Validation of an Amphibian Lifecycle Test Method for Measuring Endocrine Disruption

The Phase I study resulted in the development of an assay that tests substances that have the capacity to disturb reproductive and developmental processes during the lifecycle of a vertebrate animal by interfering with the endocrine system. The primary goal of this Phase II research project is to standardize, validate, and commercialize an amphibian lifecycle model using Xenopus as a system for the evaluation of endocrine-disrupting chemicals (EDCs) found in the workplace or in the environment. Specifically, Fort Environmental Laboratories, Inc., standardized and evaluated the use of X. tropicalis as a model system to evaluate the effect of EDCs on various aspects of the amphibian lifecycle by conducting studies with a series of known mammalian EDCs and chemicals with unknown activity. 17ß-estradiol (E2), androstene dione, and methoxychlor were used to develop and standardize the model. Results from these studies were used to generate a final study protocol from which a Guidance Document ultimately will be prepared following Phase II interlaboratory validation studies. Results from Phase I strongly indicated that chronic exposure to E2 slightly delayed development, had a marginal affect on the normalcy of early larval development, dramatically skewed sex ratios toward the female gender, induced gonadal malformations that included some intersexual development at low test concentrations, and induced liver abnormalities. Furthermore, chronic E2 exposure impaired both female and male reproductive fitness. Chronic exposure to E2 decreased breeding success in both sexes, decreased fertilization in exposed females, and decreased embryo-larval viability in the F1 progeny. Chronic exposure to androstene dione dramatically skewed sex ratios toward the male gender, induced gonadal malformations that included some intersexual development at low test concentrations, and caused liver abnormalities. Androstene dione reduced reproductive fitness in female specimens, but not male specimens. Chronic exposure to androstene dione did not markedly alter breeding success in both sexes or decrease embryo-larval viability in F1 progeny, in contrast to the effects of E2. Finally, chronic exposure to methoxychlor delayed development, including slowing the rate of metamorphosis; had a slight affect on the normalcy of larval development; caused skewed sex ratios toward the female gender; and induced gonad, liver, and thyroid abnormalities in juvenile specimens. Furthermore, chronic methoxychlor exposure impaired both female and male reproduction fitness. Increasing concerns over the widespread finding of EDCs in the environment have dramatically increased the need for standardized assays, such as the XLCA, because no other assay of this type is available today.