DNA adducts formed from 4-hydroxytamoxifen are more mutagenic than those formed by α-acetoxytamoxifen in a shuttle vector target gene replicated in human Ad293 cells

The drug tamoxifen, used to treat breast cancer, causes liver cancer in rats and endometrial cancer in women. Tamoxifen forms liver DNA adducts in both short- and long-term dosing of rodents, and DNA adducts have also been reported in tissues of women undergoing tamoxifen therapy. It is not known if the induction of endometrial cancer in women is through these DNA adducts or through the estrogenic nature of the drug. In this study, we have investigated the mutagenicity of two model reactive intermediates of tamoxifen, α-acetoxytamoxifen and 4-hydroxytamoxifen quinone methide (4-OHtamQM). These form the same DNA adducts as those found in tamoxifen-treated rats. The two compounds were used to treat the pSP189 plasmid containing the supF gene, which was replicated in Ad293 cells before being screened in indicator bacteria. Plasmid reacted with 4-OHtamQM was more likely to be mutated (2-7-fold increase) than that reacted with α-acetoxytamoxifen, despite having a lower level of DNA damage (12-20-fold less), as assayed by ³²p-postlabeling. The two compounds induced statistically different mutation spectra in the supF gene. The majority of mutations in α-acetoxytamoxifen-treated plasmid were GC →TA transversions while GC→AT transitions were formed in 4-OHtamQM-treated plasmid. 4-OHTamQM-treated DNA induced a larger proportion of multiple mutations and large deletions compared to ←-acetoxytamoxifen. Sites of mutational hotspots were observed for both compounds. In conclusion, the quantitatively minor DNA adduct of tamoxifen (dG-N²-4-hydroxytamoxifen) is more mutagenic than the major tamoxifen DNA adduct (dG-N²-tamoxifen).