PBPK modeling of zebrafish embryo for reprotoxicity assessment of valproic acid and some of its analogs

Abstract : Introduction: Understanding and predicting chemical effects on development and reproduction is a complex challenge. The zebrafish embryo is an in vitro test system increasingly used for its transparency, short development time, easy husbandry and gene homologies with humans. Objectives: To better explain and predict developmental toxic effects observed in zebrafish embryo, we are developing a generic physiologically-based pharmacokinetic (PBPK) model to predict target organ concentrations of neutral or ionizable chemicals. We present an application of the model in the assessment of the teratogenicity of valproic acid (VPA) and nine analogs. Material and methods: Quasi steady-state distribution of chemicals is modeled in ten embryo organs, and in two sub-cellular organelles: lysosomes and mitochondria. The partition coefficients between the organs or organelles and the culture medium depend on physico-chemical properties of the substances. Organ volumes grow over the first 5 days of life. Liver metabolism can be linear or saturable. For VPA and analogs, metabolic parameters were estimated by Bayesian fitting of the model to data on culture medium and embryo concentrations during repeated dosing. Results: The kinetic data were reasonably well fitted by the model (Figure 1), even though residual uncertainty was substantial. Linear clearance estimates for VPA and analogs were around 10-10 to 10-11L/min, which for an embryo volume of 3×10-7L, correspond to half-lives of about 11.5h. Embryo organ concentrations were used to calculate dose-response for general lethality and heart defect-caused deaths. Naïve dose-responses were also calculated using nominal dose. Using target organ concentrations shifts substantially the magnitude of doseresponse parameters and the relative toxicity ranking of the VPA analogs studied (Figure 2). Conclusion: The model can be used to relate zebrafish effects observed in vitro to cellular exposures. It should improve the translation of in vitro zebrafish data to humans for safety assessment.