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

Abstract

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.

Chemistry of Isoeugenol and Its Oxidation Products: Mechanism and Kinetics of Isoeugenol as a Skin Sensitizer

“…On the other hand, isoeugenol degradation was clearly associated with increased reactivity for soft and hard nucleophiles. We previously isolated and identified isoeugenol’s degradation products, including the main oxidized byproduct with superior thiol reactivity . This reactive byproduct was structurally identified as syn -7,4′-oxyneolignan ( 3a ) by comparison with the NMR data of 1,2-disubstituted-1-arylpropanes .…”

“…The degradation byproducts were isolated on silica gel using SNAP HP-Sil flash cartridges (Biotage) eluted with 100% chloroform. The major syn -7,4′-oxyneolignan 3a (1.8 g, 10.5%) and the minor diastereomer 3b (253.3 mg, 1.5%) were isolated, and their structures were confirmed by NMR (Table S1 and Figure S1), mass spectrometry, and comparison with previously reported data . The relative stereochemistry was further confirmed by computational methods as outlined in Section .…”

“…Despite the evidence from in vivo studies, the lack of experimental characterization of QM adducts complicates the validity of the QM hypothesis. We recently reported an important piece of evidence by identifying a reactive, electrophilic syn -7,4′-oxyneolignan ( 3a ), which is an oxidized dimeric derivative of isoeugenol found in a commercial isoeugenol sample (trace) and separately obtained via forced degradation (photo-induced oxidative conditions). The general structure of this oxidation compound of isoeugenol was previously reported as an olfactory receptor antagonist .…”

Synthesis and Antibacterial Properties of Oligomeric Dehydrogenation Polymer from Lignin Precursors

“…At 172.2 ppm (No. 2), the signal indicates oxidation at the γ-position to form cinnamic acid [ 33 , 34 ]. The signals at 149.8 to 147.0 ppm (No.…”

Evaluation of skin sensitization based on interleukin‑2 promoter activation in Jurkat cells

“…DCs were absent in the in vitro test; nickel and isoeugenol, having an immunosuppressive effect on T-cells alone, may exert hyperimmunogenicity when interacting with DCs; hence, the absence of DCs may have caused this false-negative result. iii) Pre/pro-hapten issue; isoeugenol is a pre/pro-hapten chemical that is prone to air oxidation and metabolism ( 22 , 23 ); a pre/pro-hapten such as isoeugenol could be a false negative in the test in the absence of a metabolic system.…”