Analysis of mephenytoin, 4‐hydroxymephenytoin and 4‐hydroxyphenytoin enantiomers in human urine by cyclodextrin micellar electrokinetic capillary chromatography: Simple determination of a hydroxylation polymorphism in man

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Abstract
Using cyclodextrin micellar electrokinetic capillary chromatography (CD-MECC), baseline separation of mephenytoin, 4-hydroxymephenytoin and 4-hydroxyphenytoin enantiomers in urine was effected with β-cyclodextrin. After single-dose administration of 100 mg of racemic mephenytoin, the 0–8 h urine was collected, and enzymatically hydrolyzed urine specimens were applied. For extensive metabolizers, a single peak for 4-hydroxymephenytoin was detected corresponding to the S-enantiomer. This peak was either very small or undectable in samples of poor metabolizers. Typically, mephenytoin could not be detected in these samples. However, application of undeglucu-ronidated extracts revealed the presence of free S-4-hydroxymephenytoin and R,S-mephenytoin and thus permitted phenotyping via both the urinary S:R enantiomeric ratio of mephenytoin and the hydroxylated metabolite. Application of enzymatically hydrolyzed and extracted urines after phenytoin administration (100 mg; 0–8 h urine collection) revealed the presence of S-4-hydroxyphenytoin. Thus, CD- MECC is shown to be a simple and attractive approach for (i) the confirmation of the stereoselectivity of the aromatic hydroxylation of mephenytoin and phenytoin, (ii) the simple and rapid differentiation between extensive and poor metabolizers for mephenytoin, and (iii) assessment of compliance.