Enzymes in addition to CYP3A4 and 3A5 mediateN-demethylation of dextromethorphan in human liver microsomes

Abstract

Both indinavir and troleandomycin (CYP3A inhibitors) are incapable of completely inhibiting dextromethorphan metabolism to 3‐methoxymorphinan in human liver microsomes. It is hypothesized that CYPs in addition to CYP3A4 and 3A5 contribute to this biotransformation. The effect of CYP‐selective inhibitors on the residual 3‐methoxymorphinan activity in human liver microsomes (i.e. in the presence of 30 μm indinavir, a selective CYP3A4 and 3A5 inhibitor) was measured to identify these enzymes. At this concentration, indinavir completely inhibited the formation of 3‐methoxymorphinan by rCYP3A4 and rCYP3A5. In addition, the formation kinetics of 3‐methoxymorphinan in rCYPs was measured. Only CYP2B6, 2C8 and 2C18 were considered likely candidates as contributors to residual 3‐methoxymorphinan activity. The residual 3‐methoxymorphinan activity was highly correlated with CYP2B6 activity as measured by CYP2B6 antibody (r2=0.90, p<0.001) and by orphenadrine (r2=0.97, p<0.001), but was not correlated (r2=0.12, p>0.05) with CYP2C8 activity. Collectively, these findings suggest that CYP2B6 is a major contributor towards residual 3‐methoxymorphinan activity, while CYP2C8 and 2C18 are either minor contributors or do not contribute to this metabolic process. Copyright © 1999 John Wiley & Sons, Ltd.

A comprehensive assay for nine major cytochrome P450 enzymes activities with 16 probe reactions on human liver microsomes by a single LC/MS/MS run to support reliablein vitroinhibitory drug–drug interaction evaluation

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Dynamical spot queries to improve specificity in P450s based multi-drugs monitoring

“…The two drug compounds present Km in the same magnitude order. The Km and Vmax are 595 uM and 26.9 pmol/min/ for pmol of P450 for CP [25], while they are 268 uM 1.74 pmol/min/for pmol of P450 for DX [26]. In fact, detection peaks registered in the voltammograms are well explained by fitting accounting for both the DX and CP contributions with a small chi square.…”

Lack of Indinavir Effects on Methadone Disposition Despite Inhibition of Hepatic and Intestinal Cytochrome P4503A (CYP3A)

“…43,44,55 Nonetheless, the aim of this investigation was not profound CYP3A inhibition, but rather CYP- and organ-specific effects. Indinavir did not appreciably inhibit CYP1A2, CYP2B6, CYP2C9 or CYP2E1 in human liver microsomes, 52,55 or clinically. 56 …”

“…Indinavir in vitro is a reversible inhibitor of expressed and human liver microsomal CYP3A isoforms, and, unlike other antiretroviral drugs (ritonavir, saquinavir, amprenavir, lopinavir and nelfinavir), is not a mechanism-based inhibitor. 43,44,52 Less information is available regarding indinavir effects on clinical drug disposition, and on CYP3A activity specifically, as indinavir is usually coadministered with ritonavir. Indinavir increased rifampicin and rifabutin AUCs 1.7- and 2-fold, respectively, increased nelfinavir and clarithromycin AUCs, and halved amprenavir clearance.…”