photosensitization

Photosensitization is a photochemical process in which a photosensitizing compound, upon absorption of light, alters nearby molecules and can cause tissue damage or other biological effects. The sensitizer may be endogenous (produced inside the body) or exogenous (introduced by drugs, chemicals, or plant-derived substances). Exposure to light, especially ultraviolet or visible wavelengths, is typically required for the effect.

When a photosensitizer absorbs light, it is promoted to an excited state and can transfer energy or electrons to surrounding substrates. The most common outcome is generation of reactive oxygen species (ROS). In Type II reactions, energy transfer produces singlet oxygen, a highly reactive form of oxygen. In Type I reactions, electron transfer yields radicals that react with oxygen or other molecules. These ROS can damage lipids, proteins, and DNA, leading to cell injury or death. The extent and location of damage depend on the photosensitizer’s chemistry and tissue distribution.

Clinical and biological contexts include phototoxicity and photoallergy. Phototoxic reactions resemble exaggerated sunburn after light exposure in the presence of a photosensitizer (for example, certain antibiotics like tetracyclines or fluoroquinolones, psoralens, chlorpromazine, or certain plant compounds). Phytophotodermatitis is a plant-derived form caused by furocoumarins in some citrus oils or vegetables. In animals with liver disease, hepatogenous photosensitization can occur due to accumulation of phylloerythrin.

In medicine, photosensitizers are deliberately used in photodynamic therapy to target tumors or microbes, combining a photosensitizer with controlled light exposure. Prevention focuses on avoiding triggering agents, minimizing sun exposure, and using protective clothing and appropriate sunscreens.