Pharmacogenetic roles of CYP2C19 and CYP2B6 in the metabolism of R- and S-mephobarbital in humans.
OBJECTIVE
We assessed the relationship between the metabolism of R- and S-mephobarbital (MPB) and genetic polymorphisms of cytochrome P450 (CYP) 2C19 and CYP2B6. Nine homozygous extensive metabolizers (homo-EMs, 2C19*1/2C19*1) of CYP2C19, ten heterozygous EMs (hetero-EMs, 2C19*1/2C19*2, 2C19*1/2C19*3) and eleven poor metabolizers (PMs, 2C19*2/2C19*2, 2C19*3/2C19*3, 2C19*2/2C19*3) recruited from a Japanese population, received an oral 200 mg-dose of racemic MPB. Blood and urine samples were collected, and R-MPB, S-MPB and the metabolites, phenobarbital (PB) and 4'-hydroxy-MPB, were measured. Each subject was also genotyped for CYP2B6 gene.


RESULTS
The mean area under the plasma concentration-time curve (AUC) of R-MPB was 92-fold greater in PMs than in homo-EMs. R/S ratios for AUC of MPB were much higher in PMs than in EMs (homo- and hetero-). The cumulative urinary excretion of 4'-hydroxy-MPB up to 24 h postdose was 21-fold less in PMs than in homo-EMs. The metabolic ratio of AUCPB/(AUCS-MPB + AUCR-MPB) was higher in PMs than in EMs (homo- and hetero-). In addition, this metabolic ratio was lower in the carriers of CYP2B6*6 compared with that in its non-carriers.


CONCLUSIONS
Our results indicate that the 4'-hydroxylation of R-MPB is mediated via CYP2C19 and that the rapid 4'-hydroxylation of R-MPB results in a marked difference in the pharmacokinetic profiles between R-MPB and S-MPB in the different CYP2C19 genotypic individuals. In addition, a minor fraction of the interindividual variability in PB formation from MPB may be explainable by the CYP2B6*6 allele.
Laboratory of Pharmacology and Toxicology, Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japan.
Kaoru
K
Kobayashi
Jun
J
Morita
Kan
K
Chiba
Atsuko
A
Wanibuchi
Miyuki
M
Kimura
Shin
S
Irie
Akinori
A
Urae
Takashi
T
Ishizaki
D000328:Adult; D019540:Area Under Curve; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics*; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D005838:Genotype; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D006900:Hydroxylation; D007564:Japan; D008297:Male; D008618:Mephobarbital / Q000493:pharmacokinetics*; D006899:Mixed Function Oxygenases / Q000235:genetics*; D015394:Molecular Structure; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics*; D010597:Pharmacogenetics; D010634:Phenobarbital / Q000097:blood / Q000652:urine; D011110:Polymorphism, Genetic / Q000235:genetics*; D013237:Stereoisomerism
D008618:Mephobarbital; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C045793:CYP2C19 protein, human; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D010089:Oxidoreductases, N-Demethylating; D010634:Phenobarbital