Previously, we established a method to assess drug metabolism capacity based on a pharmacokinetic estimation of the quantity of cytochrome P450 (CYP) in vivo (PKCYP-test) by introducing an apparent liver-to-blood free concentration gradient in vivo (qg). The qg values were determined as the ratio of in vivo-in vitro clearance. In this study, we examined the application of the PKCYP-test to the clearance of acetanilide and caffeine mediated by CYP1A2 using rat models in which the levels of CYP enzymes were reduced. Rats fed a choline-deficient diet (CD-fed) and aged rats were used as models for a low level of CYP in the liver. In both rat models, the contribution (fCYP) of CYP1A2 to the in vivo intrinsic clearance values (CLint) of acetanilide and caffeine metabolism was less than unity, suggesting that other metabolic pathways are involved in the CLint. The in vivo clearance for CYP1A2 was estimated by multiplying fCYP by CLint, then the value of qg was determined as the ratio of in vivo-in vitro clearance. We predicted the level of CYP1A2 in CD-fed and aged rats, based on the clearance of acetanilide mediated by CYP1A2, using the qg value of control rats. The clearance of caffeine mediated by CYP1A2 in CD-fed and aged rats, as estimated from the predicted level of CYP1A2, correlated with the observed values. In conclusion, we have demonstrated that the PKCYP-test can be applied to CYP1A2 for drugs metabolized by multiple CYP isozymes, and/or to models involving reduced CYP.
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