Catalysis of drug oxidation during embryogenesis in human hepatic tissues using imipramine as a model substrate

We investigated the catalysis of drug monooxygenation by human embryonic hepatic tissues at a very early stage of gestation (days 52-59). Imipramine was used as a model substrate and the metabolites generated were identified and quantified by electrospray mass spectroscopy and HPLC. The primary metabolite generated was desipramine. It was reported previously from this and other laboratories that cytochrome P-450 monooxygenase (CYP) 1A1, 1B1, 2E1, and 3A7 are each expressed in human embryonic hepatic tissues, and selective inhibitors were therefore used to elucidate their respective roles. Furafylline did not inhibit the reaction, supporting that CYP1A2 was not expressed in human embryonic hepatic tissues. Diethyldithiocarbamate also failed to inhibit the same reaction, suggesting that CYP2E1 did not play a significant role in catalyzing the reaction. Triacetyloleandomycin inhibited the reaction by approximately 90%, suggesting that CYP3A7 was primarily responsible for catalyzing the reaction. However, alpha-naphthoflavone inhibited the same reaction by approximately 70%, suggesting that CYP1A1 and/or CYP1B1 may also catalyze the reaction substantially. To explore this issue more, a cDNA-expressed human CYP3A7 (CYP3A7 SUPERSOMES) was incubated with alpha-naphthoflavone (1 microM). Generation of desipramine was inhibited by approximately 40 to 50%. The addition of the CYP3A subfamily selective inhibitor triacetyloleandomycin (1 microM) produced no statistically significant inhibition in reactions catalyzed by CYP1A1 or 1B1 SUPERSOMES. Taken together, the results indicated that CYP3A7 was the major if not sole isoform responsible for catalysis of the N-demethylation of imipramine in human hepatic tissues during embryogenesis.