P92 Short- and Long-term Effects on Cytochrome P450 Expression and Induction Following Phenytoin Exposure at the Neonatal Age

Stephanie Piekos , University of Connecticut School of Pharmacy, Storrs, CT
Liming Chen , University of Connecticut School of Pharmacy, Storrs, CT
Xiaochao Ma , Center for Pharmacogenetics, Department of Pharmaceutical Sciences, University of Pittsburgh, Pittsburgh, PA
Pengcheng Wang , University of Pittsburgh, Pittsburgh, PA
Xiao-bo Zhong , Department of Pharmaceutical Sciences, University of Connecticut School of Pharmacy, Storrs, CT
Significant interindividual variation exists in the expression and activity of several hepatic cytochrome P450 (P450s) enzymes important in the metabolism of drugs and xenobiotics, including CYP3A, CYP2B, and CYP2C isoforms. This contributes to the current difficulty of predicting patient responses to therapeutics, since the levels of P450 activity can directly affect the concentration of active drug compounds or toxic drug metabolites in the plasma. Several studies have shown that neonatal exposure to the antiepileptic drug phenobarbital produces significant permanent overexpression of Cyp3a, Cyp2b, and Cyp2c enzymes in rodents and alters the metabolism and efficacy of other drugs administered in adulthood. Therefore, the hypothesis for this study was that neonatal exposure to phenytoin, a different antiepileptic drug also capable of inducing P450s, would produce a similar permanent overexpression of hepatic P450 enzymes in adulthood. To test the hypothesis, male and female mice were treated with a single dose of phenytoin at different ages during postnatal development. Livers were then harvested either 24 hours after treatment to investigate short-term changes in P450 expression, or when mice reached Day 60 of age to investigate long-term changes in P450 expression. The mRNA expression of CYP3A11, CYP2B10, and CYP2C29 were determined using RT-PCR, and enzymatic activities for CYP3A11 and CYP2B10 were also evaluated. The extent of induction of Cyp3a11, Cyp2b10, and Cyp2c29 was much lower in neonatal mice than in adult mice following 24 hour exposure to a single dose of phenytoin. Mice given phenytoin at Day 5 had much lower expression of P450 mRNA and lower enzymatic activity following treatment than mice given phenytoin at Day 60. The extent of P450 induction increased with each age we tested (Day 10, Day 15, and Day 20), presumably as the liver became more mature. However, neonatal exposure to phenytoin did not appear to produce significant long-term overexpression of multiple P450s into adulthood as phenobarbital does. There were no significant differences in CYP3A11 or CYP2C29 mRNA expression between adult mice that received a dose of phenytoin at Day 5 of age compared to those that received vehicle treatment. While CYP2B10 was significantly overexpressed in adult male mice exposed to phenytoin at Day 5 of age, this upregulation was not reflected in enzymatic activity analyses. When adult mice were rechallenged with either phenytoin or phenobarbital at Day 60 of age, the extent of P450 induction at the mRNA level was not increased by Day 5 pre-treatment with phenytoin. This may indicate that phenobarbital and phenytoin induce P450 transcription through a different mechanism, with phenobarbital being capable of permanently altering epigenetic signatures during early postnatal development while phenytoin does not. While our data does not support our original hypothesis, this information may prompt physicians to re-evaluate which antiepileptic drug they choose to administer to neonatal patients to treat seizures and to adjust dosing depending on age.