Phase III interlaboratory study of FETAX, part 2: Interlaboratory validation of an exogenous metabolic activation system for Frog Embryo Teratogenesis Assay - Xenopus (FETAX)

Interlaboratory validation of an exogenous metabolic activation system (MAS) developed for the alternative, short-term developmental toxicity bioassay, Frog Embryo Teratogenesis Assay - Xenopus (FETAX) was performed with cyclophosphamide and caffeine. Seven study groups within six separate laboratories participated in the study in which three definitive concentration-response experiments were performed with and without the MAS in a side-by-side format for each chemical. Since both chemicals had been previously tested in FETAX, the test concentration were provided to each laboratory prior to testing. Interlaboratory coefficient of variation (CV) values for unactivated cyclophosphamide (no MAS) were 15%, 15%, 29%, and 25% for the 96-hr LC50, 96-hr EC50 (malformation), Minimum Concentration to Inhibit Growth (MCIG), and Teratogenic Index (TI) values respectively. Addition of the MAS increased the CV values of each endpoint at least 3.9-fold. Interlaboratory CV values for inactivated caffeine were 31%, 18%, 31%, and 46% for the 96-hr LC50, 96-hr EC50 (malformation), MCIG, and TI values respectively, Addition of the MAS decreased the CV values of each respective endpoint by at least 1.6-fold. Results indicated that bioactivated toxicants may be prone to greater variability in response amongst laboratories than compounds; which are detoxified. Even though more variability was noted with activated cyclophosphamide, results were within interlaboratory variation expected for other aquatic-based bioassays. Thus, results from these studies warrant the continued use and further refinement of FETAX for alternative developmental toxicity assessment.