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 he patic tissues at a very early stage of gestation (days 52-59). Imipramine w as used as a model substrate and the metabolites generated were identified and quantified by electrospray mass spectroscopy and HPLC. The primary meta bolite 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 selecti ve inhibitors were therefore used to elucidate their respective roles. Fura fylline did not inhibit the reaction, supporting that CYP1A2 was not expres sed in human embryonic hepatic tissues. Diethyldithiocarbamate also failed to inhibit the same reaction, suggesting that CYP2E1 did not play a signifi cant role in catalyzing the reaction. Triacetyloleandomycin inhibited the r eaction by approximately 90%, suggesting that CYP3A7 was primarily responsi ble for catalyzing the reaction. However, alpha-naphthoflavone inhibited th e 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-n aphthoflavone (1 mu M). Generation of desipramine was inhibited by approxim ately 40 to 50%. The addition of the CYP3A subfamily selective inhibitor tr iacetyloleandomycin (1 mu M) produced no statistically significant inhibiti on 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 tissue s during embryogenesis.