The Organisation for Economic Co-operation and Development has completed phase 2 of an international validation program for the rodent uterotrophic bioassay. The purpose of the validation program was to demonstrate the performance of two versions of the uterotrophic bioassay, the immature female rat and the adult ovariectomized rat, in four standardized protocols. This article reports the dose-response studies of the validation program; the coded single-dose studies are reported in an accompanying paper. The dose-response study design used five selected weak estrogen agonists, bisphenol A, genistein, methoxychlor, nonylphenol, and o,p -DDT. These weak agonists were administered in a prescribed series of doses to measure the performance and reproducibility of the protocols among the participating laboratories. All protocols successfully detected increases in uterine weights when the weak agonists were administered. Within each protocol, there was good agreement and reproducibility of the dose response among laboratories with each substance. Substance-specific variations were observed in the influence of the route of administration on the uterine response, the potency as related to the dose producing the first statistically significant increase in uterine weights, and the maximum increase in uterine weight. Substantive performance differences were not observed between the uterotrophic bioassay versions or among the standardized protocols, and these were judged to be qualitatively equivalent. It is noteworthy that these results were reproducible under a variety of different experimental conditions (e.g., animal strain, diet, housing, bedding, vehicle, animal age), indicating that the bioassay's performance as a screen is robust. In conclusion, both the intact, immature, and adult OVX versions, and all protocols appear to be reproducible and transferable across laboratories and are able to detect weak estrogen agonists.

The Organisation for Economic Co-operation and Development has completed phase 2 of an international validation program for the rodent uterotrophic bioassay. This portion of phase 2 assessed the reproducibility of the assay with a battery of positive and negative test substances. Positive agonists of the estrogen receptor included the potent reference estrogen 17α-ethinyl estradiol (EE), and the weak estrogen agonists bisphenol A, genistein, methoxychlor, nonylphenol, and o,p´-DDT. The negative test substance or nonagonist was n-dibutylphthalate. The test substances were coded, and prescribed doses of each test substance were administered in 16 laboratories. Two versions of the uterotrophic assay, the intact immature and the adult ovariectomized female rat, were tested and compared using four standardized protocols covering both sc and po administration. Assay reproducibility was compared using a) EE doses identical to those used in phase 1 and in parallel dose-response studies, b) single doses of the weak agonists identical to one of five doses from the dose-response studies, and c) a single dose of the negative test substance. The results were reproducible and in agreement both within individual laboratories and across the participating laboratories for the same test substance and protocol. The few exceptions are examined in detail. The reproducibility was achieved despite a variety of different experimental conditions (e.g., variations in animal strain, diet, housing protocol, bedding, vehicle, animal age). In conclusion, both versions of the uterotrophic bioassay and all protocols appear robust, reproducible, and transferable across laboratories and able to detect weak estrogen agonists. These results will be submitted along with other data for independent peer review to provide support for the validation of the uterotrophic bioassay.

Many commercial laboratory diets have detectable levels of isoflavones (e.g., phytoestrogens such as genistein [GN]) that have weak estrogenic activity both in vitro and in vivo. During validation studies of the uterotrophic bioassay, diet samples from 20 participating laboratories were collected and analyzed for three major phytoestrogens: GN, daidzein (DN), and coumestrol (CM). Soy phytoestrogens GN and DN were found at total phytoestrogen levels from 100 to 540 microg/g laboratory diet; a forage phytoestrogen, CM, ranged from nondetectable to 4 microg/g laboratory diet. The phytoestrogen levels were compared with both baseline uterine weights of the control groups and with the relative uterine weight increase of groups administered two weak estrogen agonists: bisphenol A (BPA) and nonylphenol (NP). The comparison uses a working assumption of additivity among the phytoestrogens, despite several significant qualifications to this assumption, to estimate total genistein equivalents (TGE). Some evidence was found that phytoestrogen levels in the diet > 325-350 microg/g TGE could diminish the responsiveness of the uterotrophic bioassay to weak agonists. This was especially true for the case of the intact, immature female version of the uterotrophic bioassay, where higher food consumption relative to body weight leads to higher intakes of dietary phytoestrogens versus ovariectomized adults. This dietary level is sufficient in the immature female to approach a biological lowest observable effect level for GN of 40-50 mg/kg/day. These same data, however, show that low to moderate levels of dietary phytoestrogens do not substantially affect the responsiveness of the assay with weak estrogen receptor agonists such as NP and BPA. Therefore, laboratories conducting the uterotrophic bioassay for either research or regulatory purposes may routinely use diets containing levels of phytoestrogens < 325-350 microg/g TGE without impairing the responsiveness of the bioassay.

The Organisation for Economic Co-operation and Development has completed the first phase of an international validation program for the rodent uterotrophic bioassay. This uterotrophic bioassay is intended to identify the in vivo activity of compounds that are suspected agonists or antagonists of estrogen. This information could, for example, be used to help prioritize positive compounds for further testing. Using draft protocols, we tested and compared two model systems, the immature female rat and the adult ovariectomized rat. Data from 19 participating laboratories using a high-potency reference agonist, ethinyl estradiol (EE), and an antagonist, ZM 189,154, indicate no substantive performance differences between models. All laboratories and all protocols successfully detected increases in uterine weights using EE in phase 1. These significant uterine weight increases were achieved under a variety of experimental conditions (e.g., strain, diet, housing protocol, bedding, vehicle). For each protocol, there was generally good agreement among laboratories with regard to the actual EE doses both in producing the first significant increase in uterine weights and achieving the maximum uterine response. Furthermore, the Hill equation appears to model the dose response satisfactorily and indicates general agreement based on calculated effective dose (ED)(10) and ED(50) within and among laboratories. The feasibility of an antagonist assay was also successfully demonstrated. Therefore, both models appear robust, reproducible, and transferable across laboratories for high-potency estrogen agonists such as EE. For the next phase of the OECD validation program, both models will be tested against a battery of weak, partial estrogen agonists.

The Organisation for Economic Co-operation and Development has undertaken an international validation program for the rodent uterotrophic bioassay. This validation program comprised two major parts. The first part was the development of a detailed background review document compiling the existing data on the bioassay's history, the molecular and physiologic basis for the bioassay's mechanistic relevance to detect estrogen agonists and antagonists, a review of important bioassay protocol parameters, and a review of the data generated by in vitro assays, previous uterotrophic bioassays, and developmental and reproductive assays to assess and support the overall predictivity of the uterotrophic bioassay. The second part was an extensive multiyear effort managed by a validation management group to demonstrate the operating characteristics of four protocols. The effort was conducted in two phases. The phase 1 results with the reference agonist ethinyl estradiol (EE) and antagonist ZM 189,154 has been published previously. This Environmental Health Perspectives mini-monograph is devoted to the phase 2 work using five weak estrogen agonists, bisphenol A, genistein, methoxychlor, nonylphenol, and o,p -DDT, as well as the negative substance dibutylphthalate. These data show that all protocols successfully detected increases in uterine weights when a sufficient dose level of the weak agonists was administered, whether the substances were known or provided as coded doses to the laboratory. The data with both the reference EE and all five weak agonists are reproducible over time and under a variety of different experimental conditions (e.g., animal strain, diet, housing, bedding, vehicle, animal age). In conclusion, all protocols now have sufficient data to support their validity.

The Organisation for Economic Cooperation and Development (OECD) has completed phase 1 of the Hershberger validation intended to identify in vivo activity of suspected androgens and anti-androgens. Seventeen laboratories from 7 countries participated in phase 1, and results were collated and evaluated by the OECD with the support of an international committee of experts. Five androgen-responsive tissues (ventral prostate, paired seminal vesicles and coagulating glands, levator ani and bulbocavernosus muscles, glans penis, and paired Cowper’s or bulbourethral glands) were evaluated. The standardized protocols used selected doses of a reference androgen, testosterone propionate (TP), and an antiandrogen, flutamide (FLU). All laboratories successfully detected TP-stimulated increases in androgen-responsive tissue weight and decreases in TP-stimulated tissue weights when FLU was co-administered. The standardized protocols performed well under a variety of conditions (e.g., strain, diet, housing protocol, bedding). There was good agreement among laboratories with regard to the TP doses inducing significant increases in tissue weights and the FLU doses decreasing TP-stimulated tissue weights. Several additional procedures (e.g., weighing of the dorsolateral prostate and fixation of tissues before weighing) and serum component measurements (e.g., luteinizing hormone) were also included by some laboratories to assess their potential utility. The results indicated that the OECD Hershberger protocol was robust, reproducible, and transferable across laboratories. Based on this phase 1 validation study, the protocols have been refined, and the next phase of the OECD validation program will test the protocol with selected doses of weak androgen agonists, androgen antagonists, a 5α-reductase inhibitor, and chemicals having no androgenic activity.

ObjectiveThe Organisation for Economic Co-operation and Development (OECD) has completed phase 2 of an international program to validate the rodent Hershberger bioassay.DesignThe Hershberger bioassay is designed to identify suspected androgens and antiandrogens based on changes in the weights of five androgen-responsive tissues (ventral prostate, paired seminal vesicles and coagulating glands, the levator ani and bulbocavernosus muscles, the glans penis, and paired Cowper’s or bulbourethral glands). Protocol sensitivity and reproducibility were tested using two androgen agonists (17α-methyl testosterone and 17β-trenbolone), four antagonists [procymi-done, vinclozolin, linuron, and 1,1-dichoro-2,2-bis-(p-chlorophenyl)ethylene (p,p’-DDE)], and a 5α-reductase inhibitor (finasteride). Sixteen laboratories from seven countries participated in phase 2.ResultsIn 40 of 41 studies, the laboratories successfully detected substance-related weight changes in one or more tissues. The one exception was with the weakest antiandrogen, linuron, in a laboratory with reduced sensitivity because of high coefficients of variation in all tissue weights. The protocols performed well under different experimental conditions (e.g., strain, diet, housing protocol, bedding, vehicle). There was good agreement and reproducibility among laboratories with regard to the lowest dose inducing significant effects on tissue weights.ConclusionsThe results show that the OECD Hershberger bioassay protocol is reproducible and transferable across laboratories with androgen agonists, weak androgen antagonists, and a 5α-reductase inhibitor. The next validation phase will employ coded test substances, including positive substances and negative substances having no androgenic or antiandrogenic activity.

The Organisation for Economic Co-operation and Development has initiated the development of new guidelines for the screening and testing of potential endocrine disruptors. The Hershberger assay is one of the assays selected for validation based on the need for in vivo screening to detect androgen agonists or antagonists by measuring the response of five sex accessory organs and tissues of castrated juvenile male rats: the ventral prostate, the seminal vesicles with coagulating glands, the levator ani and bulbocavernosus muscle complex, the Cowper's glands, and the glans penis. The phase 1 feasibility demonstration stage of the Hershberger validation program has been successfully completed with a single androgen agonist and a single antagonist as reference substances. The phase 2 validation program employs a range of additional androgen agonists and antagonists as well as 5alpha-reductase inhibitors. Seven Japanese laboratories have contributed phase 2 validation studies of the Hershberger assay using methyltestosterone, vinclozolin, and 2,2-bis (4-chlorophenyl)-1,1-dichloroethylene (p,p'-DDE). The methyltestosterone doses were 0, 0.05, 0.5, 5, and 50 mg/kg/day, and the vinclozolin and p,p'-DDE doses were 0, 3, 10, 30, and 100 mg/kg/day. All chemicals were orally administered by gavage for 10 consecutive days. In the antagonist version of the assay using vinclozolin and p,p'-DDE, 0.2 mg/kg/day of testosterone propionate was coadministered by subcutaneous injection. All five accessory sex preproductive organs and tissues consistently responded with statistically significant changes in weight within a narrow window. Therefore, the Japanese studies support the Hershberger assay as a reliable and reproducible screening assay for the detection of androgen agonistic and antagonistic effects.