Simultaneous assessment of the in vivo amount of CYP1A2 and CYP3A2 by the PKCYP-test using theophylline in rats

Summary: 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 alternatively defined as the ratio of the in vivo-in vitro clearance by a single CYP isoform. In this study, we examined the application of the PKCYP-test to drugs metabolized by multiple CYP isoforms in a rat model with fluctuating CYP1A2 levels using theophylline as a model drug. In control rats, the estimated qg values for each CYP1A2 and CYP3A2 based on the in vivo hepatic intrinsic clearance, in vitro Michaelis constant ( K m ) and maximal rate of metabolism ( V max ) values for liver slices agreed well. Moreover, the qg value for CYP1A2 determined by the K m and V max values for recombinant CYP1A2 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 determined 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.