Evaluation of skin irritation and corrosion potential is necessary in order to establish the safety profile of a wide array of industrial/pharmaceutical chemicals and cosmetics, which come into contact with skin. For the last 60 years, the Draize test1 has been used to predict irritancy/corrosivity potential of test substances in humans. It involves the application of test substances to the shaved skin of albino rabbits. Over the past decade, regulatory pressure and technological advancements have led to the development, and acceptance of ethical in vitro based alternatives with a high accuracy of predictability.
The United Nations Globally Harmonized System of Classification and Labelling of Chemicals (UN GHS) defines skin irritation as the production of reversible damage to the skin upon exposure to a test substance for up to 4 hours. Skin corrosion is defined as the production of irreversible damage to the skin, such as necrosis through the epidermis and into the dermis, upon exposure of a test substance for up to 4 hours. Irritants are classified into two categories by the UN GHS: irritants (category 2) and mild irritants (category 3). Corrosives are classified into category 1, which may further be classified into 3 subcategories: strong corrosives (category 1A) and intermediate corrosives (categories 1B and 1C). This classification system was established based on observations from the Draize test.
The Draize test was initially introduced by the US FDA for the testing of cosmetic products, but has since been widely used to assess irritancy/corrosivity of industrial/pharmaceutical chemicals as well. The Draize test is ethically controversial in nature due to animal welfare concerns and has demonstrated an overestimation of human responses.2 Following-up on the “3R principle” elaborated by Russell and Burch in 19593, alternative in vitro testing strategies were developed in the last 20 years, which attempt to reduce, replace and refine in vivo test methods. In vitro skin irritation and skin corrosion tests, based reconstructed human epidermis (RhE), have been developed and validated internationally.4,5 These tests have been shown to be sufficiently predictive and equivalent to the Draize test. The Organization for Economic Co-Operation and Development (OECD) has adopted regulatory accepted guidelines for RhE-based skin irritation testing (OECD 439) and skin corrosion testing (OECD 431).
In vitro skin irritation and skin corrosion testing should be incorporated using a tiered-testing approach in combination with either bottom-up, or top-down strategies. These approaches are described in the OECD guideline on Integrated Approaches to Testing and Assessment.6 A chemical suspected of being non-irritant would first undergo skin irritation testing. Alternatively, a chemical suspected of being corrosive, would first undergo skin corrosion testing. A positive skin irritation test result cannot exclude the possibility that the chemical being tested is potentially corrosive. Similarly, a negative skin corrosion test result cannot exclude the possibility that the chemical being tested is irritant. Therefore, in such cases, both skin irritation and skin corrosion tests would be required to provide adequate classification information. It is possible, when using a tiered testing approach, to completely replace in vivo testing requirements. However, for certain regulatory submissions (i.e. pesticides, chemicals requiring packing group assignment, etc.), categorization of irritant chemicals or subcategorization of corrosive chemicals may be needed. In this case, in vitro tests may not be suitable to allow categorization/subcategorization; animal testing would then be required. Nonetheless, the use of in vitro tests would certainly result in the reduction of the number of test animals used for classification in such instances.
With the availability of regulatory-accepted and validated alternative in vitro skin irritation and skin corrosion tests, there has been a strong emphasis on minimizing unnecessary animal testing. The legislation framework surrounding animal testing is constantly changing. Importantly, the testing of cosmetic products and their ingredients on animals has been banned across the European Union since 2013. Several other countries outside of the European Union have implemented similar bans over the years. Though North America seems to lag behind the EU, Canada passed bill S-214 in June 2018 toto amend its Food and Drug Act; applying such bans on cosmetic product testing. In August 2018, the state of California passed a similar bill, SB-1249, to amend the section of its Civil Code relating to animal testing.
As regulators move forward in setting the compass in a direction toward the use of in vitro alternatives, the industry will be required to follow. While this may present change, in vitro testing offers distinct advantages in terms of animal welfare, cost, time and throughput.
- Draize J.H.; Woodard G.; Calvery H.O. (1944). "Methods for the study of irritation and toxicity of substances applied topically to the skin and mucous membranes". J. Pharmacol. Exp. Ther. 82: 377–390
- JÍrová D.; Basketter D.; Liebsch M.; Bendová H.; Kejlová K.; Marriott M.; Kandárová H. (2010) “Comparison of human skin irritation patch test data with in vitro skin irritation assays and animal data”. Contact Dermatitis. 62(2):109-16.
- Russell, W.M.S. and Burch, R.L., (1959). The Principles of Humane Experimental Technique, Methuen, London.
- Organization for Economic Cooperation and Development (OECD) (2015), OECD Guidelines for the Testing of Chemicals, In Vitro Skin Irritation: Reconstructed Human Epidermis Test Method (OECD 439).
- Organization for Economic Cooperation and Development (OECD) (2014), OECD Guidelines for the Testing of Chemicals, In Vitro Skin Corrosion: Reconstructed Human Epidermis (RHE) Test Method (OECD 431).
- Organization for Economic Cooperation and Development (OECD) (2014), New Guidance Document on an Integrated Approach on Testing and Assessment (IATA) for Skin Corrosion and Irritation (No. 203).