Development of a source-to-outcome model for dietary exposures to insecticide residues: an example using chlorpyrifos

Probabilistic models of interindividual variation in exposure and response were linked to create a source-to-outcome population model. This model was used to investigate cholinesterase inhibition from dietary exposures to an insecticide (chlorpyrifos) in populations of adults and 3 year old children. A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model was used to calculate the variation in sensitivity occurring from interindividual variability in physiology, metabolism, and physical activity levels. A dietary intake model characterizes the variation in dietary insecticide exposures and variation in anthropometry in the populations. Published equations were used to describe the necessary physiology for each simulated individual based on the anthropometry from the dietary intake model. The model of the interindividual variation in response to chlorpyrifos was developed by performing a sensitivity analysis on the PBPK/PD model to determine the parameters that drive variation in pharmacodynamics outcomes (brain and red blood cell acetylcholinesterase inhibition). Distributions of interindividual variation were developed for parameters with the largest impact; the probabilistic model sampled from these distributions. The impact of age and interindividual variation on sensitivity at the doses that occur from dietary exposures, typically orders of magnitude lower than exposures assessed in toxicological studies, was assessed using the source-to-outcome model. The resulting simulations demonstrated that metabolic detoxification capacity was sufficient to prevent significant brain and red blood cell acetylcholinesterase inhibition, even in individuals with the lowest detoxification potential. Age-specific pharmacokinetic and pharmacodynamic parameters did not predict differences in susceptibility between adults and children. In the future, the approach of this case study could be used to assess the risks from low level exposures to other chemicals.