A critical commentary and updating of the guinea pig maximization test

Establishment of a threshold of toxicological concern for pharmaceutical intermediates based on historical repeat-dose data and its application in setting health based exposure limits

Availability of toxicological data for pharmaceutical intermediates (IMs) used in the manufacture of small molecules is often limited. Scarcity of data - in particular, repeat-dose toxicity (RDT) - renders the calculation of health-based exposure limits (HBELs) problematic. Establishment of HBELs, including occupational exposure limits (OELs) and permitted daily exposures (PDEs) facilitating worker and patient safety respectively, is however essential. Historic 28-day oral rodent toxicity data was analysed for 103 GSK isolated IMs. No-observed (adverse) effect levels (NO(A)ELs) and critical effects were extracted. The 5th percentile (p05) of the NO(A)EL distribution was 15 mg/kg/day. Substance specific HBELs were calculated, selecting the NO(A)EL as the Point of Departure (PoD); 99% of IMs (n = 102) were assigned an oral PDE ≥1000 μg/day and OEL ≥100 μg/m3. A default oral PDE of 1000 μg/day and OEL of 100 μg/m3 is thus proposed for IMs. Evaluation of an additional PoD - benchmark dose lower confidence limit (BMDL) - further supported the default HBELs. The default oral PDE can also serve as a threshold of toxicological concern (TTC) for IMs. Default limits can aid in setting HBELs for novel data-poor IMs, as well as supporting waiving of RDT in the future through read-across.

Use of guinea pig data to obtain starting points for skin sensitisation risk assessment - A commentary

The increasing drive to understand the likelihood of skin sensitisation from plant protection products (PPPs) in workers and the general public has resulted in recent initiatives to establish a quantitative risk assessment (QRA) methodology applicable to these products and their exposure scenarios. The effective evaluation of skin sensitising substances requires not only the identification of that toxicological hazard, but also determination of relative sensitising potency. Typically, this has been achieved by interpretation of local lymph node assay (LLNA) dose response data, delivering what is known as the EC3 value. This permitted regulatory division of skin sensitisers into defined potency sub-categories, but more importantly enabled derivation of a no expected sensitisation induction level (NESIL) as the point of departure for QRA. However, for many existing substances there is no LLNA data, only older guinea pig results exist. To avoid additional (in vivo) testing, an approach has been outlined to employ guinea pig data and existing regulatory guidelines on the determination of potency sub-categorisation to provide a guinea pig based NESIL. The approach adopts a conservative extrapolation from LLNA NESIL benchmarks to deliver points of departure as the basis for the type of QRA process already in successful use by other industries.

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.

Skin sensitization in silico protocol

The assessment of skin sensitization has evolved over the past few years to include in vitro assessments of key events along the adverse outcome pathway and opportunistically capitalize on the strengths of in silico methods to support a weight of evidence assessment without conducting a test in animals. While in silico methods vary greatly in their purpose and format; there is a need to standardize the underlying principles on which such models are developed and to make transparent the implications for the uncertainty in the overall assessment. In this contribution, the relationship between skin sensitization relevant effects, mechanisms, and endpoints are built into a hazard assessment framework. Based on the relevance of the mechanisms and effects as well as the strengths and limitations of the experimental systems used to identify them, rules and principles are defined for deriving skin sensitization in silico assessments. Further, the assignments of reliability and confidence scores that reflect the overall strength of the assessment are discussed. This skin sensitization protocol supports the implementation and acceptance of in silico approaches for the prediction of skin sensitization.

An in vivo biocompatibility study of surgical meshes made from bacterial cellulose modified with chitosan

Bacterial cellulose modified with chitosan (MBC) is an innovative biomaterial used in regenerative medicine which may potentially improve treatment outcomes mesh for hernia repair surgery by facilitating better absorption in native tissue with less risk of mesh-related infections. The aim of the present study was to evaluate the biocompatibility of mesh based on MBC, and determine whether immunological reactions occur due to hypersensitivity to the implants. Forty five Imp:WIST rats were randomly assigned to be implanted with one of three mesh types: simple polypropylene mesh (n = 15), mesh modified by bacterial cellulose only (n = 15) and MBC mesh (n = 15) and evaluated after one and three months following intramuscular implantation. For MBC mesh, basic toxicological studies, i.e. Acute Dermal Irritation, Intradermal Reactivity and Acute Sensitization (GPMT), were also carried out on 9 Imp:BN albino rabbits and 15 Imp:D-H guinea pigs. The lowest immune response and the highest degree of fibroplasia were observed for MBC mesh both after one and three months after implantation. Toxicological studies classified the tested MBC mesh as a barely perceptible irritant with no signs of sensitization or allergic reactions observed during the studies. The findings indicate that MBC mesh does not irritate, does not sensitize and does not cause hypersensitivity in the implant site, and therefore presents a low risk of provoking such reactions in humans.

Skin sensitization, false positives and false negatives: experience with guinea pig assays

The advent of the local lymph node assay (LLNA), and efforts to develop in vitro alternatives for the identification of skin sensitizing chemicals has focused attention on the issue of false positive and false negative results. In essence, the question becomes 'what is the gold standard?' In this context, attention has focused primarily on the LLNA as this is now the preferred assay for skin sensitization testing. However, for many years prior to introduction of the LLNA, the guinea pig maximization test and the occluded patch test of Buehler were the methods of choice. In order to encourage a more informed dialogue about the relative performance, accuracy and applicability of the LLNA and guinea pig tests, we have here considered the extent to which guinea pig methods were themselves subject to false positives and negative results. We describe and discuss here well-characterized examples of instances where both false negatives (including abietic acid and eugenol) or false positives (including vanillin and sulfanilic acid) have been recorded in guinea pig tests. These and other examples are discussed with particular reference to the fabrication of a gold standard dataset that is required for the validation of in vitro alternatives.

Nothing is perfect, not even the local lymph node assay: a commentary and the implications for REACH

For many regulatory authorities, the local lymph node assay (LLNA) is the preferred assay for the predictive identification of skin-sensitizing chemicals. It is the initial requirement for sensitization testing within the new REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances) regulations in the European Union. The primary reasons for the preferment of the LLNA are the animal welfare benefits it provides compared with traditional guinea-pig methods (refinement and reduction of animal usage) and the general performance characteristics of the assay with regard to overall reliability, accuracy, and interpretation. Moreover, a substantial published literature on the LLNA is available making it appropriate for use as a benchmark against which new approaches, including in vitro alternatives, can be evaluated and validated. There is, therefore, a view that the LLNA represents the 'gold standard' for skin sensitization testing. However, although this is probably correct, it is important to recognize and acknowledge that in common with all other predictive tests (whether they be validated or not), the LLNA has limitations, in addition to strengths, some of which were mentioned above. Arguably, it is the limitations (e.g., the occurrence of false positive and false negative results) of test methods that are most important to understand. With respect to the LLNA, these limitations are similar to those associated with guinea-pig skin sensitization methods. Among these are the occurrence of false positive and false negative results, susceptibility of results to changes in vehicle, and the possibility that interspecies differences may confound interpretation. In this commentary, these issues are reviewed and their impact on the utility of the LLNA for identification, classification, and potency assessment of skin sensitizers are considered. In addition, their relevance for the future development and validation of novel in vitro and in silico alternatives is explored.

Evaluation of the skin sensitizing potential of biodegradable magnesium alloys

Corroding metals made of magnesium alloys represent a new class of degradable implants for musculoskeletal surgery. These implants may be associated with skin sensitizing reactions because of the release of metal ions. This study was conducted to compare the sensitizing potential of four different magnesium alloys (AZ31, AZ91, WE43, and LAE442) to current implant materials such as titanium (TiAl6V4) and a degradable polymer (SR-PLA96). Solutions and solid chips of these materials were prepared and tested in 156 guinea pigs according to the Magnusson-Kligman test. A standard allergen (hydroxy-cinnamon-aldehyde) causing allergic erythema was used as positive control and a standard irritant (sodium-lauryl-sulfate) causing local skin irritation for less than 24 h was used as negative control. All erythema were graded immediately and 24 h after patch removal by three independent observers. Histomorphological analyses were performed on skin biopsies taken 24 h after patch removal. We found that initial erythema in animals treated with solid chips diminished within 24 h and were caused by local skin irritation. Local skin irritation was also determined in erythema remaining for 24 h after patch removal in animals treated with dissolved test materials. No allergenic reactions according to the histomorphological criteria were observed in skin biopsies. We conclude that no skin sensitizing potential were detected for standard materials as well as for all tested magnesium alloys by the used methods.

Characterization of liver injury associated with hypersensitive skin reactions induced by trichloroethylene in the guinea pig maximization test

Trichloroethylene (TCE) can induce non-dose-related hepatitis, possibly classified as delayed-type hypersensitivity (immune-mediated hepatitis), as well as dose-related toxic liver injury. However, the difference in pathophysiology between the two kinds of hepatitis remains unknown. This study aimed to characterize the liver injury associated with hypersensitive skin reactions induced by TCE in guinea pigs. As a model of dose-related acute toxic liver injury, the animals were treated with intradermal injection (ii) (0, 167, 500, 1500 or 4500 mg/kg of TCE) or dermal patch (dp) (0 or 900 mg/kg of TCE). The guinea pig maximization test (GPMT) was also carried out as a model of immune-mediated liver injury, in which the total TCE dosage was below 340 mg/kg. In the group of TCE 4500 mg/kg (ii), alanine aminotransferase (ALT) and aspartate aminotransferase (AST) increased (p<0.01), while total protein and globulin decreased (p<0.05). Evident fatty degeneration, hepatic sinusoid dilation and inflammatory cell infiltration were observed. No significant change was found in animals treated with TCE of doses below 500 mg/kg (ii) or 900 mg/kg (dp). In the GPMT, sensitization rates of TCE-induced dermal allergy were 66%. ALT, AST, lactate dehydrogenase and the relative liver weight increased significantly (p<0.05) while albumin, IgA and gamma-glutamyl transpeptidase decreased significantly (p<0.05). Lesions of ballooning changes were observed in liver pathology. Thus, TCE could cause both acute-type toxic liver injury and immune-mediated liver injury, the so-called delayed-type hypersensitivity at doses below the dosage for toxic liver injury. Interestingly, the histopathological features were quite different: fatty degeneration was most prominent in the former, and ballooning in the latter.

Skin sensitization: strategies for the assessment and management of risk

Allergic contact dermatitis (ACD) is to a considerable extent a preventable disease. Limitation can be achieved by correct identification of skin sensitizers, characterization of their potency, understanding human skin exposure and application of good risk assessment/management strategies. Various methods exist which are accurate for the predictive identification of chemicals that possess skin-sensitizing properties. These are enshrined in regulations that aim to provide a harmonized approach to hazard identification. One of the methods, the local lymph node assay, also delivers information on the relative potency of sensitizers. Efforts are continuing in the European Union and at the Organization for Economic Cooperation and Development to use elements of this information for regulatory categorization of skin sensitizers. However, greater use can be made of this potency information in the application of quantitative risk assessments. Such assessments depend also on the availability of accurate data on human skin exposure, one aspect where legislation has little role to play. Management of risks by restriction of skin exposure is, in contrast, a key point where legislation can play an important role, helping to establish a level playing field for industry and setting good standards based on the legislator's ability to access all data. Ultimately, the combination of accurate hazard identification, potency measurement, risk assessment and management, underpinned by enabling legislation, will lead to reduction of ACD. For individuals who do still develop contact allergy, avoidance of ACD should continue to be a goal, based on raising awareness of skin protection, allergen labelling and other skincare strategies.

Information derived from sensitization test methods: test sensitivity, false positives and false negatives

Predictive toxicology tests for the prospective identification of skin-sensitizing chemicals are well known and have been used for many years. However, of these, only the local lymph node assay (LLNA) has actually undergone formal independent assessment to determine the accuracy of the predictions, particularly with respect to the likelihood of false positives and false negatives. Often, efforts to increase the sensitivity of a test (reducing false negatives) tend to increase the number of false positives. In this short review, these issues are discussed in particular relation to the 3 predictive tests available in regulatory toxicology, the guinea-pig maximization test, the occluded patch test of Buehler and the LLNA. A key perspective is that no predictive test is without limitations; having a good appreciation of these limitations is necessary for making the best use of the information derived from these methods.

Identification and classification of skin sensitizers: identifying false positives and false negatives

The first step in regulatory evaluation of substances involves the identification of their intrinsic hazards, including the potential for skin sensitization. This is, quite properly, entirely different from assessment of the risks to human health, which might arise from incorporation of substances in products. EU guidance on regulations concerning the classification of skin sensitizers suggests a range of sources of information be deployed in the hazard identification process. These include chemical structure, predictive animal tests, and various types of human data. Where the information is clear-cut, then uncertainties rarely arise. However, for some materials, discordant information arises, perhaps because the substance is on the borderline of test sensitivity and classification (sensitizing materials of insufficient potency do not classified according to the EU scheme), due to conflicting results in predictive tests or for other reasons. In this study, we review data on a number of substances where a classification decision is complicated by such discordances and seek to use these examples to demonstrate how best to make a weight of evidence decision on whether a substance should, or should not, be classified as a skin sensitizer.

Development of a Monoclonal Antibody-Based Sandwich ELISA for Detection of Guinea Pig Interleukin-2

Interleukin 2 (IL-2) is a T cell proliferation factor released by Th0- and Th1-type helper T cells and is an essential cytokine for immune responses. In the present study, recombinant glutathione S-transferase (GST)-guinea pig IL-2 (GPIL-2) fusion protein was prepared by Escherichia coli (E. coli) and by using this protein as an immunogen, monoclonal antibodies (mAbs) against GPIL-2 were produced to establish a basis for a research on immune responses in guinea pigs. Three stable hybridoma cell lines were established, and specific binding of each mAb to recombinant GPIL-2 produced by E. coli and insect cells infected with recombinant baculovirus was shown by enzyme linked immunosorbent assay (ELISA) and/or immunoblot analysis. Isotype analyses of these mAbs revealed that all three mAbs were IgG1 and had kappa chain. Furthermore, assessment of their epitopes by competition binding assay indicated that the mAbs obtained in this study bound to three different epitopes. Thus, a sandwich ELISA based on the two mAbs specific to different GPIL-2 epitopes was developed for detection of GPIL-2, which had a sensitivity threshold of about 0.3 ng/ml of GPIL-2.

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.

Capacity of CD34+ progenitor-derived dendritic cells to distinguish between sensitizers and irritants

In an attempt to develop an in vitro test to identify contact sensitizers, mostly dendritic cells (DCs) derived from monocytes (Mo-DC) have been used. Less is known about the potency of DC derived from CD34+ progenitors (CD34-DC) for in vitro allergen testing. CD34+ progenitor derived DC were exposed to nine well-known allergens (one weak, three moderate and five strong allergens) and two irritants. Surface marker expression (CD86, CD83 and HLA-DR) and cytokine production (IL-6, IL-12 and TNF-alpha) were analyzed after 24 h exposure to these chemicals. All allergens tested induced a significant increase in at least one of the DC surface markers. In contrast, none of the irritants tested were able to significantly upregulate membrane marker expression in exposed DC. The level of upregulation of CD86, CD83 and HLA-DR was dependent on the nature and concentration of the chemical, but not on the classification of the allergen. Changes in cytokine production (IL-6, IL-12 and TNF-alpha) were not consistently related to exposure to an allergen. Based on these results, we conclude that the in vitro test using CD34-DC has the capacity to distinguish between allergens and irritants based on altered phenotypic characteristics.

Strong irritants masquerading as skin allergens: the case of benzalkonium chloride

Chemicals may possess a number of hazards to human health including the ability to cause skin irritation and contact allergy. Identification and characterization of these properties should fall within predictive toxicology, but information derived from human exposure, including clinical experience, is also of importance. In this context, it is of interest to review the case of benzalkonium chloride, a cationic surfactant. This chemical is a well-known skin irritant, but on occasions it has also been reported to have allergenic properties, typically on the basis of positive diagnostic patch test data. Because the accumulated knowledge concerning the properties of a chemical is employed as the basis for its regulatory classification (e.g. in Europe), as well as for informing the clinical community with respect to the diagnosis of irritant versus allergic contact dermatitis (ACD), it is important to distinguish properly which chemicals are simply irritants from those which are both irritant and allergenic on skin. A review of the information on benzalkonium chloride confirms that it is a significant skin irritant. However, both predictive test results and clinical data lead to the conclusion that benzalkonium chloride is, at most, an extremely rare allergen, except perhaps in the eye, but with many supposed cases of ACD being likely to arise from the misinterpretation of patch test data. As a consequence, this substance should not normally be regarded as, or classified as, a significant skin sensitizer.

Toxicological and metabolic investigations of the safety of N-α-Lauroyl-l-arginine ethyl ester monohydrochloride (LAE)

Studies were conducted to assess the safety of N-alpha-lauroyl-L-arginine ethyl ester monohydrochloride, (LAE), a novel food preservative, or Mirenat-N (a 25% solution of LAE in propylene glycol). Short term studies demonstrated low acute toxicity. LAE was shown to have mild dermal irritation effects but neither LAE nor Mirenat-N are skin sensitizers. LAE was demonstrated to be a severe eye irritant. In two 13-week feeding studies in rats, systemic NOAELs were established for LAE at 15,000 ppm and for Mirenat-N at 50,000 ppm. There were no signs of neurotoxicity with LAE after 13-weeks at dietary levels as high as 50,000 ppm. Embryo-fetal studies with LAE in rats and rabbits showed no developmental effects at oral gavage doses up to 2000 and 1000 mg/kg/day for rats and rabbits, respectively. NOAELs for systemic maternal effects (reduced food intake and body weights in rabbits) were 2000 mg/kg/day for rats and 300 mg/kg/day for rabbits. In a battery of 5 in vitro genotoxicity tests with LAE or Mirenat-N, neither material was observed to have genotoxic (clastogenic or mutagenic) activity. Metabolism studies with LAE show that it is rapidly metabolized to the amino acid arginine by hydrolysis of the ethyl ester and lauroyl amide functions. The arginine subsequently enters the naturally occurring urea cycle where it is further metabolized to ornithine and urea and eventually to CO(2) through normal mammalian biochemical pathways. The other product of LAE cleavage is lauric acid, which is a human dietary component found in many plant sources, and as such, would enter into normal fatty acid metabolism.

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.

Experimental study on phototoxicity and the photosensitization potential of ketoprofen, suprofen, tiaprofenic acid and benzophenone and the photocross-reactivity in guinea pigs

BACKGROUND

Ketoprofen, suprofen and tiaprofenic acid are arylpropionic anti-inflammatories. Their chemical structures share the same elements as the benzoyl radical and the tiophene ring. We experienced nine cases of ketoprofen photoallergy, seven cases of suprofen photoallergy and three cases of tiaprofenic photoallergy.

PURPOSE

To find the key structure of photosensitivity and photocross-reactivity to ketoprofen, suprofen and tiaprofenic acid.

METHODS

: Three animals were tested for phototoxicity and six animals for the photosensitization potentials of ketoprofen, suprofen, tiaprofenic acid and benzophenone, and the photocross-reactivity of the above chemicals. Test substances were applied symmetrically on both sides of the animals' backs. The animals were irradiated with 180 mJ/cm2 UVB ((1/2) MED) and 10 J/cm2 UVA on the left side. The reactions were read on days 2, 3 and 4. The photosensitization potentials of ketoprofen, suprofen, tiaprofenic acid and benzophenone were determined using the Adjuvant-Strip method. Six animals were assigned to each test group and to a control group.

RESULTS

Ketoprofen, suprofen, tiaprofenic acid and propionic acid showed negative reactions with the phototoxic test. Benzophenone showed phototoxic reactions to 40% acetone (ac.), 20% ac. and 10% ac. Therefore, we used 5% aq. benzophenone with the photosensitization test. Ketoprofen was the strongest photosensitizer (6/6) and showed photocross-reactivities to suprofen (2/6), tiaprofenic acid (3/6) and benzophenone (6/6). Suprofen was a strong photosensitizer (4/6) and showed photocross-reactivities to ketoprofen (1/4) and tiaprofenic acid (2/4), but not to benzophenone. Tiaprofenic acid was also a photosensitizer (2/6) but showed a photocross-reactivity only to benzophenone (2/2). Benzophenone was also the strongest photosensitizer (6/6), but did not photocross-react to the above three chemicals.

CONCLUSION

From the test results, it appears that benzoyl radical is the key structure for photosensitivity and the photocross-reactivity of ketoprofen, suprofen and tiaprofenic acid. The whole structure of benzophenone was needed to induce photosensitization of benzophenone. The animals that were photosensitized from the entire structure of benzophenone did not photocross-react to ketoprofen, suprofen or tiaprofenic acid.

Irritant dermatitis, irritancy and its role in allergic contact dermatitis

Irritant contact dermatitis is the clinical result of sufficient inflammation arising from release of pro-inflammatory cytokines from skin cells (principally keratinocytes) in response to (usually) chemical stimuli. Different clinical forms may arise. The three main pathophysiological changes seen are skin barrier disruption, epidermal cellular changes and cytokine release. An important role of irritancy in allergic contact dermatitis (ACD) comes from earlier animal and human studies. Evidence is outlined which is consistent with a "danger model" of ACD rather than one based on a traditional "self-nonself" immune model. In such a model an antigenic signal will produce sensitization only in the presence of a danger signal; in the absence of a danger signal tolerance will occur. We propose that the danger signal in ACD is cytokine release from nonimmune skin cells (principally keratinocytes) and that both the antigenic and "danger" signals arises from the hapten.

Joint positive control testing in guinea pig skin sensitization tests. A harmonized approach

A scheme for the performance of positive control studies within a coordinated group of laboratories was proposed (joint positive control testing). The procedure has been described, as well as the first results of the validation phase of this joint positive control testing project. Adoption of this proposal within the participating six laboratories would lead to a reduction in the number of guinea pigs required for reliability and sensitivity checks from current approximate 12 studies per year down to 2 studies, i.e., 150-300 fewer animals per year. Another benefit would be the use of a harmonized, and therefore more comparable, method to perform guinea pig tests and interpret the data. In the validation phase of joint reading of the positive control studies, the congruency of reading could clearly be demonstrated. From the experience gained up to now, it was possible to draw the conclusion that a coordinated interlaboratory approach for positive control testing was fully acceptable and an improvement with regard to animal welfare.

Thresholds in contact sensitization: immunologic mechanisms and experimental evidence in humans--an overview

The evidence from practical experience in man, from controlled human testing and from both chemical and biological mechanistic considerations, demonstrates that the thresholds for skin sensitization exist both at the level of induction as well as sensitization. For a given contact allergen, the concentration (expressed in dose per unit area), which is at the threshold, has to be defined in terms of the nature and extent of the skin contact involved, and whether it is the induction of skin sensitization or its elicitation that is involved, since the numerical values will depend heavily on these parameters. The induction and elicitation of contact allergy is influenced by several factors: the total dose of the allergen, its surface concentration, the size of area over which it is applied, antigenic potency of the substance, number of exposures, anatomical site of exposure, effect of draining lymph nodes, matrix (vehicle) of allergen, effect of occlusion, degree of percutaneous penetration, effect of diseased skin/trauma and possibly a host of other, as yet unknown, variables.

Labelling of skin sensitizers: the new European Dangerous Preparations Directive

The new Dangerous Preparations Directive (DPD, 1999/45/EC) introduces a special labelling requirement for skin sensitizers in products that are regulated under this Directive. The packaging of products containing 0.1% of a sensitizer must bear the inscription "Contains 'name of sensitizer'. May produce an allergic reaction." The aim is to protect individuals already sensitized by providing information which enables them to avoid products containing ingredients which may elicit their allergy. However, this is only of benefit where such sensitized individuals do exist in the population. Moreover, this labelling requirement does not take into account the potency of the skin sensitizer. For each sensitizer and type of skin exposure, there will be levels below which it will not elicit allergic contact dermatitis reactions in individuals who are sensitized to that chemical. We therefore propose that within the new DPD, it should be possible to override this labelling requirement with well-documented data, to ensure that information provided to the consumer on the product label is not misleading. The current implementation in the DPD of what is in principle a good idea means that further action (legislative changes; scope for derogation) is needed if the potential benefits are not to be lost.

Skin sensitisation testing--new perspectives and recommendations

Various methodological aspects of skin sensitisation testing have been explored, particularly in the context of animal welfare considerations and reliability and sensitivity of test methods. Recommendations are made for the conduct of current and proposed OECD skin sensitisation tests with respect to appropriate test configurations for the purposes of hazard identification and labelling, and the requirement for positive controls. Specifically, the following aspects of guinea pig sensitisation test methods have been addressed: (1) the number of test and control animals required; (2) the option of using joint positive controls between independent laboratories; (3) the choice of positive control chemicals; (4) the optimal conduct and interpretation of rechallenge; and (5) the requirement for pretreatment with sodium lauryl sulfate. In addition, the use of the murine local lymph node assay (LLNA) has been considered. A number of conclusions have been drawn and recommendations made as follows: In many instances, particularly with the conduct of the guinea pig maximisation test, it is acceptable to halve the number of test and control animals used. An optional scheme for the conduct of joint positive control studies within a co-ordinated group of laboratories is appropriate. Only one positive control chemical (alpha-hexyl cinnamic aldehyde) is necessary for the routine assessment of assay sensitivity. The proper conduct and interpretation of rechallenge can provide valuable information and confirmation of results in guinea pig sensitisation tests. Sodium lauryl sulfate should no longer be used as a pretreatment in the guinea pig maximisation test. The LLNA is a viable and complete alternative to traditional guinea pig test methods for the purposes of skin sensitisation hazard identification. These recommendations provide the opportunity for both animal welfare benefits and improved hazard identification.

Thresholds in contact sensitization: theoretical and practical considerations

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

Classification criteria for skin-sensitizing chemicals: a commentary

A formalized, standardized and effective mechanism for the identification of substances which possess significant skin sensitization potential is a necessary first step in the process of limiting the incidence of allergic contact dermatitis. Strategies to achieve such hazard identification are unified throughout the European Union and also have been publicised by the World Health Organization. Global harmonization of these and other approaches (e.g., in the USA) is being driven by the Organization for Economic Cooperation and Development. In this paper, the benefits and limitations of these classification systems are reviewed. A common element of all the systems is that they seek to distinguish important contact allergens from chemicals which are infrequent sensitizers. The European criteria are legal requirements in the EU member states and formal classification as a skin sensitizer leads to mandatory labelling. The most notable omission from current and proposed classification criteria relates to the relative potency of a classified skin sensitizer and the exposure dose. Such information is necessary for proper risk assessment and management measures to be implemented.

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.

Evaluation of the capacity of dendritic cells derived from cord blood CD34+ precursors to present haptens to unsensitized autologous T cells in vitro

Langerhans cells play a key role in contact hypersensitivity reactions. The application of haptens on the skin leads to many modifications of these cells, including the increase of major histocompatibility complex II expression, allogeneic stimulation potency, and migration towards lymph nodes to activate T cells. Moreover, it has been shown that Langerhans cells cultured in vitro are able to prime naive T cells in response to hapten contact. From CD34+ progenitors present in cord blood, we generated dendritic cells of which some presented the phenotypic markers of Langerhans cells. We show that these cells are able to sensitize syngeneic naive (CD45RA+) T cells to haptens such as trinitrophenyl conjugate of trinitrobenzene sulfonic acid (TNP) and fluoroscein isothiocyanate. The response to TNP is higher than to fluoroscein isothiocyanate, whereas sodium dodecyl sulfate, an irritant molecule used as a control, never caused this effect. Phenotypic analysis of cellular suspensions and experiments of cell sorting lead to the conclusion that only CD1a+ cells are able to induce a primary response of syngeneic T cells to TNP or fluoroscein isothiocyanate. Furthermore, we have shown a close relationship between the differentiation state of dendritic cells and their ability to prime T lymphocytes. Dendritic cells are able to present haptens in an efficient manner between day 10 and 14 of culturing CD34+ progenitors, whereas they were efficient in presenting alloantigens from day 6 until after day 20. This dissociation suggests the need of an active metabolic process for hapten presentation in the direct treatment of dendritic cells with haptens. This model of hapten presentation was used for a panel of fragrance molecules and other molecules considered as weaker haptens than TNP and fluoroscein isothiocyanate.

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.

Managing the excited skin syndrome: patch testing hyperirritable skin

Inflammation-modulating phenomena (IMPs), humoral and cellular, fluctuate during the course of irritant and allergic contact dermatitis influencing irritability of the skin. The patch test procedure is a biological assay, a titration of responses of IMPs which can produce hyporeactivity or hyperirritability of the skin of patients who have dermatitis (PDs) and a single patch test is a 'snapshot' of the tempo of an evolving process. The excited skin syndrome (ESS) refers to hyperirritability from clinical and patch test dermatitis creating false-positive patch test reactions which are not reproducible when dermatitis and IMPs have subsided. During ESS, the threshold for irritancy decreases and irritant reactions increase. Patch test concentrations should be determined and ESS investigated in PDs having enhanced IMPs, not in 'normal' individuals, and if a patch test result is important to a patient the test should be performed more than once. Variable reproducibility is inherent in the patch test method, but ESS can be managed by appropriate testing and retesting, and search for relevance.

A quantitative structure-toxicity relationships model for the dermal sensitization guinea pig maximization assay

We have developed quantitative structure-toxicity relationship (QSTR) models for assessing dermal sensitization using guinea pig maximization test (GPMT) results. The models are derived from 315 carefully evaluated chemicals. There are two models, one for aromatics (excluding one-benzene-ring compounds), and the other for aliphatics and one-benzene-ring compounds. For sensitizers, the models can resolve whether they are weak/moderate or severe sensitizers. The statistical methodology, based on linear discriminant analysis, incorporates an optimum prediction space (OPS) algorithm. This algorithm ensures that the QSTR model will be used only to make predictions on query structures which fall within its domain. Calculation of the similarities between a query structure and the database compounds from which the applicable model was developed are used to validate each skin sensitization assessment. The cross-validated specificity of the equations ranges between 81 and 91%, and the sensitivity between 85 and 95%. For an independent test set, specificity is 79%, and sensitivity 82%.

Flow-cytometric screening for the modulation of receptor-mediated endocytosis in human dendritic cells: implications for the development of an in vitro technique for predictive testing of contact sensitizers

The aim of this study was to explore the usefulness of human blood dendritic cells (DC) in the development of an in vitro model for predictive testing of contact sensitizers. A method was established to monitor the influence of chemicals on the intracellular targeting of antibody-crosslinked MHC class II molecules after their uptake by human DC. Using a three-colour flow-cytometric technique, freshly prepared DC were distinguished from other MHC class II-bearing cell types such as B-cells and monocytes in unseparated mononuclear cell suspensions of healthy volunteers. The assay is based on the pH-sensitivity of internalized fluorescein-coupled MHC class II specific antibodies. Quenching of fluorescence intensity due to internalization into acidic intracellular compartments was observed with untreated DC whereas internalization into less acidic structures following stimulation with strong contact sensitizers ensured that the fluorescence intensity was conserved. The usefulness of this approach for predictive testing of the preservatives MI/MCI, imidazolidinyl urea, methyl-4-hydroxy-benzoate and 2-phenoxyethanol in comparison to the strong allergen DNFB and the irritants sodium lauryl sulphate and dithranol was explored. Whereas low concentrations of MI/MCI resembled the strong allergen DNFB, high concentrations of imidazolidinyl urea were required for a moderate response. Methyl-4-hydroxy-benzoate and 2-phenoxyethanol as well as the irritants SLS and dithranol failed to induce a significant effect in this assay. The non-responsiveness to the latter compounds reflected their minor or absent capacity to induce contact hypersensitivity in humans, whereas DNFB, MI/MCI and imidazolidinyl urea are well established contact sensitizers. These data suggest that the capacity of a chemical to modulate endocytotic mechanisms in dendritic cells in vitro seems to reflect the probability of that substance acting as a hapten in vivo.

The local lymph node assay: a viable alternative to currently accepted skin sensitization tests

The prospective identification of skin sensitizing chemicals is a vital prerequisite for their proper risk management. Traditionally this has been achieved largely by the conduct of guinea pig assays such as the maximization and Buehler tests. These methods are recommended by the Organisation for Economic Cooperation and Development (OECD) and are required by the European Union (EU) for the evaluation of new substances. However, a novel mechanistically based method, the local lymph node assay (LLNA), has been the focus of substantial validation activity in recent years. This material is reviewed in this paper. It is shown that the LLNA has been validated successfully by five interlaboratory assessments as well as by comparisons with guinea pig tests and human data. The method also offers clear advantages to the user in terms of objectivity, time and cost, and delivers important animal welfare benefits. In consequence, it is recommended that the LLNA be formally adopted by the OECD in Guideline 406 and accepted by the EU and US EPA as a method suitable for the classification of the skin sensitizing potential of chemicals.

The value and limitations of rechallenge in the guinea pig maximization test

The guinea pig maximization test (GPMT) has played a primary rôle in the evaluation of potential skin contact sensitizers for 25 years. In the OECD Guideline 406 from 1993, it is specifically suggested that equivocal results from the initial challenge in the GPMT should be evaluated further with a repeated challenge. However, there exist few published rechallenge data and the guideline does not describe how rechallenge data should be interpreted. In this paper, we have used examples from published results to illustrate both the positive value and the limitations of repeated challenges, including cross challenge. Testing with modified concentrations may also help to indicate whether or not the response is allergic in nature, particularly where there has been a low level of skin reaction observed in shamtreated controls, or where a low level of skin reaction is the dominant response in the test animals. In conclusion, the data presented demonstrate that, as a tool for the investigation of skin sensitizing potential, the GPMT can benefit from an experienced scientific evaluation of rechallenge data, but that this information should not be treated in a mechanistic fashion.

Nonspecific hypersensitivity: false-positive responses with the use of Freund's complete adjuvant

While conducting a guinea pig sensitization protocol, using the maximization test, it was discovered, at challenge, that the test animals were more responsive to the vehicle (acetone) than they were to the proprietary test material. During rechallenge, conducted to clarify the specific immune status of the test animals, it was determined that they were also hyperreactive to an alternate vehicle (diethyl phthalate), to which they were naïve. This bizarre set of data is presented and it is suggested that this type of response is the prototype for the presence of false-positive responses experienced by toxicologists using this test. The test conditions imposed on the immune system by the maximization test that could result in these anomalous results are discussed. These data suggest that investigators need cautiously to interpret data that are produced by the injection of Freund's complete adjuvant (FCA).