Evaluation of a 15-Day Screening Assay Using Intact Male Rats for Identifying Steroid Biosynthesis Inhibitors and Thyroid Modulators

An in vivo screening assay using intact adult male rats has been evaluated for its ability to detect four endocrine-active compounds (EACs) via oral (gavage) administration. The test compounds included the aromatase inhibitor fadrozole (FAD), the testosterone biosynthesis inhibitor ketoconazole (KETO), and the thyroid modulators phenobarbital (PB) and propylthiouracil (PTU). Three of the test compounds (KETO, PB, and PTU) have been previously evaluated in the 15-day intact male assay with compound administration via intraperitoneal injection (ip). For the current studies, male rats were dosed for 15 days via oral gavage and euthanized on the morning of test day 15. The endpoints evaluated included final body and organ weights (liver, thyroid gland, testes, epididymides, prostate, seminal vesicles with fluid, accessory sex gland unit [ASG]), serum hormone concentrations (testosterone [T], estradiol [E2], dihydrotestosterone [DHT], luteinizing hormone [LH,] follicle stimulating hormone [FSH], prolactin [PRL], T3, T4, thyroid stimulating hormone [TSH]), and histopathology of the testis, epididymis, and thyroid gland; positive results for each endpoint are described below. In addition, an evaluation of immune system endpoints (humoral immune function, spleen and thymus weights, and spleen cell number) was conducted on a subset of animals dosed with either KETO or PB. FAD and KETO decreased the weights for the androgen-dependent tissues and caused similar patterns of hormonal alterations (decreased serum T and DHT; increased serum FSH and/or LH). In addition, KETO caused spermatid retention. For FAD and KETO, effects on thyroid parameters were not indicative of thyroid toxicity. PB and PTU caused thyroid effects consistent with thyroid modulators (increased thyroid weight, decreased serum T3 and T4, increased serum TSH, thyroid follicular cell hypertrophy/hyperplasia, and colloid depletion). In addition, PB increased relative liver weight and altered reproductive hormone concentrations (decreased serum DHT, PRL, LH; increased serum E2). Orally administered KETO and PB did not alter the primary humoral immune response to sheep red blood cells (SRBC), although spleen weights were increased at the highest doses for both compounds. In the current study, all four test substances were identified as endocrine-active. The effects that were observed in the current study via oral (gavage) compound administration were similar to the responses that were observed by the ip