Comparative Activities ofp-Nonylphenol and Diethylstilbestrol in Noble Rat Mammary Gland and Uterotrophic Assays

Abstract Colerangle and Roy (1996, Endocrine 4, 115–122) have described the apparent ability of both diethylstilbestrol (DES) and p -nonylphenol (NP) to cause extensive cell proliferation and lobular development in the mammary glands of young adult Noble rats. The chemicals were administered over 11 days via subcutaneously implanted minipumps. The dose level of DES used (0.076 mg/kg/day) was about 70 times higher than its minimum detection level in rodent uterotrophic and reproductive toxicology studies. In contrast, the lowest active dose level of NP (0.073 mg/kg/day) in the Noble rat mammary gland study was about 600 times lower than its minimum detection level in rat uterotrophic and multigeneration studies. The apparent enhanced sensitivity of the Noble rat mammary gland to the estrogenic activity of NP was considered worthy of further study. Ovariectomized Noble rat uterotrophic assays with NP (minimum detection level ∼40 mg/kg/day, 3 or 11 days, oral gavage) revealed similar assay sensitivity to that observed for earlier immature and ovariectomized Alderley Park (AP) rat uterotrophic assays of this chemical. The response of the ovariectomized Noble rat uterotrophic assay to DES and estradiol was also as expected from earlier immature AP rat assays. It is concluded that the general sensitivity to estrogens of the Noble rat and the AP rat is similar. A repeat of the Noble rat mammary gland study with DES (11 × 0.076 mg/kg/day) and NP (11 × either 0.073 or 53.2 mg/kg/day), as originally reported by Colerangle and Roy (1996), revealed a strong positive response to DES and no response to NP. It is concluded that the minimum detection level of NP as a weakly estrogenic material in the rat should be based on the results of rat uterotrophic and multigeneration studies and therefore be set at ∼40 mg/kg/day. It is also concluded that induced S-phase in the rodent mammary gland is best monitored using BRDU, as opposed to PCNA staining, and that use of subcutaneously implanted minipumps/pellets is inappropriate for risk/hazard assessment studies of chemicals already established as estrogenic in vitro and in vivo, as are NP and DES.