Integrating Mechanistic Data to Contextualize in Vivo Apical Observations Using the Developmental Neurotoxicity in Vitro Battery

Regulatory toxicity studies traditionally rely on in vivo apical endpoints for hazard characterization and risk assessment; when available, relevant and reliable in vitro assays are anticipated to aid this interpretation. Developmental neurotoxicity (DNT) related parameters such as brain morphometry can be especially challenging to interpret, based on technical challenges such as sample preparation artifacts. This study employed the Developmental Neurotoxicity In Vitro Battery (DNT-IVB), designed to provide functional insights on key neurodevelopmental processes (KNDPs), to provide context for equivocal brain morphometry effects observed in a legacy in vivo DNT study conducted with acibenzolar-S-methyl (ASM), a plant host defense inducer. ASM and its major mammalian metabolite, acibenzolar acid (AA), were tested in all 17 in vitro assays comprised within the current iteration of the DNT-IVB. In vitro testing concentrations were derived to match the existing in vivo DNT study by applying physiologically based pharmacokinetic (PBPK) modeling with adjustment for rat versus human toxicokinetics. All concentration-response DNT-IVB assay data were analyzed using the US EPA ToxCast Pipeline. ASM showed no effect on any KNDP in vitro. AA was active only in the rat network formation assay with low potency AC50 ranges of 96.1-150 µM that modelling shows will not be achieved in vivo at doses where the apical effects in question were observed. This case study demonstrates the potential of the DNT-IVB to be used as a tool to assess equivocal findings from in vivo DNT studies and contribute to a weight of evidence to support conclusions on chemical-mediated DNT.