Application of physiologically-based pharmacokinetic modeling to investigate the toxicological interaction between chlorpyrifos and parathion in the rat

Environmental exposure is usually due to the presence of multiple chemicals. In most cases, these chemicals interact with each other at both pharmacokinetic and pharmacodynamic toxicity mechanisms. In the absence of data, joint toxicity assessment of a mixture is based on default dose or response additivity. Although, the concept of additivity is mostly accepted at low dose levels, these levels need to be determined quantitatively to validate the use of additivity as an absence of any possible synergistic or antagonistic interactions at low environmental exposure levels. The doses at which interaction becomes significant define the interaction threshold. In most cases, estimation of these low-dose interaction thresholds experimentally is economically costly and challenging because of the need to use a large number of laboratory animals. Computational toxicology methods provide a feasible alternative to establish interaction thresholds. For example, a physiologically based pharmacokinetic (PBPK) model was developed to estimate an interaction threshold for the joint toxicity between chlorpyrifos and parathion in the rat. Initially, PBPK models were developed for each chemical to estimate the blood concentrations of their respective metabolite. The metabolite concentrations in blood out-put was then linked to acetylcholinesterase kinetics submodel. The resulting overall PBPK model described interactions between these pesticides at two levels in the organism: (a) the P450 enzymatic bioactivation site, and (b) acetylcholinesterase binding sites. Using the overall model, a response surface was constructed at various dose levels of each chemical to investigate the mechanism of interaction and to calculate interaction threshold doses. The overall model simulations indicated that additivity is obtained at oral dose levels below 0.08mg/kg of each chemical. At higher doses, antagonism by enzymatic competitive inhibition is the mode of interaction.