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Comparison of in vitro and ex vivo thyroid hormone synthesis inhibition results and in vivo outcomes for a series of benzothiazoles
Citation:
Hornung, M., J. Haselman, Joe Korte, Pat Kosian, K. Challis, S. Hall, AND S. Degitz. Comparison of in vitro and ex vivo thyroid hormone synthesis inhibition results and in vivo outcomes for a series of benzothiazoles. Presented at Society of Toxicology, Phoenix, AZ, March 24 - 27, 2014.
Impact/Purpose:
This abstract presents the results of a study that addresses how assays conducted along different levels along an Adverse Outcome Pathway can provide information on extrapolating in vitro assay data to in vivo outcomes. It further supports efforts to develop in vitro assays for thyroid hormone disruption that are essential to populating a battery of in vitro assays that may be used to support efforts in the EDSP21 program to prioritize chemicals with potential to disrupt thyroid hormone pathways.
Description:
Assessing how in vitro data may be used to predict adverse effects in vivo is critical as efforts are advanced to incorporate in vitro assays into a risk assessment framework. Within the context of a thyroid hormone (TH) synthesis inhibition adverse outcome pathway (AOP), in vitro, ex vivo and in vivo assays were used to study the TH disrupting potential for a series of benzothiazoles: benzothiazole (BTZ), 2-mercaptobenzothiazole (MBT), 5-chloro-2-mercaptobenzothiazole (CMBT), 2-aminobenzothiazole (ABT), 2-hydroxybenzothiazole (HBT), and 2-(methylthio)benzothiazole (MTBT). A thyroid peroxidase (TPO) inhibition assay was used to determine the activity of these chemicals in vitro. The rank order potency for TPO inhibition was MBT=CMBT>ABT>BTZ>HBT. MTBT did not inhibit TPO activity. The benzothiazoles were tested further in Xenopus laevis thyroid gland explant culture with inhibition of TH release as the endpoint. Toxicity was assessed as decreased glandular ATP. MBT inhibited TH release at non-cytotoxic concentrations and with similar potency to methimazole. The benzothiazoles with greatest potency for T4 release inhibition were MBT, CMBT, and HBT, with IC50s of 3, 30, and 133 µM, respectively, but all benzothiazoles showed some inhibitory activity. Benzothiazoles were further assessed in vivo in a 7-d X. laevis tadpole assay. MBT and CMBT were the most potent for affecting endpoints of thyroid hormone synthesis inhibition, whereas others showed little or no effect. Both MBT and CMBT significantly increased sodium iodide symporter (NIS) mRNA indicative of compensatory TSH stimulation in response to decreased circulating TH. Taken as a whole, these results indicate the utility of in vitro assays for queuing chemicals for further testing, but illustrate the need for caution in interpreting results of in vitro or ex vivo inhibition assays especially where toxicity may be a confounding factor.