Strategies for developing stimulus-responsive photosensitizers: Recent progress

Photodynamic therapy (PDT) has been employed in recent decades to eliminate one of the most lethal diseases, cancer. Within this review article, we aim to present photosensitizers that can be activated by various stimuli, focusing on developments since 2020. Over these six years of research, several reports have been published on this topic, and most can be found within. While earlier studies on PDT around 20 years ago focused on obtaining photosensitizers from pure synthetic fluorophores, today highly advanced photosensitizers that are activatable by specific enzymes or other stimuli, found in high concentrations within cancer tissue, have been achieved. Several other topics that have been given greater importance in recent years are also investigated, including type-I photosensitizers that can work in the hypoxic environment of the tumor region and several different mechanisms resulting in the intersystem crossing (ISC) ability of various photosensitizers. Following stimulus types have been involved: ROS ( 1 O 2 , H 2 O 2 , HOCl, NO), glutathione (GSH)/cysteine, acidic pH, hypoxia/azoreductase, light (photoswitchable systems), and specific enzymes (either intrinsic or extrinsic). Investigating recent progress within this field will exemplify where the field is evolving.