In vitro hepatic metabolism of CYP3A-mediated drugs quinine and midazolam in the common brush-tailed possum (Trichosurus vulpecula)

This report characterizes the P450 isoenzymes involved in quinine metabolism in liver microsomes of the common brush-tailed possum (Trichosurus vulpecula). The mean maximal velocity (Vmax) for 3-hydroxyquinine formation in possum livers was 1,512 +/- 510 pmol/mg protein/min (males) and 1,680 +/- 690 pmol/mg protein/min (females). The mean Vmax value for 3-hydroxyquinine formation in possums was approximately threefold higher than that found in human livers. The mean apparent Michaelis constant (Km) for 3-hydroxyquinine formation in possum livers was 31.9 +/- 16 micromolar in males and 16.1 +/- 5 micromolar in females. At low concentrations of quinine (40 micromolar), the quinine 3-hydroxylation was inhibited more than 90% by midazolam, 60% by troleandomycin, 40% by erythromycin, and 47% by nifedipine, all of which are CYP3A inhibitors. Other inhibitors for CYP2C9/10, CYP2D6, CYP2E1, and CYP1A1/2 showed little or no inhibition effect on 3-hydroxylation of quinine. Xenobiotic inhibition studies suggest that the liver CYP3A enzyme family or one similar to human liver CYP3A is responsible for 3-hydroxylation of quinine in possum livers. The metabolism of midazolam to 1'-hydroxy and 4-hydroxy metabolites was also studied. The in vitro metabolism of midazolam was found to be much lower in possum liver microsomes as compared to that observed in human liver microsomes. The mean Vmax values for 4-hydroxy- and 1'-hydroxymidazolam in male possums were 179 +/- 53 and 479 +/- 333 pmol/mg protein/min, respectively. For female possums, the mean Vmax values were 235 +/- 31 and 671 +/- 143 pmol/mg protein/min, respectively. These Vmax values for male possums were 23 and 8 times less (17 and 6 times less for female possums), respectively, than those observed with human liver microsomes. The present study has demonstrated that, although possums are able to metabolize both midazolam and quinine, the capacity to metabolize midazolam is considerably lower in possum livers than in human livers. This finding could be useful for the selection of alternative poisons to control populations of possums.