The peripubertal male rat assay as an alternative to the Hershberger castrated male rat assay for the detection of anti-androgens, oestrogens and metabolic modulators

A range of chemicals with various levels of activity as actual or potential endocrine disrupters have been evaluated for activity in the peripubertal male rat assay. The chemicals studied included antiandrogens (vinclozolin), cyproterone acetate, flutamide, 2,2-bis(4-chlorophenyl)-1,1-dichloroethyle ne (DDE), metabolic modulators (anastrazole, finasteride, ketoconazole) and oestrogens (butyl benzyl phthalate (BBP), methoxychlor, bisphenol A (BPA), diethylstilboestrol (DES)), the suspected anti-androgen dibutyl phthalate (DBP) and the non-oestrogen fenitrothion. Dosing extended over postnatal da ys (pnd) 22-35, 36-50, 36-55 and 22-35, with recovery to pnd 55 or 22-55. T he endpoints studied were changes in the weights of testes, epididymides, s eminal vesicles and prostate. Changes in body weight and the weights of the liver and kidney were also monitored. In some experiments changes in the d ay of prepuce separation (PPS) were also determined. Only BBP and BPA were inactive in all the assays conducted. Changes in the weight of reproductive tissues provided a sensitive indicator of activity for the remaining chemi cals with the exception of DDE, for which higher dose levels could have bee n used. However, none of the curtailed periods of exposure were able to det ect all of the agents. Diethyl stilboestrol, and to a lesser extent DBP and DDE, delayed PPS when exposure occurred over the period pnd 22-55. A compl ex dependence of the day of PPS on the period of exposure and the body weig ht of the test animals was observed, and caution is recommended when assess ing this endpoint in the presence of reductions in body weight. It is concl uded that reproductive tissue weight changes in the peripubertal male have shown sensitivity to a range of biochemical modulators, oestrogens and anti androgens, and that as such the assay warrants further evaluation. Measurem ent of delays in PPS may be of value in cases of large delays, but delays o f 1-2 days will be difficult to interpret with confidence. The present resu lts are discussed within the context of the sexually mature male rat assay described by O'Connor and the castrated male rat assay described by Hershbe rger, both of which are the subject of current international study. It is c oncluded that a decision on the usefulness of the peripubertal male rat ass ay must await the generation of further data on each of these three assays. There is an urgent need for international agreement on a list of reference endocrine disrupters and their active dose levels, with which to validate individual endocrine disruption assays and batteries of assays. Copyright ( C) 2000 John Wiley & Sons, Ltd.