Evaluation of the Male Pubertal Onset Assay to Detect Testosterone and Steroid Biosynthesis Inhibitors in CD Rats

The male pubertal onset assay has been recommended by the Endocrine Disrupter Screening and Testing Advisory Committee (EDSTAC) as an alternate Tier I screening assay to detect potential endocrine-active chemicals (EACs). Recently, this assay was evaluated by several laboratories using a variety of dosing schemes. This study used a 30-day dosing period to confirm and extend previous work on the assay's ability to detect steroid biosynthesis inhibitors. Weanling male rats were dosed by gavage from 21 to 50 days of age with vehicle (0.5% methocel) or chemicals from the following EAC classes: an androgen (testosterone propionate [TP], 0.1 or 0.4 mg/kg/day), a broad-spectrum steroid biosynthesis inhibitor (ketoconazole [KETO], 24 mg/kg/day), a 5alpha-reductase inhibitor (finasteride [FIN], 20 or 80 mg/kg/day), a moderately specific aromatase inhibitor (testolactone [TL], 220 mg/kg/day), or a highly specific aromatase inhibitor (fadrozole [FAD], 0.6 or 6.0 mg/kg/day). None of these treatments altered relative thyroid weights. However, TL, KETO, and FIN were positive for endocrine activity based on decreases in one or more reproductive or accessory sex gland organ weights. Of these three inhibitors, only TL significantly increased the age at PPS, indicating that PPS was less sensitive for detecting these EACs. Based on its profile of effects, TL may have been detected as an antiandrogen. TP and FAD were negative in this assay, even at doses that caused effects in other studies. With TP, oral administration limited assay sensitivity such that higher TP doses would be needed for detection. FAD decreased body weight gains, but did not significantly alter any other assay end points; thus, the capacity of this assay to detect aromatase inhibitors remains in question.