Structure-activity relationships for halobenzene induced cytotoxicity in rat and human hepatocytes

Halobenzenes are ubiquitous environmental contaminants, which are hepatotoxic in both rodents and humans. The molecular mechanism of halobenzene hepatotoxicity was investigated using Quantitative structure-activity relationships (QSAR) and accelerated cytotoxicity mechanism screening (ACMS) techniques in rat and human hepatocytes. The usefulness of isolated hepatocytes for prediciting in vivo xenobiotic toxicity was reassessed by correlating the LC(50) of 12 halobenzene congeners in phenobarbital (PB) induced rat hepatocytes in vitro determined by ACMS to the hepatotoxicities reported in vivo in PB-induced male Sprague-Dawely (SD) rats. A high correlation (r(2)=0.90) confirmed the application of hepatocytes as a "gold standard" for toxicity testing in vitro. QSARs were derived to determine the physico-chemcial variables that govern halobenzene toxicity in PB-induced rat, normal rat and human hepatocytes. We found that toxicity in normal rat and normal human hepatocytes both strongly correlate with hydrophobicity (logP), ease of oxidation (E(HOMO), energy of the highest molecular orbital) and on the asymmetric charge distribution according to arrangement of halogen substituents (dipole moment, mu). This suggests that halobenzene interaction with cytochrome P450 for oxidation is the metabolic activating path for toxicity and is similar in both species. In PB-induced rat hepatocytes the QSAR derivation is changed, where halobenzene toxicity strongly correlates to logP and dipole moment, but not E(HOMO). The changed QSAR suggests that oxidation is no longer the rate-limiting step in the cytotoxic mechanism when CYP2B/3A levels are increased, confirming CYP450 oxidation as the metabolic activating step under normal conditions.