Comparison of the metabolism of the three antidepressants amitriptyline imipramine and chlorimipramine in vitro in rat liver microsomes

The metabolism of the tricyclic antidepressants amitriptyline (AMI), imipramine (IMI), chlorimipramine (CMI) and some of their metabolites was studied in vitro in isolated liver microsomes of female Spraque-Dawley rats. Nine metabolites of AMI, seven metabolites of IMI, and 11 metabolites of CMI were quantitatively determined with high-performance liquid chromatography. The main metabolic reactions, mediated by an NADPH generating system, were hydroxylation, demethylation, and N-oxidation. The ratio of these reactions was different for the three drugs. AMI was hydroxylated more than CMI and CMI more than IMI. The order for demethylation was CMI>AMI=IMI, the order for N-oxidation IMI>CMI≤AMI. The substrate dependence of metabolism was investigated. Demethylation and N-oxidation increased proportionally to increasing substrate concentrations, whereas formation of hydroxylated metabolites became saturated (in the concentration range of 10−6−10−5 M). The in vitro metabolism was compared with the in vivo metabolism in humans, reflected by the plasma concentrations of these drugs and their metabolites. A good agreement in metabolic pathways was found.