Skin sensitization thresholds: determination in predictive models

Quantitative Risk Assessment of Dermal Sensitization Potential Following Use of Shampoo Products Containing the Formaldehyde Releasing Preservative DMDM Hydantoin

Historically, formaldehyde was used as a preservative in personal care products to extend product shelf-life; however, given its skin sensitization potential it has been phased out of use and replaced with formaldehyde-releasing preservatives, such as Dimethyloldimethyl hydantoin (DMDMH). A relationship has been established between positive patch test results following exposure to DMDMH and previous sensitization to formaldehyde. Upon direct contact with the skin, formaldehyde can react with skin proteins and cause an acute inflammatory reaction, which may progress to skin sensitization following repeated exposure. This quantitative risk assessment (QRA) aimed to assess the risk of skin sensitization induction following use of shampoo products containing the maximum allowable concentrations of DMDMH in formulation (1% w/v), translating to a free formaldehyde concentration of 0.02%. To determine a margin of safety (MOS) for exposure to DMDMH from use of shampoo products, consumer exposure levels (CEL) were estimated based on typical use scenarios and then benchmarked against an acceptable exposure level (AEL). The AEL was derived using a weight of evidence approach where a range of no expected sensitization induction levels (NESILs) was utilized. The MOS values for a shampoo product containing 1% DMDMH (.02% formaldehyde) was above 1 for the typical use scenario indicating a low likelihood of skin sensitization induction among healthy individuals. Thus, it can be concluded that shampoo products containing DMDMH at or below current allowable concentrations are not expected to increase the risk of skin sensitization induction.

Dose-Response Relationships and Threshold Levels in Skin and Respiratory Allergy

A literature study was performed to evaluate dose-response relationships and no-effect levels for sensitization and elicitation in skin- and respiratory allergy. With respect to the skin, dose-response relationships and no-effect levels were found for both intradermal and topical induction, as well as for intradermal and topical elicitation of allergenic responses in epidemiological, clinical, and animal studies. Skin damage or irritation may result in a significant reduction of the no-effect level for a specific compound. With respect to the respiratory tract, dose-response relationships and no-effect levels for induction were found in several human as well as animal studies. Although dose-response relationships for elicitation were found in some epidemiological studies, concentration-response relationships were present only in a limited number of animal studies. Reported results suggest that especially relatively high peak concentrations can induce sensitization, and that prevention of such concentrations will prevent workers from developing respiratory allergy. Moreover, induction of skin sensitization may result in subsequent heightened respiratory responsiveness following inhalation exposure. The threshold concentration for the elicitation of allergic airway reactions in sensitized subjects is generally lower than the threshold to induce sensitization. Therefore, it is important to consider the low threshold levels for elicitation for recommendation of health-based occupational exposure limits, and to avoid high peak concentrations. Notwithstanding the observation of dose-response relationships and no-effect levels, due to a number of uncertainties, no definite conclusions can be drawn about absolute threshold values for allergens with respect to sensitization of and elicitation reactions in the skin and respiratory tract. Most predictive tests are generally meant to detect the potential of a chemical to induce skin and/or respiratory allergy at relatively high doses. Consequently, these tests do not provide information of dose-response relationships at lower doses such as found in, for example, occupational situations. In addition, the observed dose-response relationships and threshold values have been obtained by a wide variety of test methods using different techniques, such as intradermal exposure versus topical or inhalation exposure at the workplace, or using different endpoints, which all appear important for the outcome of the test. Therefore, especially with regard to respiratory allergy, standardized and validated dose-response test methods are urgently required in order to be able to recommend safe exposure levels for allergens at the workplace.

Dose and airflow dependence of benzyl alcohol disposition on skin

The penetration of benzyl alcohol (BA) through split-thickness cadaver skin was measured in nonoccluded Franz cells placed in a fume hood. BA, dissolved in a small volume of ethanol and spiked with (14)C radiolabel, was applied to skin at nine doses ranging from 0.9 to 10600 microg/cm(2). The percentage of radioactivity penetrated after 24 h increased gradually with dose, ranging from 19.8 +/- 2.9% at the lowest dose to 29.2 +/- 3.0% at the highest. Less than 4% of the radioactivity was retained in the tissue at 24 h; the remainder was considered to be evaporated. These data and those from a previous study were analyzed in terms of a finite dose diffusion/evaporation model. The analysis showed that the increase in BA absorption with dose was consistent with a threefold increase in BA diffusivity in the stratum corneum, as its concentration increased from tracer levels to saturation. The variable diffusivity model was able to describe the combined observations from the two studies to within an rms error of 4.2% of dose. A method of estimating the diffusion model parameters independently of the experiment was found to yield good agreement with the experimentally-derived values at low and moderate doses.

Quantitative comparison of the results obtained by the multiple-dose guinea pig maximization test and the non-radioactive murine local lymph-node assay for various biocides

We compared the results of the multiple-dose guinea pig maximization test (GPMT) and the non-radioactive murine local lymph-node assay (LLNA) for various biocides. Thirteen out of 17 positive biocides in the GPMT gave positive results in the LLNA. In the GPMT, the minimum first induction doses ranged over four orders (0.00005-0.5%), while elicitation-threshold doses, which were evaluated using an optimally sensitized group of animals in the multiple-dose studies, ranged over five orders (0.00006-2.8%). In the LLNA, minimum induction doses ranged over more than three orders (0.01-30%). With respect to 13 biocides that were positive in both the GPMT and the LLNA, results were quantitatively compared. When compared after conversion to corresponding area doses (microg/cm), the minimum doses required to elicit skin reaction in guinea pigs were always lower than that for induction in mice with all biocides. Correlation between minimum induction doses from the GPMT and the LLNA seemed poor (r=0.57), while that between minimum induction doses in the LLNA and elicitation-threshold doses in the GPMT was relatively good (r=0.73). The results suggest the possibility to estimate human elicitation-threshold doses, which are definitely lacking in the process of risk assessment for skin-sensitizers, from the data of the LLNA.

Quantitative patch and repeated open application testing in methyldibromo glutaronitrile-sensitive patients

Contact allergy to methyldibromo glutaronitrile (MDBGN), often combined with phenoxyethanol (PE) (e.g., Euxyl K 400), increased throughout the 1990s in Europe. Consequently, in 2003, the European Commission banned its use in leave-on products, where its use concentration was considered too high and the non-sensitizing use concentration as yet unknown. The 2 objectives of the study are (a) to find a maximum non-eliciting concentration in a leave-on product in MDBGN/PE-sensitized patients, which could possibly also be considered safe regarding induction and (b) to find the best patch test concentration for MDBGN. We, therefore, performed a use-related test (ROAT) in patients sensitized to MDBGN/PE (n = 39) with 3 concentrations of MDBGN/PE (50, 100 and 250 p.p.m. MDBGN, respectively). A subset of these patients (n = 24) was later patch-tested with various concentrations (0.1, 0.2, 0.3 and 0.5% MDBGN, respectively). 15 patients (38%, 95% confidence interval (CI) = 23-55%) had a negative and 24 (62%; 95% CI = 45-77%) a positive overall repeated open application test (ROAT) result. 13 reacted to the lowest (50 p.p.m.), 8 to the middle (100 p.p.m.) and 3 to the highest concentration (250 p.p.m.) only. In those 13 reacting to the lowest ROAT concentration, dermatitis developed within a few days (1-7). The strength of the initial and the confirmatory patch test result, respectively, and the outcome of the ROAT were positively associated. Of the 24 patients with a use and confirmatory patch test, 15 reacted to 0.1% MDBGN, 16 to 0.2%, 17 to 0.3% and 22 to 0.5%. With the patch test concentration of 0.5%, the number of ROAT-negative patients but patch-test-positive patients increases considerably, particularly due to + reactions. A maximum sensitivity of 94% (95% CI = 70-100%) is reached with a patch test concentration of 0.2%, and is not further improved by increasing the concentration. However, the specificity decreases dramatically from 88 (95% CI = 47-100%) with 0.2% to a mere 12.5% (95% CI = 0-53%) with 0.5%. It can be concluded (a) that for MDBGN 0.2% is very likely the best patch test concentration and (b) that 50 p.p.m. in a leave-on product can elicit contact dermatitis in sensitized persons. We were, therefore, unable to find a safe, still microbicidal, concentration for leave-on products. By contrast, with other contact allergens, dose-response use tests may be able to identify a non-eliciting concentration, which could give valuable clues to a non-inducing (i.e., safe) concentration in products.

Thresholds of elicitation depend on induction conditions. Could low level exposure induce sub-clinical allergic states that are only elicited under the severe conditions of clinical diagnosis?

While numerous studies have examined dose/response relationships occurring in the experimental induction of contact allergic dermatitis, fewer have examined the effects of varying the doses of both induction and challenge. Recently published studies have however done this and they all show the same remarkable observation: the threshold of elicitation decreases as the doses used to induce the allergy increase. This has important implications. One is that it may be more complicated to determine clear threshold doses below which allergic responses are not seen. It is also proposed that normal exposure to weak allergens such as some fragrance materials may induce "sub-clinical" allergic states which will not be elicited under these same exposure conditions but which may become apparent under the more severe conditions of clinical diagnosis. This may explain why the prevalence of Patch test reactions to some fragrance materials is apparently increasing in the absence of any clearly documented "epidemic" of consumer complaints.

Proposal for a risk assessment methodology for skin sensitization based on sensitization potency data

In this paper, we propose a quantitative risk assessment methodology for skin sensitization aiming at the derivation of 'safe' exposure levels for sensitizing chemicals, used e.g., as ingredients in consumer products. Given the limited number of sensitizers tested in human sensitization tests, such as the human repeat-insult patch test (HRIPT) or the human maximization test (HMT), we used EC3 values from the local lymph node assay (LLNA) in mice because they provide the best quantitative measure of the skin sensitizing potency of a chemical. A comparison of LLNA EC3 values with HRIPT and HMT LOEL, and NOEL values was carried out and revealed that the EC3, expressed as area dose, can be used as a surrogate value for the human NOEL in risk assessment. The uncertainty/extrapolation factor approach was used to derive (a) an 'acceptable non-sensitizing area dose' (ANSAD) to protect non-allergic individuals against skin sensitization and (b) an 'acceptable non-eliciting area dose' (ANEAD) to protect allergic individuals against elicitation of allergic contact dermatitis. For ANSAD derivation, interspecies, intraspecies and time extrapolation factors are applied to the LLNA EC3. For ANEAD derivation, additional application of a variable sensitization-elicitation extrapolation factor is proposed. Values for extrapolation factors are derived and discussed, the proposed methodology is applied to the sensitizers methylchloroisothiazolinone/methylisothiazolinone, cinnamic aldehyde and nickel and results are compared to published risk assessments.

Nickel, chromium and cobalt in consumer products: revisiting safe levels in the new millennium

The transition metals nickel (Ni), chromium (Cr) and cobalt (Co) are common causes of allergic contact dermatitis (ACD). Given the high frequency with which these allergens can be associated with hand eczema in those responsible for domestic work, it has been suggested that contamination of household consumer products with these metals may be of relevance to the causation/chronicity of hand dermatitis. Dose-response studies using 48 h occlusive patch test conditions in sensitized individuals show that >/=90% of sensitized patients fail to react below 1 p.p.m., even on irritated skin. Assessment under more realistic exposure conditions has shown that in the presence of irritants and/or following repeated exposures, such individuals rarely react to levels below 10 p.p.m. On the basis of this information, it was recommended a decade ago that household (and other consumer) products should not contain more than 5 p.p.m. of each of Ni, Cr or Co and that, for an even greater degree of protection, the ultimate target level should be 1 p.p.m. The data generated since the original recommendations were made serve to reinforce the validity of these recommendations. Indeed, it is our view that typically the level of each of these transition metals should not normally exceed 1 p.p.m. Then, where consumer products meet this guideline fully, modern quantitative risk assessment shows clearly that elicitation of ACD is highly improbable, and the chance of the induction of sensitization is even lower.

Application of the risk assessment paradigm to the induction of allergic contact dermatitis

The National Academy of Science (NAS) risk assessment paradigm has been widely accepted as a framework for estimating risk from exposure to environmental chemicals (NAS, 1983). Within this framework, quantitative risk assessments (QRAs) serve as the cornerstone of health-based exposure limits, and have been used routinely for both cancer and noncancer endpoints. These methods have focused primarily on the extrapolation of data from laboratory animals to establish acceptable levels of exposure for humans. For health effects associated with a threshold, uncertainty and variability inherent in the extrapolation process is generally dealt with by the application of "uncertainty factors (UFs)." The adaptation of QRA methods to address skin sensitization is a natural and desirable extension of current practices. Based on our chemical, cellular and molecular understanding of the induction of allergic contact dermatitis, one can conduct a QRA using established methods of identifying a NOAEL (No Observed Adverse Effect Level) or other point of departure, and applying appropriate UFs. This paper describes the application of the NAS paradigm to characterize risks from human exposure to skin sensitizers; consequently, this method can also be used to establish an exposure level for skin allergens that does not present an appreciable risk of sensitization.

Evidence for immunotoxic effects of crude Ginkgo biloba L. leaf extracts using the popliteal lymph node assay in the mouse

Allergic reactions due to contact with different parts of the ancient tree Ginkgo biloba L. have repeatedly been reported. Provocation tests in patients and animal experiments have identified alkylphenols such as ginkgolic acids as causative constituents. Leaf extracts from Ginkgo are widely used to treat peripheral or cerebral circulatory disorders and Alzheimer's disease. Since alkylphenols are also present in leaves, potential allergic and other immunological hazards of such preparations have to be carefully controlled. Thus, we have evaluated if the popliteal lymph node assay (PLNA) in the mouse may represent a suitable model for the detection of constituents with immunotoxic properties in a complex mixture of biologically active agents such as plant extracts. Subplantar injection (2 mg) of a crude aqueous-ethanolic extract from Ginkgo leaves caused a significant lymphoproliferative reaction (LPR) in the ipsilateral popliteal lymph node. PLNA-active compounds in this extract could be enriched in the lipophilic phase by liquid-liquid partition between heptane and water. Chemical analysis of the heptane extract revealed the presence of a high concentration of alkylphenols (approx. 30%) and further subfractionation indicated that the enlargement of the popliteal lymph node was mainly due to the content of ginkgolic acids. This presumption was corroborated by observing a similar LPR following injection of a purified mixture of ginkgolic or hydroginkgolic acids. Thus, our experiments confirm that Ginkgo leaf extracts may contain constituents with immunotoxic properties, underlining the need to apply adequate production procedures to guarantee the completest possible removal of these compounds. The PLNA appears to represent a simple test model for the detection, characterisation and control of ingredients with potential immunotoxic side effects in complex herbal drugs.

A comparison of statistical approaches to the derivation of EC3 values from local lymph node assay dose responses

Effective risk assessment and management of allergic contact dermatitis require three key factors: adequate hazard identification, measurement of the relative potency of identified hazards and an understanding of the nature, extent and duration of exposure. Suitable methods for hazard identification, such as the murine local lymph node assay (LLNA) and the guinea-pig maximization test, are well established and conditions of human exposure normally can be well anticipated. Thus, the need is for a robust and quantitative method for the estimation of relative skin sensitizing potency. One possible approach is via the analysis of LLNA dose-response data. In the LLNA, contact allergens are defined currently as those chemicals that cause a threefold or greater increase in lymph node cell proliferative activity compared with concurrent vehicle-treated controls. It is possible to estimate the concentration of a sensitizer required to generate a threefold stimulation of proliferation in draining lymph nodes; such a concentration is known as the EC3 value. Using a variety of statistical approaches to derive EC3 values from LLNA dose-response data for 10 chemicals, it has been demonstrated that simple linear interpolation between the values either side of the threefold stimulation index provides a robust assessment of the EC3 value without the need for recourse to more sophisticated statistical techniques. Provided that the appropriate concentrations of test chemical have been selected, EC3 values obtained in this way are reproducible both within and between laboratories and form the basis for examination of the utility of this approach for the estimation of relative skin sensitizing potency.

Thresholds in contact sensitization: theoretical and practical considerations

The existence of thresholds for both the induction and the elicitation of contact sensitization is an important issue for hazard assessment in this area of toxicology. In this paper, the evidence for such thresholds has been reviewed from both theoretical and practical perspectives. When the mechanisms of skin sensitization are considered, the existence of thresholds can be shown to occur at several stages. They are both quantitative, for example in terms of the degree of protein haptenation and in the sufficiency of the signals for Langerhans cell migration and maturation, as well as qualitative, in terms of the type of immune response that is engaged. Such considerations are evidenced by a substantial body of practical observation. In humans and in animal models of skin sensitization there is abundant evidence for the existence of thresholds for both the induction and the elicitation of reactions. In addition to, and in distinction from, the experimental situation, in the general human population there is extensive evidence for threshold effects. This evidence arises directly from the observation that only a proportion of those who are exposed become sensitized (i.e. are patch test positive), and of that latter group only a proportion develop allergic contact dermatitis.

Skin sensitization risk assessment: a comparative evaluation of 3 isothiazolinone biocides

Many of the chemicals in common use possess, to some degree, a capacity to cause skin sensitization. Consequently, it is important to conduct a thorough and accurate risk assessment when it can be anticipated that such chemicals are likely to come into contact with human skin. Such risk assessments must consider the nature and extent of dermal exposure together with the sensitizing potency of the chemical. Whilst the exposure elements of the risk assessment process are often well understood, or can at least be reasonably predicted on the basis of the proposed usage of the chemical, the determination of skin sensitizing potency may be problematic. In this paper, we describe an approach to the determination of relative potency of 3 isothiazolinone biocides. Initially, the local lymph node assay (LLNA) provides an estimate of relative sensitizing potency. Where appropriate, this can be followed by confirmatory testing in the human repeat insult patch test (HRIPT). The data generated reveal that (chloro)methyl isothiazolinone (CMI) is a significantly stronger sensitizer than either methyl trimethylene isothiazolinone (MTI) or benzisothiazolinone (BIT). Furthermore, both the LLNA and the HRIPT data show consistently that BIT is a somewhat weaker skin sensitizer than MTI. As an important additional component of the safety evaluation, the ability of these sensitizing isothiazolinones to cross-react has been considered; the data indicate that the more weakly sensitizing isothiazolinones do not elicit allergic reactions in subjects sensitized to MCI. The implications for use of these biocides in various product types are discussed.

Acute irritant reactivity to sodium lauryl sulfate in atopics and non-atopics

Predictive testing of chemicals to assess their acute skin irritation potential is an important part of the assessment of their toxicological profile. It is possible, where safety and ethical considerations can be met, to do this work in groups of human volunteers. Previously, the relative responsiveness of atopics and non-atopics has been evaluated. The results showed that atopics (defined broadly by high IgE reactivity) were a little more susceptible to skin irritation, but not significantly so. In the present work, the relative reactivity of a skin atopic group versus a non-atopic group was examined in more detail. Sodium lauryl sulfate (SLS) was applied at a range of concentrations and exposure times, such that a fairly constant degree of skin irritation was produced. At various time points, the irritation response was measured by visual assessment, chromametry, laser Doppler flowmetry and transepidermal water loss. Using all of the methods of assessment, the reactions in atopics were similar to or a little less than those seen in non-atopics. The conclusion is that atopics and non-atopics will give similar results in a predictive human test for acute skin irritation. Furthermore, the pattern of response obtained from short duration exposure should be predictive of that following longer durations of (single) exposure.