Nicotine is known to be metabolised to its major metabolite cotinine by members of the cytochrome P450 monooxygenase superfamily. Although CYP2A6 has now been identified as the principal enzyme which catalyses this biotransformation, CYP2D6 is also an active nicotine C-oxidase. Some 8% of the Caucasian population have reduced or absent CYP2A6 activity; CYP2D6 may play a significant role in nicotine metabolism in these individuals. CYP2D6 is highly polymorphic - a number of studies linking CYP2D6 genotype to smoking behaviour have now been published. CYP2D6 may have an important constitutive function in neurotransmitter metabolism and CYP2D6 genotype is thought to be a critical determinant in the success of antidepressant drug treatment.
The target article below was today published in PSYCOLOQUY, a refereed journal of Open Peer Commentary sponsored by the American Psychological Association. Qualified professional biobehavioral, neural or cognitive scientists are hereby invited to submit Open Peer Commentary on it. Please email or consult the websites below for Instructions if you are not familiar with format or acceptance criteria for PSYCOLOQUY commentaries (all submissions are refereed).
To submit articles and commentaries or to seek information:
EMAIL: firstname.lastname@example.org URL: http://www.princeton.edu/~harnad/psyc.html http://www.cogsci.soton.ac.uk/psyc
INTRODUCTION FOR NICOTINE ADDICTION WORKSHOP BY MARCUS MUNAFO AND MIKE MURPHY: The ICRF General Practice Research Group in Oxford recently hosted a workshop on the theme of "Neuroscience, molecular genetics and nicotine addiction". It was chaired by Professor Neal Benowitz from the University of California at San Francisco, a specialist in the field of nicotine pharmacology spending an invited week in the University to work with the Group, and included about 30 other experts from groups around the UK and Sweden, including the ICRF Molecular Pharmacology Unit in Dundee and the ICRF Health Behaviour unit in London. Their backgrounds included neuroscience, molecular genetics, genetic epidemiology, pharmacology, psychology and clinical medicine, but they were united in a belief about the importance of tobacco control.
It is now recognised that addiction to nicotine lies at the heart of the tobacco control problem. Without the marketing of tobacco its use would dwindle. But while it is so widely promoted smoking research focuses heavily on factors affecting why people continue smoking after trying it and how to get them to stop with psychological support and the use of effective drugs. Although tobacco use is rightly viewed as a social and psychological problem we also need to see it as a pharmacological one with a potential genetic contribution to the addictive process. The need to integrate knowledge about the brain chemistry underlying addiction, the molecular genetics of these cellular processes, and how the drugs which are effective work, was the workshop's subject.
Each of the meeting's 4 sessions kicked off with a brief introduction by speakers who had precirculated draft papers. The sessions considered in turn: (1)how nicotine is metabolised and cleared from the body (2)the brain receptors in which nicotine "docks" to exert its effects (3)the brain pathways involving the neurotransmitter,dopamine,which are thought to contribute to the rewarding properties of nicotine which reinforce a smoker's desire to smoke (4)other non-dopamine pathways that are activated by nicotine and may underly such effects as nicotine withdrawal symptoms.
Understanding how the drugs which are effective (Nicotine Replacement Therapy and some antidepressants) interact with the expression of individual genotypes in the brain and elsewhere to predict success in quitting smoking will help to shed light on which bits of the brain are important in nicotine addiction at both the anatomical and cellular level. The workshop participants emerged pleased to have caught up with advances in fields which were not their speciality and with a strong sense that more enduring closer contact and cooperation will be possible in the future.
1. Nicotine, the major addictive component of tobacco, is a tertiary amine which binds nicotinic cholinergic and acetylcholine receptors in the brain. Nicotine binding leads to the release of a variety of neuro- transmitters including acetylcholine, noradrenaline, dopamine and serotonin (Benowitz 1996 and references therein).
2. The major metabolite (70-80%) of nicotine is cotinine (Figure 1), the formation of which is catalysed by a cytochrome P450 enzyme, CYP2A6 (Messina et al. 1997). Although CYP2A6 is now thought to be the principal nicotine C-oxidase, certain other P450 isozymes including CYP2B6 and CYP2D6 have also shown to be active in catalysing this reaction in recombinant systems (McCracken et al. 1992; Yamazaki et al. 1999, Table 1).
FIGURE 1: CYP2A6 catalyses the rate-limiting step in nicotine metabolism
TABLE 1: Relative nicotine C-oxidase activities of recombinant human P450s expressed in baculovirus (Yamazaki et al. 1999)
P450 Km Vmax
CYP2A6 11 11 CYP2B6 105 8.2 CYP2D6 132 8.6
3. The relative activities of CYP2A6 and CYP2D6 towards nicotine have also been compared by analysis of individual P450 isozyme expression in a panel of human liver microsomes by Western blotting and correlation of the level of expression of individual isozymes with nicotine C-oxidation rates (Yamazaki et al. 1999). The results of these experiments demonstrated that the strongest correlation was observed between CYP2A6 expression and enzyme activity. In addition, no inhibition of enzyme activity was observed with the CYP2D6 inhibitor quinidine, suggesting that CYP2D6 does not make a significant contribution to nicotine metabolism in human liver. This was further confirmed by the observation that the CYP2D6 phenotype of an individual known to be deficient in nicotine C-oxidation was that of an extensive metaboliser (Benowitz et al. 1995).
4. Considerable inter-individual variation has been observed in nicotine metabolism, and individuals with severely compromised nicotine C-oxidation activities identified (Benowitz et al. 1995). Recent evidence suggests that the principal defect in nicotine oxidation arises from genetic polymorphism at the CYP2A6 gene locus. Approximately 8% of the Caucasian population and 27% of Chinese appear to have diminished or absent CYP2A6 activity due to heterozygous or homozygous CYP2A6 polymorphisms (Oscarson et al. 1999). This ethnic variation in CYP2A6 allele frequencies is likely to go some way to rationalise the ethnic differences observed in nicotine and cotinine metabolism (Benowitz et al. 1999). It may be hypothesised that while the catalytic activity of CYP2D6 towards nicotine is negligible in the presence of CYP2A6, CYP2D6 may contribute to nicotine metabolism in the absence of CYP2A6.
5. Unlike CYP2A6, which is thought to play a relatively minor role in drug metabolism, CYP2D6 is known to be responsible for the metabolism of more than 25% of commonly prescribed drugs. The expression of CYP2D6 is also subject to genetic polymorphism, with an ever increasing number of allelic variants with differing catalytic properties described (Daly et al. 1996, Table 2). Approximately 7% of the Caucasian population are homozygous for a combination of several inactive CYP2D6 alleles (Sachse et al. 1997, Table 2). These individuals, "poor metabolisers (PMs)", express no functional CYP2D6 protein and therefore have a severely compromised ability to metabolise compounds which are CYP2D6 substrates. In addition, 1-3% of Caucasians are so-called "ultra-rapid metabolisers", carrying multiple copies of CYP2D6 arranged in tandem. This gene amplification can involve as many as 13 copies of CYP2D6 and results in extremely rapid substrate metabolism (Johansson et al. 1993). Like CYP2A6, the frequency of both CYP2D6 poor and ultra-rapid metabolisers varies in different ethnic groups (Table 3), with obvious consequences for prescribing regimens for the affected drugs.
TABLE 2: Compilation of CYP2D6 alleles
Allele name Nucleotide Protein Enzyme Allele New Old sequence sequence activity frequency changes changes [%]
*1 wt "Wild-type" None normal 36.4
*2 L 1661 G>C, 2850 C>T, R296C, down 32.4 4180 G>C S486T
*1x2 CYP2D6 duplicated 2 active UP 0.51 genes
*2xN L2 CYP2D6 duplicated N active UP 1.34 genes
*3 A 2549 A deleted Frameshift 0 2.04
*4 B 100 C>T, 1846 G>A, Frameshift 0 20.7 4180 G>C of splicing signal
*5 D CYP2D6 deleted None 0 1.95 expressed
*6 T 1707 T deleted Frameshift 0 0.93
*9 C 2613-2615 AGA K281 down 1.78 deleted deleted
*10 J/Ch 100 C>T, 1661 G>C, P34S, down 1.53 4180 G>C S486T
Alleles of CYP2D6 (updated at http://www.imm.ki.se/CYPalleles/cyp2d6.htm), found to be most frequent among white Caucasians (frequencies from Sachse et al. 1997). Consensus CYP2D6 gene nomenclature is according to Daly et al. 1996.
TABLE 3: Ethnic differences in CYP2D6 allele frequencies
Population Subjects Poor metaboli- Allele frequency Reference sample [n] sers (PMs)[%] of *MxN [%]
Koreans 152 0.0 0.3 Roh et al. 1996 Chinese 113 0.0 1.3 Johansson et al. 1994 Swedes 270 8.0 1.0 Dahl et al. 1995 Germans 589 7.0 2.0 Sachse et al. 1997 French 265 8.4 1.9 Legrand et al. 1998 S.Spaniards 217 2.8 3.5 Agundez et al. 1995 Anatolians 404 1.5 5.8 Aynacioglu et al. 1997 Saudi Arabs 101 2.0 10 McLellan et al. 1997 Blk Ethiopians 122 1.8 16 Aklillu et al. 1995 Blk Tanzanians 113 7.0 4.1 Wennerholm et al. 1999 Blk Americans 246 3.3 2.4 London et al. 1997 Nicaraguans 137 3.6 1.1 Agundez et al. 1997 Wht Americans 464 5.8 2.2 London et al. 1997
Population frequencies of CYP2D6 PM alleles and the ultrarapid CYP2D6 gene duplication in populations of different ethnic origin. a CYP2D6*MxN is a general designation of this allele; it includes duplications as well as higher amplifications (known from the literature: N = 2, 3, 4, ..., 13) of different alleles (known from the literature: M = *1, *2, *4).
6. A number of studies have addressed the issue of whether CYP2D6 genotype is associated with desire to smoke (Turgeon et al. 1995; Cholerton et al. 1996) or whether genotype may reinforce smoking behaviour in committed smokers (Boustead et al. 1997). The majority of these studies do not report a strong association between CYP2D6 genotype and smoking behaviour but were, however, mostly performed before CYP2A6 was identified as the major nicotine C-oxidase.
7. Much of the data on CYP2D6 genotype in smokers are found contained within larger studies investigating the CYP2D6 polymorphism as a susceptibility factor in lung cancer. The findings of many of these studies are contradictory, however, with meta-analysis of the available data suggesting no overall association between CYP2D6 genotype and susceptibility to lung cancer, irrespective of smoking status (Christensen et al. 1997; London et al. 1997). Many early studies were additionally complicated by the use of phenotyping rather than genotyping methods to assess CYP2D6 activity.
8. Although some positive findings have been reported relating CYP2D6 genotype to smoking history, the results of these studies have, in general, been based on the analysis of relatively small populations and on qualitative rather than quantitative assessment of smoking behaviour. It is therefore difficult to confidently correlate these variables with a quantitative assessment of CYP2D6 genotype.
9. A recent study (Saarikoski et al. 2000), however, reports a significant over-representation of the CYP2D6 ultra-rapid genotype in smokers, with a two-fold increase observed in heavy smokers compared to occasional smokers (OR=2.3, 95%CI=1.2-4.4) and a four-fold increase in heavy compared to never smokers (OR=4.2, 95%CI=1.8-9.8). Although the population tested was a mixture of cancer patients and healthy controls, when all possible confounding factors were included a significant trend was still observed suggesting increased tobacco use correlated with increased CYP2D6 metabolic activity.
10. Several independent epidemiological studies have suggested that smoking is protective against Parkinson's disease (e.g. Tzourio et al. 1997, OR=0.4). Nicotine is known to stimulate dopamine release in the substantia nigra, the area of the brain which is progressively destroyed in Parkinson's disease. CYP2D6 expression has been demonstrated in this area of the brain and a possible role for the enzyme in dopamine transport proposed (Niznik et al. 1990). CYP2D6 in vitro activity is competitively inhibited by serotonin, tryptamine and dopamine, and noncompetitively inhibited by adrenaline and noradrenaline (Martinez et al. 1997). More importantly, CYP2D6 has been shown to be active in the metabolic conversion of tyramine to dopamine (Hiroi et al. 1998), and in the metabolism of tryptamine (Martinez et al. 1997). Lack of active CYP2D6 protein in poor metabolisers may therefore result in reduced dopamine formation, a hypothesis which is supported by several epidemiological studies comparing CYP2D6 PM frequencies in PD patients and healthy controls (Smith et al. 1992; Lucotte et al. 1996). More over, CYP2D6 is known to metabolise MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; Gilham et al. 1997), a contaminant of synthetic narcotics, exposure to which results in the development of Parkinson's-like symptoms - studies in animal models have demonstrated a protective effect of nicotine on neuronal loss in the substantia nigra following exposure to MPTP (Janson & Moller 1993).
11. Further evidence that CYP2D6 may have a constitutive role in neurotransmission comes from Sweden (Bertilsson et al. 1989), where CYP2D6 PMs were shown to demonstrate increased vitality, efficiency and alertness, and from a Spanish study where PMs were found to be more anxiety-prone and less successfully socialized (Llerena et al. 1993).
12. Links between smoking and depression are well documented - depressive and schizophrenic illnesses are often exacerbated by smoking cessation (Covey et al. 1990; Borrelli et al. 1996), and the higher frequency of smokers among psychiatric patients is believed to represent a self-medication mechanism (Stefanis & Kokkevi 1986). Antidepressant drugs are routinely prescribed as adjutant therapy in therapeutic intervention to aid smoking cessation. Interestingly, many of these antidepressants including nortriptyline and fluoxetine are CYP2D6 substrates (Table 4), and in vivo concentrations of many of these drugs are significantly dependent on CYP2D6 genotype (e.g. clomipramine, Bertilsson et al. 1997; desipramine, Dahl et al. 1993; nortriptyline, Dalen et al. 1998). Therefore, the CYP2D6 polymorphism is likely to be a critical determinant of the success of antidepressant drug therapy.
TABLE 4: Drugs used in the treatment of psychiatric diseases which are CYP2D6 substrates
Class of drugs Examples
Antidepressants: Amitriptyline, Clomipramine, Citalopram, Desipramine,Fluoxetine, Fluvoxamine, Imipramine, Nortriptyline,Paroxetine, Trimipramine, Venlafaxine
Antipsychotics: Chlorpromazine, Clozapine, Haloperidol, Olanzapine, Perazine, Perphenazine, Remoxipride, Risperidone, Thioridazine, Zuclopenthixol
(Not all of these drugs are metabolised exclusively by CYP2D6)
13. CYP2A6, not CYP2D6 appears to play a significant role in nicotine metabolism, and should therefore be the primary target for treating nicotine addiction. However, some studies have associated smoking behaviour with CYP2D6 genotype - the potential impact of CYP2D6 in neurotransmitter metabolism suggests a further link with smoking addiction. In addition, CYP2D6 is known to metabolise the majority of commonly prescribed antidepressant drugs. Where these drugs are prescribed to facilitate smoking cessation, CYP2D6 genotype may influence individual metabolic capabilities and therefore be a critical determinant of therapeutic efficacy.
The authors acknowledge the financial support of the Ministry of Agriculture, Fisheries and Food (MAFF).
Agundez JAG, Ledesma MC, Ladero JM, Benitez J. Prevalence of CYP2D6 gene duplication and its repercussion on the oxidative phenotype in a white population. Clin Pharmacol Ther 1995: 57, 265- 269.
Agundez JAG, Ramirez R, Hernandez M, Llerena A, Benitez J. Molecular heterogeneity at the CYP2D gene locus in Nicaraguans: impact of gene-flow from Europe. Pharmacogenetics 1997: 7, 337-340.
Aklillu E, Persson I, Bertilsson L, Johansson I, Rodrigues F, Ingelman- Sundberg M. Frequent distribution of ultrarapid metabolisers of debrisoquine in an Ethiopian population carrying duplicated and multiduplicated functional CYP2D6 alleles. J Pharmacol Exp Ther 1996: 278, 441-446.
Aynacioglu AS, Sachse C, Bozkurt A, Kortunay S, Nacak M, Schroder T, Kayaalp OS, Roots I, Brockmoller J. Low frequency of defective alleles of cytochrome P450 enzymes 2C19 and 2D6 in the Turkish population. Clin Pharmacol Ther 1999: 66, 185-192.
Benowitz NL, Jacob P 3rd, Sachs DP. Deficient C-oxidation of nicotine. Clin Pharmacol Ther 1995: 57, 590-594.
Benowitz NL. Pharmacology of nicotine: addiction and therapeutics. Ann Rev Pharmacol Toxicol 1996: 36, 597-613.
Benowitz NL, Perez-Stable EJ, Fong I, Modin G, Herrera B, Jacob P 3rd. Ethnic differences in N-glucuronidation of nicotine and cotinine. J Pharmacol Exp Ther 1999: 291, 1196-1203.
Bertilsson L, Alm C, DeLasCarreras C, Widen J, Edman G, Schalling D. Debrisoquine hydroxylation polymorphism and personality. Lancet 1989: V, 555.
Bertilsson L, Dahl ML, Tybring G. Pharmacogenetics of antidepressants: clinical aspects. Acta Psychiatr Scand Suppl 1997: 391, 14-21.
Borrelli B, Niaura R, Keuthen NJ, Goldstein MJ, DePue JD, Murphy C, Abrams DB. Development of major depressive disorder during smoking cessation treatment. J Clin Psychiatry 1996: 57, 534-538.
Boustead C, Taber H, Idle JR, Cholerton S. CYP2D6 genotype and smoking behaviour in cigarette smokers. Pharmacogenetics 1997: 7, 411-414.
Cholerton S, Boustead C, Taber H, Arpanahi A, Idle JR. CYP2D6 genotypes in cigarette smokers and non-tobacco users. Pharmacogenetics 1996: 6, 261-263.
Christensen PM, Gotzsche PC, Brosen K. The sparteine/debrisoquine (CYP2D6) oxidation polymorphism and the risk of lung cancer: a meta- analysis. Eur J Clin Pharmacol 1997: 51, 389-393.
Covey LS, Glassman AH, Stetner F. Depression and depressive symptoms in smoking cessation. Compr. Psychiatry 1990: 31, 350 354.
Dahl ML, Iselius L, Alm C, Svensson JO, Lee D, Johansson I. Ingelman-Sundberg M. Sjoqvist F. Polymorphic 2-hydroxylation of desipramine: A population and family study. Eur J Clin Pharmacol 1993: 44, 445-450.
Dahl M-L, Johansson I, Bertilsson L, Ingelman-Sundberg M, Sjoqvist F. Ultrarapid hydroxylation of debrisoquine in a Swedish population. Analysis of the molecular genetic basis. J Pharmac Exp Ther 1995: 274, 516-520.
Dalen P, Dahl ML, Ruiz ML, Nordin J, Bertilsson L. 10-Hydroxylation of nortriptyline in white persons with 0, 1, 2, 3, and 13 functional CYP2D6 genes. Clin Pharmacol Ther 1998: 63, 444-452.
Daly AK, Brockmoller J, Broly F, Eichelbaum M, Evans WE, Gonzalez FJ, Huang J-D, Idle JR, Ingelman-Sundberg M, Ishizaki T, Jacqz- Aigrain E, Meyer UA, Nebert DW, Steen VM, Wolf CR, Zanger UM. Nomenclature for human CYP2D6 alleles. Pharmacogenetics 1996: 6, 193-201.
Gilham DE, Cairns W, Paine MJ, Modi S, Poulsom R, Roberts GC, Wolf CR. Metabolism of MPTP by cytochrome P4502D6 and the demonstration of 2D6 mRNA in human foetal and adult brain by in situ hybridization. Xenobiotica 1997: 27, 111-125.
Hiroi T, Imaoka S, Funae Y. Dopamine formation from tyramine by CYP2D6. Biochem Biophys Res Commun 1998: 249, 838-843.
Janson AM, Moller A. Chronic nicotine treatment counteracts nigral cell loss induced by a partial mesodiencephalic hemitransection: an analysis of the total number and mean volume of neurons and glia in substantia nigra of the male rat. Neuroscience 1993: 57, 931-941.
Johansson I, Lundqvist E, Bertilsson L, Dahl ML, Sjoqvist F, Ingelman- Sundberg M. Inherited amplification of an active gene in the cytochrome P450 CYP2D locus as a cause of ultrarapid metabolism of debrisoquine. Proc Natl Acad Sci USA 1993: 90, 11825-11829.
Johansson I, Oscarson M, Yue QY, Bertilsson L, Sjoqvist F, Ingelman- Sundberg M. Genetic analysis of the Chinese cytochrome P4502D locus: characterization of variant CYP2D6 genes present in subjects with diminished capacity for debrisoquine hydroxylation. Mol Pharmacol 1994: 46, 452-459.
Legrand-Andreoletti M, Stucker I, Marez D, Galais P, Cosme J, Sabbagh N, Spire C, Cenee S, Lafitte JJ, Beaune P, Broly F. Cytochrome P450 CYP2D6 gene polymorphism and lung cancer susceptibility in Caucasians. Pharmacogenetics 1998: 8, 7-14.
Llerena L, Edman G, Cobaleda J, Benitez J, Schalling D, Bertilsson L. Relationship between personality and debrisoquine hydroxylation capacity. Acta Psychiatr Scand 1993: 87, 23-28.
London SJ, Daly AK, Leathart JB, Navidi WC, Carpenter CC, Idle JR. Genetic polymorphism of CYP2D6 and lung cancer risk in African- Americans and Caucasians in Los Angeles County. Carcinogenesis 1997: 18, 1203-1214.
Lucotte G, Turpin N, Gerard N, Panserat S, Krishnamoorthy R. Mutation frequencies of the cytochrome CYP2D6 gene in Parkinson disease patients and in families. Am J Med Genet 1996: 67, 361-365.
Martinez C, Agundez JA, Gervasini G, Martin R, Benitez J. Tryptamine: a possible endogenous substrate for CYP2D6. Pharmacogenetics 1997: 7, 85-93.
McCracken NW, Cholerton S, Idle JR. Cotinine formation by cDNA expressed human cytochromes P450. Med Sci Res 1992: 20, 877-878.
McLellan RA, Oscarson M, Seidegard J, Evans DA, Ingelman- Sundberg M. Frequent occurrence of CYP2D6 gene duplication in Saudi Arabians. Pharmacogenetics 1997: 7, 187-191.
Messina ES, Tyndale RF, Sellers EM. A major role for CYP2A6 in nicotine C-oxidation by human liver microsomes. J Pharmacol Exp Ther 1997: 282, 1608-1614.
Niznik HB, Tyndale RF, Sallee FR, Gonzalez FJ, Hardwick JP, Inaba T, Kalow W. The dopamine transporter and cytochrome P45OIID1 (debrisoquine 4-hydroxylase) in brain: resolution and identification of two distinct [3H]GBR-12935 binding proteins. Arch Biochem Biophys 1990: 276, 424-432.
Oscarson M, McLellan RA, Gullsten H, Agundez JA, Benitez J, Rautio A, Raunio H, Pelkonen O, Ingelman-Sundberg M. Identification and characterisation of novel polymorphisms in the CYP2A locus: implications for nicotine metabolism. FEBS Lett 1999: 460, 321-327.
Roh HK, Dahl ML, Johansson I, Ingelman-Sundberg M, Cha YN, Bertilsson L. Debrisoquine and S-mephenytoin hydroxylation phenotypes and genotypes in a Korean population. Pharmacogenetics 1996: 6, 441-447.
Sachse C, Brockmoller J, Bauer S, Roots I. Cytochrome P450 2D6 variants in a Caucasian population: allele frequencies and phenotypic consequences. Am J Hum Genet 1997: 60, 284-295.
Saarikoski ST, Sata F, Husgafvel-Pursiainen K, Rautalahti M, Haukka J, Impivaara O, Jarvisalo J, Vainio H, Hirvonen A. CYP2D6 ultrarapid metaboliser genotype as a potential modifier of smoking behaviour. Pharmacogenetics 2000: 10, 5-10.
Smith CA, Gough AC, Leigh PN, Summers BA, Harding AE, Maraganore DM, Sturman SG, Schapira AH, Williams AC, et al. Debrisoquine hydroxylase gene polymorphism and susceptibility to Parkinson's disease. Lancet 1992: 339, 1375-1377.
Stefanis CN, Kokkevi A. Depression and drug use. Psychopathology 1986: 19, 124-131.
Turgeon J, Labbe L, Lefez C, LeBel M. Debrisoquine metabolic ratio (DMR) distribution differs among smokers and non-smokers. Clin Pharmacol Ther 1995: 57, 150
Tzourio C, Rocca WA, Breteler MM, Baldereschi M, Dartigues JF, Lopez-Pousa S, Manubens-Bertran JM, Alperovitch A. Smoking and Parkinson's disease. An age-dependent risk effect? The EUROPARKINSON Study Group. Neurology 1997: 49, 1267-1272.
Wennerholm A, Johansson I, Massele AY, Lande M, Alm C, Aden- Abdi Y, Dahl ML, Ingelman-Sundberg M, Bertilsson L, Gustafsson LL. Decreased capacity for debrisoquine metabolism among black Tanzanians: analyses of the CYP2D6 genotype and phenotype. Pharmacogenetics 1999; 9: 707-714.
Yamazaki H, Inoue K, Hashimoto M, Shimada T. Roles of CYP2A6 and CYP2B6 in nicotine C-oxidation by human liver microsomes. Arch Toxicol 1999: 73, 65-70.