ABSTRACT
PDT relies on the combined interaction between a photosensitizer, light, and oxygen in order to generate cytotoxic singlet oxygen. Light, at a specic irradiation wavelength, is absorbed by a photosensitizer in its low energy ground state. e absorbed photons promote the photosensitizer to an excited singlet state. ermal decay and emission of uorescence can return the photosensitizer to the ground state. Intersystem crossing occurs for a portion of the photosensitizer population, leading to the formation of a lower energy excited triplet state. Type I and type II processes in PDT are the two mechanisms that generate reactive species from the photosensitizer excited triplet state (Dougherty et al., 1998). In type I processes, electrons get transferred from the photosensitizer to various receptor molecules, and free radical production via the superoxide anion, hydroxyl radical, or hydrogen peroxide (Leanne and Ross, 2008). In type II processes, the excited triplet state photosensitizer interacts with molecular oxygen to produce reactive singlet oxygen. e type II reaction is thought to be more relevant to clinical PDT and singlet oxygen is an eector for destruction of target cells and tissue. Singlet oxygen diuses only small distances in biological environments, but rapidly reacts with nearby cellular components, including lipids, amino acid residues, and nucleic acids, leading to cell death. Cell death occurs via apoptosis or necrosis, with the mechanism dependent upon factors such as the localization of the photosensitizer within the cell and the amount of singlet oxygen generated (Pervaiz and Olivo, 2006). It has been suggested that photosensitizer localization in the mitochondria or endoplasmic reticulum favors apoptosis whereas photosensitizer localization in the plasma membrane or lysosome favors necrosis (Wilson and Patterson, 2008). Beyond destruction of irradiated cells, PDT also triggers local inammation and immune responses that can have broader eects against cancers in vivo (Castano et al., 2006).
