Nanoparticle toxicokinetics in the nose: an assessment of risk
In recent years there has been a dramatic increase in the number of nanomaterials being developed, thus increasing the need for hazard assessment methods beyond the capacity of toxicological screening methods using animals. Current in vitro assays have a number of shortcomings, which were addressed in this thesis. These include: (i) high dependence on immortalized cell lines, (ii) inaccurate dosimetry descriptors, (iii) poor robustness of assay systems, and (iv) hyperoxic culture conditions. A method for harvesting viable human nasal epithelial cells using a washout technique was developed to provide squamous epithelial cells for use in culture assays. However, poor proliferation of these cells in vitro limited their use in toxicological assays. A particokinetic model was developed to relate the ’delivered dose’, i.e. the number of nanoparticles reaching immortalized airway cell layers by gravitational force and diffusional mechanisms, to toxicological endpoints measured in vitro. This model was applied to a panel of rigorously characterized nanoparticles (CuO, TiC>2, polystyrene and in-house manufactured lipid nanocapsules) and the results provided compelling evidence that the delivered dose is a more appropriate dose descriptor for cell-based toxicity assays than the widely used nominal dose, as it reflects the number of particles (or their equivalent surface area) available to interact with the cell layer over a given exposure time. The results were confirmed in two airway epithelial cell lines RPMI 2650 and A549 after 6 and 24 h using two standard toxicological end-points. However, when cells were cultured in normoxic (for the respiratory tract) oxygen concentration, 13%, as opposed to the standard culture conditions of 21% they were found to be more responsive to nanoparticle exposure in terms of both production of reactive oxygen species and reduced cell viability. This suggests that standard incubation conditions of 21% oxygen provide a baseline of oxidative stress within a cell culture system that induces adaptive mechanisms and reduces their sensitivity to materials that exert adverse effects through oxidative stress.
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