Characterisation of skin penetration pathways using stimulated Raman scattering microscopy.

Understanding the mechanisms governing the penetration of substances into the skin is crucial for the development of safe and effective topical drug delivery systems and skincare products. This study examined the partitioning of model permeants into human skin, by assessing six substances with diverse logP values. We employed stimulated Raman scattering (SRS) microscopy, an ambient, label-free optical imaging technique known for its ability to provide chemical distribution with subcellular resolution. Our investigation assessed partitioning into the two primary pathways through which substances traverse the skin: the intercellular lipid matrix and the intracellular route via corneocyte cells. We observed that the partitioning behaviour was strongly influenced by the lipophilicity of the molecule, with lipophilic compounds showing greater affinity for intercellular matrix with increased lipophilicity. Conversely, hydrophilic molecules demonstrated a preference for corneocyte cells, with their affinity increasing with increased hydrophilicity. The findings contribute to our understanding of the mechanisms underlying topical delivery and offer important implications and new methods beneficial for the development of safe and effective topical products. In addition, the methods presented could be valuable to reveal changes in drug partitioning or to assess targeting approaches in diseased skin models.

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.

ADME characterization and PBK model development of 3 highly protein-bound UV filters through topical application.

Estimating human exposure in the safety assessment of chemicals is crucial. Physiologically based kinetic (PBK) models which combine information on exposure, physiology, and chemical properties, describing the absorption, distribution, metabolism, and excretion (ADME) processes of a chemical, can be used to calculate internal exposure metrics such as maximum concentration and area under the concentration-time curve in plasma or tissues of a test chemical in next-generation risk assessment. This article demonstrates the development of PBK models for 3 UV filters, specifically octyl methoxycinnamate, octocrylene, and 4-methylbenzylidene camphor. The models were parameterized entirely based on data obtained from in vitro and/or in silico methods in a bottom-up modeling approach and then validated based on human dermal pharmacokinetic (PK) data. The 3 UV filters are "difficult to test" in in vitro test systems due to high lipophilicity, high binding affinity for proteins, and nonspecific binding, for example, toward plastic. This research work presents critical considerations in ADME data generation, interpretation, and parameterization to assure valid PBK model development to increase confidence in using PBK modeling to help make safety decisions in the absence of human PK data. The developed PBK models of the 3 chemicals successfully simulated the plasma concentration profiles of clinical PK data following dermal application, indicating the reliability of the ADME data generated and the parameters determined. The study also provides insights and lessons learned for characterizing ADME and developing PBK models for highly lipophilic and protein-bound chemicals in the future.

Measurement of chemical penetration in skin using Stimulated Raman scattering microscopy and multivariate curve resolution - alternating least squares.

The mechanistic understanding of skin penetration underpins the design, efficacy and risk assessment of many high-value products including functional personal care products, topical and transdermal drugs. Stimulated Raman scattering (SRS) microscopy, a label free chemical imaging tool, combines molecular spectroscopy with submicron spatial information to map the distribution of chemicals as they penetrate the skin. However, the quantification of penetration is hampered by significant interference from Raman signals of skin constituents. This study reports a method for disentangling exogeneous contributions and measuring their permeation profile through human skin combining SRS measurements with chemometrics. We investigated the spectral decomposition capability of multivariate curve resolution - alternating least squares (MCR-ALS) using hyperspectral SRS images of skin dosed with 4-cyanophenol. By performing MCR-ALS on the fingerprint region spectral data, the distribution of 4-cyanophenol in skin was estimated in an attempt to quantify the amount permeated at different depths. The reconstructed distribution was compared with the experimental mapping of CN, a strong vibrational peak in 4-cyanophenol where the skin is spectroscopically silent. The similarity between MCR-ALS resolved and experimental distribution in skin dosed for 4 h was 0.79 which improved to 0.91 for skin dosed for 1 h. The correlation was observed to be lower for deeper layers of skin where SRS signal intensity is low which is an indication of low sensitivity of SRS. This work is the first demonstration, to the best of our knowledge, of combining SRS imaging technique with spectral unmixing methods for direct observation and mapping of the chemical penetration and distribution in biological tissues.

PBK modelling of topical application and characterisation of the uncertainty of Cmax estimate: A case study approach.

Combined with in vitro bioactivity data, physiologically based kinetic (PBK) models has increasing applications in next generation risk assessment for animal-free safety decision making. A tiered framework of building PBK models for such application has been developed with increasing complexity and refinements, as model parameters determined in silico, in vitro, and with human pharmacokinetic data become progressively available. PBK modelling has been widely applied for oral/intravenous administration, but less so on topically applied chemicals. Therefore, building PBK models for topical applications and characterizing their uncertainties in the tiered approach is critical to safety decision making. The purpose of this study was to assess the confidence of PBK modelling of topically applied chemicals following the tiered framework, using non-animal methods derived parameters. Prediction of maximum plasma concentration (Cmax) and area under the curve were compared to observed kinetics from published dermal clinical studies for five chemicals (diclofenac, salicylic acid, coumarin, nicotine, caffeine). A bespoke Bayesian statistical model was developed to describe the distributions of Cmax errors between the predicted and observed data. We showed a general trend that confidence in model predictions increases when more quality in vitro data, particularly those on hepatic clearance and dermal absorption, are available as model input. The overall fold error distributions are useful for characterizing model uncertainty. We concluded that by identifying and quantifying the uncertainties in the tiered approach, we can increase the confidence in using PBK modelling to help make safety decisions on topically applied chemicals in the absence of human pharmacokinetic data.

A Next-Generation Risk Assessment Case Study for Coumarin in Cosmetic Products.

Next-Generation Risk Assessment is defined as an exposure-led, hypothesis-driven risk assessment approach that integrates new approach methodologies (NAMs) to assure safety without the use of animal testing. These principles were applied to a hypothetical safety assessment of 0.1% coumarin in face cream and body lotion. For the purpose of evaluating the use of NAMs, existing animal and human data on coumarin were excluded. Internal concentrations (plasma Cmax) were estimated using a physiologically based kinetic model for dermally applied coumarin. Systemic toxicity was assessed using a battery of in vitro NAMs to identify points of departure (PoDs) for a variety of biological effects such as receptor-mediated and immunomodulatory effects (Eurofins SafetyScreen44 and BioMap Diversity 8 Panel, respectively), and general bioactivity (ToxCast data, an in vitro cell stress panel and high-throughput transcriptomics). In addition, in silico alerts for genotoxicity were followed up with the ToxTracker tool. The PoDs from the in vitro assays were plotted against the calculated in vivo exposure to calculate a margin of safety with associated uncertainty. The predicted Cmax values for face cream and body lotion were lower than all PoDs with margin of safety higher than 100. Furthermore, coumarin was not genotoxic, did not bind to any of the 44 receptors tested and did not show any immunomodulatory effects at consumer-relevant exposures. In conclusion, this case study demonstrated the value of integrating exposure science, computational modeling and in vitro bioactivity data, to reach a safety decision without animal data.

Application of physiologically based kinetic (PBK) modelling in the next generation risk assessment of dermally applied consumer products.

Next Generation Risk Assessment (NGRA) is a procedure that integrates new approach methodologies (NAMs) to assure safety of a product without generating data from animal testing. One of the major challenges in the application of NGRA to consumer products is how to extrapolate from the in vitro points of departure (PoDs) to the human exposure level associated with product use. To bridge the gap, physiologically based kinetic (PBK) modelling is routinely used to predict systemic exposure (Cmax or AUC) from external exposures. A novel framework was developed for assessing the exposure of new ingredients in dermally applied products based on the construction of PBK models describing consumer habits and practices, formulation type, and ADME (absorption, distribution, metabolism and excretion) properties exclusively obtained from NAMs. This framework aims to quantify and reduce the uncertainty in predictions and is closely related to the risk assessment process (i.e., is the margin of safety sufficient to cover the uncertainties in the extrapolation between the in vitro and in vivo toxicodynamics and toxicokinetics?). Coumarin, caffeine, and sulforaphane in four product types (kitchen cleaner liquid, face cream, shampoo, and body lotion) were selected to exemplify how this framework could be used in practise. Our work shows initial levels of the framework provide a conservative estimate of Cmax in most cases which can be refined using sensitivity analysis to inform the choice of follow-up in vitro experiments. These case studies show the framework can increase confidence in use of PBK predictions for safety assessment.

Applying the skin sensitisation adverse outcome pathway (AOP) to quantitative risk assessment.

As documented in the recent OECD report 'the adverse outcome pathway for skin sensitisation initiated by covalent binding to proteins' (OECD, 2012), the chemical and biological events driving the induction of human skin sensitisation have been investigated for many years and are now well understood. Several non-animal test methods have been developed to predict sensitiser potential by measuring the impact of chemical sensitisers on these key events (Adler et al., 2011; Maxwell et al., 2011); however our ability to use these non-animal datasets for risk assessment decision-making (i.e. to establish a safe level of human exposure for a sensitising chemical) remains limited and a more mechanistic approach to data integration is required to address this challenge. Informed by our previous efforts to model the induction of skin sensitisation (Maxwell and MacKay, 2008) we are now developing two mathematical models ('total haptenated protein' model and 'CD8(+) T cell response' model) that will be linked to provide predictions of the human CD8(+) T cell response for a defined skin exposure to a sensitising chemical. Mathematical model development is underpinned by focussed clinical or human-relevant research activities designed to inform/challenge model predictions whilst also increasing our fundamental understanding of human skin sensitisation. With this approach, we aim to quantify the relationship between the dose of sensitiser applied to the skin and the extent of the hapten-specific T cell response that would result. Furthermore, by benchmarking our mathematical model predictions against clinical datasets (e.g. human diagnostic patch test data), instead of animal test data, we propose that this approach could represent a new paradigm for mechanistic toxicology.

Determining epidermal disposition kinetics for use in an integrated nonanimal approach to skin sensitization risk assessment.

Development of risk assessment methods for skin sensitization in the absence of toxicological data generated in animals represents a major scientific and technical challenge. The first step in human skin sensitization induction is the transport of sensitizer from the applied dose on the skin surface to the epidermis, where innate immune activation occurs. Building on the previous development of a time course in vitro human skin permeation assay, new kinetic data for 10 sensitizers and 2 nonsensitizers are reported. Multicompartmental modeling has been applied to analyze the data and determine candidate dose parameters for use in integrated risk assessment methods: the area under the curve (AUC) and maximum concentration (C(max)) in the epidermis. A model with two skin compartments, representing the stratum corneum and viable skin (epidermis and dermis), was chosen following a formal model selection process. Estimates of the uncertainty, as well as average values of the epidermal disposition kinetics parameters, were made by fitting to the time course skin permeation data from individual skin donors. A potential reduced time course method is proposed based on two time points at 4 and 24 h, which gives results close to those from the full time course for the current data sets. The time course data presented in this work have been provided as a resource for development of predictive in silico skin permeation models.

Assuring consumer safety without animal testing: a feasibility case study for skin sensitisation.

Allergic Contact Dermatitis (ACD; chemical-induced skin sensitisation) represents a key consumer safety endpoint for the cosmetics industry. At present, animal tests (predominantly the mouse Local Lymph Node Assay) are used to generate skin sensitisation hazard data for use in consumer safety risk assessments. An animal testing ban on chemicals to be used in cosmetics will come into effect in the European Union (EU) from March 2009. This animal testing ban is also linked to an EU marketing ban on products containing any ingredients that have been subsequently tested in animals, from March 2009 or March 2013, depending on the toxicological endpoint of concern. Consequently, the testing of cosmetic ingredients in animals for their potential to induce skin sensitisation will be subject to an EU marketing ban, from March 2013 onwards. Our conceptual framework and strategy to deliver a non-animal approach to consumer safety risk assessment can be summarised as an evaluation of new technologies (e.g. 'omics', informatics), leading to the development of new non-animal (in silico and in vitro) predictive models for the generation and interpretation of new forms of hazard characterisation data, followed by the development of new risk assessment approaches to integrate these new forms of data and information in the context of human exposure. Following the principles of the conceptual framework, we have been investigating existing and developing new technologies, models and approaches, in order to explore the feasibility of delivering consumer safety risk assessment decisions in the absence of new animal data. We present here our progress in implementing this conceptual framework, with the skin sensitisation endpoint used as a case study.

Development of a modified in vitro skin absorption method to study the epidermal/dermal disposition of a contact allergen in human skin.

In vitro skin absorption methods exist in Organisation for Economic Co-operation and Development (OECD) guideline form (No. 428) and are used to estimate the degree of systemic penetration of chemicals through skin. More detailed kinetics of permeation through skin compartments are not described well by existing methods. This study was designed to assess the practical feasibility of generating compartmental (stratum corneum/epidermal/dermal) disposition and kinetic data of topically applied chemicals. For chemically induced effects initiated in the skin (e.g., skin allergy), the delivery of tissue concentrations of chemical will impact the incidence and severity of biological effect. Explicit data on the kinetics of chemical disposition in skin have not traditionally been needed for skin allergy risk assessment: current in vivo assays embody delivery implicitly. Under the 7th Amendment to the European Cosmetics Directive, in vivo assays (such as the local lymph node assay for skin sensitization) will not be permitted to assess cosmetic ingredients. New in vitro and in silico alternative approaches and ways of predicting risk of adverse effects in humans need to be developed, and new methods such as that described here provide a way of estimating delivered concentrations and the effect of formulation changes on that delivery. As we continue to deconstruct the contributing factors of skin allergy in humans, it will be useful to have methods available that can measure skin tissue compartment exposure levels delivered from different exposure use scenarios. Here we provide such a method. The method could also be used to generate useful data for developing in silico kinetic models of compartmental skin delivery and for refining data for skin delivery in relation to the evaluation of systemic toxicity.

Optimisation of the caco-2 permeability assay using experimental design methodology.

PURPOSE: This study used a Box-Behnken experimental design to optimise the experimental conditions in the Caco-2 assay for a series of p-hydroxybenzoate ester compounds (log P 1.96-5.69), as highly lipophilic compounds are not handled well in this system. METHODS: Caco-2 cells, passage 55-70, were cultured on Transwelltrade mark cell culture supports and permeability assays were performed on day 21. A three level three factorial experimental design was used to optimise the experimental conditions. RESULTS: Addition of BSA (4% w/v) in the medium increased the apparent permeability coefficients (Papp) of each of the parabens except the octyl ester. Increasing the stirring rate by 100 rpm increased Papp for all the parabens. Use of simulated intestinal fluid either increased (fasted state) or decreased (fed state) the Papp of methyl-butyl parabens. CONCLUSIONS: The optimised conditions were; 1.55% w/v BSA, 215 rpm stirring rate and 3.02 mM sodium taurocholate in the simulated intestinal fluid; where octyl paraben (log P 5.69) had an Papp of 33.93 +/- 1.84 x 10(-6) cm/s, reflecting its rapid absorption in man. This study provides a systematic optimisation of the Caco-2 permeability assay to avoid the underestimation of the intestinal permeability of compounds with log P > 3.

Xenobiotic metabolism in human skin and 3D human skin reconstructs: a review.

In this review, we discuss and compare studies of xenobiotic metabolism in both human skin and 3D human skin reconstructs. In comparison to the liver, the skin is a less studied organ in terms of characterising metabolic capability. While the skin forms the major protective barrier to environmental chemical exposure, it is also a potential target organ for adverse health effects. Occupational, accidental or intended-use exposure to toxic chemicals could result in acute or delayed injury to the skin (e.g. inflammation, allergy, cancer). Skin metabolism may play a role in the manifestation or amelioration of adverse effects via the topical route. Today, we have robust testing strategies to assess the potential for local skin toxicity of chemical exposure. Such methods (e.g. the local lymph node assay for assessing skin sensitisation; skin painting carcinogenicity studies) incorporate skin metabolism implicitly in the in vivo model system used. In light of recent European legislation (i.e. 7(th) Amendment to the Cosmetics Directive and Registration Evaluation and Authorisation of existing Chemicals (REACH)), non-animal approaches will be required to reduce and replace animal experiments for chemical risk assessment. It is expected that new models and approaches will need to account for skin metabolism explicitly, as the mechanisms of adverse effects in the skin are deconvoluted. 3D skin models have been proposed as a tool to use in new in vitro alternative approaches. In order to be able to use 3D skin models in this context, we need to understand their metabolic competency in relation to xenobiotic biotransformation and whether functional activity is representative of that seen in human skin.

Paraben transport and metabolism in the biomimetic artificial membrane permeability assay (BAMPA) and 3-day and 21-day Caco-2 cell systems.

Noncellular and cellular in vitro models for predicting intestinal absorption were used to investigate the transport and metabolism of parabens. The biomimetic artificial membrane permeability assay (BAMPA) membrane was constructed by impregnating a lipid solution on a hydrophobic filter. Caco-2 cells at passage numbers 65 to 80 were cultured in either the accelerated 3-day Biocoat system or the standard 21-day Transwell cell culture system. Paraben transport across the BAMPA system showed a parabolic relationship. The lowest log P (p-hydroxybenzoic acid) and highest log P compounds (heptyl and octyl parabens) had apparent permeabilities (Papp) less than 1.0 x 10(-6) cm/s and Papp was maximal at approximately 8.5 x 10(-6)cm/s for the intermediate log P (ethylparaben) compound. With the Biocoat, a similar parabolic relationship was found. In the 21-day Caco-2 cells, the parabens were metabolized by esterases at to p-hydroxybenzoic acid. In conclusion, the in vitro models added complementary insight into the absorption process, such as the transport route, intrinsic permeability, and extent of metabolism of the parabens. This study indicated that presystemic metabolism of orally ingested parabens to the p-hydroxybenzoic acid in the intestine may limit systemic exposure to alkyl-paraben esters in vivo.