Effect of the CYP2D6*10 genotype on venlafaxine pharmacokinetics in healthy adult volunteers

Abstract

Aims

Interindividual differences in the pharmacokinetics of venlafaxine, a new antidepressant, were shown during early clinical trials in Japan. Venlafaxine is metabolized mainly by CYP2D6 to an active metabolite, O-desmethylvenlafaxine (ODV). Therefore, the influence of the CYP2D6 genotypes on venlafaxine pharmacokinetics was examined in a Japanese population.

Methods

Twelve adult Japanese men in good health participated in this study. Genomic DNA was isolated from peripheral lymphocytes, and the CYP2D6 genotypes were determined by codon 188C/T, 1934G/A, 2938G/A and 4268G/C mutations using endonuclease tests based on PCR and by Xba I-RFLP analysis. Subjects were categorized into the following 3 groups (n=4 in each group); Group1: CYP2D6*10/*10, *5/*10, Group2: CYP2D6*1/*10, *2/*10 and Group3: CYP2D6*1/*1, CYP2D6*1/*2. Venlafaxine (25 mg, n=6; 37.5 mg, n=6) was administered orally at 09.00 h following an overnight fast. Plasma concentrations of venlafaxine and ODV were monitored by h.p.l.c. for 48 h.

Results

The C_max and AUC of venlafaxine were 184% and 484% higher in the group 1 subjects than in the group 3 subjects, and 101% and 203% higher in the group 1 than in the group 2, respectively.

Conclusions

These results suggest that CYP2D6*10 influences the pharmacokinetics of venlafaxine in a Japanese population.

Introduction

The frequency of the poor metabolizer (PM) with deficient CYP2D6 activity is reported to be 5% to 10% in Caucasians [1] and less than 1% in Japanese [2]. The CYP2D6*10 allele, which includes both the CYP2D6*10 A and CYP2D6*10B variants, is widely observed in Japanese and Chinese and has two amino acid changes (Pro34Ser and Ser486Thr) [3–7]. A similar rate of distribution of debrisoquine metabolic ratio (MR) values has been reported for the three Asian groups of Chinese, Japanese, and Koreans [8, 9]. The debrisoquine MR of the homozygous *10 subject is smaller than that of the PM, but larger than that of the homozygous *1 [6, 9].

Interindividual differences in the pharmacokinetics of venlafaxine, a new antidepressant [10], were shown during early clinical trials in Japan. Venlafaxine is metabolized into its major active metabolite O-desmethylvenlafaxine (ODV) by CYP2D6 [11]. The frequency of *10 is about 50% in Chinese [6] and 45% in Japanese (unpublished data). Therefore, in line with the results of the phase I clinical study of venlafaxine in Japan, we investigated the effect of *10 on the pharmacokinetics of venlafaxine and ODV in a Japanese population.

Methods

Study population

Twelve male healthy volunteers, age 21–31 years, participated in this study. Approve for the study was obtained from the local Institutional Review Board. Informed consent was obtained from all subjects. These subjects were categorized into the following three groups according to the CYP2D6 genotyping, Group 1: CYP2D6*10/*10, *5/*10, Group 2: CYP2D6*1/*10, *2/*10 and Group 3: CYP2D6*1/*1, CYP2D6*1/*2.

Genotyping

Genomic DNA was isolated from peripheral lymphocytes of each subject. Four different fragments were amplified to detect 188, 1938, 2938 and 4268 mutation sites by polymerase chain reaction (PCR) method, and then digested by a restriction enzyme Hph I, Mva I, Hha I and Ban II, respectively [6, 12, 13]. Xba I restriction fragment length polymorphism (RFLP) analysis was performed to detect *5 allele according to the method of Steen et al. [14]. The genotyping by 188C/T mutation was performed using two primer sets 9/10 and 9/10B according to the method of Johansson et al. [6]. CYP2D6*2 allele have gene conversion amplified only by 9/10B primers and two mutations (2938G/A, 4268G/C).

Clinical study protocol

Venlafaxine tablets containing 25.0 mg or 37.5 mg were provided by Wyeth (Japan). Six subjects received 25.0 mg and six subjects 37.5 mg at 09.00 h with 100 ml water. Blood samples (6 ml) were obtained from the median cubital vein a total of 15 times from 20 min up to 48 h after administration for the pharmacokinetics analysis. The plasma concentrations of venlafaxine and its major metabolite ODV were determined by h.p.l.c. [15]. The coefficient of variation of the assay was less than 5% at the lower limit of determination (5 ng ml⁻¹).

Non compartmental pharmacokinetic analysis method

The terminal elimination rate constant, λ_z was determined by linear regression of at least four data points from the terminal portion of plasma concentration-time plots. The area under the plasma drug concentration-time curve (AUC) was calculated using the linear trapezoidal rule up to the last measured plasma concentration Cp(last), and extrapolated to infinity by addition of the correction term Cp/λ_z. C_max, the maximum plasma drug concentration, was obtained as the measured value. The terminal elimination half-life, t_1/2, was determined by dividing ln2 by λ_z.

Statistics

All values are expressed as their mean±s.d. with 95% confidence interval (CI) for the mean difference. Statistical differences were determined between groups with anova and Kruskal–Wallis test. When a significant difference was detected, multiple comparison analysis was performed using Bonferroni/Dunn’s test. A P value less than 0.05 was considered to be statistically significant.

Results

The CYP2D6 genotype of the 12 subjects is shown in Table 1. The categorization based on codon 188 was well connected with that based on codon 4268 and 2938.

Table 1.

The CYP2D6 genotype of the 12 subjects.

The individual C_max values of venlafaxine differed by about 3-fold and individual AUC values differed by a maximum of 10-fold (Table 2). The subjects who had higher plasma concentrations of venlafaxine had lower plasma concentrations of ODV. The C_max and AUC values of venlafaxine were 184% and 484% higher for group 1 than those for group 3, and 101% and 203% higher for group 1 than for group 2. There were no significant differences between the different genotypes of CYP2D6 in the total plasma concentrations of venlafaxine and ODV.

Table 2.

Pharmacokinetic data^a of venlafaxine (VEN) and O-desmethylvenlafaxine (ODV) after administration of a single dose of venlafaxine. Values are mean±s.d. (range).

Discussion

In the past, detailed studies on the genetic polymorphism of CYP2D6 have not been conducted in Japan because the frequency of PM was reported as below 1% in the Japanese populations [2]. To date it has been impossible to explain the interindividual differences in the pharmacokinetics of venlafaxine by simply categorizing subjects on EM or PM. Therefore, we decided to take other genotypes among EM into consideration in explaining these differences. In this CYP2D6 genotyping study, we assumed the 188T, 2938G plus 4268C to be *10, and 2938 A plus 4268C, amplification using primers 9/10B to be *2 although the possibility of other alleles could not be ruled out.

In Chinese subjects, the influence of the 188C/T mutation on propranolol disposition and formation of morphine from codeine was reported [16, 17]. In Japanese subjects, however, there has been no investigation on the influence of *10 on the pharmacokinetics of other drugs with the exception of debrisoquine and sparteine [3, 8].

We have shown that the pharmacokinetics of venlafaxine is affected by the *10 genotype. However, concentrations of venlafaxine plus ODV were not different between groups and since ODV is equally active, the effect of genotype on pharmacokinetics is not expected to be translated into differences in response. The CYP2D6*10 allele may be a factor in determining interindividual differences in the pharmacokinetics of CYP2D6drug substrates in the Japanese population and, since metabolites of drugs that are CYP2D6substrates are generally inactive, may contribute to differences in drug response.