title: Parallel pathway interactions in imipramine metabolism in rats.
abstract: The in vitro metabolic inhibitions between imipramine and its metabolites were investigated in rat liver microsomes. A type of precursor-metabolite interaction similar to that shown with lidocaine was observed in imipramine metabolism. Desipramine competitively inhibited the formation of 2-hydroxyimipramine from imipramine. Similarly, imipramine inhibited the formation of 2-hydroxydesipramine from desipramine. As in the cases of those 2-hydroxylations, a competitive inhibitory relationship also existed in the N-demethylation pathways of imipramine and 2-hydroxyimipramine. Studies on age-associated alterations of the metabolic rates of imipramine and its metabolites in rats demonstrated that N-demethylation activities of imipramine and of 2-hydroxyimipramine, which showed a large sex difference (male greater than female) in young rats, decreased markedly only in old male rats, while 2-hydroxylation activities of imipramine and desipramine, with no sex difference at any age, did not show a marked alteration in either sex. These data strongly suggest that the hydroxylation pathways of imipramine and desipramine and the demethylation pathways of imipramine and 2-hydroxyimipramine are each sharing the same species of cytochrome P-450. The in vivo metabolic inhibition between imipramine and desipramine was examined by simultaneous intraportal infusion of imipramine (25 nmol/min) and desipramine (175 nmol/min). The steady-state concentration of imipramine after simultaneous infusion was increased twofold over that after infusion of imipramine alone, without any change in the free fraction in blood.
authors.affiliation: Department of Biopharmaceutics, Faculty of Pharmaceutical Sciences, Chiba University, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Chiba
authors.forename: S
authors.initials: S
authors.lastname: Fujita
authors.forename: T
authors.initials: T
authors.lastname: Suzuki
mesh_terms: D000375:Aging / Q000378:metabolism; D000818:Animals; D005260:Female; D007099:Imipramine / Q000097:blood / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D011919:Rats, Inbred Strains
chemical_list: D007099:Imipramine