Uterine carcinoma in mice treated neonatally with tamoxifen

The induction of preneoplastic and neoplastic lesions by the widely used antiestrogen Tamoxifen was studied in female mice. Outbred CD-1 mice were treated with Tamoxifen (1, 2, 5, 10, 25 or 50 microg/pup/day) for the first 5 days after birth. At 14-17 months, reproductive tract tissues were examined for pathological changes. In the ovary, corpora lutea were lacking while cysts were quite common in Tamoxifen-exposed mice at all doses; cystadenomas were seen in two mice. Structural malformations and epithelial hyperplasia of the oviduct were seen in 100% of the treated mice. Malformations of the uterus, cervix, and vagina were also seen. Excessive vaginal keratinization was not a common feature although vaginal adenosis was observed more often after Tamoxifen treatment than previously reported after similar treatment with diethylstilbestrol (DES). The most striking histological features, however, were seen in the uterus. One hundred percent of the Tamoxifen-treated mice at all doses exhibited uterine hypoplasia with focal areas of basal cell hyperplasia in the lining endometrium. Progressive cellular atypias were seen in the lining endometrium ranging from atypical hyperplasia to uterine adenocarcinoma; the highest incidence of uterine adenocarcinoma was 7/14 (50%) observed in the Tamoxifen 10 microg/pup/day dose group. No similar tumors were observed in corresponding control mice. The induction of atypical uterine hyperplasia and adenocarcinoma combined with other abnormalities observed in genital tract structure following neonatal treatment with Tamoxifen suggests the developing reproductive tract is exquisitely sensitive to perturbation by compounds with hormonal activity. These studies provide the basis for future investigation into the mechanisms of Tamoxifen's carcinogenic effects in experimental animals, and to the risk benefit analysis for the prophylactic use of Tamoxifen in healthy women who are at risk of developing breast cancer.