Algorithm for in vitro Sun Protection Factor Based on Transmission Spectrum Measurement with Concomitant Evaluation of Photostability

Abstract

In the in vitro evaluation of Sun Protection Factor (SPF), the photostability of the ultraviolet (UV) filters can have a major impact, especially for high-SPF formulations, but is generally not taken into consideration. In this study, we present a UV transmission spectrum measurement system utilizing a high-sensitivity UV photomultiplier tube with concomitant evaluation of photostability. We have developed an algorithm to estimate SPF in vitro by converting the amount of UV light transmission through the sunscreen layer into cumulative relative erythema effectiveness to obtain one minimal erythema dose. Thus, the algorithm uses the same endpoint as in vivo SPF methods, but with a photomultiplier tube as the detector instead of skin. The values obtained showed an excellent correlation with in vivo SPF values, even for high-SPF sunscreens exceeding SPF 50. This method should be suitable as an in vitro SPF testing method for regulatory purposes.

New approach for a reliable in vitro sun protection factor method Part I: Principle and mathematical aspects

“…Concerning UVB part, historically based on the in vivo method, the in vitro method initially described by L. Diffey and J. Robson that was then modified and improved to evaluate the level of UVB protection brought by the product is increasingly used and preferred due to ethical, economical, practical and reliable reasons . Unfortunately, despite all the extensive work conducted worldwide through basic in vitro method , in silico information and hybrid method , mimicking the in vivo test as far as possible …, the in vitro method for SPF assessment has not been validated not only due to the lack of reproducibility of the intra‐ and interlaboratories but also due to the lack of accuracy for in vivo / in vitro correlation on a large population of sunscreen formulations. Beyond this technical reality, awareness among people regarding this lack is clearly a reason that can explain why it is so hard to obtain a reliable method.…”

Light Source Spectrum Strongly Influences the In Vitro Estimation of Sun Protection Factor

“…It is important to note that in vitro SPF (NS) was lower than in vitro SPF (RCEE) for most samples. Although the samples used here were photostable, we have previously shown that the in vitro SPF value of photo‐unstable samples decreased with increasing UV irradiation time, with a change in the shape of the absorbance curve (13). Therefore, conventional in vitro SPF testing without consideration of photostability will in principle overestimate the in vitro SPF values of photo‐unstable samples.…”

“…In conclusion, the results of the present study indicate that careful optimization of in vitro SPF measurement is essential for the estimation of in vivo SPF. From the viewpoint of consistency with in vivo SPF, our previous studies have demonstrated the importance in principle of using in vitro SPF test conditions that properly reflect in vivo SPF test conditions (13,17). As shown in this study, this principle is also applicable to the light source spectrum.…”

Spectroscopic measurements of sunscreen protection

“…Stanfield extended this concept, adding spectroscopic measurements of applied and transmitted UV doses at discrete time points and a power curve fit that facilitated computation of the SPF [24][25][26]. Miura et al subsequently added the concept of simultaneously measuring applied and transmitted UV doses [27]. This approach represents a spectroscopic simulation of the SPF test on human skin.…”