photosensitizers

Photosensitizers are light-activated compounds used to drive chemical reactions in illuminated tissues. When exposed to oxygen, they absorb photons and reach excited states. Through intersystem crossing they form a triplet state that transfers energy to molecular oxygen, generating reactive oxygen species such as singlet oxygen or radicals. This photochemical activity enables targeted cytotoxic effects in diseased tissue while remaining largely dark-inactive. Activating wavelengths are typically in the red to near-infrared range, which improves tissue penetration.

Common photosensitizer classes include porphyrin- and chlorin-based agents such as porfimer sodium (Photofrin), benzoporphyrin derivatives (verteporfin), temoporfin (mTHPC), and chlorin e6; phthalocyanines are also widely studied. Non-porphyrin dyes like methylene blue, toluidine blue O, and rose bengal are used in specific settings, while natural products such as hypericin have been investigated. Modern generations emphasize greater selectivity and longer-wavelength absorption, using targeted delivery, prodrugs, or nanoparticle carriers.

Applications include photodynamic therapy (PDT) for cancer and dermatology, as well as ocular diseases, where light-activated cytotoxicity or vascular damage provides treatment. Antimicrobial photodynamic therapy (aPDT) employs photosensitizers to inactivate bacteria, fungi, and viruses. Photosensitizers also function as fluorescent tracers for diagnostic imaging and theranostics.

Key considerations are oxygen dependence, light dose, and tissue penetration of the activating light, along with clearance and potential skin photosensitivity after treatment. Several agents are approved for clinical use in various regions, including porfimer sodium and verteporfin for PDT and ALA-derived formulations for topical therapy.