MAJOR PATHWAY OF IMIPRAMINE METABOLISM IS CATALYZED BY CYTOCHROMES P-450 1A2 AND P-450 3A4 IN HUMAN LIVER

The metabolism of imipramine by human liver microsomes was examined us ing a combination of five strategies. Human hepatic microsomes produce d N-desmethylimipramine (84%), 2-hydroxyimipramine (10%), and 10-hydro xyimipramine (6%). Preincubation of human hepatocytes in culture with beta-naphthoflavone and macrolides exclusively induced the formation o f desmethylimpramine (552%, p < 0.05, and 234%, p < 0.003, respectivel y). Correlations were obtained between rates of imipramine demethylati on 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 prepara tions from 13 different subjects. Anti-P-450 1A2 and anti-P-450 3A ant ibodies selectively inhibited N-demethylation (80% and 60%, respective ly). N-Demethylation was completely inhibited when anti-1A2 and anti-3 A were added simultaneously. Kinetic studies with human microsomes con firm the contribution of two different enzymes in the N-demethylation. The K of 1A2 was similar to the high affinity K(m) in human liver mic rosomes, whereas the K(m) of 3A was similar to the low affinity K(m) i n human liver microsomes. P-450 1A2 was apparently more efficient than 3A4 (lower K(m) and higher V(max)) but was expressed in much lower co ncentration. Human P-450s 1A2 and 3A4 expressed in yeast efficiently p roduced 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-h ydroxylation. Interindividual variations in 3A4 hepatic content may ex plain the large variations in imipramine blood levels observed after u niform dosages and thus may explain the variations in clinical efficac y. Caution might be advised in the clinical use of tricyclic antidepre ssants when drugs are also administered that induce or inhibit P-450s 3A4 and 1A2.