Next generation risk assessment for skin sensitisation: A case study with propyl paraben

Next generation risk assessment of hair dye HC yellow no. 13: Ensuring protection from liver steatogenic effects

This study employs animal-free Next Generation Risk Assessment (NGRA) principles to evaluate the safety of repeated dermal exposure to 2.5% (w/w) HC Yellow No. 13 (HCY13) hair dye. As multiple in silico tools consistently flagged hepatotoxic potential, likely due to HCY13's trifluoromethyl group, which is known to interfere with hepatic lipid metabolism, liver steatosis was chosen as the primary mode of action for evaluation. AOP-guided in vitro tests were conducted, exposing human stem cell-derived hepatic cells to varying HCY13 concentrations over 72 h. The expression of 11 lipid metabolism-related marker genes (AHR, PPARA, LXRA, APOB, ACOX1, CPT1A, FASN, SCD1, DGAT2, CD36, and PPARG) and triglyceride accumulation, a phenotypic hallmark of steatosis, were measured. PROAST software was used to calculate in vitro Points of Departure (PoDNAM) for each biomarker. Using GastroPlus 9.9, physiologically-based pharmacokinetic (PBPK) models estimated internal liver concentrations (Cmax liver) of HCY13, ranging from 4 to 20 pM. All PoDNAM values significantly exceeded the predicted Cmax liver, indicating that HCY13 at 2.5% (w/w) is unlikely to induce liver steatosis under the assumed conditions. This research demonstrates the utility of NGRA, integrating AOP-based in vitro assays and computational models to protect human health and support regulatory decision-making without animal testing.

Regulatory frameworks for fragrance safety in cosmetics: a global overview

Fragrances play a crucial role in cosmetic products, influencing consumer preferences and enriching sensory experiences. However, making sure these products are safe, especially concerning natural and synthetic fragrances, requires robust regulatory frameworks. This review offers a global perspective on the regulatory systems governing the safety of fragrances. It begins by examining the fundamental differences between natural and synthetic fragrances, highlighting their origins and unique safety considerations. Natural fragrances, sourced from botanicals like essential oils, have a long history of human exposure. Synthetic fragrances, on the other hand, are artificially manufactured compounds that often lack sufficient safety data, thereby requiring strict regulation. Various countries have developed safety guidelines to address concerns such as skin sensitization, allergies, and health risks associated with fragrance use. This article provides a comprehensive analysis of these global regulatory frameworks, emphasizing both commonalities and disparities in safety standards for natural and synthetic fragrances. It also discusses ongoing efforts to harmonize regulations and improve fragrance safety in cosmetics. By offering this in-depth overview of regulatory approaches, the article serves as a valuable resource for cosmetics industry professionals. It provides insights into the evolving landscape of cosmetics regulations worldwide, aiding stakeholders in navigating the complexities of natural and synthetic fragrance safety and ultimately safeguarding consumer well-being.

Graphical Abstract

Skin Sensitization Potency Prediction Based on Read-Across (RAx) Incorporating RhE-Based Testing Strategy (RTSv1)-Defined Approach: RTSv1-Based RAx

In recent years, nonanimal approaches for skin sensitization have been developed in response to political, regulatory, and ethical demands. The reconstructed human epidermis (RhE)-based testing strategy (RTS)v1-defined approach (DA) is used to categorize skin sensitization potency. However, the RTSv1 DA alone cannot be used to predict potency based on EC3 values [the estimated concentration that produces a stimulation index of 3 in the local lymph node assay (LLNA)], and underpredictions have been reported. Read-across (RAx) can complement DA data and improve prediction confidence. Although case studies combining new approach methodology/DA data with RAx have been reported, they focus on a single target chemical and lack a comprehensive and robust strategy with well-examined reliability. This study developed a strategy incorporating the RTSv1 DA into RAx (RTSv1-based RAx) to predict skin sensitization potency, applying it to 43 chemicals. The predictive performance of RTSv1-based RAx was evaluated by comparing its predicted potency category and EC3 outcomes with those of RTSv1 DA and the LLNA. RTSv1-based RAx accurately predicted the Globally Harmonized System of Classification (GHS) subcategorization for 38 chemicals and determined the predicted EC3 values for 17 sensitizers within a fourfold range of LLNA-derived EC3 values. This study demonstrates that RTSv1-based RAx offers robust predictivity for both GHS subcategorization and predicted EC3 values, making it useful for quantitative risk assessment.

Deriving a Continuous Point of Departure for Skin Sensitization Risk Assessment Using a Bayesian Network Model

Regulations of cosmetic ingredients and products have been the most advanced in embracing new approach methodologies (NAMs). Consequently, the cosmetic industry has assumed a forerunner role in the development and implementation of animal-free next-generation risk assessment (NGRA) that incorporates defined approaches (DAs) to assess the skin sensitization potency of ingredients. A Bayesian network DA predicting four potency categories (SkinSens-BN) was constructed against reference Local Lymph Node Assay data for a total of 297 substances, achieving a predictive performance similar to that of other DAs. With the aim of optimally informing risk assessment with a continuous point of departure (PoD), a weighted sum of the SkinSens-BN probabilities for four potency classes (non-, weak, moderate, and strong/extreme sensitizer) was calculated, using fixed weights based on associated LLNA EC3-values. The approach was promising, e.g., the derived PoDs for substances classified as non-sensitizers did not overlap with any others and 77% of PoDs were similar or more conservative than LLNA EC3. In addition, the predictions were assigned a level of confidence based on the probabilities to inform the evaluation of uncertainty in an NGRA context. In conclusion, the PoD derivation approach can substantially contribute to reliable skin sensitization NGRAs.

Benchmarking of BMDC assay and related QSAR study for identifying sensitizing chemicals

The Bone-Marrow derived Dendritic Cell (BMDC) test is a promising assay for identifying sensitizing chemicals based on the 3Rs (Replace, Reduce, Refine) principle. This study expanded the BMDC benchmarking to various in vitro, in chemico, and in silico assays targeting different key events (KE) in the skin sensitization pathway, using common substances datasets. Additionally, a Quantitative Structure-Activity Relationship (QSAR) model was developed to predict the BMDC test outcomes for sensitizing or non-sensitizing chemicals. The modeling workflow involved ISIDA (In Silico Design and Data Analysis) molecular fragment descriptors and the SVM (Support Vector Machine) machine-learning method. The BMDC model's performance was at least comparable to that of all ECVAM-validated models regardless of the KE considered. Compared with other tests targeting KE3, related to dendritic cell activation, BMDC assay was shown to have higher balanced accuracy and sensitivity concerning both the Local Lymph Node Assay (LLNA) and human labels, providing additional evidence for its reliability. The consensus QSAR model exhibits promising results, correlating well with observed sensitization potential. Integrated into a publicly available web service, the BMDC-based QSAR model may serve as a cost-effective and rapid alternative to lab experiments, providing preliminary screening for sensitization potential, compound prioritization, optimization and risk assessment.

A novel three-dimensional Nrf2 reporter epidermis model for skin sensitization assessment

Skin sensitization assessment has progressed from the use of animal models towards the application of New Approach Methodologies (NAMs). Several skin sensitization NAMs are accepted for regulatory use, but a majority relies on submerged in vitro cell cultures that limit their applicability domain, posing challenges for testing hydrophobic chemicals and mixtures. A newly developed three-dimensional (3D) Nrf2 reporter epidermis model for skin sensitization assessment is reported. This NAM may help to overcome these limitations. The NAM combines the in vivo-like biology and exposure conditions of 3D epidermis models with the reliability, convenience, and cost-effectiveness of secreted reporter gene technology. The Keap1-Nrf2-ARE pathway was chosen as the reporter gene read-out, as it is induced by most skin sensitizers and already adopted in OECD Test guideline 442D. Immortalized human primary keratinocytes (Ker-CT) were stably transfected with the pIGB-Nrf2-SEAP vector to construct a Nrf2 reporter cell line. Ker-CT Nrf2 reporter cells showed negligible basal expression of the Secreted Embryonic Alkaline Phosphatase (SEAP) reporter, which was induced 13.5-fold by exposure to the skin sensitizer cinnamic aldehyde (CA). Co-exposure to CA and the Nrf2 inhibitor glucocorticoid clobetasol propionate significantly suppressed the CA-induced SEAP expression, confirming dependance of the SEAP expression on Nrf2 activation. Using air-liquid interface and animal constituent free culture conditions, the Ker-CT Nrf2 reporter cells differentiated to stratified 3D epidermis models with an in vivo-like skin architecture and functional skin barrier. Evaluation of a Ker-CT Nrf2 reporter cell-based 2D assay by testing 10 conventional reference chemicals showed a predictive accuracy for skin sensitization potential of 80% and 70% compared to LLNA and human data in two independent laboratories and a high intra- and interlaboratory reproducibility. Moreover, the 3D epidermis models predicted 3 sensitizing and 2 non-sensitizing reference chemicals correctly in a first proof-of-concept study. Further investigations foresee the testing of additional chemicals, including hydrophobic compounds and mixtures to confirm the potential of the 3D epidermis models to broaden the applicability domain for NAM-based skin sensitization assessment.

Non-animal approaches for photoallergenicity safety assessment: Needs and perspectives for the toxicology for the 21st century

Certain chemicals and/or their byproducts are photoactivated by UV/VIS and trigger a dermal allergenic response, clinically recognized as photoallergic contact dermatitis (PACD). It is important to identify the chemicals which are potentially photoallergenic, not only for establishing the correct differential diagnosis between PACD and other photodermatoses, but also as causative agents which should be avoided as a preventative measure. Moreover, materials with photoallergenic properties need to be correctly identified to allow thorough safety assessments for their use in finished products (e.g. cosmetics). Development of methods for predicting photoallergenicity potential of chemicals has advanced at slow pace in recent years. To date, there are no validated methods for photosensitisation potential of chemicals for regulatory purposes, although it remains a required endpoint in some regions. The purpose of this review is to explore the mechanisms potentially involved in the photosensitisation process and discuss the methods available in the literature for identification of photosensitisers. The review also explores the possibilities of further research investment required to develop human-relevant new approach methodologies (NAMs) and next generation risk assessment (NGRA) approaches, considering the current perspectives and needs of the Toxicology for the 21st Century.

Skin sensitisation prediction using read-across, an illustrative next generation risk assessment (NGRA) case study for vanillin

Skin sensitisation is a key adverse human health effect to be addressed in the safety assessment of cosmetic ingredients. Regulatory demands and scientific progress have led to the development of a Next Generation Risk Assessment (NGRA) framework, relying on the use of New Approach Methodologies (NAM) Defined Approaches (DA) and read-across instead of generating animal data. This case study illustrates the application of read-across for the prediction of the skin sensitisation potential of vanillin at the hypothetical use concentration of 0.5% in a shower gel and face cream. A three-step process was applied to select the most suitable analogues based on their protein reactivity, structural characteristics, physicochemical properties, skin metabolism profile and availability of skin sensitisation data. The applied read-across approach predicted a weak skin sensitiser potential for vanillin corresponding with a Local Lymph Node Assay EC3 value of 10%. Based on this EC3 value a point of departure of 2500 μg/cm2 was derived, resulting in an acceptable exposure level (AEL) of 25 μg/cm2. Because the consumer exposure levels (CEL) for the face cream (13.5 μg/cm2) and shower gel (0.05 μg/cm2) scenarios were lower than the AEL, the NGRA concluded both uses as safe.

[Determination of 12 typical personal care products in human urine samples by ultra performance liquid chromatography-tandem mass spectrometry]

A rapid and sensitive method based on ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was developed for the simultaneous determination of 12 typical personal care products (PCPs) in human urine. These PCPs included five paraben preservatives (PBs), five benzophenone UV absorbers (BPs), and two antibacterial agents. Accordingly, 1 mL of the urine sample was mixed with 500 μL of β-glucuronidase-ammonium acetate buffer solution (enzymatic activities are 500 units/mL) and 75 μL of a mixed internal standard working solution (internal standard contents are 7.5 ng), followed by enzymatic hydrolysis overnight (≥16 h) at 37 ℃ in a water bath. The 12 targeted analytes were enriched and cleaned up using an Oasis HLB solid phase extraction column. Separation was performed on an Acquity BEH C18 column (100 mm×2.1 mm, 1.7 μm) using an acetonitrile-water system as the mobile phase, in negative electrospray ionization (ESI-) multiple reaction monitoring (MRM) mode, for target detection and stable isotope internal standard quantification. The optimal MS conditions were established by optimizing the instrument parameters and comparing two analytical columns (Acquity BEH C18 and Acquity UPLC HSS T3) as well as different types of mobile phases (methanol or acetonitrile as the organic phase) to achieve better chromatographic separation. In order to obtain higher enzymatic and extraction efficiency, different enzymatic conditions, solid phase extraction columns, and elution conditions were investigated. The final results showed that methyl parabens (MeP), benzophenone-3 (BP-3), and triclosan (TCS) showed good linearities in the ranges of 4.00-800, 4.00-800 and 5.00-200 μg/L, respectively, the other targeted compounds showed good linearities in the ranges of 1.00-200 μg/L. The correlation coefficients were all greater than 0.999. The method detection limits (MDLs) were in the range of 0.06-1.09 μg/L, and the method quantification limits (MQLs) ranged from 0.08 to 3.63 μg/L. At three spiked levels, the average recoveries of the 12 targeted analytes ranged from 89.5% to 111.8%. The intra-day and inter-day precisions were 3.7%-8.9% and 2.0%-10.6%, respectively. The results of the matrix effect assessment showed that MeP, ethyl paraben (EtP), and benzophenone-2 (BP-2) exhibited strong matrix effects (26.7%-103.8%); propyl paraben (PrP) exhibited moderate matrix effects (79.2%-112.0%); and the other eight target analytes exhibited weak matrix effects (83.3%-113.8%). The matrix effects of the 12 targeted analytes after correction using the stable isotopic internal standard method ranged from 91.9% to 110.1%. The developed method was successfully applied to the determination of the 12 PCPs in 127 urine samples. Ten typical PCPs were detected, with the overall detection rates ranging from 1.7% to 99.7%, except for benzyl paraben (BzP) and benzophenone-8 (BP-8). The results revealed that the population in this area was widely exposed to PCPs, especially MeP, EtP and PrP; the detection rates and concentrations of these PCPs were found to be very high. Our analytical method is simple and sensitive, and it is expected to be an effective tool for biomonitoring PCPs in human urine samples as part of environmental health studies.

Incorporating integrated testing strategy (ITSv1) defined approach into read-across (RAx) in predicting skin sensitization potency: ITSv1-based RAx

Recently, due to regulatory and ethical demands, new approach methodologies (NAMs), defined approaches (DAs), and read-across (RAx) have been used in the risk assessment of skin sensitization. Integrated testing strategy (ITS)v1 DA, adopted in OECD Guideline No. 497, can be used for skin sensitization potency categorization. However, ITSv1 DA alone is not used for further refinement of the potency prediction based on EC3 (the estimated concentration that produces a stimulation index of 3 in murine local lymph node assay) values. Moreover, there is no explicit approach to incorporating NAM/DA data into RAx to fill the data gap of EC3 values with high confidence. This study developed a strategy incorporating ITSv1 DA into RAx to predict skin sensitization potency: ITSv1-based RAx. To examine the reliability of this novel strategy, a case study with lilial, a fragrance material, was performed. Based on ITSv1-based RAx, the skin sensitization potency of lilial was determined by extrapolating the EC3 value of 9.5% for the suitable analogue bourgeonal, which was close to the historical EC3 value of 8.6%. The result suggested that the strategy can refine the prediction of EC3 values with high confidence and be useful for the risk assessment of skin sensitization.

Amending the U-SENS™ skin sensitization test method for interfering auto-fluorescent chemicals

Limitations of the applicability domain of new approach methodologies (NAM) present a major challenge for the testing of cosmetic ingredients in Europe, as the regulation does not allow to resort to in vivo test method. Therefore, research focused on overcoming such limitations of established in vitro test methods is frequently conducted. Here, we address a limitation of the U-SENS™, an in vitro skin sensitization test method that addresses the key event 3 on activation of dendritic cells of the adverse outcome pathway (AOP) for skin sensitization. The applicability domain of the U-SENS™ excludes autofluorescent substances that can interfere with the measurement of the expression of CD86, i.e., the primary readout. An evaluation of several fluorochromes identified APC as most suitable for testing auto-fluorescent chemicals. Acceptance criteria were reproducibly met when using the APC-labelled antibody. Equivalent performance in terms of reproducibility and skin sensitisation hazard assessment of the standard FITC-labelled antibodies and the APC-labelled antibodies was demonstrated by testing 40 substances. Finally, the value of the expanded technical applicability domain was highlighted with a case study using sulfuretin. In conclusion, we successfully demonstrated the expansion of the U-SENS™ applicability domain to interfering auto-fluorescent chemicals by using APC-labelled antibodies.

Next generation risk assessment for skin allergy: Decision making using new approach methodologies

Our aim is to develop and apply next generation approaches to skin allergy risk assessment (SARA) that do not require new animal test data and better quantify uncertainties. Significant progress has been made in the development of New Approach Methodologies (NAMs), non-animal test methods, for assessment of skin sensitisation and there is now focus on their application to derive potency information for use in Next Generation Risk Assessment (NGRA). The SARA model utilises a Bayesian statistical approach to infer a human-relevant metric of sensitiser potency and a measure of risk associated with a given consumer exposure based upon any combination of human repeat insult patch test, local lymph node, direct peptide reactivity assay, KeratinoSens™, h-CLAT or U-SENS™ data. Here we have applied the SARA model within our weight of evidence NGRA framework for skin allergy to three case study materials in four consumer products. Highlighting how to structure the risk assessment, apply NAMs to derive a point of departure and conclude on consumer safety risk. NGRA based upon NAMs were, for these exposures, at least as protective as the historical risk assessment approaches. Through such case studies we are building our confidence in using NAMs for skin allergy risk assessment.