Tamoxifen chemoprevention of a hormone-independent tumor in the proto-neu transgenic mice model

We evaluated the effect of tamoxifen (TAM) on tumor development in proto-ne u transgenic mice that spontaneously develop mammary carcinomas overexpress ing the neu protein. These mammary carcinomas are hormone independent becau se superimposable growth of transplants was observed in females and males. Virgin transgenic mice treated with TAM from 24 weeks of age, i.e., when su bclinical mammary tumors are already present, showed a slightly accelerated tumor development. In contrast, transgenic mice treated with TAM starting at 12 weeks of age, when subclinical tumors are not Set present, showed a 5 0% reduction of tumor incidence. Light microscopy analysis of the mammary g land of these mice revealed an apparently normal ductal branching but a com plete regression of the acini, In conclusion, TAM can prevent the occurrenc e of hormone-independent breast carcinoma if given early enough to inhibit normal cells.