Singlet Oxygen in Photodynamic Therapy.

Photodynamic therapy (PDT) is a therapeutic modality that depends on the interaction of light, photosensitizers, and oxygen. The photon absorption and energy transfer process can lead to the Type II photochemical reaction of the photosensitizer and the production of singlet oxygen ( ¹ O ₂ ), which strongly oxidizes and reacts with biomolecules, ultimately causing oxidative damage to the target cells. Therefore, ¹ O ₂ is regarded as the key photocytotoxic species accountable for the initial photodynamic reactions for Type II photosensitizers. This article will provide a comprehensive review of ¹ O ₂ properties, ¹ O ₂ production, and ¹ O ₂ detection in the PDT process. The available ¹ O ₂ data of regulatory-approved photosensitizing drugs will also be discussed.