Use of Emerging Models for Developmental Toxicity Testing - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 26

just prior to normal delivery. Then, death and fetal defects, including external,
skeletal, and visceral malformations, are assessed visually or by X-ray. External
malformations including missing limbs and missing or malformed bones and visceral
malformations including development of internal organs such as heart, brain, lungs,
and so on are examined. Each specific malformation is typically recorded as a
dichotomous variable (present/absent) and each fetus may exhibit several types of
malformations. These conventional developmental toxicity tests are laborious, cost
ineffective, and time consuming. As a result, developmental toxicity is currently
assessed for only a small number of drugs during preclinical development.

3.3 USE OF EMERGING MODELS FOR
DEVELOPMENTAL TOXICITY TESTING

Although whole embryo testing has previously been performed on invertebrates,
including fruit flies and nematodes (Eisses, 1989; Hitchcock et al., 1997), these
species are not closely related to humans and they lack many mammalian organs and
enzymes. Therefore, use of Drosophila and Caenorhabditis elegans as comparative
models for assessing toxicity, including developmental toxicity, has been limited.

3.3.1 Frog Embryo Teratogenesis Assay— Xenopus

The frog embryo teratogenesis assay— Xenopus (FETAX) has recently been devel-
oped as a screening assay for assessing developmental toxicity. This test uses early-
stage South African clawed frog Xenopus embryos to assess chemical effects on
(1) mortality, (2) malformation, and (3) growth inhibition. In 1998, the U.S. EPA
requested that the Interagency Coordinating Committee on the Validation of Alter-
native Methods (ICCVAM) evaluate FETAX.
Although FETAX appears to be capable of measuring key relevant develop-
mental toxicity end points, including lethality, malformation, and growth, and
estimating the dose-response relationships for these end points, based on the available
data, the ICCVAM panel concluded that FETAX results were excessively variable,
both within and between labs. The ICCVAM panel also concluded that FETAX was
not sufficiently validated or optimized for inclusion in regulatory applications and
it recommended further standardization of the assay to improve variability and
expansion of the number of end points to identify developmental toxicants. Additional
disadvantages of FETAX include that since frog embryos are only partially trans-
parent, development and internal organ structures cannot be fully visualized. Frogs
have also been shown to exhibit toxic responses different from those present in
mammals (Fort et al., 1988, 1991, 1998).
An additional recent relevant development is that Public Law 103-43 directed
the National Institute of Environmental Health Sciences (NIEHS, NIH) to develop
and validate alternative methods that (1) reduce or eliminate use of animals in
acute or chronic toxicity testing, (2) establish criteria for the validation and
regulatory acceptance of alternative testing methods, and (3) recommend a process