Cancer Letters

Cancer Letters

Volume 195, Issue 1, 30 May 2003, Pages 1-16
Cancer Letters

Prognostic significance of tumor oxygenation in humans

https://doi.org/10.1016/S0304-3835(03)00012-0Get rights and content

Abstract

Low tissue oxygen concentration has been shown to be important in the response of human tumors to radiation therapy, chemotherapy and other treatment modalities. Hypoxia is also known to be a prognostic indicator, as hypoxic human tumors are more biologically aggressive and are more likely to recur locally and metastasize. Herein, we discuss and summarize the various methods under investigation to directly or indirectly measure tissue oxygen in vivo. Secondly, we consider the advantages and disadvantages of each of these techniques. These considerations are made in light of our specific hypotheses that hypoxia should be measured as a continuum, not a binary measurement and that moderate, not severe hypoxia is of great biological consequence.

Introduction

Hypoxia, i.e. low tissue oxygen concentration, has long been known to limit the response of tumor cells and animal tumors to radiation therapy (for review see [1], [2]). Hypoxic cells are also resistant to chemotherapy both because of their relative isolation from the blood supply and that many of the drugs are only effective against dividing cells [3]. The importance of hypoxia as a prognostic and/or predictive factor for human tumors, however, was controversial for many years. This debate was fueled by the limited success of trials designed to sensitize hypoxic cells by hyperbaric oxygen [4], [5] or nitroimidazole drugs [4], [5], [6], [7], [8], [9]. The question was asked: if hypoxia is present and clinically relevant in human cancer, why have the agents and methods tested to target hypoxic tumor cells had so little success? Several possible answers to this question have surfaced over the intervening years. Examples include: (a) the administration of inadequate drug in order to limit patient toxicity [9]; (b) inability of the agent to access hypoxic tissues due to vascular insufficiency or acute changes in blood flow [10], [11]; (c) drug hydrophilicity [12]; and/or (d) inability to compete against high tumor thiol levels [13], [14], [15]. Indeed, several of these factors may have confounded the results.

Two additional factors, the presence of intermediate hypoxia in tumors and the inclusion of patients in the trial who did not have hypoxic tumors are almost certainly important. 2-Nitroimidazoles are unable to sensitize cells that are only modestly hypoxic [16]. In Olive et al. [17], approximately 35% of the SiHa human cervical cancer xenografts had radiobiologic evidence of severe hypoxia (below 0.1% oxygen) and most of these tumors exhibited radioresistance consistent with a pO2 of approximately 2% oxygen, e.g. modest hypoxia. Pimonidazole binding demonstrated that approximately 60% of the cells in these xenografts were intermediate in oxygenation or hypoxic, supporting the radiation response data. Using a modeling approach, Wouters and Brown have suggested that cells at intermediate oxygen levels could be more important than the ‘hypoxic fraction’, e.g. severely hypoxic cells, in determining tumor response to fractionated radiation therapy [18]. Employing quantitative fluorescence methods, we have been able to show the presence and importance of intermediately hypoxic cells in clinical and preclinical models [13], [19], [20], [21], [22], [23], [24]). We have defined physiologic oxygenation as >10% oxygen, modest hypoxia as approximately 2.5% oxygen, moderate hypoxia as approximately 0.5% and severe hypoxia as approximately 0.1% oxygen. Cells that are in the moderate–modest oxygen range would be considered intermediately hypoxic.

Clinical trials attempting to modify patient outcome after radiation therapy were planned on the assumption that all of the patients had radioresistant tumors due to poor tumor oxygenation. None of the small trials (median patient number was 97, range 17–620) showed a significant improvement in patient outcome. However, a meta-analysis performed by Overgaard et al. [7] on 7000 patients showed that drug modification of tumor resistance significantly improved the loco-regional tumor control after radiotherapy (odds ratio of 1.17) and the overall survival rate (odds ratio of 1.13). These observations suggest that identifying those patients with hypoxic tumors is critical for testing any anti-hypoxia therapy. The inclusion of patients without hypoxic tumors who would not be expected to benefit from improvement of tumor oxygenation would dilute the power of the clinical study and dramatically increase the number of experimental subjects required to identify a statistically significant therapeutic benefit [25], [26]. Furthermore, the apparent benefit for any toxicity would be skewed [27].

Section snippets

Techniques to measure tumor oxygenation

In the 1980s, clinically relevant techniques were developed to assess the presence of hypoxia in individual human tumors. Recent data suggests that these methods can be used as prognostic markers to determine which patients could benefit from adjunctive anti-hypoxia therapy. Such hypoxia-specific therapies are available [22], [28], [29], [30] and, in appropriately identified patients, these treatments can be effectively and safely tested. Clearly, the more sensitive and specific the association

Invasive oxygen measurement techniques—needle electrodes

Polarographic needle electrodes provided the first evidence to conclusively identify the presence of hypoxia in human cancers. The early oxygen needle electrode studies by Kolstad, Wendling and Gatenby [40], [41], [42] demonstrated hypoxia in human rectal, cervix and head/neck tumors. However, these measurements were suspect due to the electrode's large diameter and its propensity to create tissue compression and bleeding. In the late 1980s, a smaller polarographic needle electrode made by the

‘Inverse’ hypoxia detection techniques—2-nitroimidazoles binding agents

The proposal to use 2-nitroimidazoles (originally developed as hypoxic-cell radiosensitizers) as hypoxia detection reagents was initially suggested in the late 1970s because these agents bind intracellularly in hypoxic cells [61], [62]. Such compounds, e.g. misonidazole, form covalent bonds with intracellular macromolecules, identified primarily as protein thiols by Raleigh and Koch [63]. This binding of 2-nitroimidazoles is proportionately inhibited as a function of increasing oxygen

‘Inverse’ hypoxia detection techniques—endogenous markers

With the current interest in molecular biology and cellular signaling, there has been a quest to identify an intrinsic molecule whose presence and/or level reflects tissue oxygenation. Oxygen levels have long been known to modulate the release of chemical messengers in the body; the classical example is the release of erythropoietin by the kidney under conditions of physiological or pathological hypoxia [83]. HIF is a key regulator maintaining oxygen homeostasis (for review see [57]). Many

Non-invasive hypoxia imaging

Non-invasive hypoxia imaging could be used as prognostic and predictive assays especially in patients where surgery is not clinically indicated. Several factors are important in determining the success or failure of a non-invasive imaging approach. Perhaps the most fundamental is the isotope half-life that should be tailored to the properties of the drug (Table 3). Isotopes with short half-lives have the advantage of reducing patient dose, but the disadvantage of drug clearance times which are

Conclusions

It is clear from published studies using each of the measuring techniques discussed that hypoxia is present in a subset of human tumors. However, the percentage of, for example, cervix cancer patients, having a ‘hypoxic’ tumor seems to be dependent on the technique used and how hypoxia is defined. All of these studies agree that there is substantial inter- and intra-tumoral heterogeneity within tumors of similar histology and site, emphasizing the importance of measuring hypoxia in individual

Summary

A clinically relevant method for evaluating the presence and pattern of hypoxia in human tumors can improve patient prognosis and treatment planning. Intrinsic markers, such as VEGF, CA9 or HIF-1alpha have the potential advantage that patients would only require a biopsy. It is also possible that secreted markers of hypoxia could be monitored by a blood sample. In order for these markers to be optimized, a better understanding of the microenvironmental conditions modifying their regulation is

Acknowledgments

Drs Richard Hill and Anthony Fyles for helpful discussions regarding needle electrode studies.

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