Assessment of photoprotection against pigmentation induced by visible light using a new in vitro method: correlation and new in vivo photoprotection factor against visible light

While ultraviolet radiation is the main cause of skin pigmentation, visible light (400-700 nm) has more recently been shown to be a major contributor, in particular in melanocompetent subjects. The fact that photoprotection against visible light, in particular blue light, improves a number of hyperpigmentation disorders has led to the development of sunscreens containing specific pigments that block blue light (iron oxides and titanium dioxide). To assess the efficacy of sunscreens on visible light photoprotection, an assessment method has recently been suggested by Lim et al. based on in vivo pigmentation, leading to the calculation of the visible light photoprotection factor (VL-PF). This involves measuring changes in the ITA colorimetric parameter over several days using a chromameter. However, although in vivo methods are still the most representative of real life, in vitro methods are better suited to screening suncare formulations. The purpose of this study was to assess the correlation between in vivo and in vitro methods in assessing protection against visible light-induced pigmentation.

Initially, the in vitro photoprotective properties of 10 sunscreens (9 of which contained pigments) offering very high photoprotection (≥SPF50+) were analysed using transmission measurements in the visible spectrum. Next, a single-centre, double-blind, randomised controlled study with intra-individual comparisons on 20 healthy subjects was carried out to measure the in vivo VL-PF of these sunscreens. This VL-PF was reinterpreted as a percentage using the (1-(1/VL-PF))×100 formula and named pVL-PF (0% corresponding with an untreated exposed area and 100% with full theoretical protection against visible light, equivalent to an unexposed area). The correlation between pVL-PF and the percentage of blocked light was assessed using the coefficient of determination R, for each test area, for each wavelength from 400 to 700 nm and for each wavelength range from the 400 nm wavelength. Statistical analysis was carried out using Pearson correlation.

In vitro, the transmission spectra of tinted sunscreens showed that the best protection was achieved in high-energy visible light. In vivo, twenty subjects (13 women and 7 men) with an average age of 34.9 (18 to 49) were included in the study. The pVL-PF obtained ranged from 9.7% (for the non-tinted sunscreen) to 66.4% (for one of the tinted sunscreens). A highly significant correlation was demonstrated between the in vivo visible light protection factor and the in vitro transmission measurements. Correlations were highest at 420 nm (R²=0.9910) for distinct wavelengths, and from 400 to 469 nm (R²=0.9904) for wavelength ranges starting at 400 nm. This suggests a strong linear relationship between the in vivo pVL-PF measurement of sunscreens and the percentage of visible light blocked in vitro between 400 and 469 nm, in particular at 420 nm.