Direct enantiomeric resolution of mephenytoin and its N-demethylated metabolite in plasma and blood using chiral capillary gas chromatography.
A gas chromatographic method was developed for the determination of the R- and S- enantiomers of the anticonvulsant, mephenytoin, and its N-demethylated metabolite, 5-phenyl-5- ethylhydantoin ( PEH ), in plasma and blood. Direct enantiomeric separation of mephenytoin and its internal standard was obtained using a chiral capillary column ( Chirasil -Val) followed by nitrogen specific detection. However, resolution of the enantiomers of PEH and its internal standard required propylation at the 3-position of the hydantoin ring prior to analysis. Similar linear and reproducible standard curves were obtained from both plasma and blood over the concentration range 50 ng/ml to 5 micrograms/ml, and above 100 ng/ml the reproducibility was less than 8% (coefficient of variation). Pronounced stereoselective differences in the plasma concentration--time curves for both mephenytoin and PEH were observed in a normal subject who received a single oral dose of 300 mg racemic mephenytoin. The peak plasma level of S-mephenytoin was only one-fifth that of the R-enantiomer and its elimination half-life was less than 3 h compared to over 70 h for R-mephenytoin. Similarly, S- PEH levels were barely detectable whereas concentrations of R-metabolite steadily increased over 4-6 days before slowly declining.
D000927:Anticonvulsants; D002849:Chromatography, Gas / Q000379:methods; D006207:Half-Life; D006801:Humans; D006827:Hydantoins / Q000097:blood*; D008617:Mephenytoin / Q000031:analogs & derivatives / Q000097:blood*; D013237:Stereoisomerism
D000927:Anticonvulsants; D006827:Hydantoins; C014544:ethylphenylhydantoin; D008617:Mephenytoin