Workshop 4 . 3 Application of rat medium - term bioassays for detecting carcinogenic and modifying potentials of endocrine active substances *

Two in vivo bioassay methods, a rat medium-term liver bioassay and a rat multiorgan bioassay, can be used for detecting carcinogenic or modifying potentials of endocrine active substances (EASs) on endocrine disruption (ED). The first bioassay, the rat mediumterm liver bioassay, is fundamentally based on the two-step hypothesis of liver carcinogenesis; initiation with diethylnitrosamine (DEN, 200 mg/kg b.w., ip) is followed by test chemical administrations during the second stage, in combination with 2/3 partial hepatectomy. It requires only eight weeks for animal experimental treatment and a further few weeks for quantitative analysis of immunohistochemically demonstrated gluthathione-S-transeferase placental form positive hepatic foci. A total of 313 chemicals/substances have already been analyzed, and the efficacy of the system for hepatocarcinogenesis has thereby been well established. This bioassay also provides information concerning dose responses and inhibitory potentials of test chemicals. Several possible EASs, most of them categorized as pesticides, have already been examined in this bioassay, and dose–response studies of nonylphenol, bisphenol A, and styrene have also been tested. Another bioassay, a medium-term, multiorgan bioassay system, using five different chemical carcinogens—DEN, MNU, BBN, DMH, and DHPN—has also been established for rapid detection of not only hepatocarcinogens, but also other organ-targeted carcinogens. These medium-term bioassays are particularly useful and reliable methods for detecting carcinogenic or modifying potentials of low doses of test chemicals, such as EASs, and these methods can be used for the effects of chemical mixtures of EASs.