High vascular delivery of EGF, but low receptor binding rate is observed in AsPC-1 tumors as compared to normal pancreas.

PURPOSE: Cellular receptor targeted imaging agents present the potential to target extracellular molecular expression in cancerous lesions; however, the image contrast in vivo does not reflect the magnitude of overexpression expected from in vitro data. Here, the in vivo delivery and binding kinetics of epidermal growth factor receptor (EGFR) was determined for normal pancreas and AsPC-1 orthotopic pancreatic tumors known to overexpress EGFR. PROCEDURES: EGFR in orthotopic xenograft AsPC-1 tumors was targeted with epidermal growth factor (EGF) conjugated with IRDye800CW. The transfer rate constants (k(e), K12, k21, k23, and k32) associated with a three-compartment model describing the vascular delivery, leakage rate and binding of targeted agents were determined experimentally. The plasma excretion rate, k (e), was determined from extracted blood plasma samples. K12, k21, and k32 were determined from ex vivo tissue washing studies at time points ≥ 24 h. The measured in vivo uptake of IRDye800CW-EGF and a non-targeted tracer dye, IRDye700DX-carboxylate, injected simultaneously was used to determined k23. RESULTS: The vascular exchange of IRDye800CW-EGF in the orthotopic tumor (K12 and k21) was higher than in the AsPC-1 tumor as compared to normal pancreas, suggesting that more targeted agent can be taken up in tumor tissue. However, the cellular associated (binding) rate constant (k23) was slightly lower for AsPC-1 pancreatic tumor (4.1 × 10(-4) s(-1)) than the normal pancreas (5.5 × 10(-4) s(-1)), implying that less binding is occurring. CONCLUSIONS: Higher vascular delivery but low cellular association in the AsPC-1 tumor compared to the normal pancreas may be indicative of low receptor density due to low cellular content. This attribute of the AsPC-1 tumor may indicate one contributing cause of the difficulty in treating pancreatic tumors with cellular targeted agents.

Protoporphyrin IX fluorescence contrast in invasive glioblastomas is linearly correlated with Gd enhanced magnetic resonance image contrast but has higher diagnostic accuracy.

The sensitivity and specificity of in vivo magnetic resonance (MR) imaging is compared with production of protoporphyrin IX (PpIX), determined ex vivo, in a diffusely infiltrating glioma. A human glioma transfected with green fluorescent protein, displaying diffuse, infiltrative growth, was implanted intracranially in athymic nude mice. Image contrast from corresponding regions of interest (ROIs) in in vivo MR and ex vivo fluorescence images was quantified. It was found that all tumor groups had statistically significant PpIX fluorescence contrast and that PpIX contrast demonstrated the best predictive power for tumor presence. Contrast from gadolinium enhanced T1-weighted (T1W+Gd) and absolute T2 images positively predicted the presence of a tumor, confirmed by the GFP positive (GFP+) and hematoxylin and eosin positive (H&E+) ROIs. However, only the absolute T2 images had predictive power from controls in ROIs that were GFP+ but H&E negative. Additionally, PpIX fluorescence and T1W+Gd image contrast were linearly correlated in both the GFP+ (r = 0.79, p<1×10(-8)) and H&E+ (r = 0.74, p<0.003) ROIs. The trace diffusion images did not have predictive power or significance from controls. This study indicates that gadolinium contrast enhanced MR images can predict the presence of diffuse tumors, but PpIX fluorescence is a better predictor regardless of tumor vascularity.

Comparing implementations of magnetic-resonance-guided fluorescence molecular tomography for diagnostic classification of brain tumors.

Fluorescence molecular tomography (FMT) systems coupled to conventional imaging modalities such as magnetic resonance imaging (MRI) and computed tomography provide unique opportunities to combine data sets and improve image quality and content. Yet, the ideal approach to combine these complementary data is still not obvious. This preclinical study compares several methods for incorporating MRI spatial prior information into FMT imaging algorithms in the context of in vivo tissue diagnosis. Populations of mice inoculated with brain tumors that expressed either high or low levels of epidermal growth factor receptor (EGFR) were imaged using an EGF-bound near-infrared dye and a spectrometer-based MRI-FMT scanner. All data were spectrally unmixed to extract the dye fluorescence from the tissue autofluorescence. Methods to combine the two data sets were compared using student's t-tests and receiver operating characteristic analysis. Bulk fluorescence measurements that made up the optical imaging data set were also considered in the comparison. While most techniques were able to distinguish EGFR(+) tumors from EGFR(-) tumors and control animals, with area-under-the-curve values=1, only a handful were able to distinguish EGFR(-) tumors from controls. Bulk fluorescence spectroscopy techniques performed as well as most imaging techniques, suggesting that complex imaging algorithms may be unnecessary to diagnose EGFR status in these tissue volumes.

Imaging targeted-agent binding in vivo with two probes.

An approach to quantitatively image targeted-agent binding rate in vivo is demonstrated with dual-probe injection of both targeted and nontargeted fluorescent dyes. Images of a binding rate constant are created that reveal lower than expected uptake of epidermal growth factor in an orthotopic xenograft pancreas tumor (2.3 x 10(-5) s(-1)), as compared to the normal pancreas (3.4 x 10(-5) s(-1)). This approach allows noninvasive assessment of tumor receptor targeting in vivo to determine the expected contrast, spatial localization, and efficacy in therapeutic agent delivery.

MRI-coupled fluorescence tomography quantifies EGFR activity in brain tumors.

RATIONALE AND OBJECTIVES: This report demonstrates the diagnostic potential of magnetic resonance imaging (MRI)-coupled fluorescence molecular tomography (FMT) to determine epidermal growth factor receptor (EGFR) status in brain cancer. MATERIALS AND METHODS: Two orthotopic glioma xenograft models were used in this study: one represented high EGFR expression and the other low expression. Nude mice were inoculated with cells from either one of the tumor lines or were used in a sham surgery control group. Animals were imaged using a unique MRI-FMT scanner 48 hours after intravenous injection of a near-infrared fluorophore bound to epidermal growth factor (EGF) ligand. Coronal images of fluorescence activity of the injected dye in the mouse brain were recovered using the MRI images as anatomical templates. RESULTS: In vivo images of fluorescence activity showed significant differences between animal populations, an observation confirmed by receiver operating characteristic analysis that revealed 100% sensitivity and specificity between animal groups implanted with EGFR((+)) and EGFR((-)) tumor lines. Similar performance was observed between EGFR((+)) and sham surgery control animals. CONCLUSIONS: This preclinical study suggests that MRI-FMT with fluorescent EGF provides excellent discrimination between tumors based on EGFR status. Reliable quantification of receptor status using minimally invasive techniques would be an important innovation for investigating new and existing cancer treatments that target these cellular mechanisms in research animals, and may be applied to identify receptor amplification in human brain cancer patients. This study represents the first systematic multianimal validation of receptor-specific imaging using MRI-guided fluorescence tomography.

Detecting epidermal growth factor receptor tumor activity in vivo during cetuximab therapy of murine gliomas.

RATIONALE AND OBJECTIVES: Noninvasive molecular imaging of glioma tumor receptor activity was assessed with diagnostic in vivo fluorescence monitoring during targeted therapy. The study goals were to assess the range of use for treatment monitoring and stratification of tumor types using epidermal growth factor (EGF) receptor (EGFR) status with administration of fluorescently labeled EGF and determine its utility for tumor detection compared to magnetic resonance imaging (MRI). MATERIALS AND METHODS: EGFR+ and EGFR- glioma tumor lines (human glioma [U251-GFP] and rat gliosarcoma [9L-GFP], respectively) were used to assess these goals, having a 20-fold difference between their EGF uptakes. RESULTS: Treatment with cetuximab in the EGFR+ tumor-bearing animals led to decreased EGF tumor uptake, whereas for the EGFR- tumors, no change in fluorescence signal followed treatment. This diagnostic difference in EGFR expression could be used to stratify the tumor-bearing animals into groups of potential responders and nonresponders, and receiver-operating characteristic curve analysis revealed an area under the curve (AUC) of 0.92 in separating these tumors. The nonlocalized growth pattern of U251-GFP tumors resulted in detection difficulty on standard MRI, but high EGFR expression made them detectable by fluorescence imaging (AUC = 1.0). The EGFR+ U251-GFP tumor-bearing animals could be noninvasively stratified into treated and untreated groups on the basis of fluorescence intensity difference (P = .035, AUC = 0.90). CONCLUSIONS: EGFR expression was tracked in vivo with fluorescence and determined to be of use for the stratification of EGFR+ and EGFR- tumors, the detection of EGFR+ tumors, and monitoring of molecular therapy.

Imaging tumor variation in response to photodynamic therapy in pancreatic cancer xenograft models.

PURPOSE: A treatment monitoring study investigated the differential effects of orthotopic pancreatic cancer models in response to interstitial photodynamic therapy (PDT), and the validity of using magnetic resonance imaging as a surrogate measure of response was assessed. METHODS AND MATERIALS: Different orthotopic pancreatic cancer xenograft models (AsPC-1 and Panc-1) were used to represent the range of pathophysiology observed in human beings. Identical dose escalation studies (10, 20, and 40J/cm) using interstitial verteporfin PDT were performed, and magnetic resonance imaging with T2-weighted and T1-weighted contrast were used to monitor the total tumor volume and the vascular perfusion volume, respectively. RESULTS: There was a significant amount of necrosis in the slower-growing Panc-1 tumor using high light dose, although complete necrosis was not observed. Lower doses were required for the same level of tumor kill in the faster-growing AsPC-1 cell line. CONCLUSIONS: The tumor growth rate and vascular pattern of the tumor affect the optimal PDT treatment regimen, with faster-growing tumors being relatively easier to treat. This highlights the fact that therapy in human beings shows a heterogeneous range of outcomes, and suggests a need for careful individualized treatment outcomes assessment in clinical work.

Deferoxamine iron chelation increases delta-aminolevulinic acid induced protoporphyrin IX in xenograft glioma model.

Exogenous administration of delta-aminolevulinic acid (delta-ALA) leads to selective accumulation of protoporphyrin IX (PpIX) in brain tumors, and has shown promising results in increasing extent of resection in fluorescence-guided resection (FGR) of brain tumors. However, this approach still suffers from heterogeneous staining and so some tumor margins may go undetected because of this variation in PpIX production. The aim of this study was to test the hypothesis that iron chelation therapy could increase the level of fluorescence in malignant glioma tumors. Mice implanted with xenograft U251-GFP glioma tumor cells were given a 200 mg kg(-1) dose of deferoxamine (DFO), once a day for 3 days prior to delta-ALA administration. The PpIX fluorescence observed in the tumor regions was 1.9 times the background in animal group without DFO, and 2.9 times the background on average, in the DFO pre-treated group. A 50% increase in PpIX fluorescence contrast in the DFO group was observed relative to the control group (t-test P-value = 0.0020). These results indicate that iron chelation therapy could significantly increase delta-ALA-induced PpIX fluorescence in malignant gliomas, pointing to a potential role of iron chelation therapy for more effective FGR of brain tumors.

Imaging of glioma tumor with endogenous fluorescence tomography.

Tomographic imaging of a glioma tumor with endogenous fluorescence is demonstrated using a noncontact single-photon counting fan-beam acquisition system interfaced with microCT imaging. The fluorescence from protoporphyrin IX (PpIX) was found to be detectable, and allowed imaging of the tumor from within the cranium, even though the tumor presence was not visible in the microCT image. The combination of single-photon counting detection and normalized fluorescence to transmission detection at each channel allowed robust imaging of the signal. This demonstrated use of endogenous fluorescence stimulation from aminolevulinic acid (ALA) and provides the first in vivo demonstration of deep tissue tomographic imaging with protoporphyrin IX.

Diagnostic detection of diffuse glioma tumors in vive with molecular fluorescent probe-based transmission spectroscopy.

The diffuse spread of glioma tumors leads to the inability to image and properly treat this disease. The optical spectral signature of targeted fluorescent probes provides molecular signals from the diffuse morphologies of glioma tumors, which can be a more effective diagnostic probe than standard morphology-based magnetic resonance imaging (MRI) sequences. Three orthotopic xenograft glioma models were used to examine the potential for transmitted optical fluorescence signal detection in vivo, using endogenously produced protoporphyrin IX (PpIX) and exogenously administered fluorescently labeled epidermal growth factor (EGF). Accurate quantification of the fluorescent signals required spectral filtering and signal normalization, and when optimized, it was possible to improve detection of sparse diffuse glioma tumor morphologies. The signal of endogenously produced PpIX provided similar sensitivity and specificity to MRI, while detection with fluorescently labeled EGF provided maximal specificity for tumors with high EGF receptor activity. Optical transmitted fluorescent signal may add significant benefit for clinical cases of diffuse infiltrative growth pattern glioma tumors given sufficient optimization of the signal acquisition for each patient.

Quantitative imaging of scattering changes associated with epithelial proliferation, necrosis, and fibrosis in tumors using microsampling reflectance spectroscopy.

Highly localized reflectance measurements can be used to directly quantify scatter changes in tissues. We present a microsampling approach that is used to raster scan tumors to extract parameters believed to be related to the tissue ultrastructure. A confocal reflectance imager was developed to examine scatter changes across pathologically distinct regions within tumor tissues. Tissue sections from two murine tumors, AsPC-1 pancreas tumor and the Mat-LyLu Dunning prostate tumor, were imaged. After imaging, histopathology-guided region-of-interest studies of the images allowed analysis of the variations in scattering resulting from differences in tissue ultra-structure. On average, the median scatter power of tumor cells with high proliferation index (HPI) was about 26% less compared to tumor cells with low proliferation index (LPI). Necrosis exhibited the lowest scatter power signature across all the tissue types considered, with about 55% lower median scatter power than LPI tumor cells. Additionally, the level and maturity of the tumor's fibroplastic response was found to influence the scatter signal. This approach to scatter visualization of tissue ultrastructure in situ could provide a unique tool for guiding surgical resection, but this kind of interpretation into what the signal means relative to the pathology is required before proceeding to clinical studies.

Noninvasive measurement of aminolevulinic acid-induced protoporphyrin IX fluorescence allowing detection of murine glioma in vivo.

Aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) fluorescence is studied as a contrast agent for noninvasive detection of murine glioma, using the fluorescence-to-transmission ratio measured through the cranium. Signals measured prior to administration of ALA are very similar between control animals, 9L-GFP, and U251 tumor-bearing animals. However, 2 h after ALA administration, the PpIX signal from both tumor-bearing groups is significantly higher than the control group (9L-GFP group p-value=0.016, and U251 group p-value=0.004, relative to the control group). The variance in signal from the 9L-GFP group is much larger than either the control group or the U251 group, which is consistent with higher intrinsic PpIX fluorescence heterogeneity as seen in situ at ex vivo analysis. Decreasing the skin PpIX fluorescence via intentional photobleaching using red light (635 nm) is examined as a tool for increasing PpIX contrast between the tumor-bearing and control groups. The red light bleaching is found to increase the ability to accurately quantify PpIX fluorescence in vivo, but decreases the specificity of detection between tumor-bearing and nontumor-bearing groups.

Protoporphyrin IX level correlates with number of mitochondria, but increase in production correlates with tumor cell size.

Protoporphyrin IX (PpIX) is produced in cells via the heme synthesis pathway, from the substrate aminolevulinic acid (ALA), and can be used for tumor detection, monitoring or photodynamic therapy. PpIX production varies considerably between tumor cell types, and determining the cell types and methods to optimize production is a central issue in properly utilizing this drug. A panel of eight cancer cell types was examined for PpIX production capacity, including breast, prostate, and brain cancer tumors, and the production varied up to 10-fold among cell types. A positive correlation was seen between mitochondrial content and naturally occurring PpIX prior to ALA administration, but mitochondrial content did not correlate to the yield of PpIX resulting from the addition of ALA. Interestingly, total cell size was positively correlated to the yield of PpIX from ALA administration. Addition of an iron chelator, 1,2-dimethyl-3-hydroxy-4-pyridone (L1) in combination with ALA allows the final step in the heme synthesis pathway, conversion of PpIX to heme, to be delayed, thereby further increasing the yield of PpIX. Those cell types that had the lowest ALA to PpIX production without L1 showed the largest percentage increase in production with L1. The study indicates that use of L1 in tumors with a lower innate production of PpIX with ALA alone may be the most productive approach to this combined delivery.

Increased oxygenation of intracranial tumors by efaproxyn (efaproxiral), an allosteric hemoglobin modifier: In vivo EPR oximetry study.

PURPOSE: To determine quantitatively the changes in oxygenation of intracranial tumors induced by efaproxiral, an allosteric hemoglobin modifier. Efaproxiral reduces hemoglobin-oxygen binding affinity, which facilitates oxygen release from hemoglobin into surrounding tissues and potentially increases the pO(2) of the tumors. METHODS AND MATERIALS: The study was performed on 10 male Fisher 344 rats with 9L intracranial tumors. Electron paramagnetic resonance (EPR) oximetry was used to measure quantitatively the changes in the pO(2) in the tumors. Lithium phthalocyanine (LiPc) crystals were implanted in the tumors and in the normal brain tissue in the opposite hemispheres. We monitored the cerebral pO(2) starting 7 to 10 days after the tumor cells were implanted. NMR imaging determined the position and size of tumor in the brain. After an initial baseline EPR measurement, efaproxiral (150 mg/kg) was injected intravenously over 15 minutes, and measurements of tumor and normal brain oxygen tension were made alternately at 10-minute intervals for the next 60 minutes; the procedure was repeated for 6 consecutive days. RESULTS: Efaproxiral significantly increased the pO(2) of both the intracranial tumors and the normal brain tissue on all days. The maximum increase was reached at 52.9 to 59.7 minutes and 54.1 to 63.2 minutes after injection, respectively. The pO(2) returned to baseline values at 106 to 126.5 minutes after treatment. The maximum tumor and normal tissue pO(2) values achieved after efaproxiral treatment from Day 1 through Day 6 ranged from 139.7 to 197.7 mm Hg and 103.0 to 135.9 mm Hg, respectively. The maximum increase in tumor tissue pO(2) values from Day 2 to Day 5 was greater than the maximum increase in normal tissue pO(2). CONCLUSION: We obtained quantitative data on the timing and extent of efaproxiral-induced changes in the pO(2) of intracerebral 9L tumors. These results illustrate a unique and useful capability of in vivo EPR oximetry to obtain repeated noninvasive measurements of tumor oxygenation over a number of days. The information on the dynamics of tumor pO(2) after efaproxiral administration illustrates the ability of efaproxiral to increase intracranial tumor oxygenation.

Simultaneous measurement of rat brain cortex PtO2 using EPR oximetry and a fluorescence fiber-optic sensor during normoxia and hyperoxia.

Electron paramagnetic resonance (EPR) oximetry is a promising, relatively non-invasive method of monitoring tissue partial pressure of oxygen (PtO(2)) that has proven useful in following changes in PtO(2) under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here, we report on the absolute values and changes of rat brain PtO(2) using EPR oximetry and the OxyLite, an oxygen monitor based on fluorescence quenching, at adjacent locations in the same brain. EPR oximetry utilizes an implanted oxygen-sensitive material and reports tissue PtO(2) at the surface of the material. OxyLite measures PtO(2) using the fluorescence lifetime of a chromophore fixed to the tip of an optical fiber that is inserted into tissue. Measurements were made at a depth of 2-3 mm into the cortex during normoxia and during breathing of carbogen (95% O(2):5% CO(2)) followed by a return to normoxia. We conclude that in this study (1) PtO(2) values reported by the two methods are similar but not exactly the same, (2) both methods can record a baseline and rapid changes in PtO(2), (3) changes in PtO(2) induced by increasing FiO(2) from 0.26 to 0.95 (carbogen) were similar by the two methods and (4) in some rats breathing carbogen, absolute values of PtO(2) were above the sensitive range of the OxyLite method.

Effect of tumor host microenvironment on photodynamic therapy in a rat prostate tumor model.

PURPOSE: Tumor host microenvironment plays an important role in tumor growth, metastasis, and response to cancer therapy. In this study, the influence of tumor host environment on tumor pathophysiology, photosensitizer distribution, and photodynamic therapy (PDT) treatment effect was examined in the metastatic at lymph node and lung (MatLyLu) rat prostate tumor. EXPERIMENTAL DESIGN: MatLyLu tumors implanted in different host environment [i.e., orthotopically (in the prostate) or s.c.] were compared for difference in vessel density, average vessel size, vascular permeability, tumor vascular endothelial growth factor production, and tumor oxygenation. Uptake of photosensitizer verteporfin in tumors in both sites was determined by fluorescence microscopy. To compare tumor response to PDT, both orthotopic and s.c. MatLyLu tumors were given the same doses of verteporfin and laser light treatment, and PDT-induced tumor necrotic area was measured histologically. RESULTS: Orthotopic MatLyLu tumors were found to grow faster, have higher vessel density and more permeable vasculature, have higher vascular endothelial growth factor protein levels, and have lower tumor hypoxic fraction than the s.c. tumors. Uptake of photosensitizer verteporfin in the orthotopic tumor was higher than in the s.c. tumors at 15 minutes after injection (1 mg/kg, i.v.), and became similar at 3 hours after injection. For the vascular targeting PDT treatment (0.25 mg/kg verteporfin, 50 J/cm(2) at 50 mW/cm(2), 15 minutes drug-light interval), there was no significant difference in PDT-induced tumor necrotic area between the orthotopic and s.c. tumors, with 85% to 90% necrosis in both types of tumors. However, tumor necrosis induced by the cellular targeting PDT (1 mg/kg verteporfin, 50 J/cm(2) at 50 mW/cm(2), 3 hours drug-light interval) was significantly different in the orthotopic (64%) versus the s.c. (29%) tumors. CONCLUSIONS: Tumor host environment can significantly affect photosensitizer verteporfin distribution and PDT treatment effect. Verteporfin-PDT regimen targeting tumor cells is more sensitive to such influence than the vascular targeting PDT. Our study showed the importance of tumor host environment in determining tumor physiologic properties and tumor response to PDT. To obtain clinically relevant information, orthotopic tumor model should be used in the experimental studies.

Effect of RSR13, an allosteric hemoglobin modifier, on oxygenation in murine tumors: an in vivo electron paramagnetic resonance oximetry and bold MRI study.

PURPOSE: RSR13, an allosteric modifier of hemoglobin, reduces hemoglobin-oxygen binding affinity facilitating oxygen release from hemoglobin, resulting in increases in tissue pO(2). The purpose of this study was noninvasively to monitor the time course and effect of RSR13 on tumor oxygenation, directly using in vivo electron paramagnetic resonance (EPR oximetry), and indirectly using blood oxygen level dependent magnetic resonance imaging (BOLD MRI). METHODS AND MATERIALS: The study was performed in transplanted radiation-induced fibrosarcoma tumors (RIF-1) in 18 female C3H/HEJ mice, which had two lithium phthalocyanine (LiPc) deposits implanted in the tumor when the tumors reached about 200-600 mm(3). Baseline EPR measurements were made daily for 3 days. Then, for 6 consecutive days and after an initial baseline EPR measurement, RSR13 (150 mg/kg) or vehicle (same volume) was injected intraperitoneally, and measurements of intratumoral oxygen were made at 10-min intervals for the next 60 min. In each mouse, every third day, instead of EPR oximetry, BOLD MRI measurements were made for 60 min after administration of the RSR13. RESULTS: Based on EPR measurements, RSR13 produced statistically significant temporal increases in tumor pO(2) over the 60-min time course, which reached a maximum at 35-43 min postdose. The average time required to return to the baseline pO(2) was 70-85 min. The maximum increase in tumor tissue pO(2) values after RSR13 treatment from Day 1 to Day 5 (8.3-12.4 mm Hg) was greater than the maximum tumor tissue pO(2) value for Day 6 (4.7 mm Hg, p < 0.01). The maximum increase in pO(2) occurred on Day 2 (12.4 mm Hg) after RSR13 treatment. There was little change in R(2)*, indicating that the RSR13 had minimal detectable effects on total deoxyhemoglobin and hemoglobin-oxygen saturation. CONCLUSION: The extent of the increase in tumor pO(2) achieved by RSR13 would be expected to lead to a significant increase in the effectiveness of tumor radiotherapy. The lack of a change in the BOLD MRI signal suggests that the tumor physiology was largely unchanged by RSR13. These results illustrate a unique and useful capability of in vivo EPR oximetry and BOLD MRI to obtain repeated measurements of tumor oxygenation and physiology. The dynamics of tumor pO(2) after RSR13 administration may be useful for the design of clinical protocols using allosteric hemoglobin effectors.

Comparison of EPR oximetry and Eppendorf polarographic electrode assessments of rat brain PtO2.

EPR oximetry is a promising, relatively non-invasive method for monitoring the partial pressure of oxygen in tissue (PtO2) that has proved useful in following changes under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here we report on the absolute values of rat brain PtO2 using EPR and the more widely used Eppendorf polarographic microelectrode system in the same brain. EPR used an L-band (1.2 GHz) spectrometer and implanted lithium phthalocyanine (LiPc) as the oxygen-sensitive paramagnetic material. Eppendorf measurements were made by a needle probe moved vertically through the cortex at 0.5 mm intervals in three tracks including one adjacent to the location of the LiPc. Several conclusions were drawn, including, (1) the average PtO2 measured by the two methods was similar but EPR reported a significantly higher average PtO2, (2) there was poor correlation between the values in the same animal on the same side of the brain, (3) the Eppendorf reported a larger range of values and (4) the heterogeneity of oxygen levels in the brain and the areas sampled by the two methods provide an adequate explanation for the observed differences.

Tumor pO2 assessments in human xenograft tumors measured by EPR oximetry: location of paramagnetic materials.

Radioantibody immunotherapy (RAIT) is a promising treatment modality but the effectiveness of this targeted low dose radiation varies from tumor to tumor. Since RAIT is an oxygen dependent treatment, baseline pO2 or growth-induced changes in the microenvironment may alter treatment response. In this pilot work we monitored tumor pO2 in untreated human xenograft tumors growing s.c. in nude mice. These data will be used to plan a study of the relationship between the effectiveness of RAIT and tumor pO2. Growth or treatment-induced changes in the microenvironment may alter the tumor pO2 and thus affect the response to therapy but may also affect location and microenvironment of the particulate oxygen sensor. We monitored tumor pO2 during growth and also examined the tumor histological structure overall and in the region of the paramagnetic material in the tumor at the time of necropsy.