Assessment of urinary mephenytoin metrics to phenotype for CYP2C19 and CYP2B6 activity.
OBJECTIVE
(S)-Mephenytoin is selectively metabolised to (S)-4'-hydroxymephenytoin by CYP2C19. The urinary excretion of 4'-hydroxymephenytoin reflects the activity of individual enzymes. We evaluated fractioned urinary collection and beta-glucuronidase pre-treatment in order to determine the optimal CYP2C19 metrics. We also assessed whether urinary excretion of N-desmethylmephenytoin (nirvanol) might be a useful CYP2B6 metric in in vivo studies.


METHODS
A 50-mg dose of mephenytoin was administered to 52 volunteers as a component of phenotyping cocktails in four separate studies. Urine was collected up to 166 h post-dose. Urinary excretion of 4'-hydroxymephenytoin and nirvanol was quantified by liquid chromatography-tandem mass spectrometry, and common CYP2C19 and CYP2B6 genotypes were determined.


RESULTS
Cumulative excretion of 4'-hydroxymephenytoin in urine with beta-glucuronidase treatment collected from before mephenytoin administration up to 12-16 h thereafter showed the greatest difference between CYP2C19 genotypes and the lowest intra-individual variability (7%). Renal elimination of nirvanol was highest for a *4/*4 individual and lowest for individuals carrying the *5/*5 and *1/*7 genotype, but lasted for several weeks, thus making its use in cross-over studies difficult.


CONCLUSIONS
Cumulative urinary excretion of 4'-hydroxymephenytoin 0-12 h post-administration is a sensitive and reproducible metric of CYP2C19 activity, enabling the effect of a drug on CYP2C19 to be assessed in a small sample size of n=6 volunteers. While nirvanol excretion may reflect CYP2B6 activity in vivo, it is not useful for CYP2B6 phenotyping.
Klaassen
Tobias
T
Department of Pharmacology, Clinical Pharmacology, Hospital of the University of Cologne, Gleueler Str. 24, 50931, Köln, Germany.
Jetter
Alexander
A
Tomalik-Scharte
Dorota
D
Kasel
Dirk
D
Kirchheiner
Julia
J
Jaehde
Ulrich
U
Fuhr
Uwe
U
D000328:Adult; D000927:Anticonvulsants / Q000493:pharmacokinetics / Q000652:urine*; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism*; D001711:Biotransformation; D002851:Chromatography, High Pressure Liquid; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D003627:Data Interpretation, Statistical; D006801:Humans; D008297:Male; D013058:Mass Spectrometry; D008617:Mephenytoin / Q000031:analogs & derivatives / Q000378:metabolism / Q000493:pharmacokinetics / Q000652:urine*; D008875:Middle Aged; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*; D010641:Phenotype; D010672:Phenytoin / Q000031:analogs & derivatives / Q000652:urine
D000927:Anticonvulsants; C014544:ethylphenylhydantoin; C001419:hydroxyphenytoin; D010672:Phenytoin; 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; D008617:Mephenytoin