title: Assessment of urinary mephenytoin metrics to phenotype for CYP2C19 and CYP2B6 activity.
abstract: 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.
authors: Department of Pharmacology, Clinical Pharmacology, Hospital of the University of Cologne, Gleueler Str. 24, 50931, Köln, Germany.
authors: Tobias
authors: T
authors: Klaassen
authors: Alexander
authors: A
authors: Jetter
authors: Dorota
authors: D
authors: Tomalik-Scharte
authors: Dirk
authors: D
authors: Kasel
authors: Julia
authors: J
authors: Kirchheiner
authors: Ulrich
authors: U
authors: Jaehde
authors: Uwe
authors: U
authors: Fuhr
mesh_terms: 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
chemical_list: 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