Estradiol-stimulated growth of MCF-7 tumors implanted in athymic mice: A model to study the tumoristatic action of tamoxifen

Ovariectomized athymic (nude) mice were inoculated (10⁷ cells) with the breast cancer cell line, MCF-7, into the axillary mammary fat pads. Tumors did not grow unless animals were implanted with a 1.7 mg estradiol sustained (8-week)-release cholesterol pellet. Co-implantation with tamoxifen (5 mg, 4-week release) caused an inhibition of estradiol-stimulated growth but did not cause tumor growth when implanted alone. The metabolism of [³H]tamoxifen was determined in the athymic mouse bearing MCF-7 tumors. Metabolites in the liver, uterus and tumor were determined by TLC. The principal metabolite in each of the tissues was 4-hydroxytamoxifen (by comparison of R_fs with authentic Standards). Studies with 4-hydroxytamoxifen and N-desmethyltamoxifen (the principal metabolites in patients) showed that each was effective in inhibiting estradiol-stimulated tumor growth. However, tumor growth could be reactivated by treatment with estradiol alone. In a separate experiment, tumor-implanted animals were treated with tamoxifen for 1, 2 and 6 months. Tamoxifen did not cause tumor growth. Nevertheless, tumor growth was reactivated by estradiol on each occasion. These studies confirm the tumorstatic actions of tamoxifen and strongly support the view that therapy must be given indefinitely to patients to control tumor recurrence. The athymic mouse model can be used in the future to determine the efficacy of novel antiestrogens and the development of antiestrogen drug resistance.