evaluation of the drug interaction potential and clinical development of maribavir.

The influence of long-term treatment with psychotropic drugs on cytochrome P450: the involvement of different mechanisms

This paper emphasises that besides the direct action of psychotropic drugs on cytochrome P450 (CYP) (i.e., the binding of the parent drug to the enzyme) indirect mechanisms of CYP-psychotropic interactions, namely the formation of CYP-reactive metabolite complexes and their influence on enzyme regulation, are also very important. The described interactions that are time-, drug- and CYP isoform-dependent may overlap during long-term treatment. The final result of the overlapping depends on the dosage and time interval after the last administration of a drug, which determines the concentration of the parent drug and its metabolites in the environment of the enzyme. These interactions may occur not only in the liver, but also in the brain, and may change the activity of CYP towards the metabolism of drugs, sex steroids, neurosteroids and amine neurotransmitters. The role of the CNS in the regulation of CYP by psychotropics and the significance of CYP-psychotropic interactions for pharmacological and clinical profiling of these drugs is discussed. In addition, different experimental approaches for studying CNS-acting drugs are compared.

Metabolic Ratios of Psychotropics as Indication of Cytochrome P450 2D6/2C19 Genotype

The cytochrome P450 enzymes (CYP) 2C19 and 2D6 are involved in the metabolism of many psychotropic drugs. Variability in enzyme activity results in variable metabolic capacities, affecting the metabolism of substrates. The metabolic ratio (MR) of drugs metabolized via these enzymes may therefore reflect the enzyme's activity and/or genotype. To serve as an example for different groups of medications, the selective serotonin reuptake inhibitor venlafaxine, the tricyclic antidepressant amitriptyline, and the antipsychotic risperidone were studied to examine a possible correlation between the MRs of these drugs and the CYP2C19 and/or CYP2D6 genotype. For this purpose data from routine genotyping and serum level analysis were used. The relationships between the observed metabolic ratios and CYP2D6 and/or CYP2C19 genotype were characterized using nonparametric statistical analysis. A clear correlation was observed between the CYP2D6 genotype and the metabolic ratio of venlafaxine. Genotyping of individuals with a log(MR) < -0.6 or a log(MR) > 0.2 would include all patients with an aberrant genotype but would result in a reduction of 52% of genotyping reactions. Slow metabolism of amitriptyline is correlated with a log(MR) > 0.4. Genotyping only those subjects with a log(MR) > 0.4 would result in 88% fewer genotyping reactions. For risperidone, genotyping individuals with a log(MR) > 0.4 would include all CYP2D6 poor metabolizers while reducing the number of genotyping reactions by 93%. According to these data, correlations exist between the log(MR) of venlafaxine, amitriptyline, and risperidone and the genotype of the CYP enzymes involved in their metabolism. From the ranges of log(MR) defined here, a high percentage of aberrant metabolizers can be detected even when patients are not routinely genotyped. Thus, the metabolic ratio may serve as an indication of when genotyping should be considered.

CYP2D6 and CYP2C19 genotypes and amitriptyline metabolite ratios in a series of medicolegal autopsies

In a series of 202 postmortem toxicology cases, the CYP2D6 and CYP2C19 genes were genotyped, and the concentrations of amitriptyline (AT) and six metabolites were analyzed. The polymorphic CYP2D6 and CYP2C19 genes encode enzymes participating in the metabolism of several potentially toxic drugs, and mutations in these genes may lead to adverse drug reactions, possibly even intoxications. AT was chosen as the substrate of interest because it is mainly metabolized by these enzymes, is considered relatively toxic, and ranks among the major causes of fatal drug poisoning in Finland. Our objective was to evaluate genetically determined interindividual variation in conjunction with metabolite ratios of drugs found in toxicological analysis in a series of medicolegal autopsies. Positive correlations were found between the proportion of trans-hydroxylated metabolites and the number of functional copies of CYP2D6 and between the proportion of demethylated metabolites and the number of functional copies of CYP2C19. None of the accidental or undetermined AT poisonings coincided with the CYP2D6 or CYP2C19 genotype which predicts a poor metabolizer phenotype. However, an unusually high femoral blood concentration of AT, 60mg/l, was found in one suicide case with no functional CYP2D6 genes. Our study shows a concordance of AT metabolite patterns with CYP2D6 and CYP2C19 genotypes in the presence of confounding factors typical for postmortem material. This result demonstrates the feasibility of postmortem pharmacogenetic analysis and supports the dominant role of genes in drug metabolism.

Excess fatality from desipramine and dosage recommendations

Higher case fatality rates (CFR) were previously reported from desipramine than for 3 other tricyclic antidepressants (TCAs): amitriptyline, nortriptyline, and imipramine. The database of the American Association of Poison Control Centers (AAPCC) Toxic Exposure Surveillance System (TESS) for the 20 years 1983-2002 was used to evaluate the CFR of desipramine and the other TCAs. The CFR of desipramine was 2.25-, 2.31-, and 2.62-fold the CFR for amitriptyline, nortriptyline, and imipramine, respectively (P < 0.001). Mechanisms of desipramine toxicity and its dosage recommendations are discussed. Desipramine and nortriptyline have higher distribution volumes and erythrocyte/plasma ratios than their parent compounds imipramine and amitriptyline. This implies lower therapeutic plasma levels and reduced doses for desipramine and nortriptyline compared with their parent compounds. Such adjustments have been done for nortriptyline, but not for desipramine. The authors suggest that the high CFR of desipramine might be reduced by lowering its dose, therapeutic plasma level, and maximal pill content.

The Metabolic Fate of Amitriptyline, Nortriptyline and Amitriptylinoxide in Man

Amitriptyline (AT), the most widely used tricyclic antidepressant, undergoes oxidative metabolism in the side chain with production of the secondary amine nortriptyline (NT), a primary amine, and the N-oxide amitriptylinoxide (AT-NO); in addition, direct conjugation leads to a quaternary ammonium-linked glucuronide. Hydroxylation of AT or NT at the ethylene bridge of the central seven-membered ring results in four isomeric alcohols and occurs with high stereo- and enantioselectivity, the (-)-(E)-10-hydroxy compounds usually being the major products. The disposition of the alcohols is also partially enantioselective, for instance with regard to glucuronidation and reversible oxidation to ketones. Introduction of a second hydroxy group results in isomeric glycols. Oxidative attack at an aromatic ring is a minor pathway leading to dihydrodiols and phenols. Numerous metabolites originate by combinations of reactions in the ring system and the side chain. AT-NO is by about one-third excreted in unchanged form or as 10-hydroxy derivative; the major part is reduced to AT and metabolized further. The review covers current knowledge on the enzymes participating in the individual pathways. Their quantitative importance is inferred from kinetic studies in volunteers and patients and from experiments in vitro. Clinical consequences of biochemical findings mainly derive from the impact of the polymorphic CYP2D6 mediating (-)-(E)-10-hydroxylation and from its potential inhibition by other psychoactive drugs.

Omeprazole as a CYP2C19 marker in Chinese subjects: assessment of its gene-dose effect and intrasubject variability

The objective of this study was to determine the reliability of omeprazole as a marker for CYP2C19 activity in Chinese subjects. In 27 healthy male Chinese subjects, the CYP2C19 phenotype was first determined with the standard mephenytoin hydroxylation index (HI) method. Subsequently, the subjects were randomized in a three-way crossover manner to receive an oral 40-mg dose from each of three omeprazole formulations (as part of a bioequivalence study). Multiple blood samples were obtained over 12 hours, and plasma concentrations of omeprazole, 5-hydroxyomeprazole, and omeprazole sulfone were determined by a high-performance liquid chromatography (HPLC) method. Individual CYP2C19 genotype was determined by the polymerase chain reaction/restriction fragment length polymorphism method. To assess the specificity for CYP2C19 activity, the hydroxylation metabolic ratio (MR) of omeprazole (AUC(omeprazole)/AUC(5-hydroxyomeprazole)) was compared to mephenytoin HI and related to CYP2C19 genotype status. The inter- and intrasubject variabilities of MR were also calculated, and their magnitudes were compared. The intersubject MR varied more than 20 fold. Among the subjects, there was a gene-dose effect, and the mean MR was 1.76, 3.45, and 33.08, respectively, in the homozygous extensive metabolizers (wt/wt, n = 9), heterozygous extensive metabolizers (wt/m1 or wt/m2, n = 10), and poor metabolizers (m1/m1 or m1/m2, n = 7). However, the coefficients of variation for intrasubject MR only ranged from 4.5% to 33.7% over the three periods with the three formulations. The phenotype based on MR was concordant with HI. In view of the clear gene-dose effect, concordance with mephenytoin HI, and low intrasubject variability, omeprazole MR following a 40-mg oral dose can be considered as a specific and sensitive marker for CYP2C19 activity in Chinese subjects.

Potential for drug interactions involving cytochromes P450 2D6 and 3A4 on general adult psychiatric and functional elderly psychiatric wards

AIMS

To assess the potential for interactions involving cytochromes P450 2D6 (CYP2D6) and 3A4 (CYP3A4) between drugs prescribed in a city in-patient psychiatric service.

METHODS

Prescription information was obtained from all 236 patients in general adult wards and all 87 patients in functional elderly wards under a city psychiatric service. The frequencies with which combinations of drugs expected to interact via CYP2D6 or CYP3A4 were documented and compared between these two settings.

RESULTS

All 2089 drug prescriptions, of which 1237 (59%) were administered, were analyzed. One hundred and seventy-two patients (73%) on adult wards and 59 (68%) on functional elderly wards were prescribed at least one drug metabolized by and/or inhibiting CYP2D6, the difference being nonsignificant (95% confidence interval on the difference -6.3%, 16.4%). Anticipated interactions from 62/82 CYP2D6-related combinations prescribed on adult wards (27/100 patients) and 19/30 prescribed to elderly patients (22/100 patients) were judged to be clinically important or potentially clinically important. The proportion of patients on functional elderly wards prescribed at least one drug interacting with CYP3A4 (87%) was significantly greater than that for patients on adult wards (57%, P < 0.001). The frequency of interactions involving CYP3A4 was significantly greater on functional elderly than adult wards (43/100 vs 22/100 patients, P < 0.025, 95% confidence interval on the difference 4, 38/100).

CONCLUSIONS

Our findings confirm extensive polypharmacy on general adult psychiatric and functional elderly psychiatric wards. A substantial proportion of patients were receiving combinations of drugs that interact with CYP2D6 and/or CYP3A4, many of which are known to produce clinically important interactions. Doctors practising in old age psychiatry should be aware that patients on functional elderly wards are at increased risk of clinically important CYP3A4 interactions. Psychiatrists should consider the pharmacokinetic implications of drugs prescribed for use 'as needed', because of the potential for unpredictable interactions.

Enantioselective analytical methods in pharmacokinetics with specific reference to genetic polymorphic metabolism

The new trend towards developing enantiospecific drugs has increased the interest in enantiospecific pharmacokinetics of chiral drugs, mainly in the case where only one of the two enantiomers is responsible for the pharmacological activity. Enantiospecific bioassays are also useful in investigating the pharmacokinetic behaviour of the two enantiomers when a given drug is marketed as racemate. The stability of the stereogenic centre in vitro and in vivo, as far as unidirectional and bidirectional inversions are concerned, is another reason for requiring stereospecific assay and bioassay. These assays are often complicated in order to achieve quantification, mainly for in vivo measurements, which are often in the low pg/ml range. This paper considers the enantiospecific bioassays, the methods and approaches used, the need for chemical derivatization, and the difficulties involved. It includes a specific discussion for the genetic polymorphic metabolism involving stereogenic centres.

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.

In vitro-in vivo correlations for drugs eliminated by glucuronidation: investigations with the model substrate zidovudine

AIMS

To investigate the effects of incubation conditions on the kinetic constants for zidovudine (AZT) glucuronidation by human liver microsomes, and whether microsomal intrinsic clearance (CLint) derived for the various conditions predicted hepatic AZT clearance by glucuronidation (CLH) in vivo.

METHODS

The effects of incubation constituents, particularly buffer type (phosphate, Tris) and activators (Brij58, alamethacin, UDP-N-acetylglucosamine (UDP-NAcG)), on the kinetics of AZT glucuronidation by human liver microsomes was investigated. AZT glucuronide (AZTG) formation by microsomal incubations was quantified by h.p.l.c. Microsomal CLint values determined for the various experimental conditions were extrapolated to a whole organ CLint and these data were used to calculate in vivo CLH using the well-stirred, parallel tube and dispersion models.

RESULTS

Mean CLint values for Brij58 activated microsomes in both phosphate (3.66 +/- 1.40 micro l min-1 mg-1, 95% CI 1.92, 5.39) and Tris (3.79 +/- 0.74 micro l min-1 mg-1, 95% CI 2.87, 4.71) buffers were higher (P < 0.05) than the respective values for native microsomes (1.04 +/- 0.42, 95% CI 0.53, 1.56 and 1.37 +/- 0.30 micro l min-1 mg-1, 95% CI 1.00, 1.73). Extrapolation of the microsomal data to a whole organ CLint and substitution of these values in the expressions for the well-stirred, parallel tube and dispersion models underestimated the known in vivo blood AZT clearance by glucuronidation by 6.5- to 23-fold (3.61-12.71 l h-1vs 82 l h-1). There was no significant difference in the CLH predicted by each of the models for each set of conditions. A wide range of incubation constituents and conditions were subsequently investigated to assess their effects on GAZT formation, including alamethacin, UDP-NAcG, MgCl2, d-saccharic acid 1,4-lactone, ATP, GTP, and buffer pH and ionic strength. Of these, only decreasing the phosphate buffer concentration from 0.1 m to 0.02 m for Brij58 activated microsomes substantially increased the rate of GAZT formation, but the extrapolated CLH determined for this condition still underestimated known AZT glucuronidation clearance by more than 4-fold. AZT was shown not to bind nonspecifically to microsomes. Analysis of published data for other glucuronidated drugs confirmed a trend for microsomal CLint to underestimate in vivo CLH.

CONCLUSIONS

AZT glucuronidation kinetics by human liver microsomes are markedly dependent on incubation conditions, and there is a need for interlaboratory standardization. Extrapolation of in vitro CLint underestimates in vivo hepatic clearance of drugs eliminated by glucuronidation.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or 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 with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

The impact of CYP2C19 and CYP2D6 genotypes on metabolism of amitriptyline in Japanese psychiatric patients

We investigated the effect of the CYP2C19 and CYP2D6 genotypes on the metabolism of amitriptyline (AT) in Japanese psychiatric patients. Steady-state concentrations of AT and its metabolites (nortriptyline [NT], trans-10-hydroxy-nortriptyline [EHNT], cis-10-hydroxy-nortriptyline [ZHNT], trans-10-hydroxy-amitriptyline [EHAT], and cis-10-hydroxy-amitriptyline [ZHAT]) in 50 patients were determined by high-performance liquid chromatography. Significantly higher plasma concentrations of AT corrected for dose and body weight in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 were observed (no mutated alleles vs. two mutated alleles: 36.0 +/- 18.2 vs. 64.0 +/- 25.2 ng/mL/mg/kg, p = 0.025). A significantly higher AT/NT ratio was seen in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 (no mutated alleles vs. two mutated alleles: 1.27 +/- 0.59 vs. 3.40 +/- 1.02, p = 0.001). A trend for higher NT/EHNT ratio in the subjects with two mutated alleles of CYP2D6 than in those with no mutated alleles of CYP2D6 was observed (no mutated alleles vs. two mutated alleles: 0.73 +/- 0.39 vs. 1.31 +/- 0.81, p = 0.068). A trend for higher plasma concentrations of total hydroxylated metabolites of AT (EHAT + ZHAT) corrected for dose and body weight in the subjects with two mutated alleles of CYP2C19 than in those with no mutated alleles of CYP2C19 was found (no mutated alleles vs. two mutated alleles: 9.5 +/- 5.8 vs. 17.8 +/- 8.9, p = 0.051). Therefore, the genotype of CYP2C19 is one of the important determinants of the plasma concentrations of AT and the capacity to desmethylate AT. Mother compound AT is shunted via hydroxylation pathways from AT to EHAT and ZHAT in the subjects with homozygotes of mutated alleles of CYP2C19 in order to compensate for the decreased capacity to desmethylate AT.

Decreased plasma levels of amitriptyline and its metabolites on comedication with an extract from St. John's wort ( Hypericum perforatum )

Extracts of St. John's wort ( Hypericum perforatum ) became increasingly popular as easily available remedies for mild to moderate depression. Comedication with hypericum extract was recently shown to drastically reduce plasma concentration of ciclosporin, digoxin, and indinavir. We investigated the possible interaction of hypericum extract LI160 with amitriptyline. Both antidepressants have a high probability of concomitant use. Twelve patients requiring amitriptyline treatment received a single dose of hypericum extract (900 mg) at day 1, continued by a 12-to 14-day treatment with retarded amitriptyline (75 mg twice daily). Then hypericum (900 mg/day) was added for another 14 to 16 days. Steady-state pharmacokinetics of amitriptyline were compared before and after multiple-dose treatment with hypericum extract. Furthermore, comparisons were made for single-dose kinetics of hypericum-extract ingredients hypericin, pseudohypericin, and hyperforin between the first day of concomitant treatment and LI160 alone. Multiple-dose comedication with LI160 led to a statistically significant decrease in the area under the plasma concentration-time curve within one dosing interval of amitriptyline by 22% ( p = 0.03) and nortriptyline by 41% ( p = 0.002), as well as of all hydroxylated metabolites, except for 10-E-hydroxynortriptyline. Plasma levels of amitriptyline and hydroxylated metabolites gradually decreased, whereas nortriptyline concentrations were already markedly decreased after 3 days of cotreatment with hypericum. Cumulative urinary amounts of amitriptyline and metabolites decreased to the same extent as plasma concentrations upon hypericum comedication. Induction of cytochrome P-450 enzymes or drug transporters (P-glycoprotein) by St. John's wort extract may explain this pharmacokinetic interaction. Physicians should be aware of this interaction when treating patients with amitriptyline.

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.

Relative contribution of CYP3A to amitriptyline clearance in humans: in vitro and in vivo studies

The relative contribution of cytochrome P450 3A (CYP3A) to the oral clearance of amitriptyline in humans has been assessed using a combination of in vitro approaches together with a clinical pharmacokinetic interaction study using the CYP3A-selective inhibitor ketoconazole. Lymphoblast-expressed CYPs were used to study amitriptyline N-demethylation and E-10 hydroxylation in vitro. The relative activity factor (RAF) approach was used to predict the relative contribution of each CYP isoform to the net hepatic intrinsic clearance (sum of N-demethylation and E-10 hydroxylation). Assuming no extrahepatic metabolism, the model-predicted contribution of CYP3A to net intrinsic clearance should equal the fractional decrement in apparent oral clearance of amitriptyline upon complete inhibition of the enzyme. This hypothesis was tested in a clinical study of amitriptyline (50 mg, p.o.) with ketoconazole (three 200 mg doses spaced 12 hours apart) in 8 healthy volunteers. The RAF approach predicted CYP2C19 to be the dominant contributor (34%), with a mean 21% contribution of CYP3A (range: 8%-42% in a panel of 12 human livers). The mean apparent oral clearance of amitriptyline in 8 human volunteers was decreased from 2791 ml/min in the control condition to 2069 ml/min with ketoconazole. The average 21% decrement (range: 2%-40%) was identical to the mean value predicted in vitro using the RAF approach. The central nervous system (CNS) sedative effects of amitriptyline were slightly greater when ketoconazole was coadministered, but the differences were not statistically significant. In conclusion, CYP3A plays a relatively minor role in amitriptyline clearance in vivo, which is consistent with in vitro predictions using the RAF approach.

CYP2D6 and CYP2C19 genotype-based dose recommendations for antidepressants: a first step towards subpopulation-specific dosages

OBJECTIVE

This review aimed to provide distinct dose recommendations for antidepressants based on the genotypes of cytochrome P450 enzymes CYP2D6 and CYP2C19. This approach may be a useful complementation to clinical monitoring and therapeutic drug monitoring.

METHOD

Our literature search covered 32 antidepressants marketed in Europe, Canada, and the United States. We evaluated studies which had compared pharmacokinetic parameters of antidepressants among poor, intermediate, extensive and ultrarapid metabolizers.

RESULTS

For 14 antidepressants, distinct dose recommendations for extensive, intermediate and poor metabolizers of either CYP2D6 or CYP2C19 were given. For the tricyclic antidepressants, dose reductions around 50% were generally recommended for poor metabolizers of substrates of CYP2D6 or CYP2C19, whereas differences were smaller for the selective serotonin reuptake inhibitors.

CONCLUSION

We have provided preliminary average dose suggestions based on the phenotype or genotype. This is a first attempt to apply the new pharmacogenetics to suggest dose-regimens that take the differences in drug metabolic capacity into account.

Steady-state plasma levels of nortriptyline and its hydroxylated metabolites in Japanese patients: impact of CYP2D6 genotype on the hydroxylation of nortriptyline

The authors investigated the impact of the CYP2D6 genotype on steady-state concentrations of nortriptyline (NT) and its metabolites, trans-10-hydroxynortriptyline (EHNT) and cis-10-hydroxynortriptyline in a Japanese population of psychiatric patients. Forty-one patients (20 men and 21 women) were orally administered nortriptyline hydrochloride. The allele frequencies of the CYP2D6*5 and CYP2D6*10 were 4.9% and 34.1%, respectively. Significant differences in NT concentrations corrected for dose and weight were observed between the subjects with no mutated alleles and those with one mutated allele (mean +/- SD for no mutated alleles vs. one mutated allele: 70.3 +/- 25.4 vs. 98.4 +/- 36.6 ng/mL x mg(-1) x kg(-1); t = 2.54, dcf = 33, p < 0.05) and between the subjects with no mutated alleles and two mutated alleles (no mutated alleles vs. two mutated alleles: 70.3 +/- 25.4 vs. 147 +/- 31.1 ng/mL x mg(-1) x kg(-1); t = 5.87, df = 19, p < 0.0001). Also, a significant difference in the NT/EHNT ratio, which is representative of the hydroxylation ratio of NT, was observed between the subjects with no mutated alleles and those with two mutated alleles (no mutated alleles vs. two mutated alleles: 0.82 +/- 0.30 vs. 2.71 +/- 0.84; t = 7.86, df = 19, p < 0.0001). Multiple regression analysis showed that the number of mutated alleles of CYP2D6, which was the only significant factor, accounted for 41% and 48% of the variability in log(NT corrected for dose and weight) and log(NT/EHNT), respectively.

Cytochrome P-450 activities in human and rat brain microsomes

The role of cytochrome P450 in the metabolism of dextromethorphan, amitriptyline, midazolam, S-mephenytoin, citalopram, fluoxetine and sertraline was investigated in rat and human brain microsomes. Depending on the parameters, the limit of quantification using gas chromatography-mass spectrometry methods was between 1.6 and 20 pmol per incubation, which generally contained 1500 microg protein. Amitriptyline was shown to be demethylated to nortriptyline by both rat and human microsomes. Inhibition studies using ketoconazole, furafylline, sulfaphenazole, omeprazole and quinidine suggested that CYP3A4 is the isoform responsible for this reaction whereas CYP1A2, CYP2C9, CYP2C19 and CYP2D6 do not seem to be involved. This result was confirmed by using a monoclonal antibody against CYP3A4. Dextromethorphan was metabolized to dextrorphan in rat brain microsomes and was inhibited by quinidine and by a polyclonal antibody against CYP2D6. Only the addition of exogenous reductase allowed the measurement of this activity in human brain microsomes. Metabolites of the other substrates could not be detected, possibly due to an insufficiently sensitive method. It is concluded that cytochrome P450 activity in the brain is very low, but that psychotropic drugs could undergo a local cerebral metabolism which could have pharmacological and/or toxicological consequences.

High-affinity stereoselective reduction of the enantiomers of ketotifen and of ketonic nortriptyline metabolites by aldo-keto reductases from human liver

Aldo-keto reductases (AKR) form an enzyme superfamily catalyzing the reduction of carbonyl compounds and in some cases the reverse oxidation of alcohols as well. In particular, a role in drug metabolism has been considered for the AKR1C family, but published data failed to reveal low Km drug substrates. Moreover, structure activity relationships using chemically related substrates have not been established. In the present investigation, a modified procedure was developed for the isolation of AKR1C1, 1C2, and 1C4 (dihydrodiol dehydrogenases 1, 2, and 4) from human liver cytosol along with carbonyl reductase (EC 1.1.1.184), a member of the short-chain alcohol dehydrogenase superfamily. The kinetics of NADPH-dependent reduction by the closely related enzymes AKR1C1 and 1C2 were studied with the structurally similar substrates (R)- and (S)-ketotifen and E- and Z-10-oxonortriptyline by HPLC measurement of the products. Km values varied between 2.6 and 53 microM and Vmax values between 5 and 313 mU/mg protein; substrate inhibition with Ki around 30 microM occurred in the reduction of E- and Z-10-oxonortriptyline by AKR1C1. The reactions were strictly stereospecific with production of one enantiomeric alcohol from each ketotifen enantiomer and of the (+)-enantiomers of E- and Z-10-hydroxynortriptyline. Enzymatic NADP+ -dependent oxidation of the alcohols mirrored the reduction with regard to stereochemical specificity. All four ketones were no or poor substrates of carbonyl reductase, whereas haloperidol was reduced by this enzyme with low affinity, but high efficiency.

Pharmacokinetics of selective serotonin reuptake inhibitors

The five selective serotonin reuptake inhibitors (SSRIs), fluoxetine, fluvoxamine, paroxetine, sertraline, and citalopram, have similar antidepressant efficacy and a similar side effect profile. They differ, however, in their pharmacokinetic properties. Under steady-state concentrations, their half-lives range between 1 and 4 days for fluoxetine (7 and 15 days for norfluoxetine) and between 21 (paroxetine) and 36 (citalopram) hr for the other SSRIs. Sertraline and citalopram show linear and fluoxetine, fluvoxamine, and paroxetine nonlinear pharmacokinetics. SSRIs underlie an extensive metabolism with high interindividual variability, whereby cytochrome P450 (CYP) isoenzymes play a major role. Therefore, resulting blood concentrations are highly variable between individuals. Except for N-demethylated fluoxetine, metabolites of SSRIs do not contribute to clinical actions. Therapeutically effective blood concentrations are unclear so far, although there is evidence for minimal effective and upper-threshold concentrations that should not be exceeded. Paroxetine and, to a lesser degree, fluoxetine and norfluoxetine are potent inhibitors of CYP2D6 and fluvoxamine of CYP1A2 and CYP2C19. This can give rise to drug-drug interactions that may have no effect, lead to intoxication, or improve the therapeutic response. These different pharmacokinetic properties of the five SSRIs, especially their drug-drug interaction potential, should be considered when selecting a distinct SSRI for treatment of depression or other disorders with a suggested dysfunction of the serotonergic system in the brain.

Polymorphic cytochromes P450 and drugs used in psychiatry

1. The cytochrome P450 monooxygenases, CYP2D6, CYP2C19, and CYP2C9, display polymorphism. CYP2D6 and CYP2C19 have been studied extensively, and despite their low abundance in the liver, they catalyze the metabolism of many drugs. 2. CYP2D6 has numerous allelic variants, whereas CYP2C19 has only two. Most variants are translated into inactive, truncated protein or fail to express protein. 3. CYP2C9 is expressed as the wild-type enzyme and has two variants, in each of which one amino acid residue has been replaced. 4. The nucleotide base sequences of the cDNAs of the three polymorphic genes and their variants have been determined, and the proteins derived from these genes have been characterized. 5. An absence of CYP2D6 and/or CYP2C19 in an individual produces a poor metabolizer (PM) of drugs that are substrates of these enzymes. 6. When two drugs that are substrates for a polymorphic CYP enzyme are administered concomitantly, each will compete for that enzyme and competitively inhibit the metabolism of the other substrate. This can result in toxicity. 7. Patients can be readily phenotyped or genotyped to determine their CYP2D6 or CYP2C19 enzymatic status. Poor metabolizers (PMs), extensive metabolizers (EMs), and ultrarapid metabolizers (URMs) can be identified. 8. Numerous substrates and inhibitors of CYP2D6, CYP2C19, and CYP2C9 are identified. 9. An individual's diet and age can influence CYP enzyme activity. 10. CYP2D6 polymorphism has been associated with the risk of onset of various illnesses, including cancer, schizophrenia, Parkinson's disease, Alzheimer's disease, and epilepsy.

Metabolism of tricyclic antidepressants

1. Despite the considerable advances in the treatments available for mood disorders over the past generation, tricyclic antidepressants (TCAs) remain an important option for the pharmacotherapy of depression. 2. The pharmacokinetics of TCAs are characterized by substantial presystemic first-pass metabolism, a large volume of distribution, extensive protein binding, and an elimination half-life averaging about 1 day (up to 3 days for protriptyline). 3. Clearance of tricyclics is dependent primarily on hepatic cytochrome P450 (CYP) oxidative enzymes. Although the activities of some P450 isoenzymes are largely under genetic control, they may be influenced by external factors, such as the concomitant use of other medications or substances. Patient variables, such as ethnicity and age, also affect TCA metabolism. The impact of gender and related reproductive issues is coming under increased scrutiny. 4. Metabolism of TCAs, especially their hydroxylation, results in the formation of active metabolites, which contribute to both the therapeutic and the adverse effects of these compounds. 5. Renal clearance of the polar metabolites of TCAs is reduced by normal aging, accounting for much of the increased risk of toxicity in older patients. 6. Knowledge of factors affecting the metabolism of TCAs can further the development and understanding of newer antidepressant medications.

Extrapolating in vitro data on drug metabolism to in vivo pharmacokinetics: evaluation of the pharmacokinetic interaction between amitriptyline and fluoxetine

Recently, models have been proposed to extrapolate in vitro data on the influence of inhibitors on drug metabolism to in vivo decrement in drug clearance. Many factors influence drug clearance such as age, gender, habits, diet, environment, liver disease, heredity, and other drugs. In vitro investigation of hepatic cytochrome P450 activity has generally centered on genetic influences and interactions with other drugs. This group of enzymes is involved in many, although not all, drug interactions. The interaction of amitriptyline and fluoxetine is an example. Of the different in vitro paradigms, interaction studies utilizing human liver microsomal preparations have proved to be the most generally applicable for in vitro scaling models. Assuming Michaelis-Menten conditions and applying nonlinear regression, a hybrid inhibition constant (Ki) can be generated that allows classification of the inhibitory potency of an inhibitor toward a specific reaction. This constant is largely independent of the substrate concentration, but in vivo relevance is critically dependent on the inhibitor concentration in the site of metabolic activity, the liver cell cytosol. Many lipophilic drugs are extensively bound to plasma protein but, nonetheless, demonstrate extensive partitioning into liver tissue. This is not compatible with diffusion only of the unbound drug fraction into liver cells. The introduction of a partition factor, based on data from a number of possible sources, provided a reasonable basis for the scaling of in vitro data to in vivo conditions. Many interactions could be reconstructed or predicted with greater accuracy and clinical relevance for interactions such as terfenadine or midazolam and ketoconazole. Even for less marked interactions such as amitriptyline and fluoxetine, this model provides a forecast consistent with the clinically observed range of 22-45% reduction in oral clearance, although this interaction is complicated by the presence of two inhibitors, fluoxetine and norfluoxetine. The concept of in vitro-in vivo scaling is promising and might ultimately yield a fast and more cost-effective screening for drug interactions with reduced human drug exposure and risk.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Five distinct human cytochromes mediate amitriptyline N-demethylation in vitro: dominance of CYP 2C19 and 3A4

The human cytochromes P450 (CYPs) mediating amitriptyline N-demethylation have been identified using a combination of enzyme kinetic and chemical inhibition studies. Amitriptyline was N-demethylated to nortriptyline by microsomes from cDNA transfected human lymphoblastoid cells expressing human CYPs 1A2, 2C9, 2C19, 2D6, and 3A4. CYP 2E1 showed no detectable activity. While CYP 2C19 and CYP 2D6 showed high affinity, CYP 3A4 showed low affinity; CYP 2C9 and 1A2 showed intermediate affinities. Based on these kinetic parameters and estimated relative abundance of the different CYPs in human liver, CYP 2C19 was identified as the major amitriptyline N-demethylase at low (therapeutically relevant) amitriptyline concentrations, whereas CYP 3A4 may be more important at higher amitriptyline concentrations. Chemical inhibition studies with ketoconazole and omeprazole indicate that CYP 3A4 is the major amitriptyline N-demethylase at 100 mumol/L amitriptyline, while CYP 2C19 is equally important at a substrate concentration of 5 mumol/L. The CYP 1A2 inhibitor alpha-naphthoflavone and the CYP 2C9 inhibitor sulfaphenazole produced much less inhibition of amitriptyline N-demethylation at both substrate concentrations. Quinidine produced no detectable inhibition. The kinetics of amitriptyline N-demethylation by human liver microsomes were consistent with a two enzyme model, with the high affinity component exhibiting Michaelis Menten kinetics and the low affinity component exhibiting Hill enzyme kinetics. No difference was apparent in the kinetics of amitriptyline N-demethylation in two liver samples with low levels of CYP 2C19 activity compared with two other samples with relatively normal 2C19 activity. This may reflect the importance of higher substrate concentration values in estimation of kinetic parameters in vitro.

A dual label oligonucleotide ligation assay for detection of the CYP2C19*1, CYP2C19*2, and CYP2C19*3 alleles involving time-resolved fluorometry

CYP2C19 (S-mephenytoin hydroxylase) is a polymorphically expressed enzyme. Currently, two defective alleles are known--CYP2C19*2 and CYP2C19*3. The authors have developed an oligonucleotide ligation assay to detect these two alleles. This assay combines the hybridization of one common, biotinylated capture probe and two allele-specific probes to the target DNA, with the ability of a DNA ligase to distinguish mismatched nucleotides. The probes are only ligated if they are base paired correctly to the target strand. The biotin is bound to streptavidin, and all DNA not covalently bound to the biotin-labeled capture probe, is removed in a washing procedure. The allele-specific probes are labeled with either europium or samarium, and their emission can be measured simultaneously. The ratio between the emission separates the genotypes. This method was applied on DNA from 19 whites and 21 Vietnamese living in Denmark. All genotypes determined by the assay were consistent with the results from restriction enzyme cleavage. There were 12 poor metabolizers; 10 homozygous CYP2C19*2/CYP2C19*2, one heterozygous CYP2C19*2/CYP2C19*3, and one heterozygous CYP2C19*1/CYP2C19*2. The authors conclude that this assay is well-suited for a high throughput of samples in a routine laboratory. The finding of an apparently heterozygous CYP2C19*1/CYP2C19*2 poor metabolizer, confirms that there are still unknown mutations in CYP2C19.

Examination of metabolic pathways and identification of human liver cytochrome P450 isozymes responsible for the metabolism of barnidipine, a calcium channel blocker

1. In a human liver microsomal system, barnidipine was converted into three primary metabolites, an N-debenzylated product (M-1), a hydrolyzed product of the benzyl-pyrrolidine ester (M-3) and an oxidized product of the dihydropyridine ring (M-8). 2. Involvement of CYP3A in the three primary metabolic pathways was revealed by the following studies: (a) inhibition of CYP3A, (b) a correlation study using 10 individual human liver microsomes and (c) cDNA-expression studies. The secondary metabolites, M-2 and M-4 (pyridine forms of M-1 and M-3), were most likely generated from M-8 but were unlikely from M-1 or M-3. Involvement of CYP3A in the secondary pathways of metabolism is also suggested. 3. The possibility of interactions between barnidipine and coadministered drugs was examined in vitro. The formation rate of the primary metabolites was little affected by warfarin, theophylline, phenytoin, diclofenac and amitriptyline at concentrations of 200 microM, but was inhibited by glibenclamide, simvastatin and cyclosporin A. IC50 for the latter drugs was estimated to be > 200, 200 and 20 microM respectively, which was roughly > 200, 6000 and 50 times higher than their respective therapeutic plasma levels, suggesting that interactions with cyclosporin A, a CYP3A inhibitor, are of possible clinical relevance.

Effects of genetic defects in the CYP2C19 gene on the N-demethylation of imipramine, and clinical outcome of imipramine therapy

The relationship between the genetic polymorphism of S-mephenytoin 4'-hydroxylation catalyzed by CYP2C19 and the N-demethylation of imipramine was examined in 10 Japanese depressed patients. Five patients, who were poor metabolizers of S-mephenytoin, were determined to be either homozygous for a mutation in exon 5 or heterozygous for mutations in exon 4 and exon 5 of the CYP2C19 gene. In contrast, five patients, who were extensive metabolizers, had no mutations. The demethylation index (the desipramine/imipramine ratio) was significantly lower in patients with genetic defects. Plasma levels of imipramine and 2-hydroxyimipramine normalized by the daily dose (mg) per weight (kg) were significantly higher in patients with genetic defects. This suggests that the N-demethylation of imipramine is impaired in patients with genetic defects in the CYP2C19 gene, and that genotype determination may be useful in preventing side effects induced by unexpectedly elevated levels of imipramine.

Variability of diphenhydramine N-glucuronidation in healthy subjects

The H1-antagonist diphenhydramine can undergo direct glucuronidation at its tertiary amino group with formation of a quaternary ammonium glucuronide. The intraindividual variability in the amount of N-glucuronide excretion in urine was investigated in two female volunteers who repeatedly took single doses of 25 mg diphenhydramine hydrochloride without and with concomitant administration of ascorbic acid or ammonium chloride for urine acidification. Another two female and four male subjects underwent single tests without and with additional ascorbic acid. Diphenhydramine N-glucuronide quantities in urine differed significantly among subjects and ranged between 2.7% and 14.8% of the dose within 8 h. Neither ascorbic acid nor ammonium chloride significantly influenced the quantity of N-glucuronide in urine, but ammonium chloride, that in contrast to ascorbic acid proved effective in lowering urinary pH, increased the excretion of the parent drug.

Pharmacogenetics: a laboratory tool for optimizing therapeutic efficiency

Pharmacogenetics is the study of the linkage between an individual’s genotype and that individual’s ability to metabolize a foreign compound. Differences in metabolism of therapeutics can lead to severe toxicity or therapeutic failure by altering the relation between dose and blood concentration of the pharmacologically active drug. Phenotypes exhibiting poor and ultraextensive metabolism result from genetic variance (polymorphism) of enzymes involved in metabolism. Thus, in pharmacogenetic studies one applies genotyping of polymorphic alleles encoding drug-metabolizing enzymes to the identification of an individual’s drug metabolism phenotype. This knowledge, when applied to dosing or drug selection, can avoid adverse reactions or therapeutic failure and thus enhance therapeutic efficiency. More than 25 commonly prescribed medicines are metabolized by the cytochrome P-4502D6 (CYP2D6) isoenzyme, and polymorphism of the CYP2D6 gene affects the therapeutic management of up to 17% of individuals in some ethnic groups. In this review, we summarize and update information concerning drug-metabolizing genotypes with emphasis on CYP2D6 genotyping techniques that can be applied by the clinical laboratory for linking human genetics to therapeutic management.

Pharmacokinetic-pharmacodynamic relationship of the selective serotonin reuptake inhibitors

The recently introduced antidepressants, the selective serotonin reuptake inhibitors (SSRIs) [citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline], are known for their clinical efficacy, good tolerability and relative safety. They differ from each other in chemical structure, metabolism and pharmacokinetic properties. Therapeutic drug monitoring of these compounds is not widely used, as the plasma concentration ranges within which clinical response with minimal adverse effects appears to be optimal are not clearly defined. Almost all recent assays developed for the quantitative determination of SSRIs and their metabolites in blood are based either on the separation of SSRIs by high performance liquid chromatography (HPLC) or gas chromatography (GC). Citalopram and fluoxetine have been introduced as racemic compounds. There are some differences in the pharmacological profile, metabolism and pharmacokinetics between the enantiomers of the parent compounds and their demethylated metabolites. Stereoselective chromatographic methods for their analysis in blood are now available. With regard to the SSRIs presently available, no clearcut plasma concentration-clinical effectiveness relationship in patients with depression has been shown, nor any threshold which defines toxic concentrations. This may be explained by their low toxicity and use at dosages where serious adverse effects do not appear. SSRIs vary widely in their qualitative and quantitative interaction with cytochrome P450 (CYP) isozymes in the liver. CYP2D6 is inhibited by SSRIs, in order of decreasing potency paroxetine, norfluoxetine, fluoxetine, sertraline, citalopram and fluvoxamine. This may have clinical consequences with some but not all SSRIs, when they are taken with tricyclic antidepressants. Except for citalopram and paroxetine, little is known about the enzymes which control the biotransformation of the SSRIs. There have been many reports on marked pharmacokinetic interactions between fluoxetine and tricyclic antidepressants. Fluoxetine has a stronger effect on their hydroxylation than on their demethylation. Interactions observed between fluoxetine and alprazolam, midazolam and carbamazepine seem to occur on the level of CYP3A. Fluvoxamine strongly inhibits the N-demethylation of some tricyclic antidepressants of the tertiary amine type and of clozapine. This may lead to adverse effects but augmentation with fluvoxamine can also improve response in very rapid metabolisers, as it increases the bioavailability of the comedication. Fluvoxamine inhibits with decreasing potency, CYP1A2, CYP2C19, CYP2D6 and CYP1A1, but it is also an inhibitor of CYP3A. Fluoxetine and fluvoxamine have shown to increase methadone plasma concentrations in dependent patients. Some authors warn about a combination of monoamine oxidase (MAO) inhibitors with SSRIs, as this could lead to a serotonergic syndrome. Studies with healthy volunteers suggest, however, that a combination of moclobemide and SSRIs, such as fluvoxamine, should not present serious risks in promoting a serotonin syndrome. A combination of moclobemide and fluvoxamine has successfully been used in refractory depression, but more studies are needed, including plasma-concentration monitoring, before this combined treatment can be recommended. Paroxetine is a substrate of CYP2D6, but other enzyme(s) could also be involved. Its pharmacokinetics are linear in poor metabolisers of sparteine, and non-linear in extensive metabolisers. Due to its potent CYP2D6 inhibiting properties, comedication with this SSRI can lead to an increase of tricyclic antidepressants in plasma, as shown with amitriptyline and trimipramine. CYP3A has been claimed to be involved in the biotransformation of sertraline to norsertraline. Clinical investigations (with desipramine) confirmed in vitro findings that CYP2D6 inhibition by sertraline is only moderate. (ABSTRACT TRUNCATED)

Cytochrome P450 enzymes: interpretation of their interactions with selective serotonin reuptake inhibitors. Part II

The SSRIs have been used as an example to show how one might interpret the available evidence to draw conclusions about the relationships between drugs and P450s. Under what circumstances might one apply the knowledge of such relationships? First, the clinical implications must be considered when drugs with a narrow therapeutic index are coprescribed with other drugs that may affect P450s. For example, good clinical practice demands that before a TCA is coprescribed with another drug, the physician be aware of the potential for the second drug to interact with CYP2D6. Second, it may be helpful to consider P450 enzymes when adverse events occur during polypharmacy. It may happen that a known side effect of one drug occurs. Rather than attributing this to patient sensitivity, the physician should consider the possibility that a pharmacokinetic drug interaction increased plasma drug concentration, which in turn enhanced the probability of such an occurrence. Even when a pharmacokinetic drug interaction is considered as a possible cause, an appreciation of the role of P450s may lead to the realization that an interaction was not only possible but that it was likely. Finally, copharmacy can be used intentionally to produce controlled interactions. Indeed, planned pharmacokinetic drug interactions at the level of P450s have been proposed to reduce cyclosporine dosage requirements, to reduce variability of TCA levels, and to manipulate the contribution of alternative metabolic pathways to minimize toxic effects. As long as pharmaceuticals are metabolized by the P450 system, interactions with the various isozymes will be inescapable. It is fortunate that understanding them is becoming more tractable.

Steady-state plasma concentrations of imipramine and desipramine in relation to S-mephenytoin 4'-hydroxylation status in Japanese depressive patients

The steady-state plasma concentrations of imipramine and desipramine were measured after a more than 2-week treatment with 0.39 to 1.39 mg/kg/day of imipramine hydrochloride in 28 Japanese patients with major depression who had been phenotyped simultaneously with mephenytoin (for CYP2C19-related status) and with metoprolol (for CYP2D6-related status) before initiating the antidepressant therapy. Patients consisted of five poor metabolizers (PMs) of CYP2C19 with an extensive metabolizer (EM) phenotype of CYP2D6, whereas the remainder were EMs for both of the phenotypes. The mean respective concentrations (corrected by mg/kg) of imipramine and the sum of imipramine plus desipramine were 2.4 and 1.8 times greater in the CYP2C19-related PM than in the EM group, and these two variables correlated with the log10 urinary excretion of 4'-hydroxymephenytoin (rs = -0.73 and -0.64, both p < 0.01, respectively), but not with the metabolic ratio (MR) of metoprolol/alpha-hydroxymetoprolol. The mean N-demethylation index (MR of desipramine/imipramine) was significantly (p < 0.01) less in the PM than in the EM group. This index correlated with the 4'-hydroxylation of S-mephenytoin (rs = -0.51, p < 0.01), but not with the alpha-hydroxylation of metoprolol, implying that imipramine N-demethylation is under a coregulatory pharmacogenetic control of CYP2C19, but not of CYP2D6. In conclusion, by taking into account that the incidence of the PMs of CYP2C19 is much greater (18-23%) than that of CYP2D6 (< 1%) in Japanese population, the individually predetermined assessment of the CYP2C19-mediated metabolic capacity of imipramine would be more valuable than that of the CYP2D6-mediated capacity for forecasting the steady-state concentrations of imipramine and desipramine in Japanese depressive patients, thereby attaining an individualized optimization of imipramine therapy. Obviously, a pharmacodynamic assessment study conducted simultaneously with predetermined CYP2C19 status is required for supporting this contention.

Determination of CYP2C19 phenotype in black Americans with omeprazole: correlation with genotype

BACKGROUND

Our objective was to study omeprazole as a single-dose oral probe in the determination of CYP2C19 phenotype in black subjects and to determine the correlation between phenotype and genotype.

METHODS

This single-dose, open-label outpatient study was conducted at a community-based, university-affiliated teaching hospital outpatient clinic. Study subjects were 100 healthy, unrelated black adults (age range, 18 to 50 years) who were receiving no medications. Baseline omeprazole and 2-hour postingestion omeprazole and 5'-hydroxyomeprazole concentrations were measured for phenotype determination. Identification of CYP2C19m1 genotypes were performed with use of the polymerase chain reaction.

RESULTS

Results were obtained for 28 men and 72 women. Ninety-eight subjects were found to be phenotypically extensive metabolizers and two to be poor metabolizers (one man; one smoker). Genotype determination revealed that the two poor metabolizers of omeprazole were homozygous for a single base pair mutation (m1/m1) in exon 5 of CYP2C19. Twenty-eight of the extensive metabolizers were heterozygous (m1/wt) and the remaining 70 were homozygous (wt/wt). No side effects were reported.

CONCLUSIONS

The 2% prevalence rate of poor CYP2C19 metabolizers in this healthy black population residing in the Midwestern United States is similar to that reported in white subjects and in the Shona population of Zimbabwe but much less than in Asian subjects. Omeprazole is a safe and specific probe of the CYP2C19 enzyme system that correlates well with genotype.