A physiologically based description of ethylene oxide dosimetry in the rat

A physiologically based pharmacokinetic (PB-PK) model providing a quantitative description of ethylene oxide (ETO) dosimetry in the rat was developed by integrating information on physiology, tissue solubility of ETO, and rate constants for ETO metabolism and binding. The PB-PK model consisted of nine compartments; liver, lung, testis, brain, fat, venous blood, arterial blood, richly perfused and poorly perfused tissues. The tissue: air partition coefficients of ETO, determined by vial equilibration, were similar among the various tissues (range 44-83). The rate constants for glutathione (GSH) conjugation, hydrolysis, and hemoglobin (Hb)- and DNA-binding were estimated from published data and by conducting in vivo inhalation exposure studies. The model adequately predicted the concentrations of Hb and DNA adducts, hepatic and extrahepatic GSH, and urinary N-acetyl-S-(2-hydroxyethyl)-cysteine following inhalation exposures of 1.2 to 1,200 ppm and intravenous administration of 1 to 100 mg/kg of ETO in male Fischer-344 and Sprague-Dawley rats. There was no evidence of nonlinearity in the overall elimination of ETO in the dose range examined. However, nonlinearities in the components of this first order elimination process (namely GSH conjugation, hydrolysis, exhalation) were found to occur at high exposure concentrations. Characterization of the individual metabolic pathways that affect the tissue dosimetry of ETO is important for interspecies extrapolation and risk assessment for this chemical.