Page 235 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

220 Principles and Methods

in vivo studies. In vivo studies traditionally require a major route of expo-
sure (inhalation, dermal uptake, ingestion, or systemic injection) that
reflects how the material is produced or used in the workplace and/or
marketplace. The ideal approach would be to use an animal model in
which a disease process or pathological event is linked to a mechanism of
injury that can be followed by cellular studies. If toxicity does occur in vivo,
this could act as the departure point to perform more detailed evaluation
of NM dosimetry, toxicokinetics, and toxicodynamics. These are expensive
and labor-intensive studies that should be used under select circumstances,
for example, materials that show definitive in vitro toxicity as well as in
vivo pathological outcomes. Finally, a number of animal models are avail-
able to allow one to study mechanistic pathways by live imaging procedures
[51]. These animal models are further discussed below.
While comprehensive coverage of in vivo study methods falls outside the
scope of this chapter, suffice it to mention that the ideal approach would
be to use animal models that address disease outcome in terms of a spe-
cific mechanism of injury. Rats and mice are currently the best animal
models for which an extensive database has been developed to assess
chemical toxicity. For particle exposures, the best protocols are for pul-
monary exposure, using inhalation or intratracheal instillation methods.
Inhalation is more physiologically relevant and the preferred approach for
hazard identification and generating dose-response data. However, intra-
tracheal instillation or pharyngeal aspiration are acceptable procedures
to gather proof-of-principal evidence for an in vivo toxicological outcome.
Relevant endpoints are the assessment of airway and interstitial inflam-
mation, as well as the assessment of oxidative stress markers in the lung
by bronchoalveolar lavage and histopathology. It is also possible to assess
cardiovascular pathology by inhalation exposure, such as assessing the
effect of ambient PM on atherosclerosis, blood clotting, and the generation
of cardiac arrhythmias [52]. The performance of these cardiovascular and
pulmonary studies can be further enhanced by susceptible animal models,
for example, allergen-sensitized mice to perform asthma studies, apoE
knockout animals to conduct atherosclerosis studies, or animals impaired
in one or more aspects of antioxidant defense to show accelerated disease
development either in the lung or cardiovascular system. The approach to
the performance and assessment of dermal and gastrointestinal toxicity
is described elsewhere (see reference 12).
In vivo imaging could be used to study the hierarchical oxidative
stress paradigm. One example is the use of a transgenic mouse model
in which the HO-1 promoter has been linked to a luciferase reporter [51].
The HO-l-luc Tg mouse was developed by Dr. Christopher Contag at
Stanford University by injecting a full-length HO-1 promoter-luciferase
construct into FVB mouse pronuclei [51, 53–55]. Transgenic pups were
screened by bioluminescent imaging and by HO-1 luc-specific PCR.