Generation of in vitro data to model dose dependent in vivo DNA binding of genotoxic carcinogens and its consequences: the case of estragole

Summary

Our food contains several compounds which, when tested in isolated form at high doses in animal experiments, have been shown to be genotoxic and carcinogenic. At the present state-of-the-art there is no scientific consensus on how to perform the risk assessment of these compounds when present at low levels in a complex food matrix. In order to refine the evaluation of the risks associated with these food-borne genotoxic carcinogens, information on their mode of action (MOA) at low versus high doses, on species differences in toxicokinetics and toxicodynamics, including dose- and species- dependent occurrence of DNA damage and repair, and on effects on expression of relevant enzymes, is required. For modern Toxicology it is of importance to better understand the MOA of genotoxic carcinogens to which humans are exposed daily via the diet at low doses. Genotoxic compounds can be direct acting when the compound is reactive itself and binds to the molecular target, or may need to be bioactivated before interacting with the molecular target. Bioactivation of a compound usually proceeds by phase I and/or phase II enzymatic pathways. When a genotoxic compound binds covalently to DNA it can form adducts with the four bases of the double helix. For all of these compounds the biological effect is a sum of both kinetic (absorption, distribution, metabolism and excretion) and dynamic (ultimate reaction with the molecular target) mechanisms. The model genotoxic carcinogenic compound studied in the present thesis is estragole. Estragole is an alkenylbenzene, found in herbs and spices, to which humans are exposed at low doses via the diet. Once absorbed estragole can undergo detoxification and bioactivation through phase I and II enzymatic pathways, resulting in the compound being either excreted or converted to a reactive carbocation which binds covalently to DNA. Estragole is known to produce tumors in rodents exposed to high dose levels (Miller et al., 1983) and it has been characterisedas genotoxic and carcinogenic.

The aim of the present thesis was to develop new strategies for low dose cancer risk assessment of estragole by extending the PBBK models previously defined for estragole to so-called physiologically based biodynamic (PBBD) models for DNA adduct formation, taking the approach one step closer to the ultimate endpoint of tumorformation. Such models will facilitate risk assessment because they facilitate extrapolation from high to low dose levels, between species including human and between individuals. Furthermore, building the PBBD models predicting in vivo DNA adduct formation based on only in vitro parameters contributes to the 3Rs (Replacement, Reduction and Refinement) for animal testing.