Evaluation of a 15-Day Screening Assay Using Intact Male Rats for Identifying Antiandrogens

Abstract

An in vivo screening assay using intact adult male rats has been evaluated for its ability to detect six antiandrogenic compounds via oral administration. The test compounds included cyproterone acetate (CPA), flutamide (FLUT), p,p'-DDE (DDE), di-n-butyl phthalate (DBP), linuron (LIN), and vinclozolin (VCZ). Two of the test compounds (DDE and FLUT) 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), and 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 DDE or FLUT. All six endocrine-active compounds (EACs) increased relative liver weight. FLUT and VCZ caused the typical pattern for an androgen receptor (AR) antagonist, although not all endpoints were statistically significant for VCZ: decreased ASG weights, hormonal alterations (increased T, DHT, LH, and FSH), and induced Leydig cell hypertrophy and/or hyperplasia. CPA caused effects consistent with its mixed AR antagonist/progesterone receptor agonist activity: it decreased ASG weights, caused hormonal alterations (increased T and E2; decreased FSH), and caused spermatid retention. DBP, a compound with antiandrogen-like activity via a nonreceptor mediated mechanism, caused hormonal alterations (decreased T, DHT, and E2; increased LH, FSH, and PRL) and induced general testicular degeneration. LIN, a weak AR antagonist, decreased ASG weights, caused hormonal alterations (decreased T, DHT, and LH; increased E2), and caused spermatid retention. Unlike the other AR antagonists evaluated, DDE, a weak AR antagonist, did not alter reproductive parameters. All six antiandrogens caused some effects on thyroid parameters, although only CPA, DDE, and VCZ caused results consistent with a potential thyroid-modulator. FLUT and DDE did not alter the primary humoral immune response to SRBC, spleen or thymus weights, or spleen cell number. In the current study, 5 of the six test substances were identified as endocrine-active substances consistent with their known/proposed mechanism(s) of action. 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 DDE and FLUT. This report, 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.