Diffusion limited hypoxia estimated by vascular image analysis: comparison with pimonidazole staining in human tumors

In vivo noninvasive preclinical tumor hypoxia imaging methods: a review

Tumors exhibit areas of decreased oxygenation due to malformed blood vessels. This low oxygen concentration decreases the effectiveness of radiation therapy, and the resulting poor perfusion can prevent drugs from reaching areas of the tumor. Tumor hypoxia is associated with poorer prognosis and disease progression, and is therefore of interest to preclinical researchers. Although there are multiple different ways to measure tumor hypoxia and related factors, there is no standard for quantifying spatial and temporal tumor hypoxia distributions in preclinical research or in the clinic. This review compares imaging methods utilized for the purpose of assessing spatio-temporal patterns of hypoxia in the preclinical setting. Imaging methods provide varying levels of spatial and temporal resolution regarding different aspects of hypoxia, and with varying advantages and disadvantages. The choice of modality requires consideration of the specific experimental model, the nature of the required characterization and the availability of complementary modalities as well as immunohistochemistry.

[18F]Fluoromisonidazole PET in rectal cancer

BACKGROUND

There is an increasing interest in developing predictive biomarkers of tissue hypoxia using functional imaging for personalised radiotherapy in patients with rectal cancer that are considered for neoadjuvant chemoradiotherapy (CRT). The study explores [18F]fluoromisonidazole ([18F]FMISO) positron emission tomography (PET) scans for predicting clinical response in rectal cancer patients receiving neoadjuvant CRT.

METHODS

Patients with biopsy-proven rectal adenocarcinoma were imaged at 0-45 min, 2 and 4 h, at baseline and after 8-10 fractions of CRT (week 2). The first 6 patients did not receive an enema (the non-enema group) and the last 4 patients received an enema before PET-CT scan (the enema group). [18F]FMISO production failed on 2 occasions. Static PET images at 4 h were analysed using tumour-to-muscle (T:M) SUVmax and tumour-to-blood (T:B) SUVmax. The 0-45 min dynamic PET scans were analysed using Casciari model to report hypoxia and perfusion. Akaike information criteria (AIC) were used to compare data fittings for different pharmacokinetic models. Pathological tumour regression grade was scored using American Joint Committee on Cancer (AJCC) 7.0. Shapiro-Wilk test was used to evaluate the normality of the data.

RESULTS

Five out of eleven (5/11) patients were classed as good responders (AJCC 0/1 or good clinical response) and 6/11 as poor responders (AJCC 2/3 or poor clinical response). The median T:M SUVmax was 2.14 (IQR 0.58) at baseline and 1.30 (IQR 0.19) at week 2, and the corresponding median tumour hypoxia volume was 1.08 (IQR 1.31) cm3 and 0 (IQR 0.15) cm3, respectively. The median T:B SUVmax was 2.46 (IQR 1.50) at baseline and 1.61 (IQR 0.14) at week 2, and the corresponding median tumour hypoxia volume was 5.68 (IQR 5.86) cm3 and 0.76 (IQR 0.78) cm3, respectively. For 0-45 min tumour modelling, the median hypoxia was 0.92 (IQR 0.41) min-1 at baseline and 0.70 (IQR 0.10) min-1 at week 2. The median perfusion was 4.10 (IQR 1.71) ml g-1 min-1 at baseline and 2.48 (IQR 3.62) ml g-1 min-1 at week 2. In 9/11 patients with both PET scans, tumour perfusion decreased in non-responders and increased in responders except in one patient. None of the changes in other PET parameters showed any clear trend with clinical outcome.

CONCLUSIONS

This pilot study with small number of datasets revealed significant challenges in delivery and interpretation of [18F]FMISO PET scans of rectal cancer. There are two principal problems namely spill-in from non-tumour tracer activity from rectal and bladder contents. Emphasis should be made on reducing spill-in effects from the bladder to improve data quality. This preliminary study has shown fundamental difficulties in the interpretation of [18F]FMISO PET scans for rectal cancer, limiting its clinical applicability.

Cerebral and extracerebral vulnerability to hypoxic insults after diffuse traumatic brain injury in rats

The post-traumatic brain vulnerability suggests that after traumatic brain injury (TBI), the brain may be more susceptible to posttraumatic hypoxic insults. This concept could be extended to 'peripheral' organs, as non-neurologic organ failure is common after TBI. This study aims to characterize and quantify cerebral and extracerebral tissue hypoxia with pimonidazole resulting from a standardized hypoxia-hypotension (HH) phase occurring after a diffuse experimental TBI in rats. Rats were allocated to Sham (n=5), TBI (n=7), HH (n=7) and TBI+HH (n=7) groups. Then, pimonidazole was injected and brain, liver, heart and kidneys were analysed. In the cerebral cortex, post-treatment hypoxia was higher in TBI+HH group than Sham group (p=0.003), HH group (p=0.003) and TBI group (p=0.002). Large trends in thalamus, hippocampus and striatum for the TBI+HH group compared to the other groups were observed. For the heart and liver, the 4 groups were comparable. For the kidneys, post-treatment hypoxia was higher in the TBI group compared to the Sham and HH groups, but not more than TBI+HH group. This study reveals that a posttraumatic hypoxic insult occurring after a severe TBI has major hypoxic consequences on brain structures. However, TBI by itself appears to induce renal hypoxia that is not enhanced by posttraumatic hypoxic insult.

Oncogenic but non-essential role of N-myc downstream regulated gene 1 in the progression of esophageal squamous cell carcinoma

N-myc downstream regulated gene 1 (NDRG1/Cap43/Drg-1) has previously been shown to be dysregulated in esophageal squamous cell carcinoma (ESCC). In this study, we investigated the role of NDRG1 in the neoplastic progression of ESCC using ectopic gain-of-function and loss-of-function approaches. Stable transfectants of the KYSE30 ESCC cell line with altered NDRG1 levels were generated by lentiviral transduction. Although no measurable effects on in vitro cell proliferation were observed with altered NDRG1 expression, the ectopic overexpression of NDRG1 was positively linked to recognized markers of metastasis, angiogenesis and apoptotic evasion. Accordingly, in the nude mouse xenograft model system, NDRG1 overexpression promoted the in vivo growth of KYSE30 derived xenografts, which could be attributed to the reduced apoptotic and enhanced angiogenic activities associated with this gene. These processes were mediated in part by increased NFκB activity in NDRG1 overexpressing cells. Nevertheless, no significant phenotypic changes were observed in response to NDRG1 knock-down, suggesting that this gene might not be essential for the neoplastic progression of ESCC. Taken together, our results suggest that NDRG1 may play positive but dispensable roles in the progression of esophageal squamous cell carcinoma.

Differential expression of HIF-1 in glioblastoma multiforme and anaplastic astrocytoma

Hypoxia is an important factor mediating tumor progression and therapeutic resistance, in part through proteome changes mediated by the transcription factor hypoxia-inducible factor (HIF)-1. Since glioblastoma multiforme is the epitome of a highly aggressive tumor entity, while lower-grade astrocytomas often show a prolonged clinical course, a profound difference in the extent of hypoxic tissue areas and corresponding magnitude of HIF-1 activity may exist between these entities. In this study, to address this question, serial sections of 11 glioblastomas and 10 anaplastic astrocytomas were immunostained for HIF-1α, glucose transporter (GLUT)-1, carbonic anhydrase (CA) IX (i.e., hypoxia-related markers), Ki67 (proliferation), phosphorylated ribosomal protein S6 [p-rpS6; mammalian target of rapamycin (mTOR) activity] and CD34 (microvascular endothelium). Digital scans of whole tumor sections were registered to achieve geometric correspondence for subsequent morphometric operations. HIF-1α-, GLUT-1- and CA IX-positive staining was found in all 11 glioblastomas, showing a preferential expression in tissue areas adjacent to necroses. A considerable spatial overlap between GLUT-1 and CA IX, and a colocalization of these proteins with areas of enlarged mean diffusion distances were observed. Conversely, 8 of the 10 anaplastic astrocytomas were completely negative for hypoxia-related markers. The glioblastomas also showed significantly greater heterogeneity of intercapillary distances, larger diffusion-limited tissue fractions, significantly higher mTOR activity and a trend for higher proliferation rates. Microregionally, mTOR and proliferation showed a significant spatial overlap with areas of shorter mean diffusion distances. In conclusion, diffusion-limited hypoxia, leading to the expression of hypoxia-related markers is a pivotal element of the glioblastoma phenotype and may be driven by dysregulated growth and proliferation in normoxic subregions.

Review of positron emission tomography tracers for imaging of tumor hypoxia

Hypoxia plays a critical role in tumor development and aggressiveness and is an important prognostic factor for resistance to antineoplastic treatments; therefore, it is required to measure the hypoxic level of tumor for a favorable outcome. The pretherapy information on the oxygenation status of a tumor microenvironment should also have implications for treatment selection. A diffuse distribution of hypoxia in a tumor might suggest a benefit from a systemic approach, such as a hypoxic cell cytotoxin, tirapazamine, or antigrowth factor drugs to combat the limitations of hypoxia. Alternatively, a more focal hypoxia might benefit from a local/regional approach, such as intensity-modulated radiation therapy-based radiation dose escalation to the hypoxic subvolume. This review anticipates that (18)F-FMISO ((18)F-fluoromisonodazole) and (64)Cu-ATSM-positron emission tomography (PET) will prove useful for selecting individual patients for the most appropriate treatment. The advent of new radiotracers has allowed noninvasive assessment of hypoxia, with the most extensively investigated and validated PET radiotracer for hypoxia to date being (18)F-FMISO. This article discusses the relevance and biology of hypoxia in cells and organ systems and reviews the laboratory and clinical applications of (18)F-FMISO and other agents in oncology.

Augmentation of radiation response by motesanib, a multikinase inhibitor that targets vascular endothelial growth factor receptors

BACKGROUND

Motesanib is a potent inhibitor of vascular endothelial growth factor receptors (VEGFR) 1, 2, and 3, platelet-derived growth factor receptor, and Kit receptors. In this report we examine the interaction between motesanib and radiation in vitro and in head and neck squamous cell carcinoma (HNSCC) xenograft models.

EXPERIMENTAL DESIGN

In vitro assays were done to assess the impact of motesanib on VEGFR2 signaling pathways in human umbilical vein endothelial cells (HUVEC). HNSCC lines grown as tumor xenografts in athymic nude mice were utilized to assess the in vivo activity of motesanib alone and in combination with radiation.

RESULTS

Motesanib inhibited VEGF-stimulated HUVEC proliferation in vitro, as well as VEGFR2 kinase activity. Additionally, motesanib and fractionated radiation showed additive inhibitory effects on HUVEC proliferation. In vivo combination therapy with motesanib and radiation showed increased response compared with drug or radiation alone in UM-SCC1 (P < 0.002) and SCC-1483 xenografts (P = 0.001); however, the combination was not significantly more efficacious than radiation alone in UM-SCC6 xenografts. Xenografts treated with motesanib showed a reduction of vessel penetration into tumor parenchyma, compared with control tumors. Furthermore, triple immunohistochemical staining for vasculature, proliferation, and hypoxia showed well-defined spatial relationships among these parameters in HNSCC xenografts. Motesanib significantly enhanced intratumoral hypoxia in the presence and absence of fractionated radiation.

CONCLUSIONS

These studies identify a favorable interaction when combining radiation and motesanib in HNSCC models. The data presented suggest that motesanib reduces blood vessel penetration into tumors and thereby increases intratumoral hypoxia. These findings suggest that clinical investigations examining combinations of radiation and motesanib are warranted in HNSCC.

Bystander or no bystander for gene directed enzyme prodrug therapy

Gene directed enzyme prodrug therapy (GDEPT) of cancer aims to improve the selectivity of chemotherapy by gene transfer, thus enabling target cells to convert nontoxic prodrugs to cytotoxic drugs. A zone of cell kill around gene-modified cells due to transfer of toxic metabolites, known as the bystander effect, leads to tumour regression. Here we discuss the implications of either striving for a strong bystander effect to overcome poor gene transfer, or avoiding the bystander effect to reduce potential systemic effects, with the aid of three successful GDEPT systems. This review concentrates on bystander effects and drug development with regard to these enzyme prodrug combinations, namely herpes simplex virus thymidine kinase (HSV-TK) with ganciclovir (GCV), cytosine deaminase (CD) from bacteria or yeast with 5-fluorocytodine (5-FC), and bacterial nitroreductase (NfsB) with 5-(azaridin-1-yl)-2,4-dinitrobenzamide (CB1954), and their respective derivatives.

A model to simulate tumour oxygenation and dynamic [18F]-Fmiso PET data

The microenvironment of a tumour, in particular its hypoxic status, is a crucial factor in its response to radiotherapy. Conventional techniques for measuring hypoxia are either invasive or follow surgical intervention, and thus not ideal. Positron emission tomography allows the non-invasive pre-surgical assessment of oxygen status by measuring the spatiotemporal distribution of hypoxia-specific tracers. However, the relationship between levels of uptake and the underlying oxygen tension are yet to be elucidated. Furthermore, it is not fully understood how changes in the underlying physiology affect the appearance of uptake. This paper presents a modular simulation of the tumour microenvironment, underpinned by a probability density function (PDF) to model the vasculature. The model is solved numerically, to simulate both the steady-state oxygenation of a tumour and the spatiotemporal distribution of the hypoxia-specific tracer, [18F]-fluoromisonidazole (Fmiso), in a 2D environment. The results show that using a PDF to represent the vasculature effectively captures the 'hypoxic island' appearance of oxygen-deficient tissues seen ex vivo. Simulated tissue activity curves (TACs) demonstrate the general two-stage trend of empirical data, with an initial perfusion-dominated uptake, followed by hypoxia-specific binding. In well-perfused tissue, activity follows plasma levels in early stages, with binding of Fmiso only becoming apparent at a later stage. In structurally hypoxic tissue, a more gradual initial increase in activity is observed, followed by the same accumulation slope. We demonstrate the utility of theoretical modelling of tracer uptake, by quantifying the changes in TAC structure that arise as a result of altering key physiological characteristics. For example, by decreasing either the proximity of tissue to the vasculature, or the effective diffusion coefficient of Fmiso, we can observe a shift of TAC structure from corresponding to well-perfused to avascular regions, despite wholly different underlying causes.

Evaluation of hypoxia in an experimental rat tumour model by [(18)F]fluoromisonidazole PET and immunohistochemistry

This study aimed to evaluate tumour hypoxia by comparing [¹⁸F]Fluoromisonidazole uptake measured using positron emission tomography ([¹⁸F]FMISO-PET) with immunohistochemical (IHC) staining techniques. Syngeneic rhabdomyosarcoma (R1) tumour pieces were transplanted subcutaneously in the flanks of WAG/Rij rats. Tumours were analysed at volumes between 0.9 and 7.3 cm³. Hypoxic volumes were defined using a 3D region of interest on 2 h postinjection [¹⁸F]FMISO-PET images, applying different thresholds (1.2–3.0). Monoclonal antibodies to pimonidazole (PIMO) and carbonic anhydrase IX (CA IX), exogenous and endogenous markers of hypoxia, respectively, were used for IHC staining. Marker-positive fractions were microscopically measured for each tumour, and hypoxic volumes were calculated. A heterogeneous distribution of hypoxia was observed both with histology and [¹⁸F]FMISO autoradiography. A statistically significant correlation (P<0.05) was obtained between the hypoxic volumes defined with [¹⁸F]FMISO-PET and the volumes derived from the PIMO-stained tumour sections (r=0.9066; P=0.0001), regardless of the selected threshold between 1.4 and 2.2. A similar observation was made with the CA IX staining (r=0.8636; P=0.0006). The relationship found between [¹⁸F]FMISO-PET and PIMO- and additionally CA IX-derived hypoxic volumes in rat rhabdomyosarcomas indicates the value of the noninvasive imaging method to measure hypoxia in whole tumours.

Hypoxia in head and neck cancer: How much, how important?

BACKGROUND

Hypoxia develops in tumors because of a less ordered, often chaotic, and leaky vascular supply compared with that in normal tissues. In preclinical models, hypoxia has been shown to be associated with treatment resistance and increased malignant potential. In the clinic, several reports show the presence and extent of tumor hypoxia as a negative prognostic indicator. This article reviews the biology and importance of hypoxia in head and neck cancer.

METHODS

A review of literature was carried out and combined with our own experience on hypoxia measurements using exogenous and endogenous markers.

RESULTS

Hypoxia can increase resistance to radiation and cytotoxic drugs and lead to malignant progression, affecting all treatment modalities, including surgery. Hypoxia measurements using electrodes, exogenous bioreductive markers, or endogenous markers show the presence of hypoxia in most head and neck cancers, and correlations with outcome, although limited, consistently indicate hypoxia as an important negative factor. Each hypoxia measurement method has disadvantages, and no "gold standard" yet exists. Distinctions among chronic, acute, and intermediate hypoxia need to be made, because their biology and relevance to treatment resistance differ. Reliable methods for measuring these different forms in the clinic are still lacking. Several methods to overcome hypoxia have been tested clinically, with radiosensitizers (nimorazole), hypoxic cytotoxins (tirapazamine), and carbogen showing some success. New treatments such as hypoxia-mediated gene therapy await proper clinical testing.

CONCLUSIONS

The hypoxia problem in head and neck cancer needs to be addressed if improvements in current treatments are to be made. Increased knowledge of the molecular biology of intermediate, severe, and intermittent hypoxia is needed to assess their relevance and indicate strategies for overcoming their negative influence.

Modeling and computer simulations of tumor growth and tumor response to radiotherapy

A model of tumor growth and tumor response to radiation is introduced in which each tumor cell is taken into account individually. Each cell is assigned a set of radiobiological parameters, and the status of each cell is checked in discrete intervals. Tumor proliferation is governed by the cell cycle times of tumor cells, the growth fraction, the apoptotic capacity of the tumor, and the degree of tumor angiogenesis. The response of tumor cells to radiation is determined by the radiosensitivities and the oxygenation status. Computer simulation is performed on a 3D rigid cubic lattice, starting out from a single tumor cell. Random processes are simulated by Monte Carlo methods. Short cell cycle time, high growth fraction, and tumor angiogenesis all increase tumor proliferation rates. Accelerated time-dose patterns result in lower total doses needed for tumor control, but the extent of dose reduction depends on the kinetics and the radiosensitivities of tumor cells. Tumor angiogenesis alters fully oxygenated and hypoxic fractions within the tumor and subsequently affects the radiation response. It is demonstrated for selected radiobiological parameters that the simulation tools are suitable to quantitatively assess the total doses needed for tumor control. Using the simulation tools, it is feasible to simulate time-dependent effects during fractionated radiotherapy and to compare different time-dose patterns in terms of their tumor control.

A survey on histological image analysis-based assessment of three major biological factors influencing radiotherapy: proliferation, hypoxia and vasculature

Image analysis is a rapidly evolving field with growing applications in science and engineering. In cancer research, it has played a key role in advancing techniques of major diagnostic importance, minimising human intervention and providing vital clinical information. Especially in the field of tissue microscopy, the use of computers for the automated analysis of histological sections is becoming increasingly important. This paper presents an overview of various image analysis methodologies and summarises developments in this field, with great emphasis given on the assessment of three major biological factors known to influence the outcome of radiotherapy: proliferation, vasculature and hypoxia. A brief introduction followed by a survey is provided in each of these areas.

Glucose transporter-1 (GLUT-1): a potential marker of prognosis in rectal carcinoma?

The aim of the study is to evaluate the pattern and level of expression of glucose transporter-1 (GLUT-1) in rectal carcinoma in relation to outcome as a potential surrogate marker of tumour hypoxia. Formalin-fixed tumour sections from 43 patients with rectal carcinoma, who had undergone radical resection with curative intent, were immunohistochemically stained for GLUT-1. A mean of three sections per tumour (range 1-12) were examined. Each section was semiquantitatively scored; 0, no staining; 1, <10%; 2, 10-50%; 3, >50% and a score given for the whole section, the superficial (luminal) and deep (mural) part of the tumour. Staining was seen in 70% of tumours. Increased staining was noted adjacent to necrosis and ulceration. A diffuse and patchy pattern of staining, with and without colocalisation to necrosis was seen. Patients with high GLUT-1-expressing tumours (score 3 vs 0-2) had a significantly poorer overall survival (P=0.041), which was associated with poorer metastasis-free survival with no difference in local control. No significant correlation was seen with other prognostic factors. There was a strong correlation between the score for the superficial and deep parts of the tumour (r=0.81), but a significant relationship with outcome was only found in the deep part (P=0.003 vs P=0.46). In conclusion, increased GLUT-1 expression in rectal tumours was an adverse prognostic factor and is worth further evaluation as a predictive marker of response to therapy.

Carbonic anhydrase IX expression, a novel surrogate marker of tumor hypoxia, is associated with a poor prognosis in non-small-cell lung cancer

PURPOSE

To evaluate carbonic anhydrase (CA) IX as a surrogate marker of hypoxia and investigate the prognostic significance of different patterns of expression in non-small-cell lung cancer (NSCLC).

METHODS

Standard immunohistochemical techniques were used to study CA IX expression in 175 resected NSCLC tumors. CA IX expression was determined by Western blotting in A549 cell lines grown under normoxic and hypoxic conditions. Measurements from microvessels to CA IX positivity were obtained.

RESULTS

CA IX immunostaining was detected in 81.8% of patients. Membranous (m) (P =.005), cytoplasmic (c) (P =.018), and stromal (P <.001) CA IX expression correlated with the extent of tumor necrosis (TN). The mean distance from vascular endothelium to the start of tumor cell positivity was 90 micro m, which equates to an oxygen pressure of 5.77 mmHg. The distance to blood vessels from individual tumor cells or tumor cell clusters was greater if they expressed mCA IX than if they did not (P <.001). Hypoxic exposure of A549 cells for 16 hours enhanced CA IX expression in the nuclear and cytosolic extracts. Perinuclear (p) CA IX (P =.035) was associated with a poor prognosis. In multivariate analysis, pCA IX (P =.004), stage (P =.001), platelet count (P =.011), sex (P =.027), and TN (P =.035) were independent poor prognostic factors.

CONCLUSION

These results add weight to the contention that mCA IX is a marker of tumor cell hypoxia. The absence of CA IX staining close to microvessels suggests that these vessels are functionally active. pCA IX expression is representative of an aggressive phenotype.

Similar radiation sensitivities of acutely and chronically hypoxic cells in HT 1080 fibrosarcoma xenografts

It has been suggested that chronically hypoxic tumor cells may be more radiosensitive than acutely hypoxic or even aerobic cells. In the present study we have used the fact that chronically, but not acutely, hypoxic cells that are transformed with a vector containing an enhanced green fluorescent protein (EGFP) driven by a hypoxia-responsive promoter become green (high EGFP) at low oxygen concentrations and can be viably sorted from transplanted tumors in vitro. We showed that the fluorescence of HT 1080 human fibrosarcoma cells stably transfected with this vector increases constantly with decreasing O2 concentrations (<2%, longer than 1 h, half maximum approximately 0.2% for longer than 8 h), and that cells subjected to repeated cycles of hypoxia/reoxygenation (simulating acutely hypoxic cells) showed only background fluorescence. To test the radiosensitivity of acutely and chronically hypoxic cells in tumors, we isolated high-EGFP ("chronically hypoxic") and low-EGFP cells (containing both acutely hypoxic and aerobic cells) from HT 1080 xenograft tumors by fluorescence-activated cell sorting (FACS), immediately after in situ treatment with 20 Gy (ambient or clamped), and plated the cells to determine clonogenic survival in vitro. We found that the survival of high-EGFP cells after irradiation was not affected by clamping, suggesting that all, or almost all, of these cells were fully (chronically) hypoxic. Also, the survival of the low-EGFP cells irradiated under clamped conditions (acutely hypoxic cells) was not significantly different from that of the high-EGFR cells (chronically hypoxic) cells irradiated under nonclamped (or clamped) conditions. We therefore conclude that, at least in this tumor model, the radiation sensitivity of chronically hypoxic cells is similar to that of the acutely hypoxic cells.

HIF-1A, pimonidazole, and iododeoxyuridine to estimate hypoxia and perfusion in human head-and-neck tumors

PURPOSE

Tumor hypoxia measured by microelectrodes has been shown to indicate poor patient outcome. Here we investigated four potentially more widely applicable immunohistochemical parameters of tumor oxygenation and perfusion in human head-and-neck tumors.

METHODS

Twenty patients with squamous cell carcinomas of the head and neck treated with primary surgery were injected with pimonidazole and IdUrd the evening before operation. Consecutive paraffin-embedded sections were stained for blood vessels, pimonidazole, IdUrd, and HIF-1alpha. IdUrd labeling and Ki-67 labeling around individual blood vessels were scored. The spatial relationship between HIF-1alpha and pimonidazole was studied, as well as the distribution of both markers as a function of distance from the nearest blood vessel.

RESULTS

Measurement of all four parameters (diffusion-limited fraction, pimonidazole fraction, HIF-1alpha fraction, IdUrd-negative vessels) was feasible, and a significant difference between tumors was found for all parameters. IdUrd-labeled cells were absent around some vessels, indicating lack of perfusion, because these regions were positive for Ki-67. There was a positive correlation between diffusion-limited fraction and pimonidazole area for all images from all tumors, although no correlation for mean values per tumor. Colocalization of pimonidazole and HIF-1alpha was low (0.02%-25%). Most expression profiles showed a more homogenous distribution for HIF-1alpha than pimonidazole. There was no significant correlation between the pimonidazole and HIF-1alpha fractions in the 10 tumors studied.

CONCLUSIONS

Simultaneous immunohistochemical measurements related to hypoxia and perfusion are feasible (and easily applicable) in resected human tumors. The different geographic distributions of HIF-1alpha and pimonidazole indicate that HIF-1alpha might not be suitable as a marker for chronic hypoxia. Each parameter will be correlated with outcome in a larger ongoing study on head-and-neck tumors treated with surgery with or without postoperative radiotherapy.

Tumor hypoxia and blood vessel detection: an image analysis technique for simultaneous tumor hypoxia grading and blood vessel detection in tissue sections

We have developed a multistage image analysis technique for the simultaneous segmentation of blood vessels and hypoxic regions in dual-stained tumor tissue sections. The algorithm, which is integrated in a task-oriented image analysis system developed on-site, initially uses the K-nearest neighbor classification rule in order to label the image pixels. Classification is based on a training set selected from manually drawn regions corresponding to the areas of interest. If the output image contains a significant number of misclassified pixels, the user has the option to apply a series of specific problem-designed routines (texture analysis, fuzzy c-means clustering, and edge detection) in order to improve the final segmentation result. Validation experiments indicate that the algorithm can robustly detect these biological features, even in tissue sections with a very low quality of staining. This approach has also been combined with other image analysis based procedures in order to objectively obtain quantitative measurements of potential clinical interest.

Quantitative analysis of varying profiles of hypoxia in relation to functional vessels in different human glioma xenograft lines

Tissue oxygenation influences the radiation response of tumors. To further investigate the underlying mechanisms of tumor hypoxia, the spatial distribution of hypoxic cells in relation to the vasculature was studied. In a panel of three human glioma xenograft lines (E2, E102, E106) with different growth characteristics, tumor line-specific patterns of hypoxia (pimonidazole) and (functional) vasculature (Hoechst 33342) were observed. Two of the three glioma lines showed a more homogeneous distribution of perfused vessels (E102 and E106) than the third glioma line (E2). Although all tumors showed hypoxia, the distance at which the steepest part of the gradient of the hypoxia marker was found varied significantly among the different glioma lines. The faster-growing E102 tumors had the longest distance (>300 microm). These results indicate that tumor line-specific factors, rather than vascular geometry alone, may determine the oxygenation status of a tumor. As a consequence, vascular density cannot be used as a surrogate parameter for tumor hypoxia when comparing different tumors. Additional hypoxia and perfusion markers will further improve our understanding of changes in tumor physiology at the microregional level explaining the relationship between the low oxygen levels and the response of tumors to treatment.

Green fluorescent protein is a suitable reporter of tumor hypoxia despite an oxygen requirement for chromophore formation

The oxygen requirement for chromophore formation potentially limits the use of green fluorescent protein as a reporter under hypoxic conditions. In the light of this, the applicability of a hypoxia-responsive enhanced green fluorescent protein (EGFP)-based system to the measurement of tumor hypoxia was tested in human HT 1080 fibrosarcoma cells stably transfected with a destabilized EGFP vector containing the hypoxia-responsive 5HRE-hCMVmp promoter or, as a positive control, the strong constitutive CMV promoter. After various schedules of hypoxia and reoxygenation, EGFP fluorescence of live cells was assessed by flow cytometry, and protein levels were analyzed by Western blot. Fluorescence of CMV promoter positive control cells dropped to 38+/-5% of aerobic levels after 12 hours at <0.02% oxygen, but was unaffected by higher oxygen concentrations. Following 12 hours at <0.02% oxygen, cells transfected with the hypoxia-responsive vector exhibited maximum fluorescence after 4 hours of subsequent reoxygenation, reaching 68+/-2% of the levels in CMV promoter controls under aerobic conditions. With such reoxygenation, these cells exhibited a constant increase in fluorescence between 2% and <0.02% oxygen. EGFP chromophore formation is only affected by near-anoxic oxygen concentrations. The correlation of fluorescence and oxygen concentration is restored by a 4-hour reoxygenation period due to oxidation of pre-synthesized EGFP and a delayed increase in EGFP protein synthesis.