Abstract
Aims
To study the influence of the CYP2D6*10 allele on the disposition of debrisoquine and nortriptyline.
Methods
The pharmacokinetics of debrisoquine and nortriptyline and their main metabolites were determined in ten Koreans with the CYP2D6*1/*1 (n = 5) and CYP2D6*1/*10 (n = 5) genotypes after single oral doses of 20 mg debrisoquine and 25 mg nortriptyline, respectively. The data were compared with previously published findings from 21 Caucasians with 0, one, two, three, four or 13 functional CYP2D6 genes.
Results
The AUC0−8of 4-hydroxydebrisoquine was significantly lower in Koreans with CYP2D6*1/*10 genotype compared with CYP2D6*1/*1[95% confidence interval (CI) for the ratio between means 1.17, 1.85]. No other genotype-related differences were found in the plasma kinetics of nortriptyline and debrisoquine, or their hydroxy metabolites. The AUCnortriptyline/AUC10-hydroxynortriptyline ratio did not differ between the *1/*1 and *1/*10 genotype groups (95% CI for the ratio of means 0.60, 1.26). Similarly, there was no difference between these genotypes with respect to the AUCdebrisoquine/AUC4-hydroxydebrisoquine ratio (95% CI for the ratio of mean values 0.38, 1.46). Both Korean genotype groups had similar AUCs and parent compound/metabolite AUC ratios of debrisoquine and nortriptyline to Caucasians with two functional CYP2D6 genes.
Conclusions
Heterozygosity for CYP2D6*10 decreases the CYP2D6-dependent elimination of nortriptyline and debrisoquine to only a limited degree. Further studies in subjects homozygous for CYP2D6*10 are required to elucidate fully the pharmacokinetic consequences of this CYP2D6 genotype in Orientals.
Keywords: CYP2D6, cytochrome P450, debrisoquine, interetchnic differences, nortriptyline
Introduction
Cytochrome P450 CYP2D6 activity exhibits pronounced interethnic differences in that 7% of Caucasians but only about 1% of Orientals are poor metabolizers (PM) [1, 2]. However, the distribution of the debrisoquine metabolic ratio (MR) in Oriental extensive metabolizers (EMs) indicates lower mean CYP2D6 activity [2]. This is due to the CYP2D6*10 allele, which accounts for half of the Oriental CYP2D6 alleles and results in an unstable enzyme with decreased activity [3–5]. This might result in the need for lower mean doses in Oriental populations, as suggested by some clinical observations [6–9].
The hydroxylation of the antidepressant nortriptyline to its major metabolite E-10-hydroxynortriptyline is catalysed by CYP2D6 [10]. In a study on the disposition of nortriptyline in Caucasians with 0, one, two, three, four or 13 functional CYP2D6 genes, a correlation between the number of active CYP2D6 genes and the pharmacokinetics of nortriptyline was shown [11]. Using the same panel of subjects, a similar relationship between the genotype and pharmacokinetics of debrisoquine was observed [12]. Significant differences with regard to plasma pharmacokinetics of nortriptyline were also shown in Chinese homozygous for CYP2D6*10 compared with those heterozygous or homozygous for the CYP2D6*1 allele [13].
The aim of the present study was to investigate the disposition of debrisoquine and nortriptyline in Korean subjects in relation to CYP2D6 genotype and to compare the results with those obtained in 21 Caucasians previously studied [11, 12].
Methods
Eleven unrelated healthy native Koreans (six females, five males, 19–48 years, 47–73 kg) living in Stockholm, Sweden, were included on the basis of their CYP2D6 genotype. Five were homozygous for CYP2D6*1 (debrisoquine metabolic ratio range 0.13–0.73) and five were heterozygous for CYP2D6*1 and CYP2D6*10 (debrisoquine metabolic ratio range 0.32–0.92). Only one subject homozygous for CYP2D6*10 (debrisoquine metabolic ratio 2.33) could be found and included in this study. CYP2D6*10B, *3 and *4 alleles were analysed by allele-specific polymerase chain reaction amplification as described earlier [3, 5, 14, 15]. There were no significant differences in sex, age, or bodyweight between the genotype groups. All subjects were nonsmokers and healthy as assessed by medical history, physical examination, as well as clinical laboratory safety tests.
Debrisoquine and nortriptyline pharmacokinetic data for Caucasians were obtained from two earlier studies [11, 12]. Twenty-one subjects were included (five subjects in each group of 0, one, two and three or four functional CYP2D6*1 or *2-genes, respectively, and one subject with 13 CYP2D6*2 gene copies). The selection of subjects among the heterozygous EMs was based on genotype as well as phenotype (debrisoquine metabolic ratios> 1.0). Selection for the rest of the subjects was based on genotype alone [11, 12].
The study was approved by the ethics committee at Huddinge University Hospital and performed according to the Declaration of Helsinki. All subjects gave their informed consent.
The study protocol for the Koreans was identical to that used in the two earlier studies in Caucasians [11, 12]. In brief, all subjects were given a single oral dose of 20 mg debrisoquine (Declinax®; F. Hoffmann-La Roche Ltd, Basle, Switzerland) and 25 mg nortriptyline (Sensaval®; H. Lundbeck AB, Helsingborg, Sweden) on separate occasions, with a 6-week interval, following an overnight fast. Venous blood samples were collected immediately before and 1, 2, 3, 4, 6, 8, 24, 32, 48, 72, 96 and 168 h after drug administration (and also at 12 h post nortriptyline administration). After debrisoquine intake, urine was collected in 0–12, 12–24, 24–32, 32–48, 48–72 and 72–96-h aliquots.
Drug analysis in plasma was performed by gas chromatography–mass spectrometry (debrisoquine, 4-hydroxydebrisoquine, nortriptyline and unconjugated 10-hydroxynortriptyline) and in urine by high-performance liquid chromatography (debrisoquine, the enantiomers of 4-hydroxydebrisoquine and 7- and 8-hydroxydebrisoquine). The methods have been described in detail in the two previous papers [11, 12]. The limits of quantification (LOQ) for debrisoquine and 4-hydroxydebrisoquine in plasma were 5 nm and those for nortriptyline and unconjugated 10-hydroxynortriptyline were 3 nm. The LOQ for debrisoquine and for the enantiomers of 4-hydroxydebrisoquine in urine were 5 nm and 25 nm, respectively. The between-day coefficient of variation (CV) of debrisoquine and 4-hydroxydebrisoquine in plasma varied from 3% to 11% over the concentration range of 5–500 nm. No measurements of CV for plasma nortriptyline and unconjugated 10-hydroxynortripyline were available. The between-day CV for S- and R-4-hydroxydebrisoquine in urine was 2.8% at a concentration of 2.5 µm and 5% at 50 µm. The within-day CV was 5.4% at 5 µm for both compounds in urine.
The maximum plasma concentration (Cmax) and the time to reach Cmax (tmax) were estimated from the observed concentration vs. time data. The total area under the plasma concentration vs. time curves (AUCtot) of nortriptyline and 10-hydroxynortriptyline were calculated by the trapezoidal rule and extrapolation to infinity using the last measured plasma concentration and the elimination rate constant (λz) determined by log-linear regression analysis of the terminal concentration–time points. The apparent oral plasma clearance (Cl/F) of nortriptyline was calculated as Dose/AUCtot, assuming 100% bioavailability. For debrisoquine and 4-hydroxydebrisoquine, the 0–8-h AUC (AUC0−8) was determined by the trapezoidal rule. The apparent terminal elimination half-lives (t1/2) of all compounds were calculated using linear regression analysis from the terminal linear parts of the concentration vs. time curves.
Ninety-five percent confidence intervals (CI) on the ratio of means were calculated for the pharmacokinetic parameters of the different Korean genotypes. CIs of 98.75% (with Bonferroni correction α = 0.0125, giving an overall α of 0.05) on the ratio of means were calculated for the pharmacokinetic parameters of the different ethnic groups.
Results
In the Korean subject homozygous for the CYP2D6*10 allele, the plasma concentration–time profile of debrisoquine fluctuated considerably, thus precluding calculation of pharmacokinetic parameters. The debrisoquine concentrations in this subject were in general higher than those in the other subjects, whereas the metabolite concentrations were similar. The plasma concentrations of both nortriptyline and its 10-hydroxymetabolite in this subject were low compared with the other subjects, which might reflect impaired absorption. Owing to these aberrant kinetics, this subject homozygous for CYP2D6*10 was excluded from further analysis.
There were no major differences between Koreans heterozygous for CYP2D6*10 and those homozygous for CYP2D6*1 with regard to the plasma pharmacokinetics of nortriptyline or debrisoquine, or their metabolites. The only statistically significant difference was seen in the AUC0−8 of 4-hydroxydebrisoquine, which was higher in the *1/*1 group than in the *1/*10 group (Table 1).
Table 1.
Pharmacokinetic parameters (mean ± SD) of single oral doses of nortriptyline (25 mg) and debrisoquine (20 mg) in Korean subjects with CYP2D6*1/*1 and*1/*10 genotypes
| CYP2D6 genotype | |||
|---|---|---|---|
| *1/*1 (n = 5) | *1/*10 (n = 5) | 95% CI for the ratio of mean values | |
| Nortriptyline | |||
| Cmax (nmol l–1) | 36 ± 10 | 37 ± 8 | 0.60, 1.47 |
| tmax (h) | 8.0 ± 3.4 | 5.0 ± 2.0 | 0.93, 2.76 |
| t1/2 (h) | 29.3 ± 7.8 | 27.5 ± 4.5 | 0.74, 1.46 |
| Cl/F (l h–1 kg–1) | 1.9 ± 0.3 | 1.0 ± 0.2 | 0.62, 1.53 |
| AUCtot (nmol h l–1) | 1591 ± 530 | 1672 ± 587 | 0.59, 1.52 |
| 10-hydroxynortriptyline | |||
| Cmax (nmol l–1) | 60 ± 16 | 63 ± 16 | 0.66, 1.37 |
| tmax (h) | 6.6 ± 3.4 | 5.6 ± 1.7 | 0.60, 2.02 |
| t1/2 (h) | 26.5 ± 7.4 | 27.1 ± 5.3 | 0.68, 1.36 |
| AUCtot (nmol h l–1) | 2317 ± 373 | 2143 ± 492 | 0.83, 1.44 |
| AUCNT/AUC10-OH-NT | 0.69 ± 0.22 | 0.77 ± 0.12 | 0.60, 1.26 |
| Debrisoquine | |||
| Cmax (nmol l–1) | 149 ± 87 | 137 ± 26 | 0.52, 1.77 |
| tmax (h) | 2.0 ± 0.7 | 2.0 ± 1.0 | 0.52, 2.14 |
| t1/2 (h)† | 51 | 45 | 0.44, 1.37 |
| AUC0–8 (nmol h l–1) | 827 ± 468 | 661 ± 48 | 0.59, 2.02 |
| 4-OH-debrisoquine | |||
| Cmax (nmol l–1) | 217 ± 21 | 176 ± 34 | 1.00, 1.55 |
| tmax (h)† | 2.0 ± 0.7 | 2.4 ± 1.2 | 0.44, 1.71 |
| AUC0–8 (nmol h l–1) | 1023 ± 180 | 692 ± 93 | 1.17, 1.85* |
| AUCdebr/AUC4-OH-debr | 0.83 ± 0.50 | 0.97 ± 0.17 | 0.38, 1.46 |
Significant difference. Cmax, Maximum plasma concentration; tmax, time at which Cmax occurred; t1/2, apparent terminal elimination half-life; AUCtot, area under the plasma concentration-time curve extrapolated to infinity; AUC(0,8) area under the plasma concentration-time curve during the first 8 h; Cl/F, apparent oral plasma clearance.
Terminal half-lives of debrisoquine were determined from mean data for the group.
A comparison of the pharmacokinetics of nortriptyline between Koreans and Caucasians (data from [11]) revealed no significant differences between Koreans of either genotype and Caucasians with two functional CYP2D6 genes (Table 2). On the other hand, the AUC of nortriptyline and the AUCnortriptyline/ AUC10-hydroxynortriptyline ratio were significantly lower in both Korean groups compared with Caucasians with one or no functional CYP2D6 genes, but higher than in Caucasians with three or four functional genes (Table 2).
Table 2.
Comparison of the pharmacokinetics of nortriptyline (NT) and debrisoquine (debr) between Koreans and Caucasians with different genotype groups
| CYP2D6 genotype comparison Korean vs. Caucasian | |||||
|---|---|---|---|---|---|
| Korean | Caucasian (number of functional genes)* | AUCNT | AUCNT/AUC10-OH-NT | AUCdebr | AUCdebr/AUC4-OH-debr |
| *1/*1 | 0 | 0.19, 0.69* | 0.13, 0.41* | 0.26, 0.82* | 0.00, 0.02* |
| *1/*1 | 1 | 0.25, 0.84* | 0.20, 0.58* | 0.27, 0.84* | 0.05, 0.26* |
| *1/*1 | 2 | 0.68, 2.28 | 0.54, 1.56 | 0.85, 2.67 | 0.47, 2.22 |
| *1/*1 | 3/4 | 1.03, 3.42* | 1.26, 3.64* | 1.06, 3.33* | 0.86, 4.11 |
| *1/*10 | 0 | 0.20, 0.72* | 0.15, 0.47* | 0.24, 0.75* | 0.01, 0.03* |
| *1/*10 | 1 | 0.26, 0.88* | 0.23, 0.67* | 0.24, 0.77* | 0.07, 0.35* |
| *1/*10 | 2 | 0.72, 2.40 | 0.62, 1.79 | 0.77, 2.44 | 0.63, 2.98 |
| *1/*10 | 3/4 | 1.08, 3.61* | 1.45, 4.19* | 0.97, 3.04 | 1.16, 5.53* |
The 98.75% CI for the ratio of mean values (Korean vs. Caucasian) with Bonferroni correction (α = 0.0125) is given.
0 functional CYP2D6 genes corresponds to PM, 1 to heterozygous EM, 2 to homozygous EM and 3/4 to ultrarapid metabolizers.
Similarly, no differences in the pharmacokinetics of debrisoquine were noted between Koreans of either genotype and Caucasians with two functional CYP2D6 genes (data from [12]). The AUC of debrisoquine and the AUCdebrisoquine/AUC4-hydroxydebrisoquine ratio were lower in both Korean groups compared with Caucasians with one or no functional genes. The AUC of debrisoquine was significantly higher in the Korean *1/*1 group compared with Caucasians with three or four functional genes (Table 2).
There were no differences in the urinary recovery of debrisoquine or its hydroxy metabolites between the two Korean genotype groups (data not shown). The total recovery of debrisoquine, the enantiomers of 4-hydroxydebrisoquine and 7- and 8-hydroxydebrisoquine in the two Korean groups (39% and 36%) was similar to that in Caucasians with two (48%) or three (42%) functional CYP2D6 genes [12].
Discussion
In the present work, Korean subjects with CYP2D6*1/*10 genotype had a similar capacity to metabolize debrisoquine and nortriptyline as subjects homozygous for CYP2D6*1, whether Korean or Caucasian. Thus, the presence of one copy of the CYP2D6*10 allele appears to impair the catalytic activity of CYP2D6 to only a limited degree. Unfortunately, we were not able to study more than one Korean subject homozygous for CYP2D6*10. In two previous studies on nortriptyline in which Chinese healthy volunteers were given a single dose [13], and Japanese patients were given multiple doses [16], a clear relationship between the occurrence of CYP2D6*10 allele and various parameters of E-10-hydroxylation of nortriptyline was shown. In both studies the effect of the CYP2D6*10/*10 genotype on metabolism was more pronounced than that of the CYP2D6*1/*10 genotype. The influence of the CYP2D6*10 allele on debrisoquine hydroxylation has been well documented in several Asian groups [3–5].
Debrisoquine metabolic ratios among Koreans homozygous for CYP2D6*1 (0.13–0.72) and those heterozygous for CYP2D6*10 (0.32–0.92) in the present study were similar to and representative of metabolic ratios found in a larger population of native Koreans [5] with these genotypes. Thus, the lack of difference in the pharmacokinetics of debrisoquine and nortriptyline between the two Korean genotype groups is less likely to be explained by selection of subjects in the present study. However, it cannot be excluded that minor differences might have been revealed if larger numbers of subjects had been included.
Acknowledgments
We are grateful to C. Alm RN and E. Götharsson RN for excellent assistance. This study was supported by Karolinska Institutet, the Swedish Medical Research Council (3902), the Robert Bosch Foundation and USPHS grants GM31304 (to G.R.W.) and GM60548 (to L.B.)
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