Next Generation Risk Assessment of Chemicals: In vitro and in silico approaches to work towards enough precision to make a decision

The process of chemical risk assessment (i.e. hazard identification, hazard characterisation, exposure assessment and risk characterisation) has traditionally relied on animal experiments. However, animal experiments are not only ethically debatable, but also expensive, time-consuming and they provide only limited information on the mechanism of toxicity, which is crucial in understanding the toxicity of a chemical across species. Importantly, animal experiments are not always predictive for the human situation. Risk assessments therefore often use uncertainty factors to account for differences between and within species. Traditionally, dose-response relationships derived from animal experiments are used as a starting point to derive a safe dose in humans by applying a 100-fold margin of safety. A factor of 10 is applied to account for interspecies sensitivity differences and another factor of 10 to account for intraspecies differences. Although these uncertainty factors have a scant scientific basis, they have been used for over 60 years. Next generation risk assessment encompasses a shift from the use of animal experiments towards an hypothesis driven strategy that integrates in vitro and in silico approaches. These approaches form the basis for assessing mechanisms of toxicity, specifically by using integrated risk assessment tools like adverse outcome pathways and physiologically based kinetic modelling. This next generation risk assessment can accommodate refined uncertainty factors for a more accurate risk assessment, while complying with the 3Rs principle of the reduction, refinement and replacement of animal testing. This thesis presents alternative (non-animal) methods in the framework of next generation risk assessment that can help improve decision-making about chemical risks to human health by increasing precision and reducing uncertainty. The first chapter introduces the subject of chemical risk assessment, alternative methods, uncertainty factors and why it is important to refine these uncertainty factors. Chapters 2 and 3 demonstrate the use of in vitro methods to address interspecies differences in neurotoxicity between rats and humans. Chapters 4 and 5 present an in silico and an in vitro approach, respectively, to evaluate interindividual differences between humans. Chapters 6 and 7 provide more integrated risk assessment approaches, through the use of adverse outcome pathways and physiologically based kinetic modelling. In chapter 8, chapters 2-7 are discussed and summarised.