Related Experiment Video
Updated: Sep 29, 2026
In vivo Characterization of Endocrine Disrupting Chemical Effects via Thyroid Hormone Action Indicator Mouse
Published on: October 6, 2023
Evaluation of a 15-day screening assay using intact male rats for identifying steroid biosynthesis inhibitors and
John C O'Connor1, Steven R Frame, Gregory S Ladics
1DuPont Haskell Laboratory for Health and Environmental Sciences, P.O. Box 50, Newark, Delaware 19714, USA. john.c.oconnor@usa.dupont.com
Abstract:
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], T(3), T(4), 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 T(3) and T(4), 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 route in previous studies for KETO, PB, and PTU. Overall, the sensitivity (i.e., the dose required to elicit similar magnitude responses) between the ip and oral routes of administration were similar for the three EACs that were examined by both routes of administration. This article, in addition to the > 20 compounds that have already been examined using the 15-day intact male assay, supports this assay as a viable screening assay for detecting EACs, and also illustrates that the ability to identify EACs using the intact male assay will be equivalent regardless of the route of compound administration.
Insights
This study validates an in vivo rat assay for detecting endocrine-active compounds (EACs) using oral administration. The assay effectively identified four tested compounds, showing similar results to previous injection methods.
Area of Science:
- Endocrinology and Toxicology
- In vivo screening assays
- Endocrine-active compounds (EACs) detection
Background:
- Established 15-day intact male rat assay for endocrine disruption.
- Previous studies utilized intraperitoneal (ip) injection for compound administration.
- Need to evaluate oral gavage as an alternative administration route.
Purpose of the Study:
- To assess the efficacy of an in vivo screening assay using oral gavage for detecting EACs.
- To compare oral administration results with historical ip administration data.
- To confirm the assay's reliability across different administration routes.
Main Methods:
- Adult male rats dosed for 15 days via oral gavage with fadrozole (FAD), ketoconazole (KETO), phenobarbital (PB), or propylthiouracil (PTU).
- Evaluation of organ weights (liver, thyroid, testes, accessory sex glands), serum hormone levels (testosterone, estradiol, DHT, LH, FSH, prolactin, thyroid hormones, TSH), and histopathology.
- Assessment of immune system endpoints (humoral immunity, spleen/thymus weights) for KETO and PB.
Main Results:
- All four compounds (FAD, KETO, PB, PTU) were identified as endocrine-active.
- Oral administration yielded similar results to ip administration for KETO, PB, and PTU.
- FAD and KETO affected androgen-dependent tissues and hormone levels; PB and PTU modulated thyroid parameters and reproductive hormones.
Conclusions:
- The 15-day intact male rat assay is a viable screening tool for detecting EACs via oral gavage.
- The assay's sensitivity and ability to identify EACs are equivalent regardless of oral or ip administration.
- This study supports the assay's utility for regulatory purposes and understanding chemical endocrine activity.
