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Trimethadione metabolism and microsomal monooxygenases in untreated and phenobarbital-treated rhesus monkeys
E Tanaka1, T Taniguchi, Y Sawa
1Institute of Community Medicine, University of Tsukuba, Ibaraki-Ken, Japan. einosuke@md.tsukuba.ac.jp
Abstract:
The contribution of induced cytochrome P450 (P450) isozymes (CMLa; CYP2B, CMLb; CYP2A and CMLc; CYP3A) and related enzymes to trimethadione (TMO) metabolism in phenobarbital-treated rhesus monkey were investigated. The animals received a single dose of TMO (4 mg kg-1) and plasma samples were withdrawn before this administration and again at 0.08, 0.25, 0.5, 1 and 2 h later. Phenobarbital-treatment (20 mg kg-1 day-1 for 3 days; i.p.) significantly increased the plasma dimethadione (DMO)/TMO ratios at 0.08, 0.5, 1 and 2 h one's appropriate controls. Phenobarbital treatment also increased the P450 content (1.7-fold) and activity of aniline p-hydroxylase (1.3-fold), p-nitroanisole O-demethylase (1.8-fold) and benzphetamine N-demethylase (2.3-fold). The content of CMLa, CMLb and CMLc were increased about 12.8, 2.3 and 2.7-fold by phenobarbital pretreatment, respectively. The activity of TMO N-demethylation was inhibited by anti-P450 CMLa and anti-P450 CMLb. However, the anti-P450 CMLc antibody had no effect on this activity in liver microsomes. The results of both in vivo and in vitro studies of the effects of phenobarbital treatment on TMO metabolism indicate that these effects may be attributed to the induction of CMLa. These findings suggest that plasma DMO/TMO ratio in a single blood sampling after TMO administration is very useful for determination the degree of hepatic induction in clinical study.
Insights
Phenobarbital treatment significantly increased trimethadione (TMO) metabolism in rhesus monkeys, primarily due to the induction of cytochrome P450 CMLa (CYP2B). The plasma dimethadione (DMO)/TMO ratio effectively indicates the degree of hepatic induction.
Area of Science:
- Pharmacology
- Drug Metabolism
- Biochemistry
Background:
- Cytochrome P450 (P450) enzymes play a crucial role in drug metabolism.
- Investigating the specific P450 isozymes involved in trimethadione (TMO) metabolism is essential for understanding drug interactions and efficacy.
- Phenobarbital is a known inducer of P450 enzymes, but its specific effects on TMO metabolism require detailed study.
Purpose of the Study:
- To investigate the contribution of induced cytochrome P450 (P450) isozymes (CMLa; CYP2B, CMLb; CYP2A, and CMLc; CYP3A) to trimethadione (TMO) metabolism in phenobarbital-treated rhesus monkeys.
- To assess the utility of the plasma dimethadione (DMO)/TMO ratio as a biomarker for hepatic enzyme induction.
Main Methods:
- Rhesus monkeys were administered TMO (4 mg/kg) after a 3-day pretreatment with phenobarbital (20 mg/kg/day).
- Plasma samples were collected at various time points to determine DMO/TMO ratios.
- Liver microsomes were analyzed for P450 content and enzyme activities (aniline p-hydroxylase, p-nitroanisole O-demethylase, benzphetamine N-demethylase).
- Specific antibodies against P450 isozymes were used to inhibit TMO N-demethylation activity in vitro.
Main Results:
- Phenobarbital treatment significantly increased plasma DMO/TMO ratios.
- A substantial increase in P450 content and the activity of specific P450 isozymes (CMLa, CMLb, CMLc) was observed.
- TMO N-demethylation activity was primarily inhibited by antibodies against CMLa and CMLb, with CMLa showing the most significant effect.
- The induction of CMLa was identified as the main contributor to the enhanced TMO metabolism.
Conclusions:
- Phenobarbital-induced TMO metabolism in rhesus monkeys is predominantly mediated by the CMLa (CYP2B) isozyme.
- The plasma DMO/TMO ratio serves as a reliable indicator of the extent of hepatic P450 induction in clinical settings.
- These findings have implications for understanding drug metabolism and predicting drug responses in individuals with induced P450 activity.