ADME Studies of all Three Hexabromocyclododecane (HBCD) Diastereomers in Rats

Authors: Hakk, Heldur; HUWE, JANICE - Retired ARS Employee

Submitted to: International Workshop on Brominated Flame Retardants
Publication Type: Abstract Only
Publication Acceptance Date: 5/14/2012
Publication Date: 6/3/2012
Citation: Hakk, H., Huwe, J.K. 2012. ADME Studies of all Three Hexabromocyclododecane (HBCD) Diastereomers in Rats. International Workshop on Brominated Flame Retardants. 13th Workshop on Brominated and Other Flame Retardants, June 4-6, 2012, Winnipeg, MB, Canada.

Interpretive Summary: Hexabromocyclododecane (HBCD) is a persistent flame retardant manufactured as a mixture of alpha-, beta-, and gamma-diastereomers, although gamma-HBCD comprises nearly 89% of the total. HBCD is a candidate to be included on the international list of persistent organic pollutants (POPs), which would have important implications to trade of American food products. An intriguing shift from the gamma-HBCD to alpha-HBCD occurs with increasing trophic level in the food web. Why this occurs could be due to a number of reasons rooted in the biology of the monitored organism, or from some environmental mechanism. Elucidation of the pathway of HBCD diastereomer pattern changes will be important to agriculture from a prevention or remediation standpoint, and will hinge on data produced from individual diastereomers. The goal of this research was to determine the fate of individual HBCD diastereomers in rats as a model mammal. More than half of the dose from each diastereomer was excreted in 4 days in the rank order beta > gamma > alpha. Low residue levels of each diastereomer were mainly detected in lipophilic tissues. The metabolism of each HBCD diastereomer was elucidated. The conclusion was that persistence of HBCD stereoisomers was generally low in mammals, but was isomer specific. The data support that contention that alpha-HBCD is the most persistent HBCD diastereomer, but in vivo events alone may be inadequate to explain the significant shift in diastereomeric profiles between commercial HBCD and tissue patterns. Other environmental events, including but not limited to, bacterial degradation, photolysis, interaction with various matrices, and environmental exposure will need to be elucidated that explain the observed differences. These data suggest that mammals can metabolize and eliminate the majority of HBCD consumed, but that the toxicity of, and reason for, increasing low residual levels needs to be elucidated, and acceptable tissue levels established for trade purposes.

Technical Abstract: Introduction While laboratory studies with hexabromocyclododecane (HBCD) seem to indicate that it can degraded in soil, water and sediment in a relatively short time, environmental monitoring data show that HBCD is a global, pollutant that bioaccumulates in the food chain, and has been shown to cause a wide variety of adverse effects, particularly to fish and invertebrates. Therefore, within the scope of Stockholm Convention protocols, HBCD is being considered as a persistent organic pollutant (POP). However, limited data is available on the individual diastereomers, which make up the commercial mixture, on which to make these POP criteria decisions. Therefore, the goal of the present study was to deliver each of the diastereomers present in commercial HBCD, i.e. alpha-, beta-, and gamma-, and to determine the absorption, disposition, metabolism and excretion (ADME) in rats. Results will be used to distinguish between possible routes of diastereomer profile changes observed between the commercial mixture and biotic samples. Materials and Methods Three groups male Sprague-Dawley rats (n=3; 309 ± 12 g) were administered a single, oral dose of [14C]alpha-, beta-, and gamma-HBCD (3 mg/kg in 1.0 mL corn oil). Feces and urine was collected for 4 days, at which time selected tissues were removed following exsanguination. The tissues and excreta were assayed for radioactivity by liquid scintillation counting (LSC) or tissue combustion, and then extracted to characterize metabolites by negative ion LC/MS. Enantioselectivity in tissue dispositions were evaluated by chiral (-)LC/MS with deuterated standards. Association of urinary [14C] with carrier proteins was determined by gel filtration chromatography, LSC and protein determinations by the Bradford method. Results Total urinary elimination ranged from 13.2% for alpha-HBCD, to 30.0% for beta-HBCD. Total fecal elimination was also high, i.e. 42.4% for alpha-HBCD, 53.0% for gamma-HBCD, and 59.4% for beta-HBCD. All of the urine consisted of polar metabolites, while >76% of fecal [14C] consisted of metabolites. Disposition of HBCD was lipophilic tissues (fat, GI tract, skin and liver), although levels were low (generally <1% of administered dose, except for alpha-HBCD where ~2% of dose was observed for fat and GI tract). Absorption of HBCD diastereomers was estimated to be 85.7, 87.5%, and 90.9% for gamma-, beta-, and alpha-HBCD, respectively. Free metabolites identified in feces by (-)LC/MS were mono-hydroxylated or debromohydroxylated, and no stereoisomerization was suggested. Non-extractables comprised 34-49% of fecal [14C], and were also present in liver. Conclusions Data from the present study support the conclusion that alpha-HBCD is the most persistent and resistant HBCD diastereomer to biodegradation, which may partially explain its dominance in the environmental monitoring of biological tissues, and justify its consideration as a POP. However, the metabolic differences observed in this study may be inadequate to explain the dominance of alpha-HBCD in biota. Therefore, an alternative mechanism to in vivo selective absorption, differential metabolism and stereoisomerization is suggested that would be responsible for the diastereomeric profile shift from gamma- to alpha-HBCD observed across the globe.