A mass spectrometric method for the determination of stable isotope labeled phenytoin suitable for pulse dosing studies.

A gas chromatographic mass spectrometric method has been developed for the determination in biological fluids of phenytoin (5,5-diphenylhydantoin), 5-(4-hydroxyphenyl)-5-phenylhydantoin (the major metabolite of phenytoin), and simultaneously, their stable isotope labeled analogs [5,5-diphenyl-2-13C-1,3-15N2-hydantoin and 5-(4-hydroxyphenyl)-5-phenyl-2-13C-1,3-15N2-hydantoin]. Quantification was achieved by an isotopic dilution technique: 5,5-di(pentadeuterophenyl)-hydantoin and 5-(4-hydroxy-3,5-dideuterophenyl)-5-phenyl-2-13C-1,3-15N2-hydantoin were used as internal standards. Molecular ion abundances of the permethylated derivatives were measured using a limited mass range repetitive scanning technique. The method is accurate, selective, reproducible and linear for analysis of 1.0 ml of serum and 0.5 ml of urine samples at the expected concentrations of drug (serum: 0.1-30.0 micrograms ml-1) and metabolite (serum: 0.1-10.0 micrograms ml-1; urine: 5.0-200.0 micrograms ml-1). The pharmacological equivalence of labeled and unlabeled phenytoin is demonstrated for a human volunteer. The results are discussed in the light of the further applications of the method, i.e. determination of the pharmacokinetics of a pulse dose of the labeled drug administered to patients who are taking a steady state dose of the unlabeled drug.