The optimal combination of hyperthermia and carbogen breathing to increase tumor oxygenation and radiation response

Mild temperature hyperthermia and radiation therapy: role of tumour vascular thermotolerance and relevant physiological factors

Here we review the significance of changes in vascular thermotolerance on tumour physiology and the effects of multiple clinically relevant mild temperature hyperthermia (MTH) treatments on tumour oxygenation and corresponding radiation response. Thus far vascular thermotolerance referred to the observation of significantly greater blood flow response by the tumour to a second hyperthermia exposure than in response to a single thermal dose, even at temperatures that would normally cause vascular damage. New information suggests that although hyperthermia is a powerful modifier of tumour blood flow and oxygenation, sequencing and frequency are central parameters in the success of MTH enhancement of radiation therapy. We hypothesise that heat treatments every 2 to 3 days combined with traditional or accelerated radiation fractionation may be maximally effective in exploiting the improved perfusion and oxygenation induced by typical thermal doses given in the clinic.

Hyperthermic enhancement of tumor radiosensitization strategies

Local hyperthermia of living tissue can cause significant increases in blood flow and oxygenation depending on time-temperature history. Increases in perfusion of the abnormal and insufficient vasculature found in solid tumors may increase tumor oxygenation, thereby increasing the radiation sensitivity of the tumor. We hypothesized that local heating of tumor would increase the oxygenation of the tumor tissue and allow other oxygenating agents to further modify tumor oxygenation and radiation response. In the present study the effect of moderate temperature hyperthermia (MTH) at 41.5-42.5 degrees C for 30-60 min, 250 mg/kg nicotinamide, or carbogen breathing (95% O2/5% CO2) on the radiation sensitivity of FSaII murine fibrosarcomas or R3230 AC rat adenocarcinomas was studied. Individually, these treatments increased the tumor cell sensitivity to single dose 10-15 Gy X-irradiation by 1-5 fold on average, as measured by the in vivo/in vitro tumor excision assay. The combination of tumor MTH with nicotinamide or carbogen breathing increased the radiation sensitivity by 3-5 fold in FSaII tumors and 10-30 fold in R3230 tumors with varying levels of statistical significance. Finally, the triple combination of adjuvant MTH, nicotinamide and carbogen breathing increased the radiation-induced cell death in FSaII tumors to a similar extent as the dual combinations of MTH, nicotinamide or heat, carbogen breathing. However, in R3230 AC tumors the triple adjuvant combination significantly increased radiation-induced cell killing compared to all other dual adjuvant treatments (p < 0.04). To interrogate the mechanism by which heating alters tumor physiology, nitric oxide production in tumor and endothelial cells in culture and tumor tissue after heating was studied. Heating caused an increase in nitric oxide production over a 24 h period after treatment. Subsequently, inhibiting the enzymatic production of NO with L-NAME was found to increase heat-induced growth delay of FSaII tumors. The cause and effect of increased nitric oxide production and the response of the tumor vasculature to heat are discussed in the context of the tumor radiosensitization achieved by heating, carbogen breathing and nicotinamide.

Monitoring the effect of mild hyperthermia on tumour hypoxia by Cu-ATSM PET scanning

PURPOSE

Mild hyperthermia can improve tumour oxygenation and enhance radiosensitivity. Imaging the hypoxic fraction of a tumour can guide hyperthermia treatment planning and facilitate treatment optimization. 64Cu-ATSM (Copper-diacetyl-bis(N4-methylthiosemicarbazone)) is a positron emitting compound that has been demonstrated to have rapid uptake and selective retention in hypoxic cells and has been used for imaging human and animal tumours. The purpose of the present report is to establish methodology that will allow one to use Cu-ATSM PET scanning to detect the impact of hyperthermia on tumour physiology in as little time as possible.

MATERIALS AND METHODS

EMT6 tumours (mouse mammary carcinoma) were implanted into the subcutaneous tissue of both thighs of 10 BALB/c mice (one heated, one control tumour per animal). The target thermal dose was 41.5 degrees C x 45 min. Without interrupting heating, 64Cu-ATSM (mean activity 1.8 mCi) was then injected and serial PET scans were obtained. In a sub-group of four animals, a low administered activity (approximately 0.3 mCi) 64Cu-ATSM scan was also conducted before heating to permit a direct comparison of the effects of hyperthermia on the same tumours. In another sub-group of five animals, a low activity (approximately 0.3 mCi) 64Cu-PTSM (pyruvaldehyde-bis(N*-methylthiosemicarbazone)) scan was conducted before heating, to confirm a posited correlation between perfusion and early 64Cu-ATSM uptake.

RESULTS

This study corrected for perfusion differences by dividing tumour uptake by the average early (first minute) uptake ('self-normalized uptake'). The 10 heated tumours showed a significantly (p = 0.007) lower self-normalized uptake than control tumours by 2 min. For the four mice with low activity Cu-ATSM scans performed before hyperthermia, the tumours to be heated demonstrated self-normalized uptake consistent with the unheated control tumours and which departed significantly (p < or = 0.02) from their post-hyperthermia scans by 5 min. Comparisons between scans and needle electrode surveys were performed in an additional four animals with eight tumours. For technical reasons electrode surveys were done after the end of hyperthermia-and, therefore, these animals also had comparison scans taken after hyperthermia. Reduced self-normalized uptake on scans was associated with increased pO2 on electrode surveys. These data also suggested a substantial degradation of the effect on tumour hypoxia by approximately 15-45 min after the end of mild hyperthermia.

CONCLUSION

Short imaging times of approximately 5 min with modest (approximately 4-10) numbers of mice can discriminate the effects of mild hyperthermia on tumour physiology. The long-term objective is to use this tool to identify as short and mild a hyperthermia session as possible.

Nitric oxide-mediated increase in tumor blood flow and oxygenation of tumors implanted in muscles stimulated by electric pulses

PURPOSE

Oxygen deficiency in tumors reduces the efficacy of nonsurgical treatment modalities. We tested the hypothesis that electrical stimulation of the sciatic nerve could modify the oxygenation status and the blood flow of tumors implanted in the thigh of mice.

MATERIALS AND METHODS

The sciatic nerve was electrically stimulated at 5 Hz. Local transplantable liver tumor (TLT) and fibrosarcoma (FSaII) tumor oxygen pressure (pO(2)) and perfusion measurements were carried out using electron paramagnetic resonance (EPR) oximetry and the OxyLite/OxyFlo technique. The radiosensitizing effect of the protocol was assessed by irradiating FSaII tumors with X-rays.

RESULTS

Tumor pO(2) increased from approximately 3 mm Hg to approximately 8 mm Hg, and relative tumor blood flow was increased by 241% and 162% for TLT and FSaII tumor models, respectively. The effect on the tumor oxygenation was inhibited by a nitric oxide synthase (NOS) inhibitor, and an increase in the tumor nitric oxide (NO) content was observed using EPR spin-trapping. The tumor oxygen consumption rate was decreased after the stimulation protocol. In addition, the electrical stimulation of the host tissue increased regrowth delays by a factor of 1.65.

CONCLUSIONS

This increase in tumor oxygenation is due to the temporary increase in tumor blood flow, but particularly to a decrease in the tumor oxygen consumption rate (inhibition of respiration) that is mediated by a local production of NO during the protocol. Those tumor hemodynamic changes resulted in a radiosensitizing effect.

Potentiation of radiation-induced regrowth delay by isosorbide dinitrate in FSaII murine tumors

Oxygen deficiency in tumors reduces the efficacy of nonsurgical treatment modalities such as conventional radiotherapy and chemotherapy. Since tumor perfusion is directly affected by the vascular resistance to flow of vessels feeding the tumor, vasodilator drugs might be a way to increase tumor blood flow and oxygenation. The effects of nitric oxide (NO) donor administration on tumor oxygenation, perfusion and radiation sensitivity were studied in the FSaII tumor model. Local tumor oxygenation was measured using electron paramagnetic resonance oximetry and a fiberoptic probe, OxyLite. We concomitantly measured the modulation of tumor blood flow by laser Doppler flowmetry. We determined FSaII tumor regrowth delay after isosorbide dinitrate administration and irradiation compared to carbogen breathing before irradiation and with X-rays alone. Administration of the NO donor improved the FSaII tumor pO(2) concomitant with an increase in tumor blood flow. We also demonstrated an increase in FSaII tumor radiation sensitivity after isosorbide dinitrate administration, which was similar to the effect of carbogen breathing in the same tumor model. Administration of isosorbide dinitrate could be considered in terms of improvement in tumor blood flow and a possible concomitant increase in accessibility of chemosensitizing agents to the tumor, particularly in terms of modification of the tumor response to irradiation.

Improvement of tumor oxygenation by mild hyperthermia

There is now abundant evidence that oxygenation in rodent, canine and human tumors is improved during and for up to 1-2 days after heating at mild temperatures. An increase in tumor blood perfusion along with a decline in the oxygen consumption rate appears to account for the improvement of tumor oxygenation by mild hyperthermia. The magnitude of the increase in tumor pO(2), determined with oxygen-sensitive microelectrodes, caused by mild hyperthermia is less than that caused by carbogen breathing. However, mild hyperthermia is far more effective than carbogen breathing in increasing the radiation response of experimental tumors, probably because mild hyperthermia oxygenates both (diffusion-limited) chronically hypoxic and (perfusion-limited) acutely hypoxic cells, whereas carbogen breathing oxygenates only the chronically hypoxic cells. Mild hyperthermia is also more effective than nicotinamide, which is known to oxygenate acutely hypoxic cells, in enhancing the radiation response of experimental tumors. The combination of mild hyperthermia with carbogen or nicotinamide is highly effective in reducing the hypoxic cell fraction in tumors and increasing the radiation response of experimental tumors. A primary rationale for the use of hyperthermia in combination with radiotherapy has been that hyperthermia is equally cytotoxic toward fully oxygenated and hypoxic cells and that it directly sensitizes both fully oxygenated and hypoxic cells to radiation. Such cytotoxicity and such a radiosensitizing effect may be expected to be significant when the tumor temperature is elevated to at least 42-43 degrees C. Unfortunately, it is often impossible to uniformly raise the temperature of human tumors to this level using the hyperthermia devices currently available. However, it is relatively easy to raise the temperature of human tumors into the range of 39-42 degrees C, which is a temperature that can improve tumor oxygenation for up to 1-2 days. The potential usefulness of mild hyperthermia to enhance the response of human tumors to radiotherapy by improving tumor oxygenation merits continued investigation.