Covalent binding of carcinogens to DNA, resulting in the formation of DNA adducts, is considered a necessary first step in the initiation of cancer by genotoxic compounds. The detection of DNA adducts can therefore serve as an indicator of carcinogenic potential for a compound and as a biomarker in human risk assessments. As one of the most sensitive methods available for adduct detection we have used AMS to probe the mechanisms responsible for the development of cancers in women treated with tamoxifen.
Tamoxifen, widely employed in the treatment of breast cancer has recently been approved in the US as a chemopreventive agent in healthy women at high risk of developing this disease. Although tamoxifen is well tolerated and causes relatively few side-effects, epidemiological studies have demonstrated that long-term administration leads to an increased incidence of endometrial tumours. There is also some evidence that tamoxifen treatment increases the occurrence of colon cancers in women. In rats, tamoxifen induces hepatocarcinomas via the generation of reactive metabolites which form high levels of DNA adducts in liver tissue. It is not yet clear if endometrial cancer arises as a result of a similar genotoxic mechanism or is due to the partial estrogenic agonist activity of tamoxifen in human uterus. Understanding the processes involved will enable improved risk assessments, particularly for women taking tamoxifen as a chemotherapeutic agent.
By incubating [14C]tamoxifen with individual recombinant human cytochrome P450 enzymes in the presence of DNA, we identified CYP3A4 as the major isoform able to catalyse the metabolic activation and DNA binding of tamoxifen. We have also shown that tamoxifen is capable of binding at extremely low levels (~375 adducts/1012 nucleotides) to uterine DNA of women administered a single therapeutic dose of 14C-labeled drug prior to hysterectomy. Importantly, this level of binding is 10-fold lower than that detected in the liver of rats treated with a comparable dose of [14C]tamoxifen. The ability of tamoxifen to bind to colon DNA was also revealed in an analogous study involving women undergoing surgery for colon cancer. Our AMS studies have therefore demonstrated the potential for tamoxifen to act as a human genotoxic carcinogen but given the low level of binding detected, the contribution of tamoxifen DNA adducts to the carcinogenic process must still be determined. Work was funded by the MRC and supported by NIH/NCRR Resource for Biomedical AMS Grant P41RR13461 at LLNL under auspices of the US DOE, contract #W-7405-ENG-48.
See more of AMS in Low Dose Bioscience Workshop
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)