Development and Validation of in Vitro Induction Assays for Toxic Halogenated Aromatic Mixtures: A Review

Halogenated aromatic industrial compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs) have been identified as residues in almost every component of the global ecosystem. Risk assessment of the complex mixtures of halogenated aromatics found in envi ronmental samples is complicated by analytical problems and the lack of toxicological information on individual compounds and mixtures. Research in our laboratory has focused on the develop ment and vadidation of the in vitro aryl hydrocarbon hydroxylase (AHH) induction assay in rat hepatoma H-4-II E cells in culture for quantitating individual toxic halogenated aryl hydrocarbons and their mixtures. For several PCB, PCDD, PCDF congeners, their mixed bromo/chloro analogs and reconstituted mixtures there was an excellent linear correlation between their -log ED₅₀ values for AHH induction in rat hepatoma cells and their -log ED₅₀ values for in vivo hepatic microsomal AHH induction, inhibi tion of body weight gain and thymic atrophy in the rat. It has also been shown for selected compounds that there was a good correla tion between their in vitro AHH induction potencies and their effects in guinea pigs (AHH induction, inhibition of body weight gain) and mice (immunotoxicity). This assay system has been uti lized to quantitate the “2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents” present in extracts from diverse sources including fly ash from a municipal incinerator and pyrolyzed brom inated flame retardants which contain a complex mixture of halo genated dibenzo-p-dioxins and dibenzofurans.