Unchanged cytochrome P450 3A (CYP3A) expression and metabolism of midazolam, triazolam, and dexamethasone in mdr(-/-) mouse liver microsomes

P-Glycoprotein (P-gp) and cytochrome P450 3A (CYP3A) share common substrates and expression properties, but the relationship of mdr1 deficiency to CYP3A-mediated metabolism and protein, expression is not established. The in vitro kinetic parameters of CYP3A-mediated metabolism of midazolam (MDZ), triazolam (TRZ), and dexamethasone (DEX) were studied in liver microsomes from three mdr1a(-/-) mice, one mdr1a/b(-/-) mouse, and mdr1a/b(+/+) controls. The kinetic profiles of CYP3A-mediated MDZ 4-hydroxylation were not significantly different between mdr1-deficient animals and controls. Overall mean (+/- SEM, N = 8) values were: V-max, 0.74 +/- 0.05 nmol/min/mg protein; K-m, 28.2 +/- 2.7 mu M; and estimated intrinsic clearance, 0.026 +/- 0.003 mL/min/mg protein. Likewise, rates of formation of alpha-OH- and 4-OH-TRZ (from 500 mu M TRZ), and of DEX metabolites sensitive to ketoconazole inhibition, M1 and M5 (from 20 mu M DEX), did not differ between mdr1-deficient and control animals. Immunoquantified microsomal CYP3A protein levels in mdr1a(-/-), mdr1/a/b(-/-), and mdr1a/b(-/-) mice were not different, with overall mean immunoreactive protein levels of 2.68 +/- 0.09 pmol/mu g protein. Although CYP3A and P-gp share aspects of activity and expression, disruption of the mdr1 genes does not affect CYP3A-mediated metabolism or protein expression in the mouse. BIOCHEM PHARMACOL 57;11:1227-1232, 1999. (C) 1999 Elsevier Science Inc.