Abstract

The metabolism of imipramine by human liver microsomes was examined using a combination of five strategies. Human hepatic microsomes produced N-desmethylimipramine (84%), 2-hydroxyimipramine (10%), and 10-hydroxyimipramine (6%). Preincubation of human hepatocytes in culture with beta-naphthoflavone and macrolides exclusively induced the formation of desmethylimpramine (552%, p < 0.05, and 234%, p < 0.003, respectively). Correlations were obtained between rates of imipramine demethylation and cytochrome P-450 (P-450) 1A2 (r = 0.88, p < 0.001) and P-450 3A (r = 0.80, p < 0.02) concentrations in human liver microsomal preparations from 13 different subjects. Anti-P-450 1A2 and anti-P-450 3A antibodies selectively inhibited N-demethylation (80% and 60%, respectively). N-Demethylation was completely inhibited when anti-1A2 and anti-3A were added simultaneously. Kinetic studies with human microsomes confirm the contribution of two different enzymes in the N-demethylation. The Km of 1A2 was similar to the high affinity Km in human liver microsomes, whereas the Km of 3A was similar to the low affinity Km in human liver microsomes. P-450 1A2 was apparently more efficient than 3A4 (lower Km and higher Vmax) but was expressed in much lower concentration. Human P-450s 1A2 and 3A4 expressed in yeast efficiently produced desmethylimipramine. These results suggest that P-450 1A2 and P-450 3A4 are the major enzymes involved in imipramine N-demethylation in human hepatic microsomes. Similar experiments were conducted using P-450 2D6, and they confirmed that P-450 2D6 catalyzes imipramine 2-hydroxylation. Interindividual variations in 3A4 hepatic content may explain the large variations in imipramine blood levels observed after uniform dosages and thus may explain the variations in clinical efficacy. Caution might be advised in the clinical use of tricyclic antidepressants when drugs are also administered that induce or inhibit P-450s 3A4 and 1A2.