No abstract

A very reliable and reproducible electrochemical preparative procedure to obtain oxygen-sensitive lithium phthalocyanine (LiPc) microcrystalline powder, a critical material for in vivo application of electron paramagnetic resonance (EPR) oximetry to measure the partial pressure of molecular oxygen pO 2 , is described. Important issues including the effect of preparative conditions on the resulting material and the influence of the deposition mechanism on crystal structure are investigated using cyclic voltammetry, chronoamperometry, X-ray diffraction (XRD), and high-and low-frequency EPR measurements. The electrochemical measurements reveal that electrodeposition of LiPc follows a nucleation pathway. Detailed electrocrystallization studies show that the nucleation mechanism is instantaneous and the three-dimensional growth is controlled by the diffusion of the reactant from the bulk solution. Critical evidence, for deposition potential-dependent electrochemical phase formation, is presented. The XRD studies indicate that, in certain deposition conditions, namely, deposition at potentials +0.1 and +0.2 V (Ag/AgCl), the β structure of LiPc, which is insensitive to molecular oxygen in terms of EPR oximetry, is formed in higher fraction. On the other hand, at deposition potentials +0.4 and +0.7 V, exclusively the oxygen-sensitive x form is obtained. A rapidity test showed that while at deposition potentials +0.4 and +0.7 V only the x form is obtained, the +0.4 V sample responds more quickly to oxygen than the +0.7 V sample. From the present work, a variety of LiPc microparticles, suitable for in vivo EPR oximetry applications, can be prepared.

The purpose of this study was to noninvasively monitor tumor oxygenation and redox status during hyperoxygenation treatment, such as carbogen-breathing, in a murine tumor model using in vivo electron paramagnetic resonance (EPR) spectroscopy and imaging techniques. The study was performed using implanted lithium phthalocyanine (LiPc) microcrystals as the oximetry probe and 3-carbamoylproxyl ( Since tumor oxygenation is known to enhance the efficacy of radiotherapy, numerous experimental and clinical strategies have been introduced to improve it. A recent strategy combines breathing of carbogen gas (a mixture of 95% oxygen and 5% CO 2 ) with administration of nicotinamide (1). This combination is selected to specifically target the two major types of hypoxia thought to exist in tumors: 1) chronic hypoxia, which results from diffusion limitations of oxygen; and 2) acute hypoxia, which is caused by the intermittent constriction of tumor blood vessels. Carbogen is believed to increase the oxygen content of the blood while at the same time it promotes vasodilation based on its 5% CO 2 content. Nicotinamide is believed to reduce intermittent vessel closures (2). Using a combination of both carbogen and nicotinamide, studies have shown that oxygenation, and hence the sensitivity to radiotherapy, have been significantly enhanced in both single-dose (3) and fractionated regimens (4 -6). While these and most other studies have shown beneficial effects of carbogenbreathing on tumor oxygenation, it has also been shown in a few studies that carbogen-breathing produces variable oxygenation results (7,8). For example, in a recent report, Dewhirst et al. (8) reported that carbogen had no consistent effect on tumor blood flow and was ineffective at increasing pO 2 in R3230Ac tumors in rat. Their laser Doppler flow measurements showed that the tumor blood flow effects are not global but occur at the microregional level. Thus, irrespective of the tumor model, treatment modality, or outcome, a direct quantitative determination of tumor oxygenation will greatly aid in the evaluation of the tumor microenvironment.Methods used to measure oxygen concentration (termed "oximetry") or hypoxia in tumors have received considerable attention (9 -11). These methods include polarographic oxygen electrodes, comet assay, immunochemical techniques, optical spectroscopy, 19 F magnetic resonance spectroscopy (MRS) and imaging (MRI), 31 P-MRS, and electron paramagnetic resonance (EPR) spectroscopy and imaging. However, concerns regarding invasiveness, insufficient dynamic range of measurements, requirements for repetitive measurements, and poor spatial or temporal resolution have limited the realization of the full potential of these techniques. The polarographic oxygen electrode, often considered as the gold standard, is invasive, subject to pressure artifacts, and unsuitable for repeated measurements. Magnetic resonance techniques (MRI, blood oxygen level dependent (BOLD), Overhauser-enhanced MRI (OMRI), and EPR) and positron emission tomography (...

As an extension of our previous report (Ilangovan, G.; Zweier, J. L.; Kuppusamy, P. J. Phys. Chem. B 2000, 104, 4047) on the synthesis and characterization of lithium phthalocyanine (LiPc) as an electron paramagnetic resonance (EPR) oximetry probe, we investigated the mechanism of the effect of molecular oxygen on the EPR spectrum of the LiPc. Electrochemical preparation of LiPc under potentiostatic conditions (+0.4 V) yielded microcrystals that showed extremely narrow (peak-to-peak width < 10 mG) EPR spectrum under anoxic conditions. In this study it is observed that the peak-to-peak width is particle-size-dependent. The total EPR line shape is observed to be a composite of several components present in the microcrystalline powder. Deconvolution of the composite shape showed the presence of at least three major components corresponding to different crystal sizes in the bulk sample. The mechanism of EPR line broadening of LiPc in the presence of molecular oxygen is entirely different from that of dissolved paramagnetic probes such as nitroxides. The EPR line width variations observed in the present case are interpreted using a two-spin state model, namely mobile and fixed spin states, originally proposed for trans-polyacetylene. Further, the sensitivity of EPR line width to oxygen partial pressure (pO 2 ), measured as the slope of EPR line width of LiPc versus pO 2 plot, is higher compared to previously reported values. The higher sensitivity is due to smaller crystal size, wherein the molecular oxygen can easily diffuse into microchannels of LiPc crystals. At higher pO 2 values, the oxygen-induced EPR line broadening shows a saturation behavior, which is quantitatively interpreted from the dynamics of the two-spin model. Thus, the oxygen sensitivity of the LiPc particles is strongly dependent on the size of individual particles. The results will enable selective synthesis and control of LiPc probe suitable for a given oximetry application.