Regulatory Policy Implications of 1,4-Dioxane - Environmental Investigation and Remediation - page 466

Cell lines use living animal or human tissue cells to measure responses to toxins. Cell-based in
vitro tests permit the rapid testing or large sets of replicates to improve statistical characterization
of cellular responses to environmental contaminants. Cell testing can also be considerably quicker
and less expensive than in vivo testing; moreover, cell testing provides an indication of specii c
toxicological endpoints that are not usually determined from acute toxicity tests (Bhogal et al.,
2005). In vitro systems can include tissue slices, perfused organ preparations, primary cultures,
and cell lines grown either in suspension or as adherent cultures. Cells can be isolated from tissues
for ex vivo culture by digesting tissues with enzymes that remove the extra cellular matrix, by
purifying blood to isolate white blood cells, or by placing a piece of excised tissue in a growth
medium. Most cell cultures are short-lived; after doubling their population multiple times, cells
stop dividing but generally retain viability as an established or immortalized cell line that can
sustain itself to permit testing. Cell lines can be isolated from many tissues, organs, and species,
cultured over extended periods and/or cryopreserved for future use. The use of human tissues and
cells has an obvious advantage because the need for interspecies extrapolation is avoided (Bhogal
et al., 2005).
The liver metabolizes toxins and is central to the assessment of toxicity. A compounding factor
in toxicological evaluations is that biotic and abiotic transformations of the chemical inside the
target organism may lead to the formation of reactive metabolites that are toxic (USEPA, 2006).
Cell line bioassay methods are now available that use human liver tissue and are sensitive enough to
detect the hepatotoxicity of contaminants and their metabolites in water samples using the low den-
sity lipoprotein (LDL)-uptake activity of human hepatoblastoma cells, Hep G2. The LDL-update
activity assay of Hep G2 can be used to evaluate cytotoxicity for up to 48 h with high sensitivity and
selectivity using a l uorescent plate reader (Shoji et al., 2000).
Studies of changes to the composition of proteins and activities of cells provide an approach to
eliciting the mechanisms of toxicity using the “omics.” The “omics” methods include genomics—
the study of an organism's genome including mapping its DNA sequence; proteomics—the study of
protein structures and functions; and metabonomics—proi ling metabolic changes and metabolic
products produced by exposure to toxins (Bhogal et al., 2005). The “omics” methods make it pos-
sible to develop molecular proi les to identify the key steps that trigger toxicity and cause adverse
health effects to target organs and to entire living organisms (USEPA, 2006).
A number of programs are now underway to achieve these advances, including USEPA's
Computational Toxicology Research Program, with the goal of shifting the i eld of toxicology from
a descriptive to a predictive science and thereby improving USEPA's ability to assess hazards and
characterize risks (USEPA, 2006).
10.3.3 A PPLYING C OMPUTATIONAL T OXICOLOGY TO P HYSIOLOGICALLY B ASED
P HARMACOKINETIC M ODELING
The emerging i eld of computational toxicology applies mathematical and computer models and
molecular biological and chemical approaches to explore both qualitative and quantitative relation-
ships between environmental contaminants and adverse health effects. The integration of advanced
computing methods with molecular biology and chemistry is enabling scientists to better prioritize
data, inform decision makers on chemical risk assessments, and understand a chemical's progres-
sion from the environment to the target tissue within an organism, and ultimately, mechanisms of
toxic effects. A key goal of both computational and in vitro toxicology is to reduce the uncertain-
ties in the extrapolation of effects across dose, species, and chemicals (USEPA, 2006).
In silico simulation of contaminant biotransformations following exposure and descriptions of
metabolic maps has shown great promise for improving the toxicological foundation of health risk
assessments. Knowledge of predicted metabolites and their associated toxic effects may be useful
for pollution prevention by avoiding the commercial use of those chemicals found to form toxic
metabolites, again employing the precautionary principle. For example, forecasting the probable