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Tuesday 01 February 2000

Midazolam and triazolam biotransformation in mouse and human liver microsomes: relative contribution of CYP3A and CYP2C isoforms.

By: Perloff MD, von Moltke LL, Court MH, Kotegawa T, Shader RI, Greenblatt DJ.

J Pharmacol Exp Ther 2000 Feb;292(2):618-28

Midazolam (MDZ) and triazolam (TRZ) hydroxylation, reactions considered to be cytochrome P-4503A (CYP3A)-mediated in humans, were examined in mouse and human liver microsomes. In both species, alpha- and 4-hydroxy metabolites were the principal products. Western blotting with anti-CYP3A1 antibody detected a single band of immunoreactive protein in both human and mouse samples: 0.45 +/- 0. 12 and 2.02 +/- 0.24 pmol/mg protein (mean +/- S.E., n = 3), respectively. Ketoconazole potently inhibited MDZ and TRZ metabolite formation in human liver microsomes (IC(50) range, 0.038-0.049 microM). Ketoconazole also inhibited the formation of both TRZ metabolites and of 4-OH-MDZ formation in mouse liver microsomes (IC(50) range, 0.0076-0.025 microM). However, ketoconazole (10 microM) did not produce 50% inhibition of alpha-OH-MDZ formation in mouse liver microsomes. Anti-CYP3A1 antibodies produced concentration-dependent inhibition of MDZ and TRZ metabolite formation in human liver microsomes and of TRZ metabolite and 4-OH-MDZ formation in mouse liver microsomes to less than 20% of control values but reduced alpha-OH-MDZ formation to only 66% of control values in mouse liver microsomes. Anti-CYP2C11 antibodies inhibited alpha-OH-MDZ metabolite formation in a concentration-dependent manner to 58% of control values in mouse liver microsomes but did not inhibit 4-OH-MDZ formation. Thus, TRZ hydroxylation appears to be CYP3A specific in mice and humans. alpha-Hydroxylation of MDZ has a major CYP2C component in addition to CYP3A in mice, demonstrating that metabolic profiles of drugs in animals cannot be assumed to reflect human metabolic patterns, even with closely related substrates.

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