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Environmental Factor, August 2015

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NTP panel agrees flame retardant mixture exhibits carcinogenic activity

By Ernie Hood

Headshot of June Dunnick

June Dunnick, Ph.D., was the NTP study scientist for the DE-71 project and presented the study design and outcomes to the peer review panel. (Photo courtesy of Steve McCaw)

Kenneth Portier

Kenneth Portier, Ph.D., chaired the peer review meeting, managing the remote participation by his fellow panelists and the in-person contributions from NTP staff. Portier is managing director of the Statistics and Evaluation Center at the American Cancer Society in Atlanta. (Photo courtesy of Steve McCaw)

Blyston, Brix, and Malarkey

On hand from NTP to provide technical expertise were, from left, toxicologist Chad Blystone, Ph.D., study pathologist Amy Brix, D.V.M., Ph.D., and pathologist David Malarkey, D.V.M., Ph.D. (Photo courtesy of Steve McCaw)

The National Toxicology Program (NTP) convened a panel to peer review a draft NTP technical report on toxicology and carcinogenesis studies of a pentabromodiphenyl ether (PBDE) mixture, also known as technical grade DE-71.

The peer review panel voted to accept the draft NTP conclusions that described clear evidence of carcinogenic activity in male and female rats and mice, based on study data that documented increased incidences of various liver tumors. The panel met in a June 25 virtual meeting, with the experts participating via videoconference.

PBDE persists in the environment

PBDEs were nominated for NTP study by the California Office of Environmental Health Hazard Assessment. PBDEs are known to have widespread human exposure through household dust and foods, with children often experiencing higher exposures. The Centers for Disease Control and Prevention has found that most people retain PBDEs in body tissues. They also persist in the environment, so they are considered bioaccumulative, persistent organic pollutants.

A mixture of six PBDE compounds, known as DE-71, was used in the past as an additive flame retardant, often in furniture. Because of the limited availability of individual PBDE compounds, NTP chose the DE-71 mixture for a study in rats and mice, to characterize the toxic and carcinogenic potential of PBDEs.

The 3-month toxicology studies were conducted in F344/N rats and B6C3F1/N mice. Wistar Han rats were used in the 2-year toxicology and carcinogenicity studies, reflecting a change in selected rat strain by NTP at that time.

According to the report, although production of PBDEs was voluntarily phased out in the U.S. in 2004, and the European Union also banned their sale and use, the chemicals remain in the environment from older products still in use and in products that are being discarded. PBDEs can be detected in water, wildlife, and humans, as well as in various food products, such as meat, poultry, and fish.

Clear evidence

The NTP technical report described the methods, results, and NTP conclusions on the levels of evidence for carcinogenic activity under the specific conditions of the study. Peer reviewers were asked to review and evaluate the scientific and technical elements of the study and its presentation in the draft report, and to determine whether the study’s experimental design, conduct, and findings support the NTP’s conclusions regarding the carcinogenicity and toxicity of the tested substance.

In the series of 3-month and 2-year studies, DE-71 was administered to male and female rats and mice. In the 2-year studies, exposure began during pregnancy and continued through birth into adulthood, so that the animals were exposed throughout their lifetime. NTP found that the mixture was a multispecies carcinogen. Liver tumors were the primary outcome.

The draft NTP technical report also described a variety of nonneoplastic lesions, which are noncancerous or precancerous growths that develop due to abnormal cell or tissue growth. Such lesions were seen in both species and sexes of the test animals, particularly in the liver and the thyroid gland.

(Ernie Hood is a contract writer for the NIEHS Office of Communications and Public Liaison)


peer reviewers participating by videoconference

With the peer reviewers participating by videoconference, the central table was filled with NTP personnel. The virtual meeting ran smoothly and could serve as a model for future such events. (Photo courtesy of Steve McCaw)


Paul Foster

Toxicologist Paul Foster, Ph.D., briefed the panel on the perinatal bioassays and the NTP statistical approach, which used mixed-effects models, analyzing data from each pup while accounting for within-litter correlations. (Photo courtesy of Steve McCaw)




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