Role of P450IID6, the target of the sparteine-debrisoquin oxidation polymorphism, in the metabolism of imipramine

The effects of agomelatine and imipramine on liver cytochrome P450 during chronic mild stress (CMS) in the rat

BACKGROUND

The aim of our research was to determine the effects of chronic treatment with the atypical antidepressant agomelatine on the expression and activity of liver cytochrome P450 (CYP) in the chronic mild stress (CMS) model of depression, and to compare the results with those obtained for the first-generation antidepressant imipramine.

METHODS

Male Wistar rats were subjected to CMS for 7 weeks. Imipramine (10 mg/kg ip/day) or agomelatine (40 mg/kg ip/day) was administered to nonstressed or stressed animals for 5 weeks (weeks 3-7 of CMS). The levels of cytochrome P450 mRNA, protein and activity were measured in the liver.

RESULTS

Agomelatine and imipramine produced different broad-spectrum effects on cytochrome P450. Like imipramine, agomelatine increased the expression/activity of CYP2B and CYP2C6, and decreased the CYP2D activity. Unlike imipramine, agomelatine raised the expression/activity of CYP1A, CYP2A and reduced that of CYP2C11 and CYP3A. CMS modified the effects of antidepressants at transcriptional/posttranscriptional level; however, the enzyme activity in stressed rats remained similar to that in nonstressed animals. CMS alone decreased the CYP2B1 mRNA level and increased that of CYP2C11.

CONCLUSION

We conclude the following: (1) the effects of agomelatine and imipramine on cytochrome P450 are different and involve both central and peripheral regulatory mechanisms, which implicates the possibility of drug-drug interactions; (2) CMS influences the effects of antidepressants on cytochrome P450 expression, but does not change appreciably their effects on the enzyme activity. This suggests that the rate of antidepressant drug metabolism under CMS is similar to that under normal conditions.

Time of Administration of Acute or Chronic Doses of Imipramine Affects its Antidepressant Action in Rats

The pathogenesis and therapeutics of depression are linked to the operation of the circadian system. Here, we studied the chronopharmacological action of a tricyclic antidepressant, imipramine. Male adult Wistar–Hannover rats were administered imipramine acutely or chronically in the morning or in the evening. The antidepressant action of imipramine was analyzed using the forced swim test (FST). A single dose of imipramine (30 mg/kg) in the morning, but not in the evening, reduced immobility and increased climbing in the FST. The plasma concentrations of imipramine and its metabolite, desipramine, were slightly higher in the morning than in the evening, which might explain the dosing time-dependent action of imipramine. Next, we analyzed the effect of chronic imipramine treatment. Rats received imipramine in the morning or in the evening for 2 weeks. The morning treatment resulted in larger effects in the FST than the evening treatment, and was effective at a dose that was ineffective when administered acutely. The levels of brain α-adrenergic receptors tended to decrease after chronic imipramine treatment. Imipramine might interact with noradrenergic neurons, and this interaction might chronically alter receptor expression. This alteration seemed greater in the morning than in the evening, which might explain the dosing time-dependent action of imipramine.

Pharmacogenetics of Agents Acting on the Central Nervous System

This article will review the various agents affecting the central nervous system (CNS) such as the analgesics, antidepressants, anticonvulsants, antipsychotics, and benzodiazepines. Most of the research in pharmacogenetics with the CNS agents have been conducted in the antidepressants. The cytochrome 450 IID6 isozyme system has been shown to influence the disposition of the antidepressants and antipsychotics. Amitriptyline metabolism to nortriptyline and nortriptyline conversion to its 10-OH metabolite were shown to be influenced by the IID6 isozyme. Interestingly, imipramine metabolism to desipramine is only partially related to the IID6 isozyme. Biotransformation of imipramine to its 2-OH metabolite was shown to be affected by the IID6 isozyme, but its metabolism to the 10-OH remains to be investigated. Of the antipsychotic drugs, haloperidol and thioridazine are two agents most studied. Haloperidol is converted to a reduced metabolite via a ketone reductase enzyme. The reduced metabolite is oxidized back to Haloperidol. This oxidation pathway was reported to be affected by the IID6 isozyme. Thioridazine metabolism to mesoridazine and conversion of codeine to morphine appear to be also influenced by CP-450 IID6. Other 450 isozymes are reported to be involved with other CNS agents.

Software-aided cytochrome P450 reaction phenotyping and kinetic analysis in early drug discovery

RATIONALE

Cytochrome P450 (CYP450) reaction phenotyping (CRP) and kinetic studies are essential in early drug discovery to determine which metabolic enzymes react with new drug entities. A new semi-automated computer-assisted workflow for CRP is introduced in this work. This workflow provides not only information regarding parent disappearance, but also metabolite identification and relative metabolite formation rates for kinetic analysis.

METHODS

Time-course experiments based on incubating six probe substrates (dextromethorphan, imipramine, buspirone, midazolam, ethoxyresorufin and diclofenac) with recombinant human enzymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4) and human liver microsomes (HLM) were performed. Liquid chromatography/high-resolution mass spectrometry (LC/HRMS) analysis was conducted with an internal standard to obtain high-resolution full-scan and MS/MS data. Data were analyzed using Mass-MetaSite software. A server application (WebMetabase) was used for data visualization and review.

RESULTS

CRP experiments were performed, and the data were analyzed using a software-aided approach. This automated-evaluation approach led to (1) the detection of the CYP450 enzymes responsible for both substrate depletion and metabolite formation, (2) the identification of specific biotransformations, (3) the elucidation of metabolite structures based on MS/MS fragment analysis, and (4) the determination of the initial relative formation rates of major metabolites by CYP450 enzymes.

CONCLUSIONS

This largely automated workflow enabled the efficient analysis of HRMS data, allowing rapid evaluation of the involvement of the main CYP450 enzymes in the metabolism of new molecules during drug discovery.

Drug-drug interactions in pharmacologic management of gastroparesis

BACKGROUND

Gastroparesis is a disorder characterized by delayed gastric emptying due to chronic abnormal gastric motility. The treatment of the disease often entails the co-administration of several classes of pharmacological agents. These agents may be metabolized via the same pathway. Inhibition or induction of a shared metabolic pathway leads to change in the systemic levels of prescribed drugs, possibly leading to undesired clinical outcomes.

PURPOSE

This review discusses different pharmacological treatment for gastroparesis patients and describes the potential for drug-drug interactions (DDIs) in some of the combinations that are currently used. Prokinetic agents such as metoclopramide and domperidone are the cornerstone in treatment of gastroparesis. Antiemetic agents such as promethazine and ondansetron are frequently administered to gastroparesis patients to reduce nausea and vomiting. Gastroparesis is prevalent in diabetic patients and therefore antidiabetic agents are also prescribed. Many of these co-administered drugs are metabolized via common drug metabolizing enzymes and this can trigger potential DDIs. The scientific literature was reviewed from the years 1975-2014 for original research articles and reviews that evaluated DDIs in gastroparesis. Many commonly prescribed combinations were predicted to cause potential DDIs in gastroparesis patients. This review will help inform about potential hazardous combinations. This information will hopefully lead to less adverse effects and more successful gastroparesis management.

Safe and effective medicines for all: is personalized medicine the answer?

An improvement in drug treatment and clinical outcome is one of the major challenges in clinical medicine. The development of evidence-based standards of care has led to a significant improvement, but, by definition, strictly standardized cohorts in clinical trials have to ignore individual differences. Personalized medicine is defined as the application of genomic and molecular data to better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a person's predisposition to a particular disease or condition. After the deciphering of the human genome, however, the high expectations in individualized medicine were not always fulfilled. However, personalized medicine has become indispensable in the treatment of malignant diseases and there is increasing evidence for its benefit in other areas. This article outlines the impact of pharmacogenetics and pharmacogenomics, especially with regard to personalized medicine, in major medical indications and reflects the obstacles and chances taken in current daily practice.

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.

Nortriptyline Cardiac Toxicity Resulting from a Probable Interaction with Telithromycin

Objective

To report a case of probable nortriptyline toxicity associated with a combination of telithromycin and nortriptyline.

Case Summary

A 50-year-old white woman was admitted to the hospital with new-onset palpitations after taking 5 days of telithromycin in combination with nortriptyline. On admission her electrocardiograph showed atrioventricular nodal reentry tachycardia. The patient was also hypotensive, with a blood pressure of 90/45 mm Hg. She had been taking nortriptyline 150 mg daily at bedtime for 10 years without complications. The final day of telithromycin therapy was completed, and nortriptyline was discontinued on admission. The tachycardia subsided with a dose of diltiazem.

Discussion

It is suspected that the cardiovascular symptoms observed in the patient are the result of nortriptyline toxicity caused by inhibition of CYP2D6 by telithromycin. Limited evidence shows that telithromycin may be an inhibitor of CYP2D6. An assessment of causation of the adverse effects using the Drug Interaction Probability Scale suggests a probable interaction between telithromycin and nortriptyline.

Conclusion

It is recommended that clinicians remain aware of possible interactions between telithromycin and known CYP2D6 substrates such as nortriptyline. Should telithromycin therapy be required, close monitoring of the potential adverse effects and/or plasma drug levels of patients taking interacting drugs is warranted.

Effect of psychotropic drugs on the 21-hydroxylation of neurosteroids, progesterone and allopregnanolone, catalyzed by rat CYP2D4 and human CYP2D6 in the brain

We determined the effects of psychotropic drugs on the cytochrome P450 2D (CYP2D)-mediated 21-hydroxylation of progesterone (PROG) and allopregnanolone (ALLO) with the goal of clarifying whether neurosteroid levels are affected by psychotropic drugs in the brain. PROG or ALLO was incubated with rat CYP2D4 or human CYP2D6 in the presence of typical psychotropic drugs, fluoxetine, imipramine, desipramine, mazindol, and GBR12909, and the 21-hydroxylated metabolites of PROG and ALLO were determined by high performance liquid chromatography and liquid chromatography-tandem mass spectrometry, respectively. Fluoxetine competitively inhibited CYP2D4-mediated PROG 21-hydroxylation and increased both Km and Vmax values of CYP2D6-mediated PROG 21-hydroxylation. In addition, fluoxetine competitively inhibited ALLO 21-hydroxylation mediated by CYP2D4 and CYP2D6. Imipramine, desipramine, mazindol, and GBR12909 competitively inhibited PROG 21-hydroxylation mediated by CYP2D4 and/or CYP2D6, and all psychotropic drugs inhibited ALLO 21-hydroxylation mediated by CYP2D4 and/or CYP2D6. The inhibition constants (Ki values) of imipramine, desipramine, and mazindol against the 21-hydroxylation of PROG and ALLO by CYP2D6 were lower than those by CYP2D4. These results indicate that psychotropic drugs including fluoxetine affected the metabolism of neurosteroids, such as PROG and ALLO in the brain, suggesting that the regulation of the neurosteroid levels is modified by central nervous system-active drugs that inhibit brain CYP2D isoforms.

Minimizing Polymorphic Metabolism in Drug Discovery: Evaluation of the Utility of in Vitro Methods for Predicting Pharmacokinetic Consequences Associated with CYP2D6 Metabolism

Minimizing interindividual variability in drug exposure is an important goal for drug discovery. The reliability of the selective CYP2D6 inhibitor quinidine was evaluated in a retrospective analysis using a standardized approach that avoids laboratory-to-laboratory variation. The goal was to evaluate the reliability of in vitro metabolism studies for predicting extensive metabolizer (EM)/poor metabolizer (PM) exposure differences. Using available literature, 18 CYP2D6 substrates were selected for further analysis. In vitro microsomal studies were conducted at 1 microM substrate and 0.5 microM P450 to monitor substrate depletion. An estimate of the fraction metabolized by CYP2D6 in microsomes was derived from the rate constant determined with and without 1 microM quinidine for 11 substrates. Clearance in EM and PM subjects and fractional recovery of metabolites were taken from the literature. A nonlinear relationship between the contribution of CYP2D6 and decreased oral clearance for PMs relative to EMs was evident. For drugs having <60% CYP2D6 involvement in vivo, a modest difference between EM and PM exposure was observed (<2.5-fold). For major CYP2D6 substrates (>60%), more dramatic exposure differences were observed (3.5- to 53-fold). For compounds primarily eliminated by hepatic P450 and with sufficient turnover to be evaluated in vitro, the fraction metabolized by CYP2D6 in vitro compared favorably with the in vivo data. The in vitro estimation of fraction metabolized using quinidine as a specific inhibitor provided an excellent predictive tool. Results from microsomal substrate depletion experiments can be used with confidence to select compounds in drug discovery using a cutoff of >60% metabolism by CYP2D6.

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.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

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

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Neurobiological differences on populational level

At present, 13% of world population are suffering from neuropsychiatric disorders. Prevention and treatment of these pathological conditions are very important. New research strategies are needed for the creation of effective methods for controlling of neuropsychiatric disorders. International collaborative research projects within WHO's activities in psychopharmacology and biological psychiatry with the participation of 12 countries representing different world regions have been organized and successfully finalized. The author discusses some of these in the Festschrift for Dr. Corneille Radouco-Thomas.

Clinical pharmacokinetics of drugs used to treat urge incontinence

Urge incontinence (also known as overactive bladder) is a common form of urinary incontinence, occurring alone or as a component of mixed urinary incontinence, frequently together with stress incontinence. Because of the pathophysiology of urge incontinence, anticholinergic/antispasmodic agents form the cornerstone of therapy. Unfortunately, the pharmacological activity of these agents is not limited to the urinary tract, leading to systemic adverse effects that often promote nonadherence. Although the pharmacokinetics of flavoxate, propantheline, scopolamine, imipramine/desipramine, trospium chloride and propiverine are also reviewed here, only for oxybutynin and tolterodine are there adequate efficacy/tolerability data to support their use in urge incontinence. Oxybutynin is poorly absorbed orally (2-11% for the immediate-release tablet formulation). Controlled-release oral formulations significantly prolong the time to peak plasma concentration and reduce the degree of fluctuation around the average concentration. Significant absorption occurs after intravesical (bladder) and transdermal administration, although concentrations of the active N-desethyl metabolite are lower after transdermal compared with oral administration, possibly improving tolerability. Food has been found to significantly affect the absorption of one of the controlled-release formulations of oxybutynin, enhancing the rate of drug release. Oxybutynin is extensively metabolised, principally via N-demethylation mediated by the cytochrome P450 (CYP) 3A isozyme. The pharmacokinetics of tolterodine are dependent in large part on the pharmacogenomics of the CYP2D6 and 3A4 isozymes. In an unselected population, oral bioavailability of tolterodine ranges from 10% to 74% (mean 33%) whereas in CYP2D6 extensive metabolisers and poor metabolisers mean bioavailabilities are 26% and 91%, respectively. Tolterodine is metabolised via CYP2D6 to the active metabolite 5-hydroxymethyl-tolterodine and via CYP3A to N-dealkylated metabolites. Urinary excretion of parent compound plays a minor role in drug disposition. Drug effect is based upon the unbound concentration of the so-called 'active moiety' (sum of tolterodine + 5-hydroxymethyl-tolterodine). Terminal disposition half-lives of tolterodine and 5-hydroxymethyl-tolterodine (in CYP2D6 extensive metabolisers) are 2-3 and 3-4 hours, respectively. Coadministration of antacid essentially converts the extended-release formulation into an immediate-release formulation. Knowledge of the pharmacokinetics of these agents may improve the treatment of urge incontinence by allowing the identification of individuals at high risk for toxicity with 'usual' dosages. In addition, the use of alternative formulations (controlled-release oral, transdermal) may also facilitate adherence, not only by reducing the frequency of drug administration but also by enhancing tolerability by altering the proportions of parent compound and active metabolite in the blood.

Pharmacogenetics of cytochrome p4502D6: genetic background and clinical implication

Interindividual differences in the pharmacokinetics of a number of drugs are often due to hereditary polymorphisms of drug-metabolizing enzymes. Most important is cytochrome p4502D6 (CYP2D6), also known as debrisoquine/sparteine hydroxylase. It catalyzes hydroxylation or demethylation of more than 20% of drugs metabolized in the human liver, such as neuroleptics, antidepressants, some beta-blockers and many others like codeine. About 7%-10% of Caucasians lack any CYP2D6 activity due to deletions and frame-shift or splice-site mutations of the gene. About 1%-3% of Middle-Europeans, but up to 29% of Ethiopians display gene duplications, leading to elevated so-called ultrarapid metabolization rates. Meanwhile there is now a much better understanding of the genetic background of poor, intermediate, extensive and ultrarapid metabolizers, enabling a more precise DNA genotyping-based prediction of plasma levels. Since there is evidence that deteriorated drug elimination partly accounts for drug side-effects, CYP2D6 genotyping could contribute to an individualized and therefore optimized drug therapy.

Olanzapine as an antiemetic in refractory nausea and vomiting in advanced cancer

Nausea and vomiting are difficult symptoms to manage in patients with advanced cancer. Several classes of antiemetics are available, including phenothiazines, butyrophenones, substituted benzamides and selective serotonin antagonists, as well as corticosteroids. Most patients will respond to either single agents or combinations that frequently include corticosteroids. A minority of patients will have nausea that fails to respond. The atypical antipsychotic, olanzapine, relieves nausea in some patients failing to respond to the usual antiemetics. Two case reports are presented and the rationale for olanzapine's benefit is discussed.

Contribution of human cytochrome p-450 isoforms to the metabolism of the simplest phenothiazine neuroleptic promazine

1. The aim of the present study was to identify human cytochrome p-450 isoforms (CYPs) involved in 5-sulphoxidation and N-demethylation of the simplest phenothiazine neuroleptic promazine in human liver. 2. The experiments were performed in the following in vitro models: (A). a study of promazine metabolism in liver microsomes-(a). correlations between the rate of promazine metabolism and the level and activity of CYPs; (b). the effect of specific inhibitors on the rate of promazine metabolism (inhibitors: CYP1A2-furafylline, CYP2D6-quinidine, CYP2A6+CYP2E1-diethyldithiocarbamic acid, CYP2C9-sulfaphenazole, CYP2C19-ticlopidine, CYP3A4-ketoconazole); (B). promazine biotransformation by cDNA-expressed human CYPs (Supersomes 1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2E1, 3A4); (C). promazine metabolism in a primary culture of human hepatocytes treated with specific inducers (rifampicin-CYP3A4, CYP2B6 and CYP2C inducer, 2,3,7,8-tetrachlordibenzeno-p-dioxin (TCDD)-CYP1A1/1A2 inducer). 3. In human liver microsomes, the formation of promazine 5-sulphoxide and N-desmethylpromazine was significantly correlated with the level of CYP1A2 and ethoxyresorufin O-deethylase and acetanilide 4-hydroxylase activities, as well as with the level of CYP3A4 and cyclosporin A oxidase activity. Moreover, the formation of N-desmethylpromazine was correlated well with S-mephenytoin 4'-hydroxylation. 4. Furafylline (a CYP1A2 inhibitor) and ketoconazole (a CYP3A4 inhibitor) significantly decreased the rate of promazine 5-sulphoxidation, while furafylline and ticlopidine (a CYP2C19 inhibitor) significantly decreased the rate of promazine N-demethylation in human liver microsomes. 5. The cDNA-expressed human CYPs generated different amounts of promazine metabolites, but the rates of CYP isoforms to catalyse promazine metabolism at therapeutic concentration (10 microM) was as follows: 1A1>2B6>1A2>2C9>3A4>2E1>2A6>2D6>2C19 for 5-sulphoxidation and 2C19>2B6>1A1>1A2>2D6>3A4>2C9>2E1>2A6 for N-demethylation. The highest intrinsic clearance (V(max)/K(m)) was found for CYP1A subfamily, CYP3A4 and CYP2B6 in the case of 5- sulphoxidation, and for CYP2C19, CYP1A subfamily and CYP2B6 in the case of N-demethylation. 6. In a primary culture of human hepatocytes, TCDD (a CYP1A subfamily inducer), as well as rifampicin (mainly a CYP3A4 inducer) induced the formation of promazine 5-sulphoxide and N-desmethylpromazine. 7. Regarding the relative expression of various CYPs in human liver, the obtained results indicate that CYP1A2 and CYP3A4 are the main isoforms responsible for 5-sulphoxidation, while CYP1A2 and CYP2C19 are the basic isoforms that catalyse N-demethylation of promazine in human liver. Of the other isoforms studied, CYP2C9 and CYP3A4 contribute to a lesser degree to promazine 5-sulphoxidation and N-demethylation, respectively. The role of CYP2A6, CYP2B6, CYP2D6 and CYP2E1 in the investigated metabolic pathways of promazine seems negligible.

Mechanisms of cellular distribution of psychotropic drugs. Significance for drug action and interactions

Distribution of a drug in the body is dependent on its permeation properties, the blood flow rates in various tissues, and on plasma and tissue uptake. The distribution of drugs in vivo is largely determined by uptake competitions between blood and tissues, as well as competitions among individual tissues. Basic lipophilic drugs are characterized by extensive accumulation in tissues, which leads to a high volume of distribution. Nonspecific binding to cellular membranes and uptake by acidic compartments (mainly lysosomes) are responsible for such a distribution pattern. Lysosomal trapping is an important mechanism of distribution of basic psychotropic drugs; however, the tissue distribution of the aliphatic-type phenothiazine neuroleptic promazine, tricyclic antidepressants (TADs) and selective serotonin reuptake inhibitors (SSRIs) depends more on phospholipid binding than on lysosomal trapping, whereas in the case thioridazine and perazine, lysosomal trapping is as important for the tissue uptake as is phospholipid binding. Neuroleptics and antidepressants mutually inhibit their lysosomal uptake. A decrease in the intralysosomal drug concentrations in vivo leads to a shift of the drug from organs abundant in lysosomes (lungs, liver and kidneys) to those poor in these organella, e.g., the heart, which may be of clinical importance (cardiotoxicity). The brain is not a homogenous organ, i.e., the phospholipid pattern and density of lysosomes vary in its different regions. Therefore, the contribution of the two mechanisms (lysosomal trapping and tissue binding) to total drug uptake is different in areas of the brain. Both lysosomal trapping and binding to cellular elements for psychotropics are higher in the grey matter and neurons than in the white matter and astrocytes, respectively. Lysosomal trapping and distribution interactions of psychotropics take place mainly in neurons. A decrease (via a distributive interaction) in the concentration of psychotropics in lysosomes (depot) may lead to an increase in their level in membranes and tissue fluids (i.e., in concentrations and compartments relevant to their pharmacological action) and, in consequence, to enhancement of the drug binding to neurotransmitter receptors and/or transporters.

Inhibition and possible induction of rat CYP2D after short- and long-term treatment with antidepressants

The aim of this study was to investigate the influence of tricyclic antidepressants (imipramine, amitriptyline, clomipramine, desipramine), selective serotonin reuptake inhibitors (SSRIs: fluoxetine, sertraline) and novel antidepressant drugs (mirtazapine, nefazodone) on the activity of CYP2D, measured as a rate of ethylmorphine O-deethylation. The reaction was studied in control liver microsomes in the presence of the antidepressants, as well as in microsomes of rats treated intraperitoneally for one day or two weeks (twice a day) with pharmacological doses of the drugs (imipramine, amitriptyline, clomipramine, nefazodone 10 mg kg(-1) i.p.; desipramine, fluoxetine, sertraline 5 mg kg(-1) i.p.; mirtazapine 3 mg kg(-1) i.p.), in the absence of the antidepressants in-vitro. Antidepressants decreased the activity of the rat CYP2D by competitive inhibition of the enzyme, the potency of their inhibitory effect being as follows: clomipramine (K(i) = 14 microM) > sertraline approximate, equals fluoxetine (K(i) = 17 and 16 microM, respectively) > imipramine approximate, equals amitriptyline (K(i) = 26 and 25 microM, respectively) > desipramine (K(i) = 44 microM) > nefazodone (K(i) = 55 microM) > mirtazapine (K(i) = 107 microM). A one-day treatment with antidepressants caused a significant decrease in the CYP2D activity after imipramine, fluoxetine and sertraline. After prolonged administration of antidepressants, the decreased CYP2D activity produced by imipramine, fluoxetine and sertraline was still maintained. Moreover, amitriptyline and nefazodone significantly decreased, while mirtazapine increased the activity of the enzyme. Desipramine and clomipramine did not produce any effect when administered in-vivo. The obtained results indicate three different mechanisms of the antidepressants-CYP2D interaction: firstly, competitive inhibition of CYP2D shown in-vitro, the inhibitory effects of tricyclic antidepressants and SSRIs being stronger than those of novel drugs; secondly, in-vivo inhibition of CYP2D produced by both one-day and chronic treatment with tricyclic antidepressants (except for desipramine and clomipramine) and SSRIs, which suggests inactivation of the enzyme apoprotein by reactive metabolites; and thirdly, in-vivo inhibition by nefazodone and induction by mirtazapine of CYP2D produced only by chronic treatment with the drugs, which suggests their influence on the enzyme regulation.

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 genetic basis of variability in drug responses

It is almost axiomatic that patients vary widely in their beneficial responses to drug therapy, and serious and apparently unpredictable adverse drug reactions continue to be a major public health problem. Here, we discuss the concept that genetic variants might determine much of this variability in drug response, and propose an algorithm to enable further evaluation of the benefits and pitfalls of this enticing possibility.

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.

The effect of selective serotonin reuptake inhibitors (SSRIs) on the pharmacokinetics and metabolism of perazine in the rat

The aim of this study was to investigate the effect of three selective serotonin reuptake inhibitors (SSRIs), fluoxetine, fluvoxamine and sertraline, on the pharmacokinetics and metabolism of perazine in a steady state in rats. Perazine (10 mg kg(-1), i.p.) was administered twice daily for two weeks, alone or jointly with one of the SSRIs. Concentrations of perazine and its two main metabolites (N-desmethylperazine and 5-sulfoxide) in the plasma and brain were measured 30 min and 6 and 12 h after the last dose of the drugs. Of the investigated SSRIs, fluoxetine and fluvoxamine significantly increased plasma and brain concentrations of perazine (up to 900% and 760% of the control value, respectively), their effect being most pronounced after 30 min and 6 h. Moreover, simultaneous increases in perazine metabolites concentrations and in the perazine/metabolite concentration ratios were observed. Sertraline elevated plasma and brain concentrations of perazine after 30 min. In-vitro studies with liver microsomes of rats treated chronically with perazine, SSRIs ortheir combinations showed decreased concentrations of cytochrome P-450 after perazine and a combination of perazine and fluvoxamine (vs control), and increased concentration after a combination of perazine and fluoxetine (vs perazine-treated group). Prolonged treatment with perazine did not significantly change the rate of its own metabolism. Chronic administration of fluoxetine or sertraline, alone or in a combination with perazine, accelerated perazine N-demethylation (vs control or perazine group, respectively). Fluvoxamine had a similar effect. The 5-sulfoxidation of perazine was accelerated by fluvoxamine and sertraline treatment, but the process was inhibited by administration of a combination of perazine and fluoxetine or fluvoxamine (vs control). Kinetic studies using control liver microsomes, in the absence or presence of SSRIs added in-vitro, demonstrated competitive inhibition of both N-demethylation and sulfoxidation by the investigated SSRIs. Sertraline was the most potent inhibitor of perazine N-demethylation but the weakest inhibitor of sulfoxidation. Results of in-vivo and in-vitro studies indicate that the observed interaction between perazine and SSRIs mainly involves competition for an active site of perazine N-demethylase and sulfoxidase. Moreover, increases in the concentrations of both perazine and metabolites measured, produced by the investigated drug combinations in-vivo, suggest simultaneous inhibition of another, yet to be investigated, metabolic pathway of perazine (e.g. aromatic hydroxylation).

Different effects of amitriptyline and imipramine on the pharmacokinetics and metabolism of perazine in rats

The aim of this study was to search for possible effects of imipramine and amitriptyline on the pharmacokinetics and metabolism of perazine at steady state in rats. Perazine (10 mg kg(-1), i.p.) was administered to rats twice daily for two weeks, alone or jointly with imipramine or amitriptyline (10 mg kg(-1) i.p.). Concentrations of perazine and its two main metabolites (5-sulphoxide and N-desmethylperazine) in the plasma and brain were measured at 30 min (Cmax), 6h and 12h (slow disposition phase) after the last dose of the drugs. Liver microsomes were prepared 24 h after withdrawal of the drugs. Amitriptyline increased the plasma and brain concentrations of perazine (up to 300% of the control) and N-desmethylperazine, while not affecting those of 5-sulphoxide. Imipramine only tended to increase the neuroleptic concentration in the plasma and brain. Studies with control liver microsomes showed that amitriptyline and imipramine added to the incubation mixture in-vitro, competitively inhibited N-demethylation (Ki (inhibition constant) = 16 microM and 164 microM, respectively) and 5-sulphoxidation (Ki = 57 microM and 86 microM, respectively) of perazine, amitriptyline being a more potent inhibitor of perazine metabolism, especially with respect to N-demethylation. Studies with microsomes of rats treated chronically with perazine or tricyclic antidepressants, or both, did not show significant differences in the rate of perazine metabolism between perazine- and perazine+antidepressant-treated rats. The data obtained were compared with the results of analogous experiments with promazine and thioridazine. It was concluded that elevations of perazine concentration were caused by direct inhibition of the neuroleptic metabolism by the antidepressants. Similar interactions, possibly leading to exacerbation of the pharmacological action of perazine, may be expected in man. Since the interactions between phenothiazines and tricyclic antidepressants may proceed in two directions, reduced doses of both the neuroleptic and the antidepressant are recommended when the drugs are administered jointly.

Effect of venlafaxine on imipramine metabolism

The present study was designed to determine the effect of venlafaxine on imipramine metabolism in an attempt to elucidate the potential for cytochrome P450 drug-drug interactions with venlafaxine. We examined the metabolism of a single 100-mg dose of imipramine before and after treatment with venlafaxine, 50 mg three times a day. Eight male subjects were phenotyped for CYP2D6 activity. Two subjects were poor metabolizers of dextromethophan, and data from the remaining six subjects (mean age=45.3+/-15) were analyzed. Venlafaxine increased imipramine C(max) and elevated AUC by 40%. Desipramine clearance and volume of distribution were reduced by 20% and 25%, respectively. These findings are consistent with a statistically significant, but clinically modest impact of venlafaxine on CYP2D6-metabolized substrates.

Role of cytochrome P450 2D6 (CYP2D6) in the stereospecific metabolism of E- and Z-doxepin

The tricyclic antidepressant, doxepin, is formulated as an irrational mixture of E (trans) and Z (cis) stereoisomers (85%: 15%). We examined the stereoselective metabolism of doxepin in vitro, with the use of human liver microsomes, recombinant CYP2D6 and gas chromatography-mass spectrometry. In human liver microsomes over the concentration range 5-1500 microM, the rate of Z-doxepin N-demethylation exceeded that of E-doxepin above 100 microM in two of three livers. Eadie-Hofstee plots were curvilinear indicating the involvement of several enzymes in N-demethylation. Coincubation of doxepin with 7,8-naphthoflavone and ketoconazole reduced the rates of N-demethylation of E- and Z-doxepin by 30-50% and 40-60%, respectively, suggesting the involvement of CYP1A and CYP3A4, whilst quinidine had little effect on N-demethylation. In contrast, doxepin hydroxylation was exclusively stereo-specific; E-doxepin and E-N-desmethyldoxepin were hydroxylated with high affinity in liver microsomes and by recombinant CYP2D6 (Km in the range of 5-8 microM), but there was no evidence of Z-doxepin hydroxylation. In 'metabolic consumption' experiments with liver microsomes (having measurable CYP2D6 activity) and initial substrate concentration of 1 microM, the consumption of E-doxepin was greater (P < 0.05, n = 5) than that of Z-doxepin. Quinidine inhibited the consumption of E-doxepin but did not affect the consumption of Z-doxepin. With N-desmethyldoxepin, quinidine inhibited the consumption of E-N-desmethyl-doxepin whereas Z-N-desmethyldoxepin appeared to be a terminal oxidative metabolite. In summary, CYP2D6 is a major oxidative enzyme in doxepin metabolism; predominantly catalysing hydroxylation with an exclusive preference for the E-isomers. The relatively more rapid metabolism of E-isomeric forms, and the limited metabolic pathways for the Z-isomers may explain the apparent enrichment of Z-N-desmethyldoxepin that is observed in vivo.

Pharmacokinetics and metabolism of thioridazine during co-administration of tricyclic antidepressants

1. Because of serious side-effects of thioridazine and tricyclic antidepressants (cardiotoxicity), a possible influence of imipramine and amitriptyline on the pharmacokinetics and metabolism of thioridazine was investigated in a steady state (2-week treatment) in rats. 2. Imipramine and amitriptyline (5 and 10 mg kg(-1) i.p., respectively) elevated 30 and 20 fold, respectively, the concentration of thioridazine (10 mg kg(-1) i.p.) and its metabolites (N-desmethylthioridazine, 2-sulphoxide, 2-sulphone, 5-sulphoxide) in blood plasma. Similar, yet weaker increases in the thioridazine concentration were found in the brain. Moreover, an elevation of thioridazine/metabolite ratios was observed. 3. Imipramine and amitriptyline added to control liver microsomes in vitro inhibited the metabolism of thioridazine via N-demethylation (an increase in K(m)), mono-2-sulphoxidation (an increase in K(m) and a decrease in V(max)) and 5-sulphoxidation (mainly a decrease in V(max)). Amitriptyline was a more potent inhibitor than imipramine of the thioridazine metabolism. 4. The varying concentration ratios of antidepressant/thioridazine in vivo appear to be more important to the final result of the pharmacokinetic interactions than are relative direct inhibitory effects of the antidepressants on thioridazine metabolism observed in vitro. 5. Besides direct inhibition of the thioridazine metabolism, the decreased activity of cytochrome P-450 towards 5-sulphoxidation, produced by chronic joint administration of thioridazine and the antidepressants, seems to be relevant to the observed in vivo interaction. 6. The obtained results may also point to inhibition of another, not yet investigated, metabolic pathway of thioridazine, which may be inferred from the simultaneous elevation of concentrations of both thioridazine and the measured metabolites.

Regional and cellular distribution of CYP2D subfamily members in rat brain

1. Human CYP2D6 is present in brain, metabolizes many drugs and has been implicated in Parkinson's and Alzheimer's diseases and some cancers. It is still unclear which of the six known rat CYP2D subfamily members is/are homologous to human CYP2D6. 2. In this study, RT-PCR, Southern and Western blotting and immunohistochemical techniques were used to study the distribution of CYP2D subfamily member mRNA and proteins across 10 rat brain regions. CYP2D subfamily mRNA and protein levels were correlated with brain dextromethorphan O-demethylation (DOD), a measure of human CYP2D6 and rat CYP2D1 activities. 3. The data showed a strong relationship between CYP2D1 and CYP2D1-18 with brain DOD activity. In addition, it was shown that CYP2D proteins are present in brain mitochondrial as well as microsomal membranes. CYP2D subfamily member mRNA and proteins varied across brain regions and were highly concentrated in specific cell types. 4. These data strongly suggest that CYP2D1 and not CYP2D5 mediates DOD activity in rat brain, and may be the rat homologue of human CYP2D6. The highly localized nature of CYP2D indicates that in specific neurones enzyme levels may approach hepatic levels and, hence, contribute to local alterations in brain drug metabolism.

Lysosomal trapping as an important mechanism involved in the cellular distribution of perazine and in pharmacokinetic interaction with antidepressants

Perazine, a piperazine-type phenothiazine neuroleptic, is the most frequently chosen drug for combination with antidepressants in the therapy of complex or 'treatment-resistant' psychiatric illnesses. The aim of the present study was to investigate the contribution of lysosomal trapping to the total tissue uptake of perazine, and the pharmacokinetic interaction between the neuroleptic and antidepressants. Experiments were carried out on slices of different rat organs regarded as a system with functional lysosomes. To distinguish between lysosomal trapping and tissue binding, the experiments were performed in the absence or presence of 'lysosomal inhibitors', i.e. the lysosomotropic compound ammonium chloride or [H+] ionophore monensin, which abolish the pH-gradient of lysosomes. Under steady-state conditions, the highest tissue uptake of perazine was observed for the adipose tissue, which descended in the following order: the adipose tissue>lungs>liver>heart=brain>kidneys>muscles. The contribution of lysosomal trapping to the total tissue uptake amounted to about 40% in the liver, brain and muscles, to 30% in the kidneys, and to 25% in the heart and lungs. In the adipose tissue, no lysosomotropism of perazine was observed. Of the psychotropics studied, perazine was the only drug showing such a high degree of lysosomal trapping in muscles and distinct lysosomotropic properties in the heart. Perazine and the antidepressants used, both tricyclic (imipramine, amitriptyline) and selective serotonin reuptake inhibitors (fluoxetine, sertraline), mutually decreased their tissue uptake. The potency of imipramine to decrease perazine uptake was similar to that of the 'lysosomal inhibitors'. Other antidepressants seemed to exert a somewhat weaker effect. The above interactions between perazine and antidepressants were not observed in the presence of ammonium chloride, which indicates that they proceeded at the level of lysosomal trapping. The adipose tissue in which the drug uptake was not affected by the 'lysosomal inhibitors' was not the site of such an interaction. Ammonium chloride did not affect the drug metabolism in liver slices; other tissues displayed only a negligible biotransformation of the psychotropics studied. A parallel metabolic interaction between perazine and tricyclic antidepressants took part in liver slices (i.e. perazine and antidepressants mutually inhibited their metabolic pathways), but the influence of such an interaction on the lysosomal uptake of the parent compounds in liver slices did not seem to be great. A substantial decrease in concentrations of the drugs in lysosomes (depot form) observed in vitro may lead to an increase in the concentration in vivo of the neuroleptic and antidepressants at the site of action, which, in turn, may increase the risk of cardiotoxic and anticholinergic side-effects of tricyclic antidepressants and sedative and extrapyramidal effects of the neuroleptic.

The influence of selective serotonin reuptake inhibitors on the plasma and brain pharmacokinetics of the simplest phenothiazine neuroleptic promazine in the rat

The aim of the present study was to investigate a possible impact of the three selective serotonin reuptake inhibitors (SSRIs) fluoxetine, fluvoxamine and sertraline on the pharmacokinetics of promazine in a steady state in rats. Promazine was administered twice a day for 2 weeks, alone or jointly with one of the antidepressants. Concentrations of promazine and its two main metabolites (N-desmethylpromazine and sulfoxide) in the plasma and brain were measured at 30 min and 6 and 12 h after the last dose of the drugs. All the investigated SSRIs increased the plasma and brain concentrations of promazine up to 300% of the control value, their effect being most pronounced after 30 min and 6 h. Moreover, simultaneous increases in the promazine metabolites' concentrations and in the promazine-metabolite concentration ratios were observed. In vitro studies with liver microsomes of rats treated chronically with promazine, SSRIs or their combination did not show any significant changes in the concentrations of cytochromes P-450 and b-5. However, treatment with fluoxetine, alone or in a combination with promazine, decreased the rates of promazine N-demethylation and sulfoxidation. A similar effect was observed in the case of promazine and fluvoxamine combination. Kinetic studies into promazine metabolism, carried out on control liver microsomes in the absence or presence of SSRIs added in vitro, demonstrated competitive inhibition of both N-demethylation and sulfoxidation by the antidepressants. The results of in vivo and in vitro studies indicate the following mechanisms of the observed interactions: (a) competition for an active site of promazine N-demethylase and sulfoxidase; (b) adaptive changes in cytochrome P-450, produced by chronic treatment with fluoxetine or fluvoxamine; (c) additionally, increases in the sum of concentrations of promazine+ metabolites, produced by fluoxetine and sertraline in vivo, suggest simultaneous inhibition of another, not investigated by us, metabolic pathway of promazine, e.g. hydroxylation. In conclusion, all the three SSRIs administered chronically in pharmacological doses, increase the concentrations of promazine in the blood plasma and brain of rats by inhibiting different metabolic pathways of the neuroleptic. Assuming that similar interactions occur in humans, reduced doses of phenotiazines should be considered when one of the above antidepressants is to be given jointly.

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.

Metabolic characterization of a tripeptide human immunodeficiency virus type 1 protease inhibitor, KNI-272, in rat liver microsomes

KNI-272 is a tripeptide protease inhibitor for treating human immunodeficiency virus type 1 (HIV-1). In in vitro stability studies using rat tissue homogenates, KNI-272 concentrations in the liver, kidney, and brain decreased significantly with time. Moreover, in tissue distribution studies, KNI-272 distributed highly to the liver, kidney, and small intestine in vivo. From these results and reported physiological parameters such as the tissue volume and tissue blood flow rate, we considered the liver to be the main organ which takes part in the metabolic elimination of KNI-272. Then the hepatic metabolism of KNI-272 was more thoroughly investigated by using rat liver microsomes. KNI-272 was metabolized in the rat liver microsomes, and five metabolites were found. The initial metabolic rate constant (kmetabolism) tended to decrease when the KNI-272 concentration in microsomal suspensions increased. The calculated Michaelis-Menten constant (K(m)) and the maximum velocity of KNI-272 metabolism (Vmax), after correction for the unbound drug concentration, were 1.12 +/- 0.09 micrograms/ml (1.68 +/- 0.13 microM) and 0.372 +/- 0.008 microgram/mg of protein/min (0.558 +/- 0.012 nmol/mg of protein per min), respectively. The metabolic clearance (CLint,metabo), calculated as Vmax/K(m), was 0.332 ml/mg of protein per min. Moreover, by using selective cytochrome P-450 inhibitors and recombinant human CYP3A4 fractions, KNI-272 was determined to be metabolized mainly by the CYP3A isoform. In addition, ketoconazole, a representative CYP3A inhibitor, inhibited KNI-272 metabolism competitively, and the inhibition constant (Ki) was 4.32 microM.

Metabolism of xenobiotics in the central nervous system: implications and challenges

The metabolism of drugs and other xenobiotics in situ in the brain has far-reaching implications in the pharmacological and pharmacodynamic effects of drugs acting on the CNS, particularly with respect to psychoactive drugs wherein a wide range of therapeutic response is typically seen in the patient population. An entirely functional cytochrome P450 (P450) monooxygenase system is known to exist in the rodent and human brain, wherein it is preferentially localized in the neuronal cells, which are the sites of action of psychoactive drugs. Further, bioactivation of xenobiotics, in situ, in the CNS would result in the formation of reactive, toxic metabolites in the neuronal cells that have limited regenerative capability. The presence of P450 enzymes in selective cell populations within distinctive regions of the brain that are affected in certain neurodegenerative disorders implies the potential role of P450-mediated bioactivation as a causative factor in the etiopathogenesis of these diseases. The characterization of brain-specific P450s and their regulation and localization within the CNS assume importance for understanding the potential role of these enzymes in the pathogenesis of neurodegenerative disorders and psychopharmacological modulation of drugs acting on the CNS.

Interlaboratory comparison of the assessment of P450 activities in human hepatic microsomal samples

1. Although the importance of in vitro technology in supporting drug development is widely accepted, there is no real consensus about which approaches should be taken, which substrates should be used, or on the reliability and application of in vitro data. Consequently, as part of a collaborative project to characterize human liver with respect to the major forms of cytochrome P450, an interlaboratory comparison of the analysis of samples for form-specific activities was undertaken. 2. Microsomal fractions were isolated from five different human liver samples taken from the liver bank maintained at the Royal Postgraduate Medical School (RPMS). Aliquots from the five samples were sent to the 11 collaborating laboratories for characterization using their in-house, form-specific assays for cytochrome P450 activities. Although each laboratory assayed protein concentration, total cytochrome P450 content and enzyme activities were calculated using the protein estimation generated by RPMS to eliminate this possible source of variability. 3. With the exception of one laboratory, all estimates of protein concentration were similar (coefficient of variation, CoV, 9-13%) and the rank-order of the five samples was consistent across the laboratories. There was greater variability in the estimates of total cytochrome P450 content (CoV 28-43%), although again rank order of the samples across laboratories was fairly consistent. 4. The various laboratories used a number of different probe substrates, together with a range of conditions (substrate concentration, time of incubation, amount of protein), to assay for activity of CYP1A2, CYP2C9, CYP2D6, CYP2E1 and CYP3A4. However, apart from the occasional outlier, the five samples were ranked for activity of all these forms of cytochrome P450 with a high degree of consistency by the various laboratories and the choice of substrate had no appreciable effect on the ranking of the samples. 5. While this interlaboratory comparison has shown that greater consistency in the approach to in vivo determination of drug-metabolizing activity is desirable, there was little indication that any particular approach or substrate was superior to the others.

Isozyme selective alterations of the expression of cytochrome P450 during regeneration of male rat liver following partial hepatectomy

1. During liver regeneration in the male rat, the metabolic activities of imipramine were differentially affected depending on the specific metabolic pathways. Imipramine N-demethylation was markedly reduced whereas 2-hydroxylation showed only a moderate reduction following partial hepatectomy. 2. A slight decline was observed in the hepatic microsomal content of CYP2D apoprotein, whereas a substantial decrease occurred in CYP2C11 content during liver regeneration. Since imipramine 2-hydroxylation and N-demethylation are mediated by CYP2D and 2C11 respectively, metabolic pathway-specific alterations in the activities of imipramine metabolism are explained by the isozyme selective alteration in the levels of CYPs in regenerating liver. 3. No significant effect of regeneration was observed on expression of CYP2B1 and 2E1 apoproteins. CYP3A2 apoprotein, one of the male-specific CYP isoforms, was significantly suppressed in regenerating liver showing a similar pattern of alteration to the levels of CYP2C11. The alteration pattern of the CYP1A1 level was different to the above with a moderate decline at the first day post-operation and a marked rebound thereafter. 4. In the partially hepatectomized male rate, no significant increase in androstenedione 5-alpha reductase activity, an activity predominant in the female rat, was detected. It is concluded that the pattern of alterations of hepatic oxidative metabolism during liver regeneration was not related to the functional feminization of the liver.

Regulation of CYP3A9 gene expression by estrogen and catalytic studies using cytochrome P450 3A9 expressed in Escherichia coli

Sexual dimorphism in the expression of CYP3A9, a novel form of CYP3A from rat brain, is shown for the first time in rat brain as well as in rat liver. CYP3A9 expression is female specific in rat liver as judged by its 10-fold higher expression in females than in males. CYP3A9 gene expression was inducible by estrogen treatment both in male and in female rats. Ovariectomy of adult female rats elicited a drastic reduction on the mRNA level of CYP3A9 which could be fully restored by estrogen replacement. These results suggest that estrogen may play an important role in the female-specific expression of the CYP3A9 gene. P450 3A9 recombinant protein was expressed in Escherichia coli by means of the pCWOri+ expression vector and the MALLLAVF amino terminal sequence modification. This construct gave a high level of expression (130 nmol P450 3A9/liter culture) and the recombinant protein of the modified P450 3A9 was purified to electrophoretic homogeneity with a specific content of 10.1 nmol P450/mg protein from solubilized fractions through two chromatographic steps. The purified P450 3A9 protein was active in the metabolism of imipramine, erythromycin, benzphetamine, and ethylmorphine as well as 17beta-estradiol in a reconstituted system containing lipid and rat NADPH-P450 reductase. Of special interest is the finding that P450 3A9 can catalyze the formation of desipramine with a turnover number of 4.9 nmol/min/nmol P450, suggesting the possible involvement of this isoform in the metabolism of imipramine in brain. Optimal reconstitution conditions for P450 3A9 activities required a lipid mixture (1:1:1 mixture of L-alpha-dilauroyl phosphatidylcholine, L-alpha-dioleoyl phosphatidylcholine, and phosphatidylserine) and GSH.

Adverse drug reactions and debrisoquine/sparteine (P450IID6) polymorphism in patients with fibromyalgia

OBJECTIVE

To assess the frequency of adverse drug reaction in patients with fibromyalgia in relation to medications prescribed for this condition. To evaluate the potential role of the P450IID6 phenotype in the pathogenesis of these adverse drug reactions.

METHODS

Thirty-five patients with fibromyalgia were assessed using a structured questionnaire with demographic and clinical data and perceived adverse drug reactions. A sample of 60 patients with rheumatoid arthritis and 62 patients with localized back pain served as controls. The P450IID6 phenotype was determined for each of the fibromyalgia patients.

RESULTS

Overall, 141 patients had used NSAID and 79 (56%) of them reported adverse effects. Antidepressant drugs were used by 68 patients and 35 (51%) patients had adverse effects. Muscle relaxant drugs were used by 48 patients and 15 (31%) of them reported side effects. Analgesics were used by 122 patients and 22 (18%) had experienced adverse effects. Statistical differences in the frequency of adverse effects were found with antidepressant drugs in the fibromyalgia group, compared with rheumatoid arthritis (p=0.01) and back pain (p=0.02). Four of the 35 patients (11.4%) had a metabolic ratio (M.R.) greater than 0.30 (log M.R.= -0.52) indicative of the poor metabolizers (PM) phenotype. M.R. varied from 0.005 (log M.R. = -2.30) to 4.99 (log M.R. = 0.70).

CONCLUSIONS

The problem of adverse drug reactions in fibromyalgia patients does not appear to correlate with the PM phenotype of the P450IID6 oxidative enzyme. It also is unlikely that altered xenobiotic detoxification attributable to this PM phenotype would have a significant role in the development of fibromyalgia.

Imipramine demethylation in vivo: impact of CYP1A2, CYP2C19, and CYP3A4

OBJECTIVE

To further substantiate the role of CYP1A2 and CYP3A4 for the N-demethylation in vivo. At least three different P450s appear to be responsible for the N-demethylation of imipramine to desipramine in vivo: CYP1A2, CYP2C19, and CYP3A4. The role of CYP2C19 in this regard is well documented, but for the two other P450s the evidence is either indirect or based on in vitro studies.

METHODS

Phenotypic tests for imipramine N-demethylation, CYP1A2 (caffeine testing), CYP2C19 (mephenytoin and chloroguanide [proguanil] testing), and CYP3A4 (hydrocortisone and quinidine testing) were carried out in 32 healthy young Danes; all were poor (n = 31) or extremely slow extensive metabolizers (n = 1) of sparteine.

RESULTS

By exclusion of the insignificant log-transformed variables, multiple regression analysis for In (desipramine/imipramine) showed that only in (mephenytoin S/R) correlated (p = 0.013; r2 = 0.19). For in (2-hydroxydesipramine/2-hydroxyimipramine) we found that in (mephenytoin S/R) and in (4-chlorophenylbiguanide/chloroguanide) correlated (p = 0.001; r2 = 0.41).

CONCLUSION

We did not find in vivo evidence of either CYP1A2 or CYP3A4 activity in the N-demethylation of imipramine. This could be due in part to inadequate CYP1A2 and CYP3A4 in vivo function tests.