Towards NAM-based risk assessment for developmental neurotoxic effects, illustrated with chlorpyrifos

New approach methodologies (NAMs), such as in vitro and in silico methods, provide opportunities to replace, reduce and refine animal studies for chemical risk assessment, decrease experimental lead time and increase predictivity for human health. However, starting from/including NAMs introduces uncertainties which need to be addressed and quantified to work towards regulatory implementation of NAMs within risk assessment. A theoretical risk assessment based on the developmental neurotoxicity (DNT) of chlorpyrifos and its active metabolite chlorpyrifos-oxon was performed to highlight important extrapolations and identify and quantify uncertainties when extrapolating NAM-based data to estimate human health risk. Cell viability of differentiating human neuronal progenitor cells was used as a proxy for DNT during early pregnancy. A physiologically-based kinetic (PBK) model describing early pregnancy was developed to facilitate quantitative in vitro to in vivo extrapolation (qIVIVE) to estimate the external dose leading to DNT onset. An in vitro-based Health-Based Threshold Value was derived for the onset of DNT caused by chlorpyrifos and chlorpyrifos-oxon following chlorpyrifos exposure. This value was higher than the maternal exposure associated with the onset of DNT based on epidemiological data, showing the need for improvements in the NAM-based risk assessment of chemicals suspected of DNT. Improvements relate to the selection and refinement of the in vitro endpoint and assay and defining the quantitative relationship between the selected key event and the adverse outcome and provide a starting point for future research to aid regulatory implementation.