The ALT-SPF ring study—Correlation in silico versus in vivo SPF ISO24444 and in vitro UVA-PF ISO24443

This paper presents the results of the Alt-SPF consortium ring test, obtained by the in-silico methodology, and discusses these results in the context of existing standards. To evaluate the effectiveness of the in-silico methodology in correlating with these standards, a comparison was made between the in-silico sun protection factor (SPF) and the UVA protection factor (UVA-PF) and the values derived by standard in vivo SPF (ISO24444) and in vitro UVA-PF (ISO24443). The model utilises the same algorithm as in vitro measurements of SPF and UVA-PF, with the measured UV transmission substituted by calculated transmission simulated through an applied sunscreen film. The in-silico approach necessitates quantitative UV absorbance data of all UV-filters, their photodegradation and photointeraction properties, oil/water phase synergies and a model describing the irregular distribution of the film thickness on the skin. The performance factors are calculated using an initial analytical evaluation of the 32 test formulations of the ring test, to ascertain the presence and concentration of individual UV absorbers. The filter concentrations obtained are then processed via computational analysis to calculate the SPF and UV-PF values. It has been demonstrated that, in accordance with defined statistical parameters to characterize the ability of a method to correlate with the in vivo SPF standard, the in-silico methodology is a highly reproducible and accurate tool for SPF prediction. The alignment with the lowest measured in vivo SPF values serves to ensure the safety of the end consumer and is not a weakness of the method. Additionally, it has been demonstrated that the methodology facilitates precise prediction of UVA protection in comparison to the in vitro standard ISO 24443. These calculations can be employed to engineer novel sunscreens, thereby diminishing the necessity for ethically questionable and extensive laboratory measurements. In case still in-vivo measurements are needed, the methodology can deliver the frequently required SPF/UVA-PF values and may also support functions such as those of responsible persons, including safety assessors, in their evaluations.