CYP2D6 increases toxicity of the designer drug 4-methylthioamphetamine (4-MTA)

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Toxicology 229 (2007) 236–244

CYP2D6 increases toxicity of the designer drug4-methylthioamphetamine (4-MTA)

Helena Carmo a,∗, Marc Brulport b, Matthias Hermes b, Franz Oesch c,Douwe de Boer d, Fernando Remiao a, Felix Carvalho a,

Michael R. Schon e, Niels Krebsfaenger f, Johannes Doehmer g,Maria de Lourdes Bastos a, Jan G. Hengstler b

a REQUIMTE, Toxicology Department, Faculty of Pharmacy, University of Porto, Rua Anıbal Cunha 164,4099-030 Porto, Portugal

b Centre for Toxicology, University of Leipzig, Hartelstrasse, 16-18, 04107 Leipzig, Germanyc Institute of Toxicology, University of Mainz, Germany

d Department of Clinical Chemistry, University Hospital Maastricht, The Netherlandse Department of Surgery, University of Leipzig, Germany

f Schwarz Biosciences GmbH, Department of Pharmacology and Toxicology, Monheim, Germanyg GenPharmTox BioTech AG, D-Planegg/Martinsried, Germany

Received 1 September 2006; received in revised form 25 October 2006; accepted 27 October 2006Available online 5 December 2006

Abstract

4-Methylthioamphetamine (4-MTA) belongs to a group of new amphetamine derivatives that is usually sold as “ecstasy” or “flatlin-ers” on the illicit drug market. Large interindividual differences in 4-MTA mediated toxicity have been reported in humans. Therefore,we tested whether CYP2D6 or its variant alleles as well as CYP3A4 influence the susceptibility to 4-MTA. For this purpose, we usedthe colony formation assay with Chinese hamster lung fibroblast V79 cells expressing human wild-type CYP2D6 (CYP2D6*1), thelow activity alleles CYP2D6*2, CYP2D6*9, as well as human CYP3A4. The obtained results showed that the expression of wild type

CYP2D6*1 clearly enhanced the susceptibility to the cytotoxic effects of 4-MTA compared with the parental cells devoid of CYP-dependent enzymatic activity. Toxicity in V79 CYP2D6*1 was also higher compared to the V79 cell lines expressing the low activityalleles CYP2D6*2 and CYP2D6*9. In contrast to CYP2D6, the CYP3A4 isoenzyme did not enhance 4-MTA toxicity. In conclusion,our results suggest that CYP2D6 rapid metabolizers may be more susceptible to 4-MTA toxicity than CYP2D6 poor metabolizers.© 2006 Elsevier Ireland Ltd. All rights reserved.

Hepato

Keywords: 4-Methylthioamphetamine; Designer drugs; Metabolism;

1. Introduction

4-Methylthioamphetamine (4-MTA) belongs toa group of new amphetamine derivatives that have

∗ Corresponding author. Tel.: +351 222078979;fax: +351 222003977.

E-mail address: [email protected] (H. Carmo).

0300-483X/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reservdoi:10.1016/j.tox.2006.10.024

toxicity; CYP2D6; Polymorphism

emerged on the illicit drug market for recreational drugswhere it is usually sold as tablets marketed as “ecstasy”or “flatliners”. Since its first identification in 1997 therehave been at least six cases of death associated with

4-MTA in the UK and in the Netherlands (EMCDDA,1999). In addition, one fatal and seven nonfatal intox-ications in Belgium have been reported (de Letter etal., 2001). Therefore, 4-MTA was placed in Schedule I

ed.

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f the 1971 Convention (WHO, 2001) and is submittedo control measures and criminal penalties withinhe member States of the European Union (Europeanommunities, 1999; EMCDDA, 1999).

4-MTA acts in a similar way to other amphetamineerivatives, such as p-methoxyamphetamine and 3,4-ethylenedioxymethamphetamine (MDMA, “ecstasy”)

Dukat et al., 2002; Khorana et al., 2004). It is aery selective serotonergic agent, increasing the releasef serotonin, inhibiting serotonin uptake from nervendings (Huang et al., 1992), with low affinity for nora-renaline and dopamine uptake sites and monoamineeceptors (Huang et al., 1992; Li et al., 1996), and alsonhibiting monoamine oxidase-A activity (Li et al., 1996;corza et al., 1997).

Particular concern regarding the toxicity of 4-MTAas been raised since its slow onset of action hasncouraged abusers to administer further doses assum-ng that the first dose was inadequate (EMCDDA,999; Kavanagh et al., 1999). In the reported 4-MTA-elated intoxications in humans, symptoms typical forhe sympathomimetic effects of amphetamine-like com-ounds were observed, such as tachycardia, tremors,tomach cramps, headache, and sweating (de Boer etl., 1999; de Letter et al., 2001; Elliott, 2000). Infor-ation on the toxicity of 4-MTA is still scarce. The

bility of 4-MTA to induce a hyperserotonergic statenown as the serotonin syndrome (which is character-zed by confusion, fever, shivering, diaphoresis, ataxia,yperreflexia, myoclonus, or diarrhea) (Sporer, 1995)as observed in neurotoxicity studies performed with

ats (Huang et al., 1992). Although initially consid-red as a nonneurotoxic drug, in vitro studies showedhat 4-MTA induced neurotoxicity in rat hypothalamicultured cells (Hurtado-Guzman et al., 2002). Addition-lly, in vivo studies in mice have shown that 4-MTAnduces hyperthermia, another potentially lethal con-equence of amphetamine intoxications (Carmo et al.,003).

The interindividual variation in the propensity to thedverse effects of amphetamine designer drugs has beenncreasingly noticed (de Letter et al., 2004; Henry et al.,992; O’Donohoe et al., 1998). Several studies suggesthat interindividual differences in metabolism may beesponsible for these differences (Capela et al., 2006;arvalho et al., 2002; Carvalho et al., 2004a,b,c; Eastont al., 2003; Escobedo et al., 2005; Forsling et al., 2002;ollamudi et al., 1989; Hartung et al., 2002; Jones et al.,

005).

The metabolic pathways proposed for 4-MTA inumans are represented in Fig. 1 and include: (i) oxida-ive deamination into a ketone metabolite (deamino-oxo

229 (2007) 236–244 237

4-MTA; 1-[4-(methylthio)-phenyl]propan-2-one) thatcan be further reduced to the corresponding alcohol(deamino-hydroxy 4-MTA; 1-[4-(methylthio)-phenyl]propan-2-ol) or suffer degradation of the side chaininto the 4-methylthiobenzoic acid metabolite; (ii) ringhydroxylation to a phenolic structure [(2-aminopropyl)-(methylthio)phenol]; (iii) �-hydroxylation of the sidechain to 4-methylthionorephedrine (2-amino-1-[4-(methylthio)-phenyl]propan-1-ol) (Ewald et al., 2005).The same metabolic pathways were observed in vivo inmice (Carmo et al., 2002) and in vitro in primary hepa-tocytes from man, monkey, dog, rabbit, rat, and mouse(Carmo et al., 2004).

The enzymes involved in the metabolism of 4-MTAare not yet known but it is anticipated that the cytochromeP450 isoenzymes (CYP) play a critical role in thesemetabolic reactions. Of major importance is CYP2D6that catalyses the ring hydroxylation, demethylenation,and demethoxylation of amphetamines and derivatives(Bach et al., 1999).

CYP2D6 activity shows a very high degree ofinterindividual variability, which is primarily due togenetic polymorphism that influences both enzymeexpression and function (Zanger et al., 2004). TheCYP2D6 gene is highly polymorphic, with 58 alleles andmore than 100 distinct variants listed so far at the CYPal-lele nomenclature homepage http://www.imm.ki.se/CYPalleles/. There is a marked interethnic variabilityin the alleles frequencies among populations (Bertilssonet al., 2002). CYP2D6 protein and enzymatic activityis completely absent in less than 1% of Asians and upto 10% of Caucasians as a result of the expression ofnull alleles that encode a nonfunctional protein prod-uct. These individuals are termed poor metabolizers(Zanger et al., 2004). Among the extensive metabo-lizers (EM) the enzyme activity is highly variable andcan be markedly reduced in intermediate metaboliz-ers (IM; heterozygous for a normal and a deficientallele). The frequency of the IM phenotype was esti-mated to be around 10–15% of the European population(Zanger et al., 2004). At the other extreme, the ultrarapidmetabolizers (UM) phenotype can be caused by alle-les carrying multiple gene copies (Ingelman-Sundberg,2005). The frequencies of these alleles are markedly highin Ethiopian and Saudi Arabians populations, and up to5% of the Caucasians present this phenotype (Zanger etal., 2004).

The aim of the present study was to determine if

the CYP2D6 genotype influences 4-MTA cytotoxicity.We therefore tested 4-MTA cytotoxicity in a batteryof genetically engineered V79 cell lines expressing theCYP2D6 allelic variants *1 (wild type), *2, *9 (reduced

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Fig. 1. Proposed metabolic pathways for the phase I metabolism of 4-MTA in humans. These metabolic pathways include �-hydroxylation ofthe side chain into 4-methylthionorephedrine (I), ring hydroxylation into a phenolic metabolite (2-aminopropyl)-(methylthio)phenol (II), oxida-

]propane chain

tive deamination into a ketone metabolite 1-[4-(methylthio)phenyl1-[4-(methylthio)-phenyl]propan-2-ol (IIIa), or degradation of the sid

activity alleles), and in a V79 cell line expressingCYP3A4.

2. Materials and methods

2.1. Materials

Reagents for cell culture were obtained from CambrexBio Science (Walkersville, MD, USA): Dulbecco’s modifiedEagle’s medium (4.5 g/L glucose, with l-glutamine, withoutsodium pyruvate; DMEM), phosphate-buffered saline (PBS),trypsin (2.5%), penicillin (10,000 U/mL)-streptomycin(10 mg/mL) mixture, and foetal bovine serum (FBS). 4-MTA

(HCl salt) was synthesised by REQUIMTE/ToxicologyDepartment, Faculty of Pharmacy, University of Porto. Allother chemicals were purchased from Sigma-Aldrich (St.Louis, Mo, USA) and were of the highest grade commerciallyavailable.

-2-one (III) followed by reduction into the corresponding alcoholinto 4-methylthiobenzoic acid (IIIb).

2.2. Cell culture

Chinese hamster lung fibroblast V79 cell lines wereobtained from Prof. Johannes Doehmer and Dr. Niels Kreb-sfaenger, GenPharmTox BioTech AG, Martinsried, and werepreviously characterised for stable expression, activity, andCYP content (Krebsfaenger et al., 2003). A total number of sixcell lines were tested. Two of these cell lines served as controls.The parental cell line consisted of nontransfected cells. Anadditional control was made by using a mock-transfected cellline that carries the selective marker for neomycin resistance(mockneo). Both control cell lines have no CYP-dependentenzymatic activity. The transfected cell lines were desig-

nated according to the corresponding cDNA: CYP2D6*1,CYP2D6*2, CYP2D6*9, and CYP3A4.

The cells were cultured in 75 cm2 flasks and the cellcultures were kept at 37 ◦C in a humidified atmosphere con-taining 5% CO2/95% air. The cell culture medium was DMEM

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upplemented with 10% FBS and penicillin/streptomycin100 U/100 �g/mL). The cell cultures were routinely passagedvery 48 h before reaching confluence. All the assays wereerformed on at least three separate occasions, during differ-nt culture passages, and each test substance concentration wasested in triplicate in each experiment.

.3. Colony formation assay

All cell lines were assayed before the cultures reached 100%onfluence. A cellular suspension was obtained by adding a.25% trypsin/1 mM EDTA solution to the cell culture flasksollowed by 2 min incubation at 37 ◦C and suspension of theells in cell culture medium. The cells were counted, given ontoulture dishes at a density of 200 cells/dish/9 mL DMEM andllowed to attach for 4 h at 37 ◦C. The solutions of 4-MTA werereshly prepared in PBS and were sterile filtered. For each con-entration tested (125–1000 �M), a 10 × concentrated solutionas prepared and 1 mL of this solution was added to the cell

ulture dishes 4 h after plating the V79 cells. For the controlncubations, 1 mL of PBS was added (solvent control). After4 h of exposure to the test substance, the cell culture mediumas removed and replaced with 10 mL of fresh DMEM and the

ells were kept in culture for another 7 days. Finally, the cellolonies were fixed with an ice-cold mixture of ethanol/acetone1:1), stained with a 10% aqueous solution of Giemsa blue, andhe number of colonies was counted. The number of coloniesxpressed as a percent of the corresponding value of the solventontrol incubations was used as a measure of cytotoxicity.

.4. Statistical analysis

Results are presented as mean ± S.E.M. Data were obtainedrom at least three representative experiments performed inde-endently. The means were compared by one-way analysisf variance (ANOVA) followed by the Bonferroni’s multipleomparison post hoc test, using GraphPad Prism (Graph-ad Software Inc., San Diego, CA, USA). Significance wasccepted at a p-value of less than 0.05. Details of the statisticalnalyses are described in each figure legend.

. Results

Toxicity of 4-MTA was tested in V79 cells trans-ected with CYP2D6*1, CYP2D6*2, CYP2D6*9, andYP3A4 at concentrations of 125–1000 �M. Interest-

ngly, clear differences in susceptibility were observedetween these cell lines. Cytotoxicity was increased inhe V79 cell line transfected with the human wild typeYP2D6*1 enzyme (Fig 2C) compared to the parentalell line (V79 parental; Fig. 2A) and to V79 cells trans-

ected with a neomycin resistance gene (V79 mockneo;ig. 2B). In V79 cells expressing CYP2D6*1, a signifi-ant decrease in viability compared to the solvent controlncubation was already observed at the lowest tested con-

229 (2007) 236–244 239

centration of 125 �M 4-MTA with a 17% reduction ofthe number of colonies formed when compared to thesolvent control incubations without 4-MTA. After incu-bation with 250 �M 4-MTA CYP2D6*1 caused a 21%decrease in colony formation and at 500 �M 4-MTA thedecrease was of 80% (Fig. 2C). In contrast, 250 �M 4-MTA did not cause a significant decrease in V79 parentaland V79 mockneo colony formation.

When comparing the cytotoxicity of MTA inV79 cells transfected with CYP2D6*1, CYP2D6*2,CYP2D6*9, and CYP3A4 to the parental cell line (V79parental) and to V79 cells transfected with a neomycinresistance gene (V79 mockneo) after incubation with500 �M 4-MTA, only the V79 cell line transfected withCYP2D6*1 caused a decrease in colony formation thatwas significantly higher compared to the control celllines (47% for parental cells and 38% for mockneocells versus 80% for CYP2D6*1) (Fig. 3). CYP2D6*2and CYP2D6*9 increased susceptibility to 4-MTAwith a decrease in colony formation of 60% and 70%,respectively, but this increase was not statisticallydifferent from the control cell lines (Fig. 3). In contrastto CYP2D6, CYP3A4 did not significantly alter 4-MTAtoxicity compared to V79 parental and V79 mockneo(Fig. 3). Generally, the highest 4-MTA toxicity wasobserved in V79 cells expressing CYP2D6 wild type(CYP2D6*1).

4. Discussion

Evidence of pharmacokinetic changes in amphe-tamine derivatives due to pharmacogenetic-related defi-ciency in CYP2D6 has been given both by in vitro(Ramamoorthy et al., 2001; Ramamoorthy et al., 2002;Tucker et al., 1994) and very recently by in vivo stud-ies in humans (de la Torre et al., 2005). However, theconsequences of these pharmacokinetic changes on thetoxicity induced by these drugs are still to be determined.

In the present study we compared the cytotoxicityof 4-MTA in V79 cells expressing wild-type CYP2D6(CYP2D6*1) or the low activity alleles CYP2D6*2 andCYP2D6*9. Our results show that the expression of wildtype CYP2D6*1 greatly enhances the susceptibility tothe cytotoxic effects of 4-MTA compared to the parentalcells devoid of CYP-dependent enzymatic activity. Tox-icity in V79 CYP2D6*1 was also higher compared tothe V79 cell lines expressing the low activity allelesCYP2D6*2 and CYP2D6*9. In contrast to CYP2D6,

the CYP3A4 isoenzyme did not enhance 4-MTAtoxicity.

The V79-derived cell lines provide an in vitro systemwith a high predictive value for humans in the testing of

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Fig. 2. 4-MTA cytotoxicity towards the V79 cell lines expressing different variants of CYP2D6 [parental; mockneo; CYP2D6*1 (h2D61); CYP2D6*2(h2D62); CYP2D6*9 (h2D69); and CYP3A4 (h3A4)]. Toxicity was determined by the colony formation assay (A–F). All cell lines were exposed to0.125, 0.25, 0.5, and 1 mM 4-MTA for 24 h under standard cell culture conditions. The number of colonies expressed as percent of the corresponding

ytotoxice-waylvent co

value of the solvent control incubations was used as a measure of cfrom three independent experiments. The means were compared by oncomparison post hoc test. **p < 0.01 and ***p < 0.001 compared to so

CYP2D6 polymorphisms. These cells have been exten-

sively characterised for stable expression of mRNA, forenzyme activity using the model substrate bufuralol, andfor their CYP content (Krebsfaenger et al., 2003). Theywere found to express activities close to those observed

ity (% solvent control). Data are mean ± S.E.M. and were obtainedanalysis of variance (ANOVA) followed by the Bonferroni’s multiplentrol.

in native tissue. The measurement of the CYP content

showed some variation which could also account forthe enzyme kinetic differences between the cell lines.However, this cell model was fully validated with thehydroxylation of the CYP2D6 model substrate bufuralol

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Fig. 3. Each of the V79 cell lines transfected with the variant CYPenzymes [CYP2D6*1 (h2D61); CYP2D6*2 (h2D62); CYP2D6*9(h2D69); and CYP3A4 (h3A4)] were compared to the parental andmockneo control cell lines after incubation with 500 �M 4-MTA. Thenumber of colonies expressed as percent of the corresponding valueof the solvent control incubations was used as a measure of cytotox-icity (% solvent control). Data are mean ± S.E.M. and were obtainedfomc

a(

aneMash2dm4edtIm(12te(d

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rom three independent experiments. The means were compared byne-way analysis of variance (ANOVA) followed by the Bonferroni’sultiple comparison post hoc test. **p < 0.01 compared to mockneo

ontrol cell line. &p < 0.05 compared to parental control cell line.

nd also with the selective estrogen receptor tamoxifenKrebsfaenger et al., 2003).

Interindividual differences in susceptibility tomphetamine-related designer drugs toxicity have beenoticed (Carmo et al., 2005; Colado et al., 1995; de Lettert al., 2004; Henry et al., 1992; Jones and Simpson, 1999;alpass et al., 1999; O’Donohoe et al., 1998; Tucker et

l., 1994). In a previous in vitro study large differences inusceptibility to 4-MTA cytotoxicity were found usingepatocytes from three human donors (Carmo et al.,004). This raised the hypothesis that the interindividualifferences in metabolic capacity of these hepatocytesight be responsible for the observed differences in

-MTA cytotoxicity. It has been increasingly acknowl-dged that the toxic effects of the amphetamine designerrugs are not only related to high plasma levels ofhe drug (Greene et al., 2003) but also to metabolism.n fact, several studies have implicated the oxidativeetabolism catalysed by CYP2D6 on the neurotoxic

Capela et al., 2006; Easton et al., 2003; Gollamudi et al.,989; Jones et al., 2005), hepatotoxic (Carvalho et al.,004a,b), cardiotoxic (Carvalho et al., 2004c), nephro-oxic (Carvalho et al., 2002), hyponatraemia (Forslingt al., 2002; Hartung et al., 2002), and hyperthermicEscobedo et al., 2005) effects associated with theserugs.

CYP2D6 shows a very high degree of interindivid-al variability, which is primarily due to the extensiveenetic polymorphism that influences both enzymexpression and function (Zanger et al., 2004). Concern

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has been raised that individuals who lack functionalCYP2D6 alleles may be at risk of acute amphetamineintoxications, such as increased heart rate and hyper-tension due to the higher plasma levels of the drugs.Additionally, the combination of these drugs withCYP2D6 inhibitors may give rise to pharmacokineticinteractions of toxicological relevance. Such is thecase of dextromethorphan which is sometimes foundin “ecstasy” pills (Baggott et al., 2000), selective sero-tonin reuptake inhibitors (SSRIs) such as fluoxetine andparoxetine (Segura et al., 2005), MAO inhibitors such asmoclobemide (Smilkstein et al., 1987; Vuori et al., 2003),and ritonavir that was implicated in a fatal intoxicationwith MDMA (Henry and Hill, 1998).

The hypothesis that poor metabolizers may be atincreased risk of acute intoxication comes from theassumption that the severity of the toxic effects correlateswith the serum concentrations of the drugs (Greene et al.,2003). However, this is not corroborated by several stud-ies that have shown that there is a lack of dose-responserelationship after amphetamine administration (Asgharet al., 2003; Farre et al., 2004; Mas et al., 1999). Thiscould explain why, at least in some individuals, toxicreactions may be unrelated to the amount taken, indicat-ing that other factors beyond plasma concentration playa role in the induction of acute toxic effects.

The concern regarding the influence of the differentCYP2D6 genotypes on the susceptibility to the toxiceffects of the amphetamines prompted the investigationof a possible association between the poor metabolizergenotype for CYP2D6 and the occurrence of acute intox-ication. Such an association was not found in any of thestudies reported so far thus indicating that the metabolicimpairment is not responsible for higher susceptibilityto toxicity (Gilhooly and Daly, 2002; O’Donohoe et al.,1998; Schwab et al., 1999). In the light of our findingswe hypothesise that individuals with higher metaboliccapacity for CYP2D6 may in fact be at a higher risk ofintoxication when compared to the poor metabolizers. Itwould be of great interest to investigate whether the UMphenotype (up to 5% of the Caucasians) characterized bymultiple CYP2D6 gene copies is over represented in thecases of 4-MTA and other amphetamine designer drug-related intoxications. According to our results this maybe more relevant than the genotyping of the most com-mon deficient alleles that has already been performed inprevious studies (Gilhooly and Daly, 2002; O’Donohoeet al., 1998; Schwab et al., 1999).

Recently, we have shown that toxicity of 3,4-methy-lenedioxymethamphetamine (MDMA) was clearlyincreased in cells expressing CYP2D6*1 compared tothe parental cells devoid of CYP-dependent enzymatic

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activity (Carmo et al., 2006). Formation of the highlytoxic MDMA metabolite N-methyl-�-methyldopamine(N-Me-�-MeDA) by CYP2D6*1 was responsible forincreased toxicity. Whether a similar CYP2D6 depen-dent metabolic pathway also contributes to 4-MTAtoxicity still has to be determined.

The concentrations of 4-MTA used in the presentstudy are in the range of concentrations used in sev-eral in vitro studies (Capela et al., 2006; Carvalho etal., 2004b,c; Simantov and Tauber, 1997; Stumm et al.,1999). They are higher than blood concentrations foundin human abusers. In the reported fatal intoxications with4-MTA interindividual variations in blood levels couldbe observed (between 7.7 and 29.6 �M) (Decaesteckeret al., 2001; Elliott, 2000; EMCDDA, 1999; Poortmanand Lock, 1999). However, the tissue concentrationscan be substantially higher. Post-mortem liver andbrain concentrations of 170 and 201.6 �M, respec-tively, were reported in a fatal intoxication with 4-MTA(Decaestecker et al., 2001). Moreover, autopsy findingsin other amphetamine-related intoxications have shownthat the tissue levels of the drug in the liver can be upto 18 times higher than blood concentrations (de Letteret al., 2006) and 30 times higher in the brain (Garcia-Repetto et al., 2003). It should also be noted that theseconcentrations found at autopsy are probably lower thanthe peak concentrations that are expected to occur afterdrug intake, especially in the cases where the victimsare submitted to emergency-care treatments to controlthe intoxications.

In conclusion, we have shown that CYP2D6*1 medi-ates higher levels of 4-MTA toxicity than CYP2D6*2and CYP2D6*9 as well as CYP3A4. The data presentedin this study provide evidence that the pharmacoki-netic differences in amphetamine metabolism due to thegenetic polymorphism of CYP2D6 are of major impor-tance for the expression of the cytotoxic effects of thesedrugs.

Acknowledgments

This work was supported by Fundacao Para a Cienciae a Tecnologia (POCI/SAU-FCF/57187/2004), byREQUIMTE Associated Laboratory, by a CRUP/DAADgrant (reference A-5/04) and by the German FederalMinistry of Education and Research (BMBF), fundingpriority HepatoSys, Systems Biology of the Hepatocyte.

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