TABLE 1

Alfentanil metabolism by CYP3A enzymes at therapeutic concentrations

Incubations contained 1.0 μM alfentanil and an NADPH-regenerating system in 100 mM potassium phosphate (pH 7.4). Enzyme-selective conditions were: CYP3A4 or CYP3A5 (15 pmol/ml, 12 min), CYP3A4 or CYP3A5 (7.5 pmol/ml) with added cytochrome b₅ (9 min), CYP3A4 (2.5 pmol/ml) with coexpressed cytochrome b₅ (5 min), or CYP3A7 (15 pmol/ml) with coexpressed cytochrome b₅ (30 min). Results are the mean ± S.D. (n = 3).

Enzyme	Metabolite Formation			Noralfentanil/AMX Ratio
Enzyme		Noralfentanil	AMX	Noralfentanil/AMX Ratio
	pmol min^-1 pmol^-1
CYP3A4	0.511 ± 0.008	0.059 ± 0.002	8.6 ± 0.4
CYP3A5	0.507 ± 0.012	0.024 ± 0.002	20.8 ± 1.4
CYP3A4 with added cytochrome b₅	0.707 ± 0.041	0.075 ± 0.007	9.4 ± 0.3
CYP3A5 with added cytochrome b₅	1.23 ± 0.03	0.062 ± 0.005	20.0 ± 0.6
CYP3A4 with coexpressed cytochrome b₅	2.73 ± 0.13	0.326 ± 0.029	8.4 ± 0.5
CYP3A7 with coexpressed cytochrome b₅	0.078 ± 0.004	0.0057 ± 0.0007	13.8 ± 0.9