Reduced genotoxicity of [D5-ethyl]-tamoxifen implicates alpha-hydroxylation of the ethyl group as a major pathway of tamoxifen activation to a liver carcinogen.

A proposed mechanism for the metabolic activation of tamoxifen to electrophilic species that form DNA adducts leading to liver cancer involves alpha-hydroxylation of the ethyl group in the critical first step. This mechanism predicts that tamoxifen deuterated at the alpha-position would be less genotoxic than the non-deuterated compound owing to an isotope effect that would reduce the rate of oxidative metabolism at this position. This hypothesis has now been tested with experiments conducted in rats in vivo and in human cells in vitro. It was found that the deuterated compound [D5-ethyl]-tamoxifen is significantly less genotoxic than tamoxifen. Administration of 0.06 and 0.12 mmol/kg [D5-ethyl]-tamoxifen to female Fischer rats by gavage resulted in a 2.5- and 1.7-fold reduction respectively in the levels of hepatic DNA adducts present 24 h after treatment, compared with the non-deuterated compound. Treatment of MCL-5 cells with [D5-ethyl]-tamoxifen resulted in a 2- to 3-fold decrease, compared with tamoxifen, in the number of micronuclei induced in cells arrested in cytokinesis by cytochalasin-B. Further evidence is provided by UV irradiation of the major hepatic DNA adducts isolated from tamoxifen-treated rats, which caused a large shift in the chromatographic mobility of the adducts, consistent with the presence of a 1,2-olefinic linkage in the tamoxifen residue of the adduct leading to cyclization to a phenanthrenylic compound, and consistent with this adduct having arisen from reaction with DNA at the alpha-position of tamoxifen. Taken together, these data strongly suggest that tamoxifen is metabolized to a liver carcinogen by alpha-hydroxylation of its ethyl group.