Biotransformation of tritiated fentanyl in human liver microsomes. Monitoring metabolism using phenylacetic acid and 2-phenylethanol.
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.
Tateishi
T
T
Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232-6602, USA.
Wood
A J
AJ
Guengerich
F P
FP
Wood
M
M
D000701:Analgesics, Opioid / Q000378:metabolism*; D001711:Biotransformation; D003640:Dealkylation; D016903:Drug Monitoring; D005283:Fentanyl / Q000031:analogs & derivatives / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000378:metabolism*; D010648:Phenylacetates / Q000032:analysis*; D010626:Phenylethyl Alcohol / Q000032:analysis*; D014316:Tritium
D000701:Analgesics, Opioid; D010648:Phenylacetates; D014316:Tritium; C080127:norfentanyl; C025136:phenylacetic acid; D010626:Phenylethyl Alcohol; D005283:Fentanyl