A novel procedure for in vivo imaging of the
oxygen partial pressure (pO2) in implanted
tumors is reported. The procedure uses electron paramagnetic resonance imaging (EPRI) of
oxygen-sensing nanoprobes embedded in the
tumor cells. Unlike existing methods of pO2 quantification, wherein the probes are physically inserted at the time of measurement, the new approach uses cells that are preinternalized (labeled) with the
oxygen-sensing probes, which become permanently embedded in the developed
tumor. Radiation-induced
fibrosarcoma (RIF-1) cells, internalized with nanoprobes of
lithium octa-n-butoxy-naphthalocyanine (
LiNc-BuO), were allowed to grow as a solid
tumor. In vivo imaging of the growing
tumor showed a heterogeneous distribution of the spin probe, as well as oxygenation in the
tumor volume. The pO2 images obtained after the
tumors were exposed to a single dose of 30-Gy X-radiation showed marked redistribution as well as an overall increase in tissue oxygenation, with a maximum increase 6 hr after irradiation. However, larger
tumors with a poorly perfused core showed no significant changes in oxygenation. In summary, the use of in vivo EPR technology with embedded
oxygen-sensitive nanoprobes enabled noninvasive visualization of dynamic changes in the intracellular pO2 of growing and irradiated
tumors.