Review Series on Pharmacogenetics edited by Prof M. Pirmohamed
`
`Molecular genetics of CYP2D6: Clinical relevance with focus on
`
`psychotropic drugs
`
`
`
`,Mar]’a-LiisaDahl,PerDalén&AyrrranAl-Shtrrlfaji
`‘‘
`Department of Medical Laboratory Sciences 8 Technology, Division of Clinical Pharmacology, Karolinska Institutet, H1/iddinge University Hospital,
`SE-141 86 Stockholm, Sweden
`
`Cytochrome P450 CYPZD6 is the most extensively characterized polymorphic drug-
`metabolizing enzyme. A deficiency of the CYPZD6 enzyme is
`inherited as an
`autosomal recessive trait; these subiects (7% of Caucasians, about 1% of Orientals) are
`classified as poor metabolizers. Among the rest (extensive metabolizers), enzyme
`activity is highly variable, from extremely high in ultrarapid metabolizers, to markedly
`reduced in intermediate metabolizers. The CYPZD6 gene is highly polymorphic,
`with more than 70 allelic variants described so far. Of these, more than 15 encode
`an inactive or no enzyme at all. Others encode enzyme with reduced, ‘normal’ or
`increased enzyme activity. The C YPZD6 gene shows marked interethnic variability,
`with interpopulation differences in allele frequency and existence of ‘population-
`speciflc’ allelic variants,
`for instance among Orientals and Black Africans. The
`CYPZD6 enzyme catalyses the metabolism of a large number of clinically important
`drugs
`including antidepressants, neuroleptics,
`some antiarrhythmics,
`lipophilic
`(3—adrenoceptor blockers and opioids. The present—day knowledge on the influence
`ofthe genetic variability in CYPZD6 on the clinical pharmacokinetics and therapeutic
`effects/adverse effects of psychotropic drugs is reviewed.
`
`Keywords: antidepressants, CYPZD6, dehrisoquine, neuroleptics, polymorphism
`
`.
`Introduction
`
`and books related to various aspects of this are available
`.
`for further reading [4—8].
`
`Many drugs, especially lipophilic compounds such as
`psychotropics need to be metabolized prior to excretion
`in urine. Oxidative phase I catalysed metabolism by
`cytochrome P450 (CYP) enzyines plays a major role in
`this respect In the 1960s it was Shown that the 30_ to
`40-fold \
`antidepressant nortriptyline in patients treated with the
`same dose is due to a pronounced variation in the rate of
`metabolism of the drug [1, 2]. Twin studies further
`showed that the rate of metabolism had a strong genetic
`component [2] and in 1980 the l0—hydroxylation of nor-
`triptyline was shown to be catalysed by the polymorphic
`debrisoquine/sparteine hydroxylase (CYPZD6)
`This short review deals with the molecular genetics of
`CYP2D6 and its clinical relevance. Many recent reviews
`
`Correspondence: Professor Leif Bertilsson, Department of Medical Laboratory
`Sciences & Technology, Division of Clinical Pharmacology, Karolinska lnstitutet,
`Huddinge University Hospital, SE-|4| 86 Stockholm, Sweden. Tel.:+46
`8585 8|0 7|; Fax:+46 8585 8|0 70; E-mail: Leif.Berti|sson@|abtekki.se
`
`Received 23 February 200], accepted 24 October 200].
`
`The CYPZD6 polynqorphisnq
`
`The discovery of the debrisoquine/sparteine hydroxylation
`polymorphism
`
`In 1977, the hydroxylation of the antihypertensive drug
`dcbrisoquinc was shown to be polymorphic in nature
`[9, 10]. Independently, Eiclielbaum et al.
`[11] showed
`that the oxidation of sparteine was also polymorphic. The
`metabolic ratios
`(MR; parent driig/ metabolite) of the
`two drugs were closely correlated [12], showing that
`the same enzyme, now termed CYPZD6, was responsible
`for the two metabolic reactions.
`
`of
`(PM)
`of poor metabolizers
`incidence
`The
`debrisoquine/sparteine with deficient CYPZD6 activity
`has been investigated in many populations,
`in most of
`them with a fairly small number of subjects [13, 14].
`Among 1011 Swedish Caucasians we found 69 (6.3%)
`PM of debrisoquine [15]. This incidence is very similar
`to that in other European Caucasian populations (7—10%)
`
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`In collaboration with Lou and associates in
`[13, 14].
`Beijing it was shown that the incidence of PM among
`695 Chinese was only 1.0% using the antimode established
`in Caucasian populations [15]. A si111ilar low incidence of
`
`was shifted to the right compared with Swedish EM
`(P<0.01)
`[15]. This showed that
`the mean rate of
`hydroxylation of debrisoquine was lower
`in Chinese
`EM compared with Caucasian EM. This right shift in
`
`Molecular genetics of the poor metabolizers
`
`The gene encoding the CYPZD6 enzyme is localized on
`chromosome 22 [18]. Three major mutant alleles, now
`termed CYP2D6*3, *4 and *5 [23] (Table 1), associated
`with the PM phenotype, were found early on in
`Caucasians
`[19—22].
`In
`Swedish Caucasians,
`the
`CYP2D6*4 allele occurs with a frequency of 22% and
`accounts for more than 75% of the mutant alleles in this
`
`population [23]. The CYP21)6*4 allele is almost absent
`in Chinese and this is the reason for the low incidence
`
`(1%) of PM in this population compared with 6% in
`Caucasians
`[15]. The frequency of the gene deletion
`(CYP2D6*5) on the other hand is very similar, i.e. 4—6%
`in different populations (Table l). The CYPZD6 gene
` wim
`more than 70 allelic variants described so far (http://
`www.imm.ki.se/CYPalleles/cyp2d6.htm). In addition to
`the CYPZD6 *3, *4 and *5, alleles, a large number of
`low—frequency alleles associated with the PM phenotype
`have been identified. Usually a few variants account for
`most of the mutant alleles in a population. The alleles of
`importance may, however, vary between populations (see
`below), which needs to be taken into consideration when
`applying genotyping methodology in clinical research or
`patient care.
`
`CYP2D6*10 allele [24, 25] with the SNP C188T causing
`a Pro34Ser amino acid substitution and an unstable
`
`enzyme with decreased catalytic activity [25] (Figure 1).
`The frequency of this C YP2D6*1 0 allele is similar, about
`50%,
`in Chinese, Japanese and Koreans, but extremely
`low among Caucasians (Table 1).
`Masimirembwa et al.
`[27] found a right shift of the
`MR in black Zimbabweans, similar to that found in
`Orientals. A mutated allele encoding an enzyme with
`decreased activity was subsequently identified and named
`CYP2D6* I 7. The frequency of this allele was found to be
`34% in Zimbabweans [27] (Table 1), 17% in Tanzanians
`[28], 28% in Ghanaians [29] and 9% in Ethiopians [30].
`This and many other studies demonstrate the genetic
`heterogeneity of different black populations of Africa.
`There are thus three fairly population specific alleles with
`CYP2—D6*4 in Caucasians,
`*1—0 in—Asi—ans and *17 in
`Africans. These mutations must have occurred after the
`
`separation of the respective populations from each other.
`and Orientals
`In Caucasians
`a
`close
`geno— and
`phenotype relationship has been demonstrated [23, 25,
`26]. However, in studies in Ethiopia [30], Ghana [29] and
`Tanzania [28] a lower CYP2D() activity in relation to
`genotype has been demonstrated, indicating that in Africa,
`environmental factors, e.g. infections or food constituents
`are probably of importance in addition to genetic factors.
`
`Alleles in Orientals and Afiricans encoding C YPZD6 with
`decreased activity
`
`Our early studies comparing CYP2D6 activity between
`Swedish and Chinese subjects revealed that the distribu—
`wfiono th:lVI‘Re of C inese ex ensive me a oi ers
`
`Gene duplication, multiduplication and amplification as
`causes of increased CYPZD6 activity
`
`The problems in treating PM of debrisoquine have been
`extensively discussed over the years since the discovery of
`the CYP2D6 polymorphism [14]. Much less attention has
`een given 0 a ien s a
`e o
`er ex reme, i.e. u rara i
`
`Table 1 Frequency of CYP2D6*1 or *2 alleles (causing ‘normal’ enzyme activity) and some alleles causing no or deficient CYPZD6 activity in three
`different ethnic populations.
`
`C YPZD6 alleles
`
`Functional mutation
`
`Consequence
`
`Swedish
`
`Allele frequency (%)
`Chinese
`
`Zimbabwean
`
`*1 or *2
`*3
`
`*4
`*5
`*10
`*17
`
`A2637 del
`
`G1934A
`Gene dclction
`C188T
`C1111T
`
`Frame shift
`
`Splicing defect
`No enzyme
`Unstable enzyme
`Reduced affinity
`
`69
`2
`
`22
`4
`n d
`n d
`
`43
`0
`
`0—1
`6
`51
`nd.
`
`54
`0
`
`2
`4
`6
`34
`
`nd. 2 not determined.
`
`Data are from original publications [23, 25, 27] and reviews [4, 5].
`
`l—f2
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`10
`
`25
`
`20
`
`15
`
`_. O
`
`U‘!
`
`_; U10
`
`01E3
`
`25
`
`20
`
`15
`
`10
`
`Numberofsubjects
`
`wt/wt
`
`mut/wt
`
`mut/mut
`
`Total
`
`Molecular genetics of CYPZD6
`
`duplication/multiduplication of the CYP2l)6*2 gene and
`low debrisoquine MR [34].
`In Swedish Caucasians the frequency of subjects having
`duplicated/inultiduplicated genes
`is about 1—2% [34].
`
`to 3.6% in Germany [35], 7—10% in Spain [36, 37] and
`10% on Sicily in Italy [38]. The frequency is as high as
`29% in black Ethiopians [30] and 20% in Saudi Arabians
`[39]. There is
`thus
`a European—African north—south
`gradient in the incidence of CYPZD6 gene duplication.
`It has been speculated that the high incidence in Spain
`and Italy may have an ancestry in the Arabian conquest
`in the Mediterranean area [36]. Caucasian subjects with
`a CYPZD6 gene duplication have been shown to be
`ultrarapid metabolizers of debrisoquine with MR usually
`between 0.01 and 0.15 [31, 34—36].
`In the study
`of Aklillii
`et al.
`[30], black Ethiopians with multiple
`CYPZD6 genes had higher MR, usually between 0.1
`and 1. These subjects do thus not have the ultrarapid
`metabolism of debrisoquine demonstrated for Caucasians
`with multiple genes. This might be due to environmental
`flaeters in—Africa causrnga decreased
`
`Clinical relevance of the CYPZD6
`
`p0Iy1’I‘l01'pI‘liSl’I‘l
`
`0.1
`
`1
`Metabolic ratio
`
`10
`
`Figure 1 Distribution of the debrisoquine MR (parent drug/
`4—hydroxy metabolite) in three genotype groups related to the
`CYP2D6*10 allele in 152 Korean subjects.
`wt = CYPZD6*1(or*Z) and mut= CYPZD6*10. Reproduced
`with permission from Roh et al. [26].
`
`In 1993, we described a Swedish family
`metabolizers.
`with the father and his daughter and son having 12 extra
`copies of a functional CYP2D6*2 gene in the CYPZD
`locus [31]. These subjects were ultrarapid inetabolizers of
`debrisoquine with MR 0.01—0.02 (Figure 2). In another
`family, duplication of the CYP2l)6*2 gene was also
`associated with extremely high CYPZD6 activity [31].
`This was the first demonstration of an inherited duplica-
`tion/amplification of an active gene encoding a drug
`metabolizing enzyme. We also described two patients,
`who had to be treated with extremely high doses of
`antidepressants [32, 33]. One of the patients is further
`described below. The CYPZD locus of these patients was
`found to contain a duplication of the CYP2D6*2 gene. A
`population study confirmed the association between the
`
`Since the discovery of the CYPZD6 polymorphism almost
`100 drugs have been shown to be substrates for this
`enzyme. Some of these drugs are shown in Table 2. The
`clinical importance of the polymorphism depends on a
`number of factors including whether the parent com-
`pound, metabolite(s) or both are metabolized or formed by
`CYPZD6; whether the parent compound,
`the metab-
`olite(s) or both are active; the potency ofthe active species;
`and the overall contribution of the CYP2D6—dependent
`pathway to the clearance of the drug. Furthermore, the
`therapeutic index of the drug (narrow—broad), possible
`saturation of the CYP2D6—dependent pathway,
`and
`th%ee
`to be considered. Thus, the clinical impact of CYPZD6
`dependent metabolism needs to be carefully investigated
`for each substrate. So far, the majority of in viva data on
`the role of CYPZD6 are from single—dose pharmacokinetic
`The
`studies.
`increasing
`availability of genotyping
`methods has made clinical studies in patients receiving
`therapeutic doses possible. We will here highlight these
`aspects with some examples, mainly from the field of
`psychopharmacology.
`
`Nortriptyline
`
`Nortriptyline was one of the first clinically important
`drugs to be shown to be metabolized by CYPZD6 [3, 40].
`These early studies (prior to the era of genotyping) were
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`
`Numberofindividuals
`
`Debrisoquine/4-OH-debrisoquine
`
`10
`
`100
`
`1000
`
` USedDoses of nortriptyline
`
`500
`
`100-150
`
`‘Normal dose’
`
`10-20 mg
`
`Figure 2 Distribution of the urinary debrisoquine MR in 757 healthy Swedish subjects with schematic presentation of CYPZD6
`genotypes, where a cross in an allele indicates a detrimental mutation. Also tentative doses of nortriptyline to be used in different
`genotypes are indicated. From [64].
`
`Table 2 Some drugs whose metabolism is catalysed by CYPZD6
`
`fl—/ldrenoceptor blockers
`Metoprolol
`Propranolol
`Timolol
`
`Antiarrhythmic drugs
`Encainide
`Flecainide
`
`Perhexilene
`Propafenone
`Sparteine
`
`Antidepressants
`Amitriptyline
`Clomipramine
`Desipramine
`Fluoxetine
`
`Fluvoxamine
`Imipramine
`Mians erin
`
`Nortriptyline
`Paroxetine
`Venlafaxine
`
`Nemoleptics
`Halop eridol
`Perphenazine
`Risp ericlone
`Thioridazine
`
`Zuclopenthixol
`
`Zvliscellaneous
`
`Codeine
`Debrisoquine
`Dextromethorphan
`Phenforniin
`Tolterodine
`Tramadol
`
`performed in phenotyped panels of healthy subjects and
`the results have been confirmed in 1217120 in patients as well
`as in Vitro using human liver microsomes and expressed
`enzymes. In a recent study by Dalen et al.
`[41], nor-
`triptyline was given as a single oral dose to 21 healthy
`Swedish Caucasian subjects with different CYPZD6
`genotypes. As seen in Figure 3, there was a decrease in
`the plasma concentrations of nortriptyline from subjects
`with 0 functional genes (CYP2D6*4/*4 genotype)
`to
`those with 1, 2 and 3 (gene duplication) functional genes.
`
`The plasma concentrations of the parent drug were
`extremely low in one subject with 13 CYP2D6*2
`
`10—hydroXynortriptyline showed the opposite pattern, i.e.
`highest concentrations in the subject with 13 functional
`genes and lowest in the PM (Figure 3). This study clearly
`shows the impact of the detrimental C YP2D6*4 allele as
`well as of the duplication/amplification of the C YP2D6*2
`gene on the metabolism of nortriptyline [41]. A relation-
`ship between the CYPZD6 genotype and steady—state
`plasma concentrations of nortriptyline has
`also been
`
`demonstrated in Swedish depressed patients treated with
`nortriptyline [42].
`[41]
`We used the single dose results of Dalen et Ell.
`(Figure 3)
`to simulate steady—state concentrations of
`nortriptyline in the different genotype groups
`after
`different daily doses of the drug assuming linear kinetics
`(Figure 4). A dose of 25 mg three times daily, which is
`usually recommended as
`a starting dose,
`resulted in
`concentrations near the upper limit of the recommended
`therapeutic interval
`(200—60O nivi)
`in subjects with 0
`(PM) and 1
`(heterozygotes) functional CYPZD6 genes
`(Figure 4, upper curves). Subjects with 2 or more
`functional genes
`fall below 200 nM. At
`the usually
`recommended daily dose of nortriptyline 150 mg, subjects
`with O or 1 functional genes would attain levels above
`
`FI4
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`Nortriptyline
`
`10-hydroxynortriptyline
`
`250
`
`Molecular genetics of CYP2D6
`
`Number of
`functional
`C’Y‘P2D6genes
`
`Number of
`functional
`
`CYP2D6 genes
`
`200
`
`150
`
`100
`
`50
`
`60
`
`50
`
`40
`
`30
`
`20
`
`[9
`
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`5
`5 <1)
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`24
`
`48
`
`72
`
`hydroxynortriptyline (right) in dilferent CYPZD6 genotype groups
`Figure 3 Mean plasma concentrations of nortriptyline (left) and 10-
`after a single oral dose of nortriptyline. The numerals close to the curves represent the number of fianctional CYP2D6 genes in each
`genotype group. In groups with 0-3 fianctional genes, there were fi
`ve subjects in each group while there was only one subject with
`[41]
`
`
`
`Time (h)
`
`600 nM and might therefore be at higher risk of devel-
`oping adverse drug reactions. Subjects with 2 functional
`genes, who constitute about half of Caucasian populations,
`are in the middle of the therapeutic interval (Figure 4,
`middle
`curves).
`Subjects with gene duplication or
`amplification might
`require increased doses of nor-
`triptyline,
`e.g. 75 mg three
`times daily (Figure
`4,
`bottom curves) or higher. One out of 10 patients in
`Italy and Spain, where gene duplication is common, might
`require increased doses of CYP2D6 substrates. It should be
`underscored that the curves presented in Figure 4 are
`simulated from the single dose data of Figure 3 assuming
`linear kinetics. Early studies by Alexanderson [43] showed
`that single dose data of nortriptyline can be used to predict
`
`kinetics of this drug seem to occur in extensive meta-
`bolizers when high doses of nortriptyline saturate the
`capacity for hydroxylation [44, 45].
`It should also be
`remembered that the 10—hydroxy metabolite of nortripty-
`line,
`formed by CYP2D6,
`is pharmacologically active
`although its relative contribution to the clinical effect and
`toxicity of nortriptyline has not been clearly elucidated
`[46]. The contribution of this metabolite is probably
`more important in ultrarapid metabolizers than in other
`patients.
`Using the same protocol as in the study ofDalén er al.
`[41], we investigated the influence ofthe Oriental—speciflc
`CYP2D6*10 allele on the disposition of nortriptyline in
`Chinese subjects living in Sweden [47]. Recently, Morita
`et al. [48] showed the influence of the CYP2D6*10 allele
`
`on the steady—state plasma levels of nortriptyline and its
`l0—hydroxy metabolite in Japanese depressed patients.
`studies
`From these two
`it may be concluded that
`CYP2D6*10 encodes an enzyme with decreased activity
`to metabolize nortriptyline. This effect is less pronounced
`than that of the Caucasian—specific CYP2D6*4 allele,
`which encodes no enzyme at all.
`Genotyping or phenotyping for CYP2D6 may be a
`tool
`to predict proper initial dosing of drugs such as
`nortriptyline in individual patients, especially those with
`extremely low (PM) or high
`CYP2D6 activity. This
`can be demonstrated with our experience with two
`patients for whom the dosage of nortriptyline and other
`antidepressants needed to be individualized (Figure 2)
`[37, 33, 40, R0]
`Patient 1. A ()9 year old woman was hospitalized for
`moderate to severe depression and treated with nortripty—
`line in a modest dose of 25 mg three times daily. Two days
`after the start of treatment she complained of dizziness
`[49]. After a further 6 days of treatment, she complained
`of increasing tiredness and vertigo and appeared slightly
`confused. Low clearance of nortriptyline was suspected,
`blood was
`taken for nortriptyline analysis
`and the
`dosage was decreased to 25 mg once daily. The plasma
`concentration of nortriptyline after 8 days of treatment
`with the 75 mg daily dose was 1300 nM (recommended
`plasma concentration range 200—600 nM). The concentra—
`tion on 25 mg daily for 12 days was 742 nM. When the
`dosage was further reduced to 20 mg at night, the patient
`had no side—effects and made an excellent recovery [49].
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`L. Bertilsson et al.
`
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`is also partly metabolized by CYPZD6, it was not sur-
`prising that this patient also had low plasma concentrations
`of amitriptyline and its metabolite nortriptyline when
`treated with amitriptyline. In 1993, 8 years after we flrst
`9
`
`she had a duplication of the CYPZD6 gene and thus
`3 functional genes,
`leading to increased activity of the
`enzyme [33].
`
`Other antidepressants
`
`CYPZD6 has been shown to catalyse the metabolism ofa
`number of other antidepressants including the serotonin-
`reuptake inhibitors paroxetine, fluvoxamine and fluox—
`etine as well as venlafaxine and mianserin (Table 2). As
`an example, Sindrup et al. [51] found 25-fold differences
`in plasma concentrations of paroxetine between PM and
`EM after a single oral dose of the drug. The inter-
`phenotype difference was, however, only two—fold at
`steady—state, due to paroXetine’s saturable metabolism
`catalysed by CYPZD6 m EM.
`et a-l.
`[52] also
`found two—fold higher median steady—state plasma con-
`centrations of paroxetine in heterozygous EM than in
`homozygous EM subjects, with a considerable overlap in
`the distribution of paroxetine concentrations between the
`two genotypes. Unfortunately, no PM or UM subjects
`were included in that study. The clinical significance ofthe
`CYPZD6 genotype on the steady state plasma levels of
`fluoxetine and fluvoxamine is not known. Venlafaxine
`
`undergoes CYPZD6 dependent metabolism to the active
`major metabolite O—desmethylvenlafaXine, while
`its
`N—desmethylation is catalysed by CYP3A4, and possibly
`by CYPZC19 and CYPZC9 [53, 54]. PM ofCYP2D6 had
`a more than 4-fold lower oral clearance of venlafaxine
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`
`13
`12
`
`Figure 4 SLeady—sLaLe levels of nortriptyline simulated from the
`single dose results of Dalen et al. [41] in different CYPZD6
`genotypes. The shaded area indicates the commonly
`recommended ‘therapeutic’ range of 200—6OO nM.
`
`compared to EM, mainly due to a decreased capacity to
`form the O—desmethylated metabolites [55]. In a study
`Thirty days later, after she had stopped taking nortripty—
`on 33 patients with depression treated with 225 mg
`hP€,Eh€p%EPtW%—ph€P9Wp@dW¢h the
`She had a high metabolic ratio of43 and was thus classified
`CYPZD6
`genotype
`and
`the
`ratio
`O—desmethyl—
`as a PM of debrisoquine.
`venlafaxine/venlafaxine was found, PM having extremely
`Patient 2. A 41 year old woman had been treated
`low ratios and UM high ratios compared with lio111o—
`for
`long periods with high doses
`of nortriptyline
`and heterozygous EM [56]. A significant
`relationship
`(300—5OO mg day_1) to achieve ‘therapeutic’ plasma levels
`between the CYPZD6 genotype (ie.
`the presence of
`(200—()00 nM)
`[32].
`The mean (is.d.) plasma level in
`the C YP2D6*10 allele) and the plasma kinetics of venla—
`seven samples drawn at a dose of 300 mg day_1 was
`faXine and O—desmethylvenlafaXine has also been shown
`in Japanese subjects
`[57]. A relationship between the
`291 i56 nM. The plasma concentration of unconjugated
`10—hydroxynortriptyline was about 10 times higher than
`PM phenotype and cardiovascular toxicity of venlafaxine
`that of the parent drug, which is much higher than
`has been suggested, based on four patients with PM
`usual. A debrisoquine phenotyping test
`confirmed
`phenotype which was either genetically determined or
`that
`this patient
`is an ultrarapid metabolizer of debri—
`due to inhibition of CYPZD6 activity by concomitant
`soquine (and nortriptyline) with a metabolic ratio of
`drugs [55]. The side—effects reported included palpita-
`0.07 [32]. This metabolic ratio was one of the lowest
`tions, shortness of breath and proarrhythmias due to the
`seen in a Swedish population (Figure 2). As amitriptyline
`heterogeneity in cardiac repolarization.
`
`FI6
`
`©2002 Blackwell Science Ltd Br] Clin Pharmacol, 53,
`
`I
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`ROXi|op00148038
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`Vanda Exhibit 2036 - Page 6
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`Vanda Exhibit 2036 - Page 6
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`

`
`Halopei/idol
`
`[58] gave low single oral doses (2—4 mg)
`Llerena et al.
`of lialoperidol
`to panels of six EM and six PM of
`
`than EM, the mean plasma half—life being longer (29.4 and
`16.3 h, respectively; P<0.01) and the mean clearance
`lower (1.16 and 2.49 l h_1 kg_1, respectively; P< 0.05)
`[58]. The plasma level of reduced haloperidol was also
`higher in PM than in EM [59]. In a clinical study involv-
`ing eight Caucasian patients with schizophrenia treated
`with depot haloperidol
`(decanoate),
`the dopamine D2
`receptor occupancy was determined by positron emission
`tomography 1 and 4 weeks after intramuscular injection
`of the drug [60]. One of the patients was genotypically
`a PM of debrisoquine. Of the group, he had the highest
`plasma concentration of haloperidol and also the highest
`D2 receptor occupancy.
`Two studies
`from Japan [61, 62] have shown a
`relationship between increased steady state haloperidol
`plasma concentrations and the presence of CYP2D6*10
`(ane1*5)
`1°mg
`daily oral doses of haloperidol. In a recent study in Korea
`[63],
`a
`relationship between haloperidol and reduced
`haloperidol concentrations and CYPZD6 genotype was
`established in patients receiving less than 20 mg halo-
`peridol daily, but not in patients receiving higher doses.
`We believe that the high aff1nity—low capacity CYPZD6
`plays an important role at low concentrations/ doses of
`haloperidol, while the low afflnity—high capacity CYP3A4
`becomes more important at higher doses. Thus,
`the
`importance of the CYPZD6 genotype depends on the
`dose range used. The metabolic pathway of haloperidol
`catalysed by CYPZD6 is presently not known.
`
`Other neuroleptics
`
`Also many other neuroleptics
`
`such as perphenazine,
`
`shown to be metabolized by CYPZD6 (Table 2) [64]. At
`steady state,
`the median oral clearance of perphenazine
`was tliree—fold lower in PM than in homozygous EM
`of debrisoquine [65]. Similarly,
`there was a two—fold
`difference in the clearance of zuclopenthixol in the two
`genotype groups [65]. That PM reach higher average
`plasma concentrations per dose unit of perphenazine and
`zuclopenthixol than EM has been confirmed by others
`[66, 67]. All these studies have only included patients on
`oral treatment. We have recently demonstrated that the
`difference between PM and EM in steady—state plasma
`levels of zuclopenthixol is also seen during treatment with
`intramuscular zuclopenthixol decanoate [68].
`The metabolism of risperidone to its active metabolite
`9—hydroxyrisperidone is mainly dependent on CYPZD6,
`
`Molecular genetics of CYPZD6
`
`leading to significantly higher dose—corrected steady—state
`plasma levels of the parent drug in PM than in EM [38]
`(Figure 5). However,
`the concentrations of the ‘active
`moiety’, i.e. the su111 of risperidone and 9—liydroxyrisper—
`
`(Figure 5). Thus, as the parent compound and metabolite
`are considered pharmacologically equipotent,
`the poly-
`morphic metabolism is in this case not expected to be of
`clinical significance. It
`is to be pointed out that in all
`the published studies,
`there was a large overlap in the
`steady state plasma concentrations of neuroleptics between
`the different genotype groups,
`indicating that other
`factors in addition to the CYPZD6 genotype are of
`major importance for the interindividual variability in
`pharmacokinetics.
`
`T/4empezm'c elfkcts/side-elfects and C YPZD6 genotype
`
`Although the pharmacokinetic consequences of poly-
`morphic metabolism are relatively well documented for
`a number of CYPZD6 substrates, its clinical impact with
`
`and mainly based on case reports (see above). The clearest
`example of clinical significance relates to the antianginal
`drug perhexilene. Patients with perhexilene—induced
`peripheral neuropathy achieved higher blood concentra-
`tions than patients without this adverse effect [69]. In a
`retrospective study it was shown that of 20 patients
`developing peripheral neuropathy while taking this drug,
`11 were PM (55%) and ofthe 9 EM, 6 had a debrisoquine
`MR higher than 1
`[70]. The polymorphism apparently
`contributed to the withdrawal of perhexilene from the
`market in many countries in 1980s, but in patients with
`‘normal’ debrisoquine metabolism, other factors probably
`contributed to the development of the neuropathy.
`A number of studies have explored the relationship
`between adverse
`effects of psychotropic drugs
`and
`CYPZD6 genotype. For example, Chen 8: coworkers
`
`[71]féHflda 6f‘ "
`CYPZD6 alleles
`in depressed patients with adverse
`reactions compared with those without such reactions.
`The possible association between the CYPZD6 genotype
`and neuroleptic—induced movement disorders has been
`addressed in several studies [72—79] (Table 3). The results
`have been inconsistent but tend to show a slight over-
`representation of mutated C YPZD6 alleles in patients with
`tardive dyskinesia and parkinsonism during neuroleptic
`treatment. Possible explanations for the partly inconsistent
`or negative results are that in some studies the patients
`were taking various neuroleptics including those not
`metabolized by CYPZD6, or on polytherapy with drugs
`that are potent inhibitors of CYPZD6. In a recent 1 year
`follow—up pilot study of 100 consecutive psychiatric
`in—patients genotyped for CYPZD6 on
`admission,
`
`@2002 Blackwell Science Ltd Br] Clin Pharmacol. 53.
`
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`Vanda Exhibit 2036 - Page 7
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`

`
`L. Bertilsson et al.
`
`25
`
`a
`
`80
`
`b
`
`A
`
`‘:37
`E
`L
`6
`E
`E
`@
`g
`E
`-§
`D:
`
`20
`
`15
`
`10
`
`°
`
`8
`
`9
`
`o
`
`80
`§o
`
`0
`
`3
`
`5
`
`O
`
`8
`o
`8
`:2
`
`oooogoo
`8
`80
`EM
`EM
`UM
`PM
`homozygous heterozygous
`
`:‘~ 70
`
`E”
`60
`_
`l:
`50
`2
`5
`Q 40
`L;
`E;
`E
`20
`%:
`< 10
`O
`
`30
`
`°
`
`°
`O
`
`o
`
`O
`O
`
`O
`
`0
`028
`000
`°
`0
`O
`
`EM
`UM
`EM
`PM
`homozygous heterozygous
`
`0
`0
`
`o
`
`o
`0
`o
`880
`0
`80
`
`00
`
`Figure 5 Relationship between the CYPZD6 genotype and the plasma concentration—to—dose (C/D) ratios of risperidone (left) and the
`active moiety of risperidone (risperidone plus 9—hydroxyrisperidone, right). UM=ultrarapid metabolizers, defined as a carrier of gene
`duplication; EM homozygous=extensive metabolizers homozygous for the functional CYP2D6*1 allele; EM heterozygous=EM
`heterozygous for a defective CYPZD6 gene; PM=poor metabolizers carrying two defect genes. Reproduced with permission fi:om
`Scordo at al. [38].
`
`Table 3 Studies on the relationship between CYPZD6 genotype
`and neuroleptic—induced movement disorders.
`
`Study
`
`Side—efi’ect
`
`[72]
`Arthur et alt
`Arnistrong at alt [73]
`
`Andreasson et alt [74]
`
`Kapitany et alt [75]
`Ohmori et alt [76]
`Vandel et alt [77]
`
`[ 8]
`Flameiin et alt
`Scordo et alt
`[79]
`
`Tardive dyskinesia
`Acute dystonia
`Q]-_-H;9¥H’ mox m ne-
`
`disorders
`Tardive dyskinesia
`
`Parkinsonism
`Akathisia
`
`Tardive dyskinesia
`Tardive dyskinesia
`Extrapyramidal side—
`effects
`
`Not defined
`Extrapyramidal side—
`effects
`
`Antiarrhythmic drugs have a narrow therapeutic index
`and many of them (flecainide, encainide, propafenone,
`mexiletine, N—propyl—ajmaline) are CYPZD6 substrates.
`PM might thus be expected to have an increased risk of
`concentration—dependent adverse effects. Encainide is an
`exception as
`its O—desmetyl metabolite,
`the formation
`of which is catalysed by CYPZD6,
`is 6—1O ti111es 111ore
`N9%%
`tendency)
`[81]. Although PM treated with encainide develop less
`No (non—sign
`QRS and QT prolongation than EM, both phenotype
`tendency)
`groups
`showed a
`similar antiarrhythmic response at
`comparable doses [82]. An increased frequency of fatal
`nervous system adverse effects has been reported in PM
`during propafenone treatment
`[83].
`It has also been
`suggested that determination of
`the phenotype
`(or
`genotype) of patients before therapy with N—propyl—
`ajmaline might be useful to select the individual optimum
`dose of the drug [84]. No prospective studies have,
`however, tested this hypothesis.
`
`Relationship with
`CYPZD6 genotype
`
`No
`No
`
`No
`No
`
`Yes
`No
`Yes
`
`No
`No
`
`[80] found a trend towards greater number
`Chou et al.
`of adverse drug effects from drugs primarily metabolized
`by CYPZD6, when one moves from UM to PM. The
`costs of treating the patients with extremes of CYPZD6
`activity (i.e. PM and UM) was on average 4000—6000
`dollars greater per year than the costs of treating patients
`with the EM or intermediate metabolizer (heterozygous
`EM) genotypes. The total duration of hospital stay was
`also longer for patients in the PM group. This pilot study
`needs a large clinical
`trial to confirm these preliminary
`and interesting issues.
`
`Future perspectives
`
`The pronounced interindividual variation in the rate of
`drug metabolism has been known for many years. It was
`initially only of academic
`interest, but
`today the
`pharmaceutical industry has to document the metabolism
`of a new drug in development before registration. The
`knowledge of how a drug is metabolized and which
`enzymes are involved helps to predict drug—drug inter-
`actions and how fast an individual patient may metab-
`olize a specific drug.
`
`TF8
`
`©2002 Blackwell Science Ltd Br] Clin Pharmacol, 53,
`
`I
`
`I
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`|—|22
`
`ROXi|op00148040
`
`Vanda Exhibit 2036 - Page 8
`
`Vanda Exhibit 2036 - Page 8
`
`

`
`Molecular genetics of CYPZD6
`
`laboratory have been
`The studies performed in the authors’
`supported by the Swedish Medical Research Council
`(3902),
`National
`Institutes of Health, USA (GM 60548-01A2)
`and
`Karolinska Institutet.
`
`or therapeutic failure. Further studies are required to
`With the high—throughput screening techniques avail-
`establis