Effects of catalpol on the activity of human liver cytochrome P450 enzymes.

1. Catalpol possesses numerous pharmacological activities, and however, little data available for the effects of catalpol on the activity of human liver cytochrome P450 (CYP) enzymes. 2. This study investigates the inhibitory effects of catalpol on the main human liver CYP isoforms. In this study, the inhibitory effects of catalpol on the eight human liver CYP isoforms 1A2, 2A6, 2E1, 2D6, 2C9, 2C19, 2C8 and 3A4 were investigated in human liver microsomes. 3. The results indicated that catalpol could inhibit the activity of CYP3A4, CYP2E1 and CYP2C9, with IC50 values of 14.27, 22.4 and 14.69 μM, respectively, but those other CYP isoforms were not affected. Enzyme kinetic studies showed that catalpol was not only a noncompetitive inhibitor of CYP3A4, but also a competitive inhibitor of CYP2E1 and CYP2C9, with Ki values of 7.40, 10.75 and 7.37 μM, respectively. In addition, catalpol is a time-dependent inhibitor for CYP3A4, with maximum inactivation (kinact) and 50% maximum inactivation (KI) values of 0.02 min-1 and 1.86 μM, respectively. 4. The in vitro studies of catalpol with CYP isoforms suggest that catalpol has the potential to cause pharmacokinetic drug interactions with other co-administered drugs metabolized by CYP3A4, CYP2E1 and CYP2C9. Further in vivo studies are needed in order to evaluate the significance of this interaction.