Unrecognized causative factors for the lack of in vitro metabolism reported by McKinney et al.

McKinney et al. [1] report in vitro hepatic microsomal preparations from polar bear, beluga whale, ringed seal, and rat did notmetabolize decabromodiphenyl ethane (DBDPEthane), decabromodiphenyl ether (BDE-209), or the lower brominated polybrominated diphenyl ethers (PBDE) BDE-99, -100 and -154. The lack of metabolism of the three lower PBDEs is consistent with their potential for bioaccumulation in that metabolism is generally directed toward producing more water soluble and easily excreted metabolites. Substances that are readily metabolized to water soluble metabolites are rarely accumulated. These in vitro results on BDE-209 are also consistent with that of Gebbnick et al. [2] who found no evidence of metabolism by polar bear microsomes. Nevertheless, the low and variable recoveries of BDE-209 (81 9%) and DBDPEthane (49 23%) in controls and the ‘‘depletion’’ observed in the test groups, 14 to 25% for BDE-209 and 44 to 74% for DBDPEthane, in the absence of identified metabolites suggest that other factors may be responsible. Albemarle Corporation manufactures commercial products based on BDE-209 and DBDPEthane and has extensive experience with both substances. Both are highly insoluble in aqueous media and many organic solvents and are prone to nonspecific binding to surfaces and particulates, DBDPEthane even more so than BDE-209. Low solubility compounds are not properly assayed by many in vitro systems due to their precipitation and/or adherence to walls of the vessel, which eliminates interaction with microsomal enzymes [3]. We have observed poor solubility in in vitro genetic and bi-directional cell permeability assays with both compounds. In the cell permeability assays [4], DBDPEthane was not detectable in buffer solutions at 128 nMwhile BDE-209 was detectable at 128 nM but not at 312 nM. This is consistent with McKinney et al.’s [1] pattern of recovery (higher for BDE-209 than DBDPEthane) and with their test concentration of DBDPEthane (90 nM), which was similar to that in the cell permeability assay. (McKinney et al. erroneously report their assay concentrations to be ‘‘pmol.’’) Poor solubility, precipitation, and non-specific binding are also consistent with the inability to extract 20% of a BDE-209 dose from feces [5,6] and the low (60%) recovery reported by Huwe and Smith [7]. McKinney et al. touch on binding as a potential cause of their poor recoveries, but attribute binding to formation of reactive metabolites. Formation of reactive metabolites is inconsistent with multiple mammalian repeated dose toxicity studies with no observable effects levels (NOELs) and no observable adverse effect levels (NOAELs) of 1,000mg/kg/d for both BDE-209 and DBDPEthane [8–12]. Substances that generate significant amounts of reactive metabolites do not have this toxicology profile. Both BDE-209 and DBDPEthane are poorly absorbed after oral administration [8–11]. Based on this alone, metabolism would not play large role in their pharmacokinetic profile. Conclusions that BDE-209 undergoes significant metabolism [6] were based on extraction profiles and not structural identification [9]. Studies using radiolabelled BDE209 orDBDPEthane, mentioned by McKinney et al. as a future possibility, have already been performed [9] or are in progress by Albemarle Corporation. Recognition of these facts within the research community is important so that future research time and money is directed appropriately.