A new in vitro method to predict in vivo photoprotection against visible light-induced pigmentation and a new visible light photoprotection factor

Ultraviolet radiation is the main cause of skin pigmentation, but more recently visible light (400-700nm) has been shown to be an important contributor especially in melano-competent subjects. The fact that photoprotection from visible light, in particular blue light, can improve several hyperpigmentation disorders has led to the development of sunscreens with specific pigments blocking blue light (iron oxides and titanium dioxide). To evaluate sunscreen efficacy on visible light photoprotection, an assessment method has recently been proposed by Lim et al. based on in vivo pigmentation, leading to the calculation of the visible light photoprotection factor (VL-PF). It consists of the assessment of the change in colorimetry parameter ITA over several days measured using a chromameter. However, even if in vivo methods remain the most representative of real life, in vitro methods are more suited to screening sunscreen formulations. The aim of this study was to evaluate the correlation between in vivo and in vitro methods in assessing protection against visible light-induced pigmentation. In parallel, we proposed a new index to determine an easier way to understand VL-PF.

First the in vitro protective properties of 10 sunscreens (9 of them containing pigments) with very high photoprotection (≥ SPF50+) was analyzed using transmission measurements in the visible spectrum. Then, a monocentric, double-blind, randomized controlled study with intra-individual comparisons in 20 healthy subjects was performed to measure the VL-PF in vivo of those sunscreens. This VL-PF was reinterpreted as a percentage using the formula (1-(1/VL-PF)) ×100 and named the pVL-PF (0% corresponds to an untreated exposed area and 100% corresponds to theoretical complete protection against the visible light, equivalent to an unexposed area). The correlation between the pVL-PF and the percentage of blocked light was evaluated using the coefficient of determination R, for each test area, for each wavelength from 400 to 700 nm, and for every wavelength range starting from the wavelength 400 nm. The statistical analysis was performed using the Pearson correlation.

Twenty subjects (13 women and 7 men) with a mean age of 34.9 years (range: 18-49 years) were included in the in vivo study. In vitro, the transmission spectra of the tinted sunscreens showed that the most effective protection was against the high energy visible light. In vivo, 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 in vivo visible light protection factor and in vitro transmittance measurements. The correlations were the highest at 420 nm (R² = 0.9910) for separate wavelengths, and between 400 nm and 469 nm (R² = 0.9904) for wavelength ranges. Correlations were found to be statistically significant for every wavelength from 400 to 700 nm, and for every wavelength range (p<0.001). This indicates a strong linear relationship between the sunscreens in vivo pVL-PF measurement and the in vitro percentage of blocked light.