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The successful treatment of diseases (including cancer) by the use of drugs normally rely on a selective reactivity of a drug towards the unwanted tissue. An approach that is related to chemotherapy is that of photochemotherapy, normally called photodynamic therapy (PDT). In this treatment modality, a harmless drug (in the absence of light) is first introduced into the body and allowed to selectively biodistribute into the diseased tissue. Thus selectivity is no longer based on reactivity, but on the selective retention of the drug in abnormal tissue. Once selective biodistribution has taken place, the drug is photoexcited with light in the visible or near infrared range. Drugs that absorb near infrared light are desirable due to the increased tissue penetration at these wavelengths. PDT is a relatively "benign" treatment approach in which a normally harmless drug is combined with safe, non-ionizing light in order to initiate a photochemical reaction that leads to the selective killing of the diseased tissue. The reactive intermediate is believed to be singlet oxygen, generated by an energy-transfer (type II) process between the excited state of the drug (photosensitizer) and ground state oxygen. Singlet oxygen is a highly reactive oxidizing agent that will rapidly react with aromatic double bonds found in cell membranes. Resulting cell death can be either via apoptosis or necrosis, depending on the primary reaction site and the degree of damage.
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