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Evaluation of FETAX for Developmental Toxicity

In 1998, the U.S. Environmental Protection Agency (EPA) requested that ICCVAM evaluate the frog embryo teratogenesis assay - Xenopus (FETAX). An ICCVAM-sponsored expert panel concluded that FETAX was not sufficiently validated or optimized for regulatory applications. The panel recommended that FETAX be further standardized to improve variability and that the number of endpoints assessed be increased.


Developmental toxicity testing evaluates the extent to which exposure to a chemical can interfere with normal development. This can include effects resulting from exposures of an individual’s parents before and during pregnancy, as well as effects of exposure during the individual’s infancy or childhood. Effects of developmental toxicants can be obvious or subtle, and may be apparent at birth or emerge later in the offspring's life. Current U.S. federal regulations require testing of many chemicals and products, such as pesticides, food additives, industrial chemicals, and pharmaceuticals, for their potential to cause developmental toxicity.

Most developmental toxicity test protocols use rats, rabbits, or other mammalian species. FETAX, a 96-hour test that uses early-stage whole embryos of the South African clawed frog (Xenopus laevis) to measure the potential of chemicals to cause mortality, malformation, and growth inhibition, was nominated by the EPA as a screening assay to identify potential human developmental toxicants.

In response to the nomination, ICCVAM agreed to consider the potential regulatory applications of FETAX. NICEATM prepared a background review document summarizing available data on FETAX and the extent to which the assay met ICCVAM validation and acceptance criteria for potential regulatory applications. The NICEATM background review document was considered in May 2000 by an ICCVAM-sponsored expert panel charged to review the current status and future directions for FETAX method development.

Related Reference

Hoke RA, Ankley GT. 2005. Application of frog embryo teratogenesis assay-xenopus to ecological risk assessment. Environmental Toxicology and Chemistry 24:2677-2690. View on PubMed
This review, published after the ICCVAM expert panel evaluation, provides an evaluation and critique of the current FETAX protocol from two perspectives: practical considerations relative to conducting the test and sensitivity of the assay (and associated endpoints) compared to tests with other species.