Chapter 40: Photodynamic Therapy

Photodynamic therapy (PDT) represents a therapeutic modality that requires the concerted action of three key elements: a photosensitizer (Sens), visible light and oxygen. The process includes the electronic excitation of Sens, usually to the long-lived triplet state, and the subsequent energy or electron transfer to ³O₂ with generation of reactive oxygen species (ROS), mainly singlet oxygen (¹O₂) in the ¹Δ_g configuration. This results in the oxidative damage of cells or tissue constituents in the close surroundings of Sens; most frequently, the range of action of endocellularly generated ¹O₂ is <1 µm. Out of many possible photosensitizing agents, porphyrins and their tetrapyrrolic analogs exhibit optimal PDT properties, since their chemical structure can be engineered in order to promote their preferential association with cells from diseases tissues; quite often, porphyrins can be associated with a variety of targeting agents to enhance the selectivity of the photodamage. While the main field of applications of PDT is in the oncological field for the palliative or curative treatment of solid tumors, emerging applications include localized infectious diseases, noncancerous skin lesions and age-related macular degeneration.