Photosensitizing Properties of the Topical Retinoid Drug Adapalene

Photoreactivity is an important issue for topical drugs especially when these are applied on the sun-exposed skin area. In this context, third-generation retinoids are of special interest due to their conjugated chemical structure and their use in the treatment of acne. Herein, the phototoxic potential of one of these drugs, adapalene, is established using an in vitro 3T3 Neutral Red Uptake (NRU) test. Photophysical studies demonstrate the involvement of a Type II process with an efficient formation of singlet oxygen. Interestingly, quenching of the adapalene singlet manifold by oxygen leads to an increased production of this reactive oxygen species through the tagged O₂-enhanced intersystem crossing process. Taken together, these results are relevant from a toxicological point of view as adapalene could be considered as a double-edged sword: it can be at the origin of undesired skin photosensitivity reactions or be considered as a candidate for topical photodynamic therapy.