alpha-Hydroxytamoxifen, a metabolite of tamoxifen with exceptionally high DNA-binding activity in rat hepatocytes

It has been proposed that the antiestrogen tamoxifen induces liver tumors in rats and genotoxic effects in vitro through metabolic activation involving, initially, alpha-hydroxylation of the ethyl group. To test this hypothesis, the extent of DNA adduct formation in primary rat hepatocytes treated with tamoxifen and alpha-hydroxytamoxifen was investigated. Hepatocytes from female Fischer F-344 rats were treated with 1 or 10 microM concentrations of either alpha-hydroxytamoxifen or tamoxifen. DNA was isolated and analyzed for the presence of DNA adducts by 32P postlabeling. Chromatography on polyethyleneimine cellulose thin layer chromatography and reverse-phase high performance liquid chromatography revealed that the same pattern of adducts was formed by both compounds. However, the level of adduct formation was 25 and 49 times greater with alpha-hydroxytamoxifen than with tamoxifen at 1 and 10 microM, respectively. The formation of alpha-hydroxytamoxifen as a metabolite of tamoxifen was demonstrated by mass spectrometric analysis of the extracted culture medium. alpha-Hydroxytamoxifen was found to react with DNA in the absence of metabolizing enzymes. These results demonstrate the involvement of alpha-hydroxylation in the metabolic activation of tamoxifen.