Risk management of skin sensitisers: A commentary.

Historically, allergic contact dermatitis (ACD) to chemicals encouraged hazard identification improvements, more sophisticated risk assessment and implementation of regulatory strategies, including banning of specific sensitising substances. The validation process applied to hazard identification methods demonstrates their accuracy; their use to characterise sensitiser potency facilitates quantitative and transparent risk assessment. Diagnostic patch testing at dermatology clinics worldwide delivers feedback showing where risk assessment/management has been insufficient or did not target the exposure of concern, thereby facilitating improvements. When urgent action to protect human health was required, regulations limited/banned, specific skin sensitisers. This can be seen in practice with the fragrance industry, a known source of ACD, thus requiring risk management, usually restrictions to limit allergy induction, and very rarely specific bans on ingredients. Experience and development of more sophisticated tools, e.g. to assess aggregate exposure from multitude of consumer product types, has led to repeated adaptation of risk assessment and promulgation of updated fragrance use limits. Although targeted control may not always lead to rapid change in the overall clinical picture, it is preferable to a blanket undifferentiated regulatory control of all sensitisers, resulting in unwarranted restrictions for many uses of no health concern, with consequent substantial socio-economic impacts.

Specificity of the local lymph node assay (LLNA) for skin sensitisation.

The local lymph node assay (LLNA) has provided a large dataset against which performance of non-animal approaches for prediction of skin sensitisation potential and potency can be assessed. However, a recent comparison of LLNA results with human data has argued that LLNA specificity is low, with many human non-sensitisers, particularly hydrophobic chemicals, being false positives. It has been suggested that such putative false positives result from hydrophobic chemicals causing cytotoxicity, which induces irritancy, in turn driving non-specific lymphocyte proliferation. This paper finds that the apparent reduced specificity of the LLNA largely reflects differences in definitions of the boundaries between weak skin sensitisers and non-sensitisers. A small number of LLNA false positives may be due to lymphocyte proliferation without skin sensitisation, but most alleged 'false' positives are in fact very weak sensitisers predictable from structure-activity considerations. The evidence does not support the hypothesis for hydrophobicity-induced false positives. Moreover, the mechanistic basis is untenable. Sound LLNA data, appropriately interpreted, remain a good measure of sensitisation potency, applicable across a wide hydrophilicity-hydrophobicity range. The standard data interpretation protocol enables detection of very low levels of sensitisation, irrespective of regulatory significance, but there is scope to interpret the data to give focus on regulatory significance.

Plant extracts, polymers and new approach methods: Practical experience with skin sensitization assessment.

Over the last decade, research into methodologies to identify skin sensitization hazards has led to the adoption of several non-animal methods as OECD test guidelines. However, predictive accuracy beyond the chemical domains of the individual validation studies remains largely untested. In the present study, skin sensitization test results from in vitro and in chemico methods for 12 plant extracts and 15 polymeric materials are reported and compared to available in vivo skin sensitization data. Eight plant extracts were tested in the DPRA and h-CLAT, with the 2 out of 3 approach resulting in a balanced accuracy of 50%. The balanced accuracy for the 11 plant extracts assessed in the SENS-IS was 88%. Excluding 5 polymers inconclusive in vitro, the remainder, assessed using the 2 out of 3 approach, resulted in 63% balanced accuracy. The SENS-IS method, excluding one polymeric material due to technical inapplicability, showed 68% balanced accuracy. Although based on limited numbers, the results presented here indicate that some substance subgroups may not be in the applicability domains of the method used and careful analysis is required before positive or negative results can be accepted.

Quantitative risk assessment of skin sensitising pesticides: Clinical and toxicological considerations.

Like many other consumer and occupational products, pesticide formulations may contain active ingredients or co-formulants which have the potential to cause skin sensitisation. Currently, there is little evidence they do, but that could just reflect lack of clinical investigation. Consequently, it is necessary to carry out a safety evaluation process, quantifying risks so that they can be properly managed. A workshop on this topic in 2022 discussed how best to undertake quantitative risk assessment (QRA) for pesticide products, including learning from the experience of industries, notably cosmetics, that already undertake such a process routinely. It also addressed ways to remedy the matter of clinical investigation, even if only to demonstrate the absence of a problem. Workshop participants concluded that QRA for skin sensitisers in pesticide formulations was possible, but required careful justification of any safety factors applied, as well as improvements to the estimation of skin exposure. The need for regulations to stay abreast of the science was also noted. Ultimately, the success of any risk assessment/management for skin sensitisers must be judged by the clinical picture. Accordingly, the workshop participants encouraged the development of more active skin health monitoring amongst groups most exposed to the products.

Enzymes and sensitization via skin exposure: A critical analysis.

Some proteins, including enzymes, can induce allergic sensitization of various types, including allergic sensitization of the respiratory tract. There is now an increased understanding of the role that the skin plays in the development of IgE-mediated allergy and this prompts the question whether topical exposure to enzymes used widely in consumer cleaning products could result in allergic sensitization. Here, the evidence that proteins can interact with the skin immune system and the way they do so is reviewed, together with a consideration of the experience gained over decades of the use of enzymes in laundry and cleaning products. The conclusion drawn is that although transcutaneous sensitization to proteins can occur (typically through compromised skin) resulting in IgE antibody-mediated allergy, in practice such skin contact with enzymes used in laundry and cleaning products does not appear to pose a significant risk of allergic disease. Further, the evidence summarized in this publication support the view that proteins do not pose a risk of allergic contact dermatitis.

Harnessing co-operative immune augmentation by contact allergens to enhance the efficacy of viral vaccines.

Although the development of successful vaccines against coronaviruses may be achieved, for some individuals the immune response that they stimulate may prove to be insufficient for effective host defence. The principle that a relatively strong contact allergen will have an enhancing effect on sensitization compared with a less potent contact allergen if they are co-administered, may not, at first, appear relevant to this issue. However, this augmentation effect is thought to be due to the sharing of common or complementary pathways. Here, we briefly consider aspects of the shared and complementary pathways between skin sensitization induced by exposure to a contact allergen and the immune response to viruses, with particular reference to COVID-19. The relationship leads us to explore whether this principle, which we name here as "co-operative immune augmentation" may be extended to include viral vaccination. We consider evidence that even relatively weak contact allergens, used in vaccines for other purposes, can show enhanced sensitization, which is in keeping with a co-operative augmentation principle. Finally, we consider how the potent contact allergen diphenylcyclopropenone could be employed safely as an enhancer of vaccine responses.

Fragrance inhalation and adverse health effects: The question of causation.

The toxicology of fragrance materials is largely well understood. Although most are benign, a minority have the potential to cause adverse health effects, notably allergic contact dermatitis resulting from skin sensitization. As a consequence, industry guidelines have banned certain materials and strictly limited the use of others. Recently, data have been published that have been interpreted to suggest that inhalation of fragrances is associated with the occurrence of a variety of health effects, ranging from headaches to asthma attacks. In this review, the evidence basis for these assertions is examined critically and the biological basis and mechanistic plausibility for causation by fragranced products of these health effects is explored. This review concludes that respiratory effects, including irritation and allergy appear highly unlikely to occur by this route. While some sensory/psychosomatic effects are possible, this does not explain the very high rates of adverse effects reported in the recently published questionnaire studies, which this review concludes are more likely to be attributed to methodological weaknesses. Ultimately, it is concluded that adverse health effects arising from fragrance inhalation are uncommon and remain to be identified and confirmed by methodologically rigorous epidemiological investigations supported by a convincing biological and mechanistic basis.

Addressing the conundrums of p-phenylenediamine hair dye allergy by applying Friedmann's principles of contact sensitization.

In the first conundrum, permanent hair dyeing involves the use of aromatic amines such as p-phenylenediamine (PPD), whose oxidation is pivotal to the dyeing process, but also generates potent allergens. Despite prolonged efforts by industry to search for safer alternatives, hair dyeing is still reliant on this type of aromatic amine. In the second conundrum, patch testing with 1% PPD remains the most useful screen for hair dye contact allergy. However, there is a very small but real risk of actively sensitizing the patient. Lowering the PPD concentration below 1% significantly reduces test sensitivity and diagnostic utility. Here, we argue that by applying Friedmann's principles of contact sensitization each conundrum can be addressed from a new perspective. These principles indicate that, when the exposed area of skin is small (<1 cm2 ), induction of contact allergy is sharply reduced, whereas elicitation of allergy is unaffected. Careful reflection on this principle suggests that we can predict where hair dye sensitization is most likely to occur, indicates a strategy to reduce the chance of contact sensitization occurring in consumers as a result of hair dyeing, and how we might mitigate the risk of active sensitization resulting from diagnostic patch testing.

Are skin sensitisation test methods relevant for proteins?

Tests for identification of chemical skin sensitisation hazard have been available for decades, evolving from guinea pig assays, through the first validated method, the local lymph node assay, to several validated in vitro methods. These methods successfully identify the hazard for chemicals, with an accuracy in the region of 85%. However, in some regulations, consideration may be given to their application to proteins. Here, the scientific relevance of the use of skin sensitisation tests for the assessment of the allergenic potential of proteins is reviewed and considered in the context of both: (a) what is known of the allergenic properties of proteins compared with chemicals, and (b) current understanding of the extent to which proteins actually give rise to contact allergy. There is no doubt that many foreign proteins can behave as respiratory sensitisers and food allergens, and that certain proteins can also cause cutaneous allergy via skin contact, typically mediated via immunoglobulin E (IgE) antibody. However, the absence of any specificity in predictions from existing skin sensitisation test methods, together with the lack of a suitable body of either positive or negative controls, dictates that use of these tests with proteins is without any scientific justification or predictive merit.

Quantitative risk assessment for skin sensitization: Success or failure?

Skin sensitization is unique in the world of toxicology. There is a combination of reliable, validated predictive test methods for identification of skin sensitizing chemicals, a clearly documented and transparent approach to risk assessment, and effective feedback from dermatology clinics around the world delivering evidence of the success or failure of the hazard identification/risk assessment/management process. Recent epidemics of contact allergy, particularly to preservatives, have raised questions of whether the safety/risk assessment process is working in an optimal manner (or indeed is working at all!). This review has as its focus skin sensitization quantitative risk assessment (QRA). The core toxicological principles of QRA are reviewed, and evidence of use and misuse examined. What becomes clear is that skin sensitization QRA will only function adequately if two essential criteria are met. The first is that QRA is applied rigourously, and the second is that potential exposure to the sensitizing substance is assessed adequately. This conclusion will come as no surprise to any toxicologist who appreciates the basic premise that "risk = hazard x exposure". Accordingly, use of skin sensitization QRA is encouraged, not least because the essential feedback from dermatology clinics can be used as a tool to refine QRA in situations where this risk assessment tool has not been properly used.

Can currently available non-animal methods detect pre and pro-haptens relevant for skin sensitization?

Predictive testing to characterize substances for their skin sensitization potential has historically been based on animal tests such as the Local Lymph Node Assay (LLNA). In recent years, regulations in the cosmetics and chemicals sectors have provided strong impetus to develop non-animal alternatives. Three test methods have undergone OECD validation: the direct peptide reactivity assay (DPRA), the KeratinoSens™ and the human Cell Line Activation Test (h-CLAT). Whilst these methods perform relatively well in predicting LLNA results, a concern raised is their ability to predict chemicals that need activation to be sensitizing (pre- or pro-haptens). This current study reviewed an EURL ECVAM dataset of 127 substances for which information was available in the LLNA and three non-animal test methods. Twenty eight of the sensitizers needed to be activated, with the majority being pre-haptens. These were correctly identified by 1 or more of the test methods. Six substances were categorized exclusively as pro-haptens, but were correctly identified by at least one of the cell-based assays. The analysis here showed that skin metabolism was not likely to be a major consideration for assessing sensitization potential and that sensitizers requiring activation could be identified correctly using one or more of the current non-animal methods.

Influence of vitamin C on the elicitation of allergic contact dermatitis to p-phenylenediamine.

BACKGROUND: Hair dyes represent one of the most important causes of allergic contact dermatitis resulting from the use of cosmetic products. The principal causative chemistry is associated with oxidation products of p-phenylenediamine (PPD) and closely related substances. OBJECTIVES: To examine whether prior application of the antioxidant vitamin C to the skin was able to reduce the cutaneous allergic response to PPD. METHODS: Twenty eight volunteers with a proven history of contact allergy to PPD were recruited. Each was tested with a range of PPD doses and PPD-containing hair dye on untreated skin and skin pretreated for 10 min with a vitamin C formulation. RESULTS: Pretreatment of skin sites with vitamin C led to a reduction in the intensity, or even ablation, of the cutaneous allergic reaction to PPD in ∼75% of cases as compared with untreated skin. CONCLUSIONS: The results suggest that treatment of the skin adjacent to the hair-bearing area with antioxidant could form part of a strategy to reduce the burden of cosmetic allergic contact dermatitis caused by hair dyeing.

Managing the Risk of Occupational Allergy in the Enzyme Detergent Industry.

Enzyme proteins have potential to cause occupational allergy/asthma. Consequently, as users of enzymes in formulated products, detergents manufacturers have implemented a number of control measures to ensure that the hazard does not translate into health effects in the workforce. To that end, trade associations have developed best practice guidelines which emphasize occupational hygiene and medical monitoring as part of an effective risk management strategy. The need for businesses to recognize the utility of this guidance is reinforced by reports where factories which have failed to follow good industrial hygiene practices have given rise to incidences of occupational allergy. In this article, an overview is provided of how the industry guidelines are actually implemented in practice and what experience is to be derived therefrom. Both medical surveillance and air monitoring practices associated with the implementation of industry guidelines at approximately 100 manufacturing facilities are examined. The data show that by using the approaches described for the limitation of exposure, for the provision of good occupational hygiene and for the active monitoring of health, the respiratory allergenic risk associated with enzyme proteins can be successfully managed. This therefore represents an approach that could be recommended to other industries contemplating working with enzymes.

Categorisation of protein respiratory allergens: the case of Subtilisin.

Characterisation of the relative sensitizing potency of protein and chemical allergens remains challenging, particularly for materials causing allergic sensitization of the respiratory tract. There nevertheless remains an appetite, for priority setting and risk management, to develop paradigms that distinguish between individual respiratory allergens according to perceptions of the hazards and risks posed to human health. One manifestation thereof is recent listing of certain respiratory allergens as Substances of Very High Concern (SVHC) under the provisions of REACH (Registration, Evaluation, Authorisation and restriction of Chemicals). Although priority setting is a laudable ambition, it is important the process is predicated on evidence-based criteria that are transparent, understood and owned. The danger is that in the absence of rigorous criteria unwanted precedents can be created, and confidence in the process is compromised. A default categorisation of sensitisers as SVHC requiring assessment under the authorisation process is not desirable. We therefore consider here the value and limitations of selective assignment of certain respiratory allergens as being SVHC. The difficulties of sustaining such designations in a sound and equitable way is discussed in the context of the challenges that exist with respect to assessment of potency, and information available regarding the effectiveness of exposure-based risk management.

Diisocyanates, occupational asthma and IgE antibody: implications for hazard characterization.

Sensitization of the respiratory tract by chemicals resulting in rhinitis and asthma is an important occupational health issue. Occupational asthma is associated with significant morbidity and can be fatal. Tests for the identification and characterization of chemicals with the potential to cause sensitization of the respiratory tract are lacking. In spite of sustained interest there are no validated or widely accepted methods available, and this presents toxicologists with a considerable challenge. One important constraint on the development of appropriate testing strategies has been uncertainty and controversy about the immunological mechanisms through which chemicals may induce sensitization of the respiratory tract. By analogy with protein respiratory allergy it is legitimate to consider that IgE antibody-dependent mechanisms may play a pivotal role. However, although many aspects of chemical respiratory allergy are consistent with reactions caused by IgE antibody, uncertainty remains because among patients with occupational asthma caused by chemical respiratory allergens there are commonly a proportion, and sometimes a significant proportion, of subjects that lack detectable IgE antibody. Here we consider the relevance of IgE antibody responses for the development of a chemical respiratory allergy to diisocyanates. A case is made that IgE antibody responses are, either directly or indirectly, closely associated with occupational asthma to the diisocyanates (and to other chemical respiratory allergens). As such the argument is advanced here that IgE antibody represents an appropriate readout for the characterization of chemical respiratory allergens, and that uncertainty about mode of action should no longer represent a hurdle in the development of suitable test methods.

Performance standards and alternative assays: practical insights from skin sensitization.

To encourage the development and validation of alternative toxicity test methods, the effort required for validation of test methods proposed for regulatory purposes should be minimized. Performance standards (PS) facilitate efficient validation by requiring limited testing. Based on the validated method, PS define accuracy and reliability values that must be met by the new similar test method. The OECD adopted internationally harmonized PS for evaluating new endpoint versions of the local lymph node assay (LLNA). However, in the process of evaluating a lymph node cell count alternative (LNCC), simultaneous conduct of the regulatory LLNA showed that this standard test may not always perform in perfect accord with its own PS. The LNCC results were similar to the concurrent LLNA. Discrepancies between PS, LLNA and LNCC were largely associated with "borderline" substances and the variability of both endpoints. Two key lessons were learned: firstly, the understandable focus on substances close to the hazard classification borderline are more likely to emphasise issues of biological variability, which should be taken into account during the evaluation of results; secondly, variability in the results for the standard assay should be considered when selecting reference chemicals for PS.

Dimethylfumarate: potency prediction and clinical experience.

BACKGROUND: Dimethylfumarate (DMF) was the cause of a major outbreak of allergic contact dermatitis as a consequence of its use as an antifungal agent in leather products, particularly in furniture, with what became known as 'toxic sofa dermatitis'. OBJECTIVES: To determine whether the frequency and severity of reactions to DMF arose as a function of its intrinsic potency and/or the nature and extent of exposure. METHODS: The intrinsic potency of DMF was measured with the standard local lymph node assay (LLNA), with determination of an EC3 value, which is the threshold in the LLNA and serves as an indicator of relative skin-sensitizing potency in humans. RESULTS: The EC3 value for DMF was 0.35% when tested in dimethylformamide as a vehicle, indicating that DMF is a strong, but not an extreme, skin sensitizer in this mouse model. CONCLUSIONS: DMF appears to have a sensitizing potency in the mouse that is very similar to that of formaldehyde, which is also a strong human skin sensitizer. However, the frequency and intensity of allergic contact dermatitis reactions to DMF suggest that it was the prolonged, repeated and occlusive exposure to this chemical over large skin areas, combined with the strong sensitizing potency, that generated the 'perfect storm' conditions that caused the DMF epidemic.

Dendritic cells and the assessment in vitro of skin sensitizing potential.

It is now well established that dendritic cells (DC) play pivotal roles in the initiation and orchestration of adaptive immune responses, including cutaneous immune responses to chemical allergens that drive the acquisition of skin sensitization. It is not unexpected, therefore, that a large number, and wide variety, of proposed approaches for the identification of skin sensitizing chemicals in vitro are based upon the use of cultured DC or DC-like cells. The use of DC in this context is legitimate. However, with our rapidly increasing understanding of the diversity of cutaneous DC with respect to both phenotype and function, it is timely now to review briefly the potential limitations and interpretive difficulties that are associated with the use of DC-based assays. Among the important considerations are the fact that chemical-induced changes in the characteristics and function of cultured DC will not necessarily reflect accurately the events that that support the development of skin sensitization in vivo. In addition, most DC-based assays are predicated on a view that cutaneous DC have as their primary function the initiation of adaptive immune responses. However, it is now appreciated that cutaneous DC, and in particular epidermal Langerhans cells (LC), may also play important immunoregulatory roles that serve to limit and contain skin immune responses. Notwithstanding these considerations there is reason to believe that at least some in vitro DC-based assays are of value, and indeed some are currently the subject of a formal validation process. However, it is appropriate that such assays are configured and interpreted carefully, and with an appreciation of the complexity of DC biology.