RAT-LIVER CYTOCHROME P450-MEDIATED METABOLIC-ACTIVATION OF METHOXSALEN AND STRUCTURALLY RELATED-COMPOUNDS AND ITS RELATION TO ENZYME-INHIBITION

The metabolic activation and enzyme inhibition characteristics of meth oxsalen were investigated in rat liver microsomes obtained from untrea ted animals and those treated with a number of prototypic inducers of cytochrome P450. Glutathione depletion assays have been carried out wh ich show reactive metabolite generation to be markedly increased follo wing phenobarbitone and beta-naphthoflavone induction. Moreover, isoni azid induction led to levels of glutathione depletion significantly hi gher than those seen with other forms of induction, suggesting an impo rtant role for the cytochrome P4502E1 isozyme in the metabolic activat ion process. Methoxsalen was shown to be an extremely potent inhibitor of 7-ethoxycoumarin-O-de-ethylase activity, with inhibition constants of the order of 5 mu M with microsomes obtained from untreated, pheno barbitone- and beta-naphthoflavone-induced animals. In contrast, const ants obtained with microsomes obtained from isoniazid-induced animals were found to be markedly higher. Comparisons of the inhibition of 7-e thoxy and 7-pentoxyresorufin-O-dealkylase activities by methoxsalen an d a number of structurally-related compounds have shown that a complet e tricyclic ring system and an unsaturated 4',5'-bond are structural p rerequisites in the formation of reactive metabolites which inhibit cy tochrome P450. These data implicate the furan ring system as the sourc e of these metabolites and rule out the involvement of the pyrone ring system in the inhibition process.