Dj. Wilkinson et Jr. Fry, RAT-LIVER CYTOCHROME P450-MEDIATED METABOLIC-ACTIVATION OF METHOXSALEN AND STRUCTURALLY RELATED-COMPOUNDS AND ITS RELATION TO ENZYME-INHIBITION, Journal of Pharmacy and Pharmacology, 47(1), 1995, pp. 79-84
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.