Preclinical skin sensitization testing of antihistamines: guinea pig and local lymph node assay responses

A review of the scientific basis for uncertainty factors for use in quantitative risk assessment for the induction of allergic contact dermatitis

Safety evaluations for chemicals which possess the ability to cause sensitization by skin contact have traditionally been done using an ad hoc comparative risk assessment technique. Recently, several papers have been published supporting the use of an alternative, and potentially better, quantitative risk assessment approach. While they represent a relatively new approach to risk assessment for sensitizers, quantitative methods have been used for decades to support risk assessments for systemic toxicity. Historically, these methods have involved the extrapolation of toxicity data - generally from studies in laboratory animals at relatively high doses to human exposures at lower doses. For toxicity endpoints with a threshold, this process has traditionally involved the use of uncertainty factors. For example, uncertainty factors are commonly used to extrapolate from laboratory animals to humans, and from 'average' humans to sensitive subpopulations. In the absence of data to support a different value, a default factor of 10 is widely accepted for each of these areas. Recent papers have advocated the use of a similar approach to characterize the risk of the induction of skin sensitization by allergens of varying potency and potential for skin contact. As with other forms of toxicity, a quantitative assessment of risk for allergic skin reactions can be approached by identifying a NOAEL (no observed adverse effect level) and applying appropriate uncertainty factors. Three major areas of data extrapolation have been identified: inter-individual susceptibility, the influence of vehicle or product matrix, and exposure considerations. This paper provides an overview of each of these areas with an evaluation of the available scientific database to support an uncertainty factor in the range of 1-10 for each area.

Insights into the quantitative relationship between sensitization and challenge for allergic contact dermatitis reactions

The ability of chemical or pharmaceutical agents to induce allergic contact dermatitis (ACD) is of major health and regulatory concern. As such, tests to identify their sensitizing capacity, such as the guinea pig maximization test and the more recently developed local lymph node assay, are broadly used. Ideally, for risk assessment it is useful to translate results from animal data into establishing safe or no-effect levels for occupational or environmental agents. This, of course, would require consideration of the quantitative relationships between sensitizing and challenge doses as well as other exposure conditions. In the present studies, we modeled two sensitizers, 2,4-dinitrochlorobenzene and squaric acid dibutyl ester, over a large range of concentrations using the LLNA and more traditional tests that measure both sensitization and elicitation responses. Both the sensitization and challenge phases provided similar dose-response curves, demonstrating a threshold followed by a shallow linear increase and eventual plateau at increasing doses. Extending earlier studies by P. S. Friedmann (1994, Immunotoxicology and Immunopharmacology, pp. 589-616, Raven Press, New York) in humans, we observed that the minimum dose required to elicit sensitization or challenge was not static, but rather reflected a "sliding-scale." That is, as the sensitization dose was increased, the concentration required to elicit a challenge response was decreased. Correspondingly, as the challenge dose was increased, the dose required for sensitization was lessened. Taken together, these findings indicate that there is a need to consider dose-response relationships for sensitization and challenge in establishing minimum exposure levels for chemicals that cause ACD.

Understanding fragrance allergy using an exposure-based risk assessment approach

Conducting a sound skin sensitization risk assessment prior to the introduction of new ingredients and products into the market place is essential. The process by which low-molecular-weight chemicals induce and elicit skin sensitization is dependent on many factors, including the ability of the chemical to penetrate the skin, react with protein, and trigger a cell-mediated immune response. Based on our chemical, cellular and molecular understanding of allergic contact dermatitis, it is possible to carry out a quantitative risk assessment. Specifically, by estimating the exposure to the allergen and its allergenic potency, it is feasible to assess quantitatively the sensitization risk of an ingredient in a particular product type. This paper focuses on applying exposure-based risk assessment tools to understanding fragrance allergy for 2 hypothetical products containing the fragrance allergen cinnamic aldehyde. The risk assessment process predicts that an eau de toilette leave-on product containing 1000 ppm or more cinnamic aldehyde would pose an unacceptable risk of induction of skin sensitization, while a shampoo, containing the same level of cinnamic aldehyde, would pose an acceptable risk of induction of skin sensitization, based on limited exposure to the ingredient from a rinse-off product application.

Evaluation of skin sensitization potential of melatonin and nimesulide by murine local lymph node assay

Melatonin is a good candidate for transdermal delivery considering its short plasma half life, low molecular weight and a favorable octanol:water partition coefficient. Nimesulide is a nonsteroidal anti-inflammatory agent used orally and rectally for inflammatory disorders. The objective of this study was to investigate the skin sensitization potential of melatonin and nimesulide using the standard murine local lymph node assay (LLNA). Melatonin (0.5, 2.5, 5.0 and 10.0%, w/v) and nimesulide (0.5, 2.5, 5.0 and 10.0%, w/v) dissolved in acetone:olive oil (4:1, AOO) was applied (25 microl) on the dorsal surface of each ear of female CBA/Ca mice for three consecutive days. On the sixth day, [3H]methyl thymidine was administered intravenously and the uptake of [3H]methyl thymidine (dpm) by the draining lymph nodes was determined by established methods. Dinitrochlorobenzene (DNCB, 0.25%, w/v) and para-aminobenzoic acid (PABA, 2.5%, w/v) were used as positive and negative control, respectively. The mean dpm obtained with melatonin and nimesulide treatment at all concentrations were not significantly different (P>0.05) from that of AOO. The stimulation index (SI) values of melatonin and nimesulide at different concentrations were close to 1. The results of the present study using the standard LLNA approved by US Interagency Coordinating Committee in the Validation of Alternative Methods (ICCVAM) indicate that melatonin and nimesulide are not skin sensitizers. However, since LLNA has shown false negatives with many drugs, clinical trials are certainly needed to exclude the possibility of a weak or delayed type skin sensitization reaction. Further studies using modified LLNA procedures (extended exposure, alternative vehicle systems, pre-abrasion, etc.) may be useful in identifying the weak or delayed type skin sensitization reactions.

Skin sensitisation, vehicle effects and the local lymph node assay

Accurate risk assessment in allergic contact dermatitis is dependent on the successful prospective identification of chemicals which possess the ability to behave as skin sensitisers, followed by appropriate measurement of the relative ability to cause sensitisation; their potency. Tools for hazard identification have been available for many years; more recently, a novel approach to the quantitative assessment of potency--the derivation of EC3 values in the local lymph node assay (LLNA)--has been described. It must be recognised, however, that these evaluations of chemical sensitisers also may be affected by the vehicle matrix in which skin exposure occurs. In this article, our knowledge of this area is reviewed and potential mechanisms through which vehicle effects may occur are detailed. Using the LLNA as an example, it is demonstrated that the vehicle may have little impact on the accuracy of basic hazard identification; the data also therefore support the view that testing ingredients in specific product formulations is not warranted for hazard identification purposes. However, the effect on potency estimations is of greater significance. Although not all chemical allergens are affected similarly, for certain substances a greater than 10-fold vehicle-dependent change in potency is observed. Such data are vital for accurate risk assessment. Unfortunately, it does not at present appear possible to predict notionally the effect of the vehicle matrix on skin sensitising potency without recourse to direct testing, for example by estimation of LLNA EC3 data, which provides a valuable tool for this purpose.