Guinea pig sensitization assays. An overview

Skin Sensitization Testing: The Ascendancy of Non-Animal Methods

A century ago, toxicology was an empirical science identifying substance hazards in surrogate mammalian models. Over several decades, these models improved, evolved to reduce animal usage, and recently have begun the process of dispensing with animals entirely. However, despite good hazard identification, the translation of hazards into adequately assessed risks to human health often has presented challenges. Unfortunately, many skin sensitizers known to produce contact allergy in humans, despite being readily identified as such in the predictive assays, continue to cause this adverse health effect. Increasing the rigour of hazard identification is inappropriate. Regulatory action has only proven effective via complete bans of individual substances. Since the problem applies to a broad range of substances and industry categories, and since generic banning of skin sensitizers would be an economic catastrophe, the solution is surprisingly simple—they should be subject to rigorous safety assessment, with the risks thereby managed accordingly. The ascendancy of non-animal methods in skin sensitization is giving unparalleled opportunities in which toxicologists, risk assessors, and regulators can work in concert to achieve a better outcome for the protection of human health than has been delivered by the in vivo methods and associated regulations that they are replacing.

Draize human repeat insult patch test (HRIPT): Seven decades of pitfalls and progress

Allergic contact dermatitis, a Type IV delayed hypersensitivity reaction, can result in dermatologic signs/symptoms for patients/workers. The likelihood of this phenomenon has been estimated/predicted for numerous chemicals/drugs by animal model and human patch testing protocols developed over the last century. Karl Landsteiner initiated testing with guinea pig studies; further studies based on his initial concept were in continual development. John Draize extended Landsteiner's guinea pig studies (which led to development of blood transfusions) to a human assay documenting irritant and allergic contact dermatitis potential - for drugs, chemicals, mixtures and products. We performed a literature search of major Draize derived protocols of the human repeat insult patch test (HRIPT). Our results reveal minor and major differences between protocols and lack of international standardization. Key clarification and principles post-Draize modified and improved usefulness of the HRIPT. Without a standard method of performing the HRIPT, it is problematic to generalize results of studies summarized here. Furthermore, we suggest a potential standardization procedure/protocol combining the work of the most satisfactory HRIPT methods. As the HRIPT constitutes a key parameter in current quantitative risk assessment for chemicals/drugs, such standardization should aid potential prediction of allergic contact dermatitis potential.

Free radicals in antigen formation: reduction of contact allergic response to hydroperoxides by epidermal treatment with antioxidants

BACKGROUND

For patients with allergic contact dermatitis, the main therapy is anti-inflammatory steroids, a non-specific and symptomatic treatment. In contact allergy, the antigen formation is considered to be the binding of a chemical (hapten) to a biological macromolecule, e.g. a protein. Limonene-2-hydroperoxide (Lim-OOH) is a hapten with a known allergenic effect. It is likely to bind to proteins in the skin via a radical mechanism. It might be possible to inhibit the allergic reaction by epidermal application of substances that can trap free radicals, e.g. antioxidants such as ascorbic acid or alpha-tocopherol, prior to the application of the hapten.

OBJECTIVES

To study the influence of antioxidants on the allergenic effect of Lim-OOH in sensitization experiments on guinea pigs.

METHODS

Pretreatment with the antioxidants was performed before induction to study the effect on sensitization as well as before challenge testing to study the effect on elicitation.

RESULTS

A reduction in the response rate was found both at sensitization and at elicitation. The antioxidants had no effect on cobalt allergy or on the allergenic effect of haptens that form antigens via nucleophilic-electrophilic reactions. No reduction of the effect was seen for irritants.

CONCLUSIONS

The protective effect of antioxidants in elicitation could be of practical therapeutic value, as it indicates a possibility for the treatment of patients who have become sensitized to haptens that form full antigens via a radical mechanism.

Industry experience in the identification of the immunotoxic potential of agrochemicals

During recent years immunotoxicity has been increasingly recognized as an important endpoint in rodent short-time studies. This has been documented by FDA, OECD, and just recently in a new EPA guideline. This guideline is confined to the immunosuppressive effects of chemicals. Various parameters to detect immunotoxic effects exist, including cell counts, cell subpopulation analysis, functional tests, and/or advanced pathology. Their validity in detecting immunotoxic effects has been demonstrated to different degrees. Our experience with some of these parameters is reported here. Due to the recommendation of the guideline, it is necessary to differentiate from the context of the study data between primary and secondary immunotoxicity, the latter being an unspecific sequel of toxicity to other organs. In our studies, we found examples for both mechanisms. For primary immunotoxic substances, immunosuppression is markedly more frequent than immunostimulation, although primary effects, on the whole, occur relatively seldom during toxicological screening. In both cases, we found a good correlation between cell analysis and functional parameters on one hand and pathology on the other, thus warranting that overt immunotoxicity would not remain undetected in routine studies with high dose levels. However, the higher predictivity of functional parameters and the analysis of special subpopulations is necessary for the determination of the no-effect level and for fine differentiation during the screening of comparable immunotoxic compounds. Cyclosporin A is an example for the former, and the screening of different agrochemicals is an example for the latter aspect. As verified by the collaboration studies, an advanced histopathology of lymphoid organs, combined with flow cytometry of immune competent cells and a functional assay, is able to discriminate between primary and secondary effects as well as immunosuppression and immunostimulation, and thus to identify an immunotoxic hazard.

Analyses of cutaneous fluoroquinolones photoreactivity using the integrated model for the differentiation of skin reactions

Currently available test models for the differentiation of photoallergic and photoirritant reactions are extremely time consuming and the protocols are very heterogeneous. In vitro tests are of proven value in predicting irritant or toxic effects, but these tests fail to predict chemical-induced allergic side effects. We developed test systems for this endpoint which is not easily detected by existing assays. In a previous publication we were able to discriminate between a contact sensitizer and a skin irritant with a combination of primary ear swelling analysis and cell counting of the ear-draining lymph nodes [Toxicol. Appl. Pharm. 153 (1998) 83; Arch. Toxicol. 73 (2000) 501]. This combination of tests was called the Integrated Model for the Differentiation of chemical-induced allergic and irritant Skin reactions (IMDS). In addition, it had been shown before that inclusion of UV irradiation in the local lymph node assay enables discrimination of photoallergic from photoirritant reactions after dermal application [Photodermatol. Photoimmunol. Photomed. 10 (1994) 57]. Because of the fact that fluoroquinolones are known to induce photoreactions after oral but not dermal treatment, the aim of the present study was to apply the IMDS for the fast and reliable differentiation of photoreactions due to fluoroquinolones after oral treatment. Enoxacin, lomefloxacin, ofloxacin, sparfloxacin and BAY y 3118 were tested in this system. We found a good correlation between the results of UV light-irradiated IMDS and a guinea pig model with the quinolones as far as photoirritancy was concerned. This holds true also for the photoallergic standard olaquindox and the photoirritant standard 8-methoxypsoralen. However, in contrast to the guinea pig assays the IMDS is fast and extremely predictive for the risk of both photosensitization and photoirritancy depending on the route of exposure. Thus, the UV light-irradiated IMDS turned out to be a good tool for the preclinical risk assessment procedure in terms of discriminating photoreactions. In addition, flow cytometric analyses were used to underline the fact that antigen-independent activation occurred after the induction of photoirritant reactions.

Development of a non-radioactive endpoint in a modified local lymph node assay

A murine local lymph node assay (LLNA) has been developed as an alternative to guinea pig models for contact sensitization testing. Although the LLNA appears to be a little less sensitive than the most stringent of guinea pig assays, it provides a rapid, objective, quantitative and cost-effective method for screening strong contact sensitizers and has advantages with respect to animal welfare. However, a potential disadvantage is the need for the use of radioactive material. We have reported previously that an ex vivo assay based on similar principles to the original in vivo LLNA, but using a non-radioactive endopoint, was valid for the prediction of strong sensitizers. This ex vivo assay was not sensitive enough to allow prediction of moderately potent ones. In this study, we propose a new parameter, Corrected IL-2 Index (CII), for the prediction of moderate sensitizers. To obtain CII the IL-2 release in the supernatant of the cell culture is corrected for lymph node weight ratio and ratio of CD4-positive subset. We found that CII predicted the allergenicity of moderate sensitizers, including the ones recommended by the OECD in guideline 406, such as mercaptobenzothiazole and hexyl cinnamic aldehyde. The allergenicity of metal salts, such as potassium dichromate, ammonium tetrachloroplatinate and cobalt chloride, was also predicted by the CII. We conclude that the use of CII as an index significantly increases the sensitivity of the ex vivo method so that moderate sensitizers may also be detected.

An integrated model for the differentiation of chemical-induced allergic and irritant skin reactions

Contact and photocontact allergic as well as irritant and photoirritant skin reactions represent a major problem in clinical dermatology and during the development of new pharmaceuticals. Furthermore, there is a lack of in vitro and in vivo assays that provide a clear differentiation between allergic and irritant skin reactions. Here, we describe an integrated model to differentiate between chemical-induced allergic and irritant skin reactions by measuring objective and easy-to-determine parameters within both skin and skin-draining lymph nodes. Dose-response studies with standard contact and photocontact allergens as well as irritants and photoirritants revealed that irritants predominantly induced skin inflammation, which in turn stimulated draining lymph node cell proliferation. In contrast, the induction phase of contact or photocontact allergy was characterized by marginal skin inflammation, but a marked activation and proliferation of skin-draining lymph node cells. Therefore, a differentiation index (DI) was defined describing the relation between skin-draining lymph node cell activation (lymph node cell count index) and skin inflammation (ear swelling). A DI > 1 indicates an allergic reaction pattern whereas DI < 1 demonstrates an irritant potential of a chemical. Experiments with the contact allergen oxazolone, the photocontact allergen TCSA + UVA, the irritant croton oil, and the photoirritant 8-methoxypsoralen + UVA confirmed the predictive value of DI. Furthermore, flow cytometric analysis of lymph node-derived T- and B-cell subpopulations revealed that contact sensitizer, but not irritant, induced the expression of CD69 on the surface of I-A+ cells. In conclusion, further studies with a broad range of irritants and allergens will be required to confirm general applicability.

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.

Value of the cutaneous basophil hypersensitivity (CBH) response for distinguishing weak contact sensitization from irritation reactions in the guinea pig

Numerous studies of the histology of allergic contact dermatitis reactions to potent allergens in guinea pigs and humans have indicated that there is significant tissue infiltration with basophilic leukocytes. In this study we determined whether this histologic finding could be of value in distinguishing weak sensitization reactions from primary irritation, thereby aiding in the predictive identification of weak or moderate contact allergens. Guinea pigs were sensitized by the Buehler test method. Skin reactions were graded 24, 48, and 72 h post-challenge with duplicate patch sites biopsied at the 24- or 72-h grading timepoints. The biopsies were fixed, embedded in glycol methacrylate, thin sectioned, and Giemsa stained. The number of basophils per 400 leukocytes were counted along the upper dermis just below the dermal/epidermal junction. Challenge patch sites from animals sensitized to a relatively low dose of the strong contact allergen, oxazolone, were compared with patch sites from animals challenged only with a strong irritant, sodium lauryl sulfate (SLS). Compared to normal skin (7.5 +/- 1.0 basophils/400 leukocytes +/- SEM) only the oxazolone patch sites showed significant basophil infiltration (36.8 +/- 6.5), despite the fact that the skin reactions to the low oxazolone challenge dose were relatively weak. SLS patch sites showed no basophil infiltration above normal skin levels (4.8 +/- 0.9). Subsequent blinded studies compared weak/moderate presumptive sensitization reactions (as defined by accepted visual skin grading criteria) to various chemicals (citronellal, vanillin, cinnamic aldehyde, and ethylenediamine) to primary irritation reactions to the same chemicals. In each case, low-challenge-dose sensitization sites on previously treated (induced) animals showed mean basophil infiltration (range, 11.9-69.2 basophils/400 leukocytes) significantly greater than higher-dose irritant reactions (range, 1.6-13.3). The range for normal skin was 0.2-10.2 and the range for strong patch reactions to higher concentrations of oxazolone was 59.8-209.3. These data strongly indicate that light-microscopic quantitation of the CBH response can be used to distinguish relatively weak to moderate contact sensitization reactions from primary irritation reactions to the same chemicals.

A review of the Buehler guinea pig skin sensitization test and its use in a risk assessment process for human skin sensitization

The Buehler test is a valuable procedure for screening the sensitization potential of chemicals prior to human exposure. Our experience of over 20 years has shown it to be effective in detecting strong, moderate, and most weak sensitizers. The topical exposure inherent in the Buehler test allows it to be utilized to investigate dose responses, cross reactivity between structurally related chemicals, and the sensitization potential of contaminants in raw material mixtures. For safety assessment purposes, Buehler test results provide an initial indication of the sensitization potential of the material in question under relevant, but exaggerated, exposure conditions. These results can be compared to results on benchmark chemicals to assess sensitization risk for subsequent human exposure. Optimizing the sensitivity of the Buehler test requires adherence to the published methodology and proper interpretation of the challenge and rechallenge data obtained. Adjuvant-type test methods are generally considered to be more sensitive than topical methods. However, when done properly, topical test procedures such as the Buehler test or the open epicutaneous test can accurately detect most chemicals with any realistic potential for sensitizing humans by the topical route. Moreover, from a risk assessment perspective, these topical tests avoid the problems of overestimating the weak sensitization potential of many topically applied materials or underestimating the sensitization potential of very strong sensitizers; both are potential concerns with invasive adjuvant-type test methods. The Buehler test or other topical test methods are particularly valuable for comparative sensitization risk assessment since human sensitization data on benchmark materials are all derived from topical exposure. The risk assessment is developed by comparing the guinea pig data on the new material versus relevant benchmark chemicals or formulations and also by evaluating the existing human sensitization data on the benchmark material. These data are then utilized to predict human sensitization risk from topical exposure to the new material. Confirmation of human safety can be derived from human repeat insult patch testing (HRIPT) and other clinical tests such as the product use test and the diagnostic patch test. Utilized in this manner, the Buehler test is an integral component of an overall skin sensitization safety assessment program for a new chemical or product formulation.

A risk assessment process for allergic contact sensitization

This review describes an approach that has been used to assess the skin sensitization risk of new chemicals and product formulations prior to launching the new chemical or product on the market. 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 the skin sensitization potential for a wide range of consumer products and pharmaceuticals, ranging from products with a very transient skin exposure (e.g. some paper products), to cosmetics, to products whose ingredients may be deposited on fabrics and thus result in chronic skin exposure. The risk assessment process described includes both guinea-pig (Buehler) and human skin sensitization test methodologies to evaluate inherent toxicity under relevant epicutaneous exposure conditions. Alternative guinea-pig test methods have been reported to be more sensitive than the Buehler method, particularly those employing intradermal injection of the test material in Freund's complete adjuvant (e.g. maximization test). However, by bypassing the skin barrier at induction, these methods can overstate the sensitization risk of epicutaneous exposure to weak sensitizers (Andersen and Hamann, 1983 and 1984; Matsushita et al., 1975a,b), and can understate the risk to very strong sensitizers possibly through tolerance induction (Buehler, 1985). In addition, materials are tested and classified at concentrations that may not be relevant to anticipated human exposure. The Buehler guinea-pig test data are important in assessing skin sensitization risk in the early phases of product development, where human exposure can be limited, controlled and monitored (e.g. manufacturing employees). The Buehler test can often define consumer skin sensitization risk; however, the ultimate consumer skin safety assessment should in general be developed through a series of controlled human tests; the human repeat insult patch test and, when necessary, the provocative or extended product use tests. Post-market monitoring of skin-related consumer comments is the final phase of the data gathering process. These results can be used to assess further each product and to provide valuable feedback to confirm the validity of the overall risk assessment process. Risk assessment for skin sensitization potential is seldom a simple process.(ABSTRACT TRUNCATED AT 400 WORDS)