In Vitro Screening for ToxCast Chemicals Binding to Thyroxine-Binding Globulin

Thyroid hormone (TH) carrier proteins play an important role in distributing TH to target tissue as well as maintaining the balance of free versus bound TH in the blood. Interference with the TH carrier proteins has been identified as a potential mechanism of thyroid system disruption. To address the lack of data regarding chemicals binding to these carrier proteins and displacing TH, a fluorescence-based in vitro screening assay was utilized to screen over 1,400 chemicals from the U.S. EPA's ToxCast phase1_v2, phase 2, and e1k libraries for competitive binding to one of the carrier proteins, thyroxine-binding globulin. Initial screening at a single high concentration of 100 μM identified 714 chemicals that decreased signal of the bound fluorescent ligand by 20% or higher. Of these, 297 produced 50% or greater reduction in fluorescence and were further tested in concentration-response (0.004 to 150 μM) to determine relative potency. Ten chemicals were found to have EC50 values <1 μM, 63 < 10 μM, and 141 chemicals between 10 and 100 μM. Utilization of this assay contributes to expanding the number of in vitro assays available for identifying chemicals with the potential to disrupt TH homeostasis. These results support ranking and prioritization of chemicals to be tested in vivo to aid in the development of a framework for predicting in vivo effects from in vitro high-throughput data.