Photophysical and photochemical mechanisms

Photodynamic therapy (PDT) harnesses the power of light in an elegant method to produce cytotoxic agents in a spatially and temporally controlled manner and specifically damage target cells and tissues. For photodynamic reactions to occur, the PS molecule must absorb at least one photon to be promoted to a sufficiently long-lived excited state and then induce photodynamic reactions in an oxygenated environment. Such properties guarantee that PDT has an exceptionally broad action spectrum against tumors or pathogens, and resistance occurrence is restricted to only a few exceptions that can be avoided using simple strategies. To fully understand the intricacies of the mechanisms by which PDT acts, it is clear that one must take advantage of all the basic sciences (e.g., physics, chemistry, and biology). In fact, such conceptual complexity still maintains constant scientific investigations to deeply understand the molecular basis of PDT. Curiously, it might also be one of the reasons to explain why this hundred-year-old technique is still not generally applied in clinics or taught in standard courses of pharmacology. In thischapter, we will attempt to use a multidisciplinary approach, with simple technical language and a minimum of mathematics and equations, to allow any student with minimal training in basic sciences to understand all the fundamental mechanisms of PDT.