Antidepressant Drugs: Comments on Phenotyping

Polymorphism in the Metabolism of Drugs, Including Antidepressant Drugs: Comments on Phenotyping

R.T. Coutts, Ph.D., D.Sc.

Neurochemical Research Unit, Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta Submitted: July 21, 1992 Accepted: July 23, 1993

In neurochemistry there are advantages in determining how patients are likely to react to psychoactive drugs prior to the commencement of drug therapy. Explanations of a patient's nonresponse, or unexpected adverse reactions to drugs are required. In many instances, a knowledge of the drug metabolism status of a patient can be helpful in the selection of a drug and its dosage regimen, and in the prediction of possible drug/drug interactions when two or more drugs have to be administered concomitantly. Important information on these topics may be obtained by phenotyping patients prior to drug therapy. The metabolism of various antidepressant and neuroleptic drugs is catalyzed by CYP2D6, a cytochrome P450 isozyme (also named P450IID6), whereas the metabolism ofother drugs may involve different cytochromes P450. The properties of CYP2D6 and four other isozymes (CYPlAl, CYP1A2, CYP2C8/9 and CYP3A4) are described, and substrates identified. Phenotyping of patients for CYP2D6 activity and mephenytoin hydroxylase activity is described.

Key Words: antidepressants, drug metabolism, cytochromes P450, drug/drug interactions, phenotyping, poor metabolizers, extensive metabolizers, adverse effects

INTRODUCTION

When different individuals ingest identical doses ofa drug 2), the steady state plasma concentrations will fall within the at similar time intervals, it is not uncommon to observe large desired therapeutic range and toxic effects ofthe drug will be differences in plasma drug concentrations when steady state minimal. There will also be individuals (group 3) whose is attained. Figure 1 is a diagram of what can be expected as plasma drug concentrations are in excess of the desired a result of the chronic administration of a hypothetical anti- therapeutic range at steady state (> 0.2 gg/ml in Fig. 1). With depressant drug. In some individuals (group 1), steady state these individuals, the higher the plasma drug concentration, plasma concentrations of the antidepressant will be less than the more toxic side-effects will be superimposed upon the in 0.12 ,ug/mi and subtherapeutic. In other individuals (group therapeutic effect. The result will be a complicated picture which undesired toxic effects can mask the desired pharma- cological effects, and an assessment of the efficacy of the Presented at the 15th Annual Scientific Meeting ofthe Canadian antidepressant would be difficult. Drug treatment of group 3 College of Neuropsychopharmacology, May/June 1992. responders would probably be terminated, and the drug replaced with another antidepressant, although it is possible Address reprint requests to: R.T. Coutts, Neurochemical Research Unit, Faculty of Pharmacy and Pharmaceutical Sciences, that the patient is responding well to the drug and all that is University of Alberta, Edmonton, Alberta T6G 2N8. required is a reduction in drug dosage that reduces J Psychiatr Neurosci, Vol. 19, No. 1, 1994 30 January 1994 Polymorphism in the metabolism ofdrugs 31

Table 1 Drugs whose metabolism is or may be impaired in poor 4-hydroxylators ofdebrisoquine (Adapted from Br0sen 1990; Dahl et al 1991b; Meyer et al 1990; Murray 1992) Impairment suggested by Impairment demonstrated inhibition studies * amiflamine * ajmalime * amitriptyline * ajmalicme * amphetamine * alprenolol * bufuralol * captopril * clomipramine * chlorpromazine * codeine * diphenhydramine Fig. 1. Relationship between plasma drug concentration and * debrisoquine * domperidone the probability ofa therapeutic and/or toxic response. * desipramine * flecainide undesirable toxic side-effects. Drug substitution for group 1 * dextromethorphan * fluoxetine patients would also probably result, although an increase in * dihydrohaloperidol * haloperidol drug dosage to raise the plasma steady state drug concentration to the therapeutic range may be a better remedy. * encainide * indoramine Two examples illustrate what may be encountered in * guanoxan * labetolol antidepressant therapy. A daily dose of 2.5 mg/kg of * imipramine * levomepromazine desmethylimipramine (DMI) to adult patients results in very variable steady state plasma concentrations of DMI * 4-methoxyamphetamine * lobeline (0.02 ,ug/ml to 0.88 ,ug/ml; mean 0.17 jg/ml) and its active * methoxyphenamine * mexiletine metabolite, 2-hydroxy-DMI (2-OH-DMI: 0.007 gg/ml to * metoprolol * norfluoxetine 0.13 jg/ml; mean 0.04 gg/ml) (Bock et al 1983). Since the minimum steady state therapeutic concentration of DMI is * nortriptyline * oxprenalol 0.125 ,ug/ml (Nelson et al 1982), and moderate intoxication * perhexiline * papaverine is observed when the steady state plasma concentration of * perphenazine * penbutolol DMI is > 0.40 ug/ml (Moffat 1986), it is clear that some patients receiving the 2.5 mg/kg daily dose will not respond * phenformin * pipamperone to the drug because therapeutic plasma levels are not attained, * propafenone * prajmaline whereas other patients may display toxic effects because * propranolol * timolol excessive plasma drug concentrations exist at steady state. It may be surmised that the metabolite, 2-OH-DMI, present in * sparteine * trifluoroperidol the plasma after DMI administration, would contribute to the * thioridazine * tomoxetine antidepressant and/or side-effects of DMI, but initial studies * zuclopenthixol * yohimbine did not support such a contribution and it was concluded at I that time that routine measurement of 2-OH-DMI plasma steady state plasma concentrations of CMP (0.10IOg/ml to levels was not clinically useful (Nelson et al 1983). 0.48 ug/ml; mean 0.23 gg/ml), and its active metabolite Subsequent studies revealed, however, that plasma levels of N-desmethyl-CMP (DCMP) (0.24 jg/ml to 0.96 jg/ml; 2-OH-DMI at steady state are very variable and in some mean 0.45 gg/ml) (Montgomery et al 1980). Steady state patients they can approach levels that are about one-half that therapeutic concentrations of CMP + DCMP should be in ofthe administered DMI. In these individuals, the metabolite, excess of 0.16 gg/ml for efficacy, and below 0.50 pg/ml to 2-OH-DMI, does contribute significantly to the antidepres- minimize toxic effects (Balant-Gorgia et al 1987). Since sant outcome, and total plasma drug levels (DMI + 2-OH- moderate to severe intoxication is observed when the com- DM1) are more strongly correlated with outcome than are bined concentrations of CMP and DCMP are greater than DM1 levels alone (Nelson et al 1988). Clomipramine (CMP) 0.40 ig/ml, toxic manifestations may be expected in some is another interesting antidepressant. Attaining steady state patients who are receiving the recommended daily dose of therapeutic concentrations ofthis drug usually requires daily CMP. dosing (10 mg to 150 mg) for at least one or two weeks. These examples illustrate that some individuals are exten- However, a daily dose of 150 mg results in very variable sive drug metabolizers and never attain therapeutic plasma 32 Journal ofPsychiatry & Neuroscience VoL 19, No. I., 1994

00\ X Nl N )~~~ * Minor metabolites NI Debrisoquine H2 Nw ) 5-Dehydrosparteine OH *is

N\ X~NH N\ ,

Fig. 2. Debrisoquine metabolism in human. Fig. 3. Sparteine metabolism in human. concentrations ofthe antidepressant drug or its active metab- Typical percentage yields of DBQ and hydroxylated metabolite(s). Conversely, other individuals are poor drug olites ofDBQ isolated from a 24 hour urine sample from most metabolizers who most commonly attain excessive plasma patients receiving DBQ (40 mg) are: DBQ, 30%; 4-OH- levels of the antidepressant drug despite the fact that the DBQ, 35%, 5-OH-DBQ, 1.4%; 6-OH-DBQ, 1.9%; 7-OH- amount of the administered drug is within an acceptable DBQ, 3.9%; 8-OH-DBQ, 0.9%. A minority of individuals dosage range. Studies such as these have revealed that for excreted similar amounts of the 5-, 6-, 7-, and 8-OH-DBQ certain drugs, genetic factors control drug metabolism, metabolites, but the quantity of urinary 4-OH-DBQ was resulting in large variations in plasma drug concentrations, appreciably lowered to about two percent (Idle and Smith therapeutic responses and side-effects, and have indicated 1979). Thus, metabolic 4-hydroxylation of DBQ in humans that genetic factors should be considered when drug dosage is polymorphic. The majority of patients are classified as regimens are established. It has been emphasized that when extensive metabolizers (EMs) of DBQ, while the group that the same dose of nortriptyline (NT) is administered to a excrete much lower quantities of4-OH-DBQ is composed of a > in steady state NT patient population, 30 fold difference poor metabolizers (PMs) of the drug. The term metabolic plasma concentrations is observed (Hammer and Sjoqvist ratio (MR) was coined to differentiate EMs from PMs; 1967). This leads to large variations in therapeutic efficacy (MR = percentage of dose of DBQ excreted in urine and side-effects but, despite this knowledge, it is not a com- unchanged divided by the percentage of dose of DBQ mon practice to make appropriate dosage adjustments for excreted in urine as 4-OH-DBQ). EMs of DBQ have a patients receiving NT; a maintenance dose of 75 mg/day to debrisoquine MR of < 12.6 (or a MR <1.10). 150 mg/day of NT is a common recommendation in the logl0 MR > 1.10. treatment of adults with depression, regardless of patient Conversely, in PMs, log10 are EMs of a response. In contrast, it is the common practice to monitor Most individuals also sparteine (SPT), and make significant drug dosage adjustments in patients uterine stimulating and antiarrhythmic drug, but a minority with renal failure who are administered drugs that are mainly of patients receiving this drug experienced serious overdose excreted renally, to minimize toxic reactions. effects, including diplopia, blurred vision, dizziness and headache (Eichelbaum 1982), and fetal death in pregnant Genetic influences in drug metabolism women (Newton et al 1966). Although it was never established, it is probable that the susceptible individuals were who were unable to metabolize this drug to its In the mid 1970's, two important observations were made PMs of SPT Like conceming debrisoquine (DBQ), a modestly used antihyper- major metabolites, 2- and 5-dehydro-SPT (see Fig. 3). of SPT is tensive drug, and sparteine, an alkaloidal drug with oxytocic the 4-hydroxylation of DBQ, the metabolism and antiarrythmic properties. When the metabolism of DBQ polymorphic; the MR of SPT (urine SPT concentration was investigated in humans (Allen et al 1975, 1976), many divided by (the sum of2- and 5-dehydro-SPT concentrations) metabolites were isolated and identified (see Fig. 2): the of EMs is < 20. Frequency distribution histograms of MRs major urinary metabolite in most individuals was identified of DBQ and SPT have been constructed to illustrate the as 4-hydroxydebrisoquine (4-OH-DBQ). However, in a incidence of EMs and PMs. The histogram for DBQ (see minority of patients, the amount of 4-OH-DBQ excreted in Fig. 4) is typical and was constructed from literature data the urine was significandly lower than in most patients. (Woolhouse et al 1979; Perault et al 1991). January 1994 Polymorphism in the metabolism ofdrugs 33

E PMs

W 0 EMs

u :E.0

0 6 z

3.0

Log Io Metabolic Ratio Fig. 4. Frequency distribution histogram of debrisoquine 4- Fig. 5. A model of the active site in human CYP2D6 at which hydroxylation metabolic ratios (logio) in Caucasian amitriptyline is appropriately oriented for metabolic subjects. oxidation to occur at C-10. It has been demonstrated that 4-hydroxylation of DBQ drug at which metabolic oxidation occurs is correctly posi- and the oxidation of SPT to 2- and 5-dehydro-SPT are both tioned. The distance between the drug site of metabolic catalyzed by a single enzyme which is a member of the oxidation and the quatemary N is always between 0.5 nm to cytochrome P450 superfamily. Of the 20 or so human P450 0.7 nm (Guengerich et al 1986; Meyer et al 1986). This isozymes (discussed later), two isozymes are known to be information is helpful in the prediction ofpossible metabolic genetically polymorphic. The isozyme responsible for the products ofnovel basic drugs. Another conclusion from these DBQ/SPT polymorphism is cytochrome P450 2D6, which is conformational studies is that metabolic N-dealkylation and abbreviated as "CYP2D6" according to the most recent deamination reactions should involve a cytochrome P450 recommendations forP450nomenclature (Nebert et al 1991). other than 2D6. It also follows that some drugs will not Multiple forms of the gene controlling the synthesis of possess the necessary structural criteria to be substrates of CYP2D6 exist (for review, see Gonzalez and Meyer 1991). CYP2D6 and, therefore, will be metabolized by other P450 PMs have two defective alleles while EMs are either homo- isozymes. The elimination of most tricyclic antidepressants zygous for the normal wild-type allele or heterozygous, with is mainly by CYP2D6, but other P450 isozymes are also one active allele and one defective allele. The frequency of involved. the PM phenotype is five percent to ten percent in Caucasian Based on information obtained from X-ray crystallo- populations, but is much lower in Orientals and other popu- graphic studies, Islam et al (1991) have generated another lations (Eichelbaum 1982; Nakamura et al 1985; Kalow three dimensional molecular template for substrates of 1986; Henthom et al 1989; Eichelbaum and Gross 1990). human CYP2D6. This template defines the stereochemical In addition to DBQ and SPT, CYP2D6 catalyzes the requirements for appropriate substrates in terms ofmolecular oxidation ofmany drugs used clinically (see Table 1). Pheno- volume and position of key atoms. An important feature of typic differences in the kinetics of these drugs have been the model is the optimum N+ to anionic site distance which demonstrated. is calculated to be in the 0.25 nm to 0.45 nm range. Both models to All drugs identified in Table 1 possess common structural (N+ anionic site distance, and N+ to metabolic site features and physical properties. They are lipophilic com- distance) are compatible. pounds and strong organic bases that become protonated (quaternized) at physiological pH, and they possess a planar, Human P450 isozymes usually aromatic, ring system, which, together with the positively charged N atom, is necessary to orientate the drug Molecular genetics of the cytochrome P450 superfamily correctly within the CYP2D6 protein active site where is the subject of an excellent review (Gonzalez 1990). More metabolic oxidation occurs. Guengerich et al (1986) have than 20 human P450 isozymes have been isolated, mainly proposed a model of the active site of human CYP2D6. At from liver, and characterized to a significant extent. They this site, the drug molecule adopts a conformation that orien- have been allocated to eight different gene families (1, 2, 3, tates the N+ atom towards an anionic location (a COO- 4, 11, 17, 19 and 21) of which only one (family 2) contains group) on the P450 protein while the aromatic ring aligns a large number of subfamilies, each of which is designated a itself to a relatively planar region of the protein. Application different capital letter. Gene families reflect the degree of of this concept to amitriptyline provides a model of the P450 similarity in the amino acid sequences of the cytochrome protein's active site (see Fig. 5) in which the location in the proteins. Each distinctive gene family displays less than 40% 34 Journal ofPsychiaby & Neuroscience VoL 19, No. I., 1994

tion is induced, it becomes widely distributed in human

10 10 tissue. Cigarette smoking induces the production of lAl in placenta and lung tissue (Gentest 1992). In human, CYP1A2 is expressed mainly in liver and is 11 CCHOH1H2 CH3 X 2 CH3 CH2CH3 r1NHC(CH3)3 CH2CH2CH2N\ CHCH2CH2N\ identical to phenacetin 0-deethylase. Close structural rela- R I 11 ~~~~~~~~~R tives ofCYP1A2 are expressed in animal species (rat, mouse

4' and rabbit); (Okey 1990). All are inducible by the same substances that induce CYPlA1. Typical substrates are acet- 8X H3~~~~~~N-H anilide (4-hydroxylation), 7-ethoxy- and 7-methoxy- NH resorufin (0-dealkylation), estradiol (2-hydroxylation), and C2 O phenacetin (0-deethylation). CH2CH2CH2N1CH3 R VgCH3 CYPiA2 is predominantly responsible for the major metabolic pathway of the methylxanthines, caffeine and theophylline. Studies using antibodies suggest that Fig. 6. The structures of substrates and metabolites of N-demethylation of theophylline by human liver micro- importance in drug metabolism, drug polymorphism somes, i.e. both N1-demethylation to 3-methylxanthine and phenotyping. [Arabic numbers located on the (3-MX), and N3-demethylation to 1-MX, are catalyzed by structures indicate sites at which metabolic oxidation CYP1A2. In contrast, metabolic C8-hydroxylation of occurs. Identities: I, Bufuralol; II, R = CH3, theophylline to 1,3-dimethyluric acid is mediated by other Imipramine; II, R = H, Desipramine; III, R = CH3, isozymes, probably CYP3A3 and CYP2E1 (Sarkar et al Amitriptyline; III, R = H, Nortriptyline; IV, R = CH3, 1992). Caffeine N3-demethylation is also catalyzed by Clomipramine; IV, R = H, Desmethylclomipramine; CYP1A2, while other metabolic pathways involve different V, R = CH3, Dextromethorphan; V, R = H, Dextrorphan; VI, Mephenytoin.] P450s (Berthou et al 1991; Kalow and Tang 1991). Furafyll- ine, a methylxanthine, is a selective inhibitor ofCYP1A2 but not of CYPlAl (Boobis et al 1990). amino acid similarity with all other families. All members of the same family have 40% or greater amino acid sequence CYP2C8/9 (mephenytoin hydroxylase) identity, while all members of the same subfamily have CYP2C isozymes have broad overlapping specificities greater than 59% amino acid sequence similarity (Nebert et towards various organic compounds, including steroids. al 1989). Individual P450s within a subfamily are identified Three 2C isozymes are present in human liver and two of by Arabic numbers. these, 2C8 and 2C9, have been sequenced. Both were Family 1 has two well characterized members, CYPlAl believed to catalyze the metabolic oxidation of and CYP1A2; family 2 has six subfamilies (A-F), to which S-mephenytoin to its 4-hydroxylated metabolite (see Fig. 6) the following members have been allocated: CYP2A3, but when a human CYP2C9 microsome cell line became CYP2B6, CYP2B7, CYP2B8, CYP2C6, CYP2C8, CYP2C9, available (Gentest 1992), it was found that the 2C9 isozyme CYP2C1O, CYP2D6, CYP2D7, CYP2D8, CYP2E1, did not metabolize S-mephenytoin. However, tolbutamide CYP2F1; family 3 members are CYP3A3, CYP3A4, and hexobarbital were substrates for human CYP2C9. CYP3A5; and the family 4 member is CYP4B1. Character- Because of this discrepancy, the P450 isozyme that ized members of the remaining families are CYPIIBI, 4'-hydroxylates S-mephenytoin is usually identified as CYP17A1, CYP19A1 and CYP21A2. Many other P450 mephenytoin hydroxylase, a member ofthe CYP2C subfam- family members have been isolated from various animal ily. Other substrates for mephenytoin hydroxylase (2C8 species (Gonzalez 1990). Five human P450 isozymes that and/or 2C9) are: methylphenobarbital, diazepam, have received most attention are lA1, 1A2, 2C8/9, 2D6 and N-desmethyldiazepam, flurazepam and alprenolol. It is inter- 3A4. esting that many substrates for this isozyme are weak acids. CYPIAJ and CYPIA2 Like CYP2D6, mephenytoin hydroxylase exhibits polymorphism, the only other P450 isozyme yet known to do so. Both IA subfamily isozymes are inducible by the pre- About three percent ofCaucasians and over 20 % ofJapanese administration of polycyclic aromatic compounds, such as are PMs of mephenytoin to the 4'-hydroxy metabolite 3-methylcholanthrene (3-MC), isosafrole and 2,3,7,8-tetra- (Kupfer and Preisig 1984; Wedlund et al 1984; Gonzalez chlorodibenzo-p-dioxin (TCDD) (Okey 1990). Both play an 1990). Mephenytoin polymorphism is independent of important role in carcinogenic activation. The CYPlAl debrisoquine polymorphism. isozyme is expressed in human placenta, and structurally Relatively little is known about the cytochrome P450 related isozymes are also expressed in rat, mouse and rabbit isozymes that catalyze the N-dealkylation of antidepressant (Berthou et al 1991; Kalow and Tang 1991). Normally and neuroleptic drugs. A recent investigation has revealed CYPlAl is virtually absent in humans, but when its produc- that N-demethylation of imipramine to desipramine is January 1994 Polymorphism in the metabolism ofdrugs 35

known to stimulate their own metabolism, and common B P-450 inducers such as rifampicin, phenazone (antipyrine), ethanol, polyaromatic hydrocarbons, carbamazepine and A 1 phenobarbital have no effect on the activity of CYP2D6. I |B However, CYP2D6 expression is inducible in cell culture threefold to sixfold by pretreatment with dimethylsulfoxide (DMSO) and by horse serum (Gentest 1992). CYP2D6 has B no known endogenous substrates and metabolizes only basic substrates which have specific structural requirements and are protonated at physiological pH, of which there are many examples (Table 1). as The B-adrenoceptor blocking agent bufuralol is metabolized almost exclusively at the 1'-position by CYP2D6 to produce I'-hydroxybufuralol (seeFig. 6); thisprovides avery sensitive and specific assay of CYP2D6 activity (Gentest Fig. 7. Gas chromatograms of urine extracts from I, a poor 1992). CYP2D6 has been detected in human brain (Zanger metabolizer (PM), and II, an extensive metabolizer et al 1988). (EM) who were phenotyped with dextromethorphan. Although a drug is identified as a substrate of CYP2D6, The PM subject had a metabolic ratio (MR) = 9.62, obtained by calculating the ratio of the quantity of it cannot be assumed that CYP2D6 is the only cytochrome dextromethorphan (represented by peak A) to that of P-450 isozyme involved in that drug's metabolism. If that the metabolite, dextrorphan (represented by peak B). were so, PMs of the drugs identified in Table 1 who cannot The MR of the EM subject was 0.24. synthesize CYP2D6, would be unable to metabolize these drugs to any extent. However, since PMs are able to metab- controlled in part by mephenytoin hydroxylase (Skjelbo et al olize all the drugs in Table 1 to some extent, it is clear that 1991). isozymes other than CYP2D6 must be involved. The drugs identified in Table 1 are primarily, but not exclusively, CYP2D6 metabolized by CYP2D6. Other P-450 isozymes can metab-

This isozyme, previously designated P450bufl, P450db 1, olize these drugs, but not as efficiently as can CYP2D6. If P450IlD1, P4501ID6, and P450 2D6, is the best characterized the metabolism ofany ofthe drugs listed in Table 1 is induced P450 by far (Br0sen and Gram 1989a; Eichelbaum and Gross by another drug or xenobiotic, it is due to the induction ofthe 1990; Gaedigk et al 1991). The gene locus responsible for its alternative P-450 isozyme(s). Some very poor metabolizers synthesis is located on the long arm of chromosome 22. of a drug may lack more than one P-450 isozyme capable of Human CYP2D6 cDNA has been cloned and sequenced. metabolizing that drug. This might be true for the very poor It is believed that CYP2D6 is expressed only in human, metabolizer of dextromethorphan identified in Fig. 7. although structurally analogous proteins are synthesized in Although quinidine strongly inhibits the 2D6 isozyme, it other species. The rat has five CYP2D genes, although one is not considered to be a substrate for CYP2D6. It is true that (CYP2D1) appears to be predominantly expressed in rat CYP2D6 does catalyze quinidine's 3-hydroxylation to a liver. The substrate specificity of this isozyme is similar, but small extent (Br0sen et al 1990), but quinidine is much more not identical, to CYP2D6 (Gonzalez 1990). CYP2D6 exhib- efficiently metabolized by the CYP3A4 isozyme. An ability its polymorphism. Intact isozyme is absent from the livers of to inhibit an oxidative reaction with low doses of quinidine PMs of debrisoquine. Incorrectly spliced CYP2D6 is a specific indicator of the involvement of CYP2D6. The pre-mRNAs have been identified in the livers ofdebrisoquine synthetic neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6- PM subjects, resulting in deficient 2D6 synthesis in five tetrahydropyridine), which induces a form of Parkinsonism, percent to ten percent ofCaucasians, two percent ofOrientals is a competitive inhibitor of CYP2D6 (Fonne-Pfister et al and one percent of Arabics. 1987). Possible links with toxic implications also exist Many isozymes of cytochrome P-450 are inducible by between cancer, Parkinson's disease, Balkan nephropathy various drugs or other organic chemicals, but CYP2D6 is not and systemic lupus erythematosus and PMs of debrisoquine one of them. In his review on the induction of cytochrome because of their lack of CYP2D6 (Ayesh and Idle 1985; P450 enzyme systems, Okey (1990) identifies the inducible Kaisary et al 1987; Barbeau et al 1985; Ritchie et al 1983; P-450 isozymes, but his lists do not include CYP2D6. An Baeret al 1986). A human cell preparation in which CYP2D6 inducer increases the rate oftranscription ofthe genes coding is expressed is available commercially. for the species of P-450 that it induces. Most inducers CYP3A4 stimulate the production of more than one species of P-450 and stimulate their own metabolism as well as the metabo- This isozyme is also named nifedipine oxidase. Its sub- lism of many other chemicals. No substrates of CYP2D6 are strates are diverse in structure and include lidocaine, quini- 36 Journal ofPsychiatry & Neuroscience VoL 19, No. 1, 1994

dine, nifedipine, midazolam, triazolam, erythromycin, not a metabolite under these conditions. The formation of cyclosporin A and testosterone. This isozyme does not dis- 2-OH-IMI was reduced to less than 20 % ofthe control value play polymorphism. Its biosynthesis is induced by pre- when the microsomes and IMI were incubated with serum gnenolone 16a-carbonitrile. N-Dealkylation of the novel containing antibodies against CYP2D6, but these antibodies ergot alkaloid CQA 206-291 by human liver microsomes is had no effect on the formation of 10-OH-IMI or DMI. In catalyzed by CYP3A4. This alkaloidpossesses dopaminergic addition, quinidine and levomepromazine were potent activity and is being investigated for use in the treatment of competitive inhibitors of IMI 2-hydroxylation but had no Parkinsonism (Ball et al 1992). effect on lO-hydroxylation or N-demethylation. From these observations it was again concluded that 2-hydroxylation of Ring hydroxylation and N-demethylation of tricyclic IMI was CYP2D6-mediated, but the 2D6 isozyme was not antidepressants involved in the lO-hydroxylation reaction nor to any significant extent in the demethylation of IMI to DMI. Skjelbo et The two metabolic reactions of tricyclic antidepressants al (1991) believe that mephenytoin hydroxylase (CYP2C8/9) (TCAs) most commonly observed are ring hydroxylation is partly responsible for the N-demethylation of IMI, but an (aromatic and alicyclic) and N-dealkylation. These reactions additional P450 isozyme also appears to be involved in this have been the subject matter of many metabolism studies dealkylation in view of the observation by Perel et al (1976) which have revealed that aromatic hydroxylation of TCAs is that metabolism of IMI to DMI was induced by cigarette invariably catalyzed by cytochrome CYP2D6. In contrast, smoking, and therefore cannot involve either ofthe CYP2D6 the P450 isozymes involved in N dealkylation and alicyclic orCYP2C8/9 isozymes which are not induced in this manner. ring hydroxylations have notbeen definitively identified, and In our own studies on IMI metabolism (Coutts et al 1993; controversy is evident in the literature. To illustrate this, the Ping Su et al 1993), we employed commercially available metabolic reactions identified for five popular TCAs are CYP2D6 expressed in human AHH-1 TK+/- cells. As briefly reviewed. expected, the major metabolic reaction was ring hydroxyla- Imipramine tion of IMI to 2-OH-IMI. However, a significant quantity of In man in vivo, imipramine (IMI); (see Fig. 6) is metabo- DMI was also formed, confirming that N-demethylation of lized sequentially by demethylation to desipramine (DMI) IMI is mediated to some extent by CYP2D6. A third metab- followed by ring-hydroxylation to 2-hydroxydesipramine olite, 2-OH-DMI, was also produced in very small quantities. (2-OH-DMI). The ring hydroxylation reaction is catalyzed IMI was also incubated in the presence of CYPlAl and primarily, but not exclusively, by CYP2D6, whereas the CYP3A4 expressed in human AHII-l TK+/- cells. No N-demethylation reaction is mediated mainly by a different metabolism occurred, indicating that neither isozyme was P450 isozyme, although, surprisingly, CYP2D6 is apparently capable ofcatalyzing N-demethylation or ring hydroxylation involved in N- demethylation to a small extent (Br0sen and of IMI. Gram 1989b). The DMI:IMI plasma ratio is very variable in Desipramine patients receiving IMI and depends on whether the patients are EM or PM phenotypes. PMs and EMs will both The major in vivo metabolite of DMI (see Fig. 6) in the N-demethylate IMI to DMI to a similar extent because this human is 2-OH-DMI. This hydroxylation of DMI correlates metabolic reaction is not catalyzed significantly by CYP2D6 strongly with 4-hydroxylation of debrisoquine in human in but the extent ofthe subsequent 2-hydroxylation ofDMI will vivo and in human liver microsomes (von Bahr et al 1985). vary greatly because PMs lack the necessary CYP2D6 Dramatic differences are observed in DMI blood levels in isozyme, and any 2-hydroxylation of DMI that does occur in human EM (AUC 1481 ± 707 nmol h/L) and PM (AUC 6552 PMs must rely on another, and less efficient, P450 isozyme. ± 1822 nmol h/L) phenotypes who received a single dose (25 Prior administration of quinidine reduces the clearance of mg) ofDMI and were monitored (blood and urine) for at least IMI in humans by only 35% because IMI's clearance is by 96 hours after drug administration. The quantities of drug N-demethylation as well as ring hydroxylation, and the recovered in the urine as 2-OH-DMI were also very different former is unaffected by quinidine treatment. Thus, the phar- in EM (27.40 ± 4.72 mol) and PM (5.22 ± 0.70 gmol) macokinetic properties of IMI in EMs during quinidine phenotypes (Spina et al 1987). Studies with quinidine therapy will mimic those observed in PMs who do notreceive confirmed the dependency of2-hydroxylation ofDMI on the quinidine. In contrast, there is an 85% reduction in DMI's CYP2D6 isozyme. The effect of quinidine on oral DMI clearance by EM subjects after quinidine administration pharmacokinetics was a reduction of 85% in DMI clearance because DMI's clearance is mainly by metabolic ring because of its greatly reduced metabolism to 2-OH-DMI hydroxylation. (Br0sen and Gram 1989b; Steiner et al 1987). Both DMI and Three metabolites are formed during incubation of IMI IMI undergo extensive first pass metabolism when adminis- with microsomal preparations (Br0sen et al 1991a), namely tered orally and liver CYP2D6 is saturated during the first 2-hydroxyimipramine (2-OH-IMI), lO-hydroxyimipramine pass ofeither drug. This is particularly problematic in exten- (1O-OH-IMI) and DMI. It is noteworthy that 2-OH-DMI was sive metabolizers. January 1994 Polymorphism in the metabolism ofdrugs 37

Amitriptyline mediated mainly by CYP2D6, is inhibited by this neuroleptic. The three major metabolites of amitriptyline (AT); (see Fig. 6) in the human are 10-hydroxyamitriptyline (10-OH- AT), nortriptyline (NT) and 10-hydroxynortriptyline Clinical significance (10-OH-NT) and there is no doubt that the l0-hydroxylation reaction, which creates a chiral center, is controlled mainly Clinically important toxicological implications ofgenetic by CYP2D6. There is some controversy over which P450 polymorphism in drug metabolism have been identified in isozymes catalyze the N-demethylation of AT to NT, and recent reviews by Br0sen and Gram (1989a), Meyer et al 10-OH-AT to 10-OH-NT. One study by Mellstrom et al (1990), Eichelbaum and Gross (1990), and Islam et al (1991). (1983) found that plasma clearance of AT by N-demethyla- Whether or not a genetic defect in drug metabolism will have tion did not correlate with debrisoquin 4-hydroxylation. clinical significance depends on the identity of the drug, its Thus, N-demethylation involved an isozyme other than therapeutic window, and the relative importance of the CYP2D6. From other studies involving human liver micro- metabolic pathway that is defective. For example, the dispo- somes (Mellstrom and von Bahr 1981; von Bahr et al 1982), sition of a drug that is not metabolized by CYP2D6, or is it was concluded that 10-hydroxylation of AT and metabolized only to a small extent by this enzyme, will not N-demethylation of AT to NT were regulated by different be affected by CYP2D6 phenotype. On the other hand, enzymatic mechanisms. However, Mellstrom et al (1986) CYP2D6 contributes importantly to the overall elimination subsequently reported that in nonsmokers the clearance of ofmost ofthe drugs listed in the left column ofTable 1. These AT by hydroxylation and by N-demethylation both corre- drugs will accumulate in PMs, and exaggerated pharmaco- lated with debrisoquin 4-hydroxylation (indicating that both logical effects or toxicity may result. In contrast, PMs will be reactions were catalyzed by CYP2D6), whereas in smokers poorly able to convert a pharmacologically inactive prodrug no such correlation was observed. It appears from these to its active metabolite if the prodrug is a substrate for accounts that demethylation of AT is controlled by at least CYP2D6. Codeine, for example, is a substrate for CYP2D6, two cytochrome P450 isozymes, of which one is CYP2D6 but only a minor pathway (about 10% 0-demethylation to and the other, as yet unidentified, is induced by smoking. morphine) is involved. Significant differences are observed AT hydroxylation by human liver microsomes is inhibited in plasma levels of morphine in EMs and PMs who were by NT; this inhibition is accompanied by an increase in administered codeine phosphate (100 mg), and in EMs who N-demethylation (Mellstrom and von Bahr 1981). had received quinidine prior to codeine administration In EM maximum Nortriptyline (Desmeules et al 1991). subjects the plasma concentration (Cm.) of morphine was 17.9 nmol/l, whereas NT (see Fig. 6) is a substrate for CYP2D6 which catalyzes in a PM subject Cm. was 0.60 nmol/l and in EM subjects who the formation of the major metabolite, 10-OH-NT had received quinidine, the Cm. ofmorphine was 1.5 nmol/l. (Mellstrom et al 1981; Br0sen et al 1990). A chiral center is Thus, in PM subjects and in EMs who also received quini- introduced; the E-isomer is preferentially formed, but some dine, codeine has virtually no analgesic properties. Encainide Z-isomer is also detected. In PMs of debrisoquine, there is a is another example. Encainide is actually a prodrug that stereoselective impairment ofE- I0-hydroxylation ofnortrip- apparently must be metabolized (0-demethylation by the tyline. The enantioselective nature of E-10-OH-NT forma- CYP2D6 isozyme) to be an effective antiarrhythmic drug tion has been studied in human liver microsomes and (Roden et al 1982). This necessity for metabolic activation intestinal homogenates. The (-)-enantiomer is preferentially explains why some subjects do not respond to relatively high formed in both preparations; its formation is inhibited by doses of encainide and why there are very variable peak quinidine. In contrast, less (+)-E-10-OH-NT is formed and plasma concentrations in patients after oral encainide admin- its formation is not inhibited by quinidine, which indicates istration (Wang et al 1984). that a P450 isozyme other than 2D6 is involved in (+)-E-10- It is prudent to recognize that because the metabolism of OH-NT formation (Dahl et al 199la). certain drugs is controlled by genetic factors, large variations Clomipramine in plasma concentrations of a particular drug may be observed. This, in turn, could lead to significant variations in The N-demethylation of clomipramine (CMI); (see Fig. a therapeutic response to, and side-effects of that particular 6) to its major metabolite, desmethylclomipramine (DCMI) drug. In addition, any two or more drugs that are included in is not mediated by the CYP2D6 isozyme whereas the the Table 1, ifadministered concomitantly, will compete with 8-hydroxylation of both CMI and DCMI is so mediated each other for the CYP2D6 isozyme involved in their metab- (Balant-Gorgia et al 1987). Thirteen patients who concomi- olism. Thus, steady state blood levels of each drug and their tantly received a neuroleptic (levomepromazine) with CMI half-lives will be altered from those values obtained when had elevated blood levels of DCMI and CMI (Balant-Gorgia each drug is administered alone. There is always the possi- et al 1986), indicating that metabolic 8-hydroxylation, bility that such a drug/drug interaction could lead to a toxic 38 Journal ofPsychiatry & Neuroscience VoL 19., No. I., 1994

event, and to prevent such an occurrence, the dosage regi- DBQ was significantly higher (46%) than in subjects who mens of both drugs may have to be modified. received no neuroleptic (7.5%). Haloperidol also inhibits the Because quinidine is a potent inhibitor of CYP2D6 in metabolism of some antidepressant drugs (Gram and humans, EMs of the drugs in Table 1 will be effectively Fredricson Over0 1972; Gram et al 1974). Although it was converted to PMs of these drugs when quinidine or another known to be an inhibitor of sparteine metabolism, haloperi- very potent inhibitor of CYP2D6 is administered concomi- dol did not appear to be a substrate for CYP2D6 (Gram et al tantly. It is for this reason that a patient receiving any of the 1989), but more recent studies have revealed that haloperidol drugs listed in Table 1 should never be administered quini- is metabolized by this isozyme. When known, poor and dine, orapatientreceiving quinidine should not be prescribed extensive metabolizers of DBQ were treated with haloperi- concomitantly any of the drugs listed in Table 1, unless drug dol, the plasma levels and half-life of this neuroleptic were dosage is suitably modified. significantly higher in the PMs than in the EMs, and the It is the opinion of some physicians and medical scientists clearance of haloperidol was greatly reduced in the PMs. It that phenotyping of patients for CYP2D6 activity should be is of interest that some PMs in this study suffered neurolog- a routine practice prior to the commencement of treatment ical side-effects and restlessness, whereas the EMs, who with tricyclic antidepressants or neuroleptics. However, the received the same dose of haloperidol did not. This led relevance of this practice has been the subject of much Llerena et al (1992) to suggest that known PMs of DBQ debate. In PM subjects receiving psychoactive drugs in should receive a lower dose ofhaloperidol than EMs, and that accepted therapeutic dose ranges, high plasma levels ofdrugs EMs may require increased doses if they are very rapid must be expected and there will be an increased risk of drug metabolizers ofhaloperidol. The fact thathaloperidol inhibits side-effects in many ofthese patients. However, since serious the metabolism oftricyclic antidepressants, together with the side-effects do not invariably occur in persons with elevated knowledge that haloperidol plasma levels are significantly plasma drug levels, it can be argued that a reduction of drug increased by co-administration of fluoxetine (discussed dosage in known PM subjects may not be necessary. In a below) also support the conclusion that the disposition of similar vein, the administration of a psychoactive drug in a haloperidol is dependent on the DBQ hydroxylation pheno- recommended therapeutic dose range to EM subjects is no type. Other investigators (Tyndale et al 1991) have observed guarantee that appropriate plasma levels are ever attained. that biochemical oxidation of the reduced metabolite of These examples suggest that a more appropriate practice haloperidol back to haloperidol is catalyzed by CYP2D6. would be to monitor the patient's progress regularly and A relevant case study on the subject of combined neuro- determine plasma levels of drugs at appropriate time inter- leptic/antidepressant drug therapy was reported by Balant- vals. However, the techniques used to phenotype an individ- Gorgia et al (1987). A 62 year old female patient was ual are very simple and noninvasive procedures that when receiving a normal therapeutic dose (100 mg/day) of performed, can at least forewarn the physician that the patient clomipramine for a major depressive syndrome but her is a slow metabolizer who is susceptible to an increased risk steady state plasma concentrations of CMP + DCMP (148 + of experiencing a toxic reaction to a psychoactive drug or 450 ng/ml), were appreciably above the expected values another drug. This is of particular significance to patients (CMP, 25 ng/ml to 100 ng/ml; DCMP, 100 ng/ml to who are very poor metabolizers and who have in the past 250 ng/ml). An investigation revealed that not only was the experienced quite severe drug-induced toxic effects when patient a PM of debrisoquine, she was also receiving low drugs have been administered within a dose range that would daily doses of the neuroleptic drug, levomepromazine, as be therapeutic for EM subjects. The individual with the very well as flunitrazepam, and alloperidol had been added to the high MR value of 9.62 (see Fig. 7) had experienced distress- drug regimen during the course of her treatment. ful side-effects in the past with a variety of drugs. An earlier It should be emphasized that toxic episodes are not inev- classification of this patient as a PM subject may have itable when a drug and a known inhibitor of that drug are influenced physicians to prescribe lower initial doses of coadministered. Two studies, both involving the serotonin- subsequent drugs to minimize further drug-induced distress. uptake inhibiting antidepressant fluoxetine, are illustrative. It has been revealed over the last 20 years or so that When fluoxetine, a potent inhibitor of CYP2D6 (Br0sen and neuroleptic drugs (for example, chlorpromazine, Skjelbo 1991), was added to the drug profile ofpatients who methotrimeprazine (levomepromazine) perphenazine, thio- were receiving desipramine or nortriptyline, there were ridazine and zuclopenthixol) inhibit the metabolism of other marked elevations ofthe plasma levels ofthe antidepressants psychotropic drugs including tricyclic antidepressants in (Aranow et al 1989; Ciraulo and Shader 1990). However, the humans because both groups of drugs are metabolized to a combined use of fluoxetine and desipramine is not contrain- significant extent by CYP2D6 (Dahl-Puustinen et al 1989; dicated. In fact, this combination is identified as a rapid and Spina et al 1991; Dahl et al 1991b). Toxic incidents are not effective strategy for the treatment of major depression uncommon with this drug combination. The study by Spina (Nelson et al 1991), but, even so, it would seem prudent to et al (1991) revealed that in psychiatric patients being treated monitor blood levels of both antidepressants when they are with neuroleptic drugs, the number of poor metabolizers of administered concomitantly. In another study on plasma January 1994 Polymorphism in the metabolism ofdrugs 39

concentrations ofhaloperidol before and after the addition of to be phenotyped is already receiving other drugs. Many fluoxetine, Goffet al (1991) found that efficient metabolizers drugs are known to interfere with CYP2D6 by inhibiting it, had initial low plasma levels of haloperidol, but these levels or by utilizing this isozyme in their metabolism. These drugs were significantly elevated when fluoxetine was adminis- will also interfere in the phenotyping of subjects, making the tered in addition to haloperidol. In contrast, patients who interpretation of results difficult. Ideally, the patient to be were poormetabolizers ofhaloperidol had high initial plasma phenotyped should not be receiving any drugs. Some popular drug concentrations which were relatively unaffected by the phenotyping procedures are now described. addition of fluoxetine to the drug regimen. The authors CYP2D6 phenotyping with debrisoquine concluded that the interaction between haloperidol and fluoxetine was a relatively minor event. A satisfactory procedure for determination of poor (PM) Paroxetine, like fluoxetine, is a serotonin-uptake inhibit- and extensive metabolizers (EM) of debrisoquine (DEB) is ing antidepressant. Br0sen et al (1991b) described paroxetine to ask the patient to empty the bladder, then swallow one as one of the most potent in vitro inhibitors of CYP2D6 10 mg Declinax® tablet. A zero to eight hour urine sample known when presenting an interesting case study involving is collected and an aliquot is analyzed by a suitable method a male patient with bipolar manic depressive illness. The (GC or HPLC) for its DEB and 4-OH-DEB levels (see patient was receiving paroxetine at the time he was pheno- Fig. 2). The metabolic ratio (MR) is calculated (MR = typed with sparteine and the results indicated that he was a percentage dose of DBQ excreted unchanged divided by PM. However, subsequent tests conducted during a log10 MR > 1.10 the percentage of the dose excreted as paroxetine-free period showed that he was in fact an EM but 4-OH-DBQ). Iflog1OMR>1.10, the individual is aPM. (Price- had been converted to a PM as a result of the paroxetine Evans et al 1980). treatment. This case illustrates how important diagnostic CYP2D6 phenotyping with dextromethorphan tests can be compromised when drug/drug interactions occur. Dextromethorphan ([+]-3-methoxy-N-methylmor- One observation of extreme importance is that EMs of phinan) is a phenolic methyl ether and N-methylated com- debrisoquine apparently display a higher incidence ofcancer pound that is now commonly used for phenotyping in North of the liver, intestinal tract, bladder and lung, while PM America. Its major metabolic pathway is 0-demethylation to subjects have a decreased susceptibility to these cancers dextrorphan ([+]-3-hydroxy-N-methylmorphinan) (see (Ayesh and Idle 1985; Kaisary et al 1987). This protection of Fig. 6). This metabolic reaction is catalyzed by the CYP2D6 various tissues in individuals who lack the CYP2D6 isozyme isozyme. For phenotyping, the procedure is to administer a implies that procarcinogens located in human tissues are commercially-available dextromethorphan (single active activated by the 2D6 isozyme. Since the pathogenesis ofboth ingredient) product of which many are available as over-the- bladder and lung cancerhas been linked to cigarette smoking, counter products in Canadian pharmacies, and measure the it is possible that one or more components ofcigarette smoke urinary dextromethorphan/dextrorphan ratioby HPLC orGC is/are substrate(s) for CYP2D6. Further studies are clearly urinary (Kupfer et al 1984; Woodworth et al 1987; Henthom warranted to confirm and rationalize this observation. et al 1989). It is a common practice either to administer a 30 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), is a mg dextromethorphan tablet or an equivalent volume of compound that was inadvertently synthesized in an attempt dextromethorphan syrup to the patient and collect an eight to prepare a "new heroin" (the reversed ester of meperidine) hour urine sample (overnight), or to administer a 60 mg dose for illicit use (Singer et al 1987). MPTP induces a of dextromethorphan and collect an eight to 12 hour urine Parkinsonism-like syndrome in humans and some animal sample for analysis. A suitable aliquot of the urine is first species and, as well as being a substrate for MAO-B (Singer then for al hydrolyzed (B-glucuronidase/sulfatase) analyzed its et 1987), it is a competitive inhibitor of CYP2D6. These dextromethorphan and dextrorphan concentrations by GC or observations have led to speculation that a deficiency of HPLC, from which is calculated the dextromethorphan CYP2D6 in the brain or the liver may predispose an early metabolic ratio (MR = concentration of dextromethorphan onset ofParkinson's disease. Additional studies are required, divided by a concentration of dextrorphan). The values of not only on this subject, but also to confirm that an associa- MRs obtained reportedly range from 0.0030 to 5.27 tion does exist between debrisoquine phenotype and (Henthorn et al 1989) and PMs have an MR> 0.24. However, systemic lupus erythematosus and between debrisoquine in our studies we have PMs and identified a very poor metabolizer of Balkan nephropathy (Meyer et al 1990). dextromethorphan whose MR was 9.62. There is close correlation between dextromethorphan and debrisoquine Phenotyping phenotypes (Perault et al 1991). Phenotyping is easy to perform and generally requires Mephenytoin hydroxylase activity only drug administration followed by urine collection over an appropriate time interval for analysis. However, one must A typical procedure (Kupfer and Preisig 1984) is to always be aware that drug interactions can occur ifthe patient administer racemic mephenytoin (100 mg) (see Fig. 6) to 40 Journal ofPsychiatry & Neuroscience VoL 19., No. 1, 1994

patients and collect a zero to eight hour urine. An aliquot is recently developed methemoglobinemia when administered treated with acid to hydrolyze glucuronides and the S-4'- prilocaine during dental surgery. Prilocaine is normally hydroxymephenytoin content is measured by gas chromatog- metabolized extensively to products which are ring hydrox- raphy. The elimination of 4-hydroxymephenytoin is ylated and the direct product of hydrolysis, the primary essentially stereospecific for the S-enantiomer. Virtually no aromatic amine o-toluidine, is normally a minor metabolite. mephenytoin is excreted in urine so a metabolic ratio cannot We reasoned that if this student was unable to ring hydrox- be calculated. Instead an hydroxylation index (HI) is deter- ylate prilocaine, the high plasma levels of o-toluidine that mined (HI = dose of the S-enantiomer of mephenytoin in could result would be capable of instigating methemo- ,umol divided by mol amount ofS-4-hydroxymephenytoin in globinemia. When the student was phenotyped with the zero to eight hour urine). All extensive metabolizers of dextromethorphan, her MR was 9.62 (see Fig. 7), indicating mephenytoin have an HI value of 5.6 or less. Poor that she was an extremely poor metabolizer of this substrate, metabolizers ofmephenytoin have an HI value well in excess and presumably of all drugs that are metabolized signifi- of 5.6 (range > 20 to > 700). cantly by cytochrome CYP2D6. Otherphenotyping procedures Alternative procedures for phenotyping have been CONCLUSION reported. Methoxyphenamine has been used by Roy et al (1985) for CYP2D6 phenotyping and a rapid screening process for polymorphism in both dextromethorphan and The purpose of this article is to provide information for mephenytoin metabolism has been described (Guttendorf et the clinician and scientist who prescribe and/or conduct al 1990). In the latter procedure, dextromethorphan (60 mg) research on antidepressants and other psychoactive drugs. It and (±)-mephenytoin (100 mg) were given each alone, and is obvious that much knowledge on the metabolism and then together to patients and urine samples were collected distribution of antidepressants and neuroleptic drugs has (eight to 12 hour). Dextromethorphan and dextrorphan were accumulated in the last quarter-century and our knowledge determined by TLC and then by HPLC and mephenytoin and of the enzymes involved in metabolic reactions continues to its metabolites were analyzed by chiral capillary GC. Results increase at a rapid rate. Reasonable conclusions from the indicated that dextromethorphan and (±)-mephenytoin can material presented in this article are that many drug/drug be readily given in combination for phenotyping. In the large interactions can now be anticipated, and patients who are Caucasian population studied, 6.7% were PMs of likely to experience toxic side-effects when one psychoactive dextromethorphan and 3.7% were PMs of mephenytoin. drug or a mixture of drugs is administered are easily identi- Some individuals are PMs of both dextromethorphan and fied. Those patients who are least likely to respond to mephenytoin. psychoactive drugs administered in what are generally Unexpected observations have been made during the accepted as therapeutic dose ranges can also be readily process of phenotyping. Br0sen et al (1991a) identified a recognized, if desired, even before drug administration bipolar manic depressive male who experienced severe toxic begins. Although phenotyping is a relatively simple, rapid effects when administered a modest dose of amitriptyline and inexpensive method of categorizing patients as poor or (50 mg/day). The same patient required a relatively high dose extensive metabolizers of drugs commonly used by psychi- ofimipramine (200 mg/day) in order to attain the therapeutic atrists, it is interesting to report that mutant CYP2D6 alleles plasma levels of imipramine + desipramine. The patient was and other alleles are currently being cloned and sequenced. an EM of debrisoquine and sparteine, as well as of imipra- This has resulted in the development of allele-specific poly- mine and desipramine, and therefore would be expected to merase chain reaction (PCR) assays that can be used for the be an EM of amitriptyline since all five drugs are normally routine screening ofpatients for CYP2D6 activity and there- metabolized extensively by the same cytochrome P450 iso- fore fortheirability to metabolize psychoactive drugs, as well zyme (2D6), but this patient was clearly a very poor as for the customizing of drug dosages (Heim and Meyer metabolizer of amitriptyline. A satisfactory explanation of 1990; Gonzalez and Meyer 1991; Caporaso et al 1992). As this observation has not been offered. many as 95% of DBQ PMs can be detected by PCR assay. A related observation has been made for a Ghanaian population. When sparteine was used in phenotyping, no PMs were observed (Eichelbaum and Woolhouse 1985). ACKNOWLEDGEMENT However, when debrisoquine was used to phenotype Ghana- ians, 7.1 % of the individuals tested were PMs (Woolhouse et al 1985). In our own drug metabolism studies, we have phenotyped The author wishes to acknowledge with gratitude the valu- volunteer students using dextromethorphan. One student able suggestions and encouragement of Dr. Glen B. Baker in revealed that she reacted adversely to various drugs and had the writing of this review. January 1994 Polymorphism in the metabolism ofdrugs 41

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