Measurement of allergenic potency using the local lymph node assay

A comparative study of chemical constituents and safety of Thai herbal medicated oil formula and traditional medicated oil

OBJECTIVES

Medicated oil has been traditionally used as an analgesic medicine to relieve pain by applying to the skin over painful muscles, and has been established in The National List of Essential Medicines (2556 B.E.). Traditional Medicated oil (TMO) consists of cassumunar ginger, Wan En Lueang, and Wan Ron Thong. It is prepared by frying herbs in coconut oil which is a traditional method. Thai Herbal Medicated oil formula (MOF) comprises cassumunar ginger, sweet flag, lesser galangal, wild turmeric, Wan En Lueang, and Wan Ron Thong. Its preparation by maceration with methyl salicylate is modified method to avoid the heat in the traditional preparation. Since both recipes have different methods of extraction and compositions, it is necessary to determine the chemical constituents and acute skin irritation potential of MOF and TMO.

METHODS

This study applied Gas Chromatography-Mass Spectrometry (GC-MS) for quality assessment of MOF extract and TMO extract. The mass spectra of the compounds matched with authentic standards from the NIST library. The compounds were identified by comparing their retention time and peak area, and the percentage of matched factor was more than 80%. The safety assessment on acute skin irritation used the 4 h human patch test (4 h HPT) in 36 healthy subjects.

RESULTS

The 37 compounds were found in MOF extract. The most common chemical constituents were terpene derivatives; monoterpenes and sesquiterpenes, and others, namely benzene derivatives, ester, phenylpropanoid, and other compounds. The 33 compounds were found in TMO extract and the chemical groups were similar to MOF extract. The 4 h HPT demonstrated that the MOF extract, TMO extract, methyl salicylate solvent, and coconut oil solvent, did not induce skin irritation. However, the vehicle of the formulae and the whole formulae as MOF and TMO clinically induced skin irritation and accorded with rubefacient. The characteristic as mild erythema or dryness, but not erythema with edema, could appear after the use of rubbing skin products.

CONCLUSIONS

The chemical constituents found in MOF and TMO extracts were partially different and their acute skin irritation reactions were not significantly different. A study on the efficacy of both remedies should be performed in the future, as well as a study on active chemical constituents. This study provides scientific evidences of quality and safety, including GC-MS condition for poly-herbal medicated oil standardization.

The 21st Century movement within the area of skin sensitization assessment: From the animal context towards current human-relevant in vitro solutions

In a regulatory context, skin sensitization hazard and risk evaluations of manufactured products and their ingredients (e.g. cosmetics) are mandatory in several regions. Great efforts have been made within the field of 21st Century Toxicology to provide non-animal testing approaches to assess the skin allergy potential of materials (e.g. chemicals, mixtures, nanomaterials, particles). Mechanistic understanding of skin sensitization process through the adverse outcome pathway (AOP) has promoted the development of in vitro methods, demonstrating accuracies superior to the traditional animal testing. These in vitro testing approaches are based on one of the four AOP key events (KE) of skin sensitization: formation of immunogenic hapten-protein complexes (KE-1 or the molecular initiating event, MIE), inflammatory keratinocyte responses (KE-2), dendritic cell activation (KE-3), and T-lymphocyte activation and proliferation (KE-4). This update provides an overview of the historically used in vivo methods as well as the current in chemico and in cell methods with and without OECD guideline designations to analyze the progress towards human-relevant in vitro test methods for safety assessment of the skin allergenicity potential of materials. Here our focus is to review 96 in vitro testing approaches directed to the KEs of the skin sensitization AOP.

Determination of Protein Haptenation by Chemical Sensitizers Within the Complexity of the Human Skin Proteome

Skin sensitization associated with the development of allergic contact dermatitis occurs via a number of specific key events at the cellular level. The molecular initiating event (MIE), the first in the sequence of these events, occurs after exposure of the skin to an electrophilic chemical, causing the irreversible haptenation of proteins within skin. Characterization of this MIE is a key step in elucidating the skin sensitization adverse outcome pathway and is essential to providing parameters for mathematical models to predict the capacity of a chemical to cause sensitization. As a first step to addressing this challenge, we have exposed complex protein lysates from a keratinocyte cell line and human skin tissue with a range of well characterized sensitizers, including dinitrochlorobenzene, 5-chloro-2-methylisothiazol-3-one, cinnamaldehyde, and the non (or weak) sensitizer 6-methyl coumarin. Using a novel stable isotope labeling approach combined with ion mobility-assisted data independent mass spectrometry (HDMSE), we have characterized the haptenome for these sensitizers. Although a significant proportion of highly abundant proteins were haptenated, we also observed the haptenation of low abundant proteins by all 3 of the chemical sensitizers tested, indicating that within a complex protein background, protein abundance is not the sole determinant driving haptenation, highlighting a relationship to tertiary protein structure and the amino acid specificity of these chemical sensitizers and sensitizer potency.

Stable isotope labeling method for the investigation of protein haptenation by electrophilic skin sensitizers

The risk of contact sensitization is a major consideration in the development of new formulations for personal care products. However, developing a mechanistic approach for non-animal risk assessment requires further understanding of haptenation of skin proteins by sensitizing chemicals, which is the molecular initiating event causative of skin sensitization. The non-stoichiometric nature of protein haptenation results in relatively low levels of modification, often of low abundant proteins, presenting a major challenge for their assignment in complex biological matrices such as skin. Instrumental advances over the last few years have led to a considerable increase in sensitivity of mass spectrometry (MS) techniques. We have combined these advancements with a novel dual-labeling/LC-MS(E) approach to provide an in-depth direct comparison of human serum albumin (HSA), 2,4-dinitro-1-chlorobenzene (DNCB), 5-chloro-2-methyl-4-isothiazolin-3-one (MCI), trans-cinnamaldehyde, and 6-methyl coumarin. These data have revealed novel insights into the differences in protein haptenation between sensitizers with different reaction mechanisms and sensitizing potency; the extreme sensitizers DNCB and MCI were shown to modify a greater number of nucleophilic sites than the moderate sensitizer cinnamaldehyde; and the weak/non-sensitizer 6-methyl coumarin was restricted to only a single nucleophilic residue within HSA. The evaluation of this dual labeling/LC-MS(E) approach using HSA as a model protein has also demonstrated that this strategy could be applied to studying global haptenation in complex mixtures of skin-related proteins by different chemicals.

The LLNA: A Brief Review of Recent Advances and Limitations

Allergic contact dermatitis is the second most commonly reported occupational illness, accounting for 10% to 15% of all occupational diseases. This highlights the importance of developing rapid and sensitive methods for hazard identification of chemical sensitizers. The murine local lymph node assay (LLNA) was developed and validated for the identification of low molecular weight sensitizing chemicals. It provides several benefits over other tests for sensitization because it provides a quantitative endpoint, dose-responsive data, and allows for prediction of potency. However, there are also several concerns with this assay including: levels of false positive responses, variability due to vehicle, and predictivity. This report serves as a concise review which briefly summarizes the progress, advances and limitations of the assay over the last decade.

Potency values from the local lymph node assay: application to classification, labelling and risk assessment

Hundreds of chemicals are contact allergens but there remains a need to identify and characterise accurately skin sensitising hazards. The purpose of this review was fourfold. First, when using the local lymph node assay (LLNA), consider whether an exposure concentration (EC3 value) lower than 100% can be defined and used as a threshold criterion for classification and labelling. Second, is there any reason to revise the recommendation of a previous ECETOC Task Force regarding specific EC3 values used for sub-categorisation of substances based upon potency? Third, what recommendations can be made regarding classification and labelling of preparations under GHS? Finally, consider how to integrate LLNA data into risk assessment and provide a rationale for using concentration responses and corresponding no-effect concentrations. Although skin sensitising chemicals having high EC3 values may represent only relatively low risks to humans, it is not possible currently to define an EC3 value below 100% that would serve as an appropriate threshold for classification and labelling. The conclusion drawn from reviewing the use of distinct categories for characterising contact allergens was that the most appropriate, science-based classification of contact allergens according to potency is one in which four sub-categories are identified: 'extreme', 'strong', 'moderate' and 'weak'. Since draining lymph node cell proliferation is related causally and quantitatively to potency, LLNA EC3 values are recommended for determination of a no expected sensitisation induction level that represents the first step in quantitative risk assessment.

Reactivity profiling: covalent modification of single nucleophile peptides for skin sensitization risk assessment

The molecular basis of chemical allergy is rooted in the ability of an allergen (hapten) to modify endogenous proteins. This mechanistic understanding aided development of screening assays which generate reproducible quantitative and qualitative reactivity data. Such assays use model peptides with a limited number and type of protein nucleophiles, and the data does not reflect the specificity, variety, and complexity of hapten interactions with multiple nucleophiles. Building on these developments, we extended the standardized approach to maximize the type and the amount of information that can be derived from an in chemico assay. We used a panel of six single nucleophile peptides and individually optimized the incubation conditions to favor chemical modification. Employing liquid chromatography tandem mass spectrometry (LC-MS/MS) technique, we simultaneously obtained multiple quantitative and qualitative measurements (% peptide depletion, adducts formation, and peptide dimerization for Cys-containing peptide). Using these methods, we obtained reactivity data for 36 chemicals of known skin sensitizing potency. By optimizing incubation conditions, we ensured detection of all reactive chemicals. We explored the LC-MS/MS approach to generate kinetic data for 10 chemicals allowing further characterization of reactivity and a potentially more robust quantitative reactivity descriptor. Our ultimate aim is to integrate this dataset with available physicochemical data and outputs from other predictive assays, all addressing different key steps in the induction of sensitization, to help us make decisions about the safe use of chemicals without using animal tests. The epidermal protein target sites, modification of which may be immunogenic and lead to induction of skin sensitization, are currently unknown. Increasing the understanding of this process may help further refine in chemico reactivity assays as well as aid the interpretation of the reactivity data.

The Role of Non-Covalent Protein Binding in Skin Sensitisation Potency of Chemicals

Skin sensitisation is a delayed hypersensitivity reaction caused by repeated exposure to common natural and synthetic chemical allergens. It is thought that small chemical sensitisers (haptens) are required to form a strong irreversible bond with a self protein/peptide and generate an immunogenic hapten-protein complex in order to be recognised by the immune system and stimulate T cell proliferation. The sensitisers are usually electrophilic chemicals that are directly reactive with proteins or reactive intermediates (metabolites) of chemically inert compounds (prohaptens). Sensitising chemicals are also capable of weak, non-covalent association with proteins and there is an ongoing debate about the role of weak interactions of chemicals and proteins in the chemistry of allergy. The non-covalent interactions are reversible and thus have a major impact on skin/epidermal bioavailability of chemical/reactive metabolites. We investigated the relationship between the relative level of non-covalent association to a model protein and their relative potencies as determined by the EC3 values in the murine local lymph node assay (LLNA) for a number of chemicals. Using human serum albumin as a model protein, we determined that no observable relationship exists between the two parameters for the chemicals tested. Therefore, at least for this model protein, non-covalent interactions appear not to be a key determinant of allergen potency.

Mass spectrometric identification of covalent adducts of the skin allergen 2,4-dinitro-1-chlorobenzene and model skin proteins

A large proportion of allergic skin reactions are considered to be the result of skin exposure to small organic chemicals that possess the intrinsic ability to covalently modify skin proteins, either directly or following activation. In the absence of information about specific skin protein targets, studies of chemical modifications are limited to the use of model proteins. We have previously demonstrated that selected well known skin sensitizers (2,4-dinitro-1-chlorobenzene and phenyl salicylate) have the ability to covalently modify residues selectively on the model protein, human serum albumin. In the present work, we focus on the differences in covalent binding observed for two additional model proteins, human cytokeratin 14 and human cofilin, both constituent proteins of skin. Using matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) and nano LC-MS and -MS/MS strategies, the amino acid residues targeted by 2,4-dinitro-1-chlorobenzene on the two model proteins have been identified. In contrast, a structurally related non-sensitiser (2,4-dichloro-1-nitrobenzene) and a non-sensitising irritant (benzalkonium chloride) did not covalently modify the model proteins. Detailed examination of the results for the sensitizers indicate that reactive chemicals target nucleophilic amino acids residing in specific microenvironments of the 3D protein structure that are conducive to reactivity. This observation has important implications for the development of hapten-peptide binding assays. It is envisaged that the data from such assays will be integrated with outputs from other in vitro assays in the future to give a prediction of the sensitisation potential of novel chemicals.

The skin sensitization potential of resorcinol: experience with the local lymph node assay

Resorcinol is a simple aromatic chemical (1,3-benzenediol) that has found widespread use, particularly as a coupler in hair dyes. Clinical experience clearly shows that resorcinol is a (albeit uncommon) skin sensitizer. By contrast, predictive methods, both animal and human, have previously failed to identify resorcinol as such. Here, we describe the outcome of a recent local lymph node assay performed in accordance with Organisation for Economic Co-operation and Development guideline 429, which correctly identified resorcinol as a skin sensitizer. Clear evidence of a dose response was apparent, and an EC3 value of approximately 6% was calculated. This suggests that the skin-sensitizing potency of resorcinol is approximately 2 orders of magnitude lower than that of p-phenylenediamine but similar to that of hexyl cinnamic aldehyde. These data show the importance of adherence to test guidelines and aligns the clinical experience with resorcinol with that obtained in predictive animal methods.

Investigating protein haptenation mechanisms of skin sensitisers using human serum albumin as a model protein

Covalent modification of skin proteins by electrophiles is a key event in the induction of skin sensitisation but not skin irritation although the exact nature of the binding mechanisms has not been determined empirically for the vast majority of sensitisers. It is also unknown whether immunologically relevant protein targets exist in the skin contributing to effecting skin sensitisation. To determine the haptenation mechanism(s) and spectra of amino acid reactivity in an intact protein for two sensitisers expected to react by different mechanisms, human serum albumin (HSA) was chosen as a model protein. The aim of this work was also to verify for selected non-sensitisers and irritants that no protein haptenation occurs even under forcing conditions. HSA was incubated with chemicals and the resulting complexes were digested with trypsin and analysed deploying matrix-assisted laser desorption/ionization mass spectrometry, reverse phase high performance liquid chromatography and nano-electrospray tandem mass spectrometry. The data confirmed that different residues (lysine, cysteine, histidine and tyrosine) are covalently modified in a highly selective and differential manner by the sensitisers 2,4-dinitro-1-chlorobenzene and phenyl salicylate. Additionally, non-sensitisers 2,4-dichloro-1-nitrobenzene, butyl paraben and benzaldehyde and irritants benzalkonium chloride and sodium dodecyl sulphate did not covalently modify HSA under any conditions. The data indicate that covalent haptenation is a prerequisite of skin sensitisation but not irritation. The data also suggest that protein modifications are targeted to certain amino acids residing in chemical microenvironments conducive to reactivity within an intact protein. Deriving such information is relevant to our understanding of antigen formation in the immunobiology of skin sensitisation and in the development of in vitro protein haptenation assays.

Popliteal lymph node (PLN) assay to study adjuvant effects on respiratory allergy

Different variants of the popliteal lymph node (PLN) assay have been published. Here we describe the adjuvant popliteal lymph node assay, an immune response assay to study the adjuvant activity of soluble substances as well as particulate matter. The substance to be studied for adjuvant activity is injected into the hind footpad of mice or rats together with an antigen. Adjuvant activity is determined as the increase in PLN weight and cell numbers in animals receiving antigen together with the substance under study, compared with PLN weight and cell numbers in animals given the antigen without the substance in question, and animals given the putative adjuvant alone. Because lymph node weight and cell numbers are immunologically non-specific parameters, specific immune response assays like serum antibody responses or antibody-forming cell numbers should additionally be performed. Different antigens and immune response assays may be used, depending on the research question asked. In relation to respiratory (or food) allergy, the assays should as a minimum include determination of specific IgE in serum, and preferably also IgG1 (mouse). Serum specific IgG2a antibody determination may be added to get an indication of the Th1-Th2-balance of the response. The adjuvant PLN assay, with cellular response assays performed in the draining popliteal lymph node and antibody determinations in serum, requires small amounts of test material. The assay offers a practical, sensitive and reproducible method to determine the adjuvant activity of soluble substances as well as particulate material, with the possibility to also perform mechanistic studies.

Assessment of the relative skin sensitization potency of siloranes and bis-GMA using the local lymph node assay and QSAR predicted potency

Siloranes are silicon and oxirane (epoxy) containing monomers used for new dental composite development. The siloranes 3,4-epoxycyclohexylethyl-cyclopolymethylsiloxane (Tet-Sil) and bis-3,4-epoxycyclohexylethyl-phenyl-methylsilane (Ph-Sil) have in common cycloaliphatic epoxy moieties. The epoxy group is of concern in their biocompatibility since most epoxy compounds are known skin sensitizers. The objective of this study was to determine the in vivo skin sensitization potency of the siloranes in the local lymph node assay. A comparison was made with well-known chemical allergens, bis-GMA and DNCB. Female mice (CBA/CaJ) were exposed topically (dorsum of both ears) to several doses of acetone:olive oil in the ratio of 4:1 v/v. Doses were defined by a predictive structure-activity model (QSAR) for contact sensitization. Lymph node cell (LNC) proliferation was measured on the sixth day by incorporation of radioactive thymidine into DNA of lymph node cells. The effective concentration (EC3) that produced a 3-fold stimulation in LNC proliferation relative to controls was extrapolated from dose-response curves. DNCB was a strong sensitizer (EC3 = 0.06%). The EC3 values of Ph-Sil and bis-GMA were 19% and 45%, respectively, making these weak contact sensitizers. Tet-Sil did not increase lymph node proliferation when compared with controls. In contrast to Tet-Sil, the unpolymerized monomers Ph-Sil and bis-GMA have the capacity to induce LNC proliferation, characteristic of a T-cell mediated skin contact sensitization.

A future approach to measuring relative skin sensitising potency: a proposal

Current approaches to skin sensitisation risk assessment are dependent upon the availability of information regarding two fundamental parameters. Firstly, data relating to the relative skin sensitising potency of the chemical, and secondly, information regarding likely conditions of human exposure. During the past two decades, much has been achieved in terms of refining methods capable of informing these parameters. For example, the development of the local lymph node assay (LLNA) has made it possible to predict skin sensitising hazard, and to determine relative skin sensitising potency, in a way that was not possible previously. Taken together with accurate information about predicted exposure, such potency data can be used to facilitate the derivation of effective risk assessments. However, although the LLNA provides an integrated assessment of skin sensitising activity, it does require the use of experimental animals and there is growing enthusiasm for designing robust alternative approaches that will reduce or obviate that need. Progress is being made in defining alternative experimental strategies that avoid animal use, but it is clear that accurate characterisation of skin sensitisation hazards will require the effective integration of various sources of information. For this reason, we exemplify here one possible approach that, in theory, provides a framework for not only the identification of skin sensitising chemicals, but also the estimation of relative sensitising potency. This paradigm depends upon development of an understanding of the various biological, biochemical and chemical factors that impact on the allergenic properties of chemicals and the acquisition of skin sensitisation, and an ability to measure these in vitro.

Evaluation of anex vivo murine local lymph node assay: multiple endpoint comparison

The local lymph node assay (LLNA) is used to assess the skin sensitization potential of chemicals. In the standard assay, mice are treated topically on the dorsum of both ears with test substance for 3 days. Following 2 days of rest, the initiation of the hypersensitivity response is evaluated by injecting (3)H-thymidine into a tail vein, and then measuring the levels of radioisotope incorporated into the DNA of lymph node cells draining the ears. In the current study, BALB/c mice were treated with the contact sensitizers hexylcinnamic aldehyde (HCA) and oxazolone, and the nonsensitizer methyl salicylate. The proliferative response of lymph node cells was evaluated in an ex vivo assay, in which isolated cells were cultured in vitro with (3)H-thymidine. Treatment of mice with HCA at 5-50% resulted in concentration-related increases in (3)H-thymidine incorporation, with stimulation indices ranging from 3 to 14. Low animal-to-animal variability was seen in three replicate assays testing HCA at 25%. As anticipated, the proliferative response induced by the potent sensitizer oxazolone at 0.25% was greater than HCA at all concentrations tested. Stimulation indices of 1.5 and 3 were seen in two independent experiments with methyl salicylate. These equivocal findings were likely due to the irritancy properties of the compound. Importantly, measuring ex vivo (3)H-thymidine incorporation was more sensitive than evaluating lymph node weight and cellularity, and in vitro bromodeoxyuridine incorporation. Furthermore, the results of the ex vivo LLNA were comparable to the standard assay. This study provided evidence that supports the use of an ex vivo LLNA for hazard assessment of contact hypersensitivity.

Hapten-protein binding: from theory to practical application in the in vitro prediction of skin sensitization

In view of the forthcoming European Union ban on in vivo testing of cosmetic and toiletry ingredients, following the publication of the 7th amendment to the Cosmetics Directive, the search for practical, alternative, non-animal approaches is gathering pace. For the end-point of skin sensitization, the ultimate goal, i.e. the development and validation of alternative in vitro/in silico assays by 2013, may be achieved through a better understanding of the skin sensitization process on the cellular and molecular levels. One of the key molecular events in skin sensitization is protein haptenation, i.e. the chemical modification of self-skin protein(s) thus forming macromolecular immunogens. This concept is widely accepted and in theory can be used to explain the sensitizing capacity of many known skin sensitizers. Thus, the principle of protein or peptide haptenation could be used in in vitro assays to predict the sensitization potential of a new chemical entity. In this review, we consider some of the theoretical aspects of protein haptenation, how mechanisms of protein haptenation can be investigated experimentally and how we can use such knowledge in the development of novel, alternative approaches for predicting skin sensitization potential in the future.

An European inter-laboratory validation of alternative endpoints of the murine local lymph node assay

The original local lymph node assay (LLNA) is based on the use of radioactive labelling to measure cell proliferation. Other endpoints for the assessment of proliferation are also authorized by the OECD Guideline 429 provided there is appropriate scientific support, including full citations and description of the methodology (OECD, 2002. OECD Guideline for the Testing of Chemicals; Skin Sensitization: Local Lymph Node Assay, Guideline 429. Paris, adopted 24th April 2002.). Here, we describe the outcome of the second round of an inter-laboratory validation of alternative endpoints in the LLNA conducted in nine laboratories in Europe. The validation study was managed and supervised by the Swiss Agency for Therapeutic Products (Swissmedic) in Bern. Ear-draining lymph node (LN) weight and cell counts were used to assess LN cell proliferation instead of [3H]TdR incorporation. In addition, the acute inflammatory skin reaction was measured by ear weight determination of circular biopsies of the ears to identify skin irritation properties of the test items. The statistical analysis was performed in the department of statistics at the university of Bern. Similar to the EC(3) values defined for the radioactive method, threshold values were calculated for the endpoints measured in this modification of the LLNA. It was concluded that all parameters measured have to be taken into consideration for the categorisation of compounds due to their sensitising potencies. Therefore, an assessment scheme has been developed which turned out to be of great importance to consistently assess sensitisation versus irritancy based on the data of the different parameters. In contrast to the radioactive method, irritants have been picked up by all the laboratories applying this assessment scheme.

Safety assessment of esters of p-hydroxybenzoic acid (parabens)

Parabens are widely used as preservatives in food, cosmetic and pharmaceutical products. Acute, subchronic, and chronic studies in rodents indicate that parabens are practically non-toxic. Parabens are rapidly absorbed, metabolized, and excreted. In individuals with normal skin, parabens are, for the most part, non-irritating and non-sensitizing. However, application of compounds containing parabens to damaged or broken skin has resulted in sensitization. Genotoxicity testing of parabens in a variety of in vitro and in vivo studies primarily gave negative results. The paraben structure is not indicative of carcinogenic potential, and experimental studies support these observations. Some animal studies have reported adverse reproductive effects of parabens. In an uterotrophic assay, methyl and butyl paraben administered orally to immature rats were inactive, while subcutaneous administration of butyl paraben produced a weak positive response. The ability of parabens to transactivate the estrogen receptor in vitro increases with alkyl group size. The detection of parabens in a small number of breast tumor tissue samples and adverse reproductive effects of parabens in animals has provoked controversy over the continued use of these substances. However, the possible estrogenic hazard of parabens on the basis of the available studies is equivocal, and fails to consider the metabolism and elimination rates of parabens, which are dose, route, and species dependent. In light of the recent controversy over the estrogenic potential of parabens, conduct of a reproductive toxicity study may be warranted.

Advantage of using CBA/N strain mice in a non-radioisotopic modification of the local lymph node assay

The murine local lymph node assay (LLNA) is currently recognized as a stand-alone test method for determining the skin sensitizing potential of chemicals. It has been incorporated into the official test guidelines published by some authorities, including the OECD. To avoid the use of radioisotopes, efforts have been made recently to develop non-radioisotopic modifications of the LLNA. A non-radioisotopic modification of the LLNA was developed previously using 5-bromo-2'-deoxyuridine (BrdU) incorporation (non-RI LLNA). However, the non-RI LLNA was found to be somewhat less sensitive than the standard assay. This study reports the advantage of using mice of the CBA/N strain in the non-RI LLNA to improve the sensitivity of this method. The non-RI LLNA was performed using CBA/JN and CBA/N mice exposed to one of four confirmed skin sensitizers, 2,4-dinitrochlorobenzene (DNCB), eugenol (EG), isoeugenol (IEG) or alpha-hexylcinnamic aldehyde (HCA), and to one non-sensitizer, propylene glycol (PG). The EC3 values for DNCB, IEG, EG, HCA and PG were calculated to be 0.1%, 9.6%, 40.6%, 45.5% and >50% in CBA/JN mice and 0.08%, 1.9%, 10.7%, 20.3% and >50% in CBA/N mice, respectively. The EC3 values for DNCB, IEG, EG, HCA and PG in the standard LLNA using CBA/Ca mice and radioisotopes were reported elsewhere as being 0.08%, 1.3%, 13.0%, 8.0% and >50%, respectively. The EC3 values derived from the CBA/N mice in the non-RI LLNA were nearly equivalent to the EC3 values obtained using the standard radioisotopic LLNA with CBA/Ca mice. These data suggest that the use of CBA/N mice may provide a realistic opportunity to develop a version of the LLNA that does not have a requirement for the use of radioisotopes, but which nevertheless has sensitivity approaching, or comparable to, the standard method.

Novel approach for classifying chemicals according to skin sensitizing potency by non-radioisotopic modification of the local lymph node assay

The murine local lymph node assay (LLNA) is currently recognized as a stand-alone sensitization test for determining the sensitizing potential of chemicals, and it has the advantage of yielding a quantitative endpoint that can be used to predict the sensitization potency of chemicals. The EC3 has been proposed as a parameter for classifying chemicals according to the sensitization potency. We previously developed a non-radioisotopic endpoint for the LLNA based on 5-bromo-2'-deoxyuridine (BrdU) incorporation (non-RI LLNA), and we are proposing a new procedure to predict the sensitization potency of chemicals based on comparisons with known human contact allergens. Nine chemicals (i.e. diphencyclopropenone, p-phenylenediamine, glutaraldehyde, cinnamicaldehyde, citral, eugenol, isopropyl myristate, propyleneglycol and hexane) categorized as human contact allergen classes 1-5 were tested by the non-RI LLNA with the following reference allergens: 2,4-dinitrochlorobenzene (DNCB) as a class 1 human contact allergen, isoeugenol as a class 2 human contact allergen and alpha-hexylcinnamic aldehyde (HCA) as a class 3 human contact allergen. Consequently, nine test chemicals were almost assigned to their correct allergen class. The results suggested that the new procedure for non-RI LLNA can provide correct sensitization potency data. Sensitization potency data are useful for evaluating the sensitization risk to humans of exposure to new chemical products. Accordingly, this approach would be an effective modification of LLNA with regard to its experimental design. Moreover, this procedure can be applied also to the standard LLNA with radioisotopes and to other modifications of the LLNA.

Ranking of hair dye substances according to predicted sensitization potency: quantitative structure-activity relationships

Allergic contact dermatitis following the use of hair dyes is well known. Many chemicals are used in hair dyes and it is unlikely that all cases of hair dye allergy can be diagnosed by means of patch testing with p-phenylenediamine (PPD). The objectives of this study are to identify all hair dye substances registered in Europe and to provide their tonnage data. The sensitization potential of each substance was then estimated by using a quantitative structure-activity relationship (QSAR) model and the substances were ranked according to their predicted potency. A cluster analysis was performed in order to help select a number of chemically diverse hair dye substances that could be used in subsequent clinical work. Various information sources, including the Inventory of Cosmetics Ingredients, new regulations on cosmetics, data on total use and ChemId (the Chemical Search Input website provided by the National Library of Medicine), were used in order to identify the names and structures of the hair dyes. A QSAR model, developed with the help of experimental local lymph node assay data and topological sub-structural molecular descriptors (TOPS-MODE), was used in order to predict the likely sensitization potential. Predictions for sensitization potential were made for the 229 substances that could be identified by means of a chemical structure, the majority of these hair dyes (75%) being predicted to be strong/moderate sensitizers. Only 22% were predicted to be weak sensitizers and 3% were predicted to be extremely weak or non-sensitizing. Eight of the most widely used hair dye substances were predicted to be strong/moderate sensitizers, including PPD - which is the most commonly used hair dye allergy marker in patch testing. A cluster analysis by using TOPS-MODE descriptors as inputs helped us group the hair dye substances according to their chemical similarity. This would facilitate the selection of potential substances for clinical patch testing. A patch-test series with potent, frequently used, substances representing various chemical clusters is suggested. This may prove useful in diagnosing PPD-negative patients with symptoms of hair dye allergy and would provide some clinical validation of the QSAR predictions.

Quantitative relationship between the local lymph node assay and human skin sensitization assays

The local lymph node assay (LLNA) is a new test method which allows for the quantitative assessment of sensitizing potency in the mouse. Here, we investigate the quantitative correlation between results from the LLNA and two human sensitization tests--specifically, human repeat insult patch tests (HRIPTs) and human maximization tests (HMTs). Data for 57 substances were evaluated, of which 46 showed skin sensitizing properties in human tests, whereas 11 yielded negative results in humans. For better comparability data from mouse and human tests were transformed to applied doses per skin area, which ranged over four orders of magnitude for the substances considered. Regression analysis for the 46 human sensitizing substances revealed a significant positive correlation between the LLNA and human tests. The correlation was better between LLNA and HRIPT data (n=23; r=0.77) than between LLNA and HMT data (n=38; r=0.65). The observed scattering of data points is related to various uncertainties, in part associated with insufficiencies of data from older HMT studies. Predominantly negative results in the LLNA for another 11 substances which showed no skin sensitizing activity in human maximization tests further corroborate the correspondence between LLNA and human tests. Based on this analysis, the LLNA can be considered a reliable basis for relative potency assessments for skin sensitizers. Proposals are made for the regulatory exploitation of the LLNA: four potency groups can be established, and assignment of substances to these groups according to the outcome of the LLNA can be used to characterize skin sensitizing potency in substance-specific assessments. Moreover, based on these potency groups, a more adequate consideration of sensitizing substances in preparations becomes possible. It is proposed to replace the current single concentration limit for skin sensitizers in preparations, which leads to an all or nothing classification of a preparation as sensitizing to skin ("R43") in the European Union, by differentiated concentration limits derived from the limits for the four potency groups.

Reproducible prediction of contact allergenic potency using the local lymph node assay

A considerable body of data has been accumulated which demonstrates that the local lymph node assay (LLNA) can provide a valuable estimation of the contact allergenic potency of a substance. This estimate is obtained via interpolation of the LLNA dose-response curve and is expressed as the concentration of the chemical required to evince a 3-fold stimulation of proliferation in lymph nodes draining the site of application compared to the vehicle-treated controls (EC3). It has also been shown that the EC3 estimates are reproducible and are stable over time. In the present work, we have extended this information by a demonstration of the inherent biological variability surrounding EC3 estimation, using data derived (from a single laboratory) from the testing of isoeugenol as a positive control. Isoeugenol gave EC3 values ranging from 0.5 to 2.6% (n = 29), with a mean and standard deviation of 1.2 +/- 0.6%. Given that EC3 values for a variety of contact allergens range over several orders of magnitude, these results further endorse the utility of EC3 values as a reliable indicator of human contact allergenic potency.

Use of the local lymph node assay in the evaluation of the sensitizing potential of pharmaceutical process intermediates

The murine local lymph node assay (LLNA) has recently been developed to determine the contact sensitization potential of chemicals. Since its original development, the LLNA results have been the subject of extensive comparisons with guinea pig and human data. The investigations described here were designed to explore the ability of the LLNA to identify accurately, pharmaceutical process intermediates (PIs) known to cause contact allergy in humans. To that end, 16 PIs previously tested in the guinea-pig maximization test (GPMT) were tested in the LLNA. Another PI known to be a contact sensitizer in humans was tested only in the LLNA. Cases of contact sensitization in humans were reported only for PIs that were extreme sensitizers in the GPMT and had low EC3 values (concentration of the test substance required to generate a threefold increase in lymph node cell proliferation) in the LLNA. These data provide additional evidence that the LLNA is able to discriminate skin sensitizers from chemicals that do not possess a significant skin sensitization potential and is thus a useful method for hazard identification. In addition, this method also offers important animal welfare benefits and may also be useful for risk assessment purposes.

Utility of historical vehicle-control data in the interpretation of the local lymph node assay

The accepted approach to the interpretation of local lymph node assay (LLNA) data requires comparison of responses in the test groups with background activity found in concurrent vehicle-treated controls. However, of established value in the interpretation of toxicity test data is the use of historical control values that provide one criterion against which to judge the integrity of individual experiments. Specifically, the availability of robust and relevant historical control data permits examination of whether, in any individual experiment, control values fall within the expected range. With the most commonly used vehicle employed in the LLNA, acetone/olive oil (4 : 1) (v/v), the mean values, standard deviations and normal ranges are increasingly well established for a given laboratory, although there is some variation between laboratories, particularly with regard to expected ranges. Against this background, it is possible to identify (and, if appropriate, eliminate) a concurrent vehicle-control value that falls well outside the expected range. To explore critically the potential merits of this approach, one specific example is examined in detail.

Contact allergy: the local lymph node assay for the prediction of hazard and risk

The prospective identification of potential contact allergens and their subsequent safety assessment are pivotal in successful management of this risk to human health. Although much can be learned from the chemical and physical properties of a substance, the definitive information in respect of sensitizing hazard/risk derives from an assessment of the integrated response of the immune system. In recent years, the focus for such assessments has begun to switch from the guinea pig to the mouse, notably to the local lymph node assay (LLNA). In this paper, the current value of the LLNA for hazard identification is reviewed and its regulatory status defined. Once a potential contact allergen has been identified, however, the vital clue to accurate safety evaluation is the assessment of the potency of the allergen. How this can be achieved using the LLNA and employed in safety evaluation is discussed in detail, together with practical suggestions for the deployment of such processes in regulatory toxicology.

The local lymph node assay: past, present and future

The local lymph node assay (LLNA) was developed originally as a method for the identification of chemicals that have the potential to cause skin sensitization and allergic contact dermatitis. The assay is based on an understanding that the acquisition of contact sensitization is associated with, and dependent upon, the stimulation by chemical allergens of lymphocyte proliferative responses in skin-draining lymph nodes. Those chemicals that provoke a defined level of lymph node cell (LNC) proliferation (a 3-fold or greater increase compared with concurrent vehicle controls) are classified as skin sensitizers. Following its original inception and development, the LLNA was the subject of both national and international interlaboratory collaborative trials, and of very detailed comparisons with other test methods and with human skin sensitization data. The assay has now been validated fully as a stand-alone test for the purposes of hazard identification. In recent years, there has been a growing interest also in the use of the LLNA to assess the potency of contact allergens and in risk assessment. There is reason to believe that the extent of skin sensitization achieved is associated with the vigour of LNC proliferation induced in draining nodes. Given this relationship, the relative potency of skin sensitizing chemicals is measured in the LLNA by derivation of an EC3 value, this being the concentration of chemical required to provoke a 3-fold increase in the proliferation of LNC compared with controls. Experience to date indicates that relative potency as determined using this approach correlates closely with what is known of the activity of skin sensitizing chemicals in humans. In this article, we review the development, evaluation and validation of the LLNA for the purposes of hazard identification, and the more recent application of the method for evaluation of potency in the context of risk assessment. In addition, we consider what new applications and modifications are currently being investigated.

Quantitative aspects of contact allergy to chromium and exposure to chrome-tanned leather

The potential of trivalent and hexavalent chromium to induce and elicit allergic contact dermatitis and the degree of chromium exposure from leather products are reviewed. Chromium dermatitis is often due to exposure in the occupational environment, with cement being one of the most common chromium sources. However, consumer products such as chromium(III)-tanned leather products are also an important source of chromium exposure. Apart from Cr(III), which is used for tanning, leather often also contains trace amounts of Cr(VI), which is formed by oxidation of Cr(III) during the tanning process. In a recent study of the Cr(VI) content of leather products bought on the Danish market, 35% of such articles had a Cr(VI) content above the detection limit of 3 p.p.m., ranging from 3.6 p.p.m. to 14.7 p.p.m. Leachable Cr(III) was detected at levels of 430-980 p.p.m. An examination of available dose-response studies showed that exposure to occluded patch test concentrations of 7-45 p.p.m. Cr(VI) elicits a reaction in 10% of the chromium-sensitive patients. When reviewing repeated open exposure studies, it is seen that either exposure to 5 p.p.m. Cr(VI) in the presence of 1% sodium lauryl sulfate (SLS) or exposure to 10 p.p.m. Cr(VI) alone both elicit eczema in chromium-sensitive patients. The eliciting capacity of Cr(III) has not been systematically investigated but, compared to Cr(VI), much higher concentrations are needed to elicit eczema.