Differences in Tissue Distribution of HBCD Alpha and Gamma between Adult and Developing Mice

David T. Szabo 0 2 3 Janet J. Diliberto 0 2 Janice K. Huwe 1 2 Linda S. Birnbaum 2 4 0 United States Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, Integrated Systems Toxicology Division , Research Triangle Park, North Carolina 27711 1 United States Department of Agriculture, Agriculture Research Service, Biosciences Research Laboratory , Fargo, North Dakota 58102-2765 2 Research and Development , 2733 S. Crystal Drive, 8623-P, Arlington, VA 22202 3 University of North Carolina-Chapel Hill Curriculum in Toxicology, United States Environmental Protection Agency , Research Triangle Park, North Carolina 27711 4 National Cancer Institute/National Institutes of Health , Research Triangle Park, North Carolina 27709 Hexabromocyclododecane (HBCD) is a mixture of three stereoisomers alpha (a), beta (b), and gamma (g). g-HBCD dominates the mixture (~70%), and despite a-HBCD's minor contribution to global HBCD production and usage (~10%), it is the dominant congener found in most biotic samples worldwide. Evidence of toxicity and lack of stereoisomer studies drives the importance of understanding HBCD toxicokinetics in potentially susceptible populations. The majority of public health concern has focused on hazardous effects resulting from exposure of infants and young children to HBCD due to reports on adverse developmental effects in rodent studies, in combination with human exposure estimates suggesting that nursing infants and young children have the highest exposure to HBCD. This study was designed to investigate differences in the disposition of both g-HBCD and a-HBCD in infantile mice reported to be susceptible to the HBCD commercial mixture. The tissue distribution of a-[14C]HBCD- and g-[14C]HBCD-derived radioactivity was monitored in C57BL/6 mice following a single oral dose of either compound (3 mg/kg) after direct gavage at postnatal day 10. Mice were held up to 7 days in shoebox cages after which pups were sacrificed, tissue collected, and internal dosimetry was measured. Developing mice exposed to a-HBCD had an overall higher body burden than g-HBCD at every time point measured; at 4 days postexposure, they retained 22% of the a-HBCD administered dose, whereas pups exposed to g-HBCD retained 10%. Total body burden in infantile mice after exposure to g-HBCD was increased 10-fold as compared with adults. Similarly, after exposure to a-HBCD, infantile mice contained 2.5-fold higher levels than adult. These differences lead to higher concentrations of the HBCD diastereomers at target tissues during critical windows of development. The results indicate that the toxicokinetics of the two HBCD diastereomers differ between developing and adult Disclaimer: The use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the USDA or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable. - Brominated flame retardants (BFRs) are chemicals incorporated into plastics, electrical and electronic products, textiles, and other materials to reduce flammability. Currently, tetrabromobisphenol A, polybrominated diphenyl ethers (PBDEs), and hexabromocyclododecane (HBCD) account for the largest volume of BFRs. HBCD is a lipophilic additive flame retardant mixture extensively used since the 1970s, primarily added to polystyrene insulations and building materials to reduce combustibility (de Wit, 2002). Annual demand for HBCD 10 years ago was about 17,000 metric tons (Covaci et al., 2006), and approximately half of this quantity was consumed in Europe (Hale et al., 2006). HBCD is considered a ubiquitous and global environmental contaminant and undergoes longrange transport with detectable levels found in abiotic and biotic samples including human blood and breast milk (de Wit, 2002; Law et al., 2005, 2008). Toxicity studies suggest that the HBCD commercial mixture is an enzyme inducer, endocrine disruptor, and developmental neurotoxicant. The commercial mixture has been shown to induce both phase I and II metabolic enzyme systems, specifically CYP2B and CYP3A, by interacting with the constitutive androgen receptor (CAR) and/or the pregnane-X-receptor (PXR), respectively (Germer et al., 2006). van der Ven et al. (2006) reports The Author 2011. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For permissions, please email: a decrease in circulating total thyroxin (T4) levels, increased pituitary weight, thyroid weight, and immunostaining of thyroid-stimulating hormone in the pituitary and thyroid follicular cell activation in adult female rats exposed to the commercial mixture of HBCD. Hypothyroid conditions especially during the period of brain growth can cause abnormal brain development with severe physical and/or mental retardation in the offspring (Dobbing and Sands, 1979; Koibuchi and Chin, 2000; Yen, 2001) and can cause decreases in intelligence quotient (Haddow et al., 1999). Exposure to neonatal mice at postnatal day 10 (PND 10) with the commercial mixture of HBCD has been shown to cause impairment in learning, memory, and aberrant spontaneous behavior (Eriksson et al., 2006). HBCD has been shown to directly inhibit the uptake of neurotransmitters, dopamine, and glutamate into synaptosomes in the rodent brain (Mariussen and Fonnum, 2003). Therefore, neurodevelopmental effects may be due to either a direct or indirect result of HBCD during periods of growth. The commercial mixture is composed of three diastereoisomers, denoted as alpha (a), beta (b), and gamma (c) with the c-diastereoisomer predominating (> 70%) (Heeb et al., 2005). High concentrations of HBCD in some top predators indicate persistence and biomagnification. However, recent studies showed that there is a selective predominance of a-HBCD in biota (Law et al., 2005). Due to the different physical, chemical, and biological properties of the diastereomers, there is a growing need to characterize the individual diastereomers in the commercial mixtures. To better understand the biological behavior of a chemical requires examinination of its toxicokinetic properties. Toxicokinetic information on HBCD is limited. Unfortunately, early toxicokinetic and toxicity studies contain study design flaws in which animals were administered HBCD suspensions in oil (Marcia Hardy, personal communication; Chengelis, 2001; Yu and Atallah, 1980). Undissolved particles of HBCD in oil, in addition to the adsorption to laboratory glass equipment, may result in decreased bioavailability and decreased internal absorption. These factors create further uncertainties and inconsistencies when comparing dose and effects across studies. To address the growing need for a toxicokinetic evaluation of HBCD at the stereoisomer level, our laborat (...truncated)