Bénédicte Jordan

Carbon-centered radicals as oxygen sensors for in vivo electron paramagnetic resonance: screening for an optimal probe among commercially available charcoals

It is known that some charcoals possess paramagnetic centers with an electron paramagnetic resonance (EPR) linewidth which can be broadened by oxygen. In order to identify potential candidates as sensors for in vivo EPR oximetry, we carried out a systematic study among commercially available charcoals. A total of 34 charcoals were tested. The steps used for the screening were: (1) to check the presence of paramagnetic centers in the material; (2) to measure the EPR linewidth in nitrogen and in air on the dry material and on a aqueous suspension of particles; (3) to calibrate the oxygen sensitive materials (EPR linewidth vs. pO2); (4) to test the sensitivity and stability of the response to changes of pO2 in a simple model of hypoxia induced in mice. Seventeen charcoals contained paramagnetic centers detectable by low-frequency EPR (1.1 GHz). The EPR spectrum consist of one single line which is typical of carbon-centered radicals (g-factor ∼2). Eight charcoals presented sufficient interesting EPR properties (linewidth in nitrogen <0.1 mT, linewidth in air for an aqueous suspension of particles >0.15 mT) to be further characterized in vivo. Only three charcoals presented a stable, reproducible, and sensitive response to pO2 for more than 2 months. These three coals should be considered as good candidates to be used as oxygen sensor using in vivo EPR spectroscopy.

Targeting tumor perfusion and oxygenation to improve the outcome of anticancer therapy.

Radiotherapy and chemotherapy are widespread clinical modalities for cancer treatment. Among other biological influences, hypoxia is a main factor limiting the efficacy of radiotherapy, primarily because oxygen is involved in the stabilization of the DNA damage caused by ionizing radiations. Radiobiological hypoxia is found in regions of rodent and human tumors with a tissue oxygenation level below 10u2009mmHg at which tumor cells become increasingly resistant to radiation damage. Since hypoxic tumor cells remain clonogenic, their resistance to the treatment strongly influences the therapeutic outcome of radiotherapy. There is therefore an urgent need to identify adjuvant treatment modalities aimed to increase tumor pO2 at the time of radiotherapy. Since tumor hypoxia fundamentally results from an imbalance between oxygen delivery by poorly efficient blood vessels and oxygen consumption by tumor cells with high metabolic activities, two promising approaches are those targeting vascular reactivity and tumor cell respiration. This review summarizes the current knowledge about the development and use of tumor-selective vasodilators, inhibitors of tumor cell respiration, and drugs and treatments combining both activities in the context of tumor sensitization to X-ray radiotherapy. Tumor-selective vasodilation may also be used to improve the delivery of circulating anticancer agents to tumors. Imaging tumor perfusion and oxygenation is of importance not only for the development and validation of such combination treatments, but also to determine which patients could benefit from the therapy. Numerous techniques have been developed in the preclinical setting. Hence, this review also briefly describes both magnetic resonance and non-magnetic resonance in vivo methods and compares them in terms of sensitivity, quantitative or semi-quantitative properties, temporal, and spatial resolutions, as well as translational aspects.

Microencapsulation of paramagnetic particles by pyrroxylin to preserve their responsiveness to oxygen when used as sensors for in vivo EPR oximetry

Using the broadening of the electron paramagentic resonance (EPR) linewidth of paramagnetic particles by oxygen, it is possible to make measurements of the partial pressure of oxygen in vivo. While the results obtained so far with EPR oximetry are very encouraging, several paramagnetic materials may lose their responsiveness to oxygen in tissues. This aim of this study was to provide evidence that an appropriate coating can preserve the oxygen sensitivity of paramagnetic materials in vivo. Two charcoals that have the oxygen‐sensing properties required for EPR oximetry (combined with a tendency to lose responsiveness to oxygen when placed in tissues) were coated using pyroxylin. Sensitivity to variations in pO2 was checked by inducing hypoxia in the muscles of mice injected with charcoal. While the uncoated material lost responsiveness to oxygen within few days, the particles coated with 20–30% of pyroxylin did not lose their responsiveness for more than 2 months. Magn Reson Med 42:193–196, 1999.

Targeting tumor perfusion and oxygenation modulates hypoxia and cancer sensitivity to radiotherapy and systemic therapies

Hypoxia, a partial pressure of oxygen (pO2) below physiological needs, is a limiting factor affecting the efficiency of radiotherapy. Indeed, the reaction of reactive oxygen species (ROS, produced by water radiolysis) with DNA is readily reversible unless oxygen stabilizes the DNA lesion. While normal tissue oxygenation is around 40 mm Hg, both rodent and human tumors possess regions of tissue oxygenation below 10 mm Hg, at which tumor cells become increasingly resistant to radiation damage (radiobiological hypoxia) (Gray, 1953). Because of this so-called “oxygen enhancement effect”, the radiation dose required to achieve the same biologic effect is about three times higher in the absence of oxygen than in the presence of normal levels of oxygen (Gray et al., 1953; Horsman & van der Kogel, 2009). Hypoxic tumor cells, which are therefore more resistant to radiotherapy than well oxygenated ones, remain clonogenic and contribute to the therapeutic outcome of fractionated radiotherapy (Rojas et al., 1992). Tumor hypoxia results from the imbalance between oxygen delivery by poorly efficient blood vessels and oxygen consumption by tumor cells with high metabolic activities. On the one hand, oxygen delivery is impaired by structural abnormalities present in the tumor vasculature (Munn, 2003). They include caliber variations with dilated and narrowed single branches of tumor vessels, non-hierarchical vascular networks, disturbed precapillary architecture, and incomplete vascular walls. These structural abnormalities cause numerous functional impairments, i.e. increased transcapillary permeability, increased vascular permeability, interstitial hypertension, and increased flow resistance (Boucher et al., 1996; McDonald & Baluk, 2002). It is however important to note that, although hastily formed immature tumor microvessels lack smooth muscle layer(s) and are therefore unable to provide autoregulation, it is not uncommon to find mature blood vessels with smooth muscle layers and neural junctions inside slow-growing tumors (e.g. most human tumors) (Feron, 2004). On the other hand, the altered tumor cell metabolism with elevated metabolic rates also contributes to the occurrence of hypoxic regions in tumors and further causes extracellular acidification. Tumor hypoxia occurs in two ways: chronic hypoxia (or diffusion-limited hypoxia), and acute hypoxia (or perfusion-limited or fluctuating hypoxia). Chronic hypoxia has classically been thought to result from long diffusion distances