New approach methodologies (NAM) to characterize neurotoxic chemicals for use in risk assessment: case study of four neurotoxic pesticides

Neurotoxic pesticides: Cyfluthrin (CYF), βCYF, endosulfan (ENDO) and chlorpyrifos-oxon (CPFO) were examined using new approach methodologies (NAM) to assess ToxCast in vitro neurotoxic assay activities (50% concentration–AC50s), chemical-target specificity (Z-Scores) and in vitro to in vivo extrapolation with ToxCast assay AC50s as inputs to calculate human Administered Equivalent Dose (AEDHuman mg/kg) through Integrated Chemical Environment (ICE) models. Fold differences (FD) between AEDHuman NAM predictions and adjusted (animal to human interspecies extrapolation) in vivo regulatory neurotoxicity points of departure (POD) were calculated to assess modeled predictions against measured in vivo data. Interspecies (animal to human) in vivo adjusted PODs were 0.147 mg/kg/d (CYF/βCYF), 0.180 mg/kg/d (ENDO) and 0.106 mg/kg/d (CPFO). ToxCast neurotoxicity assay results supported in vivo observations: CYF, βCYF and CPFO showed depressed neuroactivity, while ENDO showed increased neuroactivity. βCYF is known to have twice the toxicity of CYF in vivo but was less toxic in the in vitro assays. Percent assays with Z-Scores greater than 3 for CYF, βCYF, ENDO and CPFO were 100%, 100%, 67% and 71% respectively. Most AEDHuman were less than 10-FD indicating good predictivity of the in vivo adjusted PODs with ICE models. The NAMs were useful for characterizing the main neurotoxic activities and the ICE models were highly predictive of in vivo PODs to support mechanisms in risk assessment.