From cosmetics to cleaning products, skin allergy (sensitisation) is a prominent feature of our modern chemical-centric world. Put simply, this hazard is an abnormal reaction of the immune system which makes certain individuals overly sensitive to particular chemicals. Key symptoms of skin sensitisation include redness, itching and swelling of the skin – not very nice. Once primed (or sensitised) in what’s termed the induction step, the immune system makes a note to “remember” the allergen, such that a future exposure at a lower level may produce similar effects (referred to as the elicitation step).

The history

Concerted efforts to characterise skin allergy started in the 60s and led to the adoption of two validated experimental animal models: the GPMT (Guinea Pig Maximization Test) and the Buehler test (in the same species). These models used repeated skin applications of a chemical (and injection alongside an immune-priming adjuvant substance in the case of the GPMT), followed by a “challenge” application 2 weeks later, to measure the elicitation of allergic reactions.

The early literature also features skin sensitisation studies involving human subjects, chiefly maximization tests and RIPTs (Repeat Insult Patch Tests). These were set-up using induction and elicitation phases, as with the guinea pig studies, but test concentrations were much lower, tending to focus on exposure levels relevant to actual use at the time. Such studies have since been discontinued on ethical grounds. Other human data, for example from occupational studies and consumer case reports of allergy, can provide useful information that features in our safety assessments.

Since the turn of the century, the go-to test for skin sensitisation has typically been the LLNA (Local Lymph Node Assay), which targets the induction step. As well as being a (much) milder laboratory animal method, this test has the added bonus of providing more useful hazard-response insights, aiding dermal risk assessments. The multiple test concentrations used in the assay allow us toxicologists to estimate potency by calculating the so-called EC (effect concentration) 3 value for a skin sensitiser, which loosely translates as a low effect level for sensitisation in exposed humans (or no effect level in some people’s eyes).

Times they are a-changin'

Advancements in research and development in recent years, coupled with the 3Rs initiative (Replacement, Reduction and Refinement of conventional test methods), have led to the wider availability of alternative methods to animal toxicity testing, termed New Approach Methodologies (NAMs). From here things get a bit more technical – apologies in advance for the impending jargon! For skin sensitisation, NAMs focus on the Key Events (KEs) in the Adverse Outcome Pathway (AOP) [1], a series of steps developed by the OECD that outlines how we get from a chemical dermal exposure to the observed symptoms of skin allergy: binding to skin proteins (KE1), activation of skin cells (KEs 2 & 3), and production of memory cells by the immune system (KE4). Under the REACH regulation for industrial chemicals, these types of studies are flagged as the starting point for any new testing [2].

Alongside the DPRA (Direct Peptide Reactivity Assay, which addresses KE1), other key NAMs include the KeratinoSens (KE2) and the h-CLAT (human Cell Line Activation Test; KE3), both human cell culture-based test systems (no-one’s yet come up with a NAM for KE4). Just to complicate matters, these NAMs don’t work across the universe of chemicals, with physico-chemical characteristics like fat/water solubility being an important factor to consider. Typically, more than one NAM, combined as part of a weight of evidence justification, is required to provide a good enough picture of a chemical’s skin sensitisation potential. There was until recently only limited objective guidance for us poor hazard assessors, though the OECD has since released a detailed framework (Defined Approach, or DA, is the fancy term) for addressing skin sensitisation [3]. The simplest DA, the “2 out of 3” approach, basically does what it says on the tin – run assays to cover 2 different KEs (or 3, if the results disagree) and take the consensus/safety in numbers result. Other DAs cleverly combine the above chemical/cell-based NAMs with computer-driven predictions to give us a bottom line. Pushing the science even further, the new kid on the NAMs block is the GARD (Genomic Allergen Rapid Detection) assay, which uses specialist skin cells and a data-driven analysis of well-known skin sensitisers at the DNA level to make predictions about test chemicals. More NAMs are in development as we type.

Research shows that the available NAMs and combinations of them provide a strong base for replacing traditional skin sensitisation tests, with comparable if not better performance – a win for science!

If you need support with sensitisation for anything from cosmetics to cleaning products, then please don’t hesitate to get in touch with us.

 

References

[1] OECD (2012). Organisation for Economic Cooperation and Development. The Adverse Outcome Pathway for Skin Sensitisation Initiated by Covalent Binding to Proteins. Part 1: Scientific Evidence. Series on Testing and Assessment No.168. ENV/JM/MONO(2012)10/PART1. Available via https://www.oecd-ilibrary.org/environment/the-adverse-outcome-pathway-for-skin-sensitisation-initiated-by-covalent-binding-to-proteins_9789264221444-en

[2] EU Regulation (EC) No 1907/2006. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:02006R1907-20241010

[3] OECD (2023). Organisation for Economic Co-operation and Development. Test Guideline 497. Guideline on Defined Approaches for Skin Sensitisation. 4 July 2023. Available via https://www.oecd-ilibrary.org/environment/guideline-no-497-defined-approaches-on-skin-sensitisation_b92879a4-en

Contact us