Thiazopyr and thyroid disruption: case study within the context of the 2006 IPCS Human Relevance Framework for analysis of a cancer mode of action.
Thiazopyr increases the incidence of male rat thyroid follicular-cell tumors; however, it is not carcinogenic in mice. Thiazopyr is not genotoxic. Thiazopyr exerts its carcinogenic effect on the rat thyroid gland secondary to enhanced metabolism of thyroxin leading to hormone imbalance. The relevance of these rat tumors to human health was assessed by using the 2006 IPCS Human Relevance Framework. The postulated rodent tumor mode of action was tested against the Bradford Hill criteria and was found to satisfy the conditions of dose and temporal concordance, biological plausibility, coherence, strength, consistency, and specificity that fits with a well-established mode of action for thyroid follicular-cell tumors. Although the postulated mode of action could theoretically operate in humans, marked quantitative differences in the inherent susceptibility for neoplasia to thyroid hormone imbalance in rats allows for the conclusion that thiazopyr does not pose a carcinogenic hazard to humans.
Dellarco
Vicki L
VL
Office of Pesticide Programs, U.S. Environmental Protection Agency, Washington, DC, USA. dellarco.vicki@epa.gov
McGregor
Douglas
D
Berry
Sir Colin
SC
Cohen
Samuel M
SM
Boobis
Alan R
AR
D000818:Animals; D015197:Carcinogenicity Tests; D002273:Carcinogens / Q000633:toxicity*; D004195:Disease Models, Animal; D006801:Humans; D008297:Male; D051379:Mice; D009525:Niacin / Q000031:analogs & derivatives* / Q000633:toxicity; D051381:Rats; D012107:Research Design; D018570:Risk Assessment; D013045:Species Specificity; D013844:Thiazoles / Q000633:toxicity*; D013964:Thyroid Neoplasms / Q000139:chemically induced* / Q000378:metabolism
D002273:Carcinogens; D013844:Thiazoles; D009525:Niacin; C091707:thiazopyr