Challenges in evaluating sunscreens: COSSMA

The sun protection objectivation subject represents a complex issue between in silico, in vitro, in vivo and hybrid methods at least as important as the challenge of the formulation itself. Anne Charpentier explains different methods with their advantages and disadvantages.

First, it is interesting to consider what criteria mainly influence the performance of UV protection products: composition, repartition, photostability, absorbance and distribution of the inorganic and organic filters, galenic (spray, compact powder, oil, cream…), properties to form a stable, homogeneous, and resistant film, pleasant to apply. In real use conditions, the performance of sun care is impacted by other criteria such as wrinkles, skin locations, sweat, hair, application procedures and quantity.

Complex environment of tests

Technically, it seems important to increase the reproducibility and the accuracy of the in vitro and in vivo testing by implementing systematic control testing such as Bipea inter laboratory comparison tests and audit of the global process such as Suncert diagnostics and Febea audit.

The gap between standardised application versus real-life conditions of use may also be deeply studied including anti-salt, anti-sweat, or anti-sand claim substantiation. Among all of the available methods, the in-vitro SPF is strongly required by the industry and governmental organisations delivering results equivalent to the in-vivo SPF according to ISO 24444 method. As evidence, the degree of protection should be measured using standardised, reproducible testing methods and take photo-degradation into account as recommended by the European Commission.

On the ethical point of view, the application of erythema on the subjects which causes skin damages doesn’t seem to be a long-term solution for UVB assessment. Fortunately, the HDRS method or in-vitro method should propose a new perspective within the next years.

Wide range of validated methods

Nowadays, there are different sunscreen testing procedures for claiming the Sun Protection Factor (SPF): UVA Protection Factor (UVAPF), Critical Wavelength (CW), Water Resistance (WR), and more. Fortunately, mainly according to ISO (International Organization for Standardization), standards are available to harmonise these methods worldwide.

1. For in-vivo testing

SPF according to ISO 24444
UVAPF according to ISO 24442
WR procedure according to ISO 16217
WR percentage calculation according to ISO 18861

Up to now, the SPF value given on the bottle of each sun protection product must be determined by the generation of sunburns at the back of at least ten volunteers. This is not only invasive but also time consuming since volunteers must come back after 24h for evaluation of the minimal erythemal dose.

2. For in-vitro assays

UVAPF – CW according to ISO 24443
SPF according to ISO 23675

What are the main points of vigilance: The control of the topographic parameters of the substrates, the temperature of the interface substrate/sample, the homogeneous and reproducible spreading (HD-Spreadmaster is highly recommended), the drying step, the acquisition of the initial UV absorbance spectrum by a spectrophotometer (including specific calibrations), two UV absorbance measurements to calculate the final in-vitro SPF.

3. For hybrid assays

In-vivo/in-vitro hybrid diffuse reflectance spectroscopy SPF – UVAPF according to ISO 23698 (under validation).

This new hybrid measurement principle combines spectroscopic in vivo and in vitro data (HDRS – hybrid diffuse reflectance spectroscopy). The ISO technical committee TC 217 (cosmetics) is simultaneously working on a new ISO norm for the HDRS method. A company1 has developed a new system to measure UV reflectance in vivo directly on the skin, which uses UV-LED technology and a sensitive spectroscopic system. The UV-dose applied does not cause sunburn and the SPF is determined based on the measured light attenuation of the sunscreen.

4. Quantification and stability analysis of UV filters

UV filters can lead to a partial or complete loss of their effectiveness, or even to a possible transformation into a hazard substance and undesirable by-products. Laboratories work to anticipate physical and chemical degradation of raw materials based on photodegradation and new criteria (chemical and toxicological) to ensure the safety of sun care products. However, it has been demonstrated that some skin reactions due to photoallergic contact are caused by using many commonly organic sunscreens. In this context, the maximum content of UV filter in products has been limited in cosmetics by the European Commission (Annex VI-1223/2009).

Different instrumental techniques are used to analyse the UV filters: NMR spectroscopy, Raman spectroscopy, gas chromatography/mass spectrometry, high-performance liquid chromatography/UV detector.

5. Oxidative stress assays

Solar radiations penetrate the skin, and their energy is absorbed by constitutive molecules, with consecutive damages like oxidation and inflammation. Evolution brought effective defences such as pigmentation to quench radiations, anti-oxidation, and clearance processes. However, an excess of sun exposure disrupts those self-defences, leading to uncontrolled reactive oxygen species (ROS) production, DNA damages, proinflammatory response and microbial dysbiosis. Many of these mechanisms can be evaluated with controlled radiations on 3D skin models studying various biological markers and cell signalling pathways (ROS, Nrf-2 activation, heme-oxygenase-1 (HO-1) expression, caspase-3 activation, microbiota, etc.)

Many stakeholders at sun care evaluation

On a preclinical testing platform², cosmeticians and evaluation experts can retrieve specifics in-vitro tests. Other specific photo-ageing effects can be found by selecting specific claims.

In conclusion, consistent inter- and intra-laboratory harmonisation guarantees homogeneous test methods for all solar products and optimal quality across all our sites.

The beauty and dermocosmetics sectors can expect that both worldwide industries and regulatory authorities harmonise the reference methods all over the world and continue with the labelling rules. It will guarantee the appropriate respect of the human health (nano, endocrine disruptor…) and the sustainability for the nature (ecotoxicity testing, coral protection) while keeping the evolution of the high performance of the sun protection products with all the complementary functionalities the consumers can expect.