Mechanism of Action
The basis of PDT is the interaction of light with photosensitive agents to produce an energy transfer and a local chemical effect. This is broadly similar to what is seen in photosynthesis, although in this case, many photosensitizers work together to harvest light energy to produce chemical reactions. Of the many photosensitizers that have been used in PDT, each has its own unique excitation properties. Usually, the photosensitizer is excited from a ground singlet state to an excited singlet state. It then undergoes intersystem crossing to a longer-lived excited triplet state.
One of the few chemical species present in tissue with a ground triplet state is molecular oxygen. When the photosensitizer and an oxygen molecule are in proximity, an energy transfer can take place that allows the photosensitizer to relax to its ground singlet state, and create an excited singlet state oxygen molecule. Singlet oxygen is a very aggressive chemical species and will very rapidly react with any nearby biomolecules. Ultimately, these destructive reactions will kill cells through apoptosis or necrosis. PDT can be considered a form of targeted singlet oxygen chemotherapy, where the targeting is achieved with the combination of the photosensitizer (functioning as a catalyst) and intense light.
A similar example is that cattle may become photosensitive if they graze on plants that contain photosensitizing toxins, such as marigold (Tagetes).
Read more about this topic: Photodynamic Therapy
Famous quotes containing the words mechanism of, mechanism and/or action:
“The two elements the traveler first captures in the big city are extrahuman architecture and furious rhythm. Geometry and anguish. At first glance, the rhythm may be confused with gaiety, but when you look more closely at the mechanism of social life and the painful slavery of both men and machines, you see that it is nothing but a kind of typical, empty anguish that makes even crime and gangs forgivable means of escape.”
—Federico García Lorca (1898–1936)
“I’ve never known a Philadelphian who wasn’t a downright “character;” possibly a defense mechanism resulting from the dullness of their native habitat.”
—Anita Loos (1888–1981)
“I feel like a white granular mass of amorphous crystals—my formula appears to be isomeric with Spasmotoxin. My aurochloride precipitates into beautiful prismatic needles. My Platinochloride develops octohedron crystals,—with a fine blue florescence. My physiological action is not indifferent. One millionth of a grain injected under the skin of a frog produced instantaneous death accompanied by an orange blossom odor.”
—Lafcadio Hearn (1850–1904)