In vitro/in vivo scaling of alprazolam metabolism by CYP3A4 and CYP3A5 in humans

We attempted to predict the in vivo metabolic clearance of alprazolam from in vitro metabolic studies using human liver microsomes and human CYP recom binants. Good correlations were observed between the intrinsic clearance (C Lint) for 4-hydroxylation and CYP3A4 content and between the CLint for alph a -hydroxylation and CYP3A5 content in ten human liver microsomal samples. Using the recombinant CYP isoforms expressed in insect cells, the CLint for CYP3A4 was about 2-fold higher than the CLint for CYP3A5 in the case of 4- hydroxylation. However, the CLint for CYP3A5 was about 3-fold higher than t he CLint for CYP3A4 in the case of alpha -hydroxylation. The metabolic rate s for 4- and alpha -hydroxylation increased as the added amount of cytochro me b(5) increased, and their maximum values were 3- to 4-fold higher than t hose without cytochrome b(5). The values of CLint, in vivo predicted from i n vitro studies using human liver microsomes and CYP3A4 and CYP3A5 recombin ants were within 2.5 times of the observed value calculated from literature data. The average CLint value (sum of 4- and alpha -hydroxylation) obtaine d using three human liver microsomal samples was 4-fold higher than that ob tained using three small intestinal microsomal samples from the same donors , indicating the minor contribution of intestinal metabolism to alprazolam disposition. The area under the plasma concentration-time curve (AUC) of al prazolam is reported to increase following co-administration of ketoconazol e and the magnitude of the increase predicted from the in vitro K-i values and reported pharmacokinetic parameters of ketoconazole was 2.30-2.45, whic h is close to the value observed in vivo (3.19). A quantitative prediction of the AUC increase by cimetidine was also successful (1.73-1.79 vs 1.58-1. 64), considering the active transport of cimetidine into the liver. In conc lusion, we have succeeded in carrying out an in vitro/in vivo scaling of al prazolam metabolism using human liver microsomes and human CYP3A4 and CYP3A 5 recombinants. Copyright (C) 2001 John Wiley & Sons, Ltd.