May 4 - 6, 2004 Charge/Questions to the Panel
April 19, 2004
FIFRA SCIENTIFIC ADVISORY PANEL (SAP)
MAY 4-6, 2004
FIFRA SAP WEB SITE http://www.epa.gov/scipoly/sap/
OPP Docket Telephone: (703) 305-5805
Docket Number: OPP-2004-0099
TUESDAY, MAY 4, 2004
Holiday Inn Rosslyn at Key Bridge
1900 North Fort Myer Drive, Arlington, VA 22209
CONSULTATION ON DERMAL SENSITIZATION
ISSUES FOR EXPOSURES TO PESTICIDES
CHARGE/QUESTIONS TO THE PANEL
Dermal sensitization, also known as allergic contact dermatitis (ACD) is typically characterized by two phases, termed induction and elicitation. In the induction phase, the allergen is transported to regional draining lymph nodes where clonal expansion of allergen-specific T lymphocytes results. The elicitation phase results from a subsequent exposure to the allergen, in which the allergen-specific T-lymphocytes provoke a cutaneous immune response. Although several approaches have been proposed to assess threshold concentrations for induction and elicitation of ACD and risk determination for these concentrations, there is no established scientific approach within the Agency to do a quantitative risk assessment associated with ACD.
There are several accepted methods for hazard identification of dermal sensitization, including the Buehler occluded patch test, the guinea pig maximization test, and the murine local lymph node assay (LLNA). The guinea pig maximization test as well as the Buehler test, while providing reliable information on skin sensitization, are best suited for hazard identification. Several proposals have been published regarding quantitative determination of sensitization induction and elicitation thresholds.
ISSUE 1: Quantitative Risk Assessment for the Induction Phase of ACD
The Mouse Local Lymph Node Assay (LLNA) is a test method for assessing the allergic contact dermatitis (skin sensitization) potential of chemicals, specifically the induction phase of sensitization. Using the incorporation of radiolabeled thymidine or iododeoxyuridine into DNA, the LLNA measures lymphocyte proliferation in the draining lymph nodes of mice topically exposed to the test article. The stimulation index (ratio of lymphocyte proliferation in treated mice compared to controls) is used as the indicator of potential sensitization. In 1998, following review by the FIFRA SAP, the LLNA was incorporated as a screening test in OPPTS Test Guideline 870.2600 Skin Sensitization. In 1999, the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM) Immunotoxicity Working Group (IWG) endorsed the LLNA as an acceptable alternative to currently accepted guinea pig test methods for hazard identification of chemicals with potential to produce contact hypersensitivity. Following additional studies to validate the method, the LLNA was endorsed by the SAP in December 2001 as a full stand-alone assay. The OPPTS guideline 870.2600 (Skin Sensitization) has been revised to include the LLNA as a stand-alone assay for appropriate applications. The OPPTS guideline has also been harmonized with OECD's Guideline 429 for LLNA, which was adopted in April 2002. Although the LLNA has not been validated for determination of sensitization potency, approaches for determination of quantitative assessment of sensitization induction thresholds have been proposed in the scientific literature (Gerberick 2000, 2001; Griem et al., 2003).
Gerberick (2000, 2001) proposed a methodology for determination of a ‘sensitization reference dose’ for sensitizers in consumer products. The lower boundary of the potency category for a sensitizing chemical is used as the starting point, with application of uncertainty factors for interindividual variability, product matrix effects, and use pattern. This approach was applied to the fragrance component cinnamic aldehyde and the preservative methylchloroisothiazolinone/methylisothiazolinone for which both LLNA and human sensitization potency were available (Griem et al., 2003).
Griem et al (2003) proposed a quantitative approach in which identification of known human sensitizing chemicals used both an EC3 value ( defined as the concentration of a sensitizer required to generate a threefold stimulation of proliferation in draining lymph nodes) from an LLNA test and a NOAEL or LOAEL from human repeat insult patch tests (HRIPT) or human maximization tests (HMT). The reported concentrations were converted into specific and molar area doses. Comparison of the area doses of the LLNA and human test results indicated that sensitization thresholds were similar in mice and humans despite the fact that the area doses for different chemicals ranged over several orders of magnitude (Griem et al., 2003). It was concluded from this analysis that the LLNA EC3 value is a useful measure of sensitizing potency in humans, and that the EC3 value can be used as a surrogate value for the human NOAEL that can be used as a starting point in quantitative risk assessment.
Uncertainty factors to account for interspecies variation, intraspecies variation, product matrix effects, and conditions of exposure (including repeated exposures) have been proposed for use in conduct of dermal risk assessments. Griem et al. (2003) have discussed the application and magnitude of all of these uncertainty factors with respect to establishment of safe area doses for both induction and elicitation, while Felter et al (2003) have proposed the use of only the intraspecies variation factor, product matrix factor, and exposure conditions factor for determination of safe area doses for induction. The interspecies uncertainty factor is intended to account for differences in response between tests in animals and results in humans, although it has been reported (Griem et al, 2003) that sensitizing area doses are very similar between murine and human data, thus supporting a potentially reduced uncertainty factor for this area. The intraspecies uncertainty factor is used to account for inherent variability in the human population based on age, sex, genetic makeup, or health status, and is generally agreed that a factor of 10 is appropriate for this uncertainty. An uncertainty factor may also be included for vehicle matrix effects, as the matrix in which an allergen is presented to the skin may have an influence on the potential for induction of ACD. Most experimental data are generated using simple vehicles, while actual exposures are usually to more complex formulations that may contain irritants or penetration enhancers. A factor of 10 may considered in such a case, while a reduced factor may be considered for mild formulations. Finally, an uncertainty factor may be applied to account for exposure variables that may influence the potential for induction of ACD, including the site of the body exposed, the integrity of the skin, and the potential for multiple exposures. Using the above approaches, a maximum uncertainty factor of 1000 or 3000 could be derived depending on the criteria used. By contrast, a minimum uncertainty factor of 10 could be derived if results from human studies are used.
Thresholds for induction of ACD can occur following a single exposure of sufficient magnitude, after contact with a large area of skin, or as a consequence of repeated skin applications (Marzulli and Maibach). Griem et al. (2003) suggested a possible higher sensitizing potency of a chemical upon repeated exposures. This would make sense in the case of hexavalent chromium, as the significant irritancy of the chemical could lend itself to an increased sensitizing potency by allowing more chemical to penetrate the stratum corneum.
QUESTION 1: What are the strengths and weaknesses of the proposed quantitative approach for determination of induction thresholds to dermal sensitizing chemicals? What other approaches does the Panel recommend EPA consider? Which uncertainty factors does the Panel feel are the most appropriate for application to quantitative methods of induction threshold determination? What factors should be included in the determination of the magnitude of each uncertainty factor?
ISSUE 2: Quantitative Risk Assessment for the Elicitation Phase of ACD
Several proposals have been published regarding determination of elicitation thresholds in sensitized populations. The Minimum Elicitation Threshold (MET) concept has been discussed in previous publications (Nethercott et al., 1994; Zewdie, 1998; NJDEP, 1998; Basketter et al., 2003) specifically with respect to hexavalent chromium. The concept behind the MET is that there is an ‘elicitation threshold’ below which no sensitization reaction is expected; thus, the MET is analogous to an RfD (Horowitz and Finley, 1994). The setting of an MET is usually performed as a result of tests in previously sensitized individuals; thus, the MET is considered protective of elicitation reactions. However, there has not been an extensive discussion of the criteria for employing this concept for purposes of risk assessment.
What are the strengths and weaknesses of the proposed quantitative approaches for determination of elicitation thresholds to dermal sensitizing chemicals? What other approaches does the Panel recommend that EPA consider? Which uncertainty factors does the Panel feel are the most appropriate for application to quantitative methods of elicitation threshold determination? What factors should be included in the determination of the magnitude of each uncertainty factor?
ISSUE 3: Children Sensitivity
Paustenbach et al. (1992) and Felter et al. (2002) have discussed the issue of whether children are more or less at risk for development of ACD. Paustenbach et al. addressed this issue specifically for hexavalent chromium, and this paper concluded that risk to children ages 3 to 8 is not likely to be greater than adults as there is no evidence that repeated exposures to hexavalent chromium places a person at greater risk of sensitization. Felter et al. suggested that infants and children may actually be at lower risk for development of ACD based on data gathered from dinitrochlorobenzene and pentadecylcatechol (poison ivy allergen). However, it is also understood that young children may not have been exposed to different allergens as compared to adults. In addition, increased frequency of exposure in children may increase the chance of induction to different allergens.
Does the Panel agree that the available scientific data suggest no significant difference in the relative sensitivity of children vs. adults to the induction and/or elicitation of ACD? If so, please provide scientific justification for this position. If the Panel disagrees, please provide scientific justification, including supporting data and/or uncertainties in the explanation.
ISSUE 4: Case Example - Cr(VI) in treated wood
Data from murine LLNA tests as well as from human patch testing studies using hexavalent chromium are available in the scientific literature. Results of LLNA testing show EC3 values that indicate area doses that result in the induction of sensitization in the mouse, while the results of patch test studies in humans show area doses that result in elicitation of sensitization in already sensitized individuals. In the Agency’s initial assessment seeking to assess dermal sensitization risk from hexavalent chromium, the lowest dose tested (0.018 ug/cm2) from the human patch test study of Nethercott et al (1994) was selected for determination of dermal risk from hexavalent chromium. A 10x uncertainty factor (3x for use of the lowest dose tested [LOAEL] in this study, and 3x to account for the small size of the study population in the Nethercott study) was applied, resulting in a ‘safe area’ dose of 0.0018 ug/cm2. Use of the test data of Basketter et al. (2001) and Hansen et. al (2003) also result in derivation of similar ‘safe’ area doses of 0.001 and 0.003 µg/cm2 respectively. Use of the murine LLNA test data and application of an uncertainty factor of either 1000 or 3000 calculated ‘safe’ area doses of 0.01 or 0.003 µg/cm2 respectively.
QUESTION 4: Please comment on the methods used for derivation of ‘safe’ area doses using the available LLNA data and the human patch test data, including the magnitude of the applied uncertainty factors, and include a scientific rationale in support of your position. Please comment on whether it is scientifically supportable to derive separate ‘safe’ area doses for protection against induction of dermal sensitization as well as elicitation in sensitized individuals by hexavalent chromium?
For further information, please contact the Designated Federal Official for this meeting, Mr. Paul Lewis, via telephone: (202) 564-8450; fax: (202) 564-8382; or email: firstname.lastname@example.org