The inhibition study of human UDP-glucuronosyltransferases with cytochrome P450 selective substrates and inhibitors

Abstract

Human uridine-5′-diphosphoglucuronosyltransferases (UGTs) are the major phase II metabolizing enzymes. In the present study, five human UGTs (UGT1A1, 1A4, 1A6, 2B7, and 2B10) were individually expressed and used to examine the inhibition IC₅₀ values of 20 selective substrates and inhibitors of major cytochromes P450 (CYPs). The inhibition kinetics of UGT1A1 was also analyzed. The results showed that some compounds like α-naphthoflavone, paclitaxel, midazolam, cyclosporine A, and ketoconazole displayed strong inhibitions on UGT activities with their IC₅₀ values in a range of 4.1–26 µM. Especially, the IC₅₀ values were 4.1 ± 0.8 µM for ketoconazole in inhibiting UGT1A1-mediated β-estradiol-3-glucuronidation, and 4.9 ± 0.3 µM for paclitaxel towards UGT1A4-mediated midazolam-N-glucuronidation. Additionally, the IC₅₀ values of bupropion, tolbutamide, and testosterone in inhibiting UGT-mediated metabolisms were similar with the K_m values of respective CYPs. Some kinetic behaviours of UGTs were following Michaelis–Menten kinetics, while some were not.

Keywords::

• Inhibition
• UGT1A1
• UGT1A4
• cytochromes P450
• ketoconazole
• paclitaxel

Acknowledgements

The authors are very grateful for the help and cooperation from Yan Chen and Haitao Wang.

Declaration of Interest

This work was supported in part by the National High-tech R&D Program [Grant 2006AA02Z339] and [Grant 2008AA02Z314]; the Guangzhou Science and Technology Bureau [Grant 2006Z1-E4031, 2006P067]; and the Guangzhou Development District [Grant 2006Ss-P067].