Dynamics of tumor oxygenation and red blood cell flux in response to inspiratory hyperoxia combined with different levels of inspiratory hypercapnia

Hypoxia signaling in cancer: Implications for therapeutic interventions

Hypoxia is a persistent physiological feature of many different solid tumors and a key driver of malignancy, and in recent years, it has been recognized as an important target for cancer therapy. Hypoxia occurs in the majority of solid tumors due to a poor vascular oxygen supply that is not sufficient to meet the needs of rapidly proliferating cancer cells. A hypoxic tumor microenvironment (TME) can reduce the effectiveness of other tumor therapies, such as radiotherapy, chemotherapy, and immunotherapy. In this review, we discuss the critical role of hypoxia in tumor development, including tumor metabolism, tumor immunity, and tumor angiogenesis. The treatment methods for hypoxic TME are summarized, including hypoxia-targeted therapy and improving oxygenation by alleviating tumor hypoxia itself. Hyperoxia therapy can be used to improve tissue oxygen partial pressure and relieve tumor hypoxia. We focus on the underlying mechanisms of hyperoxia and their impact on current cancer therapies and discuss the prospects of hyperoxia therapy in cancer treatment.

Temporal variation in the response of tumors to hyperoxia with breathing carbogen and oxygen

The effect of hyperoxygenation with carbogen (95% O₂ + 5% CO₂) and 100% oxygen inhalation on partial pressure of oxygen (pO₂) of radiation-induced fibrosarcoma (RIF-1) tumor was investigated. RIF-1 tumors were innoculated in C3H mice, and aggregates of oximetry probe, lithium phthalocyanine (LiPc), was implanted in each tumor. A baseline tumor pO₂ was measured by electron paramagnetic resonance (EPR) oximetry for 20 minutes in anesthetized mice breathing 30% O₂ and then the gas was switched to carbogen or 100 % oxygen for 60 minutes. These experiments were repeated for 10 days. RIF-1 tumors were hypoxic with a baseline tissue pO₂ of 6.2–8.3 mmHg in mice breathing 30% O₂. Carbogen and 100% oxygen significantly increased tumor pO₂ on days 1 to 5, with a maximal increase at approximately 32–45 minutes on each day. However, the extent of increase in pO₂ from the baseline declined significantly on day 5 and day 10. The results provide quantitative information on the effect of hyperoxic gas inhalation on tumor pO₂ over the course of 10 days. EPR oximetry can be effectively used to repeatedly monitor tumor pO₂ and test hyperoxic methods for potential clinical applications.

Temporal changes in tumor oxygenation and perfusion upon normo- and hyperbaric inspiratory hyperoxia

BACKGROUND

Inspiratory hyperoxia under hyperbaric conditions has been shown to effectively reduce tumor hypoxia and to improve radiosensitivity. However, applying irradiation (RT) under hyperbaric conditions is technically difficult in the clinical setting since RT after decompression may be effective only if tumor pO2 remains elevated for a certain period of time. The aim of the present study was to analyze the time course of tumor oxygenation and perfusion during and after hyperbaric hyperoxia.

MATERIALS AND METHODS

Tumor oxygenation, red blood cell (RBC) flux for perfusion monitoring, and vascular resistance were assessed continuously in experimental rat DS-sarcomas by polarographic catheter electrodes and laser Doppler flowmetry at 1 and 2 atm (bar) of environmental pressure during breathing of pure O2 or carbogen (95 % O2 + 5 % CO2).

RESULTS

During room air breathing, the tumor pO2 followed very rapidly within a few minutes the change of the ambient pressure during compression or decompression. With O2 breathing under hyperbaric conditions, the tumor pO2 increased more than expected based on the rise of the environmental pressure, although the time course was comparably rapid. Breathing carbogen, the tumor pO2 followed with a slight delay of the pressure change, and within 10 min after decompression the baseline values were reached again. RBC flux increased during carbogen breathing but remained almost constant with pure O2, indicating a vasodilation (decrease in vascular resistance) with carbogen but a vasoconstriction (increase in vascular resistance) with O2 during hyperbaric conditions.

CONCLUSION

Since the tumor pO2 directly followed the environmental pressure, teletherapy after hyperbaric conditions does not seem to be promising as the pO2 reaches baseline values again within 5-10 min after decompression.

Spatial oxygenation profiles in tumors during normo- and hyperbaric hyperoxia

BACKGROUND

Inspiratory hyperoxia reduces tumor hypoxia, which is responsible for limited radiosensitivity of tumors. However, very little is known about the heterogeneity of intratumoral oxygenation during this supportive treatment. The study analyzes whether local hypoxia is still present during normobaric and hyperbaric inspiratory hyperoxia and whether the addition of CO2 to the inspiratory gas affects the spatial pO2 distribution.

MATERIAL AND METHODS

Tumor oxygenation of experimental DS-sarcomas in rats was assessed by polarographic needle electrodes at 1 and 2 atm (bar) environmental pressure during pure O2 or carbogen (95 % O2 + 5 % CO2) breathing. Up to 320 individual pO2 measurements were performed in a strictly oriented grid resulting in an oxygenation profile in a horizontal tumor layer.

RESULTS

In the experimental tumors used the oxygenation showed pronounced heterogeneities with closely adjacent hypoxic and oxygenated regions. This heterogeneity was still visible under normobaric hyperoxia where large confluent hypoxic regions were detectable. At 1 atm, the addition of CO2 improved tumor oxygenation significantly (at least in large tumors). At 2 atm, only very small local regions of hypoxia were detected. However, under this condition hypercapnia had no impact on tumor oxygenation.

CONCLUSIONS

The data show that even under hyperbaric hyperoxia, hypoxic regions are detectable despite the average pO2 increased by a factor of 100. The results also clearly indicate that the oxygenation pattern improves disproportionally with increasing environmental pressure.

Systematic protocol for assessment of the validity of BOLD MRI in a rabbit model of inflammatory arthritis at 1.5 tesla

BACKGROUND

Blood-oxygen-level-dependent (BOLD) MRI has the potential to identify regions of early hypoxic and vascular joint changes in inflammatory arthritis. There is no standard protocol for analysis of BOLD MRI measurements in musculoskeletal disorders.

OBJECTIVE

To optimize the following BOLD MRI reading parameters: (1) statistical threshold values (low, r > 0.01 versus high, r > 0.2); (2) summary measures of BOLD contrast (percentage of activated voxels [PT%] versus percentage signal difference between on-and-off signal intensities [diff_on_off]); and (3) direction of BOLD response (positive, negative and positive + negative).

MATERIALS AND METHODS

Using BOLD MRI protocols at 1.5 T, arthritic (n = 21) and contralateral (n = 21) knees of 21 juvenile rabbits were imaged at baseline and on days 1, 14 and 28 after a unilateral intra-articular injection of carrageenan. Nine non-injected rabbits served as external control knees (n = 18). By comparing arthritic to contralateral knees, receiver operating characteristic curves were used to determine diagnostic accuracy.

RESULTS

Using diff_on_off and positive + negative responses, a threshold of r > 0.01 was more accurate than r > 0.2 (P = 0.03 at day 28). Comparison of summary measures yielded no statistically significant difference (P > 0.05). Although positive + negative (AUC = 0.86 at day 28) and negative responses (AUC = 0.90 at day 28) for PT% were the most diagnostically accurate, positive + negative responses for diff_on_off (AUC = 0.78 at day 28) also had acceptable accuracy.

CONCLUSIONS

The most clinically relevant reading parameters included a lower threshold of r > 0.01 and a positive + negative BOLD response. We propose that diff_on_off is a more clinically relevant summary measure of BOLD MRI, while PT% can be used as an ancillary measure.

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.

Vascular space occupancy MRI during breathholding at 3 Tesla

PURPOSE

To evaluate the vasodilatory response of normal human brain and meningiomas under repeated breathholding challenges using vascular space occupancy (VASO) MRI at 3 Tesla (T).

MATERIALS AND METHODS

Five normal volunteers and five patients with meningiomas were recruited for this study. For the normal group, VASO MRI during repeated breathholds of different duration (5 to 30 s) was acquired. Patients performed a 15-s breathhold paradigm for VASO MRI. The maximum signal change and full-width at half-maximum (FWHM) were determined by curve fitting.

RESULTS

Significant VASO signal decreases in the gray matter could be detected for a breathhold period as short as 5 s. The fractional activation volume vs. breathhold duration reached a plateau around 34.21 ± 3.39% at 15 s. In the patient group, there were significant VASO signal decreases in normal gray matters and also in small areas of three large-sized meningiomas.

CONCLUSION

The 3T VASO MRI detected significant signal decreases in the gray matter, but not in the white matter, during short periods of breathholding. The fractional activation volume reached the plateau at 15-s breathhold, which is recommended for clinical application.

Effect of hyperoxia and hypercapnia on tissue oxygen and perfusion response in the normal liver and kidney

OBJECTIVE

Inhalation of air with altered levels of oxygen and carbon dioxide to manipulate tissue oxygenation and perfusion has both therapeutic and diagnostic value. These physiological responses can be measured non-invasively with magnetic resonance (MR) relaxation times. However, interpreting MR measurements is not straight-forward in extra-cranial organs where gas challenge studies have only begun to emerge. Inconsistent results have been reported on MR, likely because different organs respond differently. The objective of this study was to elucidate organ-specific physiological responses to gas challenge underlying MR measurements by investigating oxygenation and perfusion changes in the normal liver and kidney cortex.

MATERIALS AND METHODS

Gas challenges (100% O(2), 10% CO(2), and carbogen [90% O(2)+10% CO(2)]) interleaved with room air was delivered to rabbits to investigate their effect on tissue oxygenation and perfusion. Real-time fiber-optic measurements of absolute oxygen and relative blood flow were made in the liver and kidney cortex.

RESULTS

Only the liver demonstrated a vasodilatory response to CO(2). Perfusion changes to other gases were minimal in both organs. Tissue oxygenation measurements showed the liver responding only when CO(2) was present and the kidney only when O(2) was present.

CONCLUSION

This study reveals distinct physiological response mechanisms to gas challenge in the liver and kidney. The detailed characterization of organ-specific responses is critical to improving our understanding and interpretation of MR measurements in various body organs, and will help broaden the application of MR for non-invasive studies of gas challenges.

Normal tissue quantitative T1 and T2* MRI relaxation time responses to hypercapnic and hyperoxic gases

RATIONALE AND OBJECTIVES

Longitudinal (T(1)) and effective transverse (T(2)*) magnetic resonance (MR) relaxation times provide noninvasive measures of tissue oxygenation. The objective for this study was to quantify independent effects of inhaled O(2) and CO(2) on normal tissue T(1) and T(2)* in rabbit liver, kidney, and paraspinal muscle.

MATERIALS AND METHODS

Three gas challenges (100% O(2), 10% CO(2) [balance air], and carbogen [90% O(2) + 10% CO(2)]) were delivered to the rabbits in random order to isolate the effects of inspired O(2) and CO(2). During each challenge, quantitative T(1) and T(2)* maps were collected on a 1.5 Tesla MR imaging. Mean changes in T(1) (ΔT(1)) and T(2)* (ΔT(2)*) were calculated from regions of interest in each organ.

RESULTS

Greatest ΔT(1) and ΔT(2)* changes were observed in liver for 10% CO(2) and in kidney for 100% O(2). ΔT(1) and ΔT(2)* generally followed predicted patterns when transitioning from air breathing: lower T(1)/higher T(2)* with inspired O(2), higher T(1)/lower T(2)* with inspired CO(2), and variable T(1)/T(2)* changes in the presence of both (ie, carbogen). New observations also emerged: 1) between-gas-challenge transitions revealed the greatest significance in ΔT(2)* for the liver and kidney resulting from the isolation of independent O(2) and CO(2) effects; 2) ΔT(2)* provided the best sensitivity and detected both tissue oxygenation and blood volume modulation; and 3) ΔT(1) sensitivity was restricted mainly to tissue oxygenation in the absence of counteracting vasodilatation.

CONCLUSION

Robust use of MR relaxation times as noninvasive biomarkers requires an understanding of their relative sensitivity to organ-specific physiological responses.

Synergistic combination of hyperoxygenation and radiotherapy by repeated assessments of tumor pO2 with EPR oximetry

The effect of hyperoxygenation with carbogen (95% O(2) + 5% CO(2)) inhalation on RIF-1 tumor pO(2 )and its consequence on growth inhibition with fractionated radiotherapy is reported. The temporal changes in the tumor pO(2) were assessed by in vivo Electron Paramagnetic Resonance (EPR) oximetry in mice breathing 30% O(2) or carbogen and the tumors were irradiated with 4 Gy/day for 5 consecutive days; a protocol that emulates the clinical application of carbogen. The RIF-1 tumors were hypoxic with a tissue pO(2) of 5-9 mmHg. Carbogen (CB) breathing significantly increased tumor pO(2), with a maximum increase at 22.9-31.2 min on days 1-5, however, the magnitude of increase in pO(2) declined on day 5. Radiotherapy during carbogen inhalation (CB/RT) resulted in a significant tumor growth inhibition from day 3 to day 6 as compared to 30%O(2)/RT and carbogen (CB/Sham RT) groups. The results provide unambiguous quantitative information on the effect of carbogen inhalation on tumor pO(2) over the course of 5 days. Tumor growth inhibition in the CB/RT group confirms that the tumor oxygenation with carbogen was radiobiologically significant. Repeated tumor pO(2) measurements by EPR oximetry can provide temporal information that could be used to improve therapeutic outcomes by scheduling doses at times of improved tumor oxygenation.

Effect of hyperoxygenation on tissue pO2 and its effect on radiotherapeutic efficacy of orthotopic F98 gliomas

PURPOSE

Lack of methods for repeated assessment of tumor pO(2) limits the ability to test and optimize hypoxia-modifying procedures being developed for clinical applications. We report repeated measurements of orthotopic F98 tumor pO(2) and relate this to the effect of carbogen inhalation on tumor growth when combined with hypofractionated radiotherapy.

METHODS AND MATERIALS

Electron paramagnetic resonance (EPR) oximetry was used for repeated measurements of tumor and contralateral brain pO(2) in rats during 30% O(2) and carbogen inhalation for 5 consecutive days. The T(1)-enhanced volumes and diffusion coefficients of the tumors were assessed by magnetic resonance imaging (MRI). The tumors were irradiated with 9.3 Gy x 4 fractions in rats breathing 30% O(2) or carbogen to determine the effect on tumor growth.

RESULTS

The pretreatment F98 tumor pO(2) varied between 8 and 16 mmHg, while the contralateral brain had 41 to 45 mmHg pO(2) during repeated measurements. Carbogen breathing led to a significant increase in tumor and contralateral brain pO(2); however, this effect declined over days. Irradiation of the tumors in rats breathing carbogen resulted in a significant decrease in tumor growth and an increase in the diffusion coefficient measured by MRI.

CONCLUSIONS

The results provide quantitative measurements of the effect of carbogen inhalation on intracerebral tumor pO(2) and its effect on therapeutic outcome. Such direct repeated pO(2) measurements by EPR oximetry can provide temporal information that could be used to improve therapeutic outcome by scheduling doses at times of improved tumor oxygenation. EPR oximetry is currently being tested for clinical applications.

Monitoring of hemodynamic changes induced in the healthy breast through inspired gas stimuli with MR-guided diffuse optical imaging

PURPOSE

The modulation of tissue hemodynamics has important clinical value in medicine for both tumor diagnosis and therapy. As an oncological tool, increasing tissue oxygenation via modulation of inspired gas has been proposed as a method to improve cancer therapy and determine radiation sensitivity. As a radiological tool, inducing changes in tissue total hemoglobin may provide a means to detect and characterize malignant tumors by providing information about tissue vascular function. The ability to change and measure tissue hemoglobin and oxygenation concentrations in the healthy breast during administration of three different types of modulated gas stimuli (oxygen/ carbogen, air/carbogen, and air/oxygen) was investigated.

METHODS

Subjects breathed combinations of gases which were modulated in time. MR-guided diffuse optical tomography measured total hemoglobin and oxygen saturation in the breast every 30 s during the 16 min breathing stimulus. Metrics of maximum correlation and phase lag were calculated by cross correlating the measured hemodynamics with the stimulus. These results were compared to an air/air control to determine the hemodynamic changes compared to the baseline physiology.

RESULTS

This study demonstrated that a gas stimulus consisting of alternating oxygen/carbogen induced the largest and most robust hemodynamic response in healthy breast parenchyma relative to the changes that occurred during the breathing of room air. This stimulus caused increases in total hemoglobin and oxygen saturation during the carbogen phase of gas inhalation, and decreases during the oxygen phase. These findings are consistent with the theory that oxygen acts as a vasoconstrictor, while carbogen acts as a vasodilator. However, difficulties in inducing a consistent change in tissue hemoglobin and oxygenation were observed because of variability in intersubject physiology, especially during the air/oxygen or air/carbogen modulated breathing protocols.

CONCLUSIONS

MR-guided diffuse optical imaging is a unique tool that can measure tissue hemodynamics in the breast during modulated breathing. This technique may have utility in determining the therapeutic potential of pretreatment tissue oxygenation or in investigating vascular function. Future gas modulation studies in the breast should use a combination of oxygen and carbogen as the functional stimulus. Additionally, control measures of subject physiology during air breathing are critical for robust measurements.

Effect of a topical vasodilator on tumor hypoxia and tumor oxygen guided radiotherapy using EPR oximetry

We sought to reduce tumor hypoxia by topical application of a vasodilator, benzyl nicotinate (BN), and investigated its effect on the growth of tumors irradiated at times when tumor pO(2) increased. EPR oximetry was used to follow the changes in the tissue pO(2) of subcutaneous radiation-induced fibrosarcoma (RIF-1) tumors during topical applications of 1.25-8% BN formulations for 5 consecutive days. The RIF-1 tumors were hypoxic with a tissue pO(2) of 4.6-7.0 mmHg. A significant increase in tumor pO(2) occurred 10-30 min after BN application. The formulation with the minimal BN concentration that produced a significant increase in tumor pO(2) was used for the radiation study. The tumors were irradiated (4 Gy x 5) at the time of the maximum increase in pO(2) observed with the 2.5% BN formulation. The tumors with an increase in pO(2) of greater than 2 mmHg from the baseline after application of BN on day 1 had a significant growth inhibition compared to the tumors with an increase in pO(2) of less than 2 mmHg. The results indicate that the irradiation of tumors at the time of an increase in pO(2) after the topical application of the 2.5% BN formulation led to a significant growth inhibition. EPR oximetry provided dynamic information on the changes in tumor pO(2), which could be used to identify responders and non-responders and schedule therapy during the experiments.

Enhanced radiation response of a solid tumor with the artificial oxygen carrier 'albumin-heme'

Tumor-cell hypoxia is one of the main factors inducing radioresistance. Enhanced tumor oxygenation has previously been achieved in an animal model using the synthetic heme-based oxygen carrier 'albumin-heme' (recombinant human serum albumin-Fe cyclohexanoil heme; rHSA-FeP). The present study was done to determine whether rHSA-FeP enhances the radiation response in an experimental tumor model. Male Donryu rats and LY80, a variant of the syngenic liver ascites tumor, were used. A total of 1 x 10(6) cells were injected into the subfascial tissue of the right thigh. The rats were divided randomly into five groups: sham (tumor implantation and sham operation); rHSA-FeP; irradiation; rHSA + irradiation; and rHSA-FeP + irradiation. Six days after, under general anesthesia, intra-arterial administration of 10 mL/kg of either 5% rHSA solution or oxygenated rHSA-FeP solution at 2.5 mL/min was done and a dose of 20 Gy was given. There were significant differences in tumor growth between the sham and irradiation groups, and between the sham and rHSA-FeP + irradiation groups. Tumor growth delay was observed and differences were significant between the sham and irradiation groups, and between the irradiation and rHSA-FeP + irradiation groups. In the present study, rHSA-FeP itself had a slight effect on tumor growth without irradiation. Enhancing the effect of rHSA-FeP on the radiation response is responsible in part for the oxygen-carrying property of rHSA-FeP. In conclusion, rHSA-FeP is a candidate radiation-enhancing drug. Arterial infusion of rHSA-FeP may serve as a local oxygenation method that enhances the radiation effect.

Inspiratory contrast for in vivo optical imaging

We demonstrate the use of inspired oxygen and carbon dioxide as a possible route to increase contrast in optical imaging of cancerous tissue. Differential imaging in human xenograft rodent models of cancer exhibits significant variation in signal between normal and cancerous tissue. This differential cancer-specific contrast is stronger and more consistent than the conventional static contrast. This differential technique exploits the response of abnormal tumor vasculature to inhaled gases and could provide a promising alternative to supplement mainstream cancer imaging modalities such as x-rays and MRI.

Carbogen and nicotinamide increase blood flow and 5-fluorouracil delivery but not 5-fluorouracil retention in colorectal cancer metastases in patients

PURPOSE

To examine whether carbogen and nicotinamide increases 5-fluorouracil (5-FU) delivery to colorectal cancer metastases.

EXPERIMENTAL DESIGN

Six patients were scanned using positron emission tomography. Two scans were done to coincide with the start of separate chemotherapy cycles. At the second positron emission tomography session, 60 mg/kg nicotinamide was given orally 2 to 3 hours before 10-minute carbogen inhalation. In the middle of carbogen treatment, [15O]H2O (to measure regional tissue perfusion) and then [18F]5-FU (to measure 5-FU tissue pharmacokinetics) were administered.

RESULTS

Regions of interest were drawn in 12 liver metastases, 6 spleens, 6 livers, and 12 kidneys. Nicotinamide and carbogen administration increased mean blood pO2 from 93 mm Hg (95% confidence interval, 79-198) to 278 mm Hg (95% confidence interval, 241-316; P = 0.031). Regional perfusion (mL(blood)/min/mL(tissue)) increased in metastases (mean change = 52%, range -32% to +261%, P = 0.024), but decreased in kidney (mean change = -42%, range -82% to -11%, P = 0.0005) and liver (mean change = -34%, range -43% to -26%, P = 0.031). 5-FU uptake at 3.75 minutes (m(2)/mL) increased in tumor (mean change = 40%, range -39% to +196%, P = 0.06) and decreased in kidney (mean change = -25%, range -71% to 12%, P = 0.043). 5-FU delivery measured as K1 increased in tumor (mean change = 74%, range -23% to +293%, P = 0.0039). No differences were seen in [18F]5-FU tumor exposure (net area under curve) and retention.

CONCLUSION

Nicotinamide and carbogen administration can increase 5-FU delivery to colorectal cancer liver metastases. Despite an increase in perfusion and 5-FU delivery, the effects were not directly related and did not increase 5-FU retention or tissue exposure.

BOLD signal and vessel dynamics: a hierarchical cluster analysis

The aim of the present study was to analyze blood oxygenation level-dependent (BOLD) signal variation during an apnea-based task in order to assess the capability of a functional magnetic resonance imaging (fMRI) procedure to estimate cerebral vascular dynamic effects. We measured BOLD contrast by hierarchical cluster analysis in healthy subjects undergoing an fMRI experiment, in which the task paradigm was one phase of inspirational apnea (IA). By processing the time courses of the fMRI experiment, analysis was performed only on a subclass of all the possible signal patterns; basically, root mean square and absolute variation differences have been calculated. Considering the baseline value obtained by computing the mean value of the initial rest period as reference, particular voxels showed relative important variations during the IA task and during the recovery phase following the IA. We focused our interest on the signal response of voxels that would correspond mainly to white and gray matter regions and that also may be affected by the proximity of large venous vessels. The results are presented as maps of space-temporal distribution of time series variations with two levels of hierarchical clustering among voxels with low to high initial response. Furthermore, we have presented a clustering of the signal response delay, conducting to a partition and identification of specified brain sites.

Preclinical studies on how to deal with patient intolerance to nicotinamide and carbogen

BACKGROUND AND PURPOSE

Accelerated radiation carbogen nicotinamide (ARCON) therapy is generally well tolerated, but some patients experience intolerance to nicotinamide and carbogen (95% O(2)+5% CO(2)). This study investigated the effect of reducing both the nicotinamide dose and carbogen CO(2) content on radiation response.

MATERIALS AND METHODS

A C3H mouse mammary carcinoma grown in the right rear foot of female CDF1 was used and treated when at 200 mm(3). Nicotinamide was intraperitoneally injected 20 min prior to irradiation. Carbogen (CO(2) content of either 2 or 5%, balance O(2)) breathing was started 5 min before, and continued during, additional treatments. Radiation was given locally to tissues of restrained non-anaesthetised mice either as a single or fractionated (10 fractions in 12 days) schedule. The endpoints were local tumour control at 90 days, development of moist desquamation 11-23 days after treatment of normal foot skin, or tumour oxygenation measured with the Eppendorf electrode.

RESULTS

The TCD50 values in this tumour following single or fractionated radiation treatment were 52 and 71Gy, respectively. Carbogen (5% CO(2) content) breathing with every radiation treatment in the fractionated schedule significantly (Chi-squared test; P<0.05) enhanced radiation response (ER 1.25). Significant enhancements were also seen with nicotinamide given either as 10x120 mg/kg (ER 1.25), 6x120 mg/kg (ER 1.11) or 10x90 mg/kg (ER 1.18), although the 6x120 schedule was significantly less effective than 10x120. Combining nicotinamide with carbogen resulted in ERs of 1.39-1.44, and these were independent of the nicotinamide treatments. There was also no significant difference in the enhancement of tumour radiation response or improved tumour oxygenation status if the CO(2) content of the gas breathing was varied from 0% (i.e. 100% O(2)) to 2 or 5% (balance O(2)), although a CO(2) content of 2% did give a smaller enhancement of radiation-induced normal skin damage than 5%.

CONCLUSIONS

Both the nicotinamide dose, but not the frequency, and carbogen CO(2) content may be reduced in patients experience intolerance without any significant loss of sensitisation.

BOLD MRI response to hypercapnic hyperoxia in patients with meningiomas: correlation with Gadolinium-DTPA uptake rate

Because meningiomas tend to recur after (partial) surgical resection, radiotherapy is increasingly being applied for the treatment of these tumors. Radiation dose levels are limited, however, to avoid radiation damage to the surrounding normal tissue. The radiosensitivity of tumors can be improved by increasing tumor oxygen levels. The aim of this study was to investigate if breathing a hyperoxic hypercapnic gas mixture could improve the oxygenation of meningiomas. Blood oxygen level-dependent magnetic resonance imaging and dynamic Gadolinium (Gd)-DTPA contrast-enhanced MRI were used to assess changes in tumor blood oxygenation and vascularity, respectively. Ten meningioma patients were each studied twice; without and with breathing a gas mixture consisting of 2% CO(2) and 98% O(2). Values of T(2)* and the Gd-DTPA uptake rate k(ep) were calculated under both conditions. In six tumors a significant increase in the value of T(2)* in the tumor was found, suggesting an improved tumor blood oxygenation, which exceeded the effect in normal brain tissue. Contrarily, two tumors showed a significant T(2)* decrease. The change in T(2)* was found to correlate with both k(ep) and with the change in k(ep). The presence of both vascular effects and oxygenation effects and the heterogeneous response to hypercapnic hyperoxia necessitates individual assessment of the effects of breathing a hyperoxic hypercapnic gas mixture on meningiomas. Thus, the current MRI protocol may assist in radiation treatment selection for patients with meningiomas.

Blood oxygenation level-dependent MRI of cerebral gliomas during breath holding

PURPOSE

To assess the cerebrovascular responses to short breath holding of cerebral gliomas using blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI).

MATERIALS AND METHODS

Six patients with a low-grade glioma and one patient with a high-grade glioma were studied using T2*-weighted echo planar imaging (EPI) during repeated periods of 15-second or 20-second breath-holding. Tumor vascularity was evaluated using dynamic susceptibility contrast perfusion MRI.

RESULTS

Increases in BOLD signal intensity during repeated breath-holding were consistently identified in patients' normal appearing gray matter, comparable with those in healthy adults. Absence of significant BOLD signal enhancement was noted both in low-grade and high-grade gliomas, which is either due to overwhelming hypoxia in a tumor, inadequacy or absence of hypercapnia-induced vasodilatation of tumor vessels, or both. Breath-hold regulated decreases in BOLD signals occurred only in the high-grade glioma, which is most likely due to the hypercapnia-induced steal effect that redistributes blood flow from tumor regions with unresponsive neovasculature to surrounding normal tissue.

CONCLUSION

BOLD MRI during short breath holding can disclose differential cerebrovascular response between normal tissue and cerebral glioma.

Correlation of tumor oxygen dynamics with radiation response of the dunning prostate R3327-HI tumor

Our previous studies have shown that oxygen inhalation significantly reduces tumor hypoxia in the moderately well-differentiated HI subline of the Dunning prostate R3327 rat carcinoma. To test our hypothesis that modifying hypoxia could improve the radiosensitivity of these tumors, we performed experimental radiotherapy to compare the tumor response to ionizing radiation alone or in combination with oxygen inhalation. Tumor pO(2) measurements were performed on size-selected tumors several hours before radiotherapy using (19)F nuclear magnetic resonance echo planar imaging relaxometry (FREDOM) of the reporter molecule hexafluorobenzene. In common with our previous findings, the larger tumors (>3.5 cm(3)) exhibited greater hypoxia than the smaller tumors (<2 cm(3); P < 0.001), and oxygen inhalation reduced the hypoxic fraction (<10 Torr): In the larger tumors, hypoxic fraction dropped significantly from a mean baseline value of 80% to 17% (P < 0.001). The effect of oxygen administered 30 min before and during irradiation on tumor response to a single 30-Gy dose of photons was evaluated by growth delay. For the smaller tumors, no difference in growth delay was found when treatment was given with or without oxygen breathing. By contrast, breathing oxygen before and during irradiation significantly enhanced the growth delay in the larger tumors (additional 51 days). The differential behavior may be attributed to the low baseline hypoxic fraction (<10 Torr) in small tumors (20%) as a target for oxygen inhalation. There was a strong correlation between the estimated initial pO(2) value and the radiation-induced tumor growth delay (R > 0.8). Our histological studies showed a good match between the perfused vessels marked by Hoechst 33342 dye and the total vessels immunostained by anti-CD31 and indicated extensive perfusion in this tumor line. In summary, the present results suggest that the ability to detect modulation of tumor pO(2), in particular, the residual hypoxic fraction, with respect to an intervention, could have prognostic value for predicting the efficacy of radiotherapy.

ARCON: a novel biology-based approach in radiotherapy

Two mechanisms of radiotherapy resistance which are of major importance in various tumour types are tumour-cell repopulation and hypoxia. ARCON (accelerated radiotherapy with carbogen and nicotinamide) is a new therapeutic strategy that combines radiation treatment modifications, with the aim of counteracting these resistance mechanisms. To limit clonogenic repopulation during therapy, the overall duration of the radiotherapy is reduced, generally by delivering several fractions per day. This accelerated radiotherapy is combined with inhalation of hyperoxic gas to decrease diffusion-limited hypoxia, and nicotinamide, a vasoactive agent, to decrease perfusion-limited hypoxia. Preclinical studies have been done to test the enhancing effects of these three components of ARCON, individually and in combination, in several experimentally induced tumours and normal tissues. In a mouse mammary carcinoma, the tumour-control rate obtained with ARCON was the same as that with conventional treatment, but with a radiation dose almost 50% lower. Phase 1 and 2 clinical trials have shown the feasibility and tolerability of ARCON, and have produced promising results in terms of tumour control. In particular in cancers of the head and neck and bladder, the local tumour-control rates are higher than in other studies, and phase 3 trials for these tumour types are underway. In conjunction with these trials, hypoxia markers detectable by immunohistochemistry are being tested for their potential use in predictive assays to select patients for ARCON and other hypoxia-modifying therapies.

Enhanced uptake of ifosfamide into GH3 prolactinomas with hypercapnic hyperoxic gases monitored in vivo by (31)P MRS

Previously, (31)P magnetic resonance spectroscopy (MRS) has been used to detect ifosfamide (IF) in vivo and to show that breathing carbogen (5% CO(2)/95% O(2)) enhances the uptake and increases the efficacy of IF in rat GH3 prolactinomas [Rodrigues LM, Maxwell RJ, McSheehy PMJ, Pinkerton CR, Robinson SP, Stubbs M, and Griffiths JR (1997). In vivo detection of ifosfamide by (31)P MRS in rat tumours; increased uptake and cytotoxicity induced by carbogen breathing in GH3 prolactinomas. Br J Cancer 75, 62-68]. We now show that other hypercapnic and/or hyperoxic (5% CO(2) in air, 2.5% CO(2) in O(2)) gas mixtures also increase the uptake of IF into tumors, measured by (31)P MRS. All gases caused an increased uptake (C(max)) of IF compared to air breathing, with carbogen inducing the largest increase (85% (P<.02) compared to 46% with 2.5% CO(2) in O(2) (P<.004) and 48% with 5% CO(2) in air (P<.004)). The T(max) (time of maximum concentration in tumor posintravenous injection of IF) was significantly (P<.04) later in the cohort that breathed 5% CO(2) in air. The increased uptake of IF with carbogen breathing was selective to tumor tissue and there were no significant increases in any of the normal tissues studied, suggesting that any host tissue toxicity would be minimal. Carbogen breathing by patients causes breathlessness. There was no significant difference in IF uptake between breathing carbogen and 2.5% CO(2) in O(2) and, therefore, the ability of 2.5% CO(2) in O(2) to also increase IF uptake may be clinically useful as it causes less patient discomfort.

Treatment resistance of solid tumors: role of hypoxia and anemia

Hypoxia is a characteristic property of locally advanced solid tumors, resulting from an imbalance between the supply and consumption of oxygen. Major pathogenetic mechanisms for the development of hypoxia are (1) structural and functional abnormalities of the tumor microvasculature, (2) increased diffusion distances, and (3) tumor-associated and therapy-induced anemia. The oxygenation status is independent of clinical tumor size, stage, grade, and histopathological type, but is affected by the hemoglobin level. Hypoxia is intensified in anemic patients, especially in tumors with low perfusion rates. Hypoxia and anemia (most probably via worsening of tumor hypoxia) can lead to therapeutic problems, as they make solid tumors resistant to sparsely ionizing radiation and some forms of chemotherapy. In addition to more direct mechanisms involved in the development of therapeutic resistance, there are also indirect machineries that can cause barriers to therapies. These include hypoxia-driven proteome and genome changes and clonal selection. These, in turn, can drive subsequent events that are known to further increase resistance to therapy (in addition to critically affecting long-term prognosis). Treatment resistance in anemic patients can be, at least partially, prevented or overcome by anemia correction, resulting in better locoregional tumor control and overall survival of patients.