OXYGEN TENSION IN HUMAN MALIGNANT DISEASE UNDER HYPERBARIC CONDITIONS

Effects of hyperbaric oxygen and normobaric carbogen on the radiation response of the rat rhabdomyosarcoma R1H

PURPOSE

Hypoxic tumor cells are an important factor of radioresistance. Hyperbaric oxygen (HBO) and normobaric carbogen (95% oxygen, 5% carbon dioxide) increase the oxygen delivery to tumors. This study was performed to explore changes of tumor oxygenation during a course of fractionated irradiation and to determine the effectiveness of normobaric carbogen and HBO during the final phase of the radiation treatment.

METHODS AND MATERIALS

Experiments were performed on the rhabdomyosarcoma R1H growing on WAG/Rij rats. After 20 X-ray fractions of 2 Gy within 4 weeks, oxygen partial pressure (pO2) was measured using the Eppendorf oxygen electrode under ambient conditions, with normobaric carbogen or HBO at a pressure of 240 kPa. Following the 4-week radiation course, a top-up dose of 10-50 Gy was applied in 2-10 fractions of 5 Gy with or without hyperoxygenation.

RESULTS

HBO but not carbogen significantly increased the median pO2 in irradiated tumors. The radiation doses to control 50% of tumors were 38.0 Gy, 29.5 Gy, and 25.0 Gy for air, carbogen, and HBO, respectively. Both high oxygen content gas inspirations led to significantly improved tumor responses with oxygen enhancement ratios (OERs) of 1.3 for normobaric carbogen and 1.5 for HBO (air vs. carbogen: p = 0.044; air vs. HBO: p = 0.02; carbogen vs. HBO: p = 0.048).

CONCLUSION

Both normobaric carbogen and HBO significantly improved the radiation response of R1H tumors. HBO appeared to be more effective than normobaric carbogen, both with regard to tumor oxygenation and response to irradiation.

The mechanisms by which hyperbaric oxygen and carbogen improve tumour oxygenation

Hyperbaric oxygen (HBO) has been proposed to reduce tumour hypoxia by increasing the amount of dissolved oxygen in the plasma. That this actually occurs has not been verified experimentally. This study was performed to explore changes in tumour oxygenation induced by treatment with normobaric and hyperbaric oxygen and carbogen. R3230Ac mammary adenocarcinomas were implanted into Fisher 344 rats. Arterial blood gases, blood pressure and heart rate were monitored. Tumour oxygenation was measured polarographically in five sets of animals. They received either normobaric 100% oxygen, hyperbaric (3 atmospheres; atm) 100% oxygen, normobaric carbogen or hyperbaric (3 atm) carbogen (HBC) +/- bretylium. HBO reduced the mean level of low pO2 values (< 5 mmHg) from 0.49 to 0.07 (P = 0.0003) and increased the average median pO2 from 8 mmHg to 55 mmHg (P = 0.001). HBC reduced the level of low pO2 values from 0.82 to 0.51 (P = 0.002) an increased median pO2 from 2 mmHg to 6 mmHg (P = 0.05). Normobaric oxygen and carbogen did not change tumour oxygenation significantly. Sympathetic blockade with bretylium before HBC exposure improved oxygenation significantly more than HBC alone (low pO2 0.55-0.17, median pO2 4-17 mmHg). HBO and hyperbaric carbogen improved tumour oxygenation in this model, while normobaric oxygen or carbogen had no effect. Sympathetic-mediated vasoconstriction during hyperbaric carbogen caused it to be less effective than HBO. This mechanism also appeared to operate during normobaric carbogen breathing.

PO2 in irradiated versus nonirradiated tumors of mice breathing oxygen at normal and elevated pressure

PURPOSE

To determine if prior tumor irradiation influences tumor pO2 changes in mice breathing oxygen (100%) at normal and elevated pressure.

METHODS AND MATERIALS

Single-point pO2 measurements were performed in nonirradiated and previously irradiated (72 h) isotransplanted MCaIV tumors in C3H/Sed mice. Continuous recordings were performed at the same tumor locus under air breathing, followed by 100% oxygen and oxygen at three atmospheres pressure. Following decompression and induction of pentobarbital anesthesia, the procedure was repeated at the same locus. Six nonirradiated and five irradiated tumors were evaluated under the three gas breathing conditions +/- anesthesia.

RESULTS

The mean, median, and range of pO2 values did not differ under air-breathing conditions in the nonirradiated vs. previously irradiated tumors. However, prior irradiation substantially enhanced the tumor pO2 increase when the inspired gas phase was switched from air to 100% oxygen at 1 or 3 atmospheres pressure. In four of six nonirradiated tumors, 100% oxygen breathing resulted in a pO2 increase of < 4 mmHg; in the irradiated tumors, the minimum increase was 16 mmHg. Pentobarbital anesthesia did not significantly influence the results obtained.

CONCLUSION

These data indicate that the efficacy of oxygen breathing increases during tumor treatment, and suggests that oxygen breathing is a simple nontoxic method for reducing or eliminating radiobiologic hypoxia during therapy.

Noninvasive measurement of microvascular and interstitial oxygen profiles in a human tumor in SCID mice

Simultaneous measurements of intravascular and interstitial oxygen partial pressure (PO2) in any tissue have not previously been reported, despite the importance of oxygen in health and in disease. This is due to the limitations of current techniques, both invasive and noninvasive. We have optically measured microscopic profiles of PO2 with high spatial resolution in subcutaneous tissue and transplanted tumors in mice by combining an oxygen-dependent phosphorescence quenching method and a transparent tissue preparation. The strengths of our approach include the ability to follow PO2 in the same location for several weeks and to relate these measurements to local blood flow and vascular architecture. Our results show that (i) PO2 values in blood vessels in well-vascularized regions of a human colon adenocarcinoma xenograft are comparable to those in surrounding arterioles and venules, (ii) carbogen (95% O2/5% CO2) breathing increases microvascular PO2 in tumors, and (iii) in unanesthetized and anesthetized mice PO2 drops to hypoxic values at < 200 microns from isolated vessels but drops by < 5 mmHg (1 mmHg = 133 Pa) in highly vascularized tumor regions. Our method should permit noninvasive evaluations of oxygen-modifying agents and offer further mechanistic information about tumor pathophysiology in tissue preparations where the surface of the tissue can be observed.

Oxygenation of mammary tumors: from isotransplanted rodent tumors to primary malignancies in patients

As a result of a compromised and anisotropic microcirculation, isotransplanted mammary tumors in mice exhibit hypoxic and anoxic tissue areas which are heterogeneously distributed within the tumor mass. Similarly, in poorly perfused human breast cancer xenografts, hypoxia develops at early growth stages and expands with tumor growth. In contrast, breast cancer xenografts with high perfusion rates exhibit an oxygenation status comparable to that of most normal organs. There is clear experimental evidence that the efficiency of tumor blood flow in isotransplanted tumors and in xenografted human breast cancers is the principal modulator of tissue oxygenation. The pO2 distribution found in primary lesions in patients ranged from the pO2 histograms obtained in "low-flow" isotransplants and xenografts to those measured in "high-flow" breast cancer xenografts or normal tissues. From our extended and systematic clinical studies there is clear indication that the oxygenation status of human breast cancers in situ does not correlate with the clinical stage and/or histological grade of an individual tumor.

Relationships between thermal dose and heat-induced tissue and vascular damage after thermoradiotherapy of locally advanced breast carcinoma

Twenty-four patients with locally advanced breast carcinoma were given thermoradiotherapy, and heat-induced damage to tissue and vasculature was studied in relation to thermal dose. Thermometry was performed using six to eight multi-point thermistor probes. Heat-induced damage was quantified by histopathological analysis of biopsies taken from the temperature probe locations shortly after treatment. Both tumour and normal tissue were found in the biopsies. Fraction of tissue and fraction of vessels with heat-induced damage were determined for the malignant and the normal tissue compartment separately, using stereological techniques. Clear relationships were found between these parameters and the largest thermal dose achieved in one heat fraction. The data were subjected to logit analysis, and the thermal doses (eqv. min at 43 degrees C) that caused massive necrosis in 50% of the tissue were calculated to be 116 +/- 31 for the malignant tissue compartment and 205 +/- 49 for the normal tissue compartment. Similarly, the thermal doses that caused damage to 50% of the vessels were found to be 63 +/- 34 and 144 +/- 46 for the malignant and the normal tissue compartment, respectively. Thus, the tumour tissue was more sensitive to heat than was the surrounding normal tissue, irrespective of whether necrosis or vessel damage was considered. This was probably a consequence of physiological and vascular differences between the two tissue compartments. Evidence for primary and secondary cell death was found both in the malignant and the normal tissue, although primary cell death probably was of minor importance in the normal tissue. The data indicate that selective heat-inactivation of tumour tissue is possible by external microwave hyperthermia of locally advanced breast carcinoma.

Human mammary carcinomas in nude rats--a new approach for investigating oxygen transport and substrate utilization in tumor tissues

A new model is presented for the study of oxygen supply and substrate utilization in human tumor tissue. In this approach human tumor material thrives in immune-deficient nude rats. The host chosen allows the continuous evaluation of all relevant parameters. From the data obtained so far it is concluded that this model is a valid tool in investigation of the metabolic status of human tumors.

Modulation of tumour cell colony growth in soft agar by oxygen and its mechanism

A simple technique for maintaining low oxygen concentrations (0.1-20%) is described. These conditions were then used to study the effect of oxygen on colony growth of neoplastic cells in soft-agar. Physiologically low oxygen concentrations (0.1-10%) compared to 20% O2 were found to enhance plating efficiency and colony size of tumour cells. The optimal oxygen concentration for plating efficiency varied with tumour studied and may be as low as 0.1%. Having established that tumour cell colonies will grow better at 0.1-10% O2 compared to 20% O2, the mechanism by which this enhancement occurs was investigated. Observations on the effect of free radical scavengers and superoxide dismutase on plating efficiency of Ehrlich's ascites tumour cells suggests that this phenomenon occurs through oxygen toxicity mediated by superoxide anion.

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.

Advantage of reduced oxygen tension in growth of human melanomas in semi-solid cultures: quantitative analysis

A systematic study was undertaken to compare the growth characteristics of human melanomas in liquid monolayer cultures at ambient oxygen tension, and in semi-solid cultures at ambient or reduced oxygen tension. Physically dispersed single cell suspensions from 200 freshly-excised melanomas (66 primary, 134 secondary) from 169 patients were cultured in monolayers, or plated in semi-solid cultures maintained either in 5% CO2 in room air (20% O2) or in 5% CO2, 5% O2 and 90% N2, to assay tumour colony-forming units (T-CFU). Aliquots were taken at each passage of the monolayer cultures for T-CFU assay in semi-solid culture at ambient and reduced O2 concentrations. Of 200 melanomas tested, 153 (77%) grew in monolayer culture, 94 (47%) in semi-solid culture at 5% O2, and only 48 (24%) in semi-solid culture at 20% O2. The mean number (+/-s.e.) of colonies in the 94 tumours which grew in semi-solid culture at 5% O2 (29 +/- 4 per 5 x 10(5) cells plated) was significantly greater than the mean in the same tumours in semi-solid culture at 20% O2 (11 +/- 2 per 5 x 10(5) cells). Furthermore, hypoxic colonies showed a morphologically different growth pattern. There was a significant correlation (r = 0.749, P less than 0.001) between the number of colonies growing at 5% O2 and the number at 20% O2; hypoxia appeared to act both by recruiting additional T-CFU and by increasing the proliferative activity of those already present. Short-term monolayer cultured cell lines showing evidence of persistent tumour cell characteristics were successfully established from 74 tumours, and the proportions of T-CFU assayed at each passage. In 63% of cultures the proportion of T-CFU increased initially and then declined, while in the remainder it declined progressively throughout. Although monolayer cultures were successfully maintained for up to 15 passages, T-CFU became undetectable by the eighth passage and remained so thereafter.

Cellular radiosensitivity of a rat brain tumor

The response of a model brain tumor system in the rat to single doses of x irradiation has been studied. Solid tumors were exposed in situ, removed, dissociated into single cells, and grown in tissue culture. The fraciton of surviving clonogenic cells wasdetermined as a function of x-ray dose level for tumors irradiated in anesthetized, air-breathing rats, and for tumors irradiated in rats killed 5 minutes prior to irradiation bynitrogen gas asphyxiation. The parameters of the survival curve for tumors irradiatedin air-breathing rats were: DO= 295 rads; DQ= 350 rads; and n= 3.0. Similarly, thesurvival parameters for tumors irradiated in nitrogen gas asphyxiated rats were: DO=625 rads; DQ=675 rads; and n= 3.0. The survival curve of these tumor cells when assayed in in vitro tissue culture only has parameters of: DO= 225 rads; DQ= 350 rads; andn= 4.7. Data form the in vivo to in vivo tumor cell survival curves suggest that there is no significant hypoxic fraction in this brain tumor.