Peroxidase activation of tamoxifen and toremifene resulting in DNA damage and covalently bound protein adducts.

When [14C]tamoxifen was incubated with horseradish peroxidase and H2O2, two major metabolites, separated and identified by HPLC, were N-desmethyltamoxifen and tamoxifen N-oxide. Toremifene incubated in a similar system yielded N-desmethyltoremifene and toremifene N-oxide. No 4-hydroxylated metabolites were detected with either drug. When calf thymus DNA was included in peroxidase incubation mixtures, DNA damage, as assessed by 32P-postlabelling, could also be detected. The extent of damage caused by tamoxifen and toremifene was similar. The major adducts formed following incubation of DNA with tamoxifen had similar Rf values to two of the 32P-postlabelled adducts seen following dosing of rats with tamoxifen. Peroxidase was able to activate both drugs to derivatives which covalently bound to bovine serum albumin. The pH optimum for covalent binding and N-demethylation was near to pH 6.0. Results from liquid chromatography-electrospray secondary ion mass spectrometry suggest that tamoxifen and toremifene are metabolized by peroxidase to putative reactive epoxide intermediates responsible for the genotoxic effects. It is proposed that peroxidase oxidizes tamoxifen to a carbon-centred free radical which reacts with oxygen to form peroxy radicals capable of inserting an oxygen atom into tamoxifen. Lactoperoxidase and prostaglandin synthase are also able to catalyse tamoxifen N-demethylation and binding to protein. These data show that peroxidase can activate both tamoxifen and toremifene to an intermediate(s) that can damage DNA and covalently react with protein. Since it is known that women treated with tamoxifen can develop endometrial tumours, it may be relevant to determine whether activation of tamoxifen by peroxidases may contribute to its carcinogenic action at extrahepatic sites.