How to replace animal testing for skin sensitization using defined approaches

Skin sensitisation is the scientific term for an allergic reaction in the skin. If someone is allergic to a certain chemical, this means this person will have to avoid that chemical for the rest of their life - which can be quite inconvenient.
Our skin can be often exposed to cosmetics, personal products or even drugs, and so the industry producing these products needs to ensure that they will not cause allergies, or skin sensitisation.
But how can they be sure of that?

We can answer this question by understanding the adverse outcome pathway (or AOP) for skin sensitisation.
To know more about AOPs, watch our previous video.
Imagine a chemical that causes skin sensitisation. How does this happen in our body?

First of all, it needs to be able to covalently bind to our skin proteins. When this happens, this triggers a biological cascade involving keratinocytes activation, dendritic cells activation and T-cell proliferation, which ultimately causes skin sensitisation.
Now, there are assays to measure each of the events in this cascade, such as the h-CLAT for dendritic cell activation or the LLNA to measure T-cell proliferation.
Usually, the more complex the event, the more complex the assay used to measure it.

So which of these assays should we choose if we need to answer the question: is my compound a sensitiser? If we're only allowed to choose one assay, this would have to be either a test in animals, or in humans.
However, due to ethical concerns and considering the principles of the 3Rs, replacing the use of animals is encouraged, and even mandatory in some cases.
So what is the alternative we have?

If we know the biological cascade that happens in our body, why not use this knowledge to combine results from more simple assays that can help us predict whether skin sensitisation is likely to happen?
This is where defined approaches can be helpful.

A defined approach is a fixed procedure to interpret data generated with a defined set of sources to derive a prediction without expert judgement. When we combine different methods in a defined approach, we can overcome the limitation of the individual methods, providing increased confidence in the result.

Many defined approaches have been published for skin sensitisation, and some have been even recognised and approved by OECD - the Organisation for Economic Co-operation and Development.
The OECD guideline on defined approaches for skin sensitisation describes which combination of assays they consider to be acceptable for the assessment of skin sensitisation replacing the use of animals.
The guideline currently covers 2 approved defined approaches:
The 2 out of 3 - which combines results from 3 assays - DPRA, KeratinoSensTM and h-CLAT
And the integrated testing strategy (or ITS) - combining results from 2 assays (DPRA and h-CLAT) with an in silico prediction given by either Derek Nexus or the OECD QSAR Toolbox.

But what's the benefit of using these defined approaches approved by OECD? Basically these are covered by the Mutual Acceptance of Data, meaning that these results will be accepted in a harmonised way by any regulatory agency across the globe that follows OECD.

Of course, even defined approaches have limitations and in some cases our result will be inconclusive, which means an integrative assessment might need to be done, combining other information sources and an expert review.

But this is certainly a great step forward in the acceptance of alternatives to animal testing. It's the first time that a guideline describes the possibility of combining in vitro and in silico results in defined approaches, showing once again how our science can lead us to a brighter future, reducing animal testing while better protecting human health, including the health of our skin.

Music: https://www.bensound.com
Allergy image from: https://www.drbatras.com/5-things-kno...

References
https://single-market-economy.ec.euro...
https://www.oecd-ilibrary.org/environ...