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 meta bolism in liver microsomes of the common brush-tailed possum (Trichosu rus vulpecula). The mean maximal velocity (V-max) for 3-hydroxyquinine formation in possum livers was 1,512 +/- 510 pmol/mg protein/min (mal es) and 1,680 +/- 690 pmol/mg protein/min (females). The mean V-max va lue for 3-hydroxyquinine formation in possums was approximately threef old higher than that found in human livers. The mean apparent Michaeli s constant (K-m) for 3-hydroxyquinine formation in possum livers was 3 1.9 +/- 16 mu M in males and 16.1 +/- 5 mu M in females. At low concen trations of quinine (40 mu M), the quinine 3-hydroxylation was inhibit ed more than 90% by midazolam, 60% by troleandomycin, 40% by erythromy cin, and 47% by nifedipine, all of which are CYP3A inhibitors. Other i nhibitors for CYP2C9/10, CYP2D6, CYP2E1, and CYP1A1/2 showed little or no inhibition effect on 3-hydroxylation of quinine. Xenobiotic inhibi tion 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-hydrox y metabolites was also studied. The in vitro metabolism of midazolam w as found to be much lower in possum liver microsomes as compared to th at observed in human liver microsomes. The mean V-max values for 4-hyd roxy- and 1'-hydroxymidazolam in male possums were 179 +/- 53 and 479 +/- 333 pmol/mg protein/min, respectively. For female possums, the mea n V-max values were 235 +/- 31 and 671 +/- 143 pmol/mg protein/min, re spectively. These V-max values for male possums were 23 and 8 times le ss (17 and 6 times less for female possums), respectively, than those observed with human liver microsomes. The present study has demonstrat ed that, although possums are able to metabolize both midazolam and qu inine, the capacity to metabolize midazolam is considerably lower in p ossums livers than in human livers. This finding could be useful for t he selection of alternative poisons to control populations of possums.