Biotransformation of tritiated fentanyl in human liver microsomes. Monitoring metabolism using phenylacetic acid and 2-phenylethanol

Biochem Pharmacol. 1995 Nov 27;50(11):1921-4. doi: 10.1016/0006-2952(95)02088-8.


Norfentanyl has been identified previously as a urinary metabolite of fentanyl. However, at clinically relevant concentrations, norfentanyl are below the limits of detection. The use of labeled drug in metabolic studies is a standard approach to overcome the limitations imposed by metabolite concentrations that are below detection limits. Unfortunately, the available tritium-labeled fentanyl yields unlabeled norfentanyl following N-dealkylation. Thus, we have developed a technique to monitor the N-dealkylation of fentanyl using the other products of N-dealkylation. The biotransformation of fentanyl was studied in human liver microsomes. After incubation with human liver microsomes for 20 min, almost 50% of a 0.03 microM concentration of [3H]-fentanyl was metabolized to the [3H]N-dealkylated metabolite phenylacetaldehyde, which was then converted principally to [3H]2-phenylethanol and to a smaller extent to [3H]phenylacetic acid in microsomal incubates. The apparent Km,app and Vmax,app for norfentanyl formation were 82 +/- 21 microM and 4.7 +/- 0.4 nmol product formed/min/nmol cytochrome P450, respectively. Thus, this study defined methodology that can be used to evaluate the metabolism of fentanyl, both in vivo and in vitro, at clinically relevant concentrations.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Analgesics, Opioid / metabolism*
  • Biotransformation
  • Dealkylation
  • Drug Monitoring
  • Fentanyl / analogs & derivatives
  • Fentanyl / metabolism*
  • Humans
  • In Vitro Techniques
  • Microsomes, Liver / metabolism*
  • Phenylacetates / analysis*
  • Phenylethyl Alcohol / analysis*
  • Tritium


  • Analgesics, Opioid
  • Phenylacetates
  • Tritium
  • norfentanyl
  • phenylacetic acid
  • Phenylethyl Alcohol
  • Fentanyl