Spectral Transmittance Analysis of Different Sunscreens Used in Daylight Photodynamic Therapy

The protocol for daylight photodynamic therapy (DL-PDT) includes the application of sunscreen with an SPF > 30 to block UV radiation and prevent sunburn during the 2 h of exposure to sunlight. Inorganic filters such as titanium dioxide and zinc oxide are not recommended, as they block the visible light needed to activate the photosensitizer. However, some sunscreens containing only organic filters can block 60% of the absorption spectrum of Protoporphyrin IX (PpIX).

Different sunscreens commonly used to perform DL-PDT in clinical practice were investigated by measuring their spectral transmittance. Skydome irradiances were used to calculate the absorption of PpIX, combined with these sunscreens. To perform a comprehensive analysis of this activation cream in combination with them, five distinct spectral regions were delineated for examination based on specific regions of interest (ROI): R-I (λ < 475 nm), R-II (475 ≤ λ < 525 nm), R-III (525 ≤ λ < 560 nm), R-IV (560 ≤ λ < 610 nm), and R-V (610 nm ≤ λ).

The results indicate significant variations in the spectral transmittance and PpIX activation efficiency across different sunscreens. In the R-I region, which is critical for PpIX activation, the PpIX activation without sunscreen was highest at 85%, while the addition of sunscreens drastically reduced activation, ranging from 28% to 8%, depending on the sunscreen. In regions from R-II to R-V, the activation levels were consistently low across all sunscreens evaluated. They showed similar values of approximately 5% in R-II, 3% in R-III, 2% in R-IV, and 1% in R-V, slightly lower than the corresponding PpIX alone values of 7%, 4%, 3%, and 1%, respectively.

There are significant differences between sunscreens in terms of their absorption in the PpIX absorption spectrum. These differences do not depend on whether or not they contain inorganic filters but depend on the absorption spectrum of the filters contained in their formulation. These results highlight the impact of sunscreen selection on the efficacy of PpIX activation in DL-PDT.