Molecular oxygen (O2) (O2)

Molecular oxygen (O2) is a vital diatomic molecule that acts as the primary electron acceptor in aerobic respiration and a critical determinant of the tumor microenvironment's metabolic landscape (Vaupel et al., 1989, PMID: 2673568). In solid tumors, inadequate and disorganized vascularization often leads to regions of chronic or acute hypoxia, where oxygen levels fall significantly below physiological norms (Semenza, 2003, PMID: 12928690). This hypoxic state is a major driver of therapeutic resistance, particularly in radiotherapy, where oxygen is required to "fix" radiation-induced DNA damage into permanent chemical changes, a process known as the oxygen fixation hypothesis (Gray et al., 1953, PMID: 13112371). Furthermore, oxygen serves as a necessary substrate for photodynamic therapy (PDT) to generate cytotoxic singlet oxygen (Dougherty et al., 1998, PMID: 9630877). Therapeutic interventions targeting intratumoral oxygen levels include the use of hyperbaric oxygen, oxygen-carrying perfluorocarbons, and diffusion enhancers like trans-sodium crocetinate to sensitize hypoxic cells to treatment (Gainer et al., 2005, PMID: 16144464). Conversely, the steep oxygen gradient in tumors is exploited by hypoxia-activated prodrugs, which remain non-toxic in well-oxygenated tissues but are converted to active cytotoxins in the O2-depleted tumor core (Wilson & Hay, 2011, PMID: 21455171).

Other names

DioxygenO2Intratumoral oxygenTumor oxygenationMolecular oxygen within tumor tissue

Molecular oxygen acts as a radiosensitizer by reacting with radiation-induced DNA radicals to form stable, lethal peroxyl radicals, thereby preventing DNA repair (Oxygen Fixation Hypothesis; Gray et al., 1953, PMID: 13112371). It also serves as a critical substrate for photodynamic therapy, where it is converted into singlet oxygen and other reactive oxygen species (ROS) to induce cell death (Dougherty et al., 1998, PMID: 9630877). Additionally, oxygen levels modulate the activity of hypoxia-inducible factors (HIFs), which regulate genes involved in angiogenesis, metabolism, and cell survival (Semenza, 2003, PMID: 12928690).