Participation of cytochromes P4502B and P4503A in cocaine toxicity in rat hepatocytes.

The contributions of cytochromes P4502B (P4502B) and cytochrome P4503A (P4503A) to the bioactivation of cocaine in hepatocytes isolated from Sprague-Dawley rats were assessed using a number of approaches. Hepatocytes were isolated from rats pretreated with either phenobarbital or dexamethasone. Exposure to from 50 to 500 microM of either cocaine or norcocaine resulted in toxicity in hepatocytes from phenobarbital-induced rats. Hepatocytes from dexamethasone-induced rats displayed greater resistance to toxicity mediated by either compound. Although microsomes from dexamethasone- and phenobarbital-induced rats catalyzed cocaine N-demethylation at the same rate, only inhibition of P4502B activity by chloramphenicol and not inhibition of P4503A activity by troleandomycin was associated with protection against cocaine or norcocaine-mediated toxicity. Further, inhibition of P4502B was only effective in protecting against toxicity in hepatocytes isolated from phenobarbital-induced rats. The effects of phenobarbital induction in rats, dogs, and guinea pigs, and the abilities of purified P4502B proteins from rats, dogs, and rabbits to N-demethylate cocaine were investigated. Cytochromes P4502B from different species exhibited different rates of cocaine N-demethylation; microsomes from the guinea pig were able to N-demethylate cocaine at the fastest rate, followed by the dog and the rat. Expressed human P4502B6 exhibited no ability to either N-demethylate cocaine or produce cocaine- or norcocaine-mediated toxicity in lymphoblastoid cells. These results suggest that, although P4502B and P4503A both catalyze the initial oxidation of cocaine in rats, only P4502Bs are involved in further oxidations leading to toxicity. The importance of P4502Bs toward cocaine bioactivation will depend on species-specific isoform activities.