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Dietary levels of quinine in tonic water do not inhibit CYP2D6 in vivo
Food and Chemical Toxicology 41 (2003) 1199–1201 www.elsevier.com/locate/foodchemtox
Brief communication
Dietary levels of quinine in tonic water do not inhibit CYP2D6 in vivo Jennifer L. Donovan, C. Lindsay DeVane*, David Boulton, Seetal Dodd, John S. Markowitz Laboratory of Drug Disposition and Pharmacogenetics, Medical University of South Carolina, 67 President St. suite 246 N, Charleston, SC 29425, USA Received 14 November 2002; received in revised form 12 February 2003; accepted 12 March 2003
Abstract Quinine is a bitter alkaloid that is used as a flavoring agent in tonic water. Studies suggest that quinine can inhibit cytochrome P450 2D6 (CYP2D6) which could have implications for the metabolism of co-ingested drugs. We conducted a study with 11 healthy volunteers (7 men, 4 women; aged 26–54). After urinary void, each subject consumed either 1000 ml of carbonated water or 1000 ml of tonic water containing 80 mg quinine in a crossover design. Following each beverage subjects ingested an oral dose of 30 mg dextromethorphan (DM). Urine was collected for 8 h and analyzed for DM and dextrophran, its CYP2D6 mediated metabolite. The ratio of DM and its metabolite is an established measure of CYP2D6 activity. All subjects metabolized the vast majority of DM to its metabolite after both the carbonated water and the tonic water. The ratio (mean S.D.) of DM to its metabolite was 0.013 0.028 after the carbonated water and 0.032 0.067 after the quinine containing water. No significant difference in the ratios was observed between the two beverages (P> 0.05). We conclude that quinine as consumed in tonic water does not inhibit CYP2D6 activity in vivo. Thus, quinine should not alter the metabolism of CYP2D6 substrates taken concomitantly with tonic water. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Quinine; Tonic; Human; Cytochrome P450, CYP2D6
1. Introduction Quinine is a bitter alkaloid from the bark of the cinchona tree (Brasic, 1999). Historically, it has been used to treat malaria and is more recently indicated in the treatment of nocturnal leg cramping (Diener et al., 2002). Quinine is also used as a flavoring agent in tonic water and is consumed in the diet mainly as tonic water or other tonic containing mixed beverages (Brasic, 1999). Quinine, and its diastereomer quinidine (Fig. 1), have been shown to inhibit cytochrome P450 2D6 (CYP2D6), a hepatic enzyme involved in the metabolism of many prescription and over the counter drugs including antiarrythmics, b-adrenergic receptor antagonists, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), antipsychotics, opiates, anticancer agents and amphetamines (Bloomer et al., * Corresponding author. Tel.: +1-843-792-5448; fax: +1-843-7926318. E-mail address: [email protected] (C.L. DeVane).
1992). In rat liver microsomes quinine was 50 times more potent than quinidine at inhibiting CYP2D6 activity and quinine was subsequently shown to be a potent in vivo inhibitor in the rat (Kobayashi et al., 1989; Muralidharan et al., 1991a). In human liver microsomes quinidine was a far more potent inhibitor than quinine, and quinidine is now widely recognized as one of the most potent CYP2D6 inhibitors in humans (Kobayashi et al., 1989). There are only two prior clinical studies assessing the effect of quinine on CYP2D6 activity in humans, both using high doses of purified quinine salts. One study showed that administration of 750 mg quinine hydrochloride decreased the amount of 2-hydroxy desipramine (the CYP2D6 mediated metabolite of desipramine) by 54% (Steiner et al., 1988). In the same study 800 mg quinidine sulfate decreased levels of 2-hydroxy desipramine by 96%. Another study administering 300 mg quinine sulfate also demonstrated that inhibition was much less marked in humans than with quinidine (Muralidharan et al., 1991b). The potency of dietary quinine, as it is
0278-6915/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0278-6915(03)00112-1
1200
J.L. Donovan et al. / Food and Chemical Toxicology 41 (2003) 1199–1201
Fig. 1. The structures of quinine (A) and its diastereomer quinidine (B).
normally consumed in tonic water, has never been evaluated. The objective of this study was to determine if quinine, consumed by humans in tonic water alters the metabolism of drugs by inhibition of CYP2D6. The study employed normal volunteers in a crossover design. Dextromethorphan was chosen as a probe drug for CYP2D6 activity due to its known tolerability, safety, and validity as a CYP2D6 phenotypic probe in research subjects, as well as its extensive use in drug interaction studies (Hoskins et al., 1997; Liston et al., 2002; Markowitz et al., 2000).
2. Materials and methods 2.1. Beverage analysis The tonic water used in this study was of the Schweppes1 brand (Cadbury Schweppes, London, UK) and was purchased from a local supermarket. Samples for analysis were obtained from three bottles of tonic water used in this study. The quinine content was determined by HPLC using a cyanopropyl column (Supelcosil LC-PCN 15 cm4.6 mm, 5 mm, Supelco Inc., Bellefonte, PA), eluted with a mobile phase containing 20% acetonitrile in 0.1 M acetate buffer at pH 5.0 at a flow rate of 1.0 ml/min. Detection was by fluorescence with an excitation wavelength of 350 nm and an emission wavelength of 475 nm. The carbonated water used in this study was a private label from the local supermarket and did not contain detectable levels of quinine upon analysis.
subject consumed either 1000 ml of carbonated water (without quinine) or 1000 ml of the quinine containing tonic water in a crossover design. The subjects were asked to completely consume the beverage in one hour. Upon finishing the beverage each subject received a 30 mg oral dose of DM (Robitussin1 Maxium Strength Cough syrup, Whitehall-Robins Healthcare, Madison, NJ). Urine was collected for 8 h and analyzed for DM and dextrophran (DX), its CYP2D6 mediated metabolite, as described below. The molar ratio of DM and its metabolite is an established measure of CYP2D6 activity and an increase in this ratio reflects decreased metabolism by CYP2D6 (Hoskins et al., 1997). 2.3. HPLC analysis of dextromethorphan and dextrorphan DM and DX were simultaneously determined by a validated HPLC method (Hoskins et al., 1997). In brief, urine samples (0.25 ml) were adjusted to pH 5.0 and incubated overnight with 2500 U/ml b-glucuronidase. DM, DX, and the internal standard, levallorphan were extracted using 0.25 ml saturated sodium carbonate and 2 ml diethyl ether: chloroform: propan-2-ol (20:9:1 v:v:v). After centrifugation, the organic phase was removed and extracted with 0.2 ml of 0.1 M HCl and 50 ml of this aqueous phase was injected into using a Waters 2690 HPLC (Waters Corp., Milford, MA). The samples were separated on by HPLC using the column and conditions described above for beverage analysis. Detection was by fluorescence utilizing an excitation wavelength of 275 nm and an emission wavelength of 307 nm. The lower limit of quantitation was 1 ng/ml for DM, and 2 ng/ml for DX. The intra-day coefficient of variation for this assay ranged from 5 to 14% for both compounds. 2.4. Data and statistical analysis All values are reported as the mean the standard deviation. The DM ratios after administration of both carbonated water and tonic were analyzed using the paired student’s t-test. A value of less than 0.05 was used as the minimal level of statistical significance.
2.2. Subjects and study design 3. Results and discussion The study was conducted in 11 healthy volunteers (7 men, 4 women, 9 Caucasian, 1 Asian-American, 1 African-American; aged 26–54). The clinical protocol was approved by the Office of Research Integrity at the Medical University of South Carolina and all participants provided written informed consent before participation. Subjects with the poor metaboliser phenotype of CYP2D6 were excluded from the study based on prescreening with dextromethorphan as previously described (Hoskins et al., 1997). After urinary void, each
The tonic water used in this study contained 80.3 + 0.5 mg quinine/l, in agreement with the level reported by the producers (80 mg/l) and below the maximum amount allowed by the United States Food and Drug Administration (83 mg/l, (Ash & Ash, 1995). Other leading brands of tonic water tested in our laboratory also contained between 70 and 80 mg/l quinine (data not shown). This study used a relatively large amount of tonic water (1000 ml) over a short period of time (1 h),
J.L. Donovan et al. / Food and Chemical Toxicology 41 (2003) 1199–1201
and it is unlikely that this amount would be exceeded in typical consumption of tonic beverages. All subjects extensively metabolized DM to its metabolite, DX, after both the carbonated water and the tonic water. In two individuals only the metabolite was present in detectable quantities in which case the ratio of DM to its metabolite was considered to be 0.000 and these values (which indicate a lack of inhibition of CYP2D6) were included in the statistical analysis. Ratios of DM to its metabolite for all subjects after each beverage are shown in Fig. 2. None of the 11 subjects had a decrease in the DM ratio after the tonic water compared to the carbonated water. A small increase in the DM ratio was observed in 8 subjects after the tonic water while 3 subjects had no change in the DM ratio. One subject had an increase in the DM ratio from 0.093 to 0.230 after the tonic water. Although this level of inhibition was greater than observed for other subjects it still did not reach the lower limit for classification as a poor metabolizer and is unlikely to be of clinical significance. The mean ratio of DM to its metabolite was 0.013 0.028 after the carbonated water and 0.032 0.067 after the quinine containing water. The 95% confidence interval of the difference between the two beverages was -0.046 to 0.008. The difference in the DM ratios after carbonated water and tonic water was not statistically significant (P > 0.05). The data from subjects with detectable concentrations of DX were also transformed to their log values. The ratio of these log values was also not significantly different between the carbonated and tonic water (P > 0.05). A previous study with human liver microsomes showed that both quinine and quinidine inhibit CYP2D6 activity as measured by the 4-hydroxylation of debrisoquin (Kobayashi et al., 1989). However, there
Fig. 2. Ratios of dextromethorphan (DM) to its CYP2D6 mediated metabolite in 11 normal volunteers after consumption of carbonated water and tonic water containing 80 mg quinine.
1201
was a marked difference in the potency of activity using this model (IC50=3.6 mM for quinidine; 223 mM for quinine). Although a determination of the pharmacokinetics and bioavailability of quinine from tonic water is beyond the scope of this study, it is likely that plasma concentrations did not reach adequate the concentration necessary for even minimal inhibition of CYP2D6 in vivo.
4. Conclusion We conclude that quinine, consumed in normal amounts of tonic water, does not significantly inhibit CYP2D6 activity in vivo as measured by dextromethorphan metabolism. Thus, the quinine is unlikely to alter the metabolism of CYP2D6 substrates taken concomitantly with beverages containing tonic water.
References Ash M. and Ash A. 1995. Handbook of Food Additives. Brookfield, VM, Grower Publishing Unlimited. Bloomer, J.C., Woods, F.R., Haddock, R.E., Lennard, M.S., Tucker, G.T., 1992. The role of cytochrome P4502D6 in the metabolism of paroxetine by human liver microsomes. British Journal of Clinical Pharmacology 33, 521–523. Brasic, J., 1999. Should people with nocturnal leg cramps drink tonic water and bitter lemon? Psychological Reports 84, 355–367. Diener, H.C., Dethlefsen, U., Dethlefsen-Gruber, S., Verbeek, P., 2002. Effectiveness of quinine in treating muscle cramps: a doubleblind, placebo-controlled, parallel-group, multicentre trial. International Journal of Clinical Practice 56, 243–246. Hoskins, J., Shenfield, G., Gross, A., 1997. Modified high-performance liquid chromatographic method to measure both dextromethorphan and proguanil for oxidative phenotyping. Journal of Chromatography B: Biomedical Applications 696, 81–87. Kobayashi, S., Murray, S., Watson, D., Sesardic, D., Davies, D.S., Boobis, A.R., 1989. The specificity of inhibition of debrisoquine 4hydroxylase activity by quinidine and quinine in the rat is the inverse of that in man. Biochemical Pharmacology 38, 2795–2799. Liston, H.L., DeVane, C.L., Boulton, D.W., Risch, S.C., Markowitz, J.S., Goldman, J., 2002. Differential time course of cytochrome P450 2D6 enzyme inhibition by fluoxetine, sertraline, and paroxetine in healthy volunteers. Journal of Clinical Psychopharmacology 22, 169–173. Markowitz, J.S., DeVane, C.L., Boulton, D.W., Carson, S.W., Nahas, Z., Risch, S.C., 2000. Effect of St. John’s wort (Hypericum perforatum) on cytochrome P-450 2D6 and 3A4 activity in healthy volunteers. Life Sciences 66, PL133–139. Muralidharan, G., Hawes, E.M., McKay, G., Korchinski, E.D., Midha, K.K., 1991a. Quinidine but not quinine inhibits in man the oxidative metabolic routes of methoxyphenamine which involve debrisoquine 4hydroxylase. European Journal of Clinical Pharmacology 41, 471–474. Muralidharan, G., Hawes, E.M., McKay, G., Midha, K.K., 1991b. Quinine is a more potent inhibitor than quinidine in rat of the oxidative metabolic routes of methoxyphenamine which involve debrisoquine 4-hydroxylase. Xenobiotica 21, 1441–1450.
Brief communication
Dietary levels of quinine in tonic water do not inhibit CYP2D6 in vivo Jennifer L. Donovan, C. Lindsay DeVane*, David Boulton, Seetal Dodd, John S. Markowitz Laboratory of Drug Disposition and Pharmacogenetics, Medical University of South Carolina, 67 President St. suite 246 N, Charleston, SC 29425, USA Received 14 November 2002; received in revised form 12 February 2003; accepted 12 March 2003
Abstract Quinine is a bitter alkaloid that is used as a flavoring agent in tonic water. Studies suggest that quinine can inhibit cytochrome P450 2D6 (CYP2D6) which could have implications for the metabolism of co-ingested drugs. We conducted a study with 11 healthy volunteers (7 men, 4 women; aged 26–54). After urinary void, each subject consumed either 1000 ml of carbonated water or 1000 ml of tonic water containing 80 mg quinine in a crossover design. Following each beverage subjects ingested an oral dose of 30 mg dextromethorphan (DM). Urine was collected for 8 h and analyzed for DM and dextrophran, its CYP2D6 mediated metabolite. The ratio of DM and its metabolite is an established measure of CYP2D6 activity. All subjects metabolized the vast majority of DM to its metabolite after both the carbonated water and the tonic water. The ratio (mean S.D.) of DM to its metabolite was 0.013 0.028 after the carbonated water and 0.032 0.067 after the quinine containing water. No significant difference in the ratios was observed between the two beverages (P> 0.05). We conclude that quinine as consumed in tonic water does not inhibit CYP2D6 activity in vivo. Thus, quinine should not alter the metabolism of CYP2D6 substrates taken concomitantly with tonic water. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Quinine; Tonic; Human; Cytochrome P450, CYP2D6
1. Introduction Quinine is a bitter alkaloid from the bark of the cinchona tree (Brasic, 1999). Historically, it has been used to treat malaria and is more recently indicated in the treatment of nocturnal leg cramping (Diener et al., 2002). Quinine is also used as a flavoring agent in tonic water and is consumed in the diet mainly as tonic water or other tonic containing mixed beverages (Brasic, 1999). Quinine, and its diastereomer quinidine (Fig. 1), have been shown to inhibit cytochrome P450 2D6 (CYP2D6), a hepatic enzyme involved in the metabolism of many prescription and over the counter drugs including antiarrythmics, b-adrenergic receptor antagonists, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), antipsychotics, opiates, anticancer agents and amphetamines (Bloomer et al., * Corresponding author. Tel.: +1-843-792-5448; fax: +1-843-7926318. E-mail address: [email protected] (C.L. DeVane).
1992). In rat liver microsomes quinine was 50 times more potent than quinidine at inhibiting CYP2D6 activity and quinine was subsequently shown to be a potent in vivo inhibitor in the rat (Kobayashi et al., 1989; Muralidharan et al., 1991a). In human liver microsomes quinidine was a far more potent inhibitor than quinine, and quinidine is now widely recognized as one of the most potent CYP2D6 inhibitors in humans (Kobayashi et al., 1989). There are only two prior clinical studies assessing the effect of quinine on CYP2D6 activity in humans, both using high doses of purified quinine salts. One study showed that administration of 750 mg quinine hydrochloride decreased the amount of 2-hydroxy desipramine (the CYP2D6 mediated metabolite of desipramine) by 54% (Steiner et al., 1988). In the same study 800 mg quinidine sulfate decreased levels of 2-hydroxy desipramine by 96%. Another study administering 300 mg quinine sulfate also demonstrated that inhibition was much less marked in humans than with quinidine (Muralidharan et al., 1991b). The potency of dietary quinine, as it is
0278-6915/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0278-6915(03)00112-1
1200
J.L. Donovan et al. / Food and Chemical Toxicology 41 (2003) 1199–1201
Fig. 1. The structures of quinine (A) and its diastereomer quinidine (B).
normally consumed in tonic water, has never been evaluated. The objective of this study was to determine if quinine, consumed by humans in tonic water alters the metabolism of drugs by inhibition of CYP2D6. The study employed normal volunteers in a crossover design. Dextromethorphan was chosen as a probe drug for CYP2D6 activity due to its known tolerability, safety, and validity as a CYP2D6 phenotypic probe in research subjects, as well as its extensive use in drug interaction studies (Hoskins et al., 1997; Liston et al., 2002; Markowitz et al., 2000).
2. Materials and methods 2.1. Beverage analysis The tonic water used in this study was of the Schweppes1 brand (Cadbury Schweppes, London, UK) and was purchased from a local supermarket. Samples for analysis were obtained from three bottles of tonic water used in this study. The quinine content was determined by HPLC using a cyanopropyl column (Supelcosil LC-PCN 15 cm4.6 mm, 5 mm, Supelco Inc., Bellefonte, PA), eluted with a mobile phase containing 20% acetonitrile in 0.1 M acetate buffer at pH 5.0 at a flow rate of 1.0 ml/min. Detection was by fluorescence with an excitation wavelength of 350 nm and an emission wavelength of 475 nm. The carbonated water used in this study was a private label from the local supermarket and did not contain detectable levels of quinine upon analysis.
subject consumed either 1000 ml of carbonated water (without quinine) or 1000 ml of the quinine containing tonic water in a crossover design. The subjects were asked to completely consume the beverage in one hour. Upon finishing the beverage each subject received a 30 mg oral dose of DM (Robitussin1 Maxium Strength Cough syrup, Whitehall-Robins Healthcare, Madison, NJ). Urine was collected for 8 h and analyzed for DM and dextrophran (DX), its CYP2D6 mediated metabolite, as described below. The molar ratio of DM and its metabolite is an established measure of CYP2D6 activity and an increase in this ratio reflects decreased metabolism by CYP2D6 (Hoskins et al., 1997). 2.3. HPLC analysis of dextromethorphan and dextrorphan DM and DX were simultaneously determined by a validated HPLC method (Hoskins et al., 1997). In brief, urine samples (0.25 ml) were adjusted to pH 5.0 and incubated overnight with 2500 U/ml b-glucuronidase. DM, DX, and the internal standard, levallorphan were extracted using 0.25 ml saturated sodium carbonate and 2 ml diethyl ether: chloroform: propan-2-ol (20:9:1 v:v:v). After centrifugation, the organic phase was removed and extracted with 0.2 ml of 0.1 M HCl and 50 ml of this aqueous phase was injected into using a Waters 2690 HPLC (Waters Corp., Milford, MA). The samples were separated on by HPLC using the column and conditions described above for beverage analysis. Detection was by fluorescence utilizing an excitation wavelength of 275 nm and an emission wavelength of 307 nm. The lower limit of quantitation was 1 ng/ml for DM, and 2 ng/ml for DX. The intra-day coefficient of variation for this assay ranged from 5 to 14% for both compounds. 2.4. Data and statistical analysis All values are reported as the mean the standard deviation. The DM ratios after administration of both carbonated water and tonic were analyzed using the paired student’s t-test. A value of less than 0.05 was used as the minimal level of statistical significance.
2.2. Subjects and study design 3. Results and discussion The study was conducted in 11 healthy volunteers (7 men, 4 women, 9 Caucasian, 1 Asian-American, 1 African-American; aged 26–54). The clinical protocol was approved by the Office of Research Integrity at the Medical University of South Carolina and all participants provided written informed consent before participation. Subjects with the poor metaboliser phenotype of CYP2D6 were excluded from the study based on prescreening with dextromethorphan as previously described (Hoskins et al., 1997). After urinary void, each
The tonic water used in this study contained 80.3 + 0.5 mg quinine/l, in agreement with the level reported by the producers (80 mg/l) and below the maximum amount allowed by the United States Food and Drug Administration (83 mg/l, (Ash & Ash, 1995). Other leading brands of tonic water tested in our laboratory also contained between 70 and 80 mg/l quinine (data not shown). This study used a relatively large amount of tonic water (1000 ml) over a short period of time (1 h),
J.L. Donovan et al. / Food and Chemical Toxicology 41 (2003) 1199–1201
and it is unlikely that this amount would be exceeded in typical consumption of tonic beverages. All subjects extensively metabolized DM to its metabolite, DX, after both the carbonated water and the tonic water. In two individuals only the metabolite was present in detectable quantities in which case the ratio of DM to its metabolite was considered to be 0.000 and these values (which indicate a lack of inhibition of CYP2D6) were included in the statistical analysis. Ratios of DM to its metabolite for all subjects after each beverage are shown in Fig. 2. None of the 11 subjects had a decrease in the DM ratio after the tonic water compared to the carbonated water. A small increase in the DM ratio was observed in 8 subjects after the tonic water while 3 subjects had no change in the DM ratio. One subject had an increase in the DM ratio from 0.093 to 0.230 after the tonic water. Although this level of inhibition was greater than observed for other subjects it still did not reach the lower limit for classification as a poor metabolizer and is unlikely to be of clinical significance. The mean ratio of DM to its metabolite was 0.013 0.028 after the carbonated water and 0.032 0.067 after the quinine containing water. The 95% confidence interval of the difference between the two beverages was -0.046 to 0.008. The difference in the DM ratios after carbonated water and tonic water was not statistically significant (P > 0.05). The data from subjects with detectable concentrations of DX were also transformed to their log values. The ratio of these log values was also not significantly different between the carbonated and tonic water (P > 0.05). A previous study with human liver microsomes showed that both quinine and quinidine inhibit CYP2D6 activity as measured by the 4-hydroxylation of debrisoquin (Kobayashi et al., 1989). However, there
Fig. 2. Ratios of dextromethorphan (DM) to its CYP2D6 mediated metabolite in 11 normal volunteers after consumption of carbonated water and tonic water containing 80 mg quinine.
1201
was a marked difference in the potency of activity using this model (IC50=3.6 mM for quinidine; 223 mM for quinine). Although a determination of the pharmacokinetics and bioavailability of quinine from tonic water is beyond the scope of this study, it is likely that plasma concentrations did not reach adequate the concentration necessary for even minimal inhibition of CYP2D6 in vivo.
4. Conclusion We conclude that quinine, consumed in normal amounts of tonic water, does not significantly inhibit CYP2D6 activity in vivo as measured by dextromethorphan metabolism. Thus, the quinine is unlikely to alter the metabolism of CYP2D6 substrates taken concomitantly with beverages containing tonic water.
References Ash M. and Ash A. 1995. Handbook of Food Additives. Brookfield, VM, Grower Publishing Unlimited. Bloomer, J.C., Woods, F.R., Haddock, R.E., Lennard, M.S., Tucker, G.T., 1992. The role of cytochrome P4502D6 in the metabolism of paroxetine by human liver microsomes. British Journal of Clinical Pharmacology 33, 521–523. Brasic, J., 1999. Should people with nocturnal leg cramps drink tonic water and bitter lemon? Psychological Reports 84, 355–367. Diener, H.C., Dethlefsen, U., Dethlefsen-Gruber, S., Verbeek, P., 2002. Effectiveness of quinine in treating muscle cramps: a doubleblind, placebo-controlled, parallel-group, multicentre trial. International Journal of Clinical Practice 56, 243–246. Hoskins, J., Shenfield, G., Gross, A., 1997. Modified high-performance liquid chromatographic method to measure both dextromethorphan and proguanil for oxidative phenotyping. Journal of Chromatography B: Biomedical Applications 696, 81–87. Kobayashi, S., Murray, S., Watson, D., Sesardic, D., Davies, D.S., Boobis, A.R., 1989. The specificity of inhibition of debrisoquine 4hydroxylase activity by quinidine and quinine in the rat is the inverse of that in man. Biochemical Pharmacology 38, 2795–2799. Liston, H.L., DeVane, C.L., Boulton, D.W., Risch, S.C., Markowitz, J.S., Goldman, J., 2002. Differential time course of cytochrome P450 2D6 enzyme inhibition by fluoxetine, sertraline, and paroxetine in healthy volunteers. Journal of Clinical Psychopharmacology 22, 169–173. Markowitz, J.S., DeVane, C.L., Boulton, D.W., Carson, S.W., Nahas, Z., Risch, S.C., 2000. Effect of St. John’s wort (Hypericum perforatum) on cytochrome P-450 2D6 and 3A4 activity in healthy volunteers. Life Sciences 66, PL133–139. Muralidharan, G., Hawes, E.M., McKay, G., Korchinski, E.D., Midha, K.K., 1991a. Quinidine but not quinine inhibits in man the oxidative metabolic routes of methoxyphenamine which involve debrisoquine 4hydroxylase. European Journal of Clinical Pharmacology 41, 471–474. Muralidharan, G., Hawes, E.M., McKay, G., Midha, K.K., 1991b. Quinine is a more potent inhibitor than quinidine in rat of the oxidative metabolic routes of methoxyphenamine which involve debrisoquine 4-hydroxylase. Xenobiotica 21, 1441–1450.