Figure 1.

Mechanism of action on tumors in photodynamic therapy. The photosensitizer (PS) absorbs light and an electron moves to the first short-lived excited singlet state. This is followed by intersystem crossing, in which the excited electron changes its spin and produces a longer-lived triplet state. The PS triplet transfers energy to oxygen producing reactive singlet oxygen (¹O₂) and reactive oxygen species (ROS), which kill tumor cells. However, the tumor microenvironment protects the cancer cells against PDT through the interaction of its different constituents.