title: Plasma concentrations of 5-(4-hydroxyphenyl)-5-phenylhydantoin in phenytoin-treated patients.
abstract: A mass fragmentographic method was used to determine unconjugated and conjugated 5-(4-hydroxyphenyl)-5-phenylhydantonin (4-OH-DPH) in plasma of patients being treated with phenytoin (DPH). The plasma concentration of unconjugated 4-OH-DPH is stable at steady-state for DPH and the level does not change during the dose interval. Most patients (82.5%) have plasma concentrations of unconjugated 4-OH-DPH between 0.04 and 0.2 mug/ml at concentrations of DPH between 5 and 30 mug/ml. Interindividual differences in the ratio between plasma concentration of unconjugated 4-OH-DPH and DPH were noted in a wide range of doses. Dosage adjustments of DPH in patients were not associated with major changes in the plasma level of unconjugated 4-OH-DPH. The plasma concentration of conjugated 4-OH-DPH also showed interindividual differences and varied between 1.2 and 4.5 mug/ml in patients with DPH concentrations between 5 and 30 mug/ml. The data are consistent with the concept of Michaelis-Menten kinetics for DPH metabolism.
mesh_terms: D004305:Dose-Response Relationship, Drug; D004827:Epilepsy / Q000097:blood / Q000188:drug therapy*; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D006900:Hydroxylation; D010672:Phenytoin / Q000008:administration & dosage / Q000097:blood / Q000378:metabolism*; D013997:Time Factors
chemical_list: D010672:Phenytoin