THE EFFECTS OF AROMATASE INHIBITORS AND ANTIESTROGENS IN THE NUDE-MOUSE MODEL

The effects of antiestrogens, tamoxifen and ICI 182,780, and aromatase inhibitors, arimidex (anastrozole ZD1033) and letrozole (CGS 20,267), on the growth of tumors were studied in nude mice. In this model, est rogen dependent MCF-7 human breast cancer cells stably transfected wit h the aromatase gene were inoculated in four sites per mouse. Sufficie nt estrogen is produced from aromatization of androstenedione suppleme nt (0.1 mg/mouse/day) by the cells to stimulate their proliferation, t umor formation, and maintain the uterus similar to that of the intact mouse. Once the tumors reached a measurable size, the mice were inject ed with antiestrogen or inhibitor for 35-56 days. Tumor volumes were m easured weekly. At autopsy, the tumors were removed, cleaned, and weig hed. Statistical data was determined from tumor weights. Both antiestr ogens were effective in reducing tumor growth in these mice. Tamoxifen appears to be more effective than ICI 182,780, although the former st imulated the uterine weight whereas the pure antiestrogen did not. How ever, both aromatase inhibitors were more effective than the antiestro gens. Tumor regression was observed with letrozole. Thus, after-treatm ent tumor weights were less than those of a group of mice at the start of treatment. The aromatase inhibitors also reduced the weight of the uterus, suggesting that these compounds, as well as the pure antiestr ogen, may not cause endometrial proliferation, unlike tamoxifen. These aromatase inhibitors may not only benefit patients who have relapsed from tamoxifen, but may be more effective in patients as first line ag ents for suppressing the effects of estrogen.