Intracapillary HbO2 saturation in malignant tumors during normoxia and hyperoxia

The dynamic side of the Warburg effect: glycolytic intermediate storage as buffer for fluctuating glucose and O 2 supply in tumor cells

Background: Tumor cells often show altered metabolism which supports uncontrolled proliferation. A classic example is the Warburg effect: high glucose uptake and lactate production despite sufficient oxygen supply. Remarkably, tumor cells can transiently take up glucose even an order of magnitude faster when glucose is reintroduced after depletion. Regulation and significance of this high glucose uptake are investigated here. Methods: A new computational model was developed which reproduces two types of experimental data on Ehrlich ascites tumor cells: measurements by Otto Warburg of the average aerobic glycolytic rate during one hour (Warburg effect), and fast metabolic responses measured by others during the first minutes after reintroducing glucose. The model is subsequently extended with equations for glucose and O 2 transport to predict the role of metabolism during fluctuations of blood flow in tumor tissue. Results: Model analysis reveals dynamic regulation of the head section of glycolysis where glucose uptake and phosphorylation occur. The head section is disinhibited slowly when concentrations of glycolytic intermediates fall, causing glucose uptake rate to considerably exceed that found by Warburg. The head section is partially inhibited in about a minute when sufficient glucose has been taken up. Simulations predict that tumors greedily take up glucose when blood flow resumes after periods of low flow. The cells then store glucose as fructose 1,6-bisphosphate and other glycolytic intermediates. During subsequent periods of low flow that cause O 2 and glucose depletion these stores are used for ATP production and biomass. Conclusions: The powerful glycolytic system in tumors not only synthesizes ATP at high steady rates, but can also store glycolytic intermediates to buffer temporary oxygen and nutrient shortages for up to 10 minutes. The head section of glycolysis in tumor cells, disinhibited during glucose shortages, becomes very efficient at stealing glucose from other cells, even at low glucose concentrations.

The dynamic side of the Warburg effect: glycolytic intermediates as buffer for fluctuating glucose and O 2 supply in tumor cells

Background: Tumor cells show the Warburg effect: high glucose uptake and lactate production despite sufficient oxygen supply. Otto Warburg found this effect in tissue slices and in suspensions of Ehrlich ascites tumor cells. Remarkably, these ascites tumor cells can transiently take up glucose an order of magnitude faster than the steady high rate measured by Warburg for hours. Methods: The purpose of the transiently very high glucose uptake is investigated here with a computational model of glycolysis, oxidative phosphorylation and ATP consumption which reproduces short kinetic experiments on the ascites tumor cells as well as the long-lasting Warburg, Crabtree and Pasteur effects. The model, extended with equations for glucose and O 2 transport in tissue, is subsequently used to predict metabolism in tumor cells during fluctuations of tissue blood flow resulting in cycling hypoxia. Results: The model analysis suggests that the head section of the glycolytic chain in the tumor cells is partially inhibited in about a minute when substantial amounts of glucose have been taken up intracellularly; this head section of the glycolytic chain is subsequently disinhibited slowly when concentrations of glycolytic intermediates are low. Based on these dynamic characteristics, simulations of tissue with fluctuating O 2 and glucose supply predict that tumor cells greedily take up glucose when this periodically becomes available, leaving very little for other cells. The glucose is stored as fructose 1,6-bisphosphate and other glycolytic intermediates, which are used for ATP production during   O 2 and glucose shortages. Conclusions: The head section of glycolysis which phosphorylates glucose may be dynamically regulated and takes up glucose at rates exceeding the Warburg effect if glucose levels have been low for some time. The hypothesis is put forward here that dynamic regulation of the powerful glycolytic enzyme system in tumors is used to buffer oxygen and nutrient fluctuations in tissue.

Tumor oxygenation: an appraisal of past and present concepts and a look into the future : Arisztid G. B. Kovách Lecture

Since 1970, the multifactorial pathogenesis of the deficient and heterogeneous oxygenation of transplanted murine tumors and of human cancers (including parameters determining oxygen delivery, e.g., blood flow, diffusion geometry, oxygen transport capacity of the blood) has been investigated in vivo. Hypoxia and/or anoxia was quantitatively assessed and characterized using microtechniques and special preclinical tumor models. Hypoxia subtypes were identified, and critical supply conditions were theoretically analyzed. In the 1980s, first experiments on humans were carried out in cancers of the rectum and of the oral cavity. In the 1990s, the clinical investigations were carried out on cancers of the breast and of the uterine cervix, clearly showing that hypoxia is a hallmark of locally advanced human tumors. In multivariate analysis, hypoxia was found to be an independent, adverse prognostic factor for patient survival due to hypoxia-driven malignant progression and hypoxia-associated resistance to anticancer therapy.

Functional magnetic resonance (fMR) imaging of a rat brain tumor model: implications for evaluation of tumor microvasculature and therapeutic response

Functional MR (fMR) imaging techniques based on blood oxygenation level dependent (BOLD) effects were developed and applied to a rat brain tumor model to evaluate the potential utility of the method for characterizing tumor growth and regression following treatment. Rats bearing 9L brain tumors in situ were imaged during inhalation of room air and after administration of 100% oxygen + acetazolamide (ACZ) injected 15 mg/kg intravenously. Pixel-to-pixel fMR maps of normalized signal intensity change from baseline values were calculated from T2 weighted spin echo (SE) images acquired pre- and post- oxygen + ACZ administration. Resultant fMR maps were then compared to gross histological sections obtained from corresponding anatomical regions. Regions containing viable tumor with increased cellular density and localized foci of necrotic tumor cells consistent with hypoxia were visualized in the fMR images as regions with decreased signal intensities, indicating diminished oxyhemoglobin concentration and blood flow as compared to normal brain. Histological regions having peritumor edema, caused by increased permeability of tumor vasculature, were visualized in the fMR images as areas with markedly increased signal intensities. These results suggest that fMR imaging techniques could be further developed for use as a non-invasive tool to assess changes in tumor oxygenation/hemodynamics, and to evaluate the pharmacologic effect of anti-neoplastic drugs.

Intravascular oxygen distribution in subcutaneous 9L tumors and radiation sensitivity

Phosphorescence quenching was evaluated as a technique for measuring PO2 in tumors and for determining the effect of increased PO2 on sensitivity of the tumors to radiation. Suspensions of cultured 9L cells or small pieces of solid tumors from 9L cells were injected subcutaneously on the hindquarter of rats, and tumors were grown to between 0.2 and 1.0 cm in diameter. Oxygen-dependent quenching of the phosphorescence of intravenously injected Pd-meso-tetra-(4-carboxyphenyl) porphine was used to image the in vivo distribution of PO2 in the vasculature of small tumors and surrounding tissue. Maps (512 x 480 pixels) of tissue oxygen distribution showed that the PO2 within 9L tumors was low (2-12 Torr) relative to the surrounding muscle tissue (20-40 Torr). When the rats were given 100% oxygen or carbogen (95% O2-5% CO2) to breathe, the PO2 in the tumors increased significantly. This increase was variable among tumors and was greater with carbogen compared with 100% oxygen. Based on irradiation and regrowth studies, carbogen breathing increased the sensitivity of the tumors to radiation. This is consistent with the measured increase in PO2 in the tumor vasculature. It is concluded that phosphorescence quenching can be used for noninvasive determination of the oxygenation of tumors. This method for oxygen measurements has great potential for clinical application in tumor identification and therapy.

Changes in tumor oxygenation during a combined treatment with fractionated irradiation and hyperthermia: an experimental study

PURPOSE

To determine the influence of adjuvant hyperthermia on the oxygenation status of fractionated irradiated tumors.

METHODS AND MATERIALS

Oxygen partial pressure (pO2) in rat rhabdomyosarcomas (R1H) was measured sequentially at weekly intervals during a fractionated irradiation with 60Co-gamma-rays (60 Gy/20f/4 weeks) in combination with local hyperthermia (8 f(HT) at 43 degrees C, 1 h/4 weeks). Tumors were heated twice weekly with a 2450 MHz microwave device at 43 degrees C, 1 h starting 10 min after irradiation. The pO2 measurements (pO2-histograph, Eppendorf, Germany) were performed in anesthetized animals during mechanical ventilation and in hemodynamic steady state. All tumor pO2 measurements were correlated to measurements of the arterial oxygen partial pressure (paO2) determined by a blood gas analyzer.

RESULTS

The oxygenation status of R1H tumors decreased continuously from the start of the combined treatment, with increasing radiation dose and number of heat fractions. In untreated controls a median tumor pO2 of 23 +/- 2 mmHg (mean +/- SEM) was measured. Tumor pO2 decreased to 11 +/- 2 mmHg after 30 Gy + 4 HT (2 weeks), and to 6 +/- 2 mmHg after 60 Gy + 8HT (4 weeks). The increase in the frequency of pO2-values below 5 mmHg and the decrease in the range of the pO2 histograms [delta p(10/90)] further indicated that tumor hypoxia increased relatively rapidly from the start of combined treatment. After 60 Gy + 8HT 48 +/- 5% (mean +/- SEM) of the pO2-values recorded were below 5 mmHg.

CONCLUSIONS

These findings suggest that adjuvant hyperthermia to radiotherapy induces greater changes in tumor oxygenation than radiation alone [cf. (39)]. This might be of importance for the temporary application of hyperthermia in the course of a conventional radiation treatment.

Tumor oxygenation in a transplanted rat rhabdomyosarcoma during fractionated irradiation

PURPOSE

To quantify the changes in tumor oxygenation in the course of a fractionated radiation treatment extending over 4 weeks.

METHODS AND MATERIALS

Rhabdomyosarcomas R1H of the rat were irradiated with 60Co-gamma-rays with a total dose of 60 Gy, given in 20 fractions over 4 weeks. Oxygen partial pressure (pO2) in tumors was measured at weekly intervals using polarographic needle probes in combination with a microprocessor-controlled device (pO2-Histograph/KIMOC). The pO2 measurements were carried out in anesthetized animals under mechanical ventilation and in respiratory and hemodynamic steady state. Tumor pO2 values were correlated to the arterial oxygen pressure paO2, arterial pCO2, and pH determined with a blood gas analyzer.

RESULTS

Tumor oxygenation did not change significantly during the 3 weeks of irradiation (up to 45 Gy), from a median pO2 of 23 +/- 2 mmHg in untreated controls to 19 +/- 4 mmHg after the third week. The decrease of the number of pO2 values between 0 and 5 mmHg indicated that an improved oxygenation in the tumors occurred. However, with increasing radiation dose (fourth week, 60 Gy) a significant decrease in tumor oxygenation to a median pO2 of 8 +/- 2 mmHg and a rapid increase in the frequency of pO2 values (35 +/- 4%) between 0 and 5 mmHg was found.

CONCLUSION

Improved oxygenation in rhabdomyosarcomas R1H was only present in the early phase of the fractionated irradiation. Radiation does above 45 Gy led to a considerable decrease of tumor oxygenation in the later phase of irradiation.

Non-invasive determination of tumor oxygen tension and local variation with growth

PURPOSE

The objective was to develop and demonstrate a novel noninvasive technique of measuring regional pO2 in tumors. The method is based on measuring 19F nuclear magnetic resonance spin-lattice relaxation rate (R1 = 1/T1) of perfluorocarbon (PFC) emulsion discretely sequestered in a tumor.

METHODS AND MATERIALS

We have examined pO2 in the Dunning prostate tumor R3327-AT1 implanted in a Copenhagen rat. Oxypherol blood substitute emulsion was administered intravenously and became sequestered in tissue. Proton magnetic resonance imaging (MRI) showed tumor anatomy and correlated 19F MRI indicated the distribution of perfluorocarbon. Fluorine-19 spectroscopic relaxometry was used to measure pO2 in the tumor and repeated measurements over a period of 3 weeks showed the variation in local pO2 during tumor growth.

RESULTS

Perfluorocarbon initially resided in the vascularized peripheral region of the tumor: 19F nuclear magnetic resonance R1 indicated pO2 approximately 75 torr in a small tumor (approximately 1 cm) in an anesthetized rat. As the tumor grew, the sequestered PFC retained its original distribution. When the tumor had doubled in size the residual PFC was predominantly in the core of the tumor and the pO2 of this region was approximately 1 torr indicating central tumor hypoxia.

CONCLUSION

We have demonstrated a novel noninvasive approach to monitoring regional tumor pO2. Given the critical role of oxygen tension in tumor response to therapy this may provide new insight into tumor physiology, the efficacy of various therapeutic approaches, and ultimately provide a clinical technique for assessing individual tumor oxygenation.

Pathophysiological approaches to identifying tumor hypoxia in patients

The present report summarizes observations of the authors on tumor oxygenation and on techniques for characterizing tumor hypoxia in patients. Cryospectrophotometric measurements of HbO2 saturations in tumor microvessels allow for estimates of the proportion of well oxygenated tissue regions. Labeling of tissue areas at oxygen (O2) tensions (pO2) less than 10 mm Hg with misonidazole may be used for a general characterization of the oxygenation status in patient tumors rather than for the determination of the radiobiologically hypoxic cell fraction. Quantitative bioluminescence and single photon imaging make it possible to determine ATP concentrations in absolute terms with a spatial resolution at the cellular level. It is shown that the ATP distribution reflects the efficiency of the O2 supply to tumors. Since all these techniques rely on biopsy material, the measured values can be assessed in relation to the histological structure and vascular pattern of the tumors. Such a direct interrelationship is not obtained when using polarographic microelectrodes in tumor tissue. However, a novel technology, the "computerized pO2 histography" has enabled direct polarographic measurements of pO2 values in patients, in extended and systematic clinical trials. Preliminary results in cervix and breast cancers demonstrate that pO2 values are lower in tumors than in adjacent normal tissues, and that great intra- and intertumoral differences occur even among tumors of the same clinical stages and histological grades, illustrating the necessity of such a pathophysiological approach to an "individualization" of tumor therapy.

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

Frequency distributions for intracapillary HbO2 saturation were determined for two murine tumour lines (KHT, RIF-1) and two human ovarian carcinoma xenograft lines (MLS, OWI) using a cryospectrophotometric method. The aim was to search for possible relationships between HbO2 saturation status and tumour volume, tumour pH and fraction of radiobiologically hypoxic cells. Tumour pH was measured by 31P NMR spectroscopy. Hypoxic fractions were determined from cell survival curves for tumours irradiated in vivo and assayed in vitro. Tumours in the volume range 100-4000 mm3 were studied and the majority of the vessels were found to have HbO2 saturations below 10%. The volume-dependence of the HbO2 frequency distributions differed significantly among the four tumour lines; HbO2 saturation status decreased with increasing tumour volume for the KHT, RIF-1 and MLS lines and was independent of tumour volume for the OWI line. The data indicated that the rate of decrease in HbO2 saturation status during tumour growth was related to the rate of development of necrosis. The volume-dependence of tumour pH was very similar to that of the HbO2 saturation status for all tumour lines. Significant correlations were therefore found between HbO2 saturation status and tumour pH, both within tumour lines and across the four tumour lines, reflecting that the volume-dependence of both parameters probably was a compulsory consequence of reduced oxygen supply conditions during tumour growth. Hypoxic fraction increased during tumour growth for the KHT, RIF-1 and MLS lines and was volume-independent for the OWI line, suggesting a relationship between HbO2 saturation status and hypoxic fraction within tumour lines. However, there was no correlation between these two parameters across the four tumour lines, indicating that the hypoxic fraction of a tumour is not determined only by the oxygen supply conditions; other parameters may also be important, e.g. oxygen diffusivity, rate of oxygen consumption and cell survival time under hypoxic stress.

Tumor hypoxia: its impact on cancer therapy

The presence of radiation resistant cells in solid human tumors is believed to be a major reason why radiotherapy fails to eradicate some such neoplasms. The presence of unperfused regions containing hypoxic cells may also contribute to resistance to some chemotherapeutic agents. This paper reviews the evidence that radiation resistant hypoxic cells exist in solid tumors, the assumptions and results of the methods used to detect hypoxic cells, and the causes and nature of tumor hypoxia. Evidence that radiation resistant hypoxic cells exist in the vast majority of transplanted rodent tumors and xenografted human tumors is direct and convincing, but problems with the current methodology make quantitative statements about the magnitude of the hypoxic fractions problematic. Evidence that radiation resistant hypoxic cells exist in human tumors is considerably more indirect than the evidence for their existence in transplanted tumors, but it is convincing. However, evidence that hypoxic cells are a significant cause of local failure after optimal clinical radiotherapy or chemotherapy regimens is limited and less definitive. The nature and causes of tumor hypoxia are not definitively known. In particular, it is not certain whether hypoxia is a chronic or a transient state, whether hypoxic cells are proliferating or quiescent, or whether hypoxic cells have the same repair capacity as aerobic cells. A number of new methods for assessing hypoxia are reviewed. While there are still problems with all of the new techniques, some of them have the potential of allowing the assessment of hypoxia in individual human tumors.

Importance of critical metabolites and cellular interactions in the biology of microregions of tumors

Heterogeneity of cellular microenvironments and associated differences in phenotypic expression of cells in solid tumors occur as a consequence of deficiencies of vascularization. Intervascular microregions of tumors and micrometastases can be modeled by multicellular spheroids in vitro. These develop concentration gradients of critical metabolites such as oxygen, glucose, and probably also other important nutrients, hormones, and growth factors. Drug penetration may also be reduced. Concentrations of these factors decrease from the periphery towards the center, but gradients develop in the opposite direction for certain metabolic and necrotic products such as lactate and pH. Such gradients significantly modify the proliferative status of the cells, viability, clonogenicity, cell cycle distribution, antigen expression, and differentiation. Stress of oxygen and glucose deprivation induces synthesis of a specific set of proteins. Cellular interactions can also modulate phenotypic expression, including oxygen and glucose consumption rates and sensitivity to radiation and drugs. The mechanisms of these are unknown, but changes in cell-cell communication and DNA damage and repair have been demonstrated in spheroids. Microenvironmental and cellular changes may be transient. Thus, the characteristics of cells in tumor microregions may be quite different from the intrinsic cellular properties observed after growth of cells in standard culture systems. These changes may have significant effects on responses to therapeutic agents.

Cell line and growth site as relevant parameters governing tumor tissue oxygenation

Tumors commonly exhibit pronounced inter-individual and intra-individual differences in the oxygenation status. Paramount factors contributing to this variability are the tumor growth stage or size, changes in the O2 transport capacity of the arterial blood (e.g., during tumor-induced anemia) as well as inherent properties of the tumor cell line investigated and different tumor growth sites as shown in the present study. In most cases, variations of nutritive tumor blood flow and changes in the amount of arterio-venous shunt perfusion within the tumor tissue have to be considered as basic pathomechanisms for these inter-tumor variations of tissue oxygenation.

The detection and measurement of hypoxic cells in solid tumors

The presence of viable hypoxic cells in human cancers has concerned oncologists for years. Cells in tissues that are deficient in oxygen are relatively resistant to radiation inactivation and may not be accessible to some systemic chemotherapy. The premise that hypoxic tumor cells do, indeed, control the radiocurability of some cancers is supported by some clinical evidence. The presence of hypoxic regions within tumors can be directly and indirectly inferred from invasive procedures such as oxygen electrode techniques and histologic study, respectively, but such information does not significantly contribute to current prescriptions given by oncologists for tumor treatment. Novel procedures (some of which are noninvasive) for detecting hypoxic regions within solid tumors have been proposed and are based upon two recent developments: (1) the discovery that some radiosensitizing drugs become selectively bound by metabolism to the molecules of viable hypoxic cells, and (2) the growing availability of new imaging procedures based upon positron-emission tomography, single-photon emission tomography, and nuclear magnetic resonance spectroscopy. Preliminary research results from these novel procedures are reviewed, and the potential clinical impact of each is discussed.

Effectiveness of respiratory hyperoxia, of normobaric and of hyperbaric oxygen atmospheres in improving tumor oxygenation

Tumor oxygenation during respiratory hyperoxia is dependent on the tumor growth site, on the growth stage, and hence on the vascular pattern. Diffusion of O2 from the surrounding atmosphere contributes considerably to the oxygenation of subcutaneous tumors during normobaric exposure to a pure O2 atmosphere. However, hypoxia is still present in the tumor core under these conditions, and pressurization up to 4 bar is required to completely eradicate these hypoxic areas, thus enhancing the radiosensitivity of the tumors.

Heterogeneous oxygenation of rectal carcinomas in humans: a critical parameter for preoperative irradiation?

Tissue oxygenation was measured in 10 patients with differentiated adenocarcinoma in a very localized region in the middle part of the rectum (grade I - II, clinical stage II) by means of a cryophotometric micromethod. The results obtained clearly show that the oxygenation of differentiated rectal adenocarcinoma is distinctly lower than that of the normal rectal mucosa; tissue hypoxia or even anoxia are a common feature in those tumors; There exist considerable inter- individual differences among tumors of the same clinical staging and histological grading; substantial intra- individual heterogeneities in the oxygenation are evident within the same tumor and even within neighbouring microareas of the tissue. These findings imply that the commonly used classifications do not allow any conclusions concerning the oxygenation status, and probably the radiosensitivity of a tumor, respectively.

Impact of various thermal doses on the oxygenation and blood flow in malignant tumors upon localized hyperthermia

Upon localized hyperthermia at modest thermal doses an increase in tumor blood flow can be observed in many tumors which is paralleled by an improvement of the oxygenation status of the tissue. At intermediate or high thermal doses a pronounced restriction of the tumor circulation becomes obvious leading to a deterioration of the tumor oxygenation. As a consequence, a further enhancement of the thermal response of tumors relative to normal tissues has to be expected at intermediate or high thermal doses.

Tumour oxygenation under normobaric and hyperbaric conditions

Tumour oxygenation during exposure to normobaric and hyperbaric oxygen was assessed by means of a cryophotometric micromethod and a specially constructed pressure chamber. The tumours investigated were DS-carcinosarcomas implanted subcutaneously into the thigh of Sprague-Dawley rats. The animals were anaesthetised and put on a heating pad. Mean arterial blood pressure was monitored throughout the entire time of pressurisation. Cryobiopsies were removed from the tumours after exposure to O2 for 30 min at 1 bar (atmospheric pressure), 2, 3, or 4 bar. Oxyhaemoglobin saturation (HbO2) values of single red blood cells in tumour microvessels were determined photometrically in the frozen tissue samples as a quantitative measure for tumour oxygenation. Frequency distribution curves of HbO2 showed that there was a marked improvement of the O2 supply to tumour tissue in O2 atmospheres at 1 bar compared with exposure to air at 1 bar. Pressurisation in O2 atmospheres up to 2 and 3 bar did not substantially alter the distribution curve of the HbO2 values in comparison with the data sampled at 1 bar O2 exposure, yet led to a significant drop in the occurrence of HbO2 values below 5% saturation. Thus, pressurisation particularly raises those tissue O2 partial pressures (pO2) that are in a range where the radiosensitivity is critically influenced by the pO2. Pressurisation up to 4 bar caused a shift of the HbO2 distribution curve to values significantly higher than those from tumours exposed to O2 at 1 bar, with no values below 35% saturation. Using these data and a previously published model for computing tissue pO2 values it can be shown that radiobiological hypoxia is unlikely to occur in DS-carcinosarcomas exposed to an O2 atmosphere at 4 bar under the conditions chosen. Hyperbaric oxygenation is recommended as an efficient adjuvant of X irradiation, particularly in superficial tumours.

Frequency distribution histograms of oxygen tensions in multicell spheroids

Recessed oxygen sensitive microelectrodes have been used to measure oxygen tensions in spheroids from EMT6/Ro mouse mammary carcinoma cells and from V-79-171B hamster lung fibroblasts. During the experiments the spheroids were exposed to conditions similar to those prevailing during growth and previous experiments with radiation and/or drugs. Frequency distribution curves were generated from 386 steady state readings in EMT6 (N = 20) spheroids with diameters ranging from 386 to 1900 micron and from 289 steady state readings in V79 spheroids (N = 20) with diameters between 376 and 1052 micron. Small spheroids exhibit higher pO2 values than spheroids of medium and large size. Oxygen tensions in small and medium size V79 spheroids are lower than in EMT6 spheroids of the equivalent size ranges. The pO2 histograms in large spheroids of both cell types are similar to pO2 frequency distributions in solid tumors. It is concluded that spheroids may represent a valuable model to provide evidence for the way pO2 histograms are influenced by the density of capillaries, by the cellular O2 consumption and by the sampling technique.

Hypoxia and the radiation response of tumors

The effect of hypoxia on the in vivo radiation response of tumors is demonstrated both for the measurement of cell survival and growth delay following treatment. Results using these endpoints for the KHT Sarcoma are then presented to demonstrate the effect of various parameters which are expected to control tumor oxygenation: blood flow, blood pO2, blood oxygen carrying capacity and tissue consumption of oxygen. Intrinsic blood flow does not correlate with the level of hypoxia in the tumor, but acute reductions in blood flow do result in decreases in tumor oxygenation. Changing the blood PaO2 or the oxygen carrying capacity change the level of hypoxia as predicted, but only certain aspects have been studied. The intricate feedback mechanism which controls normal tissue oxygenation and compensates for any deficiencies cannot be assumed to work effectively in tumors.

Oxygen tensions in multicell spheroids of two cell lines

O2 tensions (Po2) were measured with microelectrodes in multicellular spheroids from EMT6/Ro and V-79-171-B cells. The measurements were performed in spheroids kept in flowing growth medium that was equilibrated with 5% CO2 and air at a temperature of 37 degrees C and contained 5.5 mM glucose. The recorded Po2 profiles are characterized by a diffusion-depleted zone surrounding the spheroids and by a steep drop in Po2 within the spheroids over mean distance of 220 and 188 micrometer from the surface of EMT6/Ro and V-79-171B spheroids over mean distance of 220 and 188 micrometer from the surface of EMT6/Ro and V-79-171B spheroids respectively. Smaller spheroid exhibit parabolic Po2 profiles, larger ones show a central plateau. The region of the steep decrease in Po2 corresponds to the thickness of the viable rim: the plateau region is created by the absence of O2 consumption in the central necrotic area. Po2 in the centre of EMT6/Ro spheroids decreased from 66 mmHg at a diameter of 400 micrometer to 13 mmHg at a diameter of 1000 micrometer. Under the present conditions during growth and in the experiments, values below 5 mmHg were recorded only in spheroids 1200 micrometer. Comparably low Po2 was recorded in V-79 spheroids with diameters of 650 micrometer +. In spheroids of this cell type with a diameter of 400 micrometer, Po2 was 42 mmHg. The findings provide evidence that necrosis may arise at average Po2 of 57 and 42 mmHg in EMT6/Ro and V-79-171B spheroids, respectively, grown under the conditions described.

Impact of localized microwave hyperthermia on the oxygenation status of malignant tumors

Upon heating at 40 degrees C for 30 minutes, the oxygenation of the tumor tissue significantly improved as compared with control conditions at 35 degrees C. In contradistinction to this, the tumor oxygenation significantly decreased directly after 43 degrees C- hyperthermia. A further temperature rise to 45 degrees C caused the oxygenation to drastically drop due to an almost complete cessation of nutritive blood flow. The changes in tumor oxygenation during hyperthermia seem to be predominatly mediated through changes in tumor blood flow, which showed the same directional changes.

Oxygenation of malignant tumors after localized microwave hyperthermia

The oxyhemoglobin saturation (HbO2) of single red blood cells within tumor microvessels (diameter: 3-12 micrometers) of DS-Carcinosarcoma was studied using a cryophotometric micromethod. In untreated control tumors (mean tissue temperature approx. 35 degrees C) the measured values scattered over the whole saturation range from zero to 100 sat. %, the mean being 51 sat. %. Upon heating at 40 degrees C for 30 min, the oxygenation of the tumor tissue significantly improved as compared with control conditions. After 40 degrees C-hyperthermia a mean oxyhemoglobin saturation of 66 sat. % was obtained. In contradistinction to this, after 43 degrees C-hyperthermia the tumor oxygenation was significantly lower and reached a mean HbO2 saturation value of 47 sat. %. A further temperature rise to 45 degrees C caused the oxygenation to drop drastically (mean oxyhemoglobin saturation value: 24 sat. %). This is due to a severe restriction of nutritive blood flow. The changes in tumor oxygenation after hyperthermia seem to be predominantly mediated through changes in tumor blood flow, including tumor microcirculation, which showed a similar temperature dependence. Metabolic effects probably play a minor role in the oxyhemoglobin saturation distribution within tumor microvessels.