In vitro biorelevant and in silico sunscreen performance evaluation on the basis of film Thickness frequency distribution of formulations and UV filter repartition

The present thesis consists of four studies, which aimed at improving the understanding of the mechanisms of sun protection for effective product development. To this end, in vitro tests along with in silico approach were employed for evaluating sunscreen efficacy. The findings may improve the predictability of the performance of sunscreens during the development stage to optimize their efficacy.
In the first study, we examined the use of pig ear skin as a biological substrate for SPF in vitro determination with diffuse transmission spectroscopy. The polymethylmethacrylate (PMMA) plates currently employed to this purpose mostly fail in yielding a satisfactory correlation between sun protection factor (SPF) in vitro and in vivo. The skin preparation substrate yielded SPF in vitro values that more accurately reflected the SPF in vivo than the PMMA plates. This study revealed that besides roughness, the improved affinity of the sunscreen to the skin substrate compared to PMMA plates may explain the better in vivo prediction of SPF achieved with the use of the biological substrate.
In the second study, we aimed at understanding the relationship between thickness frequency distribution of a sunscreen film formed upon application and sunscreen efficacy since sunscreen formulations with the same UV filter system were reported to produce different SPFs. We developed a method to measure the film thickness of an applied sunscreen on pig skin substrate and investigated the influence of five sunscreen vehicle and of sunscreen application on the average mean film thickness (Smean) and SPF in vitro. This work evidenced a strong influence of vehicle and application condition on sunscreen efficacy arising from differences in the film thickness. These results are of high practical importance in the route of understanding which parameters impact sun protection and subsequently how sunscreens work.
The third study relies on the second; the purpose was to quantitatively assess the role of film thickness frequency distribution for sunscreen efficacy. We developed a computational method for calculating the SPF in silico using besides the spectroscopic properties of the used UV filter mixture the complete thickness distribution of a sunscreen film obtained from topographical measurements. The investigated formulations containing the same UV filter mixture differed in their SPF in vitro and film thickness distribution. We found a very good agreement between SPF in silico and SPF in vitro demonstrating the high relevance of film thickness distribution for the interpretation of sunscreen efficacy. Integrating vehicle-dependent film parameters into tools for in silico prediction of sunscreen performance is, therefore, of high interest to improve UV efficacy predictions.
Finally, the fourth study focused on the evaluation of the repartition of an oil miscible and a water soluble UV filter in the applied sunscreen film; the UV filters should be uniformly distributed throughout the sunscreen layer for optimum efficacy. We employed confocal Raman microspectroscopy (CRM) as a highly sensitive analytical technique to precisely detect the spatial distribution of the two investigated UV filters throughout the sunscreen film applied on a pig ear substrate in three different formulations. This work revealed noticeable differences in the repartition of the two studied UV filters depending on the sunscreen vehicle, clear alcoholic spray differed from other tested oil-in-water and water-in-oil formulations. The two UV filters appeared completely disjointed in the film formed by the clear alcoholic spray formulation indicating a non-homogeneous distribution of the two UV filters in the sunscreen film. This result is of high significance as a worse repartition of UV filters in the applied film would lead to reduced photoprotection when the UV filters show a different absorbance profile which is commonly the case.
This thesis provides novel insights into the understanding of the mechanisms that influence UV efficacy. The knowledge of the behavior of sunscreens with respect to film thickness distribution and repartition of UV filters is fundamental information that allows the optimization of a sunscreen formulation during early development stage helping expedite development. This advanced understanding in combination with in vitro and in silico methodologies may improve the ability to accurately predict SPF in vivo performance with the objective of reducing clinical trials in humans and in the long run in the establishment of a validated in vitro method.