Some chemical and biochemical aspects of liver microsomal metabolism of tamoxifen.

The triarylethylene antiestrogen tamoxifen (TAM) has been shown to undergo N-demethylation, 4-hydroxylation, and N-oxidation in animals and man. We have studied the effects of drug metabolism inhibitors metyrapone and SKF 525-A, and an inducer (phenobarbital), on these processes and on overall TAM metabolism, in the presence of rat and rabbit liver microsomes. In the rabbit, metyrapone had no significant inhibitory effects. In the rat, it inhibited N-demethylation by 40% at a concentration of 1 microM, while overall TAM metabolism and 4-hydroxylation were unaffected. SKF 525-A markedly inhibited all of these processes in both species. Enzymatic N-oxidation of TAM in the rat was unaffected by either inhibitor, suggesting this to be independent of cytochrome P-450. In the rat, phenobarbital pretreatment increased N-demethylation by 105% and decreased 4-hydroxylation by 48% compared to corresponding rates in untreated animals. Together with the results obtained with the inhibitors, these findings implicate participation of alternative isoenzymatic forms of cytochrome P-450 in N-demethylation and 4-hydroxylation of TAM. In the rat, the sum of the amounts of the three metabolites found was less than the amount of TAM metabolized, implying the existence of additional biotransformation routes. Radiochromatographic analysis of extracts of incubation mixtures showed the presence of the three known metabolites, plus two additional ones. The more polar of these was spectrally and chromatographically identical to authentic 4'-hydroxytamoxifen. The other one, which was slightly less polar than TAM, was tentatively identified as TAM epoxide.