Prognostic radiology: quantitative assessment of tumor oxygen dynamics by MRI

Functional Imaging of Hypoxia: PET and MRI

Molecular and functional imaging have critical roles in cancer care. Existing evidence suggests that noninvasive detection of hypoxia within a particular type of cancer can provide new information regarding the relationship between hypoxia, cancer aggressiveness and altered therapeutic responses. Following the identification of hypoxia inducible factor (HIF), significant progress in understanding the regulation of hypoxia-induced genes has been made. These advances have provided the ability to therapeutically target HIF and tumor-associated hypoxia. Therefore, by utilizing the molecular basis of hypoxia, hypoxia-based theranostic strategies are in the process of being developed which will further personalize care for cancer patients. The aim of this review is to provide an overview of the significance of tumor hypoxia and its relevance in cancer management as well as to lay out the role of imaging in detecting hypoxia within the context of cancer.

Oxygen-Enhanced MRI Accurately Identifies, Quantifies, and Maps Tumor Hypoxia in Preclinical Cancer Models

There is a clinical need for noninvasive biomarkers of tumor hypoxia for prognostic and predictive studies, radiotherapy planning, and therapy monitoring. Oxygen-enhanced MRI (OE-MRI) is an emerging imaging technique for quantifying the spatial distribution and extent of tumor oxygen delivery in vivo. In OE-MRI, the longitudinal relaxation rate of protons (ΔR1) changes in proportion to the concentration of molecular oxygen dissolved in plasma or interstitial tissue fluid. Therefore, well-oxygenated tissues show positive ΔR1. We hypothesized that the fraction of tumor tissue refractory to oxygen challenge (lack of positive ΔR1, termed "Oxy-R fraction") would be a robust biomarker of hypoxia in models with varying vascular and hypoxic features. Here, we demonstrate that OE-MRI signals are accurate, precise, and sensitive to changes in tumor pO2 in highly vascular 786-0 renal cancer xenografts. Furthermore, we show that Oxy-R fraction can quantify the hypoxic fraction in multiple models with differing hypoxic and vascular phenotypes, when used in combination with measurements of tumor perfusion. Finally, Oxy-R fraction can detect dynamic changes in hypoxia induced by the vasomodulator agent hydralazine. In contrast, more conventional biomarkers of hypoxia (derived from blood oxygenation-level dependent MRI and dynamic contrast-enhanced MRI) did not relate to tumor hypoxia consistently. Our results show that the Oxy-R fraction accurately quantifies tumor hypoxia noninvasively and is immediately translatable to the clinic.

Principal component analysis enhances SNR for dynamic electron paramagnetic resonance oxygen imaging of cycling hypoxia in vivo

PURPOSE

Low oxygen concentration (hypoxia) in tumors strongly affects their malignant state and resistance to therapy. These effects may be more deleterious in regions undergoing cycling hypoxia. Electron paramagnetic resonance imaging (EPRI) has provided a noninvasive, quantitative imaging modality to investigate static pO2 in vivo. However, to image changing hypoxia, EPRI images with better temporal resolution may be required. The tradeoff between temporal resolution and signal-to-noise ratio (SNR) results in lower SNR for EPRI images with imaging time short enough to resolve cycling hypoxia.

METHODS

Principal component analysis allows for accelerated image acquisition with acceptable SNR by filtering noise in projection data, from which pO2 images are reconstructed. Principal component analysis is used as a denoising technique by including only low-order components to approximate the EPRI projection data.

RESULTS

Simulated and experimental studies show that principal component analysis filtering increases SNR, particularly for small numbers of sub-volumes with changing pO2 , enabling an order of magnitude increase in temporal resolution with minimal deterioration in spatial resolution or image quality.

CONCLUSION

The SNR necessary for dynamic EPRI studies with temporal resolution required to investigate cycling hypoxia and its physiological implications is enabled by principal component analysis filtering.

On the structural modification of 2-nitroimidazole-(99m)Tc(CO)(3) complex, a hypoxia marker, for improving in vivo pharmacokinetics

INTRODUCTION

A 2-nitroimidazole-(99m)Tc(CO)(3) complex reported earlier showed promise with respect to its uptake and retention in hypoxic tumor. However, significant uptake and slow clearance from liver imposed severe limitations towards advocating its possible practical utility. In an attempt to improving its liver clearance, an ether linkage, which is known to help in liver clearance, was introduced in the molecule.

METHODS

The 2-nitroimidazole iminodiacetic acid (IDA) derivative containing an ether linkage was synthesized in a five step procedure from 2-nitroimidazole. This derivative was radiolabeled using [(99m)Tc(CO)(3)(H(2)O)(3)](+) precursor complex. The corresponding Re(CO)(3) analogue was also synthesized in the macroscopic level for structural characterization. The (99m)Tc(CO)(3) complex was evaluated in an animal model bearing fibrosarcoma tumor.

RESULTS

The in vivo evaluation of the complex indicated that, as envisaged, introduction of the ether linkage has improved clearance from the liver. The complex also showed higher retention in tumor compared to the 2-nitroimidazole-IDA-(99m)Tc(CO)(3) complex reported earlier. Though the tumor to muscle ratio improved with time, the tumor to blood ratio did not show any significant improvement. Despite improved liver clearance, there was significant liver activity present even at 3h p.i. attributable to gradual accumulation of activity cleared from muscle and blood.

CONCLUSIONS

Though the introduction of ether linkage improved liver clearance of the modified 2-nitroimidazole complex, it was found that a single ether linkage was not sufficient to achieve the desirable level of clearance. Probably, a linker with multiple ether groups, such as a di- or tri-ethylene glycol spacer, may be a possible solution to this issue.

Hexafluorobenzene in comparison with perfluoro-15-crown-5-ether for repeated monitoring of oxygenation using 19F MRI in a mouse model

Hexafluorobenzene (HFB) and perfluoro-15-crown-5-ether (15C5) were compared as fluorine reporter probes of tissue oxygenation using (19)F MRI for dynamic assessment of muscle oxygenation, with special focus on muscle tissue toxicity of the probes, and consecutive alteration of animal behavior. The latter were also compared in terms of sensitivity to changes in oxygenation as well as of signal-to-noise ratio for accurate pO(2) measurements. For that purpose, mouse muscles were imaged at 11.7 T, at 2- and 36-h after intramuscular injection of HFB or 15C5. Histological analysis of the muscle tissue revealed a lack of toxicity for 15C5 from 2 up to 36-h postinjection, whereas HFB induced tissue necrosis, blood clots and thrombosis as soon as 24-h postinjection. This muscle toxicity led to a limitation in mice mobility 24-h after injection of HFB as evidenced by behavioral testing (open-field, grip strength, and catwalk tests), which was not the case after 15C5 intramuscular injection. Finally, pO(2) measurements assessed 2-h postinjection showed consistent values with both probes, evidencing cross-validation of the (19)F MRI oximetry technique for acute measurements. However, the measurement at 36-h was hampered for HFB, which showed significant lower values of muscle pO(2), whereas 15C5 was able to reliably assess muscle pO(2) at 36-h postinjection.

Quantitative tissue oxygen measurement in multiple organs using 19F MRI in a rat model

Measurement of individual organ tissue oxygen levels can provide information to help evaluate and optimize medical interventions in many areas including wound healing, resuscitation strategies, and cancer therapeutics. Echo planar (19) F MRI has previously focused on tumor oxygen measurement at low oxygen levels (pO(2)) <30 mmHg. It uses the linear relationship between spin-lattice relaxation rate (R(1)) of hexafluorobenzene (HFB) and pO(2). The feasibility of this technique for a wider range of pO(2) values and individual organ tissue pO(2) measurement was investigated in a rat model. Spin-lattice relaxation times (T(1) = 1/R(1)) of hexafluorobenzene were measured using (19) F saturation recovery echo planar imaging. Initial in vitro studies validated the linear relationship between R(1) and pO(2) from 0 to 760 mmHg oxygen partial pressure at 25, 37, and 41°C at 7 Tesla for hexafluorobenzene. In vivo experiments measured rat tissue oxygen (ptO2) levels of brain, kidney, liver, gut, muscle, and skin during inhalation of both 30 and 100% oxygen. All organ ptO(2) values significantly increased with hyperoxia (P < 0.001). This study demonstrates that (19) F MRI of hexafluorobenzene offers a feasible tool to measure regional ptO2 in vivo, and that hyperoxia significantly increases ptO2 of multiple organs in a rat model.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

Quantitative magnetic resonance fluorine imaging: today and tomorrow

Fluorine (19F) is a promising moiety for quantitative magnetic resonance imaging (MRI). It possesses comparable magnetic resonance (MR) sensitivity to proton (1H) but exhibits no tissue background signal, allowing specific and selective assessment of the administrated 19F-containing compounds in vivo. Additionally, the MR spectra of 19F-containing compounds exhibited a wide range of chemical shifts (>200 ppm). Therefore, both MR parameters (e.g., spin-lattice relaxation rate R1) and the absolute quantity of molecule can be determined with 19F MRI for unbiased assessment of tissue physiology and pathology. This article reviews quantitative 19F MRI applications for mapping tumor oxygenation, assessing molecular expression in vascular diseases, and tracking labeled stem cells.

An oxygen-consuming phantom simulating perfused tissue to explore oxygen dynamics and (19)F MRI oximetry

OBJECTIVE

This study presents a reproducible phantom which mimics oxygen-consuming tissue and can be used for the validation of (19)F MRI oximetry.

MATERIALS AND METHODS

The phantom consists of a haemodialysis filter of which the outer compartment is filled with a gelatin matrix containing viable yeast cells. Perfluorocarbon emulsions can be added to the gelatin matrix to simulate sequestered perfluorocarbons. A blood-substituting perfluorocarbon fluid is pumped through the lumen of the fibres in the filter. (19)F relaxometry MRI is performed with a fast 2D Look-Locker imaging sequence on a clinical 3T scanner.

RESULTS

Acute and perfusion-related hypoxia were simulated and imaged spatially and temporally using the phantom.

CONCLUSIONS

The presented experimental setup can be used to simulate oxygen consumption by somatic cells in vivo and for validating computational biophysical models of hypoxia, as measured with (19)F MRI oximetry.

Fluorocarbon nanoparticles as multifunctional drug delivery vehicles

The history and current status of fluorocarbon nanoparticles in biomedicine is briefly reviewed. The deficiencies of current fluorocarbon nanoparticle formulations are highlighted. Strategies to remedy such deficiencies and to functionalize fluorocarbon nanoparticles are presented. Potential applications of fluorocarbon nanoparticles as multifunctional drug delivery vehicles are discussed. The strength of fluorocarbon nanoparticles as drug delivery vehicles is that they integrate drug delivery with non-invasive MR imaging so that the biodistribution of the pharmaceutical entity (drug + delivery vehicle) can be monitored in real time. This, in turn, permits the physician to adjust treatment plan for each patient based on his/her actual response to the ongoing treatment.

Molecular imaging of hypoxia

Hypoxia, a condition of insufficient O2 to support metabolism, occurs when the vascular supply is interrupted, as in stroke or myocardial infarction, or when a tumor outgrows its vascular supply. When otherwise healthy tissues lose their O2 supply acutely, the cells usually die, whereas when cells gradually become hypoxic, they adapt by up-regulating the production of numerous proteins that promote their survival. These proteins slow the rate of growth, switch the mitochondria to glycolysis, stimulate growth of new vasculature, inhibit apoptosis, and promote metastatic spread. The consequence of these changes is that patients with hypoxic tumors invariably experience poor outcome to treatment. This has led the molecular imaging community to develop assays for hypoxia in patients, including regional measurements from O2 electrodes placed under CT guidance, several nuclear medicine approaches with imaging agents that accumulate with an inverse relationship to O2, MRI methods that measure either oxygenation directly or lactate production as a consequence of hypoxia, and optical methods with NIR and bioluminescence. The advantages and disadvantages of these approaches are reviewed, along with the individual strategies for validating different imaging methods. Ultimately the proof of value is in the clinical performance to predict outcome, select an appropriate cohort of patients to benefit from a hypoxia-directed treatment, or plan radiation fields that result in better local control. Hypoxia imaging in support of molecular medicine has become an important success story over the last decade and provides a model and some important lessons for development of new molecular imaging probes or techniques.

Use of 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl as an EPR oximetry probe: Potential for in vivo measurement of tissue oxygenation in mouse brain

Measurement of oxygen concentration and distribution in the brain is essential for understanding the pathophysiology of stroke. Low-frequency electron paramagnetic resonance (EPR) spectroscopy with a paramagnetic probe is an attractive imaging modality that potentially can be used to map O(2) concentration in the brain. We examined two nitroxides, 3-methoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [2] and 3-acetoxymethoxycarbonyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [3], as pro-imaging agents to deliver 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl [1] across the blood-brain barrier (BBB). In primary cultured neurons, nitroxide [3] but not [2] was hydrolyzed by intracellular esterases to [1], which, being anionic at physiologic pH, was well retained intracellularly. In contrast, [2] was not well retained by neurons. In vivo pharmacokinetic and pharmacodynamic studies in mice suggested that esterase-labile nitroxide [3] crossed the BBB, and was converted to [1] and retained. Retention occurred in brain tissue and not in the extensive vasculature, as evidenced by the fact that removal of blood by whole-body saline perfusion did not eliminate the nitroxide EPR signal from the brain. The EPR linewidths of [1] and [3] were more O(2)-sensitive than that of the commonly-used oximetry probe 4-oxo-2,2,6,6-tetramethylpiperidine-d(16)-1-(15)N-oxyl [4]. Moreover, we used [3] in vivo to estimate O(2) concentration in mouse brains. These results indicate that nitroxide [3] could be useful for mapping O(2) distribution in the brain following stroke.

Tissue oxygen tension measurements in the Shionogi model of prostate cancer using 19F MRS and MRI

OBJECTIVES

To investigate changes in tumour tissue oxygenation throughout the tumour growth-regression-relapse cycle in an androgen-dependent animal tumour model.

MATERIALS AND METHODS

19F T1 relaxometry of Perfluoro-15-Crown-5-Ether was used to measure in vivo partial oxygen pressure (pO2) of Shionogi tumours on a 2.35-T MR scanner. Perfluoro-15-Crown-5-Ether was administered as an emulsion injected intravenously or as a neat compound injected directly into the tumour. Non-localized, tumour 19F T1 measurements, made at multiple time points throughout the tumour cycle, were translated into pO2 levels.

RESULTS

No correlation between tumour size and pO2 values was found. Values of pO2 for growing tumours (50 +/- 30 torr) were significantly lower than for regressing and relapsing tumours after 9 days post-castration (70 +/- 10 torr, p<0.05). Maximum pO2 values (90 +/- 30 torr) were reached between fifth and eighth day post-castration, when tumour pO2 was significantly higher than both pre-castration (p<0.001) and after 9 days post-castration (p<0.05).

CONCLUSION

We demonstrate that longitudinal pO2 measurements in vivo are feasible. Values of pO2 for growing androgen-dependent tumours were significantly lower than for regressing and relapsing androgen-independent tumours. These results have potential clinical importance in optimizing the timing of chemotherapy and/or radiotherapy of hormone dependent tumours.

In vivo determination of tumor oxygenation during growth and in response to carbogen breathing using 15C5-loaded alginate capsules as fluorine-19 magnetic resonance imaging oxygen sensors

PURPOSE

The objective was to present a method for the repeated noninvasive measurement of tumor oxygenation (Po(2)) over the whole period of tumor growth.

METHODS AND MATERIALS

A mixture of tumor homogenate (GH3 prolactinoma) and alginate capsules loaded with perfluoro-15-crown-5-ether (15C5) was injected into the flanks of Wistar Furth rats. The temporal behavior of tumor Po(2) was monitored between Day 1 and 26 after injection using fluorine-19 ((19)F) magnetic resonance imaging (MRI). In addition, the response of tumor Po(2) to modifiers of the tumor microenvironment (carbogen [95% O(2)/5% CO(2)], nicotinamide, and hydralazine) was investigated.

RESULTS

An initial increase of tumor Po(2), probably reflecting neovascularization, followed by a decrease after Week 2, probably indicating tumor hypoxia or necrosis, were observed. The minimum and maximum average Po(2) +/- SEM observed were 3.3 +/- 2.0 mm Hg on Day 2 and 25.7 +/- 3.8 mm Hg on Day 13, respectively. Carbogen increased the tumor Po(2), whereas nicotinamide caused no significant change and hydralazine induced a significant decrease in tumor oxygenation.

CONCLUSIONS

A preclinical method for the repeated noninvasive determination of tumor Po(2) was presented. It might help to investigate tumor physiology and the mechanisms of modifiers of the tumor microenvironment and their role in different therapeutic approaches.

Tumor oxygen dynamics: correlation of in vivo MRI with histological findings

Tumor oxygenation has long been recognized as a significant factor influencing cancer therapy. We recently established a novel magnetic resonance in vivo approach to measuring regional tumor oxygen tension, FREDOM (Fluorocarbon Relaxometry Using Echo Planar Imaging for Dynamic Oxygen Mapping), using hexafluorobenzene (HFB) as the reporter molecule. We have now investigated oxygen dynamics in the two Dunning prostate R3327 rat tumor sublines, AT1 and H. FREDOM revealed considerable intratumoral heterogeneity in the distribution of pO(2) values in both sublines. The anaplastic faster-growing AT1 tumors were more hypoxic compared with the size-matched, well-differentiated, and slower-growing H tumors. Respiratory challenge with oxygen produced significant increases in mean and median pO(2) in all the H tumors (P<.001), but no response in half of the larger AT1 tumors (>3 cm(3)). Immunohistochemical studies using the hypoxia marker, pimonidazole, and the vascular endothelial cell marker, CD31, confirmed that the H tumors had more extensive vasculature and less hypoxia than the AT1 tumors. These results further validate the utilization of FREDOM to monitor tumor oxygenation and concur with the hypothesis that the level of hypoxia is related to tumor growth rate and poor vascularity.

Quantitative assessment of tumor oxygen dynamics: molecular imaging for prognostic radiology

One of the fundamental molecules governing the survival of mammalian cells is oxygen. Oxygen has gained particular significance in tumor developmental biology and oncology. An increasingly diverse array of methods is now available to characterize tumor oxygenation. This Prospect will consider a new method, Fluorocarbon Relaxometry using Echo planar imaging for Dynamic Oxygen Mapping (FREDOM), which we have recently developed for oximetry, examine application to a specific therapeutic example and place this technique in the context of other approaches.

Imaging of angiogenesis: from microscope to clinic

Advances in imaging are transforming our understanding of angiogenesis and the evaluation of drugs that stimulate or inhibit angiogenesis in preclinical models and human disease. Vascular imaging makes it possible to quantify the number and spacing of blood vessels, measure blood flow and vascular permeability, and analyze cellular and molecular abnormalities in blood vessel walls. Microscopic methods ranging from fluorescence, confocal and multiphoton microscopy to electron microscopic imaging are particularly useful for elucidating structural and functional abnormalities of angiogenic blood vessels. Magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), ultrasonography and optical imaging provide noninvasive, functionally relevant images of angiogenesis in animals and humans. An ongoing dilemma is, however, that microscopic methods provide their highest resolution on preserved tissue specimens, whereas clinical methods give images of living tissues deep within the body but at much lower resolution and specificity and generally cannot resolve vessels of the microcirculation. Future challenges include developing new imaging methods that can bridge this resolution gap and specifically identify angiogenic vessels. Another goal is to determine which microscopic techniques are the best benchmarks for interpreting clinical images. The importance of angiogenesis in cancer, chronic inflammatory diseases, age-related macular degeneration and reversal of ischemic heart and limb disease provides incentive for meeting these challenges.

Dynamic response of breast tumor oxygenation to hyperoxic respiratory challenge monitored with three oxygen-sensitive parameters

The simultaneous measurement of three oxygen-sensitive parameters [arterial hemoglobin oxygen saturation (SaO2), tumor vascular-oxygenated hemoglobin concentration ([HbO2]), and tumor oxygen tension (pO2)] in response to hyperoxic respiratory challenge is demonstrated in rat breast tumors. The effects of two hyperoxic gases [oxygen and carbogen (5% CO2 and 95% O2)] were compared, by use of two groups of Fisher rats with subcutaneous 13762NF breast tumors implanted in pedicles on the foreback. Two different gas-inhalation sequences were compared, i.e., air-carbogen-air-oxygen-air and air-oxygen-air-carbogen-air. The results demonstrate that both of the inhaled, hyperoxic gases significantly improved the tumor oxygen status. All three parameters displayed similar dynamic response to hyperoxic gas interventions, but with different response times: the fastest for arterial SaO2, followed by biphasic changes in tumor vascular [HbO2], and then delayed responses for pO2. Both of the gases induced similar changes in vascular oxygenation and regional tissue pO2 in the rat tumors, and changes in [HbO2] and mean pO2 showed a linear correlation with large standard deviations, which presumably results from global versus local measurements. Indeed, the pO2 data revealed hetergeneous regional response to hyperoxic interventions. Although preliminary near-infrared measurements had been demonstrated previously in this model, the addition of the pO2 optical fiber probes provides a link between the noninvasive relative measurements of vascular phenomena based on endogenous reporter molecules, with the quantitative, albeit, invasive pO2 determinations.

Correlation of tumor oxygen dynamics with radiation response of the dunning prostate R3327-HI tumor

Our previous studies have shown that oxygen inhalation significantly reduces tumor hypoxia in the moderately well-differentiated HI subline of the Dunning prostate R3327 rat carcinoma. To test our hypothesis that modifying hypoxia could improve the radiosensitivity of these tumors, we performed experimental radiotherapy to compare the tumor response to ionizing radiation alone or in combination with oxygen inhalation. Tumor pO(2) measurements were performed on size-selected tumors several hours before radiotherapy using (19)F nuclear magnetic resonance echo planar imaging relaxometry (FREDOM) of the reporter molecule hexafluorobenzene. In common with our previous findings, the larger tumors (>3.5 cm(3)) exhibited greater hypoxia than the smaller tumors (<2 cm(3); P < 0.001), and oxygen inhalation reduced the hypoxic fraction (<10 Torr): In the larger tumors, hypoxic fraction dropped significantly from a mean baseline value of 80% to 17% (P < 0.001). The effect of oxygen administered 30 min before and during irradiation on tumor response to a single 30-Gy dose of photons was evaluated by growth delay. For the smaller tumors, no difference in growth delay was found when treatment was given with or without oxygen breathing. By contrast, breathing oxygen before and during irradiation significantly enhanced the growth delay in the larger tumors (additional 51 days). The differential behavior may be attributed to the low baseline hypoxic fraction (<10 Torr) in small tumors (20%) as a target for oxygen inhalation. There was a strong correlation between the estimated initial pO(2) value and the radiation-induced tumor growth delay (R > 0.8). Our histological studies showed a good match between the perfused vessels marked by Hoechst 33342 dye and the total vessels immunostained by anti-CD31 and indicated extensive perfusion in this tumor line. In summary, the present results suggest that the ability to detect modulation of tumor pO(2), in particular, the residual hypoxic fraction, with respect to an intervention, could have prognostic value for predicting the efficacy of radiotherapy.

Modulation of the tumor vasculature functionality by ionizing radiation accounts for tumor radiosensitization and promotes gene delivery

The ultimate goal of radiotherapy is to induce irreversible damages in genetically unstable, fast-growing cancer cells while minimizing the cytotoxic effects on host tissues. The satus of the tumor vasculature is particular because it is located within the tumor but mostly arises from host cells. The aim of this study was to characterize the effects of low-dose irradiation on the function of endothelial cells lining tumor vessels. Using isolated arterioles mounted on a pressure myograph, we first documented that the nitric oxide (NO)-mediated vasorelaxation that was defective in tumor vessels was completely restored following local tumor irradiation. Immunoblot analyses revealed that this was attributable to an increase in the abundance of the endothelial NO synthase while the expression of its physiological inhibitor, caveolin-1, was reduced. We further showed that the potentiation of the NO-dependent pathway induced a marked increase in tumor blood flow and oxygenation that determined the higher sensitivity of the tumor to further irradiation. Finally, we documented that the NO-mediated effects of irradiation on the tumor vasculature increased the delivery and expression of a reporter gene into the tumor. Thus, low-dose irradiation of endothelial cells within a tumor is a key determinant of the effectiveness of radiotherapy and may offer a new strategy to increase gene and/or drug delivery to the tumor.

Dynamic breast tumor oximetry: the development of prognostic radiology

A novel pre clinical approach to evaluating tumor oxygen dynamics was recently introduced (Am. J. Clin. Oncol. 24, 462-466 (2001)). FREDOM (Fluorocarbon Relaxometry using Echo planar imaging for Dynamic Oxygen Mapping) allows maps of tumor pO(2) including 50 - 150 individual locations simultaneously to be produced with typical in plane resolution of 1.25 mm in 6.5 mins. The technique has been applied extensively in rat prostate tumors and is now demonstrated in the rat breast 13762NF adenocarcinoma. When anesthetized rats breathed 33% oxygen, mean baseline pO(2) was in the range 17 +/- 2 (se) torr to 74 +/- 4 torr with mean value for nine tumors 46 +/- 8 torr. However, small tumors (< 2.2 cm(3)) were significantly better oxygenated with mean pO(2) = 63 +/- 7 torr than large tumors (> 2.4 cm(3)) with mean pO(2) 24 +/- 5 torr (p < 0.002). Switching the inhaled gas to oxygen or carbogen produced a significant and rapid increase in mean pO(2) for both small and larger tumors (p < 0.05). Given the increasing evidence that tumor oxygenation is related to therapeutic outcome, we believe this approach to measuring tumor oxygen dynamics can be of value in predicting response to therapy, evaluating adjuvant interventions designed to modulate response to therapy, and in providing "Prognostic Radiology".

Tumor hypoxia and the progression of prostate cancer

Tumor cell hypoxia is an innate environmental factor encountered during the development of many types of human tumors, including malignant prostate tumors. For prostate cancer, however, tumor cell hypoxia may be an even more critical element in tumor development and progression. Recent evidence suggests that androgenic steroids are important regulators of blood flow to prostate tumors and suppressors of tumor cell hypoxia. In addition, because prostate tumor cells are similar to other eukaryotic cells, they have the ability to respond to hypoxic conditions with drastic changes in gene expression mediated by the upregulation of a unique transcription factor, hypoxia-inducible factor-1. This response increases cancer cells' metabolic resistance to hypoxia, and also enhances the ability of prostate cancer cells to attract a more vigorous blood supply by upregulating the expression of pro-angiogenic factors. Because such changes would, in essence, increase the potential aggressiveness of affected prostate cancer cells, it is clear that tumor hypoxia has the potential for being a very important factor in prostate cancer cell biology. This review focuses on recent studies regarding the occurrence and potential role of hypoxia in prostate cancer, including hypoxia-inducible factor-1 and its related signaling pathways.