Excretion of the principal urinary metabolites of phenytoin and absolute oral bioavailability determined by use of a stable isotope in patients with epilepsy

Background: The anticonvulsant properties of phenytoin (PHT) were discovered in 1938. Since then, it has been one of the most widely used antiepileptic drugs. It is slowly absorbed, extensively plasma protein-bound, exhibits a nonlinear, concentration-dependent pharmacokinetic profile, and has a narrow therapeutic range. Methods: We determined PHT bioavailability during steady-state therapy by 1) measurement of the two principal deconjugated PHT urinary metabolites, 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and 5-(3,4-dihydroxy-1,5-cyclohexadien-1-yl)-5-phenylhydantoin (DHD); and 2) direct determination of absolute bioavailability after simultaneous administration of an oral formulation and parenteral stable-labeled PHT (SL-PHT). Urinary metabolites were quantified by an isocratic HPLC-NI-APCI-MS method. The urinary dose recovery was calculated by dividing the molar recovery of the major PHT urinary metabolites by the molar dose received. Results: Urinary metabolite recovery was surprisingly low, 35.4% ± 15.7% in younger patients (age 21-49 years old) and 32.9% ± 15.0% in patients aged 65 to 93 years. Absolute bioavailability was 86.4% ± 19.4% and 92.5% ± 25.2%, respectively. A weak, but significant, Spearman rank correlation was observed between urinary metabolite recovery and oral bioavailability (P = 0.00924, R = 0.166). Conclusion: This weak correlation may be the result of variability in urinary versus biliary-fecal excretion of p-HPPH glucuronide. This study demonstrates that daily PHT oral absorption exhibits wide interpatient variability, which may account for fluctuations in PHT concentration over time.