AMD is caused by a growth of abnormal blood vessels over the central part of the retina, called the macular. The accepted mechanism for PDT involves the interaction of an excited state of the drug or dye with the ground state of oxygen. A molecule of the drug absorbs a photon of red light and is excited to the first excited singlet state. If this singlet state is long lived energy can be transferred from the singlet state to the triplet state through interstitial crossing. This triplet state can react with local oxygen molecules created an excited state of oxygen called singlet oxygen.
Singlet oxygen is cytotoxic and destroys nearby cells.
Since the drugs used in PDT are localized on cancer cells, this singlet oxygen reacts with the cancer cell. There are two mechanism by which the singlet oxygen can attack a cell. In cancer treatments using PDT the drug is usually localized on the outside of the cell membrane, and the singlet oxygen destroys the microvascular of the cell, thus the dies due to lack of oxygen. Alternatively, if the dye is left for a longer period of time before light activation, the dye may permeate the cell membrane and attack the mitochondria of the cell, leading to programmed cell death or apotosis. The potential for two photon excitation in PDT has recently been demonstrated by the researchers at Photogen.