Chemopreventive effects of tamoxifen in ethyl methanesulphonate-induced rat mammary carcinogenesis

Can animal models help us select specific compounds for cancer prevention trials?

Animal models provide unparalleled mechanistic insights into cancer development and potential opportunity for cancer prevention. Nevertheless, species differ markedly with regard to dietary exposures, cancer development, drug effects, and toxicity thresholds; therefore, testing in a single animal system may not predict human responses. Although replication of human cancer in animal models remains inexact, more than two decades of research have clearly yielded significant gains, as is evident in agents tested--and in certain cases, approved--for the prevention of epithelial cancers. Research efficiencies achievable through preliminary testing in genetically engineered and carcinogen-induced animal models enable us to probe genetic and signaling pathways that drive normal and neoplastic processes, and thereby figure prominently in decision trees for agent development.

Inhibitory effects of uterine endometrial carcinogenesis in Donryu rats by tamoxifen

The effects of tamoxifen (TAM) on uterine carcinogenesis were investigated in female Donryu rats. The effects were initiated by a single intrauterine treatment with N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) at a dose of 20 mg/kg body weight via the vagina at 10 weeks of age. TAM tubes (cholesterol tubes containing 50% TAM) were implanted into the backs of the rats for 13 months (full TAM group) or for the second-half of this period (half TAM group). In the control group treated with ENNG alone, various proliferative lesions were induced in the uterine endometrium and the incidence of endometrial adenocarcinomas was about 30%. In contrast, the uteri in both TAM-treated groups showed severe atrophy and the incidences of uterine proliferative lesions were limited to a few endometrial hyperplasias in the half TAM group. Most of the vaginas in both TAM-treated groups showed mucification, while cornification was common in the vaginal epithelium of controls. The ovaries demonstrated similar atrophy with cystic follicles and no corpora lutea in all groups. Other estrogen responsive endocrine organs, such as the pituitaries and adrenals, were small in the TAM-treated groups. Serum estrogen levels in the TAM-treated groups were lower than in the control group but progesterone levels did not differ. These results indicated that TAM acts as an anti-estrogen on the adult rat uterus, inhibiting the development of endometrial adenocarcinomas initiated by ENNG.