Received for publication July 11, 2007.
Revised September 13, 2007.
Accepted for publication September 17, 2007.

Evaluation of the Assumptions of an Ontogeny Model of Rat Hepatic Cytochrome P450 Activity

Abstract

We previously reported an ontogeny model of hepatic cytochrome P450 (CYP) activity that predicts in vivo CYP elimination from in vitro intrinsic clearance. The purpose of this study was to conduct investigations into key assumptions of the CYP ontogeny model using the developing rat model system. We used two developmentally dissimilar enzymes, CYP2E1 and CYP1A2, and male rats (n=4) at age groups representing critical developmental stages. Total body and liver weights and hepatic microsomal protein contents were measured. Following HPLC analysis, apparent KM and VMax estimates were calculated using nonlinear regression analysis for CYP2E1- and CYP1A2-mediated chlorzoxazone 6-hydroxylation and methoxyresorufin O-dealkylation, and VMax estimates for para-nitrophenol and phenacetin hydroxylations, respectively. Hepatic scaling factors and VMax values provided estimates for infant scaling factors (ISF). The data showed microsomal protein contents increased with postnatal age and reached adult values after postnatal day (PD) 7. Apparent KM values were similar at all developmental stages except at PD7. Developmental increases in probe substrate VMax values did not correlate with the biphasic increase in immunoquantifiable CYP. The activity of two different probe substrates for each CYP covaried as a function of age. A plot of observed ISF values as a function of age reflected the developmental pattern of rat hepatic CYP. In summation, these observations diverged from several of the model's assumptions. Further investigations are required to explain these inconsistencies and to investigate whether the developing rat may provide a predictive paradigm for paediatric risk assessment for CYP-mediated elimination processes.

Key words: cytochrome P450 function, developmental toxicology, hepatic elimination, HPLC, in vitro-in vivo scaling, pharmacokinetic modeling, toxicokinetics

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