Slide 1 of 38 Office of Pesticide Programs Drs. Vicki Dellarco & Anna Lowit Health Effects Division...

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

of 38

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

of 38

“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

of 38

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

of 38

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

of 38

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)

of 38

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?

of 38

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

of 38

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

of 38

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

of 38

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

of 38

“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

of 38

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

of 38

Case StudyCase StudyCacodylic Acid Cacodylic Acid

(Dimethylarsinic acid)(Dimethylarsinic acid)

of 38

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

of 38

DMADMAVV: MODE OF ACTION ANALYSIS: MODE OF ACTION ANALYSIS

Extensive experimental cellular and laboratory animal data

Weight of Evidence

of 38

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)

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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.

of 38

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

of 38

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)

of 38

Urothelial Cytotoxicity

?

Human Relevance of DMAHuman Relevance of DMAVV’s ’s Mode of ActionMode of Action

Metabolism to DMAIII

RegenerativeProliferation

Hyperplasia & Bladder tumors

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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

of 38

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)

of 38

• 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?

of 38

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

Slide 1 of 38 Office of Pesticide Programs Drs. Vicki Dellarco & Anna Lowit Health Effects Division...

Documents

Transcript of Slide 1 of 38 Office of Pesticide Programs Drs. Vicki Dellarco & Anna Lowit Health Effects Division...