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

Multifunctional hypoxia imaging nanoparticles: multifunctional tumor imaging and related guided tumor therapy

Hypoxia is a common feature of most solid tumors. Having a comprehensive understanding of tumor hypoxia condition is a key to tumor therapy. Many hypoxia imaging nanoparticles have been used for tumor detection. However, simple optical hypoxia imaging is not enough for tumor diagnosis. Also, the tumor therapy process needs the information about the tumor hypoxia condition. Recently, researchers developed multimodal hypoxia tumor imaging nanoparticles and multifunctional hypoxia imaging-guided tumor therapy nanoparticles. The multimodal hypoxia imaging could produce more tumor region information and engage in functional tumor imaging to better understand the tumor condition. The multifunctional hypoxia imaging-guided tumor therapy could monitor the tumor therapy process and evaluate tumor therapeutic effect. Meanwhile, many challenges and limitations are still remaining in the application of multifunctional hypoxia nanoparticles. In this review, we first introduce the types of multifunctional hypoxia imaging nanoparticles. Then we focus on multimodal hypoxia imaging nanoparticles and hypoxia imaging-guided tumor therapy nanoparticles. We also discuss the challenges and future perspectives of this field. There has not been many studies in this field for now. We hope this review would bring more researchers' attention to this field so that it would substantially contribute to tumor precise therapy.

Integrated phosphorescence-based photonic biosensor (iPOB) for monitoring oxygen levels in 3D cell culture systems

Physiological processes, such as respiration, circulation, digestion, and many pathologies alter oxygen concentration in the blood and tissue. When designing culture systems to recapitulate the in vivo oxygen environment, it is important to integrate systems for monitoring and controlling oxygen concentration. Herein, we report the design and engineering of a system to remotely monitor and control oxygen concentration inside a device for 3D cell culture. We integrate a photonic oxygen biosensor into the 3D tissue scaffold and regulate oxygen concentration via the control of purging gas flow. The integrated phosphorescence-based oxygen biosensor employs the quenching of palladium-benzoporphyrin by molecular oxygen to transduce the local oxygen concentration in the 3D tissue scaffold. The system is validated by testing the effects of normoxic and hypoxic culture conditions on healthy and tumorigenic breast epithelial cells, MCF-10A cells and BT474 cells, respectively. Under hypoxic conditions, both cell types exhibited upregulation of downstream target genes for the hypoxia marker gene, hypoxia-inducible factor 1α (HIF1A). Lastly, by monitoring the real-time fluctuation of oxygen concentration, we illustrated the formation of hypoxic culture conditions due to limited diffusion of oxygen through 3D tissue scaffolds.

The clinical importance of assessing tumor hypoxia: relationship of tumor hypoxia to prognosis and therapeutic opportunities

Tumor hypoxia is a well-established biological phenomenon that affects the curability of solid tumors, regardless of treatment modality. Especially for head and neck cancer patients, tumor hypoxia is linked to poor patient outcomes. Given the biological problems associated with tumor hypoxia, the goal for clinicians has been to identify moderately to severely hypoxic tumors for differential treatment strategies. The "gold standard" for detecting and characterizing of tumor hypoxia are the invasive polarographic electrodes. Several less invasive hypoxia assessment techniques have also shown promise for hypoxia assessment. The widespread incorporation of hypoxia information in clinical tumor assessment is severely impeded by several factors, including regulatory hurdles and unclear correlation with potential treatment decisions. There is now an acute need for approved diagnostic technologies for determining the hypoxia status of cancer lesions, as it would enable clinical development of personalized, hypoxia-based therapies, which will ultimately improve outcomes. A number of different techniques for assessing tumor hypoxia have evolved to replace polarographic pO2 measurements for assessing tumor hypoxia. Several of these modalities, either individually or in combination with other imaging techniques, provide functional and physiological information of tumor hypoxia that can significantly improve the course of treatment. The assessment of tumor hypoxia will be valuable to radiation oncologists, surgeons, and biotechnology and pharmaceutical companies who are engaged in developing hypoxia-based therapies or treatment strategies.

Why is the partial oxygen pressure of human tissues a crucial parameter? Small molecules and hypoxia

Oxygen supply and diffusion into tissues are necessary for survival. The oxygen partial pressure (pO₂), which is a key component of the physiological state of an organ, results from the balance between oxygen delivery and its consumption. In mammals, oxygen is transported by red blood cells circulating in a well-organized vasculature. Oxygen delivery is dependent on the metabolic requirements and functional status of each organ. Consequently, in a physiological condition, organ and tissue are characterized by their own unique ‘tissue normoxia’ or ‘physioxia’ status. Tissue oxygenation is severely disturbed during pathological conditions such as cancer, diabetes, coronary heart disease, stroke, etc., which are associated with decrease in pO₂, i.e. ‘hypoxia’. In this review, we present an array of methods currently used for assessing tissue oxygenation. We show that hypoxia is marked during tumour development and has strong consequences for oxygenation and its influence upon chemotherapy efficiency. Then we compare this to physiological pO₂ values of human organs. Finally we evaluate consequences of physioxia on cell activity and its molecular modulations. More importantly we emphasize the discrepancy between in vivo and in vitro tissue and cells oxygen status which can have detrimental effects on experimental outcome. It appears that the values corresponding to the physioxia are ranging between 11% and 1% O₂ whereas current in vitro experimentations are usually performed in 19.95% O₂, an artificial context as far as oxygen balance is concerned. It is important to realize that most of the experiments performed in so-called normoxia might be dangerously misleading.

Detection and characterization of tumor hypoxia using pO2 histography

Data from 125 studies describing the pretreatment oxygenation status as measured in the clinical setting using the computerized Eppendorf pO2 histography system have been compiled in this article. Tumor oxygenation is heterogeneous and severely compromised as compared to normal tissue. Hypoxia results from inadequate perfusion and diffusion within tumors and from a reduced O2 transport capacity in anemic patients. The development of tumor hypoxia is independent of a series of relevant tumor characteristics (e.g., clinical size, stage, histology, and grade) and various patient demographics. Overall median pO2 in cancers of the uterine cervix, head and neck, and breast is 10 mm Hg with the overall hypoxic fraction (pO2 <or= 2.5 mm Hg) being approx. 25%. Metastatic lesions do not substantially deviate from the oxygenation status of (their) primary tumors. Whereas normal tissue oxygenation is independent of the hemoglobin level over the range of 8-15 g/dL, hypoxia is more pronounced in anemic patients and above this range in some cancers. Identification of tumor hypoxia may allow an assessment of a tumor's potential to develop an aggressive phenotype or acquired treatment resistance, both of which lead to poor prognosis. Detection of hypoxia in the clinical setting may therefore be helpful in selecting high-risk patients for individual and/or more intensive treatment schedules.

Hypoxia: importance in tumor biology, noninvasive measurement by imaging, and value of its measurement in the management of cancer therapy

PURPOSE

The Cancer Imaging Program of the National Cancer Institute convened a workshop to assess the current status of hypoxia imaging, to assess what is known about the biology of hypoxia as it relates to cancer and cancer therapy, and to define clinical scenarios in which in vivo hypoxia imaging could prove valuable.

RESULTS

Hypoxia, or low oxygenation, has emerged as an important factor in tumor biology and response to cancer treatment. It has been correlated with angiogenesis, tumor aggressiveness, local recurrence, and metastasis, and it appears to be a prognostic factor for several cancers, including those of the cervix, head and neck, prostate, pancreas, and brain. The relationship between tumor oxygenation and response to radiation therapy has been well established, but hypoxia also affects and is affected by some chemotherapeutic agents. Although hypoxia is an important aspect of tumor physiology and response to treatment, the lack of simple and efficient methods to measure and image oxygenation hampers further understanding and limits their prognostic usefulness. There is no gold standard for measuring hypoxia; Eppendorf measurement of pO(2) has been used, but this method is invasive. Recent studies have focused on molecular markers of hypoxia, such as hypoxia inducible factor 1 (HIF-1) and carbonic anhydrase isozyme IX (CA-IX), and on developing noninvasive imaging techniques.

CONCLUSIONS

This workshop yielded recommendations on using hypoxia measurement to identify patients who would respond best to radiation therapy, which would improve treatment planning. This represents a narrow focus, as hypoxia measurement might also prove useful in drug development and in increasing our understanding of tumor biology.

Hypoxia in head and neck cancer

A high level of hypoxia in solid tumours is an adverse prognostic factor for the poor outcome of cancer patients following treatment. This review describes the status of research into finding a practical method for measuring hypoxia and treating hypoxic tumours. The application of such methodology would enable the selection of head and neck cancer treatment based on an individual's tumour oxygenation status. This individualization would include the selection not only of surgery or radiotherapy, but also of novel hypoxia-modification strategies.

Autonomous multielectrode system for monitoring the interactions of isoflavonoids with lung cancer cells

The cytotoxic effect of isoflavonoids in the development of different forms of cancer has been reported by epidemiological and dietary studies. Consequently, there is a search for an accurate and reliable method for monitoring the interactions of these chemicals with cancerous cells. We have developed and optimized a fully autonomous electrochemical biosensor for studying the role of isoflavonoids on A549 lung adenocarcinoma cell line. This advanced biosensor uses a prototype 96-electrode (DOX-96) well-type device that allows the measurement of cell respiratory activity via the consumption of dissolved oxygen. The system provides a continuous, real-time monitoring of cell activity upon exposure to naturally occurring polyphenols, specifically resveratrol, genistein, and quercetin. The system is equipped with a multipotentiostat, a 96-electrode well for measurements and cell culturing with 3 disposable electrodes fitted into each well. A comparison with classical "cell culture" techniques indicates that the biosensor provides real-time measurement with no added reagents. A detection limit of 1 x 10(4) was recorded versus 200 and 6 x 10(3) cells/well for MTT and fluorescence assays, respectively. This method was optimized with respect to cell stability, reproducibility, applied potential, cell density per well, volume/composition of cell culture medium per well, and incubation. Others include total measuring time, temperature, and sterilization procedure. This study represents a basic research tool that may allow researchers to study the type, level, and specific influence of isoflavonoids on cells.

Measurement of tumor hypoxia using single-cell methods

A growing appreciation for the importance of hypoxia in tumor progression and response to treatment has driven efforts to develop methods that could be used routinely in the clinic to identify tumors containing hypoxic cells. The ideal method would be noninvasive and could be used both before treatment to determine the presence of hypoxia and during therapy to assess tumor reoxygenation. Although this goal is being approached, there are still questions about how best to measure tumor oxygenation and whether noninvasive imaging methods can provide the necessary sensitivity. Analysis of hypoxia at the level of the individual cell can provide the following information that cannot be obtained in other ways: the degree of hypoxia, the lifetime of hypoxic cells, and the dynamic nature of hypoxia. This review will describe methods that have been used to detect hypoxia in individual cells, the relation between these measurements and patient response to treatment, and indicate where these methods can provide important additional insights into the consequences of tumor hypoxia.

The Effect of Deep Inspiration Breath-hold on Tumour Oxygenation

AIM

To investigate the influence of deep inspiration breath-hold on the oxygen tension of in-vivo tumours measured using an Eppendorf pO2 histograph.

MATERIALS AND METHODS

Patients with accessible primary or metastatic tumours > or = 2 cm diameter were entered into a protocol measuring tumour oxygenation with an Eppendorf pO2 histograph during normal breathing (NB) and deep inspiration breath-hold (DIBH). Change in oxygen tension was assessed using the Wilcoxon Signed Ranks test.

RESULTS

Thirty patients were entered in to this protocol. The median maximum tumour dimension was 4 cm. The median of the median pO2 of these tumours was 18 mmHg. Tumours were assessed during NB and DIBH. Oxygen tension measurements along 1-3 pairs of tracks per tumour (median of 2) were obtained. The median number of measurements per track was 30 for NB and 29 for DIBH (range 17-59). In six tumours, the values during NB were significantly higher than during DIBH, whereas, for six other tumours, the relationship was the opposite; for the remaining 18 patients, no significant difference was observed.

CONCLUSION

These data show heterogeneity of tumour oxygenation seen with in-situ tumours both at baseline and as a result of DIBH. No systematic change in the Eppendorf pO2 measurements was seen as a result of DIBH; however, the individual tumour responses to DIBH varied dramatically.

Comparison of the comet assay and the oxygen microelectrode for measuring tumor oxygenation in head-and-neck cancer patients

PURPOSE

To compare the Eppendorf PO2 histograph and the alkaline comet assay as methods of measuring tumor hypoxia in patients with head-and-neck squamous cell carcinomas.

MATERIALS AND METHODS

As part of a larger clinical trial, 65 patients with head-and-neck squamous cell carcinoma nodal metastasis underwent tumor oxygenation measurements with Eppendorf PO2 histographs and comet assays, performed on fine-needle aspirates at 1 and 2 min after 5 Gy. Fifty-four patients had sufficient tumor cells for comet analysis at 1 min and 26 at both 1 and 2 min. Individual cells were examined for DNA single-strand breaks by alkaline gel electrophoresis, and the distribution of values was quantified using median tail moment (MTM). Nonirradiated tumor cells from pretreatment fine-needle aspirates received 5 Gy in vitro to establish the oxygenated response.

RESULTS

There was a significant correlation between the 1- and 2-min MTM (slope = 0.77 +/- 0.03). There was no relationship between DNA damage in tumor cells irradiated in vitro and in vivo. No correlation was found between Eppendorf PO2 measurements and comet MTM. There was a statistically significant correlation between the treatment response in the node studied and comet MTMs, whereas no correlation was observed between treatment response and Eppendorf measurements.

CONCLUSIONS

Comet assays are reproducible, as shown by biopsies at 1 and 2 min. Intertumor variation in the MTM is not a result of intrinsic radiosensitivity but of tumor hypoxia. There was no correlation between Eppendorf PO2 measurements and comet MTM. Comet assays were better than Eppendorf in predicting treatment response as an end point for short-term outcome. Longer follow-up is needed to determine the role of the comet assay as a predictor for locoregional tumor control and survivals.

Oxygen-sensitive enzyme-prodrug gene therapy for the eradication of radiation-resistant solid tumours

Overwhelming clinical and experimental data demonstrate that tumour hypoxia is associated with aggressive disease and poor treatment outcome as hypoxic cells are refractive to radiotherapy and some forms of chemotherapy. However, hypoxia is rare in physiologically normal tissues representing a tumour-specific condition. To selectively target this therapeutically refractive cell population, we have combined bioreductive chemotherapy with hypoxia-directed gene therapy. We have transfected the human fibrosarcoma cell line, HT1080, with a hypoxia-regulated expression vector encoding the human flavoprotein cytochrome c P450 reductase (HRE-P450R). This conferred hypoxia-dependent sensitivity to the alkylating nitroimidazole prodrug RSU1069 in vitro, with a greater than 30-fold increase in oxic/hypoxic cytotoxicity ratio compared with controls. Xenografts of both the HRE-P450R and empty vector transfectants had comparable hypoxic fractions and were refractive to single dose radiotherapy of up to 15 Gy. However, combining a prodrug of RSU1069 with a reduced radiotherapy dose of 10 Gy represents a curative regimen (50% tumour-free survival; day 100) in the HRE-P450R xenografts. In complete contrast, 100% mortality was apparent by day 44 in the empty vector control xenografts treated in the same way. Thus, an oxygen-sensitive gene-directed enzyme prodrug therapy approach may have utility when incorporated into conventional radiotherapy and/or chemotherapy protocols for loco-regional disease in any tissue where hypoxia is a contra-indication to treatment success. doi:10.1038/sj.gt.3301702

Local hypoxia is produced at sites of intratumour injection

Intratumour injection, commonly used for gene or drug delivery but also associated with needle biopsy or insertion of invasive measuring devices, may damage tumour microvessels. To examine this possibility, SCCVII tumours grown subcutaneously in C3H mice were injected with a 26 gauge needle containing 0.1 ml of the fluorescent dye Hoechst 33342 to label cells lining the track of the needle. Hoechst-labelled cells sorted from these tumours were more sensitive to killing by hypoxic cell cytotoxins (tirapazamine, RSU-1069) and less sensitive to damage by ionizing radiation. Hoechst-labelled cells also bound the hypoxia marker pimonidazole when given by i.p. injection. Intratumour injection transiently increased hypoxia from 18 to 70% in the tumour cells adjacent to the track of the needle. The half-time for return to pre-treatment oxygenation was about 30 min; oxygenation of tumour cells along the track had recovered by 20 h after intratumour injection. This effect could have significant implications for intratumour injection of drugs, cytokines or vectors that are affected by the oxygenation status of the tumour cells as well as potential effects on biodistribution via local microvasculature.

A pilot study comparing intratumoral oxygenation using the comet assay following 2.5% and 5% carbogen and 100% oxygen

PURPOSE

Tumor hypoxia has been purported to be an important biologic factor in the failure of radical radiotherapy to achieve local control in many tumor types. This study was designed to evaluate the effect of breathing high oxygen content gas mixtures (oxygen with 0%, 2.5%, or 5% carbon dioxide) on tumor oxygenation measured using the Eppendorf polarographic oxygen electrode and the comet assay in accessible, hypoxic human tumors.

METHODS AND MATERIALS

Using Eppendorf pO2 histography to identify hypoxic tumors (median pO2 < or = 10 mmHg), eligible patients were systematically allocated either 100% oxygen (O2) or oxygen with 2.5% or 5% carbon dioxide (CO2). Tumors were treated with 6-10 Gy during which two fine needle aspirates (FNA) were obtained from different regions of the lesion, one at midway and the other at completion of the radiation exposure. Gas breathing was initiated 4 min before radiation was commenced. A 10-min interval was specified between the first and second halves of the radiation exposure to allow near maximal DNA repair prior to the second half of the radiation treatment. FNAs were performed within 2 min of cessation of radiation and the cells immediately suspended in buffered saline at 4 degrees C for analyses of hypoxic fraction using the comet assay.

RESULTS

Fifteen evaluations were performed in 13 patients with hypoxic tumors (median O2 tension 2.75 mmHg) treated with a median dose of 8 Gy. The median hypoxic fraction determined using the comet assay fell from 0.36 to 0.13 (p = 0.001, Wilcoxon signed rank test) due to the addition of high oxygen content gases.

CONCLUSIONS

In tumors defined as hypoxic using Eppendorf pO2 histography, a statistically significant reduction in the hypoxic fraction with the comet assay was found following administration of high oxygen content gases. These preliminary findings reveal a trend suggesting that 5% carbogen may reduce the hypoxic fraction by a greater margin than either 100% oxygen or 2.5% carbogen.

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.

Measurement of hypoxia using the comet assay correlates with preirradiation microelectrode pO2 histography in R3327-AT rodent tumors

Polarographic determination of tumor oxygenation by Eppendorf histography is currently under investigation as a possible predictor of radiotherapy outcome. Alternatively, the alkaline comet assay has been proposed as a radiobiological approach for the detection of hypoxia in clinical tumor samples. Direct comparisons of these methods are scarce. One earlier study with different murine tumors could not establish a correlation, whereas a weak correlation was reported for a variety of human tumors. Considering the different end points and spatial resolution of the two methods, a direct comparison for a single tumor entity appeared desirable. Anaplastic R3327-AT Dunning prostate tumors were grown on Copenhagen rats to volumes of 1-6 cm(3). Eppendorf histography (100-200 readings in 5 parallel tracks) for 8 different tumors revealed various degrees of oxygenation, with median pO(2) values ranging from 1.1 to 23 mmHg. Within 5 min after an acute exposure to 8 Gy (60)Co gamma rays, tumors were excised from killed animals and rapidly cooled to limit repair, and a single cell suspension was prepared for use with the comet assay. The resulting comet moment distributions did not exhibit two subpopulations (one hypoxic and the other aerobic), and a hypoxic fraction could not be calculated. Instead, the average comet moment distribution was taken as a parameter of overall strand break induction. Corresponding experiments with tumor cells grown in vitro allowed us to derive the relationship between the oxygen enhancement ratio (OER) for the average comet moment and oxygen partial pressure (Howard-Flanders and Alper formula). The validity of this relationship was inferred for cells exposed in situ, and the convolution of a pO(2) distribution with the formula of Howard-Flanders and Alper yielded an array of expected OER values for each tumor. The average expected OER correlated well with the average comet moment (r = 0.89, P < 0.01), and the in situ comet moment distributions could be predicted from the Eppendorf data when 50% repair was taken into account, assuming a 5-min damage half-life. The findings confirm the potential of interstitial polarography to reflect radiobiologically relevant intracellular oxygenation, but also underscore the confounding influence of differences in repair that may occur when cells are prepared from irradiated tissues for use with the comet assay.