Detection of thyroid hormone transport inhibition in the Xenopus Eleutheroembryonic Thyroid Assay (XETA)

Abstract

Abstract The Xenopus Eleutheroembryonic Thyroid Assay (XETA, OECD TG 248 was established as a non-animal alternative to the Amphibian Metamorphosis Assay (AMA, OECD TG 231) for the analysis of (anti-)thyroid activity of chemicals. Since the test employs animals which are physiologically not fully functional yet, there are limitations in the XETA to detect certain mechanisms along the HPT (Hypothalamic-Pituitary-Thyroid) axis. However, the inhibition of thyroid hormone transport should physiologically be detectable with the XETA but was not yet investigated thoroughly. Here, we tested three substances that are known to inhibit thyroid hormone transport by competitive binding to transthyretin in mammalian studies, namely pentachlorophenol (PCP, tested at 0.01, 0.1 and 1.0 mg/L), tetrabromo bisphenol A (TBBPA) and mefenamic acid (both tested at 0.25, 0.5, 1.0, 2.0 and 4.0 mg/L). X. laevis eleutheroembryos of Nieuwkoop-Faber stage 45 were exposed in 6-well plates (10 embryos in 8 mL test medium per well) for 72 hours at 21°C. Results show that especially for highly potent substances such as PCP and TBBPA, the XETA effectively detects their thyroid hormone transport inhibition potential, while for the lower potency substance mefenamic acid, a trend was visible, but a significant inhibition was not detected. Overall, the results indicate that in the XETA, the mechanism of thyroid hormone transport inhibition can be detected.