Kinetics of in vitro metabolism of methoxyphenamine in rats
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1College of Pharmacy, University of Saskatchewan, Saskatoon, Canada.
Abstract:
1. Methoxyphenamine (MP) was metabolized in vitro by rat liver preparations to O-desmethylmethoxyphenamine (O-desmethyl-MP), N-desmethylmethoxyphenamine (N-desmethyl-MP) and 5-hydroxymethoxyphenamine (5-hydroxy-MP). These metabolic pathways were inhibited by SKF 525-A and carbon monoxide, which indicates that these reactions were mediated at least partly by an NADPH-dependent cytochrome P-450 system. 2. Strain differences in the metabolism of this drug in vitro were observed in female Lewis and Dark Agouti (DA) rats, which are proposed models for human debrisoquine phenotypes. Methoxyphenamine O-demethylase and 5-hydroxylase activity in DA rats were lower than those in Lewis rats. 3. The metabolic transformation of methoxyphenamine in vitro to O-desmethyl-MP was inhibited competitively by debrisoquine and sparteine. This indicates that the cytochrome P-450 isoenzyme mediating the metabolism of MP to O-desmethyl-MP is similar to that mediating metabolism of debrisoquine and sparteine. However, no inhibition was observed with methenytoin.
Insights
Methoxyphenamine metabolism involves cytochrome P-450 enzymes, with variations observed between rat strains. This drug
Area of Science:
- Pharmacology
- Drug Metabolism
- Biochemistry
Background:
- Methoxyphenamine (MP) is a drug whose metabolic pathways are not fully elucidated.
- Understanding drug metabolism is crucial for predicting drug efficacy and toxicity.
- Cytochrome P-450 enzymes play a significant role in xenobiotic metabolism.
Purpose of the Study:
- To investigate the in vitro metabolism of methoxyphenamine in rat liver preparations.
- To identify the specific cytochrome P-450 isoenzymes involved in methoxyphenamine metabolism.
- To explore potential strain differences in methoxyphenamine metabolism using rat models for human phenotypes.
Main Methods:
- In vitro metabolism studies using rat liver preparations.
- Enzyme inhibition assays with SKF 525-A and carbon monoxide to identify P-450 involvement.
- Comparative metabolism studies in Lewis and Dark Agouti (DA) rat strains.
- Competitive inhibition studies with debrisoquine, sparteine, and methenytoin.
Main Results:
- Methoxyphenamine was metabolized to O-desmethylmethoxyphenamine, N-desmethylmethoxyphenamine, and 5-hydroxymethoxyphenamine.
- Metabolic pathways were inhibited by SKF 525-A and carbon monoxide, indicating NADPH-dependent cytochrome P-450 system involvement.
- Significant strain differences in O-demethylase and 5-hydroxylase activity were observed between Lewis and DA rats.
- Debrisoquine and sparteine competitively inhibited the O-demethylation of methoxyphenamine, suggesting involvement of the same P-450 isoenzyme.
Conclusions:
- Methoxyphenamine metabolism is mediated by the NADPH-dependent cytochrome P-450 system.
- Rat strains exhibit differential metabolic capacities for methoxyphenamine, relevant to human debrisoquine phenotypes.
- The P-450 isoenzyme responsible for methoxyphenamine O-demethylation appears similar to that involved in debrisoquine and sparteine metabolism.