FENTANYL METABOLISM BY HUMAN HEPATIC AND INTESTINAL CYTOCHROME-P450 3A4 - IMPLICATIONS FOR INTERINDIVIDUAL VARIABILITY IN DISPOSITION, EFFICACY, AND DRUG-INTERACTIONS

The synthetic opioid fentanyl undergoes extensive metabolism in humans , Systemic elimination occurs primarily by hepatic metabolism. When ad ministered as a lozenge for oral transmucosal absorption, swallowed fe ntanyl is subject to first pass metabolism in the liver and possibly s mall intestine. Little is known, however, about the identity and forma tion of human fentanyl metabolites. This investigation identified rout es of human liver microsomal fentanyl metabolism and their relative im portance, tested the hypothesis that fentanyl is metabolized by human duodenal microsomes, and identified the predominantly responsible cyto chrome P450 isoforms. A GC/MS assay using deuterated internal standard s was developed for fentanyl metabolites, Piperidine N-dealkylation to norfentanyl was the predominant pathway of river microsomal metabolis m. Amide hydrolysis to despropionytfentanyl and alkyl hydroxylation to hydroxyfentanyl were comparatively minor pathways. Hydroxynorfentanyl was identified as a minor, secondary metabolite arising from N-dealky lation of hydroxyfentanyl. Liver microsomal norfentanyl formation was significantly inhibited by the mechanism-based P450 3A4 inhibitor trol eandomycin and the P450 3A4 substrate and competitive inhibitor midazo lam, and was significantly correlated with P450 3A4 protein content an d catalytic activity, Of six expressed human P450 isoforms (P450s 1A2, 2B6, 2C9, 2D6, 2E1, and 3A4), only P450 3A4 exhibited significant fen tanyl dealkylation to norfentanyl. These results indicate the predomin ant role of P450 3A4 in the primary route of hepatic fentanyl metaboli sm, Human duodenal microsomes also catalyzed fentanyl metabolism to no rfentanyl; the average rate was approximately half that of hepatic met abolism, Rates of duodenal norfentanyl formation were diminished by tr oleandomycin and midazolam, and were significantly correlated with P45 0 3A4 activity, suggesting a prominent role for P450 3A4. These result s demonstrate that human intestinal as well as liver microsomes cataly ze fentanyl metabolism, and N-dealkylation by P450 3A4 is the predomin ant route in both organs. The fraction of fentanyl lozenge that is swa llowed likely undergoes significant intestinal, as well as hepatic, fi rst-pass metabolism. Intestinal and hepatic first-pass metabolism, as well as systemic metabolism, may be subject to individual variability in P450 3A4 expression and to drug interactions involving P450 3A4.