Blood oxygen level‐dependent (BOLD) contrast‐based functional MRI (fMRI) has been reported as a method to assess the evolution of tumor oxygenation after hyperoxic treatments, because of its sensitivity to changes in blood flow and deoxyhemoglobin content. However a number of questions remain: 1) In view of tumor heterogeneity, how good is the correlation between the MR parameters in gradient‐echo imaging (signal intensity (SI) or effective transverse relaxation time (T)) and local tumor oxygen partial pressure (pO₂)? 2) Is the magnitude of the change in SI or T a quantitative marker for variation in pO₂? 3) Is initial T a good marker for initial pO₂? To address these questions, murine tumors were imaged during respiratory challenges at 4.7 Tesla, using fiber‐optic microprobes to simultaneously acquire tumor pO₂ and erythrocyte flux. The BOLD signal response (SI and T) was temporally correlated with changes in pO₂. However, the magnitude of the signal bore no absolute relation to pO₂ across tumors, i.e., a given change in SI corresponded to a 25 mmHg pO₂ change in one tumor, but to a 100 mmHg change in another. The initial T value did not reliably predict tumor oxygenation at the beginning of the experiment. In conclusion, the major advantages of the technique include noninvasiveness, high spatial resolution, and real‐time detection of pO₂ fluctuations. Information afforded by the BOLD imaging technique is qualitative in nature and may be combined with other techniques capable of providing an absolute measure of pO₂. Magn Reson Med 48:980–986, 2002.

This review paper attempts to provide an overview of the principles and techniques that are often termed electron paramagnetic resonance (EPR) oximetry. The paper discusses the potential of such methods and illustrates they have been successfully applied to measure oxygen tension, an essential parameter of the tumor microenvironment. To help the reader understand the motivation for carrying out these measurements, the importance of tumor hypoxia is first discussed: the basic issues of why a tumor is hypoxic, why these hypoxic microenvironments promote processes driving malignant progression and why hypoxia dramatically influences the response of tumors to cytotoxic treatments will be explained. The different methods that have been used to estimate the oxygenation in tumors will be reviewed. To introduce the basics of EPR oximetry, the specificity of in vivo EPR will be discussed by comparing this technique with NMR and MRI. The different types of paramagnetic oxygen sensors will be presented, as well as the methods for recording the information (EPR spectroscopy, EPR imaging, dynamic nuclear polarization). Several applications of EPR for characterizing tumor oxygenation will be illustrated, with a special emphasis on pharmacological interventions that modulate the tumor microenvironment. Finally, the challenges for transposing the method into the clinic will also be discussed. Copyright © 2004 John Wiley & Sons, Ltd.

Purpose: The aim of this work was to study changes in the tumor microenvironment early after an antiangiogenic treatment using thalidomide (a promising angiogenesis inhibitor in a variety of cancers), with special focus on a possible “normalization” of the tumor vasculature that could be exploited to improve radiotherapy. Experimental Design: Tumor oxygenation, perfusion, permeability, interstitial fluid pressure (IFP), and radiation sensitivity were studied in an FSAII tumor model. Mice were treated by daily i.p. injection of thalidomide at a dose of 200 mg/kg. Measurements of the partial pressure of oxygen (pO2) were carried out using electron paramagnetic resonance oximetry. Three complementary techniques were used to assess the blood flow inside the tumor: dynamic contrast-enhanced magnetic resonance imaging, Patent Blue staining, and laser Doppler imaging. IFP was measured by a “wick-in-needle” technique. Results: Our results show that thalidomide induces tumor reoxygenation within 2 days. This reoxygenation is correlated with a reduction in IFP and an increase in perfusion. These changes can be attributed to extensive vascular remodeling that we observed using CD31 labeling. Conclusions: In summary, the microenvironmental changes induced by thalidomide were sufficient to radiosensitize tumors. The fact that thalidomide radiosensitization was not observed in vitro, and that in vivo radiosensitization occurred in a narrow time window, lead us to believe that initial vascular normalization by thalidomide accounts for tumor radiosensitization.

Different nitric oxide (NO)‐mediated treatments (e.g., isosorbide dinitrate, insulin and electrical stimulation of the host tissue) have been investigated for their effects on tumor oxygenation and radiation sensitivity. We further address the issue of the role played by modulation of the NO‐pathway in tumor radiosensitivity. For this purpose, the local concentration of NO was monitored after treatment in FSaII tumors and a comparison between the sensitivity of LLC tumors implanted both on eNOS^−/− and wild‐type (WT) mice was carried out. First, we demonstrate the central role played by eNOS in the radiosensitizing effect after application of insulin treatment and electrical stimulation: a significant increase in tumor NO content is induced by these treatments and the increase in tumor oxygenation, as well as the radiosensitizing effect are abolished in eNOS knock‐out mice, in contrast to WT mice. Second, by comparing the level of oxygen and NO achieved in tumors after NO‐mediated treatments and carbogen, we provide evidence that these NO‐mediated treatments are not simply acting by a single oxygen effect. These treatments induced significant regrowth delays compared to carbogen, despite a smaller increase in tumor oxygenation. For the NO‐mediated treatments, there was a direct correlation between the NO content and the radiosensitizing effect. These data strongly suggest that NO is a complementary factor additive to oxygen in determining the sensitivity to irradiation and we therefore propose that NO acts as an intrinsic radiosensitizer in vivo.