Determination of phenytoin and its major metabolites in human serum by high-performance liquid chromatography with fluorescence detection

A highly sensitive fluorometric high-performance liquid chromatographic method was developed for the simultaneous determination of phenytoin and its major metabolites [5-(3-hydroxyphenyl)-5-phenylhydantoin and 5-(4-hydroxyphenyl)-5-phenylhydantoin]. After extracting these compounds and 5-(4-methylphenyl)-5-phenylhydantoin (MPPH) as an internal standard from serum (50 mu l) with ethyl acetate, they were further converted into the corresponding fluorescent derivatives by a reaction with 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone in the presence of potassium hydrogen carbonate and dibenzo-18-crown-6 in acetonitrile. The derivatives were separated by reversed-phase chromatography on a YMC-Pack ODS-A column with a mixture of acetonitrile-50 mM phosphate buffer (pH 7.0) (4:6, v/v) as a mobile phase, and were then detected spectrofluorometrically at 448 nm with excitation at 365 nm. The detection limits for phenytoin, 5-(3-hydroxyphenyl)-5-phenylhydantoin and 5-(4-hydroxyphenyl)-5-phenylhydantoin added to serum were 0.6, 3.0 and 0.8 ng (2.4, 11 and 3.1 pmol) ml(-1) serum at a signal-to-noise ratio of three. The method was applied to determine the unbound- and total-phenytoin and the metabolites levels in the serum obtained from two healthy volunteers, after oral administration of the drug.