Dr N. Shirai, First Department of Medicine, Hamamatsu University School of Medicine, 1-20-1, Handayama, Hamamatsu, 431-3192, Japan. E-mail: firstname.lastname@example.org
Omeprazole is mainly metabolized in the liver by CYP2C19, a genetically determined enzyme, whereas rabeprazole is mainly reduced non-enzymatically and partially metabolized by CYP2C19. The therapeutic effects of rabeprazole are therefore assumed to be less affected by an individual’s CYP2C19 status.
To investigate the acid inhibitory effects and plasma levels of omeprazole and rabeprazole with reference to different CYP2C19 genotypes.
Fifteen healthy volunteers took a daily dose of 20 mg of omeprazole or rabeprazole for 8 days. On post-dose days 1 and 8, 24-h profiles of intragastric pH were recorded and plasma concentrations of omeprazole, rabeprazole and their metabolites were determined.
After single and repeated doses of omeprazole, the intragastric pH values and plasma concentrations of omeprazole and its metabolites were significantly dependent on the CYP2C19 genotype. Significant differences in the same kinetic and dynamic parameters were also observed after single doses of rabeprazole. Although the plasma levels of rabeprazole differed among the different CYP2C19 genotype groups after repeated doses, no significant differences in intragastric pH values were observed.
The acid inhibitory effects of omeprazole and rabeprazole are significantly dependent on the CYP2C19 genotype status, as well as on their intrinsic pharmacokinetic and pharmacodynamic characteristics and dosing schemes.
Proton pump inhibitors, such as omeprazole (OPZ), lansoprazole and rabeprazole (RPZ), have been widely used as acid inhibitory agents for the treatment of upper gastrointestinal diseases.1 OPZ is a substituted benzimidazole, which effectively inhibits gastric acid secretion by irreversibly binding to the proton pump (H+,K+-ATPase) in gastric parietal cells,2,3 and is mainly metabolized by a genetically determined enzyme, S-mephenytoin-4′-hydroxylase (CYP2C19), in the liver to hydroxyomeprazole (OH-OPZ).4–8 OPZ is partially metabolized by CYP3A4 to omeprazole sulphone (OPZ-SFN), which is then metabolized to hydroxyomeprazole sulphone (OH-OPZ-SFN) by CYP2C19.9 In individuals with a poor metabolizer (PM) phenotype or genotype of CYP2C19, the plasma concentration of OPZ is therefore markedly increased,4,7,8 and the pharmacodynamic effects of OPZ are assumed to be enhanced in these patients. Indeed, in a recent study, the acid inhibitory effects of a single oral dose of OPZ significantly differed among the different CYP2C19 genotype groups.10 However, in clinical practice, OPZ is administered as a repeated dosing scheme, and it is not clear whether the intragastric pH profile after such a multiple regimen also depends on the CYP2C19 genotype status.
Although RPZ is also a substituted benzimidazole, it has a metabolic profile different from that of other proton pump inhibitors: RPZ has been reported to be reduced mainly via a non-enzymatic pathway to thioether-rabeprazole (thioether-RPZ), with only minor CYP2C19 and CYP3A4 involvement.11,12 Therefore, the acid inhibitory effects of RPZ are assumed to be less influenced by the CYP2C19 phenotype or genotype status compared to those of other proton pump inhibitors, such as OPZ. Moreover, RPZ has a greater and faster acid inhibitory effect than OPZ.13
Based on the above-mentioned background knowledge, we investigated the intragastric pH levels after single and repeated doses of OPZ or RPZ in the different CYP2C19 genotype groups, as well as the metabolic disposition characteristics of each proton pump inhibitor, in order to clarify whether CYP2C19 genotyping would be a useful clinical tool for optimizing the dose and selection of these proton pump inhibitors.
SUBJECTS AND METHODS
Subjects and CYP2C19 genotypes
Blood samples were obtained from 50 healthy Japanese subjects after receiving written informed consent. DNA was extracted from each individual’s leucocytes using a commercially available kit (Genomix, Talent, Trieste, Italy). Genotyping procedures for identifying the CYP2C19 wild-type (CYP2C19*1) gene (*1) and the two mutated alleles, CYP2C19*2 (*2) and CYP2C19*3 (*3), were performed by a polymerase chain reaction-restriction fragment length polymorphism method using allele-specific primers, as described by de Morais et al.,14 with minor modifications made by Kubota et al.15Helicobacter pylori (H. pylori) infection was screened by a serological test (HM·CAP Kit, Enteric Product Inc., NY, USA).
A total of 15 H. pylori-negative healthy volunteers were invited and approved to participate in this study. Six were homozygous for the wild-type alleles in both exon 5 and 4 (*1/*1) and were classified as the homozygous extensive metabolizer (homEM) group. Another five were heterozygous for the *2 mutation without *3 mutation (*1/*2) or heterozygous for the *3 mutation without *2 mutation (*1/*3) and were classified as the heterozygous extensive metabolizer (hetEM) group. The remaining four were heterozygous for both the *2 mutation and *3 mutation (*2/*3) or homozygous for the *2 mutation without the *3 mutation (*2/*2) and were classified as the PM group (Table 1). None of the subjects consumed excessive amounts of alcohol or smoked. None of the subjects had taken any drugs for at least 1 week before or during the study. Their H. pylori-negative status was confirmed by 13C-urea breath test. Written informed consent was obtained again from each of the subjects before participation in the study. The protocol was approved in advance by the Human Institutional Review Board of Hamamatsu University School of Medicine.
Table 1. Demographic characteristics of subjects enrolled in the study with different CYP2C19 genotypes
All subjects were first given a placebo dose, and then participated in a double-blind crossover study with OPZ or RPZ. They were treated with 20 mg of OPZ (Omepral, Fujisawa Pharmaceutical Co. Ltd, Osaka, Japan) or 20 mg of RPZ (Pariet, Eisai Co. Ltd, Tokyo, Japan) in a randomized, crossover manner for two separate 8-day periods. All medications were taken once daily at 08.00 h. There was a washout period of at least 7 days between the two study periods. The 24-h intragastric pH monitoring and measurement of plasma levels of OPZ, RPZ and their metabolites (OPZ-SFN, OH-OPZ and thioether-RPZ) were performed on days 1 and 8 of each of the two study periods. Three standard meals (noon, 17.00 h and 08.00 h), prepared at the hospital, were provided for each of the subjects.
Intragastric pH measurement
After overnight fasting, a glass electrode was inserted transnasally and placed about 5 cm below the cardia under fluoroscopic guidance. The electrode was calibrated with standard buffer (pH 1.07 and 6.86) before recording the pH with a MEMORY pH METER (Chemical Ind. Co. Ltd, Tokyo, Japan). Intragastric pH recordings started soon after the oral dose of OPZ or RPZ at 08.00 h on days 1 and 8.
Sample collection and assays of OPZ, RPZ and their metabolites
Blood samples were collected in heparinized tubes before and 1, 2, 3, 5, 7, 10 and 24 h after the first and eighth doses. After collection, the blood samples were immediately centrifuged at 3000 r.p.m. for 10 min. For RPZ and its thioether metabolite, 100 μL of 1% diethylamine solution was added to the 1-mL sample of plasma, but was not required for the determination of plasma concentrations of OPZ and its hydroxy and sulphone metabolites. All samples were stored at –20 °C until assayed. Plasma levels of OPZ and RPZ were measured by high performance liquid chromatography as previously reported.16–18
Intragastric pH values are given as the median (and range), and other numerical data are given as the mean ± S.E.M. The values for the areas under the plasma concentration–time curves from 0 to 24 h (AUC) for OPZ and RPZ and their metabolites were calculated using the linear trapezoidal method. Median intragastric pH values for every hour and for 24 h were obtained from the raw pH values. Statistically significant differences in the values of intragastric pH between the different CYP2C19 genotype groups were determined by the Mann–Whitney U-test when a significant difference was observed by the Kruskal–Wallis test. Statistically significant differences in the mean AUC values for OPZ, OH-OPZ, OPZ-SFN, RPZ and thioether-RPZ among the three different CYP2C19 genotype groups were determined by one-way analysis of variance (ANOVA) with a Scheffe-type multiple comparison test. To determine whether there was a change in intragastric pH values from single to repeated doses of OPZ or RPZ, the Wilcoxon signed rank test was used. Differences in intragastric pH values between OPZ and RPZ in single and repeated doses were also determined by the Wilcoxon signed rank test within each genotype group. To determine whether the AUC values for OPZ and RPZ increased from single to repeated doses, the paired t-test was used. Statistical calculations were performed by SAS software (SAS Institute Inc., Cary, NC, USA). All P values are two-sided, and P < 0.05 was taken to indicate statistical significance.
Twenty-four-hour intragastric pH
The raw data on the median intragastric pH–time curves after the placebo and after the single and repeated doses of OPZ or RPZ in the three different genotype groups are shown in Figure 1. The median values of the mean 24-h intragastric pH after the placebo and after single and repeated doses of OPZ or RPZ in the three different genotype groups are summarized in Table 2. No significant differences in the intragastric pH values were observed among the three groups after placebo administration (Table 2).
Table 2. Intragastric pH values after single and repeated doses of omeprazole (OPZ) and rabeprazole (RPZ)
After a single dose of OPZ, a significant difference in the intragastric pH values was observed among the three groups. The median intragastric pH value of the PM group was the highest, followed by that of the hetEM group, with that of the homEM group the lowest. After repeated doses of OPZ for 8 days, a similar significant difference in the intragastric pH values was observed among the three different genotype groups. Although after the single dose of RPZ, the median intragastric pH value of the PM group was significantly higher than that of the homEM group, after repeated doses of RPZ for 8 days, no significant differences were observed among the three different genotype groups (Table 2).
Significant increases in intragastric pH values from single to repeated doses of OPZ were observed in the homEM and hetEM groups. Significant increases in intragastric pH values from single to repeated doses of RPZ were also observed in the homEM and hetEM groups. However, a significant difference in intragastric pH values between OPZ and RPZ was observed only in the homEM group after the single dose. Although there was no statistically significant difference in intragastric pH values between OPZ and RPZ after repeated doses of each proton pump inhibitor, the intragastric pH values after repeated doses of OPZ were lower than those observed with RPZ in each of the different CYP2C19 genotype groups (Table 2).
AUC values for OPZ and RPZ and their metabolites in plasma
The mean plasma concentration–time curves of OPZ, OH-OPZ and OPZ-SFN in the three different genotype groups after single and repeated doses of OPZ are shown in Figure 2, and the mean AUC values for OPZ, OH-OPZ and OPZ-SFN are listed in Table 3. Although the mean AUC values for OH-OPZ were not statistically significantly different after single and repeated doses (Table 3), the mean AUC values for OPZ after a single dose were significantly different among the three different genotype groups, with a relative ratio of 1.0, 2.0 and 10.7 in the homEM, hetEM and PM groups, respectively. The mean AUC values for OPZ after repeated doses were also significantly different, with a relative ratio of 1.0, 2.3 and 6.8 in the homEM, hetEM and PM groups, respectively. The AUC values for OPZ-SFN were also dependent on the CYP2C19 status (Table 3).
Table 3. AUC values (ng.h/mL) for omeprazole, rabeprazole and their metabolite(s)
The mean plasma concentration–time curves of RPZ and thioether-RPZ and their mean AUC values are shown in Figure 3 and given in Table 3, respectively. The mean AUC values for RPZ and thioether-RPZ after single and repeated doses were significantly different between the homEM and PM groups (Table 3). The mean AUC values for RPZ after a single dose differed among the three different genotype groups, with a relative ratio of 1.0, 2.3 and 3.3 in the homEM, hetEM and PM groups, respectively. The mean AUC values for RPZ after repeated doses also differed among the three groups, with a relative ratio of 1.0, 3.0 and 5.3 in the homEM, hetEM and PM groups, respectively. Similarly, the plasma concentrations (Figure 3) and AUC values for thioether-RPZ were dependent on the CYP2C19 status (Table 3).
Although the mean AUC values for OPZ tended to increase from single to repeated doses in the homEM and hetEM groups, no significant increase in the mean AUC values for RPZ from single to repeated doses was observed in any of the three different genotype groups.
We determined intragastric pH values after single and repeated doses of OPZ or RPZ in three different CYP2C19 genotype groups, as well as the plasma levels of the two proton pump inhibitors and their metabolites. By taking into account the data derived from single or repeated doses of the two proton pump inhibitors, we observed that the kinetic disposition and dynamic effects of these proton pump inhibitors on intragastric pH depend on the individual’s CYP2C19 genotype status.
In the homEM group, OPZ reached a maximum plasma concentration within 2 h after a single dose and was more rapidly eliminated from the systemic circulation compared with the hetEM and PM groups. After repeated doses of OPZ, the maximum plasma concentration of OPZ in the homEM group was increased, but was less than that in the other two groups. Therefore, in the homEM group, neither single nor repeated doses of 20 mg of OPZ appeared to increase intragastric pH to the levels achieved in the other two groups.
Because CYP2C19, which is mainly responsible for the metabolization of OPZ,5,9,12 is deficient in the PM group, the duration of high and sustained plasma concentrations of OPZ is presumed to be longer in the PM group than in the homEM group, thereby achieving a stronger and longer inhibition of gastric acid secretion. In the present study, the mean plasma OPZ level at 7 h post-dose in the PM group was almost identical to the peak level observed in the homEM group.
The AUC values for OPZ increased with repeated doses in the homEM and hetEM groups, as reported previously.17–19 Because OPZ is not only metabolized by CYP2C19, but also inhibits CYP2C19 activity,20 the first-pass metabolism would be decreased after a repeated dosing scheme of OPZ, and the plasma OPZ levels would then increase.19 The decrease in acid degradation of OPZ with an increase in intragastric pH after repeated doses of OPZ appeared to contribute to this increase in plasma OPZ level to some extent.19
Although RPZ is partially metabolized to demethylated RPZ by CYP2C19,11,12 it is thought that the extent of the metabolism of RPZ by CYP2C19 is less compared with that of OPZ.11,12,18 In addition, RPZ has a more rapid onset of pharmacological action (i.e. inhibition of H+,K+-ATPase) compared with OPZ,13,21,22 and is equal to or greater in potency than OPZ in in vitro and in vivo models of gastric acid inhibition.13,23 Williams et al.13 reported that 20 mg of RPZ once daily had a significantly faster onset of antisecretory activity than 20 mg of OPZ. In the present study, intragastric pH values in the homEM group after a single dose of OPZ were significantly lower than those with RPZ, but no significant differences in intragastric pH values were observed in the hetEM and PM groups. Nevertheless, based on our overall data on the interphenotypic differences in the antisecretory effects and metabolism of OPZ and RPZ, we are tempted to propose that the individual’s CYP2C19 genotype status should be taken into account when using either of these proton pump inhibitors in patients with acid-related disorders.
The AUC for RPZ has previously been reported to depend on the CYP2C19 phenotype, assessed using racemic mephenytoin for S-mephenytoin 4′-hydroxylation,11,18 where the study subjects were classified into two phenotype groups, EMs or PMs. In the present study, we classified our subjects into the three groups, homEM, hetEM and PM, on the basis of genotype testing, and documented that the intragastric pH and AUC for RPZ after a single dose were affected by the CYP2C19 genotype status. After repeated doses of RPZ, however, intragastric pH was not affected by the CYP2C19 genotype status, while the AUC for RPZ was affected by the CYP2C19 genotype status. Although we have no reasonable explanation for the discrepancy between the kinetics and dynamics of RPZ, we hypothesize that the plasma levels of RPZ would become sufficient for acid inhibition, even in the homEM group, after repeated dosing.
We also observed that the AUC for RPZ did not increase with repeated doses, as reported previously.17 The AUC for thioether-RPZ also did not increase with repeated doses because, unlike OPZ, RPZ should not inhibit CYP2C19 activity. Thioether-RPZ, which is formed non-enzymatically from RPZ, is metabolized to demethylated thioether-RPZ by CYP2C19, and therefore the AUC for thioether-RPZ is significantly higher in the PM group compared with the homEM group.
Gastro-oesophageal reflux disease (GERD) is one of the most common chronic diseases to affect adults in the USA and Western Europe.24,25 Although the mechanism of GERD is still obscure, gastric acid suppression is the most common therapeutic approach, and a stronger and longer gastric acid suppression is considered to be required for appropriate therapy of GERD compared with the traditional treatment of gastroduodenal ulcers.26 A proton pump inhibitor is frequently used in the treatment of GERD as the acid inhibitory agent, and higher healing rates have been achieved in comparison with H2-receptor antagonists.27,28 However, there still remains a subgroup of approximately 10% of patients with GERD who are resistant to acid suppression, even when treated with OPZ, 40 mg/day.29 According to Leite et al.,30 six of seven patients with GERD, who had an abnormal gastric acid secretion with 40 mg/day of OPZ, successfully attained a sufficient acid suppression with 80 mg/day. Thus, one of the conclusions that can be drawn from our findings is that many of the OPZ-resistant patients with GERD may have the homEM genotype of CYP2C19. Therefore, a greater dose of OPZ would be required to attain a clinically satisfactory acid suppression for these patients, who are resistant to the usual, constant, dose-based (not genomically tailored) proton pump inhibitor therapy that is universally conducted regardless of the patient’s CYP2C19 genotype status. On the other hand, RPZ could be used for such GERD patients instead of OPZ, because the pharmacodynamic effects (e.g. intragastric pH) of RPZ are less affected by the CYP2C19 genotype status, as observed in the present study, and RPZ has a more potent anti-inhibitory effect than OPZ.12,13,21
Finally, our results must be interpreted within the following limitations. The results were obtained using single and short-term (i.e. 8-day) repeated doses of OPZ or RPZ in a CYP2C19-genotyped, otherwise healthy, volunteer group without H. pylori infection, but not in a patient group. Thus, the therapeutic effects of OPZ or RPZ as a function of CYP2C19 genotype need to be re-evaluated in an appropriate study design in patients with upper gastrointestinal disorders with and without chronic H. pylori infection undergoing a long-term dosing regimen with these proton pump inhibitors. This study should be viewed as a preliminary basis for further studies. Nevertheless, the acid inhibitory effects of proton pump inhibitors depend not only on their pharmacokinetic and pharmacodynamic characteristics, but also on the individual’s CYP2C19 genotype status. Therefore, we anticipate that genotype testing of CYP2C19 may be a useful tool for optimizing drug dosage and the selection of proton pump inhibitors.
We thank Mr Takeo Akahoshi, Chemicals Inspection and Testing Institute, Tokyo, Japan, for the use of an L-column ODS for assaying rabeprazole and thioether-rabeprazole. This study was supported by grants-in-aid for Scientific Research from the Ministry of Education, Culture, Science and Sports of Japan (08670577 and 10672149), and a grant (No. 99-2) from the Organization for Pharmaceutical Safety and Research (OPSR), Tokyo, Japan.