An in vitro model for identifying skin-corrosive chemicals. I. Initial validation

Bryostatin and its synthetic analog, picolog rescue dermal fibroblasts from prolonged stress and contribute to survival and rejuvenation of human skin equivalents

Skin health is associated with the day-to-day activity of fibroblasts. The primary function of fibroblasts is to synthesize structural proteins, such as collagen, extracellular matrix proteins, and other proteins that support the structural integrity of the skin and are associated with younger, firmer, and more elastic skin that is better able to resist and recover from injury. At sub-nanomolar concentrations (0.03-0.3 nM), bryostatin-1 and its synthetic analog, picolog (0.1-10 nM) sustained the survival and activation of human dermal fibroblasts cultured under the stressful condition of prolonged serum deprivation. Bryostatin-1 treatment stabilized human skin equivalents (HSEs), a bioengineered combination of primary human skin cells (keratinocytes and dermal fibroblasts) on an extracellular matrix composed of mainly collagen. Fibroblasts activated by bryostatin-1 protected the structural integrity of HSEs. Bryostatin-1 and picolog prolonged activation of Erk in fibroblasts to promote cell survival. Chronic stress promotes the progression of apoptosis. Dermal fibroblasts constitutively express all components of Fas associated apoptosis, including caspase-8, an initiator enzyme of apoptosis. Prolong bryostatin-1 treatment reduced apoptosis by decreasing caspase-8 and protected dermal fibroblasts. Our data suggest that bryostatin-1 and picolog could be useful in anti-aging skincare, and could have applications in tissue engineering and regenerative medicine.

Insights on in vitro models for safety and toxicity assessment of cosmetic ingredients

According to the current European legislation, the safety assessment of each individual cosmetic ingredient of any formulation is the basis for the safety evaluation of a cosmetic product. Also, animal testing in the European Union is prohibited for cosmetic ingredients and products since 2004 and 2009, respectively. Additionally, the commercialization of any cosmetic products containing ingredients tested on animal models was forbidden in 2009. In consequence of these boundaries, the European Centre for the Validation of Alternative Methods (ECVAM) proposes a list of validated cell-based in vitro models for predicting the safety and toxicity of cosmetic ingredients. These models have been demonstrated as valuable and effective tools to overcome the limitations of animal in vivo studies. Although the use of in vitro cell-based models for the evaluation of absorption and permeability of cosmetic ingredients is widespread, a detailed study on the properties of these platforms and the in vitro-in vivo correlation compared with human data are required. Moreover, additional efforts must be taken to develop in vitro models to predict carcinogenicity, repeat dose toxicity and reproductive toxicity, for which no alternative in vitro methods are currently available. This review paper summarizes and characterizes the most relevant in vitro models validated by ECVAM employed to predict the safety and toxicology of cosmetic ingredients.

Final Report on the Safety Assessment of Lauramine and Stearamine

Lauramine and Stearamine are aliphatic amines intended for use in cosmetic formulations as antistatic agents, but no current uses have been reported. In a subchronic feeding study in rats, 3,000 ppm of Stearamine in the diet caused weight loss and increased mortality, with an accumulation of histiocytes in the mucosa of the small intestine and mesenteric lymph nodes. A diet of 500 ppm did not produce these effects. Dogs fed Stearamine at 15 mg/kg/day snowed the same histiocyte accumulation. In a 2-year chronic feeding study in rats, Stearamine at concentrations up to 500 ppm (maximum tested) showed no significant increase in the incidence of tumors. Stearamine at 0.01% in a high-fat diet promoted the carcinogenic effects of DMBA in female rats, whereas 0.1% significantly inhibited the production of tumors. Tests in animals showed Lauramine to cause severe dermal irritation and necrosis. Stearamine (in an ether vehicle) applied to the skin of albino mice caused severe hyperplasia at concentrations as low as 3 mg in a 0.2-ml application. These available data were insufficient to support the safety of Lauramine and Stearamine in cosmetic formulations. Additional data considered necessary to evaluate the safety of these ingredients include the following: impurities (especially data on nitrosamines); two different genotoxicity assays (one using a mammalian system and, if positive, a dermal carcinogenesis assay by National Toxicology Program standards may be required); and a human repeat-insult patch test. It cannot be concluded that these ingredients are safe for use in cosmetic products until the listed safety data have been obtained and evaluated.

Final Report on the Safety Assessment of Cetrimonium Chloride, Cetrimonium Bromide, and Steartrimonium Chloride

Cetrimonium Bromide, Cetrimonium Chloride, and Steartrimonium Chloride are quaternary ammonium salts used for a variety of purposes in cosmetics at concentrations of up to 10%. Cetrimonium Bromide given orally is poorly absorbed from the intestine and is excreted in feces. Cetrimonium Bromide applied dermally is absorbed into the skin, but not rapidly. Dermal irritation and sensitization and ocular irritation are seen with these quaternary ammonium salts. Cetrimonium Bromide was embryotoxic and teratogenic in mice following intraperitoneal injection of 35 mg/kg; only teratogenic effects were observed with 10 mg/kg. Embryotoxic effects consistent with maternal toxicity were seen in a rat-feeding study using 50 mg/kg/day. Dermal exposure to 2% Cetrimonium Chloride produced no evidence of teratogenieity; nor did 2.5% Steartrimonium Chloride. All mutagenesis assays used were negative. Repeated insult patch tests of concentrations of up to 0.25% Cetrimonium Chloride produced no sensitization reactions, although irritation was observed during induction. Based on the available data Cetrimonium Bromide, Cetrimonium Chloride, and Steartrimonium Chloride are considered safe for use in rinse-off cosmetic products but are safe only at concentrations of up to 0.25% in leave-on products.

Tissue-engineered skin substitutes: an overview

Extensive skin loss and chronic wounds are still a significant challenge to clinicians: even if injured epidermis is normally able to self-renew, deep injuries can cause negative regulation of the wound healing cascade, leading to chronic wound formation. Skin-autografting surgical procedures are often limited by the poor availability of healthy tissue, whereas the use of non-self-tissues for allografts presents some severe risks. Tissue-engineered skin substitutes have recently become viable as a suitable alternative to auto- and allografts. However, biologists, biochemists, and technical engineers are still struggling to produce complex skin substitutes that can readily be transplanted in large quantities. The ambitious goal is now to construct a dermoepidermal substitute that rapidly vascularizes and optimally supports a stratifying epidermal graft on a biodegradable matrix. This review analyzes these aspects in light of the available literature and the authors' experience.

Acceptance of in vitro studies by regulatory authorities

There has been relatively little progress regarding the acceptance of in vitro tests by regulatory authorities since the second Practical In Vitro Toxicology Conference in 1989. Advances have been made in the international acceptance of the use of in vitro methods to identify compounds with mutagenic potential. The recognition, in international guidelines on skin and eye irritancy studies, of the need for a hierarchical approach, including the use of in vitro methods to screen out severe irritants, has also been welcome. The reasons for the relatively slow progress are considered. In the case of repeated-dose animal toxicity studies, these represent an effective broad-spectrum approach to identifying the general toxic effects and target organs. It will be difficult to design in vitro methods capable of mirroring the complex interactions seen in the whole animal or the multitudes of potential targets for toxic effects. In vitro studies may, however, be valuable in characterizing such effects once identified from the animal studies. Similar considerations apply to teratogenicity, where possible mechanisms include disturbances in placental function or in maternal metabolism. These examples illustrate the fairly substantial scientific obstacles that exist in some areas. The relatively rapid acceptance of mutagenicity studies was, to some extent, due to their single, underlying mechanism, namely, damage to DNA. In view of these problems, effort has concentrated on local effects such as skin and eye irritancy. Even here, the in vivo response is a complex series of reactions, and there is the assumption that a battery of in vitro tests will be needed. There have been difficulties in identifying the most promising combination of tests to subject to detailed validation. In addition, the number and identity of the 'reference' chemicals to be used and the comparative data (in vivo animal data or human data) needed has proved to be a matter of much debate. In addition to these difficulties, the need for international acceptance of the validation data has necessitated the adoption of a more international perspective. However, a large international study is now underway which, it is hoped, will lead to some real progress in the eye irritancy area.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Tissue engineered human skin equivalents

Human skin not only serves as an important barrier against the penetration of exogenous substances into the body, but also provides a potential avenue for the transport of functional active drugs/reagents/ingredients into the skin (topical delivery) and/or the body (transdermal delivery). In the past three decades, research and development in human skin equivalents have advanced in parallel with those in tissue engineering and regenerative medicine. The human skin equivalents are used commercially as clinical skin substitutes and as models for permeation and toxicity screening. Several academic laboratories have developed their own human skin equivalent models and applied these models for studying skin permeation, corrosivity and irritation, compound toxicity, biochemistry, metabolism and cellular pharmacology. Various aspects of the state of the art of human skin equivalents are reviewed and discussed.

Skin tissue engineering--in vivo and in vitro applications

Significant progress has been made over the years in the development of in vitro-engineered substitutes that mimic human skin, either to be used as grafts for the replacement of lost skin or for the establishment of human-based in vitro skin models. This review summarizes these advances in in vivo and in vitro applications of tissue-engineered skin. We further highlight novel efforts in the design of complex disease-in-a-dish models for studies ranging from disease etiology to drug development and screening.

Integrated Decision-tree Testing Strategies for Skin Corrosion and Irritation with Respect to the Requirements of the EU REACH Legislation

Liverpool John Moores University and FRAME recently conducted a research project, sponsored by Defra, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This report focuses on how to maximise the use of alternative methods (both in vitro and in silico) for skin corrosion and irritation testing within a tiered testing strategy. It considers the latest developments in in vitro testing, with particular reference to the reconstituted skin models which have now been now been successfully validated and independently endorsed as suitable for both skin corrosivity and irritancy testing within the EU.

Integrated decision-tree testing strategies for skin corrosion and irritation with respect to the requirements of the EU REACH legislation

Liverpool John Moores University and FRAME recently conducted a research project, sponsored by DEFRA, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This report focuses on how to maximise the use of alternative methods (both in vitro and in silico) for skin corrosion and irritation testing within a tiered testing strategy. It considers the latest developments in in vitro testing, with particular reference to the reconstituted skin models which have now been now been successfully validated and independently endorsed as suitable for both skin corrosivity and irritancy testing within the EU.

The Tinsley LCR Databridge Model 6401 and electrical impedance measurements to evaluate skin integrity in vitro

Electrical impedance is used to confirm skin integrity for in vitro dermal regulatory testing and as a tool to evaluate skin condition to determine the irritation and corrosion potential of various chemicals and personal care products. In this experiment, samples of dermatomed human skin were mounted onto static diffusion cells (0.64 cm2) maintained at 32 degrees C. Following equilibration with 0.9% saline in the donor and receptor chambers, an impedance measurement was taken with a Tinsley LCR Databridge Model 6401 set in the resistance mode (R) and in (a) the serial-equivalent mode (SER) with an alternating current (AC) frequency of 100 hertz (Hz), (b) SER and 1000 Hz, (c) parallel-equivalent mode (PAR) and 100 Hz, and (d) PAR and 1000 Hz. With the databridge set in the SER-equivalent mode and an AC frequency of 1000 Hz, the minimum (7.2 kOmega), maximum (10.0 kOmega), and median (8.6 kOmega) impedance values exhibited a limited response range (2.8 kOmega). However, when the Tinsley 6401 was set in the PAR-equivalent mode at the lower AC frequency of 100 Hz the minimum (16.7 kOmega), maximum (134.6 kOmega), median (83.2 kOmega), and range (117.9 kOmega) of values were the highest obtained. The results confirm that the operator-selected settings on the Tinsley LCR Databridge Model 6401 affect the impedance measurement and the dynamic range of values observed for dermatomed human skin in vitro.

Rapid integrity assessment of rat and human epidermal membranes for in vitro dermal regulatory testing: correlation of electrical resistance with tritiated water permeability

An approach is presented that allows for rapid selection of robust rat and human epidermal membranes for use on in vitro dermal regulatory studies. Tritiated water (THO) permeability was correlated with electrical resistance (ER) and the results used to propose ER values to judge membrane integrity. Rat and human epidermal membranes were prepared and mounted onto in vitro glass static diffusion cells (0.64 cm(2)) maintained at 32 degrees C. THO permeability coefficients (Kp) were determined and compared with ER measurements. Electrical resistance was also determined for various in vitro cell exposure areas from 0.64 cm(2) to 2.54 cm(2). Our results show that rat epidermal membrane THO Kp values exhibited a lognormal distribution with a median value of 2.76 x 10(-3) cm/h. Human epidermal membrane THO Kp values were best described by a Weibull distribution with a median value of 1.13 x 10(-3) cm/h. The corresponding median electrical resistance measurements were 5.59 kOmega for rat and 23 kOmega for human epidermal membranes. Based on the widely used and accepted single point THO Kp thresholds of </=2.5 x 10(-3) cm/h (rat) and </=1.5 x 10(-3) cm/h (human), the corresponding ER values of >/=5.87 kOmega and >/=17.1 kOmega were calculated and proposed as acceptable benchmarks for pre-qualifying membranes. In our research exploring the relationship between ER and exposure area we report that an inverse relationship exists between ER and in vitro cell exposure area; as cell area increased, ER decreased. The use of electrical resistance provides a rapid and reliable method for evaluating the integrity of rat and human epidermal membranes for in vitro dermal kinetic testing.

Non-animal testing strategies for assessment of the skin corrosion and skin irritation potential of ingredients and finished products

The dermatotoxicologist today is faced with a dilemma. Protection of workers and consumers from skin toxicities (irritation and allergy) associated with exposure to products, and the ingredients they contain, requires toxicological skin testing prior to manufacture, transport, or marketing. Testing for skin corrosion or irritation has traditionally been conducted in animals, particularly in rabbits via the long established Draize test method. However, this procedure, among others, has been subject to criticism, both for its limited predictive capacity for human toxicity, as well as for its use of animals. In fact, legislation is pending in the European Union which would ban the sale of cosmetic products, the ingredients of which have been tested in animals. These considerations, and advancements in both in vitro skin biology and clinical testing, have helped drive an intensive effort among skin scientists to develop alternative test methods based either on in vitro test systems (e.g. using rat, pig or human skin ex vivo, or reconstructed human skin models) or ethical clinical approaches (human volunteer studies). Tools are now in place today to enable a thorough skin corrosion and irritation assessment of new ingredients and products without the need to test in animals. Herein, we describe general testing strategies and new test methods for the assessment of skin corrosion and irritation. The methods described, and utilized within industry today, provide a framework for the practicing toxicologist to support new product development initiatives through the use of reliable skin safety testing and risk assessment tools and strategies.

In vitro and in vivo assessment of the irritation potential of different spin traps in human skin

No clinical data are available on the acute cutaneous toxicity of spin traps which are frequently used in combination with the electron paramagnetic resonance (EPR) technique for detection of free radicals and reactive oxygen/nitrogen species. The purpose of this study was to evaluate the acute dermatotoxicity of the following spin traps in human skin: C-phenyl-N-tert.-butyl nitrone (PBN), C-(4-pyridinyl-N-oxide)-N-tert.-butylnitrone (POBN), 5, 5-dimethyl-l-pyrroline-N-oxide(DMPO), 5 diethoxyphosphoryl-5-methyl-l-pyrroline-N-oxide (DEPMPO), diethyldithiocarbamate (DDC) and N-methyl-D-glucamine dithiocarbamate (MGD). The corrosivity of the test substances was first assessed in human skin in vitro by measurement of transcutaneous electrical resistance (TER). In this assay all spin traps were non-corrosive at 500 mM concentration. Subsequently cutaneous irritation of the spin traps was determined at different concentrations (50, 250 and 500 mM) in human skin according to a routine four h human patch test in comparison to the standardized irritant sodium laurylsulfate (SLS, 20%). The response was evaluated clinically as well as by a biophysical method analyzing transepidermal water loss (TEWL). PBN and DEPMPO caused a transient and weak inflammatory reaction at 500 mM in four of 17 and in two of 17 volunteers, respectively. DMPO, POBN, DDC, MGID, and the iron complexes of DDC and MGD were clinically non-irritant at all concentrations tested and no delayed-acute inflammatory reactions were observed. However, the TEWL values were significantly increased by all spin traps except DMPO at 500 mM, indicating disturbed epidermal barrier function. We conclude that the spin traps investigated have a low potential to cause acute skin toxicity and may be used safely for in vivo EPR studies in human skin.

The identification and classification of skin irritation hazard by a human patch test

There exist various regulatory instruments the purpose of which is to ensure that the intrinsic toxic hazards associated with substances and preparations are identified. In the context of identification of skin irritation potential, the method is normally the Draize test. Guidance notes provided by the OECD and the EEC expect that corrosive substances will have been screened out by a variety of methods. Substances or preparations which cause a sufficient degree of skin irritation will be classified as skin irritants. The primary motivation behind the present work was to introduce the concept that it is possible to assess the hazard potential of a substance or preparation to produce skin irritation in a human study. In the example presented here, 20% sodium lauryl sulfate (SLS) has been chosen as the positive control. With the protocol currently devised, occluded patch treatment with 20% SLS for up to 4 hr produces an irritant response in just over half of the panel. An irritant response is taken as a clinically evident and significant increase in erythema, oedema or dryness--a minimum of a+ reaction on the ICDRG scale. At such a level of response with the positive control (both in terms of intensity and in proportion of the panel), it is then possible to judge and/or to determine statistically, whether the test material has produced a level of skin irritation which is similar to, greater, or lower than the positive control. In this way a human patch test protocol can form a fundamental component of a strategy for the replacement of animals in determination of skin irritation and corrosion potential. By use of a careful and progressive protocol and by comparison of test data against a positive control it is both possible and practical to classify substances and preparations in terms of their skin irritation potential using that endpoint in the species of concern, man.

The Galileo Data Bank on Toxicity Testing with In Vitro Alternative Methods. II. Toxicology Profiles of 20 Chemicals

The identification of the hazard of chemicals to man has relied on the use of several animal models. However, the availability of various cell toxicity models as alternatives to the use of animals has stimulated attempts to evaluate in vitro data for use in the prediction of human toxicity.

The cell toxicity models developed previously are capable of indicating a variety of endpoints susceptible to the activity of various chemical substances.

The in vitro data derived so far from testing a variety of types of chemicals, have been used to develop toxicology profiles for twenty chemicals, which are presented in this paper. Data have been selected from among those already entered in the Galileo Data Bank, a computerised data system containing all the available existing data derived using in vitro methods.

The role of in vitro experiments in animal welfare

The prospects of replacing animal experiments with other types of toxicological studies are considered, and the use of human data and in vitro experiments are discussed. Ongoing validation studies of in vitro methods for evaluation of acute toxicity, local irritation, target organ toxicity, tumour promotion and teratogenicity are presented.

Two-compartment model for rabbit skin organ culture

A new method was developed for rabbit skin organ culture. In a two-compartment model, skin discs were cultured on a Millicell-HA insert unit with a microporous membrane which allows transport of culture medium via the dermis into the epidermis, whereas the epidermal side remains free of direct contact with culture medium. In this relatively simple two-compartment organ culture model, rabbit skin could be cultured for 7 d in RPMI 1640 medium supplemented with fetal bovine serum, or for 2 d in RPMI 1640 medium supplemented with cofactors. The histomorphology and ultrastructure of 7-d cultured rabbit skin discs was essentially similar to that of freshly isolated rabbit skin. Keratinocytes in the stratum basale continued to divide during organ culture. The terminal differentiation of the epidermis continued in vitro as was found by the presence of keratohyalin granules, the intact stratum corneum, and keratin expression. Furthermore, glucose consumption continued until culture Day 7, but thereafter it declined rapidly. Concomitantly, degenerative changes were found. At the end of the 7-d culture period the distance between single dermal collagen fibrils had increased as compared to noncultured skin. This model of skin organ cultures can be used to study biological processes, dermal toxicity, and penetration and metabolism of xenobiotics in intact skin. Furthermore, within certain limits, processes responsible for repair and regeneration of damaged skin can also be studied in this model because the rabbit skin can be cultured for 7 d.