In chemico assessment of potential sensitizers: Stability and direct peptide reactivity of 24 fragrance ingredients

Investigation of additional suitable positive controls in the human Cell Line Activation Test

The human Cell Line Activation Test (h-CLAT) is an in vitro skin sensitization assay adopted by the OECD as Test Guideline 442E. In the h-CLAT, 2,4-dinitrochlorobenzene (DNCB) is used as a positive control; however, DNCB is considered a poisonous substance under the Poisonous and Deleterious Substances Control Act in Japan since 2014 because of its high acute toxicity. Strict control, handling, and storage are required when using DNCB, which is a burden to the users. Although the use of other suitable positive controls with historical data is accepted by the guideline, to our knowledge, there have been no reports of such substances. Therefore, in this study, we investigated suitable positive controls that can be used in addition to DNCB for the h-CLAT. Three candidates that are not considered poisonous substances, imidazolidinyl urea, hydroxycitronellal, and 2,4-dinitrofluorobenzene, were selected. To determine their suitability as positive controls, the h-CLAT was performed repeatedly for each chemical in two laboratories. For imidazolidinyl urea, the results that failed the positive control criteria were observed in both laboratories, indicating that it was inconclusive for the suitability as a positive control at the tested concentration. In contrast, all experiments with hydroxycitronellal and 2,4-dinitrofluorobenzene met the criteria and resulted in relative fluorescence intensity values of CD86/CD54, which were comparable to those for DNCB. Based on these results, hydroxycitronellal and 2,4-dinitrofluorobenzene may be used as positive controls. This study would provide valuable information for users examining other suitable positive controls in the h-CLAT.

Alternative Methods for Skin-Sensitization Assessment

Skin sensitization is a term used to refer to the regulatory hazard known as allergic contact dermatitis (ACD) in humans or contact hypersensitivity in rodents, an important health endpoint considered in chemical hazard and risk assessments. Information on skin sensitization potential is required in various regulatory frameworks, such as the Directive of the European Parliament and the Council on Registration, Evaluation and Authorization of Chemicals (REACH). The identification of skin-sensitizing chemicals previously required the use of animal testing, which is now being replaced by alternative methods. Alternative methods in the field of skin sensitization are based on the measurement or prediction of key events (KE), i.e., (i) the molecular triggering event, i.e., the covalent binding of electrophilic substances to nucleophilic centers in skin proteins; (ii) the activation of keratinocytes; (iii) the activation of dendritic cells; (iv) the proliferation of T cells. This review article focuses on the current state of knowledge regarding the methods corresponding to each of the key events in skin sensitization and considers the latest trends in the development and modification of these methods.

Synthesis of Cyclic Fragrances via Transformations of Alkenes, Alkynes and Enynes: Strategies and Recent Progress

With increasing demand for customized commodities and the greater insight and understanding of olfaction, the synthesis of fragrances with diverse structures and odor characters has become a core task. Recent progress in organic synthesis and catalysis enables the rapid construction of carbocycles and heterocycles from readily available unsaturated molecular building blocks, with increased selectivity, atom economy, sustainability and product diversity. In this review, synthetic methods for creating cyclic fragrances, including both natural and synthetic ones, will be discussed, with a focus on the key transformations of alkenes, alkynes, dienes and enynes. Several strategies will be discussed, including cycloaddition, catalytic cyclization, ring-closing metathesis, intramolecular addition, and rearrangement reactions. Representative examples and the featured olfactory investigations will be highlighted, along with some perspectives on future developments in this area.

Critical Evaluation of Low-Molecular Weight Respiratory Sensitizers and Their Protein Reactivity Potential Toward Lysine Residues

Interest in the development of methods to evaluate the respiratory sensitization potential of low-molecular weight chemicals continues, but no method has yet been generally accepted or validated. A lack of chemical reference standards, together with uncertainty regarding relevant immunological mechanisms, has hampered method development. The first key event in the development of either skin or respiratory sensitization is the formation of stable adducts of the chemical with host proteins. This event is measured in the Direct Peptide Reactivity Assay using cysteine- and lysine-containing model peptides. It is hypothesized that protein reactivity and subsequent adduct formation may represent the earliest point of divergence in the pathways leading to either skin or respiratory sensitization. Direct Peptide Reactivity Assay data for 200 chemicals were compiled and grouped into respiratory, skin and nonsensitizers. Chemicals grouping was based on extensive literature research and expert judgment. To evaluate if chemical groups represent different peptide reactivity profiles, peptide reactivity data were clustered and compared with information on protein binding mechanisms and chemical categories available via the Organization for Economic Co-operation and Development. Toolbox. Respiratory sensitizers (n = 15) showed a significant (3-fold) higher lysine reactivity than skin sensitizers (n = 129). However, this difference was driven largely by the high representation of acid anhydrides among the respiratory sensitizers that showed clear lysine selectivity. Collectively, these data suggest that preferential reactivity for either cysteine or lysine is associated primarily with chemical structure, and that lysine preference is not a unifying characteristic of chemical respiratory allergens.

Is Isoeugenol a Prehapten? Characterization of a Thiol-Reactive Oxidative Byproduct of Isoeugenol and Potential Implications for Skin Sensitization

Isoeugenol is widely used by the cosmetic and fragrance industries, but it also represents a known cause of skin sensitization adverse effects. Although devoid of a structural alert, isoeugenol has been classified as prehapten in virtue of the presence of a pre-Michael acceptor domain. Isoeugenol oxidation could theoretically lead to the generation of reactive toxic quinones, and photoinduced oxidative degradation of isoeugenol was reported to generate strongly thiol reactive byproducts. Nonetheless, the isoeugenol degradation product responsible for increased reactivity was found to be elusive. In the present study, an aged isoeugenol sample was subjected to reactivity-guided experiments to trap elusive thiol reactive species with a fluorescent nucleophile, viz. dansyl cysteamine (DCYA). The results herein presented demonstrate that photo-oxidation of isoeugenol led to the formation of a dimeric 7,4'-oxyneolignan with strong chemical reactivity, capable of nucleophilic substitution with thiols. The results were confirmed by isolation, structural characterization, and further NMR reactivity studies. Isoeugenol is already well-known as moderately reactive in thiol depletion assays, and was herein demonstrated to be capable of converting to more potent electrophilic species upon degradation, thus acting as a prehapten. The application of the reactivity-guided strategy described herein was shown to serve as an effective tool to investigate elusive skin sensitizers.