Photoactivated cell-killing amino-based flavylium compounds

Photodynamic therapy (PDT) is a well-established therapeutic for the treatment of different diseases. The growing interest of this technique required the development of new photosensitizers with better photo-features. This work reports the study of the potential of five nature-inspired amino-based flavylium compounds with different structural features as photosensitizers towards topical PDT. In terms of dark cytotoxicity the five pigments were tested towards confluent skin cells in both fibroblasts and keratinocytes. In the range of concentrations tested (6.3-100 μM), keratinocytes were more prone to growth inhibition and the IC₅₀ values for 5OH4'NMe₂, 7NEt₂st4'NMe₂ and 7NEt₂4'NH₂ were determined to be 47.3 ± 0.3 μM; 91.0 ± 0.8 μM and 29.8 ± 0.8 μM, respectively. 7NEt₂4'NMe₂, 7NEt₂st4'NMe₂ and 7NEt₂4'NH₂ showed significant fluorescence quantum yields (from 3.40 to 20.20%) and production of singlet oxygen (¹O₂). These latter chromophores presented IC₅₀ values of growth inhibition of keratinocytes between 0.9 and 1.5 µM, after 10 min of photoactivation with white light. This cellular damage in keratinocyte cells upon white light activation was accompanied with the production of reactive oxygen species (ROS). It was also found that the compounds can induce damage by either type I (ROS production) or type II (singlet oxygen) PDT mechanism, although a higher cell survival was observed in the presence of ¹O₂ quenchers. Overall, a structure-activity relationship could be established, ranking the most important functional groups for the photoactivation efficiency as follows: C7-diethylamino > C4'-dimethylamino > C2-styryl.