Photosensitizers and Their Applications

Abstract

Photodynamic therapy (PDT) is a minimally-invasive anti-tumor therapeutic modality, known for its selectivity and limited side-effects. PDT has three fundamental requirements: a nontoxic photosensitizer (PS), light of a specific wavelength, and molecular oxygen. The anti-tumor therapeutic effect is triggered by the photo-activation of the PS via irradiation of the target tumor site with light of a specific wavelength. This leads to a photochemical reaction in which the PS generates cytotoxic reactive oxygen species (ROS), which lead to tumorselective cell destruction. Despite significant advantages, the necessity of an external light source impairs the use of PDT for most tumors, due to the poor penetration of light into deeply localized tissues. Given this, researchers have focused on developing strategies to improve the efficiency of this therapy and expand it for deeper tumors and metastatic cancer. One strategy is to use either chemi-(CL) or bioluminescent (BL) reactions as intracellular excitation sources of PSs for PDT. Namely, CL/BL are processes in which light is generated due to a chemical/biochemical reaction, without any light source, and therefore, can be used to excite the PS intracellularly. More importantly, an extra step can be taken and instead of using CL reactions as excitation sources, they can be used as the basis for tumor-selective and self-activating single-molecule drugs that are both the excitation source and the PS itself. In this chapter, we will provide a critical