Parallel pathway interactions in imipramine metabolism in rats.

Area of Science:

• Pharmacology
• Drug Metabolism
• Biochemistry

Background:

• Imipramine is a tricyclic antidepressant with complex metabolic pathways.
• Understanding drug-drug and drug-metabolite interactions is crucial for optimizing therapeutic efficacy and safety.

Purpose of the Study:

• To investigate in vitro and in vivo metabolic interactions between imipramine and its primary metabolite, desipramine.
• To explore age- and sex-associated alterations in imipramine metabolism.
• To identify the specific cytochrome P-450 (CYP) isoforms involved in imipramine metabolism.

Main Methods:

• In vitro studies using rat liver microsomes to assess competitive inhibition of imipramine and desipramine metabolism.
• In vivo studies involving simultaneous intraportal infusion of imipramine and desipramine in rats.
• Analysis of metabolic pathways including 2-hydroxylation and N-demethylation.

Main Results:

• Desipramine competitively inhibited 2-hydroxyimipramine formation from imipramine, and vice versa.
• Imipramine and 2-hydroxyimipramine N-demethylation pathways also exhibited competitive inhibition.
• Age-associated decrease in N-demethylation was observed primarily in old male rats.
• These interactions suggest a shared CYP isoform for both hydroxylation and N-demethylation pathways.
• In vivo, co-infusion of imipramine and desipramine doubled imipramine's steady-state concentration.

Conclusions:

• Imipramine and desipramine metabolism involve significant precursor-metabolite interactions, likely mediated by a common CYP enzyme.
• Metabolic pathways for imipramine and its metabolites are susceptible to age-related changes, particularly in males.
• These findings highlight the importance of considering metabolic interactions for imipramine therapy.