ABSTRACT
We previously established a method for assessing in vivo drug-metabolizing 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 (Matsunaga et al., Jpn. J. Hosp. Pharm., 26: 492-504 (2000)). This method was applied to estimate the amount of CYP2C11 in rats treated with carbon tetrachloride (CCl(4)-treated rats). In this study, we estimated the amount of CYP1A2 in CCl(4)-treated rats by using acetanilide and caffeine as a probe and a model drug, respectively. In CCl(4)-treated rats, the total body clearance (CL(tot)) of acetanilide and caffeine was about one-fifth and one-eighth of that in control rats, respectively. In CCl(4)-treated rats, the amount of CYP1A2 was predicted as 0.60+/-0.06 nmol/kg from the clearance of acetanilide mediated by CYP1A2. Moreover, the clearance of caffeine mediated by CYP1A2 in CCl(4)-treated rats was estimated as 0.47+/-0.05 mL/min/kg by using the predicted amount of CYP1A2. The observed value was 0.44+/-0.03 mL/min/kg, and the predicted value was within the 95% confidence interval of the observed value. In conclusion, we have demonstrated that the PKCYP test can also be applied for estimating the amount of CYP1A2 in CCl(4)-treated rats.
ABSTRACT
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.
