In vivo measurement and imaging of tumor oxygenation using coembedded paramagnetic particulates

Tumor tissue oxygenation is an important parameter that is positively correlated to the chemo- or radiation treatment outcome f certain tumors. Hence, methods to accurately and noninvasively determine the concentration of oxygen (pO₂) in tumors will be valuable. In this study, electron paramagnetic resonance (EPR) spectroscopy, utilizing microcrystalline particulates of lithium phthalocyanine (LiPc), was used to perform repeated measurements of pO₂ as a function of tumor growth. We permanently embedded the particulates in the tumor by coimplanting them with RIF-1 tumor cells during inoculation in mice. This procedure enabled repeated measurements of oxygen concentration in the tumor to be obtained for >2 weeks during its growth phase. The particulates were stable and non-toxic to the tumor cells. Both an in vitro clonogenic assay and an in vivo tumor growth rate examination in C3H mice showed no apparent effect on cell proliferation or tumor growth rate. The measurements indicated that the pO₂ of the tumor decreased xponentially with tumor growth and reached hypoxic levels (∼4 mmHg) within 4 days after inoculation of the tumor cells. Spatial EPR imaging revealed a nonuniform distribution of the embedded particulates, which were localized mainly in the middle of the tumor volume. Oxygen mapping of the tumor, obtained by spectroscopic EPR imaging, showed significant variation of pO₂ within the tumor. In summary, EPR spectroscopy and imaging with an embedded oximetry probe enabled accurate and repeated measurements of pO₂ to be obtained in growing tumors under nonperturbing conditions.