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Unilever method predicts skin sensitisation potency via ‘population threshold’

Unilever scientists have developed a method for predicting the potency of skin sensitisers based on in vitro data.

Risk assessors often need to predict not only whether a particular substance is likely to cause skin sensitisation but also the potency of the skin sensitising effect. Potency is important in safety assessment of cosmetics, for example.

Potency can be predicted when animal test methods, particularly the mouse local lymph node assay (LLNA), are used for the prediction of skin sensitisation.

Regulators are phasing out use of animal test methods in favour of non-animal alternatives, however, and this has created a problem for risk assessors: non-animal alternatives usually do not enable prediction of potency.

The team, led by Joe Reynolds at the Unilever Safety and Environmental Assurance in the UK, developed a model based on the concept of the "population threshold".

This concept is linked to an experimental method called the human repeat insult patch test (HRIPT), the scientists say in their paper, published in Computational Toxicology.

Potency can be predicted using the HRIPT, via a weight of evidence (WOE) approach, but doing so requires application of sensitisation assessment factors (SAFs) to account for various sources of uncertainty. Human variability is one of these sources.

They describe the population threshold as "the maximum non-sensitising exposure ... across the HRIPT-eligible population. That is to say, non-sensitising to any individual in the population who would qualify as an eligible subject in the HRIPT". Using this metric negates the need for the human variability SAF, the scientists say.

The model can predict skin sensitisation potency based on data from in vitro test methods, such as the direct peptide reactivity assay (DPRA), the KeratinoSens method and the human cell line activation test (h-CLAT).

The scientists suggest that their model could form the basis for a defined approach (DA), which they call the "skin allergy risk assessment" (SARA) DA.

In September, the US Institute for In Vitro Sciences (IIVS), BASF and Givaudan announced that they would validate a modified version of the DPRA for prediction of potency.

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