Evaluation of the EDSTAC female pubertal assay in CD rats using 17β-estradiol, steroid biosynthesis inhibitors, and a thyroid inhibitor

The Endocrine Disrupter Screening and Testing Advisory Committee has recommended the female pubertal onset assay as a Tier I test to detect potential endocrine-disrupting chemicals (EDs). We evaluated this assay's ability to detect EDs acting through various mechanisms. In two similar experiments, weanling female rats were dosed for 20 days by gavage with vehicle (0.5% methocel) or the following test compounds (mg/kg/day): 17 beta-estradiol (E-2; 0.1, 2, or 4), ketoconazole (KETO; 24, 50, or 100), finasteride (FIN; 20), testolactone (TL; 220), fadrozole (FAD; 0.6, 1.2, or 6.0) or 6-propylthiouracil (PTU; 240). In vehicle-treated females, mean age at pubertal onset, as evidenced by vaginal opening (VO), varied interexperimentally from 32.3 +/- 1.6 days to 33.5 +/- 1.8 days. At 0.1 mg/kg E-2, age at VO was reduced slightly to 31.0 +/- 1.6 days, but clot significantly (alpha=0.05). Higher E-2 doses (2.0 and 4.0) reduced age at VO to 28 days. KETO delayed VO, but this delay was significant only at 100 mg/kg (39.7 +/- 2.4 days). FIN and TL had no effect on age at pubertal onset; however, FAD significantly delayed VO. PTU delayed VO to 34.2 +/- 1.1 days and altered thyroid weight, histology, and hormone levels. With each compound, significant changes in age at VO were accompanied by decreased uterine or ovarian weights. Thus, although this assay did not detect TL or lower doses of E-2 (0.1 mg/kg) or KETO (less than or equal to 50 mg/kg), it was capable of detecting EDs operating through a variety of mechanisms.