The influence of oxygenation on the 19F spin-lattice relaxation rates of fluosol-DA

Dual perfluorocarbon method to noninvasively monitor dissolved oxygen concentration in tissue engineered constructs in vitro and in vivo

Noninvasive in vivo monitoring of tissue implants provides important correlations between construct function and the observed physiologic effects. As oxygen is a key parameter affecting cell and tissue function, we established a monitoring method that utilizes (19) F nuclear magnetic resonance (NMR) spectroscopy, with perfluorocarbons (PFCs) as oxygen concentration markers, to noninvasively monitor dissolved oxygen concentration (DO) in tissue engineered implants. Specifically, we developed a dual PFC method capable of simultaneously measuring DO within a tissue construct and its surrounding environment, as the latter varies among animals and with physiologic conditions. In vitro studies using an NMR-compatible bioreactor demonstrated the feasibility of this method to monitor the DO within alginate beads containing metabolically active murine insulinoma βTC-tet cells, relative to the DO in the culture medium, under perfusion and static conditions. The DO profiles obtained under static conditions were supported by mathematical simulations of the system. In vivo, the dual PFC method was successful in tracking the oxygenation state of entrapped βTC-tet cells and the surrounding peritoneal DO over 16 days in normal mice. DO measurements correlated well with the extent of cell growth and host cell attachment examined postexplantation. The peritoneal oxygen environment was found to be variable and hypoxic, and significantly lower in the presence of metabolically active cells. The significance of the dual PFC system in providing critical DO measurements for entrapped cells and other tissue constructs, in vitro and in vivo, is discussed.

Monitoring of dissolved oxygen and cellular bioenergetics within a pancreatic substitute

This work investigated the use of nuclear magnetic resonance (NMR) spectroscopy in combination with a mathematical model of an encapsulated cell system as a method for rapidly assessing the status of a pancreatic substitute. To validate this method, an in vitro experiment was performed in which the encapsulated cells were perfused in an NMR-compatible system and the dissolved oxygen (DO) concentration of the perfusing medium was lowered from 0.20 to 0.05 mM, then returned to 0.20 mM in a stepwise fashion. The cellular metabolic activity and bioenergetics were evaluated by measuring the oxygen consumption rate (via DO sensors) and nucleotide triphosphate levels (via (31)P NMR). By incorporating a perfluorocarbon emulsion into the alginate beads, the cellular oxygenation state was monitored by measuring the average intrabead DO (AIDO) concentration by (19)F NMR. The in vitro measurements were then compared with model predictions based on the measured external DO concentration and time. Model-predicted cell growth and AIDO closely matched the experimentally acquired data. As the DO concentrations both external to and within the pancreatic substitute are needed to apply this methodology in vivo, the feasibility of measuring the DO concentration from two distinct bead populations implanted in the peritoneal cavity of mice was established. It is concluded that PFC incorporation and (19)F NMR measurements, in combination with a mechanistic model of the encapsulated system, allow the tracking of the state of a pancreatic substitute in vitro and potentially in vivo.

Sonochemically produced fluorocarbon microspheres: a new class of magnetic resonance imaging agent

With the intent of increasing the signal-to-noise ratio (SNR) of fluorine magnetic resonance imaging and enabling new applications, we have developed a novel class of agents based on protein encapsulation of fluorocarbons. Microspheres formed by high-intensity ultrasound have a gaussian size distribution with an average diameter of 2.5 microns. As with conventional emulsions, these microspheres target the reticuloendothelial system. However, our sonochemically produced microspheres, because of a high encapsulation efficiency, show increases in the SNR of up to 300% compared to commercially available emulsions. We also demonstrate an increase in the circulation lifetime of the microspheres with the bloodstream by more than 30-fold with a chemical modification of the outer surface of the microsphere. Finally, by encapsulating mixtures of fluorocarbons that undergo solid/liquid phase transitions, we can map temperature in the reticuloendothelial system, with signal changes of approximately 20-fold over a 5 degrees C range.

Rapid tissue oxygen tension mapping using 19F inversion-recovery echo-planar imaging of perfluoro-15-crown-5-ether

Fluorine-19 inversion-recovery, echo-planar imaging (IR-EPI) was used in conjunction with a new PFC emulsion, perfluoro-15-crown-5-ether, to map the spatial distribution of oxygen tension in murine liver, spleen and radiation induced fibrosarcoma (RIF-1) tumors. Intravenously administered PFC emulsions were allowed to sequester in the liver, spleen, and tumor 3 to 7 days prior to imaging experiments. Seven, 64 x 64 IR-EPIs were acquired with successively increasing inversion times (TI). A nonlinear least-squares regression algorithm was used to fit the seven two-dimensional matrices, on a pixel-by-pixel basis, to solve for the relaxation rate, R1, of the sequestered PFC. From in vitro calibration curves, the oxygen tension (pO2) was calculated from the measured R1. Oxygen tension maps were then murine liver and spleen were produced (in 2.5 min) to demonstrate the technique and changes in tissue oxygenation as a function of breathing gas (air and carbogen (95% O2-5% CO2)) are presented. Tissue pO2 maps from RIF-1 tumors (n = 5) were obtained in less than 10 min and changes in tumor pO2 were studied when the breathing gas was switched from air to carbogen. The results from tumor pO2 maps were compared with 19F MR spectroscopy measurements to check for consistency. Histogram analysis yielded an average liver and spleen pO2 of 43 torr and 26 torr for RIF-1 tumors when the animals were breathing air. Statistically significant changes in tumor oxygenation as a function of breathing gas were obtained from both pO2 maps (6 +/- 2 torr, P < 0.05) and 19F MR spectroscopy (13 +/- 3 torr, P < 0.01) as evaluated using the Student's paired t test.

Non-invasive physiology: 19F NMR of perfluorocarbons

Ever since it was shown that the 19F NMR spin-lattice relaxation rates (R1) of perfluorocarbon (PFC) emulsions are highly sensitive to oxygen tension (pO2), there has been a developing interest in the use of PFCs to probe tissue physiology. Oxygen is required for efficient function by most tissues and hypoxia leads to rapid cellular dysfunction and damage. In addition, hypoxic tumor cells are refractory to radiotherapy. Thus, the opportunity to measure tissue oxygen tension non-invasively may be significant in understanding mechanisms of tissue function and in clinical prognosis. PFC NMR parameters are also sensitive to temperature, facilitating NMR thermometry with potential applications in hyperthermia studies. I will review the development of experimental techniques, applications to specific tissues and discuss the challenges and opportunities presented by 19F NMR of perfluorocarbons.

Imaging oxygen tension in liver and spleen by 19F NMR

19F NMR imaging of the perfluorocarbon emulsion Fluosol has been used to study regional variations in oxygen tension in rat liver and spleen. We have used the linear dependence of spin lattice relaxation rate (1/T1) on the partial oxygen pressure (pO2) of Fluosol to determine the oxygen tension in the reticuloendothelial system (RES) of the liver and spleen of male Sprague-Dawley rats which have received serial infusions of Fluosol. Oxygen tension maps have been computed from 19F NMR images using a calibration obtained for Fluosol in vitro at 37.5 degrees C. The spatial resolution of the pO2 maps computed using this technique is 1.2 x 1.2 mm in 3-mm thick slices. Calculations from in vivo pO2 maps indicate an average change in the median pO2 of the RES from 118 to 80 mmHg for (n = 7) rats breathing 95% O2 and 5% CO2 (carbogen) and air, respectively.

In vivo oxygen tension and temperature: simultaneous determination using 19F NMR spectroscopy of perfluorocarbon

A novel technique is presented to measure in vivo simultaneously oxygen tension and temperature using 19F NMR spectroscopy of perfluorocarbon. This work examines the variation with oxygen tension (pO2) and temperature of the individual spin lattice relaxation rates (R1) of the 19F resonances of the perfluorocarbon emulsion Oxypherol-ET. A linear relationship between R1 and pO2 is confirmed for all the resonances at any specific temperature in the range 27-50 degrees C. Similarly, a linear relationship is determined between R1 and temperature at any specific pO2 in this temperature range. Each resonance behaves uniquely with respect to temperature and pO2 and consideration of 2 or more resonances uniquely defines pO2 and temperature simultaneously, and unambiguously. This concept is demonstrated in vivo in a murine tumor and perfused rat heart, where pO2 and temperature were both determined without prior knowledge.

A new perfluorocarbon for use in fluorine-19 magnetic resonance imaging and spectroscopy

A new perfluorocarbon, PTBD (perfluoro-2,2,2',2'-tetramethyl-4,4'-bis(1,3-dioxolane)), is described for use in 19F MR imaging and spectroscopy. Two-thirds of the molecular fluorine in PTBD resonates at a single frequency and can be imaged without the use of frequency-selective spin-echo (SE) MRI pulse sequences to suppress chemical shift artifacts. The absence of strong homonuclear spin-spin coupling to the imagable -CF3 groups in PTBD minimizes signal attenuation in 19F SE MRI due to J-modulation effects. For equimolar concentrations of perfluorocarbon, PTBD gives an approximately 17% increase in sensitivity, relative to literature results for perfluorinated amines, at short values of TE (approximately 10 ms) in 19F SE MRI. These attributes allow 19F MRI of PTBD to be performed on standard clinical imaging instrumentation (without special hardware and/or software modification) and an in vivo example in a mouse is shown. This investigation involved characterizing the MR T1 and T2 relaxation times of PTBD as well as the MR spin-lattice relaxation rate, R1 (1/T1), of PTBD as a function of dissolved oxygen concentration. The T1 and T2 relaxation times and R1 relaxation rates of perfluorooctyl bromide (PFOB) were also obtained, under similar experimental conditions, to compare and contrast PTBD with a representative perfluorocarbon that has been widely employed for 19F MRI/MRS applications.

Tissue oxygenation: a novel determination using 19F surface coil NMR spectroscopy of sequestered perfluorocarbon emulsion

This work examines the variation with oxygen tension (pO2) of the individual spin-lattice relaxation times (T1) of the 19F resonances of the perfluorocarbon emulsion Oxypherol-ET (FC-43). A linear relationship between 1/T1 and pO2 has been confirmed for all four resonances at any specific temperature. Using a saturation recovery sequence, T1 has been successfully measured using surface coil NMR spectroscopy. This has facilitated measurement of T1 in vivo in a subcutaneous murine tumor. Mice were predosed with Oxypherol-ET emulsion: following complete vascular clearance of the perfluorocarbon, 19F signal was observed specifically from material sequestered in tissue, thus avoiding flow artifacts. Comparison of the pO2 estimated from each of the 19F resonances provided an internal consistency check. A pO2 = 0.1 +/- 2.2% was determined in a Meth-A murine tumor. When the mouse breathed carbogen (95% O2, 5% CO2) no significant change in tumor pO2 was detected, whereas the pO2 in the liver showed a distinct increase.

Hydrogen/fluorine retuning tomography. Applications to 1H image-guided volume-selective 19F spectroscopy and relaxometry of perfluorocarbon emulsions in tissue

A hardware modification which permits the record of 19F images, spectra, and relaxation times with a 1H tomography bird-cager resonator is described. Changing the spectrometer frequency from 1H to 19F resonance and vice versa is possible without removing the object to be investigated. This hydrogen/fluorine retuning tomography (HYFY) technique permits studies of identical slices or volume elements with 1H as well as with 19F resonance. In particular, it is possible to localize volume elements on the basis of multislice proton images and then to investigate these volume elements with fluorine magnetic resonance by the aid of volume selection methods. For this purpose, pulse sequences for the localized and spectroscopically resolved determination of spin-lattice and transverse relaxation times have been developed. The applicability of the techniques has been demonstrated by the aid of phantom samples as well as with excised porcine organs which have been perfused with perfluorocarbon emulsions.

Perfluorocarbon imaging in vivo: a 19F MRI study in tumor-bearing mice

Multiresonance perfluorocarbon emulsions (Oxypherol and Fluosol-DA) were imaged in tumor-bearing mice using 19F spin-echo magnetic resonance imaging in vivo. Multiple thin-slice fluorine images free of chemical shift artifacts were obtained in 13 minutes and these were correlated with proton images obtained during the same experiment to delineate the anatomic distribution of perfluorocarbons. Sequential images were used to determine the time course of the distribution and the retention of the compounds in tumors and organs. 19F MR spectroscopy was used ex vivo to determine with high sensitivity the relative concentration of perfluorocarbons in different tissues and organs and to confirm the results obtained from imaging experiments. The fluorine images visually demonstrated the preferential localization of the perfluorocarbons in the liver and spleen; shortly after injection, the images also revealed the highly vascularized tumor-chest wall interface. Imaging and spectroscopy together showed that the perfluorocarbons were removed from the blood pool within hours and remained sequestered in tissues at later times; the highest concentrations were found in the spleen and liver, where the agents were retained without spectral changes for the duration of these studies. The perfluorocarbons accumulated within tumors at dose-dependent concentrations, one to two orders of magnitude smaller than those observed in the spleen and liver.

Estimation of tumor oxygenation and metabolic rate using 31P MRS: correlation of longitudinal relaxation with tumor growth rate and DNA synthesis

31P MRS longitudinal relaxation times (T1) were determined for C3H murine fibrosarcomas (FSaII), and mammary carcinomas (MCaIV). Tumors were implanted in the foot dorsum, and were 100-300 mm3 in volume. T1s were repeated after the animal was allowed to breathe 100% oxygen for 30 min and then again 36-48 hr following 30 Gy. The spectrum were obtained using an 8.5 T spectrometer with a 8 cm bore and a 1.4 cm single turn antenna coil. The 31P relaxation times for untreated tumors in air breathing animals were: 3.78 sec for phosphomonoesters, 4.37 sec for inorganic phosphate (Pi), 2.73 sec for phosphocreatine, 1.37 sec for gamma ATP, 1.14 sec for alpha ATP, and 1.18 sec for beta ATP. The Pi T1s were 4.37 and 4.70 sec in control and irradiated tumors in air breathing animals. Respiration of oxygen for 30 min reduced the T1s to 3.02 and 2.62 sec in control and irradiated tumors respectively. The Pi T1 of an anoxic tumor, determined on an in situ tumor 60 min after death was 5.93 sec. The oxygen breathing induced decrease in the T1 of Pi is unlikely to have been caused by the paramagnetic properties of oxygen alone, and suggests a component of increased magnetization transfer secondary to the ATPase reaction. Oxygen breathing following 30 Gy, resulted in a decreased growth time (800 mm3 endpoint) and an increased proportion of cells in S-phase. These results support the hypothesis that the decrease in Pi T1 measured with oxygen breathing is a measure of tumor oxygen tension and metabolic rate, and suggests that T1 measurement may indirectly predict tumor growth rate and DNA synthesis.

Physical properties of a new synthetic oxygen carrier

PFC emulsions have shown considerable promise for a variety of clinical indications, but their utility has been limited by instability outside the frozen state, extended tissue residence time, and complement activation. A new formulation from Adamantech, INc. based on F-MA and egg yolk phospholipid shows significant improvement in each of these areas. The emulsion can be stored in the non-frozen state, has a tissue half-life of 8-9 days, has excellent rheological properties, and does not activate complement.

Hyperthermia-induced inhibition of respiration and mitochondrial protein denaturation in CHL cells

Respiration of Chinese hamster lung V79 cells, as assayed by O2 consumption, increases linearly from 8 to 40 degrees C when plotted in the Arrhenius fashion but is strongly inhibited above 40 degrees C. The protein of mitochondria isolated from V79 cells undergoes structural transitions at 28 and 40 degrees C. This is supported by changes in the fluorescence excitation spectrum of conjugated pyrene maleimide and, to a lesser extent, intrinsic protein fluorophores. Electron spin resonance labelling studies with a derivative of tempo maleimide imply that extensive protein unfolding coincides with the 40 degrees C transition. The structural transition at 40 degrees C correlates well with inhibition of O2 consumption, is irreversible and is probably due to protein denaturation, while the change at 28 degrees C is reversible and has no effect on O2 consumption. Previous studies indicate the presence of a broad lipid transition extending from approximately 8 to 30 degrees C in mitochondrial membranes with all lipids being in the liquid-crystalline state above 30 degrees C. Thus, the onset of the lipid transition may induce the observed protein conformational change at 28 degrees C, but inhibition of respiration above 40 degrees C can be explained by protein denaturation alone. The region from 28 to 40 degrees C of stable protein conformation corresponds to the temperature range of V79 cell growth.

Perfluorocarbon blood substitutes

The salient physicochemical properties of the fluorocarbons are briefly reviewed, including their solubility for the physiologically important gases and their properties relevant to formulation (nonmiscibility with water). The preparations used to date are described, including their properties and compositions, with some comment about the available knowledge of the properties of the constituents. A critical review of the biological aspects and the possible uses of fluorocarbon emulsions constitutes the main body of the manuscript. Gas-transporting capabilities are considered quantitatively. The biological effects of these preparations are reviewed in in vivo, whole body systems, with some in vitro evidence where appropriate. The usefulness of these preparations investigated to date are reviewed under the broad headings of cardiovascular system, radiology, intoxications, and organ preservation. Finally, the shortcomings and potential usefulness are discussed, with recommendations for potential modifications.