We show here that hyperpolarized [1‐¹³C]pyruvate can be used to detect treatment response in a glioma tumor model; a tumor type where detection of response with ¹⁸fluoro‐2‐deoxyglucose, using positron emission tomography, is limited by the high background signals from normal brain tissue. ¹³C chemical shift images acquired following intravenous injection of hyperpolarized [1‐¹³C]pyruvate into rats with implanted C6 gliomas showed significant labeling of lactate within the tumors but comparatively low levels in surrounding brain.Labeled pyruvate was observed at high levels in blood vessels above the brain and from other major vessels elsewhere but was detected at only low levels in tumor and brain.The ratio of hyperpolarized ¹³C label in tumor lactate compared to the maximum pyruvate signal in the blood vessels was decreased from 0.38 ± 0.16 to 0.23 ± 0.13, (a reduction of 34%) by 72 h following whole brain irradiation with 15 Gy. Magn Reson Med, 2011.

Cycling hypoxia is now a well-recognized phenomenon in animal and human solid tumors. Cycling hypoxia can exist more than 100-μm distances from a microvessel, and some of these regions have been shown to exist adjacent to normal tissue. Fluctuations in pO₂ of approximately 20 mm Hg can occur with periodicities of minutes to hours and even days. These fluctuations have been attributed to changes in erythrocyte flux, perfusion, and also development of newer vascular networks. Cycling hypoxia has been shown to induce the expression of hypoxia-inducible transcription factor-1α (HIF-1α) and also confer tumor cells and tumor vascular endothelial cells with enhanced prosurvival pathways, making tumors less responsive to radiation and chemotherapy. Imaging of cycling hypoxia in tumors can provide capabilities to help plan appropriate treatment, by taking into account the magnitude and frequency of fluctuations and also their locations adjacent to normal tissue. Electron paramagnetic resonance imaging (EPRI) provides the ability to distinguish chronic and cycling hypoxic regions and has the required spatial and temporal resolutions to provide quantitative maps of tumor pO₂. EPRI can serve as a valuable tool in examining tumor pO₂ longitudinally in response to treatment and in an experimentally chosen time window to spatially map fluctuations in pO₂ noninvasively in animal models of implanted or orthotopic tumors, with a potential for human applications. Cancer Res; 70(24); 10019–23. ©2010 AACR.

MRI has provided significant clinical utility in the diagnosis of diseases and will become a powerful tool to assess phenotypic changes in genetically engineered animals. Overhauser enhanced MRI (OMRI), which is a double resonance technique, creates images of free radical distributions in small animals by enhancing the water proton signal intensity by means of the Overhauser effect. Several studies have demonstrated noninvasive assessment of reactive oxygen species generation in small animals by using low frequency electron spin resonance (ESR) spectroscopy/imaging and nitroxyl radicals. In vivo ESR signal intensities of nitroxyl radicals decrease with time after injection; and the decreases are enhanced by reactive oxygen species, generated in oxidative disease models in a site-specific manner. In this study, we show images of nitroxyl radicals with different isotopes by changing the external magnetic field for ESR irradiation between ¹⁴ N and ¹⁵ N nuclei in field-cycled OMRI. OMRI simultaneously obtained dual images of two individual chemical processes. Oxidation and reduction were monitored in a rate-dependent manner at nanometer scale by labeling membrane-permeable and -impermeable nitroxyl radicals with ¹⁴ N and ¹⁵ N nuclei. Phantom objects containing ascorbic acid-encapsulated liposomes with membrane-permeable radicals but not membrane-impermeable ones show a time-dependent decrease of the OMRI image intensity. The pharmacokinetics in mice was assessed with OMRI after radical administration. This OMRI technique with dual probes should offer significant applicability to nanometer scale molecular imaging and simultaneous assessment of independent processes in gene-modified animals. Thus, it may become a powerful tool to clarify mechanisms of disease and to monitor pharmaceutical therapy.

Structural and functional abnormalities in tumor blood vessels impact the delivery of oxygen and nutrients to solid tumors, resulting in chronic and cycling hypoxia. Although chronically hypoxic regions exhibit treatment resistance, more recently it has been shown that cycling hypoxic regions acquire prosurvival pathways. Angiogenesis inhibitors have been shown to transiently normalize the tumor vasculatures and enhance tumor response to treatments. However, the effect of antiangiogenic therapy on cycling tumor hypoxia remains unknown. Using electron paramagnetic resonance imaging and MRI in tumor-bearing mice, we have examined the vascular renormalization process by longitudinally mapping tumor partial pressure of oxygen (pO₂) and microvessel density during treatments with a multi-tyrosine kinase inhibitor sunitinib. Transient improvement in tumor oxygenation was visualized by electron paramagnetic resonance imaging 2 to 4 days following antiangiogenic treatments, accompanied by a 45% decrease in microvessel density. Radiation treatment during this time period of improved oxygenation by antiangiogenic therapy resulted in a synergistic delay in tumor growth. In addition, dynamic oxygen imaging obtained every 3 minutes was conducted to distinguish tumor regions with chronic and cycling hypoxia. Sunitinib treatment suppressed the extent of temporal fluctuations in tumor pO₂ during the vascular normalization window, resulting in the decrease of cycling tumor hypoxia. Overall, the findings suggest that longitudinal and noninvasive monitoring of tumor pO₂ makes it possible to identify a window of vascular renormalization to maximize the effects of combination therapy with antiangiogenic drugs. Cancer Res; 71(20); 6350–9. ©2011 AACR.