Pure and Applied Chemistry

Study protocols for the characterization of endocrine active compounds presented in Workshop 4 included the enhanced Organization for Economic Cooperation and Development (OECD) test guideline (TG) 407, the medium-term rat liver and rat multi-organ carcinogenicity assays, and an enhanced one-generation reproduction study.

The outcome of rat studies on flutamide and ethinylestradiol indicated that these strongly active compounds can readily be detected even with a low animal number using the enhanced OECD TG 407. Both newly added (such as male accessory sex organ weights, histology of pituitary, vagina and male mammary gland) and already included parameters contributed to the detection of endocrine effects. Thorough evaluation of the results of 20 studies conducted with 10 compounds thought to interfere with the endocrine system by different mechanisms will identify the most appropriate enhancements to the current OECD TG 407.

Medium-term rat liver and rat multi-organ carcinogenicity assays are well recognized in the International Conferences on Harmonization for Pharmaceutical Chemicals. They have been successfully used to detect carcinogenic and modifying potentials of new chemicals within a relatively short time and can be applied to endocrine active compounds. Dose-response studies on nonylphenol, bisphenol A, and styrene using the rat liver carcinogenicity assay did not reveal effects of any of these compounds on the development of preneoplastic lesions in rat liver.

The enhanced one-generation reproduction study protocol included treatment of pregnant female rats from gestation day 0 through to lactation day 21, and examination of all offspring. Half of the animals were necropsied at weaning, the remaining animals were examined for vaginal opening, preputial separation, estrous cyclicity, and sperm characteristics and were necropsied at adulthood. In a pilot study ethinylestradiol inhibited maternal fertility at dose levels similar to those effective in the uterotrophic assay.

It is recommended to rapidly evaluate the conducted enhanced OECD TG 407 studies and to enhance the current OECD TG 407 appropriately. Further compounds with different mechanisms of action should be studied in the one-generation reproduction study to further investigate the usefulness of this protocol. The established medium-term carcinogenicity assays can be used to study carcinogenic potential rapidly. Use of female animals and inclusion of carcinogens targeting at breast and uterus should be considered in order to explore further the predictibility of this model.