Nicola Gilmour

Categorization of chemicals according to their relative human skin sensitizing potency.

Although adoption of skin sensitization in vivo assays for hazard identification is likely to be successful in the next few years, this does not replace their use in potency prediction. Notably, measurement of potency of skin sensitizers in the local lymph node assay has been important. However, this local lymph node assay potency measure has not been formally assessed against a range of substances of known human sensitizing potential, because the latter is lacking. Accordingly, criteria for human data have been established that characterize 6 categories of human sensitizing potency, with 1 the most potent and 5 the least potent; category 6 represents true nonsensitizers. The literature has been searched, and 131 chemicals assigned into these categories according to their intrinsic potency judged only by the available human information. The criteria and data set generated provide a basis for examination of the capacity of nonanimal approaches for the determination of human sensitization potency.

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.

p‐Phenylenediamine allergy: the role of Bandrowski's base

p‐Phenylenediamine (PPD) is a commonly used hair‐dye and a potent skin allergen. The mechanism of sensitization is unknown, as PPD is protein unreactive. We studied Bandrowskis base (BB), a PPD trimer, as well as 1,4‐benzoquinone (BQ), a PPD hapten. PPD patch‐test positive patients were patch‐tested to BB and BQ. All tests were negative to 0.01% BQ and 0.01% BB. Five of 14 (35.7%) tested had true positive reactions to 0.1% BQ. One percent BQ was found to be irritant. Seven of 43 tested (16%) were positive to either 0.1% or 1% BB. The positive reactions to BB were weak, even when PPD reactions were strong. Mice lymph node assay gave EC3 values of 0.14% for PPD compared with 0.03% for BB. Therefore, BB is approximately 10 times more potent than PPD, taking into account the molarity. We suggest that while PPD may act as a prohapten, there is probably a spectrum of antigenic determinants in vivo. BB may be bound or metabolized by keratinocytes before it reacts with Langerhans cells.

High frequency of simultaneous sensitivity to Disperse Orange 3 in patients with positive patch tests to para-phenylenediamine.

Cross‐sensitization between para‐phenylenediamine (PPD) and Disperse Orange 3 (DO3), among other textile dyes, has frequently been reported. We evaluated the frequency of simultaneous patch test reactions to PPD and a range of textile dyes. Retrospectively, we studied 128 patients who were patch test positive to PPD and who had also been tested to textile dyes. The dyes that most commonly also reacted were DO3 (46·1%) followed by Disperse Yellow 3 (21·9%). 80% of 55 patients who had a + + or stronger reaction to PPD also reacted to DO3. Dyes that were least likely also to react were Bismarck Brown (0%), Naphthol AS (1·06%), Disperse Yellow 9 (1·06%), Disperse Blue 3 (1·56%) and Disperse Red 11 (2·13%). We interpreted the simultaneous patch test reactions to PPD and DO3 as due either to cross‐sensitivity proper, or to metabolic conversion of textile dyes in the skin to PPD.

The impact of exposure variables on the induction of skin sensitization.

Whereas many investigations of the variables associated with the elicitation of allergic contact dermatitis have been undertaken, to the point where we can begin to predict the likelihood of elicitation occurring in a given situation, the same is not true for the induction of skin sensitization. Studies have demonstrated that increasing dose has an impact; in an experimental setting, a number of variables received attention some decades ago. However, in the work reported here, the relative importance of the frequency and the duration of exposure is highlighted. In an investigation using a human repeated insult patch test, it was demonstrated that reduction of the exposure duration from 48 hr to 5 min decreased the rate of sensitization to 1% p‐phenylenediamine (PPD) from 54% to 3%. However, in an extended clinical study, it was observed that infrequent but longer duration and higher concentration exposure to PPD was significantly less likely to induce sensitization compared to more frequent, short duration, and lower concentration exposure. Detailed statistical analysis of the results indicated that the most important factor driving the induction of skin sensitization was the number of exposures.

A general population from Thailand: incidence of common allergens with emphasis on para-phenylenediamine

Background Most studies on the prevalence of allergy to the permanent hair dye chemical para‐phenylenediamine (PPD) are reported from populations of eczema patients attending patch‐test clinics, and are assumed to be much higher than in the normal population. No data exist on incidence of senitization to PPD resulting from the use of commercial hair dye preparations over a defined time period.

Biocides: Characterization of the Allergenic Hazard of Methylisothiazolinone

Biocides used in many every day products often are able to act as haptens and so may cause allergic reactions in the skin. In addition, where exposure of the respiratory tract may occur, they should also be evaluated for their ability to cause respiratory allergy. Here we have used local lymph node assay (LLNA) data to compare the relative potency of four biocides together with cytokine profiling to determine whether these biocides can induce skin and/or respiratory allergy. Formaldehyde, glutaraldehyde, 5‐chloro‐2‐methyl‐4‐isothiazolin‐3‐one and 2‐methyl‐2H‐isothiazol‐3‐one mix (3:1) (CMI/MI), and 2‐methyl‐2H‐isothiazol‐3‐one alone (MI) were tested in the LLNA in two vehicles [acetone:olive oil (AOO) and propylene glycol (PG)]. Their relative allergenic potency was measured by derivation of the EC3 value (the estimated concentration that will induce a stimulation index of 3 following topical application of chemical). In AOO, the EC3 value for the chemicals were ranked as follows: formaldehyde = MI < glutaraldehyde < CMI/MI, CMI/MI thus being the most potent allergen as it has the lowest EC3 figure. In PG, a similar rank order of biocides was achieved but the estimated potency in PG was at least 1 log lower than that in AOO. Data are available indicating that while formaldehyde is a contact allergen, glutaraldehyde is both a contact and respiratory allergen. Cytokine profiling was carried out to determine whether CMI/MI and MI also have the potential to cause sensitization of the respiratory tract. The data obtained for CMI/MI were consistent with behavior as a contact sensitizer. The MI is less strongly sensitizing than CMI/MI, being comparable to formaldehyde, and due to this weaker response it has not been possible to evaluate fully its cytokine profile, an outcome indicating it is unlikely to be a significant chemical respiratory allergen.

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

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

Causality methods in Cosmetovigilance: Comparison of Colipa and PLM versus global introspection

The European Cosmetics Regulation requires a post-marketing system for detection of undesirable effects on human health of cosmetic products. Colipa, the European Cosmetic, toiletry and perfumery association, provided guidelines for causality assessment of these effects. In addition another causality method originally designed for causality rating in Post Launch Monitoring (PLM) of novel foods has been employed to assess causality of cosmetic products. In this study these two causality schemes for consumer cosmetic products were validated against clinical assessment, using the method of global introspection (GI) in 100 reported cases. Causality assessments were performed by three experienced assessors in pharmacovigilance. In the event of discordance between the assessors, an adapted Delphi method was used. The overall Spearman correlation coefficient was 0.74 for comparison of Colipa versus GI, whereas this was 0.50 for PLM versus GI. According to current guidelines, the sensitivity was 0.95 for both the Colipa and PLM method, specificity was 0.84 for Colipa and 0.40 for PLM. From these results it can be concluded the performance of the Colipa causality method yielded better correlation to GI than PLM causality method. The factor identified from comparison of these two schemes as having greatest impact was the course of the reaction.

Protein–Hapten Binding: Challenges and Limitations for In Vitro Skin Sensitization Assays

In the search for practical alternatives to the use of animals in the predictive identification of chemicals which possess the intrinsic potential to cause sensitization of the skin, for 2 decades attention has been paid to structure–activity relationships. The work has delivered many valuable insights and has even led to the development of useful computer-based systems (e.g., DEREK). However, these efforts have been predicated very largely on an interrogation of how the chemical itself may be reactive (or may become so after metabolic intervention). Scant attention has been paid to the “substrate” with which the chemical must interact, i.e., skin protein. In this review, we consider in detail those actual protein substructures, examining what is known chemically, what may be predicted to occur, and how they can be investigated. Ultimately, understanding of the protein–hapten binding mechanisms in vitro will increase the confidence in sensitization hazard predictions using in silico tools. It could also permit development of a simple in vitro sensitization screen.