Figure 1.

Principles of photodynamic therapy (PDT). Upon injecting the inactive photosensitizing agent and its accumulation in the tumor, light of an appropriate wavelength is used to activate it (here, red light). Therefore, a change in state ensues and a triplet state is reached, with a greater energy, but shorter lifespan. The subsequent photochemical and photobiological reactions exert their effects by: (1) innate immunity recruitment and generation of a systemic inflammatory response with adaptive immunity activation, (2) direct cytotoxic effects, and (3) tumor vessels damage. Their synergistic effect results in cell death, either apoptosis, necrosis or autophagy (original illustration reproduced by courtesy of Tiberiu Popescu adapted from Castano, 2006).