Repeated assessment of orthotopic glioma pO2 by multi-site EPR oximetry: A technique with the potential to guide therapeutic optimization by repeated measurements of oxygen

Tumor hypoxia plays a vital role in therapeutic resistance. Consequently, measurements of tumor pO2 could be used to optimize the outcome of oxygen-dependent therapies, such as, chemoradiation. However, the potential optimizations are restricted by the lack of methods to repeatedly and quantitatively assess tumor pO2 during therapies, particularly in gliomas. We describe the procedures for repeated measurements of orthotopic glioma pO2 by multi-site electron paramagnetic resonance (EPR) oximetry. This oximetry approach provides simultaneous measurements of pO2 at more than one site in the glioma and contralateral cerebral tissue.The pO2 of intracerebral 9L, C6, F98 and U251 tumors, as well as contralateral brain, were measured repeatedly for five consecutive days. The 9L glioma was well oxygenated with pO2 of 27–36 mm Hg, while C6, F98 and U251 glioma were hypoxic with pO2 of 7–12 mm Hg. The potential of multi-site EPR oximetry to assess temporal changes in tissue pO2 was investigated in rats breathing 100% O2. A significant increase in F98 tumor and contralateral brain pO2 was observed on day 1 and day 2, however, glioma oxygenation declined on subsequent days.In conclusion, EPR oximetry provides the capability to repeatedly assess temporal changes in orthotopic glioma pO2. This information could be used to test and optimize the methods being developed to modulate tumor hypoxia. Furthermore, EPR oximetry could be potentially used to enhance the outcome of chemoradiation by scheduling treatments at times of increase in glioma pO2.

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► EPR oximetry using particulate probes provide repeated assessment of temporal changes in orthotopic glioma pO2; a unique capability not available with any other technique.
► Simultaneous measurement of glioma and contralateral brain pO2 by the multi-site EPR oximetry provides site specific tissue pO2 information during treatments.
► Orthotopic 9L gliomas are well oxygenated while C6, F98 and U251 glioma are hypoxic.
► The effect of 100% O2 inhalation on F98 glioma pO2 declined over days, which highlights the significance of repeated oximetry for a potential optimization of the approaches developed to modulate tumor hypoxia and rationally combine with radiotherapy to enhance the outcome.