Skin sensitization--a critical review of predictive test methods in animals and man

Selective induction of cell-associated interleukin-1alpha in murine keratinocytes by chemical allergens

Cytokines may be useful tools to discriminate between irritant and allergic contact dermatitis. In the mouse only, it has been demonstrated by other, that contact sensitizers up-regulated keratinocytes-derived interleukin-1alpha (IL-1), macrophage inflammatory protein-2 and interferon induced protein 10 mRNAs. The purpose of this study was to investigate the possibility to use in vitro IL-1 production by a murine keratinocyte cell line for preliminary screening of chemicals for their irritant and/or allergic potential. We investigated the effects of five relevant skin allergens (dinitrochlorobenzene, oxazolone, nickel sulfate, penicillin G and eugenol), two skin irritants (benzalkonium chloride, and methylsalicilate) and two compounds with no sensitizing activity (glycerol and ethanol) on IL-1 production in HEL30 cells. Twenty four hours following treatment, both IL-1 release in conditioned media and cell-associated IL-1 were measured by a specific sandwich ELISA. Under our experimental conditions, only contact sensitizers were able to increase in a dose dependent fashion cell-associated IL-1, confirming the in vivo findings. Both skin irritants and allergens induced the release of IL-1, because of the irritative properties of both chemicals, while ethanol and glycerol failed to induce changes in IL-1 production, confirming the specificity of the proposed test. Taken together, these data indicate that it may be realistic to consider potential skin allergens those chemicals which are able to increase cell-associated IL-1, to consider skin irritants those chemicals which induce only IL-1 release, and to exclude as potential allergens or irritants those chemicals which fail to induce changes in IL-1 production.

Contact hypersensitivity to dicyclohexylcarbodiimide and diisopropylcarbodiimide in female B6C3F1 mice

Dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) are two commonly used coupling reagents in protein synthesis resulting in exposure of individuals in chemical and pharmaceutical industries as well as research laboratories involved in protein synthesis and recombinant DNA techniques. The objectives of these studies were to determine the irritation and sensitizing potential of these two compounds when applied topically to B6C3F1 mice. Sensitization potential was assessed by the Mouse Ear Swelling Test (MEST) and the murine Local Lymph Node Assay (LLNA). Concentrations used in the contact hypersensitivity assays were determined by primary irritancy studies. DCC and DIC were identified as both irritants and contact sensitizers with the MEST being a more sensitive indicator of sensitization potential. The MEST identified DCC as a sensitizer at concentrations as low as 0.006% (w/v) 24 hr and 48 hr post challenge and DIC at 0.3% (w/v) and 1.5% (w/v) 24 and 48 hr post challenge, respectively. In the LLNA, the lowest concentrations yielding a significant response were 0.06% (w/v) for DCC and 10% (w/v) for DIC.

Antiproliferative activity and phototoxicity of some methyl derivatives of 5-methoxypsoralen and 5-methoxyangelicin

The in vitro antiproliferative activity and in vivo phototoxicity of some methyl derivatives of 5-methoxypsoralen and 5-methoxyangelicin, i.e. 4,4'-dimethyl-5-methoxyangelicin (compound I), 3,4'-dimethyl-5-methoxyangelicin (compound II), 4,4'-dimethyl-5-methoxypsoralen (compound III); and 3,4'-dimethyl-5-methoxypsoralen (compound IV), have been investigated. The effects of the compounds were evaluated in vitro on HL60 and A431 cells, using 5-methoxypsoralen as the reference compound. In both cell lines compound I, II and III showed better antiproliferative activity than compound IV and 5-methoxypsoralen. Scanning electron microscopy revealed that all the compounds induced the formation of blebs and blisters on a A431 cell surface. Significant variations in the nuclear area strictly related to the toxicity of the compounds have been shown in both cell lines. Skin irritancy in vivo was evaluated by mean of histopathological responses on guinea-pig skin. For each compound a damage index was determined by morphometrical analysis of empty spaces in the epidermis. Histopathology revealed skin phototoxicity of compounds which lacked erythemogenic activity by visual scoring. By coupling cytotoxicity data in vitro to skin sensitization ones in vivo, compound I proved a promising candidate for use in clinical trials since due to a high inhibitory effect on the growth of human cell lines coupled to low skin phototoxicity.

Strategies for identifying false positive responses in predictive skin sensitization tests

It is important that predictive toxicological test methods are selective for their intended endpoint and that their limitations are understood and acknowledged. The local lymph node assay (LLNA) is a relatively new predictive test for skin sensitization potential that can replace traditional guinea pig tests and offers significant scientific and animal welfare advantages. However, there has been some concern that certain irritant materials may yield false positive results, although it must be emphasized that false positives also occur in guinea pig methods. Consequently, we have examined the performance in the LLNA of a range of skin irritants, from varying chemical classes and covering a range of irritation potency. The results presented here demonstrate clearly that the majority of skin irritants are negative in the LLNA. These results are reviewed in the context of the occurrence of false positive reactions in the guinea pig maximization test and the strategies for dealing with such results are discussed. The need for careful scientific evaluation of the results in all predictive tests for sensitization is thus emphasized. In terms of specificity, the LLNA has been more fully evaluated than other predictive test methods and is at least as accurate. In terms of animal welfare, objectivity, reproducibility and reliability it is superior to other methods. In summary, all predictive skin sensitization test results should be evaluated in a scientifically rigorous manner and the additional data provided herein further support the adoption of the LLNA as a complete replacement for the traditional guinea pig methods.

The guinea-pig skin sensitization test revisited: an evaluation formula to predict possible sensitization levels for eight chemicals used in household products

In predicting human skin sensitization due to possible risky chemicals, it is not sufficient to evaluate solely the minimum induction dose (MID) or the standard challenge dose (SCD) in the Guinea Pig Maximization Test (GPMT). Nakamura et al. (1994) (Nakamura, A., Momma, J., Sekiguchi, H., Noda, T., Yamano, T., Kaniwa, M., Kojima, S., Tsuda, M., Kurokawa, Y., 1994. A new protocol and criteria for quantitative determination of sensitization potencies of chemicals by guinea pig maximization test. Contact Dermatitis 31, 72-85) previously measured the residual dose of chemicals in the products implicated in human allergic accidents, and stated that '... the level of chemical in the products (direct exposure-dose = DED) was similar to or higher than value of sensitization potency.' However, several of the chemicals listed in their article, show an even lower value of sensitization potency than the DED, although a potential correlation between results of the GPMT and the DED was seemed to be evident; a key question about the essential rule of those parameters therefore remains open. Using the data of Nakamura et al. (1994), we analyzed the functional rules of the three independent parameters, the MID, the SCD, and the DED on which the GPMT is based. Calculations of the degree of allergic reactions elicited in humans provided a range of discrimination constants (D) using the formula; D = DED/(MID*SCD). Possible human allergic accidents may be predicted when the dose of a candidate chemical in a chemical product (equal to DED) exceeds the value; D*(MID*SCD), following the correct evaluation of the MID as well as the SCD.

Skin sensitization thresholds: determination in predictive models

For many years, test methods for the prospective identification of skin sensitizing chemicals have been widely available. However, although these techniques have permitted the identification of the great majority of skin sensitizers, their use in assessing the relative potency of a particular chemical as a human contact allergen has not been well described. A primary reason for this is the inherent difficulty of such an exercise. A complex phenomenon involving interactions between the vehicle, the allergen, the skin and its inflammatory responses takes place during the induction and elicitation of sensitization. All these factors can have a profound effect on the threshold values determined for a skin sensitizer. Consequently, whether the assessment is conducted in humans or in animal models, a threshold concentration is always a function of the method of measurement as much as the potency of the allergen. Although an exhaustive review has not been carried out, this paper considers the attempts that have been made to assess relative potency by the measurement of dose-response relationships and the determination of induction and elicitation thresholds in both animal models and in humans. The latter has special relevance for regulatory toxicology and this matter is given particular attention in this article. Finally, recommendations are made: (a) that threshold concentrations for skin sensitizers should be determined on a case by case basis in relation to the likely mode of skin contact; (b) where the data are used in comparisons of skin sensitization potency, then there should be standardization of the method used for the determinations.

An interlaboratory evaluation of the Buehler test for the identification and classification of skin sensitizers

The correct identification of potential skin sensitizers is an essential first step in enabling a proper risk assessment to be made and to permit the implementation of appropriate risk management practices designed to avoid the induction of sensitization. Consequently, regulatory guidelines around the world demand that new substances are evaluated to assess their skin sensitization potential. There are two guinea pig test methods which are generally recognised, the guinea pig maximisation test (GPMT) and the occluded patch test described by Buehler. In different countries, one procedure seems to be more prevalent and acceptable to regulatory authorities than the other. Notably, in the European Union, the latest revision of the Annex V (Directive 92/32/EC) Test Method for skin sensitization asks that justification should be given in the situation where the notifier does not use the GPMT, which is the preferred method. Thus in this paper, the validity of the Buehler protocol in the context of European legislation is critically examined. Results from two laboratories are collated, showing that the method can identify significant contact allergens, particularly those which would be registered formally as such according to European legislation. It is demonstrated that minor methodological variations can be tolerated without compromising test sensitivity, but it is recommended that suitable positive control testing is the best way to ensure proper test conduct.

Dichloronitrobenzene: a reappraisal of its skin sensitization potential

Unlike the closely related chemical dinitrochlorobenzene (DNCB), which is a very strong contact allergen, dichloronitrobenzene (DCNB) has been widely regarded as a non-allergen and, as such, a useful control for its strongly sensitizing counterpart. Nevertheless, it is still an organic chemical species readily capable of penetrating skin and, rather than being regarded as completely inert, it has even been suggested to react with the immune system in such a way that it induces specific tolerance to its chemical structure. We investigated whether DCNB was in reality a non-allergen, or rather merely a weak contact sensitizer. In both a rigorously conducted guinea pig maximization test and in a modified murine local lymph node assay, DCNB was demonstrated to possess weak sensitizing activity. On this basis, DCNB cannot be regarded as inert with respect to contact allergic potential, and is therefore inappropriate as a negative control in studies of skin sensitization.

An alternative strategy to the use of guinea pigs for the identification of skin sensitization hazard

For over half a century, guinea pig methods have dominated the field of toxicology concerned with the identification of skin sensitizers. Specific protocols, for example the guinea pig maximization test (GPMT), have been pre-eminent in the identification of skin sensitization hazard for regulatory purposes. However, there are increasingly several forces driving change, not least animal use/welfare considerations. In response to this and to address the need for a rapid screen for chemical allergens, an alternative strategy has been developed. In the first instance, a chemical is assessed by a computer-based expert system. This system is constructed from some 50 rules describing the key chemically reactive substructures of known skin sensitizers. The output from the expert system is also evaluated in the light of the understanding of the skin penetration characteristics of the chemical. In this way, and without use of animals, the likelihood that a chemical represents a skin sensitization hazard is assessed based on the two key characteristics of a skin sensitizer: (1) its direct or indirect ability to react with skin protein (i.e. does it contain a structural alert?); and (2) the ability of the chemical to partition into the appropriate epidermal compartment. When the chemical does possess a structural alert and has the capacity to penetrate skin sufficiently, then it may be regarded as a potential skin sensitizer. Subsequent to this screening phase, if necessary the chemical may be assessed in the murine local lymph node assay. This assay is quicker and cheaper than traditional guinea pig assays and importantly is less stressful to the fewer animals that it requires. The assay is well validated and produces objective results which are equivalent to the GPMT in terms of identifying significant skin sensitization hazard. In this paper, the above strategy is described in more detail, focusing on its relevance to hazard identification and its value in animal welfare terms. It is concluded that the strategy provides an important opportunity for both substantial reduction and refinement of animal use in a manner which will not compromise the existing standard of classification and labelling of skin sensitization hazard in the European Union.

Safety evaluation of beta-glucanase derived from Trichoderma reesei: summary of toxicological data

Barlican, a beta-glucanase enzyme obtained from Trichoderma reesei, was produced by a fermentation process and subjected to a series of toxicological tests to document its safety for use as a feed additive. The enzyme product was examined for general oral toxicity, inhalation toxicity, irritation to eye and skin, skin sensitization and mutagenic potential. An extensive literature search on the production organism was also conducted. Furthermore, safety for target species was assessed in a 28-day oral toxicity study with broilers. A strong skin-sensitizing potential of the beta-glucanase enzyme was detected, but no other evidence of oral or inhalation toxicity, mutagenic potential, eye or skin irritancy was found. Feeding of the beta-glucanase enzyme at dietary levels up to 10,000 ppm in the 90-day subchronic toxicity study in rats did not induce noticeable signs of toxicity. In addition, no adverse effects were observed when broiler chicks were fed dietary concentrations of the beta-glucanase enzyme up to eight times the daily recommended dose. It is therefore concluded that this beta-glucanase preparation is safe for use in feed of the intended target species. However, some occupational health precautions should be taken to avoid skin contact and inhalation, as is the case for almost all enzyme proteins.

An evaluation of the suitability of benzocaine as a positive control skin sensitizer

Although they are subject to some limitations, sensitization tests such as the guinea pig maximization test (GPMT) have for many years provided a valuable basis for the identification of skin sensitization potential. Thus they have been used widely by regulatory authorities, such as those in Europe, as a means to identify significant sensitization hazards associated with new chemicals. However, the standard of performance of guinea pig sensitization assays has been demonstrated to be widely variable. Consequently, the OECD sensitization test guideline (the de facto world standard) has been updated to incorporate recommendations for action whose aim is to achieve a minimum standard of test conduct. The principle is that a test laboratory should be able to demonstrate an acceptable level of response using a moderately sensitizing chemical. A list of 3 such chemicals is provided, hexyl cinnamic aldehyde, mercaptobenzothiazole and benzocaine. It is our experience that whilst good results can readily be obtained with the first 2 of these, benzocaine is much more difficult. Using both the GPMT and the local lymph node assay (LLNA), an OECD-recommended screening test, benzocaine has given highly variable results. A range of from 0% to 60% positive in the GPMT was found and, in most tests, benzocaine would not classify as a skin sensitizer according to EU criteria. In the LLNA, from a series of 12 tests conducted in 2 laboratories, only occasional positive results were obtained. Furthermore, these positive results were not reproducible. Reasons for this variability are discussed. However, the main conclusion must be that benzocaine does not represent a useful moderately sensitizing positive control.

A critical commentary and updating of the guinea pig maximization test

The guinea pig maximization test (GPMT) of Magnusson and Kligman was published in 1969. Since then, a vast body of practical experience with the test has been accumulated. New information requires that certain aspects of the procedure be re-evaluated, especially with regard to the interpretation of challenge results. In particular, awareness of the phenomenon of hyperirritable skin (the 'angry back' phenomenon) suggests that presently used controls are not always adequate and may overstate allergenicity owing to false-positive reactions. The control group should be exposed to a chemical insult at induction which provokes an inflammatory reaction comparable to the test substance. We present strategies to distinguish irritant from allergic responses. Allergic reactions should persist on rechallenge weeks later, while nonspecific irritant reactions generally fade and are irreproducible in particular animals. Finally, when a chemical is identified as a contact sensitizer in the GPMT, that result is simply a categorical statement of a theoretical hazard. An assessment of risk is necessary to estimate the relevance of the test result to usage in the real world.

Predictive immunotoxicological test systems: suitability of the popliteal lymph node assay in mice and rats

This article reviews results obtained with popliteal lymph node assays (PLNAs) in rodents and discusses their ability to detect and analyze immunotoxic effects of drugs and other low molecular weight (LMW) chemicals. In its basic form, the PLNA measures activation of the draining lymph node of the hind paw (i.e., the PLN) after injection of a test chemical into the hind foot pad. The assay appears to be appropriate to recognize sensitizing, that is, allergenic and autoimmunogenic, chemicals, as well as nonsensitizing immunostimulatory chemicals. With modifications, PLNAs can detect immunosuppressive chemicals and distinguish sensitizing from nonsensitizing chemicals. Furthermore, modified PLNAs enable detection of known as well as unknown sensitizing metabolites, and may assist in the identification of the self-molecules that act as carriers for chemical sensitization or as targets of chemical-induced autoimmune disease. Experience with PLNAs shows that they are rapid, reproducible, and objective tests for recognition of sensitizing or otherwise immunomodulating chemicals. Because current protocols of toxicity testing are insensitive in predicting a chemical's potential to result in immunomodulation, PLNAs, when further validated, may provide welcome supplements to routine toxicity screening of chemicals, thus enhancing chemical safety.

Assessment of contact sensitivity of four thiourea rubber accelerators: comparison of two mouse lymph node assays with the guinea pig maximization test

Contact sensitivity of four thiourea rubber accelerators, diphenylthiourea (DPTU), dilaurylthiourea (DLTU), dibutylthiourea (DBTU) and diethylthiourea (DETU), was evaluated by a new sensitive mouse lymph node assay (SLNA) and the murine local lymph node assay (LLNA). The results of the SLNA and LLNA were compared with the data of the guinea pig maximization test (GPMT). In the LLNA and SLNA, the sensitizing activity was measured as a function of draining lymph node activation following application of the test chemicals. Of these four thioureas, three (DETU, DBTU and DPTU) were not classified as skin sensitizers in the LLNA. The SLNA successfully detected the sensitivity of all thioureas tested. This result indicated that the SLNA was, in these cases, more sensitive than the LLNA for identification of contact allergens. The order of sensitization potential observed from the SLNA was DPTU (greatest), DLTU, DBTU and then DETU (least). The predictions of sensitizing potential and the order of the sensitizing capacity of four thioureas by the SLNA and the GPMT are very similar.

Contact sensitization assays in guinea-pigs: are they predictive of the potential for systemic allergic reactions?

Guinea-pig assays have been used extensively to detect contact sensitizers. In contrast, almost no reliable assays are available to detect the potential for low-molecular-weight drugs and chemicals to induce systemic allergic reactions in humans. Based on clinical data, and, to some extent, on recent immunological findings, it is proposed that guinea-pig assays can predict the hazard for systemic allergic reactions in man. Seventy drugs and chemicals were compared from published results in guinea-pig assays and in the clinic. A close correlation was found with 43 substances and a relatively good one with 16 substances. Conflicting results were found with 11 substances only. However, substances known to induce systemic allergic reactions in man were all detected as weak sensitizers, at least in guinea-pigs. Guinea-pig contact sensitization assays may therefore prove useful until more suitable and specific assays are available to predict the risk for systemic allergic reactions.

The performance of the local lymph node assay with chemicals identified as contact allergens in the human maximization test

For many years, tests in the guinea pig have been the favoured option for the identification of the skin sensitization potential of chemicals. However, the mouse has been used widely in immunology research and can represent a viable alternative. A variety of murine assays have been described, including several methods based on ear swelling as an endpoint. Another option is to assess induced lymph node cell proliferation and it is this which forms the basis of the murine local lymph node assay (LLNA). The LLNA has undergone several successful interlaboratory validations and compares well with standard guinea pig assays. In the present study, the performance of the LLNA was examined with chemicals tested previously in the human maximization test (HMT). 30 chemicals, 23 of which proved positive in the HMT and seven of which were negative, have been tested. All but four of the materials found positive in the HMT also tested positive in the LLNA. Of these four, sulfanilamide and paraben esters would not classify as skin sensitizers in the guinea pig maximization test and nickel has been found to yield variable results in a number of predictive animal tests. Of the seven substances which proved negative in the HMT, six were also negative in the LLNA, the exception being sodium dodecyl sulfate. These data demonstrate that the LLNA is able to identify accurately chemicals which have the potential to cause significant allergic contact dermatitis in humans.

The identification of chemicals with sensitizing or immunosuppressive properties in routine toxicology

In the context of this paper, immunotoxicity is taken to encompass immunosuppression/immunopotentiation and allergy. Over the last 10 to 15 years, well characterized methods for the assessment of altered immune competence have been reported. This has led to proposals for tiered testing schemes. This review examines the suitability of immunotoxicity parameters for inclusion in routine 28-day studies and comments on methods that have been proposed for incorporation within the guidelines issued by the US FDA and US EPA and OECD. It is recommended that the existing OECD Guideline 407 is modified to incorporate total and differential blood cell counts, spleen and thymus weight and histopathology, and draining and distal lymph node histopathology for Tier I level testing. Data so generated will provide a reliable and accurate means of identifying at an early stage potential immunotoxic effects. Tier II testing should be carried out on a case by case basis and only assuming positive results are obtained at Tier I. An increasingly sophisticated understanding of the nature of immune responses to chemical allergens has facilitated the design of novel predictive methods for the identification of sensitizing activity. Opportunities which arise from these new developments in allergy testing such as the local lymph node assay, mouse ear swelling test, and the mouse IgE test should be monitored closely.

Risk assessment of sensitizing agents

This review describes an approach that has been used to assess the skin sensitization risk of new product ingredients prior to and after marketing. The risk assessment process utilizes a comparative toxicological approach in which data on the inherent toxicity of a material and the exposure to it through manufacturing or consumer use or foreseeable misuse are integrated and compared with data generated by 'benchmark' materials of similar chemistry or product application, or both. This approach has been valuable in providing an accurate assessment of skin sensitization potential and the basis for eventual safe marketing of a wide range of consumer household and personal care products and topical pharmaceuticals.

Morphology of popliteal lymph node responses in Brown-Norway rats

The popliteal lymph node (PLN) assay has been proposed as a tool to predict in rodents those xenobiotics likely to induce autoimmune reactions in humans. To further validate this assay and to study the mechanisms involved, histologic changes in PLNs from rats injected with streptozotocin, diphenylhydantoin, pure acetone, or 50% ethanol were compared to a local graft-versus-host (GvH) reaction. This study suggests that routine histology of PLNs is instrumental to discard primary irritants. In addition, the hypothesis of a GvH-like mechanism in positive PLN responses is supported by the finding that the reference compounds streptozotocin and diphenylhydantoin produced histologic changes similar to a "true" local GvH reaction.

A sensitive mouse lymph node assay with two application phases for detection of contact allergens

A predictive test using mice for the identification of contact sensitizing chemicals was developed. Contact sensitizing activity is measured as a function of draining lymph node activation following application of test chemical. Experimental conditions for assessment of induced lymph node cell (LNC) responses have been optimized. BALB/c mice were initially treated with intradermal injections of test chemical in Freund's complete adjuvant emulsion. Five days after intradermal injection, mice were exposed topically to chemical in vehicle on the ears daily for 3 consecutive days. Next day following the final exposure, changes in lymph node weight, total cell number in the draining lymph nodes and LNC proliferation for 24 h culture were assessed. The performance of the method was evaluated with ten sensitizing chemicals and a non-sensitizing irritant, sodium lauryl sulfate (SLS). The LNC proliferation induced by combination of intradermal injection and topical application of sensitizing chemicals was more clearly increased than that following only topical application. With the single exception of sulfanilic acid, the method developed was able to detect the sensitizing capacity of chemicals that failed to induce sensitization in the local lymph node assay. Under the conditions used, SLS did not induce measurable lymph node responses. These results suggest that the mouse lymph node assay can provide a sensitive screening test for weak to moderate sensitizers. In addition, the assay offers the advantages of objective and quantitative endpoints, and is suitable for the evaluation of colored or irritant chemicals.

Results with OECD recommended positive control sensitizers in the maximization, Buehler and local lymph node assays

The guinea pig maximization test and the Buehler occluded patch test are used widely to identify the sensitization potential of new chemicals. This information enables toxicologists and/or regulatory authorities to determine whether a chemical should be classified formally as a skin sensitizer. Both to improve and to harmonize these assessments internationally, the OECD has recommended recently that moderate rather than strong contact sensitizers are used as positive control substances. The purpose is to ensure an adequate level of sensitivity in sensitization assays performed at specific testing establishments. Results from two laboratories reported here show that the minimum acceptable standard laid down by the OECD can be achieved and indeed commonly exceeded by a substantial margin. Furthermore, results with these positive controls in a new method, the local lymph node assay, also appear to satisfy similar criteria, suggesting results from this assay, including negative data, should be acceptable for classification purposes. However, a review of the way in which results with new chemicals will be interpreted for regulatory purposes, in the context of positive control data, reveals that considerable inadequacies still exist. It is recommended that ultimately, sensitization data can only be interpreted meaningfully (i.e. to protect humans from sensitization hazards) by considering the potency of the contact allergen in the context of the sensitivity of the assay performed at the particular testing institution.

Classification of chemicals as sensitisers based on new test methods

For the last decade, classification schemes worldwide have recognised that certain chemicals may need to be categorised as skin or respiratory sensitisers. Although differing in detail, the schemes use similar criteria for designating materials as sensitisers, based on either direct evidence from exposed humans or the results of predictive guinea pig tests. In the case of respiratory sensitisation, however, there are currently no acceptable animal test methods. With an increasing understanding of cellular immunology in general, and of immune responses in skin and respiratory sensitisation in particular, several laboratories have recently been developing more objective, immunologically-based tests. For skin sensitisation, the two most promising methods are the murine local lymph node assay (LLNA) and the mouse ear swelling test (MEST). Both assays have undergone inter-laboratory validation and it has been shown that they are able to detect reliably moderate to strong sensitisers. The 1992 update of the OECD test guideline for skin sensitisation suggests the use of the LLNA or MEST as a first stage of testing; if a positive result is seen in either assay, a chemical may then be designated (and classified) as a potential sensitiser and it may not be necessary to conduct a guinea pig test. However, if a negative result is obtained, a guinea pig test must be performed. For respiratory sensitisation, although certain guinea pig models of asthma appear to be predictive of the known human response to sensitisers such as diisocyanates and acid anhydrides, the measurement of changes in serum IgE antibodies in mice treated topically with chemicals may represent a simpler and more accurate method of designating chemicals as respiratory sensitisers.

Sulphanilic acid: divergent results in the guinea pig maximization test and the local lymph node assay

The guinea pig maximization test (GPMT) has proven to be a valuable tool for the identification of the skin sensitization potential of chemicals. The method identifies a hazard which can lead in the EC to compulsory labelling of that chemical. In the present study, data on sulphanilic acid derived from the GPMT has been compared with results from a second guinea pig assay (the cumulative contact enhancement test) and the murine local lymph node assay, both of which require only topical application of chemical. Except for the GPMT, no test identified any sensitizing activity associated with exposure to sulphanilic acid. These latter results are consistent with the experience gained from substantial human exposure in an occupational setting and from which no cases of allergic contact dermatitis to sulphanilic acid have arisen over a 20-year period. In consequence, it is questioned which test protocol in practice has given the more accurate identification of sensitization hazard relevant to man.

Comparison of the local lymph node assay with the guinea-pig maximization test for the detection of a range of contact allergens

The guinea-pig maximization test (GMPT) has been in use as a method for the prediction of skin sensitization potential for over 20 years, and is widely accepted by regulatory authorities because of its reliable detection of a wide variety of potential human contact allergens. Nevertheless, the method has some limitations and drawbacks, including the use of an adjuvant, the injection of the test substance at induction thus bypassing the normal skin barrier and metabolic function, a subjective endpoint, interference by irritant and/or coloured chemicals, and a relatively long and complex protocol. To address these points, an alternative technique, the local lymph node assay (LLNA), has been proposed and has become the focus of much attention. Recent data from interlaboratory trials have shown a good level of agreement between test facilities and with existing guinea-pig data. The present work investigated the correlation between LLNA results and those derived from the GPMT for 40 chemicals covering a range of chemical types and levels of skin sensitization potential. The LLNA assay was capable of detecting chemicals that exhibit a strong sensitization potential in the GPMT. For chemicals classified as moderate sensitizers in the GPMT, the LLNA was usually positive or provided an indication of sensitizing activity (that was not sufficient to satisfy the current criteria for regarding the result as positive). Weaker sensitizers in the GPMT were usually not detected by the LLNA. With the single exception of copper chloride, non-sensitizers were not positive in the LLNA. The results support the view that the LLNA can provide a rapid and objective screening test for strong sensitizers.