Activatable Photosensitizer for Smart Photodynamic Therapy Triggered by Reactive Oxygen Species in Tumor Cells

Photodynamic therapy (PDT) is a promising approach for the treatment of different kinds of cancers as well as some other diseases. By combining spatiotemporal light irradiation with photosensitizers (PS), PDT can be easily controlled by tuning illumination time and sites of irradiation. However, how to reduce the phototoxicity of the PS to normal cells without sacrificing its effectiveness to cancer cells is still a challenge. Herein, we put forward a deactivation and reactivation strategy for PDT to reduce the undesired damage to normal cells under light irradiation. First, by chemical modification of meso-(4-pyridinyl)-substitution BODIPY with phenylboronic acid pinacol ester moiety, the masked PS ProBODIPY-2I with low generation efficiency of singlet oxygen and good water solubility can be obtained. Moreover, ProBODIPY-2I can be reactivated at tumor microenvironment by reactive oxygen species (ROS), resuming their PDT efficiency. Meanwhile, ProBODIPY-2I showed low phototoxicity for the normal cells, due to the relatively low concentration of ROS. In this way, the safety and selectivity for the PDT can be greatly improved. It is anticipated that some other tumor biomarkers, such as proton, GSH and enzymes, can be employed for the smart PDT methods.