Pharmacogenetic aspects in the metabolism of psychotropic drugs: Pharmacokinetic and clinical implications

Pharmacological

Research,

Vol. 29, No. 2, 1994

121

PHARMACOGENETIC ASPECTS IN THE METABOLISM OF PSYCHOTROPIC DRUGS: PHARMACOKINETIC AND CLINICAL IMPLICATIONS EDOARDO

SPINA

Institute of Pharmacology, Received

and ACHILLE

P. CAPUTI

University of Messina, Messina, Italy

in final

form 15

November

1993

INTRODUCTION As for many drugs, therapeutic response to psychotropic drugs varies widely among patients treated with the same dose: from no effect at all to dramatic adverse reactions. It is well documented that 20 to 30% of patients treated with these compounds, namely antidepressants and neuroleptics, do not respond to therapy [l]. Apart from the role of non-pharmacological aspects, such as psychological and social implications, this variability results from the interaction of genetic, pathophysiological and environmental factors that produce interindividual differences in pharmacokinetics and pharmacodynamics. With regard to pharmacokinetic sources of variability, it is unlikely that kinetic processes dependent on the physicochemical properties of a drug differ significantly among individuals under physiological conditions. By contrast, biochemical individuality should be expected whenever enzymatic processes are involved in the fate of drugs as in the case of drug metabolism. With a few exceptions, psychoactive drugs, such as antidepressants, antipsychotics and are highly lipophilic and therefore subject to multiple anxiolytics, biotransformation steps yielding polar metabolites that can be easily excreted in the urine [2, 31. In general, their metabolism involves phase I oxidative reactions, followed by phase II glucuronide mediated by hepatic microsomal system, conjugation. As a consequence of the large interindividual variability in metabolism, standard doses of these compounds result in pronounced differences in steady-state plasma concentrations and, therefore in therapeutic outcome [2]. Non-genetic constitutional and environmental factors, including age, sex, disease states, smoking or alcohol habits and concurrent drug administration, are important elements which influence the metabolism of psychotropic drugs. Only in recent years intensive research has elucidated the role of the genetic component in hepatic drug metabolism. This article reviews the genetic aspects of the biotransformation of psychotropic drugs. As stated in recent reviews on this topic, it is now generally Correspondence to: Dr Edoardo Spina, Settembre, 4, 98 I22 Messma, Italy. 1043-66

I g/94/020 12 1-l 7/$08.00/O

Institute

of Pharmacology,

University

of Messina,

Piazza

0 1994 The Italian Pharmacological

XX

Society

accepted that the genetically determined metabolic capacity is the major determinant of the large interindividual variability in the pharmacokinetics of psychoactive drugs and may therefore contribute, at least in part, to the unpredictability of response 14, 51.

PHARMACOGENETICS:

GENETIC POLYMORPHISM OXIDATION

IN DRUG

Pharmacogenetics is the study of genetically determined variations in drug response 16, 71. Genetic factors influence drug action by affecting pharmacokinetic and/or pharmacodynamic behaviour of an agent with subsequent differences in the intensity and duration of the expected pharmacological effect. When a drug enters the body it interacts with several enzymes and other proteins. Theoretically, genetic mutations altering the quantity and the quality of any of these proteins may occur at sites of absorption, distribution, protein binding, metabolism and excretion, as well as at drug-receptor interaction level. To date, the majority of known pharmacogenetic entities involves inherited modifications in the activity of enzymes controlling the metabolism of certain drugs [8]. In these conditions, as a consequence of decreased or retarded drug inactivation, toxic concentrations may be achieved with usual doses. The activity of drug metabolizing enzymes may be regulated by one or more genes (monogenic or polygenic control). Polygenic inheritance is more difficult to detect and to distinguish from environmental factors since it may cause a continuous variation in the population for a given metabolic parameter. By contrast, a Mendelian, monogenic trait often divides a population into two or three distinct groups. Inborn errors of drug metabolism identified in recent years concern the activity of enzymes under monogenic control and they are of two types. Some are very rare, to the extent that only 1 in 10 000 to 1 in 100 000 people might be affected, and are called “rare phenotypes”. Typical examples are the inherited sensitivity to succinylcholine [9], due to pseudocholinesterase variants, which may cause prolonged apnea in affected individuals, and the defective metabolism of phenytoin, observed in a few families [lo]. Other defects are represented by classical genetic polymorphism. By this term it is defined as a monogenic or Mendelian trait, due to multiple alleles at a single gene locus, that exists in the population in at least two phenotypes (and presumably in at least two genotypes), the rarest of which occurs with a frequency of 1% to 2% [6]. Genetic polymorphisms of drug-metabolizing enzymes give rise to distinct subgroups in the population that differ in their ability to perform certain metabolic drug reactions. If a polymorphic enzyme plays a significant role in the overall elimination of a particular drug, large differences in disposition could be observed between phenotypes as well as clinical consequences, especially if the drug has a small therapeutic index. The first described genetic polymorphism of human drug metabolism was the N-acetylation polymorphism [ 111. The molecular and clinical aspects of this genetic defect have recently been reviewed [ 121. Among psychotropic drugs, only

the metabolism of the monoamine oxidase inhibitor phenelzine and of the benzodiazepines nitrazepam and clonazepam are subject to this polymorphism. Oxidation is the most common pathway of drug biotransformation in the body. Oxidative reactions are catalysed by mixed-function oxidases or mono-oxigenases that are located in the hepatic smooth endoplasmic reticulum. The terminal oxidase is one of the multiple forms of cytochrome P450, a family of structurally related isozymes with different but overlapping substrate specificity [13, 141. There is evidence that each P450 is encoded by a separate gene [ 151. Two separate genetic polymorphisms of drug oxidation have so far been established the mephenytoin unequivocally, namely the debrisoquinelsparteine and polymorphisms (Fig. 1). Polymorphic oxidation of debr-isoquinelsparteine In the late 197Os, independent studies revealed that the individual’s capacity to oxidize the antihypertensive drug debrisoquine and the antiarrhythmic compound sparteine, expressed as the ratio of parent drug to metabolite(s) excreted in the urine after a test dose, is bimodally distributed in the population [ 16, 171. Two phenotypes can be observed, poor metabolizers (PMs) and extensive metabolizers (EMS). The frequency of the PM phenotype varies between 3 and 10% in Caucasians [18, 191 (Fig. 2), but is markedly lower in Asians [20]. Family studies demonstrated that these oxidative reactions are under monogenic control and that the PM phenotype is inherited as an autosomal recessive trait [21,22]. The molecular basis of this polymorphism has been elucidated [23]. Defective oxidation is probably due to a decreased amount or absence of a specific cytochrome P450, called P450IID6 or CYP2D6, in the livers of PMs [24]. In this phenotype, substrates of the isoenzyme are predominantly eliminated by alternative P45Os, by other non-oxidative enzymes or by renal excretion of unchanged drug. The gene encoding for CYP2D6 has been characterized and is located on the long arm of chromosome 22 [25,26]. The importance of this polymorphism is not purely academic, although the drugs leading to its discovery were soon obsolete or not marketed in several countries. In fact to a varying extent the polymorphic enzyme controls the oxidative metabolism of more than 20 commonly used drugs, including some antiarrhythmics, P-adrenergic receptor antagonists, antidepressants, neuroleptics

S-Mephenytoln

Debrcsoquine Fig. 1. Chemical of hydroxylations.

formulae

of debrisoquine

and S-mephenytoin.

The arrows indicate the sites

124

Pharmacological

0.1

1 Metabolic

10

Research.

Vol. 29, No. 2, 1994

100

ratio

Fig. 2.

Frequency distribution of debrisoquine metabolic subjects. Reprinted from ref. 19, Spina et al., 1992.

ratio

(MR)

among

137 Italian

and opiates [27-301. For some of those substrates whose major metabolic route is catalysed by CYP2D6, the polymorphic oxidation may have pharmacokinetic and therapeutic consequences, as recently reviewed [27, 29, 311. There is in fact growing evidence that PMs are at high risk of developing concentration-dependent side effects when treated with standard doses of drugs metabolized by CYP2D6. Knowledge about a patient’s oxidative status might be of practical value for dose adjustment especially if there is a narrow therapeutic range or an established concentration-effect relationship. Phenotype determination may be assessed by administration of probe drugs, such as debrisoquine, sparteine, dextromethorphan or desipramine [32, 331, followed by determination of the amount of parent drug and its metabolite(s) in the urine. This approach may be replaced by simple methods for genotyping patients with a small sample of genomic DNA by applying RFLP (restriction fragment length polymorphism) and PCR (polymerase chain reaction) analyses, without the need to administer a marker drug [34, 351. Polymorphic

oxidation

ofmephenytoin

Another genetic trait related to the activity of cytochrome P4.50 isozymes was revealed by the discovery of the polymorphic 4-hydroxylation of S-mephenytoin, a now rarely used anticonvulsant agent [36, 371. Defective oxidation is inherited as an autosomal recessive trait and occurs with a frequency of 2-5% in Caucasians, but with a much higher frequency (15-20%) in Chinese and Japanese subjects. The cytochrome P450 catalysing mephenytoin hydroxylation is categorized as belonging to the CYP2C subfamily [38]. Among psychotropic drugs, the metabolism of diazepam is associated with this polymorphism.

PHARMACOGENETICS

Tiicyclic

antidepressants

OF ANTIDEPRESSANTS

are the most commonly

used drugs in the treatment

of

Pharmucolo,~ical

Research,

Vol. 29, No. 2, 1994

125

depressive disorders. They are extensively metabolized by hepatic microsomal enzymes. In general their biotransformation includes an initial oxidative reaction (N-demethylation and/or ring hydroxylation), catalysed by cytochrome P450 isozymes, followed by conjugation with glucuronic acid [39]. The tertiary amines amitripyline (AT), imipramine (IMI) and clomipramine (CI) are demethylated to the active metabolites, nortriptyline (NT), desipramine (DMI) and desmethylclomipramine (DMCI), respectively. AT and NT are subsequently hydroxylated, mainly by IO-hydroxylation, while IMI and DMI are hydroxylated at position 2. The hydroxymetabolites are rapidly glucuronidated and excreted in the urine. The possible role of genetic factors in the disposition of tricyclics had been hypothesized already at the end of the 1960s when early pharmacokinetic studies revealed pronounced variability in steady-state plasma concentrations of these compounds among patients receiving the same oral dose [40]. This was proposed to be due mainly to interindividual differences in the activity of the hydroxylating enzymes. Subsequent twin and family studies clearly established that genetic factors were the major determinants of this variability, but the exact nature of the genetic control could not be detected [41,42]. In particular, in the classical study by Alexanderson et al. [41], given the same dose of NT to several twin pairs, there were virtually no intrapair differences in steady-state plasma concentrations for the monozygotic twins, whereas significant intrapair differences were observed in about half of the dizygotic twins. The discovery of the polymorphic debrisoquine oxidation aroused a renewed interest in the pharmacogenetics of tricyclic compounds. In recent years in vivo and in vitro studies have indicated that the hydroxylation reactions of tricyclic antidepressants are catalysed by CYP2D6, whereas N-demethylation of tertiary amines AT and IMI is not affected by the genetic polymorphism [4, 51. Pharmacokinetic studies in panels of healthy, drug-free EMS and PMs have in fact suggested that the IO-hydroxylation of AT [43] and NT [44], the 2-hydroxylation of TM1 [45] and DMI [46], and the hydroxylation of CI [47] are associated with the debrisoquine/sparteine phenotype. Significant interphenotypic differences in the kinetics of secondary amines NT and DMI have been observed: PMs reach higher peak plasma concentrations, have longer plasma half-lives, lower total and metabolic clearances and excrete less hydroxymetabolites than do EMS [44,46] (Fig. 3; Table I). Both the total plasma clearance of NT or DMI and the clearance by lo-hydroxylation or 2-hydroxylation were found to covary with the debrisoquine metabolic ratio. As additional evidence, in vitro biochemical studies showed a positive correlation between the rate of hydroxylation of NT and DMI and that of debrisoquine in human liver microsomes from different individuals [48,49]. Moreover, DMI and debrisoquine acted as competitive inhibitors of each others’ metabolisms, clearly indicating the involvement of the same P450 unit in the oxidation of the two compounds. Such differences in the pharmacokinetics of secondary amines between phenotypes may be reduced by the effects of some functional characteristics of CYP2D6, such as saturation or inhibition, that are more apparent in EMS [27]. The CYP2D6 activity is of high affinity and low capacity character which implies saturability at low concentrations of the substrate [50]. This may lead to dose

z

5

0

20

40

60 80 100 0 20 40 60 80 loo Time(h) Time(h) Fig. 3. Plasma elimination of DMI after a single oral dose of 2.5 mg in one poor (0) and one extensive (A) metabolizer of debrisoquine (left panel). Urinary excretion of 2-OH-DMI in the same subjects (poor metabolizer 0; extensive metabolizer A). Each point corresponds to the midtime of each urine sampling period (right panel). Reprinted from ref. 46, Spina et (II., 1987, with permission of the copyright holder, the C.V. Mosby Co., St. Louis, USA.

Table I Plasma and urine data of the kinetics of a single oral dose of DMI (25 mg corresponding to 82.5 pmol) in eight extensive and six poor metabolizers of debrisoquine (from ref. 46, Spina et al., 1987) Extensive metaholizers

(n=8)

Poor metaholizers

P value

(n=6)

Peak concentration (nmol I-‘)

37.8fll.8

61 .Of5.9


Area under the curve (nmol I-’ h-‘)

1481f707

6552f1822


Total clearance (I h-’ kg-‘)

0.97f0.38

0.20+_0.06

co.00 1

Metabolic clearance (1 h-’ kg-‘)

0.29f0.10

0.013+0.004


Half-life

25.4f6.9

76.6k12.7

co.00 I

1.76+ I .62

6.04k2.39


5.22f0.70


(h) Urinary excretion of DMI (pmol)* Urinary excretion of 2-OH-DMI (flmol)* *Urine was collected

21.40+4.72 for up to 96 h after the dose

pharmacolo,~iwl

Rcsecrrc~h. Vol. 29, No. 2. I994

127

dependent kinetics in EMS as shown for IMI and DMI [51-531. Saturation of the 2-hydroxylation of IMI and DMI during the first-pass through the liver has also been reported to occur in EMS but not in PMs [54]. Moreover, the activity of CYP2D6 in EMS may be significantly decreased by concurrent administration of other substrates or inhibitors of the enzyme, such as quinidine, propafenone and neuroleptics [27]. This may result in clinically important drug interactions, whose potential is greater in EMS than in PMs. On the other hand, the demethylation of TM1 and AT was found not to be associated with this polymorphism [45, 551. It is therefore likely that the demethylation and the hydroxylation reactions of tricyclic antidepressants are catalysed by different cytochrome P450 isozymes. With regard to this, a pharmacokinetic study by Skjelbo et al. [56] found evidence that the Ndemethylation of IMI is at least partly related to the polymorphic hydroxylation of S-mephenytoin. More recently, a biochemical investigation in human liver microsomes has suggested that two other isozymes of cytochrome P450, the CYPl A2 and the CYP3A4, may be involved in the demethylation of IMI [57]. The activity of the demethylation enzymes, which are not polymorphically regulated, may therefore reduce the interphenotypic variability in the disposition of tertiary amines. The potential clinical implications of these pharmacogentic concepts are quite obvious if we consider that the hydroxylation is the rate-limiting step in the elimination of tricyclics and that their therapeutic and toxic effects are concentration dependent. Depressed patients of the PM phenotype may achieve high plasma concentrations when treated with conventional doses of tricyclics and may develop severe side effects. Conversely, patients with an extremely high rate of metabolism may not reach optimal plasma levels with subsequent risk of therapeutic failure. In this regard, the debrisoquine or sparteine metabolic ratio as well as the DMI hydroxylation index, were found to be good predictors of steadystate plasma concentrations of desipramine and nortriptyline in depressed patients 15%611. Knowledge of the oxidation phenotype might therefore help in identifying the two patient groups that, being at the extreme of this variation, pose clinical problems, i.e. the PMs and the rapid EMS. While low doses of antidepressants are probably needed in PMs, ultrarapid metabolizers may require increased doses. The recent demonstration of so called amplified CYP2D6 in patients characterized by extremely rapid metabolism of tricyclic antidepressants provides the molecular basis for rational megaprescribing in such subjects [62]. In principle, a simple phenotyping test before treatment with a tricyclic compound should be of value in selecting the starting dose to enhance therapeutic efficacy and to prevent toxicity. The potential usefulness of such an approach in clinical psychiatry has so far been documented by few reports concerning patients with extreme rates of drug oxidation [63-661. However, in a large retrospective study of patients taking IMI, no association was found between the debrisoquine oxidation capacity and the frequency or intensity of side effects [67]. Controlled prospective studies are needed to evaluate the role of polymorphic oxidation in the response to tricyclic antidepressants, linking phenotype determination and plasma level monitoring to clinical outcome.

Atypical untidepr-essunts Several selective serotonin reuptake inhibitors such as fluoxetine, fluvoxamine, paroxetine and citalopram, have been introduced as antidepressants in clinical practice. A recent study has shown that the metabolism of paroxetine cosegregates with the polymorphic hydroxylation of sparteine [68]. Moreover, both the sparteine and the mephenytoin oxidation polymorphism appear to contribute to pharmacokinetic variability of citalopram, being the mephenytoin hydroxylase mainly involved in the demethylation of citalopram to desmethylcitalopram, whereas the further metabolism to didesmethylcitalopram seems to be mainly dependent on the CYP2D6 activity [69]. Although no formal studies have evaluated whether fluoxetine or fluvoxamine are substrates of the polymorphic enzymes, there is evidence that they interfere with CYP2D6 or mephenytoin hydroxylase activity [70]. Fluoxetine has been reported to be a potent inhibitor of the CYP2D6-mediated hydroxylation of tricyclic antidepressants both in vivo and in vitro [71, 721. On the other hand, in vivo observations in healthy volunteers and in depressed patients and in vitro studies in human liver preparations have suggested that fluvoxamine differentially affects the metabolism of tricyclics, by acting as a potent inhibitor of IMI demethylation (partially mediated by CYP2C isozymes), without influencing significantly the hydroxylation of DMI [73-751. Since combined treatment with selective serotonin reuptake inhibitors and tricyclic antidepressants has been proposed as a possible strategy both to treat resistant depression and to obtain a faster response [76], the potential risk of this association must be seriously taken into account [77, 781.

PHARMACOGENETICS

OF ANTIPSYCHOTICS

Compounds of the antipsychotic class belong to different chemical groups, but are characterized by similar biochemical, pharmacological and behavioural effects. As interindividual variability in the with antidepressants, there is a large biotransformation of neuroleptics resulting in pronounced differences in steadystate plasma concentrations during treatment with fixed doses [79]. With the which are largely excreted unchanged exception of substituted benzamides, through the kidney, neuroleptic drugs are metabolized by oxidative reactions and several biologically active metabolites are formed [80]. Only in recent years has the role of genetic aspects in the metabolism of antipsychotics been investigated. Some neuroleptics, such as chlorpromazine, thioridazine and haloperidol, were found to be competitive inhibitors of CYP2D6-mediated oxidation of sparteine, desipramine and bufuralol in vitro in human liver microsomes [81-831. Moreover, a high prevalence of phenotypically PMs of debrisoquine was encountered among psychiatric patients being treated chronically with neuroleptics [84-861. Administration of low doses of haloperidol, levomepromazine, thioridazine or chlorpromazine to healthy volunteers or geriatric patients resulted in a marked increase in the debrisoquine or sparteine metabolic ratio and some subjects of the EM phenotype were transformed to apparent PMs [ 19, 86-881. These data, however, had not revealed whether neuroleptics were themselves polymorphically oxidized, or were competitive inhibitors of CYP2D6 activity. Phenotyped panel

pharmacoiogicul

Kesuurc~h. Vol. 29, No. 2. 1994

129

studies have recently shown that disposition of perphenazine, zuclopenthixol and thioridazine cosegregates with that of debrisoquine [89-911. In healthy subjects the kinetic profile of these compounds after single doses markedly differs between EMS and PMs, thus explaining part of the variability in their clearance (Fig. 4). It remains to be studied which of the metabolic pathways of perphenazine and zuclopenthixol is catalysed by CYP2D6. For thioridazine, it has been suggested that the formation of mesoridazine, a side-chain sulphoxide, but not of thioridazine ring-sulphoxide, is probably mediated by CYP2D6. There is now substantial evidence that also the disposition of haloperidol is dependent on CYP2D6 isoenzyme [92]. The main metabolic pathways of haloperidol are the Ndealkylation, the aromatic hydroxylation and the reduction of the ketone group to form reduced haloperidol which can be oxidized back to haloperidol. Two studies, one in viva and the other in vitro, have indicated that the pathway involved is the reoxidation of reduced haloperidol [93,94]. Moreover, the involvement of CYP2D6 also in the metabolism of the atypical antipsychotic agent clozapine has been suggested by a recent study in human liver microsomes and in recombinant RT2D6 cells which specifically express human CYP2D6 1951. The potential clinical relevance of this polymorphism for treatment with neuroleptics has begun to be investigated in the last few years. A prospective

5.0

I

0

I

I

6

12

18

I 24

32

Time (hi Fig. 4. Serum concentration of perphenazine (mean+sD; n=6) after a single oral dose of 6 mg perphenazine in poor (0) and extensive (H) hydroxylators of debrisoquine. Reprinted from ref. 89, Dahl-Puustinen St. Louis, USA.

e/ al., 1989, with permission

of the copyright

holder, the C.V. Mosby Co.,

study has suggested that patients with defective oxidative capacity may reach very high plasma concentrations of thioridazine with subsequent side effects [96]. Two recent retrospective studies by our group have evaluated the possibility of an between the genetically determined capacity to metabolize association neuroleptics, assessed by the debrisoquine hydroxylation test, and the occurrence of acute neuroleptic-induced side effects [97, 981. An over-representation of PMs was found among psychotic patients who had experienced concentrationdependent unwanted effects, such as oversedation, postural hypotension and autonomic effects, within the first few days of treatment with antipsychotics. On the other hand, no differences in the prevalence of PMs were observed between patients who had developed acute dystonic reactions during neuroleptic administration and a control group with no history of extrapyramidal effects. It is likely that pharmacodynamic factors, such as dopamine receptor function, play a more important role in the occurrence of acute dystonia than do pharmacokinetic variables. Another important implication of pharmacogenetics for the clinical use of antipsychotics is the possibility of drug interactions with other substrates of the polymorphic enzyme. In this regard, it is well documented that some neuroleptics are potent inhibitors of the metabolism of tricyclic antidepressants [99]. This may lead to potentially dangerous pharmacokinetic interactions if compounds of these two classes of psychotropic drugs are used in combination. Besides the liver, CYP2D6 is also present and functioning in the human brain, where it seems to be associated with a dopamine transporter, so that several neuroleptics, and antidepressants as well, might be metabolized close to their site drugs, of action [ 100, 1011. With the brain being the target tissue for psychotropic it is intriguing to speculate on the relevance of this finding for the pharmacology of these compounds. In this respect, on the basis of a recently reported association between personality characteristics and debrisoquine hydroxylation capacity, it has been suggested that CYP2D6 might be involved in the production or catabolism of a hypothetical endogenous substance of importance for central nervous system activity [ 1021.

PHARMACOGENETICS

OF ANXIOLYTICS

The disposition of compounds of the benzodiazepine family, the most widely used anxiolytic drugs, may differ markedly among different individuals [103]. Diazepam, the model compound of the benzodiazepine class, undergoes Ndemethylation to yield the active metabolite desmethyldiazepam which is further metabolized by 3-hydroxylation to oxazepam. The genetic aspects of diazepam biotransformation have been studied by Bertilsson et al. [104]. They have suggested that the metabolism of both diazepam and desmethyldiazepam is partly associated with the mephenytoin oxidation polymorphism. However, owing to the wide therapeutic index of diazepam, the clinical significance of these findings is probably small. Genetic factors appear to play an important role in the metabolism of two other benzodiazepines, nitrazepam and clonazepam, which possess a nitro substituent.

Table II Association between the metabolism of psychotropic drugs and the polymorphic hydroxylation of debrisoquine/sparteine (db/sp), the polymorphic hydroxylation of mephenytoin (meph) and the polymorphic acetylation (acetyl). Antidepressants Tricyclic antidepressunts Amitriptyline Nortriptyline Imipramine Desipramine Clomipramine

dblsp db/sp db/sp, meph (?) db/sp dblsp

Atypical antidepressants Paroxetine Citalopram Fluoxetine Fluvoxamine

dblsp dblsp, meph db/sp (‘?) db/sp (?), meph (?)

Monoamine Phenelzine

acetyl

osidase inhibitors

Antipsychotics Perphenazine Zuclopenthixol Thioridazine Haloperidol Clozapine

db/sp db/sp dblsp db/sp db/sp

Anxiolytics Diazepam Nitrazepam Clonazepam

meph acetyl acetyl

They undergo metabolic reduction to the corresponding amino in turn, are acetylated to a degree determined by the activity regulated N-acetyltransferase [ 121.

derivatives which. of the genetically

CONCLUSIONS Intensive research in the last 15 years has clearly demonstrated that the oxidative metabolism of some psychotropic drugs, such as most antidepressants and certain neuroleptics, is catalysed, at least partly, by a specific cytochrome P450 isoenzyme, called CYP2D6 (Table II). This is the target of the genetic polymorphism of debrisoquine/sparteine oxidation which is expressed in the population by two phenotypes, the EMS and the PMs. Genetically determined variability in drug metabolism further explains the large interindividual

differences in the elimination kinetics and in the steady-state plasma concentrations during treatment with a fixed dose of one of these compounds. This has potential implications particularly with agents such as tricyclic antidepressants which have a relatively narrow therapeutic range and whose effects are concentration-dependent. Patients of the PM phenotype, who represent 5 to 10% of the population, are more prone to adverse effects on standard doses, whereas EMS are at risk of therapeutic failure. Knowledge of the oxidative status might help to predict individual dose range required for optimal therapy. Patients with extreme rates of drug metabolism may be detected by plasma concentration analysis of the administered drug, by phenotyping with probe drugs or by genotyping with molecular biology based techniques. At present, routine phenotyping of psychiatric patients before treatment with antidepressants or neuroleptics is probably not justified, since the clinical relevance of polymorphic oxidation is yet to be clearly defined. It should also be emphasized that the test result can not replace plasma level measurement in the titration of the dose. However, determination of metabolic capacity should be borne in mind in cases of unusual response to adequate drug treatment. It is likely that in the future the combination of conventional monitoring of plasma levels and phenotype determination will improve the utilization of psychotropic drugs subject to polymorphic oxidation.

REFERENCES 1. Baldessarini

RJ. C&morlrcr-up?

in psyhiutr_y. Cambridge:

Harvard

University

Press,

1985. 2. Potter WZ, Bertilsson

3. 4.

S. 6. 7.

8. 9. IO.

I I. 12. 13.

L, Sjoqvist F. Clinical pharmacokinetics of psychotropic drugs: fundamental and practical aspects. In: H van Praag, MH Lader, OJ Rafdetsen, EJ Sachar eds. Handbook ofbiolo~~ic~ulps~c’hiutr~. New York: Marcel Dekker, 1981: 71-134. Caccia S, Garattini S. Formation of active metabolites of psychotropic drugs. An 1990; 18: 434-59. updated review of their significance. C/in Phurmacokinet Sjoqvist F. Pharmacogenetic factors in the metabolism of tricyclic antidepressants and some neuroleptics. In: W. Kalow, ed. Pharmacogrnetics of drug metabolism. New York: Pergamon Press, 1992: 689-700. Dahl ML, Berilsson L. Genetically variable metabolism of antidepressants and neuroleptic drugs in man. Pha~muco~enetics 1993; 3: 61-70. Vogel F, Motulsky AG. Humun genetics, problems und approaches. New York: Springer, 1986. Meyer UA. Drugs in special patient groups: clinical importance of genetics in drug effects. In: KL Melmon, HF Morrelli, BB Hoffman, DW Nierenberg, eds. Clinical phurmucolo,qy. Basic principles in therapeutics. New York: McGraw-Hill, 1992: 875-94. Kalow W. Phurmaco~~enetics ofdrug metuholi.sm. New York: Pergamon Press, 1992. Kalow W, Genest K. A method for the detection of atypical forms of human serum cholinesterase. CUII ./ Riochem Physiol 1957; 35: 339-46. Kutt H. Wolk M, Scherman R, McDowell F. Insufficient parahydroxylation as a cause of diphenylidantoin toxicity. Ncur-ology 1964; 14: 542-S. Evans DAP, Manley FA, McKusick VA. Genetic control of isoniazid metabolism in man. BI- Med J 1960; 2: 485-49 1. Evans DAP. N-Acetyltransferase. Pharmucol Ther 1989; 42: 157-234. Boobis AR, Davies DS. Human cytochromes P-450. Xenobiotica 1984; 14: 151-85.

14. Nebert DW. Nelson DR, Coon MJ, Estabrook RW, Feyereisen R, Fujii-Kuriyama Y, Gonzalez FJ, Guengerich FP, Gunsalus IC, Johnson EF, Loper JC, Sato R, Waterman MR, Waxman DJ. The P450 superfamily: update on new sequences, gene mapping, and recommended nomenclature. DNA Cell Biology 1991; 10: 1-14. 15. Murray M. P4SO enzymes: inhibition mechanisms, genetic regulation and effects of liver disease. Clin Pharmar-okinet 1992; 23: 13246. 16. Mahgoub A, Idle JR, Dring LG, Lancaster R, Smith RL. Polymorphic hydroxylation of debrisoquine in man. Lancer 1977; 1: 584-6. 17. Eichelbaum M, Spannbrucker N, Steincke B, Dengler JJ. Defective N-oxidation of sparteine in man: a new pharmacogenetic defect. Eur .I Clin Pharmacnl 1979; 16: 183-7. 18. Alvan

G, Bechtel P, Iselius L, Gudnert-Remy U. Hydroxylation polymorphisms of debrisoquine and mephenytoin in European populations. Eur J Clin Pharmacol 1990; 39: 533-8.

19.

Spina E, Campo GM, Calandra S, Caputi AP, Carrillo JA, Benitez J. Debrisoquine oxidation in an Italian population: a study in healthy subjects and in schizophrenic patients. Pharmacol Res 1992; 25: 43-50. variation of drug metabolism. TIPS 1991; 12: 102-7. 20. Kalow W. Interethnic 21. Evans DAP, Mahgoub A, Sloan TP, Idle JR, Smith RL. A family and population study of the genetic polymorphism of debrisoquine oxidation in a white British population. J Med Genet 1980; 17: 102-5. 22. Steiner E, Iselius L, Alvan G, Lindsten J, Sjoqvist F. A family study of genetic and environmental factors determining polymorphic hydroxylation of debrisoquin. C/in Pharmacol 23.

Ther

1985; 38: 394-401.

polymorphism of Zanger UM. The genetic Meyer UA, Skoda RC, debrisoquine/sparteine metabolism-Molecular mechanism. Pharmarol Ther 1990; 46: 297-308.

24.

Gonzalez FJ, Skoda RC, Kimura S, Umeno M, Zanger UM, Nebert DW, Gelboi HV, Hardwick JP, Meyer UA. Characterization of the common genetic defect in humans dcficicnt in debrisoquine metabolism. Nature 1988; 331: 442-6. 25. Kimura S, Umeno M, Skoda RC, Gelboin HV, Meyer UA, Gonzalez FJ. The human debrisoquine4-hydroxylase (CYP2D) locus: sequence and identification of the polymorphic CYP2D6 gene, a related gene and pseudogene. Am J Hum Gen 1989; 45: X89-904. 26.

27. 2X. 29. 30. 31.

Eichelbaum M. Baur MP, Denglcr HJ, Osikowska-Evers BO, Tieves G, Zekorn C, Rittncr C. Chromosomal assignment of human cytochrome P-450 (debrisoquine/ sparteine type) IO chromosome 22. BrJ Clin Pharmacol 1987; 23: 455-S. Brosen K, Gram L. Clinical significance of the sparteine/debrisoquine oxidation polymorphism. Elo- J C/in Pharmacol 1989; 36: 537-47. Brosen K. Recent developments in hepatic drug oxidation. Clin Pharmacokinet 1990; 18: 220-39. Eichelbaum M, Gross AS. The genetic polymorphism of debrisoquine/sparteine metabolism - clinical aspects. Phurmacwl Ther 1990; 46: 377-94. Lennard MS. Genetic polymorphism of spartine/debrisoquine oxidation: a reappraisal. Phurmur~ol

Toxicol

Alvan

Clinical

G.

Pharmacol 32.

199

1990;

67: 273-83.

consequences

of

polymorphic

drug

oxidation.

Fundum

Clin

1; 5: 209-28.

Schmid B, Bircher J, Preisig R, Kupfer A. Polymorphic dextromethorphan metabolism: co-segregation of oxdiative 0-demethylation with debrisoquin hydroxylation. Chin Pharmur~ol Ther 19X5; 38: 618-24. 33. Dahl ML, Johansson I. Porsmyr Palmertz M, Ingelman-Sundberg M, Sjoqvist F. Analysis of the CYP2D6 gene in relation IO debrisoquin and desipramine hydroxylation in a Swedish population. C/in Pharmucol Ther 1992; 51: 12-l 7. 34. Skoda RC, Gonzalez FJ, Demierre A, Meyer UA. Two mutant alleles of the human cytochrome P4SOdbl gene (P450C2Dl) associated with genetically deficient

134

35. 36. 37. 38.

39.

40. 41. 42.

43.

44.

45. 46.

47.

48.

49.

50.

51.

52.

53. 54.

Pharmaroloc~ical

Krsuarch.

Vol. 29. No. 2. 1994

metabolism of debrisoquine and other drugs. Proc Nat/ Acad Sci USA 1988; 85: 5240-3. Heim M, Meyer UA. Genotyping of poor metabolizers of debrisoquine by alleleLancet 1990; 336: 529-32. specific PCR amplification. Kupfer A, Preisig R. Pharmacogenetics of mephenytoin: a new drug hydroxylation polymorphism in man. Eur J Clin Pharmacol 1984; 26: 753-9. Wilkinson GR, Guengerich FP, Branch RA. Genetic polymorphism of S-mephenytoin Pharmacol Thrr 1989; 43: 53-76. hydroxylation. Relling MV, Aoyama T, Gonzalez FJ, Meyer UA. Tolbutamide and mephenytoin hydroxylation by human cytochrome P45Os in the CYP2C subfamily. J P harmacol E,rp Ther 1990; 252: 42-7. Rudorfer NV, Potter WZ. Pharmacokinetics of antidepressants. In: Meltzer HI, ed. Psychopharmacology: the third generation ofprogress. New York: Raven Press, 1987: 1353-63. Hammer W, Sjoqvist F. Plasma levels of monomethylated tricyclic antidepressants during treatment with imipramine-like compounds. Life Sci 1967; 6: 1895-903. Alexanderson B, Evans PDA, Sjoqvist F. Steady-state plasma levels of nortriptyline in twins: influence of genetic factors and drug therapy. Br Med J 1969; 4: 764-8. Asberg M, Evans PDA, Sjoqvist F. Genetic control of nortriptyline kinetics in man. A study of relatives of propositi with high plasma concentrations. J Med Genet 1971; 8: 129-35. Balant-Gorgia AE, Schulz P, Dayer P, Balant L, Kubli A, Gertsch C, Garrone G. Role of oxidation polymorphism on blood and urine concentrations of amitriptyline and its metabolites in man. Arch Psychiatr Nervenkr 1982; 232: 215-22. Mellstrom B, Bertilsson L, Sawe J, Schulz HU, Sjoqvist F. E- and Z-hydroxylation of nortriptyline: relationship to polymorphic debrisoquine hydroxylation. Clin Pharmacol Ther 1981; 30: 189-93. Brosen K, Otton V, Gram LF. Imipramine demethylation and hydroxylation: impact of the sparteine oxidation phenotype. C/in Pharmacol Ther 1986; 40: 543-9. Spina E, Steiner E, Ericsson 0, Sjoqvist F. Hydroxylation of desmethylimipramine: dependence on the debrisoquin hydroxylation phenotype. Clin Pharmacol Ther 1987; 41: 314-19. Balant-Gorgia AE, Balant LP, Genet C, Dayer P, Aeschlimann JM. Importance of oxidative polymorphism and levomepromazine treatment on steady-state blood concentrations of clomipramine and its major metabolites. Eur J C/in Pharmacol 1986; 31: 449-55. von Bahr C, Birgersson C, Blanck A, Goransson M, Mellstrom B, Nilssel K. Correlation between nortriptyline and debrisoquine hydroxylation in the human liver. Life Sci 1983; 33: 63 1-6. Spina E, Birgersson C, von Bahr C, Ericsson 0, Mellstrom B, Steiner E, Sjoqvist F. Phetiotypic consistency in hydroxylation of desmethylimipramine and debrisoquine in healthy subjects and in human liver microsomes. C/in Pharmacol Ther 1984; 36: 677-82. Spina E, Koike Y. Differential effects of cimetidine and ranitidine on imipramine demethylation and desmethylimipramine hydroxylation by human liver microsomes. Eur J Clin Pharmacol 1986; 30: 23942. Cooke RG, Warsh JJ, Stancer HC, Reed KL, Persad E. The nonlinear kinetics of desipramine and of 2-hydroxydesipramine in plasma. Clin Pharmacol Ther 1984; 36: 343-9. Brosen K, Gram LF, Klysner R, Beth P. Steady-state levels of imipramine and its metabolites: significance of dose-dependent kinetics. Eur J C/in Pharmacol 1986; 30: 43-9. Nelson CJ, Jatlow PI. Nonlinear desipramine kinetics: prevalance and importance. C/in Pharmacol Ther 1987; 41: 666-70. Brosen K, Gram LF. First-pass metabolism of imipramine and desipramine: impact of the sparteine oxidation phenotype. Clin Pharmacol Ther 1988; 43: 400-6.

Pharmac~olo,qiud

Research.

Vol. 29. No. 2. 1994

I.15

55. Mellstrom B, Bertilsson L, Lou YC, Sawe J, Sjoqvist F. Amitryptyline metabolism: relationship to polymorphic debrisoquine hydroxylation. Clin Pharmacol Ther 1983; 34: 5 16-20. 56. Skjelbo E, Brosen K, Hallas J, Gram LF. The mephenytoin oxidation polymorphism is partially responsible for the N-demethylation of imipramine. Clin Pharmacol Ther 1991; 57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

70.

71.

49:

18-23.

Lemoine A, Gamier JC, Azoulay D, Kiffel L, Guengerich FB, Beaune P, Maurel P, Leroux JP. Major pathway of imipramine metabolism is catalysed by cytochromes P450 lA2 and P-450 3A4 in human liver. Molec Pharmacol 1993; 43: 827-32. Bertilsson L, Aberg-Wistedt A. The debrisoquine hydroxylation test predicts steadystate plasma levels of desipramine. Br J Clin Pharmacol 1983; 15: 388-90. Nordin C, Siwers B, Benitez J, Bertilsson L. Plasma concentrations of nortriptyline and its IO-hydroxymetabolite in depressed patients: relationship to polymorphic debrisoquine hydroxylation metabolic ratio. Br J Clin Pharmacol 1985; 19: 832-5. Brosen K, Klysner R, Gram LF, Otton SV, Beth P, Bertilsson L. Steady-state imipramine and metabolites in the concentrations of its relation to Eur J Clin Pharmacol 1986; 30: 679-84. sparteine/debrisoquine polymorphism. Spina E, Arena A, Pisani F. Urinary desipramine hydroxylation index and steady-state plasma concentrations of imipramine and desipramine. Ther Drug Monit 1987; 9: 129-33. Bertilsson L, Dahl ML, Sjoqvist F, Aberg-Wistedt A, Humble M, Johannson 1, Lundqvist E, lngelman-Sundberg M. Molecular basis for rational megaprescribing in ultrarapid hydroxylators for debrisoquine. Lancet 1993; 341: 63. Bertilsson L, Mellstrom B, Sjoqvist F, Martensson B, Asberg M. Slow hydroxylation of nortriptyline and concomitant poor debrisoquine hydroxylation: clinical implications. Lancet 1981; 1: 56&l. LL, Nordin C. Extremely rapid Bertilsson L, Aberg-Wistedt A, Gustaffsson a case report with implication for treatment with hydroxylation of debrisoquine: nortriptyline and other tricyclic antidepressants. Ther Drug Monit 1985; 7: 478-80. Balant-Gorgia AE, Balant LP, Garrone G. High blood concentrations of imipramine or clomipramine and therapeutic failure: a case report study using drug monitoring data. Ther Drug Monit 1989; 11: 415-20. Bluhm RE, Wilkinson GR, Shelton R, Branch RA. Genetically determined drugmetabolizing activity and desipramine-associated cardiotoxicity: a case report. Clin Pharmacol Ther 1993; 53: 89-95. Meyer JW, Woggon B, Kupfer A. Importance of oxidative polymorphism on clinical 1988; efficacy and side-effects of imipramine: a retrospective study. Pharmacopsychiat 21: 365-6. Sindrup SH, Brosen K, Gram LF, Hallas J, Skjelbo E, Allen GD, Cooper SM, Mellows G, Tasker TCG, Zussman BD. The relationship between paroxetine and the sparteine oxidation polymorphism. Clin Pharmacol Ther 1992; 51: 278-87. Sindrup SH, Brosen K, Hansen MGJ, Aes-Jorgensen T, Overo KF, Gram LF. Pharmacokinetics of citalorpam in relation to the sparteine and mephenytoin oxidation polymorphisms. Ther Drug Monit 1993; 15: 1I-17. Crewe HK, Lennard MS, Tucker GT, Woods FR, Haddock RE. The effect of selective serotonin re-uptake inhibitors on cytochrome P4502D6 (CYP2D6) activity in human liver microsomes. Br J Clin Pharmacol 1992; 34: 262-5. Bergstrom RF, Peyton AL, Lemberger L. Quantification and mechanism of the fluoxetine and tricyclic antidepressant interaction. C/in Phurmacol Ther 1991; 51: 23948.

72.

Skielbo E, Brosen K. Inhibitors Br J Clin Pharmucol

73.

of imipramine

metabolism

by human liver microsomes.

1992; 34: 256-61.

Spina E, Pollicino AM, Avenoso A, Campo GM, Perucca E, Caputi AP. Effect of fluvoxamine on the pharmacokinetics of imipramine and desipramine in healthy subjects. Ther Drug Monit 1993; 15: 243-6. 74. Bertschy G, Vandel S, Vandel R, Allers G, Volmat R. Fluvoxamine-tricyclic

75.

76.

77. 7X.

79. 80. 81.

82.

83.

84. 85.

86.

X7.

XX.

X9.

90.

91.

92.

93.

94.

antidepressant interaction: an accidental finding. E~tr / Clirz Pharmucol 1991; 40: I 19-20. Spina E, Pollicino AM, Avenoso A. Campo GM, Caputi AP. Fluvoxamine-induced alterations in plasma concentrations of imipramine and desipramine in depressed patients. Int J C/in Pharmacol Res 1993; 13: 167-71. Nelson JC, Mazure CM, Bowers MB, Jatlow PI. A preliminary open study of the combination of fluoxetine and desipramine for rapid treatment of major depression. Arch Gun Psyc,hiutry 199 1; 48: 303-7. Westermeyer J. Fluoxetine-induced tricyclic toxicity: extent and duration. J C/in Pharmacol I99 I ; 31: 38X-92. Spina E, Campo GM, Avenoso A, Pollicino MA, Caputi AP. Interaction between fluvoxamine and imipramine/desipramine in four patients. Ther Dvug Monit 1992; 14: 194-6. Dahl SV. Plasma level monitoring of antipsychotic drugs. Clinical utility. Clin Pharmacokinrt 1986; 11: 36-61. Jorgenson A. Metabolism and pharmacokinetics of antipsychotic drugs. Progr Drug Met& 1986; 9: 11 l-74. Inaba T, Jurima M, Mahon WA, Kalow W. In vitro inhibition of human liver cytochrome P450. Mephenytoin p-hydroxylase and sparteine monooxygenase. Drug Metah Dispos 1985; 13: 443-8. von Bahr C, Spina E, Birgersson C, Ericsson 0, Goransson M, Henthorn T, Sjoqvist F. Inhibition of desmethylimipramine 2-hydroxylation by drugs in human liver microsomes. Biochem Pharmacol198.5; 34: 2501-5. Fonne-Pfister R, Meyer UA. Xenobiotic and endobiotic inhibitors of cytochrome PBiochem 450dbl function, and target of the debrisoquine/sparteine type polymorphism. Pharmacol 1988; 37: 3829-35. Syvalahti EKG, Lindberg R, Kallio J, de Vocht M. Inhibitory effects of neuroleptics on debrisoquine oxidation in man. Br J Clin Pharmacol 1986; 22: 89-92. Derenne F, Joanne C, Vandel S, Bertschy G, Volmat R, Bechtel P. Debrisoquine oxidative phenotyping and psychiatric drug treatment. Eur J C/in Phurmacol 1989; 36: 53-8. Spina E, Martines C, Caputi AP, Cobaleda J, Pinas B, Carrillo JA, Benitez J. Eur J C/in Debrisoquine oxidation phenotype during neuroleptic monotherapy. Pharmacol 1991; 41: 467-70. Gram LF, Debruyne D, Caillard V, Boulenger JP, Lacotte J, Moulin M, Zarifian E. Substantial rise in sparteine metabolic ratio during haloperidol treatment. Br J Chin Pharmacol 1989; 27: 272-5. Kallio J, Huupponen R, Seppala M, Sako E, Iisalo E. The effect of beta-adrenoceptor antagonists and levomepromazine on the metabolic ratio of debrisoquine. Br J C/in Pharmacol 1990; 30: 638-43. Dahl-Puustinen ML, Liden A, Alm C, Bertilsson L. Disposition of perphenazine is related to polymorphic debrisoquine hydroxylation in man. Clin Pharmacol Ther 1989; 46: 78-81. Dahl ML, Ekqvist B, Widen J, Bertilsson L. Disposition of the neuroleptic zuclopenthixol cosegregates with the polymorphic hydroxylation of debrisoquine in humans. Acta Psychiatr Stand 1991; 84: 99-102. von Bahr C, Movin G, Nordin C, Liden A, Hammarlund-Udenaes M, Hedberg A, Ring H, Sjoqvist F. Plasma levels of thioridazine and metabolites are influenced by the debrisoquin hydroxylation phenotype. Clin Pharmucol Ther 1991; 49: 234-40. Llerena A, Alm C, Dahl ML, Ekqvist B, Bertilsson L. Haloperidol disposition is dependent on the debrisoquine hydroxylation phenotype. Ther Drug Monit 1992; 14: 92-7. Llerena A, Alm C, Dahl ML, Ekqvist B, Bertilsson L. Haloperidol disposition is dependent on the debrisoquine hydroxylation phenotype: increased plasma levels of the reduced metabolite in poor metabolizers. Ther Drug Monit 1992; 14: 261-4. Tyndale RF, Kalow W, Inaba T. Oxidation of reduced haloperidol to haloperidol:

95.

96.

91.

98.

99. 100.

101.

102.

103.

104.

involvement of human P4501ID6 (sparteine/debrisoquine monooxygenase). B,- .I C/in Phurmacol 1991; 31: 655-60. Fischer V, Vogels B, Maurer G, Tynes RE. The antipsychotic clozapine is metabolized by the polymorphic human microsomal and recombinant cytochrome P450 2Dh. ./ Pharmacol E.xp Ther 1992; 260: 1355-60. Meyer GW, Woggon B, Baumann P, Meyer UA. Clinical implications of slow sulphoxidation of thioridazine in a poor metabolizer of the debrisoquine type. Eur J Clin Pharmacol 1990; 39: 613-l 4. Spina E, Ancione M, Di Rosa AE, Meduri M, Caputi AP. Polymorphic debrisoquine oxidation and acute neuroleptic induced adverse effects. Eur J C/in Pharmacol 1992; 42: 347-8. Spina E, Sturiale V, Valve S, Ancione M, Di Rosa AE, Meduri M, Caputi AP. Debrisoquine oxidation phenotype and neuroleptic-induced dystonic reactions. Actu Psychiatr Stand 1992; 86: 364-6. Gram LF, Fredricson Overo K. Drug interaction: inhibitory effect of neuroleptics on metabolism of tricyclic antidepressants in man. Br Med J 1972; 1: 463-5. Fonne-Pfister R, Bargetzi MJ, Meyer UA. MPTP, the neurotoxin inducing Parkinson’s disease, is a potent competitive inhibitor of human and rat cytochrome P450 isozymes (P450buf1, P450dbl) catalyzing debrisoquine 4-hydroxylation. Biochem Biophys Res Commun 1987; 148: 1144-50. Niznik HB, Tyndale RF, Sallee FR, Gonzales FJ, Hardwich JP, Inaba T, Kalow W. The dopamine transporter and cytochrome P450IIDl (debrisoquine 4-hydroxylase) in brain: resolution and identification of two distinct ‘H GBR-12935 binding proteins. Arch Biochem Biophysics 1990; 276: 424-32. Llerena A, Edman G, Cobaleda J, Benitez J, Schalling D, Bertilsson L. Relationship between personality and debrisoquine hydroxylation capacity: suggestion of an endogenous neuroactive substrate or product of the cytochrome P4502D6. Acta Psychiatr Stand 1993; 87: 23-8. Greenblatt DJ, Shader RI. Pharmacokinetics of antianxiety agents. In: Meltzer HI, ed. Psychopharmucology: the third generation of progress. New York: Raven Press, 1987: 1377-86. Bertilsson L, Henthorne TK, Sanze E, Tybring G, Sawe J, Villen T. Importance of genetic factors in the regulation of diazepam metabolism: relationship to Smephenytoin, but not debrisoquin hdyroxylation phenotype. Clin Pharmacol Ther 1989; 45: 348-55.