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BUILDING A SCIENTIFIC FOUNDATION FOR SOUND ENVIRONMENTAL DECISIONS
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Using ToxCast™ In Vitro Screening Data for Chemical Prioritization
Incorporating Human Exposure and Dosimetry
April 30, 2010
Impact Statement
Tens of thousands of chemicals
are currently in commerce, and
hundreds more are introduced
every year. Only a small
fraction of chemicals have been
adequately assessed for
potential risk. To reduce the
number of untested chemicals
and prioritize limited testing
resources, EPA and other
government agencies are
exploring the use of
technologies that can examine
the effects of thousands of
chemicals against many key
biological processes. These
technologies use non animal (in
vitro) assays to help understand
what might happen when a
human is exposed to a
chemical. Interpreting the
relevance of in vitro data to
predict toxicity remains a
challenge for a number of reasons.
This study looks into the key
question about what amount of
human exposure is required to cause
an effect measured in the in vitro
tests. To provide insights into this
question this study experimentally
measured human hepatocyte
clearance rates and plasma protein
binding for 35 of the 309 ToxCast
Phase 1 chemicals. ToxCast is a
multi-year, multi-million dollar
effort that uses innovative
technologies to efficiently ($20K per
chemical) study the key biological
processes impacted by chemicals
which then lead to adverse health
effects. ToxCast includes over 400
in vitro high-throughput screening
assays. This study indicates that
understanding relevant exposure
conditions is important when using
high-throughput in vitro data to
identify the highest priority
Toxico kinetic
Parameters
Plasma Protein
Binding
Metabolic Stability
r i
In Vitro-to-ln Vivo
Extrapolation



ToxCast ACSI
Estimated Target
Tissue Bioactivity
Concentration
Fig. 1
Predicted Assay
Oral Equivalent
Doses
Upper Level of
Human
Exposure
Chemicals with Potential to Perturb
Cellular Pathways at Relevant
Human Exposure Levels

Provided by ToxCast
IB)
Data Generated In Vitro

Data Obtained from

Registration Documents

Computational Modeling
chemicals for further testing and risk
management.
Study Description
This study used a combination of the
ToxCast data, new human in vitro
data, and computer modeling to
identify chemicals with the potential
to perturb cellular pathways at
relevant human exposure levels.
Below is a summary of how the
study combined and assessed the
data (See Figure 1):
¦ The selected chemicals had
bioactivity in many,
sometimes hundreds, of
ToxCast assays. The
amount of a chemical that
caused a 50% change in an
assay was chosen as an
estimate of the blood
concentration needed for
bioactivity.
In order to relate chemical
exposure to blood
concentration, two
measurements were made
for each chemical - the
ability of human plasma to
bind the chemical and the
ability of human
hepatocytes to metabolize
the chemical.
Computational modeling
(with Simcvp© simulation
software) was used to
predict the amount of
chemical that would have to
be ingested in order to have
blood levels that were
similar to those that caused
effects in the ToxCast
assays.
Human oral equivalents of
ToxCast in vitro results
were compared to official
EPA estimates of chronic
1
U.S. Environmental Protection Agency
Office of Research and Development

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