Ground-state molecular oxygen (O2)

Ground-state molecular oxygen, also known as triplet oxygen, is the most stable form of the oxygen molecule and serves as the essential photophysical substrate for photodynamic therapy (PDT) (PubChem CID 977). In this therapeutic modality, a photosensitizing drug is administered and subsequently activated by light of a specific wavelength, leading to a Type II photochemical reaction (Agostinis et al., 2011). During this process, the excited photosensitizer transfers its energy to ground-state oxygen, converting it into highly reactive singlet oxygen (NCI Dictionary). Singlet oxygen then induces localized oxidative damage to cellular membranes, proteins, and DNA, resulting in cell death and vascular occlusion (Agostinis et al., 2011). This mechanism is clinically utilized to treat various malignancies, dermatological conditions like actinic keratosis, and ophthalmic diseases such as age-related macular degeneration (FDA Visudyne Label). However, the efficacy of the treatment is strictly dependent on the local concentration of ground-state oxygen, making tissue hypoxia a significant challenge in the treatment of solid tumors.

Other names

Triplet oxygenDioxygenMolecular oxygen3O2Photophysical substrate

Type II photochemical energy transfer where an excited photosensitizer transfers energy to ground-state molecular oxygen to produce reactive singlet oxygen (Agostinis et al., 2011).