title: In Vitro, Ex Vivo, and In Vivo Determination of Thyroid Hormone Modulating Activity of Benzothiazoles.
abstract: As in vitro assays are increasingly used to screen chemicals for their potential to produce endocrine disrupting adverse effects, it is important to understand their predictive capacity. The potential for a set of 6 benzothiazoles to affect endpoints related to thyroid hormone synthesis inhibition were assessed using in vitro, ex vivo, and in vivo assays. Inhibition of thyroid peroxidase (TPO) derived from pig thyroid glands was determined for benzothiazole (BTZ), 2-mercaptobenzothiazole (MBT), 5-chloro-2-mercaptobenzothiazole (CMBT), 2-aminobenzothiazole (ABT), 2-hydroxybenzothiazole (HBT), and 2-methylthiobenzothiazole (MTBT). Their rank order potency for TPO inhibition was MBT=CMBT>ABT>BTZ, whereas HBT and MTBT exhibited no inhibitory activity. The benzothiazoles were tested further in a Xenopus laevis thyroid gland explant culture assay in which inhibition of thyroxine (T4) release was the measured endpoint. In this assay all 6 benzothiazoles inhibited T4 release. The activity of the benzothiazoles for disrupting thyroid hormone activity was verified in vivo using X. laevis tadpoles in a 7-day assay. The 2 most potent chemicals for TPO inhibition, MBT and CMBT, produced responses in vivo indicative of T4 synthesis inhibition including induction of sodium iodide symporter mRNA and decreases in glandular and circulating thyroid hormones. The capability to measure thyroid hormone levels in the glands and blood by ultrahigh performance LC-MS/MS methods optimized for small tissue samples was critical for effects interpretation. These results indicate that inhibition of TPO activity in vitro was a good indicator of a chemical's potential for thyroid hormone disruption in vivo and may be useful for prioritizing chemicals for further investigation.
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota hornung.michael@epa.gov.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Hornung
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Kosian
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Jonathan T
authors.initials: JT
authors.lastname: Haselman
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Joseph J
authors.initials: JJ
authors.lastname: Korte
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Katie
authors.initials: K
authors.lastname: Challis
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Chitralekha
authors.initials: C
authors.lastname: Macherla
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Erica
authors.initials: E
authors.lastname: Nevalainen
authors.affiliation: US Environmental Protection Agency, ORD, NHEERL, Mid-Continent Ecology Division, Duluth, Minnesota.
authors.forename: Sigmund J
authors.initials: SJ
authors.lastname: Degitz
mesh_terms: D000818:Animals; D052160:Benzothiazoles / Q000633:toxicity*; D001835:Body Weight / Q000187:drug effects; D052244:Endocrine Disruptors / Q000633:toxicity*; D066298:In Vitro Techniques; D007814:Larva / Q000187:drug effects* / Q000254:growth & development; D013552:Swine; D013961:Thyroid Gland / Q000187:drug effects* / Q000378:metabolism; D013963:Thyroid Hormones / Q000378:metabolism*; D014982:Xenopus laevis / Q000254:growth & development
chemical_list: D052160:Benzothiazoles; D052244:Endocrine Disruptors; D013963:Thyroid Hormones
keywords: UHPLC-MS/MS; Xenopus; endocrine disruption; in vitro-in vivo extrapolation; thyroid