Metabolic disposition of pantoprazole, a proton pump inhibitor, in relation to S-mephenytoin 4’-hydroxylation phenotype and genotype

Objectives: To assess the possible relationship between the metabolic disposition of pantoprazole and genetically determined S-mephenytoin 4’-hydroxylation phenotype and genotype. Metho& The pharmacokinetic disposition of pantoprazole was investigated in 14 Japanese male volunteers (seven extensive and seven poor metabolizers of S-mephenytoin). All subjects received a single 40 mg oral dose of pantoprazole as the enteric-coated formulation. I2esults:An interphenotypic difference in the metabolic disposition of pantoprazole was observed: the mean values for area under the concentration-time curve (AUC), elimination half-life (t&, and apparent oral clearance were significantly (p < 0.01) greater, longer, and lower, respectively, in the poor metabolizers than in the extensive metaboliiers. The mean AUC of pantoprazole sulfone was greater (p c 0.01) in the poor metabolizers than in the extensive metabohzers, whereas the mean AUC of the main demethylated metabolite (M2) was lower (p < 0.01) in the poor metabolizers than in the extensive metabolizers. A significaut negative correlation was observed between the individual values for log,,% urinary excretion of 4’-hydroxymephenytoiu and AUC of pantoprazole ( rs = -0.816; p < 0.005). The CTP2C19 genotyping test results were found to be in a complete accordance with the phenotypes. Co~clzlsion: These data indicated that the metabolic disposition of pantoprazole is under the pharmaco- genetic control of S-mephenytoin 4’-hydroxylase (CYP2C19). (Clin Pharmacol Ther 1997;62:619-28.)

Makoto Tanaka, PhD, Tadashi Ohkubo, PhD, Koichi Otani, MD, PhD, Akihito Suzuki, MD, Sunao Kaneko, MD, PhD, Kazunobu Sugawara, PhD, Yuichi Ryokawa, BS, Hideo Hakusui, PhD, Shunji Yamamori, PhD, and Takashi Ishizaki, MD, PhD Tokyo and Hirosaki, Japan

Pantoprazole [5-(difluoromethoxy)-Z[[(3,4-di- methoxy-2-pyridyl)methyl]suhinyl]-lH-benzimidazole, similar to the prototype drug, omeprazole (Fig. l)] is a

From the Drug Metabolism and Analytical Chemistry Research Laboratory and the Medical Development Department III, Daiichi Pharmaceutical Co. Ltd., the Mitsubishi Kagaku Bio- Clinical Laboratories, Inc., and the Department of Clinical Pharmacology, Research Institute, International Medical Cen- ter of Japan, Tokyo, and the Departments of Pharmacy and Neuropsychiatry, Hirosaki University Hospital, Hirosaki.

Presented in part at the Ninety-eighth Annual Meeting of the American Society for Clinical Pharmacology and Therapeutics, San Diego, California, March 5-8, 1997.

Received for publication May 16, 1997; accepted Aug. 25, 1997. Reprint requests: Makoto Tanaka, PhD, Drug Metabolism and

Analytical Chemistry Research Laboratory, Daiichi Pharma- ceutical Co. Ltd., l-16-13 Kitakasai, Edogawa-ku, Tokyo 134, Japan.

Copyright 0 1997 by Mosby-Year Book, Inc. 0009-9236/97/$5.00 + 0 13/l/85735

substituted benzimidazole sulfoxide and a selective and long-acting proton pump inhibitor.’ In healthy white subjects, pantoprazole was well tolerated after the single and multiple intravenous and oral adminis- tration and produced a dose-dependent reduction in gastric acid output?J Pantoprazole is approved and used for the treatment of acid-related gastrointestinal disorders such as reflux esophagitis and peptic ulcers in several countries.6 Pantoprazole is currently in- volved in phase II clinical trials as an antiulcer drug in Japan.

The pharmacokinetics of omeprazole, the first- registered proton pump inhibitor, has been investi- gated extensively in humans. These studies have indicated that the in vivo metabolism of omeprazole is related to S-mephenytoin 4’-hydroxylation status.‘-l4 Recently the panel studies of other proton pump inhibitors, rabeprazole (E3810) and lansopra-

619

620 Tanaka et al. CLINICAL PHABMA COLOGY &THERAPEUTICS

DECEMBER 1997

Pantoprazole Omeprazole

Sulfone .-

Fig. 1. Chemical structures of pantoprazole and omeprazole and the proposed major meta- bolic pathways of the former proton pump inhibitor.

zole, have shown that their metabolic disposition is also under the coregulatory control of genetically de- termined S-mephenytoin 4’-hydroxylase (CYP2C19) activity.9’15

In a white population, a small proportion (ap- proximately 3%) of healthy subjects eliminate pan- toprazole more slowly than the remaining subjects, as reflected by a prolonged elimination half-life (t& and an increased area under the concentra- tion-time curve (AUC).16 These slow eliminators were considered to be the poor metabolizers of pantoprazole, among whom six were phenotyped with racemic mephenytoin retrospectively and five of them were found to be poor metabolizers of S-mephenytoin.16 Thus this preliminary finding sug- gests that the metabolism of pantoprazole is also under the genetic control of CYP2C19. However, further studies with a larger number of the poor and extensive metabolizers of S-mephenytoin are re- quired to draw a firm conclusion on the pharmaco- kinetics of pantoprazole to elucidate the clinical implications of the CYP2C19-related polymorphic metabolism. This panel study was therefore in- tended to prospectively elucidate the role of the S-mephenytoin oxidation polymorphism in the me- tabolism of pantoprazole.

METHODS Subjects. Fourteen unrelated, healthy, Japanese

male subjects participated in the study after being fully informed of the study purpose. They were phe- notyped and genotyped for S-mephenytoin 4’- hydroxylation (see Methods). Seven of the subjects were poor metabolizers of S-mephenytoin 4’-

hydroxylation, whereas the remaining seven were determined to be extensive metabolizers. The indi- vidual and mean + SD demographic data and 4’- hydroxylation capacity variables of S-mephenytoin and its genotyping status are shown in Tables I and II. None of the subjects had a history of significant medical illness or hypersensitivity to any drug. Their normal health status was judged on the basis of a physical examination with screening blood chemis- tries, urinalysis, a complete blood count, and elec- trocardiogram before the study. The study protocol was approved by the Ethics Committee of Hirosaki University Hospital, and each subject gave written informed consent before the study. They were asked to refrain from the use of any medications, including alcohol and over-the-counter drugs, for at least 2 days before and throughout the study period.

Study protocol. Each volunteer received a single oral dose of pantoprazole sodium sesquihydrate (40 mg as pantoprazole), with 120 ml water on the kinetic study day (around 8 am) as the enteric- coated tablet formulation (Daiichi Pharmaceutical Co. Ltd., Tokyo, Japan) after an overnight fast. Lunch and the evening meal were provided approx- imately 4 and 9 hours after dosing, respectively. The subjects were allowed to drink water freely. Serial venous blood samples (10 ml each) were collected at 0 (predose), %, 1, l%, 2,2%, 3, 3%, 4,5,6,8, 10, 12, and 24 hours after dosing. All samples were stored frozen at -20” C until analysis.

Pantoprazole represents the free compound of pantoprazole sodium sesquihydrate. The dose and the concentrations were expressed as the equiva- lents of pantoprazole.

CLINICAL P HARMACOLOGY &THERAPEUTICS VOLUME 62, NUMBER 6 Tanaka et al. 621

Phenotyping test. All subjects took an oral dose of 100 mg racemic mephenytoin (Mesantoin, Sandoz Inc., East Hanover, N.J.). 4’-Hydroxymephenytoin excreted in the urine 8 hours after dosing was de- termined by the capillary gas-chromatographic method as described previously.17 The S-mepheny- toin oxidation phenotype status was determined by use of the percentage amount of 4’-hydroxy- mephenytoin excreted on a molar basis relative to the 100 mg racemic dose of mephenytoin and the hydroxylation index [i.e., the dose of S-mephenytoin (50% of the racemic dose) divided by the molar amount of 4’-hydroxymephenytoin excreted in the postdose 8-hour urine]8,9,‘5 In the case of the uri- nary excretion of 4’-hydroxymephenytoin, an indi- vidual with a log,, urinary excretion percentage ~0.3 (i.e., an antimode) was considered to be a poor metabolizer of S-mephenytoin.15,17

Genotyping test. Detection of the two defective mutation (cyP2C19,1 and CyP2C19,,) alleles was performed by the polymerase chain reaction (PCR) restriction enzyme analysis. The genotyping proce- dures for the identification of CyP2C19,, were per- formed by PCR amplification with intron 4-and exon 5-specific primers for CYP2C19, digestion of the PCR products with Sma I, and analysis by gel elec- trophoresis as described previously,i8 except that 4 mmol/L magnesium chloride was used in the PCR reaction. The genotyping procedures for the detec- tion of the CYP2C19,, defect were similar to those described previously.19120 In brief, the primers for amplification of the intron 3-exon 4 junction were sense 5’-TAITATTATCTGTTAACTAATATGA-3’ and antisense 5’-ACITCAGGGCITGGTCAATA- 3’. The PCR products were digested with BamHI and analyzed by 3% agarose gel electrophoresis. PCR amplification for both defects was performed with a Perkin-Elmer 9600 thermocycler (Perkin- Elmer Corp., Norwalk, Conn.) for 35 cycles; ampli- fication consisted of denaturation at 94” C for 30 seconds, annealing at 53” C for 10 seconds, and extension at 72” C for 60 seconds. An initial dena- turation step at 94” C for 5 minutes and a final extension step at 72” C for 7 minutes were also performed. PCR products were separated by elec- trophoresis on 3% agarose gel and visualized under ultraviolet light after staining of the gel with ethidium bromide. The normal (wild-type) allele is defined as CYP2C19,,,,.

Analytical methods. The concentrations of panto- prazole and its sulfone in serum (Fig. 1) were de- termined with the use of a modification of an HPLC

Table I. Subject demographics Subject Age Weight Height

No. 0 (kg) (cm)

Poor metabolizers 1 5 6 7 9

11 12 Mean + SD

Extensive metabolizers 2 3 4 8

10 13 14 Mean ? SD

43 55 168 39 54 165 36 68 175 38 57 160 28 62 175 24 60 160 28 60 179

34 2 7 59 + 5 169 5 8

33 68 171 41 55 168 26 70 171 28 54 168 26 53 165 27 60 164 38 65 169

31 2 6 61 + 7 168 2 3

method as reported previously by Huber and Mtil- ler.21 In brief, salicylanilide was used as an internal standard. Human serum (300 ~1) was mixed with the internal standard solution (60 l&ml as 50 ul) and water (30 t~l), and the resulting mixture was centri- fuged at 12,000 rpm for 5 minutes. A 200 l~,l aliquot of the supernatant was directly applied to a column- switching HPLC system as described below.

The HPLC system consisted of three model LC-6A pumps (Shimadzu, Kyoto, Japan) and a model SPD-6A ultraviolet detector (Shimadzu). The ultraviolet detector was operated at 290 nm. A model SIL-6B autosampler (Shimadzu) and a model FCV-2AI-I column-switching valve module (Shi- madzu) were used to provide the automated on-line extraction and analysis. The extraction column (10 X 6.0 mm internal diameter) was dry-packed with LiChroprep RP-2 (25 to 40 Frn particle size; E. Merck, Darmstadt, Germany). A Hypersil octa- decylsilane column (10 X 4.6 mm internal diameter, 5 km particle size; Shandon, London, England) was used as a guard column and attached ahead of the analytical column, which was a TSK gel octadecylsi- lane 80TM column (150 X 4.6 mm internal diame- ter, 5 pm particle size; Tosoh, Tokyo, Japan). A mixture of acetonitrile/methanol (4:l; solution A) and 10 mmol/L ammonium hydrogen phosphate (pH 6.8; solution B) was used as the mobile phase at a flow rate of 1.0 ml/min. The column temperature was kept at 40” C.

With use of the assay as described above, the

622 Tanaka et al. CLINICAL l’ HARMACOLOGY & THERAPEUTICS

DECEMBER 1997

Table II. Phenotyping variables and genotyping patterns of S-mephenytoin 4’-hydroxylation in the poor and extensive metabolizers

Subject No. Urinary excretion of Log,,% urinary excretion

4’-hydroqmephenytoin (%) of 4’-hydroxymephenytoin HI Log,, HI Genotype

Poor metabolizers 1 5 6 7 9

11 12 Mean + SD

Extensive metabolizers 2 3 4 8

10 13 14 Mean t SD

0.36 -0.442 138.5 2.14 1.22 0.088 40.9 1.61 0.30 -0.529 169.1 2.23 0.26 -0.581 190.7 2.28 1.20 0.078 41.8 1.62 2.05 0.312 24.4 1.39 0.51 -0.291 97.6 1.99

0.84 2 0.67 -0.195 + 0.352 100.4 t 67.2 1.89 + 0.35

39.9 1.60 1.25 0.098 20.1 1.30 2.48 0.395 30.7 1.49 1.63 0.211 34.7 1.54 1.44 0.158 39.6 1.60 1.26 0.101 35.7 1.55 1.40 0.146 38.1 1.58 1.31 0.118

34.1 2 6.9 1.52 + 0.10 1.54 + 0.44 0.175 IO.105

ml/m2 ml/m2 mllml ml/ml mllm2 m2lm2 ml/m2

wtlm2 wtlwt wtlml wtlml wtlwt wtlm2 wtlml

HI, Hydroxylation index (i.e., dose of S-mephenytoio divided by the molar amount of 4’-hydroxymephenytoin excreted in urine over 8 hours); ml, CYP2C19,,,I; m2, C’IT2Cl9,~ wt, wild-type allele (CW??C19,,).

lower quantitation limits for both pantoprazole and its sulfone in serum were 0.02 pg/ml. The intraday and interday accuracy and precision (n = 6) for both analytes were ~8.1% at concentrations above 0.02 pg/ml. At the quantitation limit of 0.02 pg/ml, the method showed an acceptable precision and accu- racy (<19%).

The serum concentrations of the main metabolite of pantoprazole (M2), which is formed by demethy- lation at the 4-position of the pyridine ring (Fig. l), followed by conjugation with sulfate, were deter- mined by a column-switching HPLC method similar to that as described above, except that a Capcell Pak MF column (50 X 4.6 mm internal diameter, 5 km particle size; Shiseido, Tokyo, Japan) was used as an extraction column.

An attempt to synthesize the analytical reference compound of M2 was unsuccessful. Therefore the solution of M2 isolated from human urine (supplied by Byk Gulden Pharmaceuticals, Konstanz, Ger- many) was used for the validation study of the assay method. The concentration of M2 was determined by comparing the peak area of M2 with that of pantoprazole, assuming that the molar absorption coefficient of M2 is identical to that of pantoprazole.

The lower quantitation limit for M2 in serum was 0.05 &ml. The intraday and interday accuracy and precision were <13%.

Pharmacokinetic and statistical analyses. The TopFit software program was used for pharma- cokinetic analysis. The pharmacokinetic parame- ters were determined with the model-independent method. The elimination rate constant (X,) was determined by the least-squares regression of the logarithm of serum concentration-time over the terminal phase. The t,,z was calculated as 0.693/ A,. Maximum serum concentration (C,,) and the time to reach C,, were read from the observed values. The area under the concentration-time curve (AUC) was determined to the last quantifi- able serum concentration by means of the linear trapezoidal rule and extrapolated to infinity with the terminal phase rate constant. The mean resi- dence time (MRT) was calculated as the ratio of the area under the first moment curve [AUMC(O-m)] to AUC(O-a). The apparent oral clearance (CL,,,,) of pantoprazole was calculated by use of the equation: CLoral = Dose/AUC(O-m).

The data were expressed as mean values + SD throughout the test. The pharmacokinetic parame- ters for the poor and extensive metabolizer groups were compared by use of an unpaired t test. The Mann-Whitney U test was used and Spear-man’s rank correlation (rs) was assessed when appropriate. Ap value of co.05 was considered to be statistically significant.

CLINICAL I’ HABMACOLOGY & THERAPEUTICS VOLUME 62, NUMBER 6 Tanaka et al. 623

0 2 4 6 6 1012 24 A Time after dosing (hours)

0 2 4 6 8 10 12 24 0 2 4 6 8 1012 24

B Time after dosing (hours) C Time after dosing (hours)

Fig. 2. Mean + SD serum concentration-time profiles of pantoprazole (A), the main metab- olite of pantoprazole (M2) (B), and pantoprazole sulfone (C) after a 40 mg dose of oral pantoprazole in the seven poor metabolizers (solid symbols) and seven extensive metabolizers (open symbols) of S-mephenytoin 4’-hydroxylation.

RESULTS All volunteers completed the study according to

the protocol. No clinically undesirable or significant signs or symptoms attributable to the administration of pantoprazole were recognizable throughout the study period.

Phenotyping and genotyping of study subjects. The extensive metabolizers had a mean I~I SD urinary 4’-hydroxymephenytoin excretion of 34.1% + 6.9% (range, 20.1% to 39.9%), which is approximately 41-fold greater than that of the poor metabolizers (0.84% t 0.67%; range, 0.26% to 2.05%; Table II). The log,,% urinary excretion of 4’-hydroxy- mephenytoin ranged from 1.30 to 1.60 among the

seven extensive metabolizers, whereas it ranged from -0.581 to 0.312 among the seven poor metabo- lizers (Table II).

The CYP2C19 genotyping test was found to pre- dict the phenotype status completely (Table II). Of the 14 subjects, two were homozygous for the wild- type allele in both exon 5 and exon 4 (wt/wt), three were heterozygous for the CYP2C19,, @t/ml), and two were heterozygous for the CYP2C19,,,, (wt/m2) in the extensive metabolizer group, whereas two were homozygous for the CY?‘2C19,,,1 (ml/ml), four were heterozygous for the two defects (ml/m2), and one was homozygous for the CYP2C19,, (m2/m2) in the poor metabolizer group.

624 TaPzaka et al. CLINICAL P HARMACOLOGY & THERAPEUTICS

DECEMBER 1997

Table III. Pharmacokinetic parameters of pantoprazole, pantoprazole sulfone, and M2 after a 40 mg dose of oral pantoprazole administered to seven poor metabolizers and seven extensive metabolizers

Parameter Poor

metabolizers Extensive

metabolizers p Value

Pantoprazole Cm, @,g/ml) AUC (kg * hr/ml) tlj2 ON MRT (hr) CLoral (ml/min)

Pa~&Eble sulfone C,L b&4 AUC (pg * hr/ml)

M2 Cm, (i-d4 AUC (pg * hr/ml)

5.00 2 1.19 34.88 ? 8.61

6.86 + 2.36 10.25 2 3.09 20.19 t 5.09

2.43 t 0.98

0.25 t 0.04 4.72 + 1.02*

0.09 -t 0.06 0.76 t 0.81*

3.03 k 0.59 5.86 ? 2.04 1.40 ? 0.14 3.12 + 0.91

125.5 + 41.1 2.00 + 0.76

0.11 k 0.03 1.30 2 0.68

0.81 + 0.24 2.42 2 0.50

co.01 co.01 co.01 co.01 ‘co.01

NS

co.01 co.01

co.01 co.01

Data are given as mean values 2 SD. M2, Main metabolite of pantoprazole; C,,, maximum concentration; AUC, area under the serum concentration-time curve; t,,, elimination half-life;

CL oralr apparent oral clearance; NS, not significant. *AUC(O-24 hr).

Kinetic disposition ofpantoprazole. The mean ? SD serum concentration-time profiles of pantoprazole and its metabolites after the single 40 mg oral dose of pantoprazole to the poor and extensive metabo- lizer groups are shown in Fig. 2. The mean + SD pharmacokinetic parameters for each compound are presented in Table III.

An interphenotypic difference in the metabolic disposition of pantoprazole was observed as follows: the mean AUC and C,, values of pantoprazole in the poor metabolizers were about 6- and 1.7-fold greater (p < 0.01) than those in the extensive me- tabolizers. The mean t,,, and MRT were signifi- cantly longer (p < O.Ol), and the mean apparent CLora, was significantly lower (p < 0.01) in the poor metabolizers than in the extensive metabolizers (Ta- ble III). A significant negative correlation was ob- served between log,,% urinary excretion of 4’- hydroxymephenytoin and the AUC of pantoprazole in all individuals (n = 14; r, = -0.816; p < 0.005; Fig. 3).

of the most widely studied polymorphisms in drug metabolism in humans.“-Z Individuals are segre- gated into the two groups-poor and extensive me- tabolizers-depending on their ability to 4’- hydroxylate S-mephenytoin. This polymorphism shows marked interracial differences. The incidence of the poor metabolizer phenotype is much higher (17% to 23%) in Oriental populations (Japanese, Chinese, and Korean)8*17,26>27 than in white populations. 12,13p23,24,26-29 This polymorphism affects the metabolism of a number of clinically important drugs, including omeprazole, propranolol, imipra- mine, hexobarbital, mephobarbital (INN, methyl- phenobarbital), chloroguanide (INN, proguanil), and diazepam.30”5

The mean AUC of pantoprazole sulfone was sig- nificantly greater (p < 0.01) in the poor metabolizer group than in the extensive metabolizer group, whereas the mean AUC of the main metabolite M2 was significantly lower @ < 0.01) in the poor metabolizers than in the extensive metabolizers (Table III).

DISCUSSION

The polymorphic enzyme that 4’-hydroxylates S- mephenytoin has been shown to be CYP2C19.36,37 The principal defect in both white and Japanese poor metabolizers has been shown to be a G+A mutation in exon 5 of CYP2C19 (CyP2C19,,), which produces an aberrant splice site.18 This mu- tation accounts for 75% to 85% of the defective alleles in both white and Japanese poor metaboliz- ers. A second defect (cyP2C19,,) consists of a G+A mutation in exon 4 that creates a premature- stop codon and appears to be extremely rare in white persons.” However, CW2C19,,,, accounted for the remaining defective alleles in the 17 available Japanese poor metabolizers of S-mephenytoin 4’- hydroxylation as reported by de Morais et all9

A pharmacogenetic entity in the metabolism of In this study, the two inactivating mutations of the anticonvulsant drug mephenytoin has been one CYP2C19-CyP2C19,1 and C3T2C19,,,2-ac-

CLINICAL PHARMA COLOGY & THERAPEUTICS VOLUME 62, NUMBER 6 Tanaka et al. 625

r, = -0.816

p< 0.005

-1.0 -0.5 0 0.5 1.0 1.5 2.0

LoglO % Urinary Excretion of 4’-Hydroxymephenytoin

Fig. 3. Relationship between log,,% urinary excretion of 4’-hydroxymephenytoi and area under the serum concentration-time curve (AUC) of pantoprazole observed in the 14 study subjects. Correlation of coefficient (rs) was assessed with Spearman’s rank correlation test. Solid circles, Poor metabolizers of S-mephenytoin 4’-hydroxylation; open circles, extensive metabolizers of S-mephenytoin 4’-hydroxylation.

counted for 100% of the poor metabolizers who participated (Table II). Five of the sevem subjects with an extensive metabolizer phenotype were found to have the heterozygous extensive genotypes (Table II), indicating a high proportion of the het- erozygotes in the Japanese extensive metabolizers, as recently reported by Kubota et a1.38

The panel studies7-l5 have shown that the meta- bolic dispositions of omeprazole, lansoprazole, and rabeprazole sodium are under a genetic control of CYP2C19. It has been reported that in a limited number of white subjects there exist apparent poor metabolizers of pantoprazole, who showed a much lower clearance of pantoprazole compared to the remaining extensive metabolizers.3,16 Poor metabo- lizers of pantoprazole have also been observed dur- ing a phase I clinical study conducted in healthy Japanese male volunteers.39 Nevertheless, no pro- spectively phenotyped or genotyped panel study has been performed to determine the difference in the metabolic disposition of pantoprazole between poor

and extensive metabolizers of S-mephenytoin 4’- hydroxylation.

This panel study has shown that the poor metabo- lizers of S-mephenytoin 4’-hydroxylation have a lower CLoral of pantoprazole than the extensive me- tabolizers, indicating that the metabolism of panto- prazole also cosegregates with the genetic polymor- phism of S-mephenytoin 4’-hydroxylation. The negative correlation between the metabolic capacity of S-mephenytoin (i.e., log,,% urinary excretion of 4’-hydroxymephenytoin) and AUC of pantoprazole (Fig. 3) supports this contention.

The AUC values of the two homozygous and the remaining five heterozygous extensive metabolizers were in the range from 4.98 to 9.35 kg . hr/ml and 3.49 to 7.26 l.rg * hr/ml, respectively, suggesting no significant gene-dose effect for CYP2C19. However, in this regard, further studies are obviously required to reach a definitive conclusion because the number of subjects in this study was too small.

The metabolic disposition of pantoprazole was

626 Tanaka et al.

investigated in healthy subjects after an oral 80 mg dose of 14C-labeled pantoprazole.40 Pantoprazole was well absorbed from the gastrointestinal tract, with the absolute bioavailability of 81%.40 Pantopra- zole underwent extensive hepatic metabolism and about 80% of an oral dose was excreted in urine, mostly as various sulfate conjugates and to a minor extent as glucuronic acid conjugates.r6 No parent drug was excreted into urine, and approximately 18% of the dose was excreted in the feces.40 The main serum and urine metabolite of pantoprazole or M2 is formed through demethylation at the 4- position of the pyridine ring (Fig. l), followed by the conjugation with sulfate.16*41

Pantoprazole lacks the 5methyl group on the pyridine ring of omeprazole (Fig. 1). The metabo- lism of this methyl group of omeprazole is mediated by CYP2C19.sV42a However, Simon4’ reported that by use of human liver microsomes the demethyla- tion of pantoprazole would also be mediated through CYP2C19. The present study showed that the serum concentrations, C,,, and AUC of M2 were much lower in the poor metabolizers than in the extensive metabolizers (Fig. 2, B, and Table III), indicating that the demethylation at the 4-position of the pyridine ring of pantoprazole is impaired in the poor metabolizers of S-mephenytoin 4’- hydroxylation and thus that this metabolic pathway is dependent on CYP2C19.

In contrast, the pharmacokinetic parameters of pantoprazole sulfone were in opposition to those of M2 (Table III). The mean C,, and AUC values are about 2.3 and 3.6 times greater, respectively, in the poor metabolizer group than in the extensive me- tabolizer group. These results suggest that the fur- ther metabolism of pantoprazole sulfone also ap- pears to be mediated through CYP2C19, as suggested for that of omeprazole sulfone by Anders- son et a1?3*44

Our study and other panel studies7-11,14715 have shown that the large interindividual variability in plasma or serum concentrations of proton pump inhibitors can be accounted for by the genetically determined S-mephenytoin 4’-hydroxylation poly- morphism. However, the magnitude of the contri- bution of this pharmacogenetic factor to the meta- bolic disposition of proton pump inhibitors appears to vary among the different drugs. For instance, the ratios of the mean AUC values in poor metabolizers versus extensive metabolizers for pantoprazole, omeprazole, lansoprazole, and rabeprazole sodium were 6.0 (this study), 6.3,9 4.0,i5 and 1.8,9 respec-

CLINICAL P HABMACOLOGY &THERAPEUTICS DECEMBER 1997

tively. Rabeprazole showed a much smaller differ- ence in AUC between the two groups compared with the other three proton pump inhibitors, indi- cating the least contribution of CYP2C19 to its met- abolic disposition.’ However, the clinical implica- tion of this pharmacokinetic difference, in light of the CYP2C19-related pharmacogenetics among these proton pump inhibitors, remains totally ob- scure at present. Further study is required to assess whether the difference in the kinetic characteristics of these proton pump inhibitors will prove to be of any clinical significance.

We express our sincere appreciation to Mr. Takeshi Ishizuka and Ms. Tsuyumi Kitada of Mitsubishi Kagaku Bio-Clinical Laboratories, Inc., for their skillful determi- nation of pantoprazole and its metabolites in serum sam- ples. Pantoprazole sulfone and the main metabolite of pantoprazole (M2) were gifts of Dr. Karl Zech of Byk Gulden (Konstanz, Germany). We are also grateful to him for his helpful comments and suggestions.

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