* Department of Pharmacology and Toxicology, School of Medicine, University of Louisville, Louisville, Kentucky; and Biological Chemistry, Division of Biomedical Sciences, Faculty of Medicine, Imperial College London, London, SW72AZ, UK
Received December 29, 2003; accepted January 23, 2004
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ABSTRACT |
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Key Words: methyl eugenol; tamoxifen; carcinogenesis; thresholds; DNA adducts; 32P-postlabelling.
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INTRODUCTION |
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Tamoxifen is a synthetic "antiestrogen" that is in current clinical use in women for the treatment and prevention of breast cancer. Tamoxifen also has been shown to be carcinogenic in rats when given at high doses (Greaves et al., 1993). Data also are available on DNA adduct formation in the site of tumors (liver) in rats (White et al., 1992
); however, a dose-response evaluation for adduct formation and tumor incidence in rats has been not been published.
This report evaluates the dose response for DNA adduct formation from methyl eugenol and compares that threshold with the carcinogenicity threshold and also with doses to which humans are exposed. The DNA adduct formation for tamoxifen is also compared with the carcinogenicity threshold for tamoxifen in rats and the dose of tamoxifen recommended for women who are at high risk for breast cancer.
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MATERIALS AND METHODS |
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Dose-response calculations for tumor formation and DNA adduct formation from tamoxifen were prepared by the same SlideWrite software and on the same scale from data published by Greaves et al. (1993) and White et al. (1992)
. The dose recommended for women at high risk for breast cancer of 20 mg per day (PDR, 2003
) was converted to molecules/kg/day assuming a woman of 60 kg body weight and plotted on the same graph with the tamoxifen thresholds.
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RESULTS |
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DISCUSSION |
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The calculations in this report suffer somewhat from not being done in the same experiment. Although the tumor incidence and adduct data from methyl eugenol were both determined in F-344 male rats, the tamoxifen data were obtained from F-344 female rats (adducts) and Wistar-derived rats (tumors). In addition, the length of treatment for the adduct data differed from the treatment duration for adducts. For example, the tamoxifen tumor data were obtained from a two-year carcinogenicity study (the first tumors being observed after 31 weeks of treatment in the top dose group), whereas the appearance of DNA adducts was measured over a seven-day dosing regime. Furthermore, 32P-postlabelling may or may not be an accurate measure of actual adducts. Comparisons with direct analyses of adducts are planned in future reports.
Superimposing a vertical grid on Figure 1 revealed that tumor formation from methyl eugenol did not begin until there were about 30 adducts/108 nucleotides. This threshold for tumor formation compares with about 30 adducts/106 nucleotides found in B6C3F1 mice by Phillips et al. (1984). However, the data are not firm because they are in different species and from different experiments. The same is true for the tamoxifen data and, although the DNA adducts fit an exponential plot better than a linear plot, minimal values of adducts coincide with the threshold for tumor formation. It appears that a dose to the animals of about 1018.7 molecules/day of tamoxifen is required for tumor formation to begin. At this threshold for tumor formation the exponential fit for adducts predicts that 5.73 adducts/108 nucleotides are present. This would suggest that adducts below this value do not produce tumors. DNA repair is a remarkably efficient process; consequently, the thresholds seen in this report arise when those repair mechanisms are overwhelmed and tumor formation actually is seen.
Tumor formation from both methyl eugenol and tamoxifen appears to be linear with the logarithm of the dose. The data for DNA adduct formation, however, are not sufficient to distinguish between a linear response and an exponential response with the logarithm of the dose. In the case of methyl eugenol, a linear response gives a slightly better fit, but in the case of tamoxifen, an exponential response gives a slightly better fit. Resolution of this question awaits further data. In the event that DNA adducts do actually prove eventually to increase exponentially with the logarithm of the dose, our suggestion at this time is that adducts might follow a Hill-type equation. That is, as adducts form, the DNA molecule becomes more accessible to further adducts.
The publication by Carthew, P., Lee, P. N., Edwards, R. E., Heydon, R. T., Nolan, B. M., and Martin, E. A. (2001). Cumulative exposure to tamoxifen: DNA adducts and liver cancer in the rat. Arch. Toxicol. 75:375380 has just come to our attention. Those authors found that DNA adducts from tamoxifen below 180 postlabelling adducts per 108 nucleotides did not produce liver tumors in Wistar rats. The difference between that threshold and our calculated threshold of about 6 nucleotides per 108 nucleotides could be due to differences in strains of rats or dosing schedules. We apologize for overlooking this publication with which the present report is consistent and supportive.
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ACKNOWLEDGMENTS |
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NOTES |
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1 To whom correspondence should be addressed at 14300 Rose Wycombe Lane, Prospect, KY 40059. Fax: (502) 228-6779. E-mail: bwaddell{at}louisville.edu
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