Photosensitizing agent (photodynamic therapy)

Photosensitizing agents used in photodynamic therapy are compounds that remain biologically inert until they are activated by light of a specific wavelength. After administration, the photosensitizer accumulates preferentially in certain tissues, such as abnormal or rapidly proliferating cells and blood vessels, or in the lipid-rich membranes of new vasculature. When the target area is subsequently exposed to a light source tuned to the drug's absorption spectrum, the molecule absorbs the light energy and transfers it to surrounding molecular oxygen, generating reactive oxygen species, most notably singlet oxygen. These reactive species damage cellular membranes, proteins, and DNA in the immediate vicinity, causing localized cell injury or death, and can also trigger closure of small blood vessels. The rationale for this approach is that it confines tissue destruction to the site where both the drug and the light coincide, sparing tissue that received the drug but not the light, or light without sufficient drug present. This spatial and temporal control makes photodynamic therapy attractive for conditions involving abnormal vascular growth or localized tissue overgrowth, such as certain eye disorders characterized by unwanted blood vessel proliferation beneath the retina, as well as select dermatologic and oncologic applications where precise, image-guided or accessible tissue targeting is feasible.