Re: Estimation of Tamoxifen's Efficacy for Preventing the Formation and Growth of Breast Tumors

Extract

The recent report by Radmacher and Simon (1) addressed the issue of whether the effect of tamoxifen observed in the National Surgical Adjuvant Breast and Bowel Project's Breast Cancer Prevention Trial (BCPT) (2) was “mainly due to prevention of newly forming tumors or to treatment of occult disease.” On the basis of analyses in various tumor growth models, those authors (1) concluded that the reduced tumor incidence in the tamoxifen-treated group was probably due to both effects.

By contrast, our recent data (3) obtained with the use of a transgenic mice model of spontaneous development of HER2/neu-positive, estrogen-independent tumors suggest that the protective effect observed in the BCPT is due only to treatment of occult disease. Indeed, in our animal model, early treatment with tamoxifen inhibited the formation of terminal buds in the mammary gland and clearly prevented spontaneous tumors. In contrast, late treatment—when subclinical tumors were already present—had no effect, as expected for the treatment of estrogen-independent tumors.