The OECD Program to Validate the Rat Uterotrophic Bioassay: an Overview

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.