TY - JOUR
AU1 - Höckel, Michael
AU2 - Vaupel, Peter
AB - Abstract Tissue hypoxia results from an inadequate supply of oxygen (O2) that compromises biologic functions. Evidence from experimental and clinical studies increasingly points to a fundamental role for hypoxia in solid tumors. Hypoxia in tumors is primarily a pathophysiologic consequence of structurally and functionally disturbed microcirculation and the deterioration of diffusion conditions. Tumor hypoxia appears to be strongly associated with tumor propagation, malignant progression, and resistance to therapy, and it has thus become a central issue in tumor physiology and cancer treatment. Biochemists and clinicians (as well as physiologists) define hypoxia differently; biochemists define it as O2-limited electron transport, and physiologists and clinicians define it as a state of reduced O2 availability or decreased O2 partial pressure that restricts or even abolishes functions of organs, tissues, or cells. Because malignant tumors no longer execute functions necessary for homeostasis (such as the production of adequate amounts of adenosine triphosphate), the physiology-based definitions of the term “hypoxia” are not necessarily valid for malignant tumors. Instead, alternative definitions based on clinical, biologic, and molecular effects that are observed at O2 partial pressures below a critical level have to be applied. Traditionally, tumor hypoxia has been considered a potential therapeutic problem because it renders solid tumors more resistant to sparsely ionizing radiation (1–3). More recent experimental and clinical studies [reviewed in (4–10)] suggest that intratumoral oxygen levels may influence a series of biologic parameters that also affect the malignant potential of a neoplasm. Sustained hypoxia in a growing tumor may cause cellular changes that can result in a more clinically aggressive phenotype (11–15). During the process of hypoxia-driven malignant progression, tumors may develop an increased potential for local invasive growth (16,17), perifocal tumor cell spreading (11,18), and regional and distant tumor cell spreading (12,13,19–21). Likewise, intrinsic resistance to radiation and other cancer treatments may be enhanced (18,22–29). Hypoxia-induced or hypoxia-mediated changes of 1) the proteome (i.e., the complete set of proteins within a cell at a given time) of the neoplastic and stroma cells and 2) the genome of the genetically unstable neoplastic cells may explain the fact that tumor oxygenation is associated with disease progression, a link that has been demonstrated for a variety of human malignant tumor types (11–15). The first goal of this review is to compile current results from experimental and clinical studies, illustrating the interaction between hypoxia and the phenomena of malignant progression and resistance toward oncologic treatment. In an increasing number of reports on tumor oxygenation, the term “hypoxia” has been used in a somewhat careless manner without due consideration of the clear definitions for certain experimental conditions and scientific questions. Different researchers discussing the problem of tumor hypoxia may use the term “hypoxia” in different ways, thus leading to a “Babylonian confusion.” The second goal of this review is, therefore, to shed some light on the pitfalls of the casual use of the term “tumor hypoxia.” Because evidence of the fundamental biologic and clinical importance of tumor hypoxia is increasing, molecular biologists, physiologists, and clinicians should take care to communicate on the same “wavelength” and clearly define what they mean when they use the term “tumor hypoxia.” Definition and Causative Mechanisms Tissue hypoxia results from the inadequate supply of oxygen (O2) that compromises biologic functions (30). Hypoxia can be caused by a number of factors, such as 1) low O2 partial pressure (O2 tension) in arterial blood due to, e.g., pulmonary diseases or high altitude (hypoxemic hypoxia); 2) reduced ability of blood to carry O2 as a result of anemia, methemoglobin formation, or carbon monoxide poisoning (anemic hypoxia); 3) reduced tissue perfusion, generalized or local (circulatory or ischemic hypoxia); 4) deterioration of the diffusion geometry, e.g., increased diffusion distances, concurrent versus countercurrent blood flow within microvessels (diffusional hypoxia); or 5) inability of cells to use O2 because of intoxication, as in cyanide poisoning (histotoxic or cytotoxic hypoxia). Because of finely tuned regulatory processes, increases in tissue O2 consumption are generally matched by an increase in blood flow and, therefore, do not usually lead to hypoxia unless the system regulating blood flow fails to meet the increased O2 demand of the tissue in question. Biochemists usually define hypoxia as O2-limited electron transport (31). Physiologists and clinicians define hypoxia as a state of reduced O2 availability or decreased O2 partial pressures below critical thresholds, thus restricting or even abolishing the function of organs, tissues, or cells (32–34). Anoxia describes the state where no O2 is detected in the tissue (O2 partial pressure = 0 mm of mercury [mmHg]). In solid tumors, oxygen delivery to the respiring neoplastic and stromal cells is frequently reduced or even abolished by a deteriorating diffusion geometry, severe structural abnormalities of tumor microvessels, and disturbed microcirculation (35). In addition, anemia and the formation of methemoglobin or carboxyhemoglobin reduce the blood's capacity to transport O2. As a result, areas with very low (down to zero) oxygen partial pressures exist in solid tumors, occurring either acutely or chronically. These microregions of very low or zero O2 partial pressures are heterogeneously distributed within the tumor mass and may be located adjacent to regions with normal O2 partial pressures. In contrast to normal tissue, neoplastic tissue can no longer fulfill physiologic functions. Thus, tumor hypoxia cannot be defined by functional deficits, although areas of necrosis, which are often found in tumor tissue on microscopic examination, indicate the loss of vital cellular functions. Metabolic Hypoxia in Solid Tumors When an unrestricted supply of oxygen is available, for most tumors, the rate of O2 consumption (respiration rate) and adenosine triphosphate (ATP) production is comparable to that found in the corresponding normal tissue, despite the deregulated organization of cells in malignant tumors. To maintain a sufficient energy supply for membrane transport systems and synthesis of chemical compounds, an adequate supply of O2 is required. In hypoxia, the mitochondrial O2 consumption rate and ATP production are reduced, which hinders inter alia active transport in tumor cells. Specifically, major effects of the reduced production of ATP are 1) collapse of Na+ and K+ gradients, 2) depolarization of membranes, 3) cellular uptake of Cl−, 4) cell swelling, 5) increased cytosolic Ca2+ concentration, and finally, 6) decreased cytosolic pH, resulting in intracellular acidosis in tumor cells. According to the definition given above, hypoxia is present in tumors when the O2 partial pressure falls below a critical value causing the O2 consumption rate or ATP production rate of a cell or a tissue to decrease progressively. On the basis of experimental results from isolated xenografted human breast cancer tissue (36,37), tumor tissue hypoxia with reduced O2 consumption rates is expected when the O2 partial pressure in the blood at the venous end of the capillaries (end-capillary blood) falls below 45–50 mmHg (Table 1). This critical threshold, however, has been validated only under the following boundary conditions: a tumor blood flow rate of 1 mL/g per minute, a hemoglobin concentration of 140 g/L, and an arterial O2 partial pressure of 90–100 mmHg. Reducing the perfusion rate to 0.3 mL/g per minute yields an hypoxic tissue fraction of approximately 20% (48). When the hemoglobin concentration falls below 100 g/L or the normal O2 content of arterial blood decreases (hypoxemia), the relative proportion of hypoxic tissue substantially increases in the experimental tumor system described. On a global tissue level, the critical O2 partial pressure in tumors, below which the detrimental changes associated with reduced O2 consumption have been observed, is 8–10 mmHg (Table 1). Measurements of the microregional distributions of ATP by quantitative bioluminescence and photon imaging in rodent tumors have shown that the concentration of ATP is relatively constant (1.0–1.8 mM) as long as an adequate supply of oxygen (i.e., comparable to that of normal tissues or organs) can be maintained (49,50). In FSaII murine fibrosarcomas growing subcutaneously in mice, relatively constant ATP levels were present as long as the median O2 partial pressure was 10 mmHg or higher [Fig. 1 and (38)]. Similar results were obtained in rat tumors when the global ATP content was evaluated with high-performance liquid chromatography (39,40). Median O2 partial pressures of approximately 10 mmHg thus appear to represent a critical threshold for energy metabolism in FSaII tumors. At higher median O2 tensions, the levels of ATP, phosphomonoesters, and total inorganic phosphate were relatively constant, coinciding with intracellular alkalosis or neutrality and a stable ATP/inorganic phosphate ratio, energy charge, and phosphorylation potential. Median O2 partial pressures of less than 10 mmHg result in intracellular acidosis, ATP depletion, a drop in the energy supply, and increasing levels of inorganic phosphate (Fig. 1). Oxidative phosphorylation for ATP formation will continue to a cellular O2 partial pressure of 0.5–10 mmHg [Table 1 and (42–45)]. Certainly, the threshold O2 partial pressure below which oxidative phosphorylation ceases is dependent on the cell line investigated and its respiratory capacity, the type of medium and substrate chosen, the temperature and pH of the suspending medium, and even the type and accuracy of the setup used to measure O2 consumption rates. Mitochondrial oxidative phosphorylation is limited at O2 partial pressures of less than approximately 0.5 mmHg [Table 1 and (32,45)]. Above this threshold, mitochondria should function physiologically. Again, this critical threshold depends on the actual substrate supply, on the pH of the suspending medium, and on the technique used to measure O2. Cytochromes aa3 and c in ascites cells require O2 partial pressures of greater than 0.02–0.07 mmHg [Table 1 and (32,46,47)] to maintain respiration. At O2 partial pressures above this range, cytochromes are fully oxidized. Spectrophotometric measurements on living and rapidly deep-frozen tissues indicate that the same is true in vivo. From this rather rudimentary summary of critical O2 partial pressures for metabolic hypoxia, there does not appear to be a single hypoxic threshold that is generally applicable. Hypoxic thresholds range from 45–50 mmHg in end-capillary blood to 0.02 mmHg in cytochromes (see Fig. 4). Furthermore, such data on hypoxic thresholds in a given tissue do not take into consideration the existence of severe heterogeneities even on a microscopic level related to variable O2 demands and O2 supply. In this discussion of hypoxic thresholds, it is important to note that, for any functional parameter, a sharp threshold between hypoxia and normoxia does not exist and should not be expected. This review deals with the problem of hypoxia as a whole, encompassing mild, moderate, and severe hypoxia (divisions that are not well defined). Approaches in which oxygen effects have been defined under in vitro conditions by using half-maximum values (e.g., in ionizing radiation) have proven useful in some instances, such as comparing radiosensitivity of different cell lines under identical boundary conditions. However, use of half-maximal values in a more general discussion of hypoxia is not very informative because these values do not give the O2 levels at which hypoxia starts and becomes a biologic problem. Methods for Detection of Tumor Hypoxia During the past decade, the oxygenation status of solid tumors has been evaluated by investigators in many specialized centers. Despite various limitations of the techniques used, a number of key findings have been described as follows: 1) Most tumors have lower median O2 partial pressures than their tissue of origin; 2) many solid tumors contain areas of low O2 partial pressure that cannot be predicted by clinical size, stage, grade, histology, and site; 3) tumor-to-tumor variability in oxygenation is usually greater than intratumor variability in oxygenation; and 4) recurring tumors have a poorer oxygenation status than the corresponding primary tumors [reviewed in (4)]. Assessment of the tumor oxygenation status by invasive and noninvasive procedures has been reviewed [(51–54) and Table 2]. Many methods can detect tumor hypoxia. Which method is most appropriate for a particular experimental or clinical need will depend on the feasibility of the approaches available in terms of invasiveness and the degree of resolution required, on whether measurement of a direct or indirect parameter is necessary, and, of course, on financial considerations. The present “gold standard” is intratumor polarographic measurement of O2 partial pressures by using microsensor techniques that adhere to the systematic random sampling principle (53). However, no single method will probably be suitable for all situations; therefore, where possible, use of more than one technique may be advisable. In all instances, careful interpretation of the data obtained is paramount, and researchers should bear in mind the exact parameters measured and the limitations of the specific methods used. Hypoxia-Mediated Proteome Changes and Tumor Propagation The intrinsic ability of a tumor to propagate by means of local destructive growth and dissemination is the hallmark of malignant disease. The hypoxic microenvironment in solid tumors, which affects neoplastic cells and non-neoplastic stromal cells such as macrophages and fibroblasts, should have profound effects on tumor propagation if one considers the proteome changes of cells demonstrated under hypoxic conditions in vitro. The proteome changes may result from the stimulation or inhibition of gene expression and from posttranscriptional and posttranslational effects induced by hypoxia or anoxia (Fig. 2). Anoxia/hypoxia-induced proteome changes in neoplastic and stromal cells may lead to the arrest or impairment of neoplastic growth through molecular mechanisms, resulting in cellular quiescence, differentiation, apoptosis, and necrosis (97–101). Cells exposed to hypoxia are generally arrested at the G1/S-phase boundary (97). O2 partial pressures of 0.2–1 mmHg disproportionately lengthen the G1 phase or arrest cells in the G1 phase (102–104). Above this hypoxic threshold, variations in O2 partial pressure should have only negligible effects on the proliferation rate. Under anoxia, most cells are arrested immediately, regardless of their position in the cell cycle. Binding of the hypoxic marker pimonidazole to suprabasal cells in epithelia supports the hypothesis that hypoxia may act as a morphogen to induce the terminal differentiation of cells (85,100). This observation appears to have a counterpart in well-differentiated squamous cell cancer, where squamous cell differentiation is consistently observed in tumor areas several cell layers away from the nearest blood vessels (Höckel M: unpublished observations). The molecular mechanisms of hypoxia-induced terminal differentiation are largely unknown. Hypoxia can induce programmed (apoptotic) cell death in normal and neoplastic cells (101). Indeed, oncogenic transformation of cells (e.g., transfection with human papillomavirus E6/E7 genes or c-myc genes) increases their susceptibility to hypoxia-induced apoptosis (23). The level of p53 in cells increases under hypoxic conditions, and the increased level of p53 induces apoptosis by a pathway involving Apaf-1 and caspase-9 as downstream effectors (105). However, hypoxia also initiates p53-independent apoptosis pathways involving hypoxia-inducible factor-1 (HIF-1), genes of the BCL-2 family, and other unidentified genes (106,107). Below a critical energy state, hypoxia/anoxia may result in necrotic cell death, a phenomenon seen in many human tumors and experimental systems. Hypoxia-induced proteome changes, leading to cell cycle arrest, differentiation, apoptosis, and necrosis, may explain delayed recurrences, dormant micrometastases (108,109), and growth retardation in large tumor masses (110). In contrast, hypoxia-induced proteome changes in tumor and/or stromal cells may promote tumor propagation by enabling the cells to adapt to nutritional deprivation or to escape their hostile environment. Hypoxia stimulates the transcription of glycolytic enzymes, glucose transporters (GLUT1 and GLUT3), angiogenic molecules, survival and growth factors (e.g., vascular endothelial growth factor [VEGF], angiogenin, platelet-derived growth factor-β, transforming growth factor-β, and insulin-like growth factor-II), enzymes, proteins involved in tumor invasiveness (e.g., urokinase-type plasminogen activator), chaperones, and other resistance-related proteins (8,17,29,97,106,111–119). At the same time, hypoxia-induced inhibition of gene expression has been demonstrated for cell-surface integrins facilitating tumor cell detachment (120). Many hypoxia-inducible genes are controlled by a common transcription factor, HIF-1, composed of two subunits, HIF-1α and HIF-1β (121). Increased concentrations of HIF-1 in the proteome of a hypoxic cell result from increased transcription of HIF-1α and HIF-1β genes and decreased HIF-1α protein degradation, an example of hypoxia-mediated posttranslational control (122–124). Jiang et al. (111) exposed human HeLa cells to concentrations of O2 between 0.125% and 20% (with 5% CO2 added and the remainder N2) and then analyzed HIF-1 expression as a function of intracellular O2 concentration. HIF-1 DNA-binding activity and the concentrations of HIF-1α protein and HIF-1β protein increased exponentially as cells were subjected to decreasing concentrations of O2, with a half-maximal response at about 10 mmHg. Hypoxia-induced HIF-1 activation can also result in an increased production of VEGF. To determine the reduced O2 concentration required to stimulate increased levels of VEGF messenger RNA (mRNA), Chiarotto and Hill (125) determined O2 concentrations in culture medium from cervical cancer cell lines SiHa, ME-180, or HeLa cells, under distinct boundary conditions, and defined the threshold for increasing the level of VEGF mRNA above baseline as O2 pressure of approximately 1 mmHg in the gassing mixture. Nuclear factor κB (NFκB) is another transcriptional factor that can be activated by hypoxia (126). The threshold for activation of NFκB in AG1522 cells occurs after 3 hours at an O2 partial pressure of about 15 mmHg (127). Thus, the critical O2 levels necessary for hypoxia-induced gene expression are probably in the range of 1–15 mmHg (Table 3 and see Fig. 4). Below these levels, mRNA levels often rise almost exponentially to a maximum value. Whether the net phenotypic result of hypoxia-induced proteome changes of the tumor and stromal cells is neoplastic growth arrest (hypoxia/0 phenotype), growth impairment (hypoxia/– phenotype), or promotion through local, perifocal, regional, or distant tumor propagation (hypoxia/+ phenotype) may be determined by the genomic state, the degree of hypoxia, and microenvironmental epigenetic factors, in addition to hypoxia. Genomic changes in neoplastic cells that reduce the potential for cell cycle arrest, differentiation, and apoptosis may favor hypoxia-associated mechanisms that promote tumor growth and dissemination, such as stress adaptation, anaerobic metabolism, angiogenesis, tumor cell detachment and subsequent adhesion, tissue remodeling, and migration (Fig. 2). Hypoxia-Mediated Malignant Progression By clinical definition, malignant progression of a neoplasm means the increasing probability of local spread (through direct invasion of neighboring tissues and organs), perifocal spread (through migration of single neoplastic cells and microfoci into the interstitial space, lymphatic space involvement, and perineural invasion), regional spread (through metastases to the lymph nodes), and distant spread (through hematogenous metastases or dissemination into body cavities, such as the peritoneum and pleura), as well as the increasing resistance toward nonsurgical therapy concomitant with primary tumor growth. Clinical progression of a malignant tumor is a consequence of 1) an increasing neoplastic cell load, 2) microenvironment-induced (epigenetic) phenotypic changes in neoplastic and stromal cells, and 3) genotypic changes and clonal selection of neoplastic cells (135). Evidence is accumulating that hypoxia not only induces proteome changes influencing tumor propagation but also drives malignant progression through transient and persistent genomic changes in neoplastic cells (18,19,29,133–136). Hypoxia (with or without reoxygenation) promotes genomic instability (through point mutations, gene amplification, and chromosomal rearrangements) and may unveil pre-existing cryptic genetic variations (which are suppressed by chaperones in cells not experiencing permanent stress), thus increasing the number of genetic variants. Concomitantly, hypoxia exerts a strong selection pressure (Fig. 3). The importance of the hypoxia-mediated clonal expansion of tumor cells with diminished apoptotic potential has been demonstrated both experimentally and clinically (18,22,23). The increasing inability of tumor cells to activate apoptotic pathways can explain many of the clinical consequences of malignant progression, such as locoregional and distant tumor propagation and resistance to nonsurgical therapy. Survival and proliferation of occult perifocal tumor cells with diminished apoptotic potential, located in hypoxic surgical scars, appear to be major pathogenetic events in the formation of local recurrences, despite complete surgical resection of solid neoplasms with microscopically tumor-free resection margins (R0 resection; Höckel M: unpublished results). In most investigations of hypoxia-induced genomic changes, transformed cells were incubated at almost zero O2 partial pressure and then reoxygenated at atmospheric O2 partial pressure. Rice et al. (29) demonstrated that, after incubating Chinese hamster ovary cells for up to 72 hours in less than 10 parts per million (ppm) of O2, the dihydrofolate reductase gene was amplified, which led to increased methotrexate resistance. By incubating murine cells under similar conditions, Young et al. (19) observed that DNA overreplication transiently enhanced the formation of experimental metastases. To induce the transformation of a benign B16 melanoma cell phenotype to a malignant phenotype, Stackpole et al. (137) incubated monolayers of cells for 48 hours in an O2-depleted medium. After 24 hours, these cultures were severely hypoxic (<50 ppm of O2). Russo and co-workers (134,136) observed DNA breakage resulting from activation of an endogenous endonuclease in immortal rat embryo fibroblasts cultured under anoxic conditions for up to 24 hours. Reynolds et al. (133) used a mouse tumor cell line carrying a chromosomally based λ phage shuttle vector for reporting mutations. After exposing these cells to an O2 partial pressure of less than 1 mmHg for 4 hours, they detected a mutation rate that was 3.4-fold higher than the rate in similar cells cultured under standard atmospheric conditions. Giaccia and colleagues (22,23,97) used an elegant procedure to select transformed cells with reduced apoptotic potential. Specifically, they cultured transformed mouse embryonic fibroblasts and transformed human cervical epithelial cells under a reduced O2 partial pressure of less than 1 mmHg for 48 hours, followed by up to seven reoxygenation treatments. As a rule, hypoxia-induced genomic changes are detectable at an O2 partial pressure of less than 1 mmHg, which is approximately one order of magnitude lower than the O2 partial pressures associated with proteome changes (Table 3 and Fig. 4). Hypoxia-mediated clonal selection of neoplastic cells with persistent genomic changes leading, inter alia, to apoptotic insensitivity and increased angiogenic potential stabilizes and further aggravates tumor hypoxia, which in turn promotes malignant progression. Thus, hypoxia is involved in a vicious circle that is regarded as a fundamental biologic mechanism of the malignant disease, once cellular proliferation has been deregulated [(11,14) and Fig. 3]. Tumor Hypoxia and Treatment Resistance Tumor hypoxia may present a severe problem for radiation therapy (x- and γ-radiation), because radiosensitivity is progressively limited when the O2 partial pressure in a tumor is less than 25–30 mmHg (Table 3 and Fig. 4). Hypoxia-associated resistance to photon radiotherapy is multifactorial. The presence of molecular oxygen increases DNA damage through the formation of oxygen free radicals, which occurs primarily after the interaction of radiation with intracellular water (2). Thus, because of this so-called “oxygen enhancement effect,” the radiation dose required to achieve the same biologic effect is three times higher in the absence of oxygen than in the presence of normal levels of oxygen (1–3). Evidence suggests that hypoxia-induced proteome and genome changes may also have a substantial impact on radioresistance by increasing the levels of heat shock proteins or by increasing the number of cells in a tumor with diminished apoptotic potential or increased proliferation potential (18,22,23), both of which have been linked to radioresistance (24,25). Oxygen dependency has been documented for a number of anticancer agents (e.g., cyclophosphamide, carboplatin, and doxorubicin) under in vitro and in vivo conditions (138,139). However, these investigations have been qualitative, and clear hypoxic thresholds for O2-dependent anticancer agents are still not available, although they presumably exist for each agent. Thus, additional research is necessary to provide quantitative data on hypoxia-induced chemoresistance, although this information may be difficult to obtain under in vivo conditions. Multiple mechanisms are probably also involved in the hypoxia-induced resistance to chemotherapeutic agents, including an inhibition of cell proliferation (140), a hypoxia-induced decreased cytotoxicity of some agents (98,138,141), and tissue acidosis, which is often observed in hypoxic tumors with a high glycolytic rate (142). Furthermore, hypoxic stress proteins and the loss of apoptotic potential can impart resistance to certain chemotherapeutic drugs (26–29,112,114,143). Photodynamic therapy-mediated cell death requires the presence of oxygen, a photosensitizing drug, and light of the appropriate wavelength, both in vitro and in vivo [for a review, see (144)]. Reports (129,145), however, vary greatly on the extent to which photodynamic therapy with hematoporphyrin derivatives is dependent on oxygen. Cells were not killed under anoxic conditions. The critical threshold below which progressively reduced cell death was observed varied from 15 to 35 mmHg (129–131), probably because of the reduced production of singlet oxygen (1O2) species and different sensitivities to the treatment in different cell lines (Table 3 and Fig. 4). Considering the reduced effectiveness of photodynamic agents at lower O2 partial pressures, the rapid induction of tumor hypoxia by photodynamic therapy itself—either as a consequence of a photodynamic therapy-induced decrease in blood flow or as a result of oxygen consumption by the photodynamic therapy process itself—has to be considered under in vivo conditions, since it may mean that this therapy is self-limiting (129,132,146). Photodynamic therapy involving prodrugs, such as aminolevulinic acid, may be further limited because conversion of the prodrug to the active photosensitizer appears to be less effective under hypoxic conditions. Studies of cells in vitro have identified several factors that can influence the effect of hyperthermia on cell survival. Cell lines can vary substantially in their intrinsic heat sensitivity. In addition, cell cycle position, intracellular pH, nutrient deprivation, and ATP depletion can affect cell survival after a heat treatment (147,148). At 43 °C hyperthermia, hypoxia per se may not cause cell death as long as concomitant changes in the nutritional and/or bioenergetic status of the cells do not occur (149). Finally, tumor hypoxia can dramatically alter the potency of cytokines (interferon gamma and tumor necrosis factor-α) and alter interleukin 2-induced activation of lymphokine-activated killer cells [reviewed in (128)]. The potency of treatment started to decrease at O2 partial pressures of less than approximately 35 mmHg (Table 3 and Fig. 4). Tumor Hypoxia as an Adverse Prognostic Factor Gatenby et al. (150) used polarographic electrodes to investigate the oxygenation status of advanced head and neck carcinomas and, thus, to our knowledge, were the first to report substantial differences in O2 partial pressures in tumors of patients who responded to radiotherapy versus tumors of those who did not. After a new generation of polarographic O2 sensors was introduced in 1989, a prospective clinical trial was initiated in which the prognostic relevance of O2 partial pressures in advanced cancers of the uterine cervix was investigated (11,14,151,152). An interim evaluation in 1991 found that a median O2 partial pressure of 10 mmHg appeared to be a cutoff level to distinguish between hypoxic cervical cancers with poor prognosis and less hypoxic cervical cancers with statistically significantly better prognosis. The study also demonstrated that tumor oxygenation was independent of various patient and tumor characteristics, including patient age, menopausal status and parity, International Federation of Gynecology and Obstetrics (FIGO) stage, clinical tumor size, histopathology, and grade of malignancy. A Kaplan–Meier life-table analysis showed statistically significantly shorter survival and recurrence-free survival for patients with hypoxic tumors. The results were consistent with the hypothesis that radiobiologically hypoxic tumors (i.e., tumors with a reduced radiosensitivity at critically low O2 levels) are less curable. However, other mechanisms of treatment failure, such as increased locoregional and distant tumor propagation, could not be excluded (151). By the end of 1995, 103 patients with advanced cancers of the uterine cervix had entered the study. From a histopathologic examination of the surgical specimens obtained from 47 patients during radical tumor resection, hypoxic tumors had larger extensions, more frequent (occult) parametrial spread, and more lymph-vascular space involvement than nonhypoxic tumors of the same clinical stage and size. Probabilities of 5-year overall and disease-free survival calculated for patients who underwent standard primary treatment for cure were again statistically significantly lower for those with hypoxic tumors than for those with nonhypoxic tumors of similar clinical stage and size. Cox regression analysis revealed tumor oxygenation as the strongest independent prognostic factor, followed by FIGO stage (11). Of special interest was the fact that the disadvantage in outcome for patients with hypoxic tumors was independent of the mode of primary treatment (radiation therapy or radical surgery). This finding led to the hypothesis that tumor hypoxia was associated with malignant progression in advanced cancer of the uterine cervix and that hypoxia not only may counteract O2-dependent forms of therapy but also may advance tumor progression per se irrespective of treatment [(11) and Table 4]. More recently, Fyles et al. (157) also determined whether the pretreatment oxygenation status of tumors could predict disease-free survival by assessing data for 74 patients with cervical cancer who were treated with radiation. There was clear evidence that hypoxia (defined as the fraction of measured O2 partial pressures of <5 mmHg) is a statistically significant adverse prognostic factor of disease-free survival. Furthermore, Knocke et al. (154) confirmed the prognostic relevance of pretreatment tumor oxygenation status by studying 51 patients with cancer of the uterine cervix who were treated with primary radiation (Table 4). Recently, Sundfor et al. (158) also reported a poor outcome associated with low oxygen tensions in 40 advanced squamous cell carcinomas of the uterine cervix. In local recurrences of cervical cancer, oxygenation levels of the recurrent tumor were generally lower than levels in the primary tumors of comparable mass. In the cohort of patients with recurrent tumors, oxygenation measurements provided additional prognostic information. Patients with tumors that had a median O2 partial pressure of less than 4 mmHg had a statistically significantly shorter median survival time than those with median O2 partial pressure of 4 mmHg or more [Table 4 and (14)]. The pretreatment tumor oxygenation status was also assessed in patients with soft-tissue sarcomas (12,153,155,159,160). In these patients, the more hypoxic tumors were associated with a poorer survival when compared with normoxic tumors resulting from local treatment failure or distant metastases (Table 4). In a study on the association between the tissue oxygenation status and the radiation response in lymph node metastases of squamous cell carcinomas of the head and neck, Nordsmark et al. (156) showed that the most hypoxic tumors had statistically significantly lower locoregional tumor control than well-oxygenated tumors. Cox multiple regression analysis found that an O2 partial pressure of 2.5 mmHg or less was the strongest independent variable for prediction of a response to radiation therapy, when the end point was tumor control at the site where the O2 partial pressure was measured. Brizel et al. (20,155) also showed that tumor hypoxia appears to adversely affect the prognosis of patients with primary and metastatic squamous cell carcinomas of the head and neck (Table 4). How Can the Term “Tumor Hypoxia” Be Used Appropriately? By the definition and criteria presented in this review, there is clear evidence that the range of hypoxia in malignant tumors can vary widely. Critical O2 levels (hypoxic thresholds) characterize the upper limit of the hypoxic range below which activities and functions progressively become restricted. These O2 levels can encompass O2 partial pressures from 35 mmHg (start of reduced cell death in conventional photodynamic therapy or restricted efficacy of some immunotherapies) to 0.02 mmHg (below this level, cytochromes aa3 and c are no longer fully oxidized) with all other critical O2 levels for specific cellular functions or activities distributed in between (Fig. 4). We, therefore, recommend that only data describing defined functions or activities of tumor cells or characteristics of tumors be compared. Because critical O2 levels of different parameters or reactions can vary substantially, we recommend that only results from assays describing the same biologic parameter (e.g., radiation sensitivity) be tested for correlation. Otherwise, pseudocorrelations may be misinterpreted as real biologic interrelationships. Table 1. Critical O2 partial pressures below which adequate metabolic functions in solid tumors (metabolic hypoxia) cannot be maintained* Critical pO2, mmHg  Entity measured  Experimental conditions  Parameter of interest  Reference(s)  *pO2 = O2 partial pressure; mmHg = millimeters of mercury; Hb = hemoglobin; MRO2 = O2 consumption rate; TBF = tumor blood flow; ATP = adenosine triphosphate; CHO = Chinese hamster ovary.  †Median pO2.  45–50  End-capillary blood  Normoxemia, Hb content = 140 g/L;  MRO2 = 30 μL/g per min;  TBF = 1 mL/g per min  O2 consumption rate  (36,37)  8–10†  Tissue (global)  In vivo  ATP levels  (38–40)  ≈5  CHO cells  In vitro (absence of glucose)  ATP per cell  (41)  0.5–1  CHO cells  In vitro  O2 consumption rate  (42)  0.5–2  Ehrlich ascites cells  In vitro (succinate as substrate)  O2 consumption rate  (43,44)  8–10  Neuroblastoma cells  In vitro (Hanks' medium supplemented  with bovine serum albumin)  O2 consumption rate  (45)  ≈0.5  Isolated mitochondria  In vitro  O2 consumption rate  (32,45)  0.02–0.07  Cytochromes  In vitro  Oxidation status  (32,46,47)  Critical pO2, mmHg  Entity measured  Experimental conditions  Parameter of interest  Reference(s)  *pO2 = O2 partial pressure; mmHg = millimeters of mercury; Hb = hemoglobin; MRO2 = O2 consumption rate; TBF = tumor blood flow; ATP = adenosine triphosphate; CHO = Chinese hamster ovary.  †Median pO2.  45–50  End-capillary blood  Normoxemia, Hb content = 140 g/L;  MRO2 = 30 μL/g per min;  TBF = 1 mL/g per min  O2 consumption rate  (36,37)  8–10†  Tissue (global)  In vivo  ATP levels  (38–40)  ≈5  CHO cells  In vitro (absence of glucose)  ATP per cell  (41)  0.5–1  CHO cells  In vitro  O2 consumption rate  (42)  0.5–2  Ehrlich ascites cells  In vitro (succinate as substrate)  O2 consumption rate  (43,44)  8–10  Neuroblastoma cells  In vitro (Hanks' medium supplemented  with bovine serum albumin)  O2 consumption rate  (45)  ≈0.5  Isolated mitochondria  In vitro  O2 consumption rate  (32,45)  0.02–0.07  Cytochromes  In vitro  Oxidation status  (32,46,47)  View Large Table 2. Methods currently available or under development for detection of tumor hypoxia (selection)* *Histomorphometric and DNA strand break (after 2–4 Gy) assays, as indirect measures of tumor hypoxia, are not listed. pO2 = oxygen partial pressure; HbO2 = oxyhemoglobin saturation; MRI = nuclear magnetic resonance imaging; Bold = blood oxygen level dependent; PET = positron emission tomography; SPECT = single photon emission computed tomography.  1)  Invasive microsensor techniques for direct tissue pO2 measurements     • Polarographic O2 sensors [e.g., (55–57)]    • Luminescence-based optical sensors [e.g., (58,59)]  2)  Electron paramagnetic resonance oximetry (60,61)  3)  Techniques for intravascular O2 detection    • Cryospectrophotometry [HbO2 saturation (62–65)]    • Near-infrared spectroscopy [HbO2 saturation (66, 67)]    • Phosphorescence imaging (68)  4)  Nuclear magnetic resonance spectroscopy and imaging techniques    • 1H-MRI, Bold effect (69–71)    • 19F-magnetic resonance relaxometry (72–76)  5)  Noninvasive detection of sensitizer adducts    • [18F]Fluoromisonidazole [PET (77–80)]    • [123I]Iodoazomycin-arabinoside [SPECT (81–83)]  6)  Invasive immunohistochemical hypoxia marker techniques    • Misonidazole [3H-labeled (52,84–87)]    • Pimonidazole (88–90)    • Etanidazole (91–95)     • Nitroimidazole-theophylline (96)  *Histomorphometric and DNA strand break (after 2–4 Gy) assays, as indirect measures of tumor hypoxia, are not listed. pO2 = oxygen partial pressure; HbO2 = oxyhemoglobin saturation; MRI = nuclear magnetic resonance imaging; Bold = blood oxygen level dependent; PET = positron emission tomography; SPECT = single photon emission computed tomography.  1)  Invasive microsensor techniques for direct tissue pO2 measurements     • Polarographic O2 sensors [e.g., (55–57)]    • Luminescence-based optical sensors [e.g., (58,59)]  2)  Electron paramagnetic resonance oximetry (60,61)  3)  Techniques for intravascular O2 detection    • Cryospectrophotometry [HbO2 saturation (62–65)]    • Near-infrared spectroscopy [HbO2 saturation (66, 67)]    • Phosphorescence imaging (68)  4)  Nuclear magnetic resonance spectroscopy and imaging techniques    • 1H-MRI, Bold effect (69–71)    • 19F-magnetic resonance relaxometry (72–76)  5)  Noninvasive detection of sensitizer adducts    • [18F]Fluoromisonidazole [PET (77–80)]    • [123I]Iodoazomycin-arabinoside [SPECT (81–83)]  6)  Invasive immunohistochemical hypoxia marker techniques    • Misonidazole [3H-labeled (52,84–87)]    • Pimonidazole (88–90)    • Etanidazole (91–95)     • Nitroimidazole-theophylline (96)  View Large Table 3. Critical O2 tensions below which typical cellular functions in solid tumors progressively cease or anticancer treatments are impaired as a result of an inadequate O2 availability Critical O2 tension, mmHg*  Function or parameter observed  Selective reference(s)  *mmHg = millimeters of mercury.  †Experiments were partly performed with monitoring of the ambient gas mixture only. Pericellular O2 partial pressures in the medium can be substantially lower, depending on number of cells, stirring procedures, and the cell line investigated.  30–35  Effectiveness of certain (passive)  immunotherapies  (128)  15–35  Cell death with photodynamic therapy  (129–132)  25–30  Cell death on exposure to x- and γ-radiation  (2,3)  10–20  Binding of hypoxia markers  (87,95)  1–15†  Proteome changes  (22,114,121,125,127)  0.2–1  Genome changes  (19,22,23,29,97,133,134)  Critical O2 tension, mmHg*  Function or parameter observed  Selective reference(s)  *mmHg = millimeters of mercury.  †Experiments were partly performed with monitoring of the ambient gas mixture only. Pericellular O2 partial pressures in the medium can be substantially lower, depending on number of cells, stirring procedures, and the cell line investigated.  30–35  Effectiveness of certain (passive)  immunotherapies  (128)  15–35  Cell death with photodynamic therapy  (129–132)  25–30  Cell death on exposure to x- and γ-radiation  (2,3)  10–20  Binding of hypoxia markers  (87,95)  1–15†  Proteome changes  (22,114,121,125,127)  0.2–1  Genome changes  (19,22,23,29,97,133,134)  View Large Table 4. Prognostic significance of tumor hypoxia* Oxygenation parameters  End points  Tumor site  Reference(s)  *pO2 = O2 partial pressure; mmHg = millimeters of mercury.  Median pO2 of ≤22 mmHg  Disease-free survival, overall survival  Primary soft-tissue sarcomas  (153)  Median pO2 of <10 mmHg  Disease-free survival, overall survival  Locally advanced primary cancers of the uterine cervix  (11,14,151,154)    Disease-free survival, overall survival  Primary soft-tissue sarcomas  (12,155)    Disease-free survival, overall survival  Primary head and neck cancers  (20,155)    Incidence of metastases  Locally advanced primary cancers of the uterine cervix  (13)  Median pO2 of <4 mmHg  Disease-free survival, overall survival  Locally recurrent cancers of the uterine cervix  (14)  Fraction of pO2 values ≤2.5 mmHg  Locoregional control, overall survival  Head and neck cancers  (156)  Fraction of pO2 values ≤5 mmHg  Disease-free survival, overall survival  Locally advanced primary cancers of the uterine cervix  (157,158)  Oxygenation parameters  End points  Tumor site  Reference(s)  *pO2 = O2 partial pressure; mmHg = millimeters of mercury.  Median pO2 of ≤22 mmHg  Disease-free survival, overall survival  Primary soft-tissue sarcomas  (153)  Median pO2 of <10 mmHg  Disease-free survival, overall survival  Locally advanced primary cancers of the uterine cervix  (11,14,151,154)    Disease-free survival, overall survival  Primary soft-tissue sarcomas  (12,155)    Disease-free survival, overall survival  Primary head and neck cancers  (20,155)    Incidence of metastases  Locally advanced primary cancers of the uterine cervix  (13)  Median pO2 of <4 mmHg  Disease-free survival, overall survival  Locally recurrent cancers of the uterine cervix  (14)  Fraction of pO2 values ≤2.5 mmHg  Locoregional control, overall survival  Head and neck cancers  (156)  Fraction of pO2 values ≤5 mmHg  Disease-free survival, overall survival  Locally advanced primary cancers of the uterine cervix  (157,158)  View Large Fig. 1. View largeDownload slide Mean concentrations of adenosine triphosphate (ATP), phosphomonoesters (PME), and total inorganic phosphate (Pi) (left ordinate) and nominal intracellular pH (pHNMR, measured by 31P nuclear magnetic resonance) in FSaII tumors (right ordinate) as a function of median tumor O2 partial pressure (pO2) values in mmHg (millimeters of mercury) [redrawn from (38)]. Fig. 1. View largeDownload slide Mean concentrations of adenosine triphosphate (ATP), phosphomonoesters (PME), and total inorganic phosphate (Pi) (left ordinate) and nominal intracellular pH (pHNMR, measured by 31P nuclear magnetic resonance) in FSaII tumors (right ordinate) as a function of median tumor O2 partial pressure (pO2) values in mmHg (millimeters of mercury) [redrawn from (38)]. Fig. 2. View largeDownload slide Hypoxia-induced proteome changes (i.e., changes in the set of proteins within a cell at a given time) in the neoplastic and stromal cells influence tumor propagation. The phenotypic consequences of the hypoxic proteome changes in the neoplastic cells are modulated by genomic variation and microenvironmental factors besides hypoxia (tissue field effects). The net result within a tumor microregion can be regression (hypoxia/− phenotype), growth stasis (hypoxia/0 phenotype), or growth promotion and tumor dissemination (hypoxia/+ phenotype). Fig. 2. View largeDownload slide Hypoxia-induced proteome changes (i.e., changes in the set of proteins within a cell at a given time) in the neoplastic and stromal cells influence tumor propagation. The phenotypic consequences of the hypoxic proteome changes in the neoplastic cells are modulated by genomic variation and microenvironmental factors besides hypoxia (tissue field effects). The net result within a tumor microregion can be regression (hypoxia/− phenotype), growth stasis (hypoxia/0 phenotype), or growth promotion and tumor dissemination (hypoxia/+ phenotype). Fig. 3. View largeDownload slide Schematic representation of the paramount importance of hypoxia in the malignant progression of solid tumors through progressive genome changes and clonal selection of hypoxia/+ phenotypes. Tumor hypoxia is a consequence of the deregulated proliferation of malignant cells and an insufficient supply of oxygen [and other nutrients; also recently reviewed in (161,162)]. Sustained hypoxia (and intermittent reoxygenation) increases genomic instability and genomic heterogeneity. New variants adapted to survive and to proliferate under reduced O2 partial pressures within the tumor (hypoxia/+ phenotypes) are selected through clonal expansion and aggravate tumor hypoxia. Thus, a vicious circle is established, leading to the dominance of hypoxia/+ phenotypes that are resistant to therapy and are able to survive and proliferate at various sites remote from the primary tumor. Fig. 3. View largeDownload slide Schematic representation of the paramount importance of hypoxia in the malignant progression of solid tumors through progressive genome changes and clonal selection of hypoxia/+ phenotypes. Tumor hypoxia is a consequence of the deregulated proliferation of malignant cells and an insufficient supply of oxygen [and other nutrients; also recently reviewed in (161,162)]. Sustained hypoxia (and intermittent reoxygenation) increases genomic instability and genomic heterogeneity. New variants adapted to survive and to proliferate under reduced O2 partial pressures within the tumor (hypoxia/+ phenotypes) are selected through clonal expansion and aggravate tumor hypoxia. Thus, a vicious circle is established, leading to the dominance of hypoxia/+ phenotypes that are resistant to therapy and are able to survive and proliferate at various sites remote from the primary tumor. Fig. 4. View largeDownload slide Critical O2 levels that characterize the upper limit of the hypoxic range, below which activities or specific functions of tumor cells progressively change. Open bars = therapy forms; hatched bars = cellular functions; solid bars = mechanisms at the subcellular and molecular levels. The bars indicate the respective hypoxic ranges, with the lengths of the bevels showing the variation in threshold values as found by various authors for different end points. pO2 = O2 partial pressure; mmHg = millimeters of mercury. Fig. 4. View largeDownload slide Critical O2 levels that characterize the upper limit of the hypoxic range, below which activities or specific functions of tumor cells progressively change. Open bars = therapy forms; hatched bars = cellular functions; solid bars = mechanisms at the subcellular and molecular levels. The bars indicate the respective hypoxic ranges, with the lengths of the bevels showing the variation in threshold values as found by various authors for different end points. pO2 = O2 partial pressure; mmHg = millimeters of mercury. Supported by a grant from the Deutsche Krebshilfe (70–1920-Va 2). We thank Dr. Debra Kelleher for her valuable input during manuscript preparation. References 1 Gray LH, Conger AD, Ebert M, Hornsey S, Scott OC. The concentration of oxygen dissolved in tissues at the time of irradiation as a factor in radiotherapy. Br J Radiol  1953; 26: 638–48. Google Scholar 2 Hall EJ, editor. Radiobiology for the radiologist. 4th ed. Philadelphia (PA): Lippincott; 1994. Google Scholar 3 Hill RP. Cellular basis of radiotherapy. In: Tannock IF, Hill RP, editors. The basic science of oncology. 2nd ed. New York (NY): McGraw-Hill; 1992. p. 259–75. Google Scholar 4 Vaupel P, Kelleher DK, editors. Tumor hypoxia: pathophysiology, clinical significance and therapeutic perspectives. Stuttgart (Germany): Wissenschaftliche Verlagsgesellschaft; 1999. Google Scholar 5 Molls M, Vaupel P, editors. Blood perfusion and microenvironment of human tumors. Implications for clinical radiooncology. Berlin and Heidelberg (Germany) and New York (NY): Springer; 2000. Google Scholar 6 Raleigh JA, editor. Hypoxia and Its Clinical Significance. Semin Radiat Oncol  1996; 6: 1–70. Google Scholar 7 Brown JM, Giaccia AJ. The unique physiology of solid tumors: opportunities (and problems) for cancer therapy. Cancer Res  1998; 58: 1408–16. Google Scholar 8 Semenza GL. Hypoxia, clonal selection, and the role of HIF-1 in tumor progression. Crit Rev Biochem Mol Biol  2000; 35: 71–103. Google Scholar 9 Semenza GL. HIF-1: mediator of physiological and pathophysiological responses to hypoxia. J Appl Physiol  2000; 88: 1474–80. Google Scholar 10 Sutherland RM. Tumor hypoxia and gene expression—implications for malignant progression and therapy. Acta Oncol  1998; 37: 567–74. Google Scholar 11 Hockel M, Schlenger K, Aral B, Mitze M, Schaffer U, Vaupel P. Association between tumor hypoxia and malignant progression in advanced cancer of the uterine cervix. Cancer Res  1996; 56: 4509–15. Google Scholar 12 Brizel DM, Scully SP, Harrelson JM, Layfield LJ, Ean JM, Prosnitz LR, et al. Tumor oxygenation predicts the likelihood of distant metastases in human soft tissue sarcoma. Cancer Res  1996; 56: 941–3. Google Scholar 13 Sundfor K, Lyng H, Rofstad EK. Tumour hypoxia and vascular density as predictors of metastasis in squamous cell carcinoma of the uterine cervix. Br J Cancer  1998; 78: 822–7. Google Scholar 14 Hockel M, Schlenger K, Hockel S, Aral B, Schaffer U, Vaupel P. Tumor hypoxia in pelvic recurrences of cervical cancer. Int J Cancer  1998; 79: 365–9. Google Scholar 15 Walenta S, Wetterling M, Lehrke M, Schwickert G, Sundfor K, Rofstad EK, et al. High lactate levels predict likelihood of metastases, tumor recurrence, and restricted patient survival in human cervical cancers. Cancer Res  2000; 60: 916–21. Google Scholar 16 Cuvier C, Jang A, Hill RP. Exposure to hypoxia, glucose starvation and acidosis: effect on invasive capacity of murine tumor cells and correlation with cathepsin (L + B) secretion. Clin Exp Metastasis  1997; 15: 19–25. Google Scholar 17 Graham CH, Forsdike J, Fitzgerald CJ, Macdonald-Goodfellow S. Hypoxia-mediated stimulation of carcinoma cell invasiveness via upregulation of urokinase receptor expression. Int J Cancer  1999; 80: 617–23. Google Scholar 18 Hockel M, Schlenger K, Hockel S, Vaupel P. Hypoxic cervical cancers with low apoptotic index are highly aggressive. Cancer Res  1999; 59: 4525–8. Google Scholar 19 Young SD, Marshall RS, Hill RP. Hypoxia induces DNA overreplication and enhances metastatic potential of murine tumor cells. Proc Natl Acad Sci U S A  1988; 85: 9533–7. Google Scholar 20 Brizel DM, Sibley GS, Prosnitz LR, Scher RL, Dewhirst MW. Tumor hypoxia adversely affects the prognosis of carcinoma of the head and neck. Int J Radiat Oncol Biol Phys  1997; 38: 285–9. Google Scholar 21 Jang A, Hill RP. An examination of the effects of hypoxia, acidosis, and glucose starvation on the expression of metastasis-associated genes in murine tumor cells. Clin Exp Metastasis  1997; 15: 469–83. Google Scholar 22 Graeber TG, Osmanian C, Jacks T, Housman DE, Koch CJ, Lowe SW, et al. Hypoxia-mediated selection of cells with diminished apoptotic potential in solid tumours. Nature  1996; 379: 88–91. Google Scholar 23 Kim CY, Tsai MH, Osmanian C, Graeber TG, Lee JE, Giffard RG, et al. Selection of human cervical epithelial cells that possess reduced apoptotic potential to low-oxygen conditions. Cancer Res  1997; 57: 4200–04. Google Scholar 24 Samali A, Cotter TG. Heat shock proteins increase resistance to apoptosis. Exp Cell Res  1996; 223: 163–70. Google Scholar 25 Zhivotovsky B, Joseph B, Orrenius S. Tumor radiosensitivity and apoptosis. Exp Cell Res  1999; 248: 10–7. Google Scholar 26 Anthoney DA, McIlwrath AJ, Gallagher WM, Edlin AR, Brown R. Microsatellite instability, apoptosis, and loss of p53 function in drug-resistant tumor cells. Cancer Res  1996; 56: 1374–81. Google Scholar 27 Sethi T, Rintoul RC, Moore SM, MacKinnon AC, Salter D, Choo C, et al. Extracellular matrix proteins protect small cell lung cancer cells against apoptosis: a mechanism for small cell lung cancer growth and drug resistance in vivo. Nat Med  1999; 5: 662–8. Google Scholar 28 Hickman JA, Potten CS, Merritt AJ, Fisher TC. Apoptosis and cancer chemotherapy. Philos Trans R Soc Lond B Biol Sci  1994; 345: 319–25. Google Scholar 29 Rice GC, Hoy C, Schimke RT. Transient hypoxia enhances the frequency of dihydrofolate reductase gene amplification in Chinese hamster ovary cells. Proc Natl Acad Sci U S A  1986; 83: 5978–82. Google Scholar 30 West JB. Respiratory physiology—the essentials. 6th ed. Baltimore (MD), London (U.K.), and Los Angeles (CA): Williams & Wilkins; 1999. Google Scholar 31 Boyer PD, Chance B, Ernster L, Mitchell P, Racker E, Slater EC. Oxidative phosphorylation and photophosphorylation. Annu Rev Biochem  1977; 46: 955–1026. Google Scholar 32 Honig CR. Modern cardiovascular physiology. 2nd ed. Boston (MA) and Toronto (ON, Canada): Little and Brown; 1988. Google Scholar 33 Zander R, Vaupel P. Proposal for using a standardized terminology on oxygen transport to tissue. Adv Exp Med Biol  1985; 191: 965–70. Google Scholar 34 Glossary on respiration and gas exchange. J Appl Physiol  1973; 34: 549–58. Google Scholar 35 Vaupel P, Kallinowski F, Okunieff P. Blood flow, oxygen and nutrient supply, and metabolic microenvironment of human tumors: a review. Cancer Res  1989; 49: 6449–65. Google Scholar 36 Vaupel P, Fortmeyer HP, Runkel S, Kallinowski F. Blood flow, oxygen consumption, and tissue oxygenation of human breast cancer xenografts in nude rats. Cancer Res  1987; 47: 3496–503. Google Scholar 37 Kallinowski F, Schlenger KH, Runkel S, Kloes M, Stohrer M, Okunieff P, et al. Blood flow, metabolism, cellular microenvironment, and growth rate of human tumor xenografts. Cancer Res  1989; 49: 3759–64. Google Scholar 38 Vaupel P, Schaefer C, Okunieff P. Intracellular acidosis in murine fibrosarcomas coincides with ATP depletion, hypoxia, and high levels of lactate and total Pi. NMR Biomed  1994; 7: 128–36. Google Scholar 39 Kruger W, Mayer WK, Schaefer C, Stohrer M, Vaupel P. Acute changes of systemic parameters in tumour-bearing rats, and of tumour glucose, lactate, and ATP levels upon local hyperthermia and/or hyperglycaemia. J Cancer Res Clin Oncol  1991; 117: 409–15. Google Scholar 40 Vaupel P. Physiological properties of malignant tumours. NMR Biomed  1992; 5: 220–5. Google Scholar 41 Gerweck LE, Seneviratne T, Gerweck KK. Energy status and radiobiological hypoxia at specified oxygen concentrations. Radiat Res  1993; 135: 69–74. Google Scholar 42 Marshall RS, Koch CJ, Rauth AM. Measurement of low levels of oxygen and their effect on respiration in cell suspensions maintained in an open system. Radiat Res  1986; 108: 91–101. Google Scholar 43 Starlinger H, Lubbers DW. Methodical studies on the polarographic measurement of respiration and “critical oxygen pressure” in mitochondria and isolated cells with membrane-covered platinum electrodes. Pflugers Arch  1972; 337: 19–28. Google Scholar 44 Froese G. The respiration of ascites tumour cells at low oxygen concentrations. Biochim Biophys Acta  1962; 57: 509–19. Google Scholar 45 Robiolio M, Rumsey WL, Wilson DF. Oxygen diffusion and mitochondrial respiration in neuroblastoma cells. Am J Physiol  1989; 256: C1207–13. Google Scholar 46 Wilson DF, Rumsey WL, Green TJ, Vanderkooi JM. The oxygen dependence of mitochondrial oxidative phosphorylation measured by a new optical method for measuring oxygen concentration. J Biol Chem  1988; 263: 2712–8. Google Scholar 47 Chance B, Oshino N, Sugano T, Mayevsky A. Basic principles of tissue oxygen determination from mitochondrial signals. Adv Exp Med Biol  1973; 37A: 277–92. Google Scholar 48 Groebe K. Impact of anemia on the oxygenation status of tumors: a theoretical study. In: Vaupel P, Kelleher DK, editors. Tumor hypoxia: pathophysiology, clinical significance and therapeutic perspectives. Stuttgart (Germany): Wissenschaftliche Verlagsgesellschaft; 1999. p. 75–82. Google Scholar 49 Vaupel PW. Blood flow, oxygenation, tissue pH distribution and bioenergetic status of tumors. Berlin (Germany): Ernst Schering Research Foundation, Lecture 23; 1994. Google Scholar 50 Schaefer C, Okunieff P, Vaupel P. Oxygenation and bioenergetic status of murine fibrosarcomas. Adv Exp Med Biol  1992; 317: 161–7. Google Scholar 51 Mueller-Klieser W, Schlenger KH, Walenta S, Gross M, Karbach U, Hoeckel M et al. Pathophysiological approaches to identifying tumor hypoxia in patients. Radiother Oncol  1991; 20 Suppl 1: 21–8. Google Scholar 52 Chapman JD. Measurement of tumor hypoxia by invasive and non-invasive procedures: a review of recent clinical studies. Radiother Oncol  1991; 20 Suppl 1: 13–9. Google Scholar 53 Stone HB, Brown JM, Phillips TL, Sutherland RM. Oxygen in human tumors: correlations between methods of measurement and response to therapy. Summary of a workshop held November 19–20, 1992, at the National Cancer Institute, Bethesda, Maryland. Radiat Res  1993; 136: 422–34. Google Scholar 54 Chapman JD, Engelhardt EL, Stobbe CC, Schneider RF, Hanks GE. Measuring hypoxia and predicting tumor radioresistance with nuclear medicine assays. Radiother Oncol  1998; 46: 229–37. Google Scholar 55 Kallinowski F, Zander R, Hoeckel M, Vaupel P. Tumor tissue oxygenation as evaluated by computerized pO2-histography. Int J Radiat Oncol Biol Phys  1990; 19: 953–61. Google Scholar 56 Vaupel P, Schlenger K, Knoop C, Hockel M. Oxygenation of human tumors: evaluation of tissue oxygen distribution in breast cancers by computerized O2 tension measurements. Cancer Res  1991; 51: 3316–22. Google Scholar 57 Hockel M, Schlenger K, Knoop C, Vaupel P. Oxygenation of carcinomas of the uterine cervix: evaluation by computerized O2 tension measurements. Cancer Res  1991; 51: 6098–102. Google Scholar 58 Collingridge DR, Young WK, Vojnovic B, Wardman P, Lynch EM, Hill SA, et al. Measurement of tumor oxygenation: a comparison between polarographic needle electrodes and a time-resolved luminescence-based optical sensor. Radiat Res  1997; 147: 329–34. Google Scholar 59 Griffiths JR, Robinson SP. The OxyLite: a fibre-optic oxygen sensor. Br J Radiol  1999; 72: 627–30. Google Scholar 60 Swartz HM, Bacic G, Friedman B, Goda F, Grinberg O, Hoopes PJ, et al. Measurements of pO2 in vivo, including human subjects, by electron paramagnetic resonance. Adv Exp Med Biol  1994; 361: 119–28. Google Scholar 61 Halpern HJ, Yu C, Peric M, Barth E, Grdina DJ, Teicher BA. Oxymetry deep in tissues with low-frequency electron paramagnetic resonance. Proc Natl Acad Sci U S A  1994; 91: 13047–51. Google Scholar 62 Wendling P, Manz R, Thews G, Vaupel P. Heterogeneous oxygenation of rectal carcinomas in humans: a critical parameter for preoperative irradiation? Adv Exp Med Biol  1984; 180: 293–300. Google Scholar 63 Mueller-Klieser W, Vaupel P, Manz R, Schmidseder R. Intracapillary oxyhemoglobin saturation of malignant tumors in humans. Int J Radiat Oncol Biol Phys  1981; 7: 1397–404. Google Scholar 64 Rofstad EK, Fenton BM, Sutherland RM. Intracapillary HbO2 saturations in murine tumours and human tumour xenografts measured by cryospectrophotometry: relationship to tumour volume, tumour pH and fraction of radiobiologically hypoxic cells. Br J Cancer  1988; 57: 494–502. Google Scholar 65 Fenton BM, Rofstad EK, Degner FL, Sutherland RM. Cryospectrophotometric determination of tumor intravascular oxyhemoglobin saturations: Dependence on vascular geometry and tumor growth. J Natl Cancer Inst  1988; 80: 1612–9. Google Scholar 66 Sevick EM, Chance B, Leigh J, Nioka S, Maris M. Quantitation of time- and frequency-resolved optical spectra for the determination of tissue oxygenation. Anal Biochem  1991; 195: 330–51. Google Scholar 67 Hull EL, Conover DL, Foster TH. Carbogen-induced changes in rat mammary tumour oxygenation reported by near infrared spectroscopy. Br J Cancer  1999; 79: 1709–16. Google Scholar 68 Wilson DF, Cerniglia GJ. Localization of tumors and evaluation of their state of oxygenation by phosphorescence imaging. Cancer Res  1992; 52: 3988–93. Google Scholar 69 Robinson SP, Collingridge DR, Howe FA, Rodrigues LM, Chaplin DJ, Griffiths JR. Tumour response to hypercapnia and hyperoxia monitored by FLOOD magnetic resonance imaging. NMR Biomed  1999; 12: 98–106. Google Scholar 70 Stubbs M, Griffiths JR. Monitoring cancer by magnetic resonance. Br J Cancer  1999; 80 Suppl 1: 86–94. Google Scholar 71 McCoy CL, McIntyre DJ, Robinson SP, Aboagye EO, Griffiths JR. Magnetic resonance spectroscopy and imaging methods for measuring tumour and tissue oxygenation. Br J Cancer  1996; 74 Suppl 27: S226–31. Google Scholar 72 Le D, Mason RP, Hunjan S, Constantinescu A, Barker BR, Antich PP. Regional tumor oxygen dynamics: 19F PBSR EPI of hexafluorobenzene. Magn Reson Imaging  1997; 15: 971–81. Google Scholar 73 van der Sanden BP, Heerschap A, Simonetti AW, Rijken PF, Peters HP, Stuben G, et al. Characterization and validation of noninvasive oxygen tension measurements in human glioma xenografts by 19F-MR relaxometry. Int J Radiat Oncol Biol Phys  1999; 44: 649–58. Google Scholar 74 Hunjan S, Mason RP, Constantinescu A, Peschke P, Hahn EW, Antich PP. Regional tumor oximetry: 19F NMR spectroscopy of hexafluorobenzene. Int J Radiat Oncol Biol Phys  1998; 41: 161–71. Google Scholar 75 Fishman JE, Joseph PM, Carvlin MJ, Saadi-Elmandjra M, Mukherji B, Sloviter HA. In vivo measurements of vascular oxygen tension in tumors using MRI of a fluorinated blood substitute. Invest Radiol  1989; 24: 65–71. Google Scholar 76 Mason RP, Antich PP, Babcock EE, Constantinescu A, Peschke P, Hahn EW. Non-invasive determination of tumor oxygen tension and local variation with growth. Int J Radiat Oncol Biol Phys  1994; 29: 95–103. Google Scholar 77 Koh WJ, Rasey JS, Evans ML, Grierson JR, Lewellen TK, Graham MM, et al. Imaging of hypoxia in human tumors with [F-18]fluoromisonidazole. Int J Radiat Oncol Biol Phys  1991; 22: 199–212. Google Scholar 78 Koh WJ, Bergman KS, Rasey JS, Peterson LM, Evans ML, Graham MM, et al. Evaluation of oxygenation status during fractionated radiotherapy in human nonsmall cell lung cancers using [F-18]fluoromisonidazole positron emission tomography. Int J Radiat Oncol Biol Phys  1995; 33: 391–8. Google Scholar 79 Rasey JS, Koh WJ, Evans ML, Peterson LM, Lewellen TK, Graham MM, et al. Quantifying regional hypoxia in human tumors with positron emission tomography of [18F]fluoromisonidazole: a pretherapy study of 37 patients. Int J Radiat Oncol Biol Phys  1996; 36: 417–28. Google Scholar 80 Valk PE, Mathis CA, Prados MD, Gilbert JC, Budinger TF. Hypoxia in human gliomas: demonstration by PET with fluorine-18-fluoromisonidazole. J Nucl Med  1992; 33: 2133–7. Google Scholar 81 Parliament M, Urtasun R. Misonidazole labeling as a marker of cellular hypoxia. In: Molls M, Vaupel P, editors. Blood perfusion and microenvironment of human tumors. Implications for clinical radiooncology. Berlin and Heidelberg (Germany) and New York (NY): Springer; 2000. p. 89–99. Google Scholar 82 Groshar D, McEwan AJ, Parliament MB, Urtasun RC, Golberg LE, Hoskinson M, et al. Imaging tumor hypoxia and tumor perfusion. J Nucl Med  1993; 34: 885–8. Google Scholar 83 Parliament MB, Chapman JD, Urtasun RC, McEwan AJ, Golberg LE, Mercer JR, et al. Non-invasive assessment of human tumour hypoxia with 123I-iodoazomycin arabinoside: preliminary report of a clinical study. Br J Cancer  1992; 65: 90–5. Google Scholar 84 Urtasun RC, Chapman JD, Raleigh JA, Franko AJ, Koch CJ. Binding of 3H-misonidazole to solid human tumors as a measure of tumor hypoxia. Int J Radiat Oncol Biol Phys  1986; 12: 1263–7. Google Scholar 85 Cline JM, Thrall DE, Rosner GL, Raleigh JA. Distribution of the hypoxia marker CCI-103F in canine tumors. Int J Radiat Oncol Biol Phys  1994; 28: 921–33. Google Scholar 86 Cline JM, Thrall DE, Page RL, Raleigh JA. Immunohistochemical detection of a hypoxia marker in spontaneous canine tumours. Br J Cancer  1990; 62: 925–31. Google Scholar 87 Gross MW, Karbach U, Groebe K, Franko AJ, Mueller-Klieser W. Calibration of misonidazole labeling by simultaneous measurement of oxygen tension and labeling density in multicellular spheroids. Int J Cancer  1995; 61: 567–73. Google Scholar 88 Raleigh JA, Dewhirst MW, Thrall DE. Measuring Tumor Hypoxia. Semin Radiat Oncol  1996; 6: 37–45. Google Scholar 89 Azuma C, Raleigh JA, Thrall DE. Longevity of pimonidazole adducts in spontaneous canine tumors as an estimate of hypoxic cell lifetime. Radiat Res  1997; 148: 35–42. Google Scholar 90 Kennedy AS, Raleigh JA, Perez GM, Calkins DP, Thrall DE, Novotny DB, et al. Proliferation and hypoxia in human squamous cell carcinoma of the cervix: first report of combined immunohistochemical assays. Int J Radiat Oncol Biol Phys  1997; 37: 897–905. Google Scholar 91 Lord EM, Harwell L, Koch CJ. Detection of hypoxic cells by monoclonal antibody recognizing 2-nitroimidazole adducts. Cancer Res  1993; 53: 5721–6. Google Scholar 92 Lee J, Siemann DW, Koch CJ, Lord EM. Direct relationship between radiobiological hypoxia in tumors and monoclonal antibody detection of EF5 cellular adducts. Int J Cancer  1996; 67: 372–8. Google Scholar 93 Evans SM, Jenkins WT, Joiner B, Lord EM, Koch CJ. 2-Nitroimidazole (EF5) binding predicts radiation resistance in individual 9L s.c. tumors. Cancer Res  1996; 56: 405–11. Google Scholar 94 Varia MA, Calkins-Adams DP, Rinker LH, Kennedy AS, Novotny DB, Fowler WC Jr, et al. Pimonidazole: a novel hypoxia marker for complementary study of tumor hypoxia and cell proliferation in cervical carcinoma. Gynecol Oncol  1998; 71: 270–7. Google Scholar 95 Koch CJ, Evans SM, Lord EM. Oxygen dependence of cellular uptake of EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)acetamide]: analysis of drug adducts by fluorescent antibodies vs bound radioactivity. Br J Cancer  1995; 72: 869–74. Google Scholar 96 Hodgkiss RJ, Webster L, Wilson GD. Measurement of hypoxia in vivo using a 2-nitroimidazole (NITP). Adv Exp Med Biol  1997; 428: 61–7. Google Scholar 97 Giaccia AJ. Hypoxic Stress Proteins: Survival of the Fittest. Semin Radiat Oncol  1996; 6: 46–58. Google Scholar 98 Durand RE. Keynote address: the influence of microenvironmental factors on the activity of radiation and drugs. Int J Radiat Oncol Biol Phys  1991; 20: 253–8. Google Scholar 99 Moulder JE, Rockwell S. Tumor hypoxia: its impact on cancer therapy. Cancer Metastasis Rev  1987; 5: 313–41. Google Scholar 100 Haroon ZA, Raleigh JA, Greenberg CS, Dewhirst MW. Early wound healing exhibits cytokine surge without evidence of hypoxia. Ann Surg  2000; 231: 137–47. Google Scholar 101 Riva C, Chauvin C, Pison C, Leverve X. Cellular physiology and molecular events in hypoxia-induced apoptosis. Anticancer Res  1998; 18: 4729–36. Google Scholar 102 Amellem O, Loffler M, Pettersen EO. Regulation of cell proliferation under extreme and moderate hypoxia: the role of pyrimidine (deoxy) nucleotides. Br J Cancer  1994; 70: 857–66. Google Scholar 103 Koch CJ, Kruuv J, Frey HE. The effect of hypoxia on the generation time of mammalian cells. Radiat Res  1973; 53: 43–8. Google Scholar 104 Pettersen EO, Lindmo T. Inhibition of cell-cycle progression by acute treatment with various degrees of hypoxia: modifications induced by low concentrations of misonidazole present during hypoxia. Br J Cancer  1983; 48: 809–17. Google Scholar 105 Soengas MS, Alarcon RM, Yoshida H, Giaccia AJ, Hakem R, Mak TW, et al. Apaf-1 and caspase-9 in p53-dependent apoptosis and tumor inhibition. Science  1999; 284: 156–9. Google Scholar 106 Dang CV, Semenza GL. Oncogenic alterations of metabolism. Trends Biochem Sci  1999; 24: 68–72. Google Scholar 107 Shimizu S, Eguchi Y, Kosaka H, Kamiike W, Matsuda H, Tsujimoto Y. Prevention of hypoxia-induced cell death by Bcl-2 and Bcl-xL. Nature  1995; 374: 811–3. Google Scholar 108 Holmgren L, O'Reilly MS, Folkman J. Dormancy of micrometastases: Balanced proliferation and apoptosis in the presence of angiogenesis suppression. Nat Med  1995; 1: 149–53. Google Scholar 109 Demicheli R, Terenziani M, Valagussa P, Moliterni A, Zambetti M, Bonadonna G. Local recurrences following mastectomy: support for the concept of tumor dormancy. J Natl Cancer Inst  1994; 86: 45–8. Google Scholar 110 Prehn RT. The inhibition of tumor growth by tumor mass. Cancer Res  1991; 51: 2–4. Google Scholar 111 Jiang BH, Semenza GL, Bauer C, Marti HH. Hypoxia-inducible factor 1 levels vary exponentially over a physiologically relevant range of O2 tension. Am J Physiol  1996; 271: C1172–80. Google Scholar 112 Mattern J, Kallinowski F, Herfarth C, Volm M. Association of resistance-related protein expression with poor vascularization and low levels of oxygen in human rectal cancer. Int J Cancer  1996; 67: 20–3. Google Scholar 113 Sanna K, Rofstad EK. Hypoxia-induced resistance to doxorubicin and methotrexate in human melanoma cell lines in vitro. Int J Cancer  1994; 58: 258–62. Google Scholar 114 Ausserer WA, Bourrat-Floeck B, Green CJ, Laderoute KR, Sutherland RM. Regulation of c-jun expression during hypoxic and low-glucose stress. Mol Cell Biol  1994; 14: 5032–42. Google Scholar 115 Graeber TG, Peterson JF, Tsai M, Monica K, Fornace AJ Jr, Giaccia AJ. Hypoxia induces accumulation of p53 protein, but activation of a G1-phase checkpoint by low-oxygen conditions is independent of p53 status. Mol Cell Biol  1994; 14: 6264–77. Google Scholar 116 Laderoute KR, Grant TD, Murphy BJ, Sutherland RM. Enhanced epidermal growth factor receptor synthesis in human squamous carcinoma cells exposed to low levels of oxygen. Int J Cancer  1992; 52: 428–32. Google Scholar 117 Shweiki D, Itin A, Soffer D, Keshet E. Vascular endothelial growth factor induced by hypoxia may mediate hypoxia-initiated angiogenesis. Nature  1992; 359: 843–5. Google Scholar 118 Wilson RE, Keng PC, Sutherland RM. Drug resistance in Chinese hamster ovary cells during recovery from severe hypoxia. J Natl Cancer Inst  1989; 81: 1235–40. Google Scholar 119 Hartmann A, Kunz M, Kostlin S, Gillitzer R, Toksoy A, Brocker EB, et al. Hypoxia-induced up-regulation of angiogenin in human malignant melanoma. Cancer Res  1999; 59: 1578–83. Google Scholar 120 Hasan NM, Adams GE, Joiner MC, Marshall JF, Hart IR. Hypoxia facilitates tumour cell detachment by reducing expression of surface adhesion molecules and adhesion to extracellular matrices without loss of cell viability. Br J Cancer  1998; 77: 1799–805. Google Scholar 121 Wang GL, Jiang BH, Rue EA, Semenza GL. Hypoxia-inducible factor 1 is a basic-helix-loop-helix-PAS heterodimer regulated by cellular O2 tension. Proc Natl Acad Sci U S A  1995; 92: 5510–4. Google Scholar 122 Cockman ME, Maxwell PH, Ratcliffe PJ. The beta domain of pVHL is necessary for HIF-alpha regulation. In: Symposium on Tissue Hypoxia. London (U.K.): Academy of Pharmaceutical Scientists; 2000. Google Scholar 123 Lonergan KM, Iliopoulos O, Ohh M, Kamura T, Conaway RC, Conaway JW, et al. Regulation of hypoxia-inducible mRNAs by the von Hippel– Lindau tumor suppressor protein requires binding to complexes containing elongins B/C and Cu12. Mol Cell Biol  1998; 18: 732–41. Google Scholar 124 Maxwell PH, Wiesener MS, Chang GW, Clifford SC, Vaux EC, Cockman ME, et al. The tumour suppressor protein VHL targets hypoxia-inducible factors for oxygen-dependent proteolysis. Nature  1999; 399: 271–5. Google Scholar 125 Chiarotto JA, Hill RP. A quantitative analysis of the reduction in oxygen levels required to induce up-regulation of vascular endothelial growth factor (VEGF) mRNA in cervical cancer cell lines. Br J Cancer  1999; 80: 1518–24. Google Scholar 126 Koong AC, Chen EY, Giaccia AJ. Hypoxia causes the activation of nuclear factor κB through the phosphorylation of IκBα on tyrosine residues. Cancer Res  1994; 54: 1425–30. Google Scholar 127 Brown JM, Giaccia AJ. Tumour hypoxia: the picture has changed in the 1990s. Int J Radiat Biol  1994; 65: 95–102. Google Scholar 128 Chaplin DJ, Horsman MR, Trotter MJ, Siemann DW. Therapeutic significance of microenvironmental factors. In: Molls M, Vaupel P, editors. Blood perfusion and microenvironment of human tumors. Implications for clinical radiooncology. Berlin and Heidelberg (Germany) and New York (NY): Springer; 2000. p. 133–43. Google Scholar 129 Henderson BW, Fingar VH. Relationship of tumor hypoxia and response to photodynamic treatment in an experimental mouse tumor. Cancer Res  1987; 47: 3110–4. Google Scholar 130 Mitchell JB, McPherson S, DeGraff W, Gamson J, Zabell A, Russo A. Oxygen dependence of hematoporphyrin derivative-induced photo-inactivation of Chinese hamster cells. Cancer Res  1985; 45: 2008–11. Google Scholar 131 Chapman JD, Stobbe CC, Arnfield MR, Santus R, Lee L, McPhee MS. Oxygen dependency of tumor cell killing in vitro by light-activated Photofrin II. Radiat Res  1991; 126: 73–9. Google Scholar 132 Chapman JD, McPhee MS, Walz N, Chetner MP, Stobbe CC, Soderlind K, et al. Nuclear magnetic resonance spectroscopy and sensitizer-adduct measurements of photodynamic therapy-induced ischemia in solid tumors. J Natl Cancer Inst  1991; 83: 1650–9. Google Scholar 133 Reynolds TY, Rockwell S, Glazer PM. Genetic instability induced by the tumor microenvironment. Cancer Res  1996; 56: 5754–7. Google Scholar 134 Stoler DL, Anderson GR, Russo CA, Spina AM, Beerman TA. Anoxia-inducible endonuclease activity as a potential basis of the genomic instability of cancer cells. Cancer Res  1992; 52: 4372–8. Google Scholar 135 Cheng KC, Loeb LA. Genomic instability and tumor progression: mechanistic considerations. Adv Cancer Res  1993; 60: 121–56. Google Scholar 136 Russo CA, Weber TK, Volpe CM, Stoler DL, Petrelli NJ, Rodriguez-Bigas M, et al. An anoxia inducible endonuclease and enhanced DNA breakage as contributors to genomic instability in cancer. Cancer Res  1995; 55: 1122–8. Google Scholar 137 Stackpole CW, Groszek L, Kalbag SS. Benign-to-malignant B16 melanoma progression induced in two stages in vitro by exposure to hypoxia. J Natl Cancer Inst  1994; 86: 361–7. Google Scholar 138 Teicher BA, Holden SA, al-Achi A, Herman TS. Classification of antineoplastic treatments by their differential toxicity toward putative oxygenated and hypoxic tumor subpopulations in vivo in the FSaII murine fibrosarcoma. Cancer Res  1990; 50: 3339–44. Google Scholar 139 Teicher BA, Lazo JS, Sartorelli AC. Classification of antineoplastic agents by their selective toxicities toward oxygenated and hypoxic tumor cells. Cancer Res  1981; 41: 73–81. Google Scholar 140 Bedford JS, Mitchell JB. The effect of hypoxia on the growth and radiation response of mammalian cells in culture. Br J Radiol  1974; 47: 687–96. Google Scholar 141 Durand RE. The influence of microenvironmental factors during cancer therapy. In Vivo  1994; 8: 691–702. Google Scholar 142 Wike-Hooley JL, Haveman J, Reinhold HS. The relevance of tumour pH to the treatment of malignant disease. Radiother Oncol  1984; 2: 343–66. Google Scholar 143 Sakata K, Kwok TT, Murphy BJ, Laderoute KR, Gordon GR, Sutherland RM. Hypoxia-induced drug resistance: comparison to P-glycoprotein-associated drug resistance. Br J Cancer  1991; 64: 809–14. Google Scholar 144 Freitas I, Baronzio GF. Tumor hypoxia, reoxygenation and oxygenation strategies: possible role in photodynamic therapy. J Photochem Photobiol B  1991; 11: 3–30. Google Scholar 145 Moan J, Sommer S. Oxygen dependence of the photosensitizing effect of hematoporphyrin derivative in NHIK 3025 cells. Cancer Res  1985; 45: 1608–10. Google Scholar 146 Bicher HI, Hetzel FW, Vaupel P, Sandhu TS. Microcirculation modifications by localized microwave hyperthermia and hematoporphyrin phototherapy. Bibl Anat  1981; 20: 628–32. Google Scholar 147 Song CW, Choi IB, Nah BS, Sahu SK, Osborn JL. Microvasculature and perfusion in normal tissues and tumors. In: Seegenschmiedt MH, Fessenden P, Vernon CC, editors. Medical radiology—diagnostic imaging and radiation oncology, thermoradiotherapy and thermochemotherapy. Berlin and Heidelberg (Germany) and New York (NY): Springer; 1995. p. 139–56. Google Scholar 148 Vaupel PW, Kelleher DK. Metabolic status and reaction to heat of normal and tumor tissue. In: Seegenschmiedt MH, Fessenden P, Vernon CC, editors. Medical radiology—diagnostic imaging and radiation oncology, thermoradiotherapy and thermochemotherapy. Berlin and Heidelberg (Germany) and New York (NY): Springer; 1995. p. 157–76. Google Scholar 149 Gerweck LE, Richards B, Jennings M. The influence of variable oxygen concentration on the response of cells to heat or X irradiation. Radiat Res  1981; 85: 314–20. Google Scholar 150 Gatenby RA, Kessler HB, Rosenblum JS, Coia LR, Moldofsky PJ, Hartz WH, et al. Oxygen distribution in squamous cell carcinoma metastases and its relationship to outcome of radiation therapy. Int J Radiat Oncol Biol Phys  1988; 14: 831–8. Google Scholar 151 Hockel M, Knoop C, Schlenger K, Vorndran B, Baussmann E, Mitze M, et al. Intratumoral pO2 predicts survival in advanced cancer of the uterine cervix. Radiother Oncol  1993; 26: 45–50. Google Scholar 152 Hockel M, Schlenger K, Mitze M, Schaffer U, Vaupel P. Hypoxia and Radiation Response in Human Tumors. Semin Radiat Oncol  1996; 6: 3–9. Google Scholar 153 Nordsmark M, Hoyer M, Keller J, Nielsen OS, Jensen OM, Overgaard J. The relationship between tumor oxygenation and cell proliferation in human soft tissue sarcomas. Int J Radiat Oncol Biol Phys  1996; 35: 701–8. Google Scholar 154 Knocke TH, Weitmann HD, Feldmann HJ, Selzer E, Potter R. Intratumoral pO2-measurements as predictive assay in the treatment of carcinoma of the uterine cervix. Radiother Oncol  1999; 53: 99–104. Google Scholar 155 Brizel DM. Human tumor oxygenation: The Duke University Medical Center experience. In: Vaupel P, Kelleher DK, editors. Tumor hypoxia. Pathophysiology, clinical significance and therapeutic perspectives. Stuttgart (Germany): Wissenschaftliche Verlagsgesellschaft; 1999. p. 29–38. Google Scholar 156 Nordsmark M, Overgaard M, Overgaard J. Pretreatment oxygenation predicts radiation response in advanced squamous cell carcinoma of the head and neck. Radiother Oncol  1996; 41: 31–9. Google Scholar 157 Fyles AW, Milosevic M, Wong R, Kavanagh MC, Pintilie M, Sun A, et al. Oxygenation predicts radiation response and survival in patients with cervix cancer. Radiother Oncol  1998; 48: 149–56. Google Scholar 158 Sundfor K, Lyng H, Trope CG, Rofstad EK. Treatment outcome in advanced squamous cell carcinoma of the uterine cervix: relationships to pretreatment tumor oxygenation and vascularization. Radiother Oncol  2000; 54: 101–7. Google Scholar 159 Nordsmark M, Hoyer M, Keller J, Jensen OM, Nielsen OS, Overgaard J. Predictive assays in radiotherapy, the Aarhus experience. Oxygenation, cell kinetics and 31P MR spectroscopy in human soft tissue sarcomas. Radiother Oncol  1996; 46(suppl 1): S107. Google Scholar 160 Nordsmark M, Keller J, Hoyer M, Nielsen OS, Jensen OM, Overgaard J. Hypoxia in human soft tissue sarcomas associated with poor survival. Radiother Oncol  1998; 48(suppl 1): S162. Google Scholar 161 Rofstad EK. Microenvironment-induced cancer metastasis. Int J Radiat Biol  2000; 76: 589–605. Google Scholar 162 Dachs GU, Tozer GM. Hypoxia modulated gene expression: angiogenesis, metastasis and therapeutic exploitation. Eur J Cancer  2000; 36: 1649–60. Google Scholar © Oxford University Press
TI - Tumor Hypoxia: Definitions and Current Clinical, Biologic, and Molecular Aspects
JF - JNCI: Journal of the National Cancer Institute
DO - 10.1093/jnci/93.4.266
DA - 2001-02-21
UR - https://www.deepdyve.com/lp/oxford-university-press/tumor-hypoxia-definitions-and-current-clinical-biologic-and-molecular-CpRUTt91f5
SP - 266
EP - 276
VL - 93
IS - 4
DP - DeepDyve
ER -