Simultaneous Assessment of the In Vivo Amount of CYPIA2 and CYP3A2 by the PKCYP-test Using Theophylline in Rats

Recently, we developed a method for assessing in vivo drug metabolism capacity by pharmacokinetic estimation of the quantity of cytochrome P450 (CYP) in vivo (PKCYP-test), in which an apparent liver-to-blood free concentration gradient in vivo (qg) is introduced. The qg value can be alterna-tively defined as the ratio of the in vivo-in vitro clearance by a single CYP isoform. In this study, we exa-mined the application of the PKCYP-test to drugs metabolized by multiple CYP isoforms in a rat model with fluctuating CYPIA2 levels using theophylline as a model drug. In control rats, the estimated qg values for each CYPIA2 and CYP3A2 based on the in vivo hepatic in-trinsic clearance, in vitro Michaelis constant (K~) and maximal rate of metabolism ( V~. . ) values for liver slices agreed well. Moreover, the qg value for CYPIA2 determined by the K~ and V~. . values for recom-binant CYPIA2 was compatible with that based on liver slices. These qg values also agreed with that of rats pretreated with 3-methylcholanthrene. The time-course of theophylline concentrations in serum simulated by a physiologically-based pharmacokinetic model incorporating the hepatic clearance deter-mined by the PKCYP-test agreed with the observed values. These results demonstrate that the qg value in the PKCYP-test is applicable to drugs metabolized by multiple CYP isoforms.