FIGURE 4.

The mechanism of photodynamic therapy (PDT). Protoporphyrin IX (PpIX) is synthesized from 5‐aminolevulinic acid (5‐ALA) and specifically accumulates in excess in the mitochondria of tumor cells. It can be excited at low excitation wavelengths in the red (600‐740 nm) and green (450‐580 nm) visible ranges. Once the light is absorbed, PpIX returns from the excited state to the ground state while releasing energy. This results in the production of cytotoxic reactive oxygen species (ROS), such as hydroxyl radicals, singlet oxygen, hydrogen peroxide, and superoxide, in tumor cells. ROS subsequently cause damage to the mitochondria and induce apoptosis of tumor cells, leading to cell death. This mechanism is the basis of ALA‐PDT