Overview on Current Status of Alternative Methods and Testing Approaches for Skin Corrosion Testing

Abstract

The area of skin corrosion represents one of the pioneering areas for the validation of alternative test methods, in which replacement alternatives have been validated and adopted in the regulation as early as 2000 in the European Union and in 2004 at the OECD level. Current internationally agreed approaches recommend the use of Integrated Approach for Testing and Assessment (IATA) that allow to replace or minimize to the extent possible the use of in vivo animal testing while ensuring human safety (OECD Guidance document No. 203, 2014). The IATA comprises in a sequential way (1) the use of existing information, physico-chemical properties and non-testing methods, (2) a weight-of-evidence evaluation of the existing data and (3) if needed, the conduct of prospective testing. When combined within tiered testing strategies in either a top-down or a bottom-up approach for predicting the potential skin irritation and corrosion hazard of a test chemical, the currently validated and adopted in vitro methods for skin irritation and corrosion can replace the traditional in vivo Draize rabbit test (OECD, OECD guidelines for the testing of chemicals No. 404, 2015).

According to the United Nations Globally Harmonized System (UN GHS) for classification, skin corrosives are classified as category 1, and where required by a competent authority (and in case data are sufficient), in one of the three subcategories 1A, 1B and 1C (UN, Globally harmonized system of classification and labelling of chemicals (GHS), 2015). Another system used to classify corrosive substances is described in the UN model regulations for the transport of dangerous goods, in which packaging groups are assigned to skin corrosives (referred to as PG I, II and III) (UN, Volume I – Part 2. Classification, 2015). Skin corrosion effects as defined by the UN GHS classification system and by the UN transport packaging groups can be predicted by the following scientific valid in vitro test methods that have been internationally adopted:

• The rat skin transcutaneous electrical resistance (TER) test (OECD guidelines for the testing of chemicals No. 430, 2015), which allows discriminating skin corrosives from non-corrosives but does not allow subcategorization of corrosive effects.

• The reconstructed human epidermal (RhE) models (OECD guidelines for the testing of chemicals No. 431, 2016) including the commercially available EpiSkin™ Standard Model (SM), the EpiDerm™ Skin Corrosion Test (SCT), the SkinEthic™ RhE and the epiCS^® test method. In addition to discriminating corrosives from non-corrosives, these test methods also allow the discrimination between subcategory 1A and a combination of subcategories 1B-and-1C but were not adopted for the discrimination between subcategories 1B and 1C due to the limited data set of well-known in vivo corrosive subcategory 1C chemicals.

• The in vitro membrane barrier test method (OECD guidelines for the testing of chemicals No. 435, 2015), including the commercially available Corrositex^® test, which allows, in addition to the discrimination of corrosives from non-corrosives, the discrimination of all skin corrosives under the three UN GHS subcategories, i.e. 1A, 1B and 1C, even though its applicability is limited to test chemicals that are compatible with the assay and, in the EU, it was considered valid only to acids, bases and their derivates.

The applicability domain and the ability of the test methods to provide information on subcategorization play an important role in the choice of test method to be used. When limitations and domain of these in vitro tests are adequately considered, these tests can provide, together with the adopted test method for skin irritation (OECD guidelines for the testing of chemicals No. 439, 2015), sufficient information for the decision on potential of the substance to cause skin irritation and/or corrosion.

Animal testing should be used only as a last resort when discrimination between optional subcategories 1B and 1C for chemicals outside of the applicability domain of OECD TG 435 is required, or the test chemical cannot be tested with the in vitro test methods currently adopted by the OECD due to limitations or non-applicability. However, if additional testing is required, the OECD IATA should be considered in which other in vitro skin corrosion test methods not yet adopted by the OECD are suggested to be used in a weight-of-evidence manner to resolve specific optional or subcategorization issues (OECD Guidance document No. 203, 2014). This is the case for example of the EpiSkin™ test method for which the original prediction model (not adopted due to the limited data set of well-known in vivo corrosive subcategory 1C chemicals) may be considered in a weight-of-evidence approach to distinguish between the subcategories 1B from 1C. In addition, an extended exposure period of 4 h is recommended for the EpiDerm™ SCT for testing fatty amine derivatives (characterized as cationic surfactants) which were found to have a tendency to be under-predicted with the test methods falling within the OECD TG 431 (Houthoff et al., Toxicol In Vitro 29:1263–1267, 2015; see Chap. 9).