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A dihydrodiol metabolite of mephenytoin (5-dihydroxycyclohexadienyl)-5-ethyl-3-methylhydantoin and other mono- and dihydroxylated and N-demethylated metabolites were identified in urine from a male epileptic patient receiving therapy with mephenytoin (300 mg/day). Metabolites, extracted from urine before and after enzymatic hydrolysis, were derivatized with a trimethylsilyl reagent and analyzed by combined gas chromatography and mass spectrometry. Two previously unreported metabolites were characterized: 5-ethyl-5-(di-hydroxyphenyl)-3-methylhydantoin and 5-ethyl-5-(hydroxy-methoxy-phenyl)-3-methylhydantoin. The structures of several other metabolites were confirmed: N-demethylmephenytoin, 5-ethyl-5-hydroxyphenylhydantoin, 5-ethyl-5-hydroxyphenyl-3-methylhydantoin and mephenytoin dihyrodiol. The dihydrodiol metabolite was of special interest since it was probably produced via an epoxide intermediate, 5-(epoxy-cyclohexadienyl)-5-ethyl-3-methylhydantoin. Previous reports have demonstrated that epoxides of this structural class are extremely reactive compounds, capable of alkylating biologic macromolecules. Covalent binding of the mephenytoin epoxide to macromolecules may be an important factor in the production of adverse and sometimes fatal side effects observed in patients receiving long-term therapy with mephenytoin.
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