SOX71, A Biocompatible Succinyl Derivative of the Triarylmethyl Radical OX071 for In Vivo Quantitative Oxygen Mapping Using Electron Paramagnetic Resonance

PURPOSE

This study aimed to develop a biocompatible oximetric electron paramagnetic resonance (EPR) spin probe with reduced self-relaxation, and sensitivity to oxygen for a higher signal-to-noise ratio and longer relaxation times at high oxygen concentration, compared to the reference spin probe OX071.

PROCEDURES

SOX71 was synthesized by succinylation of the twelve alcohol groups of OX071 spin probe and characterized by EPR at X-Band (9.5 GHz) and at low field (720 MHz). The biocompatibility of SOX71 was tested in vitro and in vivo in mice. A pharmacokinetic study was performed to determine the best time frame for EPR imaging. Finally, a proof-of-concept EPR oxygen imaging was performed on a mouse model of a fibrosarcoma tumor.

RESULTS

SOX71 was synthesized in one step from OX071. SOX71 exhibits a narrow line EPR spectrum with a peak-to-peak linewidth of 66 mG, similar to OX071. SOX71 does not bind to albumin nor show cell toxicity for the concentrations tested up to 5 mM. No toxicity was observed after systemic delivery via intraperitoneal injection in mice at twice the dose required for EPR imaging. After the injection, the probe is readily absorbed into the bloodstream, with a peak blood concentration half an hour, post-injection. Then, the probe is quickly cleared by the kidney with a half-life of ~ 45 min. SOX71 shows long relaxation times under anoxic condition (T1e = 9.5 µs and T2e = 5.1 µs; [SOX71] = 1 mM in PBS at 37 °C, pO2 = 0 mmHg, 720 MHz). Both the relaxation rates R1e and R2e show a decreased sensitivity to pO2, leading to twice longer relaxation times under room air conditions (pO2 = 159 mmHg) compared to OX071. This is ideal for oxygen imaging in samples with a wide range of pO2. Both the relaxation rates R1e and R2e show a decreased sensitivity to self-relaxation compared to OX071, with a negligible effect of the probe concentration on R1e. SOX71 was successfully applied to image oxygen in a tumor.

CONCLUSION

SOX71, a succinylated derivative of OX071 was synthesized, characterized, and applied for in vivo EPR tumor oxygen imaging. SOX71 is highly biocompatible, and shows decreased sensitivity to oxygen and self-relaxation. This first report suggests that SOX71 is superior to OX071 for absolute oxygen mapping under a broad range of pO2 values.

The Role of Imaging Biomarkers to Guide Pharmacological Interventions Targeting Tumor Hypoxia

Hypoxia is a common feature of solid tumors that contributes to angiogenesis, invasiveness, metastasis, altered metabolism and genomic instability. As hypoxia is a major actor in tumor progression and resistance to radiotherapy, chemotherapy and immunotherapy, multiple approaches have emerged to target tumor hypoxia. It includes among others pharmacological interventions designed to alleviate tumor hypoxia at the time of radiation therapy, prodrugs that are selectively activated in hypoxic cells or inhibitors of molecular targets involved in hypoxic cell survival (i.e., hypoxia inducible factors HIFs, PI3K/AKT/mTOR pathway, unfolded protein response). While numerous strategies were successful in pre-clinical models, their translation in the clinical practice has been disappointing so far. This therapeutic failure often results from the absence of appropriate stratification of patients that could benefit from targeted interventions. Companion diagnostics may help at different levels of the research and development, and in matching a patient to a specific intervention targeting hypoxia. In this review, we discuss the relative merits of the existing hypoxia biomarkers, their current status and the challenges for their future validation as companion diagnostics adapted to the nature of the intervention.

Transiently hypoxic tumour cell turnover and radiation sensitivity in human tumour xenografts

BACKGROUND

Solid tumour perfusion can be unstable, creating transiently hypoxic cells that can contribute to radiation resistance. We investigated the in vivo lifetime of transiently hypoxic tumour cells and chronically hypoxic tumour cells during tumour growth and following irradiation.

METHODS

Hypoxic cells in SiHa and WiDr human tumour xenografts were labelled using pimonidazole and EF5, and turnover was quantified as the loss of labelled cells over time. The perfusion-modifying drug pentoxifylline was used to reoxygenate transiently hypoxic cells prior to hypoxia marker administration or irradiation.

RESULTS

Chronically hypoxic cells constantly turnover in SiHa and WiDr tumours, with half-lives ranging from 42-82 h and significant numbers surviving >96 h. Transiently hypoxic cells constitute 26% of the total hypoxic cells in WiDr tumours. These transiently hypoxic cells survive at least 24 h, but then rapidly turnover with a half-life of 34 h and are undetectable 72 h after labelling. Transiently hypoxic cells are radiation-resistant, although vascular dysfunction induced by 10 Gy of ionising radiation preferentially kills transiently hypoxic cells.

CONCLUSIONS

Transiently hypoxic tumour cells survive up to 72 h in WiDr tumours and are radiation-resistant, although transiently hypoxic cells are sensitive to vascular dysfunction induced by high doses of ionising radiation.

Cyclic Hypoxia: An Update on Its Characteristics, Methods to Measure It and Biological Implications in Cancer

Regions of hypoxia occur in most if not all solid cancers. Although the presence of tumor hypoxia is a common occurrence, the levels of hypoxia and proportion of the tumor that are hypoxic vary significantly. Importantly, even within tumors, oxygen levels fluctuate due to changes in red blood cell flux, vascular remodeling and thermoregulation. Together, this leads to cyclic or intermittent hypoxia. Tumor hypoxia predicts for poor patient outcome, in part due to increased resistance to all standard therapies. However, it is less clear how cyclic hypoxia impacts therapy response. Here, we discuss the causes of cyclic hypoxia and, importantly, which imaging modalities are best suited to detecting cyclic vs. chronic hypoxia. In addition, we provide a comparison of the biological response to chronic and cyclic hypoxia, including how the levels of reactive oxygen species and HIF-1 are likely impacted. Together, we highlight the importance of remembering that tumor hypoxia is not a static condition and that the fluctuations in oxygen levels have significant biological consequences.

Perfluorocarbons-Based 19F Magnetic Resonance Imaging in Biomedicine

Fluorine-19 (¹⁹F) magnetic resonance (MR) molecular imaging is a promising noninvasive and quantitative molecular imaging approach with intensive research due to the high sensitivity and low endogenous background signal of the ¹⁹F atom in vivo. Perfluorocarbons (PFCs) have been used as blood substitutes since 1970s. More recently, a variety of PFC nanoparticles have been designed for the detection and imaging of physiological and pathological changes. These molecular imaging probes have been developed to label cells, target specific epitopes in tumors, monitor the prognosis and therapy efficacy and quantitate characterization of tumors and changes in tumor microenvironment noninvasively, therefore, significantly improving the prognosis and therapy efficacy. Herein, we discuss the recent development and applications of ¹⁹F MR techniques with PFC nanoparticles in biomedicine, with particular emphasis on ligand-targeted and quantitative ¹⁹F MR imaging approaches for tumor detection, oxygenation measurement, smart stimulus response and therapy efficacy monitoring, et al.

A theoretical cell-killing model to evaluate oxygen enhancement ratios at DNA damage and cell survival endpoints in radiation therapy

Radio-resistance induced under low oxygen pressure plays an important role in malignant progression in fractionated radiotherapy. For the general approach to predict cell killing under hypoxia, cell-killing models (e.g. the Linear-Quadratic model) have to be fitted to in vitro experimental survival data for both normoxia and hypoxia to obtain the oxygen enhancement ratio (OER). In such a case, model parameters for every oxygen condition needs to be considered by model-fitting approaches. This is inefficient for fractionated irradiation planning. Here, we present an efficient model for fractionated radiotherapy the integrated microdosimetric-kinetic model including cell-cycle distribution and the OER at DNA double-strand break endpoint (OER_DSB). The cell survival curves described by this model can reproduce the in vitro experimental survival data for both acute and chronic low oxygen concentrations. The OER_DSB used for calculating cell survival agrees well with experimental DSB ratio of normoxia to hypoxia. The important parameters of the model are oxygen pressure and cell-cycle distribution, which enables us to predict cell survival probabilities under chronic hypoxia and chronic anoxia. This work provides biological effective dose (BED) under various oxygen conditions including its uncertainty, which can contribute to creating fractionated regimens for multi-fractionated radiotherapy. If the oxygen concentration in a tumor can be quantified by medical imaging, the present model will make it possible to estimate the cell-killing and BED under hypoxia in more realistic intravital situations.

Clinical and Statistical Considerations when Assessing Oxygen Levels in Tumors: Illustrative Results from Clinical EPR Oximetry Studies

The success of treatment for malignancies, especially those undergoing radiation therapy or chemotherapy, has long been recognized to depend on the degree of hypoxia in the tumor. In addition to the prognostic value of knowing the tumor's initial level of hypoxia, assessing the tumor oxygenation during standard therapy or oxygen-related treatments (such as breathing oxygen-enriched gas mixtures or taking drugs that can increase oxygen supply to tissues) can provide valuable data to improve the efficacy of treatments. A series of early clinical studies of tumors in humans are ongoing at Dartmouth and Emory using electron paramagnetic resonance (EPR) oximetry to assess tumor oxygenation, initially and over time during either natural disease progression or treatment. This approach has the potential for reaching the long-sought goal of enhancing the effectiveness of cancer therapy. In order to effectively reach this goal, we consider the validity of the practical and statistical assumptions when interpreting the measurements made in vivo for patients undergoing treatment for cancer.

A faster PISTOL for 1 H MR-based quantitative tissue oximetry

Quantitative mapping of oxygen tension (pO₂ ), noninvasively, could potentially be beneficial in cancer and stroke therapy for monitoring therapy and predicting response to certain therapies. Intracellular pO₂ measurements may also prove useful in tracking the health of labeled cells and understanding the dynamics of cell therapy in vivo. Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) is a relatively new oximetry technique that measures the T₁ of administered siloxanes such as hexamethyldisiloxane (HMDSO), to map the tissue pO₂ at various locations with a temporal resolution of 3.5 minutes. We have now developed a siloxane-selective Look-Locker imaging sequence equipped with an echo planar imaging (EPI) readout to accelerate PISTOL acquisitions. The new tissue oximetry sequence, referred to as PISTOL-LL, enables the mapping of HMDSO T₁ , and hence tissue pO₂ in under one minute. PISTOL-LL was tested and compared with PISTOL in vitro and in vivo. Both sequences were used to record dynamic changes in pO₂ of the rat thigh muscle (healthy Fischer rats, n = 6), and showed similar results (P > 0.05) as the other, with each sequence reporting a significant increase in pO₂ (P < 0.05) under hyperoxia compared with steady state normoxia. This study demonstrates the ability of the new sequence in rapidly and accurately mapping the pO₂ changes and accelerating quantitative ¹ H MR tissue oximetry by approximately 4-fold. The faster PISTOL-LL technique could enable dynamic ¹ H oximetry with higher temporal resolution for assesing tissue oxygentation and tracking the health of transplanted cells labeled with siloxane-based probes. With minor modifications, this sequence can be useful for ¹⁹ F applications as well.

Effects of hypoxic preconditioning on neuroblastoma tumour oxygenation and metabolic signature in a chick embryo model

Hypoxia episodes and areas in tumours have been associated with metastatic dissemination and poor prognosis. Given the link between tumour tissue oxygen levels and cellular metabolic activity, we hypothesised that the metabolic profile between metastatic and non-metastatic tumours would reveal potential new biomarkers and signalling cues. We have used a previously established chick embryo model for neuroblastoma growth and metastasis, where the metastatic phenotype can be controlled by neuroblastoma cell hypoxic preconditioning (3 days at 1% O₂). We measured, with fibre-optic oxygen sensors, the effects of the hypoxic preconditioning on the tumour oxygenation, within tumours formed by SK-N-AS cells on the chorioallantoic membrane (CAM) of chick embryos. We found that the difference between the metastatic and non-metastatic intratumoural oxygen levels was small (0.35% O₂), with a mean below 1.5% O₂ for most tumours. The metabolomic profiling, using NMR spectroscopy, of neuroblastoma cells cultured in normoxia or hypoxia for 3 days, and of the tumours formed by these cells showed that the effects of hypoxia in vitro did not compare with in vivo tumours. One notable difference was the high levels of the glycolytic end-products triggered by hypoxia in vitro, but not by hypoxia preconditioning in tumours, likely due to the very high basal levels of these metabolites in tumours compared with cells. In conclusion, we have identified high levels of ketones (3-hydroxybutyrate), lactate and phosphocholine in hypoxic preconditioned tumours, all known to fuel tumour growth, and we herein point to the poor relevance of in vitro metabolomic experiments for cancer research.

Assessing Tumor Oxygenation for Predicting Outcome in Radiation Oncology: A Review of Studies Correlating Tumor Hypoxic Status and Outcome in the Preclinical and Clinical Settings

Tumor hypoxia is recognized as a limiting factor for the efficacy of radiotherapy, because it enhances tumor radioresistance. It is strongly suggested that assessing tumor oxygenation could help to predict the outcome of cancer patients undergoing radiation therapy. Strategies have also been developed to alleviate tumor hypoxia in order to radiosensitize tumors. In addition, oxygen mapping is critically needed for intensity modulated radiation therapy (IMRT), in which the most hypoxic regions require higher radiation doses and the most oxygenated regions require lower radiation doses. However, the assessment of tumor oxygenation is not yet included in day-to-day clinical practice. This is due to the lack of a method for the quantitative and non-invasive mapping of tumor oxygenation. To fully integrate tumor hypoxia parameters into effective improvements of the individually tailored radiation therapy protocols in cancer patients, methods allowing non-invasively repeated, safe, and robust mapping of changes in tissue oxygenation are required. In this review, non-invasive methods dedicated to assessing tumor oxygenation with the ultimate goal of predicting outcome in radiation oncology are presented, including positron emission tomography used with nitroimidazole tracers, magnetic resonance methods using endogenous contrasts (R₁ and R2*-based methods), and electron paramagnetic resonance oximetry; the goal is to highlight results of studies establishing correlations between tumor hypoxic status and patients’ outcome in the preclinical and clinical settings.

Characterizing the Blood Oxygen Level-Dependent Fluctuations in Musculoskeletal Tumours Using Functional Magnetic Resonance Imaging

This study characterized the blood oxygen level-dependent (BOLD) fluctuations in benign and malignant musculoskeletal tumours via power spectrum analyses in pre-established low-frequency bands. BOLD MRI and T1-weighted imaging (T1WI) were collected for 52 patients with musculoskeletal tumours. Three ROIs were drawn on the T1WI image in the tumours' central regions, peripheral regions and neighbouring tissue. The power spectrum of the BOLD within each ROI was calculated and divided into the following four frequency bands: 0.01-0.027 Hz, 0.027-0.073 Hz, 0.073-0.198 Hz, and 0.198-0.25 Hz. ANOVA was conducted for each frequency band with the following two factors: the location of the region of interest (LoR, three levels: tumour "centre", "peripheral" and "healthy tissue") and tumour characteristic (TC, two levels: "malignant" and "benign"). There was a significant main effect of LoR in the frequencies of 0.073-0.198 Hz and 0.198-0.25 Hz. These data were further processed with post-hoc pair-wise comparisons. BOLD fluctuations at 0.073-0.198 Hz were stronger in the peripheral than central regions of the malignant tumours; however, no such difference was observed for the benign tumours. Our findings provide evidence that the BOLD signal fluctuates with spatial heterogeneity in malignant musculoskeletal tumours at the frequency band of 0.073-0.198 Hz.

A noninvasive tumor oxygenation imaging strategy using magnetic resonance imaging of endogenous blood and tissue water

PURPOSE

To present a novel imaging strategy for noninvasive measurement of tumor oxygenation using MR imaging of endogenous blood and tissue water.

THEORY AND METHODS

The proposed approach for oxygen partial pressure (pO2) estimation is based on intravoxel incoherent motion diffusion MRI and the dependence of the blood R2 relaxation rate on the inter-echo spacing measured using a multiple spin-echo Carr-Purcell-Meiboom-Gill sequence and weak-field diffusion model. The accuracy of the approach was validated by comparison with (19)F MRI oximetry.

RESULTS

The results in eight rats at 4.7 T showed that tumors have longer T1 (1980 ± 186 ms) and T2 (59 ± 9 ms) relaxation times, heterogeneous blood volume fraction (0.23 ± 0.1), oxygen saturation level (Y) (0.53 ± 0.12), and pO2 (36 ± 15 mmHg) distributions compared with normal muscle (T1 1480 ± 86 ms, T2 29 ± 2 ms, blood volume fraction 0.22 ± 0.03, Y 0.49 ± 0.06, and pO2 39 ± 5 mmHg). pO2 estimates based on the novel (1)H approach were essentially identical with (19)F observations.

CONCLUSION

The study indicates that noninvasive measurement of tumor pO2 using (1)H MRI derived multiparametric maps is feasible and could become a valuable tool to evaluate tumor hypoxia.

A review of flux considerations for in vivo neurochemical measurements

The mass transport or flux of neurochemicals in the brain and how this flux affects chemical measurements and their interpretation is reviewed. For all endogenous neurochemicals found in the brain, the flux of each of these neurochemicals exists between sources that produce them and the sites that consume them all within μm distances. Principles of convective-diffusion are reviewed with a significant emphasis on the tortuous paths and discrete point sources and sinks. The fundamentals of the primary methods of detection, microelectrodes and microdialysis sampling of brain neurochemicals are included in the review. Special attention is paid to the change in the natural flux of the neurochemicals caused by implantation and consumption at microelectrodes and uptake by microdialysis. The detection of oxygen, nitric oxide, glucose, lactate, and glutamate, and catecholamines by both methods are examined and where possible the two techniques (electrochemical vs. microdialysis) are compared. Non-invasive imaging methods: magnetic resonance, isotopic fluorine MRI, electron paramagnetic resonance, and positron emission tomography are also used for different measurements of the above-mentioned solutes and these are briefly reviewed. Although more sophisticated, the imaging techniques are unable to track neurochemical flux on short time scales, and lack spatial resolution. Where possible, determinations of flux using imaging are compared to the more classical techniques of microdialysis and microelectrodes.

Mapping in vivo tumor oxygenation within viable tumor by 19F-MRI and multispectral analysis

Quantifying oxygenation in viable tumor remains a major obstacle toward a better understanding of the tumor micro-environment and improving treatment strategies. Current techniques are often complicated by tumor heterogeneity. Herein, a novel in vivo approach that combines (19)F magnetic resonance imaging ((19)F-MRI) R 1 mapping with diffusion-based multispectral (MS) analysis is introduced. This approach restricts the partial pressure of oxygen (pO2) measurements to viable tumor, the tissue of therapeutic interest. The technique exhibited sufficient sensitivity to detect a breathing gas challenge in a xenograft tumor model, and the hypoxic region measured by MS (19)F-MRI was strongly correlated with histologic estimates of hypoxia. This approach was then applied to address the effects of antivascular agents on tumor oxygenation, which is a research question that is still under debate. The technique was used to monitor longitudinal pO2 changes in response to an antibody to vascular endothelial growth factor (B20.4.1.1) and a selective dual phosphoinositide 3-kinase/mammalian target of rapamycin inhibitor (GDC-0980). GDC-0980 reduced viable tumor pO2 during a 3-day treatment period, and a significant reduction was also produced by B20.4.1.1. Overall, this method provides an unprecedented view of viable tumor pO2 and contributes to a greater understanding of the effects of antivascular therapies on the tumor's microenvironment.

Intracellular carbonic anhydrase activity sensitizes cancer cell pH signaling to dynamic changes in CO2 partial pressure

Carbonic anhydrase (CA) enzymes catalyze the chemical equilibration among CO2, HCO3(-) and H(+). Intracellular CA (CAi) isoforms are present in certain types of cancer, and growing evidence suggests that low levels correlate with disease severity. However, their physiological role remains unclear. Cancer cell CAi activity, measured as cytoplasmic CO2 hydration rate (kf), ranged from high in colorectal HCT116 (∼2 s(-1)), bladder RT112 and colorectal HT29, moderate in fibrosarcoma HT1080 to negligible (i.e. spontaneous kf = 0.18 s(-1)) in cervical HeLa and breast MDA-MB-468 cells. CAi activity in cells correlated with CAII immunoreactivity and enzymatic activity in membrane-free lysates, suggesting that soluble CAII is an important intracellular isoform. CAi catalysis was not obligatory for supporting acid extrusion by H(+) efflux or HCO3(-) influx, nor for maintaining intracellular pH (pHi) uniformity. However, in the absence of CAi activity, acid loading from a highly alkaline pHi was rate-limited by HCO3(-) supply from spontaneous CO2 hydration. In solid tumors, time-dependence of blood flow can result in fluctuations of CO2 partial pressure (pCO2) that disturb cytoplasmic CO2-HCO3(-)-H(+) equilibrium. In cancer cells with high CAi activity, extracellular pCO2 fluctuations evoked faster and larger pHi oscillations. Functionally, these resulted in larger pH-dependent intracellular [Ca(2+)] oscillations and stronger inhibition of the mTORC1 pathway reported by S6 kinase phosphorylation. In contrast, the pHi of cells with low CAi activity was less responsive to pCO2 fluctuations. Such low pass filtering would "buffer" cancer cell pHi from non-steady-state extracellular pCO2. Thus, CAi activity determines the coupling between pCO2 (a function of tumor perfusion) and pHi (a potent modulator of cancer cell physiology).

Correlations of noninvasive BOLD and TOLD MRI with pO2 and relevance to tumor radiation response

PURPOSE

To examine the potential use of blood oxygenation level dependent (BOLD) and tissue oxygenation level dependent (TOLD) contrast MRI to assess tumor oxygenation and predict radiation response.

METHODS

BOLD and TOLD MRI were performed on Dunning R3327-AT1 rat prostate tumors during hyperoxic gas breathing challenge at 4.7 T. Animals were divided into two groups. In Group 1 (n = 9), subsequent (19) F MRI based on spin lattice relaxation of hexafluorobenzene reporter molecule provided quantitative oximetry for comparison. For Group 2 rats (n = 13) growth delay following a single dose of 30 Gy was compared with preirradiation BOLD and TOLD assessments.

RESULTS

Oxygen (100%O2 ) and carbogen (95%O2 /5%CO2 ) challenge elicited similar BOLD, TOLD and pO2 responses. Strong correlations were observed between BOLD or R2* response and quantitative (19) F pO2 measurements. TOLD response showed a general trend with weaker correlation. Irradiation caused a significant tumor growth delay and tumors with larger changes in TOLD and R1 values upon oxygen breathing exhibited significantly increased tumor growth delay.

CONCLUSION

These results provide further insight into the relationships between oxygen sensitive (BOLD/TOLD) MRI and tumor pO2 . Moreover, a larger increase in R1 response to hyperoxic gas challenge coincided with greater tumor growth delay following irradiation.

Anoxia/reoxygenation induces epithelial-mesenchymal transition in human colon cancer cell lines

Epithelial-mesenchymal transition (EMT) is considered to be a crucial event in the development of cancer metastasis. Anoxia/reoxygenation (A/R) is known to occur in cancer tissues due to angiogenesis and changes in tissue pressure that occur during tumor growth. We investigated whether A/R induces EMT in the human colon cancer cell line HT-29. Colon cancer cells were exposed to anoxia (2 h) followed by reoxygenation (4-22 h) and evaluated for EMT changes using immunofluorescence and western blot analyses. We also investigated the expression of EMT-related transcription factors (Snail and ZEB1) using RT-PCR and evaluated the expression of NF-κB using ELISA. To determine whether NF-κB is involved in A/R-induced EMT, HT-29 cells were treated with proteasome inhibitors. Colon cancer cells exposed to A/R underwent EMT morphological changes; the cancer cells acquired a spindle-shaped phenotype. The expression of E-cadherin on the cell surface and the total amount of E-cadherin proteins were reduced after A/R. The expression of EMT-related transcription factors (Snail, ZEB1) was increased after A/R. Pretreatment with proteasome inhibitors significantly attenuated the downregulation of E-cadherin induced by A/R. These results indicate that A/R induces EMT in human colon cancer cells through an NF-κB-dependent transcriptional pathway.

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.

PET hypoxia imaging with FAZA: reproducibility at baseline and during fractionated radiotherapy in tumour-bearing mice

PURPOSE

Tumour hypoxia is linked to treatment resistance. Positron emission tomography (PET) using hypoxia tracers such as fluoroazomycin arabinoside (FAZA) may allow identification of patients with hypoxic tumours and the monitoring of the efficacy of hypoxia-targeting treatment. Since hypoxia PET is characterized by poor image contrast, and tumour hypoxia undergoes spontaneous changes and is affected by therapy, it remains unclear to what extent PET scans are reproducible. Tumour-bearing mice are valuable in the validation of hypoxia PET, but identification of a reliable reference tissue value (blood sample or image-derived muscle value) for repeated scans may be difficult due to the small size of the animal or absence of anatomical information (pure PET). Here tumour hypoxia was monitored over time using repeated PET scans in individual tumour-bearing mice before and during fractionated radiotherapy.

METHODS

Mice bearing human SiHa cervix tumour xenografts underwent a PET scan 3 h following injection of FAZA on two consecutive days before initiation of treatment (baseline) and again following irradiation with four and ten fractions of 2.5 Gy. On the last scan day, mice were given an intraperitoneal injection of pimonidazole (hypoxia marker), tumours were collected and the intratumoral distribution of FAZA (autoradiography) and hypoxia (pimonidazole immunohistology) were determined in cryosections.

RESULTS

Tissue section analysis revealed that the intratumoral distribution of FAZA was strongly correlated with the regional density of hypoxic (pimonidazole-positive) cells, even when necrosis was present, suggesting that FAZA PET provides a reliable measure of tumour hypoxia at the time of the scan. PET-based quantification of tumour tracer uptake relative to injected dose showed excellent reproducibility at baseline, whereas normalization using an image-derived nonhypoxic reference tissue (muscle) proved highly unreliable since a valid and reliable reference value could not be determined. The intratumoral distribution of tracer was stable at baseline as shown by a voxel-by-voxel comparison of the two scans (R = 0.82, range 0.72-0.90). During treatment, overall tracer retention changed in individual mice, but there was no evidence of general reoxygenation.

CONCLUSION

Hypoxia PET scans are quantitatively correct and highly reproducible in tumour-bearing mice. Preclinical hypoxia PET is therefore a valuable and reliable tool for the development of strategies that target or modify hypoxia.

Revisiting the ultra-high dose rate effect: implications for charged particle radiotherapy using protons and light ions

OBJECTIVE

To reinvestigate ultra-high dose rate radiation (UHDRR) radiobiology and consider potential implications for hadrontherapy.

METHODS

A literature search of cellular UHDRR exposures was performed. Standard oxygen diffusion equations were used to estimate the time taken to replace UHDRR-related oxygen depletion. Dose rates from conventional and novel methods of hadrontherapy accelerators were considered, including spot scanning beam delivery, which intensifies dose rate.

RESULTS

The literature findings were that, for X-ray and electron dose rates of around 10(9) Gy s(-1), 5-10 Gy depletes cellular oxygen, significantly changing the radiosensitivity of cells already in low oxygen tension (around 3 mmHg or 0.4 kPa). The time taken to reverse the oxygen depletion of such cells is estimated to be over 20-30 s at distances of over 100 μm from a tumour blood vessel. In this time window, tumours have a higher hypoxic fraction (capable of reducing tumour control), so the next application of radiation within the same fraction should be at a time that exceeds these estimates in the case of scanned beams or with ultra-fast laser-generated particles.

CONCLUSION

This study has potential implications for particle therapy, including laser-generated particles, where dose rate is greatly increased. Conventional accelerators probably do not achieve the critical UHDRR conditions. However, specific UHDRR oxygen depletion experiments using proton and ion beams are indicated.

Intra- and intertumor heterogeneities in total, chronic, and acute hypoxia in xenografted squamous cell carcinomas. Detection and quantification using (immuno-)fluorescence techniques

BACKGROUND

Heterogeneously distributed hypoxia is a major characteristic of solid tumors. (Immuno-)fluorescence detection of hypoxia in experimental tumors is frequently assessed in a single central section; however, this may not necessarily be representative of the whole tumor. In order to determine whether analysis of one central section is exemplary of the whole tumor and whether different volumes have an impact on tumor oxygenation, we assessed the fractions of total (TH), chronic (CH), and acute hypoxia (AH) throughout different layers of tumors of varying volumes.

MATERIALS AND METHODS

Xenografted FaDu human squamous cell carcinomas of different volumes were investigated for intra- and intertumor heterogeneities. Tissue blocks located at the apical, central, and basal layer were sliced from individual tumors. Four serial cryosections were analyzed from each tissue block. Vital tumor tissue was explored for the distribution of Hoechst 33342 (perfusion), pimonidazole (hypoxia), and CD31 (endothelium) to assess TH, CH, and AH.

RESULTS

Fractions of TH, CH, and AH were consistently similar in the serial sections of individual tissue blocks. However, significant differences were found between the apical, central, and basal blocks that were even opposite depending on the tumor volume. Pooled data from all three tissue blocks revealed significantly higher fractions of hypoxia in the large tumors than in the small tumors.

CONCLUSION

FaDu tumors exhibit a heterogeneous and volume-dependent oxygenation status. Assessing the average fractions of TH, CH, and AH from central blocks corresponds best to the average of the entire tumor. However, information on intratumor heterogeneities is lost, especially when considering tumors of substantially different volumes.

Hypoxia imaging with the nitroimidazole 18F-FAZA PET tracer: a comparison with OxyLite, EPR oximetry and 19F-MRI relaxometry

BACKGROUND AND PURPOSE

(18)F-FAZA is a nitroimidazole PET tracer that can provide images of tumor hypoxia. However, it cannot provide absolute pO(2) values. To qualify (18)F-FAZA PET, we compared PET images to pO(2) measured by OxyLite, EPR oximetry and (19)F-MRI.

MATERIALS AND METHODS

Male WAG/Rij rats grafted with rhabdomyosarcoma were used. Tumor oxygenation was modified by gas breathing (air or carbogen). The same day of PET acquisition, the pO(2) was measured in the same tumor either by OxyLite probes (measurement at 10 different sites), EPR oximetry using low frequency EPR or (19)F-relaxometry using 15C5 on an 11.7T MR system.

RESULTS

There was a good correlation between the results obtained by PET and EPR (R = 0.93). In the case of OxyLite, although a weaker correlation was observed (R = 0.55), the trend for two values to agree was still related to the inverse function theoretically predicted. For the comparison of (18)F-FAZA PET and (19)F-MRI, no change in T(1) was observed.

CONCLUSIONS

A clear correlation between (18)F-FAZA PET image intensities and tumor oxygenation was demonstrated, suggesting that (18)F-FAZA PET is a promising imaging technique to guide cancer therapy.

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 10 mmHg 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 pO(2) 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.

Acute versus chronic hypoxia in tumors: Controversial data concerning time frames and biological consequences

BACKGROUND

Many tumors contain hypoxic regions. Hypoxia, in turn, is known to increase aggressiveness and to be associated with treatment resistance. The two most frequently described and investigated subtypes of tumor hypoxia are acute and chronic. These two subtypes can lead to completely different hypoxia-related responses within the tumor, which could have a direct effect on tumor development and response to treatment. In order to accurately assess the specific biological consequences, it is important to understand which time frames best define acute and chronic hypoxia.

MATERIALS AND METHODS

This article provides an overview of the kinetics of in vitro and in vivo acute and chronic tumor hypoxia. Special attention was paid to differentiate between methods to detect spontaneous in vivo hypoxia and to describe the biological effects of experimental in vitro and in vivo acute and chronic tumor hypoxia.

RESULTS AND CONCLUSIONS

There are large variations in reported spontaneous fluctuations in acute hypoxia that are dependent on the cell lines investigated and the detection method used. In addition to differing hypoxia levels, exposure times used to induce in vitro and in vivo experimental acute and chronic hypoxia range from 30 min to several weeks with no clear boundaries separating the two. Evaluation of the biological consequences of each hypoxia subtype revealed a general trend that acute hypoxia leads to a more aggressive phenotype. Importantly, more information on the occurrence of acute and chronic hypoxia in human tumors is needed to help our understanding of the clinical consequences.

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.

Dynamic changes in oxygenation of intracranial tumor and contralateral brain during tumor growth and carbogen breathing: a multisite EPR oximetry with implantable resonators

INTRODUCTION

Several techniques currently exist for measuring tissue oxygen; however technical difficulties have limited their usefulness and general application. We report a recently developed electron paramagnetic resonance (EPR) oximetry approach with multiple probe implantable resonators (IRs) that allow repeated measurements of oxygen in tissue at depths of greater than 10mm.

METHODS

The EPR signal to noise (S/N) ratio of two probe IRs was compared with that of LiPc deposits. The feasibility of intracranial tissue pO(2) measurements by EPR oximetry using IRs was tested in normal rats and rats bearing intracerebral F98 tumors. The dynamic changes in the tissue pO(2) were assessed during repeated hyperoxia with carbogen breathing.

RESULTS

A 6-10 times increase in the S/N ratio was observed with IRs as compared to LiPc deposits. The mean brain pO(2) of normal rats was stable and increased significantly during carbogen inhalation in experiments repeated for 3months. The pO(2) of F98 glioma declined gradually, while the pO(2) of contralateral brain essentially remained the same. Although a significant increase in the glioma pO(2) was observed during carbogen inhalation, this effect declined in experiments repeated over days.

CONCLUSION

EPR oximetry with IRs provides a significant increase in S/N ratio. The ability to repeatedly assess orthotopic glioma pO(2) is likely to play a vital role in understanding the dynamics of tissue pO(2) during tumor growth and therapies designed to modulate tumor hypoxia. This information could then be used to optimize chemoradiation by scheduling treatments at times of increased glioma oxygenation.

Strategies To Assess Hypoxic/HIF-1-Active Cancer Cells for the Development of Innovative Radiation Therapy

Local tumor recurrence and distant tumor metastasis frequently occur after radiation therapy and result in the death of cancer patients. These problems are caused, at least in part, by a tumor-specific oxygen-poor microenvironment, hypoxia. Oxygen-deprivation is known to inhibit the chemical ionization of both intracellular macro-molecules and water, etc., and thus reduce the cytotoxic effects of radiation. Moreover, DNA damage produced by free radicals is known to be more repairable under hypoxia than normoxia. Hypoxia is also known to induce biological tumor radioresistance through the activation of a transcription factor, hypoxia-inducible factor 1 (HIF-1). Several potential strategies have been devised in radiation therapy to overcome these problems; however, they have not yet achieved a complete remission. It is essential to reveal the intratumoral localization and dynamics of hypoxic/HIF-1-active tumor cells during tumor growth and after radiation therapy, then exploit the information to develop innovative therapeutic strategies, and finally damage radioresistant cells. In this review, we overview problems caused by hypoxia/HIF-1-active cells in radiation therapy for cancer and introduce strategies to assess intratumoral hypoxia/HIF-1 activity.

19F MRI oximetry: simulation of perfluorocarbon distribution impact

In (19)F MRI oximetry, a method used to image tumour hypoxia, perfluorocarbons serve as oxygenation markers. The goal of this study is to evaluate the impact of perfluorocarbon distribution and concentration in (19)F MRI oximetry through a computer simulation. The simulation studies the correspondence between (19)F measured (pO(FNMR)(2)) and actual tissue oxygen tension (pO(2)) for several tissue perfluorocarbon distributions. For this, a Krogh tissue model is implemented which incorporates the presence of perfluorocarbons in blood and tissue. That is, in tissue the perfluorocarbons are distributed homogeneously according to Gaussian diffusion profiles, or the perfluorocarbons are concentrated in the capillary wall. Using these distributions, the oxygen tension in the simulation volume is calculated. The simulated mean oxygen tension is then compared with pO(FNMR)(2), the (19)F MRI-based measure of pO(2) and with pO(0)(2), pO(2) in the absence of perfluorocarbons. The agreement between pO(FNMR)(2) and actual pO(2) is influenced by vascular density and perfluorocarbon distribution. The presence of perfluorocarbons generally gives rise to a pO(2) increase in tissue. This effect is enhanced when perfluorocarbons are also present in blood. Only the homogeneous perfluorocarbon distribution in tissue with no perfluorocarbons in blood guarantees small deviations of pO(FNMR)(2) from pO(2). Hence, perfluorocarbon distribution in tissue and blood has a serious impact on the reliability of (19)F MRI-based measures of oxygen tension. In addition, the presence of perfluorocarbons influences the actual oxygen tension. This finding may be of great importance for further development of (19)F MRI oximetry.

How can we overcome tumor hypoxia in radiation therapy?

Local recurrence and distant metastasis frequently occur after radiation therapy for cancer and can be fatal. Evidence obtained from radiochemical and radiobiological studies has revealed these problems to be caused, at least in part, by a tumor-specific microenvironment, hypoxia. Moreover, a transcription factor, hypoxia-inducible factor 1 (HIF-1), was identified as pivotal to hypoxia-mediated radioresistance. To overcome the problems, radiation oncologists have recently obtained powerful tools, such as "simultaneous integrated boost intensity-modulated radiation therapy (SIB-IMRT), which enables a booster dose of radiation to be delivered to small target fractions in a malignant tumor", "hypoxia-selective cytotoxins/drugs", and "HIF-1 inhibitors" etc. In order to fully exploit these innovative and interdisciplinary strategies in cancer therapy, it is critical to unveil the characteristics, intratumoral localization, and dynamics of hypoxia/HIF-1-active tumor cells during tumor growth and after radiation therapy. We have performed optical imaging experiments using tumor-bearing mice and revealed that the locations of HIF-1-active tumor cells changes dramatically as tumors grow. Moreover, HIF-1 activity changes markedly after radiation therapy. This review overviews 1) fundamental problems surrounding tumor hypoxia in current radiation therapy, 2) the function of HIF-1 in tumor radioresistance, 3) the dynamics of hypoxic tumor cells during tumor growth and after radiation therapy, and 4) how we should overcome the difficulties with radiation therapy using innovative interdisciplinary technologies.