Hypoxia and reoxygenation in human melanoma xenografts

Asc-1 Transporter (SLC7A10): Homology Models And Molecular Dynamics Insights Into The First Steps Of The Transport Mechanism

The alanine-serine-cysteine transporter Asc-1 regulates the synaptic availability of d-serine and glycine (the two co-agonists of the NMDA receptor) and is regarded as an important drug target. To shuttle the substrate from the extracellular space to the cytoplasm, this transporter undergoes multiple distinct conformational states. In this work, homology modeling, substrate docking and molecular dynamics simulations were carried out to learn more about the transition between the “outward-open” and “outward-open occluded” states. We identified a transition state involving the highly-conserved unwound TM6 region in which the Phe243 flips close to the d-serine substrate without major movements of TM6. This feature and those of other key residues are proposed to control the binding site and substrate translocation. Competitive inhibitors ACPP, LuAE00527 and SMLC were docked and their binding modes at the substrate binding site corroborated the key role played by Phe243 of TM6. For ACPP and LuAE00527, strong hydrophobic interactions with this residue hinder its mobility and prevent the uptake and the efflux of substrates. As for SMLC, the weaker interactions maintain the flexibility of Phe243 and the efflux process. Overall, we propose a molecular basis for the inhibition of substrate translocation of the Asc-1 transporter that should be valuable for rational drug design.

Determination of the relative biological effectiveness and oxygen enhancement ratio for micronuclei formation using high-LET radiation in solid tumor cells: An in vitro and in vivo study

We determined the relative biological effectiveness (RBE) and oxygen enhancement ratio (OER) of micronuclei (MN) formation in clamped (hypoxic) and non-clamped (normoxic) solid tumors in mice legs following exposure to X-rays and heavy ions. Single-cell suspensions (aerobic) of non-irradiated tumors were prepared in parallel and used directly to determine the radiation response for aerobic cells. Squamous cell carcinoma (SCCVII) cells were transplanted into the right hind legs of syngeneic C3H/He male mice. Irradiation doses with either X-rays or heavy ions at a dose-averaged LET (linear energy transfer) of 14-192keV/μm were delivered to 5-mm diameter tumors and aerobic single-cells in sample-tubes. After irradiation, the tumors were excised and trypsinized to observe MN in single-cells. The single-cell suspensions were used for MN formation assays. The RBE values increased with increasing LET. The maximum RBE values for the three different oxygen conditions; hypoxic tumor, normoxic tumor, and aerobic cells, were 8.18, 5.30, and 3.76 at an LET of 192keV/μm, respectively. After X-irradiation, the OERh/n values (hypoxic tumor/normoxic tumor) were lower than the OERh/a (hypoxic tumor/aerobic cells), and were 1.73 and 2.58, respectively. We found that the OER for the in vivo studies were smaller in comparison to that for the in vitro studies. Both of the OER values at 192keV/μm were small in comparison to those of the X-ray irradiated samples. The OERh/n and OERh/a values at 192keV/μm were 1.12 and 1.19, respectively. Our results suggest that high LET radiation has a large biological effect even if a solid tumor includes substantial numbers of hypoxic cells. To conclude, we found that the RBE values under each oxygen state for non-MN fraction increased with increasing LET and that the OER values for both tumors in vivo and cells in vitro decreased with increasing LET.

Malignant melanoma and radiotherapy: past myths, excellent local control in 146 studied lesions at Georgetown University, and improving future management

Introduction: Once thought to be radioresistant, emerging cellular and clinical evidence now suggests melanoma can respond to large radiation doses per fraction. Materials and Methods: We conducted a retrospective study of all patients treated with stereotactic radiosurgery and stereotactic body radiotherapy at Georgetown University Hospital from May 2002 through November 2008 and studied the classic extrapolated total dose corrected for volume (ETD_vol) model for predicting melanoma tumor response. Region-specific tumor outcomes were categorized by RECIST criteria and local control curves were estimated and analyzed when stratified by ETD_vol thresholds by use of the Kaplan–Meier method. Results: Follow-up information was available for 78 lesions (49 intracranial, 8 spinal, and 21 body) with mean follow-up period of 9.2 (range, 2–36) months. 1-year local control rates for intracranial, spinal, and body tumors were 84, 100, and 72%, respectively. Treatments in general were well-tolerated. Increased ETD_vol (p < 0.001) among intracranial sites resulted from larger (p < 0.001) doses per fraction combined with smaller (p < 0.001) tumor diameters. Intracranial 6-, 12-, and 24-month local control rates when treated above ETD_vol threshold of 230 Gy were all 90 vs. 89, 80, and 53% below this threshold. Body 6- and 12-month local control rates when treated above ETD_vol threshold of 100 Gy were 100 and 80% vs. 74 and 59% below this threshold. Discussion: By tailoring to melanoma’s unique radiobiology with large radiation doses per fraction, favorable local control was safely achieved. The ETD_vol model combines the important factor of dose per fraction in melanoma treatment with a volume correction factor to predict tumor response. Although limited sample size may have prevented reaching statistical significance for local control improvements using ETD_vol thresholds, optimal thresholds may exist to improve future tumor responses and further research is required.

Changes in the fraction of total hypoxia and hypoxia subtypes in human squamous cell carcinomas upon fractionated irradiation: evaluation using pattern recognition in microcirculatory supply units

BACKGROUND AND PURPOSE

Evaluate changes in total hypoxia and hypoxia subtypes in vital tumor tissue of human head and neck squamous cell carcinomas (hHNSCC) upon fractionated irradiation.

MATERIALS AND METHODS

Xenograft tumors were generated from 5 hHNSCC cell lines (UT-SCC-15, FaDu, SAS, UT-SCC-5 and UT-SCC-14). Hypoxia subtypes were quantified in cryosections based on (immuno-)fluorescent marker distribution patterns of Hoechst 33342 (perfusion), pimonidazole (hypoxia) and CD31 (endothelium) in microcirculatory supply units (MCSUs). Tumors were irradiated with 5 or 10 fractions of 2 Gy, 5×/week.

RESULTS

Upon irradiation with 10 fractions, the overall fraction of hypoxic MCSUs decreased in UT-SCC-15, FaDu and SAS, remained the same in UT-SCC-5 and increased in UT-SCC-14. Decreases were observed in the proportion of chronically hypoxic MCSUs in UT-SCC-15, in the fraction of acutely hypoxic MCSUs in UT-SCC-15 and SAS, and in the percentage of hypoxemically hypoxic MCSUs in SAS tumors. After irradiation with 5 fractions, there were no significant changes in hypoxia subtypes. Changes in the overall fraction of hypoxic MCSUs were comparable to corresponding alterations in the proportions of acutely hypoxic MCSUs. There was no correlation between radiation resistance (TCD(50)) and any of the investigated hypoxic fractions upon fractionated irradiation.

CONCLUSIONS

This study shows that there are large alterations in the fractions of hypoxia subtypes upon irradiation that can differ from changes in the overall fraction of hypoxic MCSUs.

Assessment of tumor radioresponsiveness and metastatic potential by dynamic contrast-enhanced magnetic resonance imaging

PURPOSE

It has been suggested that gadolinium diethylene-triamine penta-acetic acid (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) may provide clinically useful biomarkers for personalized cancer treatment. In this preclinical study, we investigated the potential of DCE-MRI as a noninvasive method for assessing the radioresponsiveness and metastatic potential of tumors.

METHODS AND MATERIALS

R-18 melanoma xenografts growing in BALB/c nu/nu mice were used as experimental tumor models. Fifty tumors were subjected to DCE-MRI, and parametric images of Ktrans (the volume transfer constant of Gd-DTPA) and ve (the fractional distribution volume of Gd-DTPA) were produced by pharmacokinetic analysis of the DCE-MRI series. The tumors were irradiated after the DCE-MRI, either with a single dose of 10 Gy for detection of radiobiological hypoxia (30 tumors) or with five fractions of 4 Gy in 48 h for assessment of radioresponsiveness (20 tumors). The host mice were then euthanized and examined for lymph node metastases, and the primary tumors were resected for measurement of cell survival in vitro.

RESULTS

Tumors with hypoxic cells showed significantly lower Ktrans values than tumors without significant hypoxia (p<0.0001, n=30), and Ktrans decreased with increasing cell surviving fraction for tumors given fractionated radiation treatment (p<0.0001, n=20). Tumors in metastasis-positive mice had significantly lower Ktrans values than tumors in metastasis-negative mice (p<0.0001, n=50). Significant correlations between ve and tumor hypoxia, radioresponsiveness, or metastatic potential could not be detected.

CONCLUSIONS

R-18 tumors with low Ktrans values are likely to be resistant to radiation treatment and have a high probability of developing lymph node metastases. The general validity of these observations should be investigated further by studying preclinical tumor models with biological properties different from those of the R-18 tumors.

Limitations of dynamic contrast-enhanced MRI in monitoring radiation-induced changes in the fraction of radiobiologically hypoxic cells in human melanoma xenografts

PURPOSE

To investigate the potential of gadopentetate dimeglumine (Gd-DTPA)-based dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) in detecting radiation-induced changes in the fraction of radiobiologically hypoxic cells in A-07 human melanoma xenografts.

MATERIALS AND METHODS

A-07 tumors were randomly assigned to an unirradiated control group or a group given a single radiation dose of 20 Gy. DCE-MRI and measurement of fraction of hypoxic cells were performed immediately before and 24 h after the radiation exposure. Tumor images of E . F (E is the initial extraction fraction of Gd-DTPA and F is blood perfusion) and lambda (lambda is proportional to extracellular volume fraction) were produced by subjecting DCE-MRI series to Kety analysis. Fraction of hypoxic cells was measured by using a radiobiological assay based on the paired survival curve method.

RESULTS

Fraction of radiobiologically hypoxic cells was higher in irradiated tumors (26.2+/-5.8%) than in unirradiated tumors (7.5+/-2.7%) by a factor of 3.5+/-1.5 (P=0.0093), whereas only minor radiation-induced changes in E . F and lambda could be detected.

CONCLUSION

DCE-MRI does not seem to offer insight into the changes in fraction of radiobiologically hypoxic cells occurring in A-07 tumors within 24 h after irradiation with 20 Gy.

Fluctuations in pO2 in irradiated human melanoma xenografts

Several studies have demonstrated that untreated tumors may show significant fluctuations in tissue oxygen tension (pO(2)). Radiation treatment may induce changes in the tumor microenvironment that alter the pO(2) fluctuation pattern. The purpose of the present study was to investigate whether pO(2) fluctuations may also occur in irradiated tumors. A-07 human melanoma xenografts were irradiated with single doses of 0, 5 or 10 Gy. Fluctuations in pO(2) were recorded with OxyLite probes prior to irradiation and 24 and 72 h after the radiation exposure. Radiation-induced changes in the tumor microenvironment (i.e. blood perfusion and extracellular volume fraction) were assessed by dynamic contrast-enhanced magnetic resonance imaging. Seventy-two hours after 10 Gy, tumor blood perfusion had decreased to approximately 40% of that prior to irradiation, whereas the extracellular volume fraction had increased by approximately 25%. Fluctuations in pO(2) were seen in most tumors, irrespective of radiation dose and time after irradiation. The mean pO(2), the number of fluctuations around the mean pO(2), the number of fluctuations around threshold pO(2) values of 1, 2, 3, 5, 7 and 10 mmHg, and the amplitude of the fluctuations were determined for each pO(2) trace. No significant differences were detected between irradiated and unirradiated tumors. The results showed that pO(2) fluctuations may occur in irradiated tumors and that the pO(2) fluctuation pattern in A-07 tumors exposed to 5 or 10 Gy is similar to that in untreated tumors. Consequently, these doses did not induce changes in the tumor microenvironment that were sufficient to cause detectable alterations in the pO(2) fluctuation pattern.

Differences in pO2 peaks of a murine fibrosarcoma between carbon-ion and X-ray irradiation

We measured and compared the oxygen partial pressure (pO(2)) profiles in experimental tumors after irradiation with carbon ions and with X-rays. The NFSa fibrosarcomas grown in the hind legs of C3H male mice received isoeffect single doses of carbon ions or X-rays. Coaxial oxygen microelectrodes of high spatial resolution were inserted into the tumor with 20 microm steps by a computerized micromanipulator. The number of pO(2) peaks that reached 15 mmHg were at least 0.45 per 3,000 microm in unirradiated tumors and significantly increased to 1.55 per 3,000 microm as early as day 1 of carbon-ion irradiation (p < 0.001). The tumors that received X-ray irradiation also significantly increased pO(2) peaks, but as late as day 3. The time course of pO(2) peak appearance in the present study coincides with a previous report where reoxygenation was measured by paired growth delay assay. The pO(2) peaks appeared selectively in peripheral regions of X-ray irradiated tumors, but they appeared rather homogeneously in the tumor after carbon-ion irradiation. It is concluded that carbon-ion irradiation reoxygenated the NFSa fibrosarcomas earlier in time and deeper in space than the X-ray irradiation did.

Improved tumor response by combining radiation and the vascular-damaging drug 5,6-dimethylxanthenone-4-acetic acid

The interaction between 5,6-dimethylxanthenone-4-acetic acid (DMXAA) and radiation was investigated in two different mouse tumor models and a normal mouse tissue. C3H mouse mammary carcinomas transplanted in the feet of CDF1 mice and KHT mouse sarcomas growing in the leg muscles of C3H/HeJ mice were used. DMXAA was dissolved in saline and injected intraperitoneally. Tumors were irradiated locally in nonanesthetized mice, and response was assessed using tumor growth for the C3H mammary carcinoma and in vivo/in vitro clonogenic cell survival for the KHT sarcoma. DMXAA alone had an antitumor effect in both tumor types, but only at doses above 15 mg/kg. DMXAA also enhanced radiation damage, and again there was a threshold dose. No enhancement was seen in the C3H mammary carcinoma at 10 mg/kg and below, while in the KHT sarcoma, doses above 15 mg/kg were necessary. This enhancement of radiation damage was also dependent on the sequence of and interval between the treatments with DMXAA and radiation. Combining radiation with DMXAA at the maximum tolerated dose (i.e., the highest dose that could be injected without causing any lethality) of either 20 mg/kg (CDF1 mice) or 17.5 mg/kg (C3H/HeJ mice) gave an additive response when the two agents were administered simultaneously. Even greater antitumor effects were achieved when DMXAA was administered 1-3 h after irradiation. However, when administration of DMXAA preceded irradiation, the effect was similar to that seen for radiation alone, suggesting that appropriate timing is essential to maximize the utility of this agent. When such conditions were met, DMXAA was found to increase the tumor response significantly in the absence of an enhancement of radiation damage in normal skin, thus giving rise to therapeutic gain.

Time course of reoxygenation in experimental murine tumors after carbon-beam and X-ray irradiation

We compared the tumor reoxygenation patterns in three different murine tumor cell lines after X-irradiation with those after carbon-beam irradiation using a heavy-ion medical accelerator (HIMAC) system. The tumors of the cell lines SCCVII, SCCVII-variant-1 and EMT6 on the hind legs of mice received local priming irradiation with a carbon-beam (8 Gy, 73 keV/microm in LET, 290 MeV/u, 6 cm SOBP) or X-rays (13 Gy, 250 kVp). After various intervals, the mice were given whole-body test irradiation (16 Gy. 250 kVp X-ray) either in air or after they were killed. The hypoxic fractions were estimated as the proportions of the surviving fractions of the tumors in killed mice to those in air-breathing mice. In the SCCVII tumors, the hypoxic fractions at 0.5 h were 50% and 21% (p < 0.05) after the priming X-irradiation and carbon-beam irradiation, respectively. In the SCCVII-variant-1 tumors, the hypoxic fractions were 85% and 82% at 0.5 h, 84% and 20% at 12 h (p < 0.01), and 21% and 31% at 24 h after X-ray and after carbon-beam irradiation, respectively. In the EMT6 tumors, the reoxygenation patterns after X-irradiation and carbon-beam irradiation were quite similar. We concluded that the reoxygenation pattern differed among the three tumor cell lines, and that reoxygenation tended to occur more rapidly after carbon-beam irradiation than after X-irradiation for SCCVII and SCCVII-variant-1 tumors.

Effects of radiation on tumor intravascular oxygenation, vascular configuration, development of hypoxia, and clonogenic survival

The underlying physiological mechanisms leading to tumor reoxygenation after irradiation have elicited considerable interest, but they remain somewhat unclear. The current study was undertaken to determine the effects of a single dose of 10 Gy gamma radiation on both tumor pathophysiology and radiobiologically hypoxic fraction. Immunohistochemical staining and perfusion markers were used to quantify tumor vasculature, uptake of the hypoxia marker EF5 to assess the distribution of hypoxia, and intravascular HbO(2) measurements to determine oxygen availability. Tumor radiosensitivity was measured by a clonogenic assay. At 24 h postirradiation, oxygen availability increased, perfused vessel numbers decreased, EF5 uptake decreased, and the radiobiologically hypoxic fraction was unchanged. Together, these results demonstrate that tumor hypoxia develops at an increased distance from perfused blood vessels after irradiation, suggesting a decrease in oxygen consumption at 24 h. By 72 h postirradiation, all physiological parameters had returned to the levels in volume-matched, nonirradiated controls. These studies clearly show that single measures of either tumor oxygenation or vascular structure are inadequate for assessing the effects of radiation on tumor clonogenicity. Although such direct measurements have previously proven valuable in predicting tumor response to therapy or oxygen manipulation, a combination of parameters is required to adequately describe the mechanisms underlying these changes after irradiation.

Microenvironment-induced cancer metastasis

PURPOSE

To present and evaluate clinical data suggesting that cancer metastasis may be induced by the microenvironment of the primary tumour and to discuss possible mechanisms of microenvironment-induced metastasis, based on a critical review of relevant data from studies of experimental tumours and cells in culture.

CONCLUSIONS

Low oxygen tension in the primary tumour is associated with metastasis in soft tissue sarcoma, cervix carcinoma and carcinoma of the head and neck. Multiple mechanisms may be involved in hypoxia-induced metastasis. Thus, hypoxia followed by reoxygenation may induce point mutations and DNA strand breakage leading to deletions, amplifications and genomic instability. Hypoxia may also provide a physiological pressure in tumours selecting for metastatic cell phenotypes. Moreover, hypoxia may induce a temporary increase in the expression of gene products involved in the metastatic cascade, either through gene amplifications or through normal physiological processes by activating oxygen sensors, hypoxia signal transduction pathways and DNA transcription factors. Low glucose concentration, high lactate concentration and low extracellular pH may induce metastasis by similar mechanisms as hypoxia. Tumour reoxygenation during radiation therapy may promote microenvironment-induced metastasis by rescuing hypoxic or nutritionally deprived metastatic cells from dying. Ionizing radiation can elicit a stress response in tumour cells similar to that elicited by hypoxia. Radiation therapy may therefore adversely affect the rate of metastasis in patients who do not achieve control of the primary tumour by enhancing the expression of gene products of importance in metastasis.

Changes in oxygen tension during radiotherapy of head and neck tumours

Increased knowledge about changes that occur in tumour oxygenation during radiotherapy and the biological factors causing these changes can be useful in the development of optimal radiation treatments. The aims of this study were a) to study changes in the oxygen tension (pO2) of human head and neck tumours during radiotherapy in relationship to changes in cell density and vascular density, and b) to investigate whether the pO2, measured before or during therapy, can be used to predict the therapeutic outcome. Preliminary data from the first 11 patients included in the study are reported. The pO2 was measured before treatment (11 patients) and once a week during therapy (8 patients), using polarographic needle electrodes. Cell density and vascular density were determined from biopsies taken after each pO2 measurement in 5 patients. Significant fluctuations in pO2 occurred during therapy. Changes in hypoxic fraction; i.e., fraction of pO2 readings below 2.5 mm Hg, 5 mm Hg or 10 mm Hg, coincided with changes in cell density, but not with changes in vascular density, which suggests that the changes in hypoxic fraction were caused by changes in oxygen consumption rather than supply. Response evaluation after a median follow-up time of 19 months showed that progressive disease occurred among the patients with highly hypoxic tumour, regardless of whether hypoxic fraction before treatment or after two weeks of radiotherapy was considered.

Monitoring of tumor reoxygenation following irradiation by 31P magnetic resonance spectroscopy: an experimental study of human melanoma xenografts

BACKGROUND AND PURPOSE

Inadequate tumor reoxygenation during radiation therapy may cause local treatment failure. This study was aimed at investigating the potential usefulness of 31P-MRS in monitoring tumor reoxygenation following radiation treatment.

MATERIALS AND METHODS

Tumors of two human melanoma xenograft lines (BEX-t and HUX-t) were exposed to 15.0 Gy, and then the fraction of radiobiologically hypoxic cells, measured by using the paired survival curve method, or tumor bioenergetic status, measured by 31P-MRS as the (PCr + NTPbeta)/Pi resonance ratio, was determined versus time after the radiation exposure.

RESULTS

Untreated BEX-t and HUX-t tumors showed similar fractions of radiobiologically hypoxic cells and similar bioenergetic status, whereas both parameters differed substantially between the lines in irradiated tumors. A close association was found between radiation-induced changes in tumor bioenergetic status and radiation-induced changes in the fraction of radiobiologically hypoxic cells.

CONCLUSION

31P-MRS is a potentially useful method for monitoring tumor reoxygenation following radiation treatment.

Oxygen tension in transplanted mouse osteosarcomas during fractionated high-LET- and low-LET radiotherapy--predictive aspects for choosing beam quality?

PURPOSE

The lower OER of high-LET radiations, compared to conventional (low-LET) radiations, has often been put forward as an argument for using high-LET radiotherapy in the management of hypoxic tumours. Among the different neutron beams used in therapy, the reactor fission neutrons have the lowest OER. The aim of the present study is to follow the variations of tumour oxygenation status during fractionated irradiation with different radiation qualities. Little information is available so far after fractionated high-LET irradiation. In addition, the RBE of reactor fission neutrons for effects on tumours and on normal tissues are compared.

MATERIAL AND METHODS

Murine OTS 64-osteosarcomas were transplanted in 102 balb-C mice and irradiated by 36 Gy of photons in fractions of 3 Gy five times a week (group P-36/3) or by 12 Gy of reactor fission neutrons in fractions of 2 Gy two times a week (group N-12/2). Irradiations started at a tumor volume of 500 to 600 mm3. A third group received no radiotherapy, but all investigations (group CG). Tumor volume and tumor oxygenation were measured once a week under therapy and during three weeks after therapy. For in vivo-evaluation of oxygen status a computerized polarographic needle electrode system (KIMOC pO2 histograph, Eppendorf) was used. The median pO2 and the hypoxic fraction (pO2 values < 5 mm Hg) of single tumors and of total groups were calculated from pooled histograms and from row data as well.

RESULTS

In correlation with the increase of tumor volume, from day 1 to day 42 of follow-up the median pO2 decreased from 20 mm to 8 mm Hg and the hypoxic fraction increased from 7% to 31%. After fractionated photon therapy a growth delay of three weeks was observed. Six weeks after beginning of the irradiation the median tumor volume had been doubled again. After fission neutron therapy growth delay continued until the end of the follow-up period. In both of the irradiated groups a significant decrease of median pO2 values and an increase of the hypoxic fraction were observed under radiotherapy. Hypoxia was more intensive after neutrons with a decrease of the median pO2 from 20 mm Hg to 1 mm Hg vs. 10 mm Hg after photon therapy and with an increase of the hypoxic fraction from 7% to 78% vs. 36% respectively. Two weeks after the end of therapy the median pO2 and the hypoxic fraction of both treated groups reached the levels prior to irradiation indicating a complete reoxygenation.

CONCLUSION

During fractionated irradiation of murine osteosarcomas with photons and reactor fission neutrons, a marked hypoxia was observed for both radiation qualities, but hypoxia was more intense during fractionated neutron irradiation. After irradiation, a complete reoxygenation occurred in both groups independently of the degree of hypoxia observed during the treatment. The RBE of reactor fission neutrons, after fractionated irradiation, was much higher for effects on murine osteosarcomas compared to their RBE observed for normal tissues in previous experiments. Present data are in agreement with our clinical observations on more than 300 patients treated with reactor fission neutrons for advanced and hypoxic tumours with various histologies.

Palliative radiotherapy of malignant melanoma with reactor fission neutron therapy (RENT): a prospective study

To study the efficacy and safety of relatively low-dosed reactor fission neutron therapy (RENT) at the research reactor of the Technical University Munich, we treated 33 superficial lesions of 20 patients with advanced malignant melanoma by neutron beam alone (n = 22), mixed neutron/electron beam (n = 5), or by neutron beam after incomplete surgery (n = 6). Median tumor volume was 17.0 cm3. Median dose for neutron beam alone was 8.0 Gy and for mixed beam 3.0 Gy n + 45.3 Gy e-. Local tumor response, local control time, survival and treatment related toxicity were followed prospectively over a time period of 52 months. Overall response rate (CR;PR) after neutron beam alone and mixed beam therapy was 64% (CR: 36%) and 100% (CR: 60%), respectively. Observed differences between complete (CR) and incomplete (PR, NC) responding lesions were as follows: median tumor volume: 2.0 vs. 51.5 cm3, local control time: 13.3 vs. 3.7 months, median survival: 19.8 vs. 9.0 months. No severe acute or late sequelae could be observed. In conclusion, low-dosed RENT is an effective and well tolerated palliative treatment of superficial malignant melanoma utilizing the biologic advantage of diminished cellular repair capacity. Because melanoma lesions of small size (< or = 6 cm3) tend to respond completely, neutron beam should be performed at an early stage.

Oxygenation of cervical cancers during radiotherapy and radiotherapy + cis-retinoic acid/interferon

PURPOSE

We have evaluated the tumor tissue pO2 in cervical cancers during radiotherapy with special emphasis on the course of the pO2 in primarily hypoxic tumors and in patients treated with radiotherapy plus 13-cis-retinoic acid/interferon-alpha-2a.

METHODS AND MATERIALS

From June 1995 through April 1997, 49 patients with squamous cell carcinoma FIGO IIB-IVA of the cervix who were treated with definitive radiotherapy with curative intent underwent polarographic measurement of tumor tissue pO2 with an Eppendorf pO2-histograph prior to and during radiation treatment. Radiotherapy consisted of external irradiation with 50.4 Gy in 28 fractions of 1.8 Gy plus high dose rate (HDR) brachytherapy. Twenty-two patients had additional treatment with 13-cis-retinoic acid (cRA, isotretinoin) and interferon-alpha-2a (IFN-alpha-2a). Therapy with cRA/IFN in these patients started 2 weeks before radiotherapy; during this induction period, cRA was administered in a dosage of 1 mg per kilogram body weight orally daily and IFN-alpha-2a in a dosage of 6x10(6) I.U. subcutaneously daily. After start of external radiotherapy (XRT), cRA/IFN was continued concomitantly with radiotherapy in reduced doses (0.5 mg cRA per kg body weight orally daily plus 3x10(6) I.U. IFN-alpha-2a subcutaneously three times weekly until the end of the radiation treatment). PO2 measurements were performed prior to radiotherapy, at 20 Gy, and at the end of radiotherapy.

RESULTS

A poor oxygenation defined as a median pO2 of 10 mm Hg or less was present in 15/38 tumors (39%) in which measurements prior to any treatment were done. Low pO2 readings below 5 mm Hg were present in 70% of all tumors prior to treatment. In 13 of 15 hypoxic tumors, pO2 measurements at 19.8 Gy were performed. In these tumors, a significant increase of the median pO2 from 6.0+/-3.1 mm Hg to 20.7+/-21.2 mm Hg was found, p<0.01. The increase in the median pO2 was more pronounced in patients with radiotherapy plus additional cRA/IFN treatment as compared to patients treated with irradiation alone (median pO2 raised from 7.0+/-3.5 mm Hg to 40.9+/-21.3 mm Hg versus 5.7+/-3.1 mm Hg to 14.7+/-17.9 mm Hg). In a multivariate analysis, both the effect of radiation dose (pretreatment versus 19.8 Gy) and the type of treatment (XRT alone versus XRT plus cRA/IFN) had significant impact on the pO2 (P = 0.003 and p = 0.04). In patients with well-oxygenated tumors (pretreatment median pO2>10 mm Hg), 20/23 (87%) achieved a clinically complete response. In patients with primarily hypoxic tumors, 6/6 patients whose primarily hypoxic tumors showed an increase of the median pO2 above 10 mm Hg at 19.8 Gy achieved a complete remission (CR). In contrast, only 4/7 patients with a low pretreatment and persisting low median pO2 achieved a CR.

CONCLUSIONS

There are evident changes in the oxygenation of cervical cancers during a course of fractionated radiotherapy. In primarily hypoxic tumors, a significant increase of the median pO2 was found. An additional treatment with cis-retinoic acid/interferon further improved the oxygenation. An impact of the different patterns of oxygenation on local control is to be evaluated.

Radiation-induced changes in phosphorus T1 values in human melanoma xenografts studied by 31P-MRS

31P-magnetic resonance spectroscopy (MRS) has been shown to be a promising method for monitoring tumor response to radiation therapy. The purpose of the work reported here was to investigate whether the usefulness of 31P-MRS might be enhanced by measurement of spin-lattice relaxation times (T1s) in addition to resonance ratios. The work was based on the hypothesis that tumors having a high probability of being controlled locally would show shortened T1s during the treatment course due to reoxygenation and development of necrosis. BEX-t human melanoma xenografts, which show efficient reoxygenation and development of necrosis following single dose irradiation, were used as tumor models. Tumors were treated with single doses of 5.0 or 15.0 Gy and the T1s of the inorganic phosphate and nucleoside triphosphate beta resonances were measured as a function of time after irradiation by using the superfast inversion recovery method. Fractional tumor water content was determined by drying excised tumors at 50 degrees C until a constant weight was reached. The T1s in irradiated tumors were either longer than or not significantly different from those in unirradiated control tumors. The increase in the T1s following irradiation coincided in time with a radiation-induced increase in tumor water content, suggesting a causal relationship. The effects of reoxygenation and development of necrosis on T1s were probably overshadowed by the effects of tumor water content. Consequently, the usefulness of 31P-MRS in monitoring tumor response to radiation therapy might not be significantly enhanced by measurement of T1s.

Effects of carbogen plus fractionated irradiation on KHT tumor oxygenation

BACKGROUND AND PURPOSE

Numerous studies have demonstrated improvements in the oxygenation of tumor cells following both irradiation and carbogen breathing. The current studies were initiated to measure the combined effects of carbogen inhalation plus single and multi-dose irradiation on tumor oxygen availability, to better define the underlying physiological relationships.

MATERIALS AND METHODS

Using KHT murine sarcomas, radiation was delivered to the tumor-bearing legs of non-anesthetized mice. Tumors were quick-frozen prior to or following single or multifraction irradiation and carbogen breathing, and intravascular HbO2 saturation profiles were determined cryospectrophotometrically.

RESULTS

HbO2 levels for blood vessels located near the tumor surface initially decreased following 10 Gy irradiation, then increased and remained elevated. Interior HbO2 levels remained unchanged. Following 2.5 Gy, HbO2 changes were minimal. At 24 h following 10 Gy, HbO2 levels were significantly increased compared to non-irradiated controls, and carbogen breathing produced no additional benefit. At 24 h following five fractions of 2 Gy, HbO2 levels throughout the tumor volume were significantly higher in carbogen breathing animals than in air breathing controls.

CONCLUSIONS

Although peripheral blood vessels demonstrated substantial improvements in oxygenation following irradiation, oxygen availability nearer the tumor center remained at very low levels. The utility of carbogen in enhancing tumor oxygen availability was maintained following five clinically relevant fractions. At higher doses, radiation-induced enhancements in HbO2 levels overshadowed the carbogen effect. For either air or carbogen breathing, a decrease in the percentage of vessels with very low oxygen content did not appear to be a major factor in the reoxygenation of the KHT tumor.

Radiation therapy for malignant melanoma

Sufficient biologic and clinical evidence now exists to refute the longstanding dogma that melanomas are uniformly radiation resistant and hence radiation therapy has little role in the management of this disease. Although surgery remains the treatment of choice for the vast majority of localized melanomas, available data indicate that radiation therapy is a viable alternative for a few subsets of patients in whom surgery would result in cosmetic or functional deformity, such as patients with large facial lentigo maligna melanomas or small or intermediate-sized uveal melanomas. Retrospective and Phase II prospective studies have revealed that elective/adjunctive radiation therapy improves the local-regional control rate in patients with thick primary lesions, nodal involvement, or mucosal melanomas. However, the impact of elective/adjunctive radiation therapy on the survival rate has yet to be determined. Radiation therapy has been established as a simple and cost-effective treatment modality for palliation of patients with symptomatic metastatic spread. The response of metastatic deposits to radiation varies with the tumor volume, total dose, and dose per fraction. The choice of optimal fractionation depends on tumor site and the patient's survival expectation. New data indicate that hyperthermia enhances the response of metastatic lesions to radiation. Ongoing research with a variety of experimental strategies may offer the possibility of further increasing the utility of radiation therapy in the management of this disease.

Reoxygenation after single irradiation in rodent tumors of different types and sizes

PURPOSE

To investigate the variation of reoxygenation patterns after single irradiation in murine tumors of different types and sizes.

METHODS AND MATERIALS

Whole-body single irradiation of 13 to 15 Gy was delivered to 10 mm RIF1 tumors of C3H/He mice, 22 mm SCCVII tumors of C3H/He mice, and 16 mm EMT6 tumors of Balb/c mice. Thereafter, changes in the hypoxic fraction with time were determined by the paired survival curve method. The data were compared with the results we had ++previously obtained with 10 mm SCCVII and 10 mm EMT6 tumors.

RESULTS

The hypoxic fraction at 1 h after the priming irradiation was 26% for 10 mm RIF1 tumors, 48% for 10 mm SCCVII tumors, and 100% for 10 mm EMT6 tumors. Thus, RIF1 and SCCVII tumors, both of which have few necrotic areas, showed rapid reoxygenation, whereas EMT6 tumors, which have large necrotic areas, reoxygenated slowly. Although the hypoxic fraction returned to the pretreatment level within 72 h in 10 mm SCCVII and 10 mm EMT6 tumors, it did not in 10 mm RIF1 tumors. In contrast, the patterns of reoxygenation were similar between 22 mm and 10 mm SCCVII tumors and between 16 mm and 10 mm EMT6 tumors.

CONCLUSION

The three tumors showed different patterns of reoxygenation. Tumors that have a low proportion of necrosis may reoxygenate rapidly. However, tumor size appeared to have less influence on the pattern of reoxygenation.

Growth rates or radiobiological hypoxia are not correlated with local metabolite content in human melanoma xenografts with similar vascular network

Investigations were carried out on two lines of human melanomas (MF; n = 12 and EE; n = 13) xenografted in nude mice. The tumours were characterised by a similar vascular supply but showed a pronounced difference in the rate of volume growth and in the radiobiologically hypoxic fraction. The distribution of ATP, glucose and lactate in the tumours was investigated using quantitative bioluminescence and single photon imaging. Concentrations of the metabolites were obtained as global values for the entire tumour mass, in regions with densely packed, structurally intact tumour cells ('viable zones'), in areas with necrosis, stromal cells and fibrous material ('necrotic zones') and in adjacent normal tissue. In all melanomas investigated glucose concentrations were significantly lower and lactate concentrations were significantly higher than in normal tissue. In contrast, no significant differences for ATP were detected. ATP and glucose concentrations were significantly less in necrotic than in viable tumour zones, whereas lactate concentrations were nearly equal in these tumour parts. Corresponding results were obtained in central versus peripheral tumour zones. There was no dependency of global or regional metabolite concentrations on tumour size within the volume range 110-1470 mm3. Based on this lack of dependency, metabolic concentrations were averaged over the whole tumour size range. Metabolite concentrations were not significantly different either globally or regionally between the two tumour entities investigated, a finding which held true for all three metabolites registered. Thus, metabolite distributions apparently mirror the similarity in vascularity of MF and EE melanomas rather than reflecting intrinsic properties with regard to tumour growth rates or susceptibility to radiation.

Tumor oxygenation in a transplanted rat rhabdomyosarcoma during fractionated irradiation

PURPOSE

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

METHODS AND MATERIALS

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

RESULTS

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

CONCLUSION

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

The effect of the overall treatment time of fractionated irradiation on the tumor control probability of a human soft tissue sarcoma xenograft in nude mice

PURPOSE

To study the impact of the overall treatment time of fractionated irradiation on the tumor control probability (TCP) of a human soft tissue sarcoma xenograft growing in nude mice, as well as to compare the pretreatment potential doubling time (Tpot) of this tumor to the effective doubling time (Teff) derived from three different schedules of irradiation using the same total number of fractions with different overall treatment times.

METHODS AND MATERIALS

The TCP was assessed using the TCD50 value (the 50% tumor control dose) as an end point. A total of 240 male nude mice, 7-8 weeks old were used in three experimental groups that received the same total number of fractions (30 fractions) with different overall treatment times. In group 1, the animals received three equal fractions/day for 10 consecutive days, in group 2 they received two equal fractions/day for 15 consecutive days, and in group 3 one fraction/day for 30 consecutive days. All irradiations were given under normal blood flow conditions to air breathing animals. The mean tumor diameter at the start of irradiation was 7-8 mm. The mean interfraction intervals were from 8-24 h. The Tpot was measured using Iododeoxyuridine (IudR) labeling and flow cytometry and was compared to Teff.

RESULTS

The TCD50 values of the three different treatment schedules were 58.8 Gy, 63.2 Gy, and 75.6 Gy for groups 1, 2, and 3, respectively. This difference in TCD50 values was significant (p < 0.05) between groups 1 and 2 (30 fractions/10 days and 30 fractions/15 days) vs. group 3 (30 fractions/30 days). The loss in TCP due to the prolongation of the overall treatment time from 10 days to 30 days was found to be 1.35-1.4 Gy/day. The pretreatment Tpot (2.4 days) was longer than the calculated Teff in groups 2 and 3 (1.35 days).

CONCLUSION

Our data show a significant loss in TCP with prolongation of the overall treatment time. This is most probably due to an accelerated repopulation of tumor clonogens. The pretreatment Tpot of this tumor model does not reflect the actual doubling of the clonogens in a protracted regimen.

The effects of clamping and reoxygenation on repopulation during fractionated irradiation

PURPOSE

Clamping of solid tumors is often used during radiobiological experiments to eliminate reoxygenation and hypoxia as confounding variables. The appropriateness of this procedure has been questioned recently.

METHODS AND MATERIALS

This article describes experiments designed to examine the tumor surviving fraction (TSF) during fractionated treatment (up to 9 x 2 Gy, every 6 h or 24 h) given under normal, unclamped conditions (aerobic), and to compare it to TSF values for treatment given under clamped conditions (anoxic). The dose per fraction for the clamped procedure was 5.5 Gy, approximately equivalent to the unclamped doses of 2 Gy (based on an assumed OER of 2.75).

RESULTS

Overall, the survival in the unclamped experiments was greater than in the clamped experiments. When the contribution of repopulation to this phenomenon was analyzed, no significant difference was found for four (KHT-C, KHT-LP1, RIF-1, B16F1) of five transplantable murine tumor cell lines used. However, for the SCC-VII squamous cell carcinoma, following small numbers of fractions there appeared to be more repopulation during the aerobic treatment schedule. Reoxygenation during the treatment was also analyzed, and the results indicate that, even with 24 h between doses, reoxygenation was insufficient to prevent an increasing hypoxic fraction during the treatment, which can explain the enhanced survival seen for the aerobic vs. the anoxic irradiation conditions.

CONCLUSION

With the fractionation schedules used in this study, there was no evidence for a difference in repopulation kinetics when the treatment was given to clamped (anoxic) or unclamped (aerobic) tumors. In all five tumors studied, reoxygenation between the 2 Gy fractions was insufficient to prevent hypoxia from affecting the response of the tumors to the treatment.

Reoxygenation in a C3H mouse mammary carcinoma. The importance of chronic rather than acute hypoxia

The role that chronic and acute hypoxia play in tumour reoxygenation after irradiation was investigated in a C3H mouse mammary carcinoma grown in the feet of female CDF1 mice. Tumours at 200 mm3 in size were locally irradiated with a priming dose of 20 Gy and then at various times after given a range of radiation doses under normal or clamped conditions. Local tumour control was determined 90 days later from which the tumour hypoxic fractions were calculated. Untreated tumours contained 23% hypoxic cells. Immediately after 20 Gy this increased to 52% and by 24 h had fallen to 10%. These reoxygenation experiments were repeated, giving either nicotinamide (1000 mg/kg; i.p. injected 30 min before each irradiation) to remove acute hypoxia, or carbogen breathing (for 5 min before and during irradiation) to decrease chronic hypoxia. With nicotinamide the normal hypoxic fraction was reduced to 7%, but after irradiation it had risen to 46% and by 24 h there was full reoxygenation with a value of 5% being observed. Carbogen breathing also decreased the normal hypoxic fraction to 6%, and immediately after irradiation this was increased to 38%. However, by 24 h it was still elevated at around 23%. These results suggest that chronic rather than acute hypoxia is necessary for reoxygenation in this tumour.

Fractionation sensitivity (alpha/beta ratio) of human melanoma xenografts

The fractionation sensitivity (alpha/beta) of ten human melanoma xenograft lines was measured in vivo and related to cellular radiation sensitivity measured in vitro. Xenografted tumours were irradiated under clamped conditions with 2, 4, 8 or 16 fractions within a constant overall treatment time of 5 days. At least three different fractional doses were used for each schedule. Tumour regrowth delay was used as endpoint. Isoeffective total doses were determined from plots of tumour regrowth delay versus total radiation dose and subjected to FE analysis for the determination of alpha/beta values. Single cell suspensions prepared by disaggregation of xenografted tumours were irradiated in vitro. Cell survival was measured by using the Courtenay soft agar colony assay. The alpha/beta ratio in vivo, divided by an oxygen enhancement ratio of 2.8, ranged from 1.0 +/- 0.2 Gy to 33.0 +/- 6.7 Gy. Seven lines showed alpha/beta values similar to or higher than those of acutely responding normal tissues. The alpha/beta values of the remaining three lines were similar to or lower than those of late responding normal tissues. The alpha/beta ratio in vivo showed statistically significant correlations to the alpha/beta ratio and the cell surviving fraction at 2.0 Gy (SF2) in vitro. High alpha/beta values in vivo corresponded to low SF2 values in vitro. Optimum radiation therapy of malignant melanoma requires an individualized treatment strategy. SF2 values measured in vitro might be useful guidelines for selection of the optimum fractionation schedule.

Radiation-induced reoxygenation in the SCCVII murine tumour: evidence for a decrease in oxygen consumption and an increase in tumour perfusion

Radiation-induced reoxygenation of the SCCVII murine tumour was examined with the goal of determining whether rapid reoxygenation in this tumour occurs as a result of cell loss, redistribution of hypoxic cells within the tumour cord, a change in oxygen consumption rate, or a change in tumour perfusion. Six hours after exposure of the tumour to 10 Gy, oxygen diffusion distance, measured using tumour cubes incubated with a fluorescent hypoxia probe, had increased by about 20%, corresponding to a reduction of about 16% in oxygen consumption rate. Cell loss and redistribution within the tumour cord were negligible at this time. Hypoxic fraction, measured using the comet assay, indicated a significant decrease from 18% hypoxic cells in unirradiated tumours to 6% in tumours examined 6 h after 10 Gy. Changes in tumour perfusion were also measured using the dual fluorescent dye method which allows detection of regions undergoing transient fluctuations in perfusion. Six hours after 10 Gy, the percentage of blood vessels labeled with only one stain had decreased from 8.6% to 4.5%. Results indicate that the rapid reoxygenation of the SCCVII tumour following exposure to 10 Gy can be attributed, not to cell loss or redistribution, but to a decrease in oxygen consumption and an increase in tumour perfusion.

Reoxygenation in the SCCVII tumor after KU-2285 sensitization plus single or fractionated irradiation

PURPOSE

Because reoxygenation of solid tumors after irradiation with a hypoxic cell sensitizer has never previously been investigated, we assessed the reoxygenation in SCCVII tumors after treatment with KU-2285 plus single or fractionated irradiation.

METHODS AND MATERIALS

KU-2285 (100 mg/kg) was injected intraperitoneally into C3H mice bearing 1-cm SCCVII tumors at 30 min before a single dose of 12 Gy or three fractions of 5 Gy at 12 h intervals. Changes of the hypoxic fraction (HF) were then evaluated by the paired survival curve method.

RESULTS

Since the radiosensitizing effect of KU-2285 was relatively persistent, the HF was only evaluable after 6 h of irradiation. The HF of untreated SCCVII tumors was 9.1%. After treatment with KU-2285 and 12 Gy, the HF was 25% at 6 h, 32% at 12 h, 24% at 24 h, and 7.6% at 72 h. The HF was lower at 6 h than that after radiation alone, but was similar at later periods. After three fractions of 5 Gy with or without KU-2285, the HF was 33% at 6 h in both groups, while it was 12% and 13% at 24 h for tumors pretreated with KU-2285 and those receiving radiation alone, respectively. However, a sensitizing effect of KU-2285 was indicated by the downward shift of the survival curves for tumors irradiated after exposure to this agent.

CONCLUSION

Reoxygenation occurred quite efficiently in tumors receiving KU-2285 and 12 Gy. After fractionated irradiation, however, reoxygenation was similar in the KU-2285-pretreatment and irradiation alone groups.

Reoxygenation and rehypoxiation in the SCCVII mouse tumor

PURPOSE

To test the hypothesis that, following preferential killing of tumor hypoxic cells, the fraction of hypoxic cells in the tumor will reestablish itself to pretreatment levels (rehypoxiation) with the same kinetics as for reoxygenation.

METHODS AND MATERIALS

Mouse squamous cell carcinoma VII (SCCVII) tumors were treated with a single dose of 10 Gy or a single dose of the bioreductive hypoxic cell cytotoxin, tirapazamine (SR 4233, 0.2 mmol/kg), which preferentially kills hypoxic cells within the tumor. Hypoxic fractions were determined by the paired survival curve technique using the in vivo-in vitro clonogenic assay 0-24 h after treatment.

RESULTS

Immediately after irradiation with 10 Gy, the hypoxic fraction of the tumors increased to 80% and rapidly returned to pretreatment levels 3-6 h later. Within 1 h of injecting tirapazamine, the hypoxic fraction fell to 0.57% (about 7% of pretreatment levels) and returned to pretreatment levels 3-5 h later.

CONCLUSION

The return to pretreatment levels of hypoxia among tumor cells surviving a single dose of radiation (reoxygenation) and of the hypoxic cell toxin tirapazamine (rehypoxiation) was rapid and occurred with similar kinetics for the two processes. These data support the hypothesis that reoxygenation and rehypoxiation are different manifestations of the same phenomenon and result from fluctuating tumor blood flow which creates acute hypoxia.

Magnetic resonance imaging of human melanoma xenografts in vivo: proton spin-lattice and spin-spin relaxation times versus fractional tumour water content and fraction of necrotic tumour tissue

Proton nuclear magnetic resonance (1H-nmr) imaging is used routinely in clinical oncology to provide macroscopic anatomical information, whereas its potential to provide physiological information about tumours is not well explored. To evaluate the potential usefulness of 1H-nmr imaging in the prediction of tumour treatment resistance caused by unfavourable microenvironmental conditions, possible correlations between proton spin-lattice and spin-spin relaxation times (T1 and T2) and physiological parameters of the tumour microenvironment were investigated. Tumours from six human melanoma xenograft lines were included in the study. 1H-nmr imaging was performed at 1.5 T using spin-echo pulse sequences. T1- and T2-distributions were generated from the images. Fractional tumour water content and the fraction of necrotic tumour tissue were measured immediately after 1H-nmr imaging. Significant correlations across tumour lines were found for T1 and T2 versus fractional tumour water content (p < 0.001) as well as for T1 and T2 versus fraction of necrotic tumour tissue (p < 0.05). Tumours with high fractional water contents had high values of T1 and T2, probably caused by free water in the tumour interstitium. Fractional water content is correlated to interstitial fluid pressure in tumours, high interstitial fluid pressure being indicative of high vascular resistance. Tumours with high fractional water contents are thus expected to show regions with radiobiologically hypoxic cells as well as poor intravascular and interstitial transport of many therapeutic agents. T1 and T2 decreased with increasing fraction of necrotic tumour tissue, perhaps because complexed paramagnetic ions were released during development of necrosis. Viable tumour cells adjacent to necrotic regions are usually chronically hypoxic. Tumours with high fractions of necrotic tissue are thus expected to contain significant proportions of radiobiologically hypoxic cells. Consequently, quantitative 1H-nmr imaging has the potential to be developed as an efficient clinical tool in prediction of tumour treatment resistance caused by hypoxia and/or transport barriers for therapeutic agents. However, much work remains to be done before this potential can be adequately evaluated. One problem is that high fractional tumour water contents result in longer T1 and T2 whereas high fractions of necrotic tumour tissue result in shorter T1 and T2; i.e. the two parameters which are indicative of treatment resistance contribute in opposite directions. Another problem is that the correlations for T1 and T2 versus fraction of necrotic tumour tissue are not particularly strong.

The hypoxic cytotoxin SR 4233 increases the effectiveness of radioimmunotherapy in mice with human non-Hodgkin's lymphoma xenografts

PURPOSE

To determine if either the hypoxic cell radiosensitizer etanidazole (SR 2508) or the hypoxic cytotoxin SR 4233 could improve the effectiveness of radioimmunotherapy.

METHODS AND MATERIALS

LC4 (an IgG1 monoclonal antibody directed toward malignant T cells) and MB-1 (an irrelevant isotype-matched control antibody) were injected intraperitoneally into severe combined immunodeficient phenotype mice with human cutaneous T cell lymphoma xenografts in order to determine the distribution of the antibodies in the tumors and normal tissues as a function of time. Computerized-pO2-histography was used to measure the median oxygen tension in the tumors. Tumor-bearing mice were treated with: (a) LC4; (b) 90Y-LC4; (c) 90Y-MB-1; (d) whole body irradiation delivered via an external 137Cs source; (e) etanidazole and 90Y-LC4; (f) SR 4233 and 90Y-LC4; (g) etanidazole; and (h) SR 4233. An additional group of mice received no treatment and served as controls. A tumor growth delay assay was used to assess the effectiveness of the different treatment regimens.

RESULTS

LC4 accumulated in the tumors to a significantly greater extent than MB-1 (p < 0.001) and reached a peak concentration in the tumors 5 days post-injection. The human cutaneous T cell lymphoma xenografts had a relatively low median oxygen tension. LC4 by itself was able to produce a minor decrease in tumor size (control vs. LC4; p = 0.001). 90Y-LC4 produced greater tumor growth delay than LC4 alone (LC4 vs. 90Y-LC4; p = 0.01); however, the Yttrium-90 caused neutropenia and weight loss. The 90Y-labeled tumor-specific and non-specific antibodies both exerted greater tumor growth delay than externally delivered whole body irradiation (p < or = 0.03) due to preferential uptake of the antibodies in the tumors. Etanidazole and SR 4233 by themselves did not significantly inhibit the growth of the tumors. Etanidazole did not significantly enhance the tumor growth delay produced by 90Y-LC4 (90Y-LC4 vs etanidazole and 90Y-LC4, p = 0.13). SR 4233, on the other hand, did enhance the tumor growth delay produced by 90Y-LC4 (90Y-LC4 vs. SR 4233 and 90Y-LC4, p = 0.046). The neutropenia and weight loss caused by 90Y-LC4 were exacerbated slightly (< 10%) by the administration of SR 4233.

CONCLUSIONS

A first generation hypoxic cytotoxin, SR 4233, was able to enhance the tumor growth delay produced by radioimmunotherapy in severe combined immunodeficient phenotype mice with human cutaneous T cell lymphoma xenografts.

Metabolic response of AH13r rat tumours to cyclophosphamide as monitored by pO2 and pH semi-microelectrodes

The composition of the microenvironment has an important influence on the cellular response to cytotoxic agents. Using pH and pO2 semi-microelectrodes, we have monitored metabolic changes in AH13r rat tumours as a function of time after subcurative chemotherapy. Prior to therapy, tumours contained large areas considered hypoxic (mean pO2 approximately 4 mmHg) and are characterised by a marked accumulation of acidic metabolites (mean pH 6.65). Administration of cyclophosphamide (40 mg/kg body weight) resulted in tumour regression to 15% of pretreatment volumes and a growth delay of 12 days. Concomitant with volume reduction, tumours became reoxygenated (mean pO2 approximately 7 mmHg), with maximum values being reached within 2-4 days, paralleled by a shift of pH to more alkaline values (0.17 U on average). These changes coincided with the development of subtotal necrosis. During early tumour regrowth, the pH and pO2 histograms returned to control values. These data corroborate and extend the results of previous studies in which noninvasive techniques had been applied for the monitoring of treatment-induced metabolic changes in malignant tumours in vivo. In addition, these results support the notion that the effectiveness of anticancer therapy might be improved by selecting and scheduling therapeutic agents in consideration of physiological changes caused by preceding courses of treatment.

Measurement of oxygen diffusion distance in tumor cubes using a fluorescent hypoxia probe

Oxygen diffusion distance was measured in solid tumor "cubes" prepared by excising the tumor from the mouse and incubating 1-2 mm sided tumor cubes in spinner culture flasks with fluorescent drugs (AF-2 or DM113) which bind to hypoxic cells. After incubation, frozen sections were prepared and examined for AF-2 or DM113 binding using a fluorescence image processing system. Alternatively, cubes were stained with a slowly penetrating fluorescent dye, Hoechst 33342, prior to disaggregation and measurement of AF-2 binding using flow cytometry. The distance from the cube surface to the AF-2 stained region (i.e., the thickness of the unstained rim) was used as an indication of oxygen diffusion distance, which depends on the square root of the tumor oxygen consumption rate. Oxygen diffusion distance was dependent on tumor type, external oxygen concentration, and temperature during incubation with AF-2, but was unaffected by tumor size up to 1.2 gm or position of the cube within the tumor. Oxygen diffusion distances (in planar geometry) measured for cubes prepared from SCCVII, RIF-1, Lewis lung or WiDr tumors were 107, 123, 153 and 193 microns, respectively. For SCCVII and WiDr tumors, the percentage of blood vessels separated by a distance greater than double the mean oxygen diffusion distance was used as an indication of the hypoxic fraction.

Repopulation between radiation fractions in human melanoma xenografts

Rate of tumor repopulation between radiation fractions was studied using two human melanoma xenograft lines (E.F. and V.N.). Tumors were given five radiation fractions under hypoxic conditions in vivo and clonogenic cells survival was measured in vitro after the last radiation fraction. Dose-response curves were established for interfraction times of 12, 24, 36, and 48 hr by varying dose per fraction from 4.2 to 11.2 Gy. Assuming an oxygen enhancement ratio of 2.8, these doses corresponded to doses of 1.5 to 4.0 Gy under aerobic conditions, that is, clinically relevant doses per fraction were used. The dose-response curves were nearly parallel and were shifted to the right with increasing interfraction time, demonstrating significant repopulation between the radiation fractions. Iso-effect analyses showed that additional radiation doses of 2.0 +/- 0.6 Gy/day (E.F.) and 2.2 +/- 0.6 Gy/day (V.N.), corresponding to doses of 0.7 +/- 0.2 Gy/day (E.F.) and 0.8 +/- 0.2 Gy/day under aerobic conditions, would be required to compensate for the repopulation. These doses were equivalent to the surviving clonogenic cells showing doubling times of 40-50 hr (E.F.) and 30-40 hr (V.N.) during the treatment period. The radioresponsiveness of the two melanoma xenograft lines was also measured. Tumors in air-breathing mice were given from 5 to 15 daily fractions of 2.0 Gy and cell survival curves were established in vitro. Theoretical survival curves, calculated from SF2 in vitro and rate of repopulation during fractionated irradiation in vivo, agreed fairly well with the measured survival curves. This suggested that the radioresponsiveness of the melanoma xenograft lines was governed by two main parameters: (a) cellular radiation sensitivity and repair capacity and (b) rate of repopulation between radiation fractions.

Should tumors be clamped in radiobiological fractionation experiments?

In radiobiological fractionation experiments tumors are often clamped during irradiation. This is done not only to facilitate the interpretation of the data by eliminating the influence of reoxygenation but also to avoid uncontrolled changes in the hypoxic fraction that are caused by anesthesia or stress from physical restraint. In this study it is shown that clamping of tumors during irradiation also affects repair, repopulation, and redistribution. From these results it is concluded that clamping of tumors during fractionated irradiation treatment does not appear to be an adequate measure to elucidate the mechanisms that determine tumor response to radiotherapy. Even some of the fundamental concepts of tumor radiobiology might contain some uncertainties, since they are often based on data resulting from clamped tumors.

Variations of the hypoxic fraction in the SCC VII tumors after single dose and during fractionated radiation therapy: assessment without anesthesia or physical restraint of mice

Variations of the hypoxic fraction (HF) after single dose (13 Gy or 4 Gy) and during fractionated (5 fractions of 4 Gy, 1 or 2 fractions per day) radiation therapy were studied in SCC VII tumors implanted subcutaneously in the hind legs of C3H/He/Jms mice using the paired survival curve method. Whole-body irradiation was delivered to tumor-bearing mice without anesthesia or physical restraint, because both are known to increase the HF artificially. The HF decreased after a single 13 Gy dose in a biphasic fashion: extremely rapidly within 1 hr and comparatively slowly during the following 12-72 hr. On the other hand, nearly no fall of HF was observed in 24 hr following a single 4 Gy dose. Also, reoxygenation was found to occur more rapidly in the interfraction period as the number of fractions of 4 Gy increased irrespective of differences of interfraction time. However, the HF just before each radiation fraction was significantly higher than the pretreatment level for both fractionated regimens. Thus, the reoxygenation patterns observed after single low and high doses of irradiation were different from each other, and reoxygenation in each interfraction period did not always proceed in a similar manner to that after single low dose irradiation. Reoxygenation was facilitated as fractionated radiation therapy proceeded, but it was not sufficient for the HF to remain at a level comparable to that before irradiation.

Reoxygenation in a rat rhabdomyosarcoma tumor following X-irradiation

The paired survival curve technique was used to characterize the rate at which the fraction of hypoxic cells in rat rhabdomyosarcoma R-1 tumors returns to the preirradiation value of 37% following a single dose of 225-kVp X rays. Tumors were administered a conditioning x-ray dose of 15-Gy, followed at 0, 3, 6, 12, 24, or 48 hr by a 5-Gy, 10-Gy, or 15-Gy dose of X rays under air-breathing conditions or under hypoxic conditions produced by nitrogen-gas asphyxiation 5 min prior to irradiation. Cellular surviving fractions were determined by the tumor excision assay following in vivo irradiation. From the ratio of the survival fractions measured for tumor cells from air-breathing and hypoxic animals, the fraction of hypoxic cells was determined as a function of time postirradiation. These results indicated that immediately following a 15-Gy dose of X rays, essentially 100% of the viable cells remaining were hypoxic. The tumors reoxygenated rapidly, returning to the preirradiation level of 37% during the first 6 hr postirradiation.

Histological study of the regrowth of a human melanoma xenograft exposed to single dose irradiation

The aim of this investigation was to find the location and the histological characteristics of cells that were the source of tumour regrowth after single dose irradiation. A human melanoma xenograft was irradiated with a single dose of 25.0 Gy which gives local tumour control in nearly 50% of treated animals. Serial histological sections were made from tumours removed during the first two weeks after irradiation. During the first week the perivascular organization of tumour parenchyma disappeared and the central part of the tumour became necrotic. The occurrence of vascular stasis, thrombosis and endothelial cell changes indicated that radiation injury to the vascular system was involved in the disappearance of the tumour cords. Tumour cells that remained histologically intact were located in subcapsular areas. The incidence of normal mitotic figures increased, and the fraction of abnormal mitoses and the incidence of micronuclei decreased 5 to 7 days before macroscopical regrowth was usually detected. It is concluded that cells which are the source of tumour regrowth were located in the subcapsular areas at the time of irradiation. Radiation injury to the tumour vascular system was an important factor in the necrotization of the tumour centre after treatment.

Hypoxic fractions of human tumors xenografted into mice: a review

Transplanted tumors in rats and mice have been used extensively in radiobiology studies examining the biological basis and therapeutic implications of hypoxia in solid tumors, and in studies evaluating new agents and regimens which may circumvent the radioprotective effects of hypoxic cells. This use of rodent tumors assumes that data obtained using rapidly-growing transplanted tumors arising in inbred rodents can be extrapolated meaningfully to the treatment of primary and/or metastatic lesions in heterozygous humans. The studies reported here examine one facet of this critical assumption by comparing the hypoxic fractions of transplanted rodent tumors with those of human tumor cell lines xenografted into immune deficient mice. No significant differences were found between the hypoxic fractions of the xenografts and those of mouse tumors. This finding eliminates several possible bases for predicting that the oxygenation of human tumors might be systematically different from that of the rodent tumors often used in preclinical radiobiology studies, and supports the hypothesis that human tumors contain hypoxic cells which may be critical in determining their response to therapy.

PLD-repair in human melanoma xenografts following single dose and fractionated irradiation

PLD-repair following single dose and fractionated irradiation was studied in vivo using five human melanoma xenograft lines. Tumours given single graded radiation doses were excised immediately after or 24 h after the radiation exposure for assay of cell survival in vitro. All melanoma lines showed PLD-repair after single dose irradiation: the PLD-repair factors, i.e. the ratio of the Do values for tumours excised 24 h after and immediately after irradiation, ranged from 1.2 +/- 0.1 to 1.4 +/- 0.1. PLD-repair following fractionated irradiation was studied by giving tumours seven fractions of 2.0 Gy over 7 days and then, after an interval of 24 h, single graded radiation doses in the range 6-21 Gy. Cell survival was assayed in vitro immediately after or 24 h after the last radiation exposure. The Do values as well as the surviving fractions were approximately equal after immediate and delayed cell seeding, i.e. none of the melanoma lines showed significant PLD-repair after fractionated irradiation. The lack of PLD-repair after fractionated irradiation was possibly a consequence of radiation-induced recruitment of quiescent tumour cells into the cell cycle. Consequently, PLD-repair is probably not a major cause of failure in the radiation therapy of malignant melanoma when treated with 2.0 Gy fractions.