Evaluation of potential sodium-iodide symporter (NIS) inhibitors using a secondary Fischer rat thyroid follicular cell (FRTL-5) radioactive iodide uptake (RAIU) assay
The Fischer rat thyroid follicular cell line, FRTL-5, endogenously expresses the sodium iodide symporter (NIS) and has been used to identify environmental chemicals that perturb thyroid hormone homeostasis by disruption of NIS-mediated uptake of iodide. Previously, a high-throughput radioactive iodide uptake (RAIU) screening assay using the novel hNIS-HEK293T-EPA cell line was used to identify and rank potential human NIS (hNIS) inhibitors in ToxCast chemical libraries. In this study, the FRTL-5 cell line was evaluated as a secondary assay utilizing a RAIU assay coupled with cell viability assays to compare and further prioritize potential NIS inhibitors. After validation, results from assay controls, as well as reference and test chemicals, demonstrated robust, highly reproducible assays. Top-ranked chemicals from ToxCast Phase 1 (ph1_v2) and Phase II screening were evaluated in both hNIS and FRTL-5 RAIU assays using newly sourced chemicals to strengthen the testing paradigm, while also allowing a species comparison. Nineteen of 29 test chemicals showed less than <1 order of magnitude difference in IC50 values between the two assays. Several chemicals including etoxazole, methoxyfenozide, oxyfluorfen, triclocarban, perfluorohexane sulfonate, mepanipyrim, and niclosamide exhibited strong NIS inhibition with minimal cytotoxicity in both assays, and are proposed for additional testing using short-term in vivo assays to characterize effects on thyroid hormone synthesis.