Cinnamyl alcohol oxidizes rapidly upon air exposure

Bioactivation of cinnamic alcohol in a reconstructed human epidermis model and evaluation of sensitizing potency of the identified metabolites

Background

Cinnamic alcohol is a natural compound, widely used in fragrances, which can cause allergic contact dermatitis. Cinnamic alcohol lacks intrinsic reactivity and autoxidation or metabolic activation is necessary for it to act as a sensitizer.

Methods

Bioactivation of cinnamic alcohol was explored using human liver microsomes, human liver S9 and SkinEthic™ Reconstructed Human Epidermis. A targeted multiple reaction monitoring mass spectrometry method was employed to study and quantify cinnamic alcohol along with eight potential phase I or phase II metabolites. The reconstructed human epidermis model, treated with cinnamic alcohol, was also analyzed with a non-targeted high-resolution mass spectrometry method to identify metabolites not included in the targeted method.

Results

Two metabolites identified with the targeted method, namely, pOH-cinnamic alcohol and pOH-cinnamic aldehyde, have not previously been identified in a metabolic in vitro system. Their reactivity toward biologically relevant nucleophiles was investigated and compared to their sensitizing potency in vivo in the murine local lymph node assay (LLNA). According to the LLNA, the pOH-cinnamic alcohol is non-sensitizing and pOH-cinnamic aldehyde is a moderate sensitizer. This makes pOH-cinnamic aldehyde less sensitizing than cinnamic aldehyde, which has been found to be a strong sensitizer in the LLNA. This difference in sensitizing potency was supported by the reactivity experiments. Cinnamic sulfate, previously proposed as a potential reactive metabolite of cinnamic alcohol, was not detected in any of the incubations. In addition, experiments examining the reactivity of cinnamic sulfate toward a model peptide revealed no evidence of adduct formation. The only additional metabolite that could be identified with the non-targeted method was a dioxolan derivative. Whether or not this metabolite, or one of its precursors, could contribute to the sensitizing potency of cinnamic alcohol would need further investigation.

Discussion

Cinnamic alcohol is one of the most common fragrance allergens and as it is more effective to patch test with the actual sensitizer than with the prohapten itself, it is important to identify metabolites with sensitizing potency. Further, improved knowledge of metabolic transformations occurring in the skin can improve prediction models for safety assessment of skin products.

Protein Haptenation and Its Role in Allergy

Humans are exposed to numerous electrophilic chemicals either as medicines, in the workplace, in nature, or through use of many common cosmetic and household products. Covalent modification of human proteins by such chemicals, or protein haptenation, is a common occurrence in cells and may result in generation of antigenic species, leading to development of hypersensitivity reactions. Ranging in severity of symptoms from local cutaneous reactions and rhinitis to potentially life-threatening anaphylaxis and severe hypersensitivity reactions such as Stephen-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), all these reactions have the same Molecular Initiating Event (MIE), i.e. haptenation. However, not all individuals who are exposed to electrophilic chemicals develop symptoms of hypersensitivity. In the present review, we examine common chemistry behind the haptenation reactions leading to formation of neoantigens. We explore simple reactions involving single molecule additions to a nucleophilic side chain of proteins and complex reactions involving multiple electrophilic centers on a single molecule or involving more than one electrophilic molecule as well as the generation of reactive molecules from the interaction with cellular detoxification mechanisms. Besides generation of antigenic species and enabling activation of the immune system, we explore additional events which result directly from the presence of electrophilic chemicals in cells, including activation of key defense mechanisms and immediate consequences of those reactions, and explore their potential effects. We discuss the factors that work in concert with haptenation leading to the development of hypersensitivity reactions and those that may act to prevent it from developing. We also review the potential harnessing of the specificity of haptenation in the design of potent covalent therapeutic inhibitors.

Electrophilic Reactivity of Sulfated Alcohols in the Context of Skin Sensitization

The surfactant sodium lauryl sulfate (SLS), although consistently positive in the murine local lymph node assay (LLNA) for skin sensitization, shows no evidence of being a human sensitizer and is often described as a false positive, lacking structural alerts for sensitization. However, there is evidence of the cinnamyl sulfate anion being the metabolite responsible for the sensitization potential of cinnamyl alcohol to humans and in animal tests. Here, manufacturing chemistry data and physical organic chemistry principles are applied to confirm that SLS is not reactive enough to sensitize, whereas sensitization to cinnamyl alcohol via cinnamyl sulfate is plausible. Sensitization data for several other primary alcohols, including geraniol, farnesol, and possibly hydrocortisone, are also consistent with this mechanism. It seems possible that biosulfation may play a wider role than has previously been recognized in skin sensitization.

Common fragrance chemicals activate dendritic cells in coculture with keratinocytes

BACKGROUND

Fragrances are important contact allergens; however, investigation of their skin sensitization potency has been challenging in new approach methods (NAMs). Many fragrance chemicals are susceptible to autoxidation or can be metabolized by enzymes constitutively expressed in skin keratinocytes. Strong sensitizers can be formed in both of these processes. Further, keratinocytes can modulate the dendritic cell (DC) activation and maturation potential, a key event in the acquisition of contact allergy.

OBJECTIVES

To evaluate the 2D coculture model consisting of keratinocytes and DCs using different weak to moderate sensitizing fragrance chemicals. Further, to investigate fragrances and related oxidation products in the in vitro model and compare to in vivo data.

METHODS

Chemicals were tested in the coculture activation test (COCAT), consisting of HaCaT keratinocytes and THP-1 cells. THP-1 cell surface expression of costimulatory and adhesion molecules (CD86 and CD54) collected after 24 h incubation with the chemicals was analysed using flow cytometry.

RESULTS

Twenty-four molecules were tested positive, three were negative (n = 27). Four pairs were evaluated, with aldehydes showing a 6- to 13-fold stronger responses compared to their corresponding alcohols.

CONCLUSIONS

Results provide insight into the activation of DC in their natural environment of keratinocytes. α,β-Unsaturated alcohols were classified as weaker sensitizers compared to their corresponding aldehydes. In sum, testing of fragrances retrieved results in good agreement with in vivo data.

Indonesian Cinnamon (Cinnamomum burmannii): Extraction, Flavonoid Content, Antioxidant Activity, and Stability in the Presence of Ascorbic Acid

Cinnamon (Cinnamomum burmannii) is a potential source used in cosmetics to prevent skin aging due to its flavonoids and antioxidant properties. This research aims to determine the best solvent for extracting flavonoids, to conduct a large-scale isolation of flavonoids, and to evaluate the effect of ascorbic acid and temperature on the extract’s stability for 16 weeks. Various solvents for small-scale extraction were screened based on the polarity index, and one of the best for use in large-scale maceration was selected based on the AlCl3 colorimetric flavonoid content and DPPH antioxidant activity. The stability test was performed by treating cinnamon with and without ascorbic acid at selected temperatures. This test measured physical stability, evaluated flavonoid content and antioxidant activity, and analyzed volatile and non-volatile compounds using GC-MS and LC-MS. The most excellent solvent to extract flavonoids was ethanol due to its high yield (21.50%), flavonoid content (0.01749 ± 8.0 × 10−5 mg QE/g extract), and antioxidant activity (IC50 0.0162 + 7.5 × 10−4 mg/mL). The ascorbic acid addition at both temperatures affected the stability of the pH and chemical constituents. The vast majority of the extract’s flavonoid content and antioxidant activity continued to increase until the end of the observation week. This study revealed that ethanol was the best extraction solvent, and ascorbic acid can be recommended as a stabilizer of cinnamon extract for use in cosmetics for further application.

Haptenation of Macrophage Migration Inhibitory Factor: A Potential Biomarker for Contact Hypersensitivity

The immunological response in contact hypersensitivity is incited by small electrophilic compounds, known as haptens, that react with endogenous proteins after skin absorption. However, the identity of hapten-modified proteins seen as immunogenic remains as yet largely unknown. In a recent study, we have for the first time identified a hapten-modified protein in the local lymph nodes of mice treated topically with the model hapten tetramethylrhodamine isothiocyanate (TRITC). The TRITC modification was located on the N-terminal proline of the protein macrophage migration inhibitory factor (MIF). The focus of the current study was to investigate the presence of the same hapten-protein conjugate in blood samples from mice treated topically with TRITC. Furthermore, TRITC modifications of the two major blood proteins, namely hemoglobin (Hb) and albumin (Alb), as well as TRITC modifications of MIF other than the N-terminal proline, were examined. Following incubation with different molar ratios of TRITC, a proteomic approach was applied to characterize conjugate formation of the three aforementioned proteins, using high resolution mass spectrometry (HRMS). The targeted screening of the TRITC-treated mice blood and lymph node samples for these sites led to the identification of only the same TRITC-MIF conjugate previously detected in the lymph nodes. No Hb and Alb conjugates were detected. Quantification of both the TRITC-modified and unmodified N-terminal peptide of MIF in blood and lymph node samples gave interesting insights of MIF’s role in murine contact hypersensitivity. Incubation of MIF with four different haptens encompassing different reactivity mechanisms and potencies, showed adduct formation at different amino acid residues, suggesting that MIF can be the preferred target for a wide variety of haptens. The present study provides essential progress toward understanding of hapten-protein conjugate formation in contact hypersensitivity and identifies hapten-modified MIF as a potential biomarker for this condition. Further investigation of MIF as a target protein can be a next step to determine if MIF is a biomarker that can be used to develop better diagnostic tools and targeted therapeutics for individuals with allergic contact dermatitis.

Thermal stability and pathways for the oxidation of four 3-phenyl-2-propene compounds

Cinnamaldehyde, cinnamyl alcohol, β-methylstyrene and cinnamic acid are four important biomass 3-phenyl-2-propene compounds. In the field of perfume and organic synthesis, their thermal stability and oxidation pathways deserve attention. This paper reports a new attempt to investigate the thermal stability and reactivity by a custom-designed mini closed pressure vessel test (MCPVT). The pressure and temperature behaviors were measured by MCPVT under nitrogen and oxygen atmosphere. The temperature of initial oxygen absorption (T a) and rapid oxidation (T R) were calculated. The results showed that four 3-phenyl-2-propene compounds were stable under nitrogen atmosphere. The T a of cinnamaldehyde, cinnamyl alcohol, β-methylstyrene, and cinnamic acid was 271.25 K, 292.375 K, 323.125 K, and 363.875 K, and their T R was 301.125 K, 332.75 K, 357.91 K, and 385.375 K, respectively. The oxidation reactivity order was derived to be cinnamaldehyde > cinnamyl alcohol > β-methylstyrene > cinnamic acid. The oxidation kinetics were determined using n versus time (n-t) plots, which showed a second-order reaction. Peroxide was determined by iodimetry, and the oxidation products were analyzed by gas chromatography-mass spectrometry (GC-MS). The results showed that the peroxide value of cinnamaldehyde, cinnamyl alcohol, β-methylstyrene, and cinnamic acid reached 18.88, 15.07, 9.62, and 4.24 mmol kg-1 at 373 K for 6 h, respectively. The common oxidation products of four 3-phenyl-2-propene compounds were benzaldehyde, benzoic acid, and epoxide, which resulted from the carbon-carbon double bond oxidation. The substituents' oxidation products were obtained from the oxidation of cinnamaldehyde, cinnamyl alcohol, and β-methylstyrene. In particular, the difference is that no oxidation products of the carboxyl group of cinnamic acid were detected. The common oxidation products of the four 3-phenyl-2-propene compounds were benzaldehyde, benzoic acid, and epoxide, which resulted from the carbon-carbon double bond oxidation. The substituents' oxidation products were caught in the oxidation of cinnamaldehyde, cinnamyl alcohol, and β-methylstyrene. In particular, the difference is that no oxidation products of the carboxyl group of cinnamic acid were detected. According to the complex oxidation products, important insights into the oxidation pathways were provided.

Insights into the Photodegradation of the Contact Allergen Fragrance Cinnamyl Alcohol: Kinetics, Mechanism, and Toxicity

Fragrances can cause general health issues, and special concerns exist surrounding the issue of skin safety. Cinnamyl alcohol (CAL) is a frequent fragrance contact allergen that has various toxic effects on indiscriminate animals. In the present study, the photodegradation transformation mechanism of CAL and toxicity evolution during this process were examined. The results showed that CAL (50 μM) can be completely degraded after 90-min ultraviolet (UV) irradiation with a degradation rate of 0.086 min^-1 . Increased toxicity on bioluminescent bacteria was observed during this process, with lethality increasing from 10.6% (0 min) to 50.2% (90 min) under UV light irradiation. Further, the photodegradation mechanisms of CAL were explored to find the reason behind the increased toxicity observed. Laser flash photolysis and quenching experiments showed that O₂^•- , ¹ O₂ , and ^• OH were mainly responsible for CAL photodegradation, together with ³ CAL* and e_aq^- . The 5 main photodegradation products were cinnamyl aldehyde, benzaldehyde, benzenepropanal, cinnamic acid, and toluene, as identified using gas chromatography-mass spectrometry and liquid chromatography-quadrupole-time-of-flight-mass spectrometry. Once exposed to air, CAL was found to be easily oxidized to cinnamyl aldehyde and subsequently to cinnamic acid by O₂^•- - or ¹ O₂ -mediated pathways, leading to increased toxicity. Benzaldehyde exhibited bioreactive toxicity, increasing the toxicity through ^• OH-mediated pathways. Theoretical prediction of skin irritation indicated that cinnamyl aldehyde (0.83), benzenepropanal (0.69), cinnamyl aldehyde (0.69), and benzaldehyde (0.70) were higher than CAL (0.63), which may cause a profound impact on an individual's health and well-being. Overall, the present study advances the understanding of the photodegradation processes and health impacts of fragrance ingredients. Environ Toxicol Chem 2021;40:2705-2714. © 2021 SETAC.

The photocatalytic destruction of cinnamic acid and cinnamyl alcohol: Mechanism and the effect of aqueous ions

Cinnamic acid was chosen as an exemplar molecule to study the effect of potential contaminants on the kinetics and mechanism of the photocatalytic destruction of hydrocarbons in aqueous solutions. We identify the principal intermediates in the photocatalytic reaction of the acid and corresponding alcohol, and propose a mechanism that explains the presence of these species. The impact of two likely contaminants of aqueous systems, sulfate and chloride ions were also studied. Whereas sulfate ions inhibit the degradation reaction at all concentrations, chloride ions, up to a concentration of 0.5 M, accelerate the removal of cinnamic acid from solution by a factor of 1.6. However, although cinnamic acid is removed, the pathway to complete oxidation is blocked by the chloride, with the acid being converted (in the presence of oxygen) into new products including acetophenone, 2-chloroacetophenone, 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane. We speculate that the formation of these products involves chlorine radicals formed from the reaction of chloride ions with the photoinduced holes at the catalyst surface. Interestingly, we have shown that the 1-(2-chlorophenyl)ethenone and 1,2-dibenzoylethane products form from 2-chloroacetophenone when irradiated with 365 nm light in the absence of the catalyst. The formation of potentially dangerous side products in this reaction suggest that the practical implementation of the photocatalytic purification of contaminated water needs to considered very carefully if chlorides are likely to be present.

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.

Comparative analyses of cuticular waxes on various organs of faba bean (Vicia faba L.)

Cuticular waxes cover the plant surface and serve as hydrophobic layer, exhibiting various wax profiles between plant species and plant organs. This paper reports comprehensive analysis of the waxes on organs exposed to air, including leaf, stem, pod pericarp, and petals (banner, wing and keel), and on seed coat enwrapped in pod pericarp of faba bean (Vicia faba). In total 7 classes of wax compounds were identified, including fatty acids, primary alcohols, alkyl esters, aldehydes, alkanes, cinnamyl alcohol esters, and alkylresorcinols. Overall, primary alcohols dominated the waxes on leaves and the seed coat enwrapped in pod pericarp, alkanes accumulated largely in stem and petals, whereas alkylresorcinols were observed in leaf, stem and pod pericarp. Organs exposed to air had higher coverage (>1.2 μg/cm²) than those on seed coat (<0.8 μg/cm²), and keel having the highest wax coverage. Meanwhile, the wax coverage on seed coat reduced during the seed development. The variations of wax coverages, compound class distributions and chain length profiles among organs suggested that wax depositions were associated with their ecophysiological functions, and the enzymes involved in wax biosynthesis also showed organ-specific.

Can the epoxides of cinnamyl alcohol and cinnamal show new cases of contact allergy?

BACKGROUND

Cinnamyl alcohol is considered to be a prohapten and prehapten with cinnamal as the main metabolite. However, many individuals who are allergic to cinnamyl alcohol do not react to cinnamal. Sensitizing epoxides of cinnamyl alcohol and cinnamal have been identified as metabolites and autoxidation products of cinnamyl alcohol.

OBJECTIVE

To investigate the clinical relevance of contact allergy to epoxycinnamyl alcohol and epoxycinnamal.

METHODS

Irritative effects of the epoxides were investigated in 12 dermatitis patients. Epoxycinnamyl alcohol and epoxycinnamal were patch tested in 393 and 390 consecutive patients, respectively. In parallel, cinnamyl alcohol and cinnamal were patch tested in 607 and 616 patients, respectively.

RESULTS

Both epoxides were irritants, but no more positive reactions were detected than when testing was performed with cinnamyl alcohol and cinnamal. Late allergic reactions to epoxycinnamyl alcohol were observed. In general, patients with late reactions showed doubtful or positive reactions to cinnamal and fragrance mix I at regular patch testing.

CONCLUSION

The investigated epoxides are not important haptens in contact allergy to cinnamon fragrance. The high frequency of fragrance allergy among patients included in the irritancy study showed the difficulty of suspecting fragrance allergy on the basis of history; patch testing broadly with fragrance compounds is therefore important.

Cinnamyl alcohol oxidation using supported bimetallic Au–Pd nanoparticles: an investigation of autoxidation and catalysis

In this study, we examine autoxidation and its role on the catalytic aerobic oxidation of cinnamyl alcohol using supported AuPd nanoparticles.

In Situ Metabolism of Cinnamyl Alcohol in Reconstructed Human Epidermis: New Insights into the Activation of This Fragrance Skin Sensitizer

Chemical modification of epidermal proteins by skin sensitizers is the molecular event which initiates the induction of contact allergy. However, not all chemical skin allergens react directly as haptens with epidermal proteins but need either a chemical (prehaptens) or metabolic (prohaptens) activation step to become reactive. Cinnamyl alcohol has been considered a model prohapten, as this skin sensitizer has no intrinsic reactivity. Therefore, the prevailing theory is that cinnamyl alcohol is enzymatically oxidized into the protein-reactive cinnamaldehyde, which is the sensitizing agent. Knowing that reconstructed human epidermis (RHE) models have been demonstrated to be quite similar to the normal human epidermis in terms of metabolic enzymes, use of RHE may be useful to investigate the in situ metabolism/activation of cinnamyl alcohol, particularly when coupled with high-resolution magic angle spinning nuclear magnetic resonance. Incubation of carbon-13 substituted cinnamyl derivatives with RHE did not result in the formation of cinnamaldehyde. The metabolites formed suggest the formation of an epoxy-alcohol and an allylic sulfate as potential electrophiles. These data suggest that cinnamyl alcohol is inducing skin sensitization through a route independent of the one involving cinnamaldehyde and should therefore be considered as a skin sensitizer on its own.

Catalytic oxidation of cinnamyl alcohol using Au-Ag nanotubes investigated by surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive detection of adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities on the same material. Here we report the application of well-defined Au-Ag nanotubes as both SERS substrates and catalysts for the oxidation of cinnamyl alcohol. The species adsorbed on the catalyst surfaces at different reaction times were analyzed by SERS. The bimetallic nanotubes prepared via a simple galvanic replacement reaction are highly active in the oxidation of cinnamyl alcohol, but do not avoid a radical-chain reaction and the cleavage of the carbon-carbon double bond. A comparison between changes in bulk composition and the nature of adsorbed species at the surface of the catalyst over time suggests that cinnamaldehyde is formed on the catalyst surface (metal-catalyzed oxidation) and benzaldehyde is probably formed in the bulk solution via a radical-chain pathway. In the presence of 2,6-di-tert-butyl-4-methylphenol, the radical-chain reaction is suppressed and the oxidation reaction produces cinnamaldehyde.

Epoxyalcohols: bioactivation and conjugation required for skin sensitization

Allylic alcohols, such as geraniol 1, are easily oxidized by varying mechanisms, including the formation of both 2,3-epoxides and/or aldehydes. These epoxides, aldehydes, and epoxy-aldehydes can be interconverted to each other, and the reactivity of them all must be considered when considering the sensitization potential of the parent allylic alcohol. An in-depth study of the possible metabolites and autoxidation products of allylic alcohols is described, covering the formation, interconversion, reactivity, and sensitizing potential thereof, using a combination of in vivo, in vitro, in chemico, and in silico methods. This multimodal study, using the integration of diverse techniques to investigate the sensitization potential of a molecule, allows the identification of potential candidate(s) for the true culprit(s) in allergic responses to allylic alcohols. Overall, the sensitization potential of the investigated epoxyalcohols and unsaturated alcohols was found to derive from metabolic oxidation to the more potent aldehyde where possible. Where this is less likely, the compound remains weakly or nonsensitizing. Metabolic activation of a double bond to form a nonconjugated, nonterminal epoxide moiety is not enough to turn a nonsensitizing alcohol into a sensitizer, as such epoxides have low reactivity and low sensitizing potency. In addition, even an allylic 2,3-epoxide moiety is not necessarily a potent sensitizer, as shown for 2, where formation of the epoxide weakens the sensitization potential.

Skin sensitizers differentially regulate signaling pathways in MUTZ-3 cells in relation to their individual potency

Background

Due to the recent European legislations posing a ban of animal tests for safety assessment within the cosmetic industry, development of in vitro alternatives for assessment of skin sensitization is highly prioritized. To date, proposed in vitro assays are mainly based on single biomarkers, which so far have not been able to classify and stratify chemicals into subgroups, related to risk or potency.

Methods

Recently, we presented the Genomic Allergen Rapid Detection (GARD) assay for assessment of chemical sensitizers. In this paper, we show how the genome wide readout of GARD can be expanded and used to identify differentially regulated pathways relating to individual chemical sensitizers. In this study, we investigated the mechanisms of action of a range of skin sensitizers through pathway identification, pathway classification and transcription factor analysis and related this to the reactive mechanisms and potency of the sensitizing agents.

Results

By transcriptional profiling of chemically stimulated MUTZ-3 cells, 33 canonical pathways intimately involved in sensitization to chemical substances were identified. The results showed that metabolic processes, cell cycling and oxidative stress responses are the key events activated during skin sensitization, and that these functions are engaged differently depending on the reactivity mechanisms of the sensitizing agent. Furthermore, the results indicate that the chemical reactivity groups seem to gradually engage more pathways and more molecules in each pathway with increasing sensitizing potency of the chemical used for stimulation. Also, a switch in gene regulation from up to down regulation, with increasing potency, was seen both in genes involved in metabolic functions and cell cycling. These observed pathway patterns were clearly reflected in the regulatory elements identified to drive these processes, where 33 regulatory elements have been proposed for further analysis.

Conclusions

This study demonstrates that functional analysis of biomarkers identified from our genomics study of human MUTZ-3 cells can be used to assess sensitizing potency of chemicals in vitro, by the identification of key cellular events, such as metabolic and cell cycling pathways.