The influence of haemoglobin affinity for oxygen on tumour radiosensitivity

Analysis of Red Blood Cells and their Components in Medical Workers with Occupational Exposure to Low-Dose Ionizing Radiation

Plenty of reports focus on the effects of low-dose radiation (LDR) on peripheral blood lymphocytes in radiation workers. However, studies on red blood cells (RBCs) in radiation workers are rarely reported. Many studies focused on investigate the hemogram of radiation staffs without detecting other components of RBCs. To explore the potential effect of LDR on RBCs, we detected the level of RBC count, hemoglobin, 2,3-disphosphoglycerate (2,3-DPG), and glutathione (GSH), and then analyzed the factors on these indices in 106 medical radiation workers. As a result, RBC count was affected by sex, age, type of work, length of service (only for females), and annual effective dose (only for males). Hemoglobin status was affected by sex, type of work, and annual effective dose (only for males). Sex, age, and type of work had no effects on the concentration of 2,3-DPG and GSH. Length of service affected 2,3-DPG concentration, and annual effective dose affected GSH level. In conclusion, chronic occupational LDR exposure may have an effect on RBC count, hemoglobin status, and the concentration of 2,3-DPG and GSH in radiation workers to some extent. However, it is still unknown how this kind of influence affects the health of radiation workers.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

A Multimodal Molecular Imaging Study Evaluates Pharmacological Alteration of the Tumor Microenvironment to Improve Radiation Response

: Hypoxic zones in solid tumors contribute to radioresistance, and pharmacologic agents that increase tumor oxygenation prior to radiation, including antiangiogenic drugs, can enhance treatment response to radiotherapy. Although such strategies have been applied, imaging assessments of tumor oxygenation to identify an optimum time window for radiotherapy have not been fully explored. In this study, we investigated the effects of α-sulfoquinovosylacyl-1,3-propanediol (SQAP or CG-0321; a synthetic derivative of an antiangiogenic agent) on the tumor microenvironment in terms of oxygen partial pressure (pO₂), oxyhemoglobin saturation (sO₂), blood perfusion, and microvessel density using electron paramagnetic resonance imaging, photoacoustic imaging, dynamic contrast-enhanced MRI with Gd-DTPA injection, and T2*-weighted imaging with ultrasmall superparamagnetic iron oxide (USPIO) contrast. SCCVII and A549 tumors were grown by injecting tumor cells into the hind legs of mice. Five days of daily radiation (2 Gy) combined with intravenous injection of SQAP (2 mg/kg) 30 minutes prior to irradiation significantly delayed growth of tumor xenografts. Three days of daily treatment improved tumor oxygenation and decreased tumor microvascular density on T2*-weighted images with USPIO, suggesting vascular normalization. Acute effects of SQAP on tumor oxygenation were examined by pO₂, sO₂, and Gd-DTPA contrast-enhanced imaging. SQAP treatment improved perfusion and tumor pO₂ (ΔpO₂: 3.1 ± 1.0 mmHg) and was accompanied by decreased sO₂ (20%-30% decrease) in SCCVII implants 20-30 minutes after SQAP administration. These results provide evidence that SQAP transiently enhanced tumor oxygenation by facilitating oxygen dissociation from oxyhemoglobin and improving tumor perfusion. Therefore, SQAP-mediated sensitization to radiation in vivo can be attributed to increased tumor oxygenation. SIGNIFICANCE: A multimodal molecular imaging study evaluates pharmacological alteration of the tumor microenvironment to improve radiation response.

Importance of hemoglobin concentration and its modification for the outcome of head and neck cancer patients treated with radiotherapy

INTRODUCTION

Hypoxia induced radioresistance has been acknowledged for decades. One of the indirect evidences of the influence of hypoxia on radiation response comes from the observations of a correlation between tumor control and hemoglobin level. This review examines the clinical data on the prognostic and predictive role of hemoglobin level and hemoglobin manipulation in radiotherapy of squamous cell carcinomas of the head and neck, a tumor type where hypoxic radioresistance have been previously documented.

THE INFLUENCE OF HEMOGLOBIN CONCENTRATION ON TUMOR OXYGENATION AND OUTCOME

The aim is to evaluate the existing literature for information of the influence of hemoglobin concentration and hemoglobin modifications on tumor oxygenation and outcome in head and neck squamous cell cancer patients. The data from several randomized trials show that while most studies have confirmed the prognostic value of hemoglobin, increasing the hemoglobin level through transfusion or erythropoietin stimulation did not result in improved outcome for patients with low initial hemoglobin levels. Clinical studies showed that smoking reduced the oxygen carrying capacity of the blood through formation of carboxyhemoglobin, and lead to poorer response to radiotherapy in smokers compared to non-smokers. Smoking also increased the risk of the development of secondary cancers.

CONCLUSION AND FUTURE PERSPECTIVES

In conclusion, low hemoglobin is a significant negative prognostic factor for radiotherapy of head and neck cancer. Correction of pre-treatment low hemoglobin by blood transfusion and/or erythropoietin stimulating agents does, however, not improve the outcome. Smoking leads to a decrease in effective hemoglobin and poorer treatment outcome. Smoking should be avoided in order to improve the therapeutic efficacy of radiotherapy and development of other smoking-related diseases and/or secondary cancers.

Microenvironmental adaptation of experimental tumours to chronic vs acute hypoxia

This study investigated long-term microenvironmental responses (oxygenation, perfusion, metabolic status, proliferation, vascular endothelial growth factor (VEGF) expression and vascularisation) to chronic hypoxia in experimental tumours. Experiments were performed using s.c.-implanted DS-sarcomas in rats. In order to induce more pronounced tumour hypoxia, one group of animals was housed in a hypoxic atmosphere (8% O₂) for the whole period of tumour growth (chronic hypoxia). A second group was acutely exposed to inspiratory hypoxia for only 20 min prior to the measurements (acute hypoxia), whereas animals housed under normal atmospheric conditions served as controls. Acute hypoxia reduced the median oxygen partial pressure (pO₂) dramatically (1 vs 10 mmHg in controls), whereas in chronically hypoxic tumours the pO₂ was significantly improved (median pO₂=4 mmHg), however not reaching the control level. These findings reflect the changes in tumour perfusion where acutely hypoxic tumours show a dramatic reduction of perfused tumour vessels (maybe the result of a simultaneous reduction in arterial blood pressure). In animals under chronic inspiratory hypoxia, the number of perfused vessels increased (compared to acute hypoxia), although the perfusion pattern found in control tumours was not reached. In the chronically hypoxic animals, tumour cell proliferation and tumour growth were significantly reduced, whereas no differences in VEGF expression and vascular density between these groups were observed. These results suggest that long-term adaptation of tumours to chronic hypoxia in vivo, while not affecting vascularity, does influence the functional status of the microvessels in favour of a more homogeneous perfusion.

A phase I study of RSR13, a radiation-enhancing hemoglobin modifier: tolerance of repeated intravenous doses and correlation of pharmacokinetics with pharmacodynamics

PURPOSE

Preclinical studies indicate that RSR13 oxygenates and radiosensitizes hypoxic solid tumors by decreasing the oxygen (O(2))-binding affinity of hemoglobin (Hb). A Phase I open-label, multicenter dose and frequency escalation study was conducted to assess the safety, tolerance, pharmacokinetics, and pharmacodynamic effect of daily RSR13 administration to cancer patients receiving concurrent palliative radiotherapy (RT).

METHODS AND MATERIALS

Eligibility criteria included the following: ECOG performance status < or =2; resting and exercise arterial oxygen saturation (SaO(2)) > or =90%; an indication for palliative RT, 20-40 Gy in 10-15 fractions. RSR13 was administered i.v. via central vein over 60 min immediately before RT. Patients received supplemental O(2) via nasal cannula at 4 L/min during RSR13 infusion and RT. Plasma, red blood cell (RBC), and urine RSR13 concentrations were assayed. The pharmacodynamic effect of RSR13 on Hb-O(2) binding affinity was quantified by multipoint tonometry and expressed as an increase in p50, defined as the partial pressure of O(2) that results in 50% SaO(2). The RSR13 dose in the first cohort was 75 mg/kg once a week for two doses; successive cohorts received higher, more frequent doses up to 100 mg/kg/day for 10 days during RT.

RESULTS

Twenty patients were enrolled in the study. Repeated daily doses of RSR13 were generally well tolerated. Two adverse events of note occurred: (1) A patient with pre-existing restrictive lung disease had transient persistent hypoxemia after the sixth RSR13 dose; (2) a patient with a recurrent glioma receiving high-dose corticosteroids had edema after the seventh RSR13 dose, likely due to the daily high-volume fluid infusions. Both patients recovered to baseline status with conservative management. Maximum pharmacodynamic effect occurred at the end of RSR13 infusion and was proportional to the RBC RSR13 concentration. After an RSR13 dose of 100 mg/kg, the peak increase in p50 averaged 8.1 mm Hg, consistent with the targeted physiologic effect, and then diminished with a half-life of approximately 5 h.

CONCLUSIONS

RSR13 was well tolerated in daily doses up to 100 mg/kg administered for 10 days during RT. The combined administration of RSR13 with 4 L/min supplemental O(2) yielded pharmacodynamic conditions in which hypoxic tumor radiosensitization can occur. Ongoing Phase II and Phase III studies are evaluating the combination of RT and RSR13 for selected indications, including primary brain tumors, brain metastases, and non-small-cell lung cancer.

RSR13, an allosteric effector of haemoglobin, and carbogen radiosensitize FSAII and SCCVII tumours in C3H mice

Pre-clinical evaluation has demonstrated that 2-[4-(((3,5-dimethylanilino)carbonyl)methyl)phenoxy]-2-methylpropi onic acid (RSR13) acts as an allosteric effector of haemoglobin (Hb). RSR13 binding to Hb results in decreased haemoglobin-oxygen (Hb-O2) affinity, improved tumour oxygenation, and enhanced radiation-induced cell killing in several experimental tumour systems. In the present work, ex vivo clonogenic survival analyses are applied in two murine tumour systems to characterize the relationship between the magnitude of decrease in Hb-O2 affinity and radiosensitization, the influence of inspired pO2 upon this effect, and the efficacy of combining RSR13 and radiation during a course of repeated radiation exposures. For FSaII tumours in C3H mice breathing air, 100 mg kg(-1) RSR13 administered intraperitoneally produced an enhancement ratio (ER) of 1.3, but there was marked desensitization at a RSR13 dose of 300 mg kg(-1) (ER 0.6). The most likely reason for the increased radioresistance was insufficient oxygen loading of Hb in the pulmonary circulation due to reduced haemoglobin-oxygen affinity because carbogen breathing combined with 300 mg kg(-1) RSR13 reversed the effect and produced an ER of 1.8. In SCCVII tumours in C3H mice irradiated with eight fractions of 2.5 Gy over 4 days, the surviving fraction was reduced to 58-67% of control values when RSR13 was combined with radiation on days 1 and 2, days 3 and 4, or days 1-4. These results confirm that combining RSR13 and irradiation within a fractionated course of clinically relevant low-dose exposures provides significant radiosensitization. Additional preclinical experimentation is needed to define better the optimum dose-scheduling conditions for clinical applications.

Oxygenation in tumors by modified hemoglobins

The effect of systemic injection of modified hemoglobin (Hb) prepared from bovine, human, or mouse Hb on tumor oxygenation was investigated. Hb was modified by (1) diisothiocyanatobenzenesulfonate (DIBS) to yield cross-linking within a tetramer; (2) glycolaldehyde (Glyal) to yield cross-linking between and within tetramers; (3) carboxymethylation (Cm) to change oxygen affinity; or (4) poly(ethylene glycol) (PEG) to yield attachment between tetramers. HGL9 (human glioma) in nude mice and FSaII (mice fibrosarcoma) in C3H mice were used as tumor models. Dose and time dependency were detected in the oxygenation effect by bovine-PEG-Hb. Internal cross-linkage prolonged the half-life in the circulation, and thus showed a significant effect. Compared to bovine-CmHb, bovine-DIBS-Hb and bovine-DIBS-CmHb were more effective. Decreasing the oxygen affinity by Cm significantly enhanced tumor oxygenation. Human-DIBS-CmHb was more effective than human-DIBS-Hb. These effects were caused by oxygen carrying capacity of modified Hbs as well as hemodynamic factors, and the injection seemed to reduce both perfusion-limited (acute) and diffusion-limited (chronic) hypoxia.

Noninvasive monitoring of carbogen-induced changes in tumor blood flow and oxygenation by functional magnetic resonance imaging

PURPOSE

The response of tumors to radiotherapy can be enhanced if carbogen (95% O2, 5% CO2) is breathed. The timing of carbogen administration is critical, and a noninvasive method of monitoring the response of individual tumors would have obvious utility. Functional gradient recalled echo (GRE) magnetic resonance imaging (MRI) techniques are sensitive to changes in the concentrations of deoxyhemoglobin, which, thus, acts as an endogenous contrast agent for oxygenation status and blood flow.

METHODS AND MATERIALS

Subcutaneous GH3 prolactinomas in three rats were imaged at 4.7 Tesla with a GRE 1H sequence [echo time (TE) = 20 ms, repetition time (TR) = 80 ms, flip angle = 45 degrees, 1 mm slice, 256 phase encode steps, 4 cm field of view, in-plane resolution 0.08 x 0.08 mm, acquisition time = 4 min]. The rats breathed air or carbogen for four periods of 20 min; three control rats breathed only air.

RESULTS

Carbogen breathing caused increases of up to 100% in the GRE image intensity of the tumors. Reversion of air breathing caused the image intensity to fall; essentially the same response was observed with the second cycle of carbogen and air breathing. Control rat tumors showed no significant change.

CONCLUSIONS

The response of tumors to carbogen can be monitored noninvasively by GRE MRI. In principle, this could be due to an increase in oxygen content of the blood, a decrease in tumor cell oxygen consumption, or an increase in tumor blood flow. The very large changes in signal intensity suggest that a blood flow increase is the most probable explanation. If this technique can be successfully applied in man, it should be possible to optimize carbogen treatment for individual radiotherapy patients, and perhaps also to enhance tumor uptake of chemotherapeutic agents.

The relationship between carbon monoxide breathing, tumour oxygenation and local tumour control in the C3H mammary carcinoma in vivo

The effect of acute carbon monoxide (CO) breathing on blood oxygenation and tumour hypoxia was related to the radiation response of the C3H/Tif mammary carcinoma. Blood gas analysis showed that CO breathing caused a time- and dose-dependent formation of carboxyhaemoglobin (HbCO), a significant left shift of the oxygen dissociation curve and a reduction in tumour blood perfusion. These factors all contributed to a marked drop in tumour oxygen supply. In agreement with this, tumour hypoxia was found to be significantly increased: Microelectrode PO2 measurements showed a clear relationship between CO concentration and the proportion of low PO2 measurements (< or = 5 mmHg). The fraction of clonogenic hypoxic cells increased from 8% in air-breathing animals to 13%, 18% and 54% with 75,220 and 660 p.p.m. CO respectively. The tumour hypoxia resulted in significant radiation modification. The local tumour control after single-dose and fractionated irradiation gave TCD50 enhancement ratios (relative to air-breathing controls) of 0.90, 0.85 and 0.89 for single dose and five or ten fractions given in 5 days (P < 0.005 for all values). For 15 fractions in 5 days with 6- 6- and 12 h intervals, the TCD50 was similar in CO- and air-breathing mice, presumably as a consequence of insufficient reoxygenation during the short inter-fraction intervals. It is concluded that elevated HbCO levels to increased tumour hypoxia and that the induced hypoxia has a significant impact on the local tumour control also after fractionated irradiation.

The prognostic value of hemoglobin and a decrease in hemoglobin during radiotherapy in laryngeal carcinoma

An association between a low Hb-content and a poor response on radiation therapy has been established in previous years, especially in patients with squamous cell carcinomas. Whether this is linked with hypoxic conditions of the tumour or associated with tumour volume, blood loss or other conditions, is not yet known for certain. Patients with small head and neck tumours seem to form an interesting group to study this issue because several factors can be ruled out. Therefore, a retrospective analysis of 306 patients with glottic or supraglottic carcinoma was performed, to identify whether the Hb-value was of prognostic importance and whether this was linked to other patient characteristics. All patients were treated with primary radiotherapy with doses ranging from 60 to 70 Gy in 6-7 weeks. It appeared that in glottic carcinoma, the disease-free survival was significantly worse in patients with a Hb-value below normal at the start or/and at the end of therapy (p = 0.09, p = 0.0012, respectively). In patients with supraglottic carcinoma, the above mentioned difference approved only for a Hb-value below normal at the end of treatment (p = 0.05). A decrease in Hb-level of greater than or equal to 0.5 mmol/l during therapy, regardless whether the Hb-values were within or beyond normal range, reduced the disease-free survival in patients with glottic carcinoma (p = 0.0125). In a multivariate analysis, the Hb-value at the end of treatment proved to be an independent prognostic factor after T- and N-classification. There was no relationship with nutritional condition of the patient, nor with the Karnofsky Performance scale.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor hypoxia, reoxygenation and oxygenation strategies: possible role in photodynamic therapy

The concept of hypoxia and its role in tumor therapy are currently under re-evaluation. Poor oxygenation is no longer visualized as an independent feature promoting necrosis and resistance to treatments, but rather as one of the several interdependent microenvironmental parameters associated with impaired blood perfusion. Tumor cells display several survival strategies and remain clonogenic for long periods in nutrient-deprived situations. Reoxygenation may cause lethal damage, improve the response to therapy, or else allow the cell variants adapted to hypoxia to resume proliferation with enhanced aggressiveness and resistance to treatment. The blood supply parameters, oxygenation status and metabolism of malignant cells are discussed here from the standpoint of tumor photodynamic therapy. The role of the tumor interstitial fluid as oxygen- and sensitizer-carrier is discussed. Techniques for assessing tumor oxygenation and for mapping hypoxic territories are described. Strategies for locally improving the oxygenation levels or for selectively destroying the hypoxic populations are outlined.

In situ tumour radiosensitization induced by clofibrate administration: single dose and fractionated studies

It is generally accepted that hypoxia is a common occurrence in many experimental and human tumours and that it is a major cause of local failure after radiotherapy. Many attempts have been made over the last years to eliminate this problem. One of the manoeuvres to improve tumour oxygenation is to manipulate the binding affinity of oxygen (O2) and haemoglobin (Hb). Previous studies have shown that some antilipidaemic drugs (clofibrate and its analogues) can reduce the Hb/O2 binding affinity and sensitize various animal tumours to radiation. The present study evaluated the ability of clofibrate to sensitize in situ a mouse carcinoma (CaNT) to radiation. Clofibrate at 1.5 mmol/kg increased the tumour radiosensitivity, when it was administered per os 2-6 h before a single radiation dose or 2 to 4 h before each of 10 fractions in 5 days. In both the single dose and fractionated regimens, the radiosensitizing effect was drug dose-dependent, but was only statistically significant at doses from 1.0 to 2.0 mmol/kg. These results suggest that clofibrate may be an effective radiosensitizer at radiation doses that are clinically relevant. Further experiments need to be carried out to evaluate clofibrate analogues for their radiosensitizing properties. Clofibrate tolerable doses in man have to be determined first in order to know if clofibrate and analogues could play a role in the clinical management of tumours where hypoxia may limit the outcome of radiotherapy.

Chemical manipulations of tissue oxygenation for therapeutic benefit

Many new approaches are currently being explored to improve cancer treatment through hypoxic cell radiosensitization. Oxygen carriers, calcium antagonists, some drugs which shift the oxygen hemoglobin curve to the right improve the oxygen availability in the tumors. An alternative way to improve radiotherapy or chemotherapy is to increase tumor hypoxia either by shifting the hemoglobin oxygen dissociation curve to the left or by using vasodilators. This paper summarizes main results obtained in that field. The relevance of data obtained from animal experiments to clinical practice will be discussed.

Altered radiosensitivity in a mouse carcinoma after administration of clofibrate and bezafibrate

We have investigated the ability of the antilipidaemia drugs clofibrate and bezafibrate, to reduce the binding affinity of haemoglobin for oxygen and to sensitize an experimental tumour (the SCCVII/St carcinoma) in the mouse to radiation. Clofibrate at a high dose (4.1 mmol/kg, p.o.) increased the P50 of the blood by about 10 mm Hg. Its effect on tumour radiosensitivity was dependent on tumour size. Highly significant sensitization, equivalent to a 40-fold reduction in the number of hypoxic cells, was seen in small tumours; but in large tumours there was much less effect. At a low dose, which is close to that currently used clinically (0.3 mmol/kg), clofibrate produced a small and barely significant increase in P50. The effect of low dose clofibrate on tumour radiosensitivity also depended on tumour size, small tumours (200 mg) being significantly sensitized, while no significant effect was seen in large tumours. Bezafibrate, at the low dose of 0.3 mmol/kg, gave a significant increase in P50 (by approximately equal to 8 mm Hg), but sensitization to radiation in small tumours was not impressive and not statistically significant. We must gain a better understanding of the mechanisms involved in these effects before applying this approach to clinical radiotherapy.

Chlorophenoxy acetic acid derivatives as hemoglobin modifiers and tumor radiosensitizers

We have studied the influence of the antilipidemia drug, clofibrate, and several structurally related analogs on the binding affinity of hemoglobin for oxygen and the radiation sensitivity of the SCCVII/St carcinoma in the mouse. Several compounds in this class reduced hemoglobin affinity in vivo; and two of these, ML1024 (etophyline clofibrate) and ML1037, were at least as effective as clofibrate at reducing hemoglobin affinity and much less toxic. When given orally at a dose of 4.1 m mole/Kg, 1/2-2 hrs before 20 Gy X rays, clofibrate gave radiosensitization in the SCCVII/St tumor equivalent to a 40-fold reduction in hypoxic fraction. ML1024 and ML1037 at a dose (3.0 m mole/Kg), which had a similar effect on hemoglobin, gave much less sensitization of the tumor. Only ML1024 produced a statistically significant effect, equivalent to a four-fold reduction in hypoxic fraction. We conclude that there are several clofibrate analogs which in relation to their toxicity are much better hemoglobin modifiers than the parent compound. They do not, however, show the same radiosensitizing effects, leading us to believe that mechanisms other than changes in hemoglobin/oxygen binding must also be involved.

Manipulations in the oxygen transport capacity of blood as a means of sensitizing tumors to radiation therapy

Tumor response to radiation is dependent not only on the quantity of hemoglobin (Hb) available for oxygen (O2) transport but also on the position of the Hb-O2 dissociation curve (Hb affinity). Previous studies have shown that administering agents which shift the Hb-O2 dissociation curve to the right (decrease Hb affinity) sensitize tumors to radiation by reducing the fraction of radiobiologically hypoxic cells. However, there may be toxicity limitations when agents aimed at altering Hb affinity are administered directly to the host. The present studies evaluated the therapeutic benefit of shifting the Hb-O2 dissociation curve in vitro prior to the transfusion of the biochemically modified RBCs into recipient hosts. Mice were given a hemolysis agent (phenylhydrazine hydrochloride, PH) prior to transfusing RBCs with normal or altered Hb affinity. A 100 mg/kg dose of PH reduced the hematocrit to approximately 60% of control 24 hr after treatment. Tumors irradiated at this time demonstrated an increased fraction of hypoxic cells. If the hematocrit was returned to normal by transfusing mice prior to irradiation, a significant but transient reduction in the hypoxic fraction was seen. Tumor response was reduced if RBCs with elevated Hb affinity, obtained by storing the erythrocytes at 4 degrees C, were used. Alternatively, tumor sensitization was noted when animals were transfused with RBCs having decreased Hb affinities. The latter was achieved by incubating the RBCs in the presence of either clofibrate or the precursors of 2,3 diphosphoglycerate (2,3 DPG). These findings further support the notion that the Hb affinity is an important parameter in determining tumor response to radiation and suggest that this factor ought to be considered when RBCs are used to transfuse anemic patients undergoing radiotherapy.

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.