Temporal variation in the response of tumors to hyperoxia with breathing carbogen and oxygen

The effect of hyperoxygenation with carbogen (95% O2 + 5% CO2) and 100% oxygen inhalation on partial pressure of oxygen (pO2) of radiation-induced fibrosarcoma (RIF-1) tumor was investigated. RIF-1 tumors were innoculated in C3H mice, and aggregates of oximetry probe, lithium phthalocyanine (LiPc), was implanted in each tumor. A baseline tumor pO2 was measured by electron paramagnetic resonance (EPR) oximetry for 20 minutes in anesthetized mice breathing 30% O2 and then the gas was switched to carbogen or 100 % oxygen for 60 minutes. These experiments were repeated for 10 days. RIF-1 tumors were hypoxic with a baseline tissue pO2 of 6.2-8.3 mmHg in mice breathing 30% O2. Carbogen and 100% oxygen significantly increased tumor pO2 on days 1 to 5, with a maximal increase at approximately 32-45 minutes on each day. However, the extent of increase in pO2 from the baseline declined significantly on day 5 and day 10. The results provide quantitative information on the effect of hyperoxic gas inhalation on tumor pO2 over the course of 10 days. EPR oximetry can be effectively used to repeatedly monitor tumor pO2 and test hyperoxic methods for potential clinical applications.