Phase III interlaboratory study of FETAX - Part 3. FETAX validation using 12 compounds with and without an exogenous metabolic activation system

FETAX (Frog Embryo Teratogenesis Assay-Xenopus) is a 96-h whole-embryo deve lopmental toxicity screening assay that can be used in ecotoxicology and in detecting mammalian developmental toxicants when an in vitro metabolic act ivation system is employed. A standardized American Society for Testing and Materials (ASTM) guide for the conduct of FETAX has been published, along with a companion atlas that helps in embryo staging and in identifying malf ormations, As part of the ASTM process, an interlaboratory validation study was undertaken to evaluate the repeatability and reliability of FETAX and to evaluate the potential teratogenic hazard of 12 compounds. Three differe nt laboratories participated in the study. All three participating laborato ries had extensive experience with the assay. FETAX intralaboratory and int erlaboratory variability, as judged by coefficients of variation, were very low. Potential teratogenic hazard was evaluated using two major criteria f rom FETAX experiments employing metabolic activation systems (MAS). These w ere the teratogenic index TI (TI = 96-h LC50/96-h EC50 (malformation)) and the minimum concentration that inhibits growth (MCIG), A compound was consi dered teratogenic by this criterion when the MCIG was significantly differe nt from controls at concentrations below the 30% level of the MAS 96-h LC50 . Based on the results of this and other studies, a decision table was cons tructed in order to evaluate additional studies. Severity of malformations caused, especially near the MAS 96-h EC50 (malformation), were also evaluat ed. Four compounds were non-teratogenic but two compounds were clearly tera togenic. The remaining six compounds were ranked as equivocal teratogens, T he results were discussed in light of the difficulty of producing an adequa te decision table. FETAX proved to yield repeatable and reliable data as lo ng as care was taken during range-finding and technicians were adequately t rained. The MAS was essential in using FETAX to predict developmental hazar d in mammals, and still requires further development. Copyright (C) 1999 Jo hn Wiley & Sons, Ltd.