Relationship between radiobiological hypoxia in tumors and electrode measurements of tumor oxygenation

What do cellular responses to acidity tell us about cancer?

The notion that invasive cancer is a product of somatic evolution is a well-established theory that can be modelled mathematically and demonstrated empirically from therapeutic responses. Somatic evolution is by no means deterministic, and ample opportunities exist to steer its trajectory towards cancer cell extinction. One such strategy is to alter the chemical microenvironment shared between host and cancer cells in a way that no longer favours the latter. Ever since the first description of the Warburg effect, acidosis has been recognised as a key chemical signature of the tumour microenvironment. Recent findings have suggested that responses to acidosis, arising through a process of selection and adaptation, give cancer cells a competitive advantage over the host. A surge of research efforts has attempted to understand the basis of this advantage and seek ways of exploiting it therapeutically. Here, we review key findings and place these in the context of a mathematical framework. Looking ahead, we highlight areas relating to cellular adaptation, selection, and heterogeneity that merit more research efforts in order to close in on the goal of exploiting tumour acidity in future therapies.

FLASH proton radiotherapy spares normal epithelial and mesenchymal tissues while preserving sarcoma response

In studies of electron and proton radiotherapy, ultrahigh dose rates of FLASH radiation therapy appear to produce fewer toxicities than standard dose rates while maintaining local tumor control. FLASH-proton radiotherapy (F-PRT) brings the spatial advantages of PRT to FLASH dose rates (>40 Gy/sec), making it important to understand if and how F-PRT spares normal tissues while providing anti-tumor efficacy that is equivalent to standard-proton radiotherapy (S-PRT). Here we studied PRT damage to skin and mesenchymal tissues of muscle and bone and found that F-PRT of the C57BL/6 murine hind leg produced fewer severe toxicities leading to death or requiring euthanasia than S-PRT of the same dose. RNAseq analyses of murine skin and bone revealed pathways upregulated by S-PRT yet unaltered by F-PRT, such as apoptosis signaling and keratinocyte differentiation in skin, as well as osteoclast differentiation and chondrocyte development in bone. Corroborating these findings, F-PRT reduced skin injury, stem cell depletion, and inflammation, mitigated late effects including lymphedema, and decreased histopathologically detected myofiber atrophy, bone resorption, hair follicle atrophy, and epidermal hyperplasia. F-PRT was equipotent to S-PRT in control of two murine sarcoma models, including at an orthotopic intramuscular site, thereby establishing its relevance to mesenchymal cancers. Finally, S-PRT produced greater increases in TGF-β1 in murine skin and the skin of canines enrolled in a phase 1 study of F-PRT versus S-PRT. Collectively, these data provide novel insights into F-PRT-mediated tissue sparing and support its ongoing investigation in applications that would benefit from this sparing of skin and mesenchymal tissues.

Tumors Resistant to Checkpoint Inhibitors Can Become Sensitive after Treatment with Vascular Disrupting Agents

Immune therapy improves cancer outcomes, yet many patients do not respond. This pre-clinical study investigated whether vascular disrupting agents (VDAs) could convert an immune unresponsive tumor into a responder. CDF1 mice, with 200 mm³ C3H mammary carcinomas in the right rear foot, were intraperitoneally injected with combretastatin A-4 phosphate (CA4P), its A-1 analogue OXi4503, and/or checkpoint inhibitors (anti-PD-1, PD-L1, or CTLA-4 antibodies), administered twice weekly for two weeks. Using the endpoint of tumor growth time (TGT5; time to reach five times the starting volume), we found that none of the checkpoint inhibitors (10 mg/kg) had any effect on TGT5 compared to untreated controls. However, CA4P (100 mg/kg) or OXi4503 (5–50 mg/kg) did significantly increase TGT5. This further significantly increased by combining the VDAs with checkpoint inhibitors, but was dependent on the VDA, drug dose, and inhibitor. For CA4P, a significant increase was found when CA4P (100 mg/kg) was combined with anti-PD-L1, but not with the other two checkpoint inhibitors. With OXi4503 (50 mg/kg), a significant enhancement occurred when combined with anti-PD-L1 or anti-CTLA-4, but not anti-PD-1. We observed no significant improvement with lower OXi4503 doses (5–25 mg/kg) and anti-CTLA-4, although 30% of tumors were controlled at the 25 mg/kg dose. Histological assessment of CD4/CD8 expression actually showed decreased levels up to 10 days after treatment with OXi4503 (50 mg/kg). Thus, the non-immunogenic C3H mammary carcinoma was unresponsive to checkpoint inhibitors, but became responsive in mice treated with VDAs, although the mechanism remains unclear.

Enhancing the radiation response of tumors but not early or late responding normal tissues using a vascular disrupting agent

INTRODUCTION

Vascular disrupting agents (VDAs) damage tumor vasculature and enhance tumor radiation response. In this pre-clinical study, we combined radiation with the leading VDA in clinical development, combretastatin A-4 phosphate (CA4P), and compared the effects seen in tumors and relevant normal tissues.

MATERIAL AND METHODS

Radiation was applied locally to tissues in CDF1 mice to produce full radiation dose-response curves. CA4P (250 mg/kg) was intraperitoneally (i.p.) injected within 30 minutes after irradiating. Response of 200 mm³ foot implanted C3H mammary carcinomas was assessed using percent tumor control at 90 days. Normal tissue effects were evaluated using early responding skin (development of moist desquamation in the foot at 11-30 days), and late responding bladder (50% reduction in reservoir function estimated by cystometry up to 9 months after treatment), and lung (20% increase in ventilation rate measured by plethysmography within 9 months). A Chi-squared test was used for statistical comparisons (significance level of p < .05).

RESULTS

The radiation dose controlling 50% of irradiated tumors was 52 Gy. This significantly decreased to 45 Gy with CA4P. The radiation doses inducing a change in skin, bladder and lung response in 50% of mice were 31 Gy, 14 Gy and 12 Gy, respectively. CA4P had no significant effect on the radiation response of any of these normal tissues.

CONCLUSIONS

VDAs significantly enhance tumor radiation response, but had absolutely no effect on the radiation response of early or late responding normal tissues.

Importance of hemoglobin concentration and its modification for the outcome of head and neck cancer patients treated with radiotherapy

INTRODUCTION

Hypoxia induced radioresistance has been acknowledged for decades. One of the indirect evidences of the influence of hypoxia on radiation response comes from the observations of a correlation between tumor control and hemoglobin level. This review examines the clinical data on the prognostic and predictive role of hemoglobin level and hemoglobin manipulation in radiotherapy of squamous cell carcinomas of the head and neck, a tumor type where hypoxic radioresistance have been previously documented.

THE INFLUENCE OF HEMOGLOBIN CONCENTRATION ON TUMOR OXYGENATION AND OUTCOME

The aim is to evaluate the existing literature for information of the influence of hemoglobin concentration and hemoglobin modifications on tumor oxygenation and outcome in head and neck squamous cell cancer patients. The data from several randomized trials show that while most studies have confirmed the prognostic value of hemoglobin, increasing the hemoglobin level through transfusion or erythropoietin stimulation did not result in improved outcome for patients with low initial hemoglobin levels. Clinical studies showed that smoking reduced the oxygen carrying capacity of the blood through formation of carboxyhemoglobin, and lead to poorer response to radiotherapy in smokers compared to non-smokers. Smoking also increased the risk of the development of secondary cancers.

CONCLUSION AND FUTURE PERSPECTIVES

In conclusion, low hemoglobin is a significant negative prognostic factor for radiotherapy of head and neck cancer. Correction of pre-treatment low hemoglobin by blood transfusion and/or erythropoietin stimulating agents does, however, not improve the outcome. Smoking leads to a decrease in effective hemoglobin and poorer treatment outcome. Smoking should be avoided in order to improve the therapeutic efficacy of radiotherapy and development of other smoking-related diseases and/or secondary cancers.

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.

Hypoxic modification of radiotherapy in squamous cell carcinoma of the head and neck--a systematic review and meta-analysis

BACKGROUND

The importance of tumour hypoxia for the outcome of radiotherapy has been under investigation for decades. Numerous clinical trials modifying the hypoxic radioresistance in squamous cell carcinoma of the head and neck (HNSCC) have been conducted, but most have been inconclusive, partly due to a small number of patients in the individual trial. The present meta-analysis was, therefore, performed utilising the results from all clinical trials addressing the specific question of hypoxic modification in HNSCC undergoing curative intended primary radiotherapy alone.

METHODS

A systematic review of published and unpublished data identified 4805 patients with HNSCC treated in 32 randomized clinical trials, applying, normobaric oxygen or carbogen breathing (5 trials); hyperbaric oxygen (HBO) (9 trials); hypoxic radiosensitizers (17 trials) and HBO and radiosensitizer (1 trial). The trials were analysed with regard to the following endpoints: loco-regional control (32 trials), disease specific survival (30 trials), overall survival (29 trials), distant metastases (12 trials) and complications to radiotherapy (23 trials).

RESULTS

Overall hypoxic modification of radiotherapy in head and neck cancer did result in a significant improved therapeutic benefit. This was most dominantly observed when using the direct endpoint of loco-regional control with an odds ratio (OR) of 0.71, 95% cf.l. 0.63-0.80; p<0.001), but this was almost mirrored in the disease specific survival (OR: 0.73, 95% cf.l. 0.64-0.82; p<0.001), and to a lesser extent in the overall survival (OR: 0.87, 95% cf.l. 0.77-0.98; p=0.03). The risk of distant metastases was not significantly influenced although it appears to be less in the tumours treated with hypoxic modification (OR: 0.87, 95% cf.l. 0.69-1.09; p=0.22), whereas the radiation related late complications were not influenced by the overall use of hypoxic modifications (OR: 1.00, 95% cf.l. 0.82-1.23; p=0.96). The improvement in loco-regional control was found to be independent of the type of hypoxic modification. The trials have used different fractionation schedules, including large doses per fraction, which may result in relatively more hypoxia and greater benefit. However, analysis of HNSCC trials using conventional fractionation only, showed that the significant effect of hypoxic modification was maintained.

CONCLUSION

The meta-analysis thus demonstrates that there is level 1a evidence in favour of adding hypoxic modification to radiotherapy of squamous cell carcinomas of the head and neck.

Identifying hypoxia in human tumors: A correlation study between 18F-FMISO PET and the Eppendorf oxygen-sensitive electrode

INTRODUCTION

Polarographic oxygen-sensitive electrodes have demonstrated prognostic significance of hypoxia. However, its routine application is limited. (18)F-FMISO PET scans are a noninvasive approach, able to measure spatial and temporal changes in hypoxia. The aim of this study was to examine the association between measures of hypoxia defined by functional imaging and Eppendorf pO(2) electrodes.

MATERIALS AND METHODS

A total of 18 patients were included, nine squamous cell carcinoma of the head and neck and nine soft tissue tumors. The tumor volume was defined by CT, MRI, (18)FDG-PET or by clinical examination. The oxygenation status of the tumors was assessed using (18)F-FMISO PET imaging followed by Eppendorf pO(2) electrode measurements. Data were compared in a 'virtual voxel', resulting in individual histograms from each tumor.

RESULTS

The percentages of pO(2) ≤ 5 mmHg ranged from 9 to 94% (median 43%) for all 18 tumors. For (18)F-FMISO PET the T/M ratio ranged from 0.70 to 2.38 (median 1.13). Analyzing the virtual voxel histograms tumors could be categorized in three groups: Well oxygenated tumors with no hypoxia and concordance between the (18)F-FMISO data and the Eppendorf measurements, hypoxic tumors likewise with concordance between the two assays and inconclusive tumors with no concordance between the assays.

CONCLUSION

This study analyzed the relationship between (18)F-FMISO PET and Eppendorf pO(2) electrode measurements by use of a virtual voxel model. There was a spectrum of hypoxia among tumors that can be detected by both assays. However no correlation was observed, and in general tumors were more hypoxic based on Eppendorf pO(2) measurements as compared to (18)F-FMISO PET.

Resolution in PET hypoxia imaging: voxel size matters

INTRODUCTION

Tumor hypoxia adversely affects treatment outcome, especially in squamous cell carcinomas (SCCs). Image guided radiotherapy (IGRT) based on PET-generated tumor hypoxia maps allows dose boosting to hypoxic sub-volumes and has received considerable interest. However, the combination of slow oxygenation-dependent tracer retention, slow clearance of unbound tracer from non-hypoxic tissue and the necessity to average signal over large non-homogenous tissue areas due to the low PET resolution remains problematic.

MATERIALS AND METHODS

To assess pitfalls inherent to low-resolution imaging we have analyzed the fine-scale distribution of a PET hypoxia tracer (autoradiograms) and tissue architecture (immunofluorescence microscopy) in sectioned experimental SCCs, and compared the results to those obtained when applying macroscopic averaging mimicking the resolution in clinical PET scanners.

RESULTS AND DISCUSSION

We show that tumor areas that would be classified as non-hypoxic based on simple PET threshold identification, often contains foci of hypoxic cells, in particular in tumors where necrosis and severely hypoxic cells are intermixed. In contrast, in a non-necrotic tumor model we found that the risk of missing hypoxic cells was greatly reduced, however, its patchy hypoxic pattern made a clear delineation of a target to boost unfeasible. We discuss the implications of these and other complicating factors in PET hypoxia-imaging and outline future strategies to overcome or circumvent them.

The impact of hypoxia on the activity of lactate dehydrogenase in two different pre-clinical tumour models

AIM

To investigate the direct relationship between tumour hypoxia and lactate dehydrogenase (Ldh) levels in serum and tumour in two different pre-clinical murine models.

MATERIALS AND METHODS

Experiments were performed in CDF1 or C3H/ Km mice implanted with a C3H mammary carcinoma and SCCVII squamous cell carcinoma, respectively. Low oxygen breathing for 1-72 h was used to increase tumour hypoxia. Ldh activity was measured in the serum and tumour cytosole with a colorimetric method. Tumour Ldha mRNA levels were assessed with RT-PCR. RESULTS; The serum Ldh in non-tumour bearing CDF1 mice and C3H/km mice was 10.592 U/ml and 1292 U/ml, respectively. For C3H mammary carcinoma bearing mice, a positive correlation between tumour volume and tumour and serum Ldh was found. Tumour Ldh in SCCVII carcinomas also increased with increasing tumour volume, but no volume dependence of serum Ldh was found. Low oxygen breathing caused a 2-3 fold increase in tumour Ldha mRNA in both tumour models. In C3H mammary carcinoma bearing mice, serum and tumour Ldh significantly increased after 48 and 72 hours of hypoxia, respectively. Low oxygen breathing did not change serum and tumour Ldh in SCCVII carcinoma bearing mice. Reoxygenation for 4 or 24 hours had no additional effect on Ldh activity in any of the models.

DISCUSSION

Serum Ldh activity can be a marker for tumour burden in certain types of cancer. The relationship between serum and tumour Ldh and tumour hypoxia has not been confirmed. However, Ldha mRNA may be a potential new marker of tumour hypoxia and should be further investigated.

Dynamics of tumor hypoxia measured with bioreductive hypoxic cell markers

Hypoxic cells are common in tumors and contribute to malignant progression, distant metastasis and resistance to radiotherapy. It is well known that tumors are heterogeneous with respect to the levels and duration of hypoxia. Several strategies, including high-oxygen-content gas breathing, radiosensitizers and hypoxic cytotoxins, have been developed to overcome hypoxia-mediated radioresistance. However, with these strategies, an increased tumor control rate is often accompanied by more severe side effects. Consequently, development of assays for prediction of tumor response and early monitoring of treatment responses could reduce both over- and undertreatment, thereby avoiding unnecessary side effects. The purpose of this review is to discuss different assays for measurement of hypoxia that can be used to detect changes in oxygen tension. The main focus is on exogenous bioreductive hypoxia markers (2-nitroimidazoles) such as pimonidazole, CCI-103F, EF5 and F-misonidazole. These are specifically reduced and bind to macromolecules in viable hypoxic cells. A number of these bioreductive drugs are approved for clinical use and can be detected with methods ranging from noninvasive PET imaging (low resolution) to microscopic imaging of tumor sections (high resolution). If the latter are stained for multiple markers, hypoxia can be analyzed in relation to different microenvironmental parameters such as vasculature, proliferation and endogenous hypoxia-related markers, for instance HIF1alpha and CA-IX. In addition, temporal and spatial changes in hypoxia can be analyzed by consecutive injection of two different hypoxia markers. Therefore, bioreductive exogenous hypoxia markers are promising as tools for development of predictive assays or as tools for early treatment monitoring and validation of potential endogenous hypoxia markers.

Strain and tumour specific variations in the effect of hypoxia on osteopontin levels in experimental models

PURPOSE

To investigate the relationship between tumour hypoxia and serum and tumour osteopontin (OPN) levels.

MATERIALS AND METHODS

Experiments were performed in CDF1 or C3H/Km mice implanted with a C3H mammary carcinoma (CDF1) or SCCVII squamous cell carcinoma (C3H/Km), respectively. Mice were either untreated or gassed with 10% oxygen for 1-72 h. Serum and tumour OPN levels were measured with an ELISA and tumour OPN mRNA levels using RT-PCR. Tumour oxygenation was estimated using the Eppendorf histograph with the percentage of pO(2) values <or=5 mm Hg (HF5) as the endpoint.

RESULTS

OPN levels were 50-fold higher in the serum of non-tumour bearing CDF1 mice compared to C3H/Km mice. A tumour related increase in serum OPN levels was observed in CDF1 but not in C3H/Km mice. Low oxygen breathing increased HF5 in both tumour models and in the C3H mammary carcinoma model both serum and tumour OPN decreased after prolonged hypoxia (24h and more). When 12h of hypoxia was followed by 24h reoxygenation there was a twofold increase in serum OPN levels. No changes were observed in the SCCVII model. No changes in tumour OPN mRNA expression were observed during hypoxia and reoxygenation in these tumour models.

CONCLUSION

Clear strain and tumour specific differences in the effect of hypoxia on OPN levels have been observed in two different mouse tumour models. These data emphasize the complexity in the relationship between poor oxygenation (and/or reoxygenation) of tumours and serum levels of OPN.

Effect of intratumoral heterogeneity in oxygenation status on FMISO PET, autoradiography, and electrode Po2 measurements in murine tumors

PURPOSE

To explore conflicting results obtained when tumor hypoxia is assessed with Eppendorf electrode Po(2) measurements and with positron emission tomography (PET) by use of [(18)F]fluoromisonidazole (FMISO).

METHODS AND MATERIALS

We compared the 2 methods in conjunction with 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG) PET, dual-tracer ex vivo autoradiography (FMISO and 2-deoxy-D-[1-(14)C]glucose (2DG)), and histology in 2 murine tumor models, the C3H mammary carcinoma and the SCCVII squamous cell carcinoma.

RESULTS

2-[(18)F]fluoro-2-deoxy-D-glucose (FDG)-PET showed tumor-to-reference tissue ratios of 3.5 in both tumor models after 2 hours. C3H mammary carcinoma reached an FMISO PET ratio of 11 after 3.5 hours. Autoradiography showed large confluent areas of FMISO and 2DG uptake. Median Po(2) was 7 mm Hg and necrotic fraction was 10% to 30%. SCCVII squamous-cell carcinoma reached an FMISO PET tumor-to-reference tissue ratio of 2 after 2.5 hours. Autoradiography showed homogeneous 2DG uptake and scattered foci of high FMISO uptake. Median Po(2) was 1 mm Hg and necrotic fraction was below 5%.

CONCLUSIONS

Ex vivo dual-tracer autoradiography documented the ability of in vivo FMISO PET to distinguish between confluent areas of either viable tissue or necrosis. Electrode Po(2) measurements could not be ascribed to specific areas in the tumors. Less uptake of FMISO in SCCVII squamous-cell carcinoma than in C3H mammary carcinoma could be caused by scattered foci versus confluent areas of viable hypoxic tissue in the 2 tumors, respectively.

Analysis of sampling volume and tissue heterogeneity on the in vivo detection of fluorescence

The effect of sampling region size and tissue heterogeneity is examined using fluorescence histogram assessment in a rat prostate tumor model with benzoporphyrin derivative fluorophore. Spatial heterogeneity in the fluorescence signal occurs on both macroscopic and microscopic scales. The periphery of the tumor is more fluorescent than the center. Fluorescence is also highest nearest the blood vessels immediately after injection, but over time this fluorescence becomes uniform through the tumor tissue. Using microscopy analysis, the fluorescence intensity histogram distributions follow a normal distribution, yet as the sampling area is increased from the micron scale to the millimeter scale, the variance of the distribution decreases. The mean fluorescence intensity is accurately measured with a millimeter size scale, but this cannot provide accurate measurements of the microscopic variance of drug in tissue. Fiber probe measurements taken in vivo are used to confirm that the variance observed is smaller than would be expected with microscopic sampling, but that the average fluorescence can be measured with fibers. Sampling tissue with fibers smaller than the intercapillary spacing could provide a way to estimate the spatial variance more accurately. In summary, sampling fiber size affects the fluorescence intensities detected and use of multiple region microscopic sampling could provide better information about the distribution of values that occur.

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.

MR perfusion measurements on pharyngeal tumors: comparison of quantification strategies

For the case of pharyngeal carcinomas, the clinical value as well as the stability of several evaluation methods of MR tomographic perfusion measurement are compared. Eighteen patients suffering from histologically proven squamous cell carcinomas were investigated by MR tomography (1.5 T, 0.2 mmol/kg Gd‐DTPA) prior to and during radiation therapy. Perfusion measurements were performed using a double‐echo FLASH sequence. Parameters describing regional blood flow, blood volume, mean transit time, and interstitial concentration of contrast medium (CM) were calculated, applying seven different combinations of correction approaches (separating the shortening of T1 and T2∗, arterial input function (AIF), and tumor shunts). Their correlations to MR independent tumor physiological parameters were analyzed (metabolic activity measurements using ¹⁸F‐FDG‐PET, polarographical pO2 measurement, tumor volume). Significant improvements of the correlation between perfusion‐dependent and other tumor physiological parameters could be achieved by decoupling the shortening of T1 and T2∗ and by applying of the tumor shunt model. Deconvolution from the AIF deteriorated the correlation. Therefore, the elimination of the T1 shortening due to interstitial CM proves to be essential for MR perfusion measurements on contrast medium uptaking lesions. Depending on the measurement conditions (temporal resolution, signal‐to‐noise ratio), the consideration of the AIF can even make the results significantly worse by introducing additional measuring errors.

PACS numbers: 87.61.‐c

Hypoxia Is Important in the Biology and Aggression of Human Glial Brain Tumors

We investigated whether increasing levels of tissue hypoxia, measured by the binding of EF5 [2-(2-nitro-1-H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide] or by Eppendorf needle electrodes, were associated with tumor aggressiveness in patients with previously untreated glial brain tumors. We hypothesized that more extensive and severe hypoxia would be present in tumor cells from patients bearing more clinically aggressive tumors. Hypoxia was measured with the 2-nitroimidazole imaging agent EF5 in 18 patients with supratentorial glial neoplasms. In 12 patients, needle electrode measurements were made intraoperatively. Time to recurrence was used as an indicator of tumor aggression and was analyzed as a function of EF5 binding, electrode values and recursive partitioning analysis (RPA) classification. On the basis of EF5 binding, WHO grade 2 tumors were characterized by modest cellular hypoxia (pO2s approximately 10%) and grade 3 tumors by modest-to-moderate hypoxia (pO2s approximately 10%- 2.5%). Severe hypoxia (approximately 0.1% oxygen) was present in 5 of 12 grade 4 tumors. A correlation between more rapid tumor recurrence and hypoxia was demonstrated with EF5 binding, but this relationship was not predicted by Eppendorf measurements.

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.

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.

Differences in pO2 peaks of a murine fibrosarcoma between carbon-ion and X-ray irradiation

We measured and compared the oxygen partial pressure (pO(2)) profiles in experimental tumors after irradiation with carbon ions and with X-rays. The NFSa fibrosarcomas grown in the hind legs of C3H male mice received isoeffect single doses of carbon ions or X-rays. Coaxial oxygen microelectrodes of high spatial resolution were inserted into the tumor with 20 microm steps by a computerized micromanipulator. The number of pO(2) peaks that reached 15 mmHg were at least 0.45 per 3,000 microm in unirradiated tumors and significantly increased to 1.55 per 3,000 microm as early as day 1 of carbon-ion irradiation (p < 0.001). The tumors that received X-ray irradiation also significantly increased pO(2) peaks, but as late as day 3. The time course of pO(2) peak appearance in the present study coincides with a previous report where reoxygenation was measured by paired growth delay assay. The pO(2) peaks appeared selectively in peripheral regions of X-ray irradiated tumors, but they appeared rather homogeneously in the tumor after carbon-ion irradiation. It is concluded that carbon-ion irradiation reoxygenated the NFSa fibrosarcomas earlier in time and deeper in space than the X-ray irradiation did.

Comparison of the biodistribution of two hypoxia markers [18F]FETNIM and [18F]FMISO in an experimental mammary carcinoma

The first aim of this study was to compare the hypoxia imaging ability of fluorine-18 fluoroerythronitroimidazole ([18F]FETNIM) with that of fluorine-18 fluoromisonimidazole ([18F]FMISO) in murine tumours of different sizes under two different oxygenation conditions. Secondly, we wanted to assess the biodistribution of the markers in normal tissues under similar conditions. Female CDF1 mice with a C3H mammary carcinoma grown on their backs were used. Tumours were size matched and animals breathed either normal air (21% O(2)) or carbogen gas (95% O(2) + 5% CO(2)). The gassing procedure was begun 5 min before the intravenous injection of either [18F]FETNIM or [18F]FMISO and continued until the mice were sacrificed at 120 min. Blood, tumour, muscle, heart, lung, liver, kidney and fat were removed, counted for radioactivity and weighed. The tumour and muscle were frozen and cut with a cryomicrotome into sections. The spatial distribution of radioactivity from the tissue sections was determined with digital autoradiography. Estimation of the necrotic fraction was made on sections from formalin-fixed tumours. Digital autoradiography showed that the whole tumour-to-muscle radioactivity uptake ratios were significantly higher in normal air-breathing mice than in carbogen-treated mice for both [18F]FETNIM (4.9+/-2.6 vs 1.8+/-0.5; P<0.01) and [18F]FMISO (4.4+/-1.0 vs 1.5+/-0.4; P<0.01). The carbogen treatment had only slight effects on the biodistribution of either marker in normal tissues. The necrotic fraction determined in tumours did not correlate with the tumour volume or with the tumour-to-muscle radioactivity uptake ratio. This study shows that the uptake of both [18F]FETNIM and [18F]FMISO correlates with the oxygenation status in tumours. In addition, our data show no significant difference in the intratumoral uptake between the two markers. However, significantly higher radioactivity uptake values were measured for [18F]FMISO than for [18F]FETNIM in normal tissues.

Oxygen depletion during and after mTHPC-mediated photodynamic therapy in RIF1 and H-MESO1 tumors

During photodynamic therapy (PDT), low oxygenation levels, induced both by oxygen consumption and by vascular occlusion, can lead to an inefficient photochemical reaction that may compromise the efficacy of PDT. In the present studies, tumor oxygenation was measured before, during and after meta-tetrahydroxyphenylchlorin (mTHPC)-mediated PDT of murine RIF1 tumors and human mesothelioma xenografts (H-MESO1). Tumor pO2 was measured in real time with Eppendorf polarography, and the extent of relative hypoxia at specific times was measured by immunohistochemical staining. Significant decreases in median pO2 values, as well as an increase in the number of values below 2.5 mmHg, were seen during and after PDT in RIF1 tumors, although there was a large intertumoral variation. Tumor pO2 values did not change significantly in H-MESO1 tumors. Staining with antibodies against the hypoxia marker EF3 showed significant increases in relative hypoxia after PDT in both tumor types compared with separate groups of untreated controls. Our results are consistent with PDT-induced oxygen depletion (reduced pO2) leading to an increase in relative hypoxia in RIF1 tumors. Extensive necrosis in the H-MESO1 tumors may have prevented the detection of PDT-induced hypoxia using the Eppendorf polarographic needle, whereas immunohistochemistry did reveal increases in relative hypoxia.

Effects of hyperbaric oxygen and normobaric carbogen on the radiation response of the rat rhabdomyosarcoma R1H

PURPOSE

Hypoxic tumor cells are an important factor of radioresistance. Hyperbaric oxygen (HBO) and normobaric carbogen (95% oxygen, 5% carbon dioxide) increase the oxygen delivery to tumors. This study was performed to explore changes of tumor oxygenation during a course of fractionated irradiation and to determine the effectiveness of normobaric carbogen and HBO during the final phase of the radiation treatment.

METHODS AND MATERIALS

Experiments were performed on the rhabdomyosarcoma R1H growing on WAG/Rij rats. After 20 X-ray fractions of 2 Gy within 4 weeks, oxygen partial pressure (pO2) was measured using the Eppendorf oxygen electrode under ambient conditions, with normobaric carbogen or HBO at a pressure of 240 kPa. Following the 4-week radiation course, a top-up dose of 10-50 Gy was applied in 2-10 fractions of 5 Gy with or without hyperoxygenation.

RESULTS

HBO but not carbogen significantly increased the median pO2 in irradiated tumors. The radiation doses to control 50% of tumors were 38.0 Gy, 29.5 Gy, and 25.0 Gy for air, carbogen, and HBO, respectively. Both high oxygen content gas inspirations led to significantly improved tumor responses with oxygen enhancement ratios (OERs) of 1.3 for normobaric carbogen and 1.5 for HBO (air vs. carbogen: p = 0.044; air vs. HBO: p = 0.02; carbogen vs. HBO: p = 0.048).

CONCLUSION

Both normobaric carbogen and HBO significantly improved the radiation response of R1H tumors. HBO appeared to be more effective than normobaric carbogen, both with regard to tumor oxygenation and response to irradiation.

The K(m) for Radiosensitization of Human Tumor Cells by Oxygen is Much Greater than 3 mmHg and is Further Increased by Elevated Levels of Cysteine

We studied the role of cysteine as an intracellular radiation protector under conditions in which both oxygen and thiols were monitored at 37 degrees C. In HCT-116 human colon cancer cells, the intracellular cysteine content affects the radiation survival dramatically at intermediate oxygen levels, but not at zero or high oxygen levels. Using a spin-through-oil method with a dual radioactive label detection system, we measured intracellular cysteine and glutathione (GSH) levels for cells in suspension culture. A comparison of the cysteine levels of monolayer cells lysed in situ and of trypsinized monolayer cells in suspension (Horan et al., Cytometry 29, 76-82, 1997) revealed that, upon trypsinization from monolayer culture and transfer to a spinner apparatus at 37 degrees C, HCT-116 cells lose most of their intracellular cysteine. Over the 60-min time course of control experiments, these cells do not recover intracellular cysteine despite the availability of cystine (the disulfide of cysteine) in the medium. When cells in spinner culture are provided with exogenous cysteine, they initially concentrate it to 10-fold the extracellular concentration, with the concentration factor decreasing to about 5-fold over the course of an hour. The intracellular GSH concentration changes little throughout this period, regardless of the changes in cysteine levels. The same apparatus was used to assess the survival of HCT-116 cells irradiated at 37 degrees C under conditions of constant pO(2) monitoring. For cells without added cysteine, the oxygen concentration for half-maximal radiation sensitivity was about 7.5 mmHg (intermediate hypoxia), more than twice the commonly accepted value (3 mmHg). At 7.5 mmHg, cells with added cysteine (intracellular concentration 3.5 mM) were almost as radioresistant as severely hypoxic cells (approximately 0.005% oxygen). Cells in parallel experiments in which the cells were grown in monolayers on glass Petri dishes had intermediate cysteine values and also intermediate radiosensitivity. We conclude that the radiation response of cells at intermediate oxygen levels is controlled predominantly by intracellular cysteine levels and that the cysteine levels commonly found in tumors may increase the K(m) for radiosensitivity to values much higher than suggested previously.

Feasibility of detecting hypoxia in experimental mouse tumours with 18F-fluorinated tracers and positron emission tomography--a study evaluating [18F]Fluoro-2-deoxy-D-glucose

The study was designed to investigate the binding of [18F]Fluoromisonidazole ([18F]FMISO) and [18F]Fluoro-2-deoxy-D-glucose ([18F]FDG) in a C3H mouse mammary carcinoma. Non-anaesthetized tumour-bearing animals breathing either normal air or carbogen (to reduce tumour hypoxia) were examined by PET after tracer injection. Tumours were identified by radioactive labelling and methods of defining regions of interest (ROI) in the tumours were investigated. Reference tissue was selected elsewhere in the mice and the ratio between mean radioactivity in tumour and reference tissue was compared. The results showed a correlation between the methods of identifying ROIs and a significantly lower tumour to reference tissue ratio for carbogen-treated mice compared with controls when using [18F]FMISO. Only one of the methods showed a significant difference in the tumour labelling between treatment groups using [18F]FDG. The study supports the contention that [18F]FMISO may be able to identify hypoxia in tumours, whereas a similar role for [18F]FDG is more doubtful.

Estimation of oxygen distribution in RIF-1 tumors by diffusion model-based interpretation of pimonidazole hypoxia and eppendorf measurements

Numerical simulations of oxygen diffusion from the capillaries in tumor tissue were used to predict the capillary oxygen supply within and near hypoxic regions of the RIF-1 tumor. A finite element method to simulate the oxygen distribution from a histology section is presented, along with a method to iteratively estimate capillary oxygen concentrations. Pathological structural data for these simulations came from sections of the tumor stained with hematoxylin and eosin and were used to define the capillary positions and shapes, while overlapping regions of low oxygen concentration were defined by the hypoxia marker pimonidazole. These simulations were used to calculate spatial maps of the oxygen concentration and were tested for their ability to reproduce Eppendorf pO(2) histograms from the same tumor line. This simulation study predicted that capillary oxygen concentrations ranged from zero to above 20 microM, with a dominant peak in the hypoxic regions showing 78% of capillaries with less than 1 microM oxygen concentration, compared to only 12% in the non-hypoxic regions. The results were not highly sensitive to the metabolic oxygen consumption rate, within the range of 2 to 16 microM/s. This numerical method for oxygen capillary simulation is readily adaptable to histology sections and provides a method to examine the heterogeneity of oxygen within the capillaries and throughout the tumor tissue section being examined.

Comparison between the comet assay and pimonidazole binding for measuring tumour hypoxia

Pimonidazole is finding increasing use in histochemical analyses of hypoxia in tumours. Whether it can identify every hypoxic cell in a tumour, and whether the usual subjective criteria used to define 'positive' cells are optimal, are less certain. Therefore, our aim was to develop an objective flow cytometry procedure for quantifying pimonidazole binding in tumours, and to validate this method by using a more direct indicator of radiobiologic hypoxia, the comet assay. SCCVII tumours in C3H mice were analysed for pimonidazole binding using flow cytometry and an iterative curve-fitting procedure, and the results were compared to the comet assay for the same cell suspensions. On average, cells defined as anoxic by flow analysis (n = 43 tumours) bound 10.8 +/- 0.95 times more antibody than aerobic cells. In samples containing known mixtures of aerobic and anoxic cells, hypoxic fractions as low as 0.5% could easily be detected. To assess the flow cytometry assay under a wider range of tumour oxygen contents, mice were injected with hydralazine to reduce tumour blood flow, or allowed to breathe various gas mixtures during the 90 min exposure to pimonidazole. Hypoxic fraction estimated by the pimonidazole binding method agreed well with the hypoxic fraction measured using the comet assay in SCCVII tumours (r2 = 0.87, slope = 0.98), with similar results in human U87 glioma cells and SiHa cervical carcinoma xenografts. We therefore conclude that this objective analysis of pimonidazole labelling by flow cytometry gives a convenient and accurate estimate of radiobiological hypoxia. Preliminary analyses of biopsies from 3 patients given 0.5 g m-2 pimonidazole also suggest the suitability of this approach for human tumours.

Radiation-induced DNA damage in tumors and normal tissues. VI. Estimation of the hypoxic fraction of experimental tumors

For several years, we have concentrated our efforts on validating the use of radiation-induced DNA strand breaks and DNA-protein crosslinks to assess the oxygenation status of tumors and normal tissues. We have demonstrated that (1) the oxygen dependence of strand break formation is identical to that of radiation-induced cell killing; (2) the oxygen dependence of DNA-protein crosslink formation is the mirror image of that of radiation-induced cell killing; and (3) the formation of these radiation-induced DNA lesions is predominantly dependent on the oxygen concentration near the DNA and is independent of the cell type, metabolic status, proliferative status, pH of the surrounding environment, and composition or properties of the proteins tightly associated with the DNA. In the present study, the hypoxic fraction of three experimental tumors was estimated using our assay of radiation-induced DNA damage. The average hypoxic fraction of a large number of tumors estimated with this assay of radiation-induced DNA damage for (1) WiDR human colorectal carcinoma xenografts (40.8 +/- 4.2%), (2) 66 mouse mammary adenocarcinoma tumors (41.8 +/- 3.1%), and (3) subcutaneous tumors grown from 9L rat brain tumor cells (95% CI =-8.2-4.2%) was not statistically different from that of a large number of tumors measured for each of these tumor models by the paired survival curve method (38.3 +/- 6. 3%, 28.9 +/- 5.5%, 95% CI = 2.2-4.4%, respectively). When the hypoxic fraction measured by the alkaline elution method on one half of an individual tumor was compared to that measured by the paired survival curve method on the other half of the same tumor, no statistical correlation was found for either 66 or WiDR tumors. Although this assay of radiation-induced DNA damage can be used effectively in the laboratory to answer a number of important questions about the oxygenation status of animal tumors and normal tissues, failure to reliably estimate the hypoxic fraction of individual tumors and technical considerations make it unlikely that the assay can be used in the clinic to estimate the hypoxic fraction of human tumors.

Hypoxia and necrosis in rat 9L glioma and Morris 7777 hepatoma tumors: comparative measurements using EF5 binding and the Eppendorf needle electrode

PURPOSE

The purpose of this study was to assess the presence of tumor hypoxia using two independent techniques: binding of the 2-nitroimidazole EF5 and Eppendorf needle electrode measurements. The distribution of tumor hypoxia was assessed with respect to tumor necrosis in corresponding histological studies.

METHODS AND MATERIALS

Each of several rats bearing a subcutaneous 9L glioma or Morris 7777 hepatoma tumor was given EF5 i.v. to a final, whole-body concentration of 100 microM. About 2.5 h later, each rat was anesthetized, and needle electrode measurements were made in the tumor along 1-5 tracks (30-200 individual measurements). At 3 h post-EF5 injection, the tumor was excised and frozen. Frozen sections were analyzed for the presence and distribution of binding of EF5 and necrosis using immunohistochemical techniques followed by staining with hematoxylin and eosin (H&E). The histochemical analysis and electrode readings in similar regions of the tumor were compared.

RESULTS

Electrode measurements were taken at 0.4-mm intervals along one-dimensional tracks, whereas EF5 binding measurements from tissue sections contained two-dimensional information at high spatial resolution ( approximately 2.5 micro). The EF5 measurements showed greater spatial heterogeneity than did the electrode measurements. In tumor regions with minimal necrosis, needle tracks with relatively high pO(2) readings were usually found to contain relatively low EF5 binding, and vice versa. Because EF5 binding is inversely related to tissue pO(2), this result was expected. The expected inverse correlation of the two techniques was most disparate in necrotic tumor regions (confirmed by H&E staining), where needle electrode measurements showed low to zero pO(2) values, but little or no EF5 binding was found.

CONCLUSION

The two methods compared in this study operate in fundamentally different ways and provide substantially different information. EF5 binding provided detailed spatial information on the distribution of hypoxia in viable tumor tissue. There was no EF5 binding in necrotic tumor tissue because cells in such tissue were unable to metabolize the drug. In contrast, output from the needle electrode method appeared to represent a "track-average" tissue pO(2) and did not distinguish between extreme hypoxia and either macroscopic or microscopic necrosis. At the present time, the importance of tumor necrosis in determining treatment response is unknown. However, our data suggest that the Eppendorf needle electrode technique will overestimate the presence of hypoxia. Both techniques are potentially limited by sampling errors in tumors with heterogeneous distributions of hypoxia.

Determining hypoxic fraction in a rat glioma by uptake of radiolabeled fluoromisonidazole

The usefulness of radiolabeled nitroimidazoles for measuring hypoxia will be clarified by defining the relationship between tracer uptake and radiobiologically hypoxic fraction. We determined the radiobiologically hypoxic fraction from radiation response data in 36B10 rat gliomas using the paired cell survival curve technique and compared the values to the radiobiologically hypoxic fraction inferred from mathematical modeling of time-activity data acquired by PET imaging of [(18)F]FMISO uptake. Rats breathed either air or 10% oxygen during imaging, and timed blood samples were taken. The uptake of [(3)H]FMISO by 36B10 cells in vitro provided cellular binding characteristics of this radiopharmaceutical as a function of oxygen concentration. The radiobiologically hypoxic fraction determined for tumors in air-breathing rats using the paired survival curve technique was 6.1% (95% CL = 4.3- 8.6%), which agreed well with that determined by modeling FMISO time-activity data (7. 4%; 95% CL = 2.5-17.3%). These results are consistent with the agreement between the two techniques for measuring radiobiologically hypoxic fraction in Chinese hamster V79 cell spheroids. In contrast, the FMISO-derived radiobiologically hypoxic fraction in rats breathing 10% oxygen was 13.1% (95% CL 7.9-8.3%), much lower than the radiobiologically hypoxic fraction of 43% determined from the radiation response data. This discrepancy may be due to the failure of FMISO to identify hypoxic cells residing at or above an oxygen level of 2-3 mmHg that will still confer substantial protection against radiation. The presence of transiently hypoxic cells in rats breathing reduced oxygen may also be under-reported by nitroimidazole binding, which is strongly dependent on time and concentration.

Oxygen tension measurements of tumors growing in mice

PURPOSE

Clinical studies using the Eppendorf histograph have shown that patients whose tumors have a low pO2 have worse local control after radiotherapy, and have higher metastatic rates. Because preclinical studies of methods of overcoming, or exploiting, hypoxia generally use transplanted tumors in mice, we have compared the oxygenation of mouse tumors with human tumors to determine the appropriateness of the transplanted mouse model for such preclinical studies.

METHODS AND MATERIALS

We evaluated the oxygenation status of subcutaneous (s.c.) tissue and of 12 intradermally (i.d.)- and 7 s.c.-growing mouse or human transplanted tumors in mice using the Eppendorf histograph, and compared the values obtained with measurements of human head and neck nodes.

RESULTS

The normal tissue pO2 profile of air-breathing mice showed a nearly Gaussian distribution (38.2+/-14.9 mmHg). Breathing 10% O2 or carbogen resulted in dramatic changes in normal tissue oxygenation. Tumors growing intradermally in the back of air-breathing mice were extremely hypoxic and resistant to expected changes in oxygenation (carbogen breathing, size, and use of anesthetics). Tumors growing s.c. in the foot showed higher oxygen profiles with marked changes in oxygenation when exposing the animals to different levels of oxygen. However, the oxygenation of the mouse tumors transplanted in either site was only a fraction of that of the majority of human tumors.

CONCLUSION

Experimental mouse tumors are markedly hypoxic, with median values of 10-20% of those of human tumors. Hence, mouse tumors are probably good models for the most hypoxic human tumors that respond poorly to radiotherapy; however, caution has to be exercised in extrapolating data from mouse to man.

A comparison in individual murine tumors of techniques for measuring oxygen levels

PURPOSE

To investigate the relationship between different techniques for measuring oxygen levels in a murine tumor model.

METHODS AND MATERIALS

Using the murine fibrosarcoma line KHT-C, five techniques of measuring oxygen levels-the Eppendorf pO2 Histograph, EF5 binding, the comet assay, a paired survival assay, and an in vivo growth delay assay-were assessed. In these experiments, three or more techniques were applied in different combinations to measure the oxygen levels in individual tumors.

RESULTS

Statistically significant correlations were observed between the hypoxic proportions calculated from the paired survival assay with those from EF5 binding. The comet assay was found to have a statistically significant correlation with the paired survival analysis and the growth delay analysis. No statistically significant correlation was found between the Eppendorf pO2 Histograph measurements and those from the other techniques, although there were weak correlations with the paired survival assay and EF5 binding. For technical reasons, a comparison was not made between EF5 binding and the growth delay assay.

CONCLUSIONS

The correlations found between EF5 binding and the comet assay with the radiobiological assays suggest that these techniques have potential for predicting outcome following radiation treatment. The lack of correlation seen between the pO2 Histograph data and the radiobiological assays is in contrast to results from early clinical trials.

Comparison between the comet assay and the oxygen microelectrode for measurement of tumor hypoxia

BACKGROUND AND PURPOSE

Hypoxic cells are present in some solid tumours and are known to limit radiocurability. To compare two measures of tumour hypoxia, 25 patients with locally advanced disease and accessible tumours or metastatic nodes were examined using an oxygen microelectrode and the alkaline comet assay.

MEASUREMENTS AND METHODS

For the comet assay, fine needle aspirate biopsies were taken immediately following a dose of 5-10 Gy. Single cells were examined for radiation-induced DNA strand breaks, and the percentage of radio-resistant hypoxic cells within the population was calculated from DNA damage histograms. For oxygen tension (pO2) measurements, multiple tracks were made using an Eppendorf oxygen microelectrode. The possibility that application of the first method might influence hypoxic fraction measurement by the second method was examined in a more controlled system by creating four tracks in murine SCC-VII tumours using an oxygen electrode, and measuring hypoxic fraction at subsequent times.

RESULTS

For 28 tumours from 25 patients, hypoxic fraction measured by comet assay correlated with the percentage of PO2 values < 5 mmHg (r2 = 0.46, P < 0.001). The mean comet hypoxic fraction was 0.36 for five tumours with a median PO2 < 10 mmHg. For the remaining 23 tumours with a median PO2 > 10 mmHg, the mean hypoxic fraction was 0.09. Advancement of an oxygen electrode through SCCVII tumours had no significant effect on hypoxic fraction measured 5 min to 24 h later using the alkaline comet assay.

CONCLUSIONS

Tumours defined as hypoxic based on a median pO2 < 10 mmHg appear to contain more than 20% radio-biologically hypoxic cells as estimated by the comet assay. In an animal tumour model, puncture of the tumour with an oxygen electrode did not influence hypoxic fraction measured using the comet assay, in agreement with the clinical data that the order in which the two methods were performed was not important.

RSR13, an allosteric effector of haemoglobin, and carbogen radiosensitize FSAII and SCCVII tumours in C3H mice

Pre-clinical evaluation has demonstrated that 2-[4-(((3,5-dimethylanilino)carbonyl)methyl)phenoxy]-2-methylpropi onic acid (RSR13) acts as an allosteric effector of haemoglobin (Hb). RSR13 binding to Hb results in decreased haemoglobin-oxygen (Hb-O2) affinity, improved tumour oxygenation, and enhanced radiation-induced cell killing in several experimental tumour systems. In the present work, ex vivo clonogenic survival analyses are applied in two murine tumour systems to characterize the relationship between the magnitude of decrease in Hb-O2 affinity and radiosensitization, the influence of inspired pO2 upon this effect, and the efficacy of combining RSR13 and radiation during a course of repeated radiation exposures. For FSaII tumours in C3H mice breathing air, 100 mg kg(-1) RSR13 administered intraperitoneally produced an enhancement ratio (ER) of 1.3, but there was marked desensitization at a RSR13 dose of 300 mg kg(-1) (ER 0.6). The most likely reason for the increased radioresistance was insufficient oxygen loading of Hb in the pulmonary circulation due to reduced haemoglobin-oxygen affinity because carbogen breathing combined with 300 mg kg(-1) RSR13 reversed the effect and produced an ER of 1.8. In SCCVII tumours in C3H mice irradiated with eight fractions of 2.5 Gy over 4 days, the surviving fraction was reduced to 58-67% of control values when RSR13 was combined with radiation on days 1 and 2, days 3 and 4, or days 1-4. These results confirm that combining RSR13 and irradiation within a fractionated course of clinically relevant low-dose exposures provides significant radiosensitization. Additional preclinical experimentation is needed to define better the optimum dose-scheduling conditions for clinical applications.

Changes in head and neck tumor hypoxic fraction during split-course radiochemotherapy

This study explored patterns of oxygen distribution in human tumors during primary radiochemotherapy. Patients with positive nodes from head and neck squamous cell carcinoma (n = 15) were investigated before therapy, before and after a 2-week respite, and at the end of the treatment. Intratumoral tissue oxygen tension (pO2) was measured with sterile polarographic needle electrodes and a computerized histography system. The 2 lowest pO2 classes, ie, 0 and 5 mm Hg, were designated the hypoxic fraction, and the mean and median pO2 were evaluated for each tumor. In the beginning, a marked variability in tissue pO2 was found. The initial size of the hypoxic fraction ranged from 0% to 61% of measured values. At the end of treatment, 4 tumors showed an increase in mean pO2, and 7 tumors a lower mean pO2 in comparison to the initial values. However, an impressive finding was that the hypoxic fraction of 9 tumors became smaller during the pause, 1 tumor showed no change, and 1 showed an increase in hypoxic fraction. The results show that there is a widely distributed tissue oxygenation with marked hypoxic zones in human neck nodes. During radiochemotherapy, tissue oxygenation changed, with great intertumor variability. A tendency toward a decrease of the hypoxic fraction after the respite could be seen. This change in tumor oxygenation during therapy needs further evaluation.

Heterogeneity of tumor oxygenation: relationship to tumor necrosis, tumor size, and metastasis

PURPOSE

Measurements of oxygenation in the transplanted rodent KHT-C and SCC-VII tumors demonstrate significant heterogeneity from tumor to tumor as is observed in human tumors. This finding suggests that heterogeneity in oxygenation between tumors is likely related to factors associated with tumor growth rather than to intrinsic genetic differences. In this study we examined whether measurements of the oxygenation of individual KHT-C tumors were related to necrosis in the tumors or to tumor size and whether the more hypoxic tumors gave rise to more metastases.

METHODS

Tumors were grown in the gastrocnemius muscle of C3H mice and tumor oxygenation was measured at defined sizes (approx. 0.35 g, 1.0 g, and 2.0 g) using an Eppendorf polarographic oxygen probe. Necrosis was assessed by examining histological sections cut from tumors used for the oxygen measurements. Metastasis was assessed by counting macroscopic lung nodules in mice sacrificed when their tumors reached a size of approximately 2 g.

RESULTS

Tumor oxygenation in individual KHT-C tumors became poorer and necrosis became more extensive as the tumors grew larger but, at a size of 0.3-0.4 g, there was no relationship between oxygenation and extent of necrosis. In general, measurements of tumor pO2 at a size of 0.3-0.4 g were predictive of tumor PO2 in the same tumor at a size of about 1 g, but by the time the tumors reached a size of about 2 g they were all very hypoxic. There was a trend suggesting a relationship between macroscopic metastases in the lung and degree of hypoxia in the KHT-C tumors but this was not statistically significant.

CONCLUSION

The results indicate that the heterogeneity of oxygenation seen in KHT-C tumors is not explained by different degrees of necrosis in the individual tumors. The lack of a correlation between increased metastasis formation and increased levels of hypoxia in the KHT-C tumors is not consistent with results reported for human tumors.

Experimental studies on the possible influence of invasive oxygen measurements on tumour radiosensitivity

The effects of tissue damage associated with invasive pO2 measurements on radiation sensitivity were investigated using a xenografted squamous cell carcinoma model. For the tumour cure experiments, single dose irradiations were given following different regimens of polarographic pO2 measurements associated with different degrees of mechanical tissue damage. With a dose of 32 Gy, 57% of animals were cured. Following 3 tracks of needle measurements, 73% of tumours were locally controlled, and 75% were cured after 8 needle tracks. The polarographic measurements gave virtually identical oxygenation data for recurrent or cured tumours (both median pO2 1.0 mmHg), respectively. There was thus no evidence of decreased radiosensitivity associated with tissue damage after invasive pO2 measurements. The pre-therapeutic oxygenation status gave no evidence for a prediction of radiation response on an individual basis.

The effect of combined nicotinamide and carbogen treatments in human tumour xenografts: oxygenation and tumour control studies

BACKGROUND AND PURPOSE

This was an investigation to study the effect of giving carbogen and nicotinamide (CON) on pO2 and the radiation response of human xenografted tumours.

MATERIALS AND METHODS

The human xenografts were two sarcomas (ENE2 and ES3) and a glioblastoma (HTZ17). Nicotinamide (500 mg/ kg, i.p.) was administered 60 min before PO2 measurements and irradiation, while carbogen was given for 5 min before and during these treatments. Tumour pO2 was measured with an Eppendorf electrode and radiation response was assessed by local tumour control following irradiation with 10 daily fractions.

RESULTS

All three xenografts were found to be poorly oxygenated (about 80% of all pO2 values were < or =2.5 mmHg). CON treatment improved the oxygenation status in all three tumours such that 65, 52 and 71% of the pO2 values were < or =2.5 mmHg in ENE2, ES3 and HTZ17, respectively. However, only in ES3 was this decrease significant. The TCD50 doses for all tumours were around 52-54 Gy. No significant improvement was seen with CON in ENE2 (TCD50 = 48 Gy) and HTZ17 (TCD50 = 56 Gy), but for the ES3 xenograft a significant decrease to 42 Gy was found.

CONCLUSIONS

The three tumours used in this study appeared to show the same level of hypoxia as measured both by pO2 and radiation response. However, only one tumour showed a significant improvement after CON treatment, suggesting that not all hypoxic human tumours might benefit from this type of therapy.

Reduction of tumour oxygenation during and after photodynamic therapy in vivo: effects of fluence rate

It has been proposed that the generation of O2 during photodynamic therapy (PDT) may lead to photochemical depletion of ambient tumour oxygen, thus causing acute hypoxia and limiting treatment effectiveness. We have studied the effects of fluence rate on pO2, in the murine RIF tumour during and after PDT using 5 mg kg(-1) Photofrin and fluence rates of 30, 75 or 150 mW cm(-2). Median pO2 before PDT ranged from 2.9 to 5.2 mmHg in three treatment groups. Within the first minute of illumination, median tumour pO2 decreased with all fluence rates to values between 0.7 and 1.1 mmHg. These effects were rapidly and completely reversible if illumination was interrupted. During prolonged illumination (20-50 J cm(-2)) pO2 recovered at the 30 mW cm(-2) fluence rate to a median value of 7.4 mmHg, but remained low at the 150 mW cm(-2) fluence rate (median pO2 1.7 mmHg). Fluence rate effects were not found after PDT, and at both 30 and 150 mW cm(-2) median tumour pO2 fell from control levels to 1.0-1.8 mmHg within 1-3 h after treatment conclusion. PDT with 100 J cm(-2) at 30 mW cm(-2) caused significantly (P = 0.0004) longer median tumour regrowth times than PDT at 150 mW cm(-2), indicating that lower fluence rate can improve PDT response. Vascular perfusion studies uncovered significant fluence rate-dependent differences in the responses of the normal and tumour vasculature. These data establish a direct relationship between tumour pO2, the fluence rate applied during PDT and treatment outcome. The findings are of immediate clinical relevance.

The relationship between tumour oxygenation determined by oxygen electrode measurements and magnetic resonance spectroscopy of the fluorinated 2-nitroimidazole SR-4554

The relationship between two methods of assessing tumour oxygenation in vivo, namely oxygen electrode measurement and magnetic resonance spectroscopy (MRS) of the fluorinated 2-nitroimidazole SR-4554, was investigated. Using three tumour models (two sites), no linear correlation was observed between 19F retention index and pO2 parameters (r < or = 0.3). Substantial retention of SR-4554 (19F retention index > 0.5) was, however, associated with low tumour pO2 (% pO2 < or = 5 mmHg = 60%). Depending on the pO2 parameters used, SR-4554 administration was shown to produce either a significant or a non-significant increase in tumour oxygenation. We conclude that measurement of SR-4554-related compound(s) by 19F-MRS has the potential to detect clinically relevant levels of tumour hypoxia.

Effects of carbogen plus fractionated irradiation on KHT tumor oxygenation

BACKGROUND AND PURPOSE

Numerous studies have demonstrated improvements in the oxygenation of tumor cells following both irradiation and carbogen breathing. The current studies were initiated to measure the combined effects of carbogen inhalation plus single and multi-dose irradiation on tumor oxygen availability, to better define the underlying physiological relationships.

MATERIALS AND METHODS

Using KHT murine sarcomas, radiation was delivered to the tumor-bearing legs of non-anesthetized mice. Tumors were quick-frozen prior to or following single or multifraction irradiation and carbogen breathing, and intravascular HbO2 saturation profiles were determined cryospectrophotometrically.

RESULTS

HbO2 levels for blood vessels located near the tumor surface initially decreased following 10 Gy irradiation, then increased and remained elevated. Interior HbO2 levels remained unchanged. Following 2.5 Gy, HbO2 changes were minimal. At 24 h following 10 Gy, HbO2 levels were significantly increased compared to non-irradiated controls, and carbogen breathing produced no additional benefit. At 24 h following five fractions of 2 Gy, HbO2 levels throughout the tumor volume were significantly higher in carbogen breathing animals than in air breathing controls.

CONCLUSIONS

Although peripheral blood vessels demonstrated substantial improvements in oxygenation following irradiation, oxygen availability nearer the tumor center remained at very low levels. The utility of carbogen in enhancing tumor oxygen availability was maintained following five clinically relevant fractions. At higher doses, radiation-induced enhancements in HbO2 levels overshadowed the carbogen effect. For either air or carbogen breathing, a decrease in the percentage of vessels with very low oxygen content did not appear to be a major factor in the reoxygenation of the KHT tumor.

Detection of hypoxic cells in a C3H mouse mammary carcinoma using the comet assay

The comet assay was used to estimate radiobiological hypoxic fraction across a full range of tumour oxygenations in C3H mammary tumours implanted into the feet of female CDF1 mice. Tumours were either clamped before irradiation or mice were allowed to breath air, 100% oxygen, carbogen or carbon monoxide for 5-35 min before and during exposure to 15 Gy. For the alkaline comet assay, tumours were excised after irradiation and individual tumour cells were analysed for DNA single-strand breaks. Hypoxic cells were defined as those cells with approximately three times fewer single-strand breaks than aerobic cells. Radiobiological hypoxic fraction was calculated by fitting DNA damage histograms to two normal distributions, representing the response of the aerobic and hypoxic populations. The percentage of hypoxic cells estimated using the comet assay was then compared with hypoxic fraction measured using a clamped tumour control assay. Carbogen and oxygen breathing reduced the normal hypoxic fraction from 14% to 2-3% in this tumour, whereas 75-660 p.p.m. carbon monoxide progressively increased the hypoxic fraction from 18% to 82%. The slope of the line comparing the two methods was 1.23 with 95% confidence limits of 1.12-1.33 (r2 = 0.994). In the SCCVII squamous cell carcinoma growing subcutaneously in C3H mice, a similar correlation was observed between hypoxic fraction measured using the comet assay and hypoxic fraction measured in the same tumour cells using the paired survival curve assay (slope = 1.20 with 95% confidence limits of 1.03-1.37). These results confirm the ability of the comet assay to provide an accurate estimate of radiobiological hypoxic fraction over a wide range of tumour oxygenations and between two tumour types.

The apparent diffusion constant measured by MRI correlates with pO2 in a RIF-1 tumor

As tissue oxygen tension (pO2) is an important variable in cancer therapy, it would be of major clinical benefit to be able to measure pO2 noninvasively. Current methods for determining pO2 in clinical settings are limited to superficial tumors. The authors measured the apparent diffusion constant (ADC) in an implanted murine fibrosarcoma (RIF-1) using magnetic resonance imaging and correlated the ADC with tissue pO2 measured by electron paramagnetic resonance oximetry. The ADC correlates significantly with tissue pO2 in this tumor (r = 0.89; n = 14) and so may provide a noninvasive index of pO2 in tumors.