slide 1 of 38 office of pesticide programs drs. vicki dellarco & anna lowit health effects...
TRANSCRIPT
Slide 1 of 38
Drs. Vicki Dellarco & Anna LowitHealth Effects Division
Office of Pesticide ProgramsOffice of Pesticide Programs
Mode of Action/ Human Relevance Analysis For Incorporating Mechanistic Data in Human Health Risk
May 2006
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Uses of Mechanistic Data Uses of Mechanistic Data in Risk Assessmentin Risk Assessment
Identify Key biological (precursor) events leading to adverse
toxicities (Mode of Action) Inform
Human relevance of animal findings Dose response extrapolation Life stage susceptibilities
Understand Common pathways of toxicity (cumulative risk
assessment) Promote
Consistent harmonized approach to risk assessment for all health endpoints
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“Mechanism of action”
(more detailed understanding at
biochemical & molecular level)
versus
“Mode of action” (identification of key & obligatory steps)
Exposure
Toxicity
Key event
Key event
Key event
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Mode of Action FrameworkMode of Action Framework
EPA’s Guidelines for Carcinogen Risk Assessment 1996 Proposed Revisions
put forth the notion of understanding mode of action versus mechanism of action
1999 Interim Guidance introduced mode of action framework
2005 Final Guidance minor rewording of MOA framework
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Mode of Action FrameworkMode of Action Framework Postulated mode of action
Identify sequence of key events on the path to cancer Experimental support
Concordance of dose-response for key events with that for tumors
Temporal relationships for key events & tumors Biological plausibility & Coherence Strength, consistency & specificity Other modes of action Identify uncertainties Conclusion
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Human Relevance Human Relevance FrameworkFramework
Risk Sciences Institute-ILSI Comparability or concordance analysis of the key events &
relevant biology between the laboratory species & humans Tumor Responses: Meek et al., 2003, Critical Reviews in
Toxicology Vol 33/Issue 6, 581-653 Reproductive, Developmental, Neurtoxocity Responses: Seed
et al., 2005 Critical Reviews in Toxicology Vol 35/Issue 8-9, 63-781
extended human relevance analysis to include mutagenic carcinogens & noncancer end points
WHO/IPCS Human Relevance Framework (in preparation)
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Human Relevance Human Relevance FrameworkFramework
Based on three analyses: Is the Weight of Evidence sufficient to establish a MOA in
animals (MOA Framework)? Are the key events in the animal MOA plausible in humans? Taking into account kinetic/dynamic factors, is the animal
MOA plausible in humans?
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Relevant or Unknown
HumanRelevance
Not Relevant in Humans 2.) Is the Animal
MoA Plausible in Humans?
Yes
No
Human Relevance Framework
Human Relevance Framework
NotSufficient
Sufficient
1.) Is the Weight of Evidence Sufficient to Establish a
MoA in Animals?
3.) Taking Into Account Kinetic and Dynamic
Factors, Is the Animal MoA Plausible in
Humans? YesNo,
Continue with the Dose
Response & Exposure
Assessment
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Assessing an Animal Mode of ActionAssessing an Animal Mode of Action
General Points: Applicable to all chemicals, to all endpoints,
and to all modes of action Evaluation of MOA for tumors or (other
adverse effects) in different organs MOA in different organs may or may not be
the same Site concordance between animals &
humans
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Assessing an Animal Mode of ActionAssessing an Animal Mode of Action
General Points: When a substance operates via a novel MOA, the
analysis is focused on the chemical & entails a detailed evaluation via the MOA Framework
When a substance produces an adverse effect consistent with an already established & peer reviewed MOA through which other chemicals have been shown to operate, the analysis is focused on the established MOA & a determination of whether the substance operates via the same key events established for the pathway
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Assessing an Animal Mode of Assessing an Animal Mode of Action for Human RelevanceAction for Human Relevance
General Points: Concordance Analysis of key events is for
the MOA & is not necessarily a chemical specific evaluation.
Chemical specific & generic information relevant to the toxicity process can be valuable
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“EPA’s Cancer Assessment Review Committee (CARC) classified atrazine as “not likely to be carcinogenic to humans”.
Vinclozolin--“Since the androgen receptor is widely conserved across species lines, antiandrogenic effects would be expected in humans.”
MOA: Inform Human Relevance
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Alachlor - “. . . a margin of exposure (MOE) approach (indicative of a non-linear dose response) should be used for the risk assessment.”
Cacodylic Acid – “. . . nonlinear default approach (i.e., derivation of a reference dose or margin of exposure) is regarded as the more appropriate dose response extrapolation approach. . .”
Chloroform – “. . . a nonlinear approach is more appropriate for low-dose extrapolation.”
MOA: Inform Dose Response Extrapolation
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Case StudyCase StudyCacodylic Acid Cacodylic Acid
(Dimethylarsinic acid)(Dimethylarsinic acid)
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DMADMAVV Mode of Action Mode of Action Science Issue Paper: “Mode of Carcinogenic
Action for Cacodylic Acid (Dimethylarsinic Acid, DMAV) and Recommendations for Dose Response Extrapolation” (July 26, 2005) http://www.epa.gov/oppsrrd1/reregistration/
cacodylic_acid/ Revised issue paper will be publicly available this
spring. EPA’s Science Advisory Board (SAB)
reviewed the special issue paper in September, 2005 Draft SAB report December 27, 2005
TMAsIII
TMAsVMethylation
Reduction
Metabolism of ArsenicMetabolism of Arsenic
Alternate steps of oxidative methylation & reduction
PesticideChemical
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DMADMAVV: MODE OF ACTION ANALYSIS: MODE OF ACTION ANALYSIS
Extensive experimental cellular and laboratory animal data
Weight of Evidence
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DMADMAVV: Available Cancer Data: Available Cancer Data
No epidemiology data Standard rodent bioassay
Bladder carcinogen in rats via feed -100 ppm (9.4 mg/kg bw per day) via drinking water- 50 & 200 ppm females more sensitive than males
Not carcinogenic in mice Up to 500 ppm in B6C3F (Gurr et al., 1989) 121 ppm in C57 XC3H/Anf or AKR (NCI 1969)
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Mode of ActionMeasurable Key Events in Target Tissue
DMAIII
Metabolite
Hyperplasia
UrothelialToxicity
RegenerativeProliferation
Urinary BladderTumors
Sustained
Urinary bladder from a female F344 rat treated with 100 ppm DMAV
BrdU Labeling
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Compensatory regeneration in rat bladder at weeks 8 & 10 following ingestion of DMAV
Cell Proliferation Response (BrDU labeling Indices)
Hour 6
Hour 24
Day 3 Week 1
Week 2
Week 2
Week 2
Week 10
Week 10
Week 20
Week 26*
Cohen et al., 2001
Cohen et al., 2001
Cohen et al., 2001
Cohen et al., 2001
Cohen et al., 2001
Cohen et al., 2001
Coh en et al., 2002
Arnold et al., 1999
Cohen et al., 2001
Arnold et al., 1999
Cohen et al., 2002
--- 1X 0.22 ± 0.04
0.42 ± 0.05 1X 0.24 ± 0.04
0.23 ± 0.04 1.4X 0.33 ± 0.11
0.44 ± 0.09 2.2X 0.96 ± 0.14 a
0.22 ± 0.03 6.2X 1.36 ± 0.13 a
0.19 ± 0.04 4.9X 0.94 ± 0.20 a
0.16 ± 0.02 3.9X 0.63 ± 0.10 a
0.22 ± 0.05 4.2X 0.93 ± 0.11 a
0.18 ± 0.03 3.4X 0.61 ± 0.10 a
0.25 ± 0.03
3.9X 0.97 ± 0.11 a
0.13 ± 0.02 1.6X 0.21 ± 0.03 a
Uncertainty expressed as ± S.E. of the mean in all studies a P<0.05 when compared to respective controls cAll results in female rats
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Dos
e R
espo
nse
Con
cord
ance
TemporalDose
(mg/kg bw/day)
Metabolism DMAVDMAIII
Urothelial Toxicity
Regenerative Proliferation
Urothelial Hyperplasia
Transitional Cell
Carcinoma
0.2(2 ppm)
+(wk 3-0.03 ± 0.01 uM)
+(wk 10-6/10, grade 3 or 4)
- - -
1(10 ppm)
+(wk 3-0.12 ± 0.02 uM)
+(wk 3-2/7, grade 3) (wk- 10; 8/10, grade 3 or 4)
slight(wk 10-1.5X inc) - -
4(40 ppm)
+(wk 3-0.28 ± 0.09 uM)
+(wk 3-7/7, grade 3) (wk 10-5/10, grade 3 or 4)
+(wk 10-4.3X inc)
+(wk 10- 4/10)
-
9.4(100 ppm)
+(wk 3-0.55 ± 0.15 uM)
+(6 hrs-6/7, grade 3) (24 hrs-4/7, grade 3 or 4)(wk 2 6/10, grade 5)(wk 10-0/10, grade 4 or 5)
+(wk 1- 2.2X inc) (wk 2-3.9X inc) (wk 10-4.2X inc)
+ (wk 8-7/10)(wk 10-9/10)
+ (papilloma first obs at wk 107; carcinoma first obs at wk 87)
Association of Key Precursor Events & Bladder Tumors in F344 Rats
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Cacodylic Acid: Key Events Cacodylic Acid: Key Events Temporal RelationshipTemporal Relationship
DMAIII DMAV
Urothelial Cytotoxicity
Regenerative Proliferation
Hyperplasia
Tumors
6 hours
1 Week
8-10 weeks
104 weeks
Urinary bladder from a female F344 treated with 100 ppm DMAV
BrdU labeling
Urinary bladder tumors
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Cacodylic Acid: Key EventsCacodylic Acid: Key EventsCytotoxicity/Regenerative ProliferationCytotoxicity/Regenerative Proliferation
Strength, Consistency & Specificity Consistency of association found in repeated
experiments within a lab & among different labs Inhibition of DMAV DMAIII reduced cytotoxicity Cessation of exposure to DMAV results in recovery
of tissue (i.e., hyperplasia) Biological Plausibility & Coherence
Regenerative proliferation associated with persistent toxicity appears to be a risk factor for bladder cancer in humans
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Characterization of Characterization of Cacodylic Acid’s GenotoxicityCacodylic Acid’s Genotoxicity
Neither DMAV or DMAIII are direct acting point/gene mutagens
Both are clastogenic but DMAIII is the more potent In vitro data only
DNA damage appears to result from an indirect mechanism (ROS/oxidative damage)
DMAIII DMAV
0%
5%
10%
15%
20%
25%
30%
35%
-0.05 0.95 1.95 2.95
Concentration ( μ )M
(%)Abberant Cells
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Chromosomal AberrationsChromosomal Aberrations For the oxidative DNA damage to be relevant to the
carcinogenic process (i.e., clonally expanded), stable chromosomal mutations must be formed
Formation of chromosomal mutations requires DNA replication because chromosomal alterations are produced by errors of replication on a damaged DNA template. frequency of chromosomal mutations will be a function of the
regenerative proliferative response. All these events--genetic errors, cytotoxicity,
stimulation in cell proliferaiton -- must occur to result in bladder tumors.
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Other Modes of Action or Key Other Modes of Action or Key EventsEvents
No other MOA with sufficient scientific support Direct DNA reactivity Formation of solids Changes in urinary chemistry & physiology
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Mode of Action ConclusionsMode of Action Conclusions
Sequence of key events leading to bladder tumors measurable & supported by robust data
Biologically plausible Uncertainties do not discount scientific
support cellular target for cytotoxicity not understood unknown cytotoxic metabolites found in urine
(after drinking water exposure)
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Urothelial Cytotoxicity
?
Human Relevance of DMAHuman Relevance of DMAVV’s ’s Mode of ActionMode of Action
Metabolism to DMAIII
RegenerativeProliferation
Hyperplasia & Bladder tumors
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Concordance Analysis of Key Events in Rats & Concordance Analysis of Key Events in Rats & Humans: Qualitative & Quantitative PlausibilityHumans: Qualitative & Quantitative Plausibility
Key Event Rats Humans
Presence of DMAIII in urine
YesYes (based on
Asi)
Persistent cytototoxicity
Yes Possible
Persistent regenerative prolif/hyperplasia
Yes Possible
Bladder Tumors Yes Possible
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DMADMAVV Mode of Action (MOA) Mode of Action (MOA)
The SAB concurred with EPA’s conclusions
1. Rat data developed for DMAV most appropriate data for quantifying cancer risk
2. MOA for the development of bladder tumors in rats established
3. The rat MOA is expected to be plausible in humans
4. The MOA supports nonlinear extrapolation of cancer risk to DMAV
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Dose response extrapolation Dose response extrapolation approachapproach
Dose response extrapolation should be based on considerations of MOA which supports nonlinearity Must be sufficient DMAIII to produce cell killing & sufficient
cell killing to lead to regenerative proliferation Cytotoxicity & enhanced proliferation need to be sustained
Frequency of chromosomal mutations dependent on enhanced
proliferation & on generation of ROS (DMAIII DMAV) Point of Departure based on cell proliferation should
be protective of DMA’s carcinogenic & promoting effects
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DoseDose Response Considerations Response Considerations Cancer Guidelines describe a two-
step dose-response process which separates Modeling the observable range of data Extrapolation to lower doses
Nonlinear extrapolation Preferred approaches
PBPK Model--internal dosimetry at the target tissue
– e.g. DMAIII
BBDR Model—predict biological effect– e.g., two stage clonal growth
Interim approach Identify a point of departure (POD)
based on benchmark dose modeling Apply uncertainty and safety factors
POD
Key event
Dose
Res
po
nse
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MOA informslow-dose
extrapolation?
MOAEstablished?
BBDR model?
1. Fit data in observable range
2. Linear extrapolation from PODUse model
RfD/RfCor MOE
Yes
Yes
Yes, nonlinear
No
No
No
Yes, linear(including mutagenic MOA)
QuantitativeDose-responseAssessment
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6 7
Mea
n R
espo
nse
dose
Hill Model with 0.95 Confidence Level
BMD10
BMDL10
Benchmark Dose Modeling: Benchmark Dose Modeling: Regenerative Proliferation Regenerative Proliferation
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Biological Event
Duration
Feeding
Duration
Drinking water
10% 1% 10% 1%
BMD (mg/kg/day)
BMDL(mg/kg/day)
BMD(mg/kg/day)
BMDL(mg/kg/day)
BMD(mg/kg/day)
BMDL(mg/kg/day)
BMD(mg/kg/day)
BMDL(mg/kg/day)
Tumor 104 weeks 7.74 5.96 6.80 2.22104
weeks1.92 1.21 0.88 0.14
Hyperplasia
10weeks
1.36 1.04 0.42 0.32104
weeks1.63 1.04 0.74 0.14
104 weeks 1.97 1.61 0.93 0.66
BrdU labeling (proliferation)
10 weeks 0.65 0.29 0.54 0.07 Not determined. Available data not suitable for modeling.
Cytotoxicity
3 weeks
0.68 0.18 0.31 0.02
No reliable dose-response data available
10 weeks 0.02 0.008 0.002 0.0007
Cacodylic Acid: Summary of benchmark dose estimates and lower 95% confidence limits for cytotoxicity, BrdU labeling index,
hyperplasia and tumor data. (Doses in mg/kg/day)
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• Estimate target tissue dose using various dose metrics (e.g., DMAV, DMAIII or TMAO concentration in urine or bladder tissue) associated with bladder tumor development using DMAV PBPK model.
• Current status: Agency developing mouse model first then scale to rats and humans
• Use PBPK model to estimate the environmental exposure to DMAV required to achieve the same target tissue dose to bladder.
PBPK Model Application to DMA Risk Assessment
DMAV
oral exposure
metabolism
elimination
Q: What human exposure to DMAV is required to produce the same target tissue dose of DMA to bladder that results in tumors in rats exposed to DMAV?
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SummarySummary
Human Relevance Framework Identify key events Assist in dose response assessment Assist in rodent to human extrapolation Promote harmonization of risk assessment
for all endpoints