Comparative assessment of different alternatives to animal models for developmental toxicity prediction using physiologically based toxicokinetic modelling approach: A case study of hexaconazole, an azole fungicide

The 21^st-century risk assessment is gradually moving from animal-based toxicity studies to in vitro alternative assays that are sustainable and ethically acceptable. Alternative assays, such as rat whole embryo culture test (WEC), mouse embryonic stem cell test (EST), zebrafish embryotoxicity test (ZET), and ToxCast assays, are widely used for screening the chemicals for developmental toxicity. However, for use in risk assessment, these assays require integration with the predictive approaches, such as physiologically based toxicokinetic (PBTK) model. Using PBTK-facilitated reverse dosimetry approach, we translated the in vitro assay concentration to human equivalent doses (HEDs) using hexaconazole (HEX, a widely used fungicide) as the model compound. For this, a rat PBTK model was developed and verified using in-house generated toxicokinetic data. Based on the rat model, human PBTK model was developed to translate the in vitro concentrations of various alternative assays into HEDs (0.16–7850 mg/kg/day). These HEDs were compared with the HED derived using the traditional approach based on rat toxicity data. The HEDs derived from the alternative assays (WEC, EST and ZET) showed poor correlation with the HED derived from the traditional approach. However, most of the HEDs derived from the ToxCast assays were close to the traditional HED. This indicated that the PBTK model-facilitated reverse dosimetry approach could derive the HEDs directly from in vitro assays when sufficient animal data is lacking.