Stereoselective Metabolism of α-, β-, and γ-Hexabromocyclododecanes (HBCDs) by Human Liver Microsomes and CYP3A4

This is the first study investigating the in vitro metabolism of alpha-, beta-, and gamma-hexabromocyclododecane (HBCD) stereoisomers in humans and providing serniquantitative metabolism data. Human liver microsomes were incubated with individual racemic mixtures and with individual stereoisomers of alpha-, beta-, and gamma-HBCDs, the hydroxylated metabolites formed were analyzed by liquid chromatography tandem mass spectrometry, and the value of the intrinsic in vitro clearance (Cl-int,Cl-vitro) was calculated. Several mono- and dihydroxylated metabolites of alpha-, beta-, and gamma-HBCDs were formed, with mono-OH-HBCDs being the major metabolites. No stereoisomerization of any of the six alpha-, beta-, and gamma-HBCD isomers catalyzed by cytochrome P450 (CYP) enzymes occurred. The value of Cl-int,Cl-vitro of alpha-HBCDs was significantly lower than that of beta-HBCDs, which, in turn, was significantly lower than that of gamma-HBCDs (p < 0.05). Such differences were explained by the significantly lower values of Cl-int,Cl-vitro each alpha-HBCD stereoisomer than those of the /3- and y-HBCD stereoisomers. In addition, significantly lower values of Cl-int,Cl-vitro of the (-) over the (+)alpha- and beta-HBCD stereoisomers, but not gamma-HBCDs, were obtained. Our data offer a possible explanation of the enrichment of alpha-HBCDs over beta- and gamma-HBCDs on the one hand and, on the other hand, of (-)alpha-HBCDs over (+)alpha-HBCDs previously reported in human samples. It also offers information about the mechanism resulting in such enrichments, the stereoisomerselective metabolism of alpha-, beta-, and gamma-HBCDs catalyzed by CYPs with the lack of stereoisomerization.