Mutations induced by alpha-hydroxytamoxifen in the lacI and cII genes of Big Blue transgenic rats.

The antiestrogen tamoxifen is widely used for the treatment of breast cancer and more recently for the prevention of breast cancer. A concern over the use of tamoxifen as a chemopreventive agent is its carcinogenicity in rat liver, through a genotoxic mechanism involving alpha-hydroxylation, esterification, and DNA adduct formation, primarily by reaction with dG. In a recent study [Gamboa da Costa et al., Cancer Lett., 176, 37-45 (2002)], we demonstrated a significant increase in the mutant frequency in the lacI gene of Big Blue rats treated with tamoxifen, and a further increase in rats administered alpha-hydroxytamoxifen. In the present study, we have assessed mutation induction by tamoxifen and alpha-hydroxytamoxifen in the liver cII gene of Big Blue rats and have characterized the types of mutations induced by alpha-hydroxytamoxifen in the liver lacI and cII genes. The mutant frequencies in the liver cII gene were 80 +/- 13 x 10(-6) in the control, 112 +/- 13 x 10(-6) in the tamoxifen-treated group (P < 0.01 vs. control), and 942 +/- 114 x 10(-6) in the alpha-hydroxytamoxifen-treated animals (P < 0.001 vs. control; P < 0.001 vs. tamoxifen). Molecular analysis of the mutants indicated that the alpha-hydroxytamoxifen-induced mutational spectrum differed significantly from the control spectrum, but was very similar to the spectrum induced by tamoxifen for both the lacI and cII genes [Davies et al., ENVIRON: Mol. Mutagen., 28, 430-433 (1996); Davies et al., Carcinogenesis, 20, 1351-1356 (1999)]. G:C --> T:A transversion was the major type of mutation induced by alpha-hydroxytamoxifen and tamoxifen, while G:C --> A:T transition was the main type of mutation in the control. These results support the hypothesis that alpha-hydroxytamoxifen is a major proximate tamoxifen metabolite causing the initiation of tumors in the liver of rats treated with tamoxifen.