With the 2016 changes to the REACH (Research, Evaluation, Authorisation and Restriction of Chemicals) Regulation, skin sensitisation tests which were historically done using in vivo methods must now rely instead on non-animal testing methods, in line with the general move away from in vivo testing. This change in assessment method came about due to not only pressure for better animal welfare considerations but also due to legislative changes and the principles of the 3Rs (Replacement, Reduction and Refinement). In line with OECD tests guidelines, several strategies have been accepted for non-animal skin sensitisation testing. These currently include six in vitro testing methods which have been fully approved as a means for skin sensitisation testing and have been translated into the internationally-recognised test guidelines set forth by the OECD (Organisation for Economic Co-Operation and Development). The currently approved testing methods cover the first three key events (KEs) of skin sensitisation; interaction with the protein, keratinocyte activation, and dendritic cell (DC) activation – although not each test is approved for each KE.
Current Testing Methods Approved for in vitro Skin Sensitisation Testing
KE3 (dendritic cell activation) currently has the most validated testing methods, with the OECD adopting three assays. Each of these in vitro testing methods works by measuring biomarkers of DC maturation, through the use of DC surrogates.
KE2 (keratinocyte activation) currently has even fewer approved in vitro testing methods with only two in vitro cell-based methods currently holding regulatory acceptance. These are the LuSens and KeratinoSens™ assays.
Aside from KE4, which currently has no in vitro testing methods approved by regulation, KE1 has the fewest approved assays, with only the Direct Peptide Reactivity Assay (DPRA) currently validated and accepted. This form of non-animal testing uses an in chemico method to assess the KE through the use of synthetic peptides.
Limitations of Non-Animal Skin Sensitisation Testing
However, despite the regulatory acceptance of some of the non-animal skin sensitisation assays, it is essential to note that each comes with its own limitation regardless of whether it is used by itself or in combination with another testing method.
Of course, these tests do still have their place and are currently accepted as a way of supporting hazard identification by differentiating between skin sensitisers and non-sensitisers. As the approval of different in vitro testing methods continues to grow, it is more than likely that non-animal-based assays will eventually replace the need for in vivo testing as long as they will be able to cover all KEs and gain regulatory approval. The current challenge is finding which one of the assays will be best placed to inform on potency and reliably replace animal-based testing methods.
