Standard and Expanded Series Patch Testing Update by the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC)

After the meeting held by the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC) back in October 2021, changes were suggested to the Spanish standard series patch testing. Hydroxyethyl methacrylate (2% pet.), textile dye mixt (6.6% pet.), linalool hydroperoxide (1% pet.), and limonene hydroperoxide (0.3% pet.) were, then, added to the series that agreed upon in 2016. Ethyldiamine and phenoxyethanol were excluded. Methyldibromoglutaronitrile, the mixture of sesquiterpene lactones, and hydroxyisohexyl 3-cyclohexene (Lyral) were alo added to the extended Spanish series of 2022.

Antioxidant Activity and Skin Sensitization of Eugenol and Isoeugenol: Two Sides of the Same Coin?

Eugenol and isoeugenol are well acknowledged to possess antioxidant and thus cytoprotective activities. Yet both compounds are also important skin sensitizers, compelling the cosmetics and fragrance industries to notify their presence in manufactured products. While they are structurally very similar, they show significant differences in their sensitization properties. Consequently, eugenol and isoeugenol have been the subject of many mechanistic studies where the final oxidation forms, electrophilic ortho-quinone and quinone methide, are blamed as the reactive species forming an antigenic complex with nucleophilic residues of skin proteins, inducing skin sensitization. However, radical mechanisms could compete with such an electrophilic-nucleophilic pathway. The antioxidant activity results from neutralizing reactive oxygen radicals by the release of the phenolic hydrogen atom. The so-formed phenoxyl radicals can then fully delocalize upon the structure, becoming potentially reactive toward skin proteins at several positions. To obtain in-depth insights into such reactivity, we investigated in situ the formation of radicals from eugenol and isoeugenol using electron paramagnetic resonance combined with spin trapping in reconstructed human epidermis (RHE), mimicking human skin and closer to what may happen in vivo. Two modes of radical initiation were used, exposing RHE to (i) horseradish peroxidase (HRP), complementing RHE metabolic capacities, and mimicking peroxidases present in vivo or (ii) solar light using a AM 1.5 solar simulator. In both experimental approaches, where the antioxidant character of both compounds is revealed, oxygen- and carbon-centered radicals were formed in RHE. Our hypothesis is that such carbon radicals are relevant candidates to form antigenic entities prior to conversion into electrophilic quinones. On this basis, these studies suggest that pro- or prehapten fingerprints could be advanced depending on the radical initiation method. The introduction of HRP suggested that eugenol and isoeugenol behave as prohaptens, while when exposed to light, a prehapten nature could be highlighted.

Synthesis and In Situ Behavior of 1,4- and 2,5-(13C) Isotopomers of p-Phenylenediamine in Reconstructed Human Epidermis Using High Resolution Magic Angle Spinning NMR

p-Phenylenediamine (PPD) has been classified as a strong skin allergen, but when it comes to toxicological concerns, benzoquinone diamine (BQDI), the primary oxidation derivative of PPD, is frequently considered and was shown to covalently bind nucleophilic residues on model peptides. However, tests in solution are far from providing a reliable model, as the cutaneous metabolism of PPD is not covered. We now report the synthesis of two ¹³C substituted isotopomers of PPD, 1,4-(¹³C)p-phenylenediamine 1 and 2,5-(¹³C)p-phenylenediamine 2, and the investigation of their reactivity in reconstructed human epidermis (RHE) using the high resolution magic angle spinning (HRMAS) NMR technique. RHE samples were first treated with 1 or 2 and incubated for 1 to 48 h. Compared to the control, spectra clearly showed only the signals of 1 or 2 gradually decreasing with time to disappear after 48 h of incubation. However, the culture media of RHE incubated with 1 for 1 and 24 h, respectively, showed the presence of both monoacetylated- and diacetylated-PPD as major products. Therefore, the acetylation reaction catalyzed by N-acetyltransferase (NAT) enzymes appeared to be the main process taking place in RHE. With the aim of increasing the reactivity by oxidation, 1 and 2 were treated with 0.5 equiv of H₂O₂ prior to their application to RHE and incubated for different times. Under these conditions, new peaks having close chemical shifts to those of PPD-cysteine adducts previously observed in solution were detected. Under such oxidative conditions, we were thus able to detect and quantify cysteine adducts in RHE (maximum of 0.2 nmol/mg of RHE at 8 h of incubation) while no reaction with other nucleophilic amino acid residues could be observed.

Autoxidized citronellol: Free radicals as potential sparkles to ignite the fragrance induced skin sensitizing pathway

Citronellol, one of the most used fragrance compounds worldwide, is one ingredient of Fragrance Mix II used to assess skin allergy to fragrances in dermatitis patients. Pure citronellol is non-allergenic. Main issue is it autoxidizes when exposed to air becoming then allergenic. The increased skin sensitizing potency of air-exposed citronellol has been attributed to the hydroperoxides detected at high concentrations in the oxidation mixtures. It has been postulated that such hydroperoxides can give rise to specific antigens, although chemical mechanisms involved and the pathogenesis are far from being unraveled. Hydroperoxides are believed to react with skin proteins through mechanisms involving radical intermediates. Here, insights on the potential radicals involved in skin sensitization to citronellol hydroperoxides are given. The employed tool is a multispectroscopic approach based on (i) electron paramagnetic resonance and spin trapping, that confirmed the formation of oxygen- and carbon-radicals when exposing reconstructed human epidermis to concentrations of hydroperoxides close to those used for patch testing patients with air-oxidized citronellol; (ii) liquid chromatography-mass spectrometry, that proved the reaction with amino acids such as cysteine and histidine, known to be involved in radical processes and (iii) density functional theory calculations, that gave an overview on the preferential paths for radical degradation.

Limonene and linalool hydroperoxides review: pros and cons for routine patch testing

Limonene and linalool are among the most common fragrance terpenes used in products of everyday life. They are pre-haptens forming hydroperoxides (Lim-OOHs, Lin-OOHs) upon oxidation inducing frequent positive patch test reactions in patients with dermatitis. Still, they are not yet routinely tested in Europe. This review evaluates the patch testing experience with Lim-OOHs and Lin-OOHs by answering key questions such as whether hydroperoxide patch testing is warranted, understand difficulties or challenges related to the reading and interpretation of hydroperoxide patch test results with currently available material, assessing their relevance. Studies are increasingly pointing out to high percentages of positive reactions in patients consecutively patch tested with these oxidized products. An association between a positive clinical history and a strong patch test reaction has been described, but problems with doubtful/irritant reactions have also been reported. Considering the high frequencies of relevant positive reactions, the incorporation of Lim-OOHs 0.3% and Lin-OOHs 1% in the baseline series could be discussed and is maybe justified. Since exposure, sensitization and elicitation limits of Lim-OOHs and Lin-OOHs in the products still need to be better determined, an assessment of previous exposure, possible sensitizations and reactions may help to improve the clinical assessment. This article is protected by copyright. All rights reserved.

Electron paramagnetic resonance and spin trapping to detect free radicals from allergenic hydroperoxides in contact with the skin: from the molecule to the tissue

A major research topic consists of revealing the contribution of radical-mediated reactions in dermatological diseases related to xenobiotic-induced stress, to succeed risk assessment procedures protecting producers and consumers. Allergic contact dermatitis is the clinically relevant consequence of skin sensitization, one of the most critical occupational and environmental health issues related to xenobiotics exposure. The first key event identified for the skin sensitization process to a chemical is its aptitude to react with epidermal proteins and form antigenic structures that will further trigger the immune response. Many chemical sensitizers are suspected to react through mechanisms involving radical intermediates. This review focuses on recent progress we have accomplished over the last few years studying radical intermediates derived from skin sensitizing chemicals by electron paramagnetic resonance in combination with the spin trapping technique. Our work is carried out "from the molecule", performing studies in solution, "to the tissue", by the development of a methodology on a reconstructed human epidermis model, very close in terms of histology and metabolic/enzymatic activity to real human epidermis, that can be used as suitable biological tissue model. The benefits are to test chemicals under conditions close to human use and real-life sensitization exposures and benefit from the 3D microenvironment. This article is protected by copyright. All rights reserved.

Bikini textile contact dermatitis: A Sherlockian approach revealing 2.4-dichlorophenol as a potential textile contact allergen

BACKGROUND

Different textile constituents may act as allergens and/or irritants and provoke textile contact dermatitis (TCD).

OBJECTIVES

To report a case of TCD caused by ethylene glycol monododecyl ether and 2.4-dichlorophenol, present in a bikini.

METHODS

A woman presented with an eczematous, pruritic rash in the area of the bikini straps and back. Patch testing was performed with the European baseline, textile, sunscreen, and photo-patch series, the bikini "as is", and ethanol and acetone extracts of the bikini. Thin-layer chromatography (TLC) of the extracts and gas chromatography-mass spectrometry (GC-MS) analysis were used to elucidate the culprit agents.

RESULTS

Positive reactions were found to the bikini "as is" and to the ethanol and acetone extracts. Patch testing with TLC strips showed a strong reaction to spots-fractions 3 and 4. GC-MS was performed to identify substances in each fraction and those suspected to be skin sensitisers were patch tested. On day (D) 4 positive reactions to ethylene glycol monododecyl ether (irritant reaction) and 2.4-dichlorophenol (++) were observed.

CONCLUSION

A myriad of chemical compounds can be found in clothing. Ethylene glycol monododecyl ether and 2.4-dichlorophenol were identified as the potential culprits of this bikini TCD.

HIGHLIGHTS

We have combined chemical analyses (thin-layer chromatography and gas chromatography-mass spectrometry) to study a case of textile contact dermatitis. As such, the presence of a myriad of chemical compounds was found in a bikini, suggesting that clothing in general may become impregnated or contaminated by a wide range of external substances that may be harmful to the skin. Textile contact dermatitis could in this case be attributed to ethylene glycol monododecyl ether (CAS No. 4536-30-5) and 2.4-dichlorophenol (CAS No. 120-83-2), the latter not yet previously described as a textile contact allergen.

Skin sensitization to fragrance hydroperoxides: interplay between dendritic cells, keratinocytes and free radicals

BACKGROUND

Skin sensitization to hydroperoxides (R-OOHs) of the commonly used fragrance terpenes limonene, linalool and citronellol is frequently reported. R-OOHs are believed to initiate the process leading to sensitization and allergic contact dermatitis through mechanisms involving radical intermediates. Thus, radical intermediates, keratinocytes and dendritic cells (DCs) may act in concert to initiate the process.

OBJECTIVES

To evaluate individual DC activation profiles by R-OOHs in the context of keratinocytes with regard to frequency, specificity and magnitude of upregulation.

METHODS

We used 2D and 3D cocultures with keratinocytes/reconstructed human epidermis (RHE) and DCs to evaluate cell surface levels of the costimulatory molecules CD86, CD80 and the adhesion molecule CD54 on cocultured DCs. Analysis of radical formation from limonene hydroperoxides in RHE was performed using electron paramagnetic resonance combined with the spin trapping technique.

RESULTS

R-OOHs induce donor-dependent DC activation. Major differences were found between the limonene-OOHs. Limonene-1-OOH was stronger with respect to both frequency and magnitude of response. Using a 3D coculture model, no DC activation was detected after topical application of 0·2% limonene-OOHs (20 µg cm^-2 ), while 1·2% limonene-1-OOH or 2% limonene-2-OOH induced DC activation. Furthermore, we demonstrated differences in the carbon and oxygen radicals formed from the limonene-OOHs using RHE, mimicking what may happen in vivo.

CONCLUSIONS

We report clear individual differences in DC maturation induced by the most important hydroperoxides. Response rates and magnitude of response both indicate that very small structural alterations in the hydroperoxides are translated into specific DC responses. In addition, we provide more insight into the amounts of hydroperoxides that can activate DCs and induce sensitization.

In Situ Alkylation of Reconstructed Human Epidermis by Methyl Methanesulfonate: A Quantitative HRMAS NMR Chemical Reactivity Mapping

Allergic contact dermatitis (ACD) is a reaction of the immune system resulting from skin sensitization to an exogenous hazardous chemical and leading to the activation of antigen-specific T-lymphocytes. The adverse outcome pathway (AOP) for skin sensitization identified four key events (KEs) associated with the mechanisms of this pathology, the first one being the ability of skin chemical sensitizers to modify epidermal proteins to form antigenic structures that will further trigger the immune system. So far, these interactions have been studied in solution using model nucleophiles such as amino acids or peptides. As a part of our efforts to better understand chemistry taking place during the sensitization process, we have developed a method based on the use of high-resolution magic angle spinning (HRMAS) NMR to monitor in situ the reactions of ¹³C substituted chemical sensitizers with nucleophilic amino acids of epidermal proteins in reconstructed human epidermis. A quantitative approach, developed so far for liquid NMR applications, has not been developed to our knowledge in a context of a semisolid nonanisotropic environment like the epidermis. We now report a quantitative chemical reactivity mapping of methyl methanesulfonate (MMS), a sensitizing methylating agent, in reconstructed human epidermis by quantitative HRMAS (qHRMAS) NMR. First, the haptenation process appeared to be much faster in RHE than in solution with a maximum concentration of adducts reached between 4 and 8 h. Second, it was observed that the concentration of cysteine adducts did not significantly increase with the dose (2.07 nmol/mg at 0.4 M and 2.14 nmol/mg at 1 M) nor with the incubation time (maximum of 2.27 nmol/mg at 4 h) compared to other nucleophiles, indicating a fast reaction and a potential saturation of targets. Third, when increasing the exposure dose, we observed an increase of adducts up to 12.5 nmol/mg of RHE, excluding cysteine adducts, for 3112 μg/cm² (1 M solution) of (¹³C)MMS. This methodology applied to other skin sensitizers could allow for better understanding of the potential links between the amount of chemical modifications formed in the epidermis in relation to exposure and the sensitization potency.

Mechanistic Insights on Skin Sensitization to Linalool Hydroperoxides: EPR Evidence on Radical Intermediates Formation in Reconstructed Human Epidermis and 13C NMR Reactivity Studies with Thiol Residues

Linalool is one of the most commonly used fragrance terpenes in consumer products. While pure linalool is considered as non-allergenic because it has a very low skin sensitization potential, its autoxidation on air leads to allylic hydroperoxides that have been shown to be major skin sensitizers. These hydroperoxides have the potential to form antigens via radical mechanisms. In order to obtain in-depth insights of such reactivity, we first investigated the formation of free radicals derived from linalool hydroperoxides in situ in a model of human reconstructed epidermis by electron paramagnetic resonance combined with spin trapping. The formation of carbon- and oxygen-centered radical species derived from the hydroperoxides was especially evidenced in an epidermis model, mimicking human skin and thus closer to what may happen in vivo. To further investigate these results, we synthesized linalool hydroperoxides containing a ¹³C-substitution at positions precursor of carbon radicals to elucidate if one of these positions could react with cysteine, its thiol chemical function being one of the most labile groups prone to react through radical mechanisms. Reactions were followed by mono- and bidimensional ¹³C NMR. We validated that carbon radicals derived from allylic hydrogen abstraction by the initially formed alkoxyl radical and/or from its β-scission can alter directly the lateral chain of cysteine forming adducts via radical processes. Such results provide an original vision on the mechanisms likely involved in the reaction with thiol groups that might be present in the skin environment. Consequently, the present findings are a step ahead toward the understanding of protein binding processes to allergenic allylic hydroperoxides of linalool through the involvement of free radical species and thus of their sensitizing potential.

Association Between Severe Acute Contact Dermatitis Due to Nigella sativa Oil and Epidermal Apoptosis

Importance

Nigella sativa oil (NSO) is widely used for cosmetic and culinary purposes. Cases of severe acute contact dermatitis due to NSO are poorly described, with no histologic description.

Objectives

To describe the clinical and histologic features of severe acute contact dermatitis due to NSO and investigate the components responsible for such eruptions.

Design, Setting, and Participants

A case series study of 3 patients with contact dermatitis admitted to the dermatology department between August 21, 2009, and February 19, 2017, was conducted. All patients had been referred to the dermatology department for acute contact dermatitis due to NSO and had patch tests performed.

Main Outcomes and Measures

Clinical and histologic features of the cutaneous eruptions, length of hospital stay, chemical analysis of NSO, and results of patch tests.

Results

Three patients (3 women; median age, 27 years [range, 20-47 years]) were included in the case series. All patients had polymorphic skin lesions spreading beyond the area of NSO application: typical and atypical targets, patches with central blisters, erythematous or purpuric plaques with a positive Nikolsky sign mimicking Stevens-Johnson syndrome, or toxic epidermal necrolysis. Two patients had pustules. They had severe impairment, with more than 15% skin detachment and fever. The results of skin biopsies showed epidermal apoptosis characterized by vacuolar alteration of the basal layer, keratinocyte apoptosis, and a moderate perivascular infiltrate of lymphocytes in the dermis. The results of patch tests using the patients' NSO were all positive. The results of gas chromatography combined with mass spectrometry performed on the NSO of 1 patient identified several constituent substances, mainly terpenes, thymoquinone, linoleic acid, and fatty acids.

Conclusions and Relevance

These cases suggest that acute contact dermatitis due to NSO may induce topically triggered epidermal apoptosis, previously described as the concept of acute syndrome of apoptotic pan epidermolysis. Thymoquinone and p-cymene may be the main agents involved in the pathophysiologic characteristics of this acute contact dermatitis. Clinicians should be aware of such severe reactions to NSO and report these cases to pharmacovigilance authorities.

Formation of methyl radicals derived from cumene hydroperoxide in reconstructed human epidermis: an EPR spin trapping confirmation by using 13C-substitution

Dermal exposure to cumene hydroperoxide (CumOOH) during manufacturing processes is a toxicological issue for the industry. Its genotoxicity, mutagenic action, ability to promote skin tumour, capacity to induce epidermal hyperplasia, and aptitude to induce allergic and irritant skin contact dermatitis are well known. These toxic effects appear to be mediated through the activation to free radical species such as hydroxyl, alkoxyl, and alkyl radicals characterised basically by electron paramagnetic resonance (EPR) and spin-trapping (ST) techniques. To be a skin sensitiser CumOOH needs to covalently bind to skin proteins in the epidermis to form the antigenic entity triggering the immunotoxic reaction. Cleavage of the O-O bond allows formation of unstable CumO^•/CumOO^• radicals rearranging to longer half-life specific carbon-centred radicals R^• proposed to be at the origin of the antigen formation. Nevertheless, it is not still clear which R^• is precisely formed in the epidermis and thus involved in the sensitisation process. The aim of this work was to elucidate in conditions closer to real-life sensitisation which specific R^• are formed in a 3D reconstructed human epidermis (RHE) model by using ¹³C-substituted CumOOH at carbon positions precursors of potentially reactive radicals and EPR-ST. We demonstrated that most probably methyl radicals derived from β-scission of CumO^• radicals occur in RHE through a one-electron reductive pathway suggesting that these could be involved in the antigen formation inducing skin sensitisation. We also describe a coupling between nitroxide radicals and β position ¹³C atoms that could be of an added value to the very few examples existing for the coupling of radicals with ¹³C atoms.

Sensitization potential and potency of terpene hydroperoxides in the cocultured activation test method

BACKGROUND

Positive patch test reactions to mixtures of oxidized terpenes containing allergenic hydroperoxides are frequently reported. However, human sensitization data for these hydroperoxides are not available.

OBJECTIVES

To analyse and evaluate the human sensitization potential and potency of hydroperoxides in vitro by using human cells.

MATERIALS/METHODS

Limonene-1-hydroperoxide, limonene-2-hydroperoxide, citronellol-7-hydroperoxide, cumene hydroperoxide, 1-(1-hydroperoxy-1-methylethyl)cyclohexene and mixtures of citronellol hydroperoxides (isomers at positions 6 and 7) and linalool hydroperoxides (isomers at positions 6 and 7) were studied. All compounds were synthesized except for cumene hydroperoxide, which was commercially available. Their potential and potency to activate dendritic cells (DCs) was evaluated by measuring the upregulation of CD86 and CD54 on THP-1 cells upon exposure in the cocultured activation test (COCAT) consisting of HaCaT cells (human keratinocyte cell line) and THP-1 monocytes (as a surrogate for DCs).

RESULTS

Hydroperoxides upregulated CD86 and/or CD54 on cocultured THP-1 cells in a concentration-dependent manner. The results are comparable with their sensitization potency ranking in predictive animal models.

CONCLUSIONS

For the first time, the human sensitization potential and potency of several hydroperoxides were determined by the use of human cells and the COCAT method.

Self-testing for contact allergy to hair dyes - a 5-year follow-up multicentre study

BACKGROUND

In 2011, a multicentre study was conducted in order to determine how hair dye manufacturers instructed consumers to perform a self-test prior to dyeing their hair, in order to identify individuals who are likely to react upon subsequent hair dyeing. A number of concerns were raised concerning the variability in instructions between products and producers, and the safety and validity of this tool.

OBJECTIVES

To perform a 5-year follow-up study in order to determine whether manufacturers still recommend a self-test, and if so, whether the procedures have been changed.

METHODS

During March 2016, a total of 40 oxidative hair dye products from 21 different manufacturers were bought in retail stores in 8 European countries.

RESULTS

The consumers were instructed to perform a self-test prior to hair dyeing for 39 of the products; however, the procedures varied greatly regarding the method of application, the amount of hair dye applied, the location and size of the application area, the number of applications, whether or not rinsing was performed after application, the reading times, and how a positive reaction was defined.

CONCLUSIONS

Self-testing is still recommended by almost all manufacturers of permanent hair dyes. There are major variations in the instructions, even in products from the same manufacturer. The previously raised concerns regarding safety and validity still remain.

In situ chemical behaviour of methylisothiazolinone (MI) and methylchloroisothiazolinone (MCI) in reconstructed human epidermis: a new approach to the cross-reactivity issue

BACKGROUND

Methylisothiazolinone (MI) [with methylchloroisothiazolinone (MCI) in a ratio of 1:3, a well-recognized allergenic preservative] was released as an individual preservative in the 2000s for industrial products and in 2005 for cosmetics. The high level of exposure to MI since then has provoked an epidemic of contact allergy to MI, and an increase in MI/MCI allergy. There are questions concerning the MI/MCI cross-reaction pattern.

OBJECTIVES

To bring a new perspective on the MI/MCI cross-reactivity issue by studying their in situ chemical behaviour in 3D reconstructed human epidermis (RHE).

METHODS

MI and MCI were synthesized with (13) C substitution at positions C-4/C-5 and C-5, respectively. Their in situ chemical behaviours in an RHE model were followed by use of the high-resolution magic angle spinning nuclear magnetic resonance technique.

RESULTS

MI was found to react exclusively with cysteine thiol residues, whereas MCI reacted with histidines and lysines. The reaction mechanisms were found to be different for MI and MCI, and the adducts formed had different molecular structures.

CONCLUSION

In RHE, different MI/MCI reactions towards different nucleophilic amino acids were observed, making it difficult to explain cross-reactivity between MI and MCI.

Reactivity of chemical sensitizers toward amino acids in cellulo plays a role in the activation of the Nrf2-ARE pathway in human monocyte dendritic cells and the THP-1 cell line

Allergic contact dermatitis resulting from skin sensitization is an inflammatory skin disease linked to the use of chemicals termed haptens. Chemical reactivity is necessary for a chemical to be a sensitizer, allowing both covalent binding to proteins and maturation of dendritic cells (DCs) by mimicking "danger signals." The aim of this study was to evaluate how the reactivity of chemical sensitizers toward amino acids translates into a biological response using the activation of the nuclear factor-erythroid 2-related factor 2 (Nrf2) pathway, which was assessed by the induction of three Nrf2 target genes (ho-1, nqo1, and il-8) and Nrf2 protein accumulation. Nrf2 activation is known to play a role in numerous detoxification mechanisms that could regulate danger signal outcomes in myeloid cells. Monocyte-derived DCs and THP-1 cells were exposed to (a) haptens with cysteine, lysine, or cysteine/lysine reactivity, (b) pro-/prehaptens, and (c) nonsensitizing molecules with reducing or oxidative properties (17 molecules in total). Chemicals were classified as "Nrf2 pathway activators" when at least two Nrf2 target genes associated with Nrf2 protein expression were induced. Results showed that most chemical sensitizers having cysteine and cysteine/lysine affinities were inducers of the Nrf2 pathway in both cell models, whereas lysine-reactive chemicals were less efficient. In THP-1 cells, the Nrf2 pathway was also activated by pro-/prehaptens. Regression analysis revealed that ho-1 and nqo1 expressions were found to be associated with chemical sensitizer reactivity to cysteine, providing evidence of the importance of chemical reactivity, as a part of danger signals, in DC biology.

Synthesis of allylic hydroperoxides and EPR spin-trapping studies on the formation of radicals in iron systems as potential initiators of the sensitizing pathway

Many terpenes used as fragrance compounds autoxidize when exposed to air, forming allylic hydroperoxides that have the potential to be skin contact allergens. To trigger the immunotoxicity process that characterizes contact allergy, these hydroperoxides are supposed to bind covalently to proteins in the skin via radical pathways. We investigated the formation of reactive radical intermediates from 7-hydroperoxy-3,7-dimethylocta-1,5-dien-3-ol and 2-hydroperoxylimonene, responsible for the sensitizing potential acquired by autoxidized linalool and limonene. Both compounds were synthesized through new short and reproducible synthetic pathways. The hydroperoxide decomposition catalyzed by Fe(II)/Fe(III) redox systems, playing a key role in degradating peroxides in vivo, was examined by spin-trapping-EPR spectroscopy. Alkoxyl and carbon-centered free radicals derived from the hydroperoxides were successfully trapped by the spin-trap 5,5-dimethyl-1-pyrroline N-oxide, whereas peroxyl radicals were characterized by spin-trapping studies with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide. Using liquid chromatography combined with mass spectrometry, we demonstrated the formation of adducts, via radical mechanisms induced by Fe(II)/Fe(III), between the hydroperoxides and N-acetylhistidine methyl ester, a model amino acid that is prone to radical reactions. Free radicals derived from these hydroperoxides can thus induce amino acid chemical modifications via radical mechanisms. The study of these mechanisms will help to understand the sensitizing potential of hydroperoxides.

Alpha-methylene-gamma-butyrolactones: versatile skin bioactive natural products

Natural products containing an alpha-methylene-gamma-butyrolactone moiety, mainly of the sesquiterpene type, are widely observed in plants, which upon coming into contact with skin, will induce major skin toxicological side effects or phytodermatitis. Indeed two main dermatological pathologies have been associated with a skin exposure to molecules containing an alpha-methylene-gamma-butyrolactone moiety: allergic contact dermatitis (ACD) and chronic actinic dermatitis (CAD). ACD is an immunologically based disease resulting from modifications of epidermal proteins by sensitizers or haptens. Indeed, alpha-methylene-gamma-butyrolactones are highly electrophilic structures that can act as Michael acceptors towards nucleophilic residues of proteins. Cysteine and lysine are the most modified residues leading, in the case of enantiomerically pure lactones, to the formation of diastereomeric adducts. This chemical enantioselectivity induces an enantiospecificity of the allergic reaction, i.e., an individual sensitized to one enantiomer will not develop clinical symptoms when exposed to the other enantiomer and vice versa. Sesquiterpene lactones have been also associated with another pathology that involves UV irradiation and DNA modifications. Interestingly, it was found that alpha-methylene-gamma-butyrolactones, in addition to their electrophilic properties, were highly photoreactive molecules able to react with thymine/thymidine to form [2 + 2] photoadducts in very high yields. In all cases a syn regioselectivity was observed, probably associated with the polarization of the exomethylenic bond. This high photoreactivity of alpha-methylene-gamma-butyrolactones towards thymidine could be an explanation of the progressive evolution of allergic contact dermatitis towards chronic actinic dermatitis.

Nuclear magnetic resonance studies on covalent modification of amino acids thiol and amino residues by monofunctional aryl 13C-isocyanates, models of skin and respiratory sensitizers: transformation of thiocarbamates into urea adducts

Exposure to aryl isocyanates, intermediates in the manufacture of polyurethanes, provokes lung sensitization and asthma but also occupational allergic contact dermatitis, sensitization occurring from a single accidental exposure. The initial step in the sensitization process is believed to be the covalent binding of the -N triple bond C triple bond O group with nucleophilic residues on proteins. While a wide knowledge exists on the reactivity of skin sensitizers toward amino acids, little is known about respiratory sensitizers such as aryl isocyanates. (13)C-Labeled monofunctional aryl isocyanates were synthesized, and their reactivities toward nucleophilic amino acids, GSH, and a model peptide were studied by (13)C and [(1)H-(13)C] NMR spectroscopy. An acetonitrile/buffer solution was used as a solvent to avoid the hampering of the follow up of the reactivity by the isocyanate hydrolysis competing reaction. The compounds reacted with thiol groups, through the formation of thiocarbamate bonds and with amino groups to form urea derivatives. The reactivity was confirmed with GSH, containing both free amino and thiol groups, and with a model peptide, particularly in the case of the reaction with lysine. The use of (13)C NMR to follow the aryl isocyanates reversible conjugation with thiol groups is also reported. Particularly, it is shown that thiocarbamate adducts can be converted into adducts of the urea kind by reaction with amino groups. These results confirmed the hypothesis by which thiol-containing peptides/proteins may act as carriers of isocyanates for possible reaction at a later time and/or place with other nucleophiles and confirmed the role of lysine as a good competing nucleophilic amino acid. The reactivity of aryl isocyanates with thiol and amino groups needs thus to be considered in their assigned sensitization processes.

Mechanistic studies on the lysine-induced N-formylation of 2,5-dimethyl-p-benzoquinonediimine

2,5-Dimethyl- p-benzoquinonediimine was used as a model to study the reactivity of p-benzoquinonediimines, the first oxidation intermediates of allergenic p-amino aromatic compounds, toward lysine, as it has been suggested that this amino acid could play a key role in the induction mechanism of allergic contact dermatitis for a number of chemicals. The use of 6-[ (13)C]lysine and Nalpha-acetyl-6-[ (13)C]lysine, in association with (13)C NMR and HPLC in tandem with mass spectrometry techniques, allowed the identification of 4-amino-2,5-dimethylformanilide, 4-amino-2,5-dimethyl[ (13)C]formanilide, and the derivative containing the amino acid covalently bound at the para position. While enzymatic N-acetylation of p-phenylenediamine (PPD) has been described in the literature, in human skin for example, to our knowledge this was the first time that N-formylation of a PPD derivative induced by the reaction with an amino acid such as lysine was observed in solution, together with the formation of an adduct with the amino acid. To afford an explanation for the lysine-induced N-formylation,we undertook mechanistic studies, and they showed that 2,5-dimethyl- p-benzoquinonediimine was involved in an oxido reduction process that is capable of deaminating the alpha-NH 2 group, even when N-acetylated, and the epsilon-NH 2 groups of lysine in an oxidative way, forming the real reactive intermediates for N-formylation. This initially unexpected behavior should be considered when investigating the reactivity of such compounds with lysine-containing peptides or proteins in the context of hapten-protein binding studies.

Synthesis and Reactivity Toward Nucleophilic Amino Acids of 2,5-[13C]-Dimethyl-p-benzoquinonediimine

2,5-[(13)C]-Dimethyl-p-benzoquinonediimine was synthesized, and its reactivity toward several nucleophilic amino acids was studied by associated (13)C and (1)H{(13)C} NMR spectroscopies, combined with HPLC in tandem with mass spectrometry. A classical electrophile-nucleophile mechanism was observed for the reaction with N-acetyl-Cys. Adducts resulted from the reaction of the amino acid thiol group with the benzoquinonediimine electrophilic positions 3 and 6 as well as with the nitrogen atom of the imino group. However, N-acetyl-Trp and N-acetyl-Lys were chemically modified in the presence of 2,5-[(13)C]-dimethyl-p-benzoquinonediimine through the involvement of oxido-reduction processes. Heteronuclear (1)H{(13)C} NMR experiments allowed the identification of known oxidation intermediates derived from N-acetyl-Trp, indicating the oxidative strength of the reaction media. An adduct resulted from the reaction between the reduced form of the benzoquinonediimine and N-acetyl-formylkynurenine, which is the most known oxidation derivative of N-acetyl-Trp. In the case of N-acetyl-Lys, 4-amino-2,5-dimethyl-[(13)C]-formanilide and its derivative with N-acetyl-Lys at position 4 were obtained. A reaction mechanism was suggested in which the epsilon-NH(2) of the amino acid reacted on the electrophilic diimine to form an enamine adduct, which could then induce an oxidative deamination of N-acetyl-Lys. Further oxido-reduction mechanisms on the N-acetyl-alpha-aminoadipate-delta-semialdehyde formed might afford N,N-acetyl-formyl glutamic semialdehyde, which was considered as the powerful reactive species toward the reduced form of 2,5-[(13)C]-dimethyl-p-benzoquinonediimine. In the presence of N-acetyl-Tyr or N-acetyl-Met, the hydrolysis of the diimine parent compound was preferred, followed by a reduction to the hydroquinone form. In this study, we have thus shown that p-benzoquinonediimines, the first oxidation derivatives of allergenic p-amino aromatic compounds, can react with nucleophilic residues on amino acids through a set of complex mechanisms and must be seriously considered as potential candidates for the formation of antigenic structures responsible for allergic contact dermatitis.

Comparison of elicitation potential of chloroatranol and atranol - 2 allergens in oak moss absolute

Chloroatranol and atranol are degradation products of chloroatranorin and atranorin, respectively, and have recently been identified as important contact allergens in the natural fragrance extract, oak moss absolute. Oak moss absolute is widely used in perfumery and is the cause of many cases of fragrance allergic contact dermatitis. Chloroatranol elicits reactions at very low levels of exposure. In oak moss absolute, chloroatranol and atranol are present together and both may contribute to the allergenicity and eliciting capacity of the natural extract. In this study, 10 eczema patients with known sensitization to chloroatranol and oak moss absolute were tested simultaneously to a serial dilution of chloroatranol and atranol in ethanol, in equimolar concentrations (0.0034-1072 microM). Dose-response curves were estimated and analysed by logistic regression. The estimated difference in elicitation potency of chloroatranol relative to atranol based on testing with equimolar concentrations was 217% (95% confidence interval 116-409%). Both substances elicited reactions at very low levels of exposure. It is concluded that the differences in elicitation capacity between the 2 substances are counterbalanced by exposure being greater to atranol than to chloroatranol and that both substances contribute to the clinical problems seen in oak moss absolute-sensitized individuals.

Identification of carbon-centred radicals derived from linalyl hydroperoxide, a strong skin sensitizer: a possible route for protein modifications

Few studies are reported on the formation of reactive carbon-centred radical species from toxic xenobiotics. In this paper the formation of carbon radicals derived from the skin sensitizer linalyl hydroperoxide is described using radical trapping and EPR studies. Radical trapping used TMIO as scavenger agent and light, heat or TPP-Fe(3+) as radical inducers. EPR spin trapping was based on the use of the parent alcohol, generating the same allyloxyl radical than the hydroperoxide by photolysis of the corresponding nitrite formed with t-BuONO, also playing the role of the spin trap. It is suggested that the generation of these carbon radical species could play an important role for the binding of the hydroperoxide with skin proteins to form antigenic structures, the first step of the skin sensitization mechanism.

Content of oak moss allergens atranol and chloroatranol in perfumes and similar products

Chloroatranol and atranol have been identified as the main allergens in the fragrance material of botanical origin, oak moss absolute. A previous study has shown that nearly all individuals sensitized to chloroatranol will elicit to 5 microg/ml. in a repeated open application test and that 50% will get a reaction to 0.15 micro g/ml under patch test conditions. Thus, chloroatranol is known as a potent allergen. The aim of the current investigation was to quantify exposure to chloroatranol and the chemically related substance atranol in some popular perfumes, eaux de parfum and eaux de toilette available on the European market. In total, 31 products were analysed by liquid chromatography-electrospray ionization-tandemmass spectrometry (LC-ESI-MS-MS) for their contents of atranol and chloroatranol. The 2 substances were found in 87% (n = 27) of the products. The median concentration of atranol in perfumes was 0.502 micro g/ml and 0.012 micro g/ml in eaux de toilette, and 0.235 micro g/ml and 0.006 micro g/ml for chloroatranol, respectively, in perfumes and eaux de toilette. Chloroatranol was found at a maximum concentration of 53 micro g/ml and atranol at one of 190 micro g/ml. The wide exposure to oak moss allergens, together with significant amounts of these potent allergens in at least half of perfumes and some eaux de toilettes explains the high frequencies of oak moss absolute allergy. It is suggested that regulations should be introduced aimed directly at these substances, and not just at oak moss absolute.

Chloroatranol, an extremely potent allergen hidden in perfumes: a dose-response elicitation study

Oak moss absolute is a long-known, popular natural extract widely used in perfumes. It is reported as the cause of allergic reactions in a significant number of those with perfume allergy. Oak moss absolute has been the target of recent research to identify its allergenic components. Recently, chloroatranol, a hitherto unknown fragrance allergen, was identified in oak moss absolute. The objective was to assess the clinical importance of chloroatranol as a fragrance allergen by characterizing its elicitation profile. 13 patients previously showing a positive patch test to oak moss absolute and chloroatranol were included, together with a control group of 10 patients without sensitization to either of the 2 materials. A serial dilution patch test was performed on the upper back with concentrations ranging from 200 to 0.0063 p.p.m. of chloroatranol in ethanol. Simultaneously, the participant performed an open test simulating the use of perfumes on the volar aspect of the forearms in a randomized and double-blinded design. A solution with 5 p.p.m. chloroatranol was used for 14 days, and, in case of no reaction, the applications were continued for another 14 days with a solution containing 25 p.p.m. All test subjects (13/13) developed an allergic reaction at the site of application of the solution containing chloroatranol. Among them, 12/13 (92%) gave a positive reaction to the 5 p.p.m. solution and 1 to 25 p.p.m. None of the controls reacted (P < 0.001). The use test was terminated at median day 4. The dose eliciting a reaction in 50% of the test subjects at patch testing was 0.2 p.p.m. In conclusion, the hidden exposure to a potent allergen widely used in perfumes has caused a highly sensitized cohort of individuals. Judged from the elicitation profile, chloroatranol is the most potent allergen present in consumer products today.

Contact allergy to oak moss: search for sensitizing molecules using combined bioassay-guided chemical fractionation, GC-MS, and structure-activity relationship analysis

In addition to pure synthetic fragrance materials several natural extracts are still in use in the perfume industry. Among them oak moss absolute, prepared from the lichen Evernia prunastri (L.) Arch., is considered a major contact sensitizer and is therefore included in the fragrance mix used for diagnosing perfume allergy. The process of preparing oak moss absolute has changed during recent years and, even though several potential sensitizers have been identified from former benzene extracts, its present constituents and their allergenic status are not clear. In the study reported here, we applied a method developed for the identification of contact allergens present in natural complex mixtures to oak moss absolute. The method is based on the combination of bioassay-guided chemical fractionation, gas chromatography-mass spectrometry analysis and structure-activity relationship studies. Our first results showed that atranol and chloroatranol, formed by transesterification and decarboxylation of the lichen depsides, atranorin and chloroatranorin, during the preparation of oak moss absolute, are strong elicitants in most patients sensitized to oak moss. Methyl-beta-orcinol carboxylate, a depside degradation product and the most important monoaryl derivative of oak moss from an olfactory standpoint, was also found to elicit a reaction in most patients.

Molecular recognition between a new pentacyclic acridinium salt and DNA sequences investigated by optical spectroscopic techniques, proton nuclear magnetic resonance spectroscopy, and molecular modeling

A pentacyclic acridine, 1H-2,3-dihydroindolizino[7,6,5-kl]acridinium chloride (1), related in structure to tetra- and pentacyclic marine natural products, has previously been shown to induce apoptosis in breast and non-small-cell lung tumor cell lines and shows significant differences in biological potency and antitumor profile from other intercalating agents based on the acridine framework. We report on the molecular recognition of the acridinium salt with DNA, quantified by optical spectroscopic methods, and have compared these results with the clinical agent amsacrine (m-AMSA). The results point to an intercalative association between 1 and G-C-rich sequences of DNA. We have synthesized a hexamer duplex d(ACGCGT)2, presenting two potential 5'-CpG recipient sites, and have investigated in detail by NMR and molecular modeling methods the orientational preferences of 1, particularly with regard to the pyrrolidine ring system. On the basis of the intermolecular nuclear Overhauser effect (NOE) data, four possible intercalation models were considered; no single model produced a significantly better fit than any of the others. The best fit to the experimental data was obtained by considering a dynamic equilibrium between the different intercalated orientations with the drug maximizing pi-overlap with the G-C base pairs at the intercalation site. We found little evidence for any degree of groove specificity imparted by the pyrrolidine ring. If these simulations have biological relevance they suggest that, at most, the agent induces only a transitory hot spot in the DNA which, evidently, is sufficient to be sensed by damage-recognition mechanisms of the cell.

Antitumour polycyclic acridines. Part 4. Physico-chemical studies on the interactions between DNA and novel tetracyclic acridine derivatives

The non-covalent interactions between a series of new tetracyclic acridine derivatives (5-11) and DNA have been studied by spectrophotometric analysis, fluorescences quenching, thermal denaturation, and circular and linear dichroism. In order to compare the extent of the DNA binding by compounds 5-11 in their neutral and cationic forms, all experiments were conducted at pH 7.4 (physiological pH) and 5.0. The results indicated that compounds 5-11 are strong DNA-binding ligands with DNA affinities comparable to that of m-AMSA (1) or even higher. They showed a stronger DNA binding activity at pH 5.0 as a result of the N-protonation of the pyridoacridine aromatic chromophore. Ethidium-DNA fluorescence assays showed an A-T base pair preference of the binding distinguishing these novel compounds from simple acridines which show a slight G-C base pair preference. Circular and linear dichroism studies indicated that the drugs bind to DNA by undergoing intercalation inside the duplex macromolecule at high DNA:drug ratios and revealed alternative binding modes at low DNA:drug ratios.

Antitumour polycyclic acridines. Part 2. Physicochemical studies on the interactions between DNA and novel polycyclic acridine derivatives

The noncovalent interactions between a series of new polycyclic acridine derivatives (1-5) and salmon testes DNA have been studied using several physicochemical techniques. These include spectrophotometric analysis, fluorescence quenching, thermal denaturation, and circular and linear dichroism. In order to compare the extent of the DNA binding by compounds 1-5 in their neutral and cationic forms, all experiments have been conducted at pH 7.4 and at pH 5.0. Other polynucleotides, including [Poly(dA-dT)]2 and [Poly(dG-dC)]2, were used in order to study the DNA base-pair binding specificity of these novel annelated acridine derivatives. The results indicate that the new polycyclic acridines display the following properties: (i) they are strong DNA-binding ligands with affinities 10- to 400-fold greater than that of acridine, 3- to 100-fold greater than that of m-AMSA (6) and 1- to 23-fold greater than that of proflavine at physiological pH (7.4); (ii) they have stronger DNA-binding activity at pH 5.0 as a result of the N-protonation of the aromatic chromophore; (iii) they bind more selectively to [Poly(dA-dT)]2 polynucleotide than to [Poly(dG-dC)]2 polynucleotide; (iv) within the series compound 3 binds to DNA less than compounds 1, 2, 4 and 5 at both pH values studied; and (v) the polycyclic acridines form a molecular complex with DNA undergoing intercalation inside the duplex macromolecule, as shown by linear and circular dichroism. Nevertheless, circular dichroism studies reveal alternative binding modes at low DNA: drug ratios.