Replacement of surgical castration by GnRH-inhibition or Leydig cell ablation in the male rat Hershberger antiandrogen assay

OECD validation of the rodent Hershberger assay using three reference chemicals; 17α-methyltestosterone, procymidone, and p,p′-DDE

The rodent Hershberger assay is being validated as an in vivo test method for detecting androgenic or antiandrogenic compounds by the Organization for Economic Cooperation and Development (OECD). As part of the international validation work, we studied 17alpha-methyltestosterone for evaluating androgenic activity, and procymidone and p,p'-DDE for evaluating antiandrogenic activity. Male Sprague-Dawley rats were castrated at postnatal day 42, and only the rats that showed preputial separation were used in this study. Seven days after castration, chemicals were administered daily by gavages to groups of rats for 10 days, as recommended by OECD phase-2 protocol. Administration of 17alpha-methyltestosterone induced increases of weights of accessory sex tissues and glands in a dose-dependent manner. Administration of procymidone and p,p'-DDE produced a dose-dependent decrease of weights of accessory sex tissues and glands in the rats co-treated with testosterone propionate (0.4 mg/kg/day) subcutaneously. Our data strongly suggested that the current protocol of OECD Hershberger assay (phase-2) should be used as a reliable method for the detection of endocrine related toxicity of other chemicals.

p,p'-DDE fails to reduce the competitive reproductive fitness in Nigerian male guppies

The ecologically highly important effect of intermale competition is assessed here in a new approach to evaluating endocrine disruption at the population level. A guppy (Poecilia reticulata) male exposed to the antiandrogenic DDT metabolite p,p'-DDE competed against an unexposed male for the opportunity to fertilize a female. Offspring were then assigned to sires using microsatellites in genetic paternity analysis. In addition, a suite of male sexual characteristics including sperm count, coloration, and sexual behavior were also measured. p,p'-DDE produced no significant harmful effect on either the male sexual characteristics or the success in siring young, although the highest sublethal dose tested was only a factor 10 below the dose producing 100% mortality. At present, data on the antiandrogenic potency of p,p'-DDE are highly ambiguous. This study supports the increasing amount of evidence that the demasculinizing action of this chemical is often weak and highly variable across populations, even within the same species.

Comparison of anti-androgenic activity of flutamide, vinclozolin, procymidone, linuron, and p, p′-DDE in rodent 10-day Hershberger assay

The rodent Hershberger assay proposed by the Organization for Economic Co-operation and Development (OECD) is in the process of the validating a test method to detecting the androgenic or anti-androgenic compounds. The aim of this study was to compare the anti-androgenic properties of flutamide, vinclozolin, procymidone, linuron, and p,p'-DDE in a 10-day Hershberger assay. In the present study, we used immature Sprague-Dawley male rats castrated at 6 weeks of age. Testosterone propionate (TP) was subcutaneously injected for 10 consecutive days at doses of 0.1, 0.2, 0.4, 0.8, or 1.6 mg/kg per day. To compare the anti-androgenic activity of test compounds, flutamide (1, 5, 10, or 20 mg/kg per day), a pure androgen antagonist was used as a positive control, and administered by oral gavage after TP (0.4 mg/kg per day) treatment. In addition, vinclozolin (25, 50, or 100 mg/kg per day), procymidone (25, 50, or 100 mg/kg per day), linuron (25, 50, or 100 mg/kg per day), and p,p '-DDE (25, 50, or 100 mg/kg per day) were also administered by oral gavage after TP (0.4 mg/kg per day) treatment. As expected, TP dose-dependently increased accessory sex organ weights, and statistically significant effects were observed at doses of 0.1 (only seminal vesicles) or 0.2mg/kg per day and above. Serum testosterone levels increased significantly at 0.4 mg/kg per day and above, while serum LH levels were decreased in a dose-dependent manner. Flutamide significantly inhibited the TP-induced re-growth of seminal vesicles, ventral prostate, and Levator ani plus bulbocavernosus muscles (LABC) at 1mg/kg per day and above, and Cowper's glands and glans penis at 5mg/kg per day and above. In contrast to accessory sex organ weights, flutamide did not affect the serum testosterone levels compared to the control at any concentration, but serum LH levels were significantly increased at doses of 10 and 20 mg/kg per day. Similar to flutamide, vinclozolin caused a statistically significant decrease in the weights of seminal vesicles (to 65 and 40% of the control), ventral prostate (to 66 and 51% of the control), LABC (to 81 and 66% of the control), and Cowper's glands (to 81 and 65% of the control) at 50 and 100 mg/kg per day, respectively. Glans penis weight was also significantly reduced (to 79% of the control), but only at 100 mg/kg per day. The most pronounced effects were observed in the procymidone treatment groups. Procymidone significantly inhibited TP-induced re-growth of accessory sex organs at 25mg/kg per day and above, whereas glans penis weight significantly decreased (to 69% of the control), but only at 100 mg/kg per day. Linuron also inhibited TP-induced re-growth of the seminal vesicles (to 72 and 53% of the control), ventral prostate (to 75 and 62% of the control), Cowper's glands (to 74 and 61% of the control) at 50 and 100 mg/kg per day, respectively. LABC (to 65% of the control) and glans penis (to 80% of the control) weights were significantly reduced, but only at 100 mg/kg per day. In case of p,p'-DDE, seminal vesicle weights were significantly decreased at 50 (to 66% of the control) and 100 mg/kg per day (to 58% of the control). In addition, ventral prostate (to 79% of the control), LABC (to 75% of the control), and Cowper's gland (to 82% of the control) weights were reduced, but only at 100 mg/kg per day. On the contrary, no statistically significant differences in serum testosterone or LH levels were observed versus the control. p,p'-DDE significantly increased liver weight in a dose-dependent manner, without affecting on body weights. Our results indicate that procymidone may act as a stronger androgen receptor (AR) antagonist than vinclozolin, linuron, or p,p'-DDE. We conclude that the 10-day Hershberger assay is a sensitive method for detecting potential anti-androgenic compounds.

Comparison of prostate gene expression and tissue weight changes as monitors of antiandrogen activity in GNRH-inhibited rats

BACKGROUND

The Hershberger assay for antiandrogens and modifiers of steroid biosynthesis uses surgically-castrated rats. We described an adaptation of the assay using the GnRH inhibitor Antarelix in place of surgical castration [Ashby J, Lefevre PA, Deghenghi R, Wallis N. Regulatory Toxicology and Pharmacology 34:188-203, 2001], and concomitantly described changes in expression of the androgen-dependent prostatic genes PBP C3, TRPM-2, and ODC as a possible complement to gravimetric analysis of the sex accessory tissues (SAT) [Nellemann C, Vinggaard AM, Dalgaard M, Hossaini A, Larsen J-J. Toxicology 163:29-38, 2001.

METHODS

The present study describes the results of combining these two modifications into a single assay. During the course of these experiments it was shown that SD rats gave similar results to AP rats and that the higher stimulatory dose of testosterone propionate (TP) used in our experiments gave stronger assay responses to FLU than the lower dose of TP used by some earlier investigators. The potent antiandrogen flutamide (FLU) and the weak antiandrogen DDE were used to evaluate this modified assay.

RESULTS

For all parameters studied (SAT weights and changes in expression of the 3 prostatic genes) FLU gave the expected positive results. The weak antiandrogen DDE gave variable and mainly non-reproducible responses. Use of DDE as a weak antiandrogen accelerated assessment of the new assay.

CONCLUSIONS

Possible reasons for this failure to detect DDE are discussed, and it is concluded that the modified assay is unsuitable for use in its present form. The use of gene expression analyses together with evaluation of SAT weights is a promising tool as an early and sensitive marker of antiandrogen action, but more work is needed on the choice of time frame as well as the selection of genes to monitor.

Concept evaluation: an assay for receptor-mediated and biochemical antiestrogens using pubertal rats

At present, assessment of chemicals for receptor-mediated antiestrogenic activity involves inhibition of uterine growth stimulated by coadministration of a reference estrogen in either ovariectomized or immature rodents. In the present paper, we describe an alternative assay for both receptor-mediated and biochemical antiestrogens. The assay involves treatment of immature rats from postnatal (pnd) 25 or 26 for either 7 or 14 days and monitors two benchmarks of puberty, the mean day of vaginal opening and the weight of the uterus, that require estrogen activity. The receptor-mediated antiestrogens ZM 189,154 and Faslodex (ICI 182,780), the aromatase inhibitor Arimidex (Anastrozole), and the GnRH inhibitor Antarelix were each effective in preventing uterine growth and in delaying vaginal opening for the course of the experiments. The 5alpha-reductase inhibitor Finasteride was inactive in the assay indicating assay specificity for antiestrogens. Delays in uterine growth were clearly evident in the 7-day experiments, but assessment of vaginal opening required the 14-day protocol. No significant changes in body weight were observed in any of the experiments. It is concluded that the assay holds promise as a simple method of detecting antiestrogens and that it is worthy of further study.

Concept evaluation: androgen-stimulated immature intact male rats as an assay for antiandrogens

The effects of the concomitant oral administration of a potent reference androgen (17alpha-methyltestosterone) and both a potent (flutamide) and a weak (p,p'-DDE) antiandrogen on intact weanling male rats are described. This protocol resulted in the inhibition by the antiandrogens of the increase in sex accessory tissue weights induced by coadministration of 17alpha-methyltestosterone. Although both flutamide and p,p'-DDE inhibited the androgen-induced growth of the levator ani/bulbocavernosus muscle complex, the Cowper's glands, and the seminal vesicles, the growth of the prostate gland was unaffected by either antiandrogen. The unresponsiveness of the prostate gland, a primary target tissue in the castrated rat antiandrogen assay, has yet to be fully explained. However, the ability of the assay to detect the activity of low dose levels of the weak antiandrogen DDE (at doses of 20 mg/kg body weight) makes the system worthy of further study as one of several alternatives. Given the rapid rate of assay/protocol exploration and refinement of this assay and its alternatives, there is the need for careful comparative studies before selecting a single bioassay for validation and regulatory use.