PBPK modeling of the metabolic interactions of carbon tetrachloride and tetrachloroethylene in B6C3F1 mice

Abstract

Potential exists for widespread human exposure to low levels of carbon tetrachloride (CT) and tetrachloroethylene (TET). These halocarbons are metabolized by the cytochrome P450 system. CT is known to inhibit its own metabolism (suicide inhibition) and to cause liver injury by generation of metabolically derived free radicals. The objective of this research was to use develop a physiologically based pharmacokinetic (PBPK) model to forcast the metabolic interactions between orally administered CT and TET in male B6C3F1 mice. Trichloroacetic acid (TCA), a stable metabolite of TET, was used as a biomarker to assess inhibition of the cytochrome P450 system by CT. Metabolic constants utilized for CT were 1.0 mg/kg/h for Vmaxc_CT and 0.3 for Km_CT (mg/l). Values for TET (based in TCA production), were 6.0 mg/kg/h for Vmaxc_TET was 3.0 mg/l for Km_TET. The rate of loss of metabolic capacity for CT (suicide inhibition) was describe as: Vmaxloss ( mg/ h)=− Kd ( RAM× RAM) , where Kd (h/kg) is a second-order rate constant, and RAM (mg/h) is the Michaelis–Menten description of the rate of metabolism of CT. For model simplicity, CT was assumed to damage the primary enzymes responsible for metabolism of CT (CYP2E1) and TET (CYP2B2) in an equal fashion. Thus, the calculated fractional loss of TET metabolic capacity was assumed to be equivalent to the calculated loss in metabolic capacity of CT. Use of a Kd value of 400 h/kg successfully described serum TCA levels in mice dosed orally with 5–100 mg/kg of CT. We report, for the first time, suicide inhibition at a very low dose of CT (1 mg/kg). The PBPK model under-predicted the degree of metabolic inhibition in mice administered 1 mg/kg of CT. This PBPK model is one of only a few physiological models available to predict the metabolic interactions of chemical mixtures involving suicide inhibition. The success of this PBPK model demonstrates that PBPK models are useful tools for examining the nature of metabolic interactions of chemical mixtures, including suicide inhibition. Further research is required to compare the inhibitory effects of inhaled CT vapors with CT administered by oral bolus dosing and determine the interaction threshold for CT-induced metabolic inhibition.