Postulated Mode of Action by mikeholy

VIEWS: 7 PAGES: 37

									Mode of Action/ Human Relevance
Analysis For Incorporating Mechanistic
Data in Human Health Risk


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




                                   May 2006

                                      Slide 1 of 38
          Uses of Mechanistic Data
            in 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
                                                    Slide 2 of 38
 “Mechanism of                       Exposure
     action”
                           Key event
  (more detailed
 understanding at
  biochemical &
 molecular level)
                           Key event

       versus

  “Mode of action‖
(identification of key   Key event
& obligatory steps)
                                 Toxicity
                                                Slide 3 of 38
        Mode 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


                                                             Slide 4 of 38
    Mode 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


                                                                Slide 5 of 38
                       Human Relevance
                         Framework
   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)

                                                                           Slide 6 of 38
                    Human Relevance
                      Framework
   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?




                                                                Slide 7 of 38
           Human Relevance
              Framework
                                               Relevant or
           1.) Is the Weight of Evidence        Unknown
              Sufficient to Establish a          Human
                   MoA in Animals?             Relevance
                                    Not
Not                               Sufficient
                       Sufficient
Relevant
in
Humans           2.) Is the Animal                           Continue with
                MoA Plausible in
                     Humans?
                                                               the Dose
              No                                             Response &
                         Yes                                  Exposure
                                                             Assessment
              3.) Taking Into Account
               Kinetic and Dynamic
               Factors, Is the Animal
                  MoA Plausible in
                No, Humans? Yes




                                                                   Slide 8 of 38
Assessing 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
                                               Slide 9 of 38
Assessing 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


                                                     Slide 10 of 38
    Assessing an Animal Mode of
    Action 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



                                                Slide 11 of 38
MOA: Inform Human Relevance


 ―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.‖



                                              Slide 12 of 38
MOA: Inform Dose Response Extrapolation

      Alachlor - ―. . . a margin of exposure (MOE)
      approach (indicative of a non-linear dose
      response) should be used for the risk
      assessment.‖
          Chloroform – ―. . . a nonlinear approach is
          more appropriate for low-dose extrapolation.‖


             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. . .‖
                                                 Slide 13 of 38
    Case Study
   Cacodylic Acid
(Dimethylarsinic acid)




                         Slide 14 of 38
           DMAV 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/cacodyl
        ic_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

                                                       Slide 15 of 38
          Metabolism of Arsenic

                           Pesticide
                           Chemical


Alternate steps of
oxidative methylation
& reduction
      Methylation
                          TMAsV
      Reduction

                            TMAsIII
DMAV: MODE OF ACTION ANALYSIS


     Weight of Evidence


    Extensive experimental cellular and
     laboratory animal data



                                           Slide 17 of 38
DMAV:             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)

                                                        Slide 18 of 38
              Mode of Action
     Measurable Key Events in Target Tissue

               DMAIII
              Metabolite


                                          Urinary bladder from a female F344 rat
              Urothelial                  treated with 100 ppm DMAV
               Toxicity

Sustained
             Regenerative
             Proliferation        BrdU Labeling




                              Urinary
              Hyperplasia
                              Bladder
                              Tumors
                                                              Slide 19 of 38
                       Compensatory regeneration in rat bladder at
                       weeks 8 & 10 following ingestion of DMAV



                           Cell Proliferation Response (BrDU labeling Indices)


Hour      Hour      Day 3      Week      Week      Week      Week      Week      Week      Week      Week
6         24                   1         2         2         2         10        10        20        26*
Cohen     Cohen     Cohen      Cohen     Cohen     Cohen     Cohen     Arnold    Cohen     Arnold    Cohen
et al.,   et al.,   et al.,    et al.,   et al.,   et al.,   et al.,   et al.,   et al.,   et al.,   et al.,
2001      2001      2001       2001      2001      2001      2002      1999      2001      1999      2002
---       0.42 ±    0.23 ±     0.44 ±    0.22 ±    0.19 ±    0.16 ±    0.22 ±    0.18 ±    0.25 ±    0.13 ±
          0.05      0.04       0.09      0.03      0.04      0.02      0.05      0.03       0.03     0.02


1X        1X        1.4X       2.2X      6.2X      4.9X      3.9X      4.2X      3.4X      3.9X      1.6X

0.22 ±    0.24 ±    0.33 ±     0.96 ±    1.36 ±    0.94 ±    0.63 ±    0.93 ±    0.61 ±    0.97 ±    0.21 ±
                                   a         a         a         a         a         a         a         a
0.04      0.04      0.11       0.14      0.13      0.20      0.10      0.11      0.10      0.11      0.03



Uncertainty expressed as ± S.E. of the mean in all studies
a
  P<0.05 when compared to respective controls
c
  All results in female rats



                                                                                                               Slide 20 of 38
             Association of Key Precursor Events & Bladder Tumors in F344 Rats
                                                                                                   Temporal
                             Dose                                                                                  Transitional
                                         Metabolism       Urothelial            Regenerative        Urothelial
                            (mg/kg                                                                                    Cell
                                        DMAVDMAIII        Toxicity             Proliferation      Hyperplasia
                            bw/day)                                                                                Carcinoma
Dose Response Concordance




                                0.2           +                +
                             (2 ppm)    (wk 3-0.03 ±   (wk 10-6/10,                   -                  -                -
                                        0.01 uM)       grade 3 or 4)

                                              +                +
                                1                      (wk 3-2/7, grade            slight
                            (10 ppm)    (wk 3-0.12 ±
                                                       3) (wk- 10; 8/10,      (wk 10-1.5X inc)
                                                                                                         -                -
                                        0.02 uM)
                                                       grade 3 or 4)


                                              +                +
                                4
                                        (wk 3-0.28 ±
                                                       (wk 3-7/7, grade               +                  +                -
                            (40 ppm)                   3) (wk 10-5/10,         (wk 10-4.3X inc )   (wk 10- 4/10)
                                        0.09 uM)
                                                       grade 3 or 4)


                                                               +
                                                       (6 hrs-6/7, grade 3)           +                                  +
                               9.4            +        (24 hrs-4/7, grade 3   (wk 1- 2.2X inc)
                                                                                                         +         (papilloma first
                            (100 ppm)   (wk 3-0.55 ±   or 4)                                       (wk 8-7/10)     obs at wk 107;
                                                       (wk 2 6/10, grade
                                                                              (wk 2-3.9X inc)
                                        0.15 uM)                                                   (wk 10-9/10)    carcinoma first
                                                       5)(wk 10-0/10,         (wk 10-4.2X inc)                     obs at wk 87)
                                                       grade 4 or 5)

                                                                                                                    Slide 21 of 38
       Cacodylic Acid: Key Events
         Temporal Relationship
       DMAIII  DMAV
                                                   Urinary bladder from a female F344
                                                       treated with 100 ppm DMAV
 6 hours   Urothelial Cytotoxicity

 1 Week    Regenerative Proliferation                         BrdU labeling




8-10 weeks Hyperplasia


104 weeks Tumors
                                     Urinary bladder
                                         tumors

                                                                      Slide 22 of 38
     Cacodylic Acid: Key Events
Cytotoxicity/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


                                                              Slide 23 of 38
             Characterization of
             Cacodylic 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




                                             Slide 24 of 38
     Chromosomal 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.


                                                                Slide 25 of 38
    Other Modes of Action or Key
              Events
   No other MOA with sufficient scientific
    support
       Direct DNA reactivity
       Formation of solids
       Changes in urinary chemistry & physiology




                                               Slide 26 of 38
          Mode 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)

                                                          Slide 27 of 38
Human Relevance of DMAV’s
     Mode of Action
       Metabolism to DMAIII

            Urothelial
           Cytotoxicity

          Regenerative
          Proliferation
                              ?

    Hyperplasia & Bladder tumors

                                   Slide 28 of 38
Concordance Analysis of Key Events in Rats &
Humans: Qualitative & Quantitative Plausibility
 Key Event            Rats       Humans
 Presence of                   Yes (based on
                      Yes
 DMAIII in urine                    Asi)
 Persistent           Yes         Possible
 cytototoxicity
 Persistent
 regenerative         Yes        Possible
 prolif/hyperplasia
 Bladder Tumors       Yes        Possible

                                            Slide 29 of 38
DMA V        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
                                              Slide 30 of 38
                 Dose response extrapolation
                          approach
   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

                                                                Slide 31 of 38
    Dose Response Considerations
   Cancer Guidelines describe a two-
    step dose-response process which
    separates
       Modeling the observable range of data
       Extrapolation to lower doses
                                                                       Key event
    Nonlinear extrapolation




                                                     Response
                                                               POD
       Preferred approaches
            PBPK Model--internal dosimetry at the
             target tissue
               – e.g. DMAIII
            BBDR Model—predict biological effect               Dose
               – e.g., two stage clonal growth
       Interim approach
            Identify a point of departure (POD)
             based on benchmark dose modeling
            Apply uncertainty and safety factors


                                                                        Slide 32 of 38
Quantitative
                                            No
Dose-response              MOA
                       Established?
Assessment
                               Yes
                                           1. Fit data in observable range
                Yes
  Use model               BBDR
                          model?           2. Linear extrapolation from POD


                                No

             Yes,
           nonlinear                             No
 RfD/RfC                MOA informs
 or MOE                   low-dose
                       extrapolation?
                                              Yes, linear
                                     (including mutagenic MOA)



                                                                    Slide 33 of 38
                  Benchmark Dose Modeling:
                   Regenerative Proliferation
                       Hill Model with 0.95 Confidence Level
                1.4

                1.2
Mean Response




                 1

                0.8

                0.6

                0.4

                0.2            BMD10
                      BMDL10
                 0
                       0       1       2   3      4   5    6   7


                                           dose                Slide 34 of 38
             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)

                                                            Feeding                                                               Drinking water

Biological                                  10%                                 1%                                      10%                                 1%
                  Duration                                                                     Duration
Event
                                 BMD                BMDL             BMD           BMDL                         BMD             BMDL             BMD           BMDL
                              (mg/kg/day)         (mg/kg/day)     (mg/kg/day)    (mg/kg/day)                 (mg/kg/day)      (mg/kg/day)     (mg/kg/day)    (mg/kg/day)

                                                                                                104
Tumor             104 weeks      7.74                5.96             6.80            2.22                       1.92             1.21           0.88               0.14
                                                                                               weeks

                    10
                                 1.36                1.04             0.42            0.32
                   weeks                                                                         104
Hyperplasia                                                                                                      1.63             1.04           0.74               0.14
                                                                                                weeks
                  104 weeks      1.97                1.61             0.93            0.66


BrdU labeling
                  10 weeks       0.65                0.29             0.54            0.07              Not determined. Available data not suitable for modeling.
(proliferation)


                     3
                                 0.68                0.18             0.31            0.02
                   weeks
Cytotoxicity                                                                                                   No reliable dose-response data available

                  10 weeks       0.02               0.008             0.002          0.0007




                                                                                                                                                     Slide 35 of 38
PBPK Model Application to DMA Risk Assessment
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?
           • Use PBPK model to estimate the environmental exposure to DMAV required
             to achieve the same target tissue dose to bladder.
                                 oral exposure
                                DMAV




                                                 metabolism


                                           elimination

• 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
                                                                                  Slide 36 of 38
                   Summary
   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 37 of 38

								
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