Integrated in vitro-in silico models for predicting in vivo developmental toxicity : facilitating non-animal based safety assessment

In chemical safety assessment, information on adverse effects after repeated dose and chronic exposure to low levels of hazardous compounds is essential for estimating human risks. At present, this information is almost solely obtained by performing animal experiments. Therefore, suitable methods to reduce, refine or replace (3Rs) repeated dose animal testing are urgently needed. At present, <em>in vitro</em> toxicity assays are able to screen compounds for toxicity, but since these tests result in <em>in vitro</em> concentration-response curves, whereas for the safety assessment of chemicals for human <em>in vivo</em> dose-response curves are needed, it is important that <em>in vitro</em> concentration-response curves can be translated to <em>in vivo</em> dose-response curves. The goal of the present project is to extrapolate <em>in vitro</em> concentration-response curves to <em>in vivo</em> dose-response curves with the help of physiologically based kinetic (PBK) models that describe the <em>in vivo</em> absorption, distribution, metabolism and excretion (ADME) processes. This is achieved by using the concentration-response curves, acquired in an appropriate <em>in vitro</em> toxicity test, as internal concentrations in the model, in order to calculate the <em>in vivo</em> dose levels that are needed to reach the internal (toxic) concentrations, by using the PBK-model. The predicted dose-response curves thus obtained can be used to determine safe exposure levels in chemical safety assessment. The endpoint used in the present study is developmental toxicity. The <em>in vitro</em> toxicity assay used is the differentiation assay of the embryonic stem cell test (EST). With the use of a rat PBK model, predicted dose-response curves for <em>in vivo</em> developmental toxicity for the rat are acquired, which are compared with experimental literature data on the <em>in vivo</em> developmental toxicity of these compounds in the rat. To obtain the dose-response curves for <em>in vivo</em> developmental toxicity in human, PBK-models describing the <em>in vivo</em> kinetics in human are used. The combined <em>in vitro-in silico</em> approach described is used for compounds belonging to the chemical class of glycol ethers or the chemical class of retinoids. This enables evaluation of whether the combined <em>in vitro - in silico</em> approach is able to predict dose-response curves for <em>in vivo</em> developmental toxicity for compounds belonging to the same chemical class, but with differences in toxic potency. The results of the research reveal the feasibility of translating <em>in vitro</em> concentration-response curves to <em>in vivo</em> dose-response curves using PBK modeling. This finding shows the possibility of using <em>in vitro</em> toxicity data in chemical risk assessment, which will, if applied in risk assessment, highly contribute to the 3Rs.