8 Principles of Drug Metabolism, With an Emphasis on Psychiatric Drugs

Principles of Drug Metabolism, with an Emphasis on Psychiatric Drugs

Ronald 7. Couffs, jian Fang, Michel Bourin, and Glen B. Baker

1. Introduction

A knowledge of how drugs are metabolized in the body is often of clinical relevance because often an administered drug is not soley responsible for observed pharmacological and toxicological effects. Regrettably, the formation of drug metabolites is often not considered in pharmacological evaluations, the assumption being that it is the drug itself that is the active species. In many instances, desirable and undesirable drug effects can be correlated with rates of biotransformations and the properties of metabolites; pharmacokinetic drug-drug interactions may also occur if a patient is receiving two or more drugs that compete for the same metabolic enzymes. In the present chapter, we will concentrate on metabolism of drugs used to treat psychiatric disorders, although the principles and protocols discussed are applicable to other classes of drugs as well.

2. General Principles of Drug Metabolism

Drug metabolism may be defined as the chemical modification of a drug in a biologic environment. The procedure is also com- monly referred to as drug biotransformation or drug detoxifica- tion. Most drugs undergo metabolic modification in the body; only a few (e.g., acetazolamide, barbital, decamethonium, hexametho- nium, penicillin G) are excreted almost quantitatively in unchanged form. Drugs that are metabolized may be converted to many products or may form only one major metabolite (e.g., metabolic conjugation of benzoic acid to hippuric acid). Normally drug

From Neuromethods, vol 33 Cell Neurohology Jechnques Eds A A Boulton, G 6 Baker, and A N Bateson 0 Humana Press Inc

255

2.56 Coutts et al.

metabolism is enzymatically controlled via oxrdases, reductases, esterases, and enzymes involved in conjugation reactions, but sometimes nonenzymatic reactions occur in the body. Nitroso compounds, for example, may be oxidized to nitro compounds in the presence of oxygen but without the involvement of an enzyme.

2.1. Purposes of Drug Metabolism

Drug metabolism in the organism has two principal functions:

1. To convert drugs to products (metabolites) that are less phar- macologically active. Otherwise a pharmacologic reaction would continue indefinitely.

2. To convert drugs to products that are much more water soluble (i.e., more polar or ionized) than the parent drug and therefore more readrly and rapidly excreted. Because of their polarity, most metabolites do not undergo tubular reabsorption in the kidney to any great extent, and are thus excreted in the urine.

Although it is true that most metabolites are less active phar- macologically and more polar than the parent drug, there are exceptions to this generalization. Some products are referred to as “active metabolites” because they possess a pharmacologic activity equal to, greater than, or different from, the parent drug itself; some may have appreciable activity. Occasionally, a metabolite is more lipophilic than the drug from which it was formed. Some sulfonamides, for example, are metabolically converted to lipophilic N-acetylated metabolites.

2.2. Sites of Drug Metabolism

Drug metabolism occurs mainly in the liver and to a lesser extent in the kidney, blood, brain, lungs, gastrointestinal tract, skin, and other tissues.

The most important metabolic reaction is drug oxidation (discussed subsequently), and this occurs mainly in the liver. The liver contains various cells, including heptocytes, where metabolic oxidation occurs. In the cytoplasm of the liver cell there are various structures, including a network of channels called smooth endoplasmic reticulum, which contain the oxidase enzymes, and the granular, or rough, endoplasmic reticulum, which is not involved in drug metabolism (its main function is protein synthesis). Metabolic reductions also occur in the liver. The most common ones are the reverse of known oxidative mechanisms (e.g., C = 0 -+ CHOH) and may require the

Principles of Drug Metabolism 257

same enzyme system. Many enzymatic reactions are reversible, but usually the reaction equilibrium favors one direction.

Metabolism of drugs is also carried out by microorganisms of the gastrointestinal tract, which tend to cause reductions rather than oxidations, and also can catalyze the hydrolysis of esters and amides.

2.3. Metabolic Pathways

Drug metabolism reactions are classified as phase I or phase II reactions (Gibson and Skett, 1988; Ciraulo et al., 1995; Benet et al., 1996). A phase I metabolic reaction is one in which a new chemical group is introduced into a drug molecule, especially by oxidative, reductive, and hydrolytic methods (Table 1). A phase II metabolic reaction is one in which a drug or phase I metabolite is conjugated by an enzymatic process with a small endogenous molecule. Glu- curonide and sulfate formation are excellent examples.

Numerous examples of phase I metabolic reactions have been observed (see Table 1); many are oxidative reactions. The primary mechanism by which oxygen is introduced into a molecule is com- plex. The reaction is catalyzed by the mixed function oxidases of the endoplasmic reticulum of the liver and other tissues. This has been referred to as the P-450 system (consisting of cytochrome P-450, cytochrome P-450 reductase, and cytochrome c reductase), which requires molecular oxygen and NADPH (or NADH) for the introduction of one atom of oxygen into the drug. The overall reaction can be simply depicted as follows:

P-450 system R-H + 0, + NADPH + H’ -R-OH + NADP + H,O (drug) (oxidized drug)

or more descriptively:

\ +e +e ---p450 -

H 0 / 2H+

----------- = R-H “txxded” to P450

Phase II metabolic reactions are conjugation reactions of drugs and drug metabolites of general formula R-XH, where X = NH, NR’, 0, or S. Most examples involve compounds of structure R-OH. Meta- bolic conjugation reactions that have been observed include:

258 Coutts et al.

Table 1 Important Phase I Metabolic Reactions

In this section, metabolic C-oxidations are emphasrzed Examples of metabolic N-oxidation and S-oxidation and metabolic hydrolyses are also provided

Oxidation The vast malority of metabolic reactions are ones of oxidation and most

that mvolve the mtroduction of an oxygen atom originatmg from atmospheric oxygen are catalyzed by CYP enzymes

1. Primary alcohols They are oxidized first to aldehydes and then to acids The alcohol to

aldehyde reaction is reversible These oxidations fare not usually catalyzed by CYP enzymes, but by alcohol dehydrogenase and aldehyde oxidase (see Table 2).

Examples

A CH,CH20H _ CH&HO - CH$OOH

Ethyl alcohol Acetaldehyde Acettc actd

6 Retlnol __ RetInaI - Retlnolc acid

2. Secondary alcohols They are similarly oxidized to ketones and this reaction 1s also reversible Examples

A CH,CH,CHOHCH,CH3 _ CH3CH2COCHZCH3

3-Pentanol 3-Pentanone

COCHN(CpH5),

&Hz

7

Dlethylproplon Reduced drethylproplon

C Haloperidol~ Reduced halopendol

When a ketone is enzymatically reduced to a secondary alcohol, an asymmetric center 1s produced and the reduction is stereoselective

3. Allphatrc and allcyclic carbon atoms They are oxidized to alcohols The oxidation 1s catalyzed by various

enzymes Examples

A CH3 -

Tolbutamide Alcohol metabolite

Princrples of Drug Metabolism 259

The alcohol metabolite undergoes further metabolism to the corresponding acid (cf reactron 1)

I3 G2$- QH3 R R

Amitnptyhne [R=CH3] IO-Hydroxyamltrlptyhne [R=CH3] Nortnptyhne [R=H] IO-Hydroxynortriptyline [R=H]

A center of asymmetry is created by the mtroduction of the lo-hydroxy group mto amltriptylme and nortrrptylme

4 Aromatic carbon atoms Compounds that contam aromatic rmgs are often metabohcally ring-oxidized

to phenols m the rmg position para to a chemical groupmg that IS attached to the aromatic rmg The reaction proceeds vta an mtermedi- ate epoxide and is catalyzed by various CYP enzymes

Examples a) Amphetamme -+ 4-hydroxyamphetamine b) Imipramme +J 2-hydroxyimipramme c) Amoxapine + 7- and 8-hydroxyamoxapme d) Clomipramine -+ 8-hydroxyclomipramme

Many other antidepressants and neuroleptics are metabolically rmg hydroxylated

CH,CH(CH3)NH2 - HO (It

/ \ CH,CH(CH,lNH, -

Amphetamine 4-Hydroryamphelamlne

Amoxaplne

260

Table 1 (continued)

Coutts et al.

5 Dealkylatron N-Alkyl, 0-alkyl, and S-alkyl compounds are metabolically dealkylated

This an oxrdatron reaction catalyzed by CYP enzymes A general reaction can be drawn

R’-X-CH,R2 + [RI-X-CHOHR*] + R’-XH + R2CH0 X-Alkylated drug Metabohte X = NH, N-alkyl, N-aryl, 0, S

Examples

CH3CH20 NHCOCH3 NHCOCH,

Phenacetln p-Hydroxyacetamllde (acetamlnophen)

cJy.xTJ-m

CH2CH2CH2N(CH3)2 iH2CH2CH2NHCH3

lmlpramine Destpramrne

F3C

0

F3C - -

\ / CH2CH(CH3)NHCH2CH3 -

0 \ / CH2CH(CH3)NH2

Fentluramlne Notfenfluramlne

Others Numerous CNS stimulants (e.g., N-methylamphetamme), anorexrants, antidepressants, anxrolytics and neuroleptrcs are secondary or tertiary bases. Tertiary bases are N-dealkylated to secondary bases which are further N-dealkylated to primary ammes

R-N’ CY

‘CH3 - R-NHGH3 - R-NH2

Tertraty amine

6 Deammatron

Secondary amine Prrmary amine

Many ammes also undergo this oxidative reaction whrch 1s generally catalyzed by CYP enzymes A general reaction can be drawn

R’R2CHNHR3 + [R’RZC(OH)NHR3] + R’COR* + R3NH, Amme drug Deammated drug

The mechanism (VZU a C-hydroxylated intermediate) 1s the same as m N-dealkylation The oxygenated C atom is directly attached to the basic N atom


Examples Amphetamine -+ 1-Phenylpropanone Haloperidol + 4-Fluorobenzoylpropiomc acid Ephedrine -+ 1-Hydroxy-1-phenylpropanone Mexrletine -+ 3,4,5-trimethoxyphenylacetaldehyde (whrch 1s then

reduced to the corresponding alcohol and oxrdrzed to the correspondmg acrd)

7. N-Oxidation This metabolic reactron 1s observed mainly with tertiary ammes which

are converted to tertiary amine N-oxides Imipramme, for example, 1s converted to rmipramme N-oxide. Some primary and secondary amines are similarly converted to hydroxylamines which are toxic.

0 lmlpramlne N-oxide Chlorpromazme S-oxide

8 S-Oxidation Many phenothrazine drugs, e.g , chlorpromazine, are S-oxidized mainly to

sulfoxrdes, but also to sulfones. The structure of the sulfoxrde metabolrte of chlorpromazme IS provided. Chlorpromazine is also metabolically ring hydroxylated, N-dealkylated, deammated and N-oxidized

Hydrolysis Some important drugs are esters or amides and they undergo meta-

bohc hydrolysis 1. Esters

They are hydrolyzed by esterases to the correspondmg acid Esterases are plentiful in the bloodstream and lrver and are also present m numerous other tissues

Examples

Acetylsakyllc acid Sallcyllc acid (active metabolite)

kH3

Mependlne

kH3

Meperidinlc actd (Inactive metabollte)

Ccontlnued)

262

Table 1 (contznued)

Coutts et al.

2. Amides Amidases are located in the liver and m other tissues Rates of hydroly-

sis of amides are slower than those of esters Examples

A GNHTr GH B ($ZHC~CH~N~C~H~J~ - (=JII-I

lprontaztd lsomcotmlc acid Lldocame Z.&Xyltdine

Table 2 Drug Metabolism Mechanisms

Phase I OXIDATION

1 Cytochrome P-450 [CYPI systems 2 Alcohol dehydrogenase + aldehyde oxidase 3. N- and S-oxidation [Flavme adenme dmucleotide (FAD)1

REDUCTION CYP systems m the absence of oxygen

HYDROLYSIS Esterases, amidases

Phase II CONJUGATION

Glucuromdation and sulfation are most important CYP Oxidations - order of importance

1 N-Dealkylation; 2 Aromatic ring hydroxylatlon, 3 Aliphatic/ahcychc C-oxidation; 4 0- and S-Dealkylation

Oxidations not catalyzed by CYP

RCH,CH,OH + NAD’ < alcohol

dehydrogenase > RCH,CHO + NADH + H’

RCH,CHO + HOH + NAD’ aldehyde

oxldase > RCH,COOH + NADH + H’


1. Conjugation with glucuronic acid. 2. Sulfate formation. 3. Acetate formation. 4. Conjugation with glycine. 5. Methylation. 6. Conjugation with glutamine. 7. Conjugation with glutathione -+ mercapturic acid formation. 8. Other conjugation reactions.

Of these reactions, conjugation with glucuronic acid (which yields a glucuronide) is the most important in humans. Reactions 2 through 5 are observed in humans, but are of less importance. The other conjugation reactions are relatively unimportant in humans.

3. Cytochrome P4.W lsozymes

Almost all known drugs and xenobiotics are metabolized in the body to some extent prior to their excretion. While many enzymes are involved in drug metabolism reactions (Table 2), the heme-containing cytochromes P-450 (CYPs) are of particular importance in the oxidative metabolism of endogenous compounds, such as steroids, and of numerous exogenous compounds including drugs, environmental chemicals, and other xenobiotics. The term 450 refers to the initial identification of CYP as a red liver pigment (I?), which produced a characteristic spectrophotometry absorption peak near 450 nm when reduced and bound to carbon monoxide (Garfinkel, 1958; Omura and Sato, 1962; Glue and Banfield, 1996).

There are at least 14 different mammalian CYP enzyme gene families (1,2,3,4,5,7,8,11,17,19,21,24,27, and 51) based on the degree of similarity in the amino acid sequences of the CYP proteins (Nelson et al., 1996). Several of these families are involved in biosynthesis and/or catabolism of endogenous substrates such as fatty acids, eicosanoids, vitamins, bile, and steroids. At least three families (l-3) are implicated in the metabolism of numerous drugs and xenobiotics (Gonzalez, 1995). Some of the gene families, especially family 2, contain subfamilies, each of which is designated a different capital letter. Members of the same subfamily have greater than 55% amino acid sequence similarity, and individual CYPs within a subfamily are distinguished by a termi- nal Arabic number (Nebert et al., 1989; Nelson et al., 1993). Although many CYP isozymes are found in human liver (Glue

264 Coutts et al.

and Clement, 19971, a relatively small number are involved in the important drug and chemical transformations. They include CYPlA2, CYP2A6, CYP2B6, CYP2C9, CYP2C19 (CYP,,), CYP2D6, CYP2E1, and CYP3A4. Of these, CYP2D6 and CYP3A4 are domi- nant in the metabolism of drugs (Guengerich, 1995). About 70% of liver cytochrome P450 proteins can be accounted for by CYPlA2 (-13% of total), CYP2A6 f-4%), CYP2B6 (cl%), CYP2C isoforms l-20%), CYP2D6 t-2%), CYP2El t-7%), and CYP3A (-30%) (Shimada et al., 1994). The four most consequential human CYP2C isozymes are 2C9 (-60% of total human 20, 2C8 c-35%), 2C18 f-4%), and 2C19 (-1%) (Goldstein et al., 1994).

CYPs are also present in brain, but at much lower overall concentrations than in liver (Kalow and Tyndale, 1992; Warner et al., 1993; Hedlund et al., 1996; Sequeira and Strobel, 19961, but CYPs may be highly expressed in localized brain areas or cell types (Hansson et al., 1990; Kempermann et al., 1994; Britto and Wedland, 1992; Wu et al., 1995). It is of interest that regulation of CYP in the liver does not necessarily predict CYP regulation in the brain. For example, inducers of hepatic CYP can increase, decrease, or have no effect on brain CYP in a region-, enzyme-, and cell-specific manner (Liccione and Maines, 1989; Schmid et al., 1989; Otton et al., 1993).

A Worldwide Web server entitled “The Directory of P450-containing Systems” has been established at <http://www.icgeb.trieste.it/ p450/>. It is readily accessible and contams up-to-date lists of mRNA base sequences and the amino acid sequences in most P450 proteins. The directory is well referenced and it provides accession numbers and cross-links to relevant sequence data banks (Degtyarenko and Fabian, 1996).

CYP2C19 (CYP,, or mephenytoin hydroxylase) and CYP2D6 (also named debrisoquine 4-hydroxylase, sparteine dehydrogenase, or debrisoquine/sparteine oxidase) exhibit genetic polymorphism (van Harten, 1993; Coutts, 1994; DeVane, 1994; Kromer and Eichelbaum, 1995; Lin et al., 1996). Such polymorphism in CYP genes results from changes in nucleotide base sequences that translate into amino acid changes in CYP enzyme protein molecules. About 3% of Caucasians and over 20% of Japanese are poor metabolizers (PMs) of mephenytom; their ability to synthesize CYP2C19 is impaired (Kupfer and Preisig, 1984). Other substrates of CYP2C19 include hexobarbital, omeprazole, diazepam, N-desmethyidiazepam, and propranolol (Cholerton et al., 1992), and PMs of mephenytoin will also be PMs of these drugs.

Prlnclples of Drug Metabolism 265

Most individuals are extensive metabolizers (EMS) of debrisoquine and sparteine, but around 5-10% of Caucasians and 2% of Orientals are PMs of debrisoquine/sparteine because they lack the ability to synthesize CYP2D6 (Gaedigk et al., 1991). Numer- ous drugs, including amitriptyline (AMI), imipramine (IMI), desipramine (DMI), methoxyphenamine, and propafenone, have been identified as important substrates of CYP2D6 (Coutts, 1994). The metabolism of these drugs will be impaired in individuals who cannot synthesize this enzyme (PMs of debrisoquine). There also exist ultrarapid metabolizers of CYP2D6 (Agundez et al., 1995b). When drugs that are substrates for CYP2D6 are administered to such individuals, the drug may be absent from the serum or present in subtherapeutic quantities. These subjects would thus require very high doses of such drugs to receive therapeutic benefits.

Other CYP enzymes, including CYPlAl (7-ethoxyresorufin 0-deethylase), CYP2A6 (coumarin 7-hydroxylase), CYP2C9 (tolbutamide hydroxylase; S-warfarin 7-hydroxylase), CYP2El (p-nitrophenol hydroxylase; chlorzoxone 6-hydroxylase), and CYP3A4 (nifedipine oxidase), are sometimes claimed to be polymorphic, but genotypic evidence is lacking in most instances. With respect to CYP2C9 polymorphism, there is considerable interest at present in this abundant CYP enzyme. Two structural variants of CYP2C9 have recently been characterized. The lack of genetic information on the polymorphism of CYPs lAl,2A6,2El, or 3A4 genes suggests that if they do exist, they will be only rarely observed. However, even if the extent to which they are eventu- ally detected involves only 0.001% of a population group, tens of thousands of people could be affected.

A knowledge of which P-450 enzyme is involved in a metabolic process is important information. If two or more drugs that are sig- nificantly metabolized by or inhibit the same CYP enzyme are administered concomitantly to a patient, then there will be compe- tition for the enzyme, and the pharmacokinetic properties of each drug may differ from those properties observed when each drug is individually administered. Lists of substrates for and /or inhibitors of CYP are given in recent reviews (Parkinson, 1996; Preskorn, 1997; Glue and Clement, 1998). It should be remembered that many drugs are substrates for more than one CYP enzyme (Table 3).

For more comprehensive information on CYP enzymes, read- ers are referred to several recent books and review articles

266 Coutts et al.

(Wrighton and Stevens, 1992; Ortiz de Mantellano, 1995; Bourrie et al., 1996; Meyer et al., 1996; Preskorn, 1996; Shen, 1997, Glue and Banfield,l996; Nemeroff et al., 1996; Lane, 1996; Ameer and Weintraub, 1997; Edge et al., 1997; Richelson, 1997; Elaker et al., 1998; Miners and Birkett, 1998; ZumBrunnen and Jann, 1998).

3.1. Phenotyping and Genotyping

Phenotyping and genotyping are patient-assessment procedures that complement each other. Phenotyping is a relatively simple method of assessing an individual’s ability to metabolize drugs that are substrates of the polymorphic enzymes, CYP2D6 or CYP2C19. In CYP2D6 phenotyping, debrisoquine, sparteine, or dextromethorphan are used as probe drugs. All are substrates of CYP2D6. A suitable dose of the probe drug is administered orally and urine is collected for an appropriate period (8-24 hr) after drug administratron. The concentrations of the administered drug and the selected metabolite in enzymatically hydrolyzed urine are determined and a metabolic ratio (MR) or log,,MR is calculated (Coutts, 1994). MR = percentage of drug excreted unchanged divided by the percentage of drug excreted as the metabolite. The debrisoquine log,,MR ranges are approximately -0.2 to -1.0 for ultrarapid metabolizers (URMs); -1.0 to +1.08 for EMS; > 1.1 to 2.0 for PMs. In the EM group, log,,MR is -1.0 to +0.5 in most homozygous EMS and >+0.5 m most heterozygous EMS, but there is overlap. The dextromethorphan log,,,MR values are generally in the range 0.0030 to 5.27 (Henthorn et al, 1989), although a log,,MR value of 9.62 has been recorded for one very poor metabolizer of dextromethorphan (Coutts, 1994). There is close correlation between dextromethorphan and debrisoquine phenotypes (Perault et al., 1991). In CYP2C19 phenotyping, the procedure is modified because virtually no mephenytoin is excreted in urine and the elimination of the metabolite, 4’-OH-mephenytoin, is stereospecific for the S-enantiomer (Kupfer and Preisig, 1984). Racemic mephenytom is administered and a O-8 h urine is collected. A hydroxylation index (HI) is determined (HI = dose of the S-enantiomer in mmol divided by mmol amount of S-4’-OH-mephenytoin). Mephenytoin EMS have an HI value of 5 6 or less; PMs have an HI well m excess of 5.6 and usually >20.

There are advantages to the phenotyping technique. It is a relatively simple, rapid, inexpensive, noninvasive, and reproducible procedure, and it normally has to be performed only once in a person’s lifetime. It could easily be conducted routinely on psy-

Prrnciples of Drug Metabolism 267

chiatry patients in a hospital setting. The major objection to its use is that the individuals being assessed must be completely drug-free. In many instances, a patient’s phenotype is required when that patient is taking drugs or even herbal products that contain chemicals, e.g., flavonoids, that would interfere in the phenotype assessment. A PM’s status may be genetically or drug induced.

Alternative, but more sophisticated, molecular techniques to iden- tify PM subjects have been developed. One genotyping assay that com- bines PCR (polymerase chain reaction) analysis with RFLP (restriction fragment length polymorphism) analysis is now in routine use and identifies the genetic mutations that confer PM status. Genotyping assays can provide complete structures of mutated alleles and of mutant CYP enzymes expressed by them. The validity of this technique is independent of a patient’s health or drug use or abuse; samples are easily collected, and patient compliance is guaranteed. PCR/RFLP procedures are very well described by Heim and Meyer (1991).

4. Importance of Drug Metabolism in the Actions of Psychiatric Drugs

It has been known for many years that the phenothiazine antipsychotics undergo extensive metabolism (Midha et al., 1987). CYP2D6 appears to play an important role in the metabolism of these drugs as well as other antipsychotics such as haloperidol and risperidone (Lam et a1.,1995; Young et a1.,1993; Jerling et al., 1996; Huang et al., 1996; von Bahr et al., 1991; Blake et al., 1995); CYP3A4 also plays a role in catalysis of several metabolic pathways of haloperidol (Fang et al., 1997). CYPlA2 contributes sig- nificantly to the metabolism of clozapine (Bertilsson et al., 1994; Pinmohamad et al., 1995; Eiermann et al., 1997), although other CYP enzymes probably also contribute to metabolism of this atypi- cal antipsychotic (Fang et a1.,1998). In studies on olanzapine metabolism using human liver micosomes, Ring et al. (1996) sug- gested that CYPlA2 catalyzes formation of N-desmethylolanzapine and 7-OH-olanzapine, CYP2D6 catalyzes 2-OH-olanzapine formation and flavin-containing monooxygenase (FM03) catalyzes formation of the N-oxide. Although numerous benzodiazepines are available, most have in common anxiolytic, sedative, and anti- convulsant properties, and the use to which a particular benzodi- azepine is put is often determined by its pharmacokinetic properties, including lipid solubility, biological half-life, and /or route of metabolism (e.g., oxidative metabolism or conjugation)

268 Coutts et al.

(Teboul and Chouinard, 1991). CYP3A plays an important role in the metabolism of benzodiazepines such as alprazolam, midazolam, and triazolam (von Moltke et al., 1993; Yasui et al., 1996; Olkolla et al., 1993; Kronbach et al., 1989; von Moltke et al., 19961, while metabolism of diazepam and desmethyldiazepam is largely dependent on CYP2C19 (Bertilsson et al., 1989; Yasumori et al., 1994). 3-Hydroxybenzodiazepines (e.g., lorazepam and oxazepam) undergo extensive phase II metabolism. With regard to metabolism of antidepressants, it is well known that extensive N-demethylation of tertiary amine tricyclics [e.g., imipramine (IMI), amitriptyline (AMI)] to secondary amines [desipramine (DMI), nortriptyline] occurs readily in the body and that these secondary amines (which are also marketed as antidepressants) are more potent inhibitors of noradrenaline (NA) reuptake, and weaker inhibitors of 5-HT reuptake than are the parent tertiary amines (Rudorfer and Potter, 1985). The tertiary and secondary amine tricyclics also differ markedly in their ability to block mus- carinic, a-adrenergic and histaminergic receptors, and their ten- dency to produce side effects associated with blockade of these receptors also differs. In recent years, it has become apparent that ring hyroxylation is an important metabolic aspect of both tertiary and secondary amine tricyclics (Potter and Manji, 1990; Young, 19911, and CYP2D6 seems to play an important role in the formation of these hydroxylated metabolites (Bertz and Grannemar, 1997). Metabolism is also an important component of the action of the “second” and “third” generation antidepressants. For example, high doses of trazodone can result in appreciable plasma concentrations of its major metabolite, m-chlorophenylpiperazine (mCPP), a potent serotoninergic agonist with a longer half-life than the parent drug (Potter and Manji, 1990). Amoxapine can be metabolized to 7- or B-hydroxyamoxapine, the former possessing antipsychotic properties and producing neuroleptic-like toxicity and the latter probably accounting for antidepressant effects (Rudorfer and Potter, 1985). The selective 5-HT (serotonm) reuptake inhibitors (SSRIs) [e.g., fluoxetine (FLU), sertraline, fluvoxamine, paroxetine, and citalopram] differ from the tertiary amine tricyclics in that the metabolites of the SSRIs, where they exist, are also selective 5-HT reuptake inhibitors and/or very weak NA reuptake inhibitors (Potter and Manji, 1990; Baumann, 1992). The SSRIs differ considerably from one another with regard to the CYPs for which they are substrates and/or inhibitors (Brosen, 1993; Kobayashi et al., 1995; Harvey and Preskorn, 1996a,b;

Pm-top/es of Drug Metabolism 269

Hamelin et al., 1996; Jeppesen, et al., 1996; Preskorn, 1996,1997; Lane, 1996; Bertz and Granneman, 1997).

It should be remembered that the phase I metabolites, particularly the hydroxylated ones, of the drugs mentioned above may well undergo extensive phase II metabolism (e.g., conjugation with glucuronic acid).

It is now accepted that many psychiatric drugs are likely to be involved in metabolic drug-drug interactions. Several of these drugs (e.g., phenothiazines, tricyclics, SSRIs) are potent inhibitors of CYP enzymes (Ciraulo and Shader, 1990a,b; Brosen and Skjelbo, 1991; Coutts, 1994; Baker et al., 1994; Spina and Perucca, 1994; Daniel, 1995; Carson, 1996; Glue and Banfield,1996; Greenblatt et al., 1996; Nemeroff et al., 1996; Preskorn, 1997; Shen, 1997) that metabolize many drugs, and thus these psychiatric drugs can be affected by and can influence the metabolism and levels of other coadministered drugs (Bergstrom et al., 1992; Richelson, 1997; Schmider et al., 1997; Sproule et al., 1997; ZumBrunnen and Jann, 1998; Baker et al., 1998). Such interactions seem to be particularly important with the SSRIs, and there are now numerous reports of pharmacokinetic drug-drug interactions involving the SSRIs (and their metabolites in some cases) with other drugs that are administered concomitantly (Ciraulo and Shader, 1990a,b; Messiha, 1993; van Harten, 1993; Taylor, 1995; Taylor and Lader, 1996; Nemeroff et al., 1996; Daniel, 1995; Ereshefsky et al., 1995; von Moltke et al., 1996; Brosen, 1993; Shen, 1997; Baumann, 1996a,b; Lane, 1996; Richelson, 1997, Sproule etal., 1997; Baker et al., 1998).

In most cases, the metabolic drug-drug interactions involve inhibition of CYP enzymes by one or more of the coadministered drugs or their metabolites. In some cases, however, CYP induction may occur. For example, several anticonvulsants (carbamazepine, phenytoin, phenobarbitol) have been reported to reduce the clearance times of coadministered drugs, presumably by induction of CYP isozymes (Arena et al., 1985; Lane, 1996; Raitasuo et al., 1993; Balant-Gorgia and Balant, 1995; Glue and Banfield, 1996; Syrek et al., 1996). CYPlA2, 2A6, 2C9,2C19,2El, and 3A4 are apparently inducible, but there is no strong evidence for the inducibility of other CYPs, including 2D6.

5. Stereoisomers and Drug Metabolism

Many drugs have structures that contain a chiral center (center of asymmetry) or a center of unsaturation or cyclicity, or these

270 Coutts et al.

steric features are introduced as a result of the drug’s metabolism. One of the resultant enantiomers (in the case of chiral drugs) or geometric isomers (resulting from the center of unsaturation or cyclicity) may possess the desired pharmacological activity, while the other may be inactive or possess a different, perhaps undesired, action (Hubbard et al., 1986; Ariens et al., 1988). A few psychiatric drugs are available in pure enantiomeric form, but numerous other drugs that are asymmetric are administered as racemates despite the fact that pharmacological activity resides primarily in one of the enantiomers. Typical examples are tranyl- cypromine (trans-phenylcyclopropylamine), methylphenidate, fenfluramine, fluoxetine, and trimipramine. When blood levels are being evaluated, it is not uncommon to measure levels of total drug, despite the fact that only one enantiomer may be active. Conventional analytical techniques often do not differentiate enantiomers, and it must not be assumed that enantiomers will be present in equal amounts. One enantiomer may very well be absorbed and/or metabolized and/or excreted at different rates than the other enantiomer (Smith, 1984; Testa, 1986; Drayer, 1988; Eichelbaum, 1992; Coutts and Baker, 1989; Jamali et al., 1989; Hutt and Tann, 1996; Eap, et al., 1997; Lane and Baker, 1997).

The question of whether or not drugs with a chiral center should be marketed as a racemate (mixture of the enantiomers) or as the individual enantiomers is one that is of concern to physicians, researchers, pharmaceutical companies, and regulatory agencies. This aspect of drug action is discussed in detail in several recent comprehensive reviews (Mutschler et al., 1990; Caldwell, 1992, 1996; Witte et al., 1993; Gibaldi, 1993; Marzo, 1994).

Several techniques are now available for routine separation and quantition of enantiomers. Space does not permit discussron of these techniques in the present chapter, but many useful review articles on this topic are available in the literature (e.g. Camillerietal , 1994, Gorbg and Gazdag, 1994; Hutt et a1.,1994; Schurig, 1994; Subert, 1994; Terabe et al., 1994; Vespalac and Bocek, 1994; Srinivas et al., 1995; Caldwell, 1996; Ducharme et al., 1996).

G.Studying Drug Metabolism In Vitro and In Vivo

A drug can be a substrate for an enzyme and/or alter the activity of the enzyme (by enzyme inhibition or induction). If a drug is a substrate for an enzyme, this does not necessarily mean it will also inhibit or induce that enzyme at clinically relevant concentrations. Conversely, a drug can be an enzyme inhibitor (e.g., qul-

Principles of Drug Metabolrsm 271

nidine for CYP2D6; Muralidharan et al., 1991) or inducer (e. g., carbamazepine and CYP3A4; Spina et al., 1995; Glue and Banfield, 1996) without being a substrate for that enzyme. A drug may be a substrate for more than one enzyme (e.g., imipramine is metabolized by CYPlA2, -2D6, -2C19, and -3A4; Glue and Banfield, 1996).

Several standardized in vitro techniques are now available that allow determination of the specific CYP isoforms involved in the metabolism of a test drug, and assessment of the potential for and/or extent of enzyme inhibition or induction (see Section 7 for further details on protocols). Studies may be conducted in animal or human liver tissue (liver slices or microsomes, cultured hepatocytes, or subcellular fractions of hepatic tissue), and human, yeast, bacterial, and insect cell systems expressing specific human CYP isoforms have also been developed (for recent reviews see Gonzales and Korzekwa, 1995; Rodrigues, 1994; Waterman et al., 1995). The incubations may be done in the presence and absence of selective CYP enzyme inhibitors (e.g., quinidine for CYP2D6; ketoconazole for CYP3A4). An alternative to using chemical inhibitors in these studies is to use enzyme-specific antibodies; however, these are often expensive and can only be used in subcellular assays, and not in whole cell assays.

It is important to be aware of a number of technical issues when conducting in vitro studies (Rodrigues, 1994; Glue and Clement, 1998; Harvey and Preskorn, 1995; Popli et al., 1995). When human liver tissue is used, factors that might alter enzyme expression (e.g., prior use of inducing drugs or smoking history, age, gen- der) should be known. The use of nonhuman liver tissue may com- plicate data interpretation because of differences in enzyme expression and substrate specificity relative to human liver tissue. Enzyme expression or concentration in normal human liver cells, or the availability of cofactors may be quite different from that observed in cultured or cloned cells or in microsomal preparations (Glue and Clement, 1998). Concentrations of drugs used (as substrates or inhibitors) should be clinically relevant. If they are too low, interactions may be missed, and if they are too high, spurious interactions may be reported. A wide range of concentrations that will include clinically relevant plasma as well as liver concentrations should be used in in vitro testing (Glue and Clement, 1998).

In vivo tests on CYPs in humans are carried out for three main purposes: to determine the presence of genetically deltermined differences in enzyme activity (genetic polymorphism), to assess the role of specific CYP enzymes in the metabolism of compounds,

272 Coutts et al.

and/or to examine possible enzyme-inducing or -inhibitory effects of test compounds (Glue and Clement, 1998).

To test if a CYP enzyme is involved in the metabolism of a drug, the drug can be coadministered with a known specific inhibitor of that enzyme. Concentrations of the test drug and metabolites in plasma and/or urine are then compared with those obtained during monotherapy. Elevated parent and/or reduced metabolite concentrations following administration of a specific inhibitor of a CYP enzyme indicate a role for that enzyme in the test drug’s metabolism. In such studies, the previous metabolizer status should be checked to exclude PMs (it is diffficult to reduce enzyme activity further with an inhibitor in someone who has genetically low baseline enzyme activity). In addition, assessment of inhibition is impos- sible in subjects who are already taking enzyme inhibitors or who have recently finished treatment with a long-acting inhibitor, so a knowledge of current or recent drug history is important. In con- trast to the use of inhibitors in this paradigm, it is not possible to assess specific enzyme involvement in the metabolism of test compounds by using inducers because of their lack of enzyme specificity (Glue and Clement, 1998).

In in vivo clinical studies to assess a test drug’s potential to produce enzyme inhibition or induction, subjects are dosed with probe substances that are enzyme-specific substrates (e.g., debrisoquine or sparteine for CYP2D6), prior to and following treatment with the test drug. Induction may be inferred by reduced concentrations of the probe substrate and/or increased production of its metabolites, and inhibition by increased concentrations of the probe and/or reduced production of its metabolites compared with baseline levels. Inhibition may be assessed using single-dose interaction studies; but to assess the full extent of inhibition or to assess induction requires at least 2 weeks of treatment with the test drug, since steady-state concentrations of an inhibitor and its metabolites may not be reached for several weeks and full induction requires synthesis of new enzyme. It may be possible to assess the effect of a test drug on multiple CYP enzymes simultaneously using a “cocktail” approach (Breimer and Schellem, 1990); metabolism of single doses of several coadministered probe substrates is measured prior to and after treatment with the test drug. Probe substrates could include dextromethorphan for CYP2D6 (and 3A4), caffeine for CYPlA2, tolbutamide for CYP2C9, and S-mephenytoin for CYP2C19 (Glue and Clement, 1998).


7, Examples of Protocols Used In Vitro in Studies on Drug Metabolism Involving CYP Enzymes

Each approach for characterizing the involvement of enzymes in drug metabolism has its advantages and disadvantages, and a combination of approaches seems to be the most reliable means of identifying the relative contribution of each isoenzyme in the metabolism of a particular compound. For example, the ability of a recombinant CYP enzyme to metabolize a drug does not necessarily mean that this enzyme plays a major role in the metabolism of the compound in human liver microsomes, as has been shown to be the case with clozapine (Fang et al., 1998).

7.1. Standard Incubation Mixture

Incubation procedures used in our laboratory are as follows: 0.1 mL reaction mixtures containing 10 PL microsomal preparation from human or animal liver or preparations of cDNA-expressed CYP isoenzymes, a cofactor-generating system consisting of B-nicotinamide adenine dinucleotide phosphate (1.3 mM>, glucose 6-phosphate (3.3 mM), glucose 6-phosphate dehydrogenase (0.4 U/mL) and MgCl, (3.3 mM) and appropriate concentrations of substrates in phosphate buffer (0.1 mM, pH 7.4) are incubated at 37°C for specified time intervals. Control incubates contain heat-inactivated microsomes or control microsomes transfected with a control vector. At the end of the incubation period, the incubation mixture is treated appropriately (e.g., adjustment of pH, use of extraction procedures) for subsequent analysis (by HPLC, CC, etc.) of the drugs and metabolites. See the following papers for specific examples: Coutts et al. (1993, 19971, Su et al. (19931, Bolaji et al. (1993), and Fang et al. (1997)

7.2. Metabolism by Recombinant CYP Enzymes

Numerous cDNA-expressed CYP enzymes have now become available from commercial sources such as Gentest Corporation (Woburn, MA) and Oxford Biochemical Research (Oxford, MI). Use of these enzymes can establish whether a particular CYP enzyme is capable of metabolizing a drug, but the ability of a CYP enzyme to catalyze metabolism does not necessarily mean that this enzyme plays a major role in whole human liver mrcrosomes. The extrapo- lation of results obtained with recombinant CYP enzymes is further complicated by the fact that activities of recombinant CYP

274 Couth et al.

enzymes can be affected by the levels of accessory proteins, such as cytochrome b, and cytochrome I’450 reductase.

7.3. ‘/Panel Study”

This approach involves measuring the rate of formation of metabolites in human liver microsomes from several subjects and correlating the metabolic rate with activities of individual CYP enzymes in the same microsomal preparations. Human liver microsomal preparations are supplied by a number of organizations such as the International Institute for the Advancement of Medicine (IIAM) (Exton, PA), Gentest Corp. (Woburn, WA), and XenoTech (Kansas City, KS). Simple linear regression can be used to correlate the rates of formation of the metabolites with activities of individual CYP enzymes of each microsomal preparation Many of the organizations provide predetermined activities of CYP enzymes for the human liver microsomes they supply.

7.4. Use of Selective Inhibitors and.Specific Antibodies

This approach determines the effects of known CYP enzyme inhibitors on the metabolism of drugs in human liver microsomes. However, most chemical inhibitors are selective for one CYP enzyme only at certain concentrations. For competitive inhibitors, specificity depends on the concentrations of both the inhibitor and the substrate as they relate to K, (inhibition constant) and Km (the substrate concentration at which the reaction velocity equals 50% Of vnlax [maximum velocity] in the absence of the inhibitor), respectively. Highly specific antibodies against selected CYP enzymes can inhibit selectively and noncompetitively metabolic reactions catalyzed by those enzymes; unfortunately, this meth- odology is restricted by the availability and high cost of specific inhibitory antibodies.

7.5. Enzyme Kinetic Studies

Enzyme kinetic parameters for the formation of metabolites can be estimated by incubating different concentrations of the drug in question with human liver microsomes. Kinetic analysis of the formation of metabolites can initially be evaluated by vrsual examination of Eadie-Hofstee plots to assess whether one or more enzymes is/are involved in the formation of a particular metabolite. The kinetic parameters (Vmax and I$,) estimated from this examination can then be used as initial estimates for a nonlinear

Principles of Drug Metabolrsm 275

regression analysis for apparent K,,, and apparent V,,, calculations, fitting the data to the equations outlined below (Ring et al., 1996).

v =v,,,*s / UC,,, + s) (1)

v = Cl,*, * s (2)

where CI,, = Vmax/K, ; CI,, = intrinsic clearance; and S = substrate concentration.

In instances where the formation of a metabolite is biphasic, as determined by inspection an Eadie-Hofstee plot, relationships (3) and (4) are used to analyze the data (Houston, 1994; Segal, 1975). Subscripts 1 and 2 represent, respectively, the high-affinity and low-affinity enzymes involved in the formation of a metabolite. Equation (3) is used when both the high- and low-affinity enzymes approach saturation conditions (S >Kml and K,,J. Equation (4) con- sists of a high-affinity, saturated enzyme and a low-affinity enzyme exhibiting linear formation kinetics (S < Kn,2) (Ring et al, 1996).

v = crv,,,, * SIIIK,, + Sl) + (Wm.& * SIIIK,,~Z + SI) (3)

v = Kllaul * Sl/[KINI + Sl + vqnU * Sl (4)

where qnt2 = vnlax4K,n2’ To estimate the percent formation of a particular metabolite

catalyzed by either the high- or low-affinity enzyme, the apparent kinetic parameters deduced for K,,,, Ifmax, and Cllntare substi- tuted into the above equations.

Inhibition of a particular metabolic pathway can be investigated in vitro by incubating different concentrations of a drug in the absence or presence of the potential inhibitors.

Data points of reaction velocities (V) at varying concentrations of the substrate and of inhibitors (I) in question are analyzed by derivative-free iterative nonlinear least-square regression. Data points are fitted to equation (5), which represents the relationship among the variables in a competitive inhibition model.

v = Lx * S/B + I(,, (1 +UK,)l (5)

Iterated variables are: Vmax, the maximum velocity; K,,, , the substrate concentration at which the reaction velocity equals 50% of Vm,, in the absence of the inhibitor; and K,, the inhibition constant.

276 Coutts et a/.

At any given concentration of a substrate and a specific inhibitor, the percent inhibition of metabolite formation, compared to the rate with no inhibitor present, can be calculated as follows:

% inhibition = (V,- V)/Vc x 100 (6)

where V, is the reaction velocity in the presence of inhibitor, and V, is the control velocity with no inhibitor present.

7.6. Prediction of In Vivo Inhibition from In Vitro Data

To predict in vivo metabolic and pharmacokinetic profiles from in vitro studies is a cost-effective and important aspect of current research. Some successful predictions have been reported with this method with simple model compounds (e.g., Von Moltke et al., 1994; 1996). For example, the in vitro partition method, which assumes distribution via passive diffusion, has yielded accurate predictions of the in vivo inhibition of the 2-hydroxylation of DMI by fluoxetine and sertraline Won Moltke et al, 1994; 1996). In these studies, in vitro metabolism of the drug of interest was performed with human liver microsomes in the presence and absence of inhibitors (known and potential) of specific CYP isoenzymes. Their in vitro/in vivo scaling model then utilizes the in vitro K,value obtained, typical clinically relevant plasma concentrations of the inhibitors, and the presumed liver/plasma partition ratio (liver concentration = plasma concentration x liver/plasma partition coefficient) to predict the degree of clearance impairment of the drug of interest by the potential inhibitors under investigation.

Although the in vitro/in vivo correlation approach appears to be a useful one, there may be complicating factors. Partitioning of lipophilic drugs between plasma and hepatic tissue in vivo probably is dependent on factors such as time after dosage, size of dose, route of administration, plasma protein binding, and plasma/red cell distribution. Partitioning may also vary among sites within the liver, as well as between extracellular water and the intracellular medium. A recent review article contains a comprehensive discussion of factors to be considered in such an approach (Bertz and Granneman, 1997).

In the protocols described above, it is necessary to measure the concentrations of drugs and their metabolites at the end of a specified incubation period. Space does not permit a comprehensive description of the analytical methods available for extraction or


quantitation, but useful reviews are available (e.g., Boulton et al., 1988; Eap and Baumann, 1996) and published manuscripts deal- ing with specific drugs often contain comprehensive details on analytical procedures.

8. Summary

A knowledge of drug metabolism is essential since metabolites may contribute to the overall therapeutic and/or side effect pro- file of the drug of interest. In addition, polypharmacy (use of multiple drugs by patients) is not uncommon, and the risk for metabolic drug-drug interactions in such patients may be high. Numerous in vitro and in vivo techniques are now available for studying metabolism of drugs and potential metabolic drug-drug interactions, and these have been described in this chapter, particularly as they relate to drugs used to treat psychiatric disorders.

Acknowledgments

Funding for the authors’ research was provided by the Alberta Heritage Foundation for Medical Research (Mental Health Research Fund), the Medical Research Foundation of Canada, and the Faculty of Medicine, University of Alberta. The authors are grateful to J. van Muyden, G. Rauw, and R. Strel for expert technical assistance and to J. van Muyden and P. Wolfaardt for assistance in typing this manuscript.

References

Agdndez, J A., Jimenez-Jimenez, F. J , Luengo, A., Bernal, M L , Molina, J A., Ayuso, L , Vazquez, A, Parra, J , Duarte, J., Coria, F , Ladero, J M , Alvarez, J. C. and Benitez, J (1995131 Association between the oxrdative polymorphism and early onset of Parkmson’s disease. Clrn. Pharmacol They. 57,291-269.

Agundez, J A, Ledesma, M. C , Ladero, J M., and Benitez, J. (1995a) Preva- lence of CYP2D6 gene duplication and its repercussion on the oxidative phenotype in a whrte population Clan Pharmacol Ther 57, 265-269

Ameer, B and Wemtraub, R A (1997) Drug interactions with grapefruit juice Cltn Pharmacokznet 33, 103-121

Arana, G W, Goff, D C , Fnedman, H., Ornsteen, M., Greenblatt, D J , Black, B and Shader R.1 (1985) Does carbamazepme-induced reduction of plasma haloperidol levels worsen psychotic symptoms? Am J Psychiatry 143,650-651

Ariens,‘E. J., Wuis, E. W., and Vermga, E. J. (1988) Stereoselectivity of bioactive xenobiotics Biochem Pharmacol 37,13-18.

Baker, G B , Coutts, R T , and Holt, A. (1994) Metabolism and chu-ahty m psychopharmacology Btol Psychratry 36, 211-213

278 Couth et al.

Baker, G B , Urichuk, L J , and Coutts, R T (1998) Drug metabolism and metabolic drug-drug interactions m psychiatry. Chrld and Adolescent Psycho- pharmacol News (suppl)., m press.

Balant-Gorgm, E. A and Balant, L I’ (19951Therapeutrc drug momtormg relevance during the drug treatment of psychiatric disorders CNS Drugs 4,243-253

Baldessarim, I? A and Cole J 0 (19951 Interactions of serotonm reuptake mhibltors with trlcyclic antidepressants - m reply. Arch Gen Psychlat 52, 784,785

Baumann, P (1996a) Pharmacokmetic-pharmacodynamlc relationship of the selective serotonm reuptake mhibltors Clm Pharmacokmet 31, 444-469

Baumann, P (1992) Clmical pharmacokmetics of citalopram and other selective serotonin reuptake inhibitors (SSRI) Int Clan Psychopharmacol 6,13-20

Baumann, I’. (1996b) Pharmacology and pharmacokmetics of citalopram and other SSRls Int Clm Psychophurmucol ll(Suppl.), 5-11

Baumann, P and Rochat, B (1995) Comparative pharmacokmetics of selective serotonm reuptake inhibitors a look behind the mirror Int Chn Psycho- pharmacol lO(Suppl.), 15-21

Benet, L Z, Kroetz, D L., and Sheiner, L B (1996) Pharmacokmetlcs the dynamics of drug absorption, distribution and ehmmation, m Goodman & Gzlman’s The Pharmacologlcul Baszs of Therupeutlcs (Hardman, J G , Llmbud, L E , Molmoff, P. B and Ruddon, R W , eds 1, McGraw-Hill, New York

Bergstrom, R F , Peyton, A L , and Lemberger, L (1992) Quantrfrcation and mechanism of the fluoxetine and tricyclic antidepressant mteraction Clan PharmacoI Ther 51, 239-248

Bertllsson, L , Carrillo, J A, Dahl, M L , Llerena, A, Alm, C , Bondesson, IJ , Lindstrom, L , Rodriguez de la Rubia, I, Ramos, S , and Bemtez, J (1994) Clozapme drsposltion covaries with CYPlA2 activity determined by a caffeine test Br J Clm Phurmacol 38,471-473

Bertllsson, L, Henthorn, T K, Sanz, E , Tybrmg, G , Sawe, J , and Vlllen, T (1989) Importance of genetic factors m the regulation of diazepam metabohsm Relationship to S-mephenytom, but not debrisoquin, hydroxylation phenotype Chn Phurmacol Ther 45,348-355

Bertz, R J and Granneman, G R. (1997) Use of m vitro and m vrvo data to estimate the likelihood of metabolic pharmacokmetic mteractlons C/In Pharmucoktnet 32,210-258

Blake, B L , Rose, R. L , Mailman, R. B, Levi, P. E , and Hodgson, E (1995) Metabolism of thloridazme by mtcrosomal monooxygenases relative roles of I’450 and flavm-contammg monooxygenase. Xenoblotlcu 25,377-393

Bolalr, 0 0, Coutts, R T , and Baker, G B (1993) Metabolism of trimipramme m vitro by human CYP2D6 isozyme Res Commun Chem Path Phurmucol 82,111-120

Boulton, A A , Baker, G B., and Coutts, R T., eds (1988) Analysis of psychiatric drugs Neuromethods, vol. 10, Humana Press, Totowa, New Jersey

Bourrre, M , Meumer, V , Berger, Y , and Fabre, G (1996) Cytochrome P450 lsoform mhlbltors as a tool for the mvestlgation of metabolic reactions catalyzed by human liver microsomes. ] Pharmacol Exp They 277,321-332

Brelmer, D D and Schellens, J H M. (1990) A ‘cocktall’ strategy to assess m viva oxrdative drug metabolism m humans Trends Pharmucol Scr 11,223-225

Brrtto, M R and Wedlund, P J (1992) Cytochrome P-450 m the bram Potential evolutionary and therapeutic relevance of locahzatlon of drug-metabohzmg enzymes Drug Metub D~spos 20,446-450


Brosen, K (1993a) Isozyme specrfrc drug oxrdatron: genetic polymorphrsm and drug-drug mteractrons. Nerd J Psychiat. 47fSuppl. 30),21-26.

Brosen, K (1993b) The pharmacogenetrcs of the selective serotonm reuptake mhrbitors. Clm Invest. 71, 1002-1009

Brosen, K and Skjelbo, E (1991) Fluoxetine and norfluoxetme are potent mhibrtors of P450IID6 - the source of the sparteme/debrrsoqume oxrdatron polymorphrsm. Br J Clwz Pharmacol 32,137,138

Caldwell, J (1992) The importance of stereochemrstry in drug action and drspositron J Clm Pharmacol. 32,925-929

Caldwell, J (1996) Importance of stereospecific bioanalytrcal monitoring in drug development J Chromatogr 719,3-13

Camrlleri, P , de Barsr, V., and Hutt, A (1994) Resolving the problem Chemrstry in Brltaln, 30,43-46

Carson, S. W (1996) Pharmacokmetrc and pharmacodynamrc drug mteractrons with polypharmacotherapy of treatment-resistant affective and obsessive- compulsive drsorders. Psychopharmacol. Bull 32,555-568

Cholerton, S., Daley, A K , and Idle, J R (1992) The role of individual human cytochromes P450 in drug metabolism and climcal response Trends Pharmacol SC1 13,434-439

Crraulo, D A, Shader, R 1, Greenblatt, D J., and Creelman, W. L , eds (1995) Drug Interactmns rn Psychfatry, Willrams & Wrlkms, Baltimore, MD

Crraulo, D. A. and Shader R. 1 (1990a) Fluoxetme drug-drug interactions I Antidepressants and antrspsychohcs J Clan Psychopharmacol 10,48-50

Crraulo, D A, and Shader R. 1. (1990b) Fluoxetme drug-drug mteractrons II ] Glut. Psychopharmacol. 10,213-217

Coutts, R. T (1994) Polymorphrsm m the metabolrsm of drugs, including antidepressant drugs comments on phenotyprng J Psychuzt Neurosci 19,30-44

Coutts, R T., Bach, M. V., and Baker, G B (1997) Metabolism of amitrrptylme with CYP2D6 expressed in a human cell line Xenobtotica 27,33-47

Coutts, R T , Su, I’., Baker, G B , and Daneshtalab, M (1993) Metabolism of rmrpramme m vitro by CYP2D6 expressed m a human cell line, and observa- tions on metabolrte stabrlity J Chromatogr Boomed Appl 6115,265-272

Coutts, R T and Baker, G B (1989) Implications of chrrahty and geometric isomerism m some psychoactive drugs and then metabohtes Chlralzty 99-120

Daniel, W (1995) Metabolrsm of psychotroprc drugs* pharmacological and clmrcal relevance Pol J Pharmacol Pharm 47,367-379

Degtyarenko, K N and Fabran, P (1996) The directory of P450-contammg systems on Worldwide Web Computer Appl. Biosciences 12,237-240.

DeVane, C L (1994) Pharmacogenetrcs and drug metabolism of newer antr- depressant agents J Clm Psychlat 55(Suppl.), 38-45

Drayer, D E (1988) Problems m therapeutic drug momtormg the dilemma of enantromerrc drugs m man Ther Drug Monrt 10, l-7

Ducharme, J , Fernandez, C , Grmenez, F., and Farmottr, R. (1996) Crrtrcal issues m choral drug analysis m biological fluids by high-performance liquid chromatography J Chromatog B Boomed Appl 686, 65-75

Eap, C. B , Bertschy, G , Powell, K , and Bauman, P (1997) Fluvoxamme and fluoxetme do not interact m the same way with the metabolism of the enantromers of methadone J. Clan Psychopharmacol 17,113-117

280 Coutts et al.

Eap, C B and Baumann, P (1996) Analytical methods for the quantitative determmatron of selective serotonm reuptake inhibitors for therapeutic drug monitoring purposes m patients J. Chromatogr 686,51-63

Edge, S C , Markowltz, J S , and DeVane, L. (1997) Clozapine drug-drug mteractions. a review of the literature Human Psychopharmacol 12,5-20

Eichelbaum, M (1992) Pharmacokmetic and pharmacodynamic consequences of stereoselective drug metabolism in man. Blochem Phurmacol 37,93-96

Eiermann, B M , Engel, G., Johansson, 1, Zanger, U M , and Bertilsson, L (1997) The mvolvement of CYPlA2 and CYP3A4 m the metabolism of clozapme Br J Clin. Pharmacol 44,439-446.

Ereshefsky, L., Riesenman, C , and Lam, Y M F (1995) Antidepressant drug mteractions and the cytochrome P450 system* the role of cytocyrome I’450 2D6 Clrn Pharmacokmet 29,10-19

Fang, J , Coutts, R T , McKenna, K F , and Baker, G B (1998) Elucidation of individual cytochrome P450 enzymes involved m the metabolism of clozapme Naunyn Schmiedeberg’s Arch Pharmacol (m press).

Fang, J , Baker, G B., Silverstone, P H , and Coutts, R T (1997) Involvement of CYP3A4 and CYP2D6 m the metabolism of haloperidol Cell Mol Neuroblol 17,227-233.

Fang J , Baker G. B , Coutts R T., and McKenna K F (1996) Elucidation of mdi- vldual cytochrome P450 isoenzymes involved in the metabolism of clozapme Proc 7th North American ISSX Meeting, San Diego, California, USA, Octo- ber 20-24,1996,

Gaedigk, A, Blum, M , Gaedigk, R., Erchelbaum, M , and Meyer, U A. (1991) Deletion of the entire cytochrome P450 CYP2D6 gene as a cause of impaired drug metabolism m poor metabohzers of the debrisoqume/sparteme polymorphism Am J Hum Genet 48,943-950

Garfmkel, D (1958) Studies on pig liver microsomes 1 Enzymatic and pigment composition of different microsomal fractions Arch Biochem Bzo- physics 77,493-509

Gibaldi, M (1993) Stereoselective and isozyme-selective drug mteractions Chrralzty 5,407-413

Gibson, G G and Skett, P (1988) lntroductlon to Drug Metabolism, Chapman and Hall, London

Glue, P. and Clement, R l? (1998) Cytochrome P-450 enzymes and drug metabohsm-basic concepts and methods of assessment Cell Mol Neurobrol (in press)

Glue, P and Banheld, C (1996) Psychiatry, psychopharmacology and P-450s Hum Psychopharmacol. 11,97-114

Goldstem, J. A , Faletto, M. B., Romkes-Sparks, M., Sullivan, T , Kitareewan, S , Raucy, J L , Laskar, J. M., and Ghanayem, B. 1 (1994) Evidence that CYP2C19 IS the malor (S)mephenytoin 4’-hydroxylase m humans Blochemlstry 33,1743-1752

Gonzales, F J, and Korzedwa, K R (1995) Cytochrome P450 expression systems Ann Rev Pharmacol Toxic01 35,369-390

Gorog, S and Gazdag, M. (1994) Enantiomeric derivatization for biomedical chromatography J Chromatogr 659,51-84

Greenblatt, D J., von Moltke, L L , Schmider, J., Harmatz, J S , and Shader, R. I. (19961 Inhibition of human cytochrome P450-3A isoforms by fluoxetine and norfluoxetine in vitro and in vivo studies J Clan Pharmacol 36, 792-798.


Guengerich, F. P (1995) Human cytochrome P-450 enzymes, m Cytuchrome P-450, 2nd Ed. (Ortiz de Montellano, P. R , ed.), Plenum Press, New York

Hamelm, B A., Turgeon, J., Vallee, F., Belanger, P M., Paquet, F , and LeBel, M. (1996) The disposition of fluoxetme but not sertralme is altered in poor metabohzers of debrisoqum. Clin Pharmacol. Ther 60,512-521

Hansson, T , Tmdberg, N , Ingelman-Sundberg, M , and Kohler, C (1990) Regional distribution of ethanol-inducible cytochrome P450 IIEl m the rat CNS. Neurosczence 34,451-463

Harvey, A. T and Preskorn, S. H. (1996a) Cytochrome P450 enzymes mterpre- tation of their interactions with selective seroltonin reuptake inhibitors Part I J Clrn. Psychopharmacol 16,273-285.

Harvey, A. T and Preskorn, S H. (1995) Interactions of serotonin reuptake mhibitors with tricychc antidepressants. Arch. Gen Psvchrat 52,783,784

Harvey, A. T. and Preskorn, S. H. (1996b) Cytochrome I’450 enzymes mterpre- tation of their mteractions with selective seroltonin reuptake inhibitors. Part II. J C11n Psychopharmacol 16,345-354

Hedlund, E , Wyss, A, Kamu, T , Backlund, M , Kohler, C , Pelto-huikko, M., Gustafsson, J -A and Warner, M (1996) Cytochrome P4502D4 m the brain specific neuronal regulation by clozapine and toluene. Mel Pharmacol 50, 342-350.

Helm, M. H. and Meyer, U. A (1991) Genetic polymorphism of debrtsoqume oxidation. restriction fragment analysis and allele-specific amplification of mutant alleles of CYP2D6 Meth Enzymol 206,173-183

Henthorn, T. K., Bemtez, J , Avram, M. J., Martinez, C., Lierena, A, Cobaleda, J., Krelcie, T C , and Gibbons, R. D (1989) Assessment of the debrisoquin and dextromethorphan phenotypmg tests by gaussian mixture distributions analysis. Chn Pharmacol Ther 45,328-333.

Houston, J B. (1994) Utility of m vitro drug metabolism data m predicting m viva metabolic clearance. Blochem Pharmacol ,47, 1469-1479

Huang, M-L., Van Peer, A., Woestenborghs, R , DeCoster, R , Heykants, J , Jansen, A. A. I, Zylicz, Z., Visscher, H W , and Jonkman, J H G (1996) Pharmacokmetics of the novel antipsychotic agent risperidone and the pro- lactm response m healthy sublects Clan Pharmacol Ther 54,257-268.

Hubbard, J. W., Canes, D , Lrm, H. K., and Midha, K K (1986) Chiral pharmacology and rts consequences for therapeutic momtormg C11n Blochem 19,107-112.

Hutt, A. J and Tan, S C (1996) Drug chirality and its cluucal significance Drugs 52,1-12

Hutt, A. J , Hadley, M. R., and Tan, S. C (1994) Enantiospecific analysis Apphca- tlons m bloanalysis and metabolism. Eur ] Drug, Metab Pharmacokm 19,241-251.

Jamalr, F , Mahvar, R., and Pasutto, F. M (1989) Enantioselective aspects of drug action and disposition. therapeutic pitfalls. ] Pharm SU 78,695-715

Jeppesen, U., Gram, L. F., Vistisen, K., Loft, S , Poulsen, H E , and Brosen, K , (1996) Dose-dependent inhibition of CYPlA2, CYP2C19, CYP2D6 by crtalopram, fluoxetme, fluvoxamine, and paroxetme Eur J Clm Pharmacol 51,73-78

Jerling, M , Dahl, M-L, Aberg-Wlstedt, A, et al (1996) The CYP2D6 genotype predicts the oral clearance of the neuroleptic agents perphenazme and zuclopenthrxol Chn Pharmacol Ther 59,423-428

Kalow, W. and Tyndale, R. F. (1992) Debrisoqume/sparteme monooxygenase and other P-450s m the brain, in Pharmacogenetlcs of Drug Metabolism. Inter-

282 Coutts et al.

nattonal Encyclopedia of Pharmacology and Therapeutics (Kalow, W , ed ), Pergamon Press, New York, pp 649-656

Kempermann, G , Knoth, R., Gebicke-Haerter, P J , Stolz, B J , and Yolk, B (1994) Cytochrome P450 in rat astrocytes m vivo and m vitro intracellular locahzation and induction by phenytom J. Neurosct Res 39,576-588

Kobayashi, K , Yamamoto, T., Chiba, K., Tani, M , Ishrzak, T , and Kurolwa, Y (1995) The effects of selective serotonin reuptake mhibitors and their metabolites on S-mephenytom 4’-hydroxylase activity m human liver microsomes BY I Clan Pharmacol 40,481-485

Kroemer, H K and Eichelbaum, M (1995) “It’s the genes, stupid “ Molecular bases and clmrcal consequences of genetic cytochrome P450 2D6 polymorphism. Life Scr 56,2285-2298

Kronbach, T , Mathys, D , Umeno, M , Gonzalez, F J , and Meyer, U A (1989) Oxrdation of midazolam and triazolam by human liver cytochrome P450IIIA4 Mol Pharmacol 36,89-96

Kopfer, A and Prersrg, R (1984) Pharmacogenetics of mephenytoin a new drug hydroxylation polymorphism m man Eur J Clan Pharmacol 26,753-759

Lam, Y W F, Jann, M W , Chang, W-H, Yu, H-S, Lm, S-K., Chen, H., and Davis, C. M. (1995) Intra- and interethnic variabihty m reduced haloperidol to haloperidol ratios 1 Clan Pharmacol 35,128-136

Lane, R M. (1996) Pharmacokinetic drug mteractron potential of selective serotonm reuptake Inhibitors. lnt Clw Psychopharmacol 11,31-61

Lane, R M and Baker, G B (19981 Chuality and drugs used m psychiatry nice to know, or need to know7 Cell Mol Neurobzol (in press)

Llccione, J J and Mames, M D (1989) Manganese-mediated increase m the rat brain mitochondrral cytochrome-P-450 and drug metabolism activity-sus- ceptibility of the striatum J Pharmacol Exp Tker 248, 222-228

Lm, K.-M, Poland, R E , Wan, Y -J Y , Smith, W , and Lesser, I M (1996) The evolving of pharmacogenetics clmrcal and ethnic perspectrves Psycho- pharmacol Bull 32,205-217

Marzo, A (1994) Incommg guidelmes on chrrahty - A challenge for pharmacokmetics m drug development Arznezmlttelforschung 44-1,6

Messiha, F S (1993) Fluoxetme. adverse effects and drug-drug interactions Clin Tox~col 31,603-630

Meyer, U A , Amrien, R , Balant, L P , Bert&son, L , Eichelbaum, M , Geuntert, T. W , Henauer, S , Jackson, P , Laux, G , Mikkelson, H , Peck, C , Pollock, B G , Proest, R , Sloqvist, F , and Denlmr-Stula, A (1996) Antidepressants and drug-metabohzmg enzymes - expert group report Acta Psychuzt Stand 93,71-79

Mrdha, K K , Hawes, E M , Hubbard, J W , Korchmski, E D , and McKay G (1987) The search for correlations between neuroleptrc plasma levels and clmical outcome a critical review, in Psychopharmacology The Thwd Genera- tlon ofProgress (Meltzer, H Y , ed 1, Raven Press, New York

Miners, J 0 and Blrkett, D J (1998) Cytochrome P4502C9* an enzyme of malor importance m human drug metabolism Br J Clan Pharmacol 45, 525-538

Muralidharan, G , Hawes, E M , McKay, G , Korchmski, E D , and Midha, K K (1991) Qumrdme but not qumme mhibits m man the oxidative metabolic routes of methoxyphenamme which mvolve debrisoquine 4-hydroxylase Eur J Pharmacol 41,471-474


Mutschler, E , Gietl, Y , Krauss, D , Martin, E., Pflugmann, G , and Weber, H (1990) Stereospecific analysis and human pharmacokinetics of the enantromers of drugs admmrstered as racemates, in Chirulity and Bzologwl AC~JZIZ~IJ (Holmstedt, B , Frank, H , and Testa, B , eds ), Alan R Liss, New York, pp.199-219

Nebert, D W , Nelson, D R , Adesnrk, M , Coon M J , Estabrook, R W , Gonzalez, F J , Guengerich, F P , Gunsalus, I C , Johnson, E. F , Kemper, B , Levm, W , Philips, 1 R , Sato, R and Waterman, M R (1989) The P450 superfamily, updated hstmg of all genes and recommended nomenclature for the chromosomal loci DNA &l-13

Nelson, D R , Kamatakl, T , Waxman, D. J , Guengerich, F L , Estabrook, R W , Feyereisen, R , Gonzalez, F J , Coon, M J , Gunsalus, I C , Gotoh, 0, Okuda, K , and Nebert, D W (1993) The P450 superfamrly update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature DNA Cell Bml 12,1-51

Nelson, D R , Koymans, L , Kamataki, T., Stegeman, J J , Feyereuen, R , Waxman, D J , Waterman, M R , Gotoh, 0, Coon, M J, Estabrook, R W, Gunsalus, I C , and Nebert, D W (1996) P450 superfamily update on new sequences, gene mapping, accession numbers and nomenclature Phavmacogenetlcs 6, l-42.

Nemeroff, C B , DeVane, C L , and Pollock, B G (1996) Newer antidepressants and the cytochrome P450 system Am J Psyck~t 153,311-320

Olkkola, K T , Aranko, K , Luurda, H , Hiller, A , Saaemvaara, L , Himberg, J J , and Neuvonen, P J (1993) A potentially hazardous interaction between erythromycm and midazolam. Clin Pharmacol Ther 53,298-305

Omura, T and Sato, R (1962) A new cytochrome in liver microsomes ] Bwl Ckem 237, PC1375-PC1376

Ortiz de Montellano, P R , ed (1996) Cytochrome P450, in, Structure, Mecka- nlsm, and Blockemlstry, 2nd ed , Plenum Press, New York

Otton, S V., Newman, A H , Cheung, S W , Tortella F , and Sellers, E M (1993) Dextrorphan formation from alkyl ether analogues of dextromethorphan m vitro Abstracts of College on Problems of Drug Abuse

Parkmson, A (1996) Biotransformation of xenobiotics, m Cusarett and D&l’s Tovrcology The Basic Sczence of Poisons, 5th edition (Klaasen, C D , ed ), McGraw-Hill, New York, NY, pp 113-186.

Perault, M C , Bouquet, S , Bertschy, G , Vandel, S , Chakroun, R , Guibert, S , and Vandel, B (1991) Debrrsoqume and dextromethorphan phenotypmg and antidepressant treatment. Tkerapie 46,1-3.

Pirmohamed, M , Williams, D., Madden, S , Templeton, E , and Park, B K (1995) Metabolism and bioactivation of clozapine by human live in vitro 1 Pkurmacol Exp Ther 272,984-990

Poph, A, Baldessarmi, R. J , and Cole, J 0. (1995) Interactions of serotonm reuptake inhibitors with tricychc antidepressants - m reply. Arch Gen Psy- cklatry, 52, 784,785

Potter, W Z and ManIl, H K. (1990) Antidepressants, metabolites, and apparent drug resistance Clan Newopkurmacol 13(Suppl. l), S45-S53

Preskorn, S H (1996) Clrnmd Pharmacology @Selectwe Serotonrn Reuptke Inklbl- tors, 1st ed , Professional Communications Inc , Caddo, OK

Preskorn, S H (1997) Clmically relevant pharmacology of selective serotonm reuptake mhlbrtors an overview with emphasis on pharmacokmetics and effects on oxidative drug metabolism Clan Pkarmacokmet 32,(Suppl. l), l-21

284 Coutts et al.

Rattasuo, V , Lehtovaara, R , and Huttunen, M 0 (1993) Carbamazepme and plasma levels of clozapme. Am J Psychiatry 150,169

Richelson, E (1997) Pharmacokinetic drug interactions of new antidepressants A review of the effects on the metabolism of other drugs Mayo Clan Proc 72, 835-847.

Ring, B J , Catlow, J , Lindsay T, Gillespie, T, Roskos, K , Cerimele, B J , Swanson, S P , Hamman, M. A, and Wrighton, S A (1996) Identiflcatlon of the human cytochromes P450 responsible for the invitro formation of the major oxidative metabolltes of the antipsychotic agent olanzapme ] Pharmacol Exp Ther 276,658-666

Rodrigues, A D (1994) Use of in vitro human metabolism studies m drug development. Bzochem Pharmacol. 48,2147-2156.

Rosholm, N F. and Brosen, K (1995) Lack of relationship between qumldine pharmacokmetics and the sparteme oxidation polymorphism Eur ] Chn Pharmacol 48,501-504

Rudorfer, M V and Potter, W Z (1985) Metabolism of drugs used m affective disorders, m Pharmacotherapy fAfJectzve Disorders, Theory and Practzce (Dewhurst, W G and Baker, G B , eds.), N Y Umversity Press, New York, pp 382-448

Schmid, B , Blrcher, J , Preisig, R and Kupfer, A (1989) Polymorphic dextromethorphan metabolum. Cosegregation of oxidative 0-demethylation with debrisoquine hydroxylation Clm Pharmacol Ther 38,618-624

Schmider, J., Greenblatt, D J., von Moltke, L L., Karsov, D , and Shader, R I (1997) Inhibition of CYP2C9 by selective serotonin reuptake inhibitors IN vztro studies of phenytom p-hydroxylation J Clan Pharrnacol 44,495-498

Schurig, V (1994) Enantiomer separation by gas chromatography on chnal sta- tionary phases J Chromafog 666,111-129

Segel, I H (1975) Enzyme Kmetrcs Behavror and Analysis #RapId Equlllbrlum and Steady-State Enzyme Systems, John Wiley and Sons, New York, NY

Sequeira, D J and Strobel, H W. (1996) In vitro metabolism of imlpramme by brain mlcosomes effects of mhibitors and exogenous cytochrome P450 reductase. Bram Res 738,24-31.

Shen, W. W (1997) The metabolism of psychoactive drugs A review of enzymatic biotransformation and mhibition Biol Psychiatry 41,814-826

Shimada, T , Yamazaki, H., Mimura, M , Inm, Y , and Guengerich, F P (1994) Intermdividual variations in human liver cytochrome P-450 enzymes involved m the oxidation of drugs, carcinogens and toxic chemicals Studies with liver mlcrosomes of 30 Japanese and 30 Caucasians J Pharmacol Exper Ther 270,414-423

Smith, D. F (1984) CRC Handbook ofStereoisomers Drugs zn Psychopharmacology, CRC Press Inc., Boca Raton, FL

Spma, A, Avenoso, A, Campo, G , Caputi, A. I’, and Perucca, E (1995) The effect of carbamazepme on the 2-hydroxylation of deslpramine Psychopharmacol 117,413-416.

Spma, E and Perucca, E (1994) New and older antidepressants a comparative review of drug mteractions CNS Drugs 2,479-497

Sproule, B. A, Naranlo, C A., Bremmer, K. E , and Hassan, P C (1997) Selec- tive serotonm reuptake inhibitors and CNS drug interactions Clw Pharmacokinet 33,454-471

Srmivas, N R , Shyu, W C , and Barbhalya, R. H (1995) Gas chromatographlc determination of enantiomers as diastereomers following pre-column


derivatization and applications to pharmacokmetic studies. a review Biomed Chromatog 9,1-9

Su, I’., Coutts, R T , Baker, G. B., and Daneshtalab, M. (1993) Analysis of imipramme and three metabolites produced by isozyme CYP2D6 expressed m a human cell line Xenobtotica 23, 1289-1298

Subert, J (1994) Progress in the separation of enantlomers of choral drugs by HPLC without their prior derivatlzation. Pharmazte 49,3-13.

Syrek, M., Wojcikowski, J , and Daniel, W. (1996) Effect of carbamazepine on the pharmacokinetics of promazine. Pool. J Pharmacol 48,601-608

Taylor, D. (1995) Selective serotonin reuptake inhibitors and tricychc antidepressants in combination Interactions and therapeutic uses. BY J Psychiatry 167,575-580

Taylor, D. and Lader, M (1996) Cytochromes and psychotropic drug mteractions BY. J Psychiat 168,529-533.

Teboul, E. and Chouinard, G. (1991) A guide to benzodlazepme selection. part 11, climcal aspects. Can J Psychiat. 36,62-73

Terabe, S., Otsuka, K., and Nishi, H (1994) Separation of enantiomers by capll- lary electrophoretic techniques J Chromatogr. A. 666,295-319

Testa, B (1986) Chiral aspects of drug metabolism Trends Pharmacol Scz 7,60-64

Van Harten, J (1993) Clinical pharmacokmetics of selective serotonm reuptake inhibitors Clin Pharmacokmet 24,203-220

Vespalec, R and Bocek, P (1994) Chiral state of the art. Electrophoresis 15, 755-762

von Bahr, C., Movin G , Nordm, C., Linden, A, Hammarlund-Udenases, M., Hedberg, A, Ring, H., and Sloquist, F. (1991) Plasma levels of thiorldazine and metabohtes are influenced by the debrisoquin hydroxylation phenotype Clin Pharmacol Ther 49, 234-240.

von Moltke, L L , Greenblatt, D J , Harmatz, J, S., and Shader, R. 1 (1993) Alprazolam metabolism m vitro studies of man, monkey, mouse and rat liver microsomes Pharmacology 47,268-276.

von Moltke, L. L., Greenblatt, D J., Harmatz, J. S , Duan, S. X., and Harrel, L. M. (1996) Triazolam biotransformation by human liver microsomes in vitro effects of metabohc inhibitors and climcal confirmation of a predicted interaction with ketoconazole. J Pharmacol Exp They 276,370-379

von Moltke, L L, Greenblatt, D. J, Duan, S. X, Schmider, J, Kudchadker, L, Fogelman, S. M , Harmatz, J S., and Shader, R 1. (1996) Phenacetm O-dee- thylation by human liver microsomes in vitro. inhibition by chemical probes, SSRI antidepressants, nefazodone, and venlafaxine. Psychopharmacology 128, 398-407.

von Moltke, L L , Greenblatt, D J , Cotreau-Bibbo, M., Duan, S X., Harmatz, J. S., and Shader, R. 1. (1994) Inhibition of desipramme hydroxylation in vitro by serotonin-reuptake mhibitor antidepressants, and by quinine and ketoconazole a model system to predict drug interactions in vivo. J Pharmacol. Exp They 268,1278-1283.

Warner, M., Stromstedt, M., Wyss, A., and Gustafsson, J-A. (1993) Regulation of cytochrome I’450 in the central nervous system J Steroid Blochem Mol Bzol 47,191-194

Waterman, M. R., Jenkins, C. M., and Prkuleva, 1. (1995) Genetically engineered bacterial cells and applications. Toxicol. Lett 82183, 807-813

286 Coutts et al.

Witte, D T , Ensmg, K , Franke, J P , and Dezeeuw, R A (1993) Development and registration of choral drugs Pharm World SCI 15, 10-16.

Wrighton, S A and Stevens, J C (1992) The human hepatic cytochromes P450 involved in drug metabolism Crdd Xemezus m Toxzcology 22, l-21

Wu, D , Otton, S. V , Kalow, W., and Sellers, E. M (1995) Effects of route of administration on dextromethorphan pharmacokmetrcs and behavloural response m the rat J Pharmacol Exp Ther 274, 1431-1437

Yasm, N , Koichi, 0, Kaneko, S , et al (1996) A kmetlc and dynamic study of oral alprazolam with and without erythromycm in humans In viva evidence for the involvement of CYP3A4 m alprazolam metabolism Clrn Pharmacol Ther 59,514-519

Yasumorr, T , Qmg-Hua, L , Yamazoe, Y , et al (1994) Lack of low Km diazepam N-demethylase m livers of poor metabohzers for S-mephenytom 4-hydroxylatlon Pharmacogenetrcs 4,323-331

Young, R C (1991) Hydroxylated metabohtes of anndepressants Psychopharmacol Bull 27,521-532

Young, D, Midha, K K , Fossler, M J., Hawes, E M , Hubbard, J W , McKay, G , and Korchmskl, E D. (1993) Effect of qmmdme on the mterconverslon kmetlcs between haloperldol and reduced haloperldol m humans implications for the mvolvement of cytochrome P4501ID6 Eur / Clm Pharmacol 44, 433-438.

Zum Brunnen, T L and Jann, M W (1998) Drug mteractrons with antlpsy- chotlc agents CNS Drugs 9,381-401

8 Principles of Drug Metabolism, With an Emphasis on Psychiatric Drugs

Documents

parent drug

administered drug

drug biotransformation

formation of drug metabolites

undesirable drug effects

drug detoxifica tion

active metabolites

products metabolites