The mechanism of tamoxifen in breast cancer prevention

Tamoxifen (TAM) is known to have a dual mechanism of action: (1) to compete with 17β-estradiol (E₂) at the receptor site and to block the promotional role of E₂ in breast cancer; and (2) to bind DNA after metabolic activation and to initiate carcinogenesis. Recent large clinical trials indicate that TAM is also an effective chemopreventive agent against breast cancer. The mechanism is unknown. Because E₂ requires activation by epoxidation to bind DNA forming DNA adducts [1], and the same is true for TAM [2], the question is whether this preventive effect of TAM against breast cancer is contributory to the possibility that TAM, as an effective competitor for epoxidation, prevents the formation of E₂ epoxide and consequently breast cancer. Evidence will be presented to show that, indeed, when incubated together with E₂ for epoxidation, TAM was able to dramatically reduce the formation of E₂ epoxide as measured by both the loss of the ability of E₂ to inhibit nuclear RNA synthesis, and the reduced binding of [³H]labeled E₂ to nuclear DNA. Identical results were obtained when TAM and estrone (E₁) were used. These results suggest that the breast cancer preventive effect of TAM is through a competitive epoxidation mechanism with E₁/E₂.