Alprazolam metabolism in vitro: studies of human, monkey, mouse, and rat liver microsomes

Biotransformation of the triazolobenzodiazepine alprazolam (ALP) was studied in vitro using hepatic microsomal preparations from human, monkey, mouse, and rat liver tissue. Two principal hydroxylated metabolites were identified: 4-hydroxy- and alpha-hydroxy-alprazolam (4-OH-ALP and alpha-OH-ALP). In all species, rates of 4-OH-ALP formation exceeded those of alpha-OH-ALP. In human liver microsomes, ratios of 4-OH-ALP/alpha-OH-ALP reaction velocities calculated at clinically relevant plasma concentrations of ALP ranged from 7 to 17, qualitatively consistent with, but numerically larger than, the ratio of the plasma levels of the two metabolites during clinical use of ALP in humans. Km values for both 4-OH-ALP (170-305 microM) and alpha-OH-ALP (63-441 microM) considerably exceeded the usual maximum plasma concentration observed in humans (200 ng/ml, 0.65 microM), consistent with the linear (dose-independent) pharmacokinetic characteristics of ALP observed in humans. Thus formation of 4-OH-ALP via hydroxylation is the major route of ALP metabolism. This pathway is probably mediated by the cytochrome P-450-3A subfamily. Factors that impair the activity of this cytochrome subtype are likely to impair clearance of ALP in vivo.