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BUILDING A SCIENTIFIC FOUNDATION FOR SOUND ENVIRONMENTAL DECISIONS
Estimating Toxicity-Related Biological Pathway Altering Doses for High-Throughput
Chemical Risk Assessment
February 13,2011
Impact Statement
Important inputs to chemical risk assessments
are estimates of the highest allowable exposure
levels that are protective of human health.
Typical acceptable exposure values such as the
Reference Dose (RfD) are based on expensive
and time consuming animal toxicity tests. Non-
animal based methods to estimate safe exposure
levels would be beneficial because there are tens
of thousands of existing chemicals with little or
no animal testing data, and hundreds more
chemicals introduced into commerce every year.
This paper presents an approach to use on data
poor chemicals to derive screening-level
estimates of allowable exposure levels. The
method uses knowledge about how chemicals
alter biological processes or pathways related to
human disease. The proposed method for high-
throughput chemical risk assessment (HTRA)
uses data from rapid chemical screens to
estimate exposures that would alter biological
pathways in a way that could potentially lead to
toxicity or disease.
Study Description
A proposed HTRA approach for chemicals is
presented that focuses on biological pathways
linked to adversity and disease. The approach
combines results from ToxCast and Tox21
High-Throughput Screening (HTS) assays with
data on metabolism and pharmacokinetic
modeling to estimate exposure levels reasonably
expected to be without risk of chemically
induced disease in human populations. The
proposed HTRA approach is essentially a five-
step process (FIGURE 1) that calculates a
Biological Pathway Altering Dose (BPAD)
useful in estimating acceptable exposure levels.
1. Identify pathways linked to adverse
outcomes: Biological pathways are a
key connection between mode of action
based risk assessment and HTS. This
approach starts by identifying known
Figure 1- Proposed Five-Step Process for
High Throughput Risk Assessment (HTRA).
Identify Biological Pathways Linked to Adverse Effects and Disease
Measure Pathway Altering Concentration in ToxCast HTS Assays
Convert to In-Life Biological Pathway Altering Dose (BPAD)
Incorporate Uncertainty and Population Variability
Calculate Lower Limit Based on 99%ile of BPAD
targets (genes, proteins) and pathways
linked to disease.
2. Measure chemical activity in
concentration-response: The next step is
to use ToxCast HTS data to determine
the concentration of a chemical that can
perturb the biological pathway in cells.
This is termed the Biological Pathway
Altering Concentration (BPAC).
3. Convert HTS concentration-response to
human dose-response: Using metabolic
measurements and pharmacokinetic
models, the BPAD is calculated.
4. Incorporate population variability and
uncertainty: All measurements and
estimates are subject to uncertainty and
population variability. The HTRA
model incorporates both of these in a
manner analogous to traditional risk
assessments.
5. Estimate lower limit for pathway
perturbation: The final step is to
estimate a lower limit from the BPAD
below which there is minimal risk of the
toxicity-related pathway being
perturbed, the BPADL99.
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CONTACT: Monica Linnenbrink, EPA Research.,
linnenbrink. monica(@,epa. gov or 919-541-1522
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Figure 2 - Liver Toxicity
Effect Levels
1e-04
Figure Key
Red Circle=BPADL99
Blue Box=Lowest Effect
Level
Gray Triangle=No Effect
Level
Red Triangle=NEL/100
Red Vertical Line=
Estimated chronic exposure
levels from food residues
1e-02
1e+00
1e+02
1e+04
Oral Dose (mg/kg/day)
Examples of this HTRA approach are presented
for Bisphenol A, a chemical widely used in
plastics, comparing estrogenicity measured by
HTS to reproductive toxicity in animal studies.
Rat reproduction tests resulted in a No Effect
Dose of 50 mg/kg/day. Adjusted for uncertainty
and variability, the no effect dose is 0.5
mg/kg/day and close to the HTRA lower limit,
or BPADL99 of 0. 16 mg/kg/day derived from
six ToxCast estrogen receptor assays.
The second HTRA example for conazole
fungicides compares rodent liver hypertrophy to
interactions with the CAR and PXR receptor
pathways. CAR and PXR respond to chemical
exposures in ways that can lead to liver disease,
and numerous ToxCast and Tox21 HTS assays
measure chemical alterations in CAR and PXR
pathways. The HTRA lower limits, or BPADL99
calculated for 14 conazoles are compared to the
No Effect Dose/100 for rodent liver hypertrophy
in FIGURE 2. Most of the BPADL99 for the
conazoles are below and within a factor of 10 of
the No Effect Dose/100.
Conclusions
This paper outlines an efficient and rapid
method for providing screening-level estimates
of acceptable exposure levels for data poor
chemicals. There are a number of extensions and
refinements that need to be carried out, but we
believe that in time, the HTRA approach can be
an important tool for addressing the backlog of
chemicals in need of toxicity assessments. In
addition, when combined with estimates of
human exposure, the HTRA approach can be
used to prioritize which chemicals need further
toxicity testing and exposure monitoring.
Background
Many commodity chemicals in commerce have
only undergone minimal safety testing as
required by current US law. Toxicity testing
conducted in animals is time-consuming,
expensive and yields limited mechanistic
information relevant to human disease. In an
effort to improve existing chemical screening,
US EPA's ToxCast and Tox21 projects are
working to develop new ways to efficiently
screen chemicals and prioritize limited testing
resources toward those that have the greatest
potential to cause hazard to human health.
Legislation to overhaul the existing Toxic
Substance Act is currently under discussion in
the US Congress. This research was performed
as part of the US EPA's Computational
Toxicology Research Program.
Reference
Judson et al (2011) "Estimating Toxicity-
Related Biological Pathway Altering Doses for
High-Throughput Chemical Risk Assessment."
Chemical Research in Toxicology, in press.
More Information: www.epa.gov/comptox
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CONTACT: Monica Linnenbrink, EPA Research.,
linnenbrink. monica(@,epa. gov or 919-541-1522
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