Cooperativity of α-naphthoflavone in cytochrome P450 3A-dependent drug oxidation activities in hepatic and intestinal microsomes from mouse and human

1. The effects of several CYP3A substrates (alpha -naphthoflavone (alpha NF), terfenadine, midazolam, erythromycin) on nifedipine oxidation and testosterone 6 beta -hydroxylation activities were investigated in hepatic and intestinal microsomes from mouse and human. 2. alpha NF (10 muM) and terfenadine (100 muM) inhibited nifedipine oxidation activities (at substrate concentration of 100 muM) in mouse hepatic microsomes to similar to 50%, but not in mouse intestinal microsomes, alpha NF (30 muM) stimulated nifedipine oxidation activities in mouse and human intestinal microsomes and in human hepatic microsomes to similar to1.3-1.8-fold. Inhibitory potencies (50% inhibition concentration, IC50) of midazolam and erythromycin for nifedipine oxidations were calculated to be similar to 90 muM in human intestinal microsomes. In contrast, testosterone (100 muM) stimulated the nifedipine oxidation activities similar to1.5-fold in hepatic and intestinal microsomes from mouse and human. 3. alpha NF showed different effects on the kinetic parameters including the Hill coefficients of nifedipine oxidation and testosterone 6 beta -hydroxylation catalysed by hepatic and intestinal microsomes from mouse and human. Cooperativity in nifedipine oxidation was increased by the addition of alpha NF to pooled human hepatic microsomes, but little effects of alpha NF could be observed in individual human intestinal microsomes. 4. These results suggest that CYP3A enzymes in liver and intestine might have different characteristics and that observations from hepatic microsomes should not be directly applicable to intestine metabolism in some cases. Studies of drug-drug interactions of CYP3A substrates are recommended to be performed using intestinal samples.